A room temperature ionic liquid consisting of N,N,N,N-cyanomethyl trimethyl ammonium (CTMA) cation and bis(trifluoromethane sulfone)imide (TFSI) anion was newly synthesized, and its electrochemical properties were investigated. This ionic liquid has a melting point of 35°C and an order of conductivity of 10−4 Scm−1. Lithium deposition/dissolution tests in 0.2moldm−3 LiTFSI/CTMATFSI electrolytes showed an improved cycle behavior compared with that of a Li electrolyte based on a tetraalkylammonium ionic liquid without a cyano group. It is suggested that such an improvement may be associated with the formation of a protective film on the lithium surface. Introducing a proper functional group is suggested to be effective to improve the interfacial properties of ionic liquid.