Editor's note: The copper precipitation reaction in lithium-ion batteries typically occurs at the negative electrode. This phenomenon can be triggered by several factors, such as overdischarge, reverse charging during manufacturing, or large current discharges caused by local short circuits. These conditions create an environment where copper ions may dissolve from the copper foil and deposit on the electrode surfaces, leading to potential safety issues.
With the widespread use of lithium-ion power batteries, many electric vehicles equipped with these batteries have been in operation for seven to eight years. During this time, various failures can occur. One such failure is copper precipitation, which has been studied through experimental analysis. Based on the findings, recommendations have been made to prevent such issues.
As shown in the figure, copper has precipitated on both the positive and negative electrodes, indicating a severe case of copper evolution. The voltage of the affected battery dropped below 1V, signaling a complete failure.
The initial investigation suggested that large current overdischarge in individual cells might be due to inconsistencies in State of Charge (SOC). To verify this, a series of experiments were conducted. The procedure included:
1. Selecting three lithium-ion power batteries and setting their SOC to 40%, 60%, and 80% respectively.
2. Connecting them in series and performing large current discharge.
3. Allowing the batteries to rest periodically and measuring their voltage and internal resistance.
4. Repeating the process multiple times.
After several high-current cycles, the battery with the lowest SOC showed a significant voltage drop. An abnormal battery was then disassembled for further inspection.
Upon dissection, a partial short-circuit point was identified. EDS analysis of the separator at this location revealed the presence of copper, confirming that a copper precipitation reaction had occurred. This suggests that when there is SOC inconsistency during high-current discharge, copper deposition can take place in real-world applications.
The process of copper precipitation is complex and influenced by multiple factors. Therefore, it is crucial to enhance the consistency of lithium-ion power batteries and implement measures to prevent such failure modes from occurring.
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