During the charging and discharging processes, prismatic lithium-ion batteries undergo expansion, generating both reversible and irreversible expansion forces. Reversible expansion is primarily caused by the intercalation and deintercalation of lithium ions as well as temperature variations, while irreversible expansion results from factors such as the formation of the solid electrolyte interphase (SEI) film, deposition of lithium metal, and gas generation. Expansion forces can lead to various detrimental effects, including capacity degradation, reduced service life, and deformation of the battery module frame. A battery system is composed of modules formed by connecting multiple cells in series and parallel. The interaction between adjacent cells can subject the module to significant expansion forces, making it necessary to investigate the influence of module design parameters on expansion forces and lifespan. In this study, a 1P5S module composed of lithium iron phosphate (LFP) batteries was selected as the research subject to examine the impact of different design parameters on the expansion forces and lifespan of the battery system. The relationships between factors such as the thickness of the foam on the end-plate side, the type of foam, and the attachment method of the foam, and the expansion forces and lifespan of the module were tested and analyzed. An orthogonal experimental design was employed to analyze the effects of multiple coupled factors, including the thickness of the foam on the end-plate side, the thickness of the foam between cells, and the type of foam, on the module parameters. The results indicate that during the module design process, foam with lower hardness, greater thickness, and attached to the large surface should be selected whenever possible. The type of foam has a significant impact on the expansion forces of the module. For the module studied in this paper, the expansion force is minimized when the thickness of the foam between cells is 1.5 mm, the thickness of the foam on the end-plate side is 1.5 mm, and the foam material is melamine.