Battery pack

A battery pack comprising:

• a battery element;
• a generator device having said battery element disposed on a flexible covering material, said flexible covering material being folded to cover said battery element, wherein three sides of said flexible covering material around said battery element are sealed;
• a rigid covering material for covering said generator device, said rigid covering material having an opening and being bonded to said flexible covering material of said generator device;
• and
• a cover fitted into the opening of said rigid covering material,
• wherein said rigid covering material has a four-layer structure having an outer packaging layer, a metal layer, an inner packaging layer, and a heat-bonding layer which are stacked on one another,
• wherein said inner packaging layer is bonded to said cover, and
• wherein said heat-bonding layer is at least one material selected from the group consisting of an ethylene-vinyl acetate copolymer, an ethylene-acrylic acid copolymer, an ethyl acrylate copolymer, a methyl acrylate copolymer, a methacrylic acid copolymer, a methyl methacrylate copolymer, polyacrylonitrile, an ethylene vinyl alcohol resin, a polyamide resin, a polyester resin, acid-modified polypropylene, and an ionomer.