Electrolytes, SEI Formation, and Binders: A Review of Non-Electrode Factors for Sodium-Ion Battery Anodes

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This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi:

10.1002/smll.201703576.

Article type: Review

Electrolytes, SEI Formation, and Binders: A Review of Non-Electrode Factors for Sodium-Ion Battery Anodes

Clement Bommier,* and Xiulei Ji* C. Bommier, X. Ji

Department of Chemistry, Oregon State University, Corvallis, OR 97331-4003, United States E-mail: [email protected]; [email protected]

Key words: Na-ion batteries, anodes, electrolytes, binders, quantitative analysis

Abstract:

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1. INTRODUCTION

In the battery community, most research efforts are geared towards the development of newer, better, and more functional materials. In many regards, this has enabled substantial progress with reversible batteries such as Li-ion batteries (LIBs),[1,2] Na-ion batteries (NIBs),[3–6] and K-ion batteries (KIBs).[7–9] New materials have shifted our perceptions of electrochemical energy storage: whereas it was once regarded as an ideal method to power small electronic devices, it is now expected to drive vehicles for hundreds of miles, and is touted as the solution to grid-level energy storage.

While this material-centric research is present in all battery fields, it is even more pronounced with emerging battery technologies, such as NIBs, and KIBs. LIBs, which have been

commercially available since the early 1990’s, have well-accepted material technologies, such as LCO, LFP, NMC, or NCA cathodes, coupled with a graphitic anode. However, being that NIBs and KIBs are relatively nascent, with most research originating within the past decade, the anode and cathode materials are still not developed enough to produce a commercial technology, which has put a premium on material research.

Yet this emphasis also comes at cost: other components of the battery, such as the electrolyte, the formation of the solid electrolyte interphase (SEI), and the binder materials are largely overlooked. Moreover, this cost is magnified when considering that such factors can significantly influence the electrochemical performance of a battery. When paired with a poor choice of electrolyte, or a binder material, an electrode material that should theoretically work well can fail, which can lead to a false negative: even though the material did not yield good electrochemical performance, the underlying factor was not the material itself, but the extrinsic factors surrounding it.

For LIBs, these factors, such as electrolytes, electrolyte additives, and binders, have been well studied for years, most often by commercial battery manufacturers. However, for NIBs, such research has not been as prolific. As such, it is the goal of this review paper to

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2. REVIEW OUTLINE & METHODS

This review intends to achieve two goals. The first one is to highlight previous research in the areas of electrolytes, SEI formation, and binder materials for NIB anodes. This includes characterization studies, theoretical investigations, and new developments – like most typical reviews. The second goal focuses on a quantitative analysis of these topics. For the

quantitative analysis, over 500 NIB-anode literature papers were reviewed, categorized, and mined for a host of different data parameters, such as capacity, long-term cycling, CE@1st

(coulombic efficiency in the first cycle), electrolyte choice, binder choice, and many more. The data were then used for a quantitative analysis, which is included in every section of this review. The analyses range from simple surveys of the type of electrolytes solvents,

electrolyte salts, and binders that are commonly used in the literature, to more in-depth analyses looking to establish trends within the data. Examples of trends explored include relationships between electrolytes and CE@1st_{, electrolytes and long-term cycling, and}

whether the addition of certain additives help improve performance.

For the long-term cycling analyses, we devised a metric called ‘Integrated Capacity’ (IC), which is the estimation of the total number of ampere-hours (Ah) that were passed through a system. The metric is necessary for the simple reason that long-term cycling protocols vary from paper to paper. Some papers do a few hundred cycles at a low current rate, while others do thousands of cycles at a high current rate. Additionally, the loading masses between papers vary greatly. As such, it would be inaccurate to draw conclusions solely based on capacity retentions during long-term cycling, without looking at the specifics of the cycling protocol. The IC metric normalizes all the long-term cycling data, thus allowing for a quantitative analysis.

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( ( ))

Figure 1. Schematic for integrated capacity. The equation represents the sum of the two

individual equations in the gray, and green areas, with the cycles, and loading mass parameters being distributed accordingly.

This allows for a straightforward way to obtain the IC in units of Ah by measuring the total area under the long-term cycling capacity data with further factoring in of the loading mass. We realize that this analysis overestimates the IC if the capacity fades quickly at the

beginning, and slowly at the end, while it underestimates the IC if the reverse is true.

However, since most batteries fade quicker at the beginning, and we are using other people’s work, we felt that to overestimate would be more appropriate than to underestimate.

The more complex analyses will only be presented as charts, and not evaluated for statistical significance, as that is something we will leave up to the readers. The raw data for such purposes can be made available by contacting the corresponding authors.

3. ELECTROLYTES FOR NIBS

In the NIB system, the electrolytes act as ionic charge carriers necessary for the

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conductivity, are important mostly for electrochemical reasons. Electrolytes with low ionic conductivities, or high viscosities tend to cycle poorly at high current rates, or below ambient temperatures. Furthermore, electrolytes resistant to breakdown into side products, often labeled under the catchall term of SEI, are also desired, as the uncontrolled formation of SEI can lead to slower transport, higher polarization, and loss of charge carriers, which are all factors contributing to a hastened cell failure. As such, a commercially viable NIB must be able to perform both of these, which makes the correct choice of electrolyte salt/solvent combination especially prescient.

3.1. Conventional electrolytes

The majority of electrolytes used for NIBs rely on sodium salts of NaClO4, NaPF6, or Na

Bis(trifluoromethane)sulfonamide (NaTFSI), dissolved in a carbonate based organic solvent, either made of a single molecule, such as propylene carbonate (PC), or a mixture of

molecules. This makes it a binary solvent in the case of two molecules, or a ternary solvent in the case of three molecules. Common examples of binary solvents mix PC with ethylene carbonate (EC), leading to EC:PC, or mix EC with molecules, such as dimethyl carbonate (DMC) or diethyl carbonate (DEC) , leading to EC:DMC, EC:DEC (Figure 2). Ternary solvents are usually a combination of the aforementioned molecules, though they are not as represented in the literature. In addition ethyl methy carbonate (EMC) can also be used, though it is much less frequent than the ‘core four’ carbonate molecules.

Data collection from over 500 papers currently present in the literature reveals the majority of electrolyte solvents to be EC:DEC, EC:PC, PC, or EC:DMC, while electrolyte salts are mostly NaClO4, with a minority of NaPF6, along with a handful of other salts (Figure 2).

(e) (f)

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Figure 2. Molecular plots of a) Ethylene carbonate. b) Propylene carbonate. c) Dimethyl

carbonate. d) Diethyl carbonate. Red atoms are oxygen, grays are carbon, and whites are hydrogen. Molecules imaged using mathematica software. Summary of the literature regarding a) The electrolyte solvents used. b) Electrolyte salts used.

While this information is useful in that it gives us a ‘snapshot’ of the current electrolytes used for NIB anodes, it does little to tell us if the current electrolyte choice is optimal. That remains a difficult task. Furthermore, attempts at a meta-analysis, whereby electrochemical performances from different papers are combined into a larger study regarding the electrolyte is also difficult, as the parameters from paper to paper change significantly.

