Behavior of lithium battery during charging and -Lithium - Ion Battery Equipment

Behavior of lithium battery during charging and discharging -Lithium - Ion Battery Equipment

The lithium-ion battery is mainly composed of positive pole, negative pole, diaphragm, electrolyte, and structural parts. On the outside of the lithium-ion battery, the electrons of the negative pole are transmitted to the positive pole through wires and loads, while inside the battery, the positive and negative poles are connected through electrolyte. When discharging, Li+diffuses from the negative pole to the positive pole through electrolyte, and is embedded in the crystal structure of the positive pole. Therefore, electrolyte is a very important link in lithium ion batteries, which has an important impact on the performance of lithium ion batteries.(Lithium - Ion Battery Equipment)

Ideally, there should be sufficient electrolyte between the positive and negative electrodes, and sufficient Li+concentration should be available during charging and discharging, so as to reduce the performance degradation caused by the concentration polarization of electrolyte. However, in the actual charging and discharging process, due to such factors as Li+diffusion speed, there will be a Li+concentration gradient at the positive and negative poles, and the Li+concentration fluctuates with the charging and discharging. Due to structural design, production process and other reasons, it will also lead to uneven distribution of electrolyte in the cell, especially in the charging process, with the expansion of the electrode, part of the "dry zone" will be formed in the cell. The existence of the "dry zone" will lead to the reduction of active substances that can participate in the charge discharge reaction, causing uneven local SoC in the battery, which will lead to accelerated local aging in the battery. M.J.Mu? When hlbauer studied the influence of lithium ion battery aging on Li distribution, he found that the positive and negative electrode plates had a certain volume expansion during charging and discharging, leading to a certain degree of volume expansion and contraction of the cell. The cell would "breathe", repeatedly "inhale" and "spit out" the electrolyte [1], so at different times, the electrolyte infiltration in the cell was also changing in real time (as shown in the figure below).

Limited by technical means, in the past, we lacked intuitive understanding of the behavior of electrolyte in the lithium ion battery during the charging and discharging process, more like studying a black box. We put forward various theories to speculate on the starting behavior. In order to study the behavior characteristics of electrolyte in the lithium-ion battery more vividly and intuitively, Toshiro Yamanaka, et al. of Kyoto University in Japan [2] studied the laminated square lithium-ion battery using Raman spectroscopy tools. The biggest feature of this study is to realize 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 a square laminated battery as the research object, EC and DEC solvents as the electrolyte, and LiClO4 as the electrolyte salt. In order to observe the behavior of electrolyte in the cell in real time, Toshiro Yamanaka introduced eight optical fibers into the laminated lithium ion battery as Raman spectrum detectors to study the electrolyte infiltration in the battery and the change of ion concentration, The arrangement of 8 optical fibers in the battery is shown in Figure c below,

The following figure shows the change trend of different ion concentrations detected by No. 7 optical fiber detector (the edge of the cell) during charging and discharging. It can be seen from the results that the concentrations of EC Li+and ClO4 – show an upward trend during charging and a downward trend during discharging. It shows that with the charging process, Li+is released from the positive electrode and enters the electrolyte, causing the increase of Li+concentration in the electrolyte.

The following figure shows the concentration change of EC Li+/EC intensity detected by eight optical fibers during charging and discharging. It can be seen from the following figure that the intensity change trend of EC Li+/EC in different parts of the battery is also different. For example, at No. 7 optical fiber (the edge of the cell), the intensity of EC Li+/EC increases with the battery charging and decreases with the discharge, while the detection result of No. 2 optical fiber (the middle of the cell) is just the opposite



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