What progress has been made in the research of solid electrolyte interface film (SEI) for NiMH No.7 battery?

　　Current research on lithium-ion batteries focuses on improving energy density, rate and power performance, cycle performance, safety performance, and reducing manufacturing costs. However, almost all research fields related to lithium-ion batteries inevitably involve Analysis and discussion of solid electrolyte interface films (SEI).

　　In 1979, PELED and others discovered that alkali metals or alkaline earth metals will immediately form an interface film after contact with electrolytes, which has ionic conductivity and electronic insulation. Its properties are similar to solid electrolytes, so the concept of SEI film was first proposed.

　　In 1983, further research by PELED and others found that the propylene carbonate (PC) solvent in the electrolyte can be reduced on the surface of the lithium metal anode to form an SEI film composed of a double-layer structure, in which the inner layer close to the electrode surface is mainly composed of closely packed inorganic substances. , and the outer layer near the electrolyte side is mainly composed of alkyl ester organic matter, and is more loose and porous in structure. However, lithium dendrites will inevitably be generated during the cycling process of lithium metal anodes, causing short-circuit explosions and other serious safety issues, which greatly hindered the commercial application of early NiMH No.7 battery.

　　Subsequently, researchers began to try to use graphite-based negative electrodes to replace lithium metal negative electrodes. Although the safety hazards caused by lithium dendrites were effectively solved, the solvated PC molecules in the electrolyte could be co-embedded 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. discovered that the ethylene carbonate (EC) solvent molecules in the electrolyte could form a relatively stable SEI film on the surface of the graphite anode, effectively inhibiting the co-intercalation of solvent molecules and solving the safety problem of lithium metal anodes. At the same time, it also improved the cycle stability, which eventually led to the successful commercialization of lithium-ion batteries represented by graphite anodes and is still in use today.

　　It can be seen that people's research and understanding of the SEI film of lithium-ion batteries plays a vital role in the entire development process of lithium-ion batteries. The production of a stable SEI film is the key to the normal charging and discharging of lithium-ion batteries and ensuring various functions. prerequisites for electrochemical performance.

　　This article reviews the formation process mechanism, influencing factors, research ideas and current situation of SEI film, and looks forward to potential future research directions as follows: studying the formation mechanism and role of SEI film on the surface of new cathode materials; exploring the formulation optimization of functional electrolyte, Study the film-forming mechanism and role of new solvents, lithium salts or additives; use in-situ analysis or theoretical calculation methods to deeply study the chemical composition and morphological structure of the SEI film; explore effective artificial SEI film construction methods and realize the SEI film structure Controllable optimization.

　　1. Formation process and reaction mechanism of SEI film

　　At present, commercial lithium-ion battery electrolytes are mainly composed of cyclic or linear carbonate solvents, lithium salts and a small amount of functional additives. As shown in Figure 1, GOODENOUGH et al. believe that the lowest unoccupied molecular orbital energy level (LUMO) of the electrolyte and The highest occupied molecular orbital energy levels (HOMO) are approximately 1.0V and 4.7V vs. Li+/Li respectively. When the lithium-ion battery is first formed and charged, the surface potential of the negative electrode material continues to decrease. When it is lower than 1.0V, the electrolyte components can be 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. Chemical composition and morphology structure of SEI film

　　Since the SEI film has an important impact on various properties of lithium-ion batteries, an ideal SEI film should have the following characteristics: ① The film-forming potential of the SEI film must be higher than the insertion or extraction potential of lithium ions, thereby effectively preventing solvent molecules Co-intercalation; ② The SEI membrane components are insoluble in the electrolyte and can remain stable within the operating voltage and temperature range of lithium-ion batteries; it has a moderate thickness and a "rigid and soft" molecular structure, so that it can adapt to the volume changes of the anode material , and can maintain the stability of the cycle structure; ③ It has high electronic insulation and lithium ion selective passability. The electronic insulation is to hinder the decomposition of more electrolytes and the formation of a thicker SEI film, and the ion conductivity is to ensure the lithium Smooth ion migration and insertion 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. As WANG et al. proposed in the latest review article, the SEI membrane is close to the electrode interface. The inner layer is mainly composed of inorganic substances such as Li2CO3, Li2O, and LiF, and the outer layer near the electrolyte interface is mainly composed of organic products such as ROLi and ROCO2Li. The inner layer structure is dense and compact, and the outer layer structure is loose and porous.

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

　　Due to its stable physical and chemical properties, carbon materials have a lithium insertion voltage slightly higher than that of metallic lithium anodes. There is no risk of lithium dendrite precipitation, and they are rich in 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. Research by YAZAMI et al. [14-15] shows that during the first charging process, the electrolyte graphite surface is first reduced to form SEI film, and then lithium ions are embedded into the layers of graphite to form lithium-embedded compounds of graphite. Therefore, graphite material properties such as particle size and specific surface area, end face and basal face, degree of crystallization and surface functional groups will have an important impact on the structural composition of the SEI film. .