However, a few studies have attempted to fill the knowledge gap of what constitutes a good electrolyte, and what does not. To that end, one of the early studies focusing on the choice of electrolyte solutions was conducted by Ponrouch et al. in 2012, looking at many different types of solvents that could potentially be used for NIBs.[10] When testing various

parameters, such as voltage windows, thermal stability, and cycling feasibility, the authors concluded that EC:PC made for a good choice of electrolyte solvent, but that using NaClO4

or NaPF6 as an electrolyte salt amounted to little difference. This study was an important

one; in 2012, the NIB literature was only at the beginning, and thus it was important to set experimental guidelines. The same research group further refined the optimal electrolyte solvent choice to EC0.45:PC0.45:DMC0.1, paired with a sodium salt. It was claimed that this

ternary mixture was able to offer a high ionic conductivity, low ion pairing, low viscosity, and more importantly, the formation of a suitable SEI layer.[11]

Sankaranarayanan et al. uses a combination of potential mean force (PMF) calculations and molecular dynamics (MD) calculations, to estimate free energies of solvation, ionic

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and EC:DMC. Curiously, the most commonly used solvent in the literature, EC:DEC, was not endorsed by the authors of the paper, and regarded as an inferior solvent choice. To further pursue their claims, the authors also performed a study comparing the electrochemical performance of TiO2 nanotubes in a NaClO4 electrolyte with various electrolyte solvents.

The experimental study supported the findings of the theoretical study, showing that the EC:EMC, and EC:DMC based electrolyte had better capacity retention at higher current rates than the EC:DEC electrolyte.[12]

The same group later followed up on this initial computational study by performing density functional theory (DFT) calculations on similar electrolyte solvents, as to obtain a more accurate picture of the free energies of solvation, density of states (DOS) calculations, and charge transfer estimations. The results were similar to those found in the first paper. EC-based electrolytes were found to be more favorable in terms of solvation energy, and charge transfer, while these were also found to be less favorable in acyclic carbonates. However, unlike the previous paper, the simulations here ranked EC:PC as the most favorable electrolyte solvent.[13]

However, despite predictions made in these earlier papers, it was also recently argued by Cresce et al. that the solvent selection for the electrolyte was of minimal importance.[14] Utilizing various physical characterization techniques, as well as DFT based continuum calculations, the authors showed that unlike Li-ions, the performance of the NIBs is significantly decoupled from the Na interaction with the solvent. As such, unlike previous literature that suggest adding EC in order to increase solvation and ionic conductivity, the authors here suggested taking a different approach: develop electrolyte solutions that lead to the most favorable interphase formation, as opposed to one made tailored to its interaction with sodium ions.

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3.2. Co-intercalating electrolytes

The electrolytes covered in the previous section are considered ‘traditional’ in the sense that they do not actively participate in the ion storage process. Their role is to deliver the Na-ions to the storage site, whether that is a carbon, an alloy, or an insertion type anode, whereby the ion desolvates from the electrolyte and travels to the storage site through solid-state diffusion. In the case of a co-intercalating electrolyte, Na-ion never completely desolvates, and the electrolyte solvent becomes a direct participant in the storage process.

It is now known that, in the case of NIBs, intercalation of the Na-ions into a graphite anodes is not possible due to unfavorable thermodynamic parameters. Among these is the

desolvation energy needed for the Na-ion to remove itself from the electrolyte, and diffuse into the solid-state graphitic structure. However, through the use of a co-intercalation electrolyte, the desolvation step is not necessary, as the Na-ion remains solvated while participating in the electrochemical reaction.

The feasibility of this concept was first demonstrated by Adelhelm et al. who demonstrated that intercalation of a graphitic structure by sodium ions was rendered possible through the use of a sodium triflate (NaCF3SO3) salt in a diglyme solvent.[16] This was confirmed

through X-ray diffraction (XRD) measurements that showed a noticeable increase in d-spacing, consistent with the co-intercalation of the Na/diglyme complex.

The development was significant for a number of reasons. First off, the ability to utilize graphite as an electrode material enables us to access most knowledge and knowhow

previously uncovered regarding the use of graphite anode in LIBs. Additionally, the use of a co-intercalating electrolyte also displays much better capacity retention during long-term cycling, as well as a coulombic efficiency on the first cycle (CE@1st) of more than 80%, which ranks it in the upper echelon of NIB anode materials.

Following the initial paper by Adelhelm et al. were many additional publications, exploring the theory and thermodynamics behind intercalation, as well as aiming to maximize

electrochemical performance.

On the theory side, the intercalation process was further elucidated through DFT and in operando simulations by Kang et al. who were able to demonstrate that the sodium insertion process occurred via a staging mechanism, with the final graphite galleries being ‘double-stacked’ with sodium ions.[17] This was further expanded by Seidl et al. using in operando XRD, TEM, and electrochemical quartz microbalance (EQCM) measurements.[18] In their work the authors also showed the staging nature of the intercalation process. Additionally, they revealed that the nature of the interaction between the Na-ions and the graphite structure was highly influenced by the coordination shell of the solvent. Too small of a coordination shell led to sluggish kinetics, and even a failure to co-intercalate.[19]_{Conversely, while too}

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steric hindrance. Similar observations were made in DFT calculations by Han et al. who showed favorable interactions between the solvent shell and the graphite, as the Na-ion was able to be solvated without causing steric hindrance, thus keeping diffusion barriers at a minimum.[20]

The focus on interactions between the solvated species and the graphite structure drew particular interest from many authors, as it was suggested that such electrolytes are ideal for use at high current rates. This was echoed in a theoretical paper by Yu et al. who shows that intercalated sodium species experience lower diffusion barriers within graphite than co-intercalated lithium species, thus allowing for a greater ease of diffusion, and consequently, better rate performance.[21] A similar conclusion was reached in a paper by Gotoh et al. Through solid state NMR measurements, the authors demonstrated that the electrolyte complexes stayed weakly attached to the graphite and were still mobile at room temperature.[22]

Experimentally, the approach has focused on testing different solvent/salt combinations, along with different types of substrate materials. Such approaches were undertaken by Kang et al. who used graphite with a NaPF6/diglyme combination,[23] Adelhelm et al. with further

glyme combinations such as triglyme (3G) tetraglyme (4G), di(propylene glycol)methyl ether (DPGDME), diethylene glycol dibutyl ether (Butyl-2G), and 1,5-dimethoxypentane (1,5-DMP),[24] Chen et al. with a NaCF3SO3/tetraglyme electrolyte,[25] and Scrosati et al. with a

NaClO4/tetraglyme electrolyte.[26] Both compounds were able to show excellent rate

retention, with capacities of close to 100 mAh g-1 at current rates of 2000 mA g-1, which is impressive considering that the capacities at lower current rates were not much higher. Many more additional co-intercalating electrolytes, which have been demonstrated intercalatable into graphite, are available, thus leaving plenty more research opportunities into the topic,[27,28] as well as applicability beyond the NIB field.[29,30]

While co-intercalating electrolytes were first used with exclusively graphitic materials, they have begun to be used with non-graphitic carbon materials as well.[31,32][33] The resulting electrochemical performance seen in these papers all echoed a common theme: good capacity retention at higher currents, along with stable long-term cycling. In the case of a paper by Pint et al. the capacity went from 145 mAh g-1 at a current rate of 1000 mA g-1, to 130 mAh g-1 at 5000 mA g-1, which represents an extremely strong capacity retention rate for such a large increase in current. Furthermore, nearly all these papers displayed long-term cycling capacity retention of greater than 90%, even if the number of cycles was well into the

thousands. With 8000 cycles at a current rate of 12,000 mA g-1, Pint et al. reported a capacity retention of 96%.

