Briefly describe whether the positive electrode lithium supplement process can replace the negative electrode lithium supplement process

　　At present, the most common method of replenishing lithium is the method of replenishing lithium in the negative electrode, which uses processes such as lithium powder and lithium foil to supplement the irreversible capacity loss of the negative electrode during the first charge. In addition, another method of replenishing lithium is the positive electrode replenishment process. A small amount of high-capacity lithium-containing oxide, such as Li5FeO4, is added to the positive electrode, and the positive electrode is used to store extra Li to supplement the Li loss during the first discharge. These two lithium supplement methods have their own advantages. Today we will discuss and compare the two lithium supplement methods together.

　　The specific capacity of pure Si can reach 4200mAh/g (Li4.4Si) in the state of fully intercalated lithium, but it is also accompanied by a volume expansion of up to 300%, which will cause particle fragmentation and differentiation of pure silicon materials during the lithium intercalation process. , The negative electrode falls off, which leads to serious capacity decline during the material cycle. In order to overcome the problem of silicon anode material, people try to make pure silicon into nanoparticles to suppress the expansion of Si particles, but in fact this strategy is not successful. Related calculations show that only when the size of pure Si particles is smaller than the unit cell It is possible to completely suppress the volume expansion of Si particles when the size is small. This is obviously impossible. Therefore, nanometerization can only reduce the volume expansion of Si anode particles. At the same time, the larger specific surface area of the nanoparticles will also cause the anode and electrolysis. The side reactions between the liquids increased significantly. In addition, another strategy is to make Si material into a "raisin bread" structure, that is, to disperse nano Si particles in a sea of graphite, and use graphite to absorb the volume expansion of Si particles during charging and discharging, but this method is also not Not perfect. First of all, the specific capacity of the material is very low. Because of the high graphite content, the specific capacity of most of this type of silicon-carbon anode is only 400-500mAh/g, and the cycle life of such silicon-carbon materials has not been too much. Many improvements.

　　Due to the above-mentioned problems of pure Si materials, people began to try to use another silicon oxide-SiOX as the negative electrode material. The bond energy of the Si-O bond is twice that of the Si-Si bond. At the same time, it is in the process of lithium insertion. In the process, Li will react with the O element in the material to form LiXO. These Li oxides then lose their activity and become a buffer layer inside the silicon oxide particles, which can inhibit the material well during the charge and discharge process. The volume expansion improves the cycle performance of the material. Since the lithium metal oxide LiXO is generated during the first lithium insertion of SiOx, the first coulombic efficiency of the silicon oxide material is only about 70%. In recent years, after many technical improvements, the first efficiency has also been tightly increased by about 80%. There is still a big gap between this and 90% of graphite materials. Therefore, in order to take advantage of the high specific capacity of SiOX materials, it is necessary to use the lithium supplement process to supplement the irreversible capacity loss during the first lithium insertion process.

　　Comparison of positive electrode lithium supplement process and negative electrode lithium supplement process

　　At present, the lithium supplement process is mainly divided into two categories; 1) the negative electrode lithium supplement process; 2) the positive electrode lithium supplement process, of which the negative electrode lithium supplement process is our most common lithium supplement method, such as lithium powder supplement lithium and lithium foil lithium supplement. They are all lithium-replenishing processes that are currently being developed by major manufacturers. The lithium supplementation process with lithium powder was first proposed by FMC. FMC developed an inert lithium powder for this purpose. The appropriate amount of lithium powder was added to the negative electrode through processes such as spraying and homogenization. Lithium foil replenishment is also an emerging lithium replenishment process in recent years. The metal lithium foil is rolled to a thickness of several microns, and then combined with the negative electrode and rolled. After the battery is injected, these metallic Li quickly react with the negative electrode and are embedded in the negative electrode material, thereby improving the first-time efficiency of the material. However, these methods have to face a problem-the "safety problem of metallic lithium". Metallic lithium is an alkali metal with high reactivity and can react violently with water, which makes metallic lithium very demanding on the environment. Both of these negative electrode lithium supplement processes have to invest a huge amount of money to transform the production line, purchase expensive lithium supplement equipment, and at the same time, in order to ensure the lithium supplement effect, the existing production process needs to be adjusted.

　　Compared with the highly difficult and high-input negative electrode lithium supplementation process, the positive electrode lithium supplementation process is much simpler. The typical positive electrode lithium supplementation process is to add a small amount of high-capacity positive electrode material to the positive electrode homogenization process. During the charging process, the excess Li element is extracted from these high-capacity positive electrode materials and embedded in the negative electrode to supplement the irreversible capacity of the first charge and discharge. For example, XinSu and others of Argonne National Laboratory in the United States, by adding 7% of Li5FeO4 (LFO) material to the LiCoO2 cathode, the first efficiency of the battery was increased by 14%, and the cycle performance of the battery was significantly improved. The theoretical specific capacity of Li5FeO4 material can reach 700mAh/g, and almost all the capacity is irreversible. After the delithiation is completed, the material is rapidly deactivated and no longer participates in the charge and discharge reaction. The delithiation equation is: Li5FeO4? 4Li++4e-+LiFeO2+O2 . Giulio Gabrielli and others from Germany have adopted a method of mixing two positive electrode active materials: LiNi0.5Mn1.5O4 and Li1+XNi0.5Mn1.5O4. Li1+XNi0.5Mn1.5O4 can provide the battery during the first charge. The extra Li makes up for the Li lost during the first lithium insertion in the negative electrode. After the lithium is completely removed, Li1+XNi0.5Mn1.5O4 is converted into fully active LiNi0.5Mn1.5O4, so this method has no effect on the composition of the positive electrode. Li1+XNi0.5Mn1.5O4 can be regarded as a cathode material that temporarily stores excess Li. By changing the ratio of Li1+XNi0.5Mn1.5O4 to LiNi0.5Mn1.5O4, the amount of Li that the anode can provide can be determined. Control to adapt to the negative electrode with different initial efficiency.

　　Process dispute: Can the positive electrode lithium supplement process replace the negative electrode lithium supplement process?

　　Through the above analysis, it is not difficult to find that the biggest advantage of the positive electrode lithium supplement process is that the process is simple, and there is no need to change the existing lithium-ion battery production process, and there is no need to modify the existing production workshop or purchase. Expensive lithium supplement equipment, more importantly, the positive electrode lithium supplementation greatly improves the safety of the lithium supplement process, but the positive electrode lithium supplement process may cause the proportion of the positive electrode active material to decrease. For example, when Li5FeO4 is used, it needs to reach 7 % Content, and these products after supplementing lithium are inactive, which affects the further improvement of the energy density of lithium-ion batteries.

　　Comparing the two methods of replenishing lithium, the author is more optimistic about the positive electrode replenishing lithium. The negative electrode lithium supplement process has strict conditions and large investment, and the use of metal lithium poses a greater safety risk. In contrast, the positive electrode lithium supplement process is simple and does not require modification of the existing production line and process, and the investment is small. There is no safety risk. The positive electrode lithium supplement process developed by Giulio Gabrielli et al. solves the problem that the lithium supplement product affects the positive electrode composition. Although the technology is currently only applied to LiNi0.5Mn1.5O4 materials, through related technology research and development, this lithium supplement It is believed that the process can also be applied to ternary materials such as NCM and NCA to improve the efficiency of the battery for the first time.