The overcharge test is to simulate the potential safety hazards of the battery when the charger voltage detection is wrong, the charger fails or the wrong charger is used.

　　The thermal runaway caused by overcharge may come from two aspects: joule heat generated by current on the one hand, and reaction heat generated by side reaction of positive and negative electrodes on the other. When the battery is overcharged, the negative pole voltage gradually increases. When the amount of lithium removal from the negative pole is too large, the process of lithium removal becomes more and more difficult, which leads to a sharp increase in the internal resistance of the battery, so a large amount of Joule heat is generated, which is more obvious when charging at a large rate. The high voltage positive oxidant in the overcharged state emits a lot of heat, and the negative electrode will also react exothermically with the electrolyte when the temperature rises. When the heat release rate is greater than the heat dissipation rate of the battery, and the temperature rises to a certain extent, thermal runaway will occur.

　　Tobishim et al. studied the overcharge performance of aluminum shell square battery with LiCoO2 and LiMn2O4 as cathode materials respectively. The research results showed that the LiCoO2 cell would explode when charged to 10V at 2C current, while the LiMn2O4 cell did not smoke, fire or explode when overcharged at 2C/10V and 3C/10V respectively, and only bulged, which showed that Mn had better overcharge resistance than Co. Leising et al. studied the influence of different graphite proportion on the overcharge performance of LiCoO2 electric core. The results showed that the overcharge performance of the electric core mainly depends on the cathode material, and does not change with the increase of graphite content. This shows that the precipitation of lithium metal on the cathode during overcharge is not the key to affect the overcharge performance, but the thermal stability of LiCoO2 with excessive delithiation or the oxidation reaction of electrolyte on its surface.

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