<figure><img src="image_36.png" /><figcaption>**Figure 30.** (a) MAS rotors of various sizes and their corresponding maximum spinning rates. (b) Bag cell and (c) plastic cell capsule designs used for *in situ* NMR. Reprinted with permission from [232]. Copyright (2017) American Chemical Society.</figcaption></figure>

high-sensitivity nuclei. This challenge is exacerbated by *in situ* measurements, in which the sensitivity is typically further reduced due to the requirement for a large-diameter detection coil and a low filling factor due to the cell components, which reduces the sensitivity further.

## Advances in science and technology to meet challenges

An important area of NMR hardware development is MAS probe design. As discussed above, fast MAS can help to remove the effects of paramagnetic interactions, which broaden and complicate solid-state NMR spectra in battery materials. Fast MAS frequencies remove these interactions more efficiently, thereby improving both the resolution and sensitivity of NMR spectra, so that more information can be obtained. Very recently, a probe capable of MAS frequencies up to 170 kHz has been demonstrated [236]. Although this ultra-fast MAS technology has not yet been widely applied, it represents a significant step forward, which can be expected to support new insights into battery materials in the future.

Advances in DFT methods for the calculation of paramagnetic NMR shifts will play a key role in addressing the issue of spectral interpretation and assignment. Although this remains a significant challenge, progress has been made in recent years, and a number of methods have been successfully demonstrated for model systems [237, 238]. Further development of these methods will enable them to be applied more widely beyond proof-of-concept studies, although due to the complex and highly system-dependent nature of the calculations, it is not clear whether paramagnetic shift calculations will become routine in the same way as for non-paramagnetic materials.

Concerning the challenge of detecting insensitive nuclei and surface or defect species, significant progress has been made in recent years through the development of dynamic nuclear polarisation (DNP). DNP transfers strong electron spin polarisation from either native or exogenous radical species to nuclei within the material of interest. Since the transfer is distance-dependent, exogenous radical species have a tendency to selectively enhance surface groups [239]. Radicals native to the material structure have also been used to enhance insensitive nuclei in the bulk structure [240, 241]. This approach holds considerable promise for the study of unstable or reactive battery materials which may not be amenable to the addition of exogenous polarising agents. Despite the promise that DNP holds, further developments are required to properly understand polarisation transfer mechanisms, and also to widen the temperature range over which it is effective, so that materials can be studied under ambient or variable-temperature conditions.
a

<figure><img src="image_37.png" /></figure>

b

<figure><img src="image_38.png" /></figure>

**Figure 31.** (a) <sup>23</sup>Na *ex situ* NMR spectra of NaMnO<sub>2</sub> at different states of charge. Resonances corresponding to α-NaMnO<sub>2</sub>, β-NaMnO<sub>2</sub>, and interfacial stacking faults are highlighted in green, blue, and red, respectively. Reprinted with permission from [233]. Copyright (2014) American Chemical Society. (b) *In situ* e<sup>23</sup>Na NMR spectra of a hard carbon anode material as a function of the state of charge. The movement of the signal from the diamagnetic (low chemical shift) regime to the paramagnetic (high chemical shift) regime shows the transition from isolated Na ions to quasi-metallic clusters. Reproduced from [77] with permission of The Royal Society of Chemistry.

**Concluding remarks**

Although widely applied as an analytical technique, solid-state NMR continues to be a highly active area of research in itself, which engenders continual developments in experimental hardware and methodology. These ongoing advances are helping to deepen and transform our understanding of battery material structures and mechanisms, so that their material properties can be optimised. For the study of Na-ion battery materials in particular, NMR has already provided considerable insight and will continue to do so as faster MAS rates become more widely available and advances in DNP methodology provide ever higher sensitivity and selectivity.

**Acknowledgments**

We acknowledge the Faraday Institution NEXGENNA project (FIRG018) and Lancaster University for financial support.
## 6.3. Nanoscale characterisation of the local physical properties of Na-ion batteries, electrodes, and interfaces

*Oleg V Kolosov*<fnref n="1" />,<fnref n="2" /> *and Yue Chen*<fnref n="1" />,<fnref n="2" />

### Status

The majority of characterisation techniques enabling the current fast development of rechargeable batteries fall into three major categories—the traditional electrochemical performance measurements, such as galvanostatic charge/discharge and cyclic voltammetry; crystal and nanostructural characterisation, such as x-ray diffraction and electron microscopy (EM); and NMR, XPS, and soft x-ray spectroscopy, which also provide chemical component analysis. The long history of battery technology development has amassed an in-depth understanding of electrochemical processes based on the characterisation of these electrochemical, structural, and chemical properties, derived from the above electroanalytical methods and their techniques. While these properties are essential, as shown in figure 32, the physical properties, including the mechanical, thermal, and electrical properties, of the materials deployed in Na-ion batteries are no less important.

Growing attention has recently been paid to revealing the nanomechanical properties of sodium nanodendrites in high-capacity Na-metal anodes [242], and the generation of interfacial stress in layered TMD [243] anodes due to the intercalation of the larger Na ions compared to Li ions. It is also well established that the Young's moduli of the SEIs on the negative electrodes define the battery's resistance to dendrite growth whereas the stability of polycrystalline cathodes crucially depends on the mechanical integrity of their components during the charge-discharge cycle expansion and their resistance to crack propagation [244]. The electrical conductivity of the electrodes and the ionic conductivity of the interfacial layers are the key limiting parameters for battery rate performance, and the thermal conductivity is another factor that defines battery operation in demanding applications. Therefore, measuring mechanical moduli, local stress, electrical conductance, and thermal transport in real-life batteries and model systems, and correlating these data with electrochemical and structural analyses will provide an essential platform for overcoming existing challenges and for the development of novel paradigms in Na-ion batteries.

### Current and future challenges

One of the major challenges in the nanoscale measurement of the physical properties of electrodes and interfaces in NIBs is linked to the wide range of length scales involved. The electrode thicknesses are typically a few tens of micrometres (μm), the individual cathode electrode particles are a few μm across, and hard carbon particles in the battery anodes are on the nm length scale. The SEI layers are several tens of nm thick, while the cathode electrolyte interphase (CEI) can be as little as a few nm thick [245]. Na-ion incorporation occurs at interatomic distances that are on the sub-nm length scale. The successful investigation of this phenomenon requires a means of characterisation that can provide local measurements of physical properties with the commensurate nanoscale lateral resolution. Another major challenge is the extreme heterogeneity of the battery components. For example, the mechanical moduli range from sub-MPa values for the outer layers of an SEI-facing electrolyte to hundreds of GPa for cathode polycrystalline particles, and the ionic conductivity of the sodium ranges from ~10⁻<fnref n="2" /> μS cm⁻<fnref n="1" /> when crossing the SEI layers at the electrode–electrolyte interface to ~10 μS cm⁻<fnref n="1" /> within the interlayers of the TMD anode. Lastly, but most importantly, the dynamic evolution of nanoscale physical properties, which are closely related to the sodium ion transport and electrolyte electrochemical/thermal decomposition, is exceptionally difficult to ‘look’ at, since sodium battery active materials and interfaces are buried deep within the volatile liquid electrolyte under battery operating conditions.

To satisfy these complex characterisation capability requirements at multiple length scales and address the electrode heterogeneity and the fluidic electrochemical environment, scanning probe microscopy (SPM) has been applied for the micro/nanoscale characterisation of the dynamic and local physical properties of battery electrodes and interfaces. In these studies, model systems are often used, e.g., highly oriented pyrolytic graphite (HOPG) [246, 247] or the physical vapor deposition/chemical vapor deposition of binder-free electrodes [245] as a proxy for the battery electrode, enabling the direct observation of SEI formation and ion intercalation through nanomorphology measurements of the electrode surface. However, it is not sufficient to only investigate the surface physical properties of these structures; tools are required that can access the inner structures of electrodes and interfacial layers. The direct nanoscale characterisation of multiple complementary physical properties of real-life batteries, for *ex situ* and *in-situ/operando* cases, remains the holy grail of Na-ion characterisation requiring an innovative solution for three-dimensional (3D) sample preparation.

<footnote n="1">¹ Department of Physics, Lancaster University, Bailrigg, 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>
<figure><img src="image_39.png" /><figcaption>**Figure 32.** Critical physical/chemical properties and their corresponding characterisation techniques.</figcaption></figure>

### Advances in science and technology to meet challenges

Among the vast number of techniques used for sodium battery interface analysis [248], SPM occupies a unique place, as it can provide both the required nanoscale spatial resolution, as well as being sensitive to the local physical properties of the studied material. Using SPM in the electrochemical environment on the model HOPG substrate allowed the study of the initial stages of SEI growth [246], revealing the dynamics of ion intercalation. The downside of SPM is that it requires nanoscale flat samples and mainly allows the investigation of surfaces rather than 3D structures. Focused ion beam (FIB) sectioning, widely used in EM measurements, is inadequate for SPM, as the sections produced are too small to be measured efficiently and, more significantly, the Ga ions can drastically change the physical properties of the studied material during sectioning. As shown in figure 33(a), a much more suitable approach is to use advanced nanosectioning using multiple Ar-ion beams, known as beam-exit cross-section polishing (BEXP), which produces a close to atomically flat surface with minimal damage, due to the non-reactive nature of Ar atoms. In the BEXP method, the open-angle tilted section adjacent to the intact surface of the sample produces a perfect area for the material-sensitive SPM, additionally expanding the interfacial area by a factor of 5–10, allowing the study of interfaces a few nm thick (such as the superlattice in figure 33(b)) with a total section area on the sub-mm length scale. By using a dedicated material-sensitive SPM contrast, such as nanomechanical via sample excitation, ultrasonic force microscopy (UFM), or tip-excitation waveguide UFM [249], the conductance of the cross-sectioned samples can reveal the local physical properties of the internal structure at nanoscale resolution. Figures 33(c)–(e) show a diagram of a commercial Ni-rich cathode polished by BEXP and the prepared sample’s cross-sectional UFM images. In figures 33(c) and (d), the cathode particles at different states of charge (SOC) in the binder and the conductive carbon black matrix can be clearly distinguished. Additionally, the crack network within the grain section is visible in the nanomechanical UFM images, which could serve as a perfect model for *operando* SPM studies of dynamic crack propagation in cathode polycrystalline particles, revealing the fundamentals of capacity degradation in sodium-transition-metal oxide cathodes.

Another approach for examining 3D structures is to use direct *in situ* sectioning of the sample through nanotomography [250], in which the tip mills out the surface along the section plane, followed by nanomechanical, nano-electric, or surface-potential measurements. Such an approach is more suitable for the sectioning of softer layers, such as the SEI, in which follow-up nanomechanical SPM probes all the layers, starting from the double layer in the electrolyte, through to the soft and then more compact SEI layers at the interface with the anode. As shown in figure 33(f), when BEXP and nanotomography are combined, the organic adsorption layer that forms on the atomically flat cut slope of the HOPG anode can be scraped by the SPM tip. This allows for the uncovering of the subsurface physical properties of electrodes and interfaces, which could be the ‘buried treasure’ in the study of Na-ion batteries.

Overall, SPM will allow the investigation of the highly important Na-ion intercalation phenomena in carbon-based [247] and other electrodes by observing both the nanoscale morphological (*operando*) dimensional change) and physical parameters (*operando*) mechanical compliance that was shown to affect the mechanical properties during atomic intercalation) [251]. Other physical parameters, such as the surface
<figure><img src="image_40.png" /><figcaption>**Figure 33.** Top (a) principle of BEXP (www.lancastermaterialanalysis.co.uk/beam-exit-cross-sectional-polishing). The Ar beam enters the side of the sample below the surface, producing a near-atomically-flat low-damage surface adjacent to the untouched surface layer with an open angle and topographically flat geometry, which is perfectly suited for the follow-up SPM characterisation of physical properties. (b) Schematics of the BEXP approach for the preparation of a sample of cathode material. (c), (d) SPM 3D rendering of a Ni-rich cathode composite electrode with superimposed nanomechanical contrast; the cathode powder particle section clearly shows cracks that developed upon the change of SOC. (e) An SPM 3D rendering of a BEXP-prepared HOPG model anode with superimposed nanomechanical contrast obtained via ultrasonic fore microscopy (UFM); the cut area has sub-nm surface roughness.</figcaption></figure>

potential and conductance changes, when measured *ex-situ*, can provide additional information in such studies.

**Concluding remarks**

The real-space physical-property nanomapping of battery electrodes and interfaces via multifunctional SPM, combined with the efficient and artefact-free sample preparation approach via BEXP and nanotomography will provide a valuable characterisation toolbox for the development of the next generation of Na-ion batteries. Furthermore, it will allow the direct investigation of SEI stability and composition, the development of more robust cathode/anode-electrolyte interfaces, and the exploration of Na-ion intercalation, enabling the selection of a winning electrode-electrolyte combination.

**Acknowledgments**

The authors wish to acknowledge the financial support of the Faraday Institution (Grant No. FIRG018), the EU Graphene Flagship Core 3 project, the EPSRC project EP/V00767X/1, and the scientific and methodology insight provided by Marta Mucientes and Nuria Tapia-Ruiz. We are also grateful to our industrial partners Bruker, Leica Microsystems, and LMA Ltd. for their scientific and financial support.
## 6.4. Dissolution benchmarking of sodium-ion electrodes

Shahin Nikman<sup>1,2</sup> and Stijn F L Mertens<sup>1,2</sup>

### Status

Loss of battery electrode inventory to dissolution processes is one of the major sources of capacity fading. In the lithium-ion field, much research has been conducted into the charge-discharge profiles and electrolyte formulations that contribute to dissolution, in particular, for electrodes based on manganese, nickel, and cobalt, which can be translated to sodium-ion chemistries (with some caveats) when similar electrode materials and electrolytes are used [252]. Na⁺ ions have a 34% larger radius than Li⁺ (1.02 vs. 0.76 Å) which causes a greater strain on the electrode materials during insertion and extraction. Sodium-ion electrodes also span a larger compositional range than current LIB materials, encompassing transition-metal oxides, alloying reactions, and conversion-reaction materials, and therefore enhance both the possible sources and consequences of electrode dissolution. While direct loss of active electrode material due to dissolution into the liquid phase is typically in the single-digit percentage of overall capacity loss, the presence of dissolved transition metals in the electrolyte is known to rapidly accelerate SEI formation, leading to near-complete passivation of the negative electrode. This so-called dissolution-migration-deposition (DMD) process is estimated to account for up to 30% of the overall capacity loss in manganese-oxide-based intercalation batteries [253].

Several strategies already exist to counteract electrode dissolution, such as heteroatom doping [254], coatings, and the use of concentrated electrolyte formulations, figure 34. These indeed improve the long-term stability, but typically at the expense of the power and/or energy densities of the electrode. Fast throughput techniques that can determine electrode dissolution rates as a function of electrode potential, time, and electrolyte formulation are therefore essential and urgent. Understanding the effect of electrolyte, solvent, and electrode composition on dissolution and battery longevity will lead to better-informed materials discovery for durable and efficient NIBs.

### Current and future challenges

Due to their propensity to dissolve manganese, LiMn₂O₄ and NaₓMnO₂ electrodes are widely used for model studies. A primary challenge is to accurately identify the cause of electrode dissolution. For example, Hunter's mechanism of Mn³⁺ disproportionation into soluble Mn<fnref n="2" />⁺ and insoluble Mn⁴⁺ was confirmed in an aqueous solution but may differ in non-aqueous, fluoride-containing carbonate-based electrolytes. Recent studies have suggested that in non-aqueous electrolytes, Mn(III) is the dominant dissolved oxidation state. Other elements may also dissolve into the electrolyte, for example, vanadium from NaV₃O₈ cathodes [255].

