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2021 roadmap for sodium-ion batteries
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ROADMAP
2021 roadmap for sodium-ion batteries
Nuria Tapia-Ruiz1,2,∗, A Robert Armstrong2,3,∗, Hande Alptekin4, Marco A Amores2,5, Heather Au4,
Jerry Barker6, Rebecca Boston2,7, William R Brant8, Jake M Brittain9,10, Yue Chen2,11,
Manish Chhowalla2,12, Yong-Seok Choi2,13,14, Sara I R Costa1,2, Maria Crespo Ribadeneyra4,
Serena A Cussen2,5,7, Edmund J Cussen2,5,7, William I F David9,10, Aamod V Desai2,3,
Stewart A M Dickson2,15, Emmanuel I Eweka16, Juan D Forero-Saboya17, Clare P Grey2,18,
John M Griffin1,2, Peter Gross2,7, Xiao Hua2,5, John T S Irvine2,15, Patrik Johansson19,20,
Martin O Jones2,21, Martin Karlsmo19, Emma Kendrick22, Eunjeong Kim2,3, Oleg V Kolosov2,11,
Zhuangnan Li2,12, Stijn F L Mertens1,2, Ronnie Mogensen8, Laure Monconduit23,24,
Russell E Morris2,3,25, Andrew J Naylor8, Shahin Nikman1,2, Christopher A O’Keefe2,18,
Darren M C Ould2,18, R G Palgrave2,13, Philippe Poizot26, Alexandre Ponrouch17, Stéven Renault26,
Emily M Reynolds2,21, Ashish Rudola6, Ruth Sayers6, David O Scanlon2,13,14,27, S Sen2,13,
Valerie R Seymour1,2, Begoña Silva´n1,2, Moulay Tahar Sougrati23,24, Lorenzo Stievano23,24,
Grant S Stone28, Chris I Thomas2,7, Maria-Magdalena Titirici4, Jincheng Tong2,12, Thomas J Wood9,
Dominic S Wright2,18 and Reza Younesi8
1 Department of Chemistry, Lancaster University, Lancaster LA1 4YB, United Kingdom
2 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom
3 EastChem School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, United Kingdom
4 Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom
5 Department of Chemical and Biological Engineering, University of Sheffield, Sheffield S1 3JD, United Kingdom
6 Faradion Limited, The Innovation Centre, 217 Portobello, Sheffield S1 4DP, United Kingdom
7 Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, United Kingdom
8 Department of Chemistry—Ångström Laboratory, Uppsala University, Box 538, SE-75121 Uppsala, Sweden
9 ISIS Neutron and Muon Spallation Source, STFC Rutherford Appleton Laboratory, Harwell, Oxford OX11 0QX, United Kingdom
10 Inorganic Chemistry Laboratory, University of Oxford, Oxford OX1 3QR, United Kingdom
11 Department of Physics, Lancaster University, Bailrigg, Lancaster LA1 4YB, United Kingdom
12 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United Kingdom
13 Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom
14 Thomas Young Centre, University College London, Gower Street, London WC1E 6BT, United Kingdom
15 School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, United Kingdom
16 Amte Power Ltd, 153A Eastern Avenue, Milton Park, Oxford OX14 4SB, United Kingdom
17 Institut de Ciéncia de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Catalonia, Spain
18 Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom
19 Department of Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden
20 Alistore-ERI, CNRS FR 3104, 15 Rue Baudelocque, Amiens 80039, France
21 Science and Technology Facilities Council, North Star Avenue, Swindon SN2 1SZ, United Kingdom
22 School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham BT15 2TT, United Kingdom
23 ICGM, University Montpellier, CNRS, Montpellier, France
24 RS2E, CNRS, Amiens, France
25 Department of Physical and Macromolecular Chemistry, Charles University, Hlavova 8, Prague 128 43, Czech Republic
26 Université de Nantes, CNRS, Institut des Matériaux Jean Rouxel, IMN, F-44000 Nantes, France
27 Diamond Light Source Ltd, Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, United
Kingdom
28 Amte Power Ltd, Denchi House, Thurso Business Park, Thurso, Caithness KW14 7XW, United Kingdom
∗ Authors to whom any correspondence should be addressed.
E-mail: n.tapiaruiz@lancaster.ac.uk and ara@st-andrews.ac.uk
Keywords: sodium ion, batteries, cathodes, electrolytes, anodes, energy materials
Abstract
Increasing concerns regarding the sustainability of lithium sources, due to their limited availability
and consequent expected price increase, have raised awareness of the importance of developing
alternative energy-storage candidates that can sustain the ever-growing energy demand.
Furthermore, limitations on the availability of the transition metals used in the manufacturing of
cathode materials, together with questionable mining practices, are driving development towards
more sustainable elements. Given the uniformly high abundance and cost-effectiveness of sodium,
© 2021 The Author(s). Published by IOP Publishing Ltd
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
as well as its very suitable redox potential (close to that of lithium), sodium-ion battery technology
offers tremendous potential to be a counterpart to lithium-ion batteries (LIBs) in different
application scenarios, such as stationary energy storage and low-cost vehicles. This potential is
reflected by the major investments that are being made by industry in a wide variety of markets and
in diverse material combinations. Despite the associated advantages of being a drop-in replacement
for LIBs, there are remarkable differences in the physicochemical properties between sodium and
lithium that give rise to different behaviours, for example, different coordination preferences in
compounds, desolvation energies, or solubility of the solid–electrolyte interphase inorganic salt
components. This demands a more detailed study of the underlying physical and chemical
processes occurring in sodium-ion batteries and allows great scope for groundbreaking advances in
the field, from lab-scale to scale-up. This roadmap provides an extensive review by experts in
academia and industry of the current state of the art in 2021 and the different research directions
and strategies currently underway to improve the performance of sodium-ion batteries. The aim is
to provide an opinion with respect to the current challenges and opportunities, from the
fundamental properties to the practical applications of this technology.
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Contents
1. Cathode materials 6
1.1. Layered transition-metal oxides: P2 | O3 materials 6
1.2. Anion redox layered transition-metal oxides 9
1.3. Polyatomic anion-based materials 12
1.4. Prussian blue and analogues 15
1.5. Organic materials 18
2. Anode materials 22
2.1. Hard carbon 22
2.2. Titanium-based oxides 25
2.3. Alloy and conversion materials 28
2.4. 2D transition-metal dichalcogenides 31
2.5. Organic materials 33
3. Computational discovery of materials 36
4. Electrolytes and SEI engineering 39
4.1. Organic electrolytes: salts 39
4.2. Organic electrolytes: solvents 41
4.3. Ionic liquid electrolytes 44
4.4. The SEI in Na-ion batteries 47
4.5. SSEs for Na-ion batteries 50
5. Testing protocols for Na-ion batteries 53
6. Advanced characterisation 56
6.1. Neutron characterisation of battery materials 56
6.2. Solid-state NMR for the characterisation of Na-ion batteries 59
6.3. Nanoscale characterisation of the local physical properties of Na-ion batteries,
electrodes, and interfaces 62
6.4. Dissolution benchmarking of sodium-ion electrodes 65
7. Scale-up and manufacturing 68
7.1. Cell performance and requirements 68
7.2. Applications and scale-up: manufacturing 71
8. Industrial targets and techno-economic analysis 74
8.1. Industrial targets 74
8.2. Techno-economic assessment 78
References 80
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Introduction
Nuria Tapia-Ruiz
Department of Chemistry, Lancaster University, Lancaster LA1 4YB, United Kingdom
The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom
Na-ion batteries (NIBs) promise to revolutionise the area of low-cost, safe, and rapidly scalable
energy-storage technologies. The use of raw elements, obtained ethically and sustainably from inexpensive
and widely abundant sources, makes this technology extremely attractive, especially in applications where
weight/volume are not of concern, such as off-grid energy storage, load levelling, and starting, lighting, and
ignition batteries, which amount to a potential worldwide demand of≈1 TWh.
Given the similarities between the fundamental working principles and materials used in NIBs and the
well-known rechargeable lithium-ion batteries (LIBs), the swift appearance of this technology in the market
is to be expected, based on the use of existing battery manufacturing lines. Furthermore, NIBs can be stored
and transported at 0 V, reducing the costs associated with expensive shipping and safety risks, and thus, they
can be commercialised worldwide.
Despite their tremendous potential, only a limited number of companies, such as Faradion (UK), Tiamat
(Europe), Altris AB (Europe), HiNa (China), and Natron Energy (USA) are devoted to sodium battery
development. These manufacturers do not follow a consensus on the choice of sodium chemistry (e.g.,
positive electrodes can be made from layered oxides, Prussian blue analogues, or vanadium-based
polyanionic compounds, and used with aqueous and non-aqueous electrolytic solutions), allowing for
market/application diversification, and importantly, highlighting the immense scope of NIB materials
research by developing new chemistries for positive and negative electrodes and electrolytes.
The multiple research prospects of NIBs have been recognised by the Faraday Institution, the UK’s
independent institute for electrochemical energy storage research, which launched NEXt-GENeration
NA-ion batteries (NEXGENNA) [1] in October 2019 as part of its research portfolio of post-lithium
batteries. The NEXGENNA consortium combines a multidisciplinary and diverse team of academics at the
forefront of research in their respective fields, and the chief players in the UK Na-ion battery industry, with
the ambition to deliver a revolution in the development of high-performance, cost-competitive, safe, and
long cycle-life Na-ion batteries for stationary and low-cost transportation applications.
This 2021 NIB roadmap contains contributions from the NEXGENNA consortium and external project
partners, as well as other relevant academics in the NIB field. The various contributions to this roadmap are
divided into eight main research themes, ranging from fundamental experimental and computational science
to large-scale industrial processing and techno-economic metrics, i.e. (a) cathode materials; (b) anode
materials; (c) computational discovery of materials; (d) electrolytes and the solid–electrolyte interphase
layer; (e) testing protocols; (f) advanced characterisation techniques, (g) manufacturing and scale-up, and
(h) industrial targets and technoeconomics, totalling eight sections.
Sections 1, 2 and 4 contain an overview of the most relevant to date inorganic and organic cathode,
anode, and electrolyte materials in NIBs, with a focus, where posNIBle, on industry-relevant families of
electrodes and electrolyte materials, but without losing sight of novel emerging materials with enough
potential to become the next generation of candidates. In parallel, supercomputing architectures and more
accurate and cost-effective quantum chemistry approaches (covered in section 3) will undoubtedly help us to
discover new materials in broad compositional ranges which otherwise would consume extensive
research time.
We include a subsection on the not-so-well-known solid–electrolyte interphase (SEI) layer in section 4),
given its critical but extremely challenging role (due to the soluble nature of its components) in the cycling
stability and performance of NIBs. The unstable behaviour of the SEI may result in unreliable data
interpretation and thus, electrochemical measurement protocols need to be established to avoid discarding
potentially interesting candidate materials or making erroneous interpretations of results (section 5).
Furthermore, characterisation of the SEI under operando or in-situ conditions will be crucial to provide
good insight into the behaviour of this layer, and to draw correlations between electrolyte and electrode
formulations, the physicochemical properties of the SEI, and battery performance. Similarly, understanding
the long- and short-range processes occurring in electrode materials during battery operation is vital to
characterise the charge-compensation mechanisms that occur in the different families of NIB materials. A
relevant assortment of advanced in-situ/operando characterisation techniques—all used in the NEXGENNA
advanced characterisation platform, is described in section 6. Sections 7 and 8 cover relevant manufacturing
aspects of NIBs. This brings into play the use of full cells, which requires additional features that are not well
considered in half-cell studies, including a good synergy between the cathode, anode, and electrolyte
components, optimal electrode formulation, and architecture, among others. Finally, we provide an overview
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of the current UK and international industry targets for NIBs, with special emphasis on the future
performance metrics targeted by Faradion in the next five years with their technology, followed by a
technoeconomic analysis. To permit a comparison, the performance and economic targets of NIBs are
presented alongside commercial Li-ion battery data.
We expect that this 2021 roadmap for NIBs will be reference reading material for researchers working in
the areas of electrochemical science, battery chemistry, energy materials, advanced characterisation,
manufacturing, and computational methods. We also recommend other recent reviews on the topic of NIBs
to the readers of this roadmap (e.g. [2–4]). Furthermore, we hope to inform key industries about current
trends to encourage further investment in this technology, and influence decision-making from funding
agencies and private investors.
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1. Cathode materials
1.1. Layered transition-metal oxides: P2 | O3materials
Jake M Brittain1,2, Thomas J Wood1 and William I F David1,2
1 ISIS Neutron and Muon Spallation Source, STFC Rutherford Appleton Laboratory, OX11 0QX, United
Kingdom
2 Inorganic Chemistry Laboratory, University of Oxford, Oxford OX1 3QR, United Kingdom
Status
LIBs have not only transformed our digital electronic world but have also enabled the future electrification of
transport and domestic energy storage. NIBs can extend and expand these opportunities, offering safer and
more environmentally sustainable options. Recent significant developments indicate that Na-ion batteries
can achieve performances that are similar to their Li-ion counterparts.
The cathode materials in all the highest-performing LIBs have two-dimensional (2D) layered
crystallographic structures that are based on the chemical substitutions of the prototypical Li-ion cathode,
LiCoO2. The most promising Na-ion battery cathode materials possess a similar but richer layered geometry.
This additional structural richness is a consequence of the larger ionic radius of the sodium ion, which not
only adopts the elongated octahedral geometry observed in Li-ion cathode materials, but also crystallises
into a trigonal prismatic configuration. This added structural versatility not only offers the opportunity to
optimise for power and energy applications but also provides research directions for improved NIB
performance that include increased capacity, higher charge and discharge rates, superior cyclability, extended
lifetimes, and improved recycling.
Many P2 and O3 layered materials have been investigated for their potential use as Na-ion cathodes.
Figure 1 not only indicates the combinatorial opportunities for cation permutation but also highlights the
substantial variations in electrochemical performance. The symbols P2, O3, and P3 in figure 1 follow the
notation developed by Delmas et al [5] to describe the prototypical crystal structures that can be adopted by
layered Na-ion cathode materials. P and O respectively denote the coordination of trigonal prismatic and
elongated octahedral Na-ion sites between (MO2) metal-oxide sheets.
Figure 2 illustrates the two principal structural types. Assuming the sheets are stacked perpendicular to
the z-direction, the oxygen atoms in each layer within the xy-plane can occupy three possible positions,
denoted A, B and C. Each MO2 sheet contains two layers of oxygen atoms that are paired above and below
the metal layer. The number of pairs required to describe the NaxMO2 layered structure completes Delmas’
notation. An elongated octahedral Na-ion coordination is typically adopted for x = 1 materials. This,
combined with three oxide pairs per unit cell (AB CA BC) gives the rhombohedral phase its O3 notation.
The standard, single -metal O3 materials in the literature include NaNiO2 [7], NaFeO2 [8], and the
nickel-manganese hybrid, NaNi1/2Mn1/2O2 [9]. Trigonal prismatic Na-ion coordination is preferred for
0.6 < x < 0.7 and these phases typically form, with a two-pair repeating unit (AB BA), leading to a P2
notation for the hexagonal phase. Na2/3MnO2 [10], Na0.7CoO2 [11], and Na2/3Ni1/3Mn2/3O2 are the standard
undoped P2 materials described in the literature [12].
Current and future challenges
Many of the current and future challenges associated with layered Na-ion batteries are linked to the
development of higher-performance cathode materials. However, the added opportunities afforded by the
two distinct sodium environments within the P2 | O3 layered materials result in additional structural
complexities upon battery cycling. The atomic, nano- and meso-structural progressions of these structural
changes are not yet adequately characterised. A fundamental and comprehensive understanding of these
structural evolutions over many thousands of cycles is necessary to improve cycling, mitigate degradation,
and increase lifetime performance. Significant variations in performance, however, can result from subtle
substitutions. The observation, for example, that the addition of small percentages of redox inactive ions,
such as Mg2+ and Ti4+, results in substantial changes in electrochemical performance, is today an unresolved
issue, but looking forward, it will be both a combinatorial challenge and a commercial opportunity [13, 14].
A focus on power performance will best be met by P2-based systems because of the unique sodium-ion
environment, which enables facile ionic mobility through rectangular faces between adjacent trigonal
prismatic environments. This diffusion mechanism is not available to Li-ion batteries, suggesting that the
optimisation of P2-based sodium batteries may lead to higher diffusion coefficients and consequently higher
charge and discharge rates than for Li-ion batteries.
The best battery capacity performances will be associated with O3-based systems, which have the highest
sodium stoichiometries. Mimicking the best over-lithiated Li-ion batteries by over-sodiation may lead to a
further improvement in performance. The requirements of intermediate applications that involve both
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Figure 1. An electrochemical summary of various layered oxide sodium-ion cathode materials, comparing voltage, capacity and
energy density. All measurements are in half-cell systems. [6] John Wiley & Sons.
Figure 2. Crystal structures of P2 (left) and O3 (right) materials. Sodium ions are in trigonal prismatic (purple) and elongated
octahedral (orange) geometries, respectively. The metal layers are shown in grey, with their directionality indicated by the grey
arrows.
power and energy optimisation can be addressed by the development of hybrid P2 | O3 materials [15].
Figure 1 indicates that there is a large combinatorial space of cation permutations that can be explored to
improve power and energy densities, cycle life, and recyclability to meet the optimisation requirements of
different industrial applications.
Perhaps most significantly, layered P2 | O3 cathodes offer the opportunity to develop sustainable ‘green’
batteries that are only manufactured from earth-abundant materials. LIBs face ethical challenges associated
with mining the metal-containing ores as well as economic issues related to material scarcity and the
environmental impact of recycling toxic chemical constituents. Fabricating high-performance NIBs from
safe, earth-abundant materials containing, for example, a combination of sodium, iron, manganese,
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magnesium, and titanium ores mitigates environmental impacts and offers a global opportunity for the
deployment of safe and affordable batteries to both the developing and the developed parts of the world.
Advances in science and technology to meet challenges
The principal research challenges facing the development of improved P2 | O3 Na-ion batteries are centred
around the combinatorial chemical exploration of the fundamental angstrom- to micron-scale structures of
layered transition-metal cathodes, and how these multiscale structures change throughout the battery
lifetime. Synchrotron x-ray and neutron scattering facilities play a central role in determining the relationship
between crystal structure and electrochemical performance. Addressing the combinatorial aspects of cation
permutation along with point-by-point measurements of structural transitions over multiple charge and
discharge cycles necessitates the development of, and access to, increasingly precise, rapid, high-resolution,
in-situ and operando diffraction, spectroscopy, and thermodynamic and electrochemical experiments. For a
detailed fundamental understanding of battery behaviour from fabrication to failure, these experimental
measurements must be accompanied by rapid computational chemical calculations.
Concluding remarks
NIB performance has improved substantially over the past decade, and layered P2 | O3 cathode technologies
are in leading positions in many of the battery metrics. The close structural similarities between P2 | O3
materials and layered lithium-ion cathodes have helped inform their scientific and commercial development.
However, the increased structural complexity of P2 | O3 materials, resulting from the ability of sodium ions
to adopt both trigonal prismatic and elongated octahedral coordination, creates many challenges but also
offers significant opportunities. The challenges result from the combinatorial complexity of the cation
permutations. The opportunities are to develop NIBs that are competitive with the best LIBs. One of the
major goals is to ‘go green’ by producing high-performance NIBs, with a minimal carbon footprint, from
sustainable and abundant resources.
Acknowledgments
The authors wish to acknowledge Faradion Ltd. for the opportunity to study its commercial battery systems
and work in detail on its layered sodium cathode materials. The authors wish to thank Dr Jerry Barker and
Dr Richard Heap in particular, for their productive and illuminating scientific discussions. This work was
supported by the Faraday Institution (Grant No. FIRG018).
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1.2. Anion redox layered transition-metal oxides
Eunjeong Kim1,3, Begoña Silva´n2,3, Nuria Tapia-Ruiz2,3 and A Robert Armstrong1,3
1 EastChem School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, United Kingdom
2 Chemistry Department, Lancaster University, Lancaster LA1 4YB, United Kingdom
3 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom
Status
Oxygen redox-based reactions have gained interest among NIB researchers, due to the prospect of obtaining
extra capacity at high voltages beyond those available from conventional transition-metal redox reactions.
The study of anionic redox reactions in sodium layered cathode materials is especially relevant, due to their
intrinsically lower energy density when compared to their Li counterparts. As with oxygen reactions observed
in Li-rich cathode materials for LIBs, these are typically kinetically limited, and thus, research efforts are
devoted to improving their reversibility [16]. To date, most studies in this area have focussed on P2-type
materials (where Na occupies trigonal prismatic sites), accounting for over 60% of the reported compounds.
Anionic redox reactions can be described in terms of band structure, where the overlap of bonding
(M–O) and antibonding (M–O)∗ orbitals leads to a continuum band which enables the removal of electrons
from oxygen ions (figure 3(b)). In contrast, separated M d and O p bands may be observed in pure cationic
redox-driven compounds (figure 3(a)). From a proof-of-concept perspective, the formation of strong
covalent bonds and consequently favourable M–O orbital overlap was first demonstrated with 4d (Ru) and
5d metals (Ir). However, for practical reasons, oxygen redox research has naturally shifted toward the use of
low-cost and non-toxic 3d metals, such as manganese-rich compounds. When up to one third of the
manganese is replaced by another more electronegative and active element (e.g., Ni2+, Fe3+, Co3+), the
strong overlap with the O 2p states favours electron transfer, often via a reductive coupling mechanism, while
the substituted inactive elements (e.g., Li+, Zn2+, Mg2+) allow O 2p states to be placed at the top of the
valence band [17]. Furthermore, the more covalent Mn–O bonds tend to stabilise the crystal structure,
thereby avoiding unwanted phase transitions while reducing the inherent structural instability caused by
Jahn–Teller active Mn3+ ions and parasitic disproportionation reactions. A challenge encountered in these
materials is their large hysteresis, typically explained by the irreversible migration of certain metals
(including Mn) towards the Na layers or within the MO2 planes [18]. This has been partially alleviated by
designing materials with undercoordinated oxygen ions (i.e., with either ‘direct’ or ‘indirect’ vacancies –
mobile Li dopants that leave the transition-metal layers upon charging), where the presence of non-bonding
oxygen states translates into a new non-bonding O band (ONB) above the (M–O) band, where extra electrons
can be located (figure 3(c)). These vacancies need to be well dispersed and thus isolated from a nearby
vacancy to avoid the formation and release of O2 gas, which is favoured by the under-bonded O ions [19].
Current and future challenges
While oxygen redox represents an effective way of enhancing energy density in NIBs, limited reversibility on
cycling needs to be overcome for most oxygen redox-active compounds. In general, the structural
transformation from P-type to O-type structures is induced by gliding of the transition-metal layers upon
excess deintercalation of Na ions [20]. The lattice stress caused by this phase transition as well as the partial
irreversibility of the structural transition are often responsible for a reduction in oxygen redox activity. In
addition, cation migration, either from transition-metal layers to Na layers, or within transition-metal layers,
may lead to large voltage hysteresis. The local coordination around oxygen anions has a direct effect on the
voltage at which the discharge plateau occurs [19]. Moreover, increasing the upper cutoff voltage beyond
∼4.1 V accelerates the removal of cations from the transition-metal layers, which may result in lattice oxygen
loss due to uncoordinated oxygen atoms [21]. Finally, since oxygen redox occurs in the high-voltage region
where classical organic electrolytes are unstable, detrimental reactions associated with the decomposition of
electrolytes are also a significant problem.
In order to gain deep insights into oxygen redox behaviour in cathode materials, spectroscopic
techniques have been heavily used (figure 4). It is important to select appropriate techniques to distinguish
bulk vs. surface phenomena, since cathode-electrolyte reactions tend to dominate the surface chemistry.
Laboratory X-ray photoelectron spectroscopy (XPS) has been widely applied to probe chemical changes in
oxygen anions, although the technique is limited to surface information. Furthermore, synchrotron X-ray
techniques, such as hard and soft X-ray absorption spectroscopy (XAS), hard X-ray photoelectron
spectroscopy (HAXPES), and resonant inelastic x-ray scattering (RIXS) have been exploited for bulk
characterisation. In addition to the depth of analysis, the different techniques provide a range of information.
In this regard, soft XAS and RIXS are promising tools for demonstrating oxygen redox, whilst hard XAS
provides indirect evidence of the participation of oxygen anions in the charge-compensation mechanism.
Additional laboratory-based characterisation techniques, such as differential electrochemical mass
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Figure 3. Band structures of layered oxides with (a) cationic redox, (b) anionic redox resulting from strong M–O covalent bonds
and (c) anionic redox resulting from undercoordinated oxygen ions. Reprinted by permission from Springer Nature Customer
Service Centre GmbH: Springer Nature, Nature Energy [16] © 2018.
Figure 4. Schematic illustration of the depth of analysis for laboratory XPS, HAXPES, XAS, and RIXS. Used under licence (http://
creativecommons.org/licenses/by/4.0/). Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer
Nature, Communications Chemistry [23] © 2019.
spectrometry, Raman spectroscopy, electron paramagnetic resonance spectroscopy (EPR) and sequential
cyclic voltammetry (CV) are often in demand to decouple oxygen redox from electrolyte decomposition.
However, ex situ techniques may be unreliable due to the reactivity and instability of the phases formed at
high states of charge. This has led to the recent suggestion that the behaviour of Li-rich layered materials may
be derived from the Mn4+→Mn7+ + 3e− redox reaction, the resulting species being too unstable to be
observed [22]. Nonetheless, there is a strong indication of oxygen redox activity, based on a combination of
ex-situ, in-situ and operando techniques.
Advances in science and technology to meet challenges
As mentioned above, the biggest problems encountered in cathode materials exhibiting oxygen redox are
capacity fade combined with large voltage hysteresis. Among the exceptional materials which exhibit
suppressed voltage hysteresis are Na2Mn3O7 [24], with ordering between Mn and intrinsic transition metal
vacancies and NaxMn1−yLiyO2 [19], with a ribbon superstructure. These examples confirm that well-defined
ordering in the transition-metal layers mitigates detrimental cation migration and phase changes. In the case
of NaxMn1−yLiyO2, repopulation of mobile Li at the original site is critical to the retention of the
superstructure. To exploit these advantages, the design of materials containing transition-metal vacancies
and/or dopants that migrate reversibly represents a promising route towards reversible oxygen redox. This
strategy should be coupled with an investigation of the mobility of transition-metal vacancies and the
associated electronic structure changes. The development of ultra-high-resolution XAS and RIXS will be
necessary to describe the complete mechanism of oxygen redox by distinguishing different oxidised oxygen
species and to provide a detailed account of transition-metal charge transfer. As an alternative strategy, the
activation of materials via partially reversible oxygen redox may be used to develop materials with enhanced
cycling performance not accessible by direct synthesis. This approach may be considered analogous to the
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activation of the Li2MnO3 component of Li-rich layered materials for LIBs [25]. To suppress oxygen release,
surface engineering methods, such as the use of coatings, namely Al-based oxides and phosphates, may be
implemented. Typically, these coatings react with the cathode material to stabilise the surface by creating a
composite of the active material and the coating compounds [26]. Furthermore, electrolytes that are stable at
very high potentials must be sought to enable an accurate study of the cathode material without interference
from parasitic electrolyte reactions. To address this, several approaches can be adopted based on high-voltage
LIBs, namely the use of electrolyte additives or super-concentrated commercial salts in organic solvents. On
the other hand, organic solvents can be replaced by more stable ionic liquids or solid electrolytes, although
these approaches are far from large-scale commercialisation [27].
