Layered Li[Li x Ni(1−3x)/2Mn(1+x)/2]O2 materials with x=0.05, 0.1, 0.15, and 0.2 were synthesized using a sol–gel method. The layered Li[Li x Ni(1−3x)/2Mn(1+x)/2]O2 structure was stabilized by a solid solution between LiNiO2 and Li2MnO3. The discharge capacity of the Li[Li x Ni(1−3x)/2Mn(1+x)/2]O2 electrodes increased with decreasing Ni content. Li[Li x Ni(1−3x)/2Mn(1+x)/2]O2 electrode with x=0.05, 0.1, 0.15, and 0.2 delivered high discharge capacities of 184, 193, 206, and 209mAhg−1 at 30°C, respectively, with excellent cycleability.