Solid solutions between Li[Li1/3Mn2/3]O2 and LiMO2 (M=Ni1/3Mn1/3Co1/3, Ni0.4Mn0.4Co0.2, Ni0.45Mn0.45Co0.1, Ni0.5Mn0.2Co0.3, and Ni0.5Mn0.3Co0.2) have been synthesized by a solid-state reaction method. The as-prepared Li1.2Ni0.13Mn0.54Co0.13O2, Li1.2Ni0.16Mn0.56Co0.08O2, Li1.2Ni0.18Mn0.58Co0.04O2, Li1.2Ni0.2Mn0.48Co0.12O2, and Li1.2Ni0.2Mn0.52Co0.08O2 solid solutions cathode materials can deliver discharge capacities of 267, 262, 253, 235, and 238mAhg−1, respectively, at a charge/discharge current density of 25 mA g−1 in the voltage range of 2.5–4.7 V. These cathodes all have initial coulombic efficiencies larger than 80%, and show capacity loss less than 0.13% per cycle while cycling at 125 mA g−1 for 50 cycles. From Rietveld refinement results and electrochemical impedance spectra (EIS) analysis, it is found that the highest charge/discharge capacity values of Li1.2Ni0.13Mn0.54Co0.13O2 cathode with the lowest Ni content among them are attributed to the lowest cation mixing. While both the Li1.2Ni0.2Mn0.48Co0.12O2 and Li1.2Ni0.2Mn0.52Co0.08O2 cathode materials with higher Ni content exhibit better rate capabilities due to their lower charge transfer resistances and higher electrical conductivities than those of other samples.