It has been reported that lithium-rich cathode materials of LIB emit singlet oxygen during charging, which chemically oxidizes electrolyte solutions, and the decomposition products form surface film on the material. However, the detailed conditions and mechanism of the surface film formation and its effect on the electrochemical reaction at the electrode/electrolyte interface have not been clarified in detail. In this study, using 0.5LiCoO2 • 0.5Li2MnO3 thin-film electrodes as the model electrodes of the lithium-rich cathode materials, the surface film formation behavior was investigated. After a constant current-constant voltage (CCCV) measurement to 4.8 V, passivation of the electrodes did not occur. On the other hand, the electrode after cyclic voltammetry (CV) up to 4.8 V showed complete passivation. The results of spectroscopic analyses revealed that decomposition products of the solvent formed thick surface film on the electrode after CV. From the results, it was concluded that the passivation surface film was formed by the simultaneous decomposition of the solvent via electrochemical oxidation at high potentials and chemical oxidation by singlet oxygen. Furthermore, the electrode with the surface film showed better cycleability than that without the surface film, indicating that it contributes to the suppression of side reactions at the electrode/electrolyte interface.