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201272 https://hdl.handle.net/2286/R.2.N.201272 http://rightsstatements.org/vocab/InC/1.0/ All Rights Reserved 2025 2027-05-01T13:34:17 199 pages Doctoral Dissertation Academic theses en Guo, Jinzhao Chan, Candace Hwa, Yoon Xu, Xin Rolston, Nicholas Arizona State University Partial requirement for: Ph.D., Arizona State University, 2025 Field of study: Materials Science and Engineering Ta-doped Li7La3Zr2O12 (LLZTO) garnet is a promising Li-ion conducting ceramic electrolyte for solid-state batteries. LLZTO is typically synthesized from oxides via solid state reaction (SSR), but use of other precursors such as pyrochlores has also received recent interest. In this dissertation, a new type of LLZTO was prepared from Ta-doped pyrochlore precursor, in a method called “pyrochlore to garnet” (P2G). Investigation of the P2G process shows that P2G LLZTO materials could achieve similar relative densities and ionic/electronic conductivities compared to SSR LLZTO but with significantly reduced sintering time needed because of the reactive sintering mechanism, which is aided by liquid phase sintering. Reflection electron energy loss spectroscopy (REELS) and X-ray photoelectron spectroscopy (XPS) also confirm that both types of LLZTO have similar bandgaps and chemical states. However, microstructure analysis shows that the P2G method results in LLZTO with average grain size of around 3 µm, which is much smaller than the grain sizes (as large as 20 µm) seen in SSR LLZTO. Energy-dispersive X-ray spectroscopy (EDS) analysis and synchrotron diffraction showed that the P2G LLZTO has better elemental homogeneity and phase purity, but there are minor tetragonal phases in SSR LLZTO, which might have potential negative effects on cycling behavior and dendrite resistance. Galvanostatic Li stripping/plating and linear sweep voltammetry measurements show that P2G LLZTO can withstand higher critical current densities (up to 0.4 mA/cm2 in bidirectional cycling and > 1 mA/cm2 for unidirectional) than those seen in SSR LLZTO. Postmortem examination reveals much less Li deposition along the grain boundaries of P2G LLZTO, particularly in the bulk of the pellet, compared to SSR LLZTO after cycling. Solid state nuclear magnetic resonance (NMR) analysis showed that the Li dendrite growth in P2G and SSR LLZTO might follow different mechanisms. All these results indicate that the liquid phase sintering in the P2G process affected the microstructure, phase purity and elemental homogeneity in LLZTO, which may explain the better resistance failure from Li dendrite penetration. The P2G method was also demonstrated to be effective for reactive sintering of LLZTO with cathode materials to make a cathode/LLZTO composite. After co-sintering, no deleterious effects, such as unwanted cation diffusion, were observed between the cathode material and LLZTO. Finally, other dopants were added to the pyrochlores to prepare mixed ion-electron conductors (MIEC), which may serve as buffer layers between the Li metal anode and LLZTO to mitigate void formation during Li stripping. This body of work demonstrates how the versatility of the P2G method and its application not only towards preparing high performance Li conducing solid electrolytes, but also to address key interfacial challenges in the integration of different components in an all-solid-state battery. Materials Science Pyrochlore-to-Garnet: A Versatile Approach for Garnet Solid Electrolytes with Improved Microstructure and Lithium Stripping/plating Behavior