Rate Dependence of the Average Crystal Structure and Electronic Structure of 0.5Li₂MnO₃-0.5LiMn_10/24Ni_7/24Co_7/24O₂ for a Lithium-Ion Battery Positive Electrode Material in Steady State

The rate dependence of the average crystal structure and electronic structure of a 0.5Li₂MnO₃-0.5LiMn_10/24Ni_7/24Co_7/24O₂ electrode was investigated during the 1st charge and discharge processes and the 10th charge and discharge processes, using neutron and synchrotron X-ray diffraction analyses. In cyclic tests at 0.1C, 1C, and 3C, the electrode could deliver a high capacity of over 200 mAh/g in the first discharge process within a voltage range of 2.5-4.8 V versus Li/Li⁺ at a 0.1C rate, but the capacity decreased at higher rates. To determine the reason for this change in characteristics, we performed X-ray and neutron diffraction analyses of electrodes following the 1st and 10th charge/discharge cycles and determined the average structure using Rietveld analysis. The results indicated that transition metals migrated and that there was higher distortion of MO₆ after 10 cycles than after 1 cycle. Such a structure change could be considered as the reason for lower capacity at higher rate. We also performed maximum-entropy method analysis, which showed that the electron density following the 10th discharge cycle was lower than that following the 1st discharge cycle. The results indicated a rate-dependent change in the transition-metal site occupancy, which affected the battery characteristics.