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

Abstract

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 f2N-) [2]. The existence of FSO2- group in f2N- might cause such a good performance. In this study, we tried to prepare new perfluoromethide, tris(fluorosulfonyl)methide (f3C-) to investigate the effect of FSO2 - group on physicochemical and electrochemical properties of onium salts composed of f3C-. The physical, thermal and electrochemical properties of the ILs were compared to another similar methide anion (tris(trifluoromethylsulfonyl)methide, Tf3C-). The Tf3C- could form ILs with conventional cations such as N-ethyl-N-methylimidazolium (C1C2im+) and N,N-dimethyl-N-methyl-N-(2-ethoxymethyl)ammonium (N2,2,1,2O1 +), however, It was quite interesting that the f3C- had a tendency to form organic ionic plastic crystals (OIPCs) rather than ILs. As far as we know, the N2,2,1,2O1 + forms ILs with various anions even PF6 - [3] however, the f3C- forms good OIPCs with N2,2,1,2O1 + (T m = 69 °C, T s-s = -29 °C). The cathodic limiting potential of a ILs composed of N-ethyl-N-methylimidazolium (C1C2im+) and f3C- was about 1.0 V shifted toward negative potential with adding Lif3C and stable lithium deposition and redissolution cycle could be successfully observed. The apparent charge transfer resistance at an interface between a Li metal and [C1C2im][f3C] was more stable over 18 months than that for [C1C2im][Tf3C], 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 [C1C2im][f3C] 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 f3C- composed of FSO2 - group as in bis(fluorosulfonyl)amide based melts. These results suggest that the FSO2 - 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.