Thermostable polymers offer excellent thermal and mechanical stabilities, and in this study, an organosoluble polyimide (PI) was employed to fabricate high-performance separators for use in advanced lithium-ion batteries (LIBs). The organosoluble PI was synthesized and its nanofibrous membrane was fabricated via electrospinning. A subsequent heat treatment induced thermal crosslinking between the PI nanofibers to improve the heat resistance of the separators. PI nanofibrous membrane exhibited excellent wettability toward the liquid electrolyte, resulting in a greatly improved rate capability and cycle life. These results suggest that organosoluble PI nanofibrous membranes have a high potential for use as separators in high-performance LIBs.