A multilevel safety defense: A review of functional separators for thermal-runaway protection in lithium-ion batteries

Over the past decades, lithium-ion batteries have been widely deployed in electric vehicles and large-scale energy-storage systems owing to their high energy density and long cycle life. However, safety hazards triggered by thermal runaway have become increasingly prominent, severely restricting their further large-scale application. As a core component that physically separates the cathode and anode while maintaining normal ion transport, the separator is critical to battery operational safety, making the development of high-safety separators a pivotal strategy to mitigate thermal runaway risks. Despite considerable research progress, most existing studies focus on isolated functional modifications or specific failure events, lacking a systematic review covering the entire evolutionary chain of thermal runaway. Based on the staged evolution of thermal runaway—from early initiation and intermediate propagation to terminal combustion—this review establishes a three-level safety defense framework of prevention, blocking, and extinguishment, and systematically summarizes recent advances in functional separators for thermal-runaway protection, spanning thermal management, dendrite regulation, mechanical reinforcement, high-temperature stability, and flame-retardant designs. Finally, key challenges in this field are identified, and perspectives on future research directions are outlined.