Numerical Simulation‐Guided Assessment of Modern Composite Phase-Change Materials for Thermal Regulation of 21700 Lithium-Ion Battery Modules under High-Rate Operation

• Simulated four advanced CPCMs for BTMS in 21700 Li-ion cells under 3C–4C discharge. • CPCM-4 achieved lowest T max (324.9 K) with strong heat absorption and phase stability. • CPCM-1 offered best ΔT control (1.5 K) due to high thermal conductivity and latent heat. • Trade-offs between conductivity and latent heat dictate CPCM suitability for BTMS. • A transient ANSYS Fluent 3D model validated against a published experimental data for reliable CPCM analysis. Effective thermal management is required to ensure safe operation of lithium-ion battery modules under high-rate discharge. In this study, a numerical model is developed to compare the thermal behaviour of a 21700 lithium-ion battery module equipped with different phase change materials under identical operating conditions. The model couples battery heat generation with solid–liquid phase change and is validated using published single-cell experimental data. Four composite phase change materials (CPCMs) are considered, each representing a different design emphasis: CPCM-1 is a conductivity-enhanced composite, CPCM-2 is a flame-retardant formulation with reduced latent heat, CPCM-3 is an elastomer-modified composite intended to improve structural stability, and CPCM-4 is a bio-reinforced expanded-graphite composite. Pure paraffin is included as a reference material. The materials are evaluated based on maximum cell temperature (T max ), temperature difference within the module (ΔT), and liquid volume fraction (LVF) evolution during discharge. The results show that CPCM-1 and CPCM-4 provide more balanced thermal regulation by limiting peak temperature and maintaining lower temperature gradients over the discharge period. CPCM-2 exhibits faster initial heat dissipation but limited thermal buffering, while CPCM-3 shows slower heat spreading associated with its lower thermal conductivity. The comparison highlights how differences in thermal conductivity, latent heat, and phase transition range influence the effectiveness of CPCMs for passive battery thermal management.