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What are the reasons for the failure of lithium iron phosphate batteries?

by:Vglory      2021-04-07
The impurity in the active material of the electrode causes the battery to fail: During the synthesis of LiFePO4, there will be a small amount of Fe?O?, Fe2P, Fe, Fe4 (P2O7) 3 and other impurities. These impurities will be reduced on the surface of the negative electrode. It will pierce the diaphragm and cause an internal short circuit. When LiFePO4 is exposed to the air for a long time, moisture will deteriorate the battery and cause the aging mechanism: amorphous iron phosphate is formed on the surface of the material at the initial stage of aging, and its local composition and structure are similar to LiFePO4 (OH); with the embedding of OH, LiFePO4 is continuously consumed, showing an increase in volume; afterwards, it recrystallizes and slowly forms LiFePO4 (OH). The impurity of Li3PO2 in LiFePO4 is electrochemically inert. The higher the impurity content of the graphite anode (mostly Al and Fe), the greater the irreversible capacity loss (the smaller the reversible capacity loss). The failure of the battery caused by the formation method: the irreversible loss of active lithium ions firstly reflects the current lithium ions consumed in the process of forming a solid electrolyte interface membrane (SEl membrane). Studies have found that increasing the formation temperature will cause more irreversible loss of lithium ions, because when the formation temperature is increased, the proportion of inorganic components in the SEl film will increase, and it will be released during the transformation process from the organic component ROCOLi to the inorganic component LiCO. The gas will cause more defects in the SEI film, and a large amount of lithium ions solvated by these defects will be embedded in the graphite negative electrode. During the formation, the composition and thickness of the SEl film formed by low-current charging are average, but time-consuming; high-current charging will cause more side reactions to occur, resulting in increased irreversible lithium ion loss, and the negative electrode interface impedance will increase, but Time-saving; at present, the formation mode of small current constant current-large current constant current and constant voltage is used more frequently, so that the advantages of both can be taken into account. The use of electrochemical methods further proved 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 caused by moisture in the processing environment: In actual processing, the battery will be in contact with the air unsuspectingly, because the positive and negative materials are mostly micron or nano-scale particles, and the solvent molecules in the electrolyte are highly electronegative. Both carboxyl groups and meta-stable carbon-carbon double bonds can easily absorb moisture in the air. The water molecules react with the lithium salt (especially LiPF6) in the electrolyte, which not only consumes the electrolyte (analysis to form PF5), but also produces acidic substance HF. Both PF5 and HF will destroy the SEI membrane, and HF will also promote the corrosion of the LifePO4 active material. Water molecules will also delithiate the lithium-intercalated graphite negative electrode, forming a lithium hydroxide mesh at the bottom of the SEI film. In addition, O2 dissolved in the electrolyte will also accelerate the aging of LiFePO4 batteries. In the process of processing, in addition to the processing technology that affects the battery performance, the critical factors that cause the failure of LiFePO4 power lithium-ion batteries include impurities in the raw materials (containing water) and the formation process, so the purity of the material, the control of the environmental humidity, and the formation Factors such as methods appear to be crucial. Disclaimer: Some pictures and content of the articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous post: Analysis of the types of lead-acid batteries and daily maintenance
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