The electrochemical behaviour of a polyethylene oxide (PEO)-based composite polymer electrolyte are studied. The crystallinity of the PEO is suppressed by using a comb-shaped polymer to improve polymer chain mobility. An amorphous comb-shaped polymer, ‘TEC-24’, with a side-chain content of 24mol%, is designed and fine silica powder is dispersed within it to enhance the mechanical properties above the melting point. The composite polymer electrolyte has an ionic conductivity of 1.6×10−4 and 1.6×10−3 Scm−1 at 30 and 90°C, respectively, with an electrochemical stability window close to 5.0V, even at 80°C (versus Li/Li+). The polymer electrolyte is evaluated using CuS as a cathode material and shows better cycle performance than that obtained with a liquid electrolyte.