What is the reason for the swelling of the soft pack lithium-ion battery

The increase in the thickness of the battery during the charging process of lithium-ion batteries is mainly attributed to the expansion of the negative electrode. The expansion rate of the positive electrode is only 2~4%. The negative electrode is usually composed of graphite, binder, and conductive carbon. The expansion rate of the graphite material itself reaches 10%. %, the important influencing factors that cause the change of graphite anode expansion rate include: SEI film formation, state of charge, process parameters and other influencing factors. Electrolyte causes gas production inside the battery is another important cause of battery swelling. No matter the battery is left at room temperature, high temperature cycle, or high temperature, it will have different degrees of swelling and gas production. During the first charge and discharge of the battery, an SEI film will be formed on the electrode surface. The formation of the negative SEI film is mainly due to the reduction and decomposition of EC. At the same time as the formation of alkyl lithium and Li2CO3, a large amount of CO and C2H4 will be generated. DMC and EMC in the solvent will also become RLiCO3 and ROLi during the film formation process, accompanied by the presence of CH4, C2H6 and C3H8 gases and CO gas. In the PC-based electrolyte, there are relatively more gases, and the most important thing is the C3H8 gas generated by the reduction of PC. The lithium iron phosphate soft-pack battery has the most severe swelling after charging at 0.1C during the first cycle. It can be seen from the above that the formation of SEI will be accompanied by the appearance of a large amount of gas, which is an unavoidable process. The presence of H2O in impurities will make the P-F bond in LiPF6 unstable and generate HF. HF will lead to the instability of the battery system, accompanied by the appearance of gas. The presence of excess H2O will consume Li+ and generate LiOH, LiO2 and H2, leading to the appearance of gas. Gas will also appear during storage and long-term charging and discharging. For sealed lithium-ion batteries, a large amount of gas will cause the battery to swell, which will affect the performance of the battery and shorten the service life of the battery. There are two important reasons for the appearance of gas in the battery during storage: (1) H2O in the battery system will cause the generation of HF and cause the destruction of SEI. The O2 in the system may cause the oxidation of the electrolyte, leading to the formation of a large amount of CO2; (2) If the SEI film formed by the first chemical conversion is unstable, the SEI film will be destroyed during the storage stage, and the re-repair of the SEI film will release hydrocarbons. Class-based gas. During the long-term charge and discharge cycle of the battery, the crystal structure of the positive electrode material changes, the uneven potential of the electrode surface and other factors cause the potential of some points to be too high, the stability of the electrolyte on the electrode surface decreases, and the electrode surface film continues to thicken The electrode interface resistance is increased, and the reaction potential is further increased, causing the decomposition of the electrolyte on the electrode surface to produce gas, and the positive electrode material may also release gas. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous: What are the advantages of soft pack batteries?