Flexible secondary battery having a bonding portion extending in a lengthwise direction

A flexible secondary battery comprising:

• an electrode assembly;
• a first sealing sheet on a first surface of the electrode assembly, a portion of the first sealing sheet extending along a lengthwise edge of the electrode assembly and folded on itself such that an inner surface of the first sealing sheet faces away from the electrode assembly;
• a second sealing sheet on a second surface of the electrode assembly opposite to the first surface, a portion of the second sealing sheet extending along the lengthwise edge of the electrode assembly and folded on itself such that an inner surface of the second sealing sheet faces away from the electrode assembly;
• and
• a pair of third sealing sheets that respectively cover a pair of side surfaces of the electrode assembly,
• wherein an inner surface of one of the third sealing sheets facing the electrode assembly is bonded to the inner surfaces of the first and second sealing sheets at where they are folded on themselves to form a first bonding portion and a second bonding portion, respectively,
• wherein the first bonding portion and the second bonding portion overlap the electrode assembly at the edge of the electrode assembly, and
• wherein the inner surface of the first sealing sheet, the inner surface of the second sealing sheet and the inner surfaces of the thirds sealing sheets comprise the same material.