鋰離子電池高安全性阻燃電解液研究進展

許高潔，王 曉，陸 迪,2，姜苗苗，黃蘇琪,3，上官雪慧,4，崔光磊

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鋰離子電池高安全性阻燃電解液研究進展

許高潔1，王 曉1，陸 迪1,2，姜苗苗1，黃蘇琪1,3，上官雪慧1,4，崔光磊1

（1中國科學(xué)院青島生物能源與過程研究所，青島儲能產(chǎn)業(yè)技術(shù)研究院，山東 青島 266101；2中國海洋大學(xué)化學(xué)化工學(xué)院，山東 青島 266100；3青島大學(xué)化學(xué)化工學(xué)院，山東 青島 266071；4中國科學(xué)院青海鹽湖研究所，青海 西寧 810008）

商品鋰離子電池在機械沖擊、熱沖擊和過充短路等濫用條件下易發(fā)生起火燃燒甚至爆炸。為了解決這一安全性問題，需要開發(fā)高安全性阻燃電解液取代傳統(tǒng)易燃燒的碳酸酯電解液。本文綜述了高安全性阻燃電解液的研究進展，首先介紹了燃燒機理、阻燃機理和阻燃測試方法，再闡述鋰離子電池對阻燃電解液的性質(zhì)要求，并對阻燃電解液進行分類探討，包括阻燃添加劑、阻燃溶劑（共溶劑）、高濃度阻燃電解液、離子液體和阻燃型凝膠聚合物電解質(zhì)。重點對這些高安全性阻燃電解液的配方、阻燃效果、適用的電池體系進行詳細(xì)闡述。最后對高安全性阻燃電解液未來的研究方向進行展望。

鋰離子電池；安全性；阻燃電解液

商品鋰離子電池具有能量和功率密度高、無記憶效應(yīng)、循環(huán)壽命長和環(huán)境友好等優(yōu)點，其應(yīng)用正迅速從消費電子品領(lǐng)域拓展到電動汽車和新能源儲能領(lǐng)域[1]。然而，近年來，隨著鋰離子電池的大規(guī)模推廣應(yīng)用，在世界范圍內(nèi)，每年都會發(fā)生大量與鋰離子電池濫用熱失控相關(guān)的安全事故，學(xué)術(shù)界和產(chǎn)業(yè)界也在不斷重視和加強對鋰離子電池安全性的探究和提高（圖1）[2-3]。造成熱失控（冒煙、起火燃燒、爆炸）的濫用條件主要有機械濫用（如擠壓、針刺）、電濫用（如過充、內(nèi)部短路）和熱濫用（如過熱沖擊）等。從鋰離子電池?zé)崾Э剡^程的鏈?zhǔn)椒磻?yīng)定性描述圖中（圖2）可以看出，電解液在鋰離子電池?zé)崾Э剡^程中扮演的角色非常關(guān)鍵[2]。商品鋰離子電池中通常采用碳酸酯基電解液，由鋰鹽（如六氟磷酸鋰LiPF6）與閃點低、高度可燃、電化學(xué)穩(wěn)定性差的碳酸酯溶劑（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）組成[4-5]。目前，從材料角度看，防止鋰離子電池?zé)崾Э仄鸹鹑紵ǖ陌踩愿倪M策略眾多，如采用阻燃電解液[4-9]、阻燃型耐熱收縮隔膜[1,7,10-15]、固態(tài)電解質(zhì)[16]、結(jié)構(gòu)穩(wěn)定性高的電極材料（如磷酸鐵鋰LFP）[7]等。這其中，發(fā)展高安全性阻燃電解液是最經(jīng)濟簡單的策略，能夠有效降低鋰離子電池?zé)崾Э厝紵L(fēng)險（概率），并極大降低熱失控帶來的人員財產(chǎn)傷害。本文將結(jié)合文獻資料首先介紹燃燒機理、阻燃機理和阻燃測試方法；再闡述鋰離子電池對阻燃電解液的性質(zhì)要求；對阻燃電解液進行分類探討論述，包括基于阻燃添加劑的阻燃電解液、基于阻燃溶劑（共溶劑）的阻燃電解液、基于高濃度鋰鹽的阻燃電解液、基于離子液體的阻燃電解液和阻燃型凝膠聚合物電解質(zhì)；最后，對高安全性阻燃電解液未來的研究方向進行展望。

圖1 鋰離子電池失效引發(fā)的安全事故及相關(guān)聯(lián)的濫用條件[2]

圖2 鋰離子電池?zé)崾Э剡^程中鏈?zhǔn)椒磻?yīng)的定性描述[2]

1 燃燒機理、阻燃機理和阻燃測試方法

1.1 燃燒機理

燃燒過程是一個復(fù)雜的化學(xué)反應(yīng)過程（圖3），需同時具備3個重要條件，即熱（heat）、氧化劑（oxidizer）和燃料（fuel），燃料是燃燒的物質(zhì)，氧化劑是產(chǎn)生氧氣讓燃料可以燃燒的物質(zhì)，而熱是驅(qū)動燃燒過程的能量[17-18]。維持燃燒通常依賴自由基的產(chǎn)生，基態(tài)O2吸收熱量產(chǎn)生反應(yīng)活性非常高的單線態(tài)氧（singlet oxygen, O2*），如式(1)所示；與此同時，有機質(zhì)（燃料）吸收熱量產(chǎn)生氫自由基（H?），如式(2)所示；O2*與H?結(jié)合產(chǎn)生氫過氧化物自由基（HOO*?），如式(3)所示；HOO*?分解產(chǎn)生氫氧自 由基（HO?），如式(4)所示[17]。這些自由基壽命 極短但反應(yīng)活性非常強，會產(chǎn)生大量的熱/火焰?？傊?，這種自由基機制意味著氫自由基（H?）、單線態(tài)氧（O2*）和氫氧自由基（HO?）在維持燃燒過程中扮演著極為重要的角色。

