The present invention concerns a flat battery comprising a package formed by a cathode, an anode, and a separator layer sandwiched between the cathode and the anode, a sealing frame extending circumferentially around said package, a first current collector contacting the anode, and a second current collector contacting the cathode. The first and second current collectors each partly cover the sealing frame in a zone being adjacent to the package. According to the invention, the battery further comprises a first polymeric jacket layer being arranged on the first current collector and a second polymeric jacket layer being arranged on the second current collector, said first and second polymeric jacket layers extending circumferentially beyond the current collectors and beyond the sealing frame and being sealed together to form an outer jacket for the battery. Furthermore, the present invention also concerns a method to produce such a battery.