Nonflammable Liquid Electrolytes for Safe Lithium Batteries

Lithium‐based batteries (lithium‐ion batteries, lithium‐metal batteries, and lithium–sulfur batteries, etc.) have become one of the most irreplaceable energy‐storage devices and shown huge application potential. However, the traditional liquid electrolytes used in lithium‐based batteries are flammable and exhibit poor electrochemical stability, which will significantly limit the further development of high energy density and high safety lithium‐based batteries. Thus, it is crucial to develop nonflammable electrolytes. In this work, the reasons why traditional liquid electrolyte is unsafe and the recent progress of nonflammable liquid electrolytes including flame‐retardant‐based electrolytes, highly concentrated electrolytes, localized high concentrated electrolytes, and ionic liquids–based electrolytes are summarized. The challenges and future perspectives toward how to decrease the fire hazard of lithium‐based batteries through liquid electrolyte design are also put forward.


Introduction
The global lithium-ion batteries (LIBs) market is projected to grow from $44.49 billion in 2021 to $193.13 billion by 2028, at a compound annual growth rate of 23.3% in forecast period.Currently, LIBs are the widely applied as energy-storage devices in consumer electronics, electric vehicles, and stationary energystorage systems due to their relatively high specific energy densities and stable/long-term cycling performance. [1,2]However, with the development of electric vehicles and energy-storage system, the LIBs cannot meet their requirements for energy density and safety.The prevailing LIBs composed of lithium transition-metal oxide cathodes and graphite anodes deliver only limited energy densities of <300 Wh kg À1 , [3] which is far lower than the goal of 500 Wh kg À1 set by electric vehicle industry. [4,5]Consequently, lithium (Li)  batteries including Li-metal batteries (LMBs), and lithium-sulfur (Li-S) batteries, [6][7][8][9] etc., have attracted tremendous attention due to their high theoretical energy densities. [10,11]owever, it has been reported that LIBs show the high fire hazard due to their flammable electrolyte, unstable lithium intercalated graphite and inflammable delithiation cathode.LMBs even display the higher fire hazard compared with that of LIBs due to the presence of lithium.Safety issues have become one of the main obstacles restricting the development of lithium-based batteries.
The uncontrollable thermal runaway caused by high temperature, overcharging, over-discharging, puncture, short circuit, etc., is the main reason for safety issues of lithium battery. [12]A series of internal exothermic reactions will occur once the thermal runaway of lithium battery starts (Figure 1).The solid electrolyte interface (SEI) which is formed by the reaction between the electrolyte and anode plays a prominent role in extending the longtime of lithium battery.However, it is reported that the SEI is unstable and begins to decompose at around 80 °C. [13]Once the SEI starts to decompose, the anode will react with the fresh electrolyte accompanying by heat release.When the temperature inside the battery increases to 130 °C, the commercial polyethylene/polypropylene separator fuses and the batteries start to be short circuit.Subsequently, the reaction between the electrolyte and the cathode occurs, which is accompanied by a large amount of heat generation. [14]Thus, once the LIBs fire starts, it is extremely difficult to extinguish.It is concluded that electrolyte is one of the most crucial factors determining the fire hazard of lithium battery for it participates in most of the exothermic reaction during the thermal runaway.
State-of-the-art commercial LIBs electrolytes adopt LiPF 6 as the electrolyte salts due to their ranking performance in comparison with other salts.However, LiPF 6 is unstable and probe to decompose to form PF 5 , which can react with the organic solvent to form flammable products. [15,16]Although organic carbonate solvents endow the excellent electrochemical performance to electrolyte, they are inflammable and unstable.Lithium intercalated anode will react with organic solvent, resulting in release of flammable gases.19] Thus, nonflammable electrolyte is crucial for safety applications of lithium battery.In this review, the recent progress of nonflammable electrolyte including flame-retardant-based electrolyte, highly concentrated electrolytes (HCEs), localized high concentrated electrolytes (LHCEs), and ionic liquids (ILs)-based electrolytes will be introduced.The physical and electrochemical performance of nonflammable electrolyte will be elucidated.The challenges and prospects of nonflammable lithium battery electrolytes will also be presented.

Nonflammable Electrolytes
At present, the ester-and ether-based electrolyte used in lithium batteries are highly flammable, which are extremely easy to cause the thermal runaway of lithium batteries in case of abuse, leading to conflagration or some other serious safety accident.Therefore, it is necessary to develop the nonflammable electrolyte to reduce the fire risk of batteries.The introduction of flame-retardant additives into liquid electrolytes is a common and effective method to improve the fire-resistance performance of batteries.

