In the production process, personnel, equipment, raw materials, methods, and the environment are the main factors that affect product quality, and the production process of LiFePO4 power batteries is no exception. Personnel and equipment belong to the category of management, so we mainly discuss the last three influences factor.

Failure of batteries caused by impurities in electrode active materials

During the synthesis process of LiFePO4, there will be a small amount of impurities such as Fe2O3, Fe2P, Fe, etc. These impurities will be reduced on the surface of the negative electrode, which may pierce the diaphragm and cause an internal short circuit. LiFePO4 is exposed to the air for a long time, and moisture will deteriorate the battery. Its aging mechanism: amorphous iron phosphate is formed on the surface of the material in the early stage of aging, and its local composition and structure are similar to LiFePO4(OH); LiFePO4 is continuously consumed, showing an increase in volume; after that, it recrystallizes slowly to form LiFePO4(OH). The Li3PO4 impurity in LiFePO4 is electrochemically inert. The higher the impurity content of the graphite anode (mainly Al and Fe), the greater the irreversible capacity loss (the smaller the reversible capacity loss).

The failure of the battery caused by the chemical method

The irreversible loss of active lithium ions is first manifested in the lithium ions consumed during the formation of solid electrolyte interfacial films (SEI films). The study found that increasing the formation temperature will cause more irreversible loss of lithium ions, because the proportion of inorganic components in the SEI film will increase when the formation temperature is increased, and the gas released during the transformation of the organic component ROCO2Li to the inorganic component Li2CO3 It will cause more defects in the SEI film , and the solvated lithium ions through these defects will intercalate into the graphite negative electrode in large quantities.

During formation, the composition and thickness of the SEI film formed by low-current charging are uniform, but it is time-consuming; high-current charging will cause more side reactions, resulting in increased irreversible lithium ion loss, and negative interface impedance will also increase, but saves energy. At present, the formation mode of small current constant current and large current constant current and constant voltage is used more, which can take into account the advantages of both. The electrochemical method was used to further prove that the activation of the battery affects the stability of the SEI film. The higher the stability of the SEI film, the lower the self-discharge rate of the battery.

Battery failure due to moisture in the production environment

In actual production, the battery will inevitably come into contact with air. Since most of the positive and negative materials are micron or nanoscale particles, and the solvent molecules in the electrolyte have electronegative carbonyl groups and metastable carbon- carbon double bonds, are easy to absorb moisture in the air.

Water molecules react with lithium salts (especially LiPF6) in the electrolyte, which not only decomposes and consumes the electrolyte (decomposes to form PF5), but also produces an acidic substance HF. However, both PF5 and HF will destroy the SEI film, and HF will also promote the corrosion of the LiFePO4 active material. The water molecules also delithiate the lithium-intercalated graphite anode, forming lithium hydroxide at the bottom of the SEI film. In addition, the dissolved O2 in the electrolyte will also accelerate the aging of LiFePO4 batteries.

In the production process, in addition to the production process affecting the performance of the battery, the main factors that cause the failure of LiFePO4 power batteries include impurities in the raw materials (including water) and the process of formation, so the purity of the material, the control of environmental humidity, the method of formation, etc. factors appear to be crucial.

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