In the previous studies, we observed the three-dimensional distribution of various Co-oxidation states in LiCoO2 (LCO) active materials. It was found that as a LCO electrode is charged, the surface of the electrode is preferentially oxidized than the inside area [1]. In this study, we researched the cross-sectional distribution of Li-ion concentration in the electrolyte of Li-ion battery by using an in-situ X-ray imaging method. In order to use this method we first created a special test cell, which models the cross-section of a positive electrode, separator, and negative electrode in a cell. While charging and discharging the cell, we observed how the cross-sectional distribution of Li-ion in the separator and negative electrode changed. Figure 1 illustrates the X-ray image of the cross section structure of the cell: the positive electrode, separator, and negative electrode and their interfaces. Figure 2 shows how the degree of X-ray transmission of the cross section changes before and 15 and 30-second after the cell is charged at 5C current. Before the cell was charged, the transmission degrees of the negative electrode and separator were the same, demonstrating that the Li-ion concentration was equally distributed across them. However, 15-second after the cell was charged, the transmission degrees in the negative electrode increased, and the degree of the electrode surface (the side close to the separator) became higher than that of the inside (the side close to the Cu collector). We expect that charging made the Li-ion concentration in the negative electrode decrease and the concentration of the surface became lower than that of the inside. As for analysis on the separator, the transmission degree on the negative electrode side of the separator became higher than that on the positive electrode side, demonstrating that the Li-ion concentration on the negative electrode side decreased after the cell was charged and that on the positive electrode side increased. Additionally, the concentration was more largely distributed 30-second after the cell was charged than 15-second after. In summary, we succeeded in applying the in-situ X-ray imaging method for identifying the Li-ion distribution in the negative electrode and separator during charging. References: [1] H. Yamashige, et al. 52th Battery Symposium, Nagoya, 9th Nov 2010.