# 2021 roadmap for sodium-ion batteries

Nuria Tapia-Ruiz, A. Robert Armstrong, Hande Alptekin, Marco Amores, Heather Au, Jerry Barker, Rebecca Boston, William Brant, Jake Brittain, Yue Chen, et al.

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Nuria Tapia-Ruiz, A. Robert Armstrong, Hande Alptekin, Marco Amores, Heather Au, et al. 2021 roadmap for sodium-ion batteries. JPhys Energy, 2021, 3 (3), pp.031503. 10.1088/2515-7655/ac01ef. hal-03327098

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ROADMAP • OPEN ACCESS

2021 roadmap for sodium-ion batteries

To cite this article: Nuria Tapia-Ruiz et al 2021 *J. Phys. Energy* 3 031503

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ROADMAP

2021 roadmap for sodium-ion batteries

Nuria Tapia-Ruiz<sup>1,2,*</sup> ID, A Robert Armstrong<sup>2,3,*</sup> ID, Hande Alptekin<sup>4</sup> ID, Marco A Amores<sup>2,5</sup>, Heather Au<sup>4</sup> ID, Jerry Barker<sup>6</sup> ID, Rebecca Boston<sup>2,7</sup>, William R Brant<sup>8</sup> ID, Jake M Brittain<sup>9,10</sup>, Yue Chen<sup>2,11</sup>, Manish Chhowalla<sup>2,12</sup>, Yong-Seok Choi<sup>2,13,14</sup> ID, Sara I R Costa<sup>1,2</sup> ID, Maria Crespo Ribadeneyra<sup>4</sup> ID, Serena A Cussen<sup>2,5,7</sup> ID, Edmund J Cussen<sup>2,5,7</sup>, William I F David<sup>9,10</sup>, Aamod V Desai<sup>2,3</sup> ID, Stewart A M Dickson<sup>2,15</sup>, Emmanuel I Eweka<sup>16</sup>, Juan D Forero-Saboya<sup>17</sup> ID, Clare P Grey<sup>2,18</sup>, John M Griffin<sup>1,2</sup> ID, Peter Gross<sup>2,7</sup>, Xiao Hua<sup>2,5</sup>, John T S Irvine<sup>2,15</sup> ID, Patrik Johansson<sup>19,20</sup> ID, Martin O Jones<sup>2,21</sup>, Martin Karlsmo<sup>19</sup> ID, Emma Kendrick<sup>22</sup> ID, Eunjeong Kim<sup>2,3</sup> ID, Oleg V Kolosov<sup>2,11</sup> ID, Zhuangnan Li<sup>2,12</sup>, Stijn F L Mertens<sup>1,2</sup> ID, Ronnie Mogensen<sup>8</sup>, Laure Monconduit<sup>23,24</sup> ID, Russell E Morris<sup>2,3,25</sup>, Andrew J Naylor<sup>8</sup> ID, Shahin Nikman<sup>1,2</sup>, Christopher A O'Keefe<sup>2,18</sup>, Darren M C Ould<sup>2,18</sup>, R G Palgrave<sup>2,13</sup>, Philippe Poizot<sup>26</sup> ID, Alexandre Ponrouch<sup>17</sup>, Stéven Renault<sup>26</sup> ID, Emily M Reynolds<sup>2,21</sup>, Ashish Rudola<sup>6</sup> ID, Ruth Sayers<sup>6</sup> ID, David O Scanlon<sup>2,13,14,27</sup>, S Sen<sup>2,13</sup>, Valerie R Seymour<sup>1,2</sup> ID, Begoña Silván<sup>1,2</sup> ID, Moulay Tahar Sougrati<sup>23,24</sup> ID, Lorenzo Stievano<sup>23,24</sup> ID, Grant S Stone<sup>28</sup>, Chris I Thomas<sup>3,7</sup> ID, Maria-Magdalena Titirici<sup>4</sup> ID, Jincheng Tong<sup>2,12</sup> ID, Thomas J Wood<sup>9</sup> ID, Dominic S Wright<sup>2,18</sup> and Reza Younesi<sup>8</sup> ID

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,

<footnote n="1">1 Department of Chemistry, Lancaster University, Lancaster LA1 4YB, United Kingdom</footnote>

<footnote n="2">2 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom</footnote>

<footnote n="3">3 EastChem School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, United Kingdom</footnote>

