Electrospun polymer membranes have great potentials as separators for lithium-ion batteries due to their high porosity. However, electrospun membranes usually suffer from low mechanical strength because of weak bonding at inter-fiber junctions. In this study, high mechanical strength over 20 MPa is achieved for poly (vinylidene fluoride) (PVDF)/poly (acrylonitrile) (PAN) blend membranes at a ratio of 5/5 after modification by a PVDF dilute solution. Moreover, good dimensional stability at 200 °C is exhibited the modified blend membranes due to the presence of continuous PAN nanofibrils. Separators based on the modified blend membranes impart lithium-ion batteries with superior discharge capacity and cyclic performance.