A model for internal pressurization in cationic solid electrolytes

Abstract

The possibility of internal pressurization in a cationic solid electrolyte is examined by analysing a three-compartment cell consisting of two solid electrolyte membranes with different ionic and electronic conductivities. The pressure generated in the central chamber is related to the ionic and electronic conductivities of the membranes, the thicknesses of the membranes as well as the current density. By identifying the appropriate equivalent circuit for the three-compartment cell, the transient problem of pressurization is also solved. Finally, the probability of degradation of sodium-β″-alumina used in Na-S cells is assessed using published values of electronic conductivity. The calculations indicate that the pressure generated can be large if the electronic conductivity of sodiumβ″-alumina in contact with molten sodium is greater than 10⁻³ (ohm m)⁻¹. Published work, however, indicates that the electronic conductivity at 350° C is ∼8×10⁻¹⁰ (ohm m)⁻¹. Therefore, degradation due to internal pressurization is unlikely. However, on the other hand, it is conceivable that certain impurities or stabilizing elements may impart higher electronic conductivities. Consequently, the chemical composition of sodium-β″-alumina may be an important factor in relation to long-term operation.