Fractal Fin Topology Optimization for PCM-Based Battery Thermal Management with Built-in Thermal Runaway Protection

With the rapid growth of lithium-ion batteries in electric vehicles and energy storage systems, efficient and reliable thermal management is increasingly important. Phase change material (PCM)-based battery thermal management systems are promising due to their high latent heat and passive operation. However, most existing studies focus solely on enhancing heat transfer by adding fins, often overlooking the associated risks to thermal runaway safety. To address this issue, a density-based topology optimization framework combined with numerical simulation is proposed, in which tree-like fractal fins are embedded within the PCM domain to simultaneously enhance heat transfer and introduce geometric thermal isolation between cells. The effects of fin-to-PCM volume ratio and discharge rates are systematically investigated under both normal operation and thermal runaway conditions. Results show that under 3C discharge, the proposed structure reduces the average battery temperature by over 5.9% compared with conventional fin configurations. During thermal runaway, it reduces the maximum temperature of the adjacent cell from 130.4 °C to 121.9 °C, corresponding to a reduction of approximately 6.5% compared with the branched-fin configuration. In addition, a fin-to-PCM volume ratio of 0.3 achieves a near-optimal balance between heat conduction enhancement and latent heat storage capacity. This study provides a systematic design strategy for safer and more efficient PCM-based battery thermal management systems.