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suffer from the same problems that traditional electrolytes do during long-term cycling, as they demonstrate much stronger capacity retention. This could mean that these electrolytes are less inclined to form SEI, do not allow Na-ions to become irreversibly trapped in the host carbon structures, or that they are better at preserving the structures of the carbon, along with the integrity of the binder. However, these are only hypotheses, and have yet to be proven. Thus far the discussion on co-intercalating electrolytes has revolved around their use with carbon materials, and for good reason: for graphitic based materials, they are needed to make intercalation possible. That being said, they can also be used for other types of anodes that are not intercalation based, such as alloying anodes, or conversion anodes. This was the basis of a paper by Tarascon et al. who explored the use of a glyme-based electrolyte with a Sn anode.[34] This combination was reported to have good performance, mostly due to the passivation resulting from the solvent; however, the details of such a process are more appropriate for a discussion about the SEI, which will be covered in a later section.

3.3. Ionic liquid electrolytes

Ionic liquid (IL) electrolytes are composed of cationic and anionic organic species, coupled with a sodium salt. These differ from traditional electrolytes, and co-intercalation

electrolytes, as those types do not contain an ionic solvent. This does not mean that the solvent is non-polar, but rather, that the solvent itself does not contain cations or anions. The potential of such electrolytes is their superior stability, such as greater thermal stability, large potential window, low vapor pressure, and high boiling points.[35–38] Whereas organic solvents, such as EC:PC, or EC:DEC, are quite volatile at room temperature, and prone to degradation in a battery, ILs are not. This is especially true regarding the anode, as the SEI layer typically forms at the low operational potentials of the anode, leading to inefficiencies, and affecting capacity retention over the long-term cycling. As such, this gives ILs an advantage with regards to safety, as flammability becomes a lesser concern,[39]_{as well as}

cycle life, since there is a lower incidence of electrolyte breakdown.

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These factors can be improved through increasing the operational temperature of the battery to temperatures well above room temperature; however, doing so also significantly adds to the operational cost of the battery, as it lowers the energy efficiency. Additionally, the synthesis of ILs can be expensive, posing further drawbacks for their use in NIBs.

Some early attempts to use ILs as NIB electrolytes were reported by Hagiwara et al.,[42,43]

who reported the use of Na-bis(fluorosulfonyl)amide (NaFSA) and Na [N-methyl-N-propyl pyrrolidinium][bis(fluorosulfonyl)amide] (Na [C3C1pyrr][FSA]) ionic liquids. These ionic liquids were tested with a NaCrO2 cathode, and found to be operational, albeit at a

temperature more than 60°C. However, the same authors did show in a later paper, that it was possible to use IL electrolytes at room temperatures, provided that the sodium

concentration be lowered from 40% to 25% mass composition,[44] which shows that in the case of IL there is no uniform recipe, as electrolytes must be developed with a specific operating temperature range in mind. Chang et al. reported the use of

Butylmethylpyrrolidinium–bis(trifluoromethanesulfonyl)imide (BMP–TFSI) with a NaFePO4

cathode at temperatures ranging from 25°C to 75°C.[45]_{When compared to capacity with}

traditional electrolytes, the capacity was significantly lower at 25°C, while that at 50°C and 75°C were much better, which underscores the importance, and the difficulty, of obtaining a functional room-temperature IL electrolyte. Passerini et al. also demonstrated the feasibility of an IL electrolyte with a cathode material using N-butyl-N-methylpyrrolidinium

bis(fluorosulfonyl)imide (PYR14FSI ),[46] and N-butyl-pyrrolidinium

bis(trifluoromethanesulfonyl)imide (PyrH4TFSI).[47] Both showed more stable long-term

cycling performance, which was attributed to the greater potential stability window of the IL electrolyte, versus that of the organic based solvents.

Other early studies into ILs that focused on characterization but did not provide electrochemical cycling data include ones by Johansson et al. [48] and Yoon et al.[49] Johansson et al. showed that an imidazolium-TFSI (Emim-TFSI) based IL electrolyte was also suitable for room temperature based NIBs, while Yoon et al. demonstrated the

effectiveness of N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (C3mpyrFSI).

With anode materials, and in full cell environments, the use of IL electrolytes has also proven to work well. Most noticeably, a paper by Nohira et al. showed that a Na[FSA]–

[C3C1pyrr][FSA] electrolyte was compatible with a hard carbon anode, yielding capacities of close to 250 mAh g-1.[50] More importantly, the same paper then furthered the concept by building a 27 Ah prismatic cell with a hard carbon anode, and NaCrO2 cathode, and the

Na[FSA]–[C3C1pyrr][FSA] electrolyte. At a current rate of 10 A, and an operating

temperature of 363 K, the battery was able to display 80% capacity retention after 500 cycles. Given the electrochemical charges contained in the battery, as well as the high capacity

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showed that a Na-bis(fluorosulfonyl)amide (Na[FSA]) electrolyte was also able to function, though the paper only reported electrochemical data obtained in a cathode half-cell.[51] Chang et al. also reported the use of N-Propyl-N-methylpyrrolidinium (PMP)–FSI in both a half cell with hard carbon, as well as a hard carbon/Na-manganese-oxide (NMO) full cell.[52] In the hard carbon half-cell, the capacity retention after 100 cycles was of 90%, while it was of only 70% with the organic based electrolyte. Furthermore, the full cell with the IL showed a capacity retention of 97% over 100 cycles, which was superior to the 61% retention for the organic electrolyte over the same number of cycles.

Regarding the effects of IL electrolytes with anode materials solely, Monti et al. reported the use of a hybrid organic/IL electrolyte composed of EC0.45:PC0.45:Pyr13TFSI0.10 in a half-cell

with a hard carbon anode.[53] The addition of the organic solvent allowed the electrolyte to have a lower operational temperature, while the IL was able to provide stability at low operational voltages. The observed capacity in the half-cell at a current rate of C/10 was of 180 mAh g-1_{, which is still respectable while not as high as hard carbon with traditional}

electrolytes.