As the rate of dissolution is proportional to the electrochemically exposed surface area, bulk characterisation techniques, such as x-ray diffraction may not be adequate. *In situ* surface-sensitive methods, such as electron energy-loss spectroscopy, extended x-ray absorption fine structure, and high-angle annular dark-field imaging show that de-intercalated λ-MnO₂ is chemically unstable [256]. However, only the surface layers break down to Li₂Mn₂O₄ and Mn₃O₄, where the average Mn oxidation state is much closer to three. The stability of the electrode therefore cannot be predicted, based on the bulk composition or the cell potential, which would indicate only an insoluble product. Such structural changes also induce stress in the host material due to changes in the lattice constant, leading to cracks and again increasing the exposed surface area. Although such methods have not yet been employed to elucidate sodium-ion electrode dissolution, the intercalation of Na⁺ induces more strain than the intercalation of Li⁺ due to its larger radius, potentially leading to more cracks, a greater exposed surface area, and hence, enhanced dissolution. Heteroatom dopants, such as iron and nickel, for example, Na₀.67Fe₀.5Ni₀.15Mn₀.65O₂ may increase stability against manganese dissolution [257], but may themselves dissolve following chemical or electrochemical pathways and increase the potential range in which dissolution takes place. The dissolution problem is further compounded in sodium-ion anodes, in which elements such as tin, antimony, phosphorus, titanium, copper, iron, or sulphur can produce soluble byproducts during operation, whereas graphite does not.

Solvents, electrolytes, and contamination from moisture or CO₂ can further accelerate the degradation of sodium cathode host materials at a much higher rate than their lithium counterparts [258]. The dissolution of ions into the electrolyte solution depends on the particular electrolyte formulation, ion solubility, and potential programming of the battery cell, which encompasses a huge array of variables. The lack of direct, real-time dissolution measurements greatly compromises the time and potential resolution of dissolution information. To date, most dissolution diagnosis has been conducted post-mortem. For example, inductively coupled plasma (ICP)-based elemental analysis of the electrolyte has been used to identify ppm-range

<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, Didcot OX11 0RA, United Kingdom</footnote>
<figure><img src="image_41.png" /><figcaption>**Figure 34.** Key dissolution reactions occurring on sodium-ion anodes: attack from reactive species in solution, anodic dissolution, and crack formation caused by structural strain. Heteroatom doping and surface coatings may have a stabilising effect.</figcaption></figure>

<figure><img src="image_42.png" /><figcaption>**Figure 35.** V-shaped flow cell direct-injection online ICP analysis design for fast-throughput dissolution analysis. The electrolyte flows past an online counter electrode (CE), passes over the working electrode (WE) surface controlled by a potentiostat and the reference electrode (RE). Any dissolution products are efficiently transported to an ICP-type instrument for nebulisation and ionised by the plasma torch. The red-framed insert is a magnified view of the contact between the flow cell and the working electrode.</figcaption></figure>

concentrations of dissolved iron from sodium iron hexacyanoferrate electrodes as a function of the surface coating strategy [259]. Energy-dispersive x-ray spectroscopy (EDS) is also commonly used, for example, to analyse the separator in order to identify trapped ions, or to analyse the negative electrode to identify DMD products from $Na_{0.67}MnO_2$ electrodes [260]. The preparation and *ex situ* handling of such samples increases experimental error and may compromise the reliability of the conclusions. Furthermore, full and half cells prepared in CR2032 coin cells do not isolate a single cause of dissolution, as the effects of electrode arrangement, cell pressure, the counter-electrode material and current collectors must also be considered.

Open questions are whether electrode dissolution is electrochemically or chemically induced, i.e., which are the most significant dissolution pathways, and their accurate quantification. Methods of detection must be highly sensitive, as dissolution only affects a small part of the overall electrode mass, and setups must rigorously exclude agents known to accelerate dissolution, such as water, as otherwise, artefacts will arise. An adequate dissolution monitoring system must therefore be carefully designed.

### Advances in science and technology to meet challenges

ICP-based instruments are routinely available, and allow highly sensitive elemental analysis, with detection limits of sub-ppb using optical emission spectroscopy (ICP-OES) or even sub-ppt using mass spectrometry (ICP-MS). Primarily used for aqueous samples, these are, to a certain degree, forgiving towards non-aqueous battery solvents and electrolyte salts (up to 0.1 M (ICP-MS) or 1 M (ICP-OES) without dilution). The observation of dissolution in non-aqueous electrolytes is challenging, due to the need for oxygen and water to be excluded from the electrolyte. Prohibitive factors include instrument corrosion from highly reactive electrolytes and the high cost of many high-quality non-aqueous electrolyte solutions.

Recent advances in electrochemical flow cell design have enabled direct observations of the dissolution of candidate electrode surfaces in non-aqueous environments under potential or galvanostatic control. Typically, the electrochemical flow cell outlet is directly connected to the ICP nebuliser by tubing, in the so-called direct-injection analysis. Successful examples include the flow-through scanning flow cell (FT-SFC) introduced by Mayrhofer *et al* [261], which incorporates a V-shaped channel design for efficient transport of dissolution products, figure 35. This type of cell has also been manufactured by 3D printing and successfully
used for fast and high-throughput dissolution detection [262]. However, intricate new cell designs are not always necessary. Markovic's group [263] fitted an ICP-MS inlet to a rotating disc electrode tip in a stationary probe rotating disc electrode (SPRDE-ICP-MS), adding the advantage of uniform and well-characterised convection. Hence, hyphenation of existing apparatus to increase time- and potential-dependent dissolution information output in accelerated degradation tests becomes possible.

While ICP-based analytical instruments identify the atomic mass/charge ratio (ICP-MS) or element-specific emission spectra (ICP-OES), valuable information about the oxidation state of the dissolved species, i.e., speciation, is lost. Other *in situ* techniques may additionally allow speciation, e.g., rotating ring-disc electrode [264], electron paramagnetic resonance, or UV-vis [265] measurement of the electrolyte in *operando* cells. However, interference from solvents, electrolytes, or other solution species must be accounted for.

**Concluding remarks**

Post-mortem analyses and *ex-situ* observations of battery electrodes limit our understanding of the degradation processes that are the main causes of capacity fading and the premature failure of batteries. Today, a detailed understanding of the atomistic causes of the dissolution of electrode materials remains elusive, even for the most basic model electrodes. The development of online analysis techniques that quantify time-resolved dissolution as a function of potential and other experimental parameters can revolutionise our understanding of battery electrode degradation, as an essential step towards its mitigation and the rational design of resilient new battery electrode materials. Advances in spectroelectrochemical cell design and analytical instrumentation can unlock detailed dissolution probing, revealing not only elemental quantities but also chemical speciation.

**Acknowledgments**

The authors gratefully acknowledge funding from ALISTORE-ERI, the Faraday Institution (FIRG018), and the Austrian Research Fund (FWF, Project I3256-N36).
# 7. Scale-up and manufacturing

## 7.1. Cell performance and requirements

*Emma Kendrick*

School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham BT15 2TT, United Kingdom

### Status

Research into Na-ion batteries (NIBs) began in the 1970s and 1980s and was usurped by LIBs in the 1990s. NIBs have recently re-emerged as a potential sustainable and lower-cost alternative to LIBs [266]. They offer an energy storage solution which is not as reliant upon geographically localised raw or critical materials. Several commercial entities have built up around this technology, with demonstrator cells and prototype battery packs, offering the potential for lithium-ion and lead-acid battery substitutions in current markets [71]. Sodium can be extracted from seawater, anode materials can be obtained from renewable sources, and cathodes generally contain iron and manganese, rather than high levels of cobalt and nickel. The manufacturing processes are expected to be the same as those for LIBs, and therefore, no investment is required in new manufacturing lines. The cost difference between LIBs and NIBs can therefore be considered to be due to materials and performance only [184]. NIBs are unlikely to ever reach the energy densities (gravimetric and volumetric) of LIBs, due to their lower operating voltages and lower active component densities [267]. The 'drop-in' manufacturing approach also creates limitations on NIB development, in terms of the design of components and cells. The thickness and design of the electrodes are limited by these tape-casting methods. Opportunities for differentiation consequently lie in high power, safe, and long-life NIB technologies. A sustainable circular-economy perspective on materials, manufacturing, and end-of-life could also offer significant advantages over current battery technologies. Alternative manufacturing methods may offer solutions for improved electrode kinetics, the engineering of more efficient electronic and ionic transport pathways, and simplified recycling. Sustainability needs to be considered within a greater holistic outlook, however; the use of low-cost and highly abundant materials may mean, for example, that the reclamation and reuse of these materials never becomes economically viable. Consideration of the hierarchy of recycling, re-use, or second life within the circular economy is required. Therefore, lifetime, in particular, is a key performance parameter for sodium ion batteries, as the value of the materials reclaimed from recycling will not be sustainable if low-cost and abundant materials are used [268].

### Current and future challenges

There are several research challenges which need to be addressed in the field of NIB technologies, one of which is long life, which is required to improve sustainability. Advancements in LIB energy density, both volumetric and gravimetric, are required for applications reliant upon this technology, and there is an immense amount of work ongoing to develop new anode and cathode materials. However, the lifespan of the cells is governed by various chemical and cell engineering optimisations relating not only to the materials but also to the design of electrodes and cells and their operation (figure 36) [269]. For NIBs, mass balance optimisation between positive and negative electrodes is critical for high-energy and long-life batteries. Each cathode and anode material has a first-cycle coulombic inefficiency related to electrolyte decomposition and irreversible sodium movement from the positive to the negative electrode. As a result, an interface (surface-electrolyte interface (SEI)) forms on the anode, causing less sodium to be available (reducing capacity) for shuttling between the positive and the negative electrodes. The mass balance and the voltages chosen therefore have a large effect upon the effectiveness of this interface formation at stabilising the electrolyte against decomposition, and consequently, the energy density of the cell [270]. The stability of the interfaces on the positive and negative electrodes is reliant on many factors, and it is known that the stability of NIB interfaces is inferior to those of LIBs due to the highly soluble nature of sodium salts. Much work is required to understand the formation and changes that occur at these surfaces and interfaces during the lifetime of the cell, and some progress has been made in this area (figure 37) [271]. These interfaces are not only important for longevity but also for safety. Dendrites and plating can occur very readily in sodium-ion cells, and this is not only controlled by the electrolyte composition and the ability to form low-resistance, stable interfaces, but also through electrode design. For example, low and inhomogeneous electronic conductivities within the electrode can lead to changes in current density across the electrode, resulting in sodium plating and dendrite growth in areas with high resistance. Improving the electronic conductivity of the electrode with conductive additives, such as carbon black, can aid electron transport but additionally leads to increased electrolyte side reactions and hence ongoing growth in the SEI. This, in turn, blocks pore networks for ionic transport and increases local resistance [272]. Understanding the combined effects of
<figure><img src="image_43.png" /><figcaption>**Figure 36.** Factors which have been shown to have an influence on the creation and maintenance of a stable surface-electrolyte interface. Reproduced from [269]. © IOP Publishing Ltd CC BY 3.0.</figcaption></figure>

<figure><img src="image_44.png" /><figcaption>**Figure 37.** Schematic diagram of the growth in the SEI layer on hard carbon, with electrolyte additives; FEC (a) and NZeo (b) taken from Nanozeolite ZSM-5 electrolyte additive for long-life sodium-ion batteries. Reproduced from [271] with the permission of The Royal Society of Chemistry.</figcaption></figure>

these changes upon the lifetime, capacity, and ultimately the degradation mechanisms will be essential to extend the lifetime of cells and make further NIB optimisations.

**Advances in science and technology to meet challenges**

In order to achieve longer lifetime NIBs, a greater understanding of the interaction of all the cell components and competing ageing and degradation processes is required. This will be different for each cell chemistry, voltage window, and electrode design. An understanding of the interfaces is of the greatest importance for both cathodes and anodes. These interfaces control the transport properties of sodium into the electrodes and influence the type and extent of microstructural changes during the ageing of the electrode. The compositional makeup of these interfaces requires optimisation through electrode and electrolyte design. Changes in these interfaces and their effects on the physical and chemical properties will also affect performance and safety. This offers opportunities for advanced, data-driven, and multiphysics models to be partnered with experimental data to provide greater in-depth information about the changes in the fundamental kinetic and thermodynamic properties of the cell for various compositions and designs.
Advances in parameterisation methods for LIBs could be translated to NIBs, offering novel, unique insights into cell construction and component optimisation [273].

Moving away from traditional LIB manufacturing processes is essential to achieve step changes in sustainable cell designs. Novel processes are required to provide improvements in electrode 3D electronic and ionic transport pathways. With improvements in electrode design, less current collector and packaging materials are required, which improves energy density and reduces waste at the end of battery life. Novel methods of embedding electronic conductors in active materials without using high-cost and highly resistive polymeric binders to adhere them to metallic current collectors could provide the step change required. In addition, designs that enable simple manufacturing and subsequent disassembly for material reclamation will enable greater value from material recovery and reuse, creating a sustainable circular economy for NIBs. The ultimate design intent would be to use no binder, however moving to renewable sources for water-soluble binders, such as those based on algae and cellulose, may also improve recyclability prospects. Designing for recycling and disassembly is a key area which can capitalise on the benefits of this set of materials and unlock the potential of NIBs.

## Concluding remarks

An economically viable NIB should be considered with respect to the sustainability potential of a circular economic model rather than purely the first-life cost of materials and manufacturing. Consideration of lifetime performance is required for both first life and second life, with recovery and recycling of cell components at the end of life. The current focus is upon matching LIB performance criteria. However, with rapidly changing markets for energy storage solutions, NIBs offer different opportunities for new cell design and manufacturing processes, allowing a holistic approach to sustainability. A focus upon developing longer-life batteries is required to achieve this. The optimisation of electrolyte and electrode designs to reduce interface resistance and dendrite growth will improve both longevity and safety, while new manufacturing processes and cell designs are required for sustainable end-of-life solutions. Materials and components can then be easily and repeatedly manufactured, reconditioned, disassembled, and remanufactured. In this way, the challenges of both the performance and economics of sodium-ion batteries can be overcome by combining novel materials, processes, and products with advanced material recovery, repurposing, and recycling.

## Acknowledgment

Innovate UK for funding (IUK Project 104179).
## 7.2. Applications and scale-up: manufacturing

Stewart A M Dickson<fnref n="1" />,<fnref n="2" /> and John T S Irvine<fnref n="1" />,<fnref n="2" />

### Status

The manufacture of batteries is often overlooked at a research level, due to the importance of finding new, higher-energy electrode materials, and solving the challenges faced in cell chemistries at smaller scales. However, when producing large-format cells, its importance in achieving the optimal cell performance is paramount. As demand increases further, together with the growing applications of LIBs, the availability of manufacturing facilities has also increased. Europe has multiple ‘Gigafactories’ in the pipeline, with examples such as Northvolt, who are expected to reach 32 GWh by 2023. Similar or larger-scale factories in China and beyond are also in the pipeline, backed by larger companies such as Tesla. As noted by the Faraday Institution, eight ‘Gigafactories’ will be needed by 2040 to satisfy UK battery demands, which requires one to be established at least every two years from 2020 [274]. Projects such as those recently proposed by AMTE Power and Britishvolt are the start of promising large-scale manufacturing of LIBs within the UK, with the hope that this can complete the chain to take materials from powder to power, thus attracting end users of battery technology by shortening supply chains [275].

NIBs are beginning to reach a technology readiness which requires large-scale production, as demonstrated by Faradion, with recent orders from Australia and India [276]. NIBs will see use in large, decentralised power applications, and small-scale deployments will be required to assist in load balancing from intermittent-power farms, such as wind and solar [277]. Factories can switch between lithium and sodium chemistries, due to their ‘drop-in’ nature, as long as the technologies remain similar enough in their production (figure 38).

This helps to reduce barriers to entry and initial costs, as the creation of new facilities is not required; however, the gap between commercial and laboratory technologies still needs to be bridged to facilitate material advances and their application in larger cells. Additionally, research institutions often lack access to facilities which can help to realise materials at larger scales. By improving this connection, especially through links between industry and academia, it is hoped that an acceleration of research towards manufacturing readiness can be achieved.

