Professor Young Min Song of the School of Electrical Engineering has, in collaboration with Professor Hyeon-Ho Jeong of the Department of Electrical Engineering and Computer Science at GIST, developed a nano-optical temperature visualization sensor capable of detecting the risk of thermal runaway in real time below 80 °C, before a battery reaches dangerous levels, and intuitively warning users. This breakthrough is expected to preemptively identify internal abnormalities in batteries and prevent serious accidents such as fires and explosions.
Batteries are indispensable energy sources for advanced technologies such as electric vehicles, wearable devices, and urban air mobility (UAM). However, safety concerns have grown as fire and explosion accidents caused by thermal runaway continue to occur. In particular, once the internal temperature of a battery exceeds 80 °C, key components such as the electrolyte and separator begin to degrade, and the temperature can rapidly surge to over 500 °C within one minute. This makes early detection and warning technologies urgently necessary.
Existing temperature sensors can only measure the area directly contacted by the thermocouple*, making it difficult to determine the overall temperature distribution. Infrared cameras also suffer from measurement accuracy limitations depending on the surface material, while technologies utilizing thermochromic materials have slow response times, making them unsuitable for real-time detection. * Thermocouple: A sensor composed of two dissimilar metal wires joined together. It detects temperature by measuring the voltage change caused by the temperature difference at the junction.
The research team focused on the unique optical modulation properties of tellurium (Te), a single-element material reported in the 1960s, and developed a thermochromic nanophotonic device using a 10-nanometer ultrathin tellurium film. Tellurium partially transitions from a solid to a quasi-liquid* state as the temperature rises from room temperature to 80 °C, and during this process, it exhibits excellent optical modulation characteristics with a refractive index change of more than 0.7 in the visible light spectrum. This enables ultra-fast temperature detection on the order of 100 millionths of a second. * Quasi-liquid: A state between a solid and a liquid, characterized by partial fluidity as temperature rises, where solid and liquid phases coexist.
To realize this, the researchers fabricated a Gires–Tournois resonator* by depositing a 10-nanometer-thick tellurium layer onto the surface of an aluminum-based battery and laminating it with a glass(SiO₂) protective layer several tens of nanometers thick.* Gires–Tournois resonator: An optical device that reflects light of a specific wavelength and induces a phase shift. It uses a thin-film structure to control light interference, allowing precise modulation of reflectance and color.
This design maximizes tellurium’s optical property changes arising from its solid-to-quasi-liquid phase transition even at relatively low temperatures below 80 °C, while the glass layer protects the tellurium from environmental degradation. The device functions without complex circuitry or an external power source, and it shows reversible behavior—its color changes when a certain temperature is reached and returns to the original color upon cooling.
The fabricated nano-optical device can precisely distinguish temperature variations between 25 °C and 80 °C through visible color change and demonstrates performance comparable to commercial thermocouples. It can also visualize the temperature distribution and heat diffusion process on a battery surface in real time with a rapid frame interval of 17 milliseconds. Furthermore, it has proven stable through dozens of heating–cooling cycles and under varying humidity conditions, maintaining its thermochromic characteristics for over nine months.
The team successfully applied the device to commercial 18650 batteries and smartphones, monitoring heat generation during charging and discharging in real time and thereby demonstrating its practical applicability. The sensor can be directly deposited on battery cells or simply attached with tape, making it easy to integrate into industrial environments. Moreover, users can readily check battery temperature with a smartphone or digital camera without specialized equipment or expert analysis, highlighting its high commercialization potential.
Professor Young Min Song emphasized, “With the recent series of battery fire incidents both domestically and internationally, ensuring safety has become increasingly important. This technology presents a new paradigm for next-generation battery safety and is expected to contribute to addressing social challenges.”
This research was supported by the Ministry of Science and ICT and the National Research Foundation of Korea through the Excellent Young Researcher Program, the Future Technology Research Lab Program, and the GIST–MIT AI International Collaboration Program. The results were published online in the renowned international materials journal Advanced Materials on July 23, 2025.
