Recent progress in electrolyte design for advanced lithium metal batteries

Lithium metal batteries (LMBs) have attracted considerable interest for use in electric vehicles and as next‐generation energy storage devices because of their high energy density. However, a significant practical drawback with LMBs is the instability of the Li metal/electrolyte interface, with concurrent parasitic reactions and dendrite growth, that leads to low Coulombic efficiency and poor cycle life. Owing to the significant role of electrolytes in batteries, rationally designed electrolytes can improve the electrochemical performance of LMBs and possibly achieve fast charge and a wide range of working temperatures to meet various requirements of the market in the future. Although there are some review papers about electrolytes for LMBs, the focus has been on a single parameter or single performance separately and, therefore, not sufficient for the design of electrolytes for advanced LMBs for a wide range of working environments. This review presents a systematic summary of recent progress made in terms of electrolytes, covering the fundamental understanding of the mechanism, scientific challenges, and strategies to address drawbacks of electrolytes for high‐performance LMBs. The advantages and disadvantages of various electrolyte strategies are also analyzed, yielding suggestions for optimum properties of electrolytes for advanced LMBs applications. Finally, the most promising research directions for electrolytes are discussed briefly.


| INTRODUCTION
2][3][4] The use of graphite with high capacity (375 mAh/g), low potential, and less volume change during the charge/ discharge process confers LIBs high energy density and long lifespan.][7][8][9][10] In addition to high energy density, high-performance energy storage together with long lifespan, fast charging capability, 11 wide range of working temperatures, good safety, and low cost are needed for a range working conditions, Figure 1. 12 The lifespan of current LIBs is ca.10-15 years in small electronics and ca.8-10 years in EVs. 13 Harsh working conditions accelerate decay in battery performance, leading to frequent battery replacements and high costs.Long lifespan batteries decrease maintenance costs.It is desired, therefore, that the next-generation energy storage devices have lifespans equivalent to or greater than that for LIBs.Fast charging capability is a core performance parameter for rechargeable batteries, especially for EVs. 14,15Conventional EVs take some hours to be fully charged, certainly longer than that for filling internal combustion engine vehicles with fuel.For practical applications, it is important, therefore, to have EV batteries that can be fully charged in <0.5 h or even a few min.Energy storage devices are also important in the oil industry and deep-sea and aerospace exploration, therefore, there is a required for batteries with wide range of working temperatures. 16,17However, current LIBs lose significant capacity and power when the temperature is <0°C because of high ion-transport resistance and a low reaction rate. 18Accelerated side reactions at elevated temperatures also lead to cell failure, resulting in poor cycling performance and decreased safety. 19These drawbacks will need to be addressed to promote advancement of next-generation energy storage devices.
Li metal is an almost "ideal" anode that has therefore received considerable research attention 20 because of a high capacity of 3860 mAh/g and the lowest chemical species redox potential of 3.04 V versus a standard hydrogen electrode. 21Lithium metal batteries (LMBs) with high energy density have been promising for nextgeneration energy storage; however, development has been limited due to the instability of the Li metal/ electrolyte interface, with concurrent parasitic reactions and dendrite growth [22][23][24] during battery cycling, resulting in poor Coulombic efficiency (CE) and reduced cycle life.Large dendrites can pierce the separator, leading to short circuit, thermal runaway, and even explosion. 25Poor electrochemical performance and reduced safety, therefore, limit the application of Li anodes.
The electrolyte is an essential component in batteries; it acts as the media for ion transport between positive and negative electrodes.Highly reactive Li metal reacts with most liquid electrolytes, consuming Li + ions.7][28][29][30] The electronically insulating and ionic conductive SEI prevents direct contact between the electrolyte and Li, resulting in stable Li stripping/plating.The microstructure, morphology, and chemical composition of SEI are closely related to the electrolyte.2][33][34][35] By tuning the electrolyte via selection and optimization of solvents, Li salts, and electrolyte additives, electrochemical performance, including the wide range of working temperatures 36,37 and fast charging capability, 38,39 is significantly improved.Therefore, high-performance LMBs can be developed via the targeted design of electrolytes.
In this review, we focus on methods for the design of electrolytes for advanced LMBs.

| ACHIEVING LONG LIFESPAN LMBs
LMBs show a shorter lifespan than LIBs.In LIBs, the conventional electrolyte component, ethylene carbonate (EC), decomposes at the graphite anode and forms stable SEI, resulting in a high CE.The Li anode also reacts with nonaqueous electrolytes to form SEI; however, large volume change during charge/discharge causes SEI fracture.Dendrite formation during battery cycling also results in the destruction of the SEI layer on the Li metal anode.Dendrites form because of sluggish Li + ion diffusion that leads to uneven Li electrodeposition. 40igher electric field at the tips tends to attract additional Li + , resulting in further growth of protrusions, finally evolving into dendrites.The volume change and dendrites cause a destruction of the original SEI and more "native" Li is exposed to the electrolyte.As a result, the electrolyte reacts with Li, resulting in continuous consumption of the electrolyte, a "thickened" SEI layer, and large internal resistance, and thus decreases CE.This continues until the battery fails operationally.Therefore, formation of a stable SEI is important to ensure stable Li stripping/plating and long lifespan in LMBs.

| Formation of a robust SEI
SEI is usually derived from Li + , solvent, and anion decomposition.Anions include fewer C atoms and more S, F, and P, which likely form inorganic compounds with high interfacial energy to passivate the Li anode and boost performance. 39,41,42Therefore, targeted methods have been reported for the formation of anion-derived SEI.
The degree of fluorination impacts the solvation structure.The partially fluorinated, asymmetric -CHF 2 group showed more electron negativity to enable strong intermolecular interactions in the solvent and better Li + solvation than an all-fluorinated, symmetric counterpart, -CF 3 . 32The F5DEE electrolyte with partial fluorination concurrently dissociates Li salt and has high ionic conductivity, fast and stable interfacial transport, high Li metal efficiency of up to 99.9% within Li||Cu half-cells, fast activation with CE > 99.3% from the second cycle in Li||Cu half-cells, and high voltage stability.Fluorination is a practically useful method to decrease the dielectric constant (ε), which significantly impacts the dissociation of Li salt. 63With lower ε, the salt fails to dissociate F I G U R E 3 (A) Raman spectra of solvents and electrolytes and schematic diagrams of Li + coordination structures in different electrolytes.Cycling stability of Li||Cu half-cells using different electrolytes at (B) 0.5 mA/cm 2 and (C) 1 mA/cm 2 .Reproduced with permission. 61Copyright 2022, Springer Nature.
completely, forming less SSIP and more CIPs and AGGs.Unlike hydrofluoroethers, which have limited salt solubility, the introduction of -F in the ether electrolyte balances solvation ability and oxidation stability and optimizes the solvation shell to form anion-derived SEI.However, current research on fluorinate-ethers is ongoing and most fluorinate-ethers are not commercially available.The complex chemical synthesis is not costeffective and is therefore a drawback for large-scale application.Ionic conductivity is determined by the number of SSIP and free anions.Fluorination of solvents can weaken the solvated interaction between Li + and solvents, leading to less SSIP and low ion conductivity.Therefore, an optimal balance between the solvation structure and ion conductivity is needed for improved battery performance.