<footnote n="4">4 Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom</footnote>

<footnote n="5">5 Department of Chemical and Biological Engineering, University of Sheffield, Sheffield S1 3JD, United Kingdom</footnote>

<footnote n="6">6 Faradion Limited, The Innovation Centre, 217 Portobello, Sheffield S1 4DP, United Kingdom</footnote>

<footnote n="7">7 Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, United Kingdom</footnote>

<footnote n="8">8 Department of Chemistry—Ångström Laboratory, Uppsala University, Box 538, SE-75121 Uppsala, Sweden</footnote>

<footnote n="9">9 ISIS Neutron and Muon Spallation Source, STFC Rutherford Appleton Laboratory, Harwell, Oxford OX11 0QX, United Kingdom</footnote>

<footnote n="10">10 Inorganic Chemistry Laboratory, University of Oxford, Oxford OX1 3QR, United Kingdom</footnote>

<footnote n="11">11 Department of Physics, Lancaster University, Bailrigg, Lancaster LA1 4YB, United Kingdom</footnote>

<footnote n="12">12 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United Kingdom</footnote>

<footnote n="13">13 Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom</footnote>

<footnote n="14">14 Thomas Young Centre, University College London, Gower Street, London WC1E 6BT, United Kingdom</footnote>

<footnote n="15">15 School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, United Kingdom</footnote>

<footnote n="16">16 Amte Power Ltd, 153A Eastern Avenue, Milton Park, Oxford OX14 4SB, United Kingdom</footnote>

<footnote n="17">17 Institut de Ciencia de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Catalonia, Spain</footnote>

<footnote n="18">18 Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom</footnote>

<footnote n="19">19 Department of Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden</footnote>

<footnote n="20">20 Alistore-ERI, CNRS FR 3104, 15 Rue Baudelocque, Amiens 80039, France</footnote>

<footnote n="21">21 Science and Technology Facilities Council, North Star Avenue, Swindon SN2 1SZ, United Kingdom</footnote>

<footnote n="22">22 School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham BT15 2TT, United Kingdom</footnote>

<footnote n="23">23 ICGM, University Montpellier, CNRS, Montpellier, France</footnote>

<footnote n="24">24 RS2E, CNRS, Amiens, France</footnote>

<footnote n="25">25 Department of Physical and Macromolecular Chemistry, Charles University, Hlavova 8, Prague 128 43, Czech Republic</footnote>

<footnote n="26">26 Université de Nantes, CNRS, Institut des Matériaux Jean Rouxel, IMN, F-44000 Nantes, France</footnote>

<footnote n="27">27 Diamond Light Source Ltd, Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom</footnote>

<footnote n="28">28 Amte Power Ltd, Denchi House, Thurso Business Park, Thurso, Caithness KW14 7XW, United Kingdom</footnote>

<footnote>* Authors to whom any correspondence should be addressed.</footnote>
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.
Contents

<table><tbody><tr><td>1. Cathode materials</td><td>6</td></tr><tr><td>1.1. Layered transition-metal oxides: P2 | O3 materials</td><td>6</td></tr><tr><td>1.2. Anion redox layered transition-metal oxides</td><td>9</td></tr><tr><td>1.3. Polyatomic anion-based materials</td><td>12</td></tr><tr><td>1.4. Prussian blue and analogues</td><td>15</td></tr><tr><td>1.5. Organic materials</td><td>18</td></tr><tr><td>2. Anode materials</td><td>22</td></tr><tr><td>2.1. Hard carbon</td><td>22</td></tr><tr><td>2.2. Titanium-based oxides</td><td>25</td></tr><tr><td>2.3. Alloy and conversion materials</td><td>28</td></tr><tr><td>2.4. 2D transition-metal dichalcogenides</td><td>31</td></tr><tr><td>2.5. Organic materials</td><td>33</td></tr><tr><td>3. Computational discovery of materials</td><td>36</td></tr><tr><td>4. Electrolytes and SEI engineering</td><td>39</td></tr><tr><td>4.1. Organic electrolytes: salts</td><td>39</td></tr><tr><td>4.2. Organic electrolytes: solvents</td><td>41</td></tr><tr><td>4.3. Ionic liquid electrolytes</td><td>44</td></tr><tr><td>4.4. The SEI in Na-ion batteries</td><td>47</td></tr><tr><td>4.5. SSEs for Na-ion batteries</td><td>50</td></tr><tr><td>5. Testing protocols for Na-ion batteries</td><td>53</td></tr><tr><td>6. Advanced characterisation</td><td>56</td></tr><tr><td>6.1. Neutron characterisation of battery materials</td><td>56</td></tr><tr><td>6.2. Solid-state NMR for the characterisation of Na-ion batteries</td><td>59</td></tr><tr><td>6.3. Nanoscale characterisation of the local physical properties of Na-ion batteries, electrodes, and interfaces</td><td>62</td></tr><tr><td>6.4. Dissolution benchmarking of sodium-ion electrodes</td><td>65</td></tr><tr><td>7. Scale-up and manufacturing</td><td>68</td></tr><tr><td>7.1. Cell performance and requirements</td><td>68</td></tr><tr><td>7.2. Applications and scale-up: manufacturing</td><td>71</td></tr><tr><td>8. Industrial targets and techno-economic analysis</td><td>74</td></tr><tr><td>8.1. Industrial targets</td><td>74</td></tr><tr><td>8.2. Techno-economic assessment</td><td>78</td></tr><tr><td>References</td><td>80</td></tr></tbody></table>
# 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
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.
# 1. Cathode materials