### Current and future challenges

Although some good parallels can be drawn from LIB manufacture, there are still limitations to how this can be applied to NIBs, due to differences in the materials used. When changing any physical property of a material, such as particle size, crystal structure, morphology etc., this also alters the processing parameters, thus impacting electrochemical performance. Similarly, changing the processing procedures of an electrode material, for example, slurry preparation or electrode modifications, can also influence both its structure and its maximum performance (figure 39) [278]. Therefore, every material must be treated differently to achieve this fine balance between every parameter.

Most of the challenges within manufacture lie at a materials level. Complex multi-step procedures can be used to produce grams of material for research purposes. However, these techniques may not be feasible when producing the kilogram amounts required for larger cells. Material morphologies, such as the creation of high-surface-area particles, will assist with electrochemical performance. Additionally, some layered sodium oxides of interest are extremely sensitive to air and moisture, which can make handling of large quantities more complex, and imposes additional requirements such as additives in slurries to improve their longevity. It is possible to modify these materials to improve their stability in air or to reduce the sodium content, but this can be detrimental to electrochemical performance [279].

Electrode slurries at the coin-cell level also have standardised formulations; little attention is paid to metrology and dry powder mixing, and they include large amounts of binder and carbon to overcome material limitations, such as poor conductivity. Reducing these amounts to increase the energy density of cells is vital at large scales, as commercial cells sometimes reach up to 98 wt.% active material. However, such changes require specific testing and optimisation of every electrode slurry. After coating, the influence of drying and the application of calendering as a technique to improve electrode density and adherence are also not routinely considered within the research environment. These steps also require individual tailoring to maximise electrochemical performance without negatively impacting electrode structure.

Although materials and processing are key to cell performance, other considerations, such as electrode balancing and geometry, tab placement, and safety aspects related to long-term storage and long-term cycling which are not routinely studied at smaller scales also become relevant. Due to the infancy of NIB technology, little research exists for many of these other areas, though it is fortunate that many of these can

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

<footnote n="2">² The Faraday Institution, Quad One, Harwell Campus, OX11 0RA, United Kingdom</footnote>
<figure><img src="image_45.png" /><figcaption>**Figure 38.** Simplified flow diagram of the construction of an NIB pouch cell.</figcaption></figure>

<figure><img src="image_46.png" /><figcaption>**Figure 39.** A pictorial overview of the structure-processing-property relationships which need to be considered when scaling up battery materials. Reprinted from [278], Copyright (2014), with permission from Elsevier.</figcaption></figure>
be directly transferred from LIB research, and challenges can be jointly tackled. Some advantages and benefits which can be discovered are, however, solely for NIBs, such as the ability to internally short a pouch cell with no performance impact, as demonstrated by Faradion [277].

### Advances in science and technology to meet challenges

In general, a better understanding is required of how to produce and process materials, as there is very little information that specifically relates to NIBs at larger scales. Ideally, techniques already used to scale up electrode materials to significant quantities could be implemented. Co-precipitation is the most widely utilised approach for materials scale up, and several researchers have produced well-defined structures of layered sodium oxides for NIBs, which required additional firing steps after the initial precipitation, which can be energy intensive [280]. Prussian blue analogues (PBAs) can also be produced by co-precipitation, and do not require further processing, reducing cost and time. Co-precipitation is not necessarily suitable for all electrode materials, depending upon their stability and composition. Therefore, new routes such as microwave synthesis could be optimised to provide alternatives.

Electrode processing is also a significant focus, as it contributes a large amount to the cost of cell production. Tape casting is the technique most widely utilised to produce electrode sheets; however, the use of N-methyl pyrrolidone (NMP) increases expense, due to the need for solvent-recovery systems. The development of newer electrode slurry combinations, particularly aqueous chemistries, would reduce this cost. Additionally, several researchers have begun to develop ‘solvent-free’ casting techniques in a bid to remove this issue entirely and have greater control of electrode morphologies, though these advances are likely to take some time to become commercially viable [281, 282].

Perhaps one of the biggest advances which will help manufacturing immensely is the use of simulation and modelling. Computational modelling of cells and packs has become much more viable in recent years and by varying parameters, can allow for rapid analysis of changes to electrode composition and cell design, obtaining incremental performance gains when applied experimentally. One example of modelling to identify improvements in pouch cells is that of tab placement, where the benefits of an opposite-ended tab design were shown to improve electrode usage and temperature management [283]. Greater practical application is required to demonstrate the benefits of these findings, and whether there are correlations with simulations.

Finally, in terms of NIBs, there needs to be a much greater understanding of the long-term performance of cells, especially as size increases, which will become better understood with time. Additionally, the use of half cells with metallic sodium does not realise true cell chemistries, which instead use hard carbon anodes, though the use of half cells is also a common practice for new LIB materials. Any performance issues or irregularities in a half-cell setup are not a true representation of what may occur in a scaled-up cell configuration; thus, a greater emphasis on testing materials in three-electrode or small full-cell setups is essential to ensure that materials can be scaled up without issue.

### Concluding remarks

Unquestionably, as the field of NIBs expands over the next decade, greater cooperation between materials development and manufacturing will be required to ensure that advances can be easily transferred from small scale to large scale. Though pouch cells have been considered in this text, there are multiple formats for batteries which should all benefit from this focus. If LIB manufacturing lines can be converted to produce NIBs, this would help to drastically reduce costs further and allow for better knowledge transfer, making NIBs an attractive low-cost option. However, there are still many NIB-specific challenges which need to be tackled, mainly by improving knowledge about the processing of electrode materials, and their inherent long-term safety, which both require greater study.

### Acknowledgment

This work was supported by the Faraday Institution's NEXGENNA project (FIRG018).
# 8. Industrial targets and techno-economic analysis

## 8.1. Industrial targets

Ashish Rudola, Ruth Sayers and Jerry Barker

Faradion Limited, The Innovation Centre, 217 Portobello, Sheffield S1 4DP, United Kingdom

### Status

Faradion Limited, established in 2011, was the first non-aqueous Na-ion battery company in the world. Since then, several Na-ion companies have been founded, covering a wide range of sodium-based chemistries. Faradion's technology utilises mixed-phase O3/P2-type Na-Mn-Ni-Ti-Mg layered oxide cathodes, hard carbon anodes, and non-aqueous electrolytes [284]. Other companies reliant on carbon-based anodes and non-aqueous electrolytes include the French company, Tiamat, whose cathodes are based on the polyanionic Na₃V₂(PO₄)₂F₃ [285], the Chinese company HiNa Battery, which utilises O3-type Na-Cu-Fe-Mn layered oxide cathodes [286], and the Swedish company Altris AB, based on Prussian blue analogue (PBA) cathodes. In contrast, US-based Natron Energy has built its Na-ion technology around aqueous solvents and PBA-based cathodes and anodes [287].

From the above, it can be seen that there is already a diverse array of Na-ion chemistries available in the industry. This is important, as different markets/applications have different requirements and might require unique solutions – the battery of choice needs just the right combination of cost ($ kWh⁻¹), energy density, stability/reliability, power rating, charge acceptance, and temperature performance. As such, figure 40 presents a Ragone plot covering several main market applications with their specific energy/power envelopes (gauged by the datasheets of existing battery technologies that are currently deployed in the market for these specific applications) [288, 289]. It should be stressed that these are just a few examples of applications—there are others with different performance requirements. We have also indicated the maximum performance of Na-ion cells from some industry players, as given by their datasheets or published reports [284, 285, 287]: for Tiamat and Natron, the values mentioned are correct as of mid-2018 and mid-2019, respectively, while for Faradion, values are accurate as of mid-2020. A Na-ion cell might be appropriate for utilisation for a particular application, as long as the application's envelope lies within its energy/power capabilities (gauged by the area below its respective dotted lines in figure 40) and it can meet other requirements such as cycle life. It is evident that current Na-ion technology could be attractive for most applications, apart from some types of consumer electronics and long-range electric vehicles.

### Current and future challenges

Referring to figure 40, it can be seen that different Na-ion chemistries are better suited for different applications. For example, Natron's aqueous Na-ion chemistry has the lowest specific energy/power—this is expected, as aqueous electrolyte-based batteries cannot match the energy densities of their non-aqueous counterparts owing to the limited voltage window of water as an electrolyte solvent. Thus, Natron's Na-ion systems would be ill-suited for higher-energy applications, such as electric vehicles. However, Natron batteries' expected low cost, exceptional cycling stability, safety and efficiency would make them attractive for different ESS applications [287]. Tiamat's use of fluorinated vanadium-based polyanionic cathodes delivers Na-ion chemistry with excellent power capabilities albeit at the expense of energy density and raw material handling and availability [285]. Polyanionic cathodes represent an attractive avenue for high power and durable Na-ion systems. The challenge is to base them on more Earth-abundant elements, such as Fe and Mn. Unfortunately, pure Fe/Mn-based polyanionic cathodes generally either demonstrate relatively low operating potentials which limit their specific energies, or rely on bulky anionic groups [290], which limit their tap densities and, in turn, volumetric energy densities [284]. On the anode side, hard carbon can be prepared from a variety of precursors, including low-cost options, and has an attractive combination of desirably low operating potentials and high capacity (current hard carbon capacities almost match those of graphite Li-ion anodes). Hence, it is unlikely to be displaced as the anode of choice for high-energy applications in the foreseeable future. Furthermore, an intense research focus on hard carbon has resulted in significant improvements, not only in its cycling stability and rate performance but also in its charge acceptance capabilities—these, along with excellent compatibility with high-flash-point electrolytes, confer a high degree of safety to Na-ion systems [284].

Faradion's Na-ion cells currently deliver a good mix of energy and power densities with attractive costs, due, in large part, to cathodes devoid of costly and scarce elements, such as Co and V [284]. Table 2 illustrates how Faradion Na-ion cells compare with two types of state-of-the-art Li-ion cells: Li-Ni-Mn-Co oxide (NMC)//graphite and LiFePO₄ (LFP) cathode//graphite chemistries.
<figure><img src="image_47.png" /></figure>

## Advances in science and technology to meet challenges

Faradion's Na-ion cells currently deliver energy and power metrics in between those of NMC/graphite and LFP/graphite cells (table 2). They can respond at 10 C (continuous discharge), which compares favourably with LFP/graphite pouch cells (1–3 C) [293]. Faradion's Na-ion chemistry could be considered as a replacement for most applications where LFP/graphite is currently deployed. Table 2 also outlines targets for an 'energy cell' and a 'power cell,' that would solidify Na-ion technology as the go-to battery choice for different applications. The 'energy cell' is based on Faradion's Na-ion chemistry and seeks to satisfy some of the more energy-demanding applications, such as mobile phones, laptops, and some EVs. For this purpose, Faradion has devised a clear roadmap to achieve these targets by enhancing reversible capacities, increasing electrode densities, and boosting coulombic efficiencies with novel electrolytes and additives. In fact, we recently showed that the rates of increase of Faradion's Na-ion specific energies (and cycle lives) are significantly faster than those of Li-ion [284].

Even though current Na-ion technology can deliver high power, for applications such as power tools or drones, both power and energy densities need to be simultaneously acceptable. For such a 'power cell,' it might be challenging to meet the power and energy requirements using polyanionic cathodes at acceptable costs. We thus anticipate layered oxide cathodes will be used for such cells, relying on a majority, or a pure, P2 phase; it is well known that many P2-based Na-ion oxide cathodes can deliver excellent rate performance. Faradion's low cell BOM (bill-of-materials) is based on the absence of expensive cobalt, the use of aluminium (as opposed to expensive copper) current collectors, and the natural abundance of sodium, resulting in active materials and electrolyte salts that are cost-effective to produce. Future cell BOMs ($ kWh⁻¹) can primarily be reduced in three ways: (a) increasing energy density; (b) large-scale industrialisation of the material supply chain with (c) increasing production volumes, all reducing cost ($) with scale. The technical approach to achieving these target energy densities (stated in table 2) is listed above. The development of material supply chains and an increase in production volumes are already happening with the ever-increasing realisation of the performance benefits of Na-ion batteries (figure 40) and the ever-rising demand.
<table><caption>**Table 2.** Performance metrics of current NMC/graphite and LFP/graphite Li-ion technologies and those of Faradion's Na-ion cells. Industry targets for a Na-ion energy cell and a Na-ion power cell are also shown.</caption><thead><tr><th rowspan="2">Parameter</th><th colspan="2">Current technology</th><th colspan="2">Na-ion energy cell: production<sup>c</sup> pouch cells</th><th colspan="2">Na-ion power cell: production<sup>d</sup> ≤12 Ah pouch cells</th></tr><tr><th>NMC//graphite: cylindrical</th><th>LFP//graphite: 25 Ah-280 Ah pouch/ prismatic</th><th>Faradion Na-ion</th><th>2 year target</th><th>5 year target</th><th>2 year target</th><th>5 year target</th></tr></thead><tbody><tr><td>Cell BOM* at equivalent cell sizes and volumes ($ kWh<sup>-1</sup>)</td><td></td><td></td><td>25%-30% lower than LFP//graphite pouch cells [282]</td><td></td><td>Future cell BOM targets are in line with maintaining 25%-30% lower than LFP/graphite, and taking into account the forecasted annual reductions in Li-ion costs [289]</td><td></td><td></td></tr><tr><td>Specific energy (Wh kg<sup>-1</sup>)</td><td>240-270</td><td>140-175</td><td>160 (32 Ah pouch)</td><td>&gt;190</td><td>&gt;210</td><td>&gt;140</td><td>&gt;160</td></tr><tr><td>Energy density (Wh l<sup>-1</sup>)</td><td>670-750</td><td>240-360</td><td>290 (32 Ah pouch)</td><td>&gt;350</td><td>&gt;380</td><td>&gt;230</td><td>&gt;260</td></tr><tr><td>Specific power (W kg<sup>-1</sup>)</td><td>340-420</td><td>175-425</td><td>1000 (2 Ah pouch)</td><td>&gt;1500</td><td>&gt;2000</td><td>&gt;2500</td><td>&gt;4000</td></tr><tr><td>Power density (W l<sup>-1</sup>)</td><td>960-1200</td><td>360-770</td><td>1300 (2 Ah pouch)</td><td>&gt;2700</td><td>&gt;3600</td><td>&gt;4000</td><td>&gt;6400</td></tr><tr><td>Cycle life<sup>a</sup> @ 80% DOD at ±1 C</td><td>600 [292]</td><td>3500-6000</td><td>3000 [284]</td><td>4000</td><td>8000</td><td>2000</td><td>4000</td></tr><tr><td>Pulse discharge</td><td>5-7 C for 10 s</td><td>4 C for 10 s</td><td>20 C for 10 s</td><td>20 C for 10 s</td><td>30 C for 10 s</td><td>30 C for 10 s</td><td>50 C for 10 s</td></tr><tr><td>T range, T<sub>min</sub> to T<sub>max</sub> (°C)</td><td>-20 to 60</td><td>-20 to 60</td><td>-30 to 60</td><td>-30 to 80</td><td>-30 to 80</td><td>-30 to 80</td><td>-30 to 80</td></tr><tr><td>RTEE<sup>b</sup> at ±C/2 (%)</td><td>91</td><td>94</td><td>&gt;93</td><td>&gt;94</td><td>&gt;95</td><td>&gt;95</td><td>&gt;95</td></tr><tr><td>Impedance (mΩ) at 1 kHz</td><td>&lt;25 (21 700 cell)</td><td>&lt;1.2 (25 Ah cell)</td><td>&lt;2.5 (12 Ah cell)</td><td>&lt;1.5 (12 Ah cell)</td><td>&lt;1.2 (12 Ah cell)</td><td>&lt;1.2 (12 Ah cell)</td><td>&lt;1 (12 Ah cell)</td></tr></tbody></table>

<footnote>*BOM: bill of materials. <sup>a</sup>Number of cycles to 20% capacity fade. <sup>b</sup>Round-trip energy efficiency, calculated from datasheets. <sup>c</sup>Production energy cell pouch: capacity ≥10 Ah. <sup>d</sup>Production power cell pouch: capacity ≤12 Ah, as power cells typically have thinner electrodes which result in lower capacities and smaller cell sizes. Note that LFP/graphite chemistry (like any battery chemistry) can deliver much higher power ratings in cylindrical cells (refer to figure 40), but in large-scale pouch cells between 25–280 Ah, the manufacturers recommend a maximum continuous discharge from 1 to 3 C [293].</footnote>
## Concluding remarks

Na-ion technology is rapidly catching up with the established Li-ion battery. It is revealed here that the energy/power densities of Faradion's Na-ion chemistry, which is devoid of scarce elements, such as Co and V, already surpass those of LFP/graphite systems. For most applications, the different types of Na-ion battery already possess the required performance attributes at the cell level, from negligible self-discharge to excellent temperature performance and the ability to be shipped/stored at 0 V [284], to name a few. Such performance must now be shown in actual demonstrations of large-scale systems, cycled according to the duty cycles required for an application in real-world settings. These demonstrations by various Na-ion companies are already underway, and with each successful demonstration, the market's confidence in this technology will progressively grow. If the near-term industrial targets for Na-ion energy and power cells can be met, mass penetration of Na-ion batteries in the market will be almost inevitable.