| Additives for SEI
The introduction of other anions mediates the solvation structure, which leads to preferred anion decomposition.7][68] With the exception of NO 3

−
, other electrolytes with different anions, including LiTFSI/LiFSI (additives), 69 LiFSI/LiClO 4 , 70 and LiTFSI/LiD-FOB, 38,71 influence the Li + solvation environment to achieve better electrochemical performance.The electrophilic anion receptor that assists Li salts to dissociate influences the Li + solvation environment.For example, tris(pentafluorophenyl)borane (TPFPB) with an electrondeficient boron atom, can directly interact with FSI -. 72 The LUMO energy level for FSI -is reduced following interaction with TPFPB through B-F and B-O interactions, showing that the reduction stability of FSI − in TPFPB-LHCE is reduced.Therefore, decomposition of FSI − is promoted.3 The suspension carbonate electrolyte induces inorganic-rich SEI, resulting in an improvement in CE to ~99.7%.Although nanoparticles including Li 2 O, 73 LiF, 74 metal-organic framework, 75 and SiO 2 76 are insoluble in common electrolytes, the presence of a solid in the electrolyte influences the local solvation environment, usually near the particle surface.These insoluble particles can be used as multifunctional additives, specifically for oxidation stability and as fire retardants. 74Particle size has a significant impact on electrolyte stability.Larger microparticles can "fail" to disperse sufficiently and generate precipitates in suspension; the resulting inhomogeneous electrolyte leads to unstable electrochemical performance.The addition of particles increases viscosity, and absorption between Li + and particles decreases ion diffusion and de-solvation.These additives improve performance by modifying the solvation environment; however, another type of additive "sacrifices" itself to form a stable SEI in the Li anode.For example, FEC usually shows the lowest LUMO in common electrolytes because of the strong electro-withdrawing property of F-containing functional groups.8][79][80] Similar to FEC, lithium difluorophosphate (LiPO 2 F 2 ) has lower LUMO compared with EC and DMC, which shows that LiPO 2 F 2 is reduced before the solvent. 81,82The introduction of LiPO 2 F 2 can generate LiF-enriched SEI to boost electrochemical performance.Vinylene carbonate (VC) is a widely recognized and used functional electrolyte additive in LIBs and LMBs.The polymerictype species decomposed from VC generates surface films on the Li anode to suppress dendrites. 836][87][88][89] Compared with nanoparticle additives and anion receptor additives, "sacrificing" additives are those most reported.However, because these are consumed during cycling, the protection mechanism can fail following long cycling.A comparative summary of CE in selected electrolytes is presented in Table 1.
CE is an important parameter for the electrochemical performance of LMBs.However, at laboratory scale, CE is not an accurate predictor of the lifespan of batteries because Li is usually in excess in LMBs.Side reactions consume Li + and reduce CE; however, excess Li can be used to finish charge/discharge and boost CE.This is one contributor to LMBs at CE values close to 100% short unexpectedly and without warning signs.To accurately determine CE, it is necessary to follow the strict criterion of limited Li (less N/P ratio) and a lean electrolyte.
Parasitic reactions between the electrolyte and the cathode can also have a highly significant impact on lifespan.However, different cathodes show differing charge/discharge mechanisms, accompanied by different side reactions, for example, nickel-rich layered transition-metal oxide cathode materials including Li-Ni x Co y Mn z O 2 (NCMxyz) and sulfur cathode.

| Preventing Al corrosion in high-voltage cathodes
Usually, cathodes are obtained via casting of active material onto a current collector.Al with high electroconductivity and low cost is widely used as a cathode current collector.Although Al has a redox potential of 1.364 V (vs.Li + /Li), the Al 2 O 3 derived via air can generate a dense, protective layer, leading to stable cycling in LMBs. 94However, Al dissolution will occur on using an NCM cathode, which has high charge/discharge potential. 95,96In commercial LIBs, trace amounts of water hydrolyze LiPF 6 to produce HF, which subsequently reacts with Al to form a stable AlF 3 film on the surface and passivate Al. 94 However, with LiTFSI, Al 3+ will interact with TFSI -to form highly soluble Al(TFSI) 3 in the electrolyte, leading to worse current collector corrosion. 97l corrosion destroys the integrity of the electrode and influences electron transfer between the current collector and active material to decrease the electron conductivity of the electrode and lead to fast capacity decay.Because the choice of Li salt has a significant impact on Al corrosion, new Li salts, including LiBOB 98 and LiD-FOB, 99,100 are reportedly used to suppress Al corrosion.However, these anticorrosion salts have limited solubility and poor conductivity in carbonate and ether electrolytes, limiting their application.Use of a noncorrosive salt, lithium (difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide (Li[N(SO 2 CF 2 H)(SO 2 CF 3 )], LiDFTFSI), as the main salt in electrolytes has been reported. 101The LiDFTFSI-based carbonated electrolyte exhibited 3.7 mS/cm ionic conductivity and high anodic stability of 5.6 V versus Li + /Li.In contrast to a typical LiTFSIbased electrolyte, the LiDFTFSI-based electrolyte has good compatibility with the Al current collector at high potential of ca.4.2 V versus Li + /Li because unstable Al (DFTFSI) 3 decomposes to form AlF 3 with strong passivation ability, protecting the Al current collector from further anodic dissolution.Therefore, there is better cycling stability in coin cells using a LiDFTFSIbased electrolyte.With current LMBs and LIBs, there are a significant number of electrolytes, but the choice of Li salts for LMB application is limited.Therefore, exploration of new Li salts is needed.However, it is important to optimize cost, safety, and electrochemical performance.
HCEs are useful for Al protection.LiFSI/DMC-based HCEs suppress Al corrosion, even at >5 V (vs.Li + /Li), while continuous Al dissolution occurs in the dilute LiFSI/DMC electrolyte at 4.3 V (vs.Li + /Li) that leads to "unlimited" overcharging, Figure 5. 48 In LiFSI/DMCbased LHCEs, there is reduced pitting of the Al current collector and CE is boosted. 54A B L E 1 Coulombic efficiency (CE) of Li||Cu in selected nonaqueous electrolytes.3][104] Similar to SEI in the anode, CEIs form on cathodes and impact the performance of secondary batteries.Currently, an optimized CEI is obtained using additives such as LiBOB, 105 LiDFOB, 106 lithium difluorobis(oxalato) phosphate (LiDFBOP), 107 and LiPO 2 F 2 108 that prefenertially oxidize at the cathode surface to form robust and stable CEI composed of inorganic-rich components.The highest occupied molecular orbital (HOMO) of VC is higher than that for EC, respectively, for VC and EC: −10.26 and −12.76 eV.VC is more readily oxidized at the positive electrode surface than EC, preferentially forming a passive layer on the electrode. 109,110FEC is reported to be a useful additive to boost the formation of CEI because the decomposition of FEC yields an F-rich passivated layer. 111These additives improve SEI and promote the formation of CEI because additives containing F, B, and P elements will eventually decompose F-, B-, and P-containing compounds to isolate and prevent direct contact between the electrolyte and the electrode.However, from another point of view, additives facilitate the formation of CEI and SEI via different mechanisms.For FEC, a lower LUMO promotes preferential reduction to form LiF-enriched SEI; however, FEC as a nonsacrificial additive boosts CEI stability via modification of the solvation structure. 112It is also possible that the compounds formed in the compact and thin CEI layers on the cathode diffuse from the anode side. 113