圖3 燃燒反應(yīng)圖及公認(rèn)的燃燒維持機制[17]

1.2 阻燃機理

1.3 阻燃測試方法

在鋰離子電池電解液領(lǐng)域，最常用的測試指標(biāo)是自熄滅時間（self-extinguishing time，SET），即一個點燃的電解液混合物樣品持續(xù)燃燒的時間。另一個重要指標(biāo)是極限氧指數(shù)（limited oxygen index，LOI），即讓電解液燃燒保持至少60 s的O2/N2混合物中O2的占比。SET值越小，LOI值越高，電解液（凝膠聚合物電解質(zhì)）越不容易燃燒。SET和LOI主要通過ASTM、UL和IEC的一些標(biāo)準(zhǔn)測試方法（如ASTM D-5306, ASTM D2863, UL-94VO, IEC 62133）來確定[17,20]。XU等[21]建議根據(jù)SET值將電解液歸為3類：如果SET小于6 s/g，定義為不燃（non-flammable）；如果SET在6～20 s/g之間，定義為阻燃（flame-retarded）；如果SET大于20 s/g，定義為可燃（flammable）。

差示掃描量熱法（differential scanning calorimetry，DSC）和絕熱加速量熱法（accelerating rate calorimeter，ARC）被用來評估阻燃電解液的熱穩(wěn)定性，即測定其放熱量和失控溫度[17,22]。目前ARC設(shè)備的生產(chǎn)商主要有HEL、THT和NETZSCH。閃點（flash point，F(xiàn)P）的測試對發(fā)展阻燃電解液也至關(guān)重要，閃點的概念為：樣品被點燃的最低溫度[1.103 bar（1 bar=105Pa）條件下][17,20]。HESS等[20]對閃點的測試方法進行了詳細(xì)論述。

2 阻燃電解液性質(zhì)要求

如果采用阻燃電解液能夠保證鋰離子電池的安全性能，犧牲一部分電化學(xué)性能是可以接受的。而通常情況下，阻燃電解液的使用也總會伴隨著鋰離子電池電化學(xué)性能的下降，尤其是循環(huán)壽命和倍率性能。HAREGEWOIN等[6]和NAGASUBRAMANIAN等[17]總結(jié)了對阻燃電解液（阻燃成分）的性質(zhì)要求：①與正負(fù)極材料兼容性好，特別是碳基負(fù)極材料；對電池電化學(xué)性能毒副作用?。虎趯旱匿囯x子電導(dǎo)率影響不大、不影響基液的閃點、易與基液混合；③能夠產(chǎn)生消除燃燒自由基的自由基，低自放熱速率（ARC測試）；④低溶劑化能力，不燃性或可燃性低，低火焰?zhèn)鞑ニ俾?；⑤無毒或低毒，燃燒產(chǎn)物無毒或低毒、黏度低、不易揮發(fā)、環(huán)境友好；⑥高電壓穩(wěn)定性；⑦能夠浸潤隔膜和電極材料；⑧放熱起始溫度高且總放熱量低（采用DSC和ARC 測試）。

3 基于阻燃添加劑的阻燃電解液

3.1 含磷元素阻燃添加劑

鋰離子電池阻燃電解液研究最早和最多的一類阻燃劑是含磷元素的有機阻燃添加劑，主要分為（鹵代）磷酸酯類阻燃添加劑、（鹵代）亞磷酸酯類阻燃添加劑、（鹵代）膦酸酯類阻燃添加劑、磷腈類阻燃添加劑等。

3.1.1 磷酸酯類阻燃添加劑

研究最早的短碳鏈烷基磷酸酯類阻燃添加 劑[19,21,23-25]，如磷酸三甲酯[trimethyl phosphate，TMP，圖4(c)]、磷酸三乙酯[triethyl phosphate，TEP，圖4(i)]、磷酸三丁酯[tributylphosphate，TBP，圖4(b)]等，捕捉燃燒自由基能力強，阻燃效果良好。但是這些烷基磷酸酯通常黏度較大且與電極材料（尤其碳基負(fù)極）兼容性差，加入基液后在提高電解液阻燃性的同時會降低電解液的離子電導(dǎo)率并極大縮短電池循環(huán)壽命。提高烷基磷酸酯電化學(xué)穩(wěn)定性的途徑有：①芳香基團（苯基）取代烷基基團，如磷酸三苯酯[triphenylphosphate，TPP，圖4(a)][23,25-30]、4-異丙基苯基二苯基磷酸酯[4-isopropyl phenyl diphenyl phosphate，IPPP，圖4(d)][31]、三（4-甲氧基苯基）磷酸酯[tri-(4-methoxythphenyl) phosphate，TMPP，圖4(e)][32]、磷酸甲苯二苯酯[cresyl diphenyl phosphate，CDP，圖4(f)][33-34]、二苯基磷酸辛酯[diphenyl octyl phosphate，DPOF，圖4(g)][35-38]；②增加烷基基團的碳含量，如磷酸三辛酯[trioctyl Phosphate，TOP，圖4(h)][34]；③采用環(huán)狀磷酸酯，如乙烯乙基磷酸酯[ethylene ethyl phosphate，EEP，圖4(j)][39]；最近，TSUBOUCHI等[40]通過雙三氟甲烷磺酰亞胺鉀（KTFSA）添加劑修飾負(fù)極SEI膜來提高TMP基（50%，體積百分?jǐn)?shù)）阻燃電解液與石墨負(fù)極的兼容性、庫侖效率提升明顯。LIU等[30]設(shè)計了一種非常新穎的熱“智能”阻燃的無紡布靜電紡絲隔膜，靜電紡絲纖維由TPP內(nèi)核和偏氟乙烯-六氟丙烯共聚物（PVDF-HFP）外殼組成，鋰離子電池正常運行時，TPP不會對電池的循環(huán)性能造成影響，但當(dāng)電池過熱時，PVDF-HFP外殼熔斷破碎，釋放出TPP阻燃劑，捕捉燃燒自由基，從而阻止鋰離子電池?zé)崾Э厝紵ā?/p>

