NOVELTY - The method for forming a battery separator comprises: introducing nanofibers to form nanofiber dispersion within an aqueous solvent; mixing nanofiber dispersion under high shear conditions; introducing microfibers to form microfiber/nanofiber dispersion within the aqueous solvent; introducing the highly sheared dispersion within a paper making machine; producing a web of microfiber/nanofiber material; and drying the web. The battery separator (18) has a thickness of 250 microns, an areal density of less than 20 g/m2 and a thermal shrinkage of less than 6%.    USE - The method is useful for forming a battery separator that is useful in an energy storage device formed from two electrodes and an electrolyte (all claimed), hybrid automobiles, rechargeable watch battery, cell phone and/or laptop, where: the separator is a single-layer lithium ion battery separator; and the energy storage device includes capacitors, super capacitors and ultra capacitors.    ADVANTAGE - The method is capable of easily, economically and efficiently forming the durable and reliable battery separator with high isotropic strength and wettability levels, uniform pore size, low shrinkage and improved chemical stability, structural integrity and dimensional stability thus preventing battery failure and improving energy loss and power discharge rate.    DETAILED DESCRIPTION - The method for forming a battery separator comprises: introducing nanofibers to form nanofiber dispersion within an aqueous solvent; mixing nanofiber dispersion under high shear conditions; introducing microfibers to form microfiber/nanofiber dispersion within the aqueous solvent; introducing the highly sheared dispersion within a paper making machine; producing a web of microfiber/nanofiber material; and drying the web. The battery separator (18): has a thickness of 250 microns, an areal density of less than 20 g/m2 and a thermal shrinkage, in 240 degrees C for one hour, of less than 6% in both the machine direction and the cross direction; and includes a combination of microfiber and nanofiber constituents. The web is treated in a calendaring procedure to produce a separator material exhibiting a thickness of less than 100 microns and a pore size of 2000 nm. The microfiber and nanofiber are fibrillated fibers, and island-in-the-sea nanofiber and microfiber. The length of microfiber is greater than 0.5 mm. The microfiber/nanofiber dispersion has a concentration of fiber solids of less than 0.5 wt.% of water. The paper making machine comprises a rotoformer, an inclined wire, and a Fourdrinier machine. The water content of the sheet is reduced using a vacuum unit. The paper making machine further comprises a woven paper machine wire for holding the fibers while removing water. The wire comprises a gauge, which is finer than 40 gauge. The calendar is heated at a temperature of greater than 200 degrees F. An average diameter of the microfibers is greater than 10 times an average diameter of the nanofibers. One fiber flows under high temperature and/or pressure and another fiber does not flow under the same temperature and/or pressure.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic exploded view of a rechargeable lithium ion battery including a battery separator.    Battery (10)    Housing (12)    Anode (14)    Cathode (16)    Battery separator. (18)