To address this safety issue, it is necessary to develop high-safety flame-retardant electrolytes to replace traditional flammable carbonate electrolytes. This article reviews the research progress of high-safety flame-retardant electrolytes. First, it introduces the combustion mechanism, flame-retardant mechanism, and flame-retardant testing methods. Then, it elaborates on the property requirements of lithium batteries for flame-retardant electrolytes and classifies them into categories, including flame-retardant additives, flame-retardant solvents (co-solvents), high-concentration flame-retardant electrolytes, ionic liquids, and flame-retardant gel polymer electrolytes. The formulations, flame-retardant effects, and applicable battery systems of these high-safety flame-retardant electrolytes are described in detail. Finally, future research directions for high-safety flame-retardant electrolytes are discussed.
Keywords: lithium battery; safety; flame-retardant electrolyte
Commercial lithium batteries possess advantages such as high energy and power density, no memory effect, long cycle life, and environmental friendliness, and their applications are rapidly expanding from consumer electronics to electric vehicles and new energy storage. However, in recent years, with the large-scale promotion and application of lithium batteries, numerous safety accidents related to lithium battery abuse and thermal runaway occur worldwide every year. The academic and industrial communities are continuously emphasizing and strengthening research and improvement on lithium battery safety (Figure 1). The abuse conditions that cause thermal runaway (smoke, fire, explosion) mainly include mechanical abuse (such as crushing, puncture), electrical abuse (such as overcharging, internal short circuits), and thermal abuse (such as overheating shock). As can be seen from the qualitative description diagram of the chain reaction in the lithium battery thermal runaway process (Figure 2), the electrolyte plays a crucial role in the lithium battery thermal runaway process. Commercial lithium-ion batteries typically use carbonate-based electrolytes, composed of lithium salts (such as lithium hexafluorophosphate LiPF6) and carbonate solvents with low flash points, high flammability, and poor electrochemical stability (such as ethylene carbonate EC, propylene carbonate PC, dimethyl carbonate DMC, diethyl carbonate DEC, and methyl ethyl carbonate EMC). Currently, from a materials perspective, there are numerous safety improvement strategies to prevent thermal runaway, fire, and explosion in lithium-ion batteries, such as using flame-retardant electrolytes, flame-retardant heat-shrinkable separators, solid-state electrolytes, and electrode materials with high structural stability (such as lithium iron phosphate LFP). Among these, developing highly safe flame-retardant electrolytes is the most economical and simplest strategy, effectively reducing the risk (probability) of thermal runaway, fire, and explosion in lithium-ion batteries, and significantly reducing personal injury and property damage caused by thermal runaway. This paper will first introduce the combustion mechanism, flame retardant mechanism, and flame retardant testing methods based on literature review; then, it will elaborate on the property requirements of lithium batteries for flame retardant electrolytes; finally, it will classify and discuss flame retardant electrolytes, including flame retardant electrolytes based on flame retardant additives, flame retardant electrolytes based on flame retardant solvents (co-solvents), flame retardant electrolytes based on high-concentration lithium salts, flame retardant electrolytes based on ionic liquids, and flame retardant gel polymer electrolytes; and finally, it will look forward to the future research directions of high-safety flame retardant electrolytes.
1. Combustion mechanism, flame retardant mechanism and flame retardant test methods
1.1 Combustion Mechanism
Combustion is a complex chemical reaction (Figure 3) that requires three important conditions: heat, oxidizer, and fuel. Fuel is the substance that burns, the oxidizer is the substance that produces oxygen so that the fuel can burn, and heat is the energy that drives the combustion process. Maintaining combustion typically depends on the presence of free radicals. Ground-state O2 absorbs heat to form highly reactive singlet oxygen (O2*), as shown in equation (1); simultaneously, organic matter (fuel) absorbs heat to form hydrogen radicals (H•), as shown in equation (2); O2* and H• combine to form hydroxyl peroxide radicals (HOO*•), as shown in equation (3); HOO*• decomposes to form hydroxyl radicals (HO•), as shown in equation (4). These free radicals have extremely short lifetimes but are highly reactive, generating a large amount of heat/flame. In summary, this free radical mechanism means that hydrogen radicals (H•), singlet oxygen (O2*), and hydroxyl radicals (HO•) play extremely important roles in maintaining combustion.