Chang et al. also reported the use of a N-propyl-N-methylpyrrolidinium (PMP)–FSI IL electrolyte with a SnO2/graphene anode in a half-cell environment.[54] When compared to

organic electrolytes, the IL electrolyte was able to demonstrate higher CE@1st, along with the most stable cycling. At 25°C and a current rate of 100 mA g-1_{, the IL electrolyte showed a}

capacity retention of 99%, while the EC:PC:FEC(fluoroethylene carbonate) organic electrolyte only had a retention of 88% after 100 cycles. When the temperature was increased to 60°C the IL electrolyte retained 96% of its capacity over 100 cycles, as compared to 58% for the best performing organic electrolyte. Moreover, this increased efficiency is coupled with the fact that the IL electrolyte displayed a higher capacity at the elevated temperature, thus showing the use of IL’s could be a way to inure the battery from the deleterious effects of temperature swings. Komaba et al. also showed the effectiveness of a N-methyl-N-propylpyridinium-bisfluorosulfonylamide (MPPFSA) IL electrolyte with a red phosphorus anode.[55] Following 80 cycles, the capacity retention of the anode with the IL electrolyte was of 93%, while over the same duration the EC:DEC electrolyte only showed a capacity retention of 30%, showing the effectiveness of IL electrolyte goes beyond just carbon materials.

Lastly, the use of IL’s with Nb-doped TiO2 half-cells,[56] along with Na2Ti3O7(NTP)/C half

cells,[57] were reported by Sakaguchi et al. and Nohira et al., respectively. In the case of the Nb-doped anodes, the use of ILs was able to show a higher capacity retention over the long-term cycling, while the NTP anode showed fewer benefits due to the instability of the structure.

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problems inherent with the ILs. As such, two approaches are recommended for further research. The first one is the development of new and novel ILs, which requires

experimental effort, and computational investigations, such as the work done by Gomez et al.[58] looking to develop new inorganic electrolytes, or through the work of Sun et al. looking to achieve the same goal, but through simulation based methods.[59]

The other research approach would be to integrate the use of ILs with traditional organic electrolytes. This would be beneficial for several reasons. First off, it would allow

researchers to utilize all the previous work on organic electrolytes. Additionally, combining organic and IL solvents could allow for ‘the best of all worlds’. The organic solvents would ensure a higher ionic conductivity, and lower viscosity, while the IL may bring about

stability, along with a lower flammability risk. Furthermore, mixing the two would allow for an overall reduction in total price, as current ILs are more expensive than the traditional electrolytes. Research focusing on such a concept is currently ongoing. In addition to the paper brought up earlier, work by Noor et al.,[60] Forsyth et al.,[61] Dokko et al.,[62] Neale et al.,[63]_{and O’Dell et al.}[64]_{all look at the effects of mixing electrolyte solvents and ion pairing}

in ILs as to achieve more favorable properties, which can then be parlayed into more favorable properties.

3.4. Quantitative Analysis of Electrolytes

For the quantitative analysis of electrolytes, we tried to answer two questions. First, are certain electrolyte solvents better for CE@1st than others, and second, does the electrolyte solvent choice affect the total IC. For the case of the electrolyte solvent choice and the IC, the analysis was further divided into four groups, where anodes were classified by their types of chemistry: carbon based, alloy based, conversion based, or insertion based. An analysis of the CE@1st versus that of the electrolyte salt used was performed, but not included, as it was heavily dominated by the presence of NaClO4, and offered little insight.

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Figure 3. Analysis of the CE@1st per solvent type. a) The stacked bar graph is grouped by CE@1st, using solvent type as a factor. b) The stacked bar graph is grouped by solvent type, using CE@1st as a factor. This allows to establish the CE@1st prevalence by solvent type, thus negating effects of under representation that occur in (a) (e.g., ether based electrolytes are under represented in the literature, thus a quantitative analysis of the CE@1st minimizes their results).

From Figure 3, we can see that PC and EC:DEC are among the worse solvents to use. Of all the papers surveyed, ones with a CE@1st of under 0.25 are disproportionally represented by PC and EC:DEC. For the best solvents, EC:DMC and ethers seem to dominate. They have the highest percentage of papers with CE@1st over 0.75, and 0.50. However, one should keep in mind that both the EC:DMC and the ethers are among the least represented solvents, so the conclusion can only be drawn with limited evidence. Additionally, when looking at the percentage of CE@1st above 0.50, there are extremely similar for EC:PC and EC:DEC, making it difficult to ascertain if EC:PC is advantageous over EC:DEC.

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Figure 4. Capacity retention over long-term cycling versus IC in ampere hours (Ah) using

electrolyte solvent as a factor for a) Carbon anode. b) Alloying anodes. c) Conversion anodes. d) Insertion anodes.

Looking at the long-term cycling versus IC, using the electrolyte solvent as a factor leads to no straightforward conclusions (Figure 4). There could be a case that EC:DMC is more advantageous for carbon anodes, but few data are available, so it is dubious at best.

Additionally, we see that EC:DMC has the greatest presence with conversion anodes, which could be something else to look into. However, we will conclude that there is no clear benefit, or any apparent stratification between the different solvent types – and that further work should be preformed, or additional data assessed to make stronger conclusions.

3.5. Future Considerations

Summarizing the quantitative analysis of the electrolyte solvents, evidence from the literature would suggest that EC:DMC, and ether-based solvents are among the most advantageous for a high CE@1st, while the type of solvent seems to have minimal effect on the total IC. The first conclusion echoes what was reported in the paper by Sankaranarayanan et al.[12] while the latter conclusion is still unclear: it is possible that solvents affect long-term cycling, but there is not enough data to support the claim, or at the very least, reject it. Furthermore, the promise of IL electrolytes, along with new work, such as water-based electrolytes,[65,66] gel-electrolytes,[67–69]_{and solid state electrolytes,}[70–72]_{and polymers,}[73]_{bear more consideration.}

4. SEI

In the earlier section, we briefly introduced the concept of SEI formation, and how it can affect CE@1st and long-term cycling. However, we looked at it solely from the perspective of it’s relationship with the electrolyte. We did not explore the mechanisms that lead to its formation, its composition, or ways to mitigate it. That will be attempted in this section. In a way, the prevalence of SEI formation, during both the CE@1st_{and the long-term cycling}

can be considered one of the most important parameters of the anode materials. Currently, the preferred method of testing and screening anode materials is in a half-cell setup with the active material on one side, and a Na metal counter-electrode on the other side. Such a setup provides an unlimited source of Na-ions for anode sodiation; thus, issues such as low

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the main sources of such an inefficiency is the SEI formation during the initial sodiation, and the slower, more chronic SEI formation that occurs during long-term cycling.

The concept of the SEI interphase has been well known for close to 40 years now: it was first introduced by Peled in 1979, where he elucidated the presence of reductive side reactions between alkali metals and non-aqueous organic electrolytes.[74]_{It was named a ‘solid}

electrolyte’ as the insoluble products on the metal surface still act as an electrolyte: they allowed the diffusion of metallic cations through Schottky defects while preventing the transfer of electrons. Specifics behind diffusion, such as crystallinity, grain boundaries, dissolution of the existing layer, and subsequent reforming, and interaction with both the active, and counter electrodes all factor into the SEI formation.

