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Deposition and dissolution of lithium metal have been investigated in an equimolar mixture of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and glyme [triglyme (G3) or tetraglyme (G4)] solvate ionic liquid. The limiting current for deposition of lithiumwas not observed probably because of the high concentration of lithium species and a decrease in the local viscosity by the liberation of glyme. On the other hand, the dissolution of lithium was limited due to an increase in the local viscosity by the formation of [Li(TFSA)2]–. The formation of solid electrolyte interphase (SEI) was suggested to form by the cathodic decomposition of the solvate ionic liquids by electrochemical quartz crystal microbalance. Lithium phosphorous oxynitride (LiPON) thin film was found to act as the artificial SEI, which prevented the cathodic decomposition of the solvate ionic liquids and enabled the deposition and dissolution of lithium. The cycle performance of deposition and dissolution of lithiumwas found to be improved by coating a Cu substratewith vapor-grown carbon fiber.

New electrochemical application using room-temperature ionic liquids (ILs) are introduced, such as lithium secondary batteries, electrochemical double layer capacitors, and novel types of electrical devices for sustainable and renewal energy society. ILs have so many combinations, owing to many cation/anion species. In this chapter, we introduce properties from fundamental (general and special physicochemical properties) to electrochemical applications of ILs. We also discuss importance of molecular design and application target of ILs.