Heat‐Activated Micro‐Expansion Hierarchical Coatings for Fire and Grit Impact Resistant Protection of Lithium‐Ion Battery Enclosures

Lithium‐ion battery thermal runaway involves coupled jet flames and particle ejection, posing severe thermo‐mechanical threats to lightweight battery enclosures. Here, a heat‐activated micro‐expansion hierarchical coating is developed to provide durable protection under such extreme conditions. Unlike conventional intumescent or ceramifiable systems, the coating undergoes only moderate volumetric expansion (∼20%) and is observed to form a dual‐phase white‐black barrier after exposure. The flame‐facing surface is characterized by a dense silica‐rich shell that resists flame erosion and particle impact, while the inner region evolves into a porous ceramic‐carbon skeleton that is proposed to contribute to thermal insulation and mechanical buffering. As a result, the coating exhibits markedly reduced heat release in cone calorimetry and sustained protection in the stringent Torch and Grit (TaG) test, maintaining backside temperatures below 250°C for 10 min. Importantly, a real‐time demonstration shows that the battery maintains stable voltage output throughout testing. This work establishes a heat‐activated micro‐expansion hierarchical protection strategy to maintain stable electrical output and circuit continuity under extreme abuse conditions.