Thermochemical Stability of β‐ and β″‐Alumina Electrolytes in Na/S Cells: II . Assessment

Abstract

The degradation of β‐ and β″‐alumina electrolytes in Na/S cells is assessed from a thermochemical viewpoint. Two probable schemes for the S‐side degradation are considered: (i) Na2O‐normalleaching and (ii) free Al2O3 liberation. The probable degradation mechanism at the Na electrode, considered in this assessment, involves reactions leading to the liberation of metallic Al. The assessment indicates that α‐ and β‐alumina with low Na2O activity are thermodynamically stable, while the other soda‐rich phases in the Na2O‐Al2O3 system are unstable to the Na2O leaching reactions. On the other hand, thermodynamic stability to the reaction with Na is favored by high Na2O activity and qualifies soda‐rich β″‐alumina and other soda‐rich compounds as stable phases. Thus, the existence of any phase in the Na2O‐Al2O3 system which would be stable at both the Na and the S electrode, is negated by the assessment.

Thermodynamic modeling of the (Al2O3+Na2O), (Al2O3+Na2O+SiO2), and (Al2O3+Na2O+AlF3+NaF) systems

“…Na 2 O activity data reported in the literature [24,29,62,71,[90][91][92][93][94][95][96][97][98][99][100][101][102][103][104] are mainly derived from electrochemical measurements using galvanic cells. Näfe [105] and Barsoum [106] presented comprehensive reviews of most of the experimental data obtained by emf measurement, detailed the measuring principle, summarized the different experimental set-up and presented possible explanations for the large discrepancy observed among the different studies.…”