Physicochemical properties such as density, viscosity, ionic conductivity, and self-diffusion coefficient of two types of room-temperature ionic liquids composed of cations with N-P bonds were investigated. One is an ionic liquid composed of cations with N-P-O bond, namely, tri(N,N-dimethylamino)-n-butoxyphosphonium bis(trifluoromethanesulfonyl)amide (TABP-TFSA) and has a relatively low viscosity (38 mPa s at 30 degrees C) due to the high internal mobility of the cation, despite its large ion size. TABP-TFSA showed a characteristic electrochemical oxidation behavior, such as a wide electrochemical potential window of stability without gas evolution. Formation of passivation film on a Pt electrode was suggested from the results of potential sweep experiments. The lithium secondary battery ([LiCoO2|Li] cell) using TABP-TFSA as the electrolyte achieved stable charge-discharge operations in over 500 cycles. For another ionic liquid with N-P bond, namely, TAEP-TFSA (TAEP(+) = tri(N-methyl-N-(n-butylamino))ethylphosphonium), less battery cycle stability and low rate performances were observed, despite the higher ionic dissociation than TABP-TFSA, probably because of the high viscosity and low electrochemical oxidative stability. It is shown that the introduction of oxygen in the cation like N-P-O produces better performances compared with the cation having N-P structure. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3613959] All rights reserved.