What is the gas after the lithium battery expands?

It's definitely harmful. Gas production must be caused by the decomposition of the electrolyte. There are two situations in which the electrolyte breaks down. The first is that the electrolyte contains impurities, such as moisture and metal impurities, which cause the electrolyte to decompose. to decompose and produce gas. The other is that the electrolyte breaks down and produces gas. The chemical window is too small, causing decomposition during the charging process, solvents such as EC and DEC in the electrolyte will produce free radicals. The direct consequence of the radical reaction is the production of low boiling point hydrocarbons, esters, ethers and CO2, etc.