1.2 Flame Retardant Mechanism
Based on the free radical mechanism of combustion, the development of highly safe flame-retardant electrolytes is mainly achieved by reducing the ability of the electrolyte to generate combustion free radicals (H•, O2*, HO•) and enhancing the ability of the electrolyte to eliminate combustion free radicals. Reducing the ability of the electrolyte to generate gaseous combustion free radicals is primarily achieved by using solvents with no flash point, high flash point, low melting point, or non-volatile properties (co-solvents); enhancing the ability of the electrolyte to eliminate combustion free radicals is primarily achieved by using flame-retardant additives or flame-retardant solvents (co-solvents). It is generally believed that the key mechanism of flame-retardant additives or flame-retardant solvents (co-solvents) is free radical capture. When heated, flame-retardant additives or solvents release a large number of free radicals that can capture combustion free radicals (H•, O2*, HO•) in the gas phase, thereby blocking the chain reaction of free radicals (Figure 3), thus making the combustion process of the electrolyte impossible or difficult to occur. For example, WANG et al. elucidated the flame-retardant mechanism of trimethyl phosphate (TMP). TMP is vaporized by external heat and reaches the flame (TMPliquidTMPgas). The gaseous TMP decomposes in the flame to produce phosphorus-containing free radicals (TMPgas[P]•). The phosphorus-containing free radicals can eliminate the hydrogen free radicals [P]•+H•[P]H that maintain the combustion chain reaction. The combustion chain reaction is maintained due to the lack of hydrogen free radicals, thereby effectively reducing the risk of combustion or explosion of the electrolyte.
1.3 Flame Retardant Test Method
In the field of lithium battery electrolytes, the most commonly used test indicator is the self-extinguishing time (SET), which is the duration of combustion of an ignited electrolyte mixture sample. Another important indicator is the limiting oxygen index (LOI), which is the percentage of O2 in an O2/N2 mixture that allows the electrolyte to burn for at least 60 seconds. A lower SET value and a higher LOI value indicate that the electrolyte (gel polymer electrolyte) is less likely to ignite. SET and LOI are primarily determined using standard test methods from ASTM, UL, and IEC (such as ASTM D-5306, ASTM D2863, UL-94VO, IEC 62133). XU et al. suggested classifying electrolytes into three categories based on SET values: SET less than 6 s/g, meaning non-flammable; SET between 6 and 20 s/g, meaning flame-retarded; and SET greater than 20 s/g, meaning flammable.
Differential scanning calorimetry (DSC) and accelerating rate calorimetry (ARC) are used to evaluate the thermal stability of flame-retardant electrolytes, specifically by determining their heat release and runaway temperature. Currently, major manufacturers of ARC equipment include HEL, THT, and NETZSCH. Flash point (FP) testing is also crucial for the development of flame-retardant electrolytes. Flash point is defined as the lowest temperature at which a sample ignites [under conditions of 1.103 bar (1 bar = 105 Pa)]. HESS et al. have provided a detailed description of flash point testing methods.
2. Requirements for the properties of flame-retardant electrolyte
If the use of flame-retardant electrolytes can ensure the safety performance of lithium batteries, sacrificing some electrochemical performance is acceptable. However, the use of flame-retardant electrolytes is usually accompanied by a decrease in the electrochemical performance of lithium batteries, especially cycle life and rate performance. HAREGEWOIN et al. and NAGASUBRAMANIAN et al. summarized the property requirements for flame-retardant electrolytes (flame-retardant components): ① Good compatibility with positive and negative electrode materials, especially carbon-based negative electrode materials; minimal toxicity and side effects on battery electrochemical performance; ② Minimal impact on the lithium-ion conductivity of the base electrolyte, does not affect the flash point of the base electrolyte, and is easily mixed with the base electrolyte; ③ Ability to generate free radicals that eliminate combustion free radicals, low self-exothermic rate (ARC test); ④ Low solvation ability, low non-flammability or flammability, low flame propagation rate; ⑤ Non-toxic or low toxicity, non-toxic or low toxicity of combustion products, low viscosity, low volatility, and environmentally friendly; ⑥ High voltage stability; ⑦ Ability to wet separator and electrode materials; ⑧ High exothermic onset temperature and low total heat release (using DSC and ARC tests).
3 Flame-retardant electrolytes based on flame-retardant additives
3.1 Phosphorus-containing flame retardant additives
The earliest and most extensively researched type of flame retardant for lithium battery flame retardant electrolytes is organic flame retardant additives containing phosphorus elements, which are mainly divided into (halogenated) phosphate ester flame retardant additives, (halogenated) phosphite ester flame retardant additives, (halogenated) phosphonate ester flame retardant additives, and phosphazene flame retardant additives.
