Battery separator is a crucial component of a lithium-ion battery (LIB); it affects the battery performance. However, the effects of the separator on commercial cylindrical LIBs have not been well studied using computational models. This paper presents a numerical study on the effects of separator design on the LIB performance. We developed a two-dimensional electrochemical-thermal coupled model for a 38120-type LiFePO4 LIB. Model results showed that separator thickness strongly impacted battery energy density: the battery energy density dropped from 148.8 to 110.6 W h/kg, while the separator thickness increased from 5 to 100 mu m. In addition, the mass transfer resistance of the separator increased with decreasing separator porosity, resulting in increased electrolyte concentration gradient. However, the correlation between separator porosity and electrolyte concentration gradient indicated that a separator porosity of 80% or greater contributed little to the resistance to mass transfer. Furthermore, the battery temperature rise and temperature difference dropped when both the separator thermal conductivity and heat capacity increased to 1 W m(-1) and 3500 J kg(-1) K-1, respectively.