## Acknowledgments

The authors thank Faradion Limited for their permission to publish the details provided in this section. We also thank the entire Faradion R&D team and our commercial partners for helping to achieve the results contained herein.
## 8.2. Techno-economic assessment

Emmanuel I Eweka¹ and Grant S Stone²

¹ Amte Power Ltd, 153A Eastern Avenue, Milton Park, Oxford OX14 4SB, United Kingdom

² Amte Power Ltd, Denchi House, Thurso Business Park, Thurso, Caithness KW14 7XW, United Kingdom

### Status

Batteries using conventional Lithium-ion chemistries (LIBs) have long since been the power source solution of choice in a range of applications due to their high volumetric and gravimetric energy densities. Energy-dense, state-of-the-art LIBs in the pouch and cylindrical formats are currently achieving energy densities of up to 250 Wh kg⁻¹ [294]. Beyond the current LIB designs, bulk all-solid-state batteries (ASSBs) are also being developed [295] to provide a significant increase in energy density (≥400 Wh kg⁻¹) and safety and to address the range anxiety with electric vehicles. However, it is not believed that the first generation will be a cost-effective solution for stationary (grid, renewables, telecoms, and domestic) applications, and timescales to market for a competitively performing product are expected to be way beyond 2025. Other ‘beyond-lithium’ chemistries, such as Li-air and rechargeable magnesium batteries, are still at very early stages of R&D [294]. Current analyses are focused on non-aqueous battery systems. For stationary and telecoms applications, size and weight are less limiting than, say, cost, safety, ease of maintenance, reliability, and cycle life. Therefore, this techno-economic assessment focuses on applications where it is believed that NIBs will be competitive, at least in the near term (5–10 years) based on their current state of development. Some of these application areas include stationary and telecoms.

Their inherently low material cost and high technology safety make NIBs a very attractive proposition for stationary applications [252]. In addition to being made in similar formats to LIBs, NIBs can be made on existing LIB manufacturing lines—significantly reducing development costs and timescales. Current high-performance LIBs are considered less accessible (and safe) for these applications, although they have been proposed for second-life use [296]. Based on economics, Pb-acid and LiFePO₄ (LFP) are the current cathode chemistries of choice for stationary applications, compared to either LiNi₁-x-yMnₓCoᵧO₂ (NMC) or LiNi₁-x-yCoₓAlᵧO₂ (NCA).

NIBs using a layered oxide, NaMO₂ (where M stands for Ni, Mn, Mg, or Ti) P2/O3-type structure and hard carbons (HCs) have been significantly advanced by Faradion and look very promising for stationary applications [297]. Low-cost NIBs based on Prussian blue analogue (PBA) anodes and cathodes and sodium vanadium fluorophosphates (NVPF) are also being developed by Natron energy and Tiamat respectively [298], but are considered less competitive for these applications, predominantly on a gravimetric and volumetric energy basis.

Using various cost models including BatPac, € kWh⁻¹ has been estimated for NIB (NaMO₂/HC), LFP and NMC (40%Ni:40%Mn:20%Cobalt), 18 650 cells, respectively [267, 296]. The BatPac model included cell-design elements, depreciation, and warranty in addition to material and processing costs. The outputs from these models indicate comparable costs for LFP cells and NIBs (and lower costs for NMC cells). The models also indicate that the € kWh⁻¹ for NIBs can be significantly decreased with volume-scale manufacturing and if the Wh kg⁻¹ and Wh l⁻¹ can be increased.

### Current and Future challenges

Global energy demand is predicted to grow by up to 40% in the next ten years, including an increase of 2500 TWh in the transport sector, and the global power capacity of solar and wind are planned to be 2 TW and 1.5 TW, respectively, in the same timescale [299]. Combined with a global push to reduce fossil-fuel reliance, this will create a massive market for energy storage. Competitive battery technologies covering a wide performance, safety, and cost matrix will be required to support this growth [300].

In the short term, the first generation of NIBs has to prove that it is a contender in the energy-storage market, in terms of competitive performance, safety, cost, and maintenance, compared to incumbent technologies, such as Pb-acid, LIBs (LFP and NMC), and redox-flow batteries. The cost of development, production, and time to market will also have to be factored in. One of the major challenges in this space will be to match or, if possible, outperform state-of-the-art LIB cathodes such as NMC on a cost/performance, (€ kWh⁻¹) basis and fit in with existing infrastructure. The majority of new stationary energy-storage installations use LIBs (currently, mostly LFP), which have benefitted from cost reductions and innovations in the EV market [300], and a bulk cost reduction could allow for a quick entrance into this application. Another major short-term challenge facing all new energy-storage markets is regulation and policy. Concerns around safety and the decisions to invest in different types of infrastructure will have a huge impact on the demand for energy storage and the associated costs of installation and use [300]. Future challenges will lie in securing and maintaining a competitive edge over other developing technologies. LIB roadmaps are looking at novel materials and architectures to increase energy density and reduce cost per
kWh. Ease of recycling is another challenge that will need to be addressed, since the financial and environmental costs of disposal increase with the scale of adoption.

### Advances in science and technology to meet challenges

In recent years, there has been a significant increase in worldwide NIB R&D activities [252, 267], driven by potential cost and safety benefits compared to LIBs, as well as compatibility with LIB manufacturing processes and applications. Next-generation cells will seek to improve energy density beyond 170 Wh kg⁻¹ [301] and improve other cell characteristics without compromising safety. This will further reduce the € kWh⁻¹ cost and make them more competitive for automotive and other applications [252, 267].

Advanced NIBs based on current-generation chemistries will improve in gravimetric and volumetric energy density, simply based on material, electrode, cell design, and engineering improvements. One area of improvement will be to increase the physical and active content and loadings and the density of the electrodes (and tap densities of active materials) to maximise the volumetric capacity. Electrolyte loadings can also be optimised to reduce cell weight.

First-cycle efficiencies (FCEs) for hard carbons are also quite low compared to that of graphite (<78% compared with >92%); investigating the use of a range of hard carbon/electrolyte combinations with higher FCEs will further increase the energy density. Hard carbons with lower average discharge voltages will increase the average cell operating voltage and the overall gravimetric and volumetric energy densities of the cells. Implementing these design and engineering optimisations could increase the energy density of first-generation NIBs to 160 Wh kg⁻¹ [301].

Next-generation cathode chemistries are also being investigated [252]. These will seek to reduce the amount of Ni in the cathode by replacing it with Fe and/or Mn to significantly reduce the cost. Lower processing costs are also an area to explore to improve € kWh⁻¹ for first-generation NIBs.

Alloying elements such as Sb, Si, Ge, As, Sb, Se, Pb, and Bi are also being investigated as alternatives to hard carbon anodes [252]; it is believed that the second generation of anodes will use these as blends with hard carbon. Titanium phosphates (e.g. NaTi₂(PO₄)₃) and titanium oxide (Na₂Ti₃O₇) insertion compounds are also being investigated as alternatives to hard carbons [252].

Compared to LIBs, the NIB anode and cathode spaces are relatively unmined, so there is still significant potential to further increase the energy density of second-generation NIBs beyond 170 Wh kg⁻¹; 190 Wh kg⁻¹ has been predicted by Barker [301].

### Concluding remarks

This short techno-economic assessment has shown that the current generation of NIBs, if advanced to mass production, can be competitive against LFP (and replace Pb-Acid) and NMC cells for stationary and telecoms applications. NIBs show massive advantages in safety, compared to either LFP or NMC cells, and have the potential for reduced cost (predominantly because Na is significantly more abundant than Li) and improved cycle life. The current generation of NIBs has the potential to exceed 150 Wh kg⁻¹ through electrode and cell design optimisation. As a new technology, NIBs also have the added advantage of being a 'drop-in' replacement for LIBs, as the manufacturing processes are compatible.

Other potential benefits include the potential to store NIBs at 0 V without impacting safety and performance; this will allow for cheaper storage and transportation.

Due to the significant worldwide research in NIB materials, advances will probably lead to second-generation energy densities of >180 Wh kg⁻¹.

However, timescales for commercialisation may be long, and the timing of introduction to the market will be crucial to displace existing technologies. Satisfying existing regulatory and policy requirements is also a factor to be considered in the timescales for the commercialisation of NIBs. If Ni can be removed from the cathode formulation, then upcycling and disposal could be potentially easy and inexpensive.
Data availability statement

All data that support the findings of this study are included within the article (and any supplementary files).

ORCID iDs

Nuria Tapia-Ruiz https://orcid.org/0000-0002-5005-7043

A Robert Armstrong https://orcid.org/0000-0003-1937-0936

Hande Alptekin https://orcid.org/0000-0001-6065-0513

Heather Au https://orcid.org/0000-0002-1652-2204

Jerry Barker https://orcid.org/0000-0002-8791-1119

William R Brant https://orcid.org/0000-0002-8658-8938

Yong-Seok Choi https://orcid.org/0000-0002-3737-2989

Sara I R Costa https://orcid.org/0000-0001-8105-207X

Maria Crespo Ribadeneyra https://orcid.org/0000-0001-6455-4430

Serena A Cussen https://orcid.org/0000-0002-9303-4220

Aamod V Desai https://orcid.org/0000-0001-7219-3428

Juan D Forero-Saboya https://orcid.org/0000-0002-3403-6066

John M Griffin https://orcid.org/0000-0002-8943-3835

John T S Irvine https://orcid.org/0000-0002-8394-3359

Patrik Johansson https://orcid.org/0000-0002-9907-117X

Martin Karlsmo https://orcid.org/0000-0002-0437-6860

Emma Kendrick https://orcid.org/0000-0002-4219-964X

Eunjeong Kim https://orcid.org/0000-0002-2941-068

Oleg V Kolosov https://orcid.org/0000-0003-3278-9643

Stijn F L Mertens https://orcid.org/0000-0002-5715-0486

Laure Monconduit https://orcid.org/0000-0003-3698-856X

Andrew J Naylor https://orcid.org/0000-0001-5641-7778

Philippe Poizot https://orcid.org/0000-0003-1865-4902

Stéven Renault https://orcid.org/0000-0002-6500-0015

Ashish Rudola https://orcid.org/0000-0001-9368-0698

Ruth Sayers https://orcid.org/0000-0003-1289-0998

Valerie R Seymour https://orcid.org/0000-0003-3333-5512

Begoña Silván https://orcid.org/0000-0002-1273-3098

Moulay Tahar Sougrati https://orcid.org/0000-0003-3740-2807

Lorenzo Stievano https://orcid.org/0000-0001-8548-0231

Chris I Thomas https://orcid.org/0000-0001-8090-4541

Maria-Magdalena Titirici https://orcid.org/0000-0003-0773-2100

Jincheng Tong https://orcid.org/0000-0001-7762-1460

Thomas J Wood https://orcid.org/0000-0002-5893-5664

Reza Younesi https://orcid.org/0000-0003-2538-8104

References

[1] NEXGENNA—Sodium-ion Batteries (Available at: https://faraday.ac.uk/research/beyond-lithium-ion/sodium-ion-batteries/)

[2] Eftekhari A and Kim D-W 2018 Sodium-ion batteries: new opportunities beyond energy storage by lithium J. Power Sources 395 336–48

[3] Perveen T, Siddiq M, Shahzad N, Ihsan R, Ahmad A and Shahzad M I 2020 Prospects in anode materials for sodium ion batteries—a review *Renew. Sustain. Energy Rev.* 119 109549

[4] Hwang J-Y, Myung S-T and Sun Y-K 2017 Sodium-ion batteries: present and future Chem. Soc. Rev. 46 3529–614

[5] Delmas C, Fouassier C and Hagenmuller P 1980 Structural classification and properties of the layered oxides Physica B 99 81–85

[6] Liu Q, Hu Z, Chen M, Zou C, Jin H, Wang S, Chou S-L, Liu Y and Dou S-X 2020 The cathode choice for commercialization of sodium-ion batteries: layered transition metal oxides versus Prussian blue analogs Adv. Funct. Mater. 30 1909530

[7] Vassilaras P, Ma X, Li X and Ceder G 2013 Electrochemical properties of monoclinic NaNiO₂ J. Electrochem. Soc. 160 A207–11

[8] Yabuuchi N, Yano M, Yoshida H, Kuze S and Komaba S 2013 Synthesis and electrode performance of O3-Type NaFeO₂-NaNi₁/₂Mn₁/₂O₂ solid solution for rechargeable sodium batteries J. Electrochem. Soc. 160 A3131–7

[9] Komaba S, Yabuuchi N, Nakayama T, Ogata A, Ishikawa T and Nakai I 2012 Study on the reversible electrode reaction of Na₁-xNi₀.₅Mn₀.₅O₂ for a rechargeable sodium-ion battery Inorg. Chem. 51 6211–20

[10] Kumakura S, Tahara Y, Kubota K, Chihara K and Komaba S 2016 Sodium and manganese stoichiometry of P2-Type Na₂/₃MnO₂ Angew. Chem., Int. Ed. 55 12760–3

[11] D’Arienzo M, Ruffo R, Scotti R, Morazzoni F, Mari C M and Polizzi S 2012 Layered Na₀.₇₁CoO₂: a powerful candidate for viable and high performance Na-batteries Phys. Chem. Chem. Phys. 14 5945–52

[12] Lu Z and Dahn J R 2001 In situ x-ray diffraction study of P2-Na₂/₃[Ni₁/₃Mn₂/₃]O₂ J. Electrochem. Soc. 148 A1225–9
[13] Li Z-Y, Gao R, Zhang J, Zhang X, Hu Z and Liu X 2016 New insights into designing high-rate performance cathode materials for sodium ion batteries by enlarging the slab-spacing of the Na-ion diffusion layer *J. Mater. Chem.* **A** 4 3453–61

[14] Yoshida H, Yabuuchi N, Kubota K, Ikeuchi I, Garsuch A, Schulz-Dobrick M and Komaba S 2014 P2-type Na$_{2/3}$Ni$_{1/3}$Mn$_{2/3-x}$Ti$_{x}$O$_{2}$ as a new positive electrode for higher energy Na-ion batteries *Chem. Commun.* **50** 3677–80

[15] Barker J and Heap R 2019 O3/P2 mixed phase sodium-containing doped layered oxide materials *Patent WO 2019/197812*

[16] Assat G and Tarascon J-M 2018 Fundamental understanding and practical challenges of anionic redox activity in Li-ion batteries *Nat. Energy* **3** 373

[17] Maitra U *et al* 2018 Oxygen redox chemistry without excess alkali-metal ions in Na$_{2/3}$[Mg$_{0.28}$Mn$_{0.72}$]O$_{2}$ *Nat. Chem.* **10** 288

