A 5 V class ceramic/polymer composite all-solid-state lithium battery was prepared. The cell configuration was [Li3PO4 coated LiNi0.5Mn1.5O4 \solid polymer electrolyte \Li\. The total cell impedance was 4 kOmega at 333 K and the discharge capacity was 100 mAh g(-1) with a discharge voltage plateau in both 4.7 and 4.1 V regions. X-ray absorption near-edge structure results indicated that both transition metal ions, Ni and Mn, involved in the oxidation/reduction processes. The cell without Li3PO4 showed a lower discharge voltage plateau (<3.5 V) than the composite one. Although the Li3PO4 film was so thin that it could be nearly removed with only 2 min of Ar etching in X-ray photoelectron spectroscopy, Li3PO4 is thought to have a function as a solid electrolyte interface between LiNi0.5Mn1.5O4 and SPE to prevent the degradation of solid polymer electrolyte. (C) 2003 The Electrochemical Society.