1.SEI Film Formation
1.1 SEI film growth process
The solid electrolyte interface (SEI) film in lithium-ion batteries is a thin film generated by the decomposition and reaction of electrolytes.
The formation process of the SEI film can be divided into two main stages: the in-situ formation stage and the subsequent reconstruction stage.
In-situ formation stage: When the lithium-ion battery is first charged, the solvent and salt in the electrolyte react with the reduction products on the electrode surface to generate a series of organic and inorganic compounds. These compounds will aggregate on the electrode surface to form an initial SEI film. In this stage, lithium ions are transferred from the electrolyte to the electrode to form metallic lithium or metal alloys.
Subsequent reconstruction stage: In the early stage of SEI film formation, it has high resistance and low ion conductivity, so the decomposition of the electrolyte continues. With the further transfer and reaction of lithium ions, the SEI film gradually reconstructs to form a more stable structure with higher ion conductivity. In this process, organic compounds can be oxidized into carbonates, polymers and oxides, and inorganic salts can form stable precipitation with lithium ions.
1.2 Degradation of SEI film
Although the solid electrolyte interface (SEI) film plays a key protective role in lithium-ion batteries, the SEI film will degrade as the battery is cycled, resulting in a decrease in battery performance.
The degradation of SEI film is affected by many factors.
Organic solvents in the electrolyte solvents can undergo electrolyte decomposition reactions, and these decomposition products can react chemically with organic and inorganic components in the SEI membrane, resulting in structural damage to the membrane and blockage of ion conduction channels. Especially under high voltage and high temperature conditions, the decomposition reaction of the electrolyte solvent is more intense, accelerating the degradation process of the SEI membrane. Metallic lithium or metal alloys on the electrode surface may corrode, peel off, or reprecipitate, and these changes can destroy the integrity and stability of the SEI membrane. For example, during the charge and discharge process, lithium dendrites may form on the lithium metal surface, resulting in the rupture and inhomogeneity of the SEI membrane. In addition, oxidation and precipitation reactions on the lithium metal surface may also lead to degradation of the SEI membrane and blockage of lithium ion transmission channels. Finally, environmental conditions such as temperature, humidity, and oxygen concentration can also affect the stability of the SEI membrane. High temperature and humidity may cause volatilization of the electrolyte and evaporation of the solvent, which in turn affects the stability of the SEI membrane. The presence of oxygen can trigger oxidation reactions of the electrolyte, resulting in damage and degradation of the SEI membrane.
1.3 Components of SEI membrane
The SEI membrane is a composite structure composed of a variety of organic and inorganic compounds. Among them, the organic components include decomposition products of electrolyte solvents, reduction products of lithium salts and polymer additives. Inorganic components include oxides, carbonates, lithium salt precipitates, etc. The electrolyte solvent undergoes decomposition reactions during the charge and discharge process of the battery to produce a variety of organic substances. These organic products mainly include carbonates, carbonate esters, polycarbonates and polymers. These organic substances form the skeleton structure of the SEI membrane through polymerization, cross-linking and rearrangement reactions, and play an important role in the transmission of lithium ions. The generation process of organic components is complex and diverse, depending on the composition of the electrolyte, the properties of the electrolyte solvent and the operating conditions of the battery. In addition, polymer additives are often introduced into the electrolyte to enhance the stability of the SEI membrane and the lithium ion conductivity. Secondly, the inorganic components in the SEI membrane also play an important role. The inorganic components mainly come from the reduction products of the electrolyte salts and the precipitates of the lithium salts. The electrolyte salts undergo reduction reactions in the battery to generate inorganic substances such as hydrides, carbonates, hydroxides, etc. The precipitates of lithium salts include compounds such as lithium carbonate, lithium hydroxide and lithium phosphate.
