Causes of lithium iron phosphate battery failure
In the production process, personnel, equipment, raw materials, methods and environment are the main factors affecting product quality, and the production process of LiFePO4 power battery is no exception, personnel and equipment belong to the category of management, so we mainly discuss these three influencing factors .
First, the impurities in the electrode active material caused by the failure of the battery
LiFePO4 in the synthesis process, there will be a small amount of Fe2O3, Fe and other impurities, these impurities will be reduced on the surface of the negative electrode, may pierce the diaphragm to trigger an internal short circuit. liFePO4 long exposure to air, moisture will cause the battery deterioration, the initial stage of aging material surface formation of amorphous iron phosphate, its local composition and structure are similar to LiFePO4(OH) ; with the embedding of OH, LiFePO4 is continuously consumed, which is expressed as an increase in volume; after which recrystallization slowly forms LiFePO4(OH). And Li3PO4 impurities in LiFePO4 is shown to be electrochemically inert. The higher the impurity content of the graphite negative electrode, the greater the irreversible capacity loss caused.
Second, the failure of the battery caused by the chemical formation method
The irreversible loss of reactive lithium ions is firstly reflected in the lithium ions consumed during the formation of the solid electrolyte interface film. It is found that increasing the formation temperature will cause more irreversible lithium ion loss because the proportion of inorganic components in the SEI film will increase when the formation temperature is increased, and the gas released during the transition from organic component ROC2Li to inorganic component Li2CO3 will cause more defects in the SEI film, through which the solventized lithium ions will be embedded in the graphite cathode in large quantities.
In the formation, the composition and thickness of the SEI film formed by low current charging is uniform, but time-consuming; high current charging will cause more side reactions to occur, resulting in increased irreversible lithium ion loss and increased anode interface impedance, but time saving; nowadays, the formation mode of low current constant current - high current constant current constant voltage is more commonly used, which can take into account the advantages of both.
Third, the failure of the battery caused by moisture in the production environment
In the actual production, the battery will inevitably come into contact with air, as the positive and negative materials are mostly micron or nano particles, and the solvent molecules in the electrolyte have large electronegativity carbonyl and sub-stable carbon-carbon double bonds, which are prone to absorb moisture in the air.
The reaction between water molecules and lithium salts in electrolyte (especially LiPF6) not only decompose and consume electrolyte (decompose to form PF5), but also produce acidic substance HF, and both PF5 and HF will destroy SEI film, and HF will also promote corrosion of LiFePO4 active material. Water molecules also partially de-lithiumize the graphite anode embedded with lithium and form lithium hydroxide at the bottom of the SEI film. In addition, the dissolved O2 in the electrolyte will also accelerate the aging of LiFePO4 cells.
In the production process, besides the production process affecting the battery performance, the main influencing factors causing the failure of LiFePO4 power cells include impurities in the raw materials (including water) and the formation process, so the purity of the material, the control of environmental humidity, the formation method and other factors appear to be crucial.