Charging lithium-ion batteries above the manufacturer's high-voltage protection, so-called overcharge, causes rapid and exothermic reactions, and then violent thermal runaway may happen in the worst case. Side (electro-) chemical reactions occurring at both positive and negative electrodes during overcharge are complicated, but those have to be explored to overcome durability and safety challenges. In-situ synchrotron X-ray absorption and diffraction technique for a lithium-ion battery of LiNi0.75Co0.15Al0.05Mg0.05O2 (NCA-Mg) and graphite was developed to detect side reactions in lithium insertion /extraction reactions at both positive and negative electrodes during overcharge toward 10 V. Ni and Co K-edge X-ray absorption near-edge structure (XANES) spectra were examined to detect the oxidation states of Ni and Co species in NCA-Mg, which correspond to lithium extraction reaction, i.e., charge reaction, occurring at the positive electrode. X-ray diffraction (XRD) of NCA-Mg and graphite was also collected, and hence the combination of XANES and XRD enables us to monitor side reactions in lithium insertion / extraction reactions at both positive and negative electrodes. Possible side-reactions in lithium insertion / extraction reactions are discussed at both positive and negative electrodes in the lithium-ion batteries at 30 and 50 degrees C. (C) 2016 The Electrochemical Society. All rights reserved.