The electrolyte is the main source of the reduction reaction reactants for the formation of the SEI film , and both the electrolyte solvent and the lithium salt will affect the composition of the SEI film. When the main components of the electrolyte are basically fixed, the performance of the SEI membrane can only be improved by other means.

The researchers found that the formation of the SEI film begins with the transfer of electrons from the cathodically polarized electrode to the solvated Li+, by the establishment of a charge exchange balance between the Li+complexed solvent molecules and the generated solvent molecule free radicals. The soluble reducing compound generated in the process is oxidized again, and the charge needs an empty molecular orbital to be transferred from the electrode to the solvent molecule complexed with the ion. When this empty orbital has a relatively high energy, the charge is only at a more negative potential. The species that can be transported, that is, reduced at higher potentials, are the most reactive species.

Therefore, people consider adding more active substances to the electrolyte to improve the performance of the SEI film, that is, film-forming additives. The active groups possessed by these additives have a strong ability to absorb electrons, which can improve the reduction potential. The reduction reaction is carried out before the ions are inserted into the graphite anode to form a passivation film, which inhibits the re-decomposition of the electrolyte and improves the performance of the SEI film.

The researchers divided film-forming additives into organic compound additives and inorganic compound additives. Organic compound additives are the main components of film-forming additives, among which, the double bond structure of unsaturated carbide ethylene carbonate (VC) makes it have lower energy and is easier to be reduced, propylene sulfite (PS) , Vinyl sulfite (ES) can also better improve the performance of lithium- ion batteries. Halogen-containing organic compound additives are also of great help in improving the performance of lithium-ion batteries. Fluorinated ethylene carbonate (FEC) was added to the silicon anode lithium- ion battery, and it was found that it can promote the formation of LiF and polycarbonate compounds, and reduce the resistance of the SEI film on the silicon surface, thereby improving the cycle performance of lithium- ion batteries.    When studying inorganic compound additives, it was found that CO2, SO2, CS2, Sx2-, N2O, etc. can react with the electrolyte to generate Li2CO3, Li2S, Li2SO4 and Li2O, etc., and can also improve the electrochemical performance of lithium- ion batteries, but these gas compounds are difficult to achieve. Dissolving in organic solvents is not conducive to production practice, so choose to use inorganic salts instead, such as Li2CO3, K2CO3, NaClO4, AgPF6, etc. Compared with organic compound additives, inorganic compound additives are not flammable, which improves the safety of lithium-ion batteries.

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