4.1. Characterization and comparisons with-Li based SEI

SEI research for LIBs has taken absolute precedence of that of NIBs. A comprehensive discussion on the LIB SEI was made available in a 2014 review by Xu.[75] However, concurrent with the NIB trends of this decade, specific NIB SEI research has begun to take place. One of the first examples was by Komaba et al. in 2011.[76] Using XPS and time-of-flight secondary ionization mass spectroscopy (TOF-SIMS) measurements, they investigated the compounds present on the surface of the carbon anode. The XPS measurements revealed the presence of the carbonate Na2CO3 and the alkyl carbonate ROCO2Na. These compounds

were similar to those found in the case of an LIB surface, which shows that while different, much can still be learned from the prior LIB research. However, the authors did reveal one critical difference between the SEI of the NIBs versus that of the LIBs. When etching the surface with Ar+_{, the oxygen signal from the NIB SEI disappeared much faster than the LIB}

SEI. A further difference was spotted from the TOF-SIMS analysis, as the NIB SEI fingerprint was skewed more towards the presence of inorganic compounds, while the LIB SEI displayed a greater presence of organic-based compounds. This shows that the layering of the SEI formation is different on the carbon anode in two types of batteries, most likely due to the different reactivities of the Na and Li metals.

The difference in reactivity of the popular organic solvents used as NIB electrolytes was investigated in a 2016 paper by Passerini et al. who shows that formation of SEI is heavily dependent on the reduction of linear carbonates, such as DMC or DEC—and that such a reduction depends on whether the metal ion present is Na+_{or Li}+_.[77]_{However, their}

conclusion was that the NIB SEI was less robust than the LIB SEI.

The differences in SEI layer formation were corroborated by Edstrom et al.[78] Using a Fe2O3

electrode, the authors showed the Na-based SEI layer was more homogenous than a Li-based SEI, which demonstrated a clear stratification between the inorganic layers, and organic layers. Additionally, the Na-based SEI eschewed the long -(CH2)x- hydrocarbon chains found

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explain for the formation of a thicker, and more homogeneous SEI layer. A later study by Brandell et al. confirmed the findings of the thicker, more homogenous SEI layer, showing that the Na SEI contains a significantly higher content of inorganic components, as compared to the Li-based SEI, which has a higher concentration of organic components.[79] This

suggests two things: first, that the Li based environment is less reactive, as the SEI is still composed of organic solvent molecules. Additionally, it also suggests that the Na counter electrode used in the experiment may play an outsized role in the formation of the SEI, as side reactions between the Na counter electrode and the electrolyte would explain the shift towards a more inorganic SEI layer.

The disparity in Na-based SEIs, as opposed to Li ones, was also the subject of a paper by Lin et al.[80] Using a systematic design-of-experiment, the authors initially cycled a fresh hard carbon electrode with a counter electrode & Na based electrolyte, switched out the Na-counter electrode and electrolyte with Li components for further cycling, and lastly, replaced the Li components again with Na components, effectively cycling hard carbon through a series of Na/Li/Na environments. The converse of this was also done, whereby a hard carbon electrode was cycled in successive Li/Na/Li environments. Over the course of the

experiments, it was found that once the SEI had been formed in a Li environment, even if an SEI had already been formed with Na, the hard carbon could no longer effectively accept Na ions. This was the case for both the Na/Li/Na and Li/Na/Li experiments. Meanwhile, formation of SEI in Na environments was inconsequential to the cycling of Li with a hard carbon electrode. These findings allowed the authors to suggest a few insights. First-off, the capacity of a hard carbon electrode for NIBs can effectively be tuned through the interplay of a Na/Li SEI formation. More broadly, for NIB anodes, it also suggests that Na-based SEIs are relatively porous, or at the very least, more porous than Li-based ones, which can lead to changes for long-term cycling.

Other NIB SEI studies that have been conducted include one by Hu et al. who used a colloidal probe microscopy to reveal the mechanical properties of the SEI.[81] Using this method, the researchers reported the composition of the SEI to be inhomogeneous on the carbon surface, leading to uneven physical and mechanical properties. As such, it

demonstrates the need to devise a method leading to homogeneous SEI formation, as such a formation would be easier to characterize, and could potentially lead to more stable cycling. Addtionally, Younesi et al. reported that in the case of Sn anodes, SEI thickness was a function of sodiation states, as thehard x-ray photoelectron spectroscopy (HAXPES) revealed different depth of SEI dependent on the state of charge.[82]_{Lastly Kaghazchi et al. analyzed}

the content and ionic conductivity values of the SEI layer on a Sn anode stemming from a EC:DEC electrolyte solution using Raman spectroscopy and DFT simulations. Their work concluded that the SEI layer was composed primarily of Na2O and Na2CO3, with the Na2O

displaying much better ionic conductivity.[83]

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Computational efforts, which have been heavily used to understand other aspects of NIB materials, such as theoretical voltages and capacities, [84][85–87] have also been used to better understand SEI formation. However, the scope and complexity of the events occurring at the carbon/SEI interface/electrolyte make it difficult to run a comprehensive simulation, it is simply too computationally expensive. As such, most work has centered on investigating subsets of the SEI: diffusion rates, breakdown pathways, and other thermodynamic parameters.

The theoretical work that has been done includes one by Greeley et al. who showed that diffusion of Na-ions through a NaF SEI is slower than Li-ion diffusion through LiF using a set of DFT simulations. [88]_{Such results predicted the experimental results subsequently seen}

by Lin et al. as the Na ions showed a more unfavorable migration through the Na-based SEI. However, and this is the important point to keep in mind with theoretical work, the

environment assumed in the simulation, and the one present in the electrochemical system are not identical.

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Figure 5. Visualization from the MC/MD simulation by Nakagoa et al. displaying the

Lastly, Shenoy et al. also explored the topic of SEI formation, using DFT simulations to map out the breakdown pathways of typical electrolyte solvents, such as EC, and additives like FEC and vinyl carbonate (VC).[90]_{In their investigation, they found the decomposition}

pathway of EC and FEC to be through a ring opening reaction. Furthermore, it was found that the energy barrier for the FEC ring opening was smaller than that for the EC ring opening, thus suggesting that FEC molecules are among the first to decompose for the SEI formation. The early decomposition of the FEC, along with its subsequent generation of NaF, which acts as an electrode passivation agent, potentially explains why adding FEC provides additional stability during cycling, and potentially better CE@1st.

4.3. SEI mitigation

While characterizing the SEI, its composition, and its reaction pathways can be helpful, it does not directly address the knowledge gap necessary for commercial: SEI growth needs to be mitigated, both during the first cycle, and during the long-term cycling. There are already a few practical solutions to achieve this, smarter cycling protocols, more rational electrode design, electrolyte additives, and better binder combinations. However, each of these solutions either comes with prescient limitations, or has unclear benefits, thus leaving the field without a clear answer on how to effectively reduce SEI formation.

