Influence of Electronic Structure and Lattice Dynamics on Oxygen Ion Transport in Solid-State Ionic Conductors

Abstract

Solid-state oxygen ion conductors are crucial for electrochemical devices such as separation membranes, solid-oxide electrolyzers, fuel cells, and sensors, serving as a technological link between renewable energy generation and consumption. Currently, these conductors are limited by slow transport rates and high operational temperatures, which pose challenges and increase costs. Developing faster conductors that operate at lower temperatures requires reducing activation energy and enhancing the pre-exponential factor in the Arrhenius equation of conductivity. However, our understanding of the fundamental processes in oxygen ion transport and methods to improve oxygen ion conductivity remain limited. This thesis focuses on understanding the fundamental mechanisms that regulate oxygen ion transport. First, the migration energy barrier in perovskite oxides is linked to an electronic energy penalty from local charge screening near the hopping ion. The energy of local electronic states is identified as a fundamental descriptor of the migration barrier. Next, migration entropy and phonon density of states (DOS) are highlighted as the main factors regulating the pre-exponential factor of oxygen ion conductivity across different materials. The phonons of oxygen ions near the hopping ion significantly contribute to migration entropy, suggesting that migration entropy can be tuned by designing the phonon dynamics of these atoms. These results imply that a widely observed correlation between increasing pre-exponential factors and activation energy arises from coupling local electronic energy states and phonons. The results are extended to the formation of oxygen vacancies and interstitials in perovskite and RuddlesdenPopper oxides. We find that defect formation energy rises with defect formation entropy, which is linked to electronic energy states interacting with phonons. In perovskite oxides, lower vacancy formation entropy is correlated with increasing oxygen phonon band center and shortening bond lengths with oxygen vacancy formation. In Ruddlesden-Popper oxides, lower interstitial formation entropy is associated with reduced octahedral tilting and local phonon changes. This thesis establishes a theoretical foundation for treating migration entropy and defect formation entropy as design variables in next-generation ionic conductors. By highlighting the impact of electronic structure and lattice dynamics on energy barriers and entropic drivers, the findings suggest new pathways for material design through the strategic separation of these factors and the intelligent design of lattice moieties in oxygen ion transport environments.