Design of Ionic Liquids for Electrochemical Applications

Zwitterionic liquids composed of a tethered cation and anion were synthesized and their thermal properties and ionic conductivity were investigated as novel ionic liquids especially for electrochemical applications. We prepared nine zwitterions in this study. In addition, this paper includes 36 kinds of zwitterions already reported in order to discuss the relationship between the zwitterion structure and their properties. Most zwitterions melt above 100°C; their melting points are generally higher than that of simple ionic liquids. When an equimolar amount of lithium salt (LiTFSI, LiBETI, LiCF3SO3, LiBF4, or LiClO4) was added to the zwitterion, the mixture showed only a glass transition temperature Tg. The Tg values of the zwitterionic liquid/salt mixture showed the lowest value of –37°C when mixed with LiTFSI. This mixture also showed the highest ionic conductivity of 8.9 × 10–4 S cm–1 at 100°C. There is a good relationship between Tg and the ionic conductivity of the zwitterionic liquid/lithium salt mixtures.