The physicochemical and electrochemical properties of three 1-alkyl-2,3,5-trimethylpyrazolium cation-based room-temperature ionic liquids with various alkyl chain lengths were investigated. The temperature dependences of density, viscosity, and ionic conductivity were obtained by precise measurements. Electrolyte properties of these room-temperature ionic liquids were also examined from the viewpoint of their uses in lithium secondary batteries ([LiCoO2 positive electrode[electrolyte]lithium metal negative electrode]). It was found that the alkyl chain length affects the charge-discharge performances of cells. (C) 2009 Elsevier B.V. All rights reserved.