Electrolyte Design for Fast-Charging Li-Ion Batteries

Improving the rate capability of Li-ion cells will enable recharge times of electric vehicles that begin to rival the refuel times of internal combustion vehicles. Ionic transport in the electrolyte is an important contributor (among many factors) to the rate capability of energy dense Li-ion cells.The addition of low-viscosity co-solvents to conventional electrolytes is the most common method to improve ionic transport. Recent work moves to replace conventional electrolytes entirely with low-viscosity solvents. Typically, low-viscosity solvents affect the long-term stability of Li-ion cells, an issue that must be addressed.The Li-ion transference number t+ has a strong influence on high-rate charging. New advances aim to significantly improve t+ using concentrated electrolytes or polyanionic species.In recent years, there has been a resurgent effort to develop new methods to measure traditionally under-reported transport properties such as diffusivity and transference number.Beyond bulk ionic transport, interactions between electrolyte and solid electrolyte interphase (SEI) layers have a significant impact on fast charging. Characterization of the SEI layer is crucial and appropriate electrolyte additives must be chosen for fast charging applications. Li-ion batteries with fast charging capabilities will push the adoption of electric vehicles (EVs). The United States Department of Energy has stated goals of achieving “Extreme Fast Charging” (XFC) – charging to 80% capacity in 15 minutes or under – by 2028. The liquid electrolyte plays an extremely important role in achieving this goal. This review considers recent progress made in electrolyte development for high-rate charge applications. Approaches such as using low-viscosity co-solvents, highly concentrated electrolytes, and large polyanions are considered. New methods for measuring electrolyte transport properties are reviewed. The importance of electrolyte additives is also discussed. Finally, the importance of considering cell lifetime in the development of electrolytes for fast charge applications is stressed. Li-ion batteries with fast charging capabilities will push the adoption of electric vehicles (EVs). The United States Department of Energy has stated goals of achieving “Extreme Fast Charging” (XFC) – charging to 80% capacity in 15 minutes or under – by 2028. The liquid electrolyte plays an extremely important role in achieving this goal. This review considers recent progress made in electrolyte development for high-rate charge applications. Approaches such as using low-viscosity co-solvents, highly concentrated electrolytes, and large polyanions are considered. New methods for measuring electrolyte transport properties are reviewed. The importance of electrolyte additives is also discussed. Finally, the importance of considering cell lifetime in the development of electrolytes for fast charge applications is stressed. One of the major research objectives for Li-ion batteries is to push the boundaries of rate capability of Li-ion cells. Ever higher charging rates are especially desired in the electric vehicle (EV) industry, where it is believed that charging rates that start to rival the refuel time of internal combustion vehicles will mitigate so-called ‘range anxiety’ and push the widespread adoption of EVs. In 2018, the US Department of Energy announced over $19 million in funding for projects related to energy storage, including the area of ‘extreme fast charging’ (XFC), with EV-specific goals of including ‘charging in under 15 minutes or less by 2028’. Figure 1A compares various charging infrastructures, showing charging time to 200 miles as a function of charging power as a representative example, including a hypothetical 400 kW XFC charger, as of 2017. Charging Li-ion cells at high rates can cause a myriad of issues. Polarizations in the various materials (electrolyte, electrodes) of the cell will limit the available capacity in a charge. Large overpotentials in lithiated graphite during fast charge can cause the electrode potential to drop below 0 V versus