The synthesis, structure, and electrochemical performance of Li[NixLi1/3-2x/3Mn2/3-x/3]O-2 for 0, x less than or equal to 1/ 2 is presented. Li[NixLi1/3-2x/3Mn2/3-x/3]O-2 is prepared by substituting Ni2+ for Li+ and Mn4+ in Li[Li1/3Mn2/3]O-2 while maintaining all the remaining Mn in the 14 oxidation state. Samples with x = 1/6, 1/4, 1/3, 5/12, and 1/2 have been investigated by X-ray diffraction (XRD) and neutron diffraction as well as by electrochemical measurements. The XRD and neutron diffraction patterns of Li[NixLi1/3-2x/3Mn2/3-x/3]O-2 (0 < x &LE; 1/ 2) show that these compounds adopt the O-3-LiCoO2-type structure when synthesized at 800&DEG;C and higher. XRD and neutron diffraction also suggest a short-range superlattice ordering of Li, Ni, and Mn in the transition-metal layer for many of the samples. When synthesized at 700&DEG;C and lower, the compounds (for x = 1/3 and 1/2) appear to adopt a spinel-type structure like LT-LiCoO2. Electrochemical studies show that Li[NixLi1/3-2x/3Mn2/3-x/3]O-2 (900&DEG;C) with x = 5/12 can deliver a stable capacity of about 160 mAh/g between 3.0 and 4.4 V vs. Li. An irreversible plateau is observed at about 4.5 V during the first charge of Li/Li[NixLi1/3-2x/3Mn2/3-x/3]O-2 cells (x = 1/6, 1/4, 1/3, and 5/12), which we believe corresponds to the simultaneous removal of lithium and oxygen from the structure. After the plateau, Li[NixLi1/3-2x/3Mn2/3-x/3]O-2 cells with x = 1/3 and 5/12 can deliver stable reversible capacities of about 230 and 225 mAh/g between 2.0 and 4.6 V. Li/Li[NixLi1/3-2x/3Mn2/3-x/3]O-2 (0, x < 1/2) synthesized at low temperatures (i.e., 600 and 700degreesC) shows dramatically different differential capacity vs. voltage behavior compared to the high temperature samples, which must be related to the structural differences between materials prepared above and below 750degreesC.