圖4 磷酸酯類阻燃添加劑結(jié)構(gòu)式

圖5 熱“智能”阻燃無紡布靜電紡絲隔膜示意圖[30]

鹵代磷酸酯含有鹵素和P兩種阻燃元素，阻燃效果更佳。廉價的磷酸三（β-氯乙基）酯[tri(β-chloromethyl) phosphate，TCEP，圖4(k)]含有氯和磷兩種阻燃元素，其分解產(chǎn)物氯乙烷不但具有阻燃性，而且具有強烈的制冷作用[41-44]。為了最大限度地減少TCEP對電池電化學(xué)性能的影響，BAGINSKA等[44]通過原位聚合的手段將TCEP包裹在核-殼結(jié)構(gòu)的聚脲醛樹脂微膠囊里。最近，ASPERN 等[45]開發(fā)了兩種氟代磷酸酯阻燃添加劑，磷酸三（2,2,3,3,3-五氟丙基）酯[tris(2,2,3,3,3-pentafluoropropyl) phosphate，5F-TPrP，圖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，圖4(m)]。少量（1%，質(zhì)量百分?jǐn)?shù)）使用5F-TPrP和HFiP能夠通過修飾正極電解質(zhì)界面膜（CEI）提高高電壓三元正極（NCM111）的循環(huán)穩(wěn)定性，5F-TPrP使用量達到20%（質(zhì)量百分?jǐn)?shù)）才能保證電解液不燃燒（約13%具有阻燃性）。

3.1.2 亞磷酸酯類阻燃添加劑

亞磷酸酯也是一類非常重要的阻燃添加 劑[24,27,45-50]，如亞磷酸三甲酯[trimethyl phosphite，TMP(i)，圖6(a)][24]、亞磷酸三苯酯[triphenylphosphite，TPP(i)，圖6(b)][27,47]、亞磷酸三乙酯[triethyl phosphite，TEP(i)，圖6(c)][46]、磷酸三丁酯[tributylphosphite，TBP(i)，圖6(d)][46]、三(2,2,2-三氟乙基)亞磷酸鹽[tris(2,2,2-trifluoroethyl) phosphite，TTFP(i)，圖6(e)][48-50]、亞磷酸三(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)，圖6(f)]等。亞磷酸酯類化合物相對于磷酸酯類化合物更穩(wěn)定，這是因為P—O單鍵反應(yīng)活性低于P==O雙鍵[46]。另外，亞磷酸酯類化合物更有利于形成穩(wěn)定性高的SEI膜，能夠通過使五氟化磷（PF5）失活而穩(wěn)定鋰鹽六氟磷酸鋰（LiPF6），也能消除電解液中游離的氫氟酸（HF）[51]。WANG等[49]將氟代亞磷酸酯TTFP(i)基的碳酸酯電解液應(yīng)用于高能量密度的鋰硫電池中，當(dāng)TTFP(i)的濃度大于10%時，碳酸酯電解液阻燃甚至不燃。最重要的是，鋰硫電池的電化學(xué)性能得到了極大的提升，10 C倍率下循環(huán)750圈容量幾乎沒有損失，而且容量高達800 mA·h/g。PIRES等[50]將TTFP(i)用作正極成膜添加劑提高富鋰正極材料的循環(huán)穩(wěn)定性，并且可以明顯改善電解液的熱穩(wěn)定性。

圖6 亞磷酸酯類阻燃添加劑結(jié)構(gòu)式

3.1.3 膦酸酯類阻燃添加劑

據(jù)報道，由于磷元素含量高，烷基膦酸酯類阻燃劑的阻燃能力高于烷基磷酸酯和烷基亞磷酸 酯[52]，常見的烷基膦酸酯阻燃劑有甲基膦酸二甲酯[dimethyl methylphosphonate，DMMP，圖7(a)][52-55]、乙基膦酸二乙酯[diethyl ethylphosphonate，DEEP，圖7(b)][52-55]。為了提高烷基膦酸酯與碳基負(fù)極的兼容性，苯基取代、鹵素取代、噻吩甲基取代的膦酸酯類阻燃劑也被陸續(xù)開發(fā)出來，如苯基膦酸二乙酯[diethyl phenylphosphonate，DPP，圖7(c)][27]、雙(2,2,2-三氟乙基)甲基膦酸酯[bis(2,2,2-trifluoroethyl) methylphosphonate，TFMP，圖7(d)][25,27,56]、雙(2,2,2-三氟乙基)乙基膦酸酯[bis(2,2,2-trifluoroethyl) ethylphosphonate，TFEP，圖7(e)][57]、2-(噻吩甲基)膦酸二乙酯[diethyl(thiophen-2-ylmethyl)phosphonate，DTYP，圖7(f)][58]。最近，ZHU等[58]通過理論計算設(shè)計開發(fā)的DTYP是一種多功能阻燃添加劑（圖8），噻吩會通過自由基聚合在正極預(yù)先形成一層離子傳導(dǎo)性優(yōu)異的CEI膜，膦酸酯中的氧通過路易酸堿配位作用可以中和消除電解液中的PF5，膦酸酯可以有效終止燃燒過程中的自由基連鎖反應(yīng)。DTYP被成功應(yīng)用于5 V高電壓鎳錳酸鋰（LiNi0.5Mn1.5O4）電池中，能夠明顯提升電池循環(huán)性能。

