Study the behavior of electrolyte during the charging and discharging process of LR6 battery

Lithium-ion batteries are mainly composed of positive electrodes, negative electrodes, diaphragms, 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 are connected by electrolytes. 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, electrolytes are very important and have 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 charging and discharging process, thereby reducing the performance degradation caused by the concentration polarization of the electrolyte. However, in the actual charging and discharging process, due to factors such as the Li+ diffusion rate, a Li+ concentration gradient will be generated at the positive and negative electrodes, and the Li+ concentration will fluctuate with charging and discharging. Due to structural design and production process, the electrolyte is unevenly distributed inside the battery cell. In particular, during the charging process, as the electrodes expand, a "dry area" will form inside the battery cell. The existence of the "dry area" leads to a reduction in the active substances that can participate in the charge and discharge reaction, causing local SoC unevenness in the battery, which in turn accelerates the local aging of the battery. M.J. Mu?hlbauer found in his study of the effect of lithium-ion battery aging on Li distribution that during the charge and discharge process, both the positive and negative electrodes expand to a certain extent, which causes the battery cell to expand and contract to a certain extent. The battery cell will "breathe" and "spit out" electrolyte repeatedly [1]. Therefore, the electrolyte infiltration in the battery cell changes in real time at different times (as shown in the figure below).

Limited by technical means, we have previously lacked an intuitive understanding of the behavior of the electrolyte inside the lithium-ion battery during the charge and discharge process. It is more like studying a black box. We have proposed various theories and speculated on its behavior. In order to study the behavior characteristics of electrolyte in lithium-ion batteries more vividly and intuitively, Toshiro Yamanaka et al. [2] of Kyoto University in Japan used Raman spectroscopy tools to study stacked square lithium-ion batteries. The biggest feature of this study is that it realizes real-time observation of the distribution of electrolyte and the change of ion concentration in the electrolyte during charging and discharging.

In the experiment, Toshiro Yamanaka used square stacked batteries as the research object, EC and DEC solvents as electrolytes, and LiClO4 as electrolyte salt. In order to observe the behavior of the electrolyte inside the battery in real time, Toshiro Yamanaka introduced 8 optical fibers as Raman spectroscopic detectors inside the stacked lithium-ion battery to study the infiltration of the electrolyte in the battery and the change of ion concentration. The arrangement of the 8 optical fibers in the battery is shown in Figure c below.

However, after 30 cycles, the Li+ concentration change trends from fiber optic detector No. 1 to fiber optic detector No. 7 became consistent, with the concentration increasing during charging and decreasing during discharging. Toshiro Yamanaka believes that this is mainly because after 30 cycles, the structure and composition of the SEI film of the negative electrode gradually stabilized, resulting in the ion impedance of each part of the battery cell becoming consistent. However, the change trend of each optical fiber detector is also different. For example, the change of ion concentration during charging and discharging at the positions of fiber optic detectors No. 3 and No. 4 is relatively small, the change of ion concentration at the position of fiber optic detector No. 2 gradually decreases, and the change of ion concentration at the position of detector No. 5 gradually increases. This shows that there is still a large inhomogeneity inside the lithium-ion battery at this time. For example, the ion concentration at position 4 changes relatively little, indicating that the reaction rate here is slow during the charge and discharge process, causing local SoC inhomogeneity, which has a negative impact on the performance of the battery.

In the process of studying the change of Li+ concentration, Toshiro Yamanaka also found that the distribution of the electrolyte also changes with charge and discharge. The following figure shows the change trend of the intensity of the solvent EC at different positions during the charge and discharge process. The change in the intensity of EC mainly reflects the amount of electrolyte at this location. As the volume of the battery cell changes, the distribution of the electrolyte in the battery cell is also constantly changing. For example, at positions 1 and 2, the concentration of EC dropped rapidly and recovered slowly twice, and at the position of detector 6, the concentration of EC dropped rapidly and rose rapidly. These changes reflect that during the charge and discharge process inside the lithium-ion battery, due to the volume expansion of the electrode, the electrolyte dries and re-wets locally in the battery cell.

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