Lithium battery capacity attenuation overcharge

　　There are many kinds of active materials that can be used as the cathode of lithium ion batteries, mainly including carbon based cathode materials, silicon based and tin based cathode materials, lithium titanate cathode materials, etc. Different types of carbon materials have different electrochemical properties. Among them, graphite has the advantages of high conductivity, excellent layered structure, and high crystallinity, which is more suitable for lithium insertion and removal. At the same time, graphite materials are affordable and have a large stock, so they are widely used.

　　When the lithium ion battery is charged and discharged for the first time, solvent molecules will decompose on the graphite surface and form a passive film called SEI. This reaction will lead to the loss of battery capacity and is an irreversible process. In the overcharge process of lithium ion battery, metal lithium deposition will occur on the negative electrode surface, which is easy to occur when the positive active material is excessive relative to the negative active material. At the same time, lithium metal deposition may also occur under high magnification conditions.

　　In general, the formation of metal lithium leads to the change of lithium battery capacity attenuation mainly due to the following aspects: First, the amount of recyclable lithium in the battery decreases; Second, lithium metal reacts with electrolyte or solvent to form other by-products; Third, lithium metal is mainly deposited between the negative electrode and the diaphragm, which causes the pore of the diaphragm to be blocked, resulting in an increase in the internal resistance of the battery. Different graphite materials have different influence mechanisms on the capacity attenuation of lithium ion batteries. The specific surface area of natural graphite is relatively high. Therefore, the capacity of lithium battery will be lost if self discharge reaction occurs. As the battery cathode, the electrochemical reaction impedance of natural graphite is also higher than that of artificial graphite. In addition, the dissociation of the layered structure of the negative electrode during the cycle, the dispersion of the conductive agent during the fabrication of the electrode sheet, and the increase of the electrochemical reaction impedance during the storage process are all important factors that lead to the capacity loss of the lithium battery.

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