Study of Discharge Characteristics of Sodium/NiCl2 Cell Using Electrochemical Impedance Spectroscopy

High temperature Na/NiCl2 cells were studied using DC polarization and electrochemical impedance spectroscopy at 300°C. Both the DC and EIS measurements showed that the area specific resistance increased with the depth of discharge, and showed no dependency on discharge current density. The ohmic and polarization resistances of the cell could be separated successfully using two- and three-electrode impedance spectroscopy. The ohmic resistance did not show strong dependency on depth of discharge, while the polarization resistance increased significantly under the same conditions. The polarization resistances of the NiCl2 cathode and the Na anode were further separated using three-electrode impedance spectroscopy method. The polarization resistance of the NiCl2 cathode under low and intermediate depth of discharge could be interpreted based on the shell model, and agreed with the effective sodium transport resistance of the NaAlCl4 secondary electrolyte in the cathode. However, at higher depth of discharge, the cell polarization appeared to be dominated by other processes.