Expansion analysis of cell-constant pressure VS constant gap

　　The expansion behavior of lithium-ion batteries during charging and discharging has two manifestations: thickness and stress. Accurate measurement of expansion thickness and expansion force can help optimize the design of the battery and improve the safety performance of the battery during use. The traditional test method of the constant gap mode is to use a steel plate fixture to fix the battery cell in the middle of the pressure plate, fix the position of the upper and lower pressure plates with bolts, and install a force sensor on the upper pressure plate to monitor the pressure change, but this method is difficult to guarantee the test time The gap between the upper and lower pressure plates is constant, and sometimes the gap will produce fluctuations of tens or even hundreds of microns. As shown in Figure 1, the red curve indicates that during the charging and discharging process of the battery, the gap change of the traditional fixture produces a fluctuation of about 65um; constant pressure mode The traditional test method is to place a constant weight on the surface of the cell, but it is difficult to freely adjust different pressures.

　　The green curve in Figure 1 represents almost no change in the gap of the SWE test system in the constant gap mode.

　　Figure 1

　　The structure diagram of the in-situ expansion test system is shown in Figure 2.

　　Figure 2

　　1. Cell expansion thickness and expansion force curve during charging and discharging

　　It can be seen from the charge and discharge curve of the battery cell and the thickness and expansion force curve in the figure that when charging, the expansion force and expansion thickness of the battery increase. When fully charged, the expansion force of the battery reaches about 160kg, and the thickness of the battery expands about 2%. The expansion force and expansion thickness of the battery core are reduced. The thickness and force curve changes during charging and discharging are not completely symmetrical, indicating that there is irreversible thickness and residual stress.

　　2. Analysis of cell expansion thickness and expansion force and differential capacity curve during charging and discharging

　　The figure shows the expansion thickness and expansion force and differential capacity curve of the cell. It can be seen from the figure that the thickness and the slope of the force curve increase correspondingly during charging. Each subsequent change in the corresponding thickness and the slope of the force curve indicates that it is due to the occurrence of It is only the phase change of the positive and negative electrode structure that the cell expansion thickness and expansion force change.

　　Lithium-ion battery (LIB) has become the main energy storage solution in modern social life. Among them, lithium iron phosphate batteries are a perfect substitute for lead-acid batteries, and are the first choice for grid-connected peak shaving, off-grid energy storage, photovoltaic energy storage, UPS, data center and other industries.