We prepared Li
xMn
0.5Ni
0.5O
2 by solid-state and solution methods, and investigated their thermodynamic stabilities, crystal and electronic structures and cathode performances. The cycle performance of Li
xMn
0.5Ni
0.5O
2 depended on its synthetic method and Li content; that is, the samples synthesized by a solid-state reaction seemed to show better cycle performance than those prepared by a solution method, and the samples with x=1.03∼1.05 exhibited larger discharge capacity and higher capacity retention regardless of the synthetic process. Crystal structure analyses using a neutron source suggested that an existence of Ni
2+ at Li
+ site and/or a higher distortion around the transition metals deteriorated the cycle performance. It was also clarified by XAFS measurements that such a distortion was localized in the Mn-O
6 octahedron. From reaction enthalpies of the materials evaluated by calorimetries, it was suggested that higher thermodynamic stability was one of the reasons for better cathode performance.
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