Ester-based immersion coolant demonstrates superior safety for energy storage systems: a comprehensive comparative analysis

• EIC shows highest thermal stability and flash point (261.3°C), ensuring superior safety. • SIC forms a protective SiO 2 layer during combustion but emits heavy smoke. • MIC exhibits high flammability and rapid mass loss, posing significant fire risk. • Comprehensive safety evaluation ranks EIC as the optimal coolant for high-safety energy storage systems. With the increasing demand for high battery energy density and thermal management, immersion liquid cooling has emerged as a promising strategy, while the safety of coolants remains an area requiring further investigation. The study systematically compares the physicochemical properties, thermal stability, combustion behavior, and char residuals of mineral oil-based coolant (MIC), silicone oil-based coolant (SIC), and ester-based coolant (EIC). The results indicated that MIC exhibited a low flash point (199.2°C), poor thermal stability, and underwent dramatic combustion after ignition. Although SIC formed a protective SiO 2 layer during combustion that inhibited further combustion, it released tremendous smoke particles. In contrast, EIC demonstrated the highest initial decomposition temperature and flash point (261.3°C), combining low flammability with excellent thermal stability. A comprehensive safety evaluation revealed that EIC offered the best overall safety. This study provides valuable insights for coolant selection in safety-critical energy storage systems.