锂离子电池高安全性阻燃电解液研究进展

许高洁，王 晓，陆 迪,2，姜苗苗，黄苏琪,3，上官雪慧,4，崔光磊



锂离子电池高安全性阻燃电解液研究进展

许高洁1，王 晓1，陆 迪1,2，姜苗苗1，黄苏琪1,3，上官雪慧1,4，崔光磊1

（1中国科学院青岛生物能源与过程研究所，青岛储能产业技术研究院，山东 青岛 266101；2中国海洋大学化学化工学院，山东 青岛 266100；3青岛大学化学化工学院，山东 青岛 266071；4中国科学院青海盐湖研究所，青海 西宁 810008）

商品锂离子电池在机械冲击、热冲击和过充短路等滥用条件下易发生起火燃烧甚至爆炸。为了解决这一安全性问题，需要开发高安全性阻燃电解液取代传统易燃烧的碳酸酯电解液。本文综述了高安全性阻燃电解液的研究进展，首先介绍了燃烧机理、阻燃机理和阻燃测试方法，再阐述锂离子电池对阻燃电解液的性质要求，并对阻燃电解液进行分类探讨，包括阻燃添加剂、阻燃溶剂（共溶剂）、高浓度阻燃电解液、离子液体和阻燃型凝胶聚合物电解质。重点对这些高安全性阻燃电解液的配方、阻燃效果、适用的电池体系进行详细阐述。最后对高安全性阻燃电解液未来的研究方向进行展望。

锂离子电池；安全性；阻燃电解液

商品锂离子电池具有能量和功率密度高、无记忆效应、循环寿命长和环境友好等优点，其应用正迅速从消费电子品领域拓展到电动汽车和新能源储能领域[1]。然而，近年来，随着锂离子电池的大规模推广应用，在世界范围内，每年都会发生大量与锂离子电池滥用热失控相关的安全事故，学术界和产业界也在不断重视和加强对锂离子电池安全性的探究和提高（图1）[2-3]。造成热失控（冒烟、起火燃烧、爆炸）的滥用条件主要有机械滥用（如挤压、针刺）、电滥用（如过充、内部短路）和热滥用（如过热冲击）等。从锂离子电池热失控过程的链式反应定性描述图中（图2）可以看出，电解液在锂离子电池热失控过程中扮演的角色非常关键[2]。商品锂离子电池中通常采用碳酸酯基电解液，由锂盐（如六氟磷酸锂LiPF6）与闪点低、高度可燃、电化学稳定性差的碳酸酯溶剂（如碳酸乙烯酯EC、碳酸丙烯酯PC、碳酸二甲酯DMC、碳酸二乙酯DEC、碳酸甲乙酯EMC）组成[4-5]。目前，从材料角度看，防止锂离子电池热失控起火燃烧爆炸的安全性改进策略众多，如采用阻燃电解液[4-9]、阻燃型耐热收缩隔膜[1,7,10-15]、固态电解质[16]、结构稳定性高的电极材料（如磷酸铁锂LFP）[7]等。这其中，发展高安全性阻燃电解液是最经济简单的策略，能够有效降低锂离子电池热失控燃烧爆炸风险（概率），并极大降低热失控带来的人员财产伤害。本文将结合文献资料首先介绍燃烧机理、阻燃机理和阻燃测试方法；再阐述锂离子电池对阻燃电解液的性质要求；对阻燃电解液进行分类探讨论述，包括基于阻燃添加剂的阻燃电解液、基于阻燃溶剂（共溶剂）的阻燃电解液、基于高浓度锂盐的阻燃电解液、基于离子液体的阻燃电解液和阻燃型凝胶聚合物电解质；最后，对高安全性阻燃电解液未来的研究方向进行展望。

图1 锂离子电池失效引发的安全事故及相关联的滥用条件[2]

图2 锂离子电池热失控过程中链式反应的定性描述[2]

1 燃烧机理、阻燃机理和阻燃测试方法

1.1 燃烧机理

燃烧过程是一个复杂的化学反应过程（图3），需同时具备3个重要条件，即热（heat）、氧化剂（oxidizer）和燃料（fuel），燃料是燃烧的物质，氧化剂是产生氧气让燃料可以燃烧的物质，而热是驱动燃烧过程的能量[17-18]。维持燃烧通常依赖自由基的产生，基态O2吸收热量产生反应活性非常高的单线态氧（singlet oxygen, O2*），如式(1)所示；与此同时，有机质（燃料）吸收热量产生氢自由基（H•），如式(2)所示；O2*与H•结合产生氢过氧化物自由基（HOO*•），如式(3)所示；HOO*•分解产生氢氧自 由基（HO•），如式(4)所示[17]。这些自由基寿命 极短但反应活性非常强，会产生大量的热/火焰。总之，这种自由基机制意味着氢自由基（H•）、单线态氧（O2*）和氢氧自由基（HO•）在维持燃烧过程中扮演着极为重要的角色。

图3 燃烧反应图及公认的燃烧维持机制[17]

