What progress have made in research of solid electrolyte interface film (SEI) for lithium-ion batteries?

What progress have made in research of solid electrolyte interface film (SEI) for lithium-ion batteries?

At present, research on lithium-ion batteries focuses on improving energy density, rate and power performance, cycle performance, safety performance, and manufacturing cost reduction. However, almost all research fields related to lithium-ion batteries need to focus on analysis and discussion of solid electrolyte interface film (SEI) inevitably.

In 1979, PELED found that an alkali metal or alkaline earth metal immediately formed an interfacial film after contact with the electrolyte. It has ionic conductivity and electronic insulation, and its properties are like those of solid electrolyte. Therefore, the concept of SEI film was first proposed.

After further research by PELED in 1983, it was found that the propylene carbonate (PC) solvent in the electrolyte can be reduced on the surface of the lithium metal anode to form a SEI film composed of a two-layer structure, wherein the inner layer close to the electrode surface is mainly composed of inorganic materials closely packed. The outer layer near the electrolyte side is mainly composed of an organic substance of an alkyl ester, and is structurally more porous and porous. However, due to the inevitable lithium dendrites during the lithium metal anode cycle, serious safety problems such as short-circuit explosions are caused, which greatly hinders the commercial application of early lithium batteries.

Subsequently, the researchers began to try to replace the lithium metal anode with a graphite-based anode. Although the safety of lithium dendrites was effectively solved, the solvated PC molecules in the electrolyte could be co-intercalated with lithium ions in the interlayer structure of graphite. It is impossible to form a stable SEI film on the graphite surface. Until 1990, DAHN found that ethylene carbonate (EC) solvent molecules in the electrolyte can form a stable SEI film on the surface of the graphite negative electrode, effectively inhibiting the co-embedding of solvent molecules, and solving the safety problem of lithium metal negative electrodes. At the same time, the cycle stability is also improved, and finally the lithium ion battery represented by the graphite type negative electrode is successfully commercialized and is still in use today.

The research and understanding process of lithium-ion battery SEI membrane plays a vital role in the development of lithium-ion battery. The production of stable SEI membrane is the normal charge and discharge of lithium-ion battery and guarantee various Prerequisites for electrochemical performance.

In this paper, the formation mechanism, influencing factors, research ideas and current status of SEI films are reviewed. The future research directions are as follows: Study on the formation mechanism and role of SEI film on the surface of new positive electrode materials; explore the formulation optimization of functional electrolytes, Study the film formation mechanism and function of new solvent, lithium salt or additive; study the chemical composition and morphology of SEI film by in-situ analysis or theoretical calculation; explore effective artificial SEI film construction method and realize SEI film structure Controllable optimization.

1. Formation process and reaction mechanism of SEI film

At present, the commercial lithium ion battery electrolyte is mainly composed of a cyclic or linear carbonate solvent, a lithium salt and a small number of functional additives. As shown in Fig. 1, GOODENOUGH et al. believe that the electrolyte has the lowest unoccupied molecular orbital energy level (LUMO) and the highest occupied molecular orbital energy levels (HOMO) are about 1.0V and 4.7V vs. Li+/Li. When the lithium ion battery is first charged, the surface potential of the negative electrode material is continuously reduced. When the lithium ion battery is lower than 1.0V, the electrolyte composition can be Reductive decomposition, in which insoluble reductive decomposition products are gradually deposited on the surface of the negative electrode material to form an SEI film.

2. Chemical composition and morphology of SEI film

Since the SEI film has an important influence on the performance of the lithium ion battery, the ideal SEI film should have the following characteristics: The film formation potential of the 1SEI film must be higher than the insertion or extraction potential of the lithium ion, thereby effectively preventing solvent molecules. Co-embedding; 2SEI membrane component is insoluble in electrolyte, can be stable in the working voltage and temperature range of lithium ion battery; has a moderate thickness and a "rigid and flexible" molecular structure, so as to adapt to the volume change of the anode material It can maintain the stability of the cyclic structure; 3 has high electronic insulation and lithium ion selective passability, electronic insulation is to hinder the decomposition of more electrolyte and the formation of thicker SEI film, ion conductivity is to protect lithium Ion migration and smooth insertion of channels.

With the increasing characterization of 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 that SEI membranes are close to the electrode interface. The inner layer is mainly composed of inorganic substances such as Li2CO3, Li2O and LiF, and the outer layer of the electrolyte interface is mainly composed of organic products such as ROLi and ROCO2Li, and the inner layer structure is compact and compact, and the outer layer structure is loose and porous.