Li+/Li0 and cause Li metal to be deposited [1.Liu Q. et al.Effects of electrolyte additives and solvents on unwanted lithium plating in lithium-ion cells.J. Electrochem. Soc. 2017; 164: A1173Crossref Scopus (34) Google Scholar], severely affecting cell lifetime. Kasnatscheew and coworkers presented an in-depth study of overpotentials in Li-ion battery positive electrodes and the subsequent effect on output capacity and energy, discussing the origins of such overpotentials [2.Kasnatscheew J. et al.Learning from overpotentials in lithium ion batteries: a case study on the LiNi1/3Co1/3Mn1/3O2 (NCM ) cathode.J. Electrochem. Soc. 2016; 163: A2943-A2950Crossref Scopus (38) Google Scholar]. Further, Li+ concentration gradients in the electrolyte phase are established when large currents are applied. In extreme cases, Li+ will be depleted at the back of the negative electrode, again limiting the capacity available under fast charging conditions. Concentration gradients and Li+ depletion will only be exacerbated as electrodes become thicker and less porous, which is desirable to increase cell energy density [3.Gallagher K.G. et al.Optimizing areal capacities through understanding the limitations of lithium-ion electrodes.J. Electrochem. Soc. 2016; 163: A138-A149Crossref Scopus (159) Google Scholar]. Developing Li-ion batteries capable of XFC requires careful design and coordination of all parts of the system, including the electrolyte, electrodes, pack design, thermal management systems, and related equipment. This review presents a focus on the role of the liquid electrolyte in enabling fast charge. Other works give excellent reviews of some of the other topics that are relevant to fast charging in Li-ion batteries. A collaboration from several US National Laboratories recently published a series of papers addressing the challenges and barriers to achieving XFC on scales ranging from the cell level to more broad infrastructure and economic challenges [4.Ahmed S. et al.Enabling fast charging – a battery technology gap assessment.J. Power Sources. 2017; 367: 250-262Crossref Scopus (56) Google Scholar, 5.Keyser M. et al.Enabling fast charging – battery thermal considerations.J. Power Sources. 2017; 367: 228-236Crossref Scopus (39) Google Scholar, 6.Meintz A. et al.Enabling fast charging – vehicle considerations.J. Power Sources. 2017; 367: 216-227Crossref Scopus (26) Google Scholar, 7.Burnham A. et al.Enabling fast charging – infrastructure and economic considerations.J. Power Sources. 2017; 367: 237-249Crossref Scopus (31) Google Scholar]. Liu and colleagues discuss electrode and separator materials design to enable XFC [8.Liu Y. et al.Challenges and opportunities towards fast-charging battery materials.Nat. Energy. 2019; 4: 540-550Crossref Scopus (59) Google Scholar]. Tomaszewska and colleagues discuss possible degradation mechanisms in fast-charge scenarios as well as difficulties above the cell level, including smart charging protocols and the importance of thermal management systems [9.Tomaszewska A. et al.Lithium-ion battery fast charging: a review.eTransportation. 2019; 1: 100011Crossref Google Scholar]. Zhu and colleagues review recent advances in both positive and negative electrode materials for fast-charging applications [10.Zhu G. et al.Fast charging lithium batteries: recent progress and future prospects.Small. 2019; 151805389Crossref Scopus (14) Google Scholar]. Recent work by Colclasure and colleagues identified ionic transport in the electrolyte as one of the main barriers to achieving XFC in Li-ion batteries, noting the need for electrolytes with greatly improved transport properties [11.Colclasure A.M. et al.Requirements for enabling extreme fast charging of high energy density Li-ion cells while avoiding lithium plating.J. Electrochem. Soc. 2019; 166: A1412-A1424Crossref Google Scholar]. An alternative approach to obtain XFC is to heat cells prior to charging, as recently promoted by Yang and colleagues [12.Yang X. et al.Asymmetric temperature modulation for extreme fast charging of lithium-ion batteries asymmetric temperature modulation for extreme fast charging of lithium-ion batteries.Joule. 