By simply mixing an inorganic electrolyte (Li3PO4) with LiCoO2 cathode sheet materials in all-solid-state lithium polymer secondary batteries (LPBs) with a LiCoO2/lithium metal system, we markedly improved the charge - discharge reversibility of such batteries in a high-voltage charge state of 4.4 V vs Li/Li+. Mixing Li3PO4 with the cathode sheet materials suppressed the growing rate of the cathode/electrolyte interfacial resistance caused by the oxidation of the electrolyte during preservation tests of the LPBs at a high cathode potential under 2% of that for the case free of Li3PO4. This made the cycle performance and charge-state preservation characteristic of a Li3PO4-mixed LPB markedly improved. It is discussed based on X-ray diffraction analysis of the LPB that the generation of Co3O4 from the LiCoO2 cathode in the high-voltage state is suppressed by the Li3PO4 mixing. (C) 2006 The Electrochemical Society.