An electrode assembly and a lithium ion secondary battery using the electrode assembly having reduced winding thickness. Active materials are coated on a first region of a negative electrode collector to which a negative electrode conductive tab is welded, and on an adjacent region overlapping the first region. Active materials are also coated on a first region of a positive electrode collector, to which a positive electrode conductive tab is welded, and an adjacent region overlapping the first region. Parts of the electrode assembly corresponding to the conductive tabs do not protrude from the electrode assembly, allowing for easy insertion of the electrode assembly into a can or pouch.