Designing a Thermochemically Active Hydrogel With In Situ‐Formed Ceramic–Carbon Hybrid Barrier for Suppressing Thermal Runaway Propagation

Preventing thermal runaway propagation (TRP) is critical for the safety of lithium‐ion batteries (LIBs). While hydrogel interlayers actively cool through water evaporation, their mechanical reinforcement with conventional fillers dilutes the water‐rich matrix and reduces enthalpy. Here, we propose a paradigm‐shifting strategy by using boric acid (BA) as a thermochemically active cross‐linker in a poly(AMPS) hydrogel. BA reconstructs the hydrogel network through dual N─H⋯O hydrogen bonding and B⋯O═C coordination, simultaneously enhancing compressive strength by 567%. Upon thermal runaway, the composite provides dual‐mode protection: (1) synergistic heat absorption (volumetric enthalpy > 2000 J cm − 3 ) via water evaporation and BA stepwise dehydration, accompanied by flame‐inhibiting SO 2 release. (2) In situ formation of a highly insulating ceramic–carbon hybrid barrier. Post‐test analysis reveals a critical BA content of 25 wt%, where tetrahedral boron coordination (63.3% BO 4 ) maximizes covalent B─O─C bonds that chemically “weld” B 2 O 3 to carbon. This trend in BO 4 fraction with BA loading quantitatively explains the exceptional TRP suppression, reducing the adjacent cell's peak temperature by over 500°C. Beyond a high‐performance material, this work establishes a general design principle: employing reactive fillers that orchestrate both multi‐stage energy management and tailored barrier synthesis, paving the way for next‐generation battery safety materials.