| Preventing shuttle effect in Li-S cells
Sulfur cathodes with high capacity and high energy density have been reported as next-generation cathodes.Because of nucleophilic side reactions between carbonate electrolytes and lithium polysulfide (LiPS) species, ether electrolytes are commonly used in Li-S cells. 114,115However, LiPS has high solubility in ether electrolytes, leading to fast capacity fade and decreased CE.In recent decades, researchers have focused on cathode modification to prevent the shuttle effect caused by high LiPS solubility. 116,117However, rational electrolyte design can also suppress the shuttle effect to induce higher capacity retention.
In HCEs and LHCEs, there is limited LiPS solubility, which significantly suppresses the shuttle effect to improve cycling stability.The 7 mol/L LiTFSI in dioxolane (DOL)/DME electrolyte prompts anion-derived SEI against formation of Li dendrites that boosts Li-S cells to exhibit excellent electrochemical performance and high cycling stability. 118Acetonitrile (ACN) is an important oxidation-tolerant organic solvent with high ionic conductivity.Despite its poor reduced stability, ACN-based HCEs can form stable SEI to show excellent cycling performance. 119ACN-based LHCEs electrolytes show limited solubility of LiPS, leading to an excellent CE and improved capacity retention.In LHCEs, TTE has been demonstrated to participate in the reactions with both electrodes to form passivated films that prevent parasitic reactions and therefore improve utilization of active material and CE through mitigating LiPS dissolution and boosting conversion kinetics from polysulfides to Li 2 S/Li 2 S 2 . 120,121ecause of the high solubility of LiPS in ether electrolytes, use of novel solvents with less LiPS solubility is a practically feasible method to boost Li-S performance.A suitable choice is ionic liquid (IL), which has low solubility for LiPS.For example, LiPS has poor solubility in solvated ILs, which leads to high CE and a longer cycle life in Li-S batteries. 122Compared with the 30% selfdischarge rate in ether-based electrolytes, a battery using a LiNO 3 -containing [PP13][TFSI]-based electrolyte shows zero self-discharge because the weak Lewis acid/basic properties of the [PP13][TFSI] inhibit the dissolution of LiPS. 123Use of weakly solvated solvents can suppress LiPS solubility to improve cycling stability.There is poor solubility with LiPS in the DOL/DME electrolyte with low polar methyl propyl ether (MPE) as a co-solvent. 124Li-S batteries using a DOL/DME/MPE electrolyte show improved cycling stability, with a capacity retention of 71.5% following 200 cycles and a high average CE of 99.44%, which is significantly greater than those of the conventional DOL/DME electrolyte of, respectively, 46.9% and 98.06%, Figure 6A,B.In low polar diethyl ether electrolytes, there is also lower solubility of LiPS, leading to improved electrochemical performance. 37However, blindly reducing the solubility of LiPS will not boost the electrochemical performance.Compared with SL (donor number [DN] = 14.8) and DOL/ DME (DME, DN = 20; DOL, DN = 17), dimethyl sulfoxide (DMSO) with greater DN (DN = 30) exerts a strong solvation effect of Li + and high solubility for LiPS, resulting in greater nucleation overpotential and a significant shuttle effect in the DMSO electrolyte. 126In an SL electrolyte with low DN, the poor conductively discharged product Li 2 S x with less solubility would precipitate, and break away from the conductive network leading to poor active materials utilization.For DOL/ DME with intermediate DN numbers, LiPS shows proper solubility, balancing nucleation of the Li 2 S x particles and the solubility of LiPS, leading to better discharge capacity.Therefore, proper solubility for LiPS is important for Li-S cells to boost the utilization of active materials.
Additionally, electron-insulated Li 2 S/Li 2 S 2 is insoluble in the electrolyte, leading to poor capacity retention. 127ollowing long cycling, volume expansion of the S cathode will destroy the integrity of the cathode and cause Li 2 S/Li 2 S 2 to detach from the conductive network, which decreases the utilization of active materials and drastically reduces the discharge capacity.A practical alternative method to solve this is to make Li 2 S/Li 2 S 2 soluble, increasing the utilization of active materials.Deep eutectic solvents (DESs) consisting of caprolactam (CPL) and acetamide show the dissociation of Li 2 S because of the strong interaction(s) between S 2− and amide hydrogen (N-H) and between Li + and carbonyl oxygen in the DES, as is shown in Figure 6C,D.Li-S batteries with this "new" electrolyte show a capacity of 1360 mAh/g at 0.1 C with 94.8% capacity retention following 40 cycles, Figure 6E. 125DES electrolytes are safe and low cost, which represent major advantages in large-scale applications.However, DES can dissolve metal oxide; 128 therefore, it can cause the dissolution of the current collector, leading to reduced cycling stability.
Because the selection of cathodes impacts the lifespan of batteries, there needs to be different focus for future research application.For high-voltage cathodes, Al corrosion and CEI decomposition will require increased attention.Additional methods will need to be developed to prevent the shuttle effect in Li-S batteries.