[18] Zheng W, Liu Q, Wang Z, Wua Z, Gu S, Cao L, Zhang K, Fransaer J and Lu Z 2020 Oxygen redox activity with small voltage hysteresis in Na$_{0.67}$Cu$_{0.28}$Mn$_{0.72}$O$_{2}$ for sodium-ion batteries *Energy Storage Mater.* **28** 300

[19] House R A *et al* 2020 Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes *Nature* **577** 502

[20] Parant J-P, Olazcuaga R, Devalette M, Fouassier C and Hagenmuller P 1971 Sur quelques nouvelles phases de formule Na$_{x}$MnO$_{2}$ ($x \le 1$) *J. Solid State Chem.* **3** 1

[21] House R A *et al* 2019 What triggers oxygen loss in oxygen redox cathode materials? *Chem. Mater.* **31** 3293

[22] Radin M D, Vinckeviciute J, Seshadri R and Van der Ven A 2019 Manganese oxidation as the origin of the anomalous capacity of Mn-containing Li-excess cathode materials *Nat. Energy* **4** 639

[23] Ma L A, Massel F, Naylor A J, Duda L-C and Younesi R 2019 Understanding charge compensation mechanisms in Na$_{0.56}$Mg$_{0.04}$Ni$_{0.19}$Mn$_{0.70}$O$_{2}$ *Commun. Chem.* **2** 125

[24] Mortemard De Boisse B, Nishimura S, Watanabe E, Lander L, Tsuchimoto A, Kikkawa J, Kobayashi E, Asakura D, Okubo M and Yamada A 2018 Highly reversible oxygen-redox chemistry at 4.1 V in Na$_{4/7-x}$[☐$_{1/7}$Mn$_{6/7}$]O$_{2}$ (☐: Mn vacancy) *Adv. Energy Mater.* **8** 1800409

[25] Wang P-F *et al* 2020 Both cationic and anionic redox chemistry in a P2-type sodium layered oxide *Nano Energy* **69** 104474

[26] Sharifi-Asl S, Lu J, Amine K and Shahbazian-Yassar R 2019 Oxygen release degradation in Li-ion battery cathode materials: mechanisms and mitigating approaches *Adv. Energy Mater.* **9** 1900551

[27] Chen S, Wen K, Fan J, Bando Y and Golberg D 2018 Progress and future prospects of high-voltage and high-safety electrolytes in advanced lithium batteries: from liquid to solid electrolytes *J. Mater. Chem.* **A** 6 11631

[28] Barpanda P, Lander L, Nishimura S and Yamada A 2018 Polyanionic insertion materials for sodium-ion batteries *Adv. Energy Mater.* **8** 17030550–26

[29] Li H, Xu M, Zhang Z, Lai Y and Ma J 2020 Engineering of polyanion type cathode materials for sodium-ion batteries: toward higher energy/power density *Adv. Funct. Mater.* **30** 2000473–1–29

[30] Chotard J-N, Rousse G, David R, Mentré O, Courty M and Masquelier C 2015 Discovery of a sodium-ordered form of Na$_{3}$V$_{2}$(PO$_{4}$)$_{3}$ below ambient temperature *Chem. Mater.* **27** 5982–7

[31] Bianchini M, Brisset N, Fauth F, Weill F, Elkaim E, Suard E, Masquelier C and Croguennec L 2014 Na$_{3}$V$_{2}$(PO$_{4}$)$_{2}$F$_{3}$ revisited: a high-resolution diffraction study *Chem. Mater.* **26** 4238–47

[32] Bianchini M, Xiao P, Wang Y and Ceder G 2017 Additional sodium insertion into polyanionic cathodes for higher-energy Na-ion batteries *Adv. Energy Mater.* **7** 1700514–1–9

[33] Liang Z, Zhang X, Liu R, Ortiz G F, Zhong G, Xiang Y, Chen S, Mi J, Wu S and Yang Y 2020 New dimorphs of Na$_{5}$V(PO$_{4}$)$_{2}$F$_{2}$ as an ultrastable cathode material for sodium-ion batteries *ACS Appl. Energy Mater.* **3** 1181–9

[34] Kim M, Kim D, Lee W, Jang H M and Kang B 2018 New class of 3.7 V Fe-based positive electrode materials for Na-ion battery based on cation-disordered polyanion framework *Chem. Mater.* **30** 6346–52

[35] Plewa A, Kulka A, Hanc E, Zająć W, Sun J, Lu L and Molenda J 2020 Facile aqueous synthesis of high performance Na$_{2}$FeM(SO$_{4}$)$_{3}$ (M=Fe, Mn, Ni) alluaudites for low cost Na-ion batteries *J. Mater. Chem.* **A** 8 2728–40

[36] Song T, Yao W, Kiadkhunthod P, Zheng Y, Wu N, Zhou X, Tunmee S, Sattayaporn S and Tang Y 2020 A low-cost and environmentally friendly mixed polyanionic cathode for sodium-ion storage *Angew. Chem., Int. Ed.* **59** 740–5

[37] Mattei G S *et al* 2020 Enumeration as a tool for structure solution: a materials genomic approach to solving the cation-ordered structure of Na$_{3}$V$_{2}$(PO$_{4}$)$_{2}$F$_{3}$ *Chem. Mater.* **32** 8981–92

[38] Nguyen L H B, Iadecola A, Belin S, Olchowka J, Masquelier C, Carlier D and Croguennec L 2020 A combined operando synchrotron X-ray absorption spectroscopy and first-principles density functional theory study to unravel the vanadium redox paradox in the Na$_{3}$V$_{2}$(PO$_{4}$)$_{2}$F$_{3}$-Na$_{3}$V$_{2}$(PO$_{4}$)$_{2}$FO$_{2}$ compositions *J. Phys. Chem. C* **124** 23511–22

[39] Liu Q, Hu Z, Chen M, Zou C, Jin H, Wang S, Chou S-L, Liu Y and Dou S-X 2020 The cathode choice for commercialization of sodium-ion batteries: layered transition metal oxides versus Prussian blue analogs *Adv. Funct. Mater.* **30** 1909530

[40] You Y, Wu X-L, Yin Y-X and Guo Y-G 2014 High-quality Prussian blue crystals as superior cathode materials for room-temperature sodium-ion batteries *Energy Environ. Sci.* **7** 1643–7

[41] Wang L *et al* 2015 Rhombohedral Prussian white as cathode for rechargeable sodium-ion batteries *J. Am. Chem. Soc.* **137** 2548–54

[42] Li W-J, Han C, Cheng G, Chou S-L, Liu H-K and Dou S-X 2019 Chemical properties, structural properties, and energy storage applications of Prussian blue analogues *Small* **15** 1900470

[43] Shen L, Jiang Y, Liu Y, Ma J, Sun T and Zhu N 2020 High-stability monoclinic nickel hexacyanoferrate cathode materials for ultrafast aqueous sodium ion battery *Chem. Eng. J.* **388** 124228

[44] Rudola A, Du K and Balaya P 2017 Monoclinic sodium iron hexacyanoferrate cathode and non-flammable glyme-based electrolyte for inexpensive sodium-ion batteries *J. Electrochem. Soc.* **164** A1098

[45] You Y, Yao H-R, Xin S, Yin Y-X, Zuo T-T, Yang C-P, Guo Y-G, Cui Y, Wan L-J and Goodenough J B 2016 Subzero-temperature cathode for a sodium-ion battery *Adv. Mater.* **28** 7243–8

[46] Guo X, Wang Z, Deng Z, Li X, Wang B, Chen X and Ong S P 2019 Water contributes to higher energy density and cycling stability of Prussian blue analogue cathodes for aqueous sodium-ion batteries *Chem. Mater.* **31** 5933–42

[47] Ojwang D O, Häggström L, Ericsson T, Ångström J and Brant W R 2020 Influence of sodium content on the thermal behavior of low vacancy Prussian white cathode material *Dalton Trans.* **49** 3570–9

[48] Chen M, Zhang Z, Liu X, Li Y, Wang Y, Fan H, Liang X and Chen Q 2020 Prussian blue coated with reduced graphene oxide as high-performance cathode for lithium-sulfur batteries *RSC Adv.* **10** 31773–9

[49] Dehghani-Sanij A R, Tarumalingam E, Dusseault M B and Fraser R 2019 Study of energy storage systems and environmental challenges of batteries *Renew. Sustain. Energy Rev.* **104** 192–208

[50] Peters J F, Baumann M, Zimmermann B, Braun J and Weil M 2017 The environmental impact of Li-ion batteries and the role of key parameters—a review *Renew. Sustain. Energy Rev.* **67** 491–506
[51] Poizot P and Dolhem F 2011 Clean energy new deal for a sustainable world: from non-CO₂ generating energy sources to greener electrochemical storage devices *Energy Environ. Sci.* 4 2003–19

[52] Poizot P, Gaubicher J, Renault S, Dubois L, Liang Y and Yao Y 2020 Opportunities and challenges for organic electrodes in electrochemical energy storage *Chem. Rev.* 120 6490–557

[53] Tournier V et al 2020 An engineered PET depolymerase to break down and recycle plastic bottles *Nature* 580 216–9

[54] Zhao Q, Lu Y and Chen J 2017 Advanced organic electrode materials for rechargeable sodium-ion batteries *Adv. Energy Mater.* 7 1601792

[55] Rajagopalan R, Tang Y, Jia C, Ji X and Wang H 2020 Understanding the sodium storage mechanisms of organic electrodes in sodium ion batteries: issues and solutions *Energy Environ. Sci.* 13 1568–92

[56] Deuchert K and Hünig S 1978 Multistage organic redox systems—a general structural principle *Angew. Chem., Int. Ed. Engl.* 17 875–86

[57] Wang H, Yuan S, Ma D, Huang X, Meng F and Zhang X 2014 Tailored aromatic carbonyl derivative polyimides for high-power and long-cycle sodium-organic batteries *Adv. Energy Mater.* 4 1301651

[58] Wu D, Zhang G, Lu D, Ma L, Xu Z, Xi X, Liu R, Liu P and Su Y 2018 Perylene diimide-diamine/carbon black composites as high performance lithium/sodium ion battery cathodes *J. Mater. Chem.* A 6 13613–8

[59] Banda H, Damien D, Nagarajan K, Hariharan M and Shaijumon M M 2015 A polyimide based all-organic sodium ion battery *J. Mater. Chem.* A 3 10453–8

[60] Lee M, Hong J, Lopez J, Sun Y, Feng D, Lim K, Chueh W C, Toney M F, Cui Y and Bao Z 2017 High-performance sodium–organic battery by realizing four-sodium storage in disodium rhodizonate *Nat. Energy* 2 861–8

[61] Wang S, Wang L, Zhu Z, Hu Z, Zhao Q and Chen J 2014 All organic sodium-ion batteries with Na₄C₈H₂O₆ *Angew. Chem.* 126 6002–6

[62] Song Z, Qian Y, Zhang T, Otani M and Zhou H 2015 Poly(benzoquinonyl sulfide) as a high-energy organic cathode for rechargeable Li and Na batteries *Adv. Sci.* 2 1500124

[63] Kim J-K, Kim Y, Park S, Ko H and Kim Y 2016 Encapsulation of organic active materials in carbon nanotubes for application to high-electrochemical-performance sodium batteries *Energy Environ. Sci.* 9 1264–9

[64] Kim H W, Kim H-J, Byeon H, Kim J, Yang J W, Kim Y and Kim J-K 2020 Binder-free organic cathode based on nitroxide radical polymer-functionalized carbon nanotubes and gel polymer electrolyte for high-performance sodium organic polymer batteries *J. Mater. Chem.* A 8 17980–6

[65] Deng W, Liang X, Wu X, Qian J, Cao Y, Ai X, Feng J and Yang H 2013 A low cost, all-organic Na-ion battery based on polymeric cathode and anode *Sci. Rep.* 3 2671

[66] Jouhara A, Dupré N, Gaillot A-C, Guyomard D, Dolhem F and Poizot P 2018 Raising the redox potential in carboxyphenolate-based positive organic materials via cation substitution *Nat. Commun.* 9 4401

[67] Franklin R E 1951 Crystallite growth in graphitizing and non-graphitizing carbons *Proc. Royal Society of London Series A. Mathematical and Physical Sciences* 209 196–218

[68] Stevens D A and Dahn J R 2001 *J. Electrochem. Soc.* 148 803–11

[69] Franklin R E 1951 Crystallite growth in graphitizing and non-graphitizing carbons *Proc. R. Soc. A* 209 196–218

[70] Au H et al 2020 *Energy Environ. Sci.* 13 3469–79

[71] Roberts S and Kendrick E 2018 The re-emergence of sodium ion batteries: testing, processing, and manufacturability *Nanotechnol. Sci. Appl.* 11 23–33

[72] Zhang B, Ghimbeu C M, Laberty C, Vix-Guterl C and Tarascon J-M 2016 *Adv. Energy Mater.* 6 1501588

[73] Komaba S, Murata W, Ishikawa T, Yabuuchi N, Ozeki T, Nakayama T, Ogata A, Gotoh K and Fujiwara K 2011 *Adv. Funct. Mater.* 21 3859–67

[74] Jafta C, Bridges C, Sun X G, Veith G, Paranthaman P and Dai S 2018 *Abstracts of Papers of the American Chemical Society* vol 256 p meeting abstract 524

[75] Gu M, Kushima A, Shao Y Y, Zhang J-G, Liu J, Browning N D, Li J and Wang C M 2013 *Nano Lett.* 13 5203–11

[76] Weaving J S, Lim A, Millichamp J, Neville T P, Ledwoch D, Kendrick E, McMillan P F, Shearing P R, Howard C A and Brett D J L 2020 *ACS Appl. Energy Mater.* 3 7474–84

[77] Stratford J M, Allan P K, Pecher O, Chater P A and Grey C P 2016 *Chem. Commun.* 52 12430–3

[78] Gotoh K, Maeda M, Nagai A, Goto A, Tansho M, Hashi K, Shimizu T and Ishida H 2006 *J. Power Sources* 162 1322–8

[79] Morita R, Gotoh K, Kubota K, Komaba S, Hashi K, Shimizu T and Ishida H 2019 *Carbon* 145 712

[80] Bray J M et al 2020 *Nat. Commun.* 11 2083

[81] Alcántara R, Ortiz G F, Lavela P, Tirado J L, Stoyanova R and Zhecheva E 2006 *Chem. Mater.* 18 2293–301

[82] Le P M L et al 2020 *Adv. Funct. Mater.* 30 2001151

[83] Eshetu G G, Diemant T, Hekmatfar M, Grugeon S, Behm R J, Laruelle S, Armand M and Passerini S 2019 *Nano Energy* 55 327–40

[84] Zhang Z, Smith K, Jervis R, Shearing P R, Miller T S and Brett D J L 2020 *ACS Appl. Mater. Interfaces* 12 35132–3514

[85] Qin Y P, Zhuang Q C, Shi Y L, Jiang L, Sun Z and Sun S G 2011 *Prog. Chem.* 23 390–400

[86] Berkes B B, Jozwiuk A, Vracar M, Sommer H, Brezesinski T and Janek J 2015 *Anal. Chem.* 87 5878–83

[87] Li J T, Fang J C, Su H and Sun S G 2011 *Prog. Chem.* 23 349–56

[88] Sottmann J, Di Michiel M, Fjellvag H, Malavasi L, Margadonna S, Vajeeston P, Vaughan G B M and Wragg D S 2017 *Angew. Chem., Int. Ed.* 56 11385–9

[89] Alptekin H, Au H, Jensen A C S, Olsson E, Goktas M, Headen T F, Adelhelm P, Cai Q, Drew A J and Titirici M 2020 Sodium storage mechanism investigations through structural changes in hard carbons *ACS Appl. Energy Mater.* 3 9918–27

[90] Goktas M, Bolli C, Berg E J, Novák P, Pollok K, Langenhorst F, Roeder M V, Lenchuk O, Mollenhauer D and Adelhelm P 2018 *Adv. Energy Mater.* 8 1702724