2019; 3: 3002-3019Abstract Full Text Full Text PDF Scopus (0) Google Scholar]. This review focuses on the importance of the electrolyte in enabling and facilitating fast charge in Li-ion cells. Approaches to improving electrolyte transport are discussed, including adding low viscosity co-solvents to improve conductivity and techniques for increasing the Li-ion transference number in solution. Methods for measuring nontrivial transport properties in Li electrolytes are also discussed. Finally, a perspective is presented on the trade-off between rate capability and lifetime in Li-ion batteries. Traditionally, the metric used to quantify a given electrolyte’s ability to transport Li+ (or any other ion) is the ionic conductivity, κ [13.Xu K. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries.Chem. Rev. 2004; 104: 4303-4417Crossref PubMed Scopus (3458) Google Scholar]. This is primarily because of the simplicity by which it can be measured in the laboratory, which is not true of other relevant transport properties, as will be discussed later. Figure 1B shows conductivity versus salt concentration for a typical electrolyte system consisting of LiPF6 in ethylene carbonate (EC):dimethyl carbonate (DMC) in a ratio of 3:7 (w/w) as an unbroken line. The dashed line indicates the desired conductivity for a fast charging application. The most common and effective approach for improving conductivity is to replace the solvents in the base system, either in part or as a whole, with solvents that better facilitate ionic transport. These solvents typically have lower viscosities than the carbonate solvents DMC and ethyl methyl carbonate (EMC), which are typically considered ‘low viscosity’ with respect to EC. In contemporary works, EC is almost always included in the electrolyte as it has traditionally been considered indispensable due to its favorable passivating properties [13.Xu K. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries.Chem. Rev. 2004; 104: 4303-4417Crossref PubMed Scopus (3458) Google Scholar,14.Xu K. Electrolytes and interphases in Li-ion batteries and beyond.Chem. Rev. 2014; 114: 11503-11618Crossref PubMed Scopus (1673) Google Scholar]. However, as will be discussed later, EC is not necessarily required when sufficient ‘enabling’ additives are present. One of the most common class of solvents used alongside typical carbonate electrolytes is aliphatic esters [e.g., methyl acetate (MA) and ethyl acetate (EA)]. Ein-Eli and coworkers first used methyl formate (MF) alongside EC in a report in 1997, showing an improvement in low-temperature cycling down to –40°C [15.Ein-Eli Y. et al.Li-ion battery electrolyte formulated for low-temperature applications.J. Electrochem. Soc. 1997; 144: 823-829Crossref Scopus (57) Google Scholar]. Using esters MA and EA as co-solvents to traditional Li-ion battery electrolytes was first investigated by Smart and colleagues and Herreyre and colleagues [16.Smart M.C. et al.Electrolytes for low-temperature lithium batteries based on ternary mixtures of aliphatic carbonates.J. Electrochem. Soc. 1999; 146: 486-492Crossref Scopus (193) Google Scholar,17.Smart M.C. et al.Improved low-temperature performance of lithium-ion cells with quaternary carbonate-based electrolytes.J. Power Sources. 2003; 119–121: 349-358Crossref Scopus (108) Google Scholar]. Different esters were added alongside binary or ternary carbonate mixtures to investigate low-temperature performance (down to –40°C in some cases) in Li-ion cells [18.Smart M.C. et al.Use of organic esters as cosolvents in electrolytes for lithium-ion batteries with improved low temperature performance.J. Electrochem. Soc. 2002; 149: A361Crossref Scopus (130) Google Scholar,19.Herreyre S. et al.New Li-ion electrolytes for low temperature applications.J. Power Sources. 