1.2 阻燃机理

1.3 阻燃测试方法

在锂离子电池电解液领域，最常用的测试指标是自熄灭时间（self-extinguishing time，SET），即一个点燃的电解液混合物样品持续燃烧的时间。另一个重要指标是极限氧指数（limited oxygen index，LOI），即让电解液燃烧保持至少60 s的O2/N2混合物中O2的占比。SET值越小，LOI值越高，电解液（凝胶聚合物电解质）越不容易燃烧。SET和LOI主要通过ASTM、UL和IEC的一些标准测试方法（如ASTM D-5306, ASTM D2863, UL-94VO, IEC 62133）来确定[17,20]。XU等[21]建议根据SET值将电解液归为3类：如果SET小于6 s/g，定义为不燃（non-flammable）；如果SET在6～20 s/g之间，定义为阻燃（flame-retarded）；如果SET大于20 s/g，定义为可燃（flammable）。

差示扫描量热法（differential scanning calorimetry，DSC）和绝热加速量热法（accelerating rate calorimeter，ARC）被用来评估阻燃电解液的热稳定性，即测定其放热量和失控温度[17,22]。目前ARC设备的生产商主要有HEL、THT和NETZSCH。闪点（flash point，FP）的测试对发展阻燃电解液也至关重要，闪点的概念为：样品被点燃的最低温度[1.103 bar（1 bar=105Pa）条件下][17,20]。HESS等[20]对闪点的测试方法进行了详细论述。

2 阻燃电解液性质要求

如果采用阻燃电解液能够保证锂离子电池的安全性能，牺牲一部分电化学性能是可以接受的。而通常情况下，阻燃电解液的使用也总会伴随着锂离子电池电化学性能的下降，尤其是循环寿命和倍率性能。HAREGEWOIN等[6]和NAGASUBRAMANIAN等[17]总结了对阻燃电解液（阻燃成分）的性质要求：①与正负极材料兼容性好，特别是碳基负极材料；对电池电化学性能毒副作用小；②对基液的锂离子电导率影响不大、不影响基液的闪点、易与基液混合；③能够产生消除燃烧自由基的自由基，低自放热速率（ARC测试）；④低溶剂化能力，不燃性或可燃性低，低火焰传播速率；⑤无毒或低毒，燃烧产物无毒或低毒、黏度低、不易挥发、环境友好；⑥高电压稳定性；⑦能够浸润隔膜和电极材料；⑧放热起始温度高且总放热量低（采用DSC和ARC 测试）。

3 基于阻燃添加剂的阻燃电解液

3.1 含磷元素阻燃添加剂

锂离子电池阻燃电解液研究最早和最多的一类阻燃剂是含磷元素的有机阻燃添加剂，主要分为（卤代）磷酸酯类阻燃添加剂、（卤代）亚磷酸酯类阻燃添加剂、（卤代）膦酸酯类阻燃添加剂、磷腈类阻燃添加剂等。

3.1.1 磷酸酯类阻燃添加剂

研究最早的短碳链烷基磷酸酯类阻燃添加 剂[19,21,23-25]，如磷酸三甲酯[trimethyl phosphate，TMP，图4(c)]、磷酸三乙酯[triethyl phosphate，TEP，图4(i)]、磷酸三丁酯[tributylphosphate，TBP，图4(b)]等，捕捉燃烧自由基能力强，阻燃效果良好。但是这些烷基磷酸酯通常黏度较大且与电极材料（尤其碳基负极）兼容性差，加入基液后在提高电解液阻燃性的同时会降低电解液的离子电导率并极大缩短电池循环寿命。提高烷基磷酸酯电化学稳定性的途径有：①芳香基团（苯基）取代烷基基团，如磷酸三苯酯[triphenylphosphate，TPP，图4(a)][23,25-30]、4-异丙基苯基二苯基磷酸酯[4-isopropyl phenyl diphenyl phosphate，IPPP，图4(d)][31]、三（4-甲氧基苯基）磷酸酯[tri-(4-methoxythphenyl) phosphate，TMPP，图4(e)][32]、磷酸甲苯二苯酯[cresyl diphenyl phosphate，CDP，图4(f)][33-34]、二苯基磷酸辛酯[diphenyl octyl phosphate，DPOF，图4(g)][35-38]；②增加烷基基团的碳含量，如磷酸三辛酯[trioctyl Phosphate，TOP，图4(h)][34]；③采用环状磷酸酯，如乙烯乙基磷酸酯[ethylene ethyl phosphate，EEP，图4(j)][39]；最近，TSUBOUCHI等[40]通过双三氟甲烷磺酰亚胺钾（KTFSA）添加剂修饰负极SEI膜来提高TMP基（50%，体积百分数）阻燃电解液与石墨负极的兼容性、库仑效率提升明显。LIU等[30]设计了一种非常新颖的热“智能”阻燃的无纺布静电纺丝隔膜，静电纺丝纤维由TPP内核和偏氟乙烯-六氟丙烯共聚物（PVDF-HFP）外壳组成，锂离子电池正常运行时，TPP不会对电池的循环性能造成影响，但当电池过热时，PVDF-HFP外壳熔断破碎，释放出TPP阻燃剂，捕捉燃烧自由基，从而阻止锂离子电池热失控燃烧爆炸。

图4 磷酸酯类阻燃添加剂结构式

图5 热“智能”阻燃无纺布静电纺丝隔膜示意图[30]

