An insulating (nonconductive) microporous nonwoven polymeric battery separator comprised of a single layer of enmeshed microfibers and nanofibers and supercalendered to extremely thin dimensions and high densities is provided. Such a separator accords the ability to not only attune the porosity and pore size to any desired level through a single nonwoven fabric, but provide further benefits in terms of further reduced pore size, reduced electrolyte level requirements, and reduced total volume of the subject battery cell itself. As a result, the inventive separator permits a high strength material with low porosity and low pore size to levels previously unattained. The separator, a battery including such a separator, the method of manufacturing such a separator, and the method of utilizing such a separator within a battery device, are all encompassed within this invention.