Concluding remarks
The anion redox reaction represents a major challenge, in terms of both understanding and successful
exploitation. However, the potential rewards in terms of increased energy density, together with the
realisation that its occurrence is more widespread than hitherto believed, make it an important area for
future development. This may take the form of materials that undergo irreversible changes as a result of
oxygen redox and thereby generate phases with enhanced electrochemical performance. Alternatively, the
developments outlined in the previous section could lead to the preparation of phases exhibiting truly
reversible oxygen redox reactions stabilised by surface coatings and/or the availability of novel electrolytes.
Progress will be assisted by the further development and increasing availability of advanced characterisation
techniques, such as RIXS, which may be combined with HAXPES and XAS to obtain information at a range
of depths.
Acknowledgment
This research is funded by the Faraday Institution (Grant No. FIRG018).
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1.3. Polyatomic anion-based materials
Xiao Hua1,3, Peter Gross2,3, Edmund J Cussen1,2,3 and Serena A Cussen1,2,3
1 Department of Chemical and Biological Engineering, University of Sheffield, Sheffield S1 3JD, United
Kingdom
2 Department of Materials Science and Engineering, The University of Sheffield, Sheffield S1 3JD, United
Kingdom
3 The Faraday Institution, Quad One, Harwell Campus, OX11 0RA, United Kingdom
Status
Na-ion battery cathodes based on polyatomic anion insertion compounds offer a rich structural chemistry
and a variety of crystallographic architectures. This class of materials provides enhanced structural stability,
improvements in safety, and the ability to tune the resulting redox potentials, offering opportunities to
extend the operating voltages to above 3.5 V vs. Na+/Na [28, 29]. In addition to single polyatomic anion
species, a search is under way for mixed anion systems that can further extend redox potential improvements.
This ability to increase the cell voltage is associated with a phenomenon known as the inductive effect, where
a judicious choice of polyatomic anion species can influence the metal–oxygen bonding, thereby tuning the
resulting operating voltage. This is illustrated in figure 5 for a series of polyatomic anion cathodes used
in NIBs.
Vanadium-containing phosphates and fluoride phosphates are among the most promising Na-ion
cathodes, owing to their structural stability and high ion mobility as well as the ability to tune the operating
voltage by varying the electronegativity of the polyatomic anions. NASICON-type Na3V2(PO4)3, for
example, can reversibly intercalate two Na ions at a potential of 3.4 V. The mixed fluoride phosphate species
Na3V2(PO4)2F3 displays a higher average operating voltage of 3.9 V, with a high theoretical capacity of
128 mAh g−1 (based again on a two-Na-ion exchange) [29]. Solving the crystal structures of these materials,
which can display polymorphism, has presented an interesting challenge to the battery materials community;
however, synchrotron x-ray diffraction studies have unveiled subtleties in polymorphic Na-ion ordering.
These can potentially change materials’ ionic conductivity, highlighting the critical need for in-depth
structural analyses to rationalise the relationship between the crystal structure and the electrochemical
properties of these materials [30, 31]. The move towards oxygen-substituted derivatives of Na3V2(PO4)2F3
has led to improvements in energy density; for example, Ceder et al have demonstrated that Na3V2(PO4)2FO2
can accommodate an additional fourth Na-ion at∼1.6 V to become Na4V2(PO4)2FO2 [32]. This exciting
development, which is due to the strong screening effect of the O2− ions, elevates the specific energy to
≈600 Wh kg−1 and presents an intriguing research challenge of increasing this redox potential.
Current and future challenges
A careful consideration of reaction conditions presents ongoing opportunities to arrive at a set of design
principles that will enable the direct targeting of a desired polymorph or particle architecture. In a recent
example, Yang et al discovered a new fluoride phosphate which crystallised in two forms, depending on the
temperature chosen: a high-calcination-temperature Pbca phase and a lower-calcination P3 phase of
Na5V(PO4)2F2 [33]. Both polymorphs exhibit high operating voltages (3.5 V and 3.4 V for the Pbca and P3
phases, respectively). In this context, a topic for future research is the exploration of dopants that could allow
access to the V4+/V5+ redox couple and further elevate the achievable capacity.
The replacement of vanadium by earth-abundant and cheaper alternatives remains an ongoing research
challenge. In the work referenced earlier, Ceder et al also demonstrated that the replacement of vanadium by
aluminium (Na3V2−zAlz(PO4)2F3, where z ⩽ 0.3) also permits additional Na insertion at low voltages of
∼1.3 V [32]. While this is impractically low, it does suggest that doping could provide a rational route to
disrupting sodium ordering and improving ion mobility. The triplite phase LiFeSO4F (which also exists in
the tavorite polymorph) has exhibited the highest redox potential for an Fe2+/Fe3+ redox couple at 3.9 V.
Interestingly, the synthesis of a sodium triplite-analogue, NaFeSO4F, has been achieved through the chemical
delithiation and subsequent chemical sodiation of the triplite phase LiFeSO4F [34]. The monoclinic C/2
symmetry, with a random distribution of Na and Fe across the two metal octahedral M1 and M2 sites
enabled by the sodiation approach, provides a redox potential of 3.7 V and a capacity close to the theoretical
limit (≈138 mAh g−1) at 0.01 C. Sluggish diffusion kinetics and low coulombic efficiencies suggest that
structural distortions are occurring; further research is required to overcome this.
Developments in synthetic control have also allowed access to high-quality, open framework alluaudite
structures, which offer high potentials and energy densities, but are often difficult to achieve with
stoichiometric control. Plewa et al have developed a new solution-based synthesis of Na2FeM(SO4)3
(M = Fe, Mn, Ni) where the presence of impurity phases is reduced by the addition of excess sodium
12
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
Figure 5. (a), (b) Increasing electronegativity of selected polyatomic anions, demonstrating the tuning of the redox potential
through the inductive effect. (c) Crystal structures of NaFePO4 and Na2FeP2O7, where iron is shown in blue, sodium in green,
phosphorus in purple, and oxygen in orange.
precursor and a glucose additive to preserve a reducing environment [35]. In situ XRD and
thermogravimetric analysis monitoring provide effective optimisation guides during synthesis.
Investigations of mixed polyanionic materials continue to offer opportunities to tune the redox potential,
as described by a recent report by Tang et al on the combined electronic inductive effect of oxalate and
sulphate in the Na2Fe(C2O4)SO4.H2O phase [36]. Further investigations of dopant effects in this system
could provide a pathway to improvements in electrochemical performance.
Advances in science and technology to meet challenges
Reliably determining the crystal structures of unknown phases remains a challenge. A recent development
embracing a materials genomic approach was reported by Khalifah et al, who have focused on the unsolved
structure of the low-temperature phase of Na3V2(PO4)2F3 [37]. A plethora of enumerated plausible trial
structures (of the order of 3000), informed by reasoned Na-ion ordering, were fitted to experimental data by
Rietveld refinement, which enabled researchers to identify a set of closely related structural models that could
reliably describe the experimental powder pattern. Subsequent density functional theory (DFT) and
structural analyses then pointed to the A21am space group as the proposed structure. This analytical
approach represents an exciting development in the determination of structures, particularly for emerging
cathode chemistries, where complex cation ordering exists.
Insights from operando investigations continue to shed light on the complex structural and redox
processes ongoing in operating batteries. One recent report which exemplifies this is a study ofvanadium
oxyfluorides, Na3V2(PO4)2F3−yOy (0⩽ y ⩽ 2), by Croguennec et al [38]. By following the desodiation
process via operando x-ray absorption spectroscopy (XAS) and combining these observations with insights
gleaned from DFT calculations, it is possible to examine the complex changes in the average local
coordination environment of vanadium across a series of compositions where vanadium redox centres
coexist. Ongoing upgrades to synchrotron beamline facilities will expand our ability to probe the highly
complex and rapid processes that occur during charge and discharge, which will enable us to better
understand materials’ structure–performance relationship.
Concluding remarks
Polyatomic anion cathodes continue to provide a route to increase operating potential by capitalising on the
inductive effect, as well as by conferring structural stability to the material. There is a trade-off in terms of
the capacities that can be achieved with the heavier polyatomic anion component (in comparison to layered
13
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
oxides), but recent work to accommodate additional Na-ions through, for example, the application of
suitable dopants, is intriguing. In addition to synthetic developments that allow access to high-quality
samples and new polymorphs, particle size and shape tailoring will enable further improvements, such as
higher power densities. In addition to a further examination of the effect of dopants, challenges remain in
developing suitable coatings and additives that will bestow stability under ambient and high-voltage
conditions.
Acknowledgments
The authors gratefully acknowledge the support of the ISCF Faraday Challenge project NEXGENNA (Grant
No. FIRG018), the EPSRC (EP/N001982/2), and the University of Sheffield for support.
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1.4. Prussian blue and analogues
William R Brant and Ronnie Mogensen
Department of Chemistry—Ångström Laboratory, Uppsala University, SE-75121 Uppsala, Sweden
Status
Prussian blue analogues (PBAs), with the general formula NaxM[M′(CN)6]1−y.zG (whereM andM′ are
transition metals, most commonly Mn, Fe, or Ni, y is the number of [M′(CN)6]n− vacancies, and G is a
neutral guest, such as H2O), are highly porous positive-electrode materials for NIBs (top of figure 6). Since
PBAs are comprised of abundant elements and can be produced at low cost, they provide the best
price-to-performance ratio [39] for a positive electrode. Thus, PBAs capitalise on the key advantages of NIBs
as cheap, sustainable, and safer alternatives to lithium-ion technology. They have become viable cathode
materials for sodium batteries within the last decade. During this time, the greatest challenge facing the
implementation of PBAs as positive electrodes has been the ability to produce highly-sodiated compounds
that are structurally stable during cycling. The structural stability and maximum capacity of the material are
severely reduced by the presence of the vacancies (y) introduced during synthesis. Due to the loss of a
reducible cation (M′) and the necessary charge compensation required to offset the negative charge of
[M′(CN)6]n−, 1 mol% of vacancies can lead to a 3–4 mol% reduction in the maximum sodium content.
Resolving this issue requires alternate synthetic approaches to the more traditionally applied co-precipitation
reactions. For someM cations, in particular, Fe, co-precipitation results in a high vacancy concentration. In
2014, You et al reported a way of producing a low-vacancy PBA from a single precursor via an acid
decomposition synthesis route for the first time [40]. This approach was subsequently used to produce the
best-performing PBA to date, which delivered 120 mAh g−1 for over 800 cycles of charging at 0.5 C
(75 mA g−1) and discharging at 2 C (300 mA g−1) [41]. Since then, extensive effort has been invested in the
exploration of differentM combinations, particle sizes, morphologies, and coatings, and improvements have
been seen for either initial capacity or total cycling life, but not both [42]. Ultimately, the goal for PBAs is to
achieve capacities of more than 160 mAh g−1 which are stable for up to 10 000 cycles. Eight thousand cycles
have been demonstrated using aqueous electrolytes (figure 6 top) [43]; however, the resulting capacity is
frequently limited to 60–80 mAh g−1. Conversely, higher capacities (>80 mAh g−1) can be achieved using
non-aqueous electrolytes due to their wider potential stability window although, the number of cycles
obtained is often an order of magnitude lower.
Current and future challenges
Two connected issues are plaguing the development of PBAs: moisture sensitivity and limited reversibility
when cycling more than one atom of sodium per formula unit. Capacity fading in PBAs has generally been
attributed to limited electronic conductivity, the presence of water, and phase transitions of the highly
sodiated compounds. Several methods have been employed to overcome their limited electronic
conductivities, such as growing PBAs directly on carbon nanotubes, enabling stable cycling down to−25 ◦C
[45]. Structural stability has seemingly been a greater challenge to overcome, since phase transitions in
sodium-rich PBAs are heavily influenced by their water content. Wang et al [41] first demonstrated that,
when completely dehydrated, Fe-based PBAs adopt a rhombohedral structure with a unit cell volume that
decreases by≈18%. This work was followed up by Rudola et al [44], who demonstrated that obtaining a
moisture-free PBA depends heavily on the temperature and pressure used for drying and that, if exposed to
air, a hydrated structure forms within minutes (bottom of figure 6). Further, a 16% volume change between
cubic Fe[Fe(CN)6] and rhombohedral Na2Fe[Fe(CN)6] leads to a lower cycling stability compared to the
hydrated compound, if the capacity and voltage window are limited in the hydrated case. Water also increases
the average voltage output [46], which may be perceived as an advantage. However, the high voltage plateau
may lie beyond the upper stability limit of water (3.9 V vs. Na+/Na). Finally, it has been shown that
desodiated PBAs have a lower affinity for water [47]. Consequently, charging the cell runs an additional risk
of releasing water into the electrolyte. Thus, the influence of water on electrochemical performance is
complicated. The presence of water is beneficial for ensuring structural stability by minimising volume
contraction during cycling. However, it limits the maximum capacity that can be utilised, as water is either
released from the structure and reacts with the electrolyte, or is oxidised inside the material at high potential.
Solving this challenge will require further consideration of the role of the electrolyte. Electrolyte–PBA
interactions will be the greatest hurdle to overcome in the future, as all synthetic design choices (figure 7) will
have to determine the optimum electrolyte combination. The synthetic possibility space for PBAs is large,
and so one must take a holistic view when designing a cell based on this material.
15
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
Figure 6. Top: structure of Na1.45Ni[Fe(CN)6]0.87 along with long-term cycling performance in an aqueous cell cycled between
0–1 V vs. Ag/AgCl at 100 mA g−1. Reprinted from [43], Copyright (2020), with permission from Elsevier. Bottom: structural
transitions introduced due to the removal and uptake of water in Na2Fe[Fe(CN)6], as determined from x-ray diffraction data [44].
Figure 7. Schematic showing some of the possible combinations used to produce PBAs for sodium-ion batteries and hypothetical
electrolyte combinations. The steps are: synthesis, where composition, core–shell morphology, particle size, and shape can be
varied, or the PBA can be grown directly onto carbon supports; handling, where one can choose to dehydrate the PBA; finally,
post-synthesis modifications, where new guest species, surface coatings, or aging are used. Two pathways described in the text are
highlighted in blue and orange.
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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
Advances in science and technology to meet challenges
Optimal material design choices for PBAs are ones that address multiple issues while maintaining stable or
synergetic interactions with the electrolyte. More advanced material modifications can be achieved, given a
rigorous understanding of how a modification influences other synthetic steps and compatibility with other
material features or the electrolyte. For example, if one elects to utilise the hydrated PBA material, a surface
coating, such as reduced graphene oxide [48] might be designed. This could keep the water from leaving the
structure and prevent soluble electrolyte decomposition products from entering the structure, while
permitting the passage of sodium. Thus, structural integrity would be maintained while preventing side
reactions with the electrolyte (figure 7, blue path). However, this solution will only function withM andM′
cations for which the voltage remains below the stability limit of water [46]. Alternatively, the electrolyte may
intentionally include small molecules, such as acetonitrile, which co-insert with sodium, fulfilling a similar
role to water in aqueous-based electrolytes (figure 7, orange path). Any potential solution must be balanced
against all other synthetic choices, such that the potential benefit that the modification brings is not offset by
a lack of compatibility with either the electrode or the electrolyte. Thus, a paradigm shift in research mindset
must be made, away from simply aiming to boost performance with a single modification, and towards
deconstructing the fundamental interactions within the material and between the material and its
environment.
Concluding remarks
PBAs are a challenging class of compounds to investigate for NIBs due to the highly tuneable nature of their
synthesis and the tangled web of chemical interactions within the material and with its environment.
However, their potential as positive electrodes in beyond lithium-ion systems is clear, in terms of both cost
and performance. Thus, the grand challenge will be to build a robust fundamental understanding of these
complex interactions, enabling material developments which foster synergies that can be established using
new advanced electrolytes.
Acknowledgments
The authors appreciate the financial support of Energimyndigheten for Project: 45517-1 and of the Strategic
Research Area StandUp for Energy.
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1.5. Organic materials
Philippe Poizot and Stéven Renault
Université de Nantes, CNRS, Institut des Matériaux Jean Rouxel, IMN, F-44000 Nantes, France
Status
Most of the materials studied for use in NIBs are typically inorganic, inherited from the abundant knowledge
developed in the field of LIBs over the last four decades. Although sodium can indeed be considered to be
highly abundant on Earth and related to low-pollution resources, that may not be the case for other elements
comprising inorganic electrode materials (especially metals). It is now well documented that commercial
(inorganic-based) batteries involve a large consumption of energy from ‘cradle to grave’ and generate
environmental pollutants, including hazardous waste and greenhouse-gas emissions [49, 50]. Hence, the
environmental impacts of large-scale battery use as well as recycling and re-use represent major challenges
that will require further attention in the future. Organic electrode materials (OEMs) may offer novel and
more sustainable storage solutions, allowing eco-design at the scale of the active material itself [51, 52]. As
they are composed of naturally abundant elements (C, H, O, N, and even S) coupled with possible
biosourcing, OEMs can be produced using low-energy and low-cost synthetic chemical routes. In addition,
organic molecular engineering gives access to great structural diversity and easy control of functional groups.
After use, OEMs can be thermally disposed of, with energy recovery, or eventually recycled [53]. In practice,
two reversible electrochemical storage mechanisms can be used (alone or combined), which are characterised
either by conventional cation charge compensation (in n-type systems) or anion charge compensation
(p-type), as illustrated in figure 8. In principle, organic materials exhibit flexible crystal structures containing
discrete entities, and bind to each other by weak interactions, such as van der Waals bonding, making the
accommodation of large cations such as Na+ or anions possible without much spatial hindrance. The first
electrochemical assessments of OEMs for NIBs date from the 1980s, but occurred at a time when lithium
chemistry quickly appeared superior, halting further development until recently. Despite their early stage of
development, during the past few years, a considerable amount of effort has also been devoted to promoting
organic NIBs alongside the development of inorganic electrode materials [54, 55]. Nevertheless, further
improvements are necessary, especially in terms of organic cathode materials (i.e., positive electrodes),
because most of the investigated compounds suffer from limited electrochemical performance.
Current and future challenges
In principle, n-type cathode materials are required to assemble NIBs, because the electrode reaction enables
reversible cation uptake/removal. To date, the best reported electrode materials that can achieve excellent
stability over thousands of cycles are based on polymerised perylene diimides (table 1, entry 1) [57, 58],
because aromatic diimides are robust redox-active organic moieties. However, they suffer from limited
working potential values which usually do not exceed 2.5 V vs. Na+/Na. Moreover, they are usually prepared
in their oxidised (desodiated) state (left-hand side of the half-reaction at the positive electrode in table 1,
entries 1–3), which restricts their practical use in sodium half-cells or requires a pre-reduction (sodiation)
step before full-cell assembly (table 1, entries 2 and 3) [59, 60]. One of the few examples of a sodiated
cathode material is the tetrasodium salt of 2,5-dihydroxyterephthalic acid (Na4DHTA), which has been
successfully introduced in a symmetric Na-ion cell configuration [61]. Again, the output voltage of the cell
was restricted to about 2 V due to the low working potential of the cathode side (2.3 V vs. Na+/Na). This
potential limitation for non-polymer n-type OEMs (such as Na4DHTA salt) is notably related to the high
electron density in the redox-active organic centre. The DHTA organic ligand exhibits at least two permanent
negative charges through the two inactive carboxylate functional groups. The latter serve as a simple and
effective way to prevent dissolution in aprotic electrolytes [51] but their electron-donating behaviour drags
the average redox potential of the whole compound down. Although polymerisation is another excellent
strategy for preventing dissolution in the electrolyte, the high-molecular-weight polymers are unlikely to be
chemically prepared in their reduced (sodiated) state. It is worth noting that the low operating potential of
n-type organic cathodes seems related to sodium chemistry itself. As first underlined by Song et al [62], the
electrochemical uptake/removal of Na+ in OEMs is negatively shifted (by several hundreds of mV) in
comparison with its lithium counterpart beyond the expected standard potential gap between Na+/Na and
Li+/Li redox couples. Moreover, the larger radius of Na+ sometimes induces sluggish electrochemical
kinetics as well as lowered utilisation of the available redox-active moieties, giving rise to diminished
reversible capacity values. In short, there is currently a lack of efficient and robust sodiated organic cathodes
with high working potential.
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Figure 8. (a) Key reversible redox-active organic systems at the molecular level, together with representative examples exhibiting
redox rocking from the aromatic to the quinoid form. X/Y can be N, O, S, P, pi-systems, carboxylate, anhydride, or amide
functional groups; and R, R′ are potentially integrated within the same cyclic structure. Note that p- and n-type structures
correspond to systems A and B, respectively, according to Hünig’s classification [56]. (b) Corresponding cell configurations
obtained by experimenting with both n- and p-type systems, shown during the discharge process. Reprinted with permission
from [52]. Copyright (2020) American Chemical Society.
Advances in science and technology to meet challenges
Efforts must be focused on molecular design to obtain high-potential sodiated cathode materials. This would
probably entail the identification of innovative n-type organic functional groups possibly guided by
simulation and modelling. Furthermore, potential gain could still be possible by considering electronic
effects acting on a redox centre of interest. Thus, the redox potential is classically increased at the molecular
level (discrete entities) by introducing electron-withdrawing groups. More recently, it was demonstrated for
a lithiated organic cathode that the electronic effects acting on a redox-active organic centre can be very
efficiently mitigated in the solid state thanks to structural effects [66]. Alternatively, other OEMs can operate
as cathodes thanks to p-type functionality with anionic compensation (figure 8). The latter, which is a redox
system rarely encountered in inorganic materials, typically works at a higher potential than n-type redox
centres, making these organic systems very appealing for Na-based batteries. For instance, several organic
p-type polymer families are known to be stable in organic liquid electrolytes. Poly(2,2,6,6-
tetramethylpiperidinyloxy methacrylate) or PTMA represents a relevant example in this field which exhibits
bipolar (n/p) electroactivity. From its initial state (centre of the half-reaction at the positive electrode in
table 1, entry 5), PTMA can electrochemically accommodate a cation at 2.3 V vs. Na+/Na (in reduction) or
an anion at 3.6 V vs. Na+/Na (in oxidation) [63]. However, in the case of the closely related
poly(2,2,6,6-tetramethylpiperidine-4-yl-1-oxyl vinyl ether) or PTVE, the cation insertion mechanism was
shown to be unstable, and steady capacity was obtained with anion insertion alone (table 1, entry 6) [64].
Such p-type functionality gives access to high-potential materials which can be paired with n-type negative
OEMs in a dual-ion cell configuration (table 1, entry 7) [65]. P-type OEMs are usually synthesised in their
reduced state, making them suitable for a positive electrode without requiring an electrochemical
pre-treatment. As monomers, they do not possess permanent negative charges to prevent dissolution in
aprotic electrolytes, but polymeric p-type OEMs seem to be the most appropriate choice of positive electrode
for Na-based batteries to achieve a combination of high potential and cycling stability. However, as
underlined in figure 8(b), the electrolyte is a reservoir of ions for charge compensation within electrode
materials which causes a depletion of ionic carriers during operation, requiring a larger volume of
electrolyte. Strictly speaking, such a cell configuration cannot be considered to be a NIB, although promising
electrochemical data upon cycling have been reported (table 1, entry 7).
19
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
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,
10
00
[5
8]
2
1
M
Na
PF
6
in
PC
≈1
.6
73
,7
0%
,2
0
[5
9]
3
0.
6
M
Na
PF
6
in
D
EG
D
M
E
≈1
.2
5
13
7,
85
%
,3
0
[6
0]
4
1
M
Na
Cl
O
4
in
EC
/D
M
C
(v
/v
1:
1)
≈2
.0
20
4,
76
%
,1
00
[6
1]
5
1
M
Na
Cl
O
4
in
EC
/D
EC
(v
/v
1:
1)
≈3
.0
22
2,
92
%
,1
00
[6
3]
6
1.
5
M
Na
Cl
O
4
in
EC
/D
EC
(v
/v
1:
1)
≈3
.4
5
12
7,
94
%
,1
00
[6
4]
7
Sa
tu
ra
te
d
Na
PF
6
in
D
O
L/
D
M
E
(v
/v
1:
1)
≈1
.8
18
0,
85
%
,5
00
[6
5]
20
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
Concluding remarks
Further work is primarily required to develop high-potential and efficient sodiated organic cathode materials
beyond p-type materials for NIB technology. A better understanding of the charge/discharge storage
mechanisms is also crucial in order to design functional batteries, because each OEM seems to behave
differently upon cycling. The use of computational techniques could be very useful in this quest. Organic
compounds will not be able to supplement inorganic materials in terms of volumetric energy density, since
their densities do not exceed 1.5 g cm−3, i.e. four to five times less than inorganic electrode materials.
Furthermore, another drawback lies in their tendency to dissolve in liquid organic electrolytes, although the
use of solid electrolytes could solve this limitation. Conversely, the implementation of soft organic polymer
electrodes enables the fabrication of flexible and thin electrochemical devices which are potentially
environmentally friendly. Finally, aqueous organic NIBs can be perceived as the ultimate solution for
promoting non-flammable, cost-effective, and green batteries even if only hybrid systems are reported at the
moment.
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2. Anode materials
2.1. Hard carbon
Heather Au, Hande Alptekin, Maria Crespo Ribadeneyra and Maria-Magdalena Titirici
Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom
Status
Hard carbon materials are the most popular choice for NIB anodes. They are produced from oxygen-rich
precursors which cannot be converted into graphite, no matter how high the carbonisation temperature.
Hard carbons consist of randomly oriented graphitic domains possessing a higher interlayer spacing than
graphite (i.e. >0.34 nm) connected by disordered carbon regions with different curvatures. In between these
disordered and more ordered domains, hard carbons have ‘closed’ pores, meaning they cannot be probed by
gas sorption. The larger the size of the graphitic crystallites, the larger the pore size in between. In addition to
curvatures and edge sites, hard carbons also contain the remaining heteroatoms (mainly oxygenated groups).