2001; 97–98: 576-580Crossref Scopus (66) Google Scholar]. The enhancement in ionic conductivity achieved by adding ester co-solvents improved low-temperature performance, but the authors also note that the ‘low molecular weight (MW)’ (i.e., short chain) esters did not show favorable solid electrolyte interphase (SEI) (see Glossary) formation on graphite electrodes, suggesting that cell lifetime may be affected by using such ester co-solvents. In more recent years, other groups have employed ester co-solvents to improve high-rate charge or low-temperature performance in Li-ion cells. Methyl butyrate (MB), when combined with electrolyte additives, showed improved low- and high-temperature performance to a standard electrolyte blend [20.Smart M.C. et al.The effect of additives upon the performance of MCMB∕LiNixCo1−xO2 Li-ion cells containing methyl butyrate-based wide operating temperature range electrolytes.J. Electrochem. Soc. 2012; 159: A739Crossref Scopus (0) Google Scholar]. Smart and colleagues tested an even wider range of ester co-solvents, finding that methyl propionate (MP) affords an ideal compromise between ionic conductivity and stability in full cells [21.Smart M.C. et al.Lithium-ion electrolytes containing ester cosolvents for improved low temperature performance.J. Electrochem. Soc. 2010; 157A1361Crossref Scopus (67) Google Scholar]. More recently, Hall and colleagues investigated the performance of several different co-solvents, including esters, as well as other classes of co-solvents such as formates, nitriles, and amides as co-solvents in an EC:EMC:DMC solvent blend in NCA/graphite (Gr)-SiO cells using high-temperature storage and ultra-high precision coulometry tests [22.Smith A.J. et al.Precision measurements of the coulombic efficiency of lithium-ion batteries and of electrode materials for lithium-ion batteries.J. Electrochem. Soc. 2010; 157: A196Crossref Scopus (195) Google Scholar,23.Hall D.S. et al.Exploring classes of co-solvents for fast-charging lithium-ion cells.J. Electrochem. Soc. 2018; 165: A2365-A2373Crossref Scopus (14) Google Scholar]. Many of these co-solvents were not compatible with the cell chemistry, showing poor storage performance and coulombic efficiency (CE), but MF was identified as a co-solvent that had decent cell performance combined with a good increase in conductivity. Figure 2 shows a comparison of many candidate solvents considered by Hall and colleagues in terms of their electrochemical stability windows (E0calc), dielectric constants (ε), and viscosities (η). Work by Ma and colleagues showed the value of esters MP, EA, and MB, and MA as co-solvents in NCA/Gr-SiO and Li[Ni0.5Mn0.3Co0.2]O2 (NMC 532)/Gr cells [24.Ma L. et al.A guide to ethylene carbonate-free electrolyte making for Li-ion cells.J. Electrochem. Soc. 2017; 164: A5008-A5018Crossref Scopus (46) Google Scholar,25.Ma X. et al.A study of highly conductive ester co-solvents in Li[Ni0.5Mn0.3Co0.2]O2/graphite pouch cells.Electrochim. Acta. 2018; 270: 215-223Crossref Scopus (8) Google Scholar]. This work showed that the use of ester co-solvents greatly improved the rate capability of these cells, however, the improvement was highly dependent on the ratio of carbonate:ester used. High-rate performance was also dependent on the electrolyte additives used. Li and colleagues characterized the lifetime of NMC532/Gr cells using 20% or 40% MA in the electrolyte, studying cycling performance at different temperatures, upper cut-off voltages, and with different additive systems [26.Li J. et al.Methyl acetate as a co-solvent in NMC532/graphite cells.J. Electrochem. Soc. 2018; 165: A1027-A1037Crossref Scopus (10) Google Scholar]. This work showed conclusively and systematically the reduction in cell lifetime that is associated with using low-MW esters in Li-ion cells. Others have studied the effect of different electrolyte solvents on the high-rate and low-temperature performance of Li-ion cells. Cho and colleagues investigated the use of acetonitrile (AN), propionitrile (PN), and butyronitrile (BN) as co-solvents and achieved cycles at rates of 3C at –20°C in NCA/Gr cells where conventional electrolytes failed [27.Cho Y.G. et al.Nitrile-assistant eutectic electrolytes for cryogenic operation of lithium ion batteries at fast charges and discharges.Energy Environ. Sci. 2014; 7: 1737-1743Crossref Google Scholar]. Recently, Hilbig and colleagues studied the optimal composition of a BN:EC:fluoroethylene carbonate (FEC)-based electrolyte to maximize ionic conductivity. An electrolyte composed of 1 M LiPF6 in BN:EC 9:1 + 3% FEC showed superior high-rate performance (5C) compared with carbonate-based electrolytes. The inclusion of some amount of EC and FEC allowed a stable SEI layer to form on the graphite electrode [28.Hilbig P. et al.Butyronitrile-based electrolytes for fast charging of lithium-ion batteries.Energies. 