Abstract: During the service life of lithium-ion batteries, the increase in cycle numbers and service time will lead to battery aging at varying degrees. Aged lithium-ion batteries are more prone to self-heating and thermal runaway (TR) under mechanical, electrical and thermal abuse conditions, with their thermal stability and safety performance both degraded. Therefore, to ensure the safety of lithium-ion batteries during operation, it is urgent to explore the degradation mechanism and analyze the differences in thermal runaway behaviors before and after aging. This paper reviews the relevant research on the degradation mechanism and thermal runaway of lithium-ion batteries in recent years. Firstly, it systematically introduces the capacity degradation laws and aging mechanisms of lithium-ion batteries under abuse conditions such as high/low temperature, overcharge and high charge-discharge rates via two aging pathways: cycle aging and calendar aging. Then, a comprehensive analysis of the thermal runaway mechanism of lithium-ion batteries is carried out, including the thermal runaway process behaviors and the main chemical reactions in each stage. Meanwhile, the research results of domestic and foreign scholars on the thermal runaway behaviors of lithium-ion batteries before and after aging under different aging pathways are emphatically compared, and the intrinsic causes leading to the differences in thermal runaway behaviors of lithium-ion batteries are explored. The deficiencies existing in the current research are put forward, and the research priorities in the future are prospected finally.

Key words: lithium-ion battery, aging, thermal runaway, safety, fire