Effect of MnO2 Dopant on Properties of Na+-β/β"-Al2O3 Solid Electrolyte Prepared by a Synthesizing-cum-sintering Process

Abstract

In order to simplify the complexity of the conventional solid-state reaction process, Na⁺-β/β″-Al₂O₃ as a fast Na⁺-ionic conductive solid electrolyte was fabricated using a synthesizing-cum-sintering process combined with the double-zeta method, which is able to distribute a small amount of Li₂O more homogeneously in the Na₂O-Al₂O₃-Li₂O system. Additionally, in order to enhance the ionic conductivity, MnO₂ was used as a dopant to increase the Na⁺-ion concentration on the conduction plane in the Na⁺-β/β″-Al₂O₃ crystal structure. The relative sintered density increased with the synthesis temperature, ultimately reaching 99.7 % after synthesis at 1400 °C. The phase formation showed an overall β″-phase fraction over 90 %. The addition of MnO₂ had a positive effect on the phase formation, but a negative influence on the relative density resulting from the grain growth promotion effect. The highest ionic conductivity was observed at
1.74 × 10^-1 S/cm (350 °C) for the sample sintered at 1600 °C with 0.5 wt.% MnO₂.