2019; 12: 2869Crossref Scopus (1) Google Scholar]. Petibon and colleagues used esters EA and MP as the sole electrolyte solvent in NMC 111/Gr cells. These solvents were ‘enabled’ by adding ~5% of an SEI-forming additive, with vinylene carbonate (VC) being used in this case. The MP-VC electrolyte performed better at 4C and above than an EC:EMC-based electrolyte [29.Petibon R. et al.The use of ethyl acetate and methyl propanoate in combination with vinylene carbonate as ethylene carbonate-free solvent blends for electrolytes in Li-ion batteries.Electrochim. Acta. 2015; 154: 227-234Crossref Scopus (20) Google Scholar]. Other investigations into different co-solvents for better rate capability include using adiponitrile (ADN) alongside EC [30.Isken P. et al.High flash point electrolyte for use in lithium-ion batteries.Electrochim. Acta. 2011; 56: 7530-7535Crossref Scopus (73) Google Scholar] and 3-methoxypropionitrile (MPN) as the sole solvent with LiTFSI salt [31.Wang Q. et al.3-Methoxypropionitrile-based novel electrolytes for high-power Li-Ion batteries with nanocrystalline Li4Ti5O12 anode.J. Electrochem. Soc. 2004; 151: 1598-1603Crossref Scopus (40) Google Scholar]. Adding low-viscosity co-solvents to conventional electrolytes remains one of the easiest and most effective methods of improving rate capability while still retaining much of the stability that is enjoyed in carbonate electrolytes. Initial reports, as well as more recent work on the use of low viscosity esters and formates to improve rate capability, have noted reduced stability compared with conventional blends of carbonate solvents (e.g., EC, EMC, and DMC). In many cases this has been attributed to these species forming SEI components with poor passivating properties. However, the authors note that in conventional electrolyte blends, it is EC as well as electrolyte additives that passivate the graphite electrode rather than the linear carbonates EMC or DMC. Indeed, in an electrolyte containing only a linear carbonate with no EC or other electrolyte additive, very poor performance is observed in full cells [32.Petibon R. et al.Electrolyte system for high voltage Li-ion cells.J. Electrochem. Soc. 2016; 163: A2571-A2578Crossref Scopus (38) Google Scholar]. Therefore, to enable the use of low-MW esters and formates in a more practical application, an approach similar to what is used in so-called ‘EC-free’ electrolytes is recommended. In these electrolytes, ~5% by weight of ‘enabling’ additives are included to passivate the electrodes on the first charge. Such electrolytes have been shown to perform extremely well in high-voltage Li-ion cells [32.Petibon R. et al.Electrolyte system for high voltage Li-ion cells.J. Electrochem. Soc. 2016; 163: A2571-A2578Crossref Scopus (38) Google Scholar]. While the lifetime issue in using low-viscosity esters and formates likely cannot be completely overcome, it can be mitigated as much as possible by using enabling additives, much like what is done in EC-free electrolytes. Recently, it has been demonstrated that improved rate capability can be achieved using highly concentrated electrolyte solutions. Yamada and colleagues showed improved rate performance using a 3.6 M LiN(SO2F)2 (LiFSI) in 1,2-dimethoxyethane (DME) electrolyte [33.Yamada Y. et al.A superconcentrated ether electrolyte for fast-charging Li-ion batteries.Chem. Commun. 