MECHANISM OF THE CATHODE REACTION IN SODIUM FERROUS CHLORIDE SECONDARY CELLS

The electrochemical behaviour of iron electrodes in molten AlCl3 : NaCl saturated with NaCl has been studied in the temperature range 175-400-degrees-C. The results suggest that the electrochemical oxidation of iron occurs most readily in the presence of solid NaCl on the electrode, and that solid FeCl2 is reduced more readily than dissolved Fe(II) in this electrolyte. A predominantly solid state mechanism is thus indicated for the charge and discharge of FeCl2 cathodes used in high energy sodium cells which operate near 250-degrees-C. Results obtained in a cell containing no liquid electrolyte provide additional support for a solid state mechanism. The participation of the solid compound Na6FeCl8, which was previously reported to be an intermediate in the charge and discharge processes, has been confirmed, but it does not appear to have any significant adverse effect on the electrochemical performance. At high temperatures (300-400-degrees-C) dissolved reagents and products can contribute increasingly to the cathode reaction mechanism.