Such a complex mixture of crystalline and disordered domains with defects allows sodium diffusion
pathways and sodium storage sites. The first structure of ‘graphitisable’ vs. ‘non-graphitisable’ carbons was
described by Rosalind Franklin in a seminal paper entitled ‘The interpretation of diffuse x-ray diagrams of
carbon’ published in Acta Crystallographica in 1950 (figure 9(a)) [67]. Interestingly, until today, the exact
structure of hard carbon has remained unknown, as this depends on the precursor and carbonisation
conditions, which result in materials with varying interlayer spacing, crystallite size, pore domains and edge
termination. Such different structures store Na ions in very different ways. Hence, it is no surprise that there
is much debate in the literature about the mechanism of Na insertion into hard carbons. Figure 9(b) provides
a typical Na insertion load curve (sodiation curve in a half-cell built with hard carbon vs. metallic Na)
showing a typical sloping region between 1 V and 0.1 V and a plateau region below 0.1 V. The first theory by
Stevens and Dahn using the ‘house of cards’ model for hard carbon attributed the slope to Na insertion into
expanded graphitic domains, and the plateau to pore filling by Na (figure 10) [68]. Yet the situation is more
complex than this, as hard carbons can exhibit different features depending on the choice of precursor,
pre-treatment, and carbonisation conditions and temperature.
Current and future challenges
The biggest challenge for hard carbons is to improve their storage capacity to match or even overtake that of
graphite in LIBs (>372 mAh g−1). The practical measured capacities for hard carbons today (mostly in coin
cells vs. metallic Na) vary from 200 to 450 mAh g−1; most of the values reported in the literature average
around 300 mAh g−1. While the theoretical energy density of the commercial graphite used in LIBs can be
calculated using the LiC6 formula, for Na in hard carbons, the situation is far more complicated, due to the
heterogenous nature of hard carbons and the unknown storage mechanism, which depend on many
structural parameters. Therefore, all the structural and morphological features of hard carbons must be
clearly determined using multiple characterisation techniques applied in concert to accurately determine the
exact features, such as the interlayer spacing (XRD), defects (Raman, positron annihilation spectroscopy),
functional groups (XPS), the nature of ordered vs. disordered domains and the interconnectivity between the
two (x-ray pair distribution function), opened vs. closed pores and pore sizes (SAXS, SANS, gas adsorption).
All these features must be precisely correlated with the electrochemical performance, which should be
accurately determined in half cells, but also using three-electrode configurations [71] to better understand
the fundamental electrochemistry happening at the working anode. Finally, operando characterisation
techniques should be employed to understand the structural and morphological changes occurring during
the intercalation of Na ions into hard carbons. Some of these techniques involve the use of XRD/SAXS to
determine the change in the interlayer spacing as proof of Na intercalation [68, 72, 73], operando XPS [74] to
determine the interaction of Na with different functional groups, in situ electrochemical TEM to observe the
interaction of Na with defects [75], operando Raman spectroscopy to understand the changes in
graphitic/amorphous structure upon (de)sodiation [76], operando NMR [77–79], MRI [80] and EPR [81] to
understand the Na chemical state/deposition inside pores at less than 0.1 V. Another major challenge in the
development of high-performance hard carbon anodes is to understand the solid–electrolyte interface [82,
83] and how the anode’s structural features, in combination with the electrolyte of choice, affect the
thickness and, more importantly, the ionic conductivity of the SEI. To probe this, we recommend operando
AFM [84] as well as electrochemical impedance spectroscopy coupled with quartz crystal microbalance
measurements [85], corroborated by electrochemical mass spectroscopy [86] and operando FTIR [87]. To
understand macroscale phenomena related to the anode volume change, operando x-ray computed
tomography [88] and electrochemical dilatometry are good options [89, 90]. Once the fundamentals are
understood, both in an individual electrode as well as during its operation, these could be used to build
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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
Figure 9. (a) Franklin’s representations of non-graphitising and graphitising carbons [67]. Reproduced with permission from
[69]. (b) A typical discharge curve for hard carbons shows two distinct regions: the sloping region and the plateau region.
Figure 10. Schematic illustration of the proposed sodium storage mechanisms; stack thicknesses and plane sizes are not to scale,
and defects and curvatures are not shown [70]. Reproduced from [70] with permission of The Royal Society of Chemistry.
models for further optimisation based on DFT [91, 92], molecular dynamics (MD) [93], and machine
learning [94] to predict the optimal structural and morphological parameters.
Advances in science and technology to meet challenges
Several research groups have made great advances in developing hard carbon anodes with outstanding
performance from low-cost precursors, based on a careful synthetic design combined with advanced ex-situ
and in situ characterisation [95]. Most debates to date have arisen from contradictory Na storage
mechanisms in such carbons. Currently, there are several published theories: (a) the ‘intercalation-filling’
model: Na+ ions intercalate into graphitic layers in the sloping region, and insert themselves into nanopores
in the plateau region [96]; (b) the adsorption-intercalation model: Na+ ions adsorb at the surface or defect
sites of the carbon electrodes within the sloping region and intercalate into the graphitic layers within the
plateau region [97]; (c) the adsorption-filling model: in the sloping region, Na+ ions adsorb at the defect
sites while filling the nanopores in the plateau region [72], and (d) the ‘three-stage’ model: defect adsorption
of Na+ ions takes place in the sloping region, but in the plateau region, the Na+ ions first intercalate into the
graphitic layers and fill the nanopores at the end [98]. More recent investigations by my research team as well
as other authors have gathered stronger evidence to propose an alternative model, with adsorption and
intercalation occurring simultaneously during the sloping region, and pore filling during the plateau region,
all the while keeping in mind the dependence on the structure and pore system of the respective carbons
(figure 10). Great progress has been achieved in using operando characterisation techniques to probe the
mechanisms of different carbon materials; in particular, operando NMR has been used to probe the metallic
nature of Na in the pores [79, 80], SAXS/WAXS [100, 101] has been used to probe the Na interaction with
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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
defects and the importance of interlayer spacing, and x-ray PDF [102] has been used to understand the
connectivity between ordered and disordered domains and the way Na travels throughout the hard carbon
from defects and graphitic domains into the pores. Finally, significant machine learning efforts have recently
been dedicated to the ‘in silico’ design of optimised carbons based on experimental characterisation
techniques [94]. At a commercial level, the Japanese company Kuraray sells KURANODE™ as a hard carbon
for LIBs and NIBs, with good performance, but few structural details. This product is often used as a
benchmark by many researchers developing optimisation strategies for NIBs at the cell level, as well as by
some of the small and medium enterprises developing NIBs.
Concluding remarks
Much progress has been made in developing hard carbon anodes with outstanding performance for NIBs.
Future optimisation could be possible via automation of the synthetic processes using robots coupled with
machine learning. Benchmarking of the best-performing hard carbons would be ideal to unify the efforts of
NIB developers and standardise protocols at an international level.
Acknowledgment
We would like to thank the EPSRC ISCF EP/R021554/2 for financial support to enable new discoveries in
hard carbons for NIBs.
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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
2.2. Titanium-based oxides
Sara I R Costa1,3, Rebecca R Boston2,3 and Nuria Tapia-Ruiz1,3
1 Department of Chemistry, Lancaster University, Lancaster LA1 4YB, United Kingdom
2 Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, United
Kingdom
3 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom
Status
Titanium-based oxides are among the most promising and versatile Na anode materials, due to their low
cost, ease of processing, and non-toxicity. These materials are safer than carbon-based anodes given their
higher operating voltage, which prevents metallic sodium plating. This occurs, however, at the cost of
delivering lower energy density. In most cases, the storage of Na+ ions occurs through a (de)insertion
mechanism driven by the Ti4+/Ti3+ redox couple. The most representative oxides include TiO2, Li4Ti5O12,
Na2Ti6O13, and Na2Ti3O7, which will be briefly described herein.
TiO2-based anodes are a promising avenue of research, due to their high structural stability and
theoretical capacity (335 mAh g−1). Microsized TiO2 is inactive in NIBs; however, reducing the particle size
to the nanoscale activates reversibility, achieving practical capacities of ca. 155 mAh g−1 [103]. The various
TiO2 polymorphs exhibit different electrochemical performances, with anatase (crystalline and amorphous)
generally being the favoured form [104]. Li4Ti5O12 (Fd-3m) has been widely studied as an anode in LIBs due
to its zero-strain behaviour [105]. In NIBs, it delivers a reversible capacity of 155 mAh g−1 by inserting three
Na+ ions into the structure at a relatively low potential (0.9 V) [106]. Among the Na–Ti–O ternary
compounds, Na2Ti6O13 (C2/m) shows high Na+ ion diffusion due to its tunnel structure and small volume
change (<1%) upon Na+ ion (de)insertion. However, it displays an impractical storage capacity of
49.5 mAh g−1 [107]. Control of the morphology, particle size, and exposed crystal facets has significantly
increased the capacity to 109 mAh g−1 (at a rate of 1 A g−1) after 2800 cycles [108]. On the other hand,
Na2Ti3O7 (P21/m) has been widely investigated in recent years due to its low working potential (0.3 V); it
delivers the highest energy density among this family of compounds (figure 11). Na2Ti3O7 exhibits a specific
capacity of 177 mAh g−1 through the reversible insertion of two Na+ ions per formula unit [109].
Current challenges to be overcome before this compound can be used in practical applications include
poor electronic conductivity and sluggish Na+ ion (de)insertion kinetics. These will be described in detail
below. Recent advances in this area include the discovery of a new Na2Ti3O7 triclinic phase (P-1) with a
similar working potential to the monoclinic phase, but with an outstanding capacity retention of 94.7%
(after 20 cycles at 20 mA g−1) [110], and the development of Na2Ti3O7/Na2Ti6O13 hybrids, which combine
the higher storage capacity and ionic conductivity of their respective phases, resulting in excellent cycling
stability and superior rate performance [111].
Current and future challenges
The main factors limiting the commercialisation of Na2Ti3O7 (NTO) in NIBs are the low electronic
conductivity and sluggish Na+ ion (de)intercalation kinetics, which result in poor rate performance and
cycling stability.
The presence of Ti4+ ions (d0) makes NTO an electrical insulator (bandgap energy≈ 3.7 eV) [112],
which hampers Na+ ion diffusion within the crystal structure (DNa+ in NTO is≈ 2–3 orders of magnitude
lower than that in hard carbon). Thus, a major area of research encompasses the development of strategies to
enhance long-term cycling stability, particularly at high current densities. These mainly involve: (a) bulk and
surface structural control through doping and the introduction of defects; (b) nanostructuring; and (c)
composite fabrication with carbon allotropes. Furthermore, the formation of an SEI layer on the surface of
the electrode and the consumption of Na+ ions by carbon additives below 1 V result in low initial coulombic
efficiency (ICE) (40%–60%). These phenomena are particularly critical in full cells, where there is limited
sodium available. Methodologies are therefore required to mitigate the initial capacity irreversibility,
although, to date, such procedures are underdeveloped. Additionally, the high reactivity of Na metal when
used in half cells contributes to the underperformance of NTO due to the formation of an unstable SEI layer,
requiring the use of three-electrode cells and full cells to gain a better insight into the Na contribution to the
overall performance.
Few studies have demonstrated the complex nature of the SEI composition. For example, Casas-Cabanas
et al reported the formation of various SEI inorganic and organic species, such as Na2CO3, alkyl carbonates
and PEO during discharge when using 1 M NaClO4 in an EC:PC electrolyte [113]. Moreover, NaCl and NaF
products have been observed before cycling, as a result of the decomposition of the NaClO4 inorganic
electrolyte salt and the Polyvinylidene fluoride (PVDF) binder, respectively. Alternative inorganic salts, such
25
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
Figure 11. Average voltage and energy density vs. gravimetric capacity for various Ti-based anodes for Na-ion batteries (NIBs).
Energy density is calculated based on the weight of active materials in positive and negative electrodes using their reversible
discharge capacity.
as NaBF4 or NaFSI and binders, such as sodium alginate or sodium carboxymethylcellulose have shown
superior cycling stability, although their performance still lags behind those of their Li analogues [114].
Therefore, the development of next-generation electrolytes and binders is crucial to further improve
battery efficiency. Another issue is the structural relaxation of the fully sodiated phase (Na4Ti3O7), attributed
to the large electrostatic repulsion in the crystal structure [115]. This phenomenon may lead to self-discharge
in full cells containing NTO, compromising cycling stability.
Advances in science and technology to meet challenges
To address the poor electronic conductivity and high Na+ ion migration barriers in NTO, different
approaches, including bulk and surface (micro)structure design and carbon composite fabrication have been
considered.
Elemental doping using small concentrations (<5 at.%) of Nb [116] and elements from the lanthanide
series (e.g. Yb) [117] has been shown to decrease the bandgap energy, enabling superior high-rate
performance in NTO. For example, Nb-doped NTO demonstrated a 0.3 eV reduction in bandgap energy
with respect to pristine NTO [116]. The enhanced electrochemical performance was attributed to
doping-induced crystal structure distortions, together with the formation of Ti3+ ions. Furthermore, oxygen
vacancies, which act as n-type defects, were claimed to contribute to the higher electrical conductivity
observed in Yb-doped NTO [117]. Similarly, surface treatments involving the direct or indirect use of
hydrogen have been used to create oxygen vacancies, conferring high capacities at fast rates [118].
Besides changes in the crystal structure, nanostructuring strategies have been extensively developed,
showing an improved electrochemical performance through more efficient Na+ ion (de)intercalation,
exploiting larger surface areas which can be further enhanced by creating continuous networks of material to
improve electronic conductivity. For instance, Anwer et al used solvothermal synthesis to create 3D networks
of 2D NTO nanosheets [119], generatingmicroflowers (figure 12(a)) with a stable capacity of 108 mAh g−1
(200 mA g−1) over 1000 cycles, while NTO nanotubes 10–20 nm in diameter (figure 12(b)) have shown a
high stable capacity of 100 mAh g−1 [120] (100 mA g−1) over 2000 cycles. Moreover, the use of carbon
coatings [115] and carbon composite mixtures [121] is a common strategy to improve the electronic
conductivity and pseudocapacitive contribution during Na+ ion storage. For instance, nanosheet-coated
carbon shell particles delivered a capacity of 110 mAh g−1 (885 mA g−1) over 1000 cycles [122].
Finally, understanding the impact of electrolyte composition on interfacial electrochemistry is crucial for
developing approaches to further prolong the cycle life of NIBs. Although this area is still in its infancy,
preliminary work has been developed to mitigate SEI film instability by improving electrolyte formulations
(e.g., by the incorporation of fluoroethylene carbonate (FEC) additives) and through surface engineering,
among others.
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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
Figure 12. Examples of nanostructures in Na2Ti3O7. (a) SEM image of microflowers. Reprinted with permission from [119].
Copyright (2017) American Chemical Society. (b) TEM image of nanotubes. [120] John Wiley & Sons.
Concluding remarks
Titanium-based oxides represent a very exciting class of anode materials due to their safety, stability, low-cost,
non-toxicity, and ease of processing. While progress has been made in terms of understanding the factors
limiting the rate performance and cycling stability, much work is still required to realise these concepts in
terms of delivering high energy and power densities in NIBs. We anticipate that the greatest advances will
arise from further enhancements in electronic conductivity (by combining several of the strategies described
here) and further exploration within this family of compounds, including composites. Designing more stable
anode/electrolyte interfaces will also be crucial to advance this area of research. This will require a better
fundamental understanding of the physicochemical properties at the interface, the behaviour and stability of
the fully sodiated phase, and the driving factors behind the poor initial coulombic efficiency.
Acknowledgments
This research is funded by the Faraday Institution (Grant No. FIRG018). N T R would like to thank Lancaster
University for their financial support. R B acknowledges the support of the Lloyd’s Register Foundation and
the Royal Academy of Engineering under the Research Fellowships scheme.
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2.3. Alloy and conversionmaterials
Lorenzo Stievano1,2, Moulay Tahar Sougrati1,2 and Laure Monconduit1,2
1 ICGM, Univ. Montpellier, CNRS, Montpellier, France
2 RS2E, CNRS, Amiens, France
Status
The development of alloy and conversion negative electrode materials for NIBs (which characteristics are
resumed in figure 13) represent a substantial fraction of the work carried out to develop a suitable anode for
application in commercial systems. While, on the one hand, the high reactivity of sodium and its low melting
point (98 ◦C), as well as the formation of dendrites during its electrodeposition, make it significantly less safe
than Li, on the other hand, in contrast to the case of LIBs, graphite does not intercalate Na+ in conventional
electrolytes, and thus alternative anodes have to be found.
Several families of materials capable of reacting electrochemically with sodium have been studied and can
be classified, as for LIBs, according to their reaction mechanism, as either insertion, alloy, or conversion
materials [123]. However, the mechanism and performance of such materials in NIBs cannot be directly
extrapolated from those observed in LIBs.
The alloying reaction is usually observed with p-block elements (Ge, Sn, Pb, P, Sb, Bi, etc) and some
transition metals (e.g., Ag, Zn and Au) which form stable alloys with Na [124]. Alloy materials (AM) usually
involve a multiple-electron exchange, thus leading to very high capacities. However, the formation of the
sodiated alloy causes a strong expansion of volume, even worse than that of LIBs, which represents a real
limitation on their application. The most interesting AMs are surely phosphorus and antimony, which lead
to the formation of Na3P (2596 mAh g−1) and Na3Sb (660 mAh g−1), respectively. Interestingly, silicon,
while very promising in LIBs, is practically inactive in NIBs. While phosphorus delivers a very high
theoretical capacity, antimony has shown the greatest affinity for sodium [125].
The family of conversion materials (CM) mainly includes metal oxides and sulphides, even though other
families, such as metal selenides, carbodiimides, halides, and phosphides have been explored [126], all of
which underwent the so-called conversion reaction [127] with Na. CMs can deliver high specific and
gravimetric capacities with excellent cycling stability, but they also face at least one critical issue related to
their low initial coulombic efficiency or high volume expansion.
The study of the electrochemical and aging mechanisms in both AMs and CMs has been rather
challenging, given that multi-phase systems are often obtained during cycling, and many of the species
formed are amorphous, nanosized, and especially metastable. The development of specific operando tools is
therefore mandatory for the study of these materials [128, 129].
Current and future challenges
The main challenges for AMs and CMs in NIBs are related to volume expansion and both the ionic and
electronic conductivities of these materials. The large reversible volume exchange during charge/discharge
cycles produces mechanical stress, which, on the one hand, rapidly leads to the electrochemical pulverisation
of the electrode material, and, on the other hand, exposes new portions of the metal surface to the electrolyte
at each cycle, leading to continuous electrolyte degradation and thus difficult stabilisation of the SEI.
Moreover, in some cases, such as that of phosphorus, the active material is intrinsically insulating, hindering
electronic percolation in the electrode. Control of these phenomena is necessary for practical applications,
and several strategies have been proposed to reduce the consequences of drastic volume changes and to boost
electronic conduction.
The simplest approach is to optimise the electrode formulation using conducting additives, electrolytes,
and binders. In the case of phosphorus, for instance, the addition of 1D (carbon nanotubes) or 2D
(graphene) nanomaterials to the electrode formulation has been shown to improve the conductivity of C/P
composites, leading to an outstanding capacity retention of 80% over 2000 cycles [130], despite noticeably
impacting capacity and energy performance. Nanostructuring is also a common approach to accommodate
the stress and strain in AMs and CMs without pulverising the electrode, and to improve the ionic and
electronic transport pathways. For instance, tin particles smaller than 10 nm allow a decrease in the strain,
effectively mitigate pulverisation, and also limit their aggregation during cycling. Such nanoparticles,
homogeneously embedded in a carbon matrix, deliver a stable capacity of 415 mAh g−1 after 500 cycles at
1 A g−1 [131]. Even tuning the morphology and porosity of the materials may allow an improvement in the
performance. An interesting way to do so is to transform bulk materials into 2D materials which can buffer
volume expansion during alkali insertion [132]. For instance, phosphorene synthesised by exfoliating
phosphorus shows a rapid charge transfer between less-conductive phosphorene interlayers. In the same
spirit, layered graphene/phosphorene electrodes achieved a reversible capacity of 2440 mAh g−1 (per gram
of P) over 100 cycles [133].
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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
Figure 13. Alloy and conversion anodes compared to other families of materials. Reproduced from [123] with permission of The
Royal Society of Chemistry.
Advances in science and technology to meet challenges
As described above, the main challenge for AMs and CMs is related to volume expansion, which results in the
continuous pulverisation of electrode materials and leads to a steep deterioration in electrochemical
performance. The consequences of the large volume expansion are worsened by continuous electrolyte
reduction, which affects both the coulombic efficiency and the cycle life.
The different strategies presented in the previous section have led to great progress in electrochemical
performance in terms of cycle life, even though a substantial decrease in volumetric energy density cannot be
avoided, especially when composites with inactive materials are prepared. Moreover, if in the early papers,
the effective behaviour of the presented materials was mainly tested in half-cells vs. Na metal, the more recent
study of these materials in realistic full cells against a real cathode material has increased, and is now rather
common (for CM, see [132]). These studies are necessary to correctly evaluate the viability of these materials,
which still cause non-negligible electrolyte decomposition and irreversible sodium trapping, and thus, low
coulombic efficiencies in both the first and the following cycles.
When compared with other types of anode material, up until now, the performance of AMs and CMs has
not beaten that of hard carbon. This is partially due to the large amount of cathode excess required to
compensate for the large initial irreversible capacity and the low coulombic efficiency in cycling. Therefore,
one can conclude that the primary challenge for AMs and CMs has still to be solved [134].
Up until now, hard carbon has remained the most relevant candidate, even though its cost is significantly
higher than that of graphite; in fact, the cost of graphite ranges between 500 and 2000 $/ton, which is similar
to the cost of some of the common precursors of hard carbon (e.g. cellulose or sucrose). Among AMs,
antimony is probably the most promising material, since it possesses a large gravimetric charge storage
capacity (660 mAh g−1) associated with an excellent rate capability and cycle life. Such excellent
performance, exceeding that of Sb in LIBs, should give it good prospects when used in composites with hard
carbon. Indeed, a good compromise between energy and lifetime cost can, in principle, be attained for such
composites.
Concluding remarks
Many studies of many AMs and CMs in NIBs have shown that most of them form Na-rich phases, and thus
produce high specific capacities and energy densities. While volume expansion is often invoked as a possible
limitation of the life span of such electrodes, several studies have shown impressively long cyclabilities,
especially for antimony-based systems. Their durability lies in the elastic softening of the sodiated antimony
phases, which enhances the ability of the electrode to absorb and mitigate the strong volume changes upon
29
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
(de)sodiation [135]. The development of composites of antimony with hard carbon seems, therefore, a viable
alternative. Even though such composites might still not allow NIBs to reach the performance of current
LIBs, their application is still suited to applications where the energy performance is not critical, such as in
stationary energy storage.
Acknowledgments
The authors gratefully acknowledge RS2E and Alistore-ERI for funding their research into Na-ion batteries.
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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
2.4. 2D transition-metal dichalcogenides
Jincheng Tong1,2, Zhuangnan Li1,2 and Manish Chhowalla1,2
1 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United
Kingdom
2 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, Didcot OX11 0RA, United
Kingdom
Status
The relatively large radius and heavy mass of sodium ions require NIB anode materials with large interlayer
spacings. The large volume expansion and pulverisation of some anode materials during cycling also pose
significant challenges for the development of high-performance NIB anodes. The rational stacking of various
2D nanosheets could allow electrode designs that would be mechanically stable over a large number of cycles.
2D materials possess attributes that may help to address these challenges. In particular, metallic 2D
materials with high conductivity may improve the reaction efficiency, and weak van der Waals bonding
between planar layers may allow the fast diffusion of Na+. Furthermore, the interlayer spacing between
nanosheets can increase the contact area with the electrolyte and shorten the Na+ diffusion length.
Electrodes fabricated by the restacking of exfoliated 2D nanosheets are mechanically flexible and can
accommodate large expansions in volume [136]. Among such electrodes, the metallic phases of 2D
transition-metal dichalcogenides (TMDs)MX2 (M =Mo, W, Ti et al; X = S, Se, Te) such as molybdenum
disulphide (MoS2) [137, 138] provide large channels (6.15 Å) for Na+ intercalation and possess a high
theoretical capacity of 670 mAh g−1, which makes them promising for use in NIB anodes [139]. The unit
cells of the semiconducting 2H phase and the metallic 1T phase of MoS2, along with an image of a uniform
solution of chemically exfoliated nanosheets are shown in figures 14(a) and (b). A cross-sectional scanning
electron microscope (SEM) image of the restacked 1T phase MoS2 nanosheets is shown in figure 14(c).
Individual nanosheets can be seen, with intercalation sites between them.
Current and future challenges
2D TMDs are promising anode materials for Na-ion batteries, but several challenges hinder their
performance. One challenge is the lack of understanding of the charge-storage mechanism. Broadly
speaking, TMD-based NIB anodes work via the intercalation/extraction of sodium ions in the layered
structure. Unlike storage mechanisms in conventional 2D materials (graphene, MXenes), a phase transition
occurs in TMD-based electrodes, accompanied by electrochemical intercalation of sodium ions into the
structure [140]. For example, in MoS2 anodes, the intercalation of ions leads to a transformation from the
semiconducting 2H phase to the metallic 1T phase [141]. This transition from 2H-MoS2 to 1T-MoS2
involves a slippage of the intralayer atomic planes, and thus largely influences the intrinsic properties of the
material, as well as its electrochemical performance. A series of other structural features in MoS2—such as
lattice distortion, structural modulation, and irreversible decomposition—are also observed during sodium
ion intercalation. Furthermore, information regarding the formation of the SEI layer on the MoS2 anode is
not well elucidated. Therefore, a better understanding of the working mechanism and intercalation
chemistry of MX2 compounds is required for better NIB design.
Advances in science and technology to meet NIB anode challenges
In-situ/operando characterisation techniques are being developed to understand the mechanisms at work in
these materials and improve their electrochemical performance. In-situ atomic force microscopy (AFM)
using a microcell has been used to study the evolution of structural changes and formation of an SEI on
MoS2. It was observed that mechanically exfoliated MoS2 wrinkles upon sodiation at 0.4 V, and that the SEI
layer forms quickly, before the intercalation of sodium ions. This measurement also showed that the SEI layer
thickness on the MoS2 electrode is≈20.4 nm± 10.9 nm [142]. In addition, aberration-corrected scanning
transmission electron microscopy (STEM) was used together with in-situ x-ray diffraction (XRD) to track
the phase transition in MoS2 [141]. It has been demonstrated that in NaxMoS2, x = 1.5 is a critical point for
reversible structural evolution. This critical point indicates that the structure of MoS2 can be partially
recovered if the concentration of intercalated sodium ions is less than 1.5 per formula unit of MoS2.
