The application and the performance of a new type of poly(ethylene oxide) (PEO)-based gel electrolyte membrane in lithium-ion battery systems are presented and discussed. The compatibility of this PEO-based gel membrane toward composite graphite-based anode and LiCoO2-based cathode films, respectively, is first evaluated. Laminated, gel-type, graphite/LiCoO2 lithium-ion prototypes are assembled using the PEO membrane as an electrolyte separator. The cycling performances of these prototypes are reported and discussed. (C) 2003 The Electrochemical Society.