First-Principles Selection of Solute Elements for Er-Stabilized Bi₂O₃ Oxide-Ion Conductor with Improved Long-Term Stability at Moderate Temperatures

Quality oxide-ion conductors are essential for clean-energy applications. Rare-earth-stabilized bismuth sesquioxide, δ-Bi₂O₃, exhibits a much greater oxide-ion conductivity at high temperatures than commonly used ZrO₂- or CeO₂-based electrolytes, but it suffers from serious conductivity degradation while annealing at moderate temperatures of ∼773 K, which is the target temperature for many applications. Here, we demonstrate that a novel set of solute elements for δ-Bi₂O₃ can significantly enhance the long-term stability at 773 K. A pure oxide-ion conductivity of 0.035 S/cm at 773 K remains unchanged during annealing for 100 h, which is five times greater than the best known solid-state oxide materials after long-term annealing. For materials design, we explore a range of chemical spaces using theoretical methods based on first-principles calculations. The order-disorder transition temperature of the anion sublattice, oxygen-ion diffusivity, and solution free energy are used as descriptors. The design concept is verified experimentally.