3.1.1 Phosphate ester flame retardant additives
Early research focused on short-chain alkyl phosphate flame retardant additives, such as trimethyl phosphate (TMP, Figure 4(c)), triethyl phosphate (TEP, Figure 4(i)), and tributyl phosphate (TBP, Figure 4(b)), which exhibit strong ability to capture combustion free radicals and good flame retardant effects. However, these alkyl phosphates typically have high viscosity and poor compatibility with electrode materials (especially carbon-based anodes). While their addition to the base electrolyte improves the flame retardancy of the electrolyte, it also reduces the ionic conductivity of the electrolyte and significantly shortens the battery cycle life. The ways to improve the electrochemical stability of alkyl phosphates include: ① replacing alkyl groups with aromatic (phenyl) groups, such as triphenylphosphate [TPP, Figure 4(a)], 4-isopropylphenyldiphenylphosphate [IPPP, Figure 4(d)], tri-(4-methoxythphenyl)phosphate [TMPP, Figure 4(e)], and cresyldiphenylphosphate [CDP, Figure 4(f)]. )], diphenyloctyl phosphate [DPOF, Fig. 4(g)]; ② the carbon content of newly added alkyl groups, such as trioctyl phosphate [TOP, Fig. 4(h)]; ③ the use of cyclic phosphate esters, such as ethyleneethyl phosphate [EEP, Fig. 4(j)]; Recently, TSUBOUCHI et al. modified the negative electrode SEI film with potassium bis(trifluoromethanesulfonylimide) (KTFSA) additive to improve the compatibility of TMP-based (50%, volume percentage) flame-retardant electrolyte and graphite negative electrode, and the coulombic efficiency was significantly improved. LIU et al. designed a novel thermally "intelligent" flame-retardant nonwoven electrospun separator. The electrospun fibers consist of a TPP core and a PVDF-HFP shell. When the lithium battery is operating normally, the TPP does not affect the battery's cycle performance. However, when the battery overheats, the PVDF-HFP shell melts and breaks, releasing the TPP flame retardant, which captures combustion free radicals, thereby preventing the lithium battery from thermal runaway combustion and explosion.
Halogenated phosphates contain both halogens and phosphorus, two flame-retardant elements, resulting in superior flame-retardant performance. Inexpensive tri(β-chloromethyl)phosphate (TCEP, Figure 4(k)) contains both chlorine and phosphorus, two flame-retardant elements; its decomposition product, chloroethane, not only possesses flame-retardant properties but also exhibits strong refrigeration applications. To minimize the impact of TCEP on battery electrochemical performance, BAGINSKA et al. encapsulated TCEP in core-shell polyurea-formaldehyde resin microcapsules using in-situ polymerization. Recently, ASPERN et al. developed two fluorophosphate flame retardant additives: tris(2.2.3.3.3-pentafluoropropyl)phosphate [5F-TPrP, Fig. 4(l)] and tris(1.1.1.3.3.3-hexafluoropropan-2-yl)phosphate [HFiP, Fig. 4(m)]. Small amounts (1% by mass) of 5F-TPrP and HFiP can improve the cycle stability of high-voltage ternary cathodes (NCM111) by modifying the cathode electrolyte interphase (CEI). A 20% (by mass) concentration of 5F-TPrP is required to ensure the electrolyte does not burn (approximately 13% exhibits flame retardancy).
3.1.2 Phosphite-based flame retardant additives
Phosphites are also a very important class of flame retardant additives, such as trimethylphosphite [TMP(i), Fig. 6(a)], triphenylphosphite [TPP(i), Fig. 6(b)], triethylphosphite [TEP(i), Fig. 6(c)], tributylphosphite [TBP(i), Fig. 6(d)], tris(2,2,2-trifluoroethyl)phosphite [TTFP(i), Fig. 6(e)], and tris(1,1,1,3,3,3-hexafluoropropan-2-yl)phosphite [THFPP(i), Fig. 6(f)], etc. Phosphite compounds are more stable than phosphate compounds because the reactivity of P-O single bonds is lower than that of P==O double bonds. In addition, phosphite compounds are more conducive to the formation of a highly stable SEI film, and can stabilize lithium hexafluorophosphate (LiPF6) by deactivating phosphorus pentafluoride (PF5), and can also eliminate free hydrofluoric acid (HF) in the electrolyte. WANG et al. [49] applied fluorinated phosphite TTFP(i)-based carbonate electrolyte to high energy density lithium-sulfur batteries. When the concentration of TTFP(i) is greater than 10%, the carbonate electrolyte is flame-retardant or even non-flammable. Most importantly, the electrochemical performance of lithium-sulfur batteries is greatly improved. The capacity is almost unaffected after 750 cycles at 10C rate, and the capacity is as high as 800 mA·h/g. PIRES et al. used TTFP(i) as a positive electrode film-forming additive to improve the cycle stability of lithium-rich positive electrode materials and can significantly improve the thermal stability of the electrolyte.
3.1.3 Phosphate ester flame retardant additives
According to reports, due to their high phosphorus content, alkyl phosphonates have higher flame retardant properties than alkyl phosphates and alkyl phosphites. Common alkyl phosphonate flame retardants include dimethyl methyl phosphonate (DMMP, Figure 7(a)) and diethyl ethyl phosphonate (DEEP, Figure 7(b)). To improve the compatibility of alkyl phosphonates with carbon-based anodes, flame retardants of phosphonates with phenyl, halogen, and thiophene methyl substitution have been developed, such as diethylphenylphosphonate (DPP, Fig. 7(c)), bis(2.2.2-trifluoroethyl)methylphosphonate (TFMP, Fig. 7(d)), bis(2.2.2-trifluoroethyl)ethylphosphonate (TFEP, Fig. 7(e)), and diethyl(thiophen-2-ylmethyl)phosphonate (DTYP, Fig. 7(f)). Recently, ZHU et al. developed DTYP, a multifunctional flame retardant additive, through theoretical calculations (Figure 8). Thiophene forms a highly conductive CEI film on the positive electrode via free radical polymerization. Oxygen in the phosphonate can neutralize and eliminate PF5 in the electrolyte through Lewis acid-base coordination. Phosphonates can effectively terminate the free radical chain reaction during combustion. DTYP has been successfully applied to 5V high-voltage nickel-manganese lithium oxide (LiNi0.5Mn1.5O4) batteries, significantly improving battery cycle performance.
