Cathode material candidates with discharge capacities greater than 200 mAh/g, excellent cycling performance, and good thermal stability can be found as solid solutions in the pseudoternary system, xLi[Mn1/2Ni1/2]O(2)center dot yLiCoO(2)center dot zLi[Li1/3Mn2/3]O-2 (x + y + z = 1). In this paper, we designate such samples by the formula Li[(Ni0.5Mn0.5)(x)Co-y(Li1/3Mn2/3)(z)]O-2, where x + y + z = 1. Samples with z not equal 0 show an extended voltage plateau near 4.5 V during the first removal of lithium that is thought to correspond to the simultaneous removal of Li and oxygen form the structure. After traversing this plateau, materials can be cycled reversibly between 4.6 and 2.5 V for many cycles with high capacity. In this paper we compare the capacity, capacity retention, rate capability, and reactivity of these high-capacity materials with z = 1/6 and 1/3 and y = 0, 1/12, 2/12, 3/12, and 5/12. The following trends are revealed: (i) capacity and capacity retention improve as z increases at fixed y, in the range 1/6 <= z <= 1/3, for cells cycled between 2.5 and 4.6 V; (ii) primary particle size increases with Co content, y, increases at fixed z; however (iii) the reactivity of charged materials (4.8 V) with electrolyte becomes more severe for samples with Co content, y > 1/4. The results here show that Li[(Ni0.5Mn0.5)(x)Co-y(Li1/3Mn2/3)(z)]O-2 samples having a composition between 1/6 <= z <= 1/3 and 1/12 <= y <= 1/4 show high discharge capacity (approximately 200 mAh/g between 2.5 -4.6 V at C/6) and excellent cycling performance (above 97% capacity retention for 80 cycles). In addition, these samples charged to 4.8 V show less reactivity with LiPF6 ethylene carbonate/diethyl carbonate electrolyte than LiCoO2 charged to only 4.2 V. Therefore, these cathode materials are suggested for energy cells used in cell phones, laptops, and so on. (c) 2005 The Electrochemical Society.