Flame-Retardant Additives
Compared with pure electrolyte, the addition of flame retardants can significantly improve the thermal stability of electrolyte, inhibit the combustion of organic solvent, and reduce the heat release as well as toxic gas release during combustion process.However, abundant flame-retardant additive will dilute the electrolyte, which has adverse impact on the physicochemical properties of the electrolyte and deteriorates performance of battery, including the battery capacity, Coulomb efficiency, and cycle stability. [20,21]Therefore, the development of flame retardant with excellent fire resistance and little adverse influence on electrochemical performance has become one of the research hotspots.The commonly used flame retardant can be divided into three categories, including the phosphorus-based compounds, fluorinebased compounds, and composite flame-retardant additives.

Phosphorus-Based Flame Retardants
Combustion of electrolyte is based on free radical chain reaction mechanism.The free radicals like H• and OH• resulted from the pyrolysis of electrolyte are highly active chemical forms that can react with other small molecules through a chain reaction, which The thermal runaway process of lithium-ion batteries (LIBs).Reproduced with permission. [54]Copyright 2021, John Wiley and Sons.b) Detailed thermal runaway process for LIBs.Reproduced with permission. [12]Copyright 2019, Elsevier.
can trigger and aggravate the combustion reaction of lithium batteries.However, the phosphorus-based flame retardants will decompose to produce PO 2 • and HPO 2 • free radicals which will capture a large number of H• and OH• free radicals generated during combustion, resulting in destroying and inhibiting the chain reaction.Thus, the fire-resistance performance of electrolyte can be enhanced through terminating the free radicals chain reaction of combustion (Figure 2b). [22,23]Furthermore, the low cost and low toxicity of phosphorus-based flame retardant benefit its application as additive in electrolyte.
[26] The widely used alkyl phosphate ester includes trimethyl phosphate (TMP), triethyl phosphate (TEPa), as well as dimethyl methyl phosphonate (DMMP).Feng et al. investigated DMMP as a flame-retardant additive for LIBs. [27]It was found that the graphite anode showed notable cycling stability as well as high initial Columbic efficiency (84%).The main reason for the excellent cycling stability was that Cl-ethylene carbonate (EC) as an electrolyte additive can promote the formation of SEI layer on the graphite surface and suppress the electrochemical reduction of DMMP.In Cui's study, an ionic conducting ethyl phosphate-polyethylene glycol-based copolymer (EPCP) was synthesized as a promising additive to prepare the nonflammable liquid electrolyte. [28]When 15 wt% EPCP was added, the liquid electrolyte was totally nonflammable in the flammability test.The lithium battery containing 15 wt% EPCP remains a discharge capacity of 130 mAh g À1 at 0.5C after 100 cycles (Figure 2c,d).Wang et al. concretely investigated the relative merits of trivalent and pentavalent phosphorus-based flame retardants as additives into liquid electrolytes, mainly focusing on TEPa and triethyl phosphite (TEPi). [29]A high-capacity retention rate of 99% can be achieved after 100 cycles for TEPa-based electrolyte.Trivalent phosphorus flame retardant can serve as SEI layer former.A low loading amount of trivalent phosphorus Reproduced with permission under the terms of the Creative Commons CC BY license. [80]Copyright 2010, the Authors.Published by MDPI.c) The rate capability of LiCoO 2 /Li cells using liquid electrolyte (LE) and EPCP15-LE.d) The self-extinguishing time (SET) test of the electrolytes with ethyl phosphate-polyethylene glycol-based copolymer (EPCP)-LE.Reproduced with permission. [28]Copyright 2017, Institute of Physics Publishing.e) The SET test of electrolytes with triethyl phosphate (TEPa) and triethyl phosphite (TEPi).Reproduced with permission. [29]Copyright 2019, Elsevier.f ) The cycling curves of electrolytes with triphenyl phosphate (TPP).Reproduced with permission. [31]Copyright 2012, Institute of Physics Publishing.g) DSC profiles of the electrolytes.Reproduced with permission. [32]Copyright 2009, Elsevier.h) The electrochemical performance of mesocarbon flame retardant can significantly facilitate the formation of a dense and stable SEI layer on the graphite anode surface.Compared with trivalent phosphorus flame retardant, the electrolytes containing pentavalent phosphorus flame retardant can sustain a higher voltage and possess a wider electrochemical window.Moreover, the pentavalent phosphorus flame retardant showed excellent affinity with cathode materials (Figure 2e).
