Why Lithium-ion Cells Compression So Matters?

Lithium-ion battery cells undergo changes during cycling. cell compression can prevent various defects, such as swelling.

The main causes of cells swelling can be divided into two categories: one is the change in the thickness of the battery pole sheet(during use）and the other is the gas generated by the decomposition of electrolytic liquid oxygen.

Compression can make the migration path inside the battery more smooth, so that the anode sheet and cathode sheet and separator more closely fit together, reduce the porosity between the materials, so as to improve the transmission rate of electrons and ions between the electrodes, and thus reduce the internal resistance. (notice: too much pressure will also increase the possibility of internal resistance, so a certain amount of pressure is needed).

Regarding the change in the thickness of the electrode sheet, the thickness of the lithium battery will change significantly during use, especially the graphite negative electrode. After high-temperature storage and cycling, lithium batteries are prone to swelling, and the thickness growth rate is between 6% and 20%, of which the positive expansion rate is low, only 4%, and the negative expansion rate is more than 20%. This is because the negative graphite forms LiCx (such as LiC24, LiC12 and LiC6, etc.) during the lithium insertion process, resulting in changes in lattice spacing and the generation of microscopic internal stresses, which causes the negative electrode to expand.

After the battery assembly is completed, a small amount of unavoidable gas will be generated during the pre-formation process, which is one of the sources of irreversible capacity loss of the battery cell. During the first charge and discharge process, the electrons and the electrolyte on the negative electrode surface undergo REDOX reaction to form gas, and SEI is formed on the graphite negative electrode surface. As the SEI thickness increases, electrons cannot penetrate, thus inhibiting the continuous oxidative decomposition of the electrolyte. However, during the use of the battery, the internal gas production will gradually increase due to impurities in the electrolyte or excessive moisture in the battery. In addition, the battery overcharge and overdischarge abuse, internal short circuit, etc., will also accelerate the gas production speed of the battery and cause the battery to fail.

For safety reason, pressure is applied to squeeze the gas to the top, and the safety valve is opened after reaching 0.6Mpa.