Measurements of a.c. impedance of sodium-sulphur cells with symmetric and asymmetric resistance behaviour

Measurements of a.c. impedance in the frequency range 10 super(-2) Hz less than or equal to f less than or equal to 10 super(4) Hz were performed on laboratory sodium-sulfur battery cells with symmetric and asymmetric internal resistance behavior. The impedance spectra of the symmetric cells show only resistive behavior in the whole frequency range studied. The resistance determined from these spectra is equal to the bulk resistance of the solid electrolyte. In contrast, the impedance of the asymmetric cells is characterized by different high-frequency capacitive and low-frequency inductive behavior depending on the value of the d.c. current applied. In this case the impedance spectra are very similar to those often observed at metal passivation processes in electrolyte solutions. It is shown that the results obtained can be qualitatively interpreted by a model including the existence of a passivating film on the beta "-Al sub(2)O sub(3)/molten sodium interface. The modification of this film during the deposition or dissolution of the sodium can be assumed as a cause for the asymmetric behavior of the internal cell resistance.