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Rationale electrode design is a more realistic option for limiting SEI formation, as it limits the tradeoffs needed to achieve the benefits. From LIB literature it has been suggested that carbon anodes with greater surface areas, and thus more defects, lead to more SEI formation, and as such, a lower CE@1st.[93,94] Though such comprehensive experiments have yet to be repeated for carbon materials in NIBs, a paper by Ji et al. did point to a similar effect, showing that increased surface area led to poorer capacities, along with decreased CE@1st_,

which was attributed to greater SEI formation.[95,96] However, the technique of minimizing surface area is not guaranteed to work for every type of anode chemistry. Anode chemistries that rely on alloying or conversion reactions are dependent on the inclusion of a high surface area/high porosity carbon material for both electrical conductivity and volume change

buffering. Thus, those types of anodes will incur low CE@1st, though such inefficiencies are necessary for the proper functioning of the anode.

Additionally, the correlation does not necessarily imply causation and in many cases, a low surface area can be linked to a low CE@1st, showing there is also the concept of non-SEI related efficiency losses. This was the subject of a paper by Mitlin et al. who elucidated the concept of ‘trapped’ Na-ions in carbon materials.[97] It was reasoned that the Na-ions form a strong enough bond with the carbon anode, and that the reaction is no longer

electrochemically reversible. This leads to a lower CE@1st without the formation of SEI. Co-incidentally, this effect is found to be magnified in high surface area materials, which can lead to the mistaking of trapped Na-ions to be a sign of SEI formation. The concept of such irreversibly trapped Na-ions was previously mentioned when discussing the computational studies: a highly favorable binding energy may be too high to allow a reversible reaction to happen.

The most popular method of dealing with SEI formation is through the addition of electrolyte additives, with the most well-known and popular one in the literature being the use of small mass % of FEC. The use of FEC as an electrolyte additive for NIBs was first reported by Komaba et al. in 2011.[76] Therein, the authors showed that the inclusion of 2 wt.% FEC to a propylene carbonate (PC) electrolyte resulted in much more stable long-term cycling. When increasing the FEC to 10 wt.%, the performance was found to be lowered. Furthermore, FEC was found to be the only electrolyte additive that has a positive effect on the cycling

performance, as the traditional LIB electrolyte additives, such as trans-difluoroetyhene carbonate (DFEC), ethylene sulfite (ES), and vinylene carbonate (VC) were found to be ineffective.

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that the addition of FEC caused a higher polarization on the electrode, which could be the sign of a thicker SEI layer. This could be because continuous SEI formation is more of an issue in alloying/conversion type anodes, or possibly due to some interplay between the carbon surface properties and the formation of the SEI.

The disparate findings on electrolyte additives, such as FEC, VC, and others, highlight one of the main problems of this research as it pertains to SEI: there is no clear answer as to whether the benefits are applicable to every type of anode chemistry. While the addition of FEC may seem helpful for one type of anode, it could also be that the anode was would have a better CE@1st, or would cycle better regardless of whether or not FEC was added. The same goes for all the other electrolyte additives. Additionally, in some cases, it has been shown useful to form thicker SEIs, has they have been claimed to improve cycling by reducing sodium inventory losses. [102] Thus, there are few clear solutions when it comes to understanding SEI and its long-term effects, Furthermore, it is difficult to compare different studies, as so many parameters vary in each one. However, this being a quantitatively based review paper, we tried anyways.

4.4. Quantitative analysis of SEI formation/mitigation

For the quantitative analysis of SEI formation/mitigation, we explore three questions. The first one probes whether the CE@1st is dependent on the type of anode chemistry. The second question looks at the effects of SEI formation when FEC is mixed in the electrolyte. Lastly, the third tries to analyze the effect of FEC addition on the IC obtained from long-term cycling.

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Figure 6. CE@1st as a function of anode chemistries. a) Stacked bar graphs grouped by CE@1st using anode chemistry as a factor. b) Stacked bar graphs grouped by anode chemistries using CE@1st as a factor.

Looking at Figure 6, we see that carbon-based anodes are disproportionately represented in the CE@1st_{group of less than 0.25. Furthermore, it appears that carbon materials and}

insertion materials habitually have the lowest CE@1st, as they have the lowest percentage of papers that report CE@1st greater than 0.50. Conversely, conversion anodes and alloying anodes seem to have the best CE@1st. This trend may not reflect the intrinsic properties of different chemistries. The cause of this may be due to the fact that many carbon materials, and insertion materials reported were made into nanostructured materials with large surface areas. In the case of organic anode material, too few data exist to make meaningful

statements.

Figure 7. Stacked bar graphs grouped by FEC using CE@1st as a factor for a) Carbon anodes. b) Alloying anodes. c) Conversion anodes. d) Insertion anodes.

(a) (b)

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Before going into the analysis of FEC and its effect on SEI formation in the first cycle, it should be noted that this simulation made the assumption that FEC addition was binary: either FEC was added, or it was not. The mass% of FEC that was added was not factored in, even though it may affect the cell processes, as demonstrated in a recent paper by Martinet et

al.[103]_{Additionally, organic anodes were not analyzed, as there was too few data present.}

Looking at the data, we can in fact see that the CE@1st for carbon anodes with FEC in the electrolyte is worse than electrolyte without FEC (Figure 7a). The same is true for insertion anodes. A much greater percentage of papers with carbon-based, and insertion anodes that do not use FEC report CE@1st of greater than 0.50, as opposed to the papers that use FEC. As such, this supports the claims made earlier by Shenoy et al.[90] that FEC is more reactive and breaks down faster. Though there is a chance this discrepancy could be due to the use of certain electrolyte solvents, the effect of FEC on carbon and insertion anodes seen here should not be discounted. For alloying and conversion anodes, the effects of FEC versus no FEC on the CE@1st seems to be minimal, and does not show significant improvement, or extremely negative effects, which can be taken as a sign of the different nature of SEI formation on alloying and conversion anodes (Figure 7b & 7c).

(a) (b)

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Figure 8. Capacity retention during long-term cycling versus IC, using FEC addition as a

binary factor for a) Carbon anodes. b) Alloying anodes. c) Conversion anodes. d) Insertion anodes.

In the case of whether FEC helps improve the long-term cycling parameters of the anode material, we can see some more definitive patterns (Figure 8). In the case of conversion, and insertion based anodes, FEC addition proves to be beneficial. For carbon anodes, the case is not as straightforward, mostly due to the lack of data on papers that did long-term cycling with FEC. Lastly, addition of FEC for alloying material also seems beneficial, though in the case of alloy there are too few data on electrolytes without FEC addition to do a meaningful comparison. Overall, the addition of FEC seems to be more helpful for long-term cycling, thus giving a tacit confirmation to the hypotheses suggesting FEC addition leads to a more stable SEI during long-term cycling, and as a result, better electrochemical performances.

