Effect of Excess Li and Cation Mixing on Electronic Structure of LiNi_0.5Mn_0.5O₂ Cathode Active Material of Lithium-Ion Battery Investigated by First-Principle Calculation

Abstract

Co-free LiNi_0.5Mn_0.5O₂ shows better thermostability and higher discharge capacity than LiCoO₂. We investigate the effects of Li content and cation mixing on electronic structure in Li_xNi_0.5Mn_0.5O₂ (x=1.00∼1.08) by Rietveld analysis and first principle calculation. The crystal structures were well refined and excess Li existed at transition-metal site without any change of the Li and Ni occupancies at Li site. It suggests that the amount of the cation mixing has no relation with Li content. From the result of structure relaxation by calculation, it was found that the structure was stabilized by cation mixing and thus cation mixing is possible to happen theoretically. This result is caused by increase of covalent bond near the cation mixing atom. In the sample of Li_1.03Ni_0.5Mn_0.5O₂ which shows good discharge performance, electron density between the transition metal and oxygen showed maximum among the investigated samples by MEM (experiment) and WIEN2k (calculation). Such result suggests that host structure stability plays an important role for the discharge performance.