Layered transition-metal oxides, LiMnxCoyNi1-x-yO2, have been considered as potential cathode materials for lithium batteries with high energy density. The crystal structures, reversible potentials and activation energies of LiMnxCoyNi1-x-yO2 are studied by means of density functional theory (DFT) calculations within generalized gradient approximation (GGA) and projector-augmented-wave (PAW) method. The larger cell volume with increasing Ni content benefits the capacity of the cathode materials. Sufficient amount of Ni content in the LiMnxCoyNi1-x-yO2 is beneficial for stabilizing the electrode voltage. Ni substitution is always beneficial to Li diffusion, whereas existence of Mn ions may hinder Li motion by increasing the activation energy. All these first-principles results reveal some general trends for the synergistic effects of TM ions and may guide finding optimal compositions in future experiments. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.025208jes] All rights reserved.