Abstract: Unavoidable electric vehicle collisions can cause defects in lithium-ion batteries (LIBs), and whether defective batteries after minor collisions can continue to be used is still unknown. In this work, we focus on the mechanical performance and electrochemical performance of defective batteries, safety boundaries, and its failure mechanism. Firstly, three typical defective cells , namely indentation, 50% offset compression and plate compression defect cells, are prepared by quasi-static loading and drop hammer impact with different indenters. These defective batteries did not exhibit voltage drops or temperature increases, indicating that no internal short circuits occurred. Subsequently, their mechanical and electrochemical responses were evaluated through quasi-static plate compression at a loading rate of 1 mm/min and 1C charge/discharge cycling, respectively. It was found that defective batteries exhibited significant deterioration in mechanical performance, including earlier onset of internal short circuit, reduced short circuit force, and decreased energy absorption capacity. Defective batteries also exhibited significant electrochemical performance degradation, with greater capacity loss during cycling compared to new batteries. Further, its degradation mechanism is explained through disassembling the cells. The separator of defective batteries exhibited significant thinning, making it more prone to rupture under secondary loading. Therefore, the mechanical failure criterion of the battery was proposed based on the separator thickness. After 500 cycles, graphite delamination was observed in the defective batteries, whereas the defective batteries without cycling only exhibited cracking. Therefore, the degradation of electrochemical performance in defective batteries is caused by the combined effects of initial defects and cyclic aging stress. The effects of loading speed and defect type on the performance of defective cells are also discussed. Defective batteries subjected to higher loading rates exhibit greater performance degradation, which is related to inertia effects. Different types of defects lead to variations in separator thickness and graphite delamination, resulting in different levels of degradation. Results are instructive for the study of safety identification and treatment of defective lithium-ion batteries.