卤代磷酸酯含有卤素和P两种阻燃元素，阻燃效果更佳。廉价的磷酸三（β-氯乙基）酯[tri(β-chloromethyl) phosphate，TCEP，图4(k)]含有氯和磷两种阻燃元素，其分解产物氯乙烷不但具有阻燃性，而且具有强烈的制冷作用[41-44]。为了最大限度地减少TCEP对电池电化学性能的影响，BAGINSKA等[44]通过原位聚合的手段将TCEP包裹在核-壳结构的聚脲醛树脂微胶囊里。最近，ASPERN 等[45]开发了两种氟代磷酸酯阻燃添加剂，磷酸三（2,2,3,3,3-五氟丙基）酯[tris(2,2,3,3,3-pentafluoropropyl) phosphate，5F-TPrP，图4(l)]和磷酸三(1,1,1,3,3,3-六氟-2-丙基)酯[tris(1,1,1,3,3,3- hexafluoropropan- 2-yl) phosphate，HFiP，图4(m)]。少量（1%，质量百分数）使用5F-TPrP和HFiP能够通过修饰正极电解质界面膜（CEI）提高高电压三元正极（NCM111）的循环稳定性，5F-TPrP使用量达到20%（质量百分数）才能保证电解液不燃烧（约13%具有阻燃性）。

3.1.2 亚磷酸酯类阻燃添加剂

亚磷酸酯也是一类非常重要的阻燃添加 剂[24,27,45-50]，如亚磷酸三甲酯[trimethyl phosphite，TMP(i)，图6(a)][24]、亚磷酸三苯酯[triphenylphosphite，TPP(i)，图6(b)][27,47]、亚磷酸三乙酯[triethyl phosphite，TEP(i)，图6(c)][46]、磷酸三丁酯[tributylphosphite，TBP(i)，图6(d)][46]、三(2,2,2-三氟乙基)亚磷酸盐[tris(2,2,2-trifluoroethyl) phosphite，TTFP(i)，图6(e)][48-50]、亚磷酸三(1,1,1,3,3,3-六氟-2-丙基)酯[tris(1,1,1,3,3,3-hexafluoropropan-2-yl) phosphite，THFPP(i)，图6(f)]等。亚磷酸酯类化合物相对于磷酸酯类化合物更稳定，这是因为P—O单键反应活性低于P==O双键[46]。另外，亚磷酸酯类化合物更有利于形成稳定性高的SEI膜，能够通过使五氟化磷（PF5）失活而稳定锂盐六氟磷酸锂（LiPF6），也能消除电解液中游离的氢氟酸（HF）[51]。WANG等[49]将氟代亚磷酸酯TTFP(i)基的碳酸酯电解液应用于高能量密度的锂硫电池中，当TTFP(i)的浓度大于10%时，碳酸酯电解液阻燃甚至不燃。最重要的是，锂硫电池的电化学性能得到了极大的提升，10 C倍率下循环750圈容量几乎没有损失，而且容量高达800 mA·h/g。PIRES等[50]将TTFP(i)用作正极成膜添加剂提高富锂正极材料的循环稳定性，并且可以明显改善电解液的热稳定性。

图6 亚磷酸酯类阻燃添加剂结构式

3.1.3 膦酸酯类阻燃添加剂

据报道，由于磷元素含量高，烷基膦酸酯类阻燃剂的阻燃能力高于烷基磷酸酯和烷基亚磷酸 酯[52]，常见的烷基膦酸酯阻燃剂有甲基膦酸二甲酯[dimethyl methylphosphonate，DMMP，图7(a)][52-55]、乙基膦酸二乙酯[diethyl ethylphosphonate，DEEP，图7(b)][52-55]。为了提高烷基膦酸酯与碳基负极的兼容性，苯基取代、卤素取代、噻吩甲基取代的膦酸酯类阻燃剂也被陆续开发出来，如苯基膦酸二乙酯[diethyl phenylphosphonate，DPP，图7(c)][27]、双(2,2,2-三氟乙基)甲基膦酸酯[bis(2,2,2-trifluoroethyl) methylphosphonate，TFMP，图7(d)][25,27,56]、双(2,2,2-三氟乙基)乙基膦酸酯[bis(2,2,2-trifluoroethyl) ethylphosphonate，TFEP，图7(e)][57]、2-(噻吩甲基)膦酸二乙酯[diethyl(thiophen-2-ylmethyl)phosphonate，DTYP，图7(f)][58]。最近，ZHU等[58]通过理论计算设计开发的DTYP是一种多功能阻燃添加剂（图8），噻吩会通过自由基聚合在正极预先形成一层离子传导性优异的CEI膜，膦酸酯中的氧通过路易酸碱配位作用可以中和消除电解液中的PF5，膦酸酯可以有效终止燃烧过程中的自由基连锁反应。DTYP被成功应用于5 V高电压镍锰酸锂（LiNi0.5Mn1.5O4）电池中，能够明显提升电池循环性能。

图7 膦酸酯类阻燃添加剂结构式

图8 DTYP分子结构式及其相关功能[58]

