THE IMPEDANCE OF SODIUM-SULFUR CELLS AT VERY LOW-FREQUENCIES

The impedance of sodiumsulphur cells as a function of state of charge has been measured in the frequency range 10-5 - 103 Hz. The impedance spectra of cells near the top of charge show a depressed semi-circle in the complex plane. As the single phase region is approached the impedance exhibits Warburg behaviour below 10 mHz, and below 0.4 mHz the impedance rises sharply. The rise in impedance at low frequencies is attributed to concentration polarisation of the sulphur electrode, which arises when the electrochemical reaction rate is controlled by the diffusion rate of sodium ions through the polysulphide electrode. This behaviour is analysed in terms of an equivalent circuit, and the significance of the resulting values for the parameters of the circuit is discussed. © 1990.