Conversion Kinetics and Ionic Conductivity in Na-β-Alumina plus YSZ (Naβ AY) Sodium Solid Electrolyte via Vapor Phase Conversion Process

Sodium ion batteries have been receiving increasing attention and may see potential revival in the near future, particularly in large-scale grid energy storage coupling with wind and solar power generation, due to the abundant sodium resources, low cost, and sufficiently high energy density. Among the known sodium ion conductors, the Na-beta ''-alumina electrolyte remains highly attractive because of its high ionic conductivity. This study focuses on the vapor phase synthesis of a Na-beta ''-Alumina + YSZ (Na beta '' AY) composite sodium electrolyte, which has higher mechanical strength and stability than conventional single phase beta ''-Alumina. The objectives are the measurement of conversion kinetics through a newly developed weight-gain based model and the determination of sodium ionic conductivity in the composite electrolyte. Starting samples contained similar to 70 vol% alpha-Alumina and similar to 30 vol% YSZ (3 mol% Y2O3 stabilized Zirconia) with and without a thin alumina surface layer made by sintering in air at 1600 degrees C. The sintered samples were placed in a powder of Na-beta ''-alumina and heat-treated at 1250 degrees C for various periods. Sample dimensions and weight were measured as a function of heat treatment time. The conversion of alpha-Alumina in the alpha-Alumina + YSZ composite into Na beta '' AY occurred by coupled diffusion of sodium ions through Na-beta ''-alumina and of oxygen ions through YSZ, effectively diffusing Na2O. From the analysis of the time dependence of sample mass and dimensions, the effective diffusion coefficient of Na2O through the sample, D-eff, was estimated to be 1.74 x 10(-7) cm(2) s(-1), and the effective interface transfer parameter, k(eff), was estimated as 2.33 x 10(-6) cm s(-1). By depositing a thin alumina coating layer on top of the bulk composite, the chemical diffusion coefficient of oxygen through single phase Na-beta ''-alumina was estimated as 4.35 x 10(-10) cm(2) s(-1). An AC impedance measurement was performed on a fully converted Na beta '' AY composite, and the conductivity of the composite electrolyte was 1.3 x 10(-1) S cm(-1) at 300 degrees C and 1.6 x 10(-3) S cm(-1) at 25 degrees C, indicating promising applications in solid state or molten salt batteries at low to intermediate temperatures.

Automated summary

Sodium ion batteries have potential applications in large-scale grid energy storage and renewable energy generation due to their low cost and high energy density. This study focuses on the synthesis and characterization of a Na-beta ''-Alumina + YSZ composite electrolyte, which shows higher mechanical strength and stability compared to conventional materials. The results indicate promising applications in solid state or molten salt batteries at low to intermediate temperatures.