[91] Olsson E, Cottom J, Au H, Guo Z, Jensen A C S, Alptekin H, Drew A J, Titirici M-M and Cai Q 2020 *Adv. Funct. Mater.* 30 1908209

[92] Bommier C, Ji X L and Greaney P A 2019 *Chem. Mater.* 31 658–77

[93] Soto F A, Marzouk A, El-Mellouhi F and Balbuena P B 2018 *Chem. Mater.* 30 3315–22

[94] Deringer V L, Merlet C, Hu Y C, Lee T H, Kattirtzi J A, Pecher O, Csanyi G, Elliott S R and Grey C P 2018 *Chem. Commun.* 54 5988–91

[95] Xie F, Xu Z, Guo Z and Titirici M-M 2020 *Prog. Energy* 2 042002

[96] Stevens D and Dahn J R 2000 *J. Electrochem. Soc.* 147 1271–3

[97] Ding J et al 2013 *ACS Nano* 7 11004–15

[98] Bommier C, Surta T W, Dolgos M and Ji X 2015 *Nano Lett.* 15 5888–92
[99] Morikawa Y, Nishimura S-I, Hashimoto R-I, Ohnuma M and Yamada A 2020 *Adv. Energy Mater.* **10** 1903176

[100] Simone V, Boulineau A, De Geyer A, Rouchon D, Simonin L and Martinet S 2016 *J. Energy Chem.* **25** 761–8

[101] Li Y Q et al 2019 *Adv. Energy Mater.* **9** 1902852

[102] Forse A C, Merlet C, Allan P K, Humphreys E K, Griffin J M, Aslan M, Zeiger M, Presser V, Gogotsi Y and Grey C P 2015 *Chem. Mater.* **27** 6848–5727

[103] Wang J, Polleux J, Lim J and Dunn B 2007 Pseudocapacitive contributions to electrochemical energy storage in $TiO_2$ (anatase) nanoparticles *J. Phys. Chem.* **111** 14925–31

[104] Xu Y, Lotfabad E, Wang H, Farbod B, Xu Z, Kohandehghan A and Mitlin D 2013 Nanocrystalline anatase $TiO_2$: a new anode material for rechargeable sodium ion batteries *Chem. Commun.* **49** 8973–5

[105] Ohzuku T, Ueda A and Yamamoto N 1995 Zero-strain insertion material of Li $[Li_{1/3}Ti_{5/3}]O_4$ for rechargeable lithium cells *J. Electrochem. Soc.* **142** 1431–5

[106] Liang Z, Lin P, Shen H, Hong L and Quan C 2012 Spinel lithium titanate ($Li_4Ti_5O_{12}$) as novel anode material for room-temperature sodium-ion battery *Chin. Phys. B* **21** 028201

[107] Rudola A, Saravanan K, Devaraj S, Gong H and Balaya P 2013 $Na_2Ti_6O_{13}$: a potential anode for grid-storage sodium-ion batteries *Chem. Commun.* **49** 7451–3

[108] Cao K, Jiao L, Pang W K, Liu H, Zhou T, Guo Z, Wang Y and Yuan H 2016 $Na_2Ti_6O_{13}$ nanorods with dominant large interlayer spacing exposed facet for high-performance Na-ion batteries *Small* **12** 2991–7

[109] Senguttuvan P, Rousse G, Seznec V, Tarascon J and Palacín M 2011 $Na_2Ti_3O_7$: lowest voltage ever reported oxide insertion electrode for sodium ion batteries *Chem. Mater.* **23** 4109–11

[110] Cao Y, Ye Q, Wang F, Fan X, Hu L, Wang F, Zhai T and Li H 2020 A new triclinic phase $Na_2Ti_3O_7$ anode for sodium-ion battery *Adv. Funct. Mater.* **30** 2003733

[111] Wu C, Hua W, Zhang Z, Zhong B, Yang Z, Feng G, Xiang W, Wu Z and Guo X 2018 Design and synthesis of layered $Na_2Ti_3O_7$ and tunnel $Na_2Ti_6O_{13}$ hybrid structures with enhanced electrochemical behavior for sodium-ion batteries *Adv. Sci.* **5** 1800519

[112] Pan H, Lu X, Yu X, Hu Y, Li H, Yang X and Chen L 2013 Sodium storage and transport properties in layered $Na_2Ti_3O_7$ for room-temperature sodium-ion batteries *Adv. Energy Mater.* **3** 1186–94

[113] Marquez M, Zarrabeitia M, Martinez E, Barrio A, Rojo T and Cabanas M 2015 Composition and evolution of the solid-electrolyte interphase in $Na_2Ti_3O_7$ electrodes for Na-ion batteries: XPS and Auger parameter analysis *ACS Appl. Mater. Interfaces* **7** 7801–08

[114] Kulova T, Kudryashova Y, Kuz'mina A, Skundin A, Stenina I, Chekannikov A, Yaroslavtsev A and Libich J 2019 Study of degradation of $Na_2Ti_3O_7$-based electrode during cycling *J. Solid State Electrochem.* **23** 455–63

[115] Xu J, Ma C, Balasubramanian M and Meng Y S 2014 Understanding $Na_2Ti_3O_7$ as an ultra-low voltage anode material for a Na-ion battery *Chem. Commun.* **50** 12564–7

[116] Chen J, Zhou X, Mei C, Xu J and Wong C 2017 Electrochimica acta improving the sodiation performance of $Na_2Ti_3O_7$ through Nb-doping *Electrochim. Acta* **224** 446–51

[117] Xia J, Zhao H, Pang W, Yin Z, Zhou B, He G, Guo Z and Du Y 2018 Lanthanide doping induced electrochemical enhancement of $Na_2Ti_3O_7$ anodes for sodium-ion batteries *Chem. Sci.* **9** 3421–5

[118] Costa S, Choi Y, Fielding A, Naylor A, Griffin J, Sofer Z, Scanlon D and Ruiz N 2020 Surface engineering strategy using urea to improve the rate performance of $Na_2Ti_3O_7$ in Na-ion batteries *Chem. Eur. J.* **27** 3875–86

[119] Anwer S, Huang Y, Liu J, Liu J, Xu M, Wang Z, Chen R, Zhang J and Wu F 2017 Nature-inspired $Na_2Ti_3O_7$ nanosheets-formed three-dimensional microflowers architecture as a high-performance anode material for rechargeable sodium-ion batteries *ACS Appl. Mater. Interfaces* **9** 11669–77

[120] Chen Z, Lu L, Li N and Sun C 2019 $Na_2Ti_3O_7$ nanotubes as anode materials for sodium-ion batteries and self-powered systems *ChemElectroChem* **6** 3085–90

[121] Dong S, Shen L, Li H, Nie P, Zhu Y, Sheng Q and Zhang X 2015 Pseudocapacitive behaviours of $Na_2Ti_3O_7$ @CNT coaxial nanocables for high-performance sodium-ion capacitors *J. Mater. Chem.* **A** **3** 21277–83

[122] Chen S, Pang Y, Liang J and Ding S 2018 Red blood cell-like hollow carbon sphere anchored ultrathin $Na_2Ti_3O_7$ nanosheets as long cycling and high rate-performance anodes for sodium-ion batteries *J. Mater. Chem.* **A** **6** 13164–70

[123] Li L, Zheng Y, Zhang S, Yang J, Shao Z and Guo Z 2018 Recent progress on sodium ion batteries: potential high-performance anodes *Energy Environ. Sci.* **11** 2310–40

[124] Zhang L, Wei T, Jiang Z, Liu C, Jiang H, Chang J, Sheng L, Zhou Q, Yuan L and Fan Z 2018 Electrostatic interaction in electrospun nanofibers: double-layer carbon protection of $CoFe_2O_4$ nanosheets enabling ultralong-life and ultrahigh-rate lithium ion storage *Nano Energy* **48** 238–47

[125] Darwiche A, Marino C, Sougrati M T, Fraisse B, Stievano L and Monconduit L 2012 Better cycling performances of bulk Sb in Na-ion batteries compared to Li-ion systems: an unexpected electrochemical mechanism *J. Am. Chem. Soc.* **134** 20805–11

[126] Zhang H, Hasa I and Passerini S 2018 Beyond insertion for Na-ion batteries: nanostructured alloying and conversion anode materials *Adv. Energy Mater.* **8** 1702582

[127] Cabana J, Monconduit L, Larcher D and Palacín M R 2010 Beyond intercalation-based Li-ion batteries: the state of the art and challenges of electrode materials reacting through conversion reactions *Adv. Mater.* **22** E170–92

[128] Brennhagen A, Cavallo C, Wragg D, Sottmann J, Koposov A Y and Fjellvåg H 2021 Understanding the (de)sodiation mechanisms in Na-based batteries through operando x-ray methods *Batter Supercaps* **4** 1039–63

[129] Rodriguez J R, Aguirre S B and Pol V G 2019 Understanding sodium-ion battery anodes through operando spectroscopic techniques *Electrochim. Acta* **319** 791–800

[130] Zhu Y, Wen Y, Fan X, Gao T, Han F, Luo C, Liou S-C and Wang C 2015 Red phosphorus–single-walled carbon nanotube composite as a superior anode for sodium ion batteries *ACS Nano* **9** 3254–64

[131] Liu Y, Zhang N, Jiao L, Tao Z and Chen J 2015 Ultrasmall Sn nanoparticles embedded in carbon as high-performance anode for sodium-ion batteries *Adv. Funct. Mater.* **25** 214–20

[132] Loaiza L C, Monconduit L and Seznec V 2020 Si and Ge-based anode materials for Li-, Na-, and K-ion batteries: a perspective from structure to electrochemical mechanism *Small* **16** 1905260

[133] Sun J, Lee H-W, Pasta M, Yuan H, Zheng G, Sun Y, Li Y and Cui Y 2015 A phosphorene–graphene hybrid material as a high-capacity anode for sodium-ion batteries *Nat. Nanotechnol.* **10** 980–5

[134] Chen L, Fiore M, Wang J E, Ruffo R, Kim D-K and Longoni G 2018 Readiness level of sodium-ion battery technology: a materials review *Adv. Sus. Syst.* **2** 1700153
[135] Allan P K, Griffin J M, Darwiche A, Borkiewicz O J, Wiaderek K M, Chapman K W, Morris A J, Chupas P J, Monconduit L and Grey C P 2016 Tracking sodium-antimonide phase transformations in sodium-ion anodes: insights from operando pair distribution function analysis and solid-state NMR spectroscopy *J. Am. Chem. Soc.* **138** 2352–65

[136] Chang Y-M, Lin H-W, Li L-J and Chen H-Y 2020 Two-dimensional materials as anodes for sodium-ion batteries *Mater. Today Adv.* **6** 100054

[137] Eda G, Yamaguchi H, Voiry D, Fujita T, Chen M W and Chhowalla M 2011 Photoluminescence from chemically exfoliated MoS₂ *Nano Lett.* **11** 5111–6

[138] Acerce M, Voiry D and Chhowalla M 2015 Metallic 1T phase MoS₂ nanosheets as supercapacitor electrode materials *Nat. Nanotechnol.* **10** 313–8

[139] Geng X M, Jiao Y C, Han Y, Mukhopadhyay A, Yang L and Zhu H L 2017 Freestanding metallic 1T MoS₂ with dual ion diffusion paths as high rate anode for sodium-ion batteries *Adv. Funct. Mater.* **27** 1702998

[140] Zhang R, Qin Y, Liu P, Jia C K, Tang Y G and Wang H Y 2020 How does molybdenum disulfide store charge: a mini review *ChemSusChem* **13** 1354–65

[141] Wang X F, Shen X, Wang Z X, Yu R C and Chen L Q 2014 Atomic-scale clarification of structural transition of MoS₂ upon sodium intercalation *ACS Nano* **8** 11394–400

[142] Lacey S D, Wan J Y, Cresc A V, Russell S M, Dai J Q, Bao W Z, Xu K and Hu L B 2015 Atomic force microscopy studies on molybdenum disulfide flakes as sodium-ion anodes *Nano Lett.* **15** 1018–24

[143] Banis M N *et al* 2018 Revealing the charge/discharge mechanism of Na–O₂ cells by *in situ* soft x-ray absorption spectroscopy spectroscopy *Energy Environ. Sci.* **11** 2073–7

[144] Zhao Y, Goncharova L V, Lushington A, Sun Q, Yadegari H, Wang B Q, Xiao W, Li R Y and Sun X L 2017 Superior stable and long life sodium metal anodes achieved by atomic layer deposition *Adv. Mater.* **29** 1606663

[145] Liedel C 2020 Sustainable battery materials from biomass *ChemSusChem* **13** 2110–41

[146] Xu Y, Zhou M and Lei Y 2018 Organic materials for rechargeable sodium-ion batteries *Mater. Today* **21** 60–78

[147] Lu Y, Zhang Q, Li L, Niu Z and Chen J 2018 Design strategies toward enhancing the performance of organic electrode materials in metal-ion batteries *Chem* **4** 2786–813

[148] Desai A V, Morris R E and Armstrong A R 2020 Advances in organic anode materials for Na-/K-ion rechargeable batteries *ChemSusChem* **13** 4866–84

[149] Deng J, Luo W-B, Chou S-L, Liu H-K and Dou S-X 2018 Sodium-ion batteries: from academic research to practical commercialization *Adv. Energy Mater.* **8** 1701428

[150] Liu Y *et al* 2018 Activating aromatic rings as Na-ion storage sites to achieve high capacity *Chem* **4** 2463–78

[151] Lee S, Kwon J E, Hong J, Park S Y and Kang K 2019 The role of substituents in determining the redox potential of organic electrode materials in Li and Na rechargeable batteries: electronic effects vs. substituent-Li/Na ionic interaction *J. Mater. Chem. A* **7** 11438–43

[152] Ma C, Wu L, Jin Z, Zhao X-Y, Liu Y-S, Bai Y-L, Sun H, Wang K-X and Chen J-S 2018 Thiophene derivative as a high electrochemical active anode material for sodium-ion batteries: the effect of backbone sulfur *Chem. Mater.* **30** 8426–30

[153] Wan F, Wu X-L, Guo J-Z, Li J-Y, Zhang J-P, Niu L and Wang R-S 2015 Nanoeffects promote the electrochemical properties of organic Na₂C₈H₄O₄ as anode material for sodium-ion batteries *Nano Energy* **13** 450–7

[154] Haldar S, Kaleeswaran D, Rase D, Roy K, Ogale S and Vaidhyanathan R 2020 Tuning the electronic energy level of covalent organic frameworks for crafting high-rate Na-ion battery anode *Nanoscale Horiz.* **5** 1264–73

[155] Jain A, Shin Y and Persson K A 2016 Computational predictions of energy materials using density functional theory *Nat. Rev. Mater.* **1** 15004

[156] Mayo M and Morris A J 2017 Structure prediction of Li–Sn and Li–Sb intermetallics for lithium-ion batteries anodes *Chem. Mater.* **29** 5787–95

[157] Liu Y, Guo B, Zou X, Li Y and Shi S 2020 Machine learning assisted materials design and discovery for rechargeable batteries *Energy Storage Mater.* **31** 434–50

[158] Hinuma Y, Meng Y S and Ceder G 2008 Temperature-concentration phase diagram of P₂–Na₅CoO₂ from first-principles calculations *J. Phys. Rev. B* **77** 224111

[159] Sun Y, Guo S and Zhou H 2019 Adverse effects of interlayer-gliding in layered transition-metal oxides on electrochemical sodium-ion storage *Energy Environ. Sci.* **12** 825–40

[160] Schmidt T and Kümmel S 2016 The influence of one-electron self-interaction on d-electrons *Computation* **4** 33

[161] Dove M T and Fang H 2016 Negative thermal expansion and associated anomalous physical properties: review of the lattice dynamics theoretical foundation *Rep. Prog. Phys.* **79** 066503

