Defects in lithium-ion batteries: From origins to safety risks

Lithium-ion batteries are currently the most widely used energy storage devices due to their superior energy density, long lifespan, and high efficiency. However, the manufacturing defects, caused by production flaws and raw material impurities can accelerate battery degradation. In extreme cases, these defects may result in severe safety incidents, such as thermal runaway. Metal foreign matter is one of the main types of manufacturing defects, frequently causing internal short circuits in lithium-ion batteries. Among these, copper particles are the most common contaminants. This paper addresses the safety risks posed by manufacturing defects in lithium-ion batteries, analyzes their classification and associated hazards, and reviews the research on metal foreign matter defects, with a focus on copper particle contamination. Furthermore, we summarize the detection methods to identify defective batteries and propose future research directions to address metal foreign matter defects.