## 9 Industrial Applications (_Operando_ Observation of Electrolyte Distribution)  
Understanding transport characteristics such as the electrolyte distribution during battery operation is essential for the optimization of the operating conditions of storage batteries, especially in automotive applications requiring high output performance and high-frequency charge-discharge durability. The dynamic behavior of the ion distribution (salt concentration) in the electrolyte during storage battery operation cannot be observed with any imaging method except for CXI due to its high sensitivity, as shown in Table 7. Experimental results have demonstrated that the salt concentration distribution in the electrolyte of a lithium-ion battery (LIB) and the electrolyte stratification behavior of a lead-acid battery (LAB) can be quantitatively visualized and analyzed with a CXI system. Figure 15: Sectional images obtained by CXI of (**a**) CH\({}_{4}\) hydrate and (**b**) natural gas hydrate with ice (white part) at 193 K. Reprinted with permission from [60, 61]. Copyright 2011 and 2012, American Chemical Society. A LIB model cell connected to a galvanostat (GS) was installed in the object beam path of a CXI system for in situ observation during charging and discharging (Figure 16a). A two-pole cell composed of a LiFePO4 composite electrode was used as the working electrode (WE), a lithium metal was used as the counter electrode (CE), and 1.0 M LiClO\({}_{4}\) dissolved in carbonate solvent (EC:DEC = 1:2 \(v/v\)) was used as the electrolyte (Figure 16b). A continuous phase map of the electrolyte was obtained using 35-keV SR with an exposure time of 1 s during charging and discharging. Figure 17a shows the voltage profile during constant-current charging at 0.4 mA/cm\({}^{2}\) (C/2 rate) [71]. Phase shift \(\Delta p\) is defined as the change from the initial state and corresponds to the change in electrolyte density from the initial state due to charging. As charging progressed, \(\Delta p\) gradually shifted location (increasing near the WE side and decreasing near the CE side) and eventually reached to within the electrolyte (Figure 17b). In addition, the positive \(\Delta p\) on the WE side changed with charging, indicating that the electrolyte density on the WE side increased. Figure 17c shows line profiles of the \(\Delta p\) change along the x-axis between the WE and CE at state-of-charge (SOC) points A-F in Figure 17a. The profile slope gradually increases until point D (SOC = 20%); it then becomes linear and remains unchanged. These results mean that, at a low charge rate (C/2 rate), the \(\Delta p\) change in the electrolyte reached a steady state (balanced diffusion and migration) within 3000 s.  
Figure 18 shows voltage profiles of the charge/discharge curve during five cycles of fast (2.4 mA/cm\({}^{2}\), 3C rate) charge/discharge (Figure 18a), phase maps (\(\Delta p\) change images) of the electrolyte region during charging (points A to F) and discharging (points G to L) in the first cycle (Figure 18b), and line profiles along the x-axis during charging and discharging (Figure 18c). \(\Delta p\) changes reversibly with charging and discharging, and the dynamic change in the salt concentration distribution with charging and discharging can be visualized. Unlike with low-rate (C/2 rate) charging/discharging, with high-rate (3C rate) charging/discharging, the upper or lower voltage limit was reached before the line profile reached linearity. Moreover, \(\Delta p\) changed in the opposite direction in the vicinity of the electrode as the Li\({}^{+}\) migration direction reversed while the \(\Delta p\) gradient was still present in the electrolyte bulk (with a growing concentration gradient in the electrolyte) (Figure 18c).