Probing complex ionic dynamics on the nanoscale via energy discovery platforms

Abstract

Ionic dynamics underpin the functionalities of a broad spectrum of materials and devices ranging from energy storage and conversion, to sensors and catalytic devices. Electrochemical reactivity and ionic transport in these systems is, however, a complex process, controlled by the interplay of charge injection and field-controlled and diffusion-controlled transport, which are often very sensitive to the environmental conditions, microstructures and defect structures of the material. In addition, single characterization techniques are usually limited in detecting the mechanisms at the full range of the environmental conditions, and multiple length scales involved. To overcome these challenges, in this thesis, energy discovery platforms were developed, which are microfabricated lateral devices that enable multiple in-situ microscopy, spectroscopy, and functional characterization techniques to be performed on a single set-up. They provide easy control of external conditions including temperature (25 °C – 200 °C), humidity (0% – 90% at 25 °C), ambient gas (air and nitrogen), as well as applied electric field (1 V – 30 V). Through microfabrication, the composition, microstructure, lattice strain (-9.0% – 3.5%) and the presence and absence of triple phase boundaries of the material was also controlled, as needed. Multiple in-situ techniques, i.e. time-resolved Kelvin probe force microscopy and electrochemical impedance spectroscopy, were performed on the same sample to cover a full range of ionic response. Structural and compositional analysis were performed via a variety of techniques including atomic force microscopy, X-ray diffraction, scanning transmission electron microscopy, electron energy loss spectroscopy and atom probe tomography. In addition, theoretical calculations were performed to simulate and model the experimental phenomena, thus providing insight into the ionic dynamics. The comprehensive structure-property relationship study of these materials not only showcases the feasibility of energy discovery platforms in complex ionic dynamics study on the nanoscale, but also facilitate material and device design with better performance and reliability.