It is inevitable having capacity attenuation and loss during battery cycling. Therefore, scholars in various fields have fully studied the mechanism of lipo battery capacity loss to improve battery capacity and performance. Currently, it is known that the main factors that cause the capacity attenuation of li ion batteries include the SEI passivation film on the anode and cathode surface, the deposition of metal lipo, the dissolution of active electrode materials, the occurrence of oxidation-reduction reactions or side reactions, structural changes and phase changes, etc.
During the overcharging process of li ion batteries, lithium metal deposition will occur on the anode surface. This situation tends to occur when the positive electrode active material is excessive relative to the negative electrode active material. At the same time, lipo metal deposition may also occur under high magnification conditions.
In general, the reasons for the change of lipo battery capacity attenuation caused by lipo mental include the following aspects. First, the reduction of the recyclable lipo in the battery. Second, the side reactions of lipo metal with electrolytes or solvents form other by-products. Third, lipo metal is mainly deposited between the anode and the separator, which causes the blockage of separator pores and leads to an increase in the internal resistance of the battery.
The overcharge of the cathode electrode mainly occurs when the proportion of the cathode electrode material is too low. This will lead to the capacity imbalance between the electrodes, resulting in irreversible loss of lipo battery capacity. In addition, the coexistence and continuous accumulation of oxygen and combustible gases decomposed from the cathode material and the electrolyte may bring safety risks to the use of the lipo battery.
If the charging voltage of the lipo battery is too high, it will cause an oxidation reaction in the electrolyte. In addition, some by-products will be generated to block the electrode micropores and hinder the migration of lipo ions, and resulting in the change of cyclic capacity attenuation. The electrolyte concentration is inversely proportional to the changing trend of electrolyte stability. The higher the electrolyte concentration, the lower the stability of the electrolyte, which will affect the capacity of li ion batteries. During the charging process, the electrolyte will be consumed. Therefore, it is necessary to be replenished during assembly, which will reduce the active material of the battery and affect the initial capacity.
The electrolyte includes electrolytes, solvents and additives. The properties will affect the service life, specific capacity, rate, charge and discharge performance, and safety performance of the battery. The decomposition of electrolyte and solvent will cause the loss of battery capacity. The SEI film will be generated by solvents and other substances on the surface of the anode electrode surface during the first charge and discharge, which will cause irreversible capacity loss. However, it is inevitable. If there are impurities such as water or hydrogen fluoride in the electrolyte, the electrolyte LiPF6 may be decomposed at a high temperature. Also, the generated product reacts with the cathode electrode material and affects the battery capacity. At the same time, some of the products will react with the solvent and affect the stability of the SEI film on the anode electrode surface, which will cause the performance attenuation of li ion batteries.
In general conditions, li ion batteries will have a capacity loss. This process is called self-discharge, and it is divided into reversible capacity loss and irreversible capacity loss. The oxidation rate of the solvent has a direct influence on the self-discharge rate. The anode and cathode active materials may react with the solute during the charging process, resulting in capacity imbalance and irreversible attenuation of li ion migration. If the self-discharge situation exists for a long time, it will lead to the deposition of metal lipo, and further cause the attenuation of the anode and cathode electrode capacity.
During the charging process, the active material of the cathode electrode of the battery is unstable, which will cause it to react with the electrolyte and affect the battery capacity. Among them, the structural defects of the cathode electrode material, the high charging voltage, and the carbon black content are the main factors that affect the battery capacity.