After extensive research, it has been found that the properties of SEI films greatly affect the performance of Li-ion batteries. During the formation process, the amount of the SEI film formed represents the amount of lithium in the consumed lithium-ion battery, which directly determines the capacity of the lithium- ion battery. Therefore, in the process of forming the SEI film, the less lithium is consumed, the better.

During the cycle of lithium-ion batteries, if the electron isolation properties of the SEI film are poor, the electrons will come into contact with the electrolyte, and the reduction reaction will proceed further, consuming the lithium content in the battery, so that the SEI film is continuously generated, resulting in the cycle of lithium- ion batteries poor life.

The SEI film will fall off and thicken during the cycle. The SEI film fragments generated during the shedding enter the electrolyte, and electrophoresis occurs under the action of voltage, especially during high-rate discharge, the generated fragments will be deposited on the electrode surface. At the same time, the SEI film of the negative electrode will be significantly thickened during the high-rate cycling process of the lithium-ion battery. These two phenomena increase the surface resistance of the electrode, which affects the extraction of lithium ions, which in turn affects the rate performance of lithium ion batteries.

During the fast charging process of lithium- ion batteries , if the speed of lithium ions passing through the SEI film is slower than the deposition speed of lithium on the negative electrode , lithium dendrites will be continuously generated with the charging and discharging cycle, which may lead to short-circuiting of lithium- ion batteries, resulting in combustion and explosion; At the same time, when the SEI film is incompletely formed or decomposed, the lithium intercalated in the negative electrode will react with the electrolyte and the binder to release heat, and the reaction heat increases with the increase of the amount of intercalated lithium , which greatly affects the safety of the battery. The SEI film has a direct or indirect impact on the battery performance, and the formation of an SEI film that meets the requirements is of great help in improving the battery performance.

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