3. Influence of surface properties of graphite materials on the formation process of SEI film

Due to the stable physical and chemical properties of the carbon material, the lithium insertion voltage is slightly higher than that of the lithium metal anode, there is no risk of lithium dendrite precipitation, and the reserves are abundant and the cost is low, which is very suitable as a negative electrode material for lithium ion batteries. Graphite is the most commercially used carbon anode material, which is a two-dimensional layered structure composed of single-layer graphene. The research of YAZAMI et al. [14-15] shows that the surface of electrolyte graphite is firstly reduced during the first charging process. SEI film, followed by lithium ion intercalation between the layers of graphite to form graphite lithium intercalation compounds, so graphite material properties such as particle size and specific surface area, end and base surface, degree of crystallization and surface functional groups will have an important impact on the SEI film structure.

4. Effect of electrolyte composition on the formation process of SEI film

The SEI film is mainly formed by the reductive decomposition of various components in the electrolyte. Therefore, the composition of the electrolyte has an important influence on the morphology and composition of the SEI film. BOYER et al. studied the effect of the relative proportion of ethylene carbonate (EC) and dimethyl carbonate (DMC) solvent on the composition of SEI film by theoretical calculations. The results show that EC can form EC-free radicals by single electron reduction on the graphite surface, which further occurs. 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 by more unsolvated EC molecules, the reaction of EC to reduction to form carbonate Limited, it is easier to form a thinner and dense SEI film.

5. Effect of chemical conversion process on the formation process of SEI film

The formation process of the SEI film generally involves first vacuuming the assembled lithium ion battery, then injecting the electrolyte under a certain pressure by using an inert gas, and immersing the aging for a suitable time so that the electrolyte fully wets the electrode or the pore of the membrane, and then 0.02 The battery is charged with a small current density of ~0.2C. The formation process parameters include formation voltage, current density, temperature, etc., and the formation voltage mainly affects the film formation reaction path, and the formation temperature and current density mainly affect the rate of the film formation reaction. AN et al. [25] showed that the decomposition reactions of electrolytes are different under different charging voltages. When the anode 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 reduction decomposition starts only below 0.8V. RODRIGUES et al [26] found that using an 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

At present, there have been many types of electrolyte solvents, lithium salts or additives that have been reported, but there are dozens of them applied to commercial battery products. The reason for this is that the electrolyte formulations reported in the literature generally focus only on the improvement of the single performance of lithium-ion batteries, and cannot meet the comprehensive performance indicators when used as battery products. The most typical example is that the lithium hexafluorophosphate is poorly used as a lithium salt, and the disadvantages such as sensitivity to moisture are obvious, but no other commercial lithium salt has been found. Therefore, the original intention of electrolyte formulation optimization is not to maximize performance in one aspect, but to find the optimal balance of overall performance. Taking the VC additive as an example, in order to improve the normal temperature cycle performance of the cell product, when the addition amount of VC is increased from 0.5% to 2% to 3% to 5% or more, a thicker SEI film of 100 nm or more is formed, so that phosphoric acid is formed. The normal temperature cycle life of iron-lithium/graphite battery products has been increased from 2000 to 3000 weeks, but at the cost of high internal resistance of the battery. The low-temperature cycle life is less than 50 weeks due to poor kinetics of lithium intercalation at -20 °C. Only use no more than 3% of VC additives, and other additives that reduce the impedance and increase the ion mobility of the SEI membrane.

7, the conclusion

Because the formation process of SEI film is complicated and affected by many factors, it is very difficult to systematically study SEI film, but SEI film is an indispensable component of lithium ion battery, combined with lithium ion battery production enterprise for electrolyte formula and advanced 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, and lithium-rich lithium manganate materials, new positive electrode materials with high specific capacity and high voltage characteristics have been continuously developed. For these new positive electrode materials, surface SEI films 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 a SEI film structure or composition like the surface of the negative electrode during the charging process, but the formation of the SEI film on the positive electrode surface is mainly chemical reaction or electrochemical reaction, and the positive and negative SEI film Problems such as interaction and influence are unknown.

(2) With the development of electrochemical systems for lithium-ion batteries, performance requirements such as working voltage, operating temperature, cycle life, and safety are becoming higher and higher. Traditional carbonate electrolyte formulations have been unable to meet application requirements, and electrolysis Optimization of liquid formulations and the search for new types of new solvents, lithium salts or additives with excellent performance have become a top priority. In this process, the mechanism and role of SEI film formation in various new electrolyte compositions will be pointed out for the development of electrolyte formulations.

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

(4) At present, the commercial production of lithium-ion batteries can only form SEI film on the surface of the electrode through chemical charging. The process controllability is low, and the orientation control of the SEI film morphology structure cannot be achieved. Only by optimizing the electrolyte composition or forming Process parameters are continuously tested and errored. Therefore, if a method for controlling the growth of artificial SEI film on the surface of the electrode can be constructed, it is undoubtedly important to develop the SEI film structure that satisfies the performance requirements of the battery.

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