Abstract: A process is disclosed for producing a non-woven, boron nitride fiber mat, suitable for use as an electric cell separator in a lithium-sulfide battery. Molten boron oxide is centrifugally spun into strands and attenuated by an annular gas stream into fibers which are compacted at a controlled relative humidity into a mat. The mat of fibers passes through a needler, which repeatedly drives arrays of needles into the mat from either one or both sides, to reorient and intertwine the fibers, producing additional mechanical bonds thereamong. The needled mat then is heated in an anhydrous ammonia atmosphere to convert boron oxide in the fibers into boron nitride (BN). In an alternate embodiment the boron oxide fibers within the mat are converted into boron nitride before being passed through the needler.