| ACHIEVING FAST CHARGE
In LMBs, charge transfer at the Li anode can be conveniently divided into the following steps: (1) Li + transfers from the cathode to the anode in the electrical field, (2) Li + de-solvates at the anode surface, and (3) desolvated Li + receives electrons and forms Li.The electrochemical reaction rate is controlled by concentration polarization, Step 1, and electrochemical polarization, Steps 2 and 3, as shown in Figure 7A. 23Because electron transfer is fast, rapid Li + transport and facile desolvation are key to achieving fast charge in LMBs. 131

| High ion conductivity
High ion conductivity is a prerequisite for fast charge.Many simple Li salts, for example, Li 2 O, LiCl, and LiF, have limited solubility in nonaqueous solvents, because of which high ion conductivity cannot be achieved.LiPF 6 has low dissociation energy and good solubility in carbonate solvents, resulting in good ionic conductivities at ambient temperature, for example, 10.8 mS/cm for 1 mol/L LiPF 6 EC/DMC at 25°C.0 Copyright 2020, American Chemical Society.(C) Digital photograph of different molarity electrolyte droplet contact angles on the S cathode.Reproduced with permission. 129Copyright 2020, American Chemical Society.(D) Hopping mechanism in the SL electrolyte.Color code: Li = purple; oxygen = red; carbon = gray; anion = light blue; and sulfur = yellow.Reproduced with permission. 130Copyright 2019, American Chemical Society.DME, dimethoxyethane; DOL, dioxolane.and soluble in organic solvents, showing adequate ionic conductivity of 12.16 mS/cm for 1 mol/L LiTFSI DOL/ DME at 25°C, high thermal stability, and resistance toward hydrolysis.However, LiTFSI-based electrolytes are corrosive toward Al current collectors, leading to poor cycling stability.Given the cost and electrochemical performance, LiPF 6 is the "first choice" in many commercial LIBs.Solvents need to have a high ε to sufficiently dissociate Li salts to form SSIP and free anions to yield high ionic conductivity.Therefore, the ion conductivity of an aqueous electrolyte is greater than that of a nonaqueous electrolyte because of the greater ε for water.Designed electrolytes should have low viscosity to facilitate ion transport.Compared with other solvents, carbonate and ether solvents have low viscosity for rapid ion transport and greater ε for dissociation of Li salts.Therefore, these are used widely in practical research. 26he viscosity of HCEs is greater than that for commonly used electrolytes (1 mol/L), leading to lower ion conductivity, Figure 7B.Although less Li salt dissolved in electrolyte provides lower ion conductivity, dilute electrolytes, including 0.1 mol/L LiTFSI in DOL/DME, show required ion transport in LMBs. 129The 0.1 mol/L LiTFSI DOL/DME electrolyte with low viscosity and excellent wetting on sulfur cathodes (Figure 7C) shows significantly greater rate capability and better cell stability than the standard 1 mol/L electrolyte.However, many SSIPs in dilute electrolytes cannot generate anion-derived SEI; fewer Li + ions easily induce a concentration gradient and accelerate the growth of dendrites.

| High Li + transference number
Charge transfer is accomplished via transport of Li + and anions in electrolytes.Because of differing radii, cations and anions have different transport rates, leading to different transference numbers for cations and anions.A smaller ionic radius for Li + exerts strong Coulombic attraction toward solvent molecules, leading to a solvation sheath around Li + ions.Therefore, Li + always migrates with a solvation sheath, causing sluggish Li + transport. 26Current liquid electrolytes usually have a Li transference number (t Li+ ) < 0.5.In HCEs, despite high viscosity, fewer SSIPs lead to a high t Li+ = 0.73. 118In SLbased HCEs, where two oxygen atoms of the sulfonyl group coordinate with two neighboring Li + and TFSI -to form ionic clusters, as shown in Figure 7D, Li + shows a hopping diffusion mechanism in which Li + are transferred from one coordinating site (on either SL or anions) to another vacant site through ligand exchange in the labile Li + coordination chains.Therefore, there is a high t Li+ = 0.5 in the SL electrolyte. 130Despite the wide electrochemical window and excellent thermal stability in IL electrolyte, the cations and anions in IL compete with Li + /anions from electrolytes for charge transportation, resulting in a lower value of t Li+ .For zwitterionic, for example, a borate-type zwitterion-LiTFSI electrolyte 132 , and pyrrolidinium zwitterionic 133 , in which anion and cation groups are on the same monomer unit, ion transport of cations and anions in IL is limited, leading to a higher value of t Li+ compared with traditional IL. "Immobilizing" anions are used to improve t Li+ ; however, this method is useful only in (quasi) solid-state electrolytes, or single ionconductive electrolytes to yield a high t Li+ . 134,135owever, improving t Li+ in liquid electrolyte is a less researched field.

| Facile de-solvation for fast charge
The Li + will de-solvate at the anode surface, then Li + receive an electron and finish plating.The strong solvation effect in the electrolyte causes sluggish desolvation and greater nucleation barrier, leading to greater overpotential and poor rate performance during charge/discharge. 126However, in de-solvation research, a commonly used electrode is obtained by slurry coating on an Al current collector, which shows nonuniformity in density, porosity, and tortuosity, that impacts mass transport in the electrolyte.Therefore, it is difficult to understand the reason for improved performance is promoting mass transport or facilitating de-solvation.Use of a single-particle electrode in a three-electrode electrochemical cell can obviate the influence of mass transfer.The TFSI − anion preferentially solvates Li + rather than PF 6 − ; however, this results in a lower Li + binding energy that provides for more facile de-solvation and faster interfacial kinetics.Exchange current densities are therefore ca. 100 greater in electrolytes containing the TFSI − anion compared with those containing PF 6 − alone.
At a high rate, the sluggish interfacial kinetics cause poor rate performance in the LiPF 6 electrolyte of ~75 mAh/g at 10 C, compared with the LiTFSI + LiPF 6 electrolyte of ca.125 mAh/g at 10 C. 136 Facile de-solvation is not directly related to ion conductivity.An ACN-based HCE with lower ionic conductivity than a dilute electrolyte allows ultrafast charging in Li||graphite half-cells because the Li binding energies decrease in HCEs, leading to facile de-solvation. 39ast charge will be necessary for the anticipated future demand for EVs.Although high ion conductivity, a high t Li+ , and facile de-solvation are necessary for fast charge, the significant Li dendrites originating from greater current also represent a major challenge in fast charging LMBs.Therefore, formation of a stable SEI is also a prerequisite for fast charge.

