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This dissertation is focused on studying the effects of mixed-anion sublattice on Li+-ion conduction. Mixed-anion sublattice comprises multiple anion species in a structural framework which increases the degrees of freedom in tuning structural properties due to the differences in the anion radii, electronegativity, and polarizability. Thus, properties such as ionic vs. covalent bonding, local polyhedral distortions, and anion site disorder can be tuned for fast Li+-ion transport. Utilizing strategic synthesis of mixed-anion materials with compatible chemistries and advanced characterization techniques, structure-property correlations are established to understand the origin of fast Li+-ion conduction. The mixed-anion materials examined are the argyrodite Li6-xPS5-xClBrx, Li3PO4 – LiI, and 2LiX-GaF3 (where X = Cl-, Br- or I-). Solid-state synthesis accompanied by high-energy milling is utilized to synthesize these materials. X-ray and neutron diffraction are employed to study long-range structures whereas nuclear magnetic resonance (NMR) and Pair Distribution Function (PDF) analysis are used to determine local structures. Additionally, NMR relaxometry and EIS spectroscopy are utilized to study ion dynamics and transport and correlate these properties with refined structures. In the argyrodite structural framework, mixed-site occupancy of Cl-, Br-, and S2- is shown to enhance lithium transport properties by increasing the frequency and lowering the activation energy of lithium jumps. In the Li3PO4 – LiI composite, I- anions are used to interrupt the ordered PO43- network in Li3PO4, which destabilizes Li+-PO43- interaction and liberates Li+-ions with enhanced ionic conductivity. A similar mechanism is found in the 2LiX-GaF3 (where X = Cl-, Br- or I-) by the formation of complex GaXaFb anions that weaken lithium-anion interactions, yielding high ionic conductivity. In the Li1.05Hf0.05Ta1.95PO8 – 0.15LiF, the addition of LiF is shown to stabilize the Hf-doped LiTa2PO8 and improve the grain boundary transport properties. Overall, anion diversification is shown to be effective to enhance ionic conductivity by flattening the energy landscape for ion conduction. Additionally, metastable phases are observed that exhibit enhanced performance with this strategy. The fundamental understanding of lithium-ion transport mechanism in anion engineered framework in this study provides guidelines for designing high-performance materials as solid electrolytes for all-solid-state batteries and can be applied to materials beyond Li+-ion conductors, such as Mg2+, Ca2+, Zn2+, K+, and Na+-ion conductors.xx Furthermore, the mixed-anion strategy can be used to make new metastable phases that may exhibit unprecedented functionality.