In this study, as spun porous polyacrylonitrile (PAN) micro/nanofiber membranes were produced by electrospinning a ternary system of PAN/N, N-dimethylformamide (DMF)/water. The initiation of interconnected porous structure was due to the spinodal phase separation that happened during the electrospinning. In a further step, non-porous PAN micro/nanofiber membranes were prepared with a similar diameter. Afterward, these membranes were utilized as Li ion battery (LIB) separators. The impacts of membrane porosity on physical, mechanical and electrochemical properties of the porous PAN micro/nanofiber membranes have been discovered and compared with the non-porous PAN micro/nanofiber membranes and the commercial celgard polypropelene (Celgard PP) separators. The Brunauer-Emmett-Teller (BET) specific surface area of porous PAN micro/nanofiber membrane was 52.6 m(2) g(-1). This was 3 times higher than that of non-porous PAN micro/nanofiber membrane under the same conditions. The air permeability value of the porous PAN micro/nanofiber membrane was higher than the non-porous PAN micro/nanofiber membrane and the commercial Celgard PP separator because of higher porosity (83%). The results indicated that the electrolyte-soaked porous PAN micro/nanofiber membrane had greatest ionic conductivity due to considerable porosity and high electrolyte uptake. Also Li(Ni1,3Co1/3Mn1/3)o(2) (NCM)/separator/Li metal half-cell with porous PAN micro/nanofiber membrane had higher discharge capacity than the other cells (130 mAhg(-1) at C/10 rate).