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 LixNi0.5Mn1.5O4, 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 LiNi0.5Mn1.5O4 (Li1), Li1/2Ni0.5Mn1.5O4 (Li1/2) and Ni0.5Mn1.5O4 (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 LiNi0.5Mn1.5O4 electrode. The obtained rate constants can be used to characterize and optimize the rate capability of the electrode.