Two-dimensional distribution of sodium polysulfide composition in sulfur electrodes of sodium-sulfur cells

Distributions of the sodium polysulfide composition in the sulfur electrode of the soldium-sulfur cell and the current density in the cell are investigated using a two-dimensional and two-component (ionic and electronic) conduction model. The following points are clarified from the results of calculations and experiences. (i) The polarization of a vertically supported cell at the end of charge start partially from the upper surface of the solid electrolyte due to the gravitational drop of the sodium polysulfide melt, but the polarization at the end of discharge is axially almost uniform. (ii) A duel mat design of which a porous graphite mat has a high electric resistance adjacent to the electrolyte is preferable for improving the rechargeability. (iii) An oriented mat of which the radial resistivity is smaller than the axial resistivity is able to reduce the cell resistance. (iv) The larger the current density is, the larger the nonuniformity of the melt composition becomes, and the sulfur utilization is reduced by the high current density operation (v) Voids of the melt at the electrolyte surface accelerate the degradation of the solid electrolyte and reduce the sulfur utilization. (vi) Voids at the metal container side accelerate the corrosion of the metal container.