Candidate cathode materials of cobalt-incorporated and lithium-enriched Li(1+x)Ni0.25Co0.15Mn0.6O(2.175+x/2) (x=0.225-0.65) have been prepared by a coprecipitation method and a solid-state reaction. We systematically investigated the effect of both cobalt presence and lithium concentration on the structure, physical properties, and electrochemical behavior of the studied samples. The electrochemical performance of the cobalt-containing compounds showed much less dependence on the variation in the lithium amounts compared to the cobalt-free counterpart. The study demonstrated that even with cobalt incorporation, proper lithium content is the key to desirable cathode materials with nanostructured primary particles that are indispensable to achieve high capacity and high rate capability and, therefore, both improved energy and power densities for lithium-ion batteries. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3418608] All rights reserved.