The preparation of LiCo y Mn x Ni1−x−y O2 from LiOH·H2O, Ni(OH)2 and γ-MnOOH in air was studied in detail. Single-phase LiCo y Mn x Ni1−x−y O2 (0≦y≦0.3 and x=0.2) is obtained by heating at 830–900°C. The optimum heating temperatures are 850°C for y=0–0.1 and 900°C for y=0.2–0.3. Excess lithium (1≦z≦1.11 for y=0.2) and the Co doping level (0.05≦y≦0.2) do not significantly affect the discharge capacity of Li z Co y Mn0.2Ni0.8−y O2. The doping of Co into LiMn0.2Ni0.8O2 accelerates the oxidation of the transition metal ion, and suppresses partial cation mixing. Since the valence of the manganese ion in LiMn0.2Ni0.8O2 is determined to be 4, the formation of a solid solution between LiCo y Ni1−y O2 and Li2MnO3 is confirmed.