A method for detecting the position of a separator relative to electrodes in a secondary lithium battery includes the steps of: providing a secondary lithium battery including a positive electrode, a negative electrode, a X-ray sensitive separator located between the electrodes, and a can or pouch housing the electrodes and separator, the X-ray sensitive separator comprising a microporous membrane having a X-ray detectable element dispersed therein or thereon, the X-ray detectable element comprising at least 2 and no greater than 20 weight % of the membrane; subjecting the secondary lithium battery to X-ray radiation; determining the position of the separator relative to the electrodes; and approving or rejecting the secondary lithium battery based upon the position of the separator relative to the electrodes.