Ultra-Fast Oxygen Conduction through Vacancy-Mediated Mechanism in Sillén Oxychlorides
Authors: Meng Jun , Sheikh Md Sariful , Schultz Lane , Nachlas William O. , Jacobs Ryan , Liu Jian , Morgan Dane 
Published: December 2024

Ionic conductors play an essential role in a range of 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 >60k oxygen-containing compounds present in the Materials Project database. Our analysis is built on the fundamental hypothesis that the structure of a material is the key factor in governing its oxygen ion conduction characteristics. By calculating the structural similarity of the >60k oxygen-containing materials vs. the well-known oxygen-conducting fluorite BiO2, removing materials unlikely to be stable or synthesizable, and flagging materials with structure families distinct from those of known oxygen conductors, we identified the MBi2O4X (M=rare-earth element, X=halogen element) as a novel family of fast oxygen vacancy conductor. MBi2O4X adopts a triple-fluorite layered structure, where oxygen ions may diffuse via interstitial- or vacancy-mediated mechanisms. Our ab initio simulations of the representative material LaBi2O4Cl (LBC) reveal migration barriers of approximately 0.1 eV for oxygen vacancies and 0.6-0.8 eV for oxygen interstitials. The ultra-low barrier for oxygen vacancy diffusion suggests the potential for LBC to have fast oxygen ion conduction at low temperatures approaching room temperature, which could potentially be realized by aliovalently doping LBC to create vacancies. Hence, we investigated Sr2+ replacement for La3+ to create extrinsic oxygen vacancies, resulting in Sr-doped LBC La0.78Sr0.22Bi2O3.89Cl (SLBC). Our ab initio calculations indicate the formation of oxygen vacancies become thermodynamically favorable, and the ultra-low migration barrier is sustained in SLBC. Our simulations show the oxygen-deficient SLBC achieves conductivity higher than 0.1 S/cm over a broad temperature range from 800°C down to 100°C, demonstrating potentially outstanding low-temperature oxygen transport.
To experimentally investigate the oxygen conductivity in LBC and SLBC, we have synthesized pure LBC powder and SLBC power by a flux synthesis method. Experimental results demonstrate the total conductivity of SLBC are comparable to many state-of-the-art fast oxygen conducting electrolyte materials, and exceeds the performance of the widely used YSZ and LSGM at lower temperatures. Though the experimental activation barrier of SLBC is higher, and conductivity lower, than the extreme values predicted for vacancy-mediated diffusion from our simulations. We believe the key to realizing the exceptional predicted low temperature oxygen conductivity of LBC resides in creation of extrinsic oxygen vacancies in LBC (e.g., through aliovalent doping) and potentially through microstructural engineering, including grain size refinement. Realizing the full predicted potential oxygen mobility of LBC presents an opportunity to expand the use of oxygen-based energy conversion technologies to lower temperatures.