AG3 battery.Effect of fast charging on the performance of graphite anode and its mechanism of action

　　With the rapid development of mobile phone battery fast charging technology, the charging rate of mobile phones is getting faster and faster. While enjoying the benefits of fast charging of mobile phones, we still have a small question in our hearts. What is the impact of fast charging on lithium-ion batteries? Does lifespan have an impact? First, let’s understand what the charging rate is. Generally speaking, lithium-ion batteries use rate to describe the charging rate of the battery. For example, 1C rate refers to the battery being fully charged within 1 hour, and 2C rate refers to 0.5h. The battery can be fully charged within 10 seconds, which means that the greater the rate, the faster the charging rate, and the charging time is the reciprocal of the charging rate. After understanding the definition of charging rate, let’s learn about the impact of fast charging on lithium-ion batteries.

　　Generally speaking, fast charging will cause the internal resistance of lithium-ion batteries to increase and the capacity to decrease. We will now understand the mechanism of this. At present, the main anode material for commercial lithium-ion batteries is graphite materials. The use of graphite materials has largely solved the problem of dendrite precipitation in the metal lithium anode, greatly improved the safety of lithium-ion batteries, and enabled the commercialization of lithium-ion batteries. application. Currently, common graphite anode materials include natural graphite, artificial graphite and other types. During the charging process of lithium-ion batteries, Li+ migrates from the positive electrode to the negative electrode and is embedded in the layered structure of the graphite material, forming a LiC6 compound, making the negative electrode Appears golden yellow. The lithium insertion process of the negative electrode mainly includes the diffusion of Li+ in the electrolyte and SEI film, charge exchange on the surface of the negative electrode, and the diffusion of Li in the solid phase. These processes will directly affect the charging rate of the lithium-ion battery. During the charging process of lithium-ion batteries, the negative electrode will produce concentration polarization and electrochemical polarization, which will cause the potential of the negative electrode to be lower than its actual steady-state potential. As the charging rate increases, the polarization will further increase. , on the one hand, this will aggravate the occurrence of side reactions, on the other hand, it will cause the formation of metallic lithium coating and lithium dendrites on the surface of the negative electrode, causing safety issues and capacity reduction of lithium-ion batteries.

　　L. Somerville of Argonne National Laboratory in the United States (the largest research center of the U.S. Department of Energy) studied the impact of charging rate on graphite anodes. Research shows that in the rate range from 0.7C to 4C, the degradation of battery performance is mainly related to the increase in the thickness of the SEI film, while the composition of the SEI does not change significantly. However, when charging at a rate of 6C, the composition of the SEI film changes significantly, which also leads to a sharp increase in the internal resistance of the lithium-ion battery. Further research also found that in rolled batteries, the SEI film in the middle part of the cell is thicker and has a different composition than the SEI film in other parts. This may be due to the local temperature caused by uneven battery infiltration during the production process. Caused by the increase in SEI film thickness and composition, unevenness in the thickness and composition of the SEI film will cause the battery to undergo chemical denaturation of the binder at a 6C charging rate, causing the active material to fall off the current collector.

　　In the experiment, L.Somerville used a commercial NMC/graphite 18650 battery to restore the working state of the graphite anode in commercial lithium-ion batteries as much as possible. The charging rate is set to 0.7, 2, 4, and 6C, the discharge rate is C/3, and the battery temperature is controlled at 25°C.

　　By disassembling the cycled battery, as the charging rate increases, the color of the negative electrode also changes. At the charging rate of 0.7C and 2C, the electrode shows a uniform color. When the charging rate is increased to 4C, the color of the electrode changes. The middle position appears gray, and at a magnification of 6C, not only the electrode in the middle part appears gray, but the active material in the middle part also falls off. SEM scanning found that at the magnification of 0.7C and 2C, the electrode surface appeared in an original state. However, at the magnification of 4C, some bright spots began to appear on the surface of the electrode. At the magnification of 6C, the surface state of the electrode was obviously different. Due to As the thickness of the SEI film increases, it becomes difficult to distinguish individual graphite particles.

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