Date of Award

8-2016

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Materials Science and Engineering

Committee Member

Dr. Kyle S. Brