AG10 battery.Research and analysis on the behavior of electrolyte during charging and discharging of lithium-ion batteries

　　In lithium-ion batteries, electrolyte is a very important part and has an important impact on the performance of lithium-ion batteries. Ideally, there should be sufficient electrolyte between the positive and negative electrodes, and there should be sufficient Li+ concentration during the charge and discharge process, thereby reducing the performance degradation caused by concentration polarization of the electrolyte.

　　Lithium-ion batteries are mainly composed of positive electrodes, negative electrodes, separators, electrolytes, and structural parts. Outside the lithium-ion battery, electrons from the negative electrode are conducted to the positive electrode through wires and loads, while inside the battery, the positive and negative electrodes The two are connected through the electrolyte. During discharge, Li+ diffuses from the negative electrode to the positive electrode through the electrolyte and is embedded in the crystal structure of the positive electrode. Therefore, in lithium-ion batteries, electrolyte is a very important part and has an important impact on the performance of lithium-ion batteries. Ideally, there should be sufficient electrolyte between the positive and negative electrodes, and there should be sufficient Li+ concentration during the charge and discharge process, thereby reducing the performance degradation caused by concentration polarization of the electrolyte. However, during the actual charging and discharging process, due to factors such as the Li+ diffusion speed, a Li+ concentration gradient will occur in the positive and negative electrodes, and the Li+ concentration fluctuates with charging and discharging. Due to reasons such as structural design and production process, the electrolyte will be unevenly distributed inside the battery core. Especially during the charging process, as the electrode expands, some "dry areas" will be formed inside the battery core. The existence of the "dry area" reduces the number of active materials that can participate in the charge and discharge reactions, causing local SoC unevenness in the battery, which leads to accelerated local aging in the battery. M.J. Muhlbauer, in studying the impact of aging of lithium-ion batteries on Li distribution, found that due to the certain volume expansion of both the positive and negative electrode plates during the charging and discharging process, the battery core also has a certain degree of volume expansion and contraction. It will repeatedly "inhale" and "spit out" the electrolyte like "breathing", so at different times, the infiltration of the electrolyte in the battery core also changes in real time (as shown in the figure below).

　　Limited by technical means, in the past we lacked an intuitive understanding of the behavior of the electrolyte inside the lithium-ion battery during the charge and discharge process. It was more like studying a black box. We proposed various theories to speculate on the behavior. In order to study the behavioral characteristics of electrolytes in lithium-ion batteries more vividly and intuitively, ToshiroYamanaka et al. [2] from Kyoto University in Japan used Raman spectroscopy tools to study laminated square lithium-ion batteries. The biggest feature of this research is that Real-time observation of the distribution of electrolyte and changes in ion concentration in the electrolyte during charging and discharging.

　　In the experiment, ToshiroYamanaka used square laminated batteries as the research object. The electrolyte used EC and DEC solvents, and LiClO4 as the electrolyte salt. In order to observe the behavior of the electrolyte inside the battery core in real time, ToshiroYamanaka inside the laminated lithium-ion battery Eight optical fibers were introduced as detectors of the Raman spectrum to study the infiltration of the electrolyte in the battery and the changes in ion concentration. The arrangement of the eight optical fibers in the battery is shown in Figure c below.

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