The electrode was prepared by 140 mg Na 3 Fe 2 Li 3 F 12 mixed with 50 mg Super C65, 10 mg Knyar 761 (PVDF) and 0.95 ml N-Methyl-2-pyrrolidon. The obtained slurry was cast on a Cu-foil with a thickness of 100 µm and dried at 60°C for 12 h in a climate chamber. 10 mm diameter electrodes were stamped out of the copper foil and were dried again under reduced pressure of (10 -4 mbar) for 12 h. Free-standing carbon electrodes were prepared by mixing 90 wt% of the microporous YP80F carbon (from Kuraray) with 5 wt% carbon black (C65 from Imerys), and 5 wt% of polytetrafluoroethylene (60% dispersion in water from 3 M Chemicals) in isopropanol.