However, its structure cannot be restored once it is decomposed into NaxS and metallic Mo. In-situ
spectroscopy has been used to study the growth of Na dendrites and SEI formation on TMD-based anodes.
Synchrotron-based soft XAS has been used to investigate discharge products by probing states near the Fermi
level [143]. XAS has also been used to reveal the the local electronic and chemical structure of surfaces and
interfaces. These types of analysis could be extended to 2D TMD electrodes to study local changes in their
atomic and electronic structures during charging and discharging. Similarly, the Rutherford backscattering
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Figure 14. (a) Structures of semiconducting 2H- and metallic 1T-phase MoS2. (b) Photograph of chemically exfoliated metallic
MoS2 nanosheets suspended in water. (c) SEM image of 1T-phase MoS2 electrodes obtained by restacking exfoliated nanosheets.
The inset shows the spacing between the restacked nanosheets, which could facilitate the intercalation and de-intercalation of Na
ions. Reprinted with permission from [137]. Copyright (2011) American Chemical Society. Reprinted by permission from
Springer Nature Customer Service Centre GmbH: Springer Nature, Nature Nanotechnology [138]. © 2015.
spectrometry (RBS) technique can be employed to determine the composition and thickness of the SEI that
forms on anodes during electrochemical cycling [143].
Concluding remarks
In summary, the field of 2D TMDs is in a nascent phase. While 2D TMDs are interesting, several key
challenges remain. Furthermore, much of the work on Na-ion batteries has been done on the
semiconducting 2H phase of TMDs. This phase is hydrophobic and lacks conductivity, resulting in sluggish
ionic diffusion. In contrast, the metastable metallic phase of TMDs has been less studied. The exfoliation of
metallic TMDs and the restacking of 2D nanosheets into electrodes are likely to provide electrodes with
better intercalation efficiency and faster kinetics.
Acknowledgment
This work was supported by the Faraday Institution (Grant No. FIRG018).
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2.5. Organic materials
Aamod V Desai1,2, Russell E Morris1,2,3 and A Robert Armstrong1,2
1 EastChem School of Chemistry, University of St. Andrews, KY16 9ST, United Kingdom
2 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom
3 Department of Physical and Macromolecular Chemistry, Charles University, Hlavova 8, Prague 128 43,
Czech Republic
Status
OEMs currently lie at the periphery of rechargeable battery technology. However, with the increased
emphasis on sustainability, electrochemically active organic molecules, which, in principle, can be derived
from biomass or recycled from commercial waste, are the subject of increased interest [144]. Although the
suitability of organic polymers as electrodes for alkaline metal-ion batteries was reported in the early years of
secondary battery development, it is only during the last decade that research in this area has witnessed a
resurgence, with the successful application of small redox-active organic molecules for reversible ion
insertion in LIBs and subsequently NIBs, for example, disodium terephthalate (Na2C8O4H4) and its
analogues [145]. This recent progress has coincided with the growing applications of small organic molecules
in the preparation of several classes of solid-state materials. The features of structural diversity and the ability
to modulate their electronic properties on demand, make organic systems attractive candidates to explore for
battery applications.
Redox reactions in these materials typically involve molecules that have an initial neutral electronic state,
but then acquire a net charge [146]. This is either accompanied by the rearrangement of pi-bonds in the
backbone, or other mechanisms that stabilise the redox state. Moieties such as carboxylates are well suited for
anodes, on account of their low-voltage redox reaction (figure 15(a)) [147]. Since bare organic molecules
have high solubility in polar solvents, usually, such active molecules are considered for use in coordination
compounds, which are relatively stable solid-state materials. Organic chemistry permits the tailoring of the
the local molecular environment to alter electrochemical features, such as the working voltage of the material
and the density of active sites. Also, over the last few years, new redox chemistries and active moieties, such as
azo (figure 15(b)) and imine have been identified and tested for charge storage at lower voltages. Although
their working principle is similar to that of carboxylates, these groups are usually the central part of the
molecule, unlike carboxylates, which tend to be terminal groups. Apart from these well-studied mechanisms,
a few organic systems have shown reversible ion insertion using different pathways [147]. In addition to the
goal of practical implementation, there is an interest in developing a fundamental understanding of the
unexplored active groups.
Current and future challenges
Organic anode materials have considerable room for improvement, in terms of bothelectrochemical
properties and material development (figure 16). Organic molecules show redox activity at specific active
sites, which leaves a significant portion of the molecule unused. This directly contributes to lower theoretical
capacities. An important aspect of the study of these materials is the improvement of cycling stability. The
use of organic electrolytes in non-aqueous batteries leads to the formation of inactive side products or the
solubility of small, active organic molecules for long-term cycling. The formation of a solid–electrolyte
interphase (SEI) gives notably low coulombic efficiencies for the initial cycles, which is particularly relevant
for practical applications [148]. Another feature of OEMs is their low density, resulting in significantly lower
volumetric energy density. However, making their structures compact may result in an increase of volume
expansion during (de)insertion of bulkier species, such as Na+ ions. Most organic materials have poor
electronic conductivity, requiring significant amounts of conductive carbon either as physical mixtures or as
composites with the active solid, which directly adds to the non-contributing weight of the electrode.
Although there is immense scope to modulate structural features, the carbonyl and imine groups only
accommodate one Na+ ion per site, and the stabilisation of the redox state requires a conjugated backbone
(figure 15(a)). On the other hand, azo bonds can insert two Na+ ions (figure 15(b)), but their working
potential is seen to be much higher, which reduces the overall energy density of the battery.
Apart from electrochemical performance, an in-depth understanding of the activity and the factors that
influence that activity are yet to be fully realised. These are crucial to meet the upcoming targets for the
applications of rechargeable batteries. In this context, the production costs and scalability of this class of
materials also have room for more research. In addition, the thermal and chemical stability of organic
molecules is limited and requires more development to support practical implementation.
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Figure 15. (a) Representation of Na+ ion insertion in a typical organic redox-active molecule—disodium
naphthalene-2,6-dicarboxylate (Na2NDC). (b) Figure showing electrochemical reaction observed for azo-bonded organic
molecules.
Figure 16. Schematic showing aspects of contemporary research in the domain of organic anode materials, including broad
objectives.
Advances in science and technology to meet challenges
A major theme of research in this area has been to leverage the versatility of organic chemistry for molecular
engineering and examine new molecules by varying their atomic or structural properties (figure 16). Among
various molecules, carboxylates bonded to aromatic systems are the most popular moieties. Strategies to
extend pi-conjugation or vary the secondary functional groups have shown the potential to both improve
cycling performance and stabilise redox states. In certain cases, upon careful design, the extended
pi-conjugation has participated in the electrochemical reaction, leading to the insertion of excess charges
[149]. The role of secondary functional groups is to modulate the electronic properties of the molecule and
provide additional interaction sites for Na+ ions. It is observed that in structurally similar molecules,
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electron donor groups lower the working voltages, while electron-withdrawing groups tend to increase the
redox potentials [150]. Heteroatom (N-atom) substitution in the backbone has resulted in higher capacities
and smoother kinetics by virtue of favourable interactions with Na+ ions. In systems with sulphur (S) atoms,
the capacity is seen to increase on account of the additional interaction sites, and the electronic conduction is
also improved [151]. Apart from molecular features, bulk properties are known to affect the cycling
performance of electrode materials. The morphology and particle size of the active organic material are seen
to play significant roles in the practical capacities and kinetics of ion insertion [152].
Although atomic features lead to the modulation of electrochemical properties, even the conventionally
studied molecules still show issues of cycling stability and volume expansion. In recent years, various porous
materials displaying long-range order based on redox-active small organic molecules have been tested with
the aim of circumventing the issues of solubility and stability. These include metal–organic frameworks
(MOFs) and covalent organic frameworks (COFs), which are seen to be stable to cycling and can
accommodate bulky Na+ ions [153]. However, research into porous materials has largely been confined to
the preparation of electrochemically active hierarchical carbonaceous matter; and very few examples have
used the pristine materials.
Concluding remarks
Organic battery materials are likely to contribute significantly to the pursuit of sustainable technologies. As
anodes in NIBs, small organic molecules with redox-active groups such as carboxylate, azo, and imine have
shown much promise. Several aspects of these compounds, such as their cycling stability, volumetric
densities, and poor electronic conductivity, require more research to compete with benchmark materials.
Much progress is anticipated through the incorporation of the knowledge gained and the extensive testing of
novel solid-state materials based on the use of organic molecules as battery electrodes. The rich diversity of
organic chemistry presents an opportunity to explore untapped redox-active groups and expand our
understanding of molecular engineering to support the delivery of high-performance materials. In addition
to fundamental objectives, practical parameters such as scalability and production costs also need to be
addressed.
Acknowledgment
This research is funded by the Faraday Institution (Grant No. FIRG018).
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3. Computational discovery of materials
Yong-Seok Choi1,2,3 and David O Scanlon1,2,3,4
1 Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United
Kingdom
2 Thomas Young Centre, University College London, Gower Street, London WC1E 6BT, United Kingdom
3 The Faraday Institution, Harwell Campus, Didcot OX11 0RA, United Kingdom
4 Diamond Light Source Ltd, Diamond House, Harwell Science and Innovation Campus, Didcot,
Oxfordshire OX11 0DE, United Kingdom
Status
In the last two decades, advances in supercomputing architectures and the development of more accurate and
cost-effective quantum chemistry approaches have enabled the computational exploration of new materials
to take place in a more rapid and rigorous manner. DFT calculations have emerged as a key component in
the computational discovery of materials. To accelerate the discovery of materials in such a vast chemical
compositional space, numerous DFT-based approaches have been reported and can be classified into three
types [154]: (a) chemical substitution, (b) crystal-structure prediction, and (c) data mining.
Most new materials have been discovered by chemical substitution, i.e., a simple element replacement in
known structural prototypes. This is particularly effective for Na-ion battery (NIB) materials, as these
materials share similar atomic structures with their LIB counterparts. Representative examples include
layered oxide cathodes of NaxTMO2 (TM = transition metals) and solid electrolytes of Na10MP2S12 (M = Si,
Ge, Sn). In contrast, crystal-structure prediction, e.g., random structure prediction and evolutionary
algorithms, begin with random atomic configurations within sensible constraints on bond lengths, followed
by local optimisations that find the lowest-energy structures. In general, evolutionary algorithms should
offer better efficiency, as they improve candidate guesses over time by comparing the lowest-energy solutions
found at every iteration. These two methods have proven to be effective in predicting the intermediate phases
of alloying electrodes of NIBs, such as Sn and Sb [155].
Data mining has now emerged as a promising method for facilitating materials discovery, as it can
establish criteria for new material formations and construct quantitative structure–property relationships for
predicting their properties. In conjunction with high-throughput algorithms, this approach has recently
been applied to predict new potential cathodes and solid electrolytes for NIBs [156]. However, due to the
high dimensionality of the problem and the relatively small number of known battery materials in databases,
the predictive ability of the results obtained remains controversial. Continuous research effort has been
devoted to improving the accuracy of models [156]. All the structure-prediction methods mentioned above
can be supported by various open-source structure-prediction tools (figure 17).
Current and future challenges
While various structure-prediction methods exist, the exploration of NIB materials has mostly been limited
to structures with existing prototypes reported in LIBs. This is because of the difficulties involved in
predicting previously unknown structures, which is especially tough for NIB materials that have
characteristic features, including:
Large elemental phase space
The exploration of potential NIB materials often faces increased complexity necessitated by the large number
of elemental combinations, which are sometimes even more numerous than their LIB counterparts. For
instance, NaxTMO2 can feature a plethora of redox-active transition metals (TMs) (Ti, V, Cr, Mn, Fe, Co, Ni,
and Cu) compared to LixTMO2 where only Co-, Ni-, and Mn-redox are available [158]. Furthermore,
continuous efforts to optimise their stacking sequence (e.g., P2- and O3-type) have introduced various
dopants (e.g., Ni, Cu, Mg, Zn) [158], making the compositional space much more complicated. Such broad
elemental combinations lead to a significant number of candidate chemical formulae that need to be
considered for the prediction of high-performance cathodes.
Materials stable only at elevated temperatures
Among the various synthesis routes reported to date, high-temperature sintering has become one of the most
common ways to fabricate electrodes and solid electrolytes with high Na+ conductivities. The resultant
structures are often only stable at elevated temperatures. The discovery of high-temperature metastable
materials is challenging with standard DFT calculations alone, as they do not include the vibrational entropic
contributions or the associated temperature effects. To predict materials at a finite temperature, it is therefore
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Figure 17. Schematic of the procedures used for computational materials discovery in previous studies. Publicly accessible tools
for structure prediction are also listed for reference. Reprinted figure with permission from [157], Copyright (2008) by the
American Physical Society.
necessary to perform additional phonon calculations that can describe atomic vibrations, which carries a
significant computational cost.
Strong self-interaction errors in correlated TM oxides
Anionic redox in TM oxides has emerged as a new paradigm to increase the energy density of NIB cathodes.
However, accurate prediction of the redox activity of TM oxides has been challenging for standard DFT
calculations. This is because conventional generalised gradient approximations of DFT cannot correct the
self-interaction error in O and, in particular, in TMs with highly correlated d-orbitals [159]. To counteract
the self-interaction error inherent in DFT, the use of computationally intensive methods, such as the
DFT+ U approach or hybrid density functionals, is essential.
Advances in science and technology to meet these challenges
Advances in computational resources and theoretical methods have progressed greatly recently, such that the
difficult challenges of discovering novel NIB materials are now becoming tractable. There have been
important developments in the consideration of temperature effects, such as the quasi-harmonic
approximation and the highly efficient ab initiomolecular dynamics [160]. Another recent direction in the
study of temperature effects has targeted many-body approaches, such as the path-integral Monte Carlo and
self-consistent harmonic approximations. The application of these methods provides a route for evaluating
the thermal properties of materials, allowing the prediction of potential NIB materials that are stable at high
temperatures. In order to accurately describe strongly localised d-orbitals, various novel functionals (e.g., the
DFT+ U approach and hybrid density functionals) have been suggested. To fully remove the self-interaction
errors, many-body approaches, such as quantumMonte Carlo and random-phase approximation, are
becoming achievable using today’s supercomputing architectures.
Challenges remain, however, in exploring the potential phase space of multicomponent Na-ion systems,
which is still prohibitively large to search fully. Therefore, to provide accurate descriptions of materials and at
the same time maintain a competitive computational cost, researchers should take inspiration from the
generations of chemical knowledge in the battery arena. The use of chemical heuristics has been shown to
have great potential for narrowing down phase space. For example, the constraints of charge neutrality and
electronegativity balance can significantly reduce the total number of candidate materials by two orders of
magnitude [161]. Recent efforts to reduce the number of candidates have developed several less expensive
and less accurate ways to estimate the properties of NIB materials: Mulliken electronegativity and the
solid-state energy scale were used to estimate the electrochemical stability window (ESW) [161],
Voronoi–Dirichlet partitioning and the bond valence sum were used to estimate the ionic conductivity [162],
and ‘cationic potential’ was used to estimate the stacking sequence of cathodes [163].
Another persistent issue in battery materials is the environmental sustainability of the constituent
elements, of which the drive to reduce expensive and toxic Co in LIBs is the prime example. Therefore,
further improvements in the NIB arena should also be driven by using only earth-abundant and non-toxic
elements, with an eye to our responsibility to be environmentally friendly.
Concluding remarks
Recent advances in computational methods and increasing computational power have provided researchers
with a new toolkit with which to explore a vast chemical space for targeted materials discovery. The
systematic integration of these tools can pave the way for the discovery of future NIB materials (figure 18).
Major challenges remain, however, in the assessment of the properties of NIB materials. Despite the various
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Figure 18. Integration of structure-prediction methods into the discovery of materials for future NIBs.
computational approaches developed to date, calculations for structures that can only form at high
temperatures or strongly correlated NIB materials are still cost intensive. This severely slows down the
exploration of chemical space, which hampers the prediction of new game-changing NIB materials. The
development of new strategies to circumvent this bottleneck will thus be critical.
Acknowledgments
Y C and D O S are indebted to the Faraday Institution NEXGENNA project (FIRG018) for financial support.
The writing of this roadmap section was supported by University College London (UCL).
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4. Electrolytes and SEI engineering
4.1. Organic electrolytes: salts
Darren M C Ould1,2, Christopher A O’Keefe1,2, Clare P Grey1,2 and Dominic S Wright1,2
1 Department of Chemistry, Lensfield Road, Cambridge CB2 1EW, United Kingdom
2 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom
Status
NIBs offer a cheaper and more sustainable large-scale energy storage solution, compared to well-established
LIBs. However, contrary to the latter, which primarily use 1 M LiPF6 in ethylene carbonate/dimethyl
carbonate (EC/DMC) as a standard electrolyte solution, a leading electrolyte is yet to be established for NIBs
[165–168]. This can be explained in part by the infancy of the field as well as the fact that electrolyte
development is overlooked in favour of electrode research. Nevertheless, there is an urgent need to find
high-performance electrolytes for NIBs, since they play a leading role in the overall lifetime, rate capability,
and safety of the battery. Principally, this is due to the formation of a solid–electrolyte interphase (SEI) at the
anode, which results from the degradation of the electrolyte and/or solvent during the charging/discharging
process. Although well characterised for LIBs [168], the SEI composition for NIBs is poorly understood and
remains one of the most critical areas for future research efforts.
To be an effective organic electrolyte, the sodium salt must adhere to a number of criteria: (a) chemical,
electrochemical, and thermal stability; (b) high ionic conductivity; (c) low cost; (d) low
toxicity/environmental benignity, and (e) simple, scalable synthesis. High ionic conductivity promotes facile,
rapid exchange of sodium ions between the two electrodes, whereas a large highest occupied molecular
orbital (HOMO) energy level ensures the stability of the salt. Such strict requirements naturally limit the
possible choices of salts available; the most widely used anions are shown below, in figure 19. Traditionally,
these anions have strong electron-withdrawing groups surrounding the central atom, which create a
delocalised negative charge, and they are weakly coordinated to the Na+ cation. Generally, sodium salts have
higher melting points than their lithium counterparts, which is advantageous not only for easier drying but
also safety, since the former have greater thermal stability.
Current and future challenges
Among the electrolytes shown in figure 19, the most-studied electrolyte in NIBs is NaClO4 [167] although its
strong oxidising nature prohibits its commercial use due to safety concerns. In addition, NaClO4 is known to
be difficult to dry. Similarly to LIBs, the most promising salt to date for NIBs is NaPF6; however, its high
moisture sensitivity can promote the formation of HF via hydrolysis, leading to further electrolyte
degradation and major safety concerns. As a result of this moisture sensitivity, the quality of commercially
purchased NaPF6 is problematic, and NaF formation occurs. NaBF4 and NaOTf
(OTf= trifluoromethanesulfonate) are poor choices, as they have strong cation–anion interactions, and it
has been shown that NaOTf has reduced conductivity and electrochemical stability compared to NaClO4 and
NaPF6 [164].
An interesting emerging class of electrolytes which has previously been used in ionic liquids contains
NaTFSI (TFSI= bis(trifluoromethane)sulfonimide) and NaFSI (FSI= bis(fluorosulfonyl)imide), which are
attractive due to their reduced toxicity and high thermal stabilities. However, these cannot be used as single
salts, as they induce corrosion on the Al current collector, as well as incurring high cost. It is rare for new
electrolytes to enter the field, but the DFOB (DFOB= difluoro(oxalato)borate) and BOB
(bis(oxalato)borate) anions have recently both shown promising results in terms of reduced toxicity and low
cost of synthesis [169]. Additionally, NaBOB has proved to be a non-flammable electrolyte when used in
trimethyl phosphate (TMP) solvent; but in both cases, more work is required to optimise their performance
[170]. Clearly, developing a salt that meets all the requirements outlined above is challenging, and future
research efforts should focus on anions that have easily tuneable groups, which would allow for the
fundamental properties of the salt to be simply modified.
Another challenge is to characterise the SEI. Few studies exist to address this, but x-ray photoelectron
spectroscopy (XPS) and time-of-flight secondary-ion mass spectrometry (ToF-SIMS) analysis have
confirmed the presence of Na2CO3 and ROCO2Na species [171]. These degradation salts are often more
soluble than their lithium counterparts, and the Na+ inductive effect on the solvents/anions should be
weaker than for Li+. This could, in turn, affect the reduction products formed at the anode.
Advances in science and technology to meet challenges
In recent times, the use of computational modelling and simulation has become a much more powerful
method for the development of high-performance electrolytes. The early modelling of organic liquid
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Figure 19. Structure of the anions currently used in sodium-ion batteries. Sodium is the cation in all cases.
electrolytes often focussed on the conduction mechanism, with little investigation into how the bulk ions
themselves interacted. However, with advances in modern computer power, more recent studies have now
started to quantitatively address the role of ion pairs, although currently, the high-level theory is too
demanding for this to be a feasible method for the fast screening of electrolytes. In addition, computational
methods can assist in better understanding the mechanism of SEI formation, where calculations have
indicated that sodium electrolytes have faster desolvation of the cation at the electrolyte/electrode interface
than that of Li+ in analogous systems. DFT may also be used to simulate spectra (such as IR and Raman) of
the degradation products in the SEI to aid the identification of the species present [172].
In addition to computational developments, there have been spectroscopic advances which may be
utilised to address the challenges described above. Modern advances in dynamic nuclear
polarisation-enhanced NMR spectroscopy (DNP-NMR) have recently been used as a method to aid
characterisation of the SEI in LIBs. By using lithium metal as the polarisation source, selective observation of
the low-concentration SEI components was possible. An intimate knowledge of the SEI composition and
distribution is crucial in understanding how dendrites form, and is critical for preventing short circuits and
improving the overall safety of the battery. A major advantage of this technique is that it allows for the direct
probing of interfaces to reveal structural information, and it can be envisioned that this DNP-NMR
spectroscopy technique can readily be applied to NIBs to provide further insights into SEI formation and
composition [173]. While there have been several investigations into the structure of the SEI for NIBs, more
work is necessary to understand the interplay between electrolyte chemistry, SEI composition, and stability.
Concluding remarks
In contrast to LIBs, which are a mature technology, much work is still required to unleash the full potential of
NIBs. Electrolytes play a key role, as unwanted side reactions and degradation products significantly affect
the overall performance and lifetime of the battery. The current range of sodium electrolytes has significant
drawbacks, such as oxidising potential, moisture sensitivity, cost, and toxicity, meaning there is an urgent
need to develop novel electrolytes to overcome these shortcomings. With increased research into, and
development of, novel electrolytes, these challenges can be addressed, and this roadmap highlights key areas
where focus should be applied. Principally, this is in characterising the degraded salts in the SEI as well as
producing electrolytes which can easily be chemically modified to tune their fundamental properties. With
this increased focus, NIBs undoubtedly have a prosperous future, rivaling that of their lithium counterparts.
Acknowledgment
This research is funded by the Faraday Institution (Grant No. FIRG018).
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4.2. Organic electrolytes: solvents
Martin Karlsmo1 and Patrik Johansson1,2
1 Department of Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden
2 Alistore-ERI, CNRS FR 3104, 15 Rue Baudelocque, 80039 Amiens, France
Status
In many ways, both R&D and the practical deployment of NIB organic electrolytes follow the development of
LIB electrolytes [175–177]. This is logical, as the operating conditions and application areas are more or less
the same, and thus, similar properties are needed; i.e., readily dissolved salts to create a large concentration of
charge carriers offering fast ion transport with high fluidity and low viscosity. The latter also assists in the
proper wetting of the separator and the porous electrodes. Additionally, high thermal and chemical stabilities
are a must, to allow for usage in various climates and vs. different electrodes, as are wide ESWs matching the
electrodes. Finally, all must be available at a low cost. As of today, this means linear and cyclic carbonates,
such as DMC, EC and PC [174], or ethers, such as glymes, in particular, diglyme [174, 175].
While much of NIB R&D follows from earlier LIB developments, there are some distinct practical
differences to be found. One is that linear carbonates, such as DMC, used to lower the melting point and
viscosity of LIB electrolytes, create soluble products upon reduction in a NIB, and hence, unstable SEIs at the
anode, a problem that can be overcome by smart additive usage [176]. Another is that graphite is not capable
of intercalating naked Na+ ions and thus, it is not commonly used for NIBs. Therefore, PC, one of the
original LIB electrolyte solvents, is frequently used for NIBs (as opposed to LIBs, as it exfoliates graphite)
[174, 176, 177]. There is also a fundamental difference between the use of Na salts and Li salt: for the former,
both the ion–ion and ion–solvent interactions are reduced to ca. 80% of the latter [178]—which paves the
way for faster desolvation dynamics [179] which, at the cell level, manifests as improved power rates [174,
176, 177]. As with any other battery technology based on organic electrolytes, improved safety without
making any departure from performance is key—and here, new NIB electrolytes, in particularm solvents,
should evolve out of the LIB shadow. Another driving force should be to simplify recycling—and this, for
example, means the use of F-free solvents/electrolytes [170].
Current and future challenges
Even if challenged by both solid-state electrolytes (SSEs) offering performance and safety advantages and
aqueous electrolytes offering lower cost and better overall sustainability, organic solvent-based electrolytes
are still likely to be the main route for years to come. They can be used in their own right and/or in hybrid
electrolytes, with polymers or ionic liquids, to improve performance and safety [180] or to facilitate better
SSE/electrode contact. A major challenge is to create a holistic and Na+-centred (and not Li+-derived)
approach to the salt(s)–solvent(s)–additive(s) recipe for a standard functional NIB electrolyte [181]. This
could also reduce the complexity and offer better control and understanding of e.g., solubility issues and
ion-transport advantages. Today, the latter is, at the phenomenon level, a power performance advantage
compared to LIBs, which is coupled to lower solvent desolvation energies [179] and the presence of fewer
contact ion-pairs [178]. However, the reduction in strong interactions renders the organic products from
solvent reduction more soluble and thus less stable, and creates/results in additional inorganic SEIs. Another
large challenge comes directly from the+0.3 V shift in Ered of Na+/Na vs. Li+/Li. For the same final cell
voltage, there is a corresponding more stringent demand on the electrolyte/solvent Eox limit—which is not a
minor issue, given the interest in high-voltage cathodes. The fact that the electrolyte must be developed not
only in terms of ESW, but also to match both positive and negative electrodes, is a large challenge, as there is
not yet a dominant electrochemical couple for NIBs. Improved fluidity, important for low-temperature
performance as well as for the proper wetting of electrodes, must be achieved without compromising safety
by using volatile solvents. Another path of research is highly concentrated electrolytes (HCEs), which
improve safety, widen the ESWs, and utilise non-vehicular ion transport, which leads to improved cation
transference numbers. For HCEs, Na-based systems have an earlier onset of some of these properties,
compared to Li-based systems [179], which is cost-saving, as less salt is needed. However, the high viscosities
of the highest concentrations of salt-based HCEs, i.e., those of very low solvent content, are a challenge for
manufacturing and the cell formation stages.
