Experimental Study on Silicon Nitride–Based Composite Phase‐Change Materials for Thermal Management of Retired Power Battery Modules

The thermal safety of power batteries has become a critical technical challenge restricting the development of the new energy automobile industry. Hence, an efficient battery thermal management system (BTMS) is essential. Cooling strategies based on phase‐change materials (PCMs) have emerged as highly competitive cooling solutions due to their unique advantages. As heat transfer media, the performance parameters of PCMs directly influence battery modules’ thermal management and safety performance. To enhance the thermal conductivity, electrical insulation, and mechanical strength of pure paraffin (PA) in practical applications, silicon nitride (Si 3 N 4 )–based composite PCMs (CPCMs) were proposed in this study. The application of these materials for power battery thermal management was systematically studied. PA served as the PCM matrix, and Si 3 N 4 was employed as the thermal and electrical insulation additive. Additionally, expanded graphite (EG) was utilized as the three‐dimensional thermal skeleton and high thermal conductivity additive, and epoxy resin (ER) as the strengthening component. A creative PA/EG/Si 3 N 4 CPCMs possessing different proportions was developed. The effects of Si 3 N 4 mass ratio on melting point, latent heat, thermal conductivity, thermal stability, volume resistivity, and mechanical properties were systematically analyzed. Furthermore, the root causes of these macroscopic performance changes were identified through scanning electron microscopy (SEM) and X‐ray diffraction (XRD). Combined with the results of macroscopic properties and microscopic characterization, CPCM‐5 (containing 8 wt% Si 3 N 4 ) was identified as the optimal composition. To verify its thermal management effectiveness, the decommissioned 18650 Li(Ni 0.8 Co 0.1 Mn 0.1 )O 2 /C (NCM811) lithium‐ion power module was utilized as the subject of this research. Three thermal management methods containing natural convection, CPCM‐1 (0 wt% Si 3 N 4 ) and CPCM‐5 (8 wt% Si 3 N 4 ) were applied to battery modules to evaluate and compare thermal management performance. The results indicated that the CPCM‐5 cooling system’s maximum temperature was 48.8°C during the 2.0 C uninterrupted charge–discharge cycle, which was 20.7 and 7.9°C lower than those of the natural convection and CPCM‐1 cooling systems, respectively. The maximum temperature difference of the CPCM‐5 system was 1.3°C, whereas those for the natural convection and CPCM‐1 systems were 3.6 and 2.4°C, respectively. CPCM‐5 was demonstrated to maintain battery temperature within the optimal range even under high discharge rate conditions. Thus, the Si 3 N 4 –enhanced CPCM thermal management system demonstrated superior average temperature control and temperature uniformity. This study is expected to contribute novel insights and empirical evidence for improving the thermal safety performance of high‐energy density power and energy storage battery systems.