2013; 49: 11194-11196Crossref PubMed Scopus (0) Google Scholar]. With typical salt concentrations (1.0 M), electrolytes in DME do not allow reversible intercalation of Li+ in graphite electrodes, but using the highly concentrated electrolyte mitigated the issue [34.Yamada Y. et al.Electrochemical lithium intercalation into graphite in dimethyl sulfoxide-based electrolytes: effect of solvation structure of lithium ion.J. Phys. Chem. C. 2010; 114: 11680-11685Crossref Scopus (83) Google Scholar]. Yamada and colleagues credit a high transference number, low polarization at the electrolyte/graphite interface, higher concentrations of Li+ at the interface, and an excellent ion conducting SEI for the improved rate capability of this concentrated electrolyte. Yamada and colleagues adopted this approach in a concentrated AN/LiFSI electrolyte, again showing improved rate performance compared with carbonate-based electrolytes [35.Yamada Y. et al.Unusual stability of acetonitrile-based superconcentrated electrolytes for fast-charging lithium-ion batteries.J. Am. Chem. Soc. 2014; 136: 5039-5046Crossref PubMed Scopus (363) Google Scholar]. One may assume that the increase in viscosity and corresponding decrease in conductivity (Figure 1) from the addition of large amounts of salt will impede Li+ transport in highly concentrated electrolytes. If the solvation structure is unchanged and the motion of Li+ remains primarily vehicular in nature, this will be true. This understanding makes the improved rate capability observed in the AN/LiFSI electrolyte, for example, seem counterintuitive. However, recent work suggests that alternative Li+ solvation structures and transport mechanisms arise in highly concentrated electrolytes. While in conventional dilute solutions, the Li ion and corresponding anion of the Li salt primarily exist as solvent separated ion pairs (SSIP). However, as the salt concentration is increased into the highly concentrated regime (>2.0 M), contact ion pairs and higher order agglomerates will begin to form (Figure 3). This alternative solvation structure may give rise to an alternative Li+ conduction mechanism described as a ligand exchange reaction where the cation diffuses by a ‘cascade’ of association and disassociation reactions, leading to fast Li+ conduction in these electrolytes [36.Okoshi M. et al.Theoretical analysis of carrier ion diffusion in superconcentrated electrolyte solutions for sodium-ion batteries.J. Phys. Chem. B. 2018; 122: 2600-2609Crossref PubMed Scopus (8) Google Scholar]. Similar structures have also been studied in even more highly concentrated ‘water-in-salt’ electrolytes [37.Borodin O. et al.Liquid structure with nano-heterogeneity promotes cationic transport in concentrated electrolytes.ACS Nano. 2017; 11: 10462-10471Crossref PubMed Scopus (68) Google Scholar]. Alternative SEI compositions due to the high degree of coordination of solvent molecules as well as the abundance of Li salt anions in superconcentrated electrolytes may also impact the rate capability [38.Zheng J. et al.Research progress towards understanding the unique interfaces between concentrated electrolytes and electrodes for energy storage applications.Adv. Sci. 2017; 4: 1-19Crossref Scopus (117) Google Scholar]. Figure 3A shows an illustration of the differences between the solution structure in the dilute regime and the superconcentrated regime and Figure 3B shows the corresponding improvement in rate capability in the concentrated LiFSI/AN electrolyte. A recent review of highly concentrated electrolytes stressed that there is a lack of understanding and adequate experimental evidence of the true Li+ conduction mechanism in concentrated solutions, indicating the need for more work in this area [39.Yamada Y. et al.Advances and issues in developing salt-concentrated battery electrolytes.Nat. Energy. 2019; 4: 269-280Crossref Scopus (9) Google Scholar]. From a more practical perspective, it is noted that unpublished studies of this particular electrolyte in the authors’ laboratory show that it leads to unsafe cells and therefore researchers need to exercise extreme care. Beyond the work by Yamada and coworkers, several others have found that concentrated electrolytes enable excellent rate capability in Li-ion cells [40.Nakanishi A. et al.Sulfolane-based highly concentrated electrolytes of lithium bis(trifluoromethanesulfonyl)amide: ionic transport, Li ion coordination and Li-S battery performance.J. Phys. Chem. 