A glass fiber separator material is disclosed. The separator is composed of a mass of intermeshed glass fibers substantially all of which have a fiber diameter not greater than about 20 νm, and at least 5 percent w/w of which have a fiber diameter less than 1 νm, and, distributed through the glass fibers, and from 0.2 percent w/w to 20 percent w/w of cellulose fibrils. The fibrils are from a slurry having a Canadian freeness sufficiently low that the separator material has a tensile strength greater than on otherwise identical separator where glass fibers having an average diameter greater than 1 νm replace the cellulose fibrils.