## 1.1. Layered transition-metal oxides: P2 | O3 materials

Jake M Brittain¹,², Thomas J Wood¹ and William I F David¹,²

¹ISIS Neutron and Muon Spallation Source, STFC Rutherford Appleton Laboratory, OX11 0QX, United Kingdom

²Inorganic 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
<figure><img src="image_4.png" /><figcaption>**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.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**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.</figcaption></figure>

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,
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).
## 1.2. Anion redox layered transition-metal oxides

*Eunjeong Kim*¹,³, *Begoña Silván*²,³, *Nuria Tapia-Ruiz*²,³ and *A Robert Armstrong*¹,³

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
<figure><img src="image_6.png" /><figcaption>**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.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**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.</figcaption></figure>

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₋ᵧLiᵧO₂ [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₋ᵧLiᵧO₂, 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
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).
### 1.3. Polyatomic anion-based materials

Xiao Hua<fnref n="1" />,<fnref n="3" />, Peter Gross<fnref n="2" />,<fnref n="3" />, Edmund J Cussen<fnref n="1" />,<fnref n="2" />,<fnref n="3" /> and Serena A Cussen<fnref n="1" />,<fnref n="2" />,<fnref n="3" />

#### 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⁻<fnref n="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 O<fnref n="2" />⁻ ions, elevates the specific energy to ≈600 Wh kg⁻<fnref n="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 (Na₃V₂₋₂Alz(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 t favorite polymorph) has exhibited the highest redox potential for an Fe<fnref n="2" />⁺/Fe<fnref n="3" />⁺ 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⁻<fnref n="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

<footnote n="1">¹ Department of Chemical and Biological Engineering, University of Sheffield, Sheffield S1 3JD, United Kingdom</footnote>

<footnote n="2">² Department of Materials Science and Engineering, The University of Sheffield, Sheffield S1 3JD, United Kingdom</footnote>

<footnote n="3">³ The Faraday Institution, Quad One, Harwell Campus, OX11 0RA, United Kingdom</footnote>
<figure><img src="image_8.png" /><figcaption>**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.</figcaption></figure>

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 of vanadium oxyfluorides, Na₃V₂(PO₄)₂F₃₋₄O_y (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
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.
## 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 $Na_xM[M'(CN)_6]_{1-y}.zG$ (where M and M' 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 $H_2O$), 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 some M 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 different M 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.
<figure><img src="image_9.png" /><figcaption>**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].</figcaption></figure>

Synthesis

Handling

Post-synthesis
modification

Electrolyte

<figure><img src="image_10.png" /><figcaption>**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.</figcaption></figure>
**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 with *M* and *M'* 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.
## 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 ($Na_4DHTA$), 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 $Na_4DHTA$ 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.
<figure><img src="image_11.png" /></figure>