　　4. Effect of electrolyte composition on SEI film formation process

　　The SEI membrane is mainly formed by the reduction and decomposition of various components in the electrolyte. Therefore, the composition of the electrolyte has an important impact on the morphology, structure and composition characteristics of the SEI membrane. 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-free radicals through single electron reduction on the graphite surface, which further occurs. The multi-electron reduction reaction forms carbonate or bicarbonate, and when the EC content in the electrolyte is relatively high, since the graphite surface is covered with more unsolvated EC molecules, EC is reduced to form carbonate reaction. Restricted, it is easier to form a thinner and dense SEI film.

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

　　The formation process of SEI film is generally to evacuate the assembled lithium-ion battery first, then use inert gas to inject electrolyte under a certain pressure, and let it stand for aging for an appropriate time to allow the electrolyte to fully infiltrate the electrode or separator pores, and then use 0.02 The battery is charged with a smaller current density of ~0.2C. 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] have shown that under different charging voltages, the decomposition reactions of the electrolyte are different. When the negative electrode is above 1.0V vs. Li+/Li, only the lithium salt will decompose to produce a small amount of LiF, while the solvent or additive molecules will Reductive decomposition begins only 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. Research ideas on electrolyte and SEI membrane

　　There are already tens of thousands of reported types of electrolyte solvents, lithium salts or additives, but only a few dozen are actually used in commercial battery products. The reason is that the electrolyte formulas reported in the literature generally only focus on improving the single performance of lithium-ion batteries and cannot meet the comprehensive performance indicators of battery products. The most typical example is that when used as a lithium salt, lithium hexafluorophosphate has obvious shortcomings such as poor thermal stability and sensitivity to moisture. However, no other commercial lithium salt that can replace it has been found so far. Therefore, the original intention of optimizing the electrolyte formula is not to maximize performance in one aspect, but to find a balance point with optimal overall performance. Taking the research on VC additives as an example, in order to improve the normal temperature cycle performance of battery products, when the amount of VC added is increased from 0.5% to 2% to more than 3% to 5%, a thicker SEI film of more than 100nm will be formed, making the 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 that the internal resistance of the battery is very high. Due to the poor lithium insertion reaction kinetics during low-temperature cycling at -20°C, the low-temperature cycle life is less than 50 weeks. Finally, Only no more than 3% VC additives can be used, and other additives that can reduce impedance and increase ion mobility of the SEI membrane can be used on this basis.

　　7. Conclusion

　　Since the formation process of the SEI film is complex and affected by many factors, it is very difficult to conduct systematic research on the SEI film. However, the SEI film is an essential component of lithium-ion batteries. Combined with the electrolyte formula and advanced technology of lithium-ion battery manufacturers, To meet 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, new cathode materials with high specific capacity and high voltage characteristics such as high-nickel ternary materials and lithium-rich lithium manganese oxide materials are continuously developed. For these new cathode materials, SEI film on the surface Research on the formation mechanism and its impact on electrochemical performance is becoming more and more urgent. At present, there are a small number of studies showing that during the formation charging process, the positive electrode surface can also form an SEI film structure or composition similar to the negative electrode surface. However, the formation of the SEI film on the positive electrode surface is mainly a chemical reaction or an electrochemical reaction, and there is a difference between the positive and negative SEI films. The interactions and impacts among them are still unknown.

　　(2) With the development of lithium-ion battery electrochemical systems, performance requirements such as operating voltage, operating temperature, cycle life, and safety are becoming higher and higher. The traditional carbonate electrolyte formula can no longer meet application needs. Electrolysis It is urgent to optimize the liquid formula and seek new solvents, lithium salts or additives with excellent performance. In this process, exploring the SEI film-forming mechanism and role of various new electrolyte components will point out the direction for the development of electrolyte formulas.

　　(3) The SEI film is nanoscale, and its morphology and structure are always changing along with the charging and discharging process of lithium-ion batteries. The chemical composition of its surface is very sensitive to environmental conditions, so traditional ex-situ characterization is used. It is difficult to conduct systematic research and analysis on the SEI film due to lack of technology. There is an urgent need to construct some effective in-situ analysis methods to achieve real-time, dynamic and accurate detection of changes in the SEI film under the working conditions of lithium-ion batteries. In addition, experimental methods should be combined with theoretical calculation methods. For example, theoretical calculation results of material surface states and reactivity will provide important theoretical guidance for the research of SEI films.

　　(4) Currently, commercially produced lithium-ion batteries can only form an SEI film on the electrode surface through formation charging. The process controllability is low and it is impossible to achieve directional control of the SEI film morphology and structure. It can only be achieved by optimizing the electrolyte composition or formation. Process parameters are subject to continuous trial and error. Therefore, if a method for controllably growing an artificial SEI film on the electrode surface can be constructed and an SEI film structure that meets battery performance requirements can be easily prepared, it will undoubtedly be of great significance to the development of lithium-ion battery technology.

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