Patent ID: 8802269

Claim:
A flat nonaqueous secondary battery, comprising: an exterior case having an opening, the exterior case serving as a positive terminal; a seal case fitted with the opening, the seal case serving as a negative terminal; an insulating gasket provided between the exterior case and the seal case, the exterior case being insulate from the seal case, an electrode assembly disposed in a space formed by the exterior case and the seal case, the electrode assembly including: a plurality of positive electrodes and a plurality of negative electrodes alternately stacked upon each other, each positive electrode including a positive electrode body and a positive electrode current collector tab protruding from the positive electrode body in planar view, the positive electrode current collector tabs connected with each other, an inner surface of the exterior case being electrically connected to the positive electrode current collector tabs such that an outer surface of the exterior case serves as the positive terminal, an inner surface of the seal case being electrically connected to the negative electrodes such that the seal case serves as the negative terminal; and separators made of microporous membrane of a thermoplastic resin, each of the separators being located between one of the positive electrodes and one of the negative electrodes, each separator being disposed to sandwich and cover one of the positive electrodes, the separator as sandwiching the positive electrode having a joint formed by welding together at least a portion of a periphery of the separator, the positive electrode current collector tab extending toward the outside of the separator, the exterior case serving as a positive terminal is electrically connected to the positive electrodes through the connected positive electrode current collector tabs, wherein, when the separator and the positive electrode are measured along a cross section in their thickness direction, a ratio A/B of a shortest distance A between an end of the joint of the separator, the end facing the positive electrode, and an outer peripheral surface of the positive electrode sandwiched by the separator to a thickness B of the electrode is not smaller than 1.