Li1+x Ni1/6Co1/6Mn4/6O2.25+x/2 (0.1≤ x ≤0.7) cathode materials have been synthesized by a simple carbonate co-precipitation method. The effects of the lithium content on the structure, physical property, and electrochemical performance of the samples have been investigated. As the increase of lithium content, the Li1+x Ni1/6Co1/6Mn4/6O2.25+x/2 evolves from a mixture of hexagonal R−3m, monoclinic C2/m, and spinel Fd−3m structure to a mixture of hexagonal and monoclinic structure, accompanied with less cation mixing between Li+ and Ni2+. The increase of x value also affects the size of the primary particles and the roughness of the secondary particles. The Li1+x Ni1/6Co1/6Mn4/6O2.25+x/2 with appropriate lithium content, e.g., x =0.3, thereby small particle size, high pellet density, and low cation mixing, brings on the largest discharge capacity, which is more than 288mAhg−1 in the voltage range of 2.0–4.8V at 25mAg−1, and the lowest irreversible capacity loss of 47mAhg−1 among these cathode materials.