The failure principle of lithium iron phosphate battery
Lithium iron phosphate batteries are often overcharged in the process of use. Relatively speaking, there are fewer cases of overdischarge. The heat released during overcharge or overdischarge is easy to accumulate inside the battery. It will further increase the temperature of the battery, affect the service life of the battery, and increase the possibility of fire or explosion of the battery. Even under normal charging and discharging conditions, as the number of cycles increases, the capacity inconsistency of the single cells in the battery system will increase, and the battery with the lowest capacity will also undergo the process of charging and overdischarging.
Although LiFePO4 has the best thermal stability compared to other cathode materials under different charging conditions, overcharging will also cause LiFePO4 power batteries to be used The hidden dangers of insecurity in the process. In the overcharged state, the solvent in the organic electrolyte is more prone to oxidative decomposition. Among the commonly used organic solvents, will ethylene carbonate (EC) preferentially undergo oxidative decomposition on the surface of the positive electrode? Since the lithium intercalation potential of the graphite negative electrode (to the lithium potential) is very low, there is a great possibility of lithium precipitation in the graphite negative electrode. One of the main reasons for battery failure under overcharged conditions is the internal short circuit caused by lithium crystal branches piercing the diaphragm. The failure mechanism of lithium plating on the surface of the graphite negative electrode caused by overcharge shows that the overall structure of the graphite negative electrode has not changed, but there are lithium crystal branches and surface film. The reaction of lithium and electrolyte causes the surface film to increase continuously, which not only consumes More active lithium also makes it more difficult for lithium to diffuse to the negative electrode of the stoneware, which in turn will further promote the deposition of lithium on the surface of the negative electrode, resulting in a further decrease in capacity and coulombic efficiency. In addition, metal impurities (especially Fe) are generally considered to be one of the main reasons for battery overcharge failure. The oxidation-reduction of Fe during the overcharge/discharge cycle of lithium iron phosphate batteries is theoretically possible, and the reaction mechanism is given: when overcharge occurs, Fe is first oxidized to Fe2﹢, Fe2﹢ is further oxidized to Fe3﹢, and then Fe2﹢ and Fe3﹢ diffuse from the positive electrode side to the negative electrode side. Fe3﹢ is finally reduced to Fe2﹢, and Fe2﹢ is further reduced to form Fe; during the overcharge/discharge cycle, Fe crystal branches will form at the positive and negative electrodes at the same time, which will cause thorns. The Fe bridge is formed through the diaphragm, causing a micro short circuit of the battery. The obvious phenomenon accompanying the battery short circuit is the continuous increase in temperature after overcharge. During overdischarge, the potential of the negative electrode will rise rapidly, and the rise of the potential will cause the destruction of the SEl film on the surface of the negative electrode (the part of the SEl film rich in inorganic compounds is more likely to be oxidized), which will cause additional decomposition of the electrolyte , Resulting in a loss of capacity. More importantly, the negative current collector Cu box will oxidize. The oxidation product Cuo in the SEI film of the negative electrode will increase the internal resistance of the battery and cause the capacity loss of the battery. He et al. studied the overdischarge process of the LifePO4 power battery in detail. The results show that the negative current collector Cu foil can be oxidized to Cur during overdischarge, and Cu﹢ is further oxidized to Cu*, and then they diffuse to the positive electrode and can occur in the positive electrode. The reduction reaction is Cu﹢→Cu→Cu﹢2, so that Cu branches will be formed on the side of the positive electrode, which will pierce the separator and cause a short circuit inside the battery. Also, due to overdischarge, the battery temperature will continue to rise.
Overcharge of LiFePO4 power battery may cause electrolyte oxidative decomposition, lithium evolution, formation of Fe crystal branches; overdischarge may cause SEI damage, capacity degradation, Cu shrinkage oxidation, and even formation of Cu crystal branches .