[162] Davies D W, Butler K, Jackson A, Morris A, Frost J, Skelton J and Walsh A 2016 Computational screening of all stoichiometric inorganic materials *Chem* **1** 617–27

[163] Meutzner F *et al* 2018 Computational analysis and identification of battery materials *Phys. Sci. Rev.* **4** 1–32

[164] Zhao C *et al* 2020 Rational design of layered oxide materials for sodium-ion batteries *Science* **370** 708–11

[165] Ponrouch A, Monti D, Boschin A, Steen B, Johansson P and Palacín M R 2015 Non-aqueous electrolytes for sodium-ion batteries *J. Mater. Chem.* **A** **3** 22–42

[166] Lin Z, Xia Q, Wang W, Li W and Chou S 2019 Recent research progresses in ether- and ester-based electrolytes for sodium-ion batteries *InfoMat.* **1** 376–89

[167] Eshetu G G, Elia G A, Armand M, Forsyth M, Komaba S, Rojo T and Passerini S 2020 Electrolytes and interphases in sodium-based rechargeable batteries: recent advances and perspectives *Adv. Energy Mater.* **10** 2000093

[168] Bommier C and Ji X 2018 Electrolytes, SEI formation, and binders: a review of nonelectrode factors for sodium-ion battery anodes *Small* **14** 1703576

[169] Xu K 2014 Electrolytes and interphases in Li-ion batteries and beyond *Chem. Rev.* **114** 11503–618

[170] Chen J, Huang Z, Wang C, Porter S, Wang B, Lie W and Liu H K 2015 Sodium-difluoro(oxalato)borate (NaDFOB): a new electrolyte salt for Na-ion batteries *Chem. Commun.* **51** 9809–12

[171] Mogensen R, Colbin S, Menon A S, Björklund E and Younesi R 2020 Sodium bis(oxalato)borate in trimethyl phosphate: a fire-extinguishing, fluorine-free, and low-cost electrolyte for full-cell sodium-ion batteries *ACS Appl. Energy Mater.* **3** 4974–82

[172] Vidal-Abarca C, Lavela P, Tirado J L, Chadwick A V, Alfredsson M and Kelder E 2012 Improving the cyclability of sodium-ion cathodes by selection of electrolyte solvent *J. Power Sources* **197** 314–8

[173] Åvall G, Mindemark J, Brandell D and Johansson P 2018 Sodium-ion battery electrolytes: modeling and simulations *Adv. Energy Mater.* **8** 1703036
[174] Hope M A, Rinkel B L D, Gunnarsdóttir A B, Märker K, Menkin S, Paul S, Sergeyev I V and Grey C P 2020 Selective NMR observation of the SEI–metal interface by dynamic nuclear polarisation from lithium metal *Nat. Commun.* **11** 2224

[175] Ponrouch A, Dedryvère R, Monti D, Demet A E, Ateba Mba J M, Croguennec L, Masquelier C, Johansson P and Palacín M R 2013 Towards high energy density sodium ion batteries through electrolyte optimization *Energy Environ. Sci.* **6** 2361–9

[176] Westman K et al 2018 Diglyme based electrolytes for sodium-ion batteries ACS Appl. Energy Mater. **1** 2671–80

[177] Cometto C, Yan G, Mariyappan S and Tarascon J-M 2019 Means of using cyclic voltammetry to rapidly design a stable DMC-based electrolyte for Na-ion batteries *J. Electrochem. Soc.* **166** A3723–30

[178] (Available at: www.faradion.co.uk/) (Accessed 15 September 2020)

[179] Jónsson E and Johansson P 2012 Modern battery electrolytes: ion–ion interactions in Li⁺/Na⁺ conductors from DFT calculations *Phys. Chem. Chem. Phys.* **14** 10774–9

[180] Åvall G and Johansson P 2020 A novel approach to ligand-exchange rates applied to lithium-ion battery and sodium-ion battery electrolytes *J. Chem. Phys.* **152** 234104

[181] Monti D, Ponrouch A, Palacín M R and Johansson P 2016 Towards safer sodium-ion batteries via organic solvent/ionic liquid based hybrid electrolytes *J. Power Sources* **324** 712–21

[182] Monti D, Jónsson E, Boschin A, Palacín M R, Ponrouch A and Johansson P 2020 Towards standard electrolytes for sodium-ion batteries: physical properties, ion solvation and ion-pairing in alkyl carbonate solvents *Phys. Chem. Chem. Phys.* **22** 22768–77

[183] Huang J et al 2020 *Operando* decoding of chemical and thermal events in commercial Na(Li)-ion cells via optical sensors *Nat. Energy* **5** 674–83

[184] Yan G, Mariyappan S, Rousse G, Jacquet Q, Deschamps M, David R, Mirvaux B, Freeland J W and Tarascon J-M 2019 Higher energy and safer sodium ion batteries via an electrochemically made disordered Na₃V₂(PO₄)₂F₃ material *Nat. Commun.* **10** 585

[185] Vaalma C, Buchholz D, Weil M and Passerini S 2018 A cost and resource analysis of sodium-ion batteries *Nat. Rev. Mater.* **3** 18013

[186] Che H, Chen S, Xie Y, Wang H, Amine K, Liao X-Z and Ma Z-F 2017 Electrolyte design strategies and research progress for room-temperature sodium-ion batteries *Energy Environ. Sci.* **10** 1075–101

[187] Matsumoto K, Hwang J, Kaushik S, Chen C Y and Hagiwara R 2019 Advances in sodium secondary batteries utilizing ionic liquid electrolytes *Energy Environ. Sci.* **12** 3247–87

[188] Rakov D A, Chen F, Ferdousi S A, Li H, Pathirana T, Simonov A N, Howlett P C, Atkin R and Forsyth M 2020 Engineering high-energy-density sodium battery anodes for improved cycling with superconcentrated ionic-liquid electrolytes *Nat. Mater.* **19** 1096–101

[189] Jeong S, Hasa I and Passerini S 2019 Ionic liquid-based electrolytes for sodium-ion batteries: tuning properties to enhance the electrochemical performance of manganese-based layered oxide cathode ACS Appl. Mater. Interfaces **11** 22278–89

[190] Usui H, Domi Y, Fujiwara K, Shimizu M, Yamamoto T, Nohira T, Hagiwara R and Sakaguchi H 2017 Charge–discharge properties of a Sn₄P₃ negative electrode in ionic liquid electrolyte for Na-ion batteries ACS Energy Lett. **2** 1139–43

[191] Manohar C V, Mendes T C, Kar M, Wang D, Xiao C, Forsyth M, Mitra S and MacFarlane D R 2018 Ionic liquid electrolytes supporting high energy density in sodium-ion batteries based on sodium vanadium phosphate composites *Chem. Commun.* **54** 3500

[192] Wei S, Choudhury S, Xu J, Nath P, Tu Z and Archer L A 2017 Highly stable sodium batteries enabled by functional ionic polymer membranes *Adv. Mater.* **29** 1605512

[193] Peled E and Menkin S 2017 Review—SEI: past, present and future *J. Electrochem. Soc.* **164** A1703–19

[194] Moshkovich M, Gofer Y and Aurbach D 2001 Investigation of the electrochemical windows of aprotic alkali metal (Li, Na, K) salt solutions *J. Electrochem. Soc.* **148** E155

[195] Komaba S, Ishikawa T, Yabuuchi N, Murata W, Ito A and Ohsawa Y 2011 Fluorinated ethylene carbonate as electrolyte additive for rechargeable Na batteries ACS Appl. Mater. Interfaces **3** 4165–8

[196] Iermakova D I, Dugas R, Palacín M R and Ponrouch A 2015 On the comparative stability of Li and Na metal anode interfaces in conventional alkyl carbonate electrolytes *J. Electrochem. Soc.* **162** A7060–6

[197] Ma L A, Naylor A J, Nyholm L and Younesi R 2020 Strategies of mitigating dissolution of solid electrolyte interphases in sodium-ion batteries *Angew. Chem., Int. Ed.* **60** 4855–63

[198] Mogensen R, Brandell D and Younesi R 2016 Solubility of the solid electrolyte interphase (SEI) in sodium ion batteries ACS Energy Lett. **1** 1173–8

[199] Philippe B, Valvo M, Lindgren F, Rensmo H and Edström K 2014 Investigation of the electrode/electrolyte interface of Fe₂O₃ composite electrodes: Li vs Na batteries *Chem. Mater.* **26** 5028–41

[200] Chinnam P R and Wunder S L 2013 Self-assembled Janus-like multi-ionic lithium salts form nano-structured solid polymer electrolytes with high ionic conductivity and Li⁺ ion transference number *J. Mater. Chem.* **A** **1** 1731–9

[201] Rao R P, Chen H M, Wong L L and Adams S 2017 Na₃⁺ₓMₓP₁⁻ₓS₄ (M = Ge⁴⁺, Ti⁴⁺, Sn⁴⁺) enables high rate all-solid-state Na-ion batteries Na₂⁺₂δFe₂⁻₆(SO₄)₃|Na₃⁺ₓMₓP₁⁻ₓS₄|Na₂Ti₃O₇ *J. Mater. Chem.* **A** **5** 3377–88

[202] Duchardt M, Ruschewitz U, Adams S, Dehnen S and Roling B 2018 Vacancy-controlled Na⁺ superion conduction in Na₁₁Sn₂PS₁₂ *Angew. Chem., Int. Ed.* **57** 1351–5

[203] Zhang Z Z *et al* 2020 Na₃Zr₂Si₂PO₁₂: a stable Na⁺-ion solid electrolyte for solid-state batteries ACS Appl. Energy Mater. **3** 7427–37

[204] He S N, Xu Y L, Chen Y J and Ma X N 2020 Enhanced ionic conductivity of an F⁻-assisted Na₃Zr₂Si₂PO₁₂ solid electrolyte for solid-state sodium batteries *J. Mater. Chem.* **A** **8** 12594–602

[205] Noi K, Suzuki K, Tanibata N, Hayashi A and Tatsumisago M 2018 Liquid-phase sintering of highly Na⁺ ion conducting Na₃Zr₂Si₂PO₁₂ ceramics using Na₃BO₃ additive *J. Am. Ceram. Soc.* **101** 1255–65

[206] Zhang Z Z *et al* 2019 Correlated migration invokes higher Na⁺-ion conductivity in NaSICON-type solid electrolytes *Adv. Energy Mater.* **9** 1902373

[207] Amores M, Baker P J, Cussen E J and Corr S A 2018 Na₁.₅La₁.₅TeO₆: Na⁺ conduction in a novel Na-rich double perovskite *Chem. Commun.* **54** 10040–3

[208] Zhang Z Z *et al* 2017 A self-forming composite electrolyte for solid-state sodium battery with ultralong cycle life *Adv. Energy Mater.* **7** 1601196

[209] Cheng M, Qu T, Zi J, Yao Y, Liang F, Ma W, Yang B, Dai Y and Lei Y 2020 A hybrid solid electrolyte for solid-state sodium ion batteries with good cycle performance *Nanotechnology* **31** 425401

[210] Tchitchekova D S, Monti D, Johansson P, Bardé F, Randon-Vitanova A, Palacín M R and Ponrouch A 2017 On the reliability of half-cell tests for monovalent (Li⁺, Na⁺) and divalent (Mg²⁺, Ca²⁺) cation based batteries *J. Electrochem. Soc.* **164** A1384–92

[211] Dugas R, Forero-Saboya J D and Ponrouch A 2019 Methods and protocols for reliable electrochemical testing in post-Li batteries (Na, K, Mg, and Ca) *Chem. Mater.* **31** 8613–28
[212] Rudola A, Aurbach D and Balaya P 2014 A new phenomenon in sodium batteries: voltage step due to solvent interaction *Electrochem. Commun.* **46** 56–59

[213] Dugas R, Ponrouch A, Gachot G, David R, Palacin M R and Tarascon J M 2016 Na reactivity toward carbonate-based electrolytes: the effect of FEC as additive *J. Electrochem. Soc.* **163** A2333–9

[214] Lee S E and Tang M H 2019 Electroactive decomposition products cause erroneous intercalation signals in sodium-ion batteries *Electrochem. Commun.* **100** 70–73

[215] Kim Y-J, Lee H, Noh H, Lee J, Kim S, Ryou M-H, Lee Y M and Kim H-T 2017 Enhancing the cycling stability of sodium metal electrodes by building an inorganic–organic composite protective layer *ACS Appl. Mater. Interfaces* **9** 6000–6

[216] Luo W, Lin C-F, Zhao O, Noked M, Zhang Y, Rubloff G W and Hu L 2017 Ultrathin surface coating enables the stable sodium metal anode *Adv. Energy Mater.* **7** 1601526

[217] Schafzahl L, Hanzu I, Wilkening M and Freunberger S A 2017 An electrolyte for reversible cycling of sodium metal and intercalation compounds *ChemSusChem* **10** 401–8

[218] Lee J, Lee Y, Lee J, Lee S-M, Choi J-H, Kim H, Kwon M-S, Kang K, Lee K T and Choi N-S 2017 Ultraconcentrated sodium bis(fluorosulfonyl)imide-based electrolytes for high-performance sodium metal batteries *ACS Appl. Mater. Interfaces* **9** 3723–32

[219] Rupp R and Vlad A 2019 On the reliability of sodium metal anodes: the influence of neglected parameters *J. Electrochem. Soc.* **166** A3122–31

[220] Conder J and Villevieille C 2019 How reliable is the Na metal as a counter electrode in Na-ion half cells? *Chem. Commun.* **55** 1275–8

[221] Hwang J, Takeuchi K, Matsumoto K and Hagiwara R 2019 NASICON vs. Na metal: a new counter electrode to evaluate electrodes for Na secondary batteries *J. Mater. Chem. A* **7** 27057–65

[222] Lee S E and Tang M H 2019 Reliable reference electrodes for nonaqueous sodium-ion batteries *J. Electrochem. Soc.* **166** A3260–4

[223] Goonetilleke D, Pramudita J C, Hagan M, Al Bahri O K, Pang W K, Peterson V K, Groot J, Berg H and Sharma N 2017 *J. Power Sources* **343** 446–57

[224] Dolotko O, Senyshyn A, Mühlbauer M J, Nikolowski K and Ehrenberg H 2014 *J. Power Sources* **255** 197–203

[225] Sharma N, Yu D H, Zhu Y, Wu Y and Peterson V K 2017 *J. Power Sources* **342** 562–9

[226] Pang W K, Kalluri S, Peterson V K, Dou S X and Guo Z 2014 *Phys. Chem. Chem. Phys.* **16** 25377–85

[227] Somerville J W et al 2019 *Energy Environ. Sci.* **12** 2223–32

[228] Gutierrez A, Dose W M, Borkiewicz O, Guo F, Avdeev M, Kim S, Fister T T, Ren Y, Bareño J and Johnson C S 2018 *J. Phys. Chem. C* **122** 23251–60

[229] Tapia-Ruiz N, Dose W M, Sharma N, Chen H, Heath J, Somerville J W, Maitra U, Saiful Islam M and Bruce P G 2018 *Energy Environ. Sci.* **11** 1470–9

[230] Krogstad M J, Rosenkranz S, Wozniak J M, Jennings G, Ruff J P C, Vaughy J T and Osborn R 2020 *Nat. Mater.* **19** 63–68

[231] Weber T and Simonov A S 2012 *Z. Kristallogr.* **227** 238–47

[232] Pecher O, Carretero-González J, Griffith K J and Grey C P 2017 *Chem. Mater.* **29** 213–42

[233] Billaud J, Clément R J, Armstrong A R, Canales-Vázquez J, Rozier P, Grey C P and Bruce P G 2014 *J. Am. Chem. Soc.* **136** 17243–8

[234] Smiley D L and Goward G R 2016 *Chem. Mater.* **28** 7645–56

[235] Xiang Y et al 2020 *Nat. Nanotechnol.* **15** 883–90

[236] Samoson A 2019 *J. Magn. Reson.* **306** 167–72

[237] Kim J, Middlemiss D S, Chernova N A, Zhu B Y X, Masquelier C and Grey C P 2010 *J. Am. Chem. Soc.* **132** 16825–40

