Expansion: the first effect of silicon negative
Pure Si can achieve 4200 mAh/g (Li4.4Si) in the fully embedded state of lithium, but it is also accompanied by a volume expansion of up to 300%, which causes particle breakage and differentiation of pure silicon material during lithium insertion. The negative electrode is dropped, which leads to a serious decline in capacity during material circulation. In order to overcome the problem of silicon anode materials, attempts have been made to make pure silicon nanoparticles to inhibit the expansion of Si particles, but in fact this strategy is not successful, and related calculations show that only when the particle size of pure Si particles is smaller than the unit cell It is impossible to completely suppress the volume expansion of Si particles in the size, which is obviously impossible. Therefore, the nanocrystallization only reduces the volume expansion of the Si anode particles, and the larger specific surface area of ​​the nanoparticles also causes the anode and the electrolysis. The side reaction between the liquids is significantly increased. In addition, another strategy is to make the Si material into a "raisin bread" structure, that is, disperse the nano Si particles in the graphite ocean, and use graphite to absorb the volume expansion of the Si particles during charging and discharging, but the method is also Imperfect, first of all, the specific capacity of the material is very low. Due to the high graphite content, most of these silicon-carbon negative electrodes have a specific capacity of only 400-500 mAh/g, and the cycle life of such silicon-carbon materials is not too high. More improvement.
Due to the above problems in 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, and the process of intercalating lithium. In the process, Li reacts with the O element in the material to form LiXO. These Li oxides subsequently lose their activity and become a buffer layer inside the particles of the oxidized silicon oxide, so that the material can be suppressed very well during the charge and discharge process. The volume expands to improve the cycle performance of the material. Due to the formation of lithium metal oxide LiXO during the first intercalation of SiOx, the first coulombic efficiency of the oxysilylene material is only about 70%. In recent years, after many technical improvements, the first efficiency has also increased by about 80%. This is still far from the 90% of the graphite material. Therefore, in order to take advantage of the high specific capacity of the SiOX material, it is necessary to supplement the irreversible capacity loss in the first lithium insertion process by means of the lithium supplementation process.
Comparison of positive lithium supplementation process and negative electrode lithium supplementation process
At present, the lithium-replenishing process is mainly divided into two categories: 1) negative lithium supplementation process; 2) positive electrode lithium supplementation process, in which the negative lithium supplementation process is our most common lithium-replenishing method, such as lithium powder lithium and lithium foil lithium. It is a lithium-reinforcing process that is being developed by major manufacturers. Lithium powder lithium was first proposed by FMC. FMC developed inert lithium powder for this purpose, and added an appropriate amount of lithium powder to the negative electrode by spraying and homogenizing. Lithium-ion foil lithium is also a new lithium-reinforcing process in recent years. The metal lithium foil is crushed to a thickness of several micrometers, and then composited and laminated with a negative electrode. After the battery is injected, these metals Li react rapidly with the negative electrode and are embedded in the negative electrode material, thereby improving the first efficiency of the material. However, these methods have to face a problem - "safety of lithium metal", metal lithium is a highly reactive alkali metal, which can react with water violently, making lithium metal very environmentally demanding, which makes These two negative lithium-reinforcing processes have to invest huge amounts of money to renovate the production line, purchase expensive lithium-replenishing equipment, and also need to adjust the existing production process in order to ensure the lithium-replenishing effect.
Compared with the high-difficulty and high-input lithium-retaining process, the positive lithium is more simple. The typical positive lithium-replenishing process is to add a small amount of high-capacity positive electrode material to the positive electrode during the homogenization process. During the charging process, 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 et al. of the Argonne National Laboratory in the United States increased the first-time efficiency of the battery by 14% by adding 7% Li5FeO4 (LFO) material to the LiCoO2 positive electrode, and significantly improved the cycle performance of the battery. The theoretical specific capacity of Li5FeO4 material can reach 700mAh/g, and almost all the capacity is irreversible. After delithiation, the material is rapidly deactivated and no longer participates in charge and discharge reaction. Lithium removal equation: Li5FeO4®4Li++4e-+LiFeO2+O2 .
Giulio Gabrielli et al. from Germany adopted a method of mixing two kinds of positive active materials: LiNi0.5Mn1.5O4 and Li1+XNi0.5Mn1.5O4, and Li1+XNi0.5Mn1.5O4 can provide during the first charging of the battery. The additional Li compensates for the loss of Li during the first lithium insertion of the negative electrode. Li1+XNi0.5Mn1.5O4 is converted to fully active LiNi0.5Mn1.5O4 after complete delithiation, so the method has no effect on the composition of the positive electrode. Li1+XNi0.5Mn1.5O4 can be regarded as a positive electrode material in which excess Li is temporarily stored. By changing the ratio of Li1+XNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4, the amount of Li which can be additionally supplied to the positive electrode can be performed. Control to accommodate different negatives for the first time efficiency.
Image from reference 2
Through the above analysis, it is not difficult to find that the biggest advantage of the positive lithium supplementation process is that the process is simple, there is no need to change the existing lithium ion battery production process, and there is no need to modify the existing production workshop, no need to purchase Expensive lithium-retaining equipment, more importantly, the lithium supplementation of the positive electrode greatly improves the safety of the lithium-reinforcing process, but in the process of lithium supplementation, the proportion of active material of the positive electrode may decrease. For example, when using Li5FeO4, it is required to reach 7 The content of %, and the product after lithium supplementation is inactive, thus affecting the further increase of the energy density of the lithium ion battery.
Comparing the two methods of lithium supplementation, the author is more optimistic about the positive lithium. The process of lithium-receiving of the negative electrode is harsh, the investment is large, and the use of metal lithium causes a large safety risk. In contrast, the lithium-filling process of the positive electrode is simple, and it is not necessary to modify the existing production line and process, and the investment is small. There is no safety risk, and the positive lithium-reinforcing process developed by Giulio Gabrielli et al. solves the problem that the lithium-incorporating product affects the positive electrode composition. Although the technology is currently only applied to the LiNi0.5Mn1.5O4 material, this lithium is supplemented by related technologies. The process is believed to be able to apply ternary materials such as NCM and NCA to improve the battery's first efficiency.
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