Effect of local structural changes on rate capability of LiNi_0.5Mn_1.5O_4-δ cathode material for lithium ion batteries

High voltage spinel LiNi_0.5Mn_1.5O₄ is a promising cathode material for the practical application in high energy density lithium ion batteries. With a simple strategy of cooling rate, the degree of ordering in disordered spinel LiNi_0.5Mn_1.5O_4-δ structure is controlled at 0.5 °C min^-1 for slow-cooled spinel and at 5 °C min^-1 for fast-cooled spinel. From XRD and FTIR analyses and charge/discharge profiles, it is confirmed that the fast-cooled spinel develops more disordered structure than the slow-cooled spinel, retaining more oxygen vacancies and more abundant Mn³⁺. By using in situ XRD analysis, the more disordered spinel follows the extended solid-solution reaction in favor of Li⁺ transport, which results in an improved rate capability. Moreover, through ex situ XAS analysis and the more quantitative results from EXAFS fitting, the local structural changes in the vicinity of Mn and Ni cations are investigated at various C-rates and smaller structural distortion in more disordered spinel is proposed to enhance the rate capability.