During the formation of the SEI film, various reduction reactions are carried out by the electrolyte solvent, lithium salt, additives, and trace air impurities. This series of reactions is not only affected by the inherent properties of substances such as reduction potential, reduction activation energy, and exchange current density, but also by other factors such as temperature, electrolyte salt concentration, and reduction current. The combined effect of these factors complicates the formation process of SEI films, and the formation mechanism is difficult to clearly understand.

It is generally believed that the formation of SEI film is divided into two processes: first, the negative electrode of the battery is polarized, and the organic electrolyte solution components undergo reductive decomposition to form new chemical products; then, the newly generated products are precipitated on the surface of the negative electrode to form SEI film. In the study of the formation mechanism of SEI film, the controversial points mainly focus on the reduction reaction process, especially the reduction reaction process of the electrolyte solvent molecules. The process of these reduction reactions is the process of electron transfer to salt and solvent in the electrolyte solution obtained according to battery anode polarization, thermodynamic simulation, ion particle size, etc.

There are three possibilities for this process:

one is that the individual are directly inserted into the negative electrode graphite layer during the reduction reaction to complete the intercalation; the other is that the reduction reaction of the solvent molecule is a one-electron reaction, generating an intermediate product —Radical anion, which is further decomposed and combined with lithium ions to form a precipitated substance, which becomes a component of the SEI film, and according to the assumption, when lithium ions and solvent molecules are co-inserted, the solvent molecules undergo a two-electron reduction reaction, through weak The van der Waals forces combine lithium ions and solvent molecules to co-intercalate the graphite layer to form an intermediate product - a ternary graphite intercalation compound , which is subsequently reduced to form an SEI film; the third is the direct transfer of negative electrons to the anion salt, which is combined with Lithium ions directly generate inorganic salt precipitation.

Structure and properties of SEI films

The SEI film is a fragile thin-layer structure, and the fully formed SEI film has high lithium ion conductivity and negligible electronic conductivity, sufficient flexibility, and sufficient robustness. The electronic isolation property of the SEI film prevents further reduction reaction of the electrolyte on the surface of the negative electrode; the ionic conductivity property allows lithium ions to intercalate into the negative electrode through the SEI film; the sufficient strength and flexibility of the SEI film can avoid the negative electrode material during the lithium ion de-intercalation process. The volume change ruptures the SEI membrane. At the same time, there is a sufficiently large molecular force between the SEI film and the anode surface, which can avoid subsequent further polarization reactions.

Since there are many reactants for the formation of SEI film, the components of the electrolyte are not fixed, and the reaction conditions are also different, there are many kinds of products in the reduction reaction. The composition of the SEI film measured by different research groups is different, but the overall There are some common laws, such as: when there are fluoride salts such as LiAsF6, LiPF6, LiBF4 in the electrolyte, the fluoride salt will precipitate in the form of LiF or LixPFy after reduction reaction ; carbonate and lithium salts in the electrolyte The reaction is precipitated in the form of Li2CO3, ROCO2Li or other organic compounds; after the two-electron reduction reaction of ethylene carbonate in the electrolyte, ( CH2OCO2Li ) 2 will precipitate on the SEI film; when the content of propylene carbonate in the electrolyte is high , ROCO2Li precipitation occurs in the outer layer of the SEI film and so on.

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