Layered high manganese cathode materials Li1+n [Ni m Co m Mn1−2m ]O2 (n =0.46 and 0.36; m =0.15, 0.20 and 0.25) are prepared by coprecipitation method. The manganese (m <0.25) and lithium (n >0.3) content in the system are controlled at a high level. Structural and electrochemical feature changes of the resulted cathode materials have been studied thoroughly. The results indicate that the excess lithium in the cathode materials system tends to stabilize the high-Mn layered structure and improve electrochemical performance. The results reveal that samples of m =0.15 with a high manganese content exhibit the highest capacity and best cycling efficiency. The improved electrochemical performance is considered to be owing to the slow activation of Li2MnO3, which is good for stabilizing the structure.