An improved battery separator and an energy cell comprising the improved battery separator are provided. The improved battery separator comprises a porous membrane having at least one of the following properties when wet with electrolyte: has no or low volume; has no or low mass; soaks as much liquid electrolyte as possible; blocks or removes any harmful substances in the electrolyte; never melts at any high temperature; does not react with the cathode or the anode under any conditions; has a mechanical strength equal to or greater than steel; is an electronic insulator under any conditions; and blocks metal dendrite growth.