Battery separators have drawn considerable attention due to their vital roles for the safety and electrochemical performance of lithium-ion batteries. In this paper, the cellulose/polysulfonamide composite membrane was successfully fabricated from a mixture of microfibrillar cellulose and polysulfonamide via a facile papermaking process. And its potential application was explored as a high performance lithium-ion battery separator by characterizing their electrolyte wettability, heat tolerance, and electrochemical properties. Lithium cobalt oxide/graphite cells using the separator displayed better capacity retention ratios of 85% after 100 cycles and superior rate capability compared with those of a commercial polypropylene separator. Furthermore, the lithium iron phosphate/lithium half cell using cellulose/ polysulfonamide separator exhibited stable charge discharge capability even at 120 degrees C. It was demonstrated that the composite separator possessed an enhanced thermal dimensional stability. This research provides a promising new strategy for large-scale fabrication of high performance lithium-ion battery membranes.