The battery is often overcharged in the process of use. Relatively speaking, the overdischarge situation is less. The heat released during the overcharge or overdischarge process is easy to accumulate inside the battery, which will further increase the battery temperature. , affecting the service life of the battery and increasing the possibility of the battery catching fire or exploding. Even under normal charge-discharge conditions, as the number of cycles increases, the capacity inconsistency of the single cells inside the battery system will increase, and the battery with the lowest capacity will experience the process of overcharge and overdischarge.

Although the thermal stability of LiFePO4 is the best compared to other cathode materials under different charging states, overcharging will also cause unsafe hidden dangers in the use of LiFePO4 power batteries. In the overcharged state, the solvent in the organic electrolyte is more likely to undergo oxidative decomposition, and ethylene carbonate (EC) will preferentially undergo oxidative decomposition on the surface of the positive electrode in common organic solvents. Since the lithium intercalation potential (to lithium potential) of the graphite negative electrode 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 dendrites piercing the separator. The failure mechanism of lithium plating on the surface of graphite anode due to overcharge was analyzed. The results show that there is no change in the overall structure of the graphite negative electrode, but there are lithium dendrites and surface films. The reaction between lithium and the electrolyte causes the continuous increase of the surface film, which not only consumes more active lithium, but also allows lithium to diffuse into the graphite . The anode becomes more difficult, which in turn further promotes the deposition of lithium on the anode surface, resulting in a further decrease in capacity and Coulombic efficiency.

In addition to this, metal impurities (especially Fe) are generally considered to be one of the main reasons for battery overcharge failure. The failure mechanism of LiFePO4 power batteries under overcharged conditions was systematically studied. The results show that the redox of Fe is theoretically possible during overcharge/discharge cycles, 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+ are removed from the positive electrode. One side diffuses to the negative side, Fe3+ is finally reduced to Fe2+, and Fe2+ is further reduced to form Fe; during the overcharge/discharge cycle, Fe crystal dendrites will be formed on the positive and negative electrodes at the same time, which will pierce the diaphragm to form Fe bridges, resulting in microscopic changes in the battery. Short circuit, the obvious phenomenon that accompanies the micro-short circuit of the battery is the continuous increase of temperature after overcharging.

During overdischarge, the potential of the negative electrode will increase rapidly, and the increase of the potential will cause the destruction of the SEI film on the surface of the negative electrode (the part rich in inorganic compounds in the SEI film is more easily oxidized), which in turn will cause additional decomposition of the electrolyte , resulting in a loss of capacity. More importantly, the anode current collector Cu foil is oxidized. The oxidation product Cu2O of the Cu foil was detected in the SEI film of the negative electrode, which would increase the internal resistance of the battery and cause the capacity loss of the battery.

The overdischarge process of LiFePO4 power battery is studied in detail. The results show that the negative current collector Cu foil can be oxidized to Cu+ during overdischarge, and Cu+ is further oxidized to Cu2+, and then they diffuse to the positive electrode, and the reduction reaction can occur at the positive electrode, so that the Cu crystal dendrites It will form on the positive side, pierce the separator, and cause a micro short circuit inside the battery. Also due to overdischarge, the battery temperature will continue to rise.

Overcharging of LiFePO4 power batteries may lead to oxidative decomposition of electrolyte, lithium precipitation, and formation of Fe crystal dendrites ; while overdischarge may cause SEI damage, resulting in capacity attenuation, Cu foil oxidation , and even the formation of Cu crystal dendrites.

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