1. Aging of battery materials

　　The materials inside the lithium battery mainly include positive and negative active substances, adhesives, conductive agents, collectors, diaphragms and electrolytes. During the use of lithium batteries, these materials will be accompanied by a certain degree of decay and aging. Tang Zhiyuan and others believed that the capacity attenuation factors of lithium manganate battery include: dissolution of cathode material, phase change of electrode material, decomposition of electrolyte, formation of interface facial mask and collector corrosion.

　　The change mechanism of the positive pole, negative pole and electrolyte of the battery in the cycle was systematically and deeply analyzed. The author believes that the formation and subsequent growth of the negative electrode SEI film will be accompanied by the irreversible loss of active lithium, and the SEI film does not have a real solid electrolyte function. Except for lithium ions, the diffusion and migration of other substances will lead to gas generation and particle breakage. In addition, the change of material volume and the precipitation of lithium metal during the cycle will also lead to capacity loss.

　　Aurbach et al. disassembled the positive and negative electrode plates of lithium cobaltate battery after cycling at 25 and 40 ℃. SEM, XRD and FTIR test results showed that both positive and negative active materials were lost. Li Yang et al. analyzed the electrical performance of the lithium iron phosphate power battery with 6000 cycles. The capacity retention rate was 84.87%, the AC internal resistance increased by 18.25%, and the DC internal resistance increased by 66%. The author disassembled the recycled battery and conducted performance test and SEM analysis of button type battery respectively. It was found that the performance of the negative electrode material declined rapidly after the cycle. It was believed that the expansion of the negative electrode volume and the thickening of the SEI film were the main influencing factors.

　　2 Charging and discharging system

　　The charging and discharging system mainly includes three aspects: charging and discharging mode, magnification and cut-off conditions. In terms of charging mode, American scientist Mas once proposed the concept of optimal charging curve. He believed that the optimal charging current of the battery gradually decreased with the extension of charging time: I=I0e- α t。 Where: I is the charging current that can be received; I0 is the maximum initial current when t=0; T is the charging time; α Is the attenuation constant.

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