A process is disclosed for producing a non-woven, boron nitride-bonded boron nitride fiber mat, suitable for use as an electric cell separator in a lithium-sulfide battery. A web having a combination of structural boron nitride (BN) fibers and interstitial, thermally bondable boron oxide (B.sub.2 O.sub.3) fibers is passed through the nip of a pair of cooperating calender rolls at an appropriate temperature and pressure to soften the boron oxide binder throughout the web to fuse the BN fibers together. The interstitial boron oxide then is heated in an anhydrous ammonia atmosphere to convert it into boron nitride.