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 1 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 1C rate and good rate capability.