Thermal management properties of BN/paraffin/expanded graphite composite phase change materials for cylindrical 21700 Li-ion battery

• Seven PA/EG/BN CPCMs were prepared using physical mixing and dispersion methods. • Battery thermal management performance of CPCMs is tested at two discharge rates under 35℃. • CPCM4 has excellent thermal stability and cycling properties at 2C under 35℃. • CPCM4 can maintain the battery surface temperature below 50.9℃ with a maximum temperature differential of just 3.2℃ at 3C under 35℃. Effective thermal management is critical for lithium-ion battery (LIB) performance, safety, and longevity in electric vehicles (EVs). This study develops novel composite phase change materials (CPCMs) using boron nitride (BN), paraffin (PA), and expanded graphite (EG). Following comprehensive characterization (SEM, XRD, DSC, TGA), 21700-type LIBs underwent charge/discharge tests at 2C/3C rates under 35 °C ambient conditions. Results show all CPCMs effectively regulate maximum temperature (T max ), maintaining it below critical thresholds (50 °C at 2C; 60 °C at 3C). The optimized formulation (1 wt% BN/89 wt% PA/10 wt% EG) demonstrates exceptional thermal regulation under harsh 3C discharge, limiting T max to 50.9 °C—merely 0.9 °C above ideal—representing 7.3 % and 25.6 % reductions versus BN-free PA/EG (90/10) and natural air cooling, respectively. Remarkably, this CPCM achieves superior thermal homogeneity with a maximum temperature differential (ΔT max ) of 3.2 °C, outperforming the BN-free composite and natural convection by 69.5 % and 75 %. Cycling tests confirm operational stability, maintaining consistent T max (52.3 °C) over six consecutive 3C cycles. These metrics reflect 24.2 % higher temperature regulation efficiency than conventional air-cooling systems, underscoring BN-enhanced CPCMs as high-performance, scalable solutions for next-generation EV battery thermal management systems (BTMS).