The lithium ion secondary battery (LIB) is essential device for electric vehicle (EV). However, the fundamental mechanisms of ion transport phenomena in LiB during operation, which determine battery performance, are yet to be elucidated. In this study, inside the operating LIB was visualized by using low energy X-ray microscopy technique. As a result, ion concentration distribution in the negative electrode and the separator were measured quantitatively.

Figure 1 shows schematic image of experimental apparatus. Using a low energy X-ray is quite effective to measure the slight difference of ion concentration because the amount of X-ray absorption is greater than that of conventional X-ray. In order to observe the ion concentration distribution in a micro-scale thin layer of the battery, present study achieved high magnification (1 mm/pixel resolution) by irradiating the cone-shaped soft X-ray beam. Cells for visualization, positive electrode, negative electrode, and separator, were cut with the dimension of 6 mm x 20 mm by a cutting machine. Those were sandwiched by aluminum and copper current collector in a plastic frame. After injection of the electrolyte, cell was sealed by covering with the stainless plates that have X-ray transparent windows. All of assembling process was performed in Ar filled glove box.

In this study, graphite was employed for negative electrode. Graphite electrode was prepared by mixing graphite (particle size 20 μm), conductive assistant, and binder (polyvinylidene fluoride). Active material for positive electrode was LiMn₂O₄. Separator was polypropylene microporous membrane. The electrolyte was 1 M LiPF₆ / EC:DEC(3:7).

Figure 2 shows X-ray transparent image of the graphite negative electrode and the separator. Since Mn significantly absorbs low energy X-rays in the positive electrode, it could not be visualized. The intensity of X-ray transmission indicates density and concentration of the materials that obeys Beer-Lambert law. Thus, by calculating the ratio of X-ray intensity between initial condition and charge/discharge experiments, ion concentration change that is caused by ion transport phenomena can be observed qualitatively. Based on the calibration experiment, quantitative ion concentration distributions were derived from the X-ray intensity ratio distribution.

Figure 3 shows quantitative ion concentration distribution in an operating LIB. Constant current discharge was performed (1C, 2C, and 3C). As the discharge rate increases, the ion concentration gradient in the negative electrode becomes steep. Under the discharge condition of 3 C, the maximum ion concentration in the negative electrode reached about 2.5 M. These results suggest that the ion transport resistance increased at higher discharge rate condition because the diffusivity decreases in the higher concentration electrolyte.

As mentioned above, using the low energy X-ray microscopy technique, we have succeeded in quantitative measurement of the ion concentration distribution in an operating LIB. In order to improve battery performance, it is important to design an optimum structure (electrode thickness, porosity, etc.) and increases ion transport efficiency in the deep portion of the electrode.