A polymer electrolyte based on microporous poly(vinylidene fluoride-co-hexafluoropropane) (PVdF-HFP) film was studied for use in lithium ion batteries. The microporous PVdF-HFP (Kynar 2801) matrix was prepared from a cast of homogeneous mixture of PVdF-HFP and solvents such as ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). After evaporation of DMC and EMC, a sold film of the PVdF-HFP and the EC mixture was obtained. EC-rich phase started its formation in the PVdF-HFP/EC film at EC content of about 60 wt.% based on the total weight of PVdF-HFP and EC. The formation of the new phase resulted in the abrupt increase of the porosity of the PVdF-HFP matrix from 32 to 62%. The ionic conductivity of the film soaked in I M LiPF6-EC/DMC = 1/1 was significantly increased from order of 10(-4) S/cm to order of 10(-3) S/cm. at the EC content of 60 wt.%. Thermal and spectroscopic investigations showed that most of the EC interact with PVdF-HFP with the EC content being below 60 wt.%. MCMB/polymer electrolyte/LiCoO2 cells employing the microporous PVdF-HFP polymer film showed stable charging/discharging characteristics at I C rate and good rate capability. (C) 2003 Elsevier Science Ltd. All rights reserved.