Figure 8. Molecular structure of DTYP and its related functions
3.1.4 Phosphazene Flame Retardant Additives
Phosphazene compounds are composite flame retardant additives, mainly including small-molecule cyclic phosphorus and nitrogen compounds and high-molecule linear phosphorus and nitrogen compounds (Figure 9). A key characteristic of phosphazene flame retardant additives is that a small amount (5%–15%, by mass) is sufficient to achieve flame retardant or non-flammable effects on the electrolyte, and they exhibit good compatibility with electrode materials and minimal impact on the electrochemical performance of lithium batteries. Phosphazene flame retardant additives used for electrolyte flame retardancy are mainly classified into the following categories: hexaalkoxycyclotriphosphazenes, such as hexamethoxycyclotriphosphazene [hexamethox-ycyclotriphosphazene, HMPN, Figure 9(a)], hexa(methoxyethoxyethoxy)cyclotriphosphazene [hexa(methoxyethox-yethoxy)cyclotriphosphazene, MEEtrimer, Figure 9(b)], unsaturated alkoxycyclotriphosphazenes [AL-7, Figure 9(c)], hexa(2.2.2-triphosphazene)... Hexakis (2,2,2-trifluoroethoxy)cyclotriphosphazene (HFEPN, Fig. 9(d)) and monoalkoxy pentafluorocyclotriphosphazene, such as ethoxypentafluorocyclotriphosphazene (PFPN, Fig. 9(e)) and phenoxypentafluorocyclotriphosphazene (FPPN), are also cyclotriphosphazene. [Fig. 9(f)], 4-methoxy-phenoxypentafluorocyclotriphosphazene [(4-methoxy)-phenoxypentafluorocyclotriphosphazene, 4-MPPFPP, Fig. 9(g)], 2-chloro-4-methoxy-phenoxypentafluorocyclotriphosphazene [(2-chloro-4-methoxy)-phenoxypentafluorocyclotriphosphazene, 2-Cl-4-MPPFPP, Fig. 9(h)]; linear polyphosphazenes, such as poly[bis(methoxyethoxyethoxy)phosphazene], MEEP, Fig. 9(i)], poly[bis(ethoxyethoxyethoxy)phosphazene], EEEP, Fig. 9(j)]; triethoxyphosphazene-N-phosphoryldiethylester, PNP, Fig. 9(k)]. Due to their high electrochemical oxidation window, cyclotriphosphazene flame retardant additives have numerous applications in next-generation high-voltage lithium-ion batteries, such as those using high-voltage lithium cobalt oxide cathode materials or 5V high-voltage lithium nickel manganese oxide (LiNi0.5Mn1.5O4) materials. Recently, XU et al. combined 7% flame retardant additive PFPN with functional additives, which significantly improved the long-cycle performance of 5V high-voltage LiNi0.5Mn1.5O4/graphite full cells (300 cycles at 1C rate, capacity retention of 87.4%), while also making the electrolyte non-flammable (this flame-retardant electrolyte formulation can withstand 5 ignition tests without being ignited, Figure 10).
3.1.5 Other phosphorus-containing flame retardant additives
Reported phosphorus-containing flame retardant additives include tris(4-fluorophenyl)phosphine [TFPP, Fig. 11(a)], hexamethylphosphoramide [HMPA, Fig. 11(b)], bis(N,N-diethyl)(2-methoxyethoxy)methylphosphonamidate [DEMEMPA, Fig. 11(c)], phosphoryl-rich flame-retardant ions—novel flame-retardant lithium salts [Li[P(DPC)3], Fig. 11(d)], and poly(ethyl phosphate-ethylene glycol) copolymers [EPCP, Fig. 11(e)]. Notably, recently, WANG et al. embedded EO fragments with excellent ionic conductivity into phosphate esters to form flame-retardant EPCP oligomers. When the oligomer content was 15%, the electrolyte was completely non-flammable, and the ionic conductivity of the electrolyte was not affected by the addition of EPCP. In addition, EPCP is beneficial to the stability of the electrode material interface. Therefore, the cycle performance and rate performance of lithium batteries (LiCoO2/Li) using lithium cobalt oxide cathodes are improved.
