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

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, Na5Y0.5Gd0.5Si4O12, 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.