A porous membrane based on poly(vinylidene fluoride) (PVdF) is prepared by the addition of a non-solvent before the phase-inversion step and it is stretched uni-axially to achieve high mechanical strength without sacrificing high ionic conductivity. The tensile strength of the PVdF membrane is 23 MPa without stretching, which is around twice that of a PVdF membrane made without addition of non-solvent before phase inversion. The former membrane can be stretched by up to 350% at room temperature and there by its tensile strength is increased to 126 MPa. In addition, the ionic conductivity is greatly enhanced due to an increase in porosity. The discharge capacity of the cell based on the stretched PVdF membrane is around 90% of the initial discharge capacity after 200 cycles.