Layer-structured LiMn0.5-xCo2xNi0.5-xO2 was prepared as cathode material for lithium-ion batteries. The structures of the layered materials and the oxidation states of the elements in the compounds were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. Adsorbed oxygen was detected on the surface of material. With the increase of Co content in LiMn0.5-xCo2xNi0.5-xO2, the oxidation state of Ni, Mn, Co, and O became higher gradually while the amount of oxygen adsorbed on the surface of LiMn0.5-xCo2xNi0.5-xO2 grains reduced obviously. Electrochemical evaluation showed that addition of Co in LiMn0.5-xCo2xNi0.5-xO2 is beneficial to its rate performance. The variations of the electronic structure of Ni, Mn, and O may be responsible for the improvement of the rate capability in LiMn0.5-xCo2xNi0.5-xO2 with addition of Co. (C) 2004 The Electrochemical Society.