The impact and diagnostic strategies of foreign matter defects in prismatic power batteries

• Evolution process of foreign matter defects in the lithium-ion cell is revealed. • Foreign matter defects of different materials, sizes, and positions are compared. • Self-induced stable internal short circuit of lithium-ion cells is reproduced. • A diagnostic method for ultra-early micro internal short circuits is proposed. Foreign matter defects remain a persistent challenge in large-scale battery manufacturing, posing substantial safety risks to lithium-ion power batteries. To examine the effects of realistic foreign matter defects in prismatic power batteries, this study intentionally introduced contaminants of varying sizes and materials during production-line assembly and monitored full-process data from manufacturing through cycle-life testing. By combining in-situ production monitoring with post-mortem disassembly and characterization at multiple stages, the evolution and safety implications of foreign matter defects in prismatic cells were elucidated. Experimental results demonstrate that foreign matter defects in prismatic power batteries can trigger micro internal short circuits characterized by stable leakage behavior in the late stage of the observed period, albeit with notable randomness in occurrence. Analysis of cycle charge–discharge data identified distinctive fault features suitable for early, high-reliability diagnosis. These findings provide mechanistic insights into contaminant-induced failure modes and support improved quality control and diagnostic strategies in battery production.