圖7 膦酸酯類阻燃添加劑結(jié)構(gòu)式

圖8 DTYP分子結(jié)構(gòu)式及其相關(guān)功能[58]

3.1.4 磷腈類阻燃添加劑

磷腈類化合物，是復(fù)合型阻燃添加劑，主要包括小分子環(huán)狀磷氮化合物和高分子線性磷氮化合物（圖9）[8,21,59-72]。磷腈類阻燃添加劑的主要特點是少量添加（5%～15%，質(zhì)量百分?jǐn)?shù)）即可達到使電解液阻燃或不燃的效果，且與電極材料兼容性好，對鋰離子電池的電化學(xué)性能影響小。用于電解液阻燃的磷腈類阻燃添加劑主要分為以下幾類：六烷氧基環(huán)三磷腈，如六甲氧基環(huán)三磷腈[hexamethox- ycyclotriphosphazene，HMPN，圖9(a)][21,59]、六(甲氧基乙氧基乙氧基)環(huán)三磷腈[hexa(methoxyethox- yethoxy)cyclotriphosphazene，MEE trimer，圖9(b)][60]、不飽和烷氧基環(huán)三磷腈[AL-7，圖9(c)][61]、六(2,2,2-三氟乙氧基)環(huán)三磷腈[hexakis(2,2,2- trifluoroethoxy)cyclotriphosphazene，HFEPN，圖9(d)][62]；單烷氧基五氟環(huán)三磷腈，如乙氧基五氟環(huán)三磷腈[(ethoxy)pentafluorocyclotriphosphazene，PFPN，圖9(e)][8,63-65]、苯氧基五氟環(huán)三磷腈[pentafluoro(phenoxy)cyclotriphosphazene，F(xiàn)PPN，圖9(f)][66-68]、4-甲氧基-苯氧基五氟環(huán)三磷腈[(4-methoxy)-phenoxy pentafluorocyclotriphosphazene，4-MPPFPP，圖9(g)][69]、2-氯-4-甲氧基-苯氧基五氟環(huán)三磷腈[(2-chloro-4-methoxy)-phenoxy pentafluorocycl- otriphosphazene，2-Cl-4-MPPFPP，圖9(h)][70]；線性聚磷腈，如聚[雙(甲氧基乙氧基乙氧基)磷腈][poly[bis(methoxyethoxyethoxy)phosphazene]，MEEP，圖9(i)][60]、聚[雙(乙氧基乙氧基乙氧基)磷腈][poly[bis(ethoxyethoxyethoxy)phosphazene]，EEEP，圖9(j)][71]；磷腈小分子[triethoxyphosphazen--phosphoryldiethylester，PNP，圖9(k)][72]。由于 環(huán)三磷腈類阻燃添加劑具有較高的電化學(xué)氧化窗口，在下一代高電壓鋰離子中的應(yīng)用案例較多，如采用高電壓鈷酸鋰正極材料或采用5 V高電壓鎳錳酸鋰（LiNi0.5Mn1.5O4）材料的鋰離子電池[8,62-68]。最近，XU等[8]將7%阻燃添加劑PFPN與功能添加劑聯(lián)用，即能大幅度提高5 V高電壓LiNi0.5Mn1.5O4/石墨全電池的長循環(huán)性能（1 C倍率下循環(huán)300圈，容量保持率87.4%），又能讓電解液具有不燃性（該阻燃電解液配方可以經(jīng)受5次點火實驗而不被點燃，圖10）。

3.1.5 其它含磷元素阻燃添加劑

已報道的含磷阻燃添加劑還有三(4-氟苯基)磷化氫[tris(4-fluorophenyl) phosphine，TFPP，圖11(a)][48]、六甲基磷酰三胺[hexamethylphosphoramide，HMPA，圖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，圖11(c)][74]、富含磷?；淖枞茧x子-新型阻燃鋰鹽[Li[P(DPC)3]，圖11(d)][75]、聚(磷酸乙酯-乙二醇)共（寡）聚物[EPCP，圖11(e)][9]。值得注意的是，最近，WANG等[9]將離子傳導(dǎo)性優(yōu)異的EO片段嵌入到磷酸酯中形成具有阻燃性EPCP寡聚物，該寡聚物添加量為15%時，電解液完全不燃，且電解液的離子電導(dǎo)率不受EPCP加入影響。另外EPCP還有利于電極材料界面的穩(wěn)定，因此，采用鈷酸鋰正極的鋰離子電池（LiCoO2/Li）循環(huán)性能和倍率性能均有所提升。

圖9 磷腈類阻燃添加劑結(jié)構(gòu)式

圖10 添加7% PFPN阻燃添加劑的阻燃效果（自熄滅5次）[8]