Advances in science and technology to meet challenges
When the key performance indicators (KPIs) and the targets to be reached for NIBs by e.g., 2030 are listed,
few are directly dependent on the solvent used, but very many are indirectly limited by the electrolyte. The
first, most obvious KPI is the ionic conductivity, but the target, 1 mS cm−1, has already been reached. The
next is the cell voltage—intimately coupled to the electrolyte/solvent Eox limit, as outlined above, while the
less obviously connected KPIs are cycle-life and cost. However, for the NIB cycle life, the stability of the
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Figure 20. Some of the more popular NIB electrolyte solvents/co-solvents/additives (colour code: red= oxygen, grey= carbon,
green= fluorine, blue= nitrogen, white= hydrogen).
electrolyte/solvent, both in bulk and at the interfaces/interphases, is key. This is why different operando
techniques enabling the study of NIB cells represent the most important technological advances needed. One
such technology is based on embedding optical fibre sensors into the jelly-roll and, while it does not affect
cell performance, it enables the monitoring of not only pressure and temperature changes as a function of the
state-of-charge but also the origin of heat contributions and how different additives affect these properties, as
elegantly demonstrated for a HC//NVPF NIB i.e. based on hard carbon as anode and Na3V2(PO4)2F3 as
cathode [182]. As a baseline electrolyte, this study used commercially available NP30 (1 M NaPF6 in
EC:DMC 50:50), a direct analogue to the LP30 electrolyte, and standard 18 650 cells and protocols. This
points to several things that are needed to advance the science and technology of NIBs; commercially
available electrolytes of high purity at a cost meeting KPI targets; standardised cells and protocols, both for
electrolyte and electrode studies (at present, a chicken-and-egg problem, as no fixed-star materials exist, and
different aspects are in focus using different setups), and understanding the roles/actions of additives in
NIBs, as these may not be the same as those in LIBs—the differences in SEI formation being the most
obvious. If a proper understanding of the limitations imposed by electrolytes, test protocols, and the actions
of the additives is pursued, the growing community of NIB researchers should soon have a wealth of
unambiguous data. While there is room for rethinking and emphasising the differences rather than the
similarities between Na+/NIBs and Li+/ LIBs, NIBs should be using the field of LIB development to the
maximum extent possible for real NIB leapfrogging.
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Concluding remarks
While the R&D directions of NIBs have largely followed those of LIBs, they are slowly finding their own path,
including applications where the emphasis is on complementarity rather than replacement. The somewhat
overemphasised similarity of Na+ and Li+ dissolved in solvent(s) has, until today, promoted evolution,
leaving NIBs always trailing behind LIBs. However, as an example, it should be possible to use the weaker
solvent interactions and generally larger first solvation shells of Na+ as design criteria for novel NIB
electrolytes. Likewise, the problematic large solubility in the electrolyte of solvent reduction compounds
should be a proper R&D target, similarly to e.g., the polysulphide issue in Li-S batteries. As we are seeing
various operando, high-throughput, and robotic screening technologies starting to be applied for battery
research, these could all be used to evaluate NIB electrolyte solvents and perhaps also enable the tailoring of
the electrolyte to different operating conditions and applications.
Acknowledgments
The funding received from the European Union’s Horizon 2020 research and innovation programme under
Grant Agreement No. 646433 (NAIADES), the Swedish Research Council, the Swedish Energy Agency
(#37671-1), and the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning
(FORMAS), are all gratefully acknowledged. The many fruitful discussions within ALISTORE-ERI, and
especially with M Rosa Palacín, have been most valuable. P J is also grateful for the continuous support from
several of the Chalmers Areas of Advance: Materials Science and Energy.
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4.3. Ionic liquid electrolytes
S Sen1,2 and R G Palgrave1,2
1 Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United
Kingdom
2 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom
Status
NIBs have attracted attention in parallel to Li-ion technology due to their cost-effectiveness and the high
earth abundance of sodium [71, 183, 184]. Despite the commercialisation of NIBs by several companies
(Faradion [185], Tiamat [185, 186], SHARP labs [185, 186]), thermal runaway with volatile electrolytes and
insufficient cyclability at room temperature demand more serious electrolyte research.
Ionic liquids (ILs) are promising alternative electrolytes due to their non-flammability and wide
operating electrochemical window. They are glass-forming molten salts, consisting of versatile combinations
of aromatic cations and weakly coordinating anions. The structures of the ions and their interactions
(hydrogen bond, electrostatic, and Van der Waals) [187] govern their physicochemical properties, (phase
transitions, viscosities, thermal stability, and HOMO-LUMO (lowest unoccupied molecular orbital) gap)
[187] and electrochemistry (electrochemical window, ionicity, ionic conductivity, ionic
association/dissociation, and the double-layer structure at the electrode–electrolyte interface) [187, 188]. In
the realm of NIBs, ILs offer specific additional advantages, such as reduced corrosion of the Al current
collector, a wide range of operating temperatures, and suitable electrode–electrolyte interface processes
(homogeneous SEI formation, a conductive interface, and a dendrite-free anode–electrolyte interface) [185]
(figure 21). The wider electrochemical window of ILs permits a wide variety of cathode materials for NIBs.
Only anion decomposition defines oxidative stability and SEI composition. ILs offer a less resistive, more
uniform, and consistent SEI throughout the battery cycle with hard carbon and alloy-based anodes [186].
The composition of an IL-derived SEI (with an HC anode) (SEIs formed with polyolefin, SCO) is distinctly
different from those formed in organic electrolytes (SEIs formed with organic and inorganic sodium
carbonates) [186]. Reduced aluminium corrosion in ILs is notable, even with salts that corrode the Al
current collector in organic solvents [186]. Several imidazolium-, pyrrolidinium-, phosphonium-, and
ammonium-cation-based ionic liquids have been explored in NIBs (e.g., [C2C1Im][FSA], [C2C1Im][BF4],
[C3C1Pyrr][FSA], [C3C1Pyrr][TFSI], N2(20201)3][FSA], [P111i4][FSA], [C4C1pyrr][DCN],
[C4C1Im][SO3CF3]. CnC1im+: 1-alkyl-3-methylimidazolium, CnC1pyrr+: N-alkyl-N-methylpyrrolidinium,
CnC1pip+: N-alkyl-N-methylpiperidinium, and Nnnnn+: tetraalkylammonium) [186].
Current and future challenges
Despite several advantages, NIB research is falling behind the current state of the art of LIB technology.
Safety and the battery cycle life are significantly influenced by the electrolyte formulation and various
interfacial issues, including uncontrolled dendrite formation due to the use of sodium metal, unstable and
increasingly resistive electrolyte interface SEI formation due to electrolyte decomposition, cracked and
thicker SEIs, pronounced dissolution of SEIs, and the significant reactivity of sodiummetal in the majority of
volatile organic electrolytes.
Ionic liquids serve as green solvents. They mitigate safety issues and offer wider oxidative stability and
relatively lower solvent degradation at the electrodes. However, the major challenges of ionic liquids are their
relatively high cost, tedious purification methods, and the hygroscopic nature of widely used imidazolium-
and pyrrolidinium-based ionic liquids. This is despite their excellent stability and wider temperature
operability. Their relatively high viscosity and inferior ionic conductivity limit their battery cycling
performance at room temperature, as compared to conventional organic electrolytes. The ionic liquids offer
larger interfacial resistance and slower Na+ transfer at the sodium metal anode, compared to organic
electrolytes. In NIBs, the advantages of ionic liquids are only fully utilised at elevated temperatures (∼90 ◦C)
in half cells. Another important challenge with ionic liquids is to achieve a unity sodium transference number
and reduce concentration polarisation. The majority of ionic liquid electrolytes are made at a maximum of
1 M concentration, a concentration at which unity transference numbers are difficult to achieve. Recently,
super-concentrated ionic liquid-salt systems have been shown to offer relatively stable SEI properties and
near-unity transference numbers. However, the availability of a wider range of IL structures with high salt
dissolution capability is still limited. Designing electrolytes to achieve high transference numbers and
reasonably good ionic conductivity is an unavoidable challenge for ILs in NIBs. Although minimal, the
current-collector corrosion is not fully mitigated by the widely used fluorinated anion-based ionic liquids.
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Figure 21. Benefits of ionic liquids in NIB applications.
Figure 22. (a) Structures of ionic liquids [C3C1pyrr][FSA] as widely applied for various cathode and anode materials. (b)
Half-cell Na-ion battery performances of anodes and cathodes with a [C3C1pyrr][FSA] ionic liquid electrolyte. (c) Full-cell
Na-ion battery performance with ionic liquid in comparison to conventional organic liquid electrolytes. NMO:
Na0.6Co0.1Mn0.9O2, NVO: Na2.55V6O16. NVP: Na3V2(PO4)3, HC: hard carbon, EIL: ionic liquid electrolyte, Estand: standard
organic electrolyte. The data presented in (b) and (c) are directly taken from [186].
Advances in science and technology to meet challenges
ILs consisting of various combinations of ammonium and phosphonium-based cations and fluorinated
and non-fluorinated anions have been explored in NIB applications (see figure 22(a) for structures).
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Figures 22(b) and (c) present a survey of the battery performance of half cell (with sodium metal as the
anode) and full cell (with a sodium intercalation compound as the anode) NIBs, respectively, employing
pyrrolidinium-based IL electrolytes with a wide range of cathodes and anodes. Impressively, ionic liquids
offer superior cyclability, rate capability, and powder density, compared to organic electrolytes. Despite the
higher viscosity and inferior transference number, these results strongly encourage the investigation of IL
chemistry in NIBs. The reasons for such exciting performance improvements are: the enhanced Na+
diffusion at the interface, a compact SEI, dendrite-free sodium deposition/dissolution, the lower reactivity of
ionic liquids with sodium metal, and reduced Al corrosion at elevated temperatures (figure 21). For example,
highly conducting and stable SEI layers have been reported for an HC anode with an IL electrolyte,
Na[FSA]-[C3C1Pyrr][FSA] [186], compared to NaClO4-EC/DEC. The homogeneity and stability of the SEI
of Na[FSA]-[C3C1pyrr][FSA] improves the cyclability of several anodes, such as Sn4P3 [a cyclability of 112%
at 200 cycles] [190]. Full cells employing an IL electrolyte display similar benefits (figure 22(c)). A very
energy-dense (368 Wh kg−1) prismatic full cell has been developed using a Na[FSA]-[C3C1pyrr][FSA]
electrolyte, a Na3V2(PO4)3 cathode, and an HC anode to deliver better capacity retention (75%) than that of
a Na[ClO4]-EC/PC electrolyte (figure 22(c)) [191]. A highly stable passivation layer with negligible dendrite
formation was observed when a vinyl substituted imidazolium IL was used as electrolyte additive [192].
Super-concentrated 50 mol% NA[FSA]-[C3C1pyrr][FSA] exhibited dendrite-free sodium
deposition/dissolution onto the SEI with a nanoengineered Na+ conducting interface made from
(Na)x(FSA)y components [187].
Concluding remarks
In conclusion, ionic liquid electrolytes significantly improveme the safety of sodium ion batteries, widen the
operating temperature and electrochemical window, and offer impressive cyclability and interface chemistry.
However, insufficient literature is available to report the applicability of a wider range of electrodes in a wider
range of ILs. Fine correlations between structure and critical interface electrochemistry are still
under-represented in NIB research. Some additional concerns include cost, recyclability, tedious purification
methods, and the high viscosities of ILs. Binary IL-organic/aqueous solvents (IL–water) and non-fluorinated
cheap anion-based ILs could improve cost-effectiveness and decrease viscosity if they could be developed
without sacrificing safety aspects. Interestingly, NIBs with ILs still exhibit good specific energy (near to that
of LIBs) and better power than supercapacitors at elevated temperatures, which are difficult to achieve with
organic solvents. The possibility of tuning the molecular structure of ionic liquids and a detailed
understanding of the mechanical properties and solubility of SEIs with ILs are to be explored to enhance the
performance of NIBs with ILs.
Acknowledgment
This research is funded by the Faraday Institution (Grant No. FIRG018).
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4.4. The SEI in Na-ion batteries
Andrew J Naylor and Reza Younesi
Department of Chemistry—Ångström Laboratory, Uppsala University, Box 538, SE-75121 Uppsala, Sweden
Status
An SEI may be formed through the decomposition reactions of an electrolyte at the interface with the anode
of a battery [193]. The low electrode potential experienced by the anode causes the polar aprotic
non-aqueous electrolytes employed to be thermodynamically unstable. The formation of a durable SEI is,
however, essential for a kinetically stable battery and to obtain a long cycle life. This nm-scale interphase
layer often has the characteristics of a solid electrolyte and can comprise both organic and inorganic,
amorphous, and crystalline products derived from the electrolyte components. Ideally, it passivates the
surface of the anode, being highly electronically insulating, impermeable to solvent molecules, insoluble, and
inert, but offers ionic conductivity to allow the diffusion of cations (Na+ ions in an NIB) between the
electrode material and the electrolyte. The SEI should preferably be formed only within the initial cycles to
prevent capacity losses and electrolyte depletion.
It has been shown through photoelectron spectroscopy (PES) measurements that the SEI in NIBs can
vary in thickness and composition, depending on the state of charge [113]. The composition of the
electrolyte is the main factor that influences the SEI composition and properties (figure 23), though other
parameters including electrode-surface composition, current density, temperature, and potential limits can
have an impact. The typical decomposition species formed from carbonate-based electrolytes in NIBs can be
divided into inorganic species (mainly NaF and Na2CO3 but also Na2O and NaOH in the presence of oxygen
and water contamination) and organic species (sodium alkyl carbonates (ROCO2Na) and polyethylene oxide
oligomers) [194, 195].
Despite the fact that the SEI has been extensively studied in LIBs, resulting in the tuning of electrolyte
formulations to provide maximum performance, it is still thought to be one of the major contributors to
battery ageing [168]. However, while the concept of the SEI and its characteristics are largely similar for
Na-ion batteries, many notable differences exist between the two technologies, even though they often use
highly analogous materials [166]. A fundamental understanding of SEI formation mechanisms and
properties is required to design an electrolyte that will lead to the commercialisation of NIBs. Here, we
address the challenges of the SEI in NIBs, which can have such an important impact on their performance
and ageing.
Current and future challenges
In NIBs, the main challenge that needs to be to overcome to stabilise the SEI is its solubility [197, 198]. While
SEI dissolution has not been studied in great detail for LIBs, it is now a major focus of NIB research. The
continuous dissolution of the SEI leads to its reformation in subsequent cycles with excessive irreversible
capacity losses, poor coulombic efficiencies, and eventual failure. Because the charge density and hence the
dissociation energy of sodium salts is lower than that of lithium salts, the solubility of SEI components (e.g.,
NaF vs. LiF) formed in NIBs is generally greater compared to that in LIBs, and this holds especially true for
aqueous solutions [196]. Extensive SEI dissolution has been demonstrated for carbonate-based electrolytes
and carbonaceous electrodes [197]. It has been determined that capacity loss through such dissolution is
dependent on electrolyte composition and electrode surface area. It has even been debated whether the most
soluble organic species can be considered to be SEI components at all [194, 195].
Additional challenges in characterising the SEI arise due to its high solubility. Most commonly,
spectroscopic and imaging analyses are performed ex situ on electrodes after the drying/washing of samples.
Such practices may inadvertently alter the composition of the SEI. The reports of an overall more inorganic
SEI for NIBs compared to LIBs [199] could also be caused either by the leaching of organic species in the cells
or their dissolution during the washing step.
The formation of high-resistance carbonates on the surface of sodium metal in half cells with continuous
stripping/deposition can lead to a complex evolution of resistance. The lack of a reliable reference electrode
and the consequential use of sodium metal in half cells results in poor SEI formation at the working
electrode, and hence, inferior performance. The diffusion of decomposition products from the sodium
electrode via the electrolyte to the working electrode (‘crosstalk’) is probably responsible. These issues
prevent the extrapolation of performance from half cell to full cell; however, no such problems have been
reported for full-cell configurations [196]. Nevertheless, information from studies published using half cells
can still provide valuable insights and lead to a greater understanding of the challenges at hand.
Advances in science and technology to meet challenges
The low thickness and chemical complexity of the SEI in any battery system make it particularly difficult to
characterise. Numerous techniques have been used, including PES, electron microscopy, electrochemical
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Figure 23. FTIR spectra obtained from Li or Na metal electrodes freshly prepared and stored for 24 h (OCV) or after cyclic
voltammetry at 25 ◦C in (a) LP30 (1 M LiPF6 in EC:DMC 1:1 wt. ratio), (b) 1 M NaPF6 in EC:DMC 1:1 wt. ratio, and (c) 1 M
NaPF6 in EC:PC:DMC 45:45:10 wt.% [196]. Reproduced from [196]. CC BY-NC-SA 3.0.
Figure 24. (a), (b) Galvanostatic cycling combined with pause testing of carbon electrodes in lithium and sodium half cells with
1 M (Li/Na)PF6 in EC:DEC (1:1) electrolyte. (c) C 1s HAXPES spectra (two photon energies) of sodiated electrodes from sodium
half cells after 72 and 168 h pauses. (d) Percentage charge capacity loss for different systems and pause lengths. Reprinted with
permission from [198]. Copyright (2016) American Chemical Society.
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quartz-crystal microbalances (EQCMs), electrochemical impedance spectroscopy (EIS), time-of-flight
secondary-ion mass spectrometry (ToF-SIMS), and vibrational spectroscopies. However, innovative methods
and advanced characterisation techniques are required to gain a deep understanding of the highly soluble SEI
in NIBs.
Electrochemical ‘pausing testing’ can be used to investigate dissolution [197, 198], whereby a cell is
switched to its open-circuit voltage (OCV) for a fixed duration after a period of cycling to determine how
much capacity is lost through self-discharge (figure 24). Such experiments have shown that the capacity loss
is higher in NIBs than in LIBs.
PES is very sensitive to surfaces and chemicals and is one of the techniques most commonly used to study
SEI layers in all kinds of battery systems. Synchrotron hard x-ray photoelectron spectroscopy (HAXPES)
measurements can be used to probe to a depth of 50 nm and have shown that the SEI thickness decreases
with increasing pause durations (figure 24) [197, 198]. HAXPES measurements with large probing depths
may allow for the elimination of electrode washing, an issue already discussed, although other parameters,
including the pre-disassembly relaxation time, should also be considered. The future development of in situ
and operando PES techniques may enable the direct measurement of SEI layer formation at an electrode
during cycling.
While infrared spectroscopic (FT-IR) analysis of the electrolyte composition from cycled cells has already
been demonstrated [195], it is underutilised for detecting soluble SEI species. Complementary techniques,
such as gas chromatography mass spectrometry (GC-MS) and inductively coupled plasma (ICP) methods
may also provide valuable insights. However, the extraction of electrolytes from cycled cells and data
interpretation can present challenges.
The development of novel electrolyte components (salts and solvents) will almost certainly result in the
formation of more stable SEIs and improved battery performance. However, given such development, one
must bear in mind that cost-effective, non-toxic, and safe electrolytes will be required for the
commercialisation of NIBs.
Concluding remarks
While relatively little is known about SEIs in NIBs and their formation mechanisms, particularly compared
to their lithium counterparts, there is a growing body of knowledge on the topic as a result of increasing
interest over recent years. Progress is inhibited, though, by the challenges posed by their complex nature.
Their high solubility compared to the SEIs in LIBs causes difficulties in determining their composition and in
preparing samples for ex situ characterisation. Furthermore, the lack of a suitably stable reference electrode
and the issues of high resistance and crosstalk when employing sodium metal in half cells lead to
complications in assessing the performance of materials. Novel electrolyte formulations will advance NIB
technology towards commercialisation. However, a robust comprehension of the SEI achieved through the
use of advanced characterisation techniques and innovative electrochemical testing methods will allow for
the targeted design of electrolytes and a stable SEI.
Acknowledgments
The authors would like to acknowledge the financial support provided by the Å Forsk Foundation via Grant
No. 19-638, by the Swedish Energy Agency via Project Number 48198-1 and by S T and U P for Energy.
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4.5. SSEs for Na-ion batteries
Chris I Thomas1,3, Peter Gross1,3, Marco A Amores2,3, Serena A Cussen1,2,3 and Edmund J Cussen1,2,3
1 Department of Materials Science and Engineering, The University of Sheffield, Sheffield S1 3JD, United
Kingdom
2 Department of Chemical and Biological Engineering, University of Sheffield, Sheffield S1 3JD, United
Kingdom
3 The Faraday Institution, Quad One, Harwell Campus, OX11 0RA, United Kingdom
Status
An ideal solid-state sodium electrolyte should deliver minimal ohmic losses and (electro)chemical stability
with respect to both anode and cathode materials. Both of these parameters must be retained throughout the
cycling processes and lifetime of the battery. Delivering such performance requires a combination of
transport properties, chemical passivity, and processability, which represent a major challenge for materials
science.
Sodium electrolytes have been a long-standing challenge, and Na+ mobility is generally less facile than
for Li+ analogues. Conduction through solid polymers, such as Na salts in polyethylene oxide [200], can
realise around 10−4 S cm−1 at room temperature and the relatively low melting point of Na makes operation
above ambient temperature more fraught than in analogous Li batteries. The increased size and mass of Na+
compared to Li+ are both implicated in the more sluggish kinetics of the heavier group 1 metal, but it should
be recalled that the heavier Ag+ ion shows superionic conduction in a range of materials. Here, the
polarisability of Ag+ offers an advantage over the harder Na+ cation. This points towards the potential of
using polarisable anions, such as sulphide, in building a lattice for fast Na+ conduction.
SSEs based on the material Na3PS4 have shown that the replacement of P5+ by Ge4+, Ti4+, or Sn4+ ions
enables an increase in carrier concentration in the phases Na3+xMxP1−xS4. The introduction of dopants
causes a tetragonal distortion away from the body-centred cubic parent phase, Na3PS4, leading to ordering of
the Na ions. Unlike other ionic conducting systems, such as Li-stuffed garnets, this is not associated with a
large decrease in Na+ conduction, and computational simulations indicate that vacancy generation can be
realised in the tetragonal phase, and that the conductivity of Na3.1Sn0.1P0.9S4 is similar to that realised in solid
salt:polymer composites [201]. The efficacy of the approach is further demonstrated by the
room-temperature Na+ conductivity of 4 cm−1 afforded in the related phase, Na11Sn2PS12, where a higher
concentration of SnS4 tetrahedral units delivers a complex three-dimensional pathway for Na+ conduction
involving all five crystallographic Na positions, and additionally, two low-energy interstices [202].
Similar conductivities have been realised in the NASICON structural family of oxide-based materials
shown in figure 25, exemplified by series such as Na1+xZr2SixP3−xO12, where the carrier concentration is
adjusted by the classic approach of using an aliovalent chemical substitution, in this case, replacing P5+ with
M4+ ions. In Na3Zr2Si2PO12 [203], the conductivity can be increased towards 4 mS cm−1 through control of
the material’s microstructure and densification processes.
Current and future challenges
The deployment of these materials into battery technologies faces the great challenge of interface
management. Addressing this requires a better understanding of the role of electrolytes in interfacial
processes. The minimisation of interfacial resistance is vital to avoid ohmic losses, which both reduce the
energy storage capacity and lead to potentially hazardous heating. The challenge is to develop an interface
which shows low resistance whilst retaining (electro)chemical stability during the multiple processes of
battery cycling involving a change in potential and the (de-)swelling of electrodes. Meeting these challenges
requires moving beyond the scale of material manipulation via crystal chemistry.
The NASICON phase has been explored via different synthetic routes, and these lead to variations in
conductivity spanning an order of magnitude. An interesting development is the use of a ‘fluoride-assisted’
route to deliver a multi-phase sample with the composition Na3Zr2Si2PO12+x NaF [204]. The presence of
NaF leads to an increased grain size, which reaches a maximum for x = 0.7. As the grain size increases, the
conductivity is enhanced from 0.23× 10−3 to 1.4× 10−3 S cm−1 as shown in figure 26. This is accompanied
by a negligible change in the activation energy, indicating that the increase in grain size and the associated
reduction in grain boundary volume is the likely cause for this enhancement in conductivity. Similar
enhancements of conductivity up to 10−3 S cm−1 have been achieved using liquid-phase sintering [205] to
aid the densification of Na3Zr2Si2PO12 at 900 ◦C rather than the 1200 ◦C necessary for conventional ceramic
synthesis.
Many solid electrolytes have limited thermodynamic stability and react with metallic sodium. An
examination of the interface between Na3Zr2Si2PO12 and a metallic Na anode has shown that the reduction
of the electrolyte is kinetically limited; the initial reduction of Zr and Si is self-limiting and the electrolyte can
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Figure 25. The fast Na+ conducting material Na3Zr3Si2PO12 contains Na+ (black spheres) in the three coordination
environments indicated by orange Na–O bonds. The ZrO6 and PO4 units are represented by grey octahedra and green tetrahedra,
respectively.
be plated and stripped with Na in a symmetric cell as long as the current density is kept below the
0.2 mA cm−2 limit for Na dendrite growth [203]. A relatively high charge rate of 2 C has been demonstrated
using sulphide-based electrolytes Na3+xMxP1−xS4 (M = Ge, Ti, Sn), although this was achieved using a
Na2Ti3O7 anode to avoid the problem of Na dendrite growth.
Advances in science and technology to meet challenges
The enhancement of electrolyte performance requires an intimate understanding of the mechanisms of Na+
migration. A recent analysis of the effect of varying the Na content in the NASICON phases,
Na1+xZr2SixP3−xPO12, has indicated a more complex distribution of Na than expected, which implies that
ionic conduction must proceed via a correlated mechanism with an organised movement of multiple ions,
rather than the more widely anticipated hopping of individual ions onto vacant sites [206]. These insights
arise from combining computational simulations of molecular dynamics and advanced analysis of neutron
scattering data to deliver insights that could not come from either computation or experiment alone. Similar
complementarity is seen in the characterisation of the lattice dynamics of the novel perovskite-phase
Na1.5La1.5TeO6 via both impedance and muon spin relaxation [207]. This combination of bulk and local
probes shows how the local barrier for Na+ migration is significantly lower than that indicated by the total
impedance, which suggests that chemical modification may deliver useful increases in conductivity in this
structural family.
