In this study, we prepare a kind of solid polymer electrolyte (SPE) based on N-ethyl-N'-methyl imidazolium tetrafluoroborate (EMIBF4), LiBF4 and poly(vinylidene difluoride-co-hexafluoropropylene) [P(VdF-HFP)] copolymer. The resultant SPE displays high thermal stability above 300 degrees C and high room temperature ionic conductivity near to 10(-3) S cm(-1). Its electrochemical properties are improved with incorporation of a zwitterionic salt 1-(1-methyl-3-imidazolium)propane-3-sulfonate (MIm3S). When the SPE contains 1.0 wt% of the MIm3S, it has a high ionic conductivity of 1.57 x 10(-3) S cm(-1) at room temperature, the maximum lithium ions transference number of 0.36 and the minimum apparent activation energy for ions transportation of 30.9 kJ mol(-1). The charge-discharge performance of a Li4Ti5O12/SPE/LiCoO2 cell indicates the potential application of the as-prepared SPE in lithium ion batteries. (C) 2010 Elsevier Ltd. All rights reserved.