The instant application relates to an X-ray sensitive battery separator for a secondary lithium battery and a method for detecting the position of a separator in a secondary lithium battery. The X-ray sensitive battery separator includes a microporous membrane having an X-ray detectable element therein, thereon, or added thereto. The X-ray detectable element constitutes less than 20% by weight of the microporous membrane or separator. The method for detecting the position of a separator in a battery, cell, stack, jellyroll, can, or the like includes the following steps: (1) providing a battery, cell, stack, jellyroll, or the like including an X-ray sensitive battery separator; (2) subjecting the battery, cell, stack, jellyroll, or the like to X-ray radiation; and (3) thereby detecting the position of said separator in said battery, cell, stack, jellyroll, or the like.