These kinds of detailed insight need to be extended from isolated electrolyte materials to look at the
interplay of chemistry and microstructure in delivering stable interfaces. This can be illustrated by two
differing approaches to using composites to deliver solid-state batteries capable of extended cycling lifetimes.
A solid-state battery with excellent stability at up to 10 000 cycles, even at the high charging rate of 10 C
has been realised by wetting the electrolyte interface with ionic liquids [208]. The solid electrolyte is an
intimate composite of Na3Zr2Si2PO12 combined with adventitious Na3La(PO4)3 introduced by an
unsuccessful attempt to substitute La3+ into the Zr4+ site. The introduction of this impurity phase increases
the density of the ceramic and so the presence of the passive impurity counterintuitively enhances the
conductivity of the electrolyte composite beyond that of the pure Na3Zr2Si2PO12 phase. The capacity of
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Figure 26. Formation of a composite between Na3Zr2Si2PO12 and NaF leads to increasing densification. The conductivity of the
resultant material (a) increases to a peak value of 1.4 mS cm−1 when the density is maximised, as shown in (b).
90 mAh g−1 from the Na3V2(PO4)3 cathode combined with a metallic sodium anode is modest, but the
stability and lifetime of the device show the efficacy of this approach for interface management. A similar
capacity can be realised by casting ceramic particles in the NASICON phase in a polyvinylidene
fluoride-hexafluoropropylene polymer to combine the conductivity of the ceramic with the mechanical
properties of the polymer [209]. This solution-cast composite delivers a flexible electrolyte with a
room-temperature conductivity of 2.25× 10−3 S cm−1, which matches that of the pure ceramic, whilst
maintaining the ease of processing associated with the polymer.
Concluding remarks
Both oxide- and sulphide-based Na+ electrolytes are now reaching the desired conductivity to enable
solid-state batteries. The challenges in developing viable battery technologies lie in managing the interfaces
between the solid electrolyte and the electrodes, and recent proof-of-concept studies have demonstrated the
efficacy of an electrode/electrolyte composite approach. These examples serve as useful demonstrations of
how chemical synthesis and materials processing must proceed in tandem to deliver cycling longevity from
the management of electrolyte interfaces. Understanding the roles of the components and how they stabilise
the interfaces will require a coordinated synthetic, experimental, and computational approach. There will
also be a key role for operandomeasurements of crystal structure, lattice dynamics, and ion mobility in
developing long-term electrochemical cycling in an all-solid-state sodium battery.
Acknowledgments
The authors gratefully acknowledge the support of the ISCF Faraday Challenge projects NEXGENNA (Grant
No. FIRG018) and SOLBAT (Grant No. FIRG007), the EPSRC (EP/N001982/2), and the University of
Sheffield for support.
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5. Testing protocols for Na-ion batteries
Juan D Forero-Saboya and Alexandre Ponrouch
Institut de Ciéncia de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Catalonia,
Spain
Status
For all battery chemistries, a reliable electrochemical setup is essential. Half-cell configurations with typically
two or three electrodes, using lithium metal as the counter electrode (CE) and reference electrode (RE), are
standard setups for Li cells. However, several requirements need to be fulfilled to achieve reliable results
(figure 27) [210, 211]. The CE must have high capacity, fast reaction kinetics, and should not contaminate
the electrolyte solution with reaction byproducts. Meanwhile, the RE should ideally be nonpolarisable (a
current flow should not affect its potential) and should have a reliable and stable potential. Although these
conditions are satisfactorily met in Li half-cells, several studies point to various issues associated with the use
of Na-metal CEs and REs [196, 212, 213].
Most of these issues are rooted in the low stability (and high solubility) of the SEI formed on the Na
metal electrode (see the ‘electrolyte/interface, SEI layer’ section, by Reza Younesi). Considering the difficulty
of developing new battery chemistries, in which both electrodes and electrolytes must be studied in parallel,
the use of reliable electrochemical protocols is crucial in order not to discard potentially interesting material
candidates or to make erroneous interpretations of the results. In this section, the most common issues
arising from the use of Na metal CEs and REs are described, and recent strategies to overcome them are
introduced.
Current and future challenges
The instability of the Na metal–electrolyte interface has several consequences for any electrochemical test
involving Na metal as both the CE and the RE. First, the constant SEI dissolution/reformation results in the
contamination of the electrolyte with soluble/gaseous species. When alkyl carbonate-based solutions are in
contact with Na metal, a growing amount of CO, CO2, methane (CH4) and ethylene (C2H4) is generated
(figure 28(a)). The presence of soluble compounds, such as dimethyl 2,5-dioxahexane dicarboxylate
(DMDOHC), has been observed in the solution, even after 5 d [213]. While the nature/amount of such
contaminants is dependent on the electrolyte formulation, their impact on the working electrode interface
and electrochemistry is clear. For instance, the impedances of hard carbon (HC)||HC (2-electrode) and
HC||Na (3-electrode) cells left at OCV for 10 d indicate that the HC interface becomes more and more
resistive when Na metal is present [196]. When comparing cyclic voltammograms of 1 M NaClO4 in PC, Lee
et al concluded that electrolyte decomposition products (formed on Na surface) diffuse to the working
electrode (WE), where they are oxidised. Such parasitic reactions can obscure true measurements of the
electrochemical performances of a given material [214]. Pfeifer et al observed that carbonate electrolytes
(e.g., DMC, EC, PC) containing either NaClO4 or NaPF6 react readily in contact with Na, producing
coloration or cloudiness of the electrolyte due to side reactions [302].
Sodium-ion intercalation into Li4Ti5O12 was investigated using Na metal and activated carbon (AC) as
CEs, with a significant cyclability improvement in the latter case. This was ascribed to a different passivation
layer being formed, depending on the CE used, and possibly to more severe PVDF binder decomposition
when Na is present. The highly inhomogeneous plating of Na, together with an increase in the Na metal
impedance as a result of time and cycling, could also result in significant limitations on the cyclability of half
cells (figures 28(b)) and (c)). Thus, when tests are performed on two-electrode half cells, the real
performance of the active materials being tested as WEs can be significantly underestimated.
Finally, as the coulombic efficiency of Na plating/stripping is usually very low, the stripping of Na from
the CE commonly involves two types of metallic Na: the freshly deposited one (Naplated) and the Na bulk
originally present as a disk or a foil. This can lead to an artificial voltage step that arises during the reduction
of the active material used as the WE (figure 28(d)). Such a step can only be seen in two-electrode cells and
was ascribed to the higher overpotential for Nabulk when compared with Naplated stripping [212, 213]. Once
again, such an electrochemical response is only associated with the CE, and could be misleading.
Advances in science and technology to meet challenges
Several strategies have been developed to circumvent the problems associated with Na metal. One of the
main approaches has been to control the nature and properties of passivation layers. Artificial passivation
layers using freestanding composite layers consisting of Al2O3 particles and liquid electrolyte-swollen
poly(vinylidene fluoride-co-hexafluoropropylene) polymers were developed, leading to an improved cycle
life of Na||Na symmetric cells. However, evidence of dendrite formation and/or a cell impedance increase
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Figure 27. Scheme of a half-cell configuration with the main properties required for reliable CEs and REs. Reproduced from
[210]. CC BY 3.0.
Figure 28. (a) GC/FTIR Gram–Schmidt reconstruction of gas from the reaction between Na metal and EC:DMC (1:1 w/w)-NaPF6
1 M, without FEC (a) after 5 d, (b) after 48 d, with 3% of FEC (c) after 5 d, (d) after 48 d. Reproduced from [213]. CC BY-NC-SA
3.0. (b) Charge/discharge curves (0.05 mA cm−2) of symmetric cells cycled at 25 ◦C using 0.1 M LiTFSI or NaTFSI in EC0.5:PC0.5.
Reproduced from [210]. CC BY 3.0. (c) Nyquist diagrams of impedance measurements over time of symmetric Na/Na cell at
25 ◦C. Reproduced from [196]. CC BY-NC-SA 3.0. (d) Illustration of an artificial voltage step phenomenon observed in a
two-electrode cell due to the Na CE (and RE). Reprinted from [212], Copyright (2014), with permission from Elsevier.
could still be seen at 0.5 mA cm−2 [215]. A similar layer using 2.8 nm-thick Al2O3 produced via
low-temperature plasma-enhanced atomic-layer deposition was found to significantly increase the cyclability
of Na symmetric cells up to 0.5 mA cm−2 [216].
Another avenue used to tune the SEI stability is to adjust the electrolyte formulation. Since this approach
has already been discussed in section 4.4, here, we will focus on the impact of a few selected electrolytes on
the reliability of Na-metal CEs and REs. Fluoroethylene carbonate (FEC) is a popular electrolyte additive in
Na-ion batteries which limits the Na reactivity towards the electrolyte and helps in minimising the
irreversibility of cathode materials. However, this does not come for free, and significant Na metal
impedance increases can be measured together with a continuous release of small quantities of gases, even
after 5 d [213]. Glyme-based electrolytes are among the most successful for Na metal. Schafzahl et al
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demonstrated that improved cyclability and coulombic efficiency (reaching 97.7%) could be achieved in
NaFSI/dimethoxyethane (DME) when compared with a carbonate-based electrolyte [217].
Superconcentrated glyme-based electrolytes were also investigated, reaching a coulombic efficiency for Na
plating/stripping of 99.3% at 5 M NaFSI in DME [218].
Beyond the electrolyte formulation, several parameters can affect Na plating and stripping. Rupp and
Vlad systematically investigated the roles of the electrolyte, separator, electrode preparation, cell architecture,
cycling procedure, and the nature of the current collector [219]. The use of NaCF3SO3 in diglyme with a
current density of 2 mA cm−2, a Celgard 2325 separator and a high-density polyethylene gasket (to give
better control of the current lines) led to a Na plating/stripping coulombic efficiency of 99.74%. Careful Na
metal preparation could also result in limited surface contamination and an improved morphology [220].
While some of the strategies presented above could improve the Na plating/stripping coulombic
efficiency and cyclability, the stability of the SEI (and its impedance) and the absence of soluble/gaseous
contaminants remain to be ascertained. A radically different approach, relying on alternative CEs and REs, is
also considered. A promising example consists of using the NASICON-type Na3V2(PO4)3 (NVP) as an
intercalation-type CE [221]. Half cells with NVP as the CE exhibited stable cycling over a long period with
low polarisation compared to Na as the CE. Measurements could also be performed at temperatures above
the Na melting point, and due to the very flat voltage plateau of NVP, a two-electrode cell configuration
could be assembled with a reliable potential measurement. Since NVP operates within the ESW of most
electrolytes, it is also likely that such a CE does not produce undesirable contaminants. However, similarly to
the use of capacitive electrodes (e.g., activated carbon), the sizing of the CE as a function of the expected
capacity of the WE must be carefully considered [211].
The potential of the RE in a sodium cell is determined by the activity of the Na+ ions in its vicinity,
unlike the case for aqueous systems, where REs are commonly reversible to anions. As mentioned previously,
SEI instability on Na metal has been shown to cause unreliable voltage measurements [210]. Lee and Tang
evaluated alternative REs for Na-ion batteries: tin alloy, nickel hexacyanoferrate, activated carbon (AC) and
silver ion [222]. Unfortunately, continuous potential drifts were recorded for tin alloy, nickel
hexacyanoferrate and AC REs. Although silver ion (Ag+/Ag) REs were found to be stable, they required the
presence of a separate compartment and a porous glass frit to avoid the migration of silver cations; thus, they
were not suitable for compact cell designs, such as Swagelok cells.
Concluding remarks
The use of Na metal for the CE and RE in a half-cell configuration results in several issues: electrolyte
contamination, an increase in cell impedance, unstable potential, an artificial voltage step, and premature cell
failure, among others. Most of these issues are rooted in the poor stability of the Na metal interface in most
organic electrolytes. While several parameters can be tuned to mitigate such interfacial instability (e.g.,
electrolyte formulation, Na pre-treatment, cell geometry, etc.), there is currently no protocol which
guarantees that reliable results can be obtained using Na metal. Since highly unsafe Na metal is mostly used
in half-cells for material testing, one may wonder if research efforts need to be dedicated to making this
challenging electrode work properly. Alternative CEs and REs such as NVP, AC, and silver-based electrodes
are very attractive candidates for reliable electrochemical setups. While care must be taken when using such
electrodes, their use can easily and routinely be integrated into most battery laboratories to obtain more
reliable, and perhaps unexpectedly better results, than with Na half cells.
Acknowledgments
Funding from the European Union’s innovation program H2020 is acknowledged:
H2020-MSCA-COFUND-2016 (DOC-FAM, Grant Agreement No. 754397). A Ponrouch is grateful to the
Spanish Ministry for Economy, Industry and Competitiveness Severo Ochoa Programme for Centres of
Excellence in R&D (SEV-2015-0496).
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6. Advanced characterisation
6.1. Neutron characterisation of battery materials
Emily M Reynolds1,2 and Martin O Jones1,2
1 Science and Technology Facilities Council, North Star Avenue, Swindon SN2 1SZ, United Kingdom
2 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom
Status
Neutrons scatter from elemental nuclei and consequently interact weakly with condensed matter. This
property offers considerable advantages in addressing the challenges associated with characterising the
complex structure and dynamics of Na battery materials. Neutrons have a highly penetrating nature and can
probe magnetic structure, resolve light elements and distinguish elements close to each other in atomic
number. As a result, neutron diffraction (ND) can provide an insight into transition-metal (TM)
distributions, magnetic structure, and Na diffusion pathways. Total scattering can distinguish between locally
ordered TM distributions, observe local structural changes due to oxygen redox, characterise the structure of
nanoparticles, and quantify disorder in hard carbon anodes. Inelastic techniques such as inelastic neutron
scattering (INS) and quasi-elastic neutron scattering (QENS) can probe dynamics including Na diffusion,
and techniques such as reflectometry and small-angle neutron scattering (SANS) allow us to observe the
formation of the SEI layer and characterise the change in thickness/porosity of the SEI and electrode
materials. Imaging techniques including radiography, tomography, and Bragg edge tomography can reveal
the evolution of the components during cycling, and muon spectroscopy can probe the dynamics and
energetics of Na diffusion. In addition to these specific techniques, neutrons remain a superior
characterisation technique for operando studies of commercial and custom batteries due to their highly
penetrating nature. Insight into the structural changes on cycling has revealed the mechanisms responsible
for capacity fade [223], cycling-induced cation mixing [224], intermediate phases [224], dependence on cycle
rate [225], and has led to the optimisation of commercial materials [226].
While the capabilities of neutron characterisation are vast, there is still scope for improvement through
advances in facilities, instruments, and devices. For example, higher flux will increase temporal and spatial
resolutions, and permit operando studies at higher cycling rates with an improved signal-to-noise (S/N) ratio.
Operando neutron experiments are extremely challenging but are essential for a full understanding of battery
operation and failure. Current abilities may be improved by developing a simultaneous characterisation
facility that combines neutron scattering with electron paramagnetic resonance (EPR), NMR and X-ray
spectroscopy techniques, optimising and improving existing sample environment cells, and developing new
cells for scattering techniques currently without operando capabilities.
Current and future challenges
Disorder is a key feature in the most of the promising electrode materials, whether it be in structure, cation
and anion distribution, stacking faults, or defects. Distinguishing between disordered states and correlating
these to electrochemical properties is very challenging; however, cation disorder can improve Na transport,
prevent unwanted phase changes, and reduce volume changes during cycling [227–229]. Furthermore, the
structure of amorphous hard carbon (one of the most promising anode materials), and its Na storage
mechanisms are difficult to characterise due to the lack of long-range order. While neutron characterisation,
specifically total scattering, is well-equipped to address this, the routine characterisation of disordered
materials remains a challenge. The analysis of such data often requires additional techniques to generate
disordered models, either driven by physical/thermodynamic constraints, such as the Monte Carlo method,
or driven by data, such as the reverse Monte Carlo. In addition, operando total scattering remains
challenging, as any additional components from the battery setup interfere severely with data analysis.
Neutron characterisation excels in operando studies, despite the low neutron flux, which limits temporal
and spatial resolutions. For diffraction, this limits the cycling rate, spatial resolution across an electrode, and
the data quality achievable. For other techniques, such as quasi-elastic scattering (QENS), it has inhibited
in-situ studies altogether. Cell design is also complex, as the requirements for good quality data and
electrochemical cycling comparable to commercial battery operation are different and often incompatible.
In general, understanding and characterising the formation of the SEI on electrode materials, its chemical
composition, evolution on cycling, and impact on diffusion are huge challenges for the battery community
and are still in their infancy. Neutron reflectometry is an excellent technique for studying buried interfacial
structures, and the highly penetrating nature of neutrons means that SEI formation can be observed in-situ,
which is essential, as the interfacial structures are usually very sensitive to air. A major challenge for this
technique is the development of an operando cell that presents a thin electrode with an extremely smooth
surface.
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Figure 29. The upper part of the image is a representation of the ISIS neutron and muon source, depicting the production of
neutrons involving a linear accelerator (linac), synchrotron, and target. The lower part of the image shows the main neutron
techniques and how they are relevant to Na battery materials.
Advances in science and technology to meet challenges
Advances in instrumentation, such as guides and detectors used to obtain a higher flux in the sample will
improve the temporal resolution and data quality of operando studies. This can also allow operando
experiments for techniques where the current time resolution is too poor, such as QENS, as well as
improving spatial resolution for imaging studies. Advances in cell development will also benefit operando
studies, with the potential to reduce data collection times, allow the performance of simultaneous
multicharacterisation studies, and facilitate total-scattering operando experiments. The development of
operando total scattering is a significant challenge, as additional components from operando and/or
multicharacterisation cells interfere heavily with pair distribution function (PDF) data collection and
analysis; thus, new cells need to be developed with components that are either easily subtracted or have a
small contribution to the total scattering.
Alternatively, the temporal resolution may be improved through the development of stroboscopic
measurement techniques, whereby multiple identical experiments are carried out and the data are carefully
time marked to allow the data from the individual experiments to be combined, enhancing the S/N ratio and
resolution through overlapping data sets.
Advances in sample environments and equipment are essential, in particular, for in-situ and operando
experimentation, which enable simultaneous data collection from complementary techniques—so-called
‘multimodal characterisation’. Examples include simultaneously collecting NMR or EPR data whilst
performing neutron diffraction, or the combination of two different neutron techniques, such as diffraction
and imaging, a facility that is currently under development. The development of new multimodal
characterisation sample environments that can be utilised with different instrumentations and across
techniques with the same sample is vital. A full understanding of the physical phenomena that underpin
functional properties can only be obtained through this type of experimentation.
While advances in flux and sample environments will benefit disorder studies, data-analysis tool
development will allow heavily disordered systems to be understood. It would be helpful to improve analysis
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tools such as 3D-PDF [230, 231] that can identify Na sites when disordered, component analysis that can
separate multiple phases in the PDF, and machine-learning algorithms that can be used to identify
potentially interesting structures, including disordered ones.
Concluding remarks
Neutron characterisation techniques deliver significant capabilities to the energy materials community,
giving the possibility of full, multimodal characterisation of all functional properties and related structural
and spectroscopic phenomena. The development of new sample environments to permit these multimodal
studies across different instrumentations and techniques is essential to support progress in this area. Equally,
investment in neutron facilities to improve instrumentation, detectors, and neutron flux will result in
advances in the temporal and spatial resolution of neutron-scattering techniques. This combination of
developments will facilitate a comprehensive understanding of battery systems and thus promote the
creation of new and improved battery systems.
Acknowledgment
The authors wish to acknowledge the financial support of the Faraday Institution (Grant No. FIRG018).
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6.2. Solid-state NMR for the characterisation of Na-ion batteries
Valerie R Seymour1,2 and John M Griffin1,2
1 Department of Chemistry, Lancaster University, Lancaster LA1 4YB, United Kingdom
2 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom
Status
In order to develop and optimise new Na-ion battery materials, it is crucial to understand the underlying
structural chemistry in detail. In general, diffraction techniques are the go-to methods for studying
crystalline battery materials, and these continue to be the ‘gold standard’ in delivering atomic-level pictures
of the long-range structure. However, many structures and mechanisms involve short-range phenomena
such as disorder, dynamics, and defects that can be challenging to characterise by diffraction. In this regard,
solid-state NMR stands out as a powerful complementary approach due to its sensitivity to the local chemical
environment. Indeed, solid-state NMR has no requirement for long-range order, making it applicable to the
study of crystalline, disordered, and amorphous materials. It is also sensitive to dynamics over a wide range
of timescales, making it useful for studying ion migration and diffusion.
Solid-state NMR experiments on batteries can be carried out ex situ on materials extracted from cells, or
in situ using bespoke cells that are designed to fit inside the NMR detection coil (figure 30) [232]. Ex situ
experiments have the benefit that they are compatible with magic-angle spinning (MAS), meaning that
high-resolution results can be obtained. In situ experiments have the advantage that they give a better chance
of observing any short-lived or metastable states, since the cell is not disassembled for analysis.
Solid-state NMR studies of Na-ion battery materials are not yet as prevalent as for Li-ion battery
materials, but both ex situ and in situ approaches have been applied in a number of studies. For NaMnO2, ex
situmeasurements were used to follow changes in the domain structure during cycling. The spectra revealed
signals associated with the α- and β-phases, as well as the stacking-fault interfaces between these domains
(figure 31(a)) [233]. During desodiation, the peak corresponding to the β-phase reduced faster than the
interfacial resonance, indicating preferential extraction from this phase and/or loss of long-range order. For
the candidate cathode material Na2FePO4F, 23Na 2D exchange NMR experiments showed that the Na ions
move between crystallographic sites in the structure at a frequency of approximately 200 Hz [234]. Ex situ
measurements showed the formation of the fully oxidised NaFePO4F phase during cycling, suggesting a
two-phase desodiation mechanism. NMR performed on in situ 23Na has been applied to hard carbon-anode
materials, where it clearly revealed structural transitions during sodiation and desodiation [77]. At low
sodiation levels, the isolated Na ions showed a characteristically diamagnetic chemical shift; however, at high
sodiation levels, a marked change to paramagnetic chemical shifts signified the growth of quasi-metallic
clusters (figure 31(b)). Recently, in situ 23Na NMR and MRI have also been applied to observe the growth of
dendritic and mossy microstructures on Na metal anodes [235]. The results revealed a two-step mechanism
in which nucleating microstructures led to an increasing overpotential which triggered a rapid breakdown of
the electrolyte, causing cell failure.
Current and future challenges
One of the key challenges in the solid-state NMR of battery materials is the presence of strong paramagnetic
interactions arising from the unpaired electron spin density of transition-metal ions that are present in many
systems. These interactions can result in large resonance shifts and extensive line broadening which
complicate the acquisition and interpretation of NMR spectra. In most cases, the resolution is maximised by
performing measurements at fast MAS rates (⩾60 kHz) and using moderate or low magnetic fields. However,
even under these conditions, the resolution can still be compromised, making it difficult to identify all the
species that are present. In addition, the assignment of spectral resonances is often not straightforward. For
extended diamagnetic systems, plane-wave DFT codes can usually predict NMR parameters with sufficient
accuracy to allow assignment to a model structure. However, due to the greater complexity of paramagnetic
structures, progress in the development of accurate DFT methods for the calculation of NMR parameters has
been slower, and spectral assignment still largely relies on empirical trends.
Another challenge is the intrinsically low sensitivity of some NMR-active nuclei, which hinders or
precludes their observation. For some nuclei (e.g., 17O or 2H) the natural abundance is so low that their
observation is rarely possible under normal conditions. Isotopic enrichment of these nuclei is possible, but is
very costly and can require modification of the synthesis procedure, depending on the isotopically enriched
precursor available. Sensitivity is also a significant challenge in the detection of surface and defect species,
which play an important electrochemical role in many systems. Since NMR is quantitative, the detection of
such low-concentration species within a bulk sample can be very challenging, even when they contain
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Figure 30. (a) MAS rotors of various sizes and their corresponding maximum spinning rates. (b) Bag cell and (c) plastic cell
capsule designs used for in situ NMR. Reprinted with permission from [232]. Copyright (2017) American Chemical Society.
high-sensitivity nuclei. This challenge is exacerbated by in situmeasurements, in which the sensitivity is
typically further reduced due to the requirement for a large-diameter detection coil and a low filling factor
due to the cell components, which reduces the sensitivity further.
Advances in science and technology to meet challenges
An important area of NMR hardware development is MAS probe design. As discussed above, fast MAS can
help to remove the effects of paramagnetic interactions, which broaden and complicate solid-state NMR
spectra in battery materials. Fast MAS frequencies remove these interactions more efficiently, thereby
improving both the resolution and sensitivity of NMR spectra, so that more information can be obtained.
Very recently, a probe capable of MAS frequencies up to 170 kHz has been demonstrated [236]. Although
this ultra-fast MAS technology has not yet been widely applied, it represents a significant step forward, which
can be expected to support new insights into battery materials in the future.
Advances in DFT methods for the calculation of paramagnetic NMR shifts will play a key role in
addressing the issue of spectral interpretation and assignment. Although this remains a significant challenge,
progress has been made in recent years, and a number of methods have been successfully demonstrated for
model systems [237, 238]. Further development of these methods will enable them to be applied more widely
beyond proof-of-concept studies, although due to the complex and highly system-dependent nature of the
calculations, it is not clear whether paramagnetic shift calculations will become routine in the same way as
for non-paramagnetic materials.
Concerning the challenge of detecting insensitive nuclei and surface or defect species, significant progress
has been made in recent years through the development of dynamic nuclear polarisation (DNP). DNP
transfers strong electron spin polarisation from either native or exogenous radical species to nuclei within
the material of interest. Since the transfer is distance-dependent, exogenous radical species have a tendency
to selectively enhance surface groups [239]. Radicals native to the material structure have also been used to
enhance insensitive nuclei in the bulk structure [240, 241]. This approach holds considerable promise for the
study of unstable or reactive battery materials which may not be amenable to the addition of exogenous
polarising agents. Despite the promise that DNP holds, further developments are required to properly
understand polarisation transfer mechanisms, and also to widen the temperature range over which it is
effective, so that materials can be studied under ambient or variable-temperature conditions.
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Figure 31. (a) 23Na ex situ NMR spectra of NaMnO2 at different states of charge. Resonances corresponding to α-NaMnO2,
β-NaMnO2, and interfacial stacking faults are highlighted in green, blue, and red, respectively. Reprinted with permission from
[233]. Copyright (2014) American Chemical Society. (b) In situ e 23Na NMR spectra of a hard carbon anode material as a
function of the state of charge. The movement of the signal from the diamagnetic (low chemical shift) regime to the paramagnetic
(high chemical shift) regime shows the transition from isolated Na ions to quasi-metallic clusters. Reproduced from [77] with
permission of The Royal Society of Chemistry.
