A fin-enhanced phase change material (PCM) system was introduced for cylindrical lithium-ion batteries. Experiments were performed to explore the performance of the systems during discharging. The working time of the PCM-Fin system is improved by 75%, 68%, and 61% compared to that of the system without fins under the heat production rate of 10, 12.5, and 15 W, accordingly. Simulations were performed by ANSYS Fluent to explore the influence of the geometric parameters (thickness, length, and number) and materials (nylon, titanium, steel, Al alloy, and copper) of the fins on the thermal performance. A function considering both the improvement in thermal performance and the increase in system weight was defined to assess the overall performance. Results indicate that fins made of Al alloy with the number of 8, a length of 7.5 mm, and a thickness of 0.5 mm give the best performance. Besides, the working time of the PCM-Fin system is 2150, 2490, 2940, and 3570 seconds for the coefficient of heat transfer of 5, 10, 15, and 20 W m(-2) K-1, respectively, which is increased by 14%, 32%, 56%, and 90%, compared to that of the adiabatic condition, demonstrating the effectiveness of the PCM-Fin system.