Enhancing Safety in Lithium Batteries: A Review on Functional Separators Controlling Substance and Heat During Thermal Runaway

Abstract In past few decades, the rapid advancement of lithium battery technology has revolutionized our lives by powering portable electronic devices and vehicles. However, the increasing risk of thermal runaway (TR) poses significant challenges to their wider application, particularly regarding the detrimental effects on electric vehicles and large‐scale energy storage. The separator, serving as a barrier between cathode and anode, is critical in preventing active materials to generate Joule heat, thereby playing a vital role in ensuring safety. As the understanding of TR mechanism deepens, it is evident that numerous exothermic reactions between substances in the battery are closely linked to separator. Consequently, the functional design and development of thermal runaway‐blocking separators (TR‐blocking separators) are regarded as key strategies for mitigating TR. However, the intricate relationships between these reactions and TR remain poorly understood, making it challenging to gain progress in targeted design of functional separators. This review presents an in‐depth analysis on regulating the internal side reactions of batteries to minimize heat release during TR, and summarize the advancements of TR‐blocking separators in enhancing safety at different TR stages. Furthermore, insights into TR‐blocking separators are presented based on previous studies of various unfavorable exothermic reactions. Additionally, future directions are suggested for TR‐blocking separators.