| ACHIEVING WIDE WORKING TEMPERATURE
Wide working temperatures represent an important parameter for advanced batteries.However, changes in temperature have a significant impact on ion transport and interfacial reaction kinetics, which impose strict design requirements for electrolytes.

| Achieving low-temperature electrolytes
Sluggish ion transport at low temperatures causes sluggish electrochemical kinetics, greater overpotential, and reduced electrochemical performance.At an ultralow temperature (<−20°C), electrolytes undergo a phase change from liquid to solid, reducing ion conductivity.Therefore, use of a solvent with a lower melting point (T m ) can increase ion conductivity at low temperatures to boost low-temperature electrochemical performance. 137,138

| Antifreezing solvent and liquefied gas electrolyte
Carbonate and ether electrolytes are commonly selected for LMBs.Compared with ether electrolytes (DOL, T m = −95°C; DME, T m = −58°C), carbonate electrolytes show higher T m (EC, T m = 34°C; propylene carbonate, T m = −55°C; ethyl methyl carbonate, T m = −55°C), which is not beneficial for boosting lowtemperature electrochemical performance. 139Antifreeze solvents with lower T m can improve ionic conductivity at low temperatures and boost the electrochemical performance. 37Diethyl ether (DEE) has lower T m (−117°C) and can maintain high ion conductivity at low temperatures.On use in LMBs with a high-loading sulfurized polyacrylonitrile cathode and an N/P capacity ratio of 1, batteries with a DEE electrolyte retain 84% and 76% of room-temperature (25°C, RT) capacity when cycled at, respectively, −40°C and −60°C.Liquefied gas shows ultralow viscosity and ultralow T m , which makes it an ideal solvent for stable Li deposition at low temperature as shown in Figure 8A.With the use of liquified fluoromethane (FM) as an antifreezing solvent for low-temperature LMBs, no Li dendrites with a high CE of 97% can be practically achieved at 25°C in LMBs (Figure 8B) and improved capacity, Figure 8C,D. 140here is low solubility of Li salt in liquefied gas solvents (0.2 mol/L LiTFSI in FM), resulting in lower ionic conductivity.With ACN and THF additives that can dissociate LiTFSI well, liquefied gas electrolytes show greater salt solubility (1.2 mol/L LiTFSI in ACN-FM) and excellent ionic conductivity of 5.8 mS/cm at −60°C and 4.8 mS/cm at −78°C. 92,141Therefore, LMBs with 1.2 mol/L LiTFSI-AN-FM electrolyte can deposit at −60°C with a high CE of 98.4% without dendrites.Using 1,1,1,2-tetrafluoroethane and pentafluoroethane as safe liquefied gases, the electrolyte maintains >3 mS/cm ionic conductivity from −78°C to 80°C. 142

| Facile de-solvation boosts low-temperature performance
Although traditional ether electrolytes show high ion conductivity at low temperatures, they do not show better performance at low temperatures.For example, at temperatures below 0°C, there is a large Li nucleation barrier and an uneven Li deposition in the DOL/DME electrolyte. 143This reported finding indicates that the low-temperature design of battery systems is more complex than simply ensuring an electrolyte with adequate ionic conductivity at low temperatures.At low temperatures, a strong de-solvation barrier becomes the rate-limiting step, causing poor Li deposition dynamics, as shown in Figure 9A. 145Therefore, it is practically significant to facilitate Li + de-solvation at low temperatures.
Because solvents directly govern the de-solvation energy, selection of a solvent with low T m and weakly solvated power leads to stable Li deposition at low temperatures.In DEE with low T m (−116°C), Raman spectra and MD findings show that significant AGGs exist in the DEE electrolyte, confirming that DEE is a solvent with weak solvated ability. 37In contrast, the DOL/DME electrolyte shows strong interaction between the solvent and Li + .A stronger de-solvated barrier in the DOL/DME electrolyte increases the local charge-transfer impedance, leading to poor low-temperature electrochemical performance.The DEE-based electrolyte shows lower de-solvation energy, facilitating interface reaction kinetics.There is a high CE in coin cells with a DEEbased electrolyte at low temperatures.Dimethoxymethane (DMM) shows the anomeric effect, which thermodynamically favors a gauche-gauche conformation that cannot chelate Li + , leading to weak solvated ability, as shown in Figure 9B,C. 146Lower de-solvation energy promotes more uniform Li deposition and high plating/stripping efficiency in the DMM electrolyte at low temperatures, as shown in Figure 9D. 144dditionally, a reported method to facilitate desolvation at low temperatures involves the use of LHCEs. 147With LHCEs, the inert co-solvent usually has low ε that cannot influence the Li + solvation environment in HCEs.In solvents with high ε, highly polar molecules boost the dipole-dipole force, leading to high T m .Use of an inert dilute co-solvent breaks the strong interaction between the highly polar molecules and decreases the T m value for the electrolyte, widening the liquid-phase range.CIPs and AGGs in LHCEs promote facile de-solvation and generate anion-derived SEI.Therefore, LHCEs show excellent low-temperature electrochemical performance.4.2 mol/L LiFSI-FEC/methyl(2,2,2-trifluoroethyl) carbonate (FEMC) dissolves in diluted solvent 1,1,2,2tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane (D2), forming 1.28 mol/L LiFSI in FEC/FEMC/D2 with wide liquid range from −125°C to 70°C.This electrolyte shows high ion conductivity of >1 × 10 −2 mS/cm at −80°C. 36Based on quantum chemistry computations, introduction of FEC/FEMC into a nonpolar solvent D2 reduces the solvation energy.Therefore, there is better electrochemical performance at low temperatures.Other LHCEs also show better low-temperature performance; for example, LMBs with SL-based LHCEs can cycle at −10°C. 90

| Designed high-temperature electrolyte
Traditional ether and carbonate electrolytes are volatile, leading to high pressure inside batteries cycled at high temperatures.Long cycles at high temperatures cause battery expansion and even explosion.Therefore, selected nonvolatile solvents have been reported as electrolytes for LMBs at high temperatures.ILs with nonvolatility and high thermal stability are highly suitable for high-temperature LMBs. 16For example, the phosphonium IL trihexyldecylphosphonium bis (trifluoromethanesulfonyl)imide/1.6 mol/L LiTFSI electrolyte has a high anodic stability of 5 V (vs.Li + /Li) at 100°C together with stable cycling stability. 148High temperature decreases the viscosity of the IL electrolyte, promoting ion conductivity.The DESs are also not volatile and have good thermal stability.The Li||LiMn 2 O 4 cell with a LiTFSI/Nmethylacetamide/FEC-based DES polymer electrolyte showed a capacity retention of 86.1% following 200 cycles at 0.2 C and satisfactory cycling stability at elevated temperatures. 149Although IL and DES electrolytes are advantageous, including wide electrochemical window and high thermal stability, these do not stabilize the Li anode to promote stable Li deposition and lead to poor cycling stability.The high viscosity also limits application in LMBs.