3.2 Other types of flame retardant additives
Other important types of flame retardant additives include silane-based flame retardant additives [Fig. 12(a)~(d)], triazine-based flame retardant additives [Fig. 12(e)~(g)], ionic liquid-based flame retardant additives [Fig. 12(h)~(l)], fluoroalkoxycarbon (fluoroether) flame retardant additives [Fig. 12(m)], bisphenol-based flame retardant additives [Fig. 12(n)], perfluoroalkane [Fig. 11(o)], and allyltris(2,2,2-trifluoroethyl)carbonate [ATFEC]. Silane-based (organosilicon) organic flame retardants are characterized by excellent thermal stability, low flammability, low toxicity, high conductivity, and high decomposition voltage, making them a focus of attention in the field of novel lithium-ion battery electrolytes. QIN et al. reviewed the research progress of organosilicon electrolytes. Taking vinyl-tris-(2-methoxyethoxy)silane [VTMS, Fig. 12(a)] as an example, VTMS is an environmentally friendly flame retardant additive that helps form an effective SEI film to prevent the decomposition of propylene carbonate (PC) on the graphite surface. It also has high thermal stability, low viscosity, and minimal negative impact on the electrochemical performance of lithium batteries (LiCoO2 system). YIM et al. encapsulated decafluoro-3-methoxy-2-trifluoromethylpentane [1.1.1.2.2.3.4.5.5.5-decafluoro-3-methoxy-4-(trifluoromethyl)-pentane, DMTP, Fig. 12(m)] with cross-linked polymethyl methacrylate (PMMA) and fixed it on a polyethylene microporous membrane by coating with PVDF-HFP adhesive. When the battery overheats, the PMMA shell breaks and releases the DMTP flame retardant, which contributes to battery safety [Fig. 13(a)]. Recently, JIANG et al. designed a flame-retardant composite electrolyte with self-cooling function [Figure 13(b)], whose core component is perfluoro-2-methyl-3-pentanone [PFMP, Figure 12(o)]. Although this electrolyte can improve the thermal stability of the ternary cathode material (NCM111) of lithium batteries, its electrochemical compatibility with the electrode material still needs to be improved.
Figure 12 Structural formulas of other types of flame retardant additives
Figure 13(a) PMMA polymer microcapsules encapsulated with DMTP are fixed onto a polyethylene diaphragm using PVDF-HFP adhesive; (b) Composite electrolyte with dual protection mechanism.
4 Flame-retardant electrolytes based on flame-retardant solvents (co-solvents)
4.1 Phosphorus-containing flame-retardant solvents (co-solvents)
Phosphorus-containing compounds, when used as flame retardant additives, may not meet the requirements of battery use under certain conditions. Therefore, it is necessary to increase the amount of phosphorus-containing compounds used as flame retardant co-solvents for electrolytes. However, in general, various types of phosphorus-containing compounds have poor compatibility with electrode materials, making them difficult to use as solvents.
In the early stages of flame-retardant electrolyte research, alkyl phosphate flame-retardant co-solvents such as TMP [Fig. 4(c)], TEP [Fig. 4(i)], and TPP [Fig. 4(a)] were commonly used. However, these alkyl phosphates had poor compatibility with graphite anodes. In contrast, fluorophosphate compounds had better compatibility with electrode materials (beneficial for stabilizing the SEI film), better flame-retardant effects (lower dosage), and lower viscosity, making them more attractive as electrolyte co-solvents. Examples include tris(2,2,2-trifluoroethyl)phosphate [TFP, Fig. 14(a)], bis(2,2,2-trifluoroethyl)methylphosphate [BMP, Fig. 14(b)], and (2,2,2-trifluoroethyl)diethylphosphate [BMP, Fig. 14(b)]. e, TDP, Fig. 14(c)], tris(2.2-difluoroethyl) phosphate, TFHP, Fig. 14(d)], tripropyl phosphate, TprP, Fig. 14(e)], tris(3.3.3-trifluoropropyl) phosphate, 3F-TPrP, Fig. 14(f)], tris(2.2.3.3-tetrafluoropropyl) phosphate, 4F-TPrP, Fig. 14(g)], tris(2.2.3.3.3-pentafluoropropyl) phosphate, 5F-TPrP, Fig. 4(l)]. Recently, MURMANN et al. studied the flame retardant effect and electrochemical performance of TPrP with different degrees of fluorination. The results showed that when 5F-TPrP with the highest degree of fluorination was used as a solvent at a concentration of 30%, the electrolyte was completely non-flammable and had almost no impact on the cycling performance of the graphite/NCM111 full cell. ZHANG et al. found that, in addition to being used as a flame retardant solvent, phosphite TTFP(i) [Figure 6(e)] could also inhibit the exfoliation of graphite materials by propylene carbonate (PC), improving the cycling stability of the full cell in PC-based electrolytes, especially under high temperature conditions.