Concluding remarks
Although widely applied as an analytical technique, solid-state NMR continues to be a highly active area of
research in itself, which engenders continual developments in experimental hardware and methodology.
These ongoing advances are helping to deepen and transform our understanding of battery material
structures and mechanisms, so that their material properties can be optimised. For the study of Na-ion
battery materials in particular, NMR has already provided considerable insight and will continue to do so as
faster MAS rates become more widely available and advances in DNP methodology provide ever higher
sensitivity and selectivity.
Acknowledgments
We acknowledge the Faraday Institution NEXGENNA project (FIRG018) and Lancaster University for
financial support.
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6.3. Nanoscale characterisation of the local physical properties of Na-ion batteries,
electrodes, and interfaces
Oleg V Kolosov1,2 and Yue Chen1,2
1 Department of Physics, Lancaster University, Bailrigg, Lancaster LA1 4YB, United Kingdom
2 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom
Status
The majority of characterisation techniques enabling the current fast development of rechargeable batteries
fall into three major categories—the traditional electrochemical performance measurements, such as
galvanostatic charge/discharge and cyclic voltammetry; crystal and nanostructural characterisation, such as
x-ray diffraction and electron microscopy (EM); and NMR, XPS, and soft x-ray spectroscopy, which also
provide chemical component analysis. The long history of battery technology development has amassed an
in-depth understanding of electrochemical processes based on the characterisation of these electrochemical,
structural, and chemical properties, derived from the above electroanalytical methods and their techniques.
While these properties are essential, as shown in figure 32, the physical properties, including the mechanical,
thermal, and electrical properties, of the materials deployed in Na-ion batteries are no less important.
Growing attention has recently been paid to revealing the nanomechanical properties of sodium
nanodendrites in high-capacity Na-metal anodes [242], and the generation of interfacial stress in layered
TMD [243] anodes due to the intercalation of the larger Na ions compared to Li ions. It is also well
established that the Young’s moduli of the SEIs on the negative electrodes define the battery’s resistance to
dendrite growth whereas the stability of polycrystalline cathodes crucially depends on the mechanical
integrity of their components during the charge–discharge cycle expansion and their resistance to crack
propagation [244]. The electrical conductivity of the electrodes and the ionic conductivity of the interfacial
layers are the key limiting parameters for battery rate performance, and the thermal conductivity is another
factor that defines battery operation in demanding applications. Therefore, measuring mechanical moduli,
local stress, electrical conductance, and thermal transport in real-life batteries and model systems, and
correlating these data with electrochemical and structural analyses will provide an essential platform for
overcoming existing challenges and for the development of novel paradigms in Na-ion batteries.
Current and future challenges
One of the major challenges in the nanoscale measurement of the physical properties of electrodes and
interfaces in NIBs is linked to the wide range of length scales involved. The electrode thicknesses are typically
a few tens of micrometres (µm), the individual cathode electrode particles are a few µm across, and hard
carbon particles in the battery anodes are on the nm length scale. The SEI layers are several tens of nm thick,
while the cathode electrolyte interphase (CEI) can be as little as a few nm thick [245]. Na-ion incorporation
occurs at interatomic distances that are on the sub-nm length scale. The successful investigation of this
phenomenon requires a means of characterisation that can provide local measurements of physical
properties with the commensurate nanoscale lateral resolution. Another major challenge is the extreme
heterogeneity of the battery components. For example, the mechanical moduli range from sub-MPa values
for the outer layers of an SEI-facing electrolyte to hundreds of GPa for cathode polycrystalline particles, and
the ionic conductivity of the sodium ranges from∼10−2 µS cm−1 when crossing the SEI layers at the
electrode–electrolyte interface to∼10 µS cm−1 within the interlayers of the TMD anode. Lastly, but most
importantly, the dynamic evolution of nanoscale physical properties, which are closely related to the sodium
ion transport and electrolyte electrochemical/thermal decomposition, is exceptionally difficult to ‘look’ at,
since sodium battery active materials and interfaces are buried deep within the volatile liquid electrolyte
under battery operating conditions.
To satisfy these complex characterisation capability requirements at multiple length scales and address
the electrode heterogeneity and the fluidic electrochemical environment, scanning probe microscopy (SPM)
has been applied for the micro/nanoscale characterisation of the dynamic and local physical properties of
battery electrodes and interfaces. In these studies, model systems are often used, e.g., highly oriented
pyrolytic graphite (HOPG) [246, 247] or the physical vapor deposition/chemical vapor deposition of
binder-free electrodes [245] as a proxy for the battery electrode, enabling the direct observation of SEI
formation and ion intercalation through nanomorphology measurements of the electrode surface. However,
it is not sufficient to only investigate the surface physical properties of these structures; tools are required that
can access the inner structures of electrodes and interfacial layers. The direct nanoscale characterisation of
multiple complementary physical properties of real-life batteries, for ex situ and in-situ/operando cases,
remains the holy grail of Na-ion characterisation requiring an innovative solution for three-dimensional
(3D) sample preparation.
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Figure 32. Critical physical/chemical properties and their corresponding characterisation techniques.
Advances in science and technology to meet challenges
Among the vast number of techniques used for sodium battery interface analysis [248], SPM occupies a
unique place, as it can provide both the required nanoscale spatial resolution, as well as being sensitive to the
local physical properties of the studied material. Using SPM in the electrochemical environment on the
model HOPG substrate allowed the study of the initial stages of SEI growth [246], revealing the dynamics of
ion intercalation. The downside of SPM is that it requires nanoscale flat samples and mainly allows the
investigation of surfaces rather than 3D structures. Focused ion beam (FIB) sectioning, widely used in EM
measurements, is inadequate for SPM, as the sections produced are too small to be measured efficiently and,
more significantly, the Ga ions can drastically change the physical properties of the studied material during
sectioning. As shown in figure 33(a), a much more suitable approach is to use advanced nanosectioning
using multiple Ar-ion beams, known as beam-exit cross-section polishing (BEXP), which produces a close to
atomically flat surface with minimal damage, due to the non-reactive nature of Ar atoms. In the BEXP
method, the open-angle tilted section adjacent to the intact surface of the sample produces a perfect area for
the material-sensitive SPM, additionally expanding the interfacial area by a factor of 5–10, allowing the study
of interfaces a few nm thick (such as the superlattice in figure 33(b)) with a total section area on the sub-mm
length scale. By using a dedicated material-sensitive SPM contrast, such as nanomechanical via sample
excitation, ultrasonic force microscopy (UFM), or tip-excitation waveguide UFM [249], the conductance of
the cross-sectioned samples can reveal the local physical properties of the internal structure at nanoscale
resolution. Figures 33(c)–(e) show a diagram of a commercial Ni-rich cathode polished by BEXP and the
prepared sample’s cross-sectional UFM images. In figures 33(c) and (d), the cathode particles at different
states of charge (SOC) in the binder and the conductive carbon black matrix can be clearly distinguished.
Additionally, the crack network within the grain section is visible in the nanomechanical UFM images, which
could serve as a perfect model for operando SPM studies of dynamic crack propagation in cathode
polycrystalline particles, revealing the fundamentals of capacity degradation in sodium–transition-metal
oxide cathodes.
Another approach for examining 3D structures is to use direct in situ sectioning of the sample through
nanotomography [250], in which the tip mills out the surface along the section plane, followed by
nanomechanical, nano-electric, or surface-potential measurements. Such an approach is more suitable for
the sectioning of softer layers, such as the SEI, in which follow-up nanomechanical SPM probes all the layers,
starting from the double layer in the electrolyte, through to the soft and then more compact SEI layers at the
interface with the anode. As shown in figure 33(f), when BEXP and nanotomography are combined, the
organic adsorption layer that forms on the atomically flat cut slope of the HOPG anode can be scraped by the
SPM tip. This allows for the uncovering of the subsurface physical properties of electrodes and interfaces,
which could be the ‘buried treasure’ in the study of Na-ion batteries.
Overall, SPM will allow the investigation of the highly important Na-ion intercalation phenomena in
carbon-based [247] and other electrodes by observing both the nanoscale morphological (operando
dimensional change) and physical parameters (operandomechanical compliance that was shown to affect the
mechanical properties during atomic intercalation) [251]. Other physical parameters, such as the surface
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Figure 33. Top (a) principle of BEXP (www.lancastermaterialanalysis.co.uk/beam-exit-cross-sectional-polishing). The Ar beam
enters the side of the sample below the surface, producing a near-atomically-flat low-damage surface adjacent to the untouched
surface layer with an open angle and topographically flat geometry, which is perfectly suited for the follow-up SPM
characterisation of physical properties. (b) Schematics of the BEXP approach for the preparation of a sample of cathode material.
(c), (d) SPM 3D rendering of a Ni-rich cathode composite electrode with superimposed nanomechanical contrast; the cathode
powder particle section clearly shows cracks that developed upon the change of SOC. (e) An SPM 3D rendering of a
BEXP-prepared HOPG model anode with superimposed nanomechanical contrast obtained via ultrasonic fore microscopy
(UFM); the cut area has sub-nm surface roughness.
potential and conductance changes, when measured ex-situ, can provide additional information in such
studies.
Concluding remarks
The real-space physical-property nanomapping of battery electrodes and interfaces via multifunctional SPM,
combined with the efficient and artefact-free sample preparation approach via BEXP and nanotomography
will provide a valuable characterisation toolbox for the development of the next generation of Na-ion
batteries. Furthermore, it will allow the direct investigation of SEI stability and composition, the
development of more robust cathode/anode–electrolyte interfaces, and the exploration of Na-ion
intercalation, enabling the selection of a winning electrode–electrolyte combination.
Acknowledgments
The authors wish to acknowledge the financial support of the the Faraday Institution (Grant No. FIRG018),
the EU Graphene Flagship Core 3 project, the EPSRC project EP/V00767X/1, and the scientific and
methodology insight provided by Marta Mucientes and Nuria Tapia-Ruiz. We are also grateful to our
industrial partners Bruker, Leica Microsystems, and LMA Ltd. for their scientific and financial support.
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6.4. Dissolution benchmarking of sodium-ion electrodes
Shahin Nikman1,2 and Stijn F L Mertens1,2
1 Department of Chemistry, Lancaster University, Lancaster LA1 4YB, United Kingdom
2 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, Didcot OX11 0RA,
United Kingdom
Status
Loss of battery electrode inventory to dissolution processes is one of the major sources of capacity fading. In
the lithium-ion field, much research has been conducted into the charge–discharge profiles and electrolyte
formulations that contribute to dissolution, in particular, for electrodes based on manganese, nickel, and
cobalt, which can be translated to sodium-ion chemistries (with some caveats) when similar electrode
materials and electrolytes are used [252]. Na+ ions have a 34% larger radius than Li+ (1.02 vs. 0.76 Å) which
causes a greater strain on the electrode materials during insertion and extraction. Sodium-ion electrodes also
span a larger compositional range than current LIB materials, encompassing transition-metal oxides,
alloying reactions, and conversion-reaction materials, and therefore enhance both the possible sources and
consequences of electrode dissolution. While direct loss of active electrode material due to dissolution into
the liquid phase is typically in the single-digit percentage of overall capacity loss, the presence of dissolved
transition metals in the electrolyte is known to rapidly accelerate SEI formation, leading to near-complete
passivation of the negative electrode. This so-called dissolution–migration–deposition (DMD) process is
estimated to account for up to 30% of the overall capacity loss in manganese-oxide-based intercalation
batteries [253].
Several strategies already exist to counteract electrode dissolution, such as heteroatom doping [254],
coatings, and the use of concentrated electrolyte formulations, figure 34. These indeed improve the
long-term stability, but typically at the expense of the power and/or energy densities of the electrode. Fast
throughput techniques that can determine electrode dissolution rates as a function of electrode potential,
time, and electrolyte formulation are therefore essential and urgent. Understanding the effect of electrolyte,
solvent, and electrode composition on dissolution and battery longevity will lead to better-informed
materials discovery for durable and efficient NIBs.
Current and future challenges
Due to their propensity to dissolve manganese, LiMn2O4 and NaxMnO2 electrodes are widely used for model
studies. A primary challenge is to accurately identify the cause of electrode dissolution. For example, Hunter’s
mechanism of Mn3+ disproportionation into soluble Mn2+ and insoluble Mn4+ was confirmed in an
aqueous solution but may differ in non-aqueous, fluoride-containing carbonate-based electrolytes. Recent
studies have suggested that in non-aqueous electrolytes, Mn(III) is the dominant dissolved oxidation state.
Other elements may also dissolve into the electrolyte, for example, vanadium from NaV3O8 cathodes [255].
As the rate of dissolution is proportional to the electrochemically exposed surface area, bulk
characterisation techniques, such as x-ray diffraction may not be adequate. In situ surface-sensitive methods,
such as electron energy-loss spectroscopy, extended x-ray absorption fine structure, and high-angle annular
dark-field imaging show that de-intercalated λ-MnO2 is chemically unstable [256]. However, only the
surface layers break down to Li2Mn2O4 and Mn3O4, where the average Mn oxidation state is much closer to
three. The stability of the electrode therefore cannot be predicted, based on the bulk composition or the cell
potential, which would indicate only an insoluble product. Such structural changes also induce stress in the
host material due to changes in the lattice constant, leading to cracks and again increasing the exposed
surface area. Although such methods have not yet been employed to elucidate sodium-ion electrode
dissolution, the intercalation of Na+ induces more strain than the intercalation of Li+ due to its larger
radius, potentially leading to more cracks, a greater exposed surface area, and hence, enhanced dissolution.
Heteroatom dopants, such as iron and nickel in, for example, Na0.67Fe0.5Ni0.15Mn0.65O2 may increase stability
against manganese dissolution [257], but may themselves dissolve following chemical or electrochemical
pathways and increase the potential range in which dissolution takes place. The dissolution problem is
further compounded in sodium-ion anodes, in which elements such as tin, antimony, phosphorus, titanium,
copper, iron, or sulphur can produce soluble byproducts during operation, whereas graphite does not.
Solvents, electrolytes, and contamination from moisture or CO2 can further accelerate the degradation of
sodium cathode host materials at a much higher rate than their lithium counterparts [258]. The dissolution
of ions into the electrolyte solution depends on the particular electrolyte formulation, ion solubility, and
potential programming of the battery cell, which encompasses a huge array of variables. The lack of direct,
real-time dissolution measurements greatly compromises the time and potential resolution of dissolution
information. To date, most dissolution diagnosis has been conducted post-mortem. For example, inductively
coupled plasma (ICP)-based elemental analysis of the electrolyte has been used to identify ppm-range
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Figure 34. Key dissolution reactions occurring on sodium-ion anodes: attack from reactive species in solution, anodic dissolution,
and crack formation caused by structural strain. Heteroatom doping and surface coatings may have a stabilising effect.
Figure 35. V-shaped flow cell direct-injection online ICP analysis design for fast-throughput dissolution analysis. The electrolyte
flows past an online counter electrode (CE), passes over the working electrode (WE) surface controlled by a potentiostat and the
reference electrode (RE). Any dissolution products are efficiently transported to an ICP-type instrument for nebulisation and
ionised by the plasma torch. The red-framed insert is a magnified view of the contact between the flow cell and the working
electrode.
concentrations of dissolved iron from sodium iron hexacyanoferrate electrodes as a function of the surface
coating strategy [259]. Energy-dispersive x-ray spectroscopy (EDS) is also commonly used, for example, to
analyse the separator in order to identify trapped ions, or to analyse the negative electrode to identify DMD
products from Na0.67MnO2 electrodes [260]. The preparation and ex situ handling of such samples increases
experimental error and may compromise the reliability of the conclusions. Furthermore, full and half cells
prepared in CR2032 coin cells do not isolate a single cause of dissolution, as the effects of electrode
arrangement, cell pressure, the counter-electrode material and current collectors must also be considered.
Open questions are whether electrode dissolution is electrochemically or chemically induced, i.e., which
are the most significant dissolution pathways, and their accurate quantification. Methods of detection must
be highly sensitive, as dissolution only affects a small part of the overall electrode mass, and setups must
rigorously exclude agents known to accelerate dissolution, such as water, as otherwise, artefacts will arise. An
adequate dissolution monitoring system must therefore be carefully designed.
Advances in science and technology to meet challenges
ICP-based instruments are routinely available, and allow highly sensitive elemental analysis, with detection
limits of sub-ppb using optical emission spectroscopy (ICP-OES) or even sub-ppt using mass spectrometry
(ICP-MS). Primarily used for aqueous samples, these are, to a certain degree, forgiving towards non-aqueous
battery solvents and electrolyte salts (up to 0.1 M (ICP-MS) or 1 M (ICP-OES) without dilution). The
observation of dissolution in non-aqueous electrolytes is challenging, due to the need for oxygen and water
to be excluded from the electrolyte. Prohibitive factors include instrument corrosion from highly reactive
electrolytes and the high cost of many high-quality non-aqueous electrolyte solutions.
Recent advances in electrochemical flow cell design have enabled direct observations of the dissolution of
candidate electrode surfaces in non-aqueous environments under potential or galvanostatic control.
Typically, the electrochemical flow cell outlet is directly connected to the ICP nebuliser by tubing, in the
so-called direct-injection analysis. Successful examples include the flow-through scanning flow cell (FT-SFC)
introduced by Mayrhofer et al [261], which incorporates a V-shaped channel design for efficient transport of
dissolution products, figure 35. This type of cell has also been manufactured by 3D printing and successfully
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used for fast and high-throughput dissolution detection [262]. However, intricate new cell designs are not
always necessary. Markovic’s group [263] fitted an ICP-MS inlet to a rotating disc electrode tip in a stationary
probe rotating disc electrode (SPRDE-ICP-MS), adding the advantage of uniform and well-characterised
convection. Hence, hyphenation of existing apparatus to increase time- and potential-dependent dissolution
information output in accelerated degradation tests becomes possible.
While ICP-based analytical instruments identify the atomic mass/charge ratio (ICP-MS) or
element-specific emission spectra (ICP-OES), valuable information about the oxidation state of the dissolved
species, i.e., speciation, is lost. Other in situ techniques may additionally allow speciation, e.g., rotating
ring–disc electrode [264], electron paramagnetic resonance, or UV–vis [265] measurement of the electrolyte
in operando cells. However, interference from solvents, electrolytes, or other solution species must be
accounted for.
Concluding remarks
Post-mortem analyses and ex-situ observations of battery electrodes limit our understanding of the
degradation processes that are the main causes of capacity fading and the premature failure of batteries.
Today, a detailed understanding of the atomistic causes of the dissolution of electrode materials remains
elusive, even for the most basic model electrodes. The development of online analysis techniques that
quantify time-resolved dissolution as a function of potential and other experimental parameters can
revolutionise our understanding of battery electrode degradation, as an essential step towards its mitigation
and the rational design of resilient new battery electrode materials. Advances in spectroelectrochemical cell
design and analytical instrumentation can unlock detailed dissolution probing, revealing not only elemental
quantities but also chemical speciation.
Acknowledgments
The authors gratefully acknowledge funding from ALISTORE-ERI, the Faraday Institution (FIRG018), and
the Austrian Research Fund (FWF, Project I3256-N36).
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7. Scale-up andmanufacturing
7.1. Cell performance and requirements
Emma Kendrick
School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham BT15 2TT,
United Kingdom
Status
Research into Na-ion batteries (NIBs) began in the 1970s and 1980s and was usurped by LIBs in the 1990s.
NIBs have recently re-emerged as a potential sustainable and lower-cost alternative to LIBs [266]. They offer
an energy storage solution which is not as reliant upon geographically localised raw or critical materials.
Several commercial entities have built up around this technology, with demonstrator cells and prototype
battery packs, offering the potential for lithium-ion and lead-acid battery substitutions in current markets
[71]. Sodium can be extracted from seawater, anode materials can be obtained from renewable sources, and
cathodes generally contain iron and manganese, rather than high levels of cobalt and nickel. The
manufacturing processes are expected to be the same as those for LIBs, and therefore, no investment is
required in new manufacturing lines. The cost difference between LIBs and NIBs can therefore be considered
to be due to materials and performance only [184]. NIBs are unlikely to ever reach the energy densities
(gravimetric and volumetric) of LIBs, due to their lower operating voltages and lower active component
densities [267]. The ‘drop-in’ manufacturing approach also creates limitations on NIB development, in terms
of the design of components and cells. The thickness and design of the electrodes are limited by these
tape-casting methods. Opportunities for differentiation consequently lie in high power, safe, and long-life
NIB technologies. A sustainable circular-economy perspective on materials, manufacturing, and end-of-life
could also offer significant advantages over current battery technologies. Alternative manufacturing methods
may offer solutions for improved electrode kinetics, the engineering of more efficient electronic and ionic
transport pathways, and simplified recycling. Sustainability needs to be considered within a greater holistic
outlook, however; the use of low-cost and highly abundant materials may mean, for example, that the
reclamation and reuse of these materials never becomes economically viable. Consideration of the hierarchy
of recycling, re-use, or second life within the circular economy is required. Therefore, lifetime, in particular,
is a key performance parameter for sodium ion batteries, as the value of the materials reclaimed from
recycling will not be sustainable if low-cost and abundant materials are used [268].
Current and future challenges
There are several research challenges which need to be addressed in the field of NIB technologies, one of
which is long life, which is required to improve sustainability. Advancements in LIB energy density, both
volumetric and gravimetric, are required for applications reliant upon this technology, and there is an
immense amount of work ongoing to develop new anode and cathode materials. However, the lifespan of the
cells is governed by various chemical and cell engineering optimisations relating not only to the materials but
also to the design of electrodes and cells and their operation (figure 36) [269]. For NIBs, mass balance
optimisation between positive and negative electrodes is critical for high-energy and long-life batteries. Each
cathode and anode material has a first-cycle coulombic inefficiency related to electrolyte decomposition and
irreversible sodium movement from the positive to the negative electrode. As a result, an interface
(surface–electrolyte interface (SEI)) forms on the anode, causing less sodium to be available (reducing
capacity) for shuttling between the positive and the negative electrodes. The mass balance and the voltages
chosen therefore have a large effect upon the effectiveness of this interface formation at stabilising the
electrolyte against decomposition, and consequently, the energy density of the cell [270]. The stability of the
interfaces on the positive and negative electrodes is reliant on many factors, and it is known that the stability
of NIB interfaces is inferior to those of LIBs due to the highly soluble nature of sodium salts. Much work is
required to understand the formation and changes that occur at these surfaces and interfaces during the
lifetime of the cell, and some progress has been made in this area (figure 37) [271]. These interfaces are not
only important for longevity but also for safety. Dendrites and plating can occur very readily in sodium-ion
cells, and this is not only controlled by the electrolyte composition and the ability to form low-resistance,
stable interfaces, but also through electrode design. For example, low and inhomogeneous electronic
conductivities within the electrode can lead to changes in current density across the electrode, resulting in
sodium plating and dendrite growth in areas with high resistance. Improving the electronic conductivity of
the electrode with conductive additives, such as carbon black, can aid electron transport but additionally
leads to increased electrolyte side reactions and hence ongoing growth in the SEI. This, in turn, blocks pore
networks for ionic transport and increases local resistance [272]. Understanding the combined effects of
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Figure 36. Factors which have been shown to have an influence on the creation and maintenance of a stable surface–electrolyte
interface. Reproduced from [269]. © IOP Publishing Ltd CC BY 3.0.
Figure 37. Schematic diagram of the growth in the SEI layer on hard carbon, with electrolyte additives; FEC (a) and NZeo
(b) taken from Nanozeolite ZSM-5 electrolyte additive for long-life sodium-ion batteries. Reproduced from [271] with the
permission of The Royal Society of Chemistry.
these changes upon the lifetime, capacity, and ultimately the degradation mechanisms will be essential to
extend the lifetime of cells and make further NIB optimisations.
Advances in science and technology to meet challenges
In order to achieve longer lifetime NIBs, a greater understanding of the interaction of all the cell components
and competing ageing and degradation processes is required. This will be different for each cell chemistry,
voltage window, and electrode design. An understanding of the interfaces is of the greatest importance for
both cathodes and anodes. These interfaces control the transport properties of sodium into the electrodes
and influence the type and extent of microstructural changes during the ageing of the electrode. The
compositional makeup of these interfaces requires optimisation through electrode and electrolyte design.
Changes in these interfaces and their effects on the physical and chemical properties will also affect
performance and safety. This offers opportunities for advanced, data-driven, and multiphysics models to be
partnered with experimental data to provide greater in-depth information about the changes in the
fundamental kinetic and thermodynamic properties of the cell for various compositions and designs.
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Advances in parameterisation methods for LIBs could be translated to NIBs, offering novel, unique insights
into cell construction and component optimisation [273].
Moving away from traditional LIB manufacturing processes is essential to achieve step changes in
sustainable cell designs. Novel processes are required to provide improvements in electrode 3D electronic
and ionic transport pathways. With improvements in electrode design, less current collector and packaging
materials are required, which improves energy density and reduces waste at the end of battery life. Novel
methods of embedding electronic conductors in active materials without using high-cost and highly resistive
polymeric binders to adhere them to metallic current collectors could provide the step change required. In
addition, designs that enable simple manufacturing and subsequent disassembly for material reclamation
will enable greater value from material recovery and reuse, creating a sustainable circular economy for NIBs.
The ultimate design intent would be to use no binder, however moving to renewable sources for
water-soluble binders, such as those based on algae and cellulose, may also improve recyclability prospects.
Designing for recycling and disassembly is a key area which can capitalise on the benefits of this set of
materials and unlock the potential of NIBs.
Concluding remarks
An economically viable NIB should be considered with respect to the sustainability potential of a circular
economic model rather than purely the first-life cost of materials and manufacturing. Consideration of
lifetime performance is required for both first life and second life, with recovery and recycling of cell
components at the end of life. The current focus is upon matching LIB performance criteria. However, with
rapidly changing markets for energy storage solutions, NIBs offer different opportunities for new cell design
and manufacturing processes, allowing a holistic approach to sustainability. A focus upon developing
longer-life batteries is required to achieve this. The optimisation of electrolyte and electrode designs to
reduce interface resistance and dendrite growth will improve both longevity and safety, while new
manufacturing processes and cell designs are required for sustainable end-of-life solutions. Materials and
components can then be easily and repeatedly manufactured, reconditioned, disassembled, and
remanufactured. In this way, the challenges of both the performance and economics of sodium-ion batteries
can be overcome by combining novel materials, processes, and products with advanced material recovery,
repurposing, and recycling.
Acknowledgment
Innovate UK for funding (IUK Project 104179).
