We investigated the relationship between physical properties and electrochemical performances for Li1+yNixCo1-2xMnxO2 positive active materials. The improvement of rate capability by excess Li was related to the increased electrical conductivity that was accompanied by Ni3+ increase. Electrochemical impedance measurements indicated that the charge-transfer resistance decreases with increased excess Li content (electrical conductivity) for Li1+yNixCo1-2xMnxO2. The value of charge-transfer resistance was compared with the simulated value by the equivalent circuits' model considering the volume resistivity of positive active material. (c) 2007 Elsevier B.V. All rights reserved.