Lithium- and manganese-rich layered electrode materials, represented by the general formula xLi2MnO3·(1 − x)LiMO2 in which M is Mn, Ni, and Co, are of interest for both high-power and high-capacity lithium ion cells. In this paper, the synthesis, structural and electrochemical characterization of xLi2MnO3·(1 − x)LiMn0.333Ni0.333Co0.333O2 electrodes over a wide compositional range (0 ≤ x ≤ 0.7) is explored. Changes that occur to the compositional, structural, and electrochemical properties of the electrodes as a function of x and the importance of using a relatively high manganese content and a high charging potential (>4.4 V) to generate high capacity (>200 mAh/g) electrodes are highlighted. Particular attention is given to the electrode composition 0.3Li2MnO3·0.7LiMn0.333Ni0.333Co0.333O2 (x = 0.3) which, if completely delithiated during charge, yields Mn0.533Ni0.233Co0.233O2, in which the manganese ions are tetravalent and, when fully discharged, LiMn0.533Ni0.233Co0.233O2, in which the average manganese ...