In addition, phenyl phosphate ester including triphenyl phosphates (TPPs), diphenyl octyl phosphates (DPOF), and diphenyl methyl phosphates (CDP) also exhibit excellent fire-resistance performance. [30]Ping et al. studied the effect of TPP on cathode symmetric LIBs. [31]When TPP concentration was no more than 20%, the NCA symmetric cells with TPP exhibited almost the same capacity retention.However, the large addition amount of TPP over 20% resulted in the significant increase in impedance and really inferior capacity retention (Figure 2f ).Shim et al. concretely investigated the comprehensive performance LIBs containing different addition amount of DPOF. [32]It was found that DPOF can significantly improve the decomposition temperature of the electrolyte.But when the DPOF content was more than 30 wt%, the liquid electrolyte exhibited bad electrochemical performance with poor rate performance and cycling stability (Figure 2g).In Wang's study, cresyl diphenyl phosphate (CDP) was investigated as the electrolyte additive for the LiCoO 2 /CDP-electrolyte/Li and Li/CDP-electrolyte/C halfcells. [33]Both the thermal stability of LiCoO 2 cathode and graphite anode was notably improved with the addition of 5%-15% CDP to the electrolyte, while the electrochemical properties almost did not deteriorate.
Since the high content of phosphorus-based flame retardant causes the deterioration in the battery performance, it is necessary to develop additives with higher-flame-retardant efficiency and lower addition amount.Due to the synergistic effect of phosphorus and nitrogen elements, the phosphorus-nitrogen compounds exhibit excellent fire-resistance performance.Fei et al. reported two kinds of two phosphazene compounds as liquid electrolyte additives based on the methoxyethoxyethoxyphosphazene oligomer and the corresponding high polymer, which can reduce the fire risk of LIBs and maintain a high energy efficiency at the same time. [34]The flammability of the propylene carbonate electrolyte containing 25 wt% of high polymeric poly[bis (methoxyethoxyethoxy)phosphazene] was significantly decreased by 90%, while the ionic conductivity still remained good (2.5 Â 10 À3 S cm À1 ).In Wu' study, a novel phosphazenic compound called triethoxyphosphazen-N-phosphoryldiethylester (PNP) was served as additive to enhance flame-resistance performance of LIBs. [35]Compared with the original electrolyte, the self-extinguishing time (SET) of liquid electrolyte containing 10% PNP was notably reduced by 40%, indicating that PNP exhibited a strong inhibitory effect on the flammability of electrolyte.The prepared LiMn 2 O 4 /Li half-cells showed outstanding electrochemical performance with high Coulombic efficiency of 94% and excellent cycling stability, which can still remain 90% of the initial capacity after 100 cycles.For LiFePO 4 /Li half-cells, there was undiscernible capacity decay after 100 cycles.The discharge capacity increased from 130 to 135 mAh g À1 after 100 cycles (Figure 2h).

Fluorine-Based Flame Retardants
Fluorine-based flame retardant is another important kind of additive to enhance the thermal stability and flame-resistance performance of liquid electrolytes.The flame-retardant mechanism of fluorine-based compounds can be simply summarized into three sections, including absorbing heat, cooling down and terminating the chain reaction. [21]Because the bond energy of the C─F bond is low, the fluorine-based flame retardant is easy to decompose under high temperature, which can absorb large amount of heat.Moreover, the density of the generated hydrofluoric acid (HF) gas is higher than air, thereby HF gas can isolate oxygen from combustibles, resulting in suppressing the combustion reaction.In addition, large amount of H• and HO• free radicals will be formed during combustion of electrolyte.The generated HF gas can react with HO• free radical to form F• free radical.Then, the F• free radical can further react with the H• free radical to generate HF, thus terminating or retarding the combustion through stopping the free chain reaction between H• and HO• radicals. [20]ue to its excellent comprehensive properties including good compatibility and high thermal stability, fluorine-based flame retardant has been widely used in electrolyte. [36]Most of the reported fluorine-based flame retardants can be divided into ethers-based molecular and ethers-based molecular (Figure 3a).Furthermore, the existence of fluorine element in the electrolyte can contribute to the formation of a dense and stable SEI layer on the anode electrode surface (Figure 2b).There have been plenty of reports in the recent years, mainly focusing on the organic fluorides, such as fluoroethylene carbonate (FEC), methyl-nonafluorobutyl ether, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE), bis(2,2,2-trifluoroethyl) ether, 