Polyimide (PI) nanofiber-based nonwovens have been fabricated via electrospinning for the separators of lithium-ion batteries (LIBs). Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and hot oven tests show that the PI nanofiber-based nonwovens are thermally stable at a high temperature of 500 °C while the commercial Celgard membrane exhibits great shrinkage at 150 °C and even goes melting over 167 °C, indicating a superior thermal stability of PI nanofiber-based nonwovens than that of the Celgard membrane. Moreover, the PI nanofiber-based nonwovens exhibit better wettability for the polar electrolyte compared to the Celgard membrane. The PI nanofiber-based nonwoven separators are also evaluated to have higher capacity, lower resistance and higher rate capability compared to the Celgard membrane separator, which proves that they are ideal candidates for separators of high-performance rechargeable LIBs.