We report a new concept for making poly(ethylene oxide) (PEO) based composite gel polymer electrolyte (CGPE) lithium batteries by demonstrating the fabrication of a lithium/sulfur (Li/S) cell, in which the CGPE is in situ formed by gelling a 50PEO-50SiO(2) (wt) composite membrane with a gel-forming liquid electrolyte. The composite membrane can be either coated onto the sulfur cathode to form an electrode-supporting electrode-membrane-assemble (EMA) or cast onto a Teflon substrate to form a freestanding membrane. Attributed to the presence of a large amount of inorganic SiO2 filler, the membrane can instantly absorb and retain various types of electrolyte solvents and ionic liquids without shrinkage. Using a gel-forming liquid electrolyte, the CGPE Li/S cell can be facilely prepared by heating an electrolyte-activated cell. In this paper, we demonstrate the preparation of a freestanding and highly flexible 50PEO-50SiO(2) composite membrane, and discuss the electrochemical characteristics of its resulting CGPE and the CGPE Li/S cell. (C) 2013 The Electrochemical Society.