vulnerable surfaces of feedthroughs employed in electrochemical cells or batteries , particularly miniaturized , high energy density primary batteries for implantable medical devices , are provided with protective coatings to protect from degradation by the cell electrolyte or deposition of conductive materials bridging the feedthrough pin and ferrule . a liquid polyimide coating is applied to the vulnerable surfaces and cured into a substantially uniformly thick polyimide coating that tenaciously adheres to the vulnerable surfaces during subsequent welding and molding assembly steps . a further insulator is preferably molded in situ of a polymer adhere wells to the polyimide coating during molding . gaps between the insulator and the polyimide coating are advantageously minimized .