Abstract

This study presents a numerical investigation of a slit fin-enhanced phase change material (PCM)-based battery thermal management system (BTMS) for an 18650 cylindrical LiNixCoγMnzO2 lithium-ion battery. The proposed design modifies the conventional solid rectangular external fins by introducing four longitudinal slit fins with uniformly distributed rectangular through-thickness slot cutouts along the fin height. This modification increases the PCM-fin interfacial contact area and creates additional natural convective heat dissipation pathways from the PCM region to the ambient environment while maintaining the same BTMS envelope, PCM thickness, fin count, housing geometry, and material selection as the validated rectangular-fin baseline. The lumped-capacitance thermal model was used for battery heat generation, while the enthalpy-porosity approach was employed to model PCM melting. Simulations were performed in ANSYS Fluent 2024/R2 at 1C, 3C, 5C, and 7C discharge rates at an ambient temperature of 308.15 K. Paraffin wax PCM with a latent heat of approximately 240,000 J/kg was used. The rectangular fin model was first validated against the baseline study, achieving an average cell wall temperature error of 1.03% and a maximum error of 1.47% at 5C, while the total temperature and liquid fraction deviations remained below 0.73%, confirming the reliability of the numerical model. Mesh independence and temporal convergence studies further confirmed that the selected 0.50 mm polyhedral mesh and 0.5 s time step provided accurate and stable results. The results demonstrate that the slit fin geometry provides metric-dependent improvements in PCM utilization, thermal protection duration, and high-rate latent-heat activation rates. At 1C, both configurations remained well below the 318.15 K safety threshold, but the slit fin configuration maintained approximately 0.7 K lower total temperature at 2500 s and delayed PCM melting by about 300 s compared with rectangular fins, preserving more latent heat capacity for later thermal loading. At 3C, the slit fin design extended the thermal protection duration from 1650 s to 2500 s, corresponding to a 51.5% improvement, and increased PCM latent heat utilization from LF = 0.42 to LF = 0.49, representing a 16.7% increase. At 5C, slit fins initiated PCM melting approximately 3.5 times earlier, around 100 s, compared with 350–400 s for rectangular fins, and reached LF = 0.50 at 620 s, whereas rectangular fins reached only LF = 0.37 at 1480 s. This corresponds to approximately 2.87 times faster PCM utilization and 35.1% greater PCM melting. At 7C, the slit fin system again showed stronger PCM engagement, corresponding to 35.7% greater PCM utilization. Temperature and liquid fraction contours confirmed that the slit openings intensify localised PCM melting near the heat source, improve heat spreading through the PCM domain, and support natural convection-assisted melting. Overall, the slit-fin geometry provides a geometry-based enhancement for PCM utilization and thermal protection without changing system size or material selection for PCM-based BTMSs, improving latent heat utilization and thermal protection without increasing system size, PCM volume, or material complexity.

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