Abstract: The LiNi_0.7Mn_0.3O₂ cathode material was synthesized under different sintering atmosphere via coprecipitation and high temperature sintering method. The morphology, crystal structure, oxygen concentration, electrochemical performance and electrochemical impedance spectroscopy for LiNi 0.7Mn0.3O2 cathode material were characterized and tested by means of scanning electron microscopy (SEM), powder X -ray diffraction (XRD), oxygen analyzer, galvanostatic charge -discharge tests and electrochemical workstation. The results show that under the condition of whole sintering time in high purity oxygen atmosphere, the obtained material exhibits the best spherical morphology, the most complete layer structure, the best electrochemical performance and the smallest internal impedance. The initial discharge specific capacity of the material is as high as 191.0 mAh/g at 0.2 C within the voltage range of 2.75~4.35 V, the capacity retention ratio is 95.2 % after 70 cycles. But when the materials are synthesized in other sintering atmosphere, the initial discharge specific capacity are all below 184.0 mAh/g, and the capacity retention ratio are also all below 85 % after 70 cycles.