The microporous polyacrylonitrile (PAN) nonwoven separators have been developed by using electrospun nano-fibers with homogeneous diameter of 380 and 250nm. The physical, electrochemical and thermal properties of the PAN nonwovens were characterized. The PAN nonwovens possessed homogeneous pore size distribution with similar pore size to the conventional microporous membrane separator. Moreover, the PAN nonwovens showed higher porosities, lower gurley values and better wettabilities than the conventional polyolefin microporous separator. Cells with the PAN nonwovens showed better cycle lives and higher rate capabilities than that of a cell with conventional one. Any internal short circuit was not observed for the cells with the PAN nonwovens during charge–discharge test. Hot oven tests for the charged cells up to 4.2V have revealed that the PAN nonwoven was thermally stable at 120°C, but showed shrinkage of about 26% isotropically after the test at 150°C for 1h. The Celgard membrane showed uniaxial shrinkage of about 30% along the machine direction at 150°C for 1h.