We studied microporous poly(acrylonitrile-methyl methacrylate), AMMA, membrane as the separator of Li/LiMn2O4 cell. The porous AMMA membrane was prepared by the phase inversion method with N,N-dimethylformamide (DMF) as the solvent and water as the non-solvent. We observed that morphology of the resulting membrane was strongly affected by the concentration of polymer solution: low concentration produced finger-like pores with dense skin on two surfaces of the membrane, while high concentration yielded open voids with dense layer on the other surface of the membrane. Regardless of their morphology, both membranes could be rapidly wetted by the liquid electrolyte (1.0m LiBF4 dissolved in 1:3 wt.% mixture of ethylene carbonate (EC) and γ-butyrolactone (GBL)), and could be swollen at elevated temperatures, which resulted in the formation of a microporous gel electrolyte (MGE). It was shown that the resulting MGE not only had high ionic conductivity and but also had good compatibility with metal lithium even at 60°C. Cyclic voltammetric test showed that the MGE had an electrochemical window of 4.9V versus Li+/Li. At room temperature, the Li/MGE/LiMn2O4 cell showed excellent cycliability with a specific capacity of 121–125mAhg−1 LiMn2O4. It was shown that even at 60°C good mechanical strength of the MGE remained. Therefore, the MGE is suitable for the application of battery separator at elevated temperatures.