In the process determination test, the test items include the air permeability and thickness change of the diaphragm; whether the cell thickness meets the requirements for casing; and whether the electrode sheet is broken.
As a core component of lithium-ion battery packs, the battery separator plays a crucial role in isolating the positive and negative electrodes from electron conduction while allowing lithium ions to pass back and forth between them. The microporous structure of the separator is the important channel for these ions to travel between the positive and negative electrodes, and its permeability directly affects the battery performance. Separator permeability refers to the amount of gas that permeates through the separator under a certain time and pressure. If the separator has poor permeability, it will affect the transfer of lithium ions between the positive and negative electrodes, and consequently affect the charging and discharging of the lithium-ion battery.
The testing process is as follows: The battery separator is fixed, air pressure is applied to one side of the separator, the pressure drop and the time taken are measured, and the air permeability of the separator is tested. The shorter the time, the better the air permeability. During the hot pressing process, the separator may be severely compressed, resulting in large changes in separator thickness, which can cause micropores to become blocked. Visually, the separator will turn transparent. This indicates that the hot pressing shaping exceeds the limits for the cell's application and will affect lithium-ion transport. The wound cell is shown in the figure.
If the electrode sheets are brittle, powder may fall off or even break at the bends in the cell during hot pressing, which can restrict electron transport and increase the battery's internal resistance. Therefore, hot pressing of the cell must also prevent this from happening. Both of these aspects require that the hot pressing pressure and time be as low as possible.
