Overcharge is an aggressive abuse condition that can lead to thermal runaway of a lithium-ion cell. Understanding the failure mechanism due to overcharge is critical for designing safer lithium-ion chemistries. With the help of in situ high-energy X-ray diffraction (XRD), we are able to detect the temperature difference between the cathode and the anode during the overcharge abuse of 18650 cells. In this paper, the lattice constants of electrode current collectors (Al for the cathode and Cu for the anode) are calculated by fitting the XRD patterns, and the temperature variations of the cathode and anode are quantified by the thermal expansion of the Al and Cu foils. Based on these results, we report that during the overcharge abuse of an 18650-size cell, using graphite as the anode and LiNi0.8Co0.15Al0.05O2 as the cathode, the temperature of cathode increases as the voltage reaches 4.16 V, corresponding to the occurrence of the H2-to-H3 phase transition in the cathode material.