Ultra-Fast Oxygen Conduction in Sillén Oxychlorides

Ionic conductors play an essential role in diverse energy technologies, enabling the conversion of chemical energy into electricity and vice versa. Recent efforts have been devoted to improving the conductivity of oxygen-active materials, which play a crucial role in enhancing the efficiency of fuel cells, solid-oxide air batteries, electrolyzers, membranes, sensors, and more. In this work, we performed a structure-similarity analysis of &gt;60k oxygen-containing compounds, and identified the MBi₂O₄X (M=rare-earth element, X=halogen element) as a novel family of fast oxygen vacancy conductors. MBi₂O₄X adopts a triple fluorite layered structure, where oxygen ions may diffuse via interstitial- or vacancy-mediated mechanisms. <i>Ab initio</i>studies of the representative material LaBi₂O₄Cl (LBC) reveal migration barriers of 0.1 eV for oxygen vacancies and 0.6-0.8 eV for oxygen interstitials. With 2.8% oxygen vacancies, single crystal LBC is predicted by<i> ab initio</i> molecular dynamic simulations to exhibit an ionic conductivity of 0.3 S/cm at 25 °C. Furthermore, intrinsic LBC displays substantial ionic conductivity above 700°C, attributed to the spontaneously formed Frenkel pairs at elevated temperatures, which are the dominant defect type in LBC. The ultra-low barrier for oxygen vacancy diffusion indicates the potential of oxygen-deficient LBC to facilitate fast oxygen ion conduction even at room temperature, achievable through aliovalent doping to create vacancies. To experimentally verify the oxygen conductivity in intrinsic and oxygen-deficient LBC, we have synthesized LBC and Sr-doped LBC by a flux synthesis method. Experimental results demonstrate that both LBC and Sr-doped LBC achieve comparable or higher oxygen conductivity than yttria-stabilized zirconia (YSZ) and (La,Sr)(Ga,Mg)O₃ (LSGM) below 400 °C, with lower activation energies. However, a large discrepancy between the experimentally observed and computationally predicted conductivities is observed, where experiments show higher activation energy and significantly lower conductivity than predicted for oxygen-deficient LBC. We believe the key to realizing the exceptional predicted room temperature oxygen conductivity of LBC resides in creation of extrinsic oxygen vacancies in LBC (e.g., through aliovalent doping) and potentially through microstructural refinement to reduce grain boundary effects. Realizing the full potential of the ultra-fast oxygen conduction in LBC presents an opportunity to expand the use of oxygen-based energy conversion technologies to lower temperatures.