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 organ 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.

On the one hand, the formation temperature affects the reaction rate of the chemical reaction that forms the SEI film and the corresponding products; on the other hand, when the temperature rises, some components of the SEI film will decompose, causing the SEI film to rupture and further consume the lithium stock to generate New SEI membrane.

During the formation of the SEI film, the EC directly generates ROCO2Li through the reduction reaction, and then ROCO2Li is converted into Li2CO3, while gas is generated. The higher the temperature, the more intense this process is, the more gas is generated, the more defect points are formed on the SEI film, and the thicker the formed SEI film is. This provides more avenues for the co-intercalation of lithium ions and solvated solvent molecules, thus further deepening the passivation of the SEI film on graphite and increasing the irreversible capacity loss of the battery.

The battery cycle performance was tested by linear sweep and cyclic voltammetry at different temperatures, and it was found that the areal capacity of the battery decreased by 17% after the first cycle at 60 °C, and decreased by 40% at 25 °C, and measured in the subsequent cycling process. The surface capacity is also higher at 60°C. Due to the rapid formation of the SEI film at 60 °C, the structure is flat and uniform and the composition is mainly stable Li2CO3, which reduces the damage of the graphite surface and improves the surface capacity. The pretreatment at 60°C reduced the decomposition of graphite and increased the capacity of the graphite electrode by 28% compared with that at 25°C.

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