Abstract: The structure and cycle behavior of LiNi_1/3Li_1/9Mn_5/9O₂ were studied by means of XRD,SAED and synchrotron X-ray diffraction,respectively.The results show that LiNi_1/3Li_1/9Mn_5/9O₂ can be indexed based on the α-NaFeO₂ single phase and confirmed to be in good agreement with \sqrt 3 a_hex.×\sqrt 3 a_hex.×c_hex.superstructure.When the cell was charged to 3.8 V with lithium de-intercalation,the electrostatic repulsion between adjacent oxygen layers gradually increased and became largest near 3.8 V.At 4.6 V the rapid decrease of lattice constant c revealed that the most of the Li⁺ extracted from the Li-layer,accompanying with the oxidation of Ni²⁺ ions.When the charge voltage located between 4.6 and 4.8 V,c axis increased while a axis remained almost constant.The fact suggested that some Li⁺ de-intercalated from the transition metal layer,but the transition metal oxidation states did not change at 4.6-4.8 V.