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These electrolytes start with a typical superconcentrated electrolyte formulation such as LiFSI/DMC, but then are ‘diluted’ by an inert low-viscosity solvent [43.Chen S. et al.High-voltage lithium-metal batteries enabled by localized high-concentration electrolytes.Adv. Mater. 2018; 301706102Crossref PubMed Scopus (109) Google Scholar,44.Yu L. et al.A localized high-concentration electrolyte with optimized solvents and lithium difluoro(oxalate)borate additive for stable lithium metal batteries.ACS Energy Lett. 2018; 3: 2059-2067Crossref Scopus (35) Google Scholar] that does not participate in solvation. These LHCEs retain the unique solvation structure of highly concentrated electrolytes, but with a lower solution viscosity. While work on this electrolyte system is primarily focused on enabling Li metal anodes, some work has indicated similar rate performance to more typical highly concentrated electrolytes [45.Ren X. et al.Localized high-concentration sulfone electrolytes for high-efficiency lithium-metal batteries.Chem. 2018; 4: 1877-1892Abstract Full Text Full Text PDF Scopus (67) Google Scholar]. Much more work is required to fully understand the ion transport mechanism and rate capability with this class of electrolytes. The transference number defines the fraction of ionic current carried by a given ion in an electrolyte, with the sum of positive and negative transference numbers adding up to one; t+ + t− = 1. For Li-ion batteries, it is desirable that the transference number for Li+ be as close to 1 as possible, allowing Li+ to carry as much of the ionic current as possible. However, in carbonate electrolytes t+ is typically around 0.35–0.4 [46.Valøen L.O. Reimers J.N. Transport properties of LiPF6-based Li-ion battery electrolytes.J. Electrochem. Soc. 2005; 152: A882Crossref Scopus (403) Google Scholar]. It has been shown that the Li-ion transference number of the electrolyte has a significant influence on rate capability [46.Valøen L.O. Reimers J.N. Transport properties of LiPF6-based Li-ion battery electrolytes.J. Electrochem. Soc. 2005; 152: A882Crossref Scopus (403) Google Scholar, 47.Doyle M. et al.The importance of the lithium on transference number in lithium/polymer cells.Electrochim. Acta. 1994; 39: 2073-2081Crossref Scopus (265) Google Scholar, 48.Diederichsen K.M. et al.Promising routes to a high Li+ transference number electrolyte for lithium ion batteries.ACS Energy Lett. 2017; 2: 2563-2575Crossref Scopus (105) Google Scholar]. Figure 1C shows the effect of transference number t+ on the accessible state of charge versus charging rate (C-rate) in a system with constant conductivity (details can be found in [48.Diederichsen K.M. et al.Promising routes to a high Li+ transference number electrolyte for lithium ion batteries.ACS Energy Lett. 2017; 2: 2563-2575Crossref Scopus (105) Google Scholar]). Even for electrolytes that have relatively low conductivities, an improvement in the transference number can significantly improve the rate capability of cells. However, for high-rate charge applications, both a high transference number and high ionic conductivity are desired. The reason for typically low transference numbers in carbonate-based electrolytes is a result of the large solvation shells that form around Li+, making the effective radii of these ions quite large [48.Diederichsen K.M. et al.Promising routes to a high Li+ transference number electrolyte for lithium ion batteries.ACS Energy Lett. 2017; 2: 2563-2575Crossref Scopus (105) Google Scholar]. The most popular and widely adopted approaches to improve t+ are to: (i) use concentrated electrolytes as discussed briefly earlier [49.Suo L. et al.A new class of solvent-in-salt electrolyte for high-energy rechargeable metallic lithium batteries.Nat. Commun. 2013; 4: 1-9Crossref Scopus (1100) Google Scholar], or (ii) increase t+ by replacing the common Li salts (e.g., LiPF6 or LiBF4) with salts that have larger ‘bulky’ anions [50.Berhaut C.L. et al.LiTDI as electrolyte salt for Li-ion batteries: transport properties in EC/DMC.Electrochim. 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