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).
<table><caption>**Table 1.** Selected organic positive electrode materials tested in non-aqueous electrolytes, either in Na half-cell or full-cell configurations. Note that specific capacity values are reported per mass of positive active material, except for entry #4, which is expressed per mass of negative active material. PTCDF: 3,4,9,10-perylenetetracarboxylic diimide; TPA: terephthalic acid; DHTPA: 2,5-dihydroxyterephthalic acid; PTMA: poly(2,2,6,6-tetramethylpiperidine-4-yl-1-oxyl methacrylate); PTVE: poly(2,2,6,6-tetramethylpiperidine-4-yl-1-oxyl ether); PTPAn: polytriphenylamine; PAQS: poly(anthraquinonyl sulphide); EC/DMC: ethylene carbonate/dimethyl carbonate; DME: 1,2-dimethoxyethane; PC: propylene carbonate; DEGDME: Diethylene glycol dimethyl ether; DEC: diethyl carbonate; DOL: 1,3-dioxolane.</caption><thead><tr><th>#</th><th>Half-reaction at the positive electrode</th><th>Half-reaction at the negative electrode</th><th>Electrolyte</th><th>Output voltage (V)</th><th>Specific capacity (mAh g<sup>-1</sup>), capacity retention, cycles, ref.</th></tr></thead><tbody><tr><td>1</td><td>Poly(2-methyl-4-cyano-3,5-dimethyl-1H-pyrazole)<br/>PTCDI</td><td>Na<sup>+</sup> → Na<sup>-</sup></td><td>1 M NaPF<sub>6</sub> in EC/DMC (w/w 1:1)</td><td>2.2</td><td>111, 88%, 5000 [57] /100, 100%, 1000 [58]</td></tr><tr><td>2</td><td>PTCDI</td><td>Na<sup>+</sup> → Na<sup>-</sup></td><td>1 M NaPF<sub>6</sub> in EC/DMC (w/w 1:1)</td><td>2.2</td><td>111, 88%, 5000 [57] /100, 100%, 1000 [58]</td></tr><tr><td>3</td><td>PTCDI</td><td>Na<sup>+</sup> → Na<sup>-</sup></td><td>1 M NaPF<sub>6</sub> in EC/DMC (w/w 1:1)</td><td>2.2</td><td>111, 88%, 5000 [57] /100, 100%, 1000 [58]</td></tr><tr><td>4</td><td>PTCDI</td><td>Na<sup>+</sup> → Na<sup>-</sup></td><td>1 M NaPF<sub>6</sub> in EC/DMC (w/w 1:1)</td><td>2.2</td><td>111, 88%, 5000 [57] /100, 100%, 1000 [58]</td></tr><tr><td>5</td><td>PTCDI</td><td>Na<sup>+</sup> → Na<sup>-</sup></td><td>1 M NaPF<sub>6</sub> in EC/DMC (w/w 1:1)</td><td>2.2</td><td>111, 88%, 5000 [57] /100, 100%, 1000 [58]</td></tr><tr><td>6</td><td>PTCDI</td><td>Na<sup>+</sup> → Na<sup>-</sup></td><td>1 M NaPF<sub>6</sub> in EC/DMC (w/w 1:1)</td><td>2.2</td><td>111, 88%, 5000 [57] /100, 100%, 1000 [58]</td></tr><tr><td>7</td><td>PTCDI</td><td>Na<sup>+</sup> → Na<sup>-</sup></td><td>1 M NaPF<sub>6</sub> in EC/DMC (w/w 1:1)</td><td>2.2</td><td>111, 88%, 5000 [57] /100, 100%, 1000 [58]</td></tr></tbody></table>
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.
## 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
<figure><img src="image_12.png" /><figcaption>**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.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**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.</figcaption></figure>

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
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.
## 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 $Ti^{4+}/Ti^{3+}$ redox couple. The most representative oxides include $TiO_2$, $Li_4Ti_5O_{12}$, $Na_2Ti_6O_{13}$, and $Na_2Ti_3O_7$, which will be briefly described herein.

$TiO_2$-based anodes are a promising avenue of research, due to their high structural stability and theoretical capacity (335 mAh g$^{-1}$). Microsized $TiO_2$ 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 $TiO_2$ polymorphs exhibit different electrochemical performances, with anatase (crystalline and amorphous) generally being the favoured form [104]. $Li_4Ti_5O_{12}$ (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, $Na_2Ti_6O_{13}$ ($C2/m$) 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, $Na_2Ti_3O_7$ ($P2_1/m$) 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). $Na_2Ti_3O_7$ 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 $Na_2Ti_3O_7$ 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 $Na_2Ti_3O_7/Na_2Ti_6O_{13}$ 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 $Na_2Ti_3O_7$ (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 $Ti^{4+}$ ions ($d^0$) makes NTO an electrical insulator (bandgap energy ≈ 3.7 eV) [112], which hampers $Na^+$ ion diffusion within the crystal structure ($D_{Na^+}$ 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 $Na_2CO_3$, alkyl carbonates and PEO during discharge when using 1 M $NaClO_4$ in an EC:PC electrolyte [113]. Moreover, NaCl and NaF products have been observed before cycling, as a result of the decomposition of the $NaClO_4$ inorganic electrolyte salt and the Polyvinylidene fluoride (PVDF) binder, respectively. Alternative inorganic salts, such
<figure><img src="image_14.png" /><figcaption>**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.</figcaption></figure>