| Designed wide working temperature electrolyte
Synthesis of an electrolyte with wide working temperature is practically difficult because the electrolyte should not only have wide liquid range but also mitigate the impact caused by high and low temperatures on SEI/CEI.Several methods have been reported for achieving LMBs with wide working temperature.

| Liquefied gas electrolytes for wide temperature ranges
Because liquefied gases use pressurized hermetically sealed cells, they maintain the liquid phase at high temperatures.Liquefied gas electrolytes show excellent conductivity of >4 mS/cm over a temperature range of −78°C to 75°C, leading to a high CE of 99.4% and stable cycling of 4.5 V (vs.Li + /Li) LMBs in the temperature range of 55°C to -60°C, as shown in Figure 10A. 141Liquefied 1,1,1,2-tetrafluoroethane and pentafluoroethane electrolytes show stable cycling of Li|| NMC622 batteries from −60°C to 55°C. 142However, liquefied gas electrolytes are only suitable for special needs because pressurized hermetically sealed cells increase the cost significantly and decrease portability and flexibility.

| LHCEs for wide operating temperature
LHCEs show facile de-solvation at low temperatures and modify the solvation structure of Li + to generate inorganic-enriched SEI, which maintains stability at high temperatures.Therefore, LHCEs are practically suitable for LMBs with wide working temperature.LHCE (1.28 mol/L LiFSI in FEC/FEMC/D2) shows high ionic conductivity in the temperature range −125°C to 70°C. 36he Li||LiNi 0.8 Co 0.15 Al 0.05 O 2 battery using D2-LHCEs at −85°C shows 56% of RT capacity.The LHCE of 1.4 mol/L LiFSI in DMC/EC/TTE enables NCM811based LIBs to show excellent cycling stability at RT and 60°C, good rate capability under fast charging and discharging up to 3 C, and superior low-temperature discharge performance down to −30°C with a capacity retention of 85.6% at 0.2 C. 151 Although the addition of dilute hydrofluoroether solvents decreases the viscosity and T m for electrolytes and facilitates anion-derived SEI formation, volatile hydrofluoroethers limit the ultrahigh temperature (>100°C) performance of LHCEs.

| Additives for electrolytes with wide working temperature
A change of temperature impacts SEI/CEI.The organic component in SEI/CEI is not stable and decomposes at elevated temperatures, creating a porous SEI/CEI layer instead of a film SEI/CEI coated on the electrode surface that exposes more lithium/cathode material to the electrolyte and leads to electrolyte decomposition. 152,153ow temperatures cause uneven deposited Li morphology and a rough surface of the Li anode, leading to unstable CE. 143,145 Therefore, synthesis of stable SEI/CEI is important for practical achievement of LMBs with low, high, and wide operating temperature.Use of additives is a practically useful method to synthesize stable SEI/CEI to obtain LMBs with wide working temperature.Li-S batteries with FEC as additives in 6.5 mol/L LiTFSI electrolyte show outstanding cycle performance over temperatures ranging from −10°C to 90°C. 154This superior battery performance is attributed to the LiFrich SEI formed on the Li metal anode that suppresses the continuous growth of Li dendrites.Carbonate electrolyte containing a LiPO 2 F 2 additive shows low T m and high boiling point and improved cycle ability and rate capability over the temperature range of −40°C to 90°C. 150FEC and lithium difluorobis(oxalato) phosphate (LiDFOP), 155 lithium oxalyldifluoroborate (LiODFB), 156 LiDFBOP, 157,158 and LiBOB 159,160 can boost electrochemical performance over a wide temperature range as shown in Figure 10B-D.Table 2 presents a comparison summary of selected LMBs working at various temperatures.
In future design of LMBs with low, high, and wide working temperature, in addition to the electrolyte, other factors should also be considered, such as the cathode, lithium salt, additives, separator, and binder.Because these will all influence battery performance, a change in temperature will magnify the effects and impact the electrochemical performance and lifespan.

| IMPROVING ENERGY DENSITY
Energy density is dependent on the capacity and the operational voltage.Currently, the method used to practically improve energy density is to increase the electrode capacity but boost the cut-off voltage to widen operational voltage window, which can also improve the energy density. 161,162For example, boosting the cut-off voltage of the LiCoO 2 cathode to 4.6 V (vs.Li + /Li) leads to a ca.40% improvement in the specific energy density. 163However, current traditional electrolytes with lower anodic stability (1 mol/L LiPF 6 in EC ~4.3 V vs. Li + /Li, 1 mol/L LiTFSI in DOL/DME < 4.0 V vs. Li + /Li) are not suitable for high voltage cathode, limiting any further increase in energy density.There are several methods to improve the anodic voltage of electrolytes.

