Polyethylene oxide (PEO)-based polymer electrolytes with BaTiO3 as a filler have been examined as electrolytes in 4V class lithium polymer secondary batteries. A mixture of 90wt.% LiN(CF3SO2)2–10wt.% LiPF6 was found to be the best candidate as the salt in PEO, and showed high electrical conductivity, good corrosion resistance to the aluminum current collector and low interfacial resistance between the lithium metal anode and the polymer electrolyte. The cyclic performance of the cell, Li/[PEO10–(LiN(CF3SO2)2–10wt.% LiPF6)]–10wt.% BaTiO3/LiNi0.8Co0.2O2/Al, showed good charge–discharge cycling performance. The observed capacity fading on charging up to 4.2V at 80°C in the cell was about 0.28% per cycle in the first 30 cycles, compared to that of 0.5% for the polymer electrolyte without LiPF6 in the lithium salt.