[238] Mondal A, Gaultois M W, Pell A J, Iannuzzi M, Grey C P, Hutter J and Kaupp M 2018 *J. Chem. Theory Comput.* **14** 377–94

[239] Rossini A J, Zagdoun A, Lelli M, Lesage A, Copéret C and Emsley L 1942–51 *Acc. Chem. Res.* **46**

[240] Jardón-Álvarez D, Reuveni G, Harchol A and Leskes M 2020 *J. Phys. Chem. Lett.* **11** 5439–45

[241] Harchol A, Reuveni G, Ri V, Thomas B, Carmieli R, Herber R H, Kim C and Leskes M 2020 *J. Phys. Chem. C* **124** 7082–90

[242] Liu Q et al 2020 *In situ* observation of sodium dendrite growth and concurrent mechanical property measurements using an environmental transmission electron microscopy–atomic force microscopy (ETEM-AFM) platform *ACS Energy Lett.* **5** 2546–59

[243] Li Z, Jiang K, Khan F, Goswami A, Liu J, Passian A and Thundat T 2019 Anomalous interfacial stress generation during sodium intercalation/extraction in MoS₂ thin-film anodes *Sci. Adv.* **5** eaav2820

[244] Choi J U, Voronina N, Sun Y-K and Myung S-T 2020 Recent progress and perspective of advanced high-energy Co-less Ni-rich cathodes for Li-ion batteries: yesterday, today, and tomorrow *Adv. Energy Mater.* **10** 2002027

[245] Chen Y, Niu Y, Lin C, Li J, Lin Y, Xu G, Palmer R E and Huang Z 2020 Insight into the intrinsic mechanism of improving electrochemical performance via constructing the preferred crystal orientation in lithium cobalt dioxide *Chem. Eng. J.* **399** 125708

[246] Liu T et al 2019 *In situ* quantification of interphasial chemistry in Li-ion battery *Nat. Nanotechnol.* **14** 50–56

[247] Seidl L, Bucher N, Chu E, Hartung S, Martens S, Schneider O and Stimming U 2017 Intercalation of solvated Na-ions into graphite *Energy Environ. Sci.* **10** 1631–42

[248] Shadike Z, Zhao E, Zhou Y-N, Yu X, Yang Y, Hu E, Bak S, Gu L and Yang X-Q 2018 Advanced characterization techniques for sodium-ion battery studies *Adv. Energy Mater.* **8** 1702588

[249] Inagaki K, Kolosov O V, Briggs G A D and Wright O B 2000 Waveguide ultrasonic force microscopy at 60 MHz *Appl. Phys. Lett.* **76** 1836–8

[250] Luria J, Kutes Y, Moore A, Zhang L, Stach E A and Huey B D 2016 Charge transport in CdTe solar cells revealed by conductive tomographic atomic force microscopy *Nat. Energy* **1** 16150

[251] Grier E J, Kolosov O, Petford-Long A K, Ward R C C, Wells M R and Hjorvarsson B 2000 Structural changes to epitaxial (0001) holmium layers during hydrogen loading *J. Phys. D: Appl. Phys.* **33** 894–900

[252] Yabuuchi N, Kubota K, Dahbi M and Komaba S 2014 Research development on sodium-ion batteries *Chem. Rev.* **114** 11636–82

[253] Bhandari A and Bhattacharya J 2017 Review—manganese dissolution from spinel cathode: few unanswered questions *J. Electrochem. Soc.* **164** A106–27

[254] Zhang D, Meng X-M, Zheng Y-M, Wang X-M, Xu S-D, Chen L and Liu S-B 2020 The critical role of titanium cation in the enhanced performance of P2-Na₀.₅Ni₀.₂₅Mn₀.₆₀Ti₀.₁₅O₂ cathode material for sodium-ion batteries *Phys. Chem. Chem. Phys.* **22** 19992–8

[255] Zhu L, Ding G, Xie L, Yang Q, Yang X and Cao X 2020 Facile preparation of NaV₃O₈/polytriphenylamine composites as cathode materials towards high-performance sodium storage *Int. J. Energy Res.* **44** 3215–23

[256] Liu T et al 2019 Correlation between manganese dissolution and dynamic phase stability in spinel-based lithium-ion battery *Nat. Commun.* **10** 4721
[257] Luo C, Langrock A, Fan X, Liang Y and Wang C 2017 P2-type transition metal oxides for high performance Na-ion battery cathodes *J. Mater. Chem.* A **5** 18214–20

[258] Tang Y, Zhang W, Xue L, Ding X, Wang T, Liu X, Liu J, Li X and Huang Y 2016 Polypyrrole-promoted superior cyclability and rate capability of NaxFe[Fe(CN)6] cathodes for sodium-ion batteries *J. Mater. Chem.* A **4** 6036–41

[259] You Y, Dolocan A, Li W and Manthiram A 2019 Understanding the air-exposure degradation chemistry at a nanoscale of layered oxide cathodes for sodium-ion batteries *Nano Lett.* **19** 182–8

[260] Lyu Y, Yu J, Wu J, Effat M B and Ciucci F 2019 Stabilizing Na-metal batteries with a manganese oxide cathode using a solid-state composite electrolyte *J. Power Sources* **416** 21–28

[261] Ranninger J, Wachs S J, Möller J, Mayrhofer K J J and Berkes B B 2020 On-line monitoring of dissolution processes in nonaqueous electrolytes—a case study with platinum *Electrochem. Commun.* **114** 106702

[262] Nikman S, Zhao D, Gonzalez-Perez V, Hoster H E and Mertens H F L 2021 Surface or bulk Real-time manganese dissolution detection in a lithium-ion cathode *Electrochim. Acta* **386** 138373

[263] Lopes P P, Zorko M, Hawthorne K L, Connell J G, Ingram B J, Strmcnik D, Stamenkovic V R and Markovic N M 2018 Real-time monitoring of cation dissolution/deintercalation kinetics from transition-metal oxides in organic environments *J. Phys. Chem. Lett.* **9** 4935–40

[264] Chen J-S, Wang L-F, Fang B-J, Lee S-Y and Guo R-Z 2006 Rotating ring-disk electrode measurements on Mn dissolution and capacity losses of spinel electrodes in various organic electrolytes *J. Power Sources* **157** 515–21

[265] Zhao L, Chénard E, Çapraz Ö, Sottos N R and White S R 2018 Direct detection of manganese ions in organic electrolyte by UV-vis spectroscopy *J. Electrochem. Soc.* **165** A345–8

[266] Delmas C 2018 Sodium and sodium-ion batteries: 50 years of research *Adv. Energy Mater.* **8** 1703137

[267] Chayambuka K, Mulder G, Danilov D L and Notten P H L 2020 From Li-ion batteries toward Na-ion chemistries: challenges and opportunities *Adv. Energy Mater.* **10** 2001310

[268] Harper G et al 2019 Recycling lithium-ion batteries from electric vehicles *Nature* **575** 75–86

[269] Pathan T S, Rashid M, Walker M, Widanage W D and Kendrick E 2019 Active formation of Li-ion batteries and its effect on cycle life *J. Phys. Energy* **1** 044003

[270] Kishore B, Chen L, Dancer C and Kendrick E 2020 Electrochemical formation protocols for maximising the life-time of a sodium ion battery *Chem. Commun.* **56** 12925–8

[271] Chen L, Kishore B, Walker M, Dancer C and Kendrick E 2020 Nanozeolite ZSM-5 electrolyte additive for long life sodium-ion batteries *Chem. Commun.* **56** 11609–12

[272] Ledwoch D, Robinson J B, Gastol D, Smith K, Shearing P R, Brett D J L and Kendrick E 2020 Hard carbon composite electrodes for sodium-ion batteries with nanozeolite and carbon black additives *Batter. Supercaps* **4** 163–72

[273] Chen C, Brosa Planella F, O'Regan K, Gastol D, Widanage W D and Kendrick E 2020 Development of experimental techniques for parameterization of multi-scale lithium-ion battery models *J. Electrochem. Soc.* **167** 080534

[274] The Faraday Institution 2019 *The Gigafactory Boom: The Demand for Battery Manufacturing in the UK* (available at: https://faraday.ac.uk/wp-content/uploads/2019/08/Faraday_Insights-2_FINAL.pdf)

[275] Jolly J 2020 UK's first car battery 'gigafactory' to be built by two startups *The Guardian* (available at: https://www.theguardian.com/business/2020/may/20/uk-first-car-battery-gigafactory-amte-power-britishvolt)

[276] Faradion 2020 Press release: faradion announces major partnership for developing sodium-ion batteries for commercial vehicles in India (available at: www.faradion.co.uk/faradion-announces-partnership-for-developing-sodium-ion-batteries-in-india/) (Accessed 3 August 2020)

[277] Bauer A, Song J, Vail S, Pan W, Barker J and Lu Y 2018 The scale-up and commercialization of nonaqueous Na-ion battery technologies *Adv. Energy Mater.* **8** 1702869

[278] Haselrieder W, Ivanov S, Tran H Y, Theil S, Froböse L, Westphal B, Wohlfahrt-Mehrens M and Kwade A 2014 Influence of formulation method and related processes on structural, electrical and electrochemical properties of LMS/NCA-blend electrodes *Prog. Solid State Chem.* **42** 157–74

[279] Wang P-F, You Y, Yin Y-X and Guo Y-G 2018 Layered oxide cathodes for sodium-ion batteries: phase transition, air stability, and performance *Adv. Energy Mater.* **8** 1701912

[280] Wang H, Liao X-Z, Yang Y, Yan X, He Y-S and Ma Z-F 2016 Large-scale synthesis of $NaNi_{1/3}Fe_{1/3}Mn_{1/3}O_2$ as high performance cathode materials for sodium ion batteries *J. Electrochem. Soc.* **163** A565–70

[281] Ludwig B, Zheng Z, Shou W, Wang Y and Pan H 2016 Solvent-free manufacturing of electrodes for lithium-ion batteries *Sci. Rep.* **6** 23150

[282] Schälicke G, Landwehr I, Dinter A, Pettinger K-H, Haselrieder W and Kwade A 2020 Solvent-free manufacturing of electrodes for lithium-ion batteries via electrostatic coating *Energy Technol.* **8** 1900309

[283] Samba A, Omar N, Gualous H, Capron O, Van Den Bossche P and Van Mierlo J 2014 Impact of tab location on large format lithium-ion pouch cell based on fully coupled tree-dimensional electrochemical-thermal modeling *Electrochim. Acta* **147** 319–29

[284] Rudola A et al 2021 Commercialisation of high energy density sodium-ion batteries: Faradion's journey and outlook *J. Mater. Chem.* A **9** 8279–302

[285] Broux T et al 2019 High rate performance for carbon-coated $Na_3V_2(PO_4)_2F_3$ in Na-ion batteries *Small Methods* **3** 1800215

[286] Rong X et al 2020 Na-ion batteries: from fundamental research to engineering exploration *Energy Storage Sci. Technol.* **9** 515–22

[287] Datasheet 2019 Natron energy blue tray 4000 *Distributed at the Battery Show* (Novi, MI, 9–12 September)

[288] (a) Datasheet. Toshiba rechargeable battery SCiB high power 2.9 Ah cell (available at: www.scib.jp/en/download/ToshibaRechargeableBattery-en.pdf) (Accessed 17 September 2020); (b) Datasheet Amaron Quanta 12V200Ah (available at: www.quanta.in/images/pdf/Quanta-SF-Brochure.pdf) (Accessed 17 September 2020); (c) Datasheet ENERpower Shandong goldencell electronics technology model HTCFR18650-1800mAh-3.2V (available at: https://enerpower.de/wp/wp-content/uploads/2019/07/Technical-Specifications-HTCFR18650-1800mAh-3.2V-EN.pdf) (Accessed 17 September 2020); (d) Datasheets A123 Nanophosphate® high power lithium ion cell ANR26650M1-B (available at: https://a123batteries.com/product_images/uploaded_images/26650.pdf, https://www.evlithium.com/A123-Battery/632.html, https://www.batteryspace.com/prod-specs/6610.pdf) (Accessed 17 September 2020); (e) Datasheet AA portable power Li-ion polymer battery 3.7V 10000 mAh PL-9059156 (available at: www.batteryspace.com/prod-specs/3.7V9059156.pdf) (Accessed 17 September 2020); (f) Datasheet Panasonic lithium ion NCR18650B (available at: www.batteryspace.com/prod-specs/NCR18650B.pdf) (Accessed 17 September 2020); (g) Datasheet LG Chem lithium ion INR21700 M50 18.20Wh (available at: http://queenbattery.com.cn/index.php?controller=attachment&id=attachment=111) (Accessed 17 September 2020)
[289] Reuning A et al 2019 Investigation of battery cells for prospective use in electric cars *PCIM Europe 2019; Int. Exhibition and Conf. for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management* pp 1–7

[290] Jin T, Li H, Zhu K, Wang P-F, Liu P and Jiao L 2020 Polyanion-type cathode materials for sodium-ion batteries *Chem. Soc. Rev.* **49** 2342–77

[291] Henze V 2019 Battery pack prices fall as market ramps up with market average at $156/kWh in 2019 *BloombergNEF* (available at: https://about.bnef.com/blog/battery-pack-prices-fall-as-market-ramps-up-with-market-average-at-156-kwh-in-2019/?sf113554299=1) (Accessed 17 September 2020)

[292] Popp H, Zhang N, Jahn M, Arrinda M, Ritz S, Faber M, Sauer D U, Azais P and Cendoya I 2020 Ante-mortem-analyse sowie elektrische, thermische und Alterungstests von State-of-the-Art cylindrical lithium-ion cells *Elektrotech. Inf. Tech.* **137** 169–76

[293] (a) Datasheet Pylontech energy type cell PF25N (available at: http://en.pylontech.com.cn/service/download) (Accessed 17 September 2020); (b) Datasheet CATL LiFePO4 lithium battery 3.2V120Ah (available at: www.evlithium.com/CATL-Battery.html) (Accessed 17 September 2020); (c) Datasheet EVE energy LFP power battery LF280 (available at: www.dcmax.com.tw/LF280(3.2V280Ah).pdf) (Accessed 17 September 2020)

[294] 2019 Inventing the batteries of the future, research needs and future actions, batteries 2030+ roadmap, second draft (available at: https://battery2030.eu/digitalAssets/817/c_817144-l_1-k_battery-2030_roadmap-v2.1.pdf) (Accessed July 2020)

[295] Schnell J, Tietz F, Singer C, Hofer A, Billota N and Reinharta G 2019 Prospects of production technologies and manufacturing costs of oxide-based all-solid-state lithium batteries *Energy Environ. Sci.* **12** 1818–33

[296] Peters J F, Cruz A P and Weil M 2019 Exploring the economic potential of sodium-ion batteries *Batteries* **5** 10

[297] Barker J and Heap R J 2014 Doped nickelate compounds GB 250389A

[298] Sodium-ion battery (available at: https://en.wikipedia.org/wiki/Sodium-ion_battery) (Accessed June 2020)

[299] International Energy Agency 2019 IEA World Energy Outlook 2019 (Paris: International Energy Agency)

[300] International Energy Agency 2020 Energy storage (available at: www.iea.org/reports/energy-storage) (Accessed August 2020)

[301] Barker J, Heap R J, Roche N, Tan C, Sayers R, Whitley J and Lui Y 2015 The commercialization of high energy density Na-ion battery technology 2nd Int. Symp. on Sodium Batteries (*Phoenix, October*)

[302] Pfeifer K, Arnold S, Becherer J, Das C, Maibach J, Ehrenberg H and Dsoke S 2019 Can metallic sodium electrodes affect the electrochemistry of sodium-ion batteries? Reactivity issues and perspectives *ChemSusChem* **12** 3312–9