With the rapid and large-scale popularization of electric vehicles, people have higher and higher requirements for the convenience of travel, which puts forward higher requirements for the charging time of electric vehicles. Increasing the charging rate is an effective way to shorten the charging time. However, large-rate charging not only rapid decay of capacity but also leads to a serious decline in battery safety. In this article, the deep aging of lithium-ion batteries under the condition of large rate charging is concluded, and the discharge heat generation, heat resistance, and overcharge resistance of batteries in the process of adiabatic are involved, and the evolution mechanism of battery safety is revealed through characterization tests.
Influence of deep fast charge aging on battery safety
In the adiabatic discharge process, the total heat release during the adiabatic discharge process is lower than that of fresh batteries due to the severe decay of the capacity of the lithium-ion battery after deep aging. However, large-rate charging leads to a significant increase in the impedance of the battery, and the temperature rise rate of the battery during the discharge process increases significantly.
In the adiabatic thermal runaway test, compared with fresh batteries, the self-generated thermal onset temperature and thermal runaway trigger temperature of aging batteries are severely reduced, which means that the thermal stability of the battery is severely reduced. In addition, the maximum temperature and maximum temperature rise rate of the battery thermal runaway decrease, which is caused by the loss of active reactive substances due to high-rate charging. Further, by testing the thermal runaway of the half-cell, it is found that the thermal stability of the anode is severely reduced by large rate charging, and the change of thermal runaway trigger temperature is the result of the joint action of anode-electrolyte and anode-cathode reaction and changes the trigger mode of internal short circuit.
In the overcharge process, due to the loss of the anode capacity of the battery, due to the large rate charge, the number of lithium ions that can be embedded is reduced, and the polarization of the aging battery is serious, so the initial voltage of overcharge of the aging battery is high, while lithium precipitation occurs earlier, and the lithium precipitation potential shifts to the low voltage. After the inflection point voltage, as the overcharge progresses, the voltage continues to rise until it reaches the voltage plateau. Due to the degradation of the active substances inside the battery during the aging process, the reaction heat release is low, and the duration of this stage is longer than that of fresh batteries. After that, to the stage of thermal runaway, the duration of aging batteries is significantly shorter than that of fresh batteries. High-rate charging reduces the active material at the positive electrode, forcing a reduction in lithium ions intercalated from the positive electrode.
During this process, lithium-related side reactions and electrolyte decomposition occur violently, releasing a large amount of heat, and resulting in thermal runaway triggers. The large rate charge significantly reduces the thermal runaway trigger temperature of the battery and reduces the resistance of the battery. Further, the heat release of the battery is analyzed. Due to the large rate charge, the resistance of the battery decreases, and the energy required to trigger thermal runaway decreases. Moreover, the large rate of charging makes the side reaction heat production of the battery in the overcharge process more serious, and the contribution ratio of side reaction heat release to thermal runaway trigger increases significantly.
Through the disassembly characterization analysis of batteries, lithium precipitation is the main cause of battery degradation during large-rate charging. The precipitated lithium metal reacts with the electrolyte to thicken the SEI film. In this process, the consumption of electrolytes and the thickening of the SEI film significantly increase the resistance of lithium-ion transport, and the impedance of the battery increases significantly. Therefore, the temperature rise rate of aging batteries during adiabatic discharge is significantly higher than that of fresh batteries.
In addition, the thickened SEI film has low thermal stability, and the decomposition of the SEI film also contributes to the increase of temperature rise rate in the later stage of discharge. During the adiabatic thermal runaway, the decrease in the thermal stability of the SEI film leads to a significant decrease in the starting temperature of the battery’s self-generated heat. Moreover, the thermal stability of the side reaction products is low, the activation energy is reduced, and the thermal runaway of the battery is more easily triggered. However, in the process of large-rate charging aging, the consumption of active substances reduces the maximum temperature and temperature rise rate of thermal runaway and reduces thermal hazards. In addition, the large rate charge makes the lithium precipitation potential shift to the low voltage, the side reaction is easier to trigger, and the heat release is obvious. As a result, the proportion of heat of side reactions increased significantly for the triggering of thermal runaway. Ability to cause thermal runaway at lower temperatures.
Conclusion
Fast charge cycling can lead to severe degradation, which induces cell performance and safety to decrease significantly. Here in this article, we have concluded the findings of how fast charge aging of a battery can be dangerous.
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