Flexible secondary battery

A flexible secondary battery, comprising:

• an electrode assembly;
• a first external cover that is located on a first surface of the electrode assembly;
• and
• a second external cover that is located on a second surface of the electrode assembly, the second external cover and the first external cover being connected to each other by a hinge side, and the second external cover and the first external cover including a sealing portion where edges of the first external cover and the second external cover are attached to each other to seal the electrode assembly,
• wherein at least one of the first external cover and the second external cover has a three-layer structure formed of a first insulation layer, a second insulation layer, and a metal layer between the first insulation layer and the second insulation layer, the at least one of the first external cover and the second external cover including a stress-relief pattern in the metal layer of the at least one of the first external cover and the second external cover, the stress-relief pattern extending across the at least one of the first external cover and the second external cover in a center portion, the stress-relief pattern including patterns in a form of wrinkles that relieve a compression stress or an expansion stress when the at least one of the first external cover and the second external cover of the flexible secondary battery is bent concavely or convexly, the stress-relief pattern extending in a direction perpendicular to a length direction of the flexible secondary battery and parallel to the hinge side connecting the first and second external covers.