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], generating microflowers (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.
<figure><img src="image_15.png" /><figcaption>**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.</figcaption></figure>

**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.
## 2.3. Alloy and conversion materials

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 $Na_3P$ (2596 mAh g⁻¹) and $Na_3Sb$ (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].
<figure><img src="image_16.png" /><figcaption>**Figure 13.** Alloy and conversion anodes compared to other families of materials. Reproduced from [123] with permission of The Royal Society of Chemistry.</figcaption></figure>

**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
(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.
## 2.4. 2D transition-metal dichalcogenides

Jincheng Tong$^{1,2}$, Zhuangnan Li$^{1,2}$ and Manish Chhowalla$^{1,2}$

$^{1}$ 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 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
<figure><img src="image_17.png" /><figcaption>**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.</figcaption></figure>

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).
## 2.5. Organic materials

Aamod V Desai<fnref n="1" />,<fnref n="2" />, Russell E Morris<fnref n="1" />,<fnref n="2" />,<fnref n="3" /> and Robert Armstrong<fnref n="1" />,<fnref n="2" />

### 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 π-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 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.

<footnote n="1">¹ EastChem School of Chemistry, University of St. Andrews, KY16 9ST, United Kingdom</footnote>

<footnote n="2">² The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom</footnote>

<footnote n="3">³ Department of Physical and Macromolecular Chemistry, Charles University, Hlavova 8, Prague 128 43, Czech Republic</footnote>
<figure><img src="image_18.png" /><figcaption>**Figure 15.** (a) Representation of $Na^+$ ion insertion in a typical organic redox-active molecule—disodium naphthalene-2,6-dicarboxylate ($Na_2NDC$). (b) Figure showing electrochemical reaction observed for azo-bonded organic molecules.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>**Figure 16.** Schematic showing aspects of contemporary research in the domain of organic anode materials, including broad objectives.</figcaption></figure>

### 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 $π$-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 $π$-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,
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).
## 3. Computational discovery of materials

Yong-Seok Choi<fnref n="1" />,<fnref n="2" />,<fnref n="3" /> and David O Scanlon<fnref n="1" />,<fnref n="2" />,<fnref n="3" />,<fnref n="4" />

### 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

<footnote n="1">¹ Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom</footnote>

<footnote n="2">² Thomas Young Centre, University College London, Gower Street, London WC1E 6BT, United Kingdom</footnote>

<footnote n="3">³ The Faraday Institution, Harwell Campus, Didcot OX11 0RA, United Kingdom</footnote>

<footnote n="4">⁴ Diamond Light Source Ltd, Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom</footnote>
<figure><img src="image_20.png" /><figcaption>**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.</figcaption></figure>

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 initio* molecular 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 quantum Monte 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
<figure><img src="image_21.png" /><figcaption>**Figure 18.** Integration of structure-prediction methods into the discovery of materials for future NIBs.</figcaption></figure>

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).
# 4. Electrolytes and SEI engineering

## 4.1. Organic electrolytes: salts

Darren M C Ould<fnref n="1" />,<fnref n="2" />, Christopher A O'Keefe<fnref n="1" />,<fnref n="2" />, Clare P Grey<fnref n="1" />,<fnref n="2" /> and Dominic S Wright<fnref n="1" />,<fnref n="2" />

### 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

<footnote n="1">¹ Department of Chemistry, Lensfield Road, Cambridge CB2 1EW, United Kingdom</footnote>

<footnote n="2">² The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom</footnote>
<figure><img src="image_22.png" /><figcaption>**Figure 19.** Structure of the anions currently used in sodium-ion batteries. Sodium is the cation in all cases.</figcaption></figure>

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).
## 4.2. Organic electrolytes: solvents

