Lithium- ion battery formation process includes constant current, constant voltage and intelligent charging. The polarization phenomenon of constant current charging is serious, and its initial current is low, the current at the end of charging is high, the charging time is long, the energy waste is serious, and the battery life will be reduced. During constant voltage charging, the current is relatively large in the initial stage, and the electromotive force of the battery will gradually increase until the charging current gradually drops to zero. Compared with the former, constant voltage charging has less energy consumption and shorter charging time, and the charging performance is closer to the optimal charging curve; however, it is difficult to compensate for changes in battery terminal voltage during charging, and improper selection of charging voltage will damage the battery.

Intelligent charging is to dynamically track the acceptable charging current of the battery during the charging process. The charging power supply can automatically adjust the charging parameters according to the state of the battery, so that the charging current is always maintained within the allowable range to protect the battery. But in practice it is difficult to dynamically track the acceptable charging current of the battery. To sum up, in practical applications, more constant current and constant voltage charging methods are used, that is, constant current charging is used in the initial stage, and constant voltage charging is used when the battery terminal voltage rises to a certain value until the battery is fully charged.

The discharge method of the battery is relatively simple, generally it is constant current discharge, and the voltage will gradually decrease until the set voltage. Since the SEI film starts to be formed in the charging stage, and most of the SEI film is formed during the first charge and discharge process, the parameter control at this stage plays a decisive role in the performance of the generated SEI film. The three most important parameters are respectively are: charging current, formation temperature, cut-off voltage.

When graphite is used as the negative electrode active material, there are two types of reactions for the formation of SEI films: two-electron reactions, that is, two electrons participate in the reaction at the same time, and it is easier to generate inorganic lithium salt components; single-electron reactions, that is, only need A reaction that can take place when one electron participates, it is easier to generate organo lithium salt components at this time.

In the initial stage of SEI film formation, a large number of electrons gather on the surface of graphite particles, and it is easier to have a two-electron reaction with film-forming additives and lithium ions. Therefore, the generated SEI film is mainly composed of inorganic lithium salts; in the later stage of film formation, electrons need to pass through The formed SEI film can combine and react with film-forming additives and lithium ions, so the number of electrons reaching the reaction point is reduced, and single-electron reaction is more likely to occur, and the resulting SEI film is dominated by organic lithium salts. Therefore, under the action of different charging currents, the composition and structure of the SEI film are different.

The half-cells at two different current densities of 0.312 and 1.248uA/cm2 were characterized by EIS, in-situ FTIR and TEM. The analysis found that the formation current density at room temperature chemically affected the formation of the SEI film on the carbon negative electrode: when the current density was low, Li2CO3 is generated at the initial stage of discharge, and lithium alkyl carbonate is generated at the end of discharge.

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