3.1.4 磷腈类阻燃添加剂

磷腈类化合物，是复合型阻燃添加剂，主要包括小分子环状磷氮化合物和高分子线性磷氮化合物（图9）[8,21,59-72]。磷腈类阻燃添加剂的主要特点是少量添加（5%～15%，质量百分数）即可达到使电解液阻燃或不燃的效果，且与电极材料兼容性好，对锂离子电池的电化学性能影响小。用于电解液阻燃的磷腈类阻燃添加剂主要分为以下几类：六烷氧基环三磷腈，如六甲氧基环三磷腈[hexamethox- ycyclotriphosphazene，HMPN，图9(a)][21,59]、六(甲氧基乙氧基乙氧基)环三磷腈[hexa(methoxyethox- yethoxy)cyclotriphosphazene，MEE trimer，图9(b)][60]、不饱和烷氧基环三磷腈[AL-7，图9(c)][61]、六(2,2,2-三氟乙氧基)环三磷腈[hexakis(2,2,2- trifluoroethoxy)cyclotriphosphazene，HFEPN，图9(d)][62]；单烷氧基五氟环三磷腈，如乙氧基五氟环三磷腈[(ethoxy)pentafluorocyclotriphosphazene，PFPN，图9(e)][8,63-65]、苯氧基五氟环三磷腈[pentafluoro(phenoxy)cyclotriphosphazene，FPPN，图9(f)][66-68]、4-甲氧基-苯氧基五氟环三磷腈[(4-methoxy)-phenoxy pentafluorocyclotriphosphazene，4-MPPFPP，图9(g)][69]、2-氯-4-甲氧基-苯氧基五氟环三磷腈[(2-chloro-4-methoxy)-phenoxy pentafluorocycl- otriphosphazene，2-Cl-4-MPPFPP，图9(h)][70]；线性聚磷腈，如聚[双(甲氧基乙氧基乙氧基)磷腈][poly[bis(methoxyethoxyethoxy)phosphazene]，MEEP，图9(i)][60]、聚[双(乙氧基乙氧基乙氧基)磷腈][poly[bis(ethoxyethoxyethoxy)phosphazene]，EEEP，图9(j)][71]；磷腈小分子[triethoxyphosphazen--phosphoryldiethylester，PNP，图9(k)][72]。由于 环三磷腈类阻燃添加剂具有较高的电化学氧化窗口，在下一代高电压锂离子中的应用案例较多，如采用高电压钴酸锂正极材料或采用5 V高电压镍锰酸锂（LiNi0.5Mn1.5O4）材料的锂离子电池[8,62-68]。最近，XU等[8]将7%阻燃添加剂PFPN与功能添加剂联用，即能大幅度提高5 V高电压LiNi0.5Mn1.5O4/石墨全电池的长循环性能（1 C倍率下循环300圈，容量保持率87.4%），又能让电解液具有不燃性（该阻燃电解液配方可以经受5次点火实验而不被点燃，图10）。

3.1.5 其它含磷元素阻燃添加剂

已报道的含磷阻燃添加剂还有三(4-氟苯基)磷化氢[tris(4-fluorophenyl) phosphine，TFPP，图11(a)][48]、六甲基磷酰三胺[hexamethylphosphoramide，HMPA，图11(b)][73]、二(,-二乙基)甲氧基乙氧基甲基磷酰胺[bis(N,N-diethyl)(2-methoxyethoxy) methylphos- phonamidate，DEMEMPA，图11(c)][74]、富含磷酰基的阻燃离子-新型阻燃锂盐[Li[P(DPC)3]，图11(d)][75]、聚(磷酸乙酯-乙二醇)共（寡）聚物[EPCP，图11(e)][9]。值得注意的是，最近，WANG等[9]将离子传导性优异的EO片段嵌入到磷酸酯中形成具有阻燃性EPCP寡聚物，该寡聚物添加量为15%时，电解液完全不燃，且电解液的离子电导率不受EPCP加入影响。另外EPCP还有利于电极材料界面的稳定，因此，采用钴酸锂正极的锂离子电池（LiCoO2/Li）循环性能和倍率性能均有所提升。

图9 磷腈类阻燃添加剂结构式

图10 添加7% PFPN阻燃添加剂的阻燃效果（自熄灭5次）[8]

