How is the SEI film formed on the electrode surface?

Li metal-deep analysis of the formation of SIE films on electrode surfaces

On the surface of metal, carbon material or oxide electrode, with the redox decomposition reaction of electrolyte, electrolyte products form a new deposition layer. Some products become permanent deposits and form a passivation layer on the electrode surface. This insulating layer is called solid electrolyte interface (SEI film), which has low electronic conductivity and high ionic conductivity, and its behavior is similar to that of solid electrolyte [10,11] these holes in SE layer allow Li + to pass through the electrode freely, while other electrolyte components cannot enter and exit.

In addition to conducting Li + at the electrolyte electrode interface, SEI film can also facilitate the transmission of Li + ions under uniform current distribution, reduce concentration polarization and overpotential, and maintain consistent grain size and chemical composition. In order to make the lithium battery have a long life, the SEI layer must be firmly adhered to the electrode surface and have relatively stable physical and chemical properties. In order to improve the cycle performance, stability and cycle life of lithium-ion batteries, we must have a basic understanding of the formation mechanism of SEI layer.

Formation mechanism of SEI film
Formation mechanism of SEI film

Since the standard reduction potential of lithium is much lower than that of solvent, the reduction reaction may occur when the electrolyte contacts metal lithium [2]. In order to form a low soluble and stable SEI layer, it is very important to select an appropriate electrolyte component with high standard reduction potential and charge density.

The electrochemical reaction through SEI membrane is carried out in the following three steps:

1) Electron transfer is carried out through the electrolyte SEI film interface.

2) Li + passes through SEI film.

3) Charge transfer occurs at the SEI layer active material interface.

Generally speaking, step 2 is the rate control step. In this step, an additional layer of material is formed on the first SEI layer on the lithium metal surface. This is because when direct contact occurs, the solvent component of the electrolyte first fills the holes of the first SEI film, and a continuous reduction reaction occurs, accompanied by the transfer of electrons from lithium to the solvent. The oxidation reaction on the positive electrode interface also takes place in a similar form. When the electrolyte contacts the surface of the electrode active material, the SEI film will be formed rapidly and filled, gradually reducing the concentration of SEI phase. The first layer of SEI film is very thin and the filling rate is low. Because the later generated components co-exist with the first layer components, the whole system can be described as a porous and disordered system.

In the actual tycorun lithium battery unit, the SEI layer is formed in several early cycles. When the electrochemical performance of the battery tends to be stable, the formation of SEI film gradually slows down, its chemical stability is enhanced, and the decomposition reaction of electrolyte is limited.

The composition and formation of SEI film will vary with the electrode and electrolyte system. SEI film is composed of thick organic layer and thin inorganic layer. Various in-situ and non in-situ techniques, such as FTIR, XPS, AFM, DSC-TGA, Dems, EDS and EQCM, have been used to detect the composition of SEI membrane. Using different methods, the detected SEI layer components will be slightly different. This is because each device has different sensitivity to the composition of SEI membrane. Therefore, it is very necessary to use different methods for component analysis.

The three common types of interfaces of lithium secondary batteries are cathode electrolyte interface, cathode electrolyte interface and collector electrolyte interface. The SEI film is easier to form on the negative surface, and the SEI formed on the positive surface is not common and thinner [6]. The formation of SEI is the main factor causing irreversible lithium ion intercalation and desorption (initial coulomb efficiency) and affecting the capacity of negative electrode and positive electrode. The electrode electrolyte interface reaction will also cause self discharge. SEI film is formed on the surface of collector electrolyte. Because most active materials are porous, SEI films will be formed when the electrolyte passes through the particle gap and directly contacts with the collector.

Read more: what is a lithium rechargeable battery diaphragm?

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