The current research on lithium-ion batteries focuses on improving energy density, rate and power performance, cycle performance, safety performance, and reducing manufacturing costs. However, it is inevitable to involve in almost all research fields related to lithium-ion batteries Analysis and discussion of solid electrolyte interface membrane (SEI).

　　Subsequently, researchers began to try to replace lithium metal anodes with graphite anodes. Although it effectively solved the safety hazards caused by lithium dendrites, the solvated PC molecules in the electrolyte can be co-intercalated with lithium ions into the interlayer structure of graphite. , Unable to form a stable SEI film on the graphite surface. Until 1990, DAHN et al. found that the ethylene carbonate (EC) solvent molecules in the electrolyte can form a relatively stable SEI film on the surface of the graphite negative electrode, effectively inhibiting the co-intercalation of solvent molecules, and solving the safety problem of lithium metal negative electrodes. At the same time, the cycle stability is improved, and finally the lithium-ion battery represented by the graphite negative electrode has been successfully commercialized and is still in use today.

　　It can be seen that the process of people’s research and understanding of SEI films for lithium-ion batteries has played a vital role in the development of the entire lithium-ion battery. Stabilizing the production of SEI films is the ability of lithium-ion batteries to be able to charge and discharge normally and to ensure various Prerequisites for electrochemical performance.

　　This paper summarizes the formation process mechanism, influencing factors, research ideas and current status of SEI film, and prospects for future potential research directions are as follows: study the formation mechanism and function of SEI film on the surface of new cathode materials; explore the optimization of functional electrolyte formulations, Study the film formation mechanism and effects of new solvents, lithium salts or additives; use in-situ analysis or theoretical calculation methods to in-depth study the chemical composition and morphological structure of SEI films; explore effective artificial SEI film construction methods and realize SEI film structure Controllable optimization.

　　1. The formation process and reaction mechanism of SEI film

　　At present, the electrolyte of commercial lithium-ion batteries is mainly composed of cyclic or linear carbonate solvents, lithium salts and a small amount of functional additives. The highest occupied molecular orbital energy level (HOMO) is about 1.0V and 4.7Vvs.Li+/Li respectively. When the lithium-ion battery is first formed and charged, the surface potential of the negative electrode material decreases continuously. When it is lower than 1.0V, the electrolyte composition can be reduced. Reductive decomposition, in which insoluble reductive decomposition products will gradually deposit on the surface of the negative electrode material to form an SEI film.

　　2. The chemical composition and morphology of SEI film

　　Since the SEI film has an important impact on the performance of lithium ion batteries, the ideal SEI film should have the following characteristics: ①The film formation potential of the SEI film must be higher than the insertion or extraction potential of lithium ions, so as to effectively prevent solvent molecules Co-intercalation; ②SEI film components are insoluble in the electrolyte and can remain stable within the working voltage and temperature range of lithium-ion batteries; it has a moderate thickness and a "rigid and flexible" molecular structure, which can adapt to the volume change of the negative electrode material. , And can maintain the stability of the cycle structure; ③It has a high degree of electronic insulation and selective passage of lithium ions. The electronic insulation is to prevent the decomposition of more electrolyte and the formation of a thicker SEI film, and the ion conductivity is to protect the lithium Smooth ion migration and embedding channels.

　　With more and more characterization studies on the chemical composition of SEI membranes, various researchers have formed some consensus on the basic chemical composition and structure of SEI membranes. Just as Wang et al. proposed in the latest review article that SEI membranes are close to the electrode interface The inner layer is mainly inorganic substances such as Li2CO3, Li2O, LiF, and the outer layer near the electrolyte interface is mainly organic products such as ROLi, ROCO2Li, and the inner structure is dense and compact, and the outer structure is loose and porous.

　　3. The influence of graphite material surface characteristics on the formation process of SEI film

　　Carbon materials have stable physical and chemical properties, and their lithium insertion voltage is slightly higher than that of metal lithium anodes. There is no risk of lithium dendrites precipitation, and they have abundant reserves and low cost. They are very suitable as anode materials for lithium-ion batteries. Graphite is currently the most commercially used carbon anode material. It is a two-dimensional layered structure composed of a single layer of graphene. The research of YAZAMI et al. [14-15] showed that during the first charging process, the graphite surface of the electrolyte is first reduced to form SEI film, and then lithium ions are intercalated between graphite layers to form graphite lithium intercalation compounds. Therefore, graphite material characteristics such as particle size and specific surface area, end and basal surface, crystallinity and surface functional groups will have an important impact on the structure and composition of SEI film. .

　　4. The influence of electrolyte composition on the formation process of SEI film

　　SEI film is mainly formed by the reduction and decomposition of various components in the electrolyte, so the composition of the electrolyte has an important influence on the morphology and composition characteristics of the SEI film. BOYER et al. studied the influence of the relative ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC) solvents on the composition of the SEI film through theoretical calculations. The results showed that EC can form EC-radicals on the graphite surface through single-electron reduction, and further occur. The multi-electron reduction reaction forms carbonate or bicarbonate, and when the EC content in the electrolyte is relatively high, because the graphite surface is covered by more unsolvated EC molecules, the EC is reduced to form carbonate. Limited, it is easier to form a thinner and dense SEI film.

