The structure, synthesis, and electrochemical behavior of layered Li[NixLi(1/3-2x/3)Mn(2/3-x/3)O2 for x = 1/3, 5/12, and 1/2 are reported for the first time, Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2 is derived from Li2MnO3 or Li[Li1/3Mn2/3]O-2 by substitution of Li+ and Mn4+ by Ni2+ while maintaining all the remaining Mn atoms in the 4(+) oxidation state. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)O-2 with x = 5/12 can deliver steady capacities of 150 and 160 mAh/g at 30 and 55 degreesC, respectively, between 3.0 and 4.4 V using a current density of 30 mA/g. Differential scanning calorimetry experiments on charged electrodes of Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2 for x = 5/12 indicate that this material should be safer than LiCoO2. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2 with x = 1/3, 5/12, and 1/2 can be cycled between 2.0 and 4.6 V to give capacities of about 200, 180, and 160 mAh/g, respectively, at 30 degreesC. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2 with x = 1/3 gives a capacity of 220 mAh/g at 55 degreesC between 2.0 and 4.6 V using a current density of 30 mA/g.