2,2-dimethoxy-4-(trifluoromethyl)-1,3-dioxolane, and so on. [36,37]Chen et al. mixed TMP with HFE as flame-retardant additive to prepare the nonflammable electrolyte.The electrolyte not only shows the excellent fireresistance performance, but also does enable a stable and dendritefree cycling with lithium plating/stripping. [38]The prepared lithiumÀsulfur batteries still exhibited high Coulombic efficiency about 99.1% after more than 500 cycles.Moreover, the battery possessed outstanding cycling stability and rate performance with a high capacity up to 3.41 mAh cm À2 after 70 cycles (Figure 3d).Zhao et al. developed a novel fluorinated ether served as liquid electrolyte, which can endow the LIBs with excellent electrochemical performance, including high voltage stability (5.5 V), large lithium ion transference number (0.75), as well as stable Coulombic efficiency of 99.2% after 500 cycles. [39]In Benmayza's study, the impact of the FEC as cosolvent on the electrochemical performance and thermal properties of LIBs was investigated. [36]ompared with normal EC as cosolvent, the addition of FEC notable enhanced the thermal stability of the LiNi 0.8 Co 0.15 Al 0.015 O 2 with higher onset temperature increased from 196 to 220 °C and lower exothermic heat reduced by 15% in differential scanning calorimetry (DSC) test.However, the electrochemical performance of cells containing FEC at high C rates slightly deteriorates with a decrease of 10%.In Bao's study, a fluorinated solvent design principle was put forward.The F 4 DEE and F 5 DEE display the high-Li-metal Coulombic efficiency (CE), fast CE activation, and oxidative stability (Figure 3e). [40]In Yang's study, FEC and ethyl difluoroacetate (EFA) was chosen as additive to develop a nonflammable functional electrolyte. [41]The prepared LMBs exhibited excellent electrochemical properties.Compared with traditional carbonate-based electrolyte, the cycling performance of the prepared cells was significant enhanced, which was mainly attributed to the promotion of the formation of stable interfacial films on the electrode surface.Moreover, the capacity retention of the LMBs using FEC and EFA displays an enhanced cycling retention of 74% (150 cycles) at a charging cutoff voltage of 4.4 V as well as higher average Coulombic efficiency of 99.9% (Figure 3c,f,g).
However, fluorine-based flame retardant still has some obvious disadvantages, which restricts its application.On the one side, the fluorine-based flame retardant will decompose to generate HF gas at high temperature due to the weak C─F bond, while the released HF gas is toxic and easy to cause suffocation accidents in conflagration.On the other side, fluorine-based flame retardants can also decompose to produce toxic organic compounds under high temperature, which are difficult to degrade in the environment and harmful to human health. [42]A new generation of fluorine-based flame retardant has yet to be developed.

Composite Flame Retardants
Although both phosphorus-based and fluorine-based flame retardants can significantly enhance the safety performance of liquid electrolyte, these conventional flame retardants still have some obvious shortcomings.The phosphorus-based flame retardants show high viscosity and poor compatibility with electrode materials.The large addition amount of phosphorus-based flame retardants will also decrease the ionic conductivity of the electrolyte, shorten the cycle life of cells and rapidly worsen the battery performance. [43]The decomposition of fluorine-based flame retardants will release toxic HF gas and organic compounds, which is harmful to the environment and humans.Thus, the concept of composite flame retardant is proposed to combine the advantages of different kinds of flame retardants and offset their negative effects. [44]The composite flame retardants are composed of two or more flame-retardant elements, which can show better comprehensive properties with lower addition amount due to the synergistic effect of different elements. [21]The PO 2 • and HPO 2 • and F• free radicals, etc., will be released during pyrolysis of composite flame retardants.Those PO 2 • and HPO 2 • and F• free radicals will also wipe out a large number of free radicals generated in the chain reaction during combustion, resulting in retarding the combustion.Copyright 2018, Springer Nature.c) Long cycling performance of fluorinated electrolyte.Reproduced with permission. [41]Copyright 2021, Elsevier.d) Coulombic efficiency of Li-depositionÀstripping using Cu electrodes.Reproduced with permission. [38]Copyright 2019, American Chemical Society.e) Design principles of fluorinated 1,2-diethoxyethane (DEE) electrolyte.Reproduced with permission. [40]Copyright 2022, Springer Nature.f,g) Illustration of the CEI on the cathode surfaces.Reproduced with permission. [41]Copyright 2021, Elsevier.