Among phosphonates, DMMP is the most widely used flame-retardant solvent (co-solvent) for electrolytes [Figure 7(a)]. XIANG et al. found that DMMP-based flame-retardant electrolytes have good compatibility with Li4Ti5O12 anode materials, and this flame-retardant electrolyte was successfully used in high-energy-density, high-voltage LiNi0.5Mn1.5O4/Li4Ti5O12 full-cell systems. ZENG et al. developed a flame-retardant electrolyte suitable for LiFePO4/SiO full-cell systems using DMMP as the main solvent. WU et al. dissolved lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the main salt in a novel phosphate ester main solvent, dimethyl(2-methoxyethoxy)methylphosphonate [dimethyl(2-methoxyethoxy)methylphosphonate, DMEMMP, Figure 14(h)]. This flame-retardant electrolyte has good compatibility with lithium metal sheets and is suitable for LiFePO4/Li battery systems. There are few reports on phosphazene compounds as solvents (cosolvents) for flame-retardant electrolytes [Fig. 14(i)-(j)]. ROLLINS et al. reported a fluorohexaalkoxycyclotriphosphazene [FM-2. Fig. 14(i)] cosolvent, which can improve the electrochemical stability window, thermal stability and safety performance, and is beneficial to the stability of the SEI film. This flame-retardant electrolyte has been successfully applied to the graphite/(lithium manganese oxide + ternary material) full battery system. When the amount used is 20%, it can significantly improve the cycle performance of the full battery.
4.2 Other types of flame-retardant solvents (co-solvents)
Fluorinated ethers and fluorinated carbonates are characterized by high or no flash point. These compounds are used as flame-retardant solvents by diluting highly volatile and flammable co-solvents, thus accounting for a large proportion (typically greater than 70%) in flame-retardant electrolytes. Furthermore, due to the electron-withdrawing effect of fluorine, these fluorinated solvent molecules are more easily reduced on the carbon-based anode surface, optimizing the SEI film, improving the electrochemical compatibility of the flame-retardant electrolyte and electrode materials, and enhancing battery performance. Fluorinated ethers reported in the literature for use in flame-retardant electrolytes for lithium batteries include: methylnonafluorobutyl ether (MFE, Fig. 15(a)), ethylnonafluorobutyl ether (EFE, Fig. 15(b)), DMTP (or TMMP, Fig. 12(m)), 1.1.2.2-tetrafluoroethyl-2.2.3.3-tetrafluoropropyl ether (F-EPE, Fig. 15(c)), 1.1.2.2-tetrafluoroethyl-2.2.2-trifluoroethyl ether (HFE, Fig. 15(d)), and hexafluoroisopropylmethyl ether (HFPM, Fig. 15(e)). Representative compounds of fluorocarbonates include: fluorocyclic carbonates [F-AEC, Fig. 15(f)], trifluoroethylmethyl carbonate [3.3.3-fluoroethylmethylcarbonate, F-EMC, Fig. 15(g)], TFPOM-C [4-(2.2.3.3-tetrafluoropropo-xymethyl)-1.3-dioxolan-2-one, Fig. 15(h)], and TFTFMP-C [4-(2.3.3.3-tetrafluoropropo-xymethyl)-1.3-dioxolan-2-one, Fig. 15(h)]. [uoro-2-trifluoromethyl-propyl)-1,3-dioxolan-2-one, Fig. 15(i)], bis(tetrafluoropropyl)carbonate [bis(2.2.3.3-tetrafluoro-propyl)carbonate, BTFP-C, Fig. 15(j)], bis(pentafluoropropyl)carbonate [bis(2.2.3.3.3-pentafluoro-propyl)carbonate, BPFP-C, Fig. 15(k)]. Recently, FAN et al. developed a high-voltage-resistant, non-flammable perfluorinated electrolyte by combining fluorinated ether HFE, fluorinated carbonate F-EMC, and FEC (fluoroethylene carbonate) [Fig. 16(a)-(c)], which has achieved great success in high-nickel ternary materials [NCM811, Fig. 16(d)] and 5V lithium cobalt phosphate [LCP, Fig. 16(e)] lithium metal batteries. Compared to non-perfluorinated electrolytes, this non-flammable electrolyte can protect lithium metal sheets, suppress the formation of lithium dendrites, and reduce the risk of battery short circuits [Fig. 16(f)]. In addition, sulfolane [sulfolane, TMS, Fig. 15(l)] and adiponitrile (ADN), which have high flash points and high oxidation stability, are also used as the main solvents for flame-retardant electrolytes.
