A series of LiNi0.5Mn0.5-xCoxO2 (0 <= x <= 0.5) compounds was prepared by a solid state reaction, and their structure, surface state and electrochemical characteristics were also investigated by XRD, XPS, EIS and charge-discharge cycling. The non-equivalent substitution of cobalt for manganese induced an increase in the average valence of nickel, thereby shrinking in the lattice volume. Moreover, Co non-equivalent substitution could not only reduce the impurity content but also significantly decreased the charge transfer resistance, thereby improving the rate capabilities. (c) 2005 Elsevier Ltd. All rights reserved.