Fluorine-containing phosphates are commonly used as additives to develop nonflammable electrolytes with excellent fire-resistance performance due to the synergistic effect between F and P elements, including tris(2,2,2-trifluoroethyl) phosphate, bis(2,2,2-trifluoroethyl)-methyl phosphate, as well as tris(2,2,2trifluoroethyl) phosphite (TTFP), etc.The existence of F on the structure of fluorine-containing phosphates can not only significantly reduce the fire risk of electrolyte, but also enhance the electrochemical performance of the electrolyte. [45,46]Zhu et al. synthesized diethyl(thiophen-2-ylmethyl)phosphonate (DTYP) as a novel electrolyte additive used in high voltage LIBs. [47]he liquid electrolyte with 0.5% DTYP showed excellent fireresistance performance with a shorter SET decreased from 88 to 77s.The capacity retention of the high voltage LIBs with 0.5% DTYP was notably increased from 18% to 85% after 280 cycles at 1C at 60 °C (Figure 4b).In Gu's study, tris(2,2,2-trifluoroethyl) phosphate was used as cosolvent and mixed with γ-butyrolactone to formulate the nonflammable electrolyte. [48]e prepared electrolyte exhibited better safety performance and electrochemical properties than the traditional commercial electrolyte.The prepared full cells possessed higher capacity retention (90.8%) at 1C under room temperature after 200 cycles.Moreover, the capacity retention of batteries at 60 °C (1C) after 100 cycles was also increased to 86.7%.In Zheng's study, a novel nonflammable fluorinated cyclic phosphate solvent 2-(2,2,2trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEP) was designed and synthesized based on the fused chemical structure of cyclic carbonates.The prepared electrolyte showed excellent fire-resistance performance with a zero SET.The TFEP solvent can promote the formation of a dense and stable SEI film on the graphite anode surface, which can endow the graphite anode with highly reversible and stable cycling.TFEP can also benefit the formation of a polymeric CEI layer to prevent electrolyte oxidation and transition-metal dissolution (Figure 4f,g). [49]Similar to fluorine-containing phosphates, fluorinated phosphazene also has excellent flame retardancy with high flame-retardant  [47] Copyright 2018, Royal Society of Chemistry.c) Combustion test of electrolytes with 3% hexafluorocyclotriphosphazene (HFPN).Reproduced with permission. [49]Copyright 2020, Springer Nature.d) Schematic diagrams of HNPN-containing electrolyte.e) Cycling performance of HNPN-based electrolyte.Reproduced with permission. [50]Copyright 2021, American Chemical Society.f ) Schematic illustrations of the ring-opening polymerization of 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEP) for formation of the cathode-passivation layer.g) LSV of TFEP/fluorinated ethyl methyl carbonate electrolyte using an Al electrode.Reproduced with permission. [49]opyright 2020, Springer Nature.efficiency.Moreover, the existence of F in its chemical structure can improve the electrochemical performance and overall cycling stability of batteries.The commonly used fluorinated phosphazene in lithium-based batteries as flame-retardant additives includes ethoxy(pentafluoro)cyclotriphosphazene (PFPN), PNP, hexafluorocyclotriphosphazene (HFPN), (trifluoroethoxy)pentafluorocyclotriphosphazene, and so on.Li et al. used HFPN containing rich N, P, and F elements as a cosolvent to synthesize the high-voltage, nonflammable ether-based electrolyte.The addition of HFPN had little effect on the physical properties of the electrolyte, while the flammability was significantly improved.Moreover, HFPN can also contribute to the formation of the stable, dense, and homogeneous SEI layer on the lithium-metal anode surface.The capacity retention of the high-voltage LMBs can still maintain 95% after 100 cycles (Figure 4d,e). [50]In Sayah's study, three kinds of fluorinated phosphazene derivatives were investigated as flame-retardant additives in LIBs, including hexafluorocyclotriphosphazene (FR1), (ethoxy)pentafluorocyclotriphosphazene (FR2), and pentafluoro(phenoxy)cyclotriphosphazene (FR3). [51]The 3% of FR1 can make the electrolytes nonflammable, while the FR2 and FR3 need 5% and 15%, respectively.Compared with traditional commercial electrolyte, both FR1 and FR2 had little impact on the transport properties of electrolytes.Furthermore, the initial capacity of graphite// LiMn 1.5 Ni 0.5 O 4 (LMNO) cells containing FR1 and FR2 remained unchanged after 100 cycles with a loss of only 5% (Figure 4c).In Li's study, PFPN was investigated as additive with highflame-retardant efficiency for the liquid electrolyte. [52]ompared with initial liquid electrolyte, the electrolyte with 5 vol% of PFPN showed excellent fire-resistance performance with a shorter SET of 12.38 s g À1 and higher critical oxygen index value of 22.9, while 5 5 vol% of PFPN almost had no effect on the capacity of cathode.Moreover, the N and F elements can promote the formation of a more stable dense passivation SEI layer, resulting in the excellent cycling stability and capacity retention.PFPN can also decline the charge-transfer resistance of batteries and notably enhance the electrochemistry performance of the LiCoO 2 electrode at low temperature.
