A room temperature ionic liquid consisting of NNNN-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 degreesC and an order of conductivity of 10(-4) S Cm-1. Lithium deposition/dissolution tests in 0.2 mol dm(-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. (C) 2004 Elsevier B.V. All rights reserved.