　　5. The influence of chemical conversion process on the formation process of SEI film

　　The formation process of forming the SEI film is generally to vacuum the assembled lithium ion battery, and then use inert gas to inject the electrolyte under a certain pressure, and leave it for a proper time to allow the electrolyte to fully infiltrate the pores of the electrode or diaphragm, and then add 0.02 ~0.2C lower current density to charge the battery. The formation process parameters include formation voltage, current density, temperature, etc. The formation voltage mainly affects the film-forming reaction path, while the formation temperature and current density mainly affect the rate of the film-forming reaction. AN et al. [25] showed that under different charging voltages, the decomposition reaction of the electrolyte is different. When the negative electrode is above 1.0Vvs.Li+/Li, only the lithium salt will decompose to produce a small amount of LiF, while the solvent or additive molecules are The reductive decomposition starts below 0.8V. RODRIGUES et al. [26] found that using ionic liquid electrolyte and increasing the formation temperature to 90°C will make the SEI film formed on the graphite surface thicker and have better thermal stability, but the rate performance will decrease.

　　6, electrolyte and SEI membrane research ideas

　　There are already tens of thousands of reported electrolyte solvents, lithium salts or additives, but only dozens of them are actually used in commercial battery products. The reason is that the electrolyte formulations reported in the literature generally only focus on improving the single performance of lithium-ion batteries, and cannot meet the comprehensive performance indicators when used as battery products. The most typical example is the poor thermal stability of lithium hexafluorophosphate when used as a lithium salt, and its shortcomings such as sensitivity to moisture are very obvious. However, no other commercial lithium salt that can replace it has been found so far. Therefore, the original intention of electrolyte formulation optimization is not to maximize performance in a certain aspect, but to find the balance point for the best overall performance. Taking the study of VC additives as an example, in order to improve the room temperature cycle performance of battery cell products, when the addition amount of VC is increased from 0.5% to 2% to more than 3% to 5%, a thicker SEI film of 100nm or more will be formed, which makes phosphoric acid The normal temperature cycle life of iron-lithium/graphite battery products has been increased from 2000 weeks to 3000 weeks, but the price is high internal resistance of the battery. Due to the poor kinetics of the lithium insertion reaction during low-temperature cycles at -20°C, the low-temperature cycle life is less than 50 weeks. Only use no more than 3% of VC additives, and then use other additives that can reduce the impedance and increase the ion mobility of the SEI membrane.

　　7. Conclusion

　　Because the formation process of SEI film is complicated and affected by many factors, it is very difficult to carry out systematic research on SEI film. However, SEI film is also an indispensable component of lithium ion battery, combined with lithium ion battery manufacturer’s advanced electrolyte formulation and advanced technology. With the needs of SEI technology, future research directions in this field may focus on the following aspects.

　　(1) In order to obtain higher energy density, high nickel ternary materials, lithium-rich lithium manganate materials and other new cathode materials with high specific capacity and high voltage characteristics have been continuously developed. For these new cathode materials, the surface SEI film The formation mechanism and its influence on electrochemical performance are becoming more and more urgent. At present, a small number of studies have shown that the positive electrode surface can also form an SEI film structure or composition similar to the negative electrode surface during the chemical conversion charging process. The interaction and influence between the two are in an unknown state.

　　(2) With the development of the electrochemical system of lithium-ion batteries, performance requirements such as working voltage, working temperature, cycle life, and safety are getting higher and higher. Traditional carbonate-based electrolyte formulations can no longer meet application requirements. The optimization of liquid formulations and the search for new new solvents, lithium salts or additives with excellent performance have become a top priority. In this process, exploring the mechanism and function of SEI film formation of various new electrolyte components will point out the direction of the development of electrolyte formulations.

　　(3) The SEI film belongs to the nanometer scale, and along with the charging and discharging process of the lithium ion battery, its morphology and structure are always changing, and the chemical composition of its surface is very sensitive to environmental conditions, so traditional ex-situ characterization is used. It is difficult to systematically research and analyze SEI membranes with technology. It is urgent to construct some effective in-situ analysis methods to realize real-time, dynamic and accurate detection of changes in SEI membranes under the working conditions of lithium-ion batteries. In addition, experimental methods should be combined with theoretical calculation methods. For example, the theoretical calculation results of the surface state of the material and the reaction activity will provide important theoretical guidance for the study of SEI films.

　　(4) At present, the commercial production of lithium-ion batteries can only form SEI film on the electrode surface through chemical conversion charging. The process is low in controllability, and the directional control of the morphology and structure of the SEI film cannot be achieved. Process parameters undergo constant trial and error. Therefore, if a method of controllable growth of artificial SEI film on the electrode surface can be constructed, and an SEI film structure that meets the battery performance requirements can be easily prepared, it will undoubtedly be of great significance to the development of lithium-ion battery technology.