Elucidating the Driving Force of Relaxation of Reaction Distribution in LiCoO2 and LiFePO4 Electrodes Using X-ray Absorption Spectroscopy

The reaction distribution in the composite electrodes used in lithium-ion batteries greatly affects battery performances, including rate capability and safety. In this study, the generation of the reaction distribution and its relaxation in cross sections of LiCoO2 and LiFePO4 composite electrodes were analyzed using microbeam X-ray absorption spectroscopy. The reaction distribution immediately after delithiation could be observed clearly with different oxidation states of the transition metal (i.e., different concentrations of lithium ions). The distribution in the Li1–xCoO2 electrodes disappeared, whereas that in the Li1–xFePO4 electrodes remained unchanged even after 15 h of relaxation. After comparing the potential profile of both types of electrodes, it is suggested that the potential difference between the more delithiated area and the less delithiated area in the composite electrode is the primary driving force for the relaxation.