Abstract: Provided is a battery assembly that includes an electrically-conductive housing (510A, 510B), one or more battery modules (512) electrically coupled to a busbar (522), the one or more battery modules and busbar being received in the housing. A non-woven core layer (506) is disposed between the busbar and electrically-conductive housing, the non-woven core layer comprising a plurality of fibers, the plurality of fibers comprising 60-100 wt % of oxidized polyacrylonitrile fibers. The non-woven core layer can exhibit a breakdown voltage of at least 0.9 kV at ambient conditions after exposure to 500° C. for 5 minutes.