5. Flame-retardant electrolyte based on high-concentration lithium salt
"High-concentration electrolytes" are a type of electrolyte system that has attracted much attention. Their lithium salt concentration reaches as high as 4 mol/L, far exceeding that of ordinary electrolytes (typically 1 mol/L). In high-concentration electrolytes, almost all solvents directly coordinate with lithium ions, giving them several unique advantages: high oxidation/reduction stability, facilitating the formation of highly stable interfacial films on graphite or lithium metal anodes, high thermal stability, flame retardancy or non-flammability, and passivation of the positive electrode Al current collector under high voltage. Phosphate esters, used as flame retardant additives or solvents, suffer from poor compatibility with anode materials (carbon-based or lithium metal anodes), severely limiting their application. The development of the "high-concentration electrolyte" concept has led to the direct use of phosphate esters (TMP, TFEP, TEP) as primary solvents (or even single solvents) in lithium-ion or lithium metal batteries. For example, WANG et al. found that a high-concentration electrolyte of 5.3 mol/L LiFSI/TMP exhibited good compatibility with graphite, with almost no capacity decay in the graphite/Li half-cell after 1000 cycles. This was attributed to the formation of a highly effective passivation film on the graphite anode by the high-concentration electrolyte (Figure 17). This high-concentration electrolyte was successfully applied to a high-voltage 5V graphite/lithium nickel manganese oxide full-cell system. Furthermore, a high-concentration electrolyte using carbonate as the sole solvent (1:1.1.LiFSI/DMC) also possesses flame-retardant properties and excellent electrochemical compatibility, significantly improving the room-temperature and high-temperature cycling performance of the high-voltage 5V graphite/lithium nickel manganese oxide full-cell system. Recently, ALVARADO et al. developed a high-concentration electrolyte (3 mol/kg LiFSI/TMS) using high-flash-point sulfolane as the sole solvent, which enabled the MCMB/lithium nickel manganese oxide full-cell to run for 1000 cycles. Developing flame-retardant, high-concentration electrolytes for use in next-generation high-voltage lithium-ion batteries or lithium metal batteries with high energy density will be a key research direction in the future.
6 Flame-retardant electrolytes based on ionic liquids
Ionic liquids consist of two parts: anions and cations. Common anions include , , TFSI-, and FSI-, while common cations include pyrroles, imidazoles, piperidines, and quaternary ammonium salts. Ionic liquids are characterized by extremely low volatility, non-flammability, a wide electrochemical stability window, strong solubility, and high thermal stability. They are suitable for use in high-voltage electrolytes and for preparing flame-retardant electrolytes to improve the safety of lithium batteries. For example, experiments by CHANCELIER et al. have demonstrated the high thermal stability and flame-retardant properties of two ionic liquids: 1-butyl-2,3-methylimidazoliumbis(trifluoromethanesulfonyl)imide [C1C4ImTFSI, Figure 18(a)] and N-methyl-N-butylpyrrole-di(trifluoromethanesulfonyl)imide [PYR14TFSI or BMP-TFSI, Figure 12(i)]. Nevertheless, due to the high viscosity of pure ionic liquids and their poor wettability with separator and electrode materials, the migration of lithium ions is greatly restricted. In addition, most ionic liquids have poor compatibility with carbon-based anodes, making it difficult for pure ionic liquids to be used directly as electrolytes in lithium batteries. For example, KIM et al. [132] directly applied pure N-methyl-N-butylpyrrolidiniumbis(fluorosulfonyl)imide [PYR14FSI, Fig. 18(b)]-based flame-retardant electrolyte to a non-carbon-based anode full battery system (LiFePO4/Li4Ti5O12), but the rate performance was poor. In fact, ionic liquids are usually mixed with solvents such as carbonates, sulfones, or fluorinated ethers to prepare flame-retardant high-performance electrolytes. Pyrrole ionic liquids used in combination with carbonates to prepare flame-retardant electrolytes include PYR14TFSI or BMP-TFSI [Fig. 12(i)], N-propyl-N-methylpyrrolidiniumbis(trifluoromethanesulfonyl)imide [N-propyl-N-methylpyrrolidiniumbis(trifluoromethanesulfonyl)imide, PYR13TFSI, Fig. 18(c)], and N-ethyl-2-methoxypyrroliniumbis(fluorosulfonyl)imide [N-ethyl-2-methoxypyrroliniumbis(fluorosulfonyl)imide, E(OMe)Pyrl-FSI, Fig. 18(d)]. KIM et al. reported that the electrolyte obtained by mixing E(OMe)Pyrl-FSI with carbonate solvents exhibits excellent flame-retardant properties and can ensure stable operation of the LiFePO4/Li system at a high temperature of 60°C (Fig. 19). Representative piperidine ionic liquids that are mixed with carbonates include N-methyl-N-propylpiperidiniumbis(trifluoromethanesulfonyl)imide [PP13TFSI, Fig. 18(e)] and 1-ethyl-1-methylpiperidiniumbis(trifluoromethanesulfonyl)imide [EMP-TFSI, Fig. 18(f)]. In addition, there are also application cases of quaternary ammonium salt ionic liquids, such as the mixed use of diethylmethyl-(2-methoxyethyl)ammoniumbis(trifluoromethylsulfonyl)azanide [N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammoniumbis(trifluoromethylsulfonyl)azanide, DMMA-TFSI, Fig. 18(g)] and sulfolane [Fig. 15(l)] in the NCM/graphite system, and the quaternary ammonium salt ionic liquids in Fig. 18(h) to (i) and the flame retardant solvent methyl nonafluorobutyl ether [MFE, Fig. 15(a)] in the LiFePO4/Li system.
Figure 18 shows the structural formulas of some ionic liquid cations and anions.
