We have previously indicated that the improvement of the interface degradation between a positive electrode and an ethylene oxide-based solid polymer electrolyte (SPE) is an important problem requiring a solution for all-solid-state lithium secondary batteries. Therefore, Li3PO4 thin-film on LiCoO2 powder was prepared by the spray coating technique. The observed Li3PO4 film was so thin (approximately 20nm thick) that its contribution to the increase in interfacial resistance was small. However, it played the role of an oxidation barrier for the SPE. A [Li3PO4-coated LiCoO2|SPE|Li] cell exhibited reversibility up to 4.6V with a 200mAhg−1 discharge capacity. The energy density of 760mWhg−1 is one of the highest values obtained for all-solid-state lithium secondary batteries.