Thermochromic Gires‐Tournois Resonators with Tellurium for Battery Thermal Runaway Warning

Abstract Effective temperature monitoring is crucial for preventing battery fires caused by thermal runaway, ensuring human safety, and providing timely warnings. While thermochromic materials offer intuitive, real‐time temperature visualization, their slow response times remain them unsuitable for battery monitoring. A thermochromic Gires‐Tournois (GT) resonator specifically designed for rapid and accurate battery temperature detection in the critical range below 80 °C is introduced, where thermal runaway risks can be effectively mitigated. Central to this design is an ultrathin (10 nm) thermo‐responsive tellurium film, paired with a protective glass layer and an underlying metallic mirror. This thermochromic GT resonator exhibits reversible temperature detection over multiple cycles, actively responding to temperature changes through partial melting of tellurium, which alters its complex refractive index—a property discovered in the 1960s but now harnessed for this novel application. Notably, the resonator monitors both specific temperature points and overall heat transfer across the battery surface, achieving sub‐second response times in an untethered manner. These findings position the thermochromic GT resonator as a promising platform for direct, intuitive, and compact temperature monitoring in energy storage systems.