Effect of water/water vapor on microstructure and phase stability of (Y0.25Bi0.75)2O3solid electrolytes

Abstract

Highly conductive cubic (Y0.25Bi0.75)2O3tends to transform to rhombohedral (Y0.25Bi0.75)2O3when annealed at 600°C for more than 200 h. Although the rhombohedral phase of (Y0.25Bi0.75)2O3was known to be the stable phase at temperatures ≤600°C, it was found that the annealed (Y0.25Bi0.75)2O3was not thermodynamically stable in the water-containing environment. From x-ray diffraction and transmission electron microscopy analysis, it was observed that the annealed (Y0.25Bi0.75)2O3easily decomposed into monoclinic α-Bi2O3and yttrium hydroxide at a temperature as low as 50°C. The monoclinic α-Bi2O3further reacted with CO2and formed Bi2O2CO3. Consequently, the annealed (Y0.25Bi0.75)2O3degraded and became flaky powder. Scanning electron microscopy micrographs of water-reacted (Y0.25Bi0.75)2O3also showed surface swelling and peeling. Such surface deterioration was caused by a large volume increase during the water reaction. Similar reaction was also observed when the annealed (Y0.25Bi0.75)2O3was exposed in the humidified air at 300°C. As the temperature was raised to 500°C, little reaction was observed between water vapor and (Y0.25Bi0.75)2O3. The better stability of (Y0.25Bi0.75)2O3at elevated temperature was observed.