A highly cross-linked polyimide-poly(ethylene oxide) copolymer has been synthesized that is capable of holding large volumes of liquid component, simultaneously maintaining good dimensional stability. An amine end capped oligomer was made that was imidized in solution, followed by reaction with a triisocyanate in the presence of desired additives at ambient temperature. Polymer films are able to hold over 4 times their weight in room temperature ionic liquid (RTIL) or carbonate solvent. Electrolytes were studied that contained varying amounts of RTIL, lithium trifluoromethanesulfonimide (LiTFSi), and alumina nanoparticles. Electrochemical stability of these electrolytes with lithium metal electrodes was studied by galvanic cycling and impedance spectroscopy. Improved cycling stability and decreased interfacial resistance were observed when increasing amounts of RTIL and LiTFSi were added. The addition of small amounts of alumina further decreased interfacial resistance by nearly an order of magnitude. During the course of the study, cycling stability increased from < 3 to >1000 h at 60 degrees C and 0.25 mA/cm(2) current density. (C) 2008 The Electrochemical Society.