Sodium-ion-conductivity of high-density Na₅YSi₄O₁₂ and Na₅GdSi₄O₁₂ synthesized by the microwave-heated reflux wet method

Abstract

Na₅YSi₄O₁₂ and Na₅GdSi₄O₁₂ are expected to serve as a chemically stable solid electrolyte for electrochemical devices. To improve its ionic conductivity, studies have been conducted on synthesis conditions aimed at increasing its density. In this study, anticipating that the precipitation of uniform, fine particles would lead to higher density, we synthesized Na₅YSi₄O₁₂ and Na₅GdSi₄O₁₂ using a wet chemical synthesis method involving microwave-heated circulation. These results clearly demonstrate that microwave heating is effective in increasing density and contributes significantly to improving ionic conductivity. In Na₅YSi₄O₁₂ synthesis, microwave heating was also effective in suppressing the formation of secondary phases; however, in the case of Na₅GdSi₄O₁₂, a single Na₅GdSi₄O₁₂ phase was obtained regardless of whether microwave heating was applied. Sintering at 1050 °C was necessary to achieve high density in Na₅YSi₄O₁₂ (90 %). Na₅GdSi₄O₁₂ exhibited a high density of 94 % at 900 °C but melted at 1050 °C. As an attempt to increase the density of Na₅YSi₄O₁₂ through low-temperature sintering, its solid solution, Na₅Y_0.5Gd_0.5Si₄O₁₂, was sintered at 1000 °C. It exhibited a density of 96 %, with an ionic conductivity of 1.7 × 10⁻³ S/cm at 30 °C and an activation energy of 0.28 eV.