Some common flame retardants used in liquid electrolytes and their electrochemical performance are listed in Table 1.All the three kinds of flame retardants endow liquid electrolytes with excellent fire-resistance performance including much better thermal stability and lower SET when igniting.The ideal flame retardant for liquid electrolyte should also possess good physical properties including the suitable electrical conductivity, viscosity, boiling point, density, and solubility.Furthermore, the additives also need to exhibit excellent compatibility with positive and negative electrodes, which can ensure the stable electrochemical performance of cells.

Highly Concentrated Liquid Electrolytes and Local Highly Concentrated Liquid Electrolytes
The electrolyte needs to have good Li-ion dissolution, transport, and desolvation capabilities.Thus, the current commercial electrolyte is usually 1 M lithium salt dissolved in a mixed solution of high viscosity and high dielectric constant organic solvents (EC, PC, etc.) and low viscosity and high dielectric constant organic solvents (dimethyl carbonate [DMC], diethyl carbonate, etc.). [53]owever, commercial electrolytes are flammable and show insufficient voltage resistance.In typical 1 M electrolyte, most of the solvent molecules are "free," which means they have no coordination with Li salt.The free solvents will increase the flammability and instability of electrolyte. [54]To improve the aforementioned properties, HCE with less free solvents or no free solvents have been developed.The HCE with less or no free solvents will decrease the flammability of electrolyte through decreasing the amount of volatile gaseous solvents.Also, the Li solvation shell in HCE is different with that in commercial electrolyte.The content of salt anions in Li solvation shell in HCE will increase significantly, most of anions are even in the state of aggregation, resulting in a higher voltage resistance.Hence, the HCE can avoid the degradation of Al current collector.The position of larger amplitude in the lowest unoccupied molecular orbital moves from the solvent to the aggregated salt, resulting in reductive decomposition of the salt prior to the solvent at low potentials.Thus, main compositions of SEI formed on anode are reduction productions of anions not the solvent.The SEI with higher content of anions reduction products show a more compact structure and a better stability, which will contribute to long cycling performance. [55]The high concentration LiN(SO 2 F) 2 (LiFSA)/DMC electrolyte reported by Yamada shows good electrochemical performances.In Figure 5b-d, the 100th capacity of Li||LNMO cells incorporated with 1:1.1 LiFSA/DMC is much higher than that with commercial electrolyte.It is significant that the long cycling stability of Li||LNMO cell at the current density of 2 C and the linear sweep voltammetry (LSV) of electrolyte increases with increased concentration of LiFSA.The electrolyte with enhanced LSV can inhibit the degradation of Al current collector efficiently.The flammability of electrolyte decreases with increased concentration. [56]lthough HCE shows lots of advantages, their high viscosity, high cost, and relatively low ionic conductivity.To solve the aforementioned drawbacks, the localized high concentration electrolytes (LHCE) have been put forward (Figure 5g).The diluents used to prepare the LHCE usually are miscible with solvent but insoluble toward Li salts.The diluents like fluorinated solvents are free in LHCE.Li salts and solvent molecules in LHCE still show a well-constructed 3D network which is similar to that in HCE due to the strong coordination but concentration of Li salt in LHCE decreases due to the diluent (Figure 5a). [57]luorine-based solvents are one of the most studied diluents due to their nonflammability and stable SEI formation. [58]specially, the LHCE composed by a high salt-concentrated phosphates and fluorine-based diluents, and they are noncombustible.As shown in Figure 5e, the LHCE made up of lithium difluoro(oxalato)borate-TEPa-1,1,2,2-tetrafluoroethyl-2,2,3,3tetrafluoropropyl ether (HFE) cannot be ignited. [59]Except for high safety, the LHCE also show high LSV, enhanced cycling stability, low viscosity, and better wettability compared with that of the HCE. [60]The Li||LiNi 1/3 Mn 1/3 Co 1/3 O 2 incorporated with LHCE displays a high Coulombic efficiency (99.5%) and excellent capacity retention (>80% after 700 cycles) (Figure 5h). [61]It is also reported that LHCE can resist the shuttle effect of the polysulfides in lithium-sulfide battery.the capacity retention of lithium-sulfide batteries is SET: self-extinguishing time.