| High anodic stability solvents
The electrochemical window for solvents govern the operational range of electrolytes.Therefore, replacing ether and carbonate solvents with solvents of intrinsic wide electrochemical window improves the anodic stability of electrolytes.The sulfonyl group with strong electronwithdrawing ability shows low HOMO, boosting oxidation stability; therefore, sulfone solvents show greater anodic stability. 26,164Sulfonamide-based electrolytes show higher anodic voltage of >4.7 V (vs.Li + /Li).LiCoO 2 cathodes with this electrolyte retain, respectively, 89% and 85% of capacity following 200 and 100 cycles at cut-off voltages of 4.55 V (vs.Li + /Li) and 4.6 V (vs.Li + /Li). 165The 1 mol/L LiFSI/ N,N-dimethyltrifluoromethane-sulfonamide (DMTMSA) electrolyte enables stable cycling of NCM811 cathodes at 4.7 V (vs.Li + /Li) with a high specific capacity of >230 mAh/g and an average CE of >99.65% over 100 cycles. 166Fluorinated sulfone solvents are also synthesized for high-voltage cathodes. 167Following fluorination, the sulfone solvents have excellent anodic stability of >6.0 V (vs.Li + /Li). 168However, fluorination leads to lower solubility of Li salt and lower ion conductivity; the use of a co-solvent to dissociate Li salts will decrease the anodic stability.Therefore, the high solubility of Li salt in high anodic stability solvents is important.SL with high voltage stability of >5.0 V (vs.Li + /Li) leads to high solubility of Li salt, which makes it a suitable choice for high-voltage electrolytes. 49The 1 mol/L LiFSI SL-based electrolyte shows a high decomposition voltage of >5 V (vs.Li + /Li) as shown in Figure 11A, improving cycling stability and capacity retention in the graphite||LiNi 0.5 Mn 1.5 O 4 full cell. 49Ls and DESs with wide electrochemical window are suitable for high-voltage electrolytes.Piperidinium and pyrrolidinium ILs possess a high anodic stability of >5.5 V (vs.Li + /Li). 170,171Solvated IL obtained by directly mixing tetra(ethylene glycol) dimethyl ether with LiTFSI (1:1, molar ratio) has higher anodic stability of 5 V (vs.Li + /Li) as shown in Figure 11B because of the donation of lone pairs of ether oxygen atoms to the Li + cation, resulting in the decrease of the HOMO energy level of a glyme molecule. 169For dual-anion DES based on succinonitrile (SN) and LiTFSI/LiDFOB, no apparent redox peaks are detected in the range 0-5 V (vs.Li + /Li), indicating its wide electrochemical window. 172Table 3 presents the oxidation stability and electrochemical performance of selected electrolytes.However, these solvents have high viscosity, resulting in poor ion conductivity and reduced wettability toward electrodes and separators.For phosphate solvents, ILs, and DES, the reduced ability to form stable SEI limits large-scale application.The high viscosity of these solvents also leads to poor electrochemical performance.
Introduction of F into an ester or ether results in a decrease in energy levels for both HOMO and the LUMO.The lower HOMO leads to higher resistance against oxidation. 26The fluorinated DTDL electrolyte shows high oxidation stability up to 5.5 V (vs.Li + /Li) as shown in Figure 11C. 61Following fluorination, the FDMB electrolyte shows higher oxidation stability of >6 V (vs.Li + /Li) compared with 4 V (vs.Li + /Li) for the DMEbased electrolyte.

| HCEs for high anodic stability
HCEs can boost anodic stability, and likely the best example is the "water in salt" electrolyte, which expands the narrow electrochemical window of the aqueous electrolyte (1.23 V) to >3.0 V. 177 Following solvation with Li + , HOMO of the solvent molecules decreases, leading to stronger resistance against oxidation.However, with common electrolytes (1 mol/L), free solvent molecules are predominant and there is limited improvement in oxidation stability.With an increase in the salt concentration, the amount of free solvent molecules decreases, leading to high oxidation stability.The concept of "solvent in salt" in various organic solvents, including carbonate 48 and ether, 118 confirms that high salt concentrations can practically boost oxidation stability. 90Additionally, in LHCE, because diluted solvent also show high anodic stability, LHCEs maintains stability against oxidation. 90,176

| IMPROVING THE SAFETY OF LMBs
Low risk and high safety are design prerequisites for LMBs.Common risks in LMBs are caused by gas evolution and thermal runaway. 178

| Gas evolution mechanism
Gas evolution occurs due to oxygen release from cathode materials and decomposition of the electrolyte. 179A high degree of delithiation at high voltage causes structural destruction; then, lattice oxygen release accelerates chemically oxides electrolytes.Electrolyte decomposition at the electrode is the main reason for gas evolution.In the anode, EC, which is the carbonated electrolyte that is used the most, is reduced via a one-electron ring-opening reaction to form a lithium alkyl carbonate radical that dimerizes to form lithium ethylene decarbonate and releases ethylene gas, Figure 12A. 180The reduction of DMC proceeds via a one-electron reaction forming a lithium alkyl carbonate radical that decomposes into lithium methyl carbonate (LMC) and ethane gas, Figure 12A.LMC can react in two ways: it can (1) decompose to release CO 2 gas and form lithium methoxide and (2) react with water to form a mixture   181,183 In LiTFSI, the CF 3 SO 2 − fragment decomposes to CF 3 − and SO 2 , and then the trifluoromethyl interacts with H to form CF 3 H. 184 Accumulation of oxygen and flammable gases leads to a significant volume change in cells, especially with pouch cells, leading to poor electrode contact and decreased CE.Electrolyte decomposition is inevitable; a useful strategy is to construct stable CEI/SEI at initial cycling, avoiding further decomposition of electrolytes.Electrochemical mass spectrometry can be used to obtain detailed data regarding gas species and amounts.Differential electrochemical mass spectrometry enables the detection of species in real time, to quantify gas evolution and concurrently determine cell potential and cell current.Internal shorting causes overheating of batteries or thermal runaway.This is a dangerous phenomenon because the flammable carbonate/ether electrolytes will act as fuel, with the oxygen released and the high temperature providing conditions for combustion and explosion. 152,185,186Therefore, nonflammable solvents and flame-retardant additives are used to improve battery safety.

| Fluorinated solvent for safe LMBs
Because C-F is more stable than C-H and C-O, fluorination improves thermal stability and nonflammability.In addition,  200 Nanoparticles are also used as fireretardant additives including LiF nanobox. 74Because nonflammable solvents as electrolytes cannot produce stable SEI to show better cycling stability, use of fireretardant additives in traditional electrolytes to balance cycling stability and safety is the best choice.

| LOW-COST ELECTROLYTES IN LMBs
Cost is a significant design factor in the large-scale application of LMBs.Although IL electrolytes show hightemperature performance, nonflammability, and high stability against oxidation, their high-cost limits practical large-scale applications.DES is a better cost choice.A drawback, however, is that the electrochemical performance of DES electrolytes needs to be improved.Fluorinated ether electrolytes improve stability against oxidation and modulate the Li + solvation environment to show better cycling performance.However, a complex synthesis and purification increases the cost.HCEs have F I G U R E 13 (A) Flammability tests for 1 mol/L and TMP-based high-concentration electrolyte.Reproduced with permission. 187opyright 2018, Royal Society of Chemistry.(B) Thermal stability and flammability test for ionic liquid and conventional ester electrolyte.Reproduced with permission. 93Copyright 2020, Wiley-VCH.(C) Chemical structures for the TFA-based electrolyte.Flammability test results for the traditional electrolyte (EL) and TFA-based EL of 1 mol/L LiPF 6 in PC/TFA and self-extinguishing time (SET).Reproduced with permission. 188Copyright 2021, Wiley-VCH.DEC, diethyl carbonate; EC, ethylene carbonate; TMP, trimethyl phosphate.
been reportedly successful at high voltage, a wide range of temperatures, high cycling stability, and fast charging.A drawback is that the need for high concentrations of Li salt increases the cost.In LHCEs, hydrofluoroethers that are volatile and insoluble in water are released into the environment, contributing to global warming. 201Because of stable C-F bonds, fluorinated compounds are practically difficult to decompose and will contaminate the local environment.Compared with improved solvents, use of additives represents a better method because of the need for a smaller number of additives to induce improved performance.It appears, therefore, that it is more economical to boost battery performance via incremental design incorporation of components in the electrolyte and not replacing the skeletal composition.This is evidenced by the fact that the reported research has focused on additives and not developments in solvents and Li salts.