圖11 其它含磷阻燃添加劑結(jié)構(gòu)式

3.2 其它類型阻燃添加劑

其它類型阻燃添加劑主要有硅烷類阻燃添加劑[圖12(a)~(d)][76-80]、三嗪類阻燃添加劑[圖12(e)~(g)][81-82]、離子液體類阻燃添加劑[圖12(h)~(l)][83-85]、氟代烷氧烴（氟醚類）阻燃添加劑[圖12(m)][86]、雙酚類阻燃添加劑[圖12(n)][87]、全氟烷酮[圖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]。硅烷類（有機硅）有機阻燃劑的主要特點是熱穩(wěn)定性優(yōu)異、可燃性低、毒性小、電導(dǎo)率和分解電壓高，在鋰離子電池新型電解液領(lǐng)域備受關(guān)注，QIN等[76]綜述了有機硅電解液的研究進展。以乙烯基-三-(2-甲氧基乙氧基)硅烷[vinyl-tris-(methoxydiethoxy)silane，VTMS，圖12(a)][77]為例，VTMS是一種環(huán)境友好的阻燃添加劑，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，且熱穩(wěn)定性高，黏度低，對鋰離子電池（LiCoO2體系）電化學(xué)性能負(fù)面影響較小。YIM等[86]將十氟-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，圖12(m)]用交聯(lián)型聚甲基丙烯酸甲酯（PMMA）包裹，并通過PVDF-HFP黏結(jié)劑涂覆固定在聚乙烯微孔膜上，在電池過熱時，PMMA外殼破碎釋放DMTP阻燃劑包括電池安全[圖13(a)]。最近，JIANG等[88]設(shè)計了一種具有自冷卻功能的阻燃復(fù)合電解液[圖13(b)]，其核心組分是全氟-2-甲基-3-戊酮[perfluoro-2-methyl-3-pentanone，PFMP，圖12(o)]。盡管該電解液能夠提高鋰離子電池三元正極材料（NCM111）的熱穩(wěn)定性，但是其與電極材料的電化學(xué)兼容性還有待提高。

圖12 其它類型阻燃添加劑結(jié)構(gòu)式

圖13 （a）將包裹有DMTP的PMMA聚合物微膠囊用PVDF-HFP黏結(jié)劑固定在聚乙烯隔膜上[86]；（b）具有雙重保護 機制的復(fù)合電解液[88]

4 基于阻燃溶劑（共溶劑）的阻燃電解液

4.1 含磷元素阻燃溶劑（共溶劑）

含磷的化合物作為阻燃添加劑，在某些條件下其阻燃效果可能達不到電池的使用需求，因此需要增加含磷化合物的使用量作為電解液的阻燃共溶劑，但是總體上各種類的含磷化合物與電極材料兼容性較差，作為溶劑使用難度較大。

在阻燃電解液研究的早期，主要采用烷基磷酸酯類阻燃共溶劑的有TMP[圖4(c)][19,21]、TEP[圖4(i)][21,90]、TPP[圖4(a)][91-92]，但這些烷基磷酸酯與石墨負(fù)極兼容性差；相比之下，氟代磷酸酯類化合物與電極材料兼容性更好（利于穩(wěn)定SEI膜）、阻燃效果更佳（用量少）、黏度低，作為電解液共溶劑更受關(guān)注：如三(2,2,2-三氟乙基)磷酸酯[tris(2,2,2-trifluoroethyl)phosphate，TFP，圖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，圖14(b)][94-95]，(2,2,2-三氟乙基)-二乙基磷酸酯[(2,2,2-trifluoroethyl) diethyl phosphate，TDP，圖14(c)][94-95]，三(2,2,2-二氟乙基)磷酸酯[tris(2,2-difluoroethyl) phosphate， TFHP，圖14(d)][96]，磷酸三丙酯[tripropyl phosphate， TPrP，圖14(e)][99]，三(3,3,3-三氟丙基)磷酸酯[tris(3,3,3-trifluoropropyl) phosphate，3F-TPrP，圖14(f)][99]，三(2,2,3,3-四氟丙基)磷酸酯[tris(2,2,3,3-tetrafluoropropyl) phosphate，4F-TPrP，圖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，圖4(l)][99]。最近，MURMANN等[99]研究不同氟化程度的TPrP的阻燃效果和電化學(xué)性能，結(jié)果顯示，氟化程度最高的5F-TPrP作為溶劑使用量為30%時電解液完全不燃且?guī)缀醪粫κ? NCM111全電池的循環(huán)性能造成影響。ZHANG 等[100]發(fā)現(xiàn)，除了作為阻燃溶劑，亞磷酸酯TTFP(i)[圖6(e)]還能夠抑制碳酸丙烯酯（PC）對石墨材料的剝離，提高全電池在PC基電解液中的循環(huán)穩(wěn)定性，尤其是高溫條件下。

