The effect of Co doping in Li[Li-0.1 Ni0.35-x/2COxMn0.55-x/2]O-2 materials on the electrochemical properties has been investigated. Layered Li[Li0.1Ni0.35-x/2CoxMn0.55-x/2]O-2 (x=0-0.3) materials considered to be a solid solution of LiNi0.5Mn0.5O2, LiCoO2, and Li2MnO3 were synthesized using a sol-gel method. The Co doping of x=0.1 showed improved capacity and rate capability comparing to undoped Li[Li0.1Ni0.35Mn0.55]O-2. Area-specific impedance (ASI) and AC impedance measurement show that Co-doped electrode has the lower impedance value during cycling. It is considered that the higher capacity and superior rate capability of Co-doping sample, especially for the Li[Li0.1Ni0.3Co0.1Mn0.5]O-2, would be ascribed to the reduced resistance of the electrode during cycling. (C) 2003 Elsevier B.V. All rights reserved.