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7.2. Applications and scale-up: manufacturing
Stewart A M Dickson1,2 and John T S Irvine1,2
1 School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, United Kingdom
2 The Faraday Institution, Quad One, Harwell Campus, OX11 0RA, United Kingdom
Status
The manufacture of batteries is often overlooked at a research level, due to the importance of finding new,
higher-energy electrode materials, and solving the challenges faced in cell chemistries at smaller scales.
However, when producing large-format cells, its importance in achieving the optimal cell performance is
paramount. As demand increases further, together with the growing applications of LIBs, the availability of
manufacturing facilities has also increased. Europe has multiple ‘Gigafactories’ in the pipeline, with examples
such as Northvolt, who are expected to reach 32 GWh by 2023. Similar or larger-scale factories in China and
beyond are also in the pipeline, backed by larger companies such as Tesla. As noted by the Faraday
Institution, eight ‘Gigafactories’ will be needed by 2040 to satisfy UK battery demands, which requires one to
be established at least every two years from 2020 [274]. Projects such as those recently proposed by AMTE
Power and Britishvolt are the start of promising large-scale manufacturing of LIBs within the UK, with the
hope that this can complete the chain to take materials from powder to power, thus attracting end users of
battery technology by shortening supply chains [275].
NIBs are beginning to reach a technology readiness which requires large-scale production, as
demonstrated by Faradion, with recent orders from Australia and India [276]. NIBs will see use in large,
decentralised power applications, and small-scale deployments will be required to assist in load balancing
from intermittent-power farms, such as wind and solar [277]. Factories can switch between lithium and
sodium chemistries, due to their ‘drop-in’ nature, as long as the technologies remain similar enough in their
production (figure 38).
This helps to reduce barriers to entry and initial costs, as the creation of new facilities is not required;
however, the gap between commercial and laboratory technologies still needs to be bridged to facilitate
material advances and their application in larger cells. Additionally, research institutions often lack access to
facilities which can help to realise materials at larger scales. By improving this connection, especially through
links between industry and academia, it is hoped that an acceleration of research towards manufacturing
readiness can be achieved.
Current and future challenges
Although some good parallels can be drawn from LIB manufacture, there are still limitations to how this can
be applied to NIBs, due to differences in the materials used. When changing any physical property of a
material, such as particle size, crystal structure, morphology etc., this also alters the processing parameters,
thus impacting electrochemical performance. Similarly, changing the processing procedures of an electrode
material, for example, slurry preparation or electrode modifications, can also influence both its structure and
its maximum performance (figure 39) [278]. Therefore, every material must be treated differently to achieve
this fine balance between every parameter.
Most of the challenges within manufacture lie at a materials level. Complex multi-step procedures can be
used to produce grams of material for research purposes. However, these techniques may not be feasible
when producing the kilogram amounts required for larger cells. Material morphologies, such as the creation
of high-surface-area particles, will assist with electrochemical performance. Additionally, some layered
sodium oxides of interest are extremely sensitive to air and moisture, which can make handling of large
quantities more complex, and imposes additional requirements such as additives in slurries to improve their
longevity. It is possible to modify these materials to improve their stability in air or to reduce the sodium
content, but this can be detrimental to electrochemical performance [279].
Electrode slurries at the coin-cell level also have standardised formulations; little attention is paid to
metrology and dry powder mixing, and they include large amounts of binder and carbon to overcome
material limitations, such as poor conductivity. Reducing these amounts to increase the energy density of
cells is vital at large scales, as commercial cells sometimes reach up to 98 wt.% active material. However, such
changes require specific testing and optimisation of every electrode slurry. After coating, the influence of
drying and the application of calendering as a technique to improve electrode density and adherence are also
not routinely considered within the research environment. These steps also require individual tailoring to
maximise electrochemical performance without negatively impacting electrode structure.
Although materials and processing are key to cell performance, other considerations, such as electrode
balancing and geometry, tab placement, and safety aspects related to long-term storage and long-term
cycling which are not routinely studied at smaller scales also become relevant. Due to the infancy of NIB
technology, little research exists for many of these other areas, though it is fortunate that many of these can
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Figure 38. Simplified flow diagram of the construction of an NIB pouch cell.
Figure 39. A pictorial overview of the structure-processing-property relationships which need to be considered when scaling up
battery materials. Reprinted from [278], Copyright (2014), with permission from Elsevier.
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be directly transferred from LIB research, and challenges can be jointly tackled. Some advantages and
benefits which can be discovered are, however, solely for NIBs, such as the ability to internally short a pouch
cell with no performance impact, as demonstrated by Faradion [277].
Advances in science and technology to meet challenges
In general, a better understanding is required of how to produce and process materials, as there is very little
information that specifically relates to NIBs at larger scales. Ideally, techniques already used to scale up
electrode materials to significant quantities could be implemented. Co-precipitation is the most widely
utilised approach for materials scale up, and several researchers have produced well-defined structures of
layered sodium oxides for NIBs, which required additional firing steps after the initial precipitation, which
can be energy intensive [280]. Prussian blue analogues (PBAs) can also be produced by co-precipitation, and
do not require further processing, reducing cost and time. Co-precipitation is not necessarily suitable for all
electrode materials, depending upon their stability and composition. Therefore, new routes such as
microwave synthesis could be optimised to provide alternatives.
Electrode processing is also a significant focus, as it contributes a large amount to the cost of cell
production. Tape casting is the technique most widely utilised to produce electrode sheets; however, the use
of N-methyl pyrrolidone (NMP) increases expense, due to the need for solvent-recovery systems. The
development of newer electrode slurry combinations, particularly aqueous chemistries, would reduce this
cost. Additionally, several researchers have begun to develop ‘solvent-free’ casting techniques in a bid to
remove this issue entirely and have greater control of electrode morphologies, though these advances are
likely to take some time to become commercially viable [281, 282].
Perhaps one of the biggest advances which will help manufacturing immensely is the use of simulation
and modelling. Computational modelling of cells and packs has become much more viable in recent years
and by varying parameters, can allow for rapid analysis of changes to electrode composition and cell design,
obtaining incremental performance gains when applied experimentally. One example of modelling to identify
improvements in pouch cells is that of tab placement, where the benefits of an opposite-ended tab design
were shown to improve electrode usage and temperature management [283]. Greater practical application is
required to demonstrate the benefits of these findings, and whether there are correlations with simulations.
Finally, in terms of NIBs, there needs to be a much greater understanding of the long-term performance
of cells, especially as size increases, which will become better understood with time. Additionally, the use of
half cells with metallic sodium does not realise true cell chemistries, which instead use hard carbon anodes,
though the use of half cells is also a common practice for new LIB materials. Any performance issues or
irregularities in a half-cell setup are not a true representation of what may occur in a scaled-up cell
configuration; thus, a greater emphasis on testing materials in three-electrode or small full-cell setups is
essential to ensure that materials can be scaled up without issue.
Concluding remarks
Unquestionably, as the field of NIBs expands over the next decade, greater cooperation between materials
development and manufacturing will be required to ensure that advances can be easily transferred from
small scale to large scale. Though pouch cells have been considered in this text, there are multiple formats for
batteries which should all benefit from this focus. If LIB manufacturing lines can be converted to produce
NIBs, this would help to drastically reduce costs further and allow for better knowledge transfer, making
NIBs an attractive low-cost option. However, there are still many NIB-specific challenges which need to be
tackled, mainly by improving knowledge about the processing of electrode materials, and their inherent
long-term safety, which both require greater study.
Acknowledgment
This work was supported by the Faraday Institution’s NEXGENNA project (FIRG018).
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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
8. Industrial targets and techno-economic analysis
8.1. Industrial targets
Ashish Rudola, Ruth Sayers and Jerry Barker
Faradion Limited, The Innovation Centre, 217 Portobello, Sheffield S1 4DP, United Kingdom
Status
Faradion Limited, established in 2011, was the first non-aqueous Na-ion battery company in the world. Since
then, several Na-ion companies have been founded, covering a wide range of sodium-based chemistries.
Faradion’s technology utilises mixed-phase O3/P2-type Na–Mn–Ni–Ti–Mg layered oxide cathodes, hard
carbon anodes, and non-aqueous electrolytes [284]. Other companies reliant on carbon-based anodes and
non-aqueous electrolytes include the French company, Tiamat, whose cathodes are based on the polyanionic
Na3V2(PO4)2F3 [285], the Chinese company HiNa Battery, which utilises O3-type Na–Cu–Fe–Mn layered
oxide cathodes [286], and the Swedish company Altris AB, based on Prussian blue analogue (PBA) cathodes.
In contrast, US-based Natron Energy has built its Na-ion technology around aqueous solvents and
PBA-based cathodes and anodes [287].
From the above, it can be seen that there is already a diverse array of Na-ion chemistries available in the
industry. This is important, as different markets/applications have different requirements and might require
unique solutions – the battery of choice needs just the right combination of cost ($ kWh−1), energy density,
stability/reliability, power rating, charge acceptance, and temperature performance. As such, figure 40
presents a Ragone plot covering several main market applications with their specific energy/power envelopes
(gauged by the datasheets of existing battery technologies that are currently deployed in the market for these
specific applications) [288, 289]. It should be stressed that these are just a few examples of
applications—there are others with different performance requirements. We have also indicated the
maximum performance of Na-ion cells from some industry players, as given by their datasheets or published
reports [284, 285, 287]: for Tiamat and Natron, the values mentioned are correct as of mid-2018 and
mid-2019, respectively, while for Faradion, values are accurate as of mid-2020. A Na-ion cell might be
appropriate for utilisation for a particular application, as long as the application’s envelope lies within its
energy/power capabilities (gauged by the area below its respective dotted lines in figure 40) and it can meet
other requirements such as cycle life. It is evident that current Na-ion technology could be attractive for most
applications, apart from some types of consumer electronics and long-range electric vehicles.
Current and future challenges
Referring to figure 40, it can be seen that different Na-ion chemistries are better suited for different
applications. For example, Natron’s aqueous Na-ion chemistry has the lowest specific energy/power—this is
expected, as aqueous electrolyte-based batteries cannot match the energy densities of their non-aqueous
counterparts owing to the limited voltage window of water as an electrolyte solvent. Thus, Natron’s Na-ion
systems would be ill-suited for higher-energy applications, such as electric vehicles. However, Natron
batteries’ expected low cost, exceptional cycling stability, safety and efficiency would make them attractive for
different ESS applications [287]. Tiamat’s use of fluorinated vanadium-based polyanionic cathodes delivers
Na-ion chemistry with excellent power capabilities albeit at the expense of energy density and raw material
handling and availability [285]. Polyanionic cathodes represent an attractive avenue for high power and
durable Na-ion systems. The challenge is to base them on more Earth-abundant elements, such as Fe and
Mn. Unfortunately, pure Fe/Mn-based polyanionic cathodes generally either demonstrate relatively low
operating potentials which limit their specific energies, or rely on bulky anionic groups [290], which limit
their tap densities and, in turn, volumetric energy densities [284]. On the anode side, hard carbon can be
prepared from a variety of precursors, including low-cost options, and has an attractive combination of
desirably low operating potentials and high capacity (current hard carbon capacities almost match those of
graphite Li-ion anodes). Hence, it is unlikely to be displaced as the anode of choice for high-energy
applications in the foreseeable future. Furthermore, an intense research focus on hard carbon has resulted in
significant improvements, not only in its cycling stability and rate performance but also in its charge
acceptance capabilities—these, along with excellent compatibility with high-flash-point electrolytes, confer a
high degree of safety to Na-ion systems [284].
Faradion’s Na-ion cells currently deliver a good mix of energy and power densities with attractive costs,
due, in large part, to cathodes devoid of costly and scarce elements, such as Co and V [284]. Table 2 illustrates
how Faradion Na-ion cells compare with two types of state-of-the-art Li-ion cells: Li–Ni–Mn–Co oxide
(NMC)//graphite and LiFePO4 (LFP) cathode//graphite chemistries.
74
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
Figure 40. Ragone plot illustrating the current performance of Na-ion cells of some industry players [284, 285, 287], along with
the two-year target for Faradion’s Na-ion cells. The requirements of some applications have been illustrated as well. For this
purpose, the applications’ envelopes were constructed using datasheets of commercially available batteries that are currently being
used for these applications, as indicated in the legend [288]. Acronyms used: ESS= energy storage system (for different
grid-storage applications); 2W= two-wheeler; 3W= three-wheeler; eVs= electric vehicles; SCiB= super charge ion battery;
PbA= lead-acid; LiPo= lithium-ion polymer; NCA= Li–Ni–Co–Al oxide//graphite; DOD= depth-of-discharge.
Advances in science and technology to meet challenges
Faradion’s Na-ion cells currently deliver energy and power metrics in between those of NMC//graphite and
LFP//graphite cells (table 2). They can respond at 10 C (continuous discharge), which compares favourably
with LFP//graphite pouch cells (1–3 C) [293]. Faradion’s Na-ion chemistry could be considered as a
replacement for most applications where LFP//graphite is currently deployed. Table 2 also outlines targets for
an ‘energy cell’ and a ‘power cell,’ that would solidify Na-ion technology as the go-to battery choice for
different applications. The ‘energy cell’ is based on Faradion’s Na-ion chemistry and seeks to satisfy some of
the more energy-demanding applications, such as mobile phones, laptops, and some EVs. For this purpose,
Faradion has devised a clear roadmap to achieve these targets by enhancing reversible capacities, increasing
electrode densities, and boosting coulombic efficiencies with novel electrolytes and additives. In fact, we
recently showed that the rates of increase of Faradion’s Na-ion specific energies (and cycle lives) are
significantly faster than those of Li-ion [284].
Even though current Na-ion technology can deliver high power, for applications such as power tools or
drones, both power and energy densities need to be simultaneously acceptable. For such a ‘power cell,’ it
might be challenging to meet the power and energy requirements using polyanionic cathodes at acceptable
costs. We thus anticipate layered oxide cathodes will be used for such cells, relying on a majority, or a pure,
P2 phase; it is well known that many P2-based Na-ion oxide cathodes can deliver excellent rate performance.
Faradion’s low cell BOM (bill-of-materials) is based on the absence of expensive cobalt, the use of aluminium
(as opposed to expensive copper) current collectors, and the natural abundance of sodium, resulting in active
materials and electrolyte salts that are cost-effective to produce. Future cell BOMs ($ kWh−1) can primarily
be reduced in three ways: (a) increasing energy density; (b) large-scale industrialisation of the material
supply chain with (c) increasing production volumes, all reducing cost ($) with scale. The technical approach
to achieving these target energy densities (stated in table 2) is listed above. The development of material
supply chains and an increase in production volumes are already happening with the ever-increasing
realisation of the performance benefits of Na-ion batteries (figure 40) and the ever-rising demand.
75
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
Ta
bl
e2
.P
er
fo
rm
an
ce
m
et
ric
so
fc
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or
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m
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ce
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ce
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ll
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∗
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len
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es
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lu
m
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($
kW
h−
1 )
25
%
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lo
we
rt
ha
n
LF
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/g
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po
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ce
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[2
82
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fo
re
ca
ste
d
an
nu
al
re
du
ct
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ns
in
Li
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co
sts
[2
89
]
Sp
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rg
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cp
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z
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ll)
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∗ B
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rg
ef
ro
m
1t
o
3C
[2
93
].
76
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
Concluding remarks
Na-ion technology is rapidly catching up with the established Li-ion battery. It is revealed here that the
energy/power densities of Faradion’s Na-ion chemistry, which is devoid of scarce elements, such as Co and V,
already surpass those of LFP//graphite systems. For most applications, the different types of Na-ion battery
already possess the required performance attributes at the cell level, from negligible self-discharge to
excellent temperature performance and the ability to be shipped/stored at 0 V [284], to name a few. Such
performance must now be shown in actual demonstrations of large-scale systems, cycled according to the
duty cycles required for an application in real-world settings. These demonstrations by various Na-ion
companies are already underway, and with each successful demonstration, the market’s confidence in this
technology will progressively grow. If the near-term industrial targets for Na-ion energy and power cells can
be met, mass penetration of Na-ion batteries in the market will be almost inevitable.
Acknowledgments
The authors thank Faradion Limited for their permission to publish the details provided in this section. We
also thank the entire Faradion R&D team and our commercial partners for helping to achieve the results
contained herein.
77
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
8.2. Techno-economic assessment
Emmanuel I Eweka1 and Grant S Stone2
1 Amte Power Ltd, 153A Eastern Avenue, Milton Park, Oxford OX14 4SB, United Kingdom
2 Amte Power Ltd, Denchi House, Thurso Business Park, Thurso, Caithness KW14 7XW, United Kingdom
Status
Batteries using conventional Lithium-ion chemistries (LIBs) have long since been the power source solution
of choice in a range of applications due to their high volumetric and gravimetric energy densities.
Energy-dense, state-of-the-art LIBs in the pouch and cylindrical formats are currently achieving energy
densities of up to 250 Wh kg−1 [294]. Beyond the current LIB designs, bulk all-solid-state batteries (ASSBs)
are also being developed [295] to provide a significant increase in energy density (>400 Wh kg−1) and safety
and to address the range anxiety with electric vehicles. However, it is not believed that the first generation
will be a cost-effective solution for stationary (grid, renewables, telecoms, and domestic) applications, and
timescales to market for a competitively performing product are expected to be way beyond 2025. Other
‘beyond-lithium’ chemistries, such as Li-air and rechargeable magnesium batteries, are still at very early
stages of R&D [294]. Current analyses are focused on non-aqueous battery systems. For stationary and
telecoms applications, size and weight are less limiting than, say, cost, safety, ease of maintenance, reliability,
and cycle life. Therefore, this techno-economic assessment focuses on applications where it is believed that
NIBs will be competitive, at least in the near term (5–10 years) based on their current state of development.
Some of these application areas include stationary and telecoms.
Their inherently low material cost and high technology safety make NIBs a very attractive proposition for
stationary applications [252]. In addition to being made in similar formats to LIBs, NIBs can be made on
existing LIB manufacturing lines—significantly reducing development costs and timescales. Current
high-performance LIBs are considered less accessible (and safe) for these applications, although they have
been proposed for second-life use [296]. Based on economics, Pb-acid and LiFePO4 (LFP) are the current
cathode chemistries of choice for stationary applications, compared to either LiNi1–x–yMnxCoyO2 (NMC) or
LiNi1–x–yCoxAlyO2 (NCA).
NIBs using a layered oxide, NaMO2 (whereM stands for Ni, Mn, Mg, or Ti) P2/O3-type structure and
hard carbons (HCs) have been significantly advanced by Faradion and look very promising for stationary
applications [297]. Low-cost NIBs based on Prussian blue analogue (PBA) anodes and cathodes and sodium
vanadium fluorophosphates (NVPF) are also being developed by Natron energy and Tiamat respectively
[298], but are considered less competitive for these applications, predominantly on a gravimetric and
volumetric energy basis.
Using various cost models including BatPac, € kWh−1 has been estimated for NIB (NaMO2/HC), LFP
and NMC (40%Ni:40%Mn:20%Cobalt), 18 650 cells, respectively [267, 296]. The BatPac model included
cell-design elements, depreciation, and warranty in addition to material and processing costs. The outputs
from these models indicate comparable costs for LFP cells and NIBs (and lower costs for NMC cells). The
models also indicate that the € kWh−1 for NIBs can be significantly decreased with volume-scale
manufacturing and if the Wh kg−1 and Wh l−1 can be increased.
Current and Future challenges
Global energy demand is predicted to grow by up to 40% in the next ten years, including an increase of 2500
TWh in the transport sector, and the global power capacity of solar and wind are planned to be 2 TW and 1.5
TW, respectively, in the same timescale [299]. Combined with a global push to reduce fossil-fuel reliance, this
will create a massive market for energy storage. Competitive battery technologies covering a wide
performance, safety, and cost matrix will be required to support this growth [300].
In the short term, the first generation of NIBs has to prove that it is a contender in the energy-storage
market, in terms of competitive performance, safety, cost, and maintenance, compared to incumbent
technologies, such as Pb-acid, LIBs (LFP and NMC), and redox-flow batteries. The cost of development,
production, and time to market will also have to be factored in. One of the major challenges in this space will
be to match or, if possible, outperform state-of-the-art LIB cathodes such as NMC on a cost/performance,
(€ kWh−1) basis and fit in with existing infrastructure. The majority of new stationary energy-storage
installations use LIBs (currently, mostly LFP), which have benefitted from cost reductions and innovations in
the EV market [300], and a bulk cost reduction could allow for a quick entrance into this application.
Another major short-term challenge facing all new energy-storage markets is regulation and policy.
Concerns around safety and the decisions to invest in different types of infrastructure will have a huge
impact on the demand for energy storage and the associated costs of installation and use [300]. Future
challenges will lie in securing and maintaining a competitive edge over other developing technologies. LIB
roadmaps are looking at novel materials and architectures to increase energy density and reduce cost per
78
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
kWh. Ease of recycling is another challenge that will need to be addressed, since the financial and
environmental costs of disposal increase with the scale of adoption.
Advances in science and technology to meet challenges
In recent years, there has been a significant increase in worldwide NIB R&D activities [252, 267], driven by
potential cost and safety benefits compared to LIBs, as well as compatibility with LIB manufacturing
processes and applications. Next-generation cells will seek to improve energy density beyond 170 Wh kg−1
[301] and improve other cell characteristics without compromising safety. This will further reduce the
€ kWh−1 cost and make them more competitive for automotive and other applications [252, 267].
Advanced NIBs based on current-generation chemistries will improve in gravimetric and volumetric
energy density, simply based on material, electrode, cell design, and engineering improvements. One area of
improvement will be to increase the physical and active content and loadings and the density of the
electrodes (and tap densities of active materials) to maximise the volumetric capacity. Electrolyte loadings
can also be optimised to reduce cell weight.
First-cycle efficiencies (FCEs) for hard carbons are also quite low compared to that of graphite (<78%
compared with >92%); investigating the use of a range of hard carbon/electrolyte combinations with higher
FCEs will further increase the energy density. Hard carbons with lower average discharge voltages will
increase the average cell operating voltage and the overall gravimetric and volumetric energy densities of the
cells. Implementing these design and engineering optimisations could increase the energy density of
first-generation NIBs to 160 Wh kg−1 [301].
Next-generation cathode chemistries are also being investigated [252]. These will seek to reduce the
amount of Ni in the cathode by replacing it with Fe and/or Mn to significantly reduce the cost. Lower
processing costs are also an area to explore to improve € kWh−1 for first-generation NIBs.
Alloying elements such as Sb, Si, Ge, As, Sb, Se, Pb, and Bi are also being investigated as alternatives to
hard carbon anodes [252]; it is believed that the second generation of anodes will use these as blends with
hard carbon. Titanium phosphates (e.g. NaTi2(PO4)3) and titanium oxide (Na2Ti3O7) insertion compounds
are also being investigated as alternatives to hard carbons [252].
Compared to LIBs, the NIB anode and cathode spaces are relatively unmined, so there is still significant
potential to further increase the energy density of second-generation NIBs beyond 170 Wh kg−1; 190
Wh kg−1 has been predicted by Barker [301]
Concluding remarks
This short techno-economic assessment has shown that the current generation of NIBs, if advanced to mass
production, can be competitive against LFP (and replace Pb-Acid) and NMC cells for stationary and
telecoms applications. NIBs show massive advantages in safety, compared to either LFP or NMC cells, and
have the potential for reduced cost (predominantly because Na is significantly more abundant than Li) and
improved cycle life. The current generation of NIBs has the potential to exceed 150 Wh kg−1 through
electrode and cell design optimisation. As a new technology, NIBs also have the added advantage of being a
‘drop-in’ replacement for LIBs, as the manufacturing processes are compatible.
Other potential benefits include the potential to store NIBs at 0 V without impacting safety and
performance; this will allow for cheaper storage and transportation.
Due to the significant worldwide research in NIB materials, advances will probably lead to
second-generation energy densities of >180 Wh kg−1.
However, timescales for commercialisation may be long, and the timing of introduction to the market
will be crucial to displace existing technologies. Satisfying existing regulatory and policy requirements is also
a factor to be considered in the timescales for the commercialisation of NIBs. If Ni can be removed from the
cathode formulation, then upcycling and disposal could be potentially easy and inexpensive.
79
J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al
Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).
ORCID iDs
Nuria Tapia-Ruiz https://orcid.org/0000-0002-5005-7043
A Robert Armstrong https://orcid.org/0000-0003-1937-0936
Hande Alptekin https://orcid.org/0000-0001-6065-0513
Heather Au https://orcid.org/0000-0002-1652-2204
Jerry Barker https://orcid.org/0000-0002-8791-1119
William R Brant https://orcid.org/0000-0002-8658-8938
Yong-Seok Choi https://orcid.org/0000-0002-3737-2989
Sara I R Costa https://orcid.org/0000-0001-8105-207X
Maria Crespo Ribadeneyra https://orcid.org/0000-0001-6455-4430
Serena A Cussen https://orcid.org/0000-0002-9303-4220
Aamod V Desai https://orcid.org/0000-0001-7219-3428
Juan D Forero-Saboya https://orcid.org/0000-0002-3403-6066
John M Griffin https://orcid.org/0000-0002-8943-3835
John T S Irvine https://orcid.org/0000-0002-8394-3359
Patrik Johansson https://orcid.org/0000-0002-9907-117X
Martin Karlsmo https://orcid.org/0000-0002-0437-6860
Emma Kendrick https://orcid.org/0000-0002-4219-964X
Eunjeong Kim https://orcid.org/0000-0002-2941-068
Oleg V Kolosov https://orcid.org/0000-0003-3278-9643
Stijn F L Mertens https://orcid.org/0000-0002-5715-0486
Laure Monconduit https://orcid.org/0000-0003-3698-856X
Andrew J Naylor https://orcid.org/0000-0001-5641-7778
Philippe Poizot https://orcid.org/0000-0003-1865-4902
Stéven Renault https://orcid.org/0000-0002-6500-0015
Ashish Rudola https://orcid.org/0000-0001-9368-0698
Ruth Sayers https://orcid.org/0000-0003-1289-0998
Valerie R Seymour https://orcid.org/0000-0003-3333-5512
Begoña Silva´n https://orcid.org/0000-0002-1273-3098
Moulay Tahar Sougrati https://orcid.org/0000-0003-3740-2807
Lorenzo Stievano https://orcid.org/0000-0001-8548-0231
Chris I Thomas https://orcid.org/0000-0001-8090-4541
Maria-Magdalena Titirici https://orcid.org/0000-0003-0773-2100
Jincheng Tong https://orcid.org/0000-0001-7762-1460
Thomas J Wood https://orcid.org/0000-0002-5893-5664
Reza Younesi https://orcid.org/0000-0003-2538-8104
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