Phase transition kinetics of LiNi_0.5Mn_1.5O ₄ electrodes studied by in situ X-ray absorption near-edge structure and X-ray diffraction analysis

In situ time-resolved X-ray absorption near-edge structure (XANES) and X-ray diffraction (XRD) measurements are applied to track the phase transition processes of Li_xNi_0.5Mn_1.5O₄, which is one of the most promising positive electrode materials for lithium ion batteries with its high redox potential of 4.7 V vs. Li/Li⁺ and good rate capability. Thanks to the high time resolution brought by a strong synchrotron X-ray beam, the XANES and XRD measurements separately capture the three phases involved in nearly the same potential region, namely LiNi _0.5Mn_1.5O₄ (Li1), Li_1/2Ni _0.5Mn_1.5O₄ (Li1/2) and Ni_0.5Mn _1.5O₄ (Li0), and the phase transition kinetics under fast charging-discharging conditions is elucidated. The detailed kinetic analysis shows that the phase transitions are apparently expressed by the first-order kinetics and that the transition between the Li1 and Li1/2 phases is faster than that between the Li1/2 and Li0 phases, which leads to kinetically asymmetric behavior of the LiNi_0.5Mn_1.5O₄ electrode. The obtained rate constants can be used to characterize and optimize the rate capability of the electrode.