Flat organic electrolyte secondary battery

A flat organic electrolyte secondary battery comprising:

• a negative electrode including an oxide capable of reversibly absorbing and desorbing lithium ions as a negative electrode active material;
• a positive electrode capable of reversibly absorbing and desorbing lithium ions;
• an organic electrolyte;
• a separator interposed between said negative electrode and said positive electrode;
• a sealing plate in contact with said negative electrode, said sealing plate serving as a negative electrode terminal;
• a positive electrode can in contact with said positive electrode, said positive electrode can serving as a positive electrode terminal;
• and
• a gasket interposed between said positive electrode can and said sealing plate, said gasket comprising tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin having a heat deformation temperature of 70° C. or more at a load of 0.45 MPa and a heat deformation temperature of 60° C. or less at a load of 1.82 MPa.