Figure 19 Flame retardancy test of binary electrolytes with ionic liquid E(OMe)Pyrl-FSI and carbonate solvent and their cycling performance at 60℃ in LiFePO4/Li battery system.
7 Flame-retardant gel polymer electrolyte
Flame-retardant polymer electrolytes are mainly divided into flame-retardant solid polymer electrolytes and flame-retardant gel polymer electrolytes. This article focuses on flame-retardant gel polymer electrolytes containing liquid solvents. As early as 1998, AKASHI et al. prepared a PAN-based flame-retardant gel polymer electrolyte by optimizing the ratio of polyacrylonitrile (PAN, Figure 20(a)), carbonate solvent, and LiPF6. Thermogravimetric analysis showed that LiPF6 lowered the carbonization temperature of the PAN-based gel polymer electrolyte and added carbonaceous material remaining after combustion. The correlation between the linear combustion rate and the carbonization temperature indicated that the flame-retardant properties of the PAN-based gel polymer electrolyte originated from the carbonaceous layer formed on the surface. LU et al. prepared a non-flammable gel polymer electrolyte suitable for the LiFePO4/Li system using a polytetrafluoroethylene microporous membrane (PTFE) as a support, cross-linked poly(ethylene glycol) and poly(glycidyl methacrylate) block copolymer [PEG-b-PGMA, Fig. 20(b)] as a filler, and LiTFSI as a lithium salt. Its ionic conductivity at 25°C reached 1.30 × 10⁻³ S/cm. LI et al. cross-linked polyaryletherketone (PAEK) nonwoven fabric with polyethylene glycol dimethacrylate (PEGDMA) to obtain a flame-retardant gel polymer electrolyte [PAEKNW-SPE, Fig. 20(c)], with a room temperature ionic conductivity of 1.20 × 10⁻³ S/cm. Recently, BAIK et al. prepared a non-flammable gel polymer electrolyte with high thermal stability and a wide electrochemical stability window [up to 5V (vs. Li/Li⁺)] by cross-linking perfluoropolyether [FluorolinkE10H, Fig. 20(d)]. Flame-retardant gel polymer electrolytes based on ionic liquids have also been reported. LEE et al. successfully applied a gel polymer electrolyte based on poly(1-methyl-3-(2-acryloyloxy-hexyl)imidazolium tetrafluoroborate) [PIL, Fig. 20(e)] to LiCoO2 batteries. LUO et al. [154] developed a flame-retardant gel polymer electrolyte based on phenolic epoxy resin oligomeric ionic liquid [imidazolium type, OIL, Fig. 20(f)] and PVDF-HFP, which has high thermal stability (150℃, thermal shrinkage <1%) and room temperature ionic conductivity as high as 2.0×10-3S/cm. In 2018, GUO et al. prepared a flame-retardant gel polymer electrolyte membrane (IL-GPE, Figure 21) by mixing the ionic liquid 1-ethyl-2,3-methylimidazolium triluoromethanesufonate [EMITFSI, Figure 20(g)], the inorganic fast ion conductor Li1.5Al0.5Ge1.5(PO4)3 (LAGP), and PVDF-HFP. This membrane was used for high-safety lithium metal batteries. IL-GPE exhibits excellent compatibility with lithium metal and no lithium dendrites appear, which can significantly improve the cycle performance of the LiFePO4/Li system.
Figure 20. Polymers or ionic liquids in flame-retardant gel polymer electrolytes.
Figure 21 An organic-inorganic composite gel polymer electrolyte (ILGPE) and its flame retardancy test.
8 Outlook
The development of high-safety flame-retardant electrolytes is a crucial step in the large-scale application of lithium batteries. Currently, the use of flame-retardant electrolytes generally improves the safety performance of lithium batteries, but at the cost of some electrochemical performance. In summary, the future development of flame-retardant electrolytes shows the following trends.
(1) Develop multifunctional flame retardant additives or flame retardant solvents with excellent electrode interface compatibility, that is, not only have flame retardant effect, but also have interfacial film-forming function; or combine flame retardants and functional film-forming additives to play a synergistic role.
(2) Study the properties of the flame-retardant electrolyte and electrode interface through in-situ characterization methods, study its redox decomposition mechanism, and provide feedback guidance for the optimization and improvement of the flame-retardant electrolyte; strengthen the study of the flame-retardant mechanism of each flame-retardant electrolyte system.
(3) Develop practical technology for polymer microcapsule encapsulation of flame retardants to ensure that flame retardants are only used in extreme runaway situations and verify this technology in high-capacity batteries.
(4) Develop more high-concentration flame-retardant electrolyte systems with excellent electrode interface compatibility.
(5)开发新型低黏度阻燃型离子液体基阻燃电解液;或开发性能优异的阻燃型凝胶聚合物电解质。
(6)不能单纯只评估阻燃电解液的阻燃效果，还需在大容量电池中全面评估阻燃电解液的实用性，包括电化学性能测试、滥用性测试、热安全性测试(ARC设备)等。
