Evolution characteristics of metal foreign matter defects and their influence on the K-value of production lines
Defects arising during the battery manufacturing process can significantly compromise the safety of battery production. Among these, the intrusion of metal foreign bodies into the production line poses a risk of spontaneous internal short circuits (ISCs) or even thermal runaway. Despite the critical nature of this issue, research on the evolution mechanism of battery defects, particularly those involving small metal foreign bodies, remains scant. This study introduces hundred-micron diameter copper particles into batteries to simulate the intrusion of metal foreign bodies in the production line, thereby generating defective batteries. It then analyzes the ISC current characteristics of these batteries,disassembles them to examine the microstructure of the ISC regions, and develops a simulation model to assess the potential distribution within these regions. The study comprehensively elucidates the influence and mechanisms of defects on the critical detection indicator K-value (voltage drop rate) in the production line, with verification conducted on high-capacity batteries in an actual pilot line. The findings contribute to enhancing defect detection accuracy and preventing potential safety hazards. The results demonstrate that the intrusion of metal foreign bodies, such as copper particles, can precipitate ISC in two distinct modes: cathode-particle-anode and cathode-anode, each indicating different current paths and potential distributions. The ISC current generates a potential gradient in the cathode, reducing the particle's potential and hindering further dissolution. Consequently, the expansion of the ISC region ceases, stabilizing both ISC modes under K-value test conditions. The ISC severity is comparable across both modes, with current levels ranging between 0.1～1 mA. Given the stability of the ISC current, its impact on the K-value can be quantified via the differential voltage slope. The K-value increase attributable to ISC is inversely proportional to battery capacity; in a 100 Ah battery, a 1 mA ISC current has a limited effect on the K-value, increasing it by only 0.01 mV/h. To maintain defect detection precision on the production line, both the K-value detection threshold and the reference values for normal batteries should be adjusted downward as battery capacity increases.
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