图11 其它含磷阻燃添加剂结构式

3.2 其它类型阻燃添加剂

其它类型阻燃添加剂主要有硅烷类阻燃添加剂[图12(a)~(d)][76-80]、三嗪类阻燃添加剂[图12(e)~(g)][81-82]、离子液体类阻燃添加剂[图12(h)~(l)][83-85]、氟代烷氧烃（氟醚类）阻燃添加剂[图12(m)][86]、双酚类阻燃添加剂[图12(n)][87]、全氟烷酮[图11(o)][88]、烯丙基三(2,2,2-三氟乙基)碳酸酯[allyl tris(2,2,2-trifluoroethyl) carbonate，ATFEC][89]。硅烷类（有机硅）有机阻燃剂的主要特点是热稳定性优异、可燃性低、毒性小、电导率和分解电压高，在锂离子电池新型电解液领域备受关注，QIN等[76]综述了有机硅电解液的研究进展。以乙烯基-三-(2-甲氧基乙氧基)硅烷[vinyl-tris-(methoxydiethoxy)silane，VTMS，图12(a)][77]为例，VTMS是一种环境友好的阻燃添加剂，利于形成有效的SEI膜阻止碳酸丙烯酯（PC）在石墨表面的分解，且热稳定性高，黏度低，对锂离子电池（LiCoO2体系）电化学性能负面影响较小。YIM等[86]将十氟-3-甲氧基-2-三氟甲基戊烷[1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)- pentane，DMTP，图12(m)]用交联型聚甲基丙烯酸甲酯（PMMA）包裹，并通过PVDF-HFP黏结剂涂覆固定在聚乙烯微孔膜上，在电池过热时，PMMA外壳破碎释放DMTP阻燃剂包括电池安全[图13(a)]。最近，JIANG等[88]设计了一种具有自冷却功能的阻燃复合电解液[图13(b)]，其核心组分是全氟-2-甲基-3-戊酮[perfluoro-2-methyl-3-pentanone，PFMP，图12(o)]。尽管该电解液能够提高锂离子电池三元正极材料（NCM111）的热稳定性，但是其与电极材料的电化学兼容性还有待提高。

图12 其它类型阻燃添加剂结构式

图13 （a）将包裹有DMTP的PMMA聚合物微胶囊用PVDF-HFP黏结剂固定在聚乙烯隔膜上[86]；（b）具有双重保护 机制的复合电解液[88]

4 基于阻燃溶剂（共溶剂）的阻燃电解液

4.1 含磷元素阻燃溶剂（共溶剂）

含磷的化合物作为阻燃添加剂，在某些条件下其阻燃效果可能达不到电池的使用需求，因此需要增加含磷化合物的使用量作为电解液的阻燃共溶剂，但是总体上各种类的含磷化合物与电极材料兼容性较差，作为溶剂使用难度较大。

在阻燃电解液研究的早期，主要采用烷基磷酸酯类阻燃共溶剂的有TMP[图4(c)][19,21]、TEP[图4(i)][21,90]、TPP[图4(a)][91-92]，但这些烷基磷酸酯与石墨负极兼容性差；相比之下，氟代磷酸酯类化合物与电极材料兼容性更好（利于稳定SEI膜）、阻燃效果更佳（用量少）、黏度低，作为电解液共溶剂更受关注：如三(2,2,2-三氟乙基)磷酸酯[tris(2,2,2-trifluoroethyl)phosphate，TFP，图14(a)][93-98]，二(2,2,2-三氟乙基)-甲基磷酸酯[bis(2,2,2-trifluoroethyl) methyl phosphate BMP，图14(b)][94-95]，(2,2,2-三氟乙基)-二乙基磷酸酯[(2,2,2-trifluoroethyl) diethyl phosphate，TDP，图14(c)][94-95]，三(2,2,2-二氟乙基)磷酸酯[tris(2,2-difluoroethyl) phosphate， TFHP，图14(d)][96]，磷酸三丙酯[tripropyl phosphate， TPrP，图14(e)][99]，三(3,3,3-三氟丙基)磷酸酯[tris(3,3,3-trifluoropropyl) phosphate，3F-TPrP，图14(f)][99]，三(2,2,3,3-四氟丙基)磷酸酯[tris(2,2,3,3-tetrafluoropropyl) phosphate，4F-TPrP，图14(g)][99]，三(2,2,3,3,3-五氟丙基)磷酸酯[tris(2,2,3,3,3-pentafluoropropyl) phosphate，5F-TPrP，图4(l)][99]。最近，MURMANN等[99]研究不同氟化程度的TPrP的阻燃效果和电化学性能，结果显示，氟化程度最高的5F-TPrP作为溶剂使用量为30%时电解液完全不燃且几乎不会对石墨/ NCM111全电池的循环性能造成影响。ZHANG 等[100]发现，除了作为阻燃溶剂，亚磷酸酯TTFP(i)[图6(e)]还能够抑制碳酸丙烯酯（PC）对石墨材料的剥离，提高全电池在PC基电解液中的循环稳定性，尤其是高温条件下。

图14 含磷元素阻燃溶剂（共溶剂）结构式

膦酸酯中作为电解液阻燃溶剂（共溶剂）应用最多的是DMMP[图7(a)][91,101-105]。XIANG等[102]发现DMMP基阻燃电解液与Li4Ti5O12负极材料兼容性良好，该阻燃电解液被成功用于高能量密度高电压LiNi0.5Mn1.5O4/Li4Ti5O12全电池体系中。ZENG等[104]以DMMP为主溶剂开发出适用于LiFePO4/SiO全电池体系的阻燃型电解液。WU等[105]将双三氟甲烷磺酰亚胺锂（LiTFSI）作为主盐溶解于一种新型磷酸酯主溶剂中，二甲基(2-甲氧基乙氧基)甲基磷酸酯[dimethyl(2-methoxyethoxy) methylphosphonate，DMMEMP，图14(h)]，该阻燃型电解液与金属锂片兼容性良好，适用于LiFePO4/Li电池体系。磷腈类化合物作为阻燃电解液溶剂（共溶剂）的报道较少[图14(i)～(j)][21,106-107]，ROLLINS等[106]报道了一种氟代六烷氧基环三磷腈[FM-2，图14(i)]共溶剂，能够提高电化学稳定窗口、热稳定性和安全性能高，利于稳定SEI膜，该阻燃电解液被成功应用于石墨/（锰酸锂+三元材料）全电池体系中，当使用量为20%时，可以明显改善全电池的循环性能。

