Ionic Liquids and Organic Ionic Plastic Crystals Based on Tris(fluorosulfonyl)methide

Ionic liquids (ILs) composed of low coordinating anions such as perfluoroanions have been studied as a unique electrolyte for a lithium battery for past two decades.[1] The best performance of a lithium battery system using only ILs was achieved by bis(fluorosulfonyl)amide (FSI - , FSA - or f 2 N - ) [2]. The existence of FSO 2 - group in f 2 N - might cause such a good performance. In this study, we tried to prepare new perfluoromethide, tris(fluorosulfonyl)methide (f 3 C - ) to investigate the effect of FSO 2 - group on physicochemical and electrochemical properties of onium salts composed of f 3 C - . The physical, thermal and electrochemical properties of the ILs were compared to another similar methide anion (tris(trifluoromethylsulfonyl)methide, Tf 3 C - ). The Tf 3 C - could form ILs with conventional cations such as N-ethyl-N-methylimidazolium (C 1 C 2 im + ) and N,N-dimethyl-N-methyl-N-(2-ethoxymethyl)ammonium (N 2,2,1,2O1 + ), however, It was quite interesting that the f 3 C - had a tendency to form organic ionic plastic crystals (OIPCs) rather than ILs. As far as we know, the N 2,2,1,2O1 + forms ILs with various anions even PF 6 - [3] however, the f 3 C - forms good OIPCs with N 2,2,1,2O1 + (T m = 69 °C, T s-s = -29 °C). The cathodic limiting potential of a ILs composed of N-ethyl-N-methylimidazolium (C 1 C 2 im + ) and f 3 C - was about 1.0 V shifted toward negative potential with adding Lif 3 C and stable lithium deposition and redissolution cycle could be successfully observed. The apparent charge transfer resistance at an interface between a Li metal and [C 1 C 2 im][f 3 C] was more stable over 18 months than that for [C 1 C 2 im][Tf 3 C], which indicated that the existence of FSO2- group in a perfluoroanion must be importance to maintain a stable SEI on the lithium metal as previously reported in a bis(fluorosulfonyl)amide based melts.[3] Furthermore, the apparent charge transfer resistance at a charged LiCoO2 composite electrode (4.2 V vs Li/Li + ) in the [C 1 C 2 im][f 3 C] was almost the same value during over 500 charge and discharge cycles, which suggested that the good electrochemical stability not only for a lithium metal but also for the charged cathode were achieved with using the new methide f 3 C - composed of FSO 2 - group as in bis(fluorosulfonyl)amide based melts. These results suggest that the FSO 2 - group have an essential moiety to design an novel perfluoroanions for a lithium battery electrolyte. The f3C - provided not only ILs as a novel lithium battery electrolyte but also OIPCs and the molecular design rules, which have not yet been clearly understood. Acknowledgment This study was financially supported by JST, ALCA-SPRING project. References [1] H. Matsumoio. in Electrolytes for Lithium-ion Batteries , T. R. Jow, K. Xu., O. Borodin and M. Ue, Eds., Springer. 2014, ch. 4, and references therein. [2] H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M. Kono, J. Power Sources, 160(2), (2006) 1308. [3] H. Matsumoto, H. Sakaebe, K. Tatsumi, ECS Transactions , 16 (35), (2009)59. Figure 1