The reaction distribution in lithium ion batteries is a critical problem for high power applications such as hybrid vehicles.We have studied the cross-sectional distribution of LiPF₆ concentration in the electrolyte of lithium ion batteries to clarify the reaction distribution in lithium ion batteries under rapid charge-discharge condition.In our previous studies, we observed the cross-sectional distribution of LiPF₆ in a separator and a negative electrode while charging and discharging in a special test cell by using an in-situ X-ray imaging method. It was found that LiPF₆ concentration distribution changed as the X-ray transmission strength changed.

In this study, we researched the cross-sectional distribution of LiPF₆ in the positive electrode as well as the separator and the negative electrode. In order to use this imaging method for observation in the positive electrode, we reviewed the width of a special test cell and X-ray energy. The special test cell assumed to be an assembly of the positive electrode, the separator, and the negative electrode for the observation of the cross-section in lithium ion battery. The cell was fixed by a fastening plate, and the electrolyte was injected on the cell under an Ar-gas atmosphere. The cell functioned as well as normal some. While charging and discharging the cell, we observed how the cross-sectional distribution of LiPF₆ concentration in the separator, the negative electrode and the positive electrode at TOYOTA BL of SPring-8 in Hyogo, Japan.

Fig.1 shows the degree of X-ray transmission of the cross section before and 3 and 10-second after the cell discharging at 25C current. Before the cell was charged, the transmission degree of the separator, the negative electrode and the positive electrode were the same, demonstrating that the LiPF₆ concentration was equally distributed across them. However, 3-second after the cell charge, the transmission degree in the positive electrode and the separator on the positive side increased (a), in the negative electrode and the separator on the negative side decreased (b). We expected that discharging made the LiPF₆ concentration in positive electrode low and concentration in negative electrode high. As for the analysis of the positive electrode, the transmission degree on the separator side of the positive electrode became lower than that on the Al current collector side, demonstrating that LiPF₆ concentration on separator side became higher than that on the Al current collector side after the cell was discharged. As for the analysis of the negative electrode, the transmission degree decreased over the whole negative electrode equally, demonstrating that LiPF₆ concentration increased after the cell was discharged. As for the analysis on the separator, the transmission degree on the negative electrode side became lower than that of the positive electrode side, demonstrating that the LiPF₆ concentration of negative electrode side became higher than that on the positive electrode side after the cell was discharged.Additionally, the concentration was more largely distributed 10-second after the cell was charged than that of 3-second later.

In summary, we succeeded in applying the in-situ X-ray imaging method for identifying the LiPF₆ concentration in the positive electrode, the negative electrode and the separator during discharging.