| CONCLUSIONS AND PERSPECTIVES
Next-generation LMBs with higher energy density will evolve for long lifespan, wide range of working temperatures, and fast charge for boosted energy storage.Targeted electrolyte design will play a vital role in achieving high-performance LMBs by eliminating Li dendrites and achieving stable Li plating/stripping.Evolution will be highly dependent on an improved understanding of the fundamental mechanisms of electrolyte in different working conditions.Importantly, high ion conductivity together with facile de-solvation is a practical prerequisite for LMBs with fast charge and low working temperature.De-solvation energy is linked to the solvent used; weakening the solvated interaction between Li + and solvent molecules can lead to lower desolvated barriers.Current methods include use of weakly solvated solvents and fluorinated ether electrolytes.HCEs and LHCEs also show facile de-solvation for fast charge and working at low temperatures, because abound CIPs and AGGs result in lower de-solvation energy.For high working temperatures and high anodic voltages, together with increased safety, the electrolyte should have high thermal and high electrochemical stability.Because it is practically difficult to synthesize an electrolyte with a single solvent or a single salt to meet all the requirements for high-performance of LMBs, cosolvents and additives can be used to balance ion conductivity, anodic stability, thermal stability, and solvated environment.
For continued evolution of high-performance LMBs, it is important to form a stable SEI.SEI consists of organic and inorganic compounds.Organic compounds may dissolve in the electrolyte and result in poor passivation ability; inorganic compounds including LiF and Li 2 O with high interfacial energy can passivate the Li anode to show stable Li deposition.Because of this, the present design methods focus on the generation of inorganic-enriched SEI.However, SEI with reduced organic compounds shows higher Young's modulus, and is fragile and not able to form a homogeneous film on the anode.
More detailed knowledge of SEI is necessary for continued development of Li anodes.In situ advanced characterization techniques will be needed to capture the real-time dynamics of SEI and will include transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance, and nuclear magnetic resonance (NMR).HCE, LHCE, and fluorinated ether electrolyte form stable anion-derived SEI and boost electrochemical performance by modifying the Li + solvation environment and generating sufficient AGGs and CIPs.The Li + solvation environment is therefore very important in the formation of stable SEI in LMBs.Current technologies to characterize solvation shells include Raman and NMR.In Raman, information is obtained through characteristic vibrational peaks for the anion.With NMR, which is more sensitive to the coordination species in the solvation shell, the upfield or downfield shift indicates an increase or decrease in electron density around Li + .
Use of a solid-state electrolyte is a practical alternative solution for high-performance LMBs.A carefully designed solid-state electrolyte with significant value for Young's modulus will eliminated Li dendrites to result in stable cycling.Polymer-based solid-sate electrolytes are more advantageous in flexible devices.However, Li dendrite growth at grain boundaries can occur because of the high electronic conductivity of inorganic solidstate electrolyte and reduce the electrochemical performance of solid-state batteries.The high contact resistance in inorganic solid-state electrolytes and the low ion conductivity at RT in polymer-based solid-state electrolytes can lead to poor electrochemical performance and lower energy density.Therefore, more research is needed in both liquid and solid-state electrolytes.
Although the "ideal" electrolyte is water as an environmentally benign solvent, protic solvents are not compatible with Li anodes and therefore hinder practical application.Chemical wastes from the preparation of electrodes and waste electrode materials and electrolytes could pose a threat to a sustainable environment.Therefore, future research needs to be focused on designs to improve the environmentally friendly nature of LMBs.
LMBs are a complex system.Based on a "wood barrel" principle, it is necessary to identify and solve "short board," for example, cathode, anode, electrolyte, and separator, to evolve to advanced forms.We firmly believe that the large application of LMBs will be successful with the continuous efforts of researchers.
Fluorinated sulfonimide salts LiFSI and LiTFSI, which include a highly conjugated anionic center (-SO 2 -N − -SO 2 -), are generally well dissociated F I G U R E 7 (A) Schematic showing three steps for charge transfer at the Li anode.(B) Ionic conductivity of 0.1, 1, 2, 3, and 5 mol/L DOL/DME-based electrolyte.Reproduced with permission.

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I G U R E 8 (A) Cyclic voltammetry curves for 0.1 mol/L LiTFSI in FM at 25°C and −60°C.(B) CE for Li plating/stripping over 400 cycles using FM-and conventional liquid-based electrolytes at 25°C.(C) Discharge capacity at different temperatures and C values.(D) Voltage versus discharge capacity at different temperatures at 0.1 C. Reproduced with permission. 140Copyright 2017, Science.CE, Coulombic efficiency; FM, fluoromethane.

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I G U R E 9 (A) Illustration of the relationship between Li deposition morphology and solvated ability of solvents at low temperatures.(B) Binding energies of the Li + -solvent complex.left: Li + -DME; right: Li + -DMM.(C) Raman spectra of DMM and DME electrolytes.(D) Plating/stripping profile for CE determination in 1 mol/L LiFSI DME and 1 mol/L LiFSI DMM at 0.25 mA/cm 2 at −40°C, and Aurbach CE tests of the DMM electrolyte at various temperatures.Reproduced with permission. 144Copyright 2022, Wiley-VCH.CE, Coulombic efficiency; DME, dimethoxyethane; DMM, dimethoxymethane.

T A B L E 2
Abbreviation: LMB, lithium metal battery.

T A B L E 3
Oxidation stability and electrochemical performance of selected electrolytes.ElectrolyteOxidation stability (Li + /Li)
61Copyright 2022, Springer Nature.DEC, diethyl carbonate; DME, dimethoxyethane; EC, ethylene carbonate; HOMO, highest occupied molecular orbital; LOMO, lowest unoccupied molecular orbital; SL, sulfolane.Li salt also generates gas.In the presence of moisture, LiPF 6 will hydrolyze to generate HF, which reacts with Li and Al to form LiF and AlF 3 and H 2 gas, Figure12C.H 2 O derived from solvent decomposition can accelerate LiPF 6 hydrolysis.The