In-Situ Visualization of Li-Ion Secondary Battery Using Soft X-Ray Microscopy

The lithium ion secondary battery is essential device for electric vehicle (EV) and hybrid electric vehicle (HEV). However, fundamental mechanism of the transport phenomena in an electrode under charge/discharge condition needs to be elucidated in order to improve the battery performance. In this study, in-situ visualization of the Li-ion battery was performed by applying a soft X-ray microscopy technique. As a result, ion concentration distribution in the negative electrode was able to be investigated. Density reduction that was caused by Li intercalation was also observed.

Figure 1 shows schematic image of experimental apparatus. Using a low energy X-ray (soft X-ray) is quite effective to visualize the light element like Li because the amount of X-ray absorption is greater than that of conventional X-ray. In order to observe the transport phenomena 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 punched out 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, two active materials, hard carbon or graphite, were employed for negative electrode. Hard carbon negative electrode was prepared by mixing hard carbon (particle size 10 μm), conductive assistant, and binder (polyvinylidene fluoride). 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(1:1).

Figure 2(a) shows X-ray transparent image of the hard carbon negative electrode. 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 (I_x0) and charge/discharge experiments (I_x), concentration distribution of anion and cation that is caused by transport phenomena can be observed under operating condition.

Figure 2(b) and (c) show distribution of I_x/I_x0 in the separator and negative electrode. Constant current charge/discharge was performed (3C and 10C). During the charge process, I_x/I_x0 shows apparent gradient profile that increases toward the negative electrode side. On the other hand, I_x/I_x0 shows an opposite trend during discharge process. Increase of I_x/I_x0 during the charge process means a decrease of concentration of Li⁺ and PF₆^- in the electrolyte. It is considered that the ion concentration in the negative electrode was decreased because Li⁺ and PF₆^- were drawn to the positive electrode by the electric field during the charge process. Thus the change of ion concentration through the charge/discharge process can be observed in Figure 2(b) and (c). In the open circuit condition after charging, Li had inserted in the hard carbon. However, it is difficult to visualize the change of I_x/I_x0. The amount of Li intercalation was small to cause clear difference of I_x/I_x0 compared with Li⁺ and PF₆^- in the electrolyte.

Figure 3(a) shows the time variation (Initial to 7^th) of X-ray transparent images of graphite negative electrode. Lithium intercalation changes interlayer distance of the graphite. Thus, the insertion process can be observed by the deformation or density reduction of the negative electrode. As shown in Figure 3(a), swelling of the negative electrode during 0.5 C charge was observed successfully. I_x/I_x0 changes greater than that of hard carbon negative electrode (Figure 3(b)). Thus, Li intercalation can be detected easily. It is considered that Li intercalation occurred not depending on the position because I_x/I_x0 increased uniformly. The expansion rate 1.1 calculated from the experimental result agrees well with the theoretical rate 1.09 that was derived from the ratio between initial interlayer distance (0.34 nm) to inserted distance (0.37nm).

As mentioned above, using the soft X-ray microscopy technique, we have succeeded in visualization of the transfer phenomena in the Li-ion secondary battery during charging and discharging. By calculating the ratio of X-ray intensity (I_x/I_x0), Li⁺ and PF6^- distributions in the hard carbon negative electrode were obtained clearly. Li intercalation into the graphite negative electrode was also observed. These results suggest that transport phenomena in the electrode can be evaluated quantitatively by the soft X-ray microscopy technique.