Dynamic simulation of the charge-discharge characteristics of the sodium-sulphur cell

A dynamic model of the porous sulphur electrode including diffusion of the reactant is developed to simulate charge-discharge characteristics of the sodium-sulphur cell and transient behaviour of the cell after termination of operation. From the results of investigations the following are clarified: (1) sodium ion concentration on the solid electrolyte side is low during charge, on the other hand, it is high during discharge; (2) an insulating film of sulphur is formed at the surface of the solid electrolyte in the deeply charged stage. This results in an increase of the cell resistance and thus restricts charge acceptance; (3) the resistance in the single phase region is larger than that in the two phase region; and (4) open circuit voltage in the single phase region, observed after termination of operation, is not equal to the equilibrium value but gradually approaches it according to the concentration relaxation. These characteristics agree well with experimental results.