Surface modification of the electrode improved the performance of the LiNiO2 electrode charged to 4.6V. An amorphous carbon thin film was uniformly deposited on the electrode surface using dc plasma chemical vapor deposition. This surface treatment suppressed electrolyte decomposition during the charge process. When a coin-type cell using this electrode was charged to 4.6V, the charge–discharge efficiency was 84% in the first cycle and 96–98% through the second to fifth cycles. This treatment suggests a new approach to the development of the positive electrode for lithium-ion secondary batteries.