4.5. Additional issues with SEI formation

While surface engineering can enhance CE@1st, and additives can improve long-term cycling, there are a few things to remember about SEI going forward. First off, SEI

formation, especially during the first cycle, is dependent on cell chemistry. Additionally, the pervasive belief that inefficiencies are solely due to SEI is not entirely true: carbon and insertion anode seem to be very adept at irreversibly trapping sodium ions. Lastly, the use of current organic solvents will result in SEI formation. This leaves two possible solutions: either find new solvents that are more cathodically stable, and do not degrade as readily, or find additives that limit the degradation of the current solvents. The former would be more ideal, but it is also the less realistic, though a recent study by Nohira et al. did find 90% capacity retention of 1000 cycles using an ionic liquid electrolyte.[50]

The latter solution would be using additives such as FEC in ways to create a more stable SEI as to improve the long-term cycling prospects. Because there is so many unknowns about the current state of the SEI both experimentally and computationally, this latter method does not have a very clear research direction and thus a high throughput experimental screening of electrolyte additives might prove to be appropriate in this instance.

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half-cell setup is typically a favorite of initial research, as it allows the research to focus solely on the properties of the material at hand. In the half-cell approach, it is assumed that the sodium metal is at best not reactive, and at worse negligibly reactive, with the electrolyte. However, this was shown to be untrue in studies by both Ponrouch et al.[104] and Ji et al.[105], as they showed that conventional carbonate-based solvents react with sodium metal to a quite significant extent. The study by Ji et al. went a step further to demonstrate that electrolyte degradation was in fact the main source behind the fading of the carbon anode in half-cells. Furthermore, it was also shown that this fading was able to be reversed if a new piece of sodium metal, along with fresh electrolyte, was added to couple the ‘faded’ carbon anode. This work shows NIB researchers two things: that the electrolyte, and its conditions, play a large role in determining the electrochemical properties of a battery, and that testing the long-term cycling viability of a material in a half-cell is inherently short-sighted due to the

problem with electrolyte degradation.

5. BINDERS

In addition to the electrolyte, which makes the electrochemical reaction possible, another important external component to the operation of a battery is the binder used to hold the electrode material onto the carbon collector. Unless the electrode material is a naturally self-standing material and can be used as a monolith anode, something will need to fix the electrochemically active mass to the current collector. Should the electrochemical material ever become detached from the current collector, it is rendered inactive, and can no longer access the electrons needed to make the storage reaction proceed.

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Figure 9. a) Types of binders used in anode materials for NIBs. b) Histogram of the mass%

of binders used. c) Binder type per anode chemistry. d) Mass % of binders per anode chemistry.

As we can see from the Figure 9, the most commonly used binder material in the literature is polyvinylidene fluoride (PVDF), which is usually included into the electrode slurry in 10 wt.% by mass. This is not too surprising, as the most commonly used binder for commercial LIB electrodes has been PVDF, though it typically comprises 5% or less of the total mass. Using 10 wt% is common amongst academic labs, but not industry. However, there are subgroupings within the literature, which bear more exploration.

(a) (b)

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One of the noticeable subgroupings occurs with alloying, and conversion type anodes. While the use of binders is proportionally the same for carbon and insertion type anodes, alloying and conversion anodes show to have a much higher use of the carboxymethyl cellulose (CMC) binder. This highlights the fact that electrochemical needs for different types of battery chemistry differ, and as such, the binder needed for the optimal electrode changes as well. Furthermore, the lower prevalence of PVDF for alloying and conversion anode should come as no surprise. In the case of LIBs, it is well known that PVDF binder is not conducive to the performance of alloying type anodes.[107]

Additionally, as for the mass percentage of the binder used, it is much different for alloy based anodes. A greater number of papers reported higher binder mass percentages for the alloy based anodes. This is due to the fact that, since alloys have a greater volume change upon sodiation than the other types of anodes, more binder is needed to prevent the

pulverization of the electrodes. Looking at the recent research, it is suggested that, like the case of the alloy and conversion materials, switching away from PVDF binder and towards ones, such as CMC or polyacrylic acid (PAA) might be promising.

5.1. Reactions with electrolyte to form SEI

First it is well known from the LIB literature that PVDF partially decomposes by reactions during electrochemical cycling, which negatively affects the CE@1st and long-term cycling of the battery.[108]_{This was the subject of a study by Villevieille et al. who found that the}

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Figure 10. Schematic representation of the breakdown of SEI formation due to the presence

of PVDF binder, with either no FEC present, or FEC present. Reproduced with permission.

One such early example was reported by Komaba et al. who demonstrated that the use of CMC binder leads to a much more stable long-term cycling as compared to PVDF.[110] Additionally, CMC has the advantage of being water soluble, which makes it easier and safer to process, as opposed to PVDF that is processed in more toxic and volatile N-Methyl-2-pyrrolidone (NMP). Komaba et al. [111] and Huang et al.[112] also made a similar observation through the use of a PAA binder with a Sn based anode, which also showed superior

performance, as compared to PVDF. In both papers, it was hypothesized that the PAA binder led to improved cycling due to the formation of a more stable passivation layer, as well as a lowered internal resistance. However, it should be noted that any improvements afforded by the use of a PAA binder are entirely dependent on the type of anode used. In the case of co-intercalating anode, Maibach et al. showed that PAA binders led poorer performances than expected.[113]

5.2. Importance of binders for alloying anodes

While it becomes clear that the impact of the binder choice is insignificant on cycle life for hard carbon materials, the case is different for alloying type anodes. In alloying type anodes, it has been shown that the choice in the binder material has an outsized effect on the

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collector, or the binder is so strong that it becomes restrictive of the particle growth, leading to cracking or an incomplete alloying process. Furthermore, given the poor electrical conductivity of the alloying particles, it is also critical that the binder be somewhat

conductive. The effect of the binder on the electrochemical performance of an alloying type anode was demonstrated by Liu et al. who show the drastic differences in capacity of an Sn based anode when using a PVdF, CMC and Poly(9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic ester) (PFM) binder.[114] The PFM binder, which is more conductive than the other two, shows the best electrochemical performance with the Sn based anode. Meanwhile the CMC binder shows an average performance, while the PVdF binder displays very little capacity when directly compared to the other two binders. Similar results were also

demonstrated by Sun et al. who showed the improved performance in conversion-type Fe2O3

and NiO based conversion anodes, both of which displayed enhanced cycling performance through the use of CMC binder.[115]

Figure 11. a) Cycling data with a Sn based anode comparing different binder types, where

Goodenough et al. also reported the use of a cross-linked chitosan binder for use with a Sb based alloy.[116] Instead of using CMC, the authors relied on chitin and its deacetylated derivative of chitosan, as the basis for the binder. Using a glutaraldehyde additive, they were able to cross link the linear carbohydrate polymer. Comparing the electrochemical

performance, the cross-linked binder showed a capacity retention of 96.5% at a current rate of 660 mA g-1_{over 100 cycles. Meanwhile the non-cross-linked binder only showed a retention}

of 81.3 % over the same number of cycles. Upon further investigation, it was concluded that

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the cross-linking led to a stronger mechanical integrity of the electrode during the volume changes, while the polymer material itself led to low polarization, thus enabling the formation of a stable SEI layer.

