NaxCo0.5Mn0.5O2, single phase, which is the starting material of layered LixCo0.5Mn0.5O2, was obtained by optimized synthesis conditions. The layered LixCo0.5Mn0.5O2 with alpha-NaFeO2 structure obtained from NaxCo0.5Mn0.5O2 by ion exchanging showed an electrode potential of 4.5 V vs. Li/Li+, higher than 4.0 V for those end members, LiCoO2 and LiMnO2, and lower than 5.0 V of spinel-type LiCoMnO4. The specific capacity of the layered LixCo0.5Mn0.5O2 was increased with an increase in a Li+ content of the products and showed a maximum with a composition of Li0.8Co0.5Mn0.5O2.