NEW THEORY AND MODEL OF CHARGE TRANSPORT IN TERNARY AND HIGHER ORDER SPINELS

273 pagesPolarons are a central component of applications made from transition metal oxides, and are the backbone of exotic behavior in oxides. As a carrier “trapping” process, the transport of electronic charge by polaron hopping is sluggish, and can be the critical limitation on performance. Therefore, understanding how to model polaron charge transport has importance to a wide range of functional materials. Accurately modelling the mechanisms of charge transport in complex oxides would enable design principles for enhancing their electronic and electrochemical properties for energy applications. But the conventionally-used small-polaron hopping model, developed in 1961, has not been rigorously tested for ternaries and higher order oxides. The conventional model was developed for binary oxides, so the role of mixed cation activation energies, varied oxidation states, and continuity of conduction pathways was not accounted for. Progress in this area has been impeded by factors such as the lack of high-quality samples and the lack of reliable and accessible characterization techniques. To overcome these challenges and with the aim of developing a new charge transport model, a systematic approach is followed: Firstly, a characterization method is developed using synchrotron-based X-ray Emission Spectroscopy (XES) and shown that this technique provides complete structural information and is more reliable than other commonly used characterization methods. Secondly, using XES as a tool to determine the number of active species, a spinel system having a single type of active species (CoxMn3-xO4) is explored to ascertain how charges flow in this system. Lastly, using a more complicated spinel oxide system (having multiple type of active species, MnxFe3-xO4), the limitations of the conventional transport model are highlighted and consequently a new model is introduced for small-polaron charge transport. Based on these findings, new parameters are added to the old electronic conductivity equation, and it is shown that a near perfect correlation can be made between experiment and theory for the electronic conductivity. Lastly, the significance of the newly developed XES characterization method and the new charge transport model is highlighted by presenting two studies: (1) Using XES, the origin of plasmon behavior in CuFeS2 nanocrystals is ascertained and the plasmon peak intensity is shown to be correlated to the Fe2+ concentration in the system; and (2) Based on better understanding of charge conduction in spinels, new design principles are introduced to improve the electrochemical performance of spinels. A hybrid core@shell nanoparticle structure (Fe3O4@MnFe2O4) is designed to have a controlled lattice strain and an epitaxial interface, which is shown to exceed the supercapacitor performance of the unstrained–MnFe2O4 electrode in energy density by ~33%, power density by ~28%, and specific capacitance by ~48%. This work provides a starting point to using strain engineering as a novel approach for designing high performance energy storage devices. Finally, the significance of the newly developed XES characterization method and the new charge transport model is highlighted by presenting two studies: Firstly, using XES, the origin of plasmon behavior in CuFeS2 nanocrystals is ascertained and the plasmon peak intensity is shown to be correlated to the Fe2+ concentration in the system. Secondly, based on better understanding of charge conduction in spinels, a hybrid core@shell nanoparticle structure (Fe3O4@MnFe2O4) is designed with a controlled lattice strain which is shown to exceed the supercapacitor performance of the unstrained–MnFe2O4 electrode in energy density by ~33%, power density by ~28%, and specific capacitance by ~48%.2023-09-1