4.2 其它类型阻燃溶剂（共溶剂）

氟代醚类和氟代碳酸酯类有机化合物的特点是闪点高或者是没有闪点。该类化合物作为阻燃溶剂是通过稀释高挥发和易燃性共溶剂起作用，所以在阻燃电解液中占比较大（通常大于70%）[51]。另外，借助氟元素的吸电子效应，该类氟代化合物溶剂分子更容易在碳基负极表面还原，优化SEI膜，改进阻燃电解液与电极材料的电化学兼容性，提高电池的性能。文献报道过的用于锂电池阻燃电解液的氟代醚有：甲基九氟丁醚[methyl nonafluorobutyl ether ，MFE，图15(a)][108-110]，乙基九氟丁醚[ethyl nonafluorobutyl ether，EFE，图15(b)][94-95]，DMTP[或称为TMMP，图12(m)][111-112]，1,1,2,2-四氟乙基2,2,3,3-四氟丙醚[1,1,2,2-tetrafluoroethyl-2,2,3,3- tetrafluoropropyl ether，F-EPE，图15(c)][113-115]，1,1,2,2-四氟乙基-2,2,2-三氟乙基醚[1,1,2,2- tetrafluoroethyl 2,2,2-trifluoroethyl ether，HFE，图15(d)][116]，六氟异丙基甲醚[1,1,1,3,3,3-hexafluoroi- sopropyl methyl ether，HFPM，图15(e)][117]。氟代碳酸酯的代表性化合物有：氟代环状碳酸酯[F-AEC，图15(f)][113]，三氟乙基甲基碳酸酯[3,3,3-fluoroethylmethyl carbonate，F-EMC，图15(g)][113,116]，TFPOM-C[4-(2,2,3,3-tetrafluoropropo- xymethyl)-1,3-dioxolan-2-one，图15(h)][118]， TFTFMP- C[4-(2,3,3,3-tetrafluoro-2-trifluoromethyl-propyl)- 1,3- dioxolan-2-one，图15(i)][118]，双（四氟丙基）碳酸酯[bis(2,2,3,3-tetrafluoro-propyl) carbonate，BTFP-C，图15(j)][118]，双（五氟丙基）碳酸酯[bis(2,2,3,3,3- pentafluoro-propyl) carbonate，BPFP-C，图15(k)][118]。最近，FAN等[116]将氟代醚HFE、氟代碳酸酯F-EMC和FEC（氟代碳酸乙烯酯）联用开发了一种耐高电压的不燃全氟电解液[图16(a)～(c)]，在高镍三元材料[NCM811，图16(d)]和5 V磷酸钴锂[LCP, 图16(e)]金属锂电池中取得了巨大的成功。相比非全氟电解液，该不燃电解液能够保护金属锂片，抑制锂枝晶的产生，减小电池短路风险[图16(f)]。另外，闪点较高，氧化稳定性高的环丁砜[sulfolane，TMS，图15(l)][119-121]，己二腈（adiponitrile，ADN）[122]也被用作阻燃电解液的主溶剂。

图15 其它类型阻燃溶剂（共溶剂）结构式

图16 氟代醚（HFE）和氟代碳酸酯（F-EMC和FEC）组成的不燃全氟电解液阻燃效果及其在金属锂电池中的应用[116]

5 基于高浓度锂盐的阻燃电解液

“高浓度电解液”是一类备受关注的电解液体系，其锂盐浓度高达 4 mol/L，远远高于普通电解液锂盐浓度（通常为1 mol/L）[4]。在高浓度电解液中，几乎所有溶剂都与锂离子直接配位，使其具有一些特殊的优点：高氧化/还原稳定性、利于在石墨负极或金属锂负极形成高稳定界面膜、高热稳定性、阻燃或者不燃，及高电压下钝化正极Al集流 体[4,123-125]。磷酸酯类化合物作为阻燃添加剂或阻燃溶剂都存在与负极材料（碳基负极或金属锂负极）兼容性差的问题，其应用收到严重的限制。“高浓度电解液”概念的发展促使磷酸酯类化合物（TMP[126-128]、TFEP[129]、TEP[130-131]）直接作为主溶剂（甚至单一溶剂）用于锂离子电池或金属锂电池中。例如，WANG等[126]发现5.3 mol/L LiFSI/TMP高浓度电解液与石墨的兼容性好，石墨/Li半电池循环1000圈容量几乎没有衰减，这得益于高浓度电解液能够在石墨负极形成一层非常有效的钝化膜（图17），该高浓度电解液被成功应用于高电压5 V 石墨/镍锰酸锂全电池体系。另外，碳酸酯作为单一溶剂的高浓度电解液（1∶1.1，LiFSI/DMC）也具有阻燃特性[123]，并具有优异的电化学兼容性，能够显著提高高电压5 V 石墨/镍锰酸锂全电池体系的室温循环性能和高温循环性能。最近，ALVARADO等[125]开发了以高闪点环丁砜为单一溶剂的高浓度电解液（3 mol/kg LiFSI/TMS），其能够让MCMB/镍锰酸锂全电池运行1000次循环。发展阻燃型高浓电解液用于高能量密度的下一代高电压锂离子电池或金属锂电池将是未来的重点研究方向。

