Dependence of Thermodynamic Stability, Crystal and Electronic Structures and Battery Characteristic on Synthetic Condition and Li Content for LixMn0.5Ni0.5O2 as a Cathode Active Material of Li-Ion Battery

We prepared LixMn0.5Ni0.5O2 by solid-state and solution methods, and investigated their thermodynamic stabilities, crystal and electronic structures and cathode performances. The cycle performance of LixMn0.5Ni0.5O2 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 similar to 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 Ni2+ 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.