This study investigates the concealed effect of separator porosity on the electrochemical performance of lithium-ion batteries (LIBs) in thin and thick electrode configuration. The effect of the separator is expected to be more pronounced in cells with thin electrodes due to its high volumetric/resistance ratio within the cell. However, the electrochemical analyses show similar power performance regardless of the separator porosity in the thin electrode configuration. In contrast, for cells with thick electrodes, separator porosity significantly impacts the direct current-internal resistance (DC-IR) and the capacity retention at a high rate. This behavior is attributed to ion concentration gradients in the upper regions of thick electrodes, while Li+ transfer to lower regions is hampered as the electrode thickness increases. These findings suggest that the intrinsic properties of individual cell components, such as separator porosity, are highly dependent on the overall cell design. Moreover, while high-porosity separators enhance power performance, particularly in thick electrode configurations, they exhibit lower thermal stability and tensile strength. In conclusion, this study highlights the need for an integrated approach to optimizing separator characteristics, considering both electrochemical performance and safety trade-offs in LIBs.