5.3. Quantitative Analysis

For the quantitative analysis regarding the binders, we focused on the relationship between the binder type, and the CE@1st, the same analysis, but also sectioned off by anode

chemistry, and lastly, the IC of the individual anode chemistries as a function of the binder type.

Figure 12. Stacked bar graphs grouped by a) CE@1st_{with binder type as a factor. b) Binder}

type with CE@1st as a factor.

From Figure 12, we can see good evidence that the selection of binder material matters for CE@1st. Papers that utilized either CMC, or no binder material were the most likely to display CE@1st greater than 0.75. By contrast, PVDF binders reported the lower percentage of CE@1st_{of greater than 0.50. Thus, while PVDF may be the most commonly used anode,}

it appears that it is not the most useful in order to achieve a high CE@1st_.

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(a) (b)

(c) (d)

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Further breaking down the CE@1st by binder type, and anode chemistry, we can gather a few additional observations. First off, for carbon materials, there is no clear evidence that the use of PVDF is better, or worse, than the use of CMC. However, self-standing carbon anodes do show a high percentage of CE@1st_{in excess of 0.50. In the case of alloy and conversion}

materials, there seems to be a slight advantage for self-standing anodes. The biggest apparent difference for alloy and conversion anodes is the inferiority of PDVF compared to all the other types of binders. Lastly, the insertion type of anodes show a surprisingly better

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Figure 14. Capacity retention during long-term cycling versus IC as a function of binder

material for a) Carbon anodes. b) Alloying anodes. c) Conversion anodes. d) Insertion anodes.

Analyzing the IC as a function of both anode chemistry, and binder type, we can make some tentative conclusions. In the case of the carbon anodes, the PVDF binder seems to work the best, though there is not as much data on other binders for a strong comparison. In the case of the alloy, conversion, and insertion anodes, the best binder for long-term cycling appears to be CMC. This is especially true with conversion and insertion anodes, where the PVDF is substantially worse. For the alloy anodes, there are a lot of examples of PVDF working well, though the densest cluster at the higher capacity retentions is still that of CMC. Self-standing anodes perform well for carbon materials, but do not show as clear of benefits for the three other types of anode chemistry.

When comparing these results with those from Figure 14, we can see somewhat of a pattern emerging. For carbon anodes, the binder with the best CE@1st was the self-standing anode – those with no binder. The self-standing anodes also seem to have a slight advantage in

(a) (b)

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capacity retention. In the case of the alloy and conversion anode, the best binder for a high CE@1st was the CMC, which also happens to be the best binder for the long-term cycling. Curiously, while the self-standing binder was also beneficial for the CE@1st in alloy and conversion anodes, it was not as useful for long-term cycling, most likely due to the increased structural stresses due to the large volume changes. Thus, solely looking at carbon, alloy, and conversion anodes, we can hypothesize that a binder favorable to a high CE@1st_{is likely to}

favor more stable long-term cycling – except in the case of self-standing alloying and conversion anodes. Unfortunately, in the case of insertion anode, no relationships between binder type and long-term cycling and CE@1st could be established. The CMC binder, which performs well for long-term cycling was not the best for CE@1st_{, while there was not enough}

data to make a meaningful comparison regarding the PAA binder. 5.4. Additional considerations

Concluding this section on binders, we would like to stress that binder should be considered as more than something with which to hold the electrode particles on the current collector. In the case of amorphous carbon anode, the use of binders such as PVdF can cause

inefficiencies in the 1st cycle, and lead to poor cycle life. Alternatives, such as CMC, polyacrylic acid (PAA) or sodium alginate should be considered. The world of polymer chemistry is rich with plenty of different polymers, some even conductive, which have yet to be tested as a binder for electrode materials. Additionally, the role of the binder material for alloying and conversion type anodes becomes even more important, as it directly plays in to the capacity of the anode material and can make all the difference between a failed anode and a functional one, even if the active material stays exactly the same.

6. CONCLUSIONS

In this review article, we hope to have demonstrated some of the additional complexities imbued in the development of NIB anode materials. While most of the community focuses directly on the materials, their structure-property relationships, and their electrochemical performances, a host of outside factors, such as electrolyte choice, SEI formation, and binder selection, can strongly impact the observed results.

By highlighting some of the literature, as well as performing our own quantitative analysis, we hope to leave the reader with a few conclusions. On the issue of electrolyte selection, we have shown that the selection of the solvent plays a greater role in the SEI formation/cycling inefficiencies than the selection of the salt. In short, the differences between NaClO4 and

NaPF6 are negligible, but those between EC:DEC and EC:PC are less so. Additionally, our

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For SEI formation, the amount of literature available on Na-based SEI formation is much less than what is available in the LIB literature, which limits our understanding. Furthermore, we have also revealed that carbon-based, and insertion-based anodes, show considerably lower CE@1st than alloy, and conversion based anodes. While this could be construed as evidence that the SEI formation chemistry is different in carbon, and insertion anodes, when compared to alloying, and conversion ones, it can also be taken as a sign that carbon and insertion anodes are more conducive to irreversibly trapping Na ions. This would have the same effect lowering the efficiency on the first cycle, while not forming SEI in the form of electrolyte breakdown products.

On the issue of FEC and SEI mitigation, we have shown that FEC helps little in increasing CE@1st for alloy and conversion anodes, while making it worse for carbon and insertion anodes. However, the inclusion of FEC strongly helps long-term cycling retention, as

measured through our devised metric of ‘integrated capacity’ in the case of alloy, conversion, and insertion anodes. These two observations taken together support the common hypothesis that FEC induces greater SEI formation, thus lowering CE@1st, but also forms a more stable SEI, which is beneficial for long-term cycling. Unfortunately, the inclusion of FEC as an electrolyte additive does little to improve the CE@1st or long-term cycling performance of carbon based anodes.

Regarding binder materials, we included literature that shows some of the deleterious effects of poor binder selection. Binders, while seemingly an innocuous part of the electrode, can have negative impacts by either reacting with the electrolyte, or being incompatible with the type of anode chemistry being used. Through the quantitative analysis, we demonstrated that CMC binders, and self-standing anode – those with no binders – were the most likely to obtain a high CE@1st. When the effects of binder selection were analyzed as a function of the anode chemistry, it was found that CMC binders offered the best capacity retention during long-term cycling for alloy, conversion, and insertion anodes. However, in the case of carbon anodes, CMC did not show any benefits for long-term cycling, and in fact seemed to perform worse than PVDF binders, and self-standing anodes.

Additionally, with the case of binder materials, the wealth of knowledge acquired in the LIB field should also be considered as a guide for future development. Whereas most of the binders for NIB based anodes revolve around PVdF, CMC, and PAA, there have been many more novel, and tailored binders in the LIB field. Such examples include the use of algae-based binders,[117] catecholic binders,[118] chitosan,[119] gel mixtures,[120] polyrotaxanes additives,[121] gum Arabic,[122] karaya gum,[123] and conductive binders.[124,125] While these binders are not expected to behave in the exact same manner during electrochemical cycling as their LIB counterparts, specific material property, such as conductivity, moduli, and