Breaking the Energy Cascades: Stage-Specific Mitigation of Thermal Runaway Propagation in Lithium-Ion Battery Systems

Thermal safety remains a critical challenge for lithium-ion battery systems in electric vehicles and large-scale energy storage systems. Thermal runaway propagation is a stage-evolving, multiphysics-coupled energy cascade governed by triggering pathways, heat-transfer modes, and combustion behavior. This review reorganizes passive mitigation strategies according to the dominant hazards in the early, intermediate, and late stages of thermal runaway. Thermal buffering, thermal blocking, and fire suppression are integrated into a single framework. Major triggering pathways, including overcharge, external heating, and internal short circuits, are first summarized, followed by analysis of the stage-dependent evolution of solid-state conduction, vent-driven convection, and radiative and flame-mediated transport during propagation. Representative protective materials, including phase change materials, low-thermal-conductivity barriers, and fire suppressants, are then critically evaluated in terms of mechanism, trade-off, and stage-specific function. Beyond conventional layered barriers, future protection systems should evolve toward integrated, multifunctional intelligent platforms combining phase-change buffering, high-temperature insulation, and event-triggered suppression. Coupled with artificial-intelligence-assisted design and digital-twin-enabled validation, such platforms offer a promising route toward scalable battery thermal-safety architectures.