Lithium-ion batteries (LIBs) have emerged among many types of rechargeable batteries as a potential candidate due to their high energy density, longer cycle life and range. The performance of Lithium-ion batteries is very sensitive due to high temperatures arising out of high charging/discharging and high ambient temperatures. Despite numerous efforts, effective thermal management solutions that ensure uniform temperature distribution and prevent thermal runaway remain a significant challenge, especially for prismatic batteries. In the present work, thermal management through passive mode employing phase change materials (PCM) with optimized cell pack geometries has been undertaken. Numerical studies through ANSYS have been done on a single cell and extended to a battery module with six cells. Computational fluid dynamics (CFD) based simulations were conducted exploring various PCMs, both organic and inorganic types, with varying thicknesses and under different ambient temperatures. Results indicate that for a pack of 6 cells under 8C discharge rate, optimal PCM thickness of 4 mm significantly reduces peak temperatures and enhances thermal uniformity across the battery pack. The optimized thickness with an inorganic PCM at an ambient temperature of 300 K demonstrated up to a 10% reduction in maximum temperature compared to conventional setups. Among the organic and the inorganic PCMs, the inorganic was found to be more effective. Furthermore, the impact of ambient temperature was found to be more severe.