much higher in LHCE than in electrolyte at sulfide loading of 1.3 mg cm 3 (Figure 5i). [62]

ILs
ILs are type of organic molten salt at room temperature with adjustable physicochemical and electrochemical properties.The typical composition of ILs includes cations (imidazolium, pyrazolium, thiazolium, pyrrolidinium, pyridinium, piperidinium, ammonium, phosphonium, and metal ions) and anions (BF 4 À , PF 6 À , TFSA À , and FSA À ) (Figure 6a). [63]So far, ILs have been widely applied as electrolytes for energy-storage devices due to their high-room-temperature ionic conductivity, wide LSV, good thermal stability, and noninflammability.The room-temperature ionic conductivity of ILs ranges from 0.1 to 18 mS cm À1 .The LSV of ILs is within 4-6 V. [64] For example, imidazolium-based ILs show an LSV of 4 V, while piperidinium-and pyrrolidinium-based ILs display LSV at %6 V.The ILs are usually thermal stable below 300 °C and show no solvent evaporation before pyrolysis.Thus, the electrolytes composed of IL display high fire safety. [64,65]However, the high viscosity and poor cyclability of ILs limit their application as electrolytes.The viscosity of ILs is typically 30-50 cp, which is much higher than commercial liquid electrolyte.The viscosity of ILs mixed with lithium salt can even reach as high as several hundreds of cp. [66]The ILs with high viscosity show poor wettability with electrode materials and separator.In addition, it is reported that the lithium battery using IL as electrolyte can only cycles stably from 10 to 100 cycles. [64,67]To overcome aforementioned drawbacks, the dicationic ILs (DILs), multiple ILs, and ILs/organic solvent are developed.
DILs are a subclass of ILs that contain two cationic centers in their structure.[73][74] It is reported that the electrochemical stability window of imidazolium-based ILs is narrow and ionic conductivity of quaternary ammonium-based ILs is low.The designed imidazolium-trialkylammonium-based DIL displays a high electrochemical stability window like quaternary ammonium-based ILs and relatively high conductivity like imidazolium based ILs (Figure 6b). [71]Multiple ionic liquids electrolytes (MILEs) are a class of IL electrolytes that are formed by  [82] Copyright 2018, Elsevier.b) The voltage versus capacity curves of HCEs. [56]c) The capacity retention of the half-cells using different concentrations of LiFSA/dimethyl carbonate (DMC) electrolytes.Reproduced with permission. [56]Copyright 2016, Springer Nature.d) Oxidation stability of an aluminum electrode with different concentrations of LiFSA/DMC electrolytes.e) Combustion test of commercial electrolyte and LHCE.Reproduced with permission. [59]Copyright 2022 Springer Nature.f ) Cycling performance curve using 1.2 m LiFSI/DMC-bis(2,2,2-trifluoroethyl) ether (LiFSI/DMC-BTFE).Reproduced with permission. [61]opyright 2018, John Wiley and Sons.g) Viscosity of HCEs and LHCEs electrolytes.Reproduced with permission. [82]Copyright 2018, Elsevier.h) Coulombic efficiency of Li-metal plating/stripping using Li||Cu cells in 1.2 m LiFSI/DMC-BTFE.Reproduced with permission. [61]Copyright 2018, John Wiley and Sons.i) Long-term cycling of the Li-S cells.Reproduced with permission. [62]Copyright 2021, John Wiley and Sons.