图17 高安全性电池电解液设计理念[126]

6 基于离子液体的阻燃电解液

离子液体由阴、阳离子两部分组成, 阴离子通常有、、TFSI-、FSI-等，阳离子通常有吡咯类、咪唑类、哌啶类和季铵盐类等。离子液体具有挥发性极小、不燃、电化学稳定窗口宽、溶解能力强、热稳定性高等特点，既适合应用于高电压电解液，又适合制备阻燃型电解液，提高锂离子电池安全性[4,132-133]。例如，CHANCELIER 等[133]实验证实两种离子液体的高热稳定和阻燃特性，即1-丁基- 2,3-甲基咪唑-二(三氟甲基磺酰)亚胺[1-butyl-3-methylimidazolium bis(trifluoromethanesu- lfonyl)imide，C1C4ImTFSI，图18(a)]和-甲基--丁基吡咯-二(三氟甲基磺酰)亚胺[PYR14TFSI 或BMP-TFSI，图12(i)]。尽管如此, 由于纯离子液体黏度大，且与隔膜、电极材料的浸润性差，锂离子的迁移受到极大限制；另外，大多数的离子液体与碳基负极的兼容性差，因而，纯离子液体较难作 为电解液直接用于锂离子电池。例如，KIM等[132]将纯-甲基--丁基吡咯-双氟磺酰亚胺[-butyl-- methylpyrrolidinium bis(fluorosulfonyl) imide，PYR14FSI，图18(b)]基阻燃型电解液直接应用于非碳基负极全电池体系（LiFePO4/Li4Ti5O12），但倍率性能差。实际上，离子液体通常与碳酸酯类[134-141]、砜类[142]或氟代醚类[143]等溶剂混合使用来制备阻燃型高性能电解液。与碳酸酯混合使用配制阻燃型电解液的吡咯类离子液体有PYR14TFSI 或BMP-TFSI[图12(i)][134-135]、-丙基--甲基吡咯-二(三氟甲基磺酰)亚胺[-propyl--methylpyrrolidiniumbis (trifluoromethanesulfonyl) imide ，PYR13TFSI，图18(c)][136]、-乙基-2-甲氧基吡咯-双氟磺酰亚胺[-ethyl-2-methoxypyrrolinium bis(fluorosulfonyl) imide，E(OMe)Pyrl-FSI，图18(d)][137]。KIM等[137]报道E(OMe)Pyrl-FSI与碳酸酯溶剂混合后电解液阻燃效果优异，且能保证LiFePO4/Li体系60 ℃高温的稳定运行（图19）。与碳酸酯混合的代表性哌啶类离子液体有-甲基--丙基哌啶-二(三氟甲基磺酰)亚胺[-methyl--propylpiperidinium bis (trifluoromethanesulfonyl) imide，PP13TFSI，图18(e)][138-140]、1-乙基-1-甲基哌啶-二(三氟甲基磺酰)亚胺[1-ethyl-1-methyl piperidinium bis(trifluorome thanesulfonyl)imide，EMP-TFSI，图18(f)][141]。另外，季铵盐类离子液体也有应用案例，如二乙基甲基-(2-甲氧乙基)铵基-双(三氟甲基磺酰)亚胺[,-diethyl--methyl--(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)azanide，DMMA-TFSI，图18(g)]与环丁砜[图15(l)]混合使用于NCM/石墨体系[142]、图18(h)～(i)的季铵盐类离子液体与阻燃溶剂甲基九氟丁醚[MFE，图15(a)]混合使用于LiFePO4/Li体系[143]。

图18 一些离子液体阳离子和阴离子的结构式

图19 离子液体E(OMe)Pyrl-FSI与碳酸酯溶剂的二元电解液阻燃性测试及其在LiFePO4/Li电池体系中60 ℃高温的循环性能[137]

