Review—Understanding Thermal Runaway in Lithium-Ion Batteries: Trigger, Mechanism, and Early Warning Strategies

Safety issues have hindered the rapid development of lithium-ion batteries for use in energy storage and vehicles, especially the frequent battery thermal runaway (TR) accidents. The TR of lithium-ion batteries can result in fire and explosion. Understanding the thermal runaway mechanisms and triggers is key to optimizing early warning strategies. Here, we provide a comprehensive review from three aspects: trigger, mechanism, and early warning strategy. By analyzing typical incidents, both external abuses and internal defects are identified as key triggers of TR. The energy release mechanisms during TR are explored through multi-physics coupling models, leading to the development of a TR safety-phase diagram. The primary exothermic reactions and heat generation pathways are summarized, with a focus on the contribution of side reactions in various material systems. Furthermore, early warning strategies are reviewed, including single-signal and multi-physics characteristic signal analysis, highlighting the technical challenges for future TR safety predictions. This review enhances the understanding of TR mechanisms and is crucial for advancing battery safety.