Analysis of the main reasons for the capacity degradation of lithium batteries

　　It is an inevitable phenomenon that capacity attenuation and loss occur during battery cycling. Therefore, in order to improve battery capacity and performance, many scholars have studied the mechanism of lithium battery capacity loss. At present, it is known that the main factors that cause the capacity degradation of lithium-ion batteries include the formation of SEI passivation film on the surface of the positive and negative electrodes, the deposition of metal lithium, the dissolution of electrode active materials, the occurrence of anode and cathode redox reactions or side reactions, structural changes and phase changes, etc. At present, the capacity decay changes of lithium-ion batteries and their causes are still under constant research.

　　(1) Overcharge

　　When a lithium-ion battery is charged and discharged for the first time, a passivation film called SEI is formed. This reaction will cause the loss of battery capacity and is an irreversible process. During the overcharging process of lithium ion batteries, metal lithium deposition occurs on the surface of the negative electrode, which is likely to occur when the positive electrode active material is excessive relative to the negative electrode active material. At the same time, under high-rate conditions, metal lithium deposition may also occur.

　　Generally speaking, metallic lithium will cause: first, the amount of recyclable lithium in the battery is reduced; second, the side reaction of metallic lithium with electrolyte or solvent, forming other by-products; third, metallic lithium is mainly deposited on the negative electrode and separator In between, the pores of the diaphragm are blocked, resulting in an increase in the internal resistance of the battery.

　　The positive electrode overcharge mainly occurs when the proportion of the positive electrode material is too low. This situation will cause the capacity imbalance between the electrodes, resulting in irreversible loss of the lithium battery capacity, and the coexistence of oxygen and combustible gas from the decomposition of the positive electrode material and the electrolyte and continuous accumulation may bring safety hazards to the use of lithium batteries.

　　(2) Decomposition of electrolyte

　　The electrolyte includes electrolytes, solvents and additives, and its properties will affect the battery's service life, specific capacity, rate charge and discharge performance, and safety performance. The decomposition of electrolyte and solvent in the electrolyte will cause the loss of battery capacity. During the first charge and discharge, the formation of SEI film on the surface of the negative electrode by solvents and other substances will cause irreversible capacity loss, but this is inevitable. In general, the occurrence of side reactions between the electrolyte and the positive and negative electrodes of the battery, as well as the by-products produced, are the main factors that cause the degradation of the battery capacity.

　　(3) Self-discharge

　　Lithium-ion batteries generally suffer from capacity loss. This process is called self-discharge and can be divided into reversible capacity loss and irreversible capacity loss.

　　The solvent oxidation rate has a direct effect on the self-discharge rate. In addition, the diaphragm leakage will also cause a loss of capacity, but this possibility is low. If the self-discharge phenomenon exists for a long time, it will lead to the deposition of metal lithium, and further cause the attenuation change of the positive and negative electrode capacity.

　　(4) Electrode instability

　　During the charging process, the active material of the positive 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 positive electrode material and the excessively high charging potential are the main factors that affect the battery capacity.

　　LiFePO4 has a wide range of sources, is cheap, and has good stability and safety performance. It can reach a theoretical specific capacity of 170mAh/g, and its specific power and specific energy are similar to LiCoO2, and can achieve good compatibility with electrolyte solutions. Therefore, It is widely used in the positive electrode of lithium battery.

　　Using this material, the main factors affecting the battery capacity include the following two points: one is due to side reactions between the positive and negative electrodes, resulting in the reduction of recyclable lithium, which severely disrupts the balance between the positive and negative electrodes; Factors such as structural deterioration, electrode delamination, material dissolution, particle segregation, etc., lead to loss of active materials, thereby affecting battery capacity.

　　Lithium-ion battery (LIB) has become the main energy storage solution in modern social life. Among them, lithium iron phosphate batteries are a perfect substitute for lead-acid batteries, and are the first choice for grid-connected peak shaving, off-grid energy storage, photovoltaic energy storage, UPS, data center and other industries.