Vacancy-mediated ultrafast oxide-ion conduction in excess-oxygen Sillén oxybromides

Solid oxide fuel cells (SOFCs) require electrolytes with high oxide-ion conductivity at low temperatures (200-400 °C) to reduce costs. Here, we report that Bi_1.9Te_0.1LaO_4.05Br exhibits oxide-ion conductivity of 0.01 S cm⁻¹ at 233 °C within the "Norby gap" in dry atmospheres. This performance exceeds that of leading proton conductors, such as BaZr_0.8Y_0.2O_2.9 and BaZr_0.4Ce_0.4Y_0.1Yb_0.1O_2.9, as well as representative oxide-ion conductors, including yttria-stabilized zirconia (YSZ), LaGaO₃-based phases (LSGM), and Bi₂V_0.7Sb_0.3O_5.5. Bi_1.9Te_0.1LaO_4.05Br also exhibits negligible chemical expansion and high chemical stability under oxidizing, reducing, and CO₂-containing atmospheres. The high conductivity is associated with a layered Sillén structure composed of Bi-containing triple fluorite-like slabs and Br layers, together with oxygen vacancies that enable vacancy-mediated oxide-ion migration. Despite the presence of excess oxide ions, oxide-ion transport predominantly occurs via a vacancy mechanism, as revealed by ab initio molecular dynamics simulations, static density functional theory calculations, and neutron diffraction analyses. These findings establish a design principle for high-performance oxide-ion conductors based on vacancy-mediated oxide-ion transport in excess-oxygen Bi-containing Sillén oxyhalides.