These inorganic substances play a role in strengthening the structure and improving ion conductivity in the SEI membrane. They can fill the pores of the SEI membrane, increase the density of the membrane, and provide channels for ion transmission. It should be noted that the specific composition and structure of the SEI membrane are affected by many factors, including the working conditions of the battery, the composition and concentration of the electrolyte, the characteristics of the electrode material, etc. Researchers use different electrolyte solvents, additives and electrode materials, as well as optimize battery design and operating conditions to regulate the composition and performance of the SEI membrane in order to improve the cycle life and safety of lithium-ion batteries.
2.Improvement of SEI membrane and the latest research progress
2.1 Carbon anode materials
The surface functional groups of carbon negative electrode materials can react chemically with solvents and salts in the electrolyte to promote the formation of SEI film. For example, the functional groups of fluorine-containing compounds can react with solvents to generate fluorides, which in turn form a stable SEI film with lithium salts.
Surface defects of carbon anode materials (such as holes, cracks, etc.) can serve as the starting point for the formation of SEI film. These defects provide active sites, which are conducive to the adsorption and aggregation of electrolyte decomposition products, thereby forming SEI film. In addition, surface defects can also provide more reaction sites to promote the growth and stability of SEI film.
In order to improve the performance and stability of SEI film, researchers have improved carbon negative electrode materials through surface modification, conductive additives and nanostructure regulation. For example, a protective layer is applied to the surface to enhance the compatibility and corrosion resistance of carbon negative electrode materials with electrolytes. This protective layer can be composed of polymers, oxides, metals or other compounds, which can prevent harmful substances in the electrolyte from further corroding the carbon negative electrode material, while promoting the formation and stability of SEI film. In addition, adding conductive agents such as carbon black, conductive polymers, etc. can improve the conductivity of carbon negative electrode materials and promote the formation and stability of SEI film.
2.2 Organic electrolyte
Electrolyte is an important component of lithium-ion batteries. Its composition and properties have a significant impact on the formation and stability of SEI membrane. The selection of electrolyte solvent directly affects the formation and stability of SEI membrane. Some organic solvents have high reducibility and easy decomposition, which can easily lead to the degradation of SEI membrane. Therefore, the selection of stable solvent is crucial for the stability of SEI membrane. Lithium salt in electrolyte is an important component of SEI membrane formation. The selection and concentration of lithium salt will affect the chemical composition and structure of SEI membrane. The selection of lithium salt should consider its solubility, stability and the effect on the formation and stability of SEI membrane. Suitable lithium salt can promote the formation of SEI membrane and improve its stability and ion conductivity.
Additives are an important means to optimize the performance of electrolytes. The type and concentration of additives can adjust the properties of electrolytes and the formation of SEI membranes. For example, the addition of fluorine-containing additives can promote the formation of SEI membranes, increase their stability and ion conductivity, and introduce fluorine atoms on the surface of SEI membranes to enhance their compatibility with electrolytes and electrodes, thereby improving the stability of SEI membranes. Polymer additives can also improve the mechanical stability of SEI membranes and reduce cracking and peeling of membranes. Thiol and thiophenol additives can react with lithium salts in SEI membranes to form compounds such as sulfides and sulfates. These compounds can enhance the stability and ion conductivity of SEI membranes, reduce the degradation of SEI membranes and increase resistance. Some multifunctional additives can simultaneously have multiple functions to improve the performance of SEI membranes. For example, phosphorus-containing additives can enhance the chemical stability and mechanical strength of SEI membranes, inhibit the decomposition of electrolyte solvents and the capacity decay of batteries. The performance and stability of SEI membranes can be improved by optimizing the formulation and components of electrolytes.
3.Future development direction
With the continuous improvement of the performance requirements for lithium-ion batteries, the research on improving and optimizing SEI membranes will continue to receive attention. Future development directions include developing more stable electrolytes with high lithium ion conductivity, designing electrode materials with excellent electrochemical properties and mechanical stability, exploring new SEI formation mechanisms and regulation strategies, developing efficient SEI membrane evaluation methods and testing technologies, and promoting the application of artificial intelligence and machine learning in SEI membrane research.