During the production process, personnel, equipment, raw materials, methods, and environment are the main factors affecting the quality of lithium iron phosphate battery products. This is no exception in the production of LiFePO4 power batteries. Personnel and equipment fall within the scope of management, so we mainly discuss the latter three influencing factors.
Impurities in the electrode active material causing battery failure:
During the synthesis of LiFePO4, there will be a small amount of impurities. These impurities can be reduced on the surface of the negative electrode, which may pierce the separator and cause an internal short circuit. If LiFePO4 is exposed to the air for a long time, moisture will also cause the battery to deteriorate, leading to an aging mechanism. In the early stage of aging, amorphous iron phosphate forms on the surface of the material, and its local composition and structure are similar to LiFePO4. However, as OH is embedded, LiFePO4 is continuously consumed, resulting in volume expansion, and then LiFePO4(OH) is gradually formed. The Li3PO2 impurity in LiFePO4 is electrochemically inert. The higher the impurity content in the graphite negative electrode, the greater the irreversible capacity loss.
The formation method causing battery failure:
The irreversible loss of active lithium ions is first reflected in the lithium ions consumed during the formation of the solid electrolyte interface film (SEI film). Studies have found that increasing the formation temperature will cause more irreversible lithium ion loss because the proportion of inorganic components in the SEI film increases when the formation temperature rises, which creates more gaps in the SEI film. Through these gaps, a large number of solvated lithium ions will be embedded in the graphite negative electrode. During formation, the SEI film formed by small current charging has a more uniform composition and thickness, but it takes more time; while large current charging will cause more side reactions, resulting in greater irreversible lithium ion loss, but it is more time-saving. This further proves that the activation of the battery affects the stability of the SEI film, and the higher the stability of the SEI film, the lower the self-discharge rate of the battery.
Moisture in the production environment causing battery failure:
In actual production, batteries inevitably come into contact with the air. Since the positive and negative electrode materials are mostly micrometer or nanometer-sized particles, the solvent molecules in the electrolyte are prone to absorb moisture from the air. Water molecules react with the lithium salt in the electrolyte (especially LiPF6), not only consuming the electrolyte but also generating acidic substances. These will damage the SEI film and accelerate the corrosion process of the LiFePO4 active material.
In the production process, in addition to the production process affecting battery performance, the main factors causing the failure of LiFePO4 power batteries include impurities in raw materials (including water), the formation process, and the production environment. Therefore, factors such as material purity, control of environmental humidity, and formation methods are of crucial importance.
