A battery separator is a porous membrane that separates the positive and negative electrodes while maintaining a good ionic flow. A method of making separators made of polyetherimide through a phase-inversion process is described. The membranes prepared by this process showed uniform porous structures as indicated by the scanning electron microscope (SEM) images. The membranes also exhibited satisfactory tensile properties and enabled high ionic conductance when saturated with a liquid electrolyte for lithium-ion batteries. Coin cells with this type of separators were prepared. Stable cycling performance and improved rate capabilities were observed when tested at room temperature.