圖14 含磷元素阻燃溶劑（共溶劑）結(jié)構(gòu)式

膦酸酯中作為電解液阻燃溶劑（共溶劑）應(yīng)用最多的是DMMP[圖7(a)][91,101-105]。XIANG等[102]發(fā)現(xiàn)DMMP基阻燃電解液與Li4Ti5O12負(fù)極材料兼容性良好，該阻燃電解液被成功用于高能量密度高電壓LiNi0.5Mn1.5O4/Li4Ti5O12全電池體系中。ZENG等[104]以DMMP為主溶劑開發(fā)出適用于LiFePO4/SiO全電池體系的阻燃型電解液。WU等[105]將雙三氟甲烷磺酰亞胺鋰（LiTFSI）作為主鹽溶解于一種新型磷酸酯主溶劑中，二甲基(2-甲氧基乙氧基)甲基磷酸酯[dimethyl(2-methoxyethoxy) methylphosphonate，DMMEMP，圖14(h)]，該阻燃型電解液與金屬鋰片兼容性良好，適用于LiFePO4/Li電池體系。磷腈類化合物作為阻燃電解液溶劑（共溶劑）的報道較少[圖14(i)～(j)][21,106-107]，ROLLINS等[106]報道了一種氟代六烷氧基環(huán)三磷腈[FM-2，圖14(i)]共溶劑，能夠提高電化學(xué)穩(wěn)定窗口、熱穩(wěn)定性和安全性能高，利于穩(wěn)定SEI膜，該阻燃電解液被成功應(yīng)用于石墨/（錳酸鋰+三元材料）全電池體系中，當(dāng)使用量為20%時，可以明顯改善全電池的循環(huán)性能。

4.2 其它類型阻燃溶劑（共溶劑）

氟代醚類和氟代碳酸酯類有機化合物的特點是閃點高或者是沒有閃點。該類化合物作為阻燃溶劑是通過稀釋高揮發(fā)和易燃性共溶劑起作用，所以在阻燃電解液中占比較大（通常大于70%）[51]。另外，借助氟元素的吸電子效應(yīng)，該類氟代化合物溶劑分子更容易在碳基負(fù)極表面還原，優(yōu)化SEI膜，改進阻燃電解液與電極材料的電化學(xué)兼容性，提高電池的性能。文獻報道過的用于鋰電池阻燃電解液的氟代醚有：甲基九氟丁醚[methyl nonafluorobutyl ether ，MFE，圖15(a)][108-110]，乙基九氟丁醚[ethyl nonafluorobutyl ether，EFE，圖15(b)][94-95]，DMTP[或稱為TMMP，圖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(xiàn)-EPE，圖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，圖15(d)][116]，六氟異丙基甲醚[1,1,1,3,3,3-hexafluoroi- sopropyl methyl ether，HFPM，圖15(e)][117]。氟代碳酸酯的代表性化合物有：氟代環(huán)狀碳酸酯[F-AEC，圖15(f)][113]，三氟乙基甲基碳酸酯[3,3,3-fluoroethylmethyl carbonate，F(xiàn)-EMC，圖15(g)][113,116]，TFPOM-C[4-(2,2,3,3-tetrafluoropropo- xymethyl)-1,3-dioxolan-2-one，圖15(h)][118]， TFTFMP- C[4-(2,3,3,3-tetrafluoro-2-trifluoromethyl-propyl)- 1,3- dioxolan-2-one，圖15(i)][118]，雙（四氟丙基）碳酸酯[bis(2,2,3,3-tetrafluoro-propyl) carbonate，BTFP-C，圖15(j)][118]，雙（五氟丙基）碳酸酯[bis(2,2,3,3,3- pentafluoro-propyl) carbonate，BPFP-C，圖15(k)][118]。最近，F(xiàn)AN等[116]將氟代醚HFE、氟代碳酸酯F-EMC和FEC（氟代碳酸乙烯酯）聯(lián)用開發(fā)了一種耐高電壓的不燃全氟電解液[圖16(a)～(c)]，在高鎳三元材料[NCM811，圖16(d)]和5 V磷酸鈷鋰[LCP, 圖16(e)]金屬鋰電池中取得了巨大的成功。相比非全氟電解液，該不燃電解液能夠保護金屬鋰片，抑制鋰枝晶的產(chǎn)生，減小電池短路風(fēng)險[圖16(f)]。另外，閃點較高，氧化穩(wěn)定性高的環(huán)丁砜[sulfolane，TMS，圖15(l)][119-121]，己二腈（adiponitrile，ADN）[122]也被用作阻燃電解液的主溶劑。

圖15 其它類型阻燃溶劑（共溶劑）結(jié)構(gòu)式

圖16 氟代醚（HFE）和氟代碳酸酯（F-EMC和FEC）組成的不燃全氟電解液阻燃效果及其在金屬鋰電池中的應(yīng)用[116]

5 基于高濃度鋰鹽的阻燃電解液

“高濃度電解液”是一類備受關(guān)注的電解液體系，其鋰鹽濃度高達 4 mol/L，遠(yuǎn)遠(yuǎn)高于普通電解液鋰鹽濃度（通常為1 mol/L）[4]。在高濃度電解液中，幾乎所有溶劑都與鋰離子直接配位，使其具有一些特殊的優(yōu)點：高氧化/還原穩(wěn)定性、利于在石墨負(fù)極或金屬鋰負(fù)極形成高穩(wěn)定界面膜、高熱穩(wěn)定性、阻燃或者不燃，及高電壓下鈍化正極Al集流 體[4,123-125]。磷酸酯類化合物作為阻燃添加劑或阻燃溶劑都存在與負(fù)極材料（碳基負(fù)極或金屬鋰負(fù)極）兼容性差的問題，其應(yīng)用收到嚴(yán)重的限制?！案邼舛入娊庖骸备拍畹陌l(fā)展促使磷酸酯類化合物（TMP[126-128]、TFEP[129]、TEP[130-131]）直接作為主溶劑（甚至單一溶劑）用于鋰離子電池或金屬鋰電池中。例如，WANG等[126]發(fā)現(xiàn)5.3 mol/L LiFSI/TMP高濃度電解液與石墨的兼容性好，石墨/Li半電池循環(huán)1000圈容量幾乎沒有衰減，這得益于高濃度電解液能夠在石墨負(fù)極形成一層非常有效的鈍化膜（圖17），該高濃度電解液被成功應(yīng)用于高電壓5 V 石墨/鎳錳酸鋰全電池體系。另外，碳酸酯作為單一溶劑的高濃度電解液（1∶1.1，LiFSI/DMC）也具有阻燃特性[123]，并具有優(yōu)異的電化學(xué)兼容性，能夠顯著提高高電壓5 V 石墨/鎳錳酸鋰全電池體系的室溫循環(huán)性能和高溫循環(huán)性能。最近，ALVARADO等[125]開發(fā)了以高閃點環(huán)丁砜為單一溶劑的高濃度電解液（3 mol/kg LiFSI/TMS），其能夠讓MCMB/鎳錳酸鋰全電池運行1000次循環(huán)。發(fā)展阻燃型高濃電解液用于高能量密度的下一代高電壓鋰離子電池或金屬鋰電池將是未來的重點研究方向。