blending two or more different ILs together.One advantage of MILEs is that they can compromise the beneficial properties and drawbacks of different ILs.For example, the ionic conductivity of ternary electrolytes (LiX/PYR 13 FSI/PYR 14 TFSI) are even higher than that of PYR 13 FSI/lithium salt (Figure 6c). [75]omposite electrolytes consisting of ILs and organic solvents are a class of electrolytes that combine the unique properties of ILs with the low cost and ease of use of organic solvents.The use of organic solvents in composite electrolytes can enhance their conductivity and reduce their viscosity, while the addition of ILs can improve their stability, wide electrochemical window, and thermal properties.[78][79] The ionic conductivity of Pyr 13 TFSI/LiTFSI composite electrolyte increases by 100% after incorporating of 6.5 wt% of organic solvents (Figure 6f ). [76]

Summary and Outlook
LIBs show advantages of high energy density, excellent cycle performance, and have been widely used in 3C electronic products, electric vehicles and energy-storage power stations.However, the fire hazard of lithium-based batteries has become an important obstacle restricting their development.This is because that organic liquid electrolytes adopted in the commercial LIBs are flammable and they will increase the risks of the thermal runaway, the combustion and even the explosion of LIBs during charging and discharging process.Designing of nonflammable liquid electrolytes is an effective method to solve aforementioned drawbacks.State-of-the-art nonflammable liquid electrolytes include the phosphorus-based electrolyte, fluorine-based electrolyte, phosphorus-and fluorine-based composite electrolytes, and HCE-, LHCE-, and ILs-based electrolytes.
Nonflammable phosphorus-based electrolyte can be achieved by incorporating a certain amount of phosphorus flame Figure 6.a) The typical cationic and anionic species of ionic liquids (ILs) electrolyte. [64,83]b) Temperature dependence of ionic conductivity for imidazolium-trialkylammonium dicationic ILs (DILs) electrolytes.c) Ionic conductivity versus temperature dependence of multiple ionic liquids electrolytes (MILEs).Reproduced with permission. [71]Copyright 2018, Elsevier.d) The thermal stability of ILs.e) Flammability test of ILs electrolyte and commercial electrolyte.Reproduced with permission. [84]Copyright 2019, Springer Nature.f ) Viscosities of the hybrid ILs electrolytes.g) Ionic conductivities of the hybrid ILs electrolytes.Reproduced with permission. [76]Copyright 2016, Elsevier.h) Schematic diagrams of Li plating in ILs electrolytes.Reproduced with permission. [85]Copyright 2021 John Wiley and Sons.
retardants.However, adding phosphorus-based flame retardants will affect battery cycle stability due to their poor compatibility with anode and high viscosity.Therefore, it is necessary to improve the efficiency of phosphorus-based flame retardants to reduce the impact of their addition on battery performance or to improve their compatibility with anodes.Thus, designing composite flame-retardant additives will be a good choice.Except for increasing the efficiency, an in-depth understanding of underlying microscopic electrochemical process in the elec-trode||electrolyte interfaces also needs to be done, which will provide the guideline for nonflammable phosphorus-based electrolyte design.
Fluorine-based electrolytes show good compatibility and high flame retardancy, and contribute to the formation of a dense and stable SEI layer on the anode surface.However, fluorine-based solvents are easy to decompose to generate active HF and they are expensive.P-F composite flame-retardant-based electrolytes combine the advantages of F-based and P-based flame retardant.However, they are even more expensive.Thus, the strategy to developing low cost fluorine-based electrolytes needs to be put forward.
The HCE and LHCE show advantages of high LSV, nonflammability, and high electrochemical stability.They are one of the most promising approaches to developing high safe and high stable electrolytes.It is reported that some of the nonflammable LHCE composited of phosphorus-based solvent and fluorine-based solvent even display better electrochemical performance than traditional organic liquid electrolytes.However, HCE are expensive due the high content of lithium salt.Diluents used in LHCE are also mainly high-cost fluorine-based solvent.Thus, the further exploration of LHCE needs to be done to meet the industry requirements.In addition, the salt in HCE may function as strong oxidation agent; thus, the in-depth investigation of thermal stability of charged cathodes in contact with HCE should also be done.
ILs-based electrolytes display high-room-temperature ionic conductivity, wide electrochemical window, good thermal stability, designable molecular structure, and noninflammability.However, ILs shows disadvantages of high viscosity and poor wettability.Incorporating cosolvent is an efficient method to reduce the viscosity of ILs and improve the wettability.Thus, novel ILs system needs to be developed.
The commonly used method for studying flammability of electrolyte is SET test experiment.However, except for electrolyte, the anodes, cathodes, and separator of lithium-based batteries are also high fire hazard.A more comprehensive and standard evaluation method on studying the fire hazard of battery should be established.