Anion Sublattice Engineering via Fluorine Doping to Enhance δ‐Bi ₂ O ₃ Stability for Low‐Temperature Solid Oxide Electrochemical Cells

Abstract Solid oxide electrochemical cells (SOCs) are promising next‐generation, eco‐friendly, and efficient energy conversion devices. However, their high operating temperatures hinder commercialization, primarily due to the lack of highly durable and active materials for low‐temperature operation. Herein, a highly stable and conductive δ‐Bi ₂ O ₃ ‐based ionic conductor is introduced, in which unoccupied oxygen sites are mediated by F ⁻ ions to enhance structural stability and conductivity. The optimized material exhibits an exceptional ionic conductivity of 0.228 S cm ⁻¹ at 600 °C, representing a more than 70‐fold increase compared to conventional Y‐doped zirconia, while maintaining excellent long‐term stability. Density functional theory calculations reveal that F ⁻ incorporation stabilizes the disordered anion sublattice, reinforcing the cation–anion bonding strength and enhancing the structural symmetry of the δ‐cubic fluorite structure. When integrated into a composite oxygen electrode, the developed ionic conductor enables superior electrochemical performances in SOCs, achieving 0.98 W cm ⁻² in fuel cell mode and 0.63 A cm ⁻² at 1.3 V in electrolysis mode at 600 °C. These findings provide insights into the rational design of stable and active materials for high‐performance SOCs, facilitating efficient operation at reduced temperatures and advancing their practical viability.