An unresolved question for the layered oxides is: what is the optimum value of y in the formula LiNiyMnyCo1-2yO2 for energy storage at moderate reaction rates? Here we report a systematic study of the specific capacity, rate capability and cycle life of LixNiyMnyCo1-O-2y(2) (y = 0.5, 0.45, 0.4, and 0.333). The voltage of the Li/y = 0.333 couple crosses over those of lower cobalt content for x < 0.55, as the Co redox begins to get involved. This early involvement of cobalt, rather than just Ni, leads to a slightly smaller specific capacity for y = 0.333 than for LiNi(y)Mn(y)Co(1-2)yO(2) with y > 0.333 when charging above 4 V. Overall the y = 0.4 material has the optimum properties, having the highest theoretical capacity, less of the expensive cobalt and yet rate capabilities and capacity retention comparable to the y = 0.333 material. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3562212] All rights reserved.