The crystal chemistry and electrochemical performance of the layered LiNi0.5-yMn0.5-yCo2yO2 oxide cathodes for the entire range of 0 <= 2y <= I have been investigated. Compositions with 0.33 <= 2y <= 0.5 are found to exhibit a combination of high capacities (> 170 mAh/g at 3.0-4.5 V and C/5 rate) and good cyclability (> 90% retention in 50 cycles). The electrochemical rate capability is found to decrease with decreasing Co content 2y. While no obvious correlation is found with the electrical conductivity, the rate capability bears a clear relationship to the chemical lithium extraction rate, which decreases with decreasing Co content due to an increasing cation disorder. Additionally, the lithium extraction rate is found to influence the structure of the chemically delithiated end members Ni0.5-yMn0.5-yCo2yO2-delta; the structure changes from P3 to 01 to 03 with decreasing Co content 2y. (c) 2005 Elsevier B.V. All rights reserved.