Superionically conducting β′′-Al2O3 thin films processed using flame synthesized nanopowders

Abstract

Commercial β′′-Al₂O₃ solid electrolytes for Na⁺ batteries are exclusively produced in tubular form, in part, due to processing difficulties. High sintering temperatures combined with Na₂O loss and excessive grain growth complicate processing of β′′-Al₂O₃ thin films (<100 μm) which could potentially reduce cell resistance, increase energy density, and even permit room temperature operations. In this study, we use high surface area flame made nanopowders at selected compositions to drive densification to produce β′′-Al₂O₃ thin films with controlled microstructures and Na₂O loss to maintain a high β′′-Al₂O₃ fraction. We show that addition of TiO₂ and ZrO₂ dramatically enhances the sintering kinetics, resulting in dense (>95%), thin (≤50 μm) β′′-Al₂O₃ films at the lowest sintering temperature ever reported (1320 °C/2 h). The sintered films also offer superionic conductivity (>1 mS cm⁻¹) at room temperature. Symmetrical cell (Na/β′′-Al₂O₃/Na) testing further comfirms its potential utility in Na batteries using β′′-Al₂O₃ solid electrolytes.