圖17 高安全性電池電解液設(shè)計理念[126]

6 基于離子液體的阻燃電解液

離子液體由陰、陽離子兩部分組成, 陰離子通常有、、TFSI-、FSI-等，陽離子通常有吡咯類、咪唑類、哌啶類和季銨鹽類等。離子液體具有揮發(fā)性極小、不燃、電化學(xué)穩(wěn)定窗口寬、溶解能力強、熱穩(wěn)定性高等特點，既適合應(yīng)用于高電壓電解液，又適合制備阻燃型電解液，提高鋰離子電池安全性[4,132-133]。例如，CHANCELIER 等[133]實驗證實兩種離子液體的高熱穩(wěn)定和阻燃特性，即1-丁基- 2,3-甲基咪唑-二(三氟甲基磺酰)亞胺[1-butyl-3-methylimidazolium bis(trifluoromethanesu- lfonyl)imide，C1C4ImTFSI，圖18(a)]和-甲基--丁基吡咯-二(三氟甲基磺酰)亞胺[PYR14TFSI 或BMP-TFSI，圖12(i)]。盡管如此, 由于純離子液體黏度大，且與隔膜、電極材料的浸潤性差，鋰離子的遷移受到極大限制；另外，大多數(shù)的離子液體與碳基負(fù)極的兼容性差，因而，純離子液體較難作 為電解液直接用于鋰離子電池。例如，KIM等[132]將純-甲基--丁基吡咯-雙氟磺酰亞胺[-butyl-- methylpyrrolidinium bis(fluorosulfonyl) imide，PYR14FSI，圖18(b)]基阻燃型電解液直接應(yīng)用于非碳基負(fù)極全電池體系（LiFePO4/Li4Ti5O12），但倍率性能差。實際上，離子液體通常與碳酸酯類[134-141]、砜類[142]或氟代醚類[143]等溶劑混合使用來制備阻燃型高性能電解液。與碳酸酯混合使用配制阻燃型電解液的吡咯類離子液體有PYR14TFSI 或BMP-TFSI[圖12(i)][134-135]、-丙基--甲基吡咯-二(三氟甲基磺酰)亞胺[-propyl--methylpyrrolidiniumbis (trifluoromethanesulfonyl) imide ，PYR13TFSI，圖18(c)][136]、-乙基-2-甲氧基吡咯-雙氟磺酰亞胺[-ethyl-2-methoxypyrrolinium bis(fluorosulfonyl) imide，E(OMe)Pyrl-FSI，圖18(d)][137]。KIM等[137]報道E(OMe)Pyrl-FSI與碳酸酯溶劑混合后電解液阻燃效果優(yōu)異，且能保證LiFePO4/Li體系60 ℃高溫的穩(wěn)定運行（圖19）。與碳酸酯混合的代表性哌啶類離子液體有-甲基--丙基哌啶-二(三氟甲基磺酰)亞胺[-methyl--propylpiperidinium bis (trifluoromethanesulfonyl) imide，PP13TFSI，圖18(e)][138-140]、1-乙基-1-甲基哌啶-二(三氟甲基磺酰)亞胺[1-ethyl-1-methyl piperidinium bis(trifluorome thanesulfonyl)imide，EMP-TFSI，圖18(f)][141]。另外，季銨鹽類離子液體也有應(yīng)用案例，如二乙基甲基-(2-甲氧乙基)銨基-雙(三氟甲基磺酰)亞胺[,-diethyl--methyl--(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)azanide，DMMA-TFSI，圖18(g)]與環(huán)丁砜[圖15(l)]混合使用于NCM/石墨體系[142]、圖18(h)～(i)的季銨鹽類離子液體與阻燃溶劑甲基九氟丁醚[MFE，圖15(a)]混合使用于LiFePO4/Li體系[143]。

圖18 一些離子液體陽離子和陰離子的結(jié)構(gòu)式

圖19 離子液體E(OMe)Pyrl-FSI與碳酸酯溶劑的二元電解液阻燃性測試及其在LiFePO4/Li電池體系中60 ℃高溫的循環(huán)性能[137]

7 阻燃型凝膠聚合物電解質(zhì)

