(1) With the increase in iron powder injection amount, both the capacity retention rate and coulombic efficiency of the batteries showed a systematic decline, indicating that the degree of performance degradation is positively correlated with impurity content. Specifically, the battery group injected with 2 g of iron powder exhibited the most significant performance deterioration, with a sharp drop in capacity retention and coulombic efficiency, accompanied by a notable increase in impedance. This suggests that a higher content of metallic impurities strongly accelerates the electrochemical degradation of the battery. In contrast, the battery group injected with 0.5 g of iron powder showed a more gradual performance decline, reflecting a relatively slow degradation process. These results clearly demonstrate that a larger amount of introduced metal impurities leads to more severe negative impacts on battery performance and lifespan.
(2) Impedance testing revealed that iron powder injection directly affects the increase in total impedance of the battery, and the greater the injection amount, the more significant the impedance fluctuations. These changes are closely related to variations in the battery's electrochemical reaction rate, lithium-ion transport rate, and other parameters, further validating the significant impact of the micro-short circuit effect on battery performance.
(3) Disassembly and SEM characterization results further confirm that the micro-short circuit effect induced by iron powder injection leads to degradation of the battery’s internal structure. Particularly with higher injection amounts, significant burn marks and material corrosion were observed on the cathode materials of the battery. These results indicate that micro-short circuits not only cause performance degradation but may also induce irreversible damage to the battery's internal materials.