7 阻燃型凝胶聚合物电解质

阻燃型聚合物电解质主要分为阻燃型固态聚合物电解质[144-147]和阻燃型凝胶聚合物电解质[148-157]。本文主要关注含液体溶剂的阻燃型凝胶聚合物电解质。早在1998年，AKASHI等[148]通过优化聚丙烯腈[polyacrylonitrile，PAN，图20(a)]、碳酸酯溶剂和LiPF6的比例制备出一种PAN基的阻燃性凝胶聚合物电解质。热重分析表明LiPF6降低了PAN基凝胶聚合物电解质的碳化温度点并增加了燃烧后残留的碳质材料。线性燃烧速率与碳化温度点的相关性表明，PAN基凝胶聚合物电解质的阻燃特性源于表面形成的碳质层。LU等[149]以聚四氟乙烯微孔膜（PTFE）为支撑体，交联型聚（乙二醇）和聚（甲基丙烯酸缩水甘油酯）嵌段共聚物[PEG-b-PGMA，图20(b)]为填料，LiTFSI为锂盐制备出一种适用于LiFePO4/Li体系的不燃型凝胶聚合物电解质，其25 ℃离子电导率高达1.30×10-3S/cm。LI等[150]用聚乙二醇二甲基丙烯酸酯（PEGDMA）将聚芳醚酮（PAEK）无纺布交联，获得一种阻燃型凝胶聚合物电解质[PAEKNW-SPE，图20(c)]，其室温离子电导率为1.20×10-3S/cm。最近，BAIK等[151]将全氟聚醚[Fluorolink E10H，图20(d)]交联后制备出一种不燃、热稳定性高、电化学稳定窗口宽[高达5 V (vs. Li/Li+)]的凝胶聚合物电解质。基于离子液体的阻燃型凝胶聚合物电解质也有报道[153-157]。LEE等[153]成功将基于聚（1-甲基-3-（2-丙烯酰氧基己基）咪唑鎓-四氟硼酸盐）[poly(1-methyl-3-(2-acryloyloxy- hexyl) imidazolium tetrafluoroborate)，PIL，图20(e)]的凝胶聚合物电解质应用于LiCoO2电池中。LUO等[154]开发了一种基于酚醛环氧树脂寡聚离子液体[咪唑类，OIL，图20(f)]与PVDF-HFP的阻燃型凝胶聚合物电解质，其热稳定高（150 ℃，热收缩＜1%），室温离子电导率高达2.0×10-3S/cm。2018年GUO等[157]将离子液体1-乙基-2,3-甲基咪唑二(三氟甲基磺酰)亚胺[1-ethyl-3-methylimidazolium triluoromethanesufonate，EMITFSI，图20(g)]、无机快离子导体Li1.5Al0.5Ge1.5(PO4)3（LAGP）和PVDF-HFP混合制备一种阻燃型凝胶聚合物电解质膜（IL-GPE，图21）用于高安全性的金属锂电池，IL-GPE与金属锂兼容优异无锂枝晶产生，能够显著提高LiFePO4/Li体系循环性能。

图20 阻燃型凝胶聚合物电解质中的聚合物或离子液体

图21 一种有机无机复合的凝胶聚合物电解质（ILGPE）及其阻燃测试[126]

8 展 望

高安全性阻燃电解液的发展是锂离子电池大规模推广应用过程中至关重要的一环。目前，阻燃型电解液的使用通常能够提高锂离子电池的安全性能，但会牺牲一部分锂离子电池的电化学性能。综合全文所述，未来阻燃电解液的发展呈现以下趋势。

（1）发展电极界面兼容性优异的多功能阻燃添加剂或阻燃溶剂，即不仅具有阻燃效果，而且具有界面成膜功能；或将阻燃剂与功能成膜添加剂联用，发挥协同作用。

（2）通过原位表征手段研究阻燃电解液与电极界面性质，研究其氧化还原分解机制，反馈指导阻燃电解液优化改进；加强各阻燃电解液体系的阻燃机理研究。

（3）发展聚合物微胶囊包裹阻燃剂的实用技术，保证阻燃剂只在极端失控情况下起作用，并在大容量电池中验证此技术。

（4）发展更多电极界面兼容性优异的高浓度阻燃电解液体系。

（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。

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Research progress of high safety flame retardant electrolytes for lithium-ion batteries

XU Gaojie1, WANG Xiao1, LU Di1,2, JANG Miaomiao1, HUANG Suqi1,3, SHANGGUAN Xuehui1,4, CUI Guanglei1

(1Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao Industrial Energy Storage Research Institute, Qingdao 266101, Shandong, China;2College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, Shandong, China;3School of Chemistry and Chemical Engineering, Qingdao University, Qingdao 266071, Shandong, China;4Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, Qinghai, China)

Commercial lithium-ion batteries (LIBs) are easy to catch fire and even explode under abuse conditions such as mechanical shock, thermal shock, overcharge, and short circuit,. To resolve this safety concern, it is necessary to develop high safety flame retardant electrolytes replacing the highly flammable conventional carbonate-based electrolytes. This review presents the research progress of high safety flame retardant electrolytes for LIBs. Firstly, the mechanisms of combustion and flame retardant, together with the methods of flame retardant evaluation are introduced. Then, the LIBs demands on properties of flame retardant electrolytes are described, and flame retardant electrolytes are discussed by classifications: flame retardant additives; flame retardant solvents/cosolvents; highly concentrated electrolytes; ionic liquids; and flame retardant gel polymer electrolyte. The formulations, flame retardant effects, and applicable battery systems of these high safety flame retardant electrolytes are mainly focused. Finally, future research directions of high safety flame retardant electrolytes are prospected.

lithium-ion batteries; safety; flame retardant electrolytes

10.12028/j.issn.2095-4239.2018.0153

TQ 028.8

A

2095-4239（2018）06-1040-20

2018-08-18；

2018-09-01。

国家自然科学基金项目（51625204，51502319），山东省自然科学基金项目（ZR2016BQ18）。

许高洁（1987—），男，助理研究员，主要研究方向为锂离子电池电解液，E-mail：xugj@qibebt.ac.cn；

崔光磊，研究员，主要研究方向为电化学储能材料及器件，E-mail：cuigl@qibebt.ac.cn。