Structurally ordered oxides exhibit a broad range of structural, compositional, and functional properties, which can be further tuned or even drastically transformed by means of judicious elemental doping, strain and defect engineering. This talk will highlight our most recent effort aiming to modify complex oxides through heteroepitaxy for tunable O2- or Li+ transport, which have profound implications in energy conversion and storage devices. The creation, clustering, and ordering of oxygen vacancies (VOs) in perovskite-structured oxides (ABO3, B being a transition metal) give rise to a special class of materials, such as Brownmillerite (BM) structured SrCoO2.5, SrFeO2.5 (BM-SFO), and rhombohedral structured SrCrO2.8 (R-SCrO). We show that by varying synthesis and processing conditions, the orientation of OVCs can be selectively controlled on the same lattice matched substrate. Optical ellipsometry, in-plane transport, 18O isotope exchange, and scanning transmission electron microscopy studies reveal that these configurations offer distinct different physical and oxygen transport properties. A topotactic phase transition from BM-SFO (R-SCrO) to perovskite SrFeO3 (SrCrO3) can be promoted, delayed, or prohibited based on the interfacial strain conditions, highlighting the importance of interface engineering in designing robust and efficient ion conducting materials. In another example, I will present the epitaxial growth and in situ TEM studies of LiCoO2 with or without overlayers to understand the Li transport processes and device failure mechanisms. We show that while orienting Li containing planes of LiCoO2 can effectively tune the Li ion transport characteristics, it does not modify its stability against Li metal (mimicking a Li dendrite), which preferentially attack the LiCoO2 crystal structure along the [001] direction, enabling to Li propagation across Li-containing planes. 

Biography: Dr. Yingge Du is a senior staff scientist in the Materials Group of the Physical and Computational Sciences Directorate. He joined PNNL in 2007 and became a staff member in 2010. He served as technical lead in the acquisition and commissioning of a new state-of-the-art oxide molecular beam epitaxy (MBE) system for the EMSL user facility located at PNNL. His current research focuses on growth and characterization of epitaxial metal oxide films and superlattices for energy conversion and storage applications. Dr. Du is the recipient of a 2016 DOE Early Career Award. This award will support him to conduct basic research that aims to understand, predict, and ultimately control cation/anion ordering and topotactic phase transitions occurring in transition metal oxide thin films. Achieving so will enable energy materials (e.g., catalysts, electrodes, and electrolytes) with desired functionalities to be designed, synthesized, stabilized, and harnessed for technological benefits.