Martin Karlsmo<fnref n="1" /> and Patrik Johansson<fnref n="1" />,<fnref n="2" />

### 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 particular 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 $E_{red}$ of $Na^+/Na$ vs. $Li^+/Li$. For the same final cell voltage, there is a corresponding more stringent demand on the electrolyte/solvent $E_{ox}$ 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⁻<fnref n="1" />, has already been reached. The next is the cell voltage—intimately coupled to the electrolyte/solvent $E_{ox}$ 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

<footnote n="1">¹ Department of Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden</footnote>

<footnote n="2">² Alistore-ERI, CNRS FR 3104, 15 Rue Baudelocque, 80039 Amiens, France</footnote>
<figure><img src="image_23.png" /><figcaption>**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).</figcaption></figure>

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.
**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.
### 4.3. Ionic liquid electrolytes

S Sen<fnref n="1" />,<fnref n="2" /> and R G Palgrave<fnref n="1" />,<fnref n="2" />

#### 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 Nₙₙₙ⁺: 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 sodium metal 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.

<footnote n="1">¹ Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom</footnote>

<footnote n="2">² The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom</footnote>
<figure><img src="image_24.png" /><figcaption>**Figure 21.** Benefits of ionic liquids in NIB applications.</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>**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, E<sub>IL</sub>: ionic liquid electrolyte, E<sub>stand</sub>: standard organic electrolyte. The data presented in (b) and (c) are directly taken from [186].</figcaption></figure>

**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).
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)ᵧ components [187].

**Concluding remarks**

In conclusion, ionic liquid electrolytes significantly improve 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).
## 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 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
<figure><img src="image_26.png" /><figcaption>**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.</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>**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.</figcaption></figure>
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.
## 4.5. SSEs for Na-ion batteries

Chris I Thomas<fnref n="1" />,<fnref n="3" />, Peter Gross<fnref n="1" />,<fnref n="3" />, Marco A Amores<fnref n="2" />,<fnref n="3" />, Serena A Cussen<fnref n="1" />,<fnref n="2" />,<fnref n="3" /> and Edmund J Cussen<fnref n="1" />,<fnref n="2" />,<fnref n="3" />

### 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⁻<fnref n="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⁻<fnref n="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⁻<fnref n="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⁻<fnref n="3" /> to 1.4 × 10⁻<fnref n="3" /> S cm⁻<fnref n="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⁻<fnref n="3" /> S cm⁻<fnref n="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

<footnote n="1">¹ Department of Materials Science and Engineering, The University of Sheffield, Sheffield S1 3JD, United Kingdom</footnote>

<footnote n="2">² Department of Chemical and Biological Engineering, University of Sheffield, Sheffield S1 3JD, United Kingdom</footnote>

<footnote n="3">³ The Faraday Institution, Quad One, Harwell Campus, OX11 0RA, United Kingdom</footnote>
<figure><img src="image_28.png" /><figcaption>**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.</figcaption></figure>

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
<figure><img src="image_29.png" /><figcaption>**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).</figcaption></figure>

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 *operando* measurements 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.
# 5. Testing protocols for Na-ion batteries

Juan D Forero-Saboya and Alexandre Ponrouch

Institut de Ciència 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 (Na plated) 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 Na bulk when compared with Na plated 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
<figure><img src="image_30.png" /><figcaption>**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.</figcaption></figure>

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /><figcaption>**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.</figcaption></figure>

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*
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).
# 6. Advanced characterisation

## 6.1. Neutron characterisation of battery materials

*Emily M Reynolds*<fnref n="1" />,<fnref n="2" /> and *Martin O Jones*<fnref n="1" />,<fnref n="2" />

### 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.

<footnote n="1">¹ Science and Technology Facilities Council, North Star Avenue, Swindon SN2 1SZ, United Kingdom</footnote>

<footnote n="2">² The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom</footnote>
<figure><img src="image_35.png" /><figcaption>**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.</figcaption></figure>

### 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
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).
## 6.2. Solid-state NMR for the characterisation of Na-ion batteries

Valerie R Seymour<fnref n="1" />,<fnref n="2" /> and John M Griffin<fnref n="1" />,<fnref n="2" />

### 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 situ* measurements 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

<footnote n="1">¹ Department of Chemistry, Lancaster University, Lancaster LA1 4YB, United Kingdom</footnote>

<footnote n="2">² The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom</footnote>