A lithium secondary battery includes an electrode assembly having at least one tab which projects from one side of the electrode assembly, an external finishing material surrounding and sealing the tab except a distal end portion thereof and the electrode assembly, and polymer films interposed between the external finishing material and the tab to improve adhesion therebetween. When the tab has a thickness no less than a predetermined thickness, the tab is formed to have a sectional shape which ensures that an inside angle defined by one of upper and lower surfaces of the tab and an adjacent side surface of the tab becomes an obtuse angle, and the polymer films are formed to have protuberances on the inner surfaces thereof, so as to improve bondability between the tab and the polymer films.