阻燃型聚合物電解質(zhì)主要分為阻燃型固態(tài)聚合物電解質(zhì)[144-147]和阻燃型凝膠聚合物電解質(zhì)[148-157]。本文主要關(guān)注含液體溶劑的阻燃型凝膠聚合物電解質(zhì)。早在1998年，AKASHI等[148]通過優(yōu)化聚丙烯腈[polyacrylonitrile，PAN，圖20(a)]、碳酸酯溶劑和LiPF6的比例制備出一種PAN基的阻燃性凝膠聚合物電解質(zhì)。熱重分析表明LiPF6降低了PAN基凝膠聚合物電解質(zhì)的碳化溫度點并增加了燃燒后殘留的碳質(zhì)材料。線性燃燒速率與碳化溫度點的相關(guān)性表明，PAN基凝膠聚合物電解質(zhì)的阻燃特性源于表面形成的碳質(zhì)層。LU等[149]以聚四氟乙烯微孔膜（PTFE）為支撐體，交聯(lián)型聚（乙二醇）和聚（甲基丙烯酸縮水甘油酯）嵌段共聚物[PEG-b-PGMA，圖20(b)]為填料，LiTFSI為鋰鹽制備出一種適用于LiFePO4/Li體系的不燃型凝膠聚合物電解質(zhì)，其25 ℃離子電導(dǎo)率高達1.30×10-3S/cm。LI等[150]用聚乙二醇二甲基丙烯酸酯（PEGDMA）將聚芳醚酮（PAEK）無紡布交聯(lián)，獲得一種阻燃型凝膠聚合物電解質(zhì)[PAEKNW-SPE，圖20(c)]，其室溫離子電導(dǎo)率為1.20×10-3S/cm。最近，BAIK等[151]將全氟聚醚[Fluorolink E10H，圖20(d)]交聯(lián)后制備出一種不燃、熱穩(wěn)定性高、電化學(xué)穩(wěn)定窗口寬[高達5 V (vs. Li/Li+)]的凝膠聚合物電解質(zhì)?；陔x子液體的阻燃型凝膠聚合物電解質(zhì)也有報道[153-157]。LEE等[153]成功將基于聚（1-甲基-3-（2-丙烯酰氧基己基）咪唑鎓-四氟硼酸鹽）[poly(1-methyl-3-(2-acryloyloxy- hexyl) imidazolium tetrafluoroborate)，PIL，圖20(e)]的凝膠聚合物電解質(zhì)應(yīng)用于LiCoO2電池中。LUO等[154]開發(fā)了一種基于酚醛環(huán)氧樹脂寡聚離子液體[咪唑類，OIL，圖20(f)]與PVDF-HFP的阻燃型凝膠聚合物電解質(zhì)，其熱穩(wěn)定高（150 ℃，熱收縮＜1%），室溫離子電導(dǎo)率高達2.0×10-3S/cm。2018年GUO等[157]將離子液體1-乙基-2,3-甲基咪唑二(三氟甲基磺酰)亞胺[1-ethyl-3-methylimidazolium triluoromethanesufonate，EMITFSI，圖20(g)]、無機快離子導(dǎo)體Li1.5Al0.5Ge1.5(PO4)3（LAGP）和PVDF-HFP混合制備一種阻燃型凝膠聚合物電解質(zhì)膜（IL-GPE，圖21）用于高安全性的金屬鋰電池，IL-GPE與金屬鋰兼容優(yōu)異無鋰枝晶產(chǎn)生，能夠顯著提高LiFePO4/Li體系循環(huán)性能。

圖20 阻燃型凝膠聚合物電解質(zhì)中的聚合物或離子液體

圖21 一種有機無機復(fù)合的凝膠聚合物電解質(zhì)（ILGPE）及其阻燃測試[126]

8 展 望

高安全性阻燃電解液的發(fā)展是鋰離子電池大規(guī)模推廣應(yīng)用過程中至關(guān)重要的一環(huán)。目前，阻燃型電解液的使用通常能夠提高鋰離子電池的安全性能，但會犧牲一部分鋰離子電池的電化學(xué)性能。綜合全文所述，未來阻燃電解液的發(fā)展呈現(xiàn)以下趨勢。

（1）發(fā)展電極界面兼容性優(yōu)異的多功能阻燃添加劑或阻燃溶劑，即不僅具有阻燃效果，而且具有界面成膜功能；或?qū)⒆枞紕┡c功能成膜添加劑聯(lián)用，發(fā)揮協(xié)同作用。

（2）通過原位表征手段研究阻燃電解液與電極界面性質(zhì)，研究其氧化還原分解機制，反饋指導(dǎo)阻燃電解液優(yōu)化改進；加強各阻燃電解液體系的阻燃機理研究。

（3）發(fā)展聚合物微膠囊包裹阻燃劑的實用技術(shù)，保證阻燃劑只在極端失控情況下起作用，并在大容量電池中驗證此技術(shù)。

（4）發(fā)展更多電極界面兼容性優(yōu)異的高濃度阻燃電解液體系。

（5）開發(fā)新型低黏度阻燃型離子液體基阻燃電解液；或開發(fā)性能優(yōu)異的阻燃型凝膠聚合物電解質(zhì)。

（6）不能單純只評估阻燃電解液的阻燃效果，還需在大容量電池中全面評估阻燃電解液的實用性，包括電化學(xué)性能測試、濫用性測試、熱安全性測試（ARC設(shè)備）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。

國家自然科學(xué)基金項目（51625204，51502319），山東省自然科學(xué)基金項目（ZR2016BQ18）。

許高潔（1987—），男，助理研究員，主要研究方向為鋰離子電池電解液，E-mail：xugj@qibebt.ac.cn；

崔光磊，研究員，主要研究方向為電化學(xué)儲能材料及器件，E-mail：cuigl@qibebt.ac.cn。