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-Bi₂O₃-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.