Layered Li[Li0.17Mn0.33Co0.5-yNiy]O-2 (0 <= y <= 0.5) cathodes belonging to a solid solution series between layered Li[Li1/3Mn2/3]O-2 and LiCo1-zNizO2 were synthesized at 900 and 750 degrees C and characterized as cathode materials for lithium-ion batteries. The observed lithium content in the samples decreases with increasing Ni content due to a tendency to form Ni2+ and volatilization of lithium during synthesis. The first charge and discharge capacities decrease with increasing Ni content. For example, the first discharge capacity drops from 269 mAh/g for the y=0 sample to 194 mAh/g for the y=0.5 sample. The decrease in the discharge capacity with increasing Ni content is due to a decrease in the lithium content in the transition metal layer and a consequent decrease in the amount of oxygen lost irreversibly during the first charge as indicated by a structural and chemical characterization of the chemically delithiated samples. (c) 2007 The Electrochemical Society.