Polymer blends based on poly(vinylidene fluoride-trifluoroethylene)/poly(ethylene oxide), P(VDF-TrFE)/PEO, with different PEO contents and molecular weights have been prepared for use as Li-ion battery separator membranes. The electrolyte uptake strongly depends on PEO content within the polymer blend. Thermal, mechanical and electrical properties of the membranes are dependent both on PEO content and molecular weight. A thermal degradation mechanism is also proposed as PEO content has a large influence on the activation energy and thermal degradation of P(VDF-TrFE). After electrolyte uptake, all polymer blends exhibit high ionic conductivity at room temperature, the highest value being 0.7 mS cm??? 1 which was obtained for the 40/60 membrane with PEO with Mw = 10 kDa. From the point of view of lithium-ion battery application, polymer blends with 10 kDa molecular weight PEO are more adequate due to its excellent mechanical properties and high ionic conductivity.