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. The X-ray detectable element constitutes less than 0.1% by weight of the microporous membrane. The method for detecting the position of a separator in a battery includes the following steps: (1) providing a battery including an X-ray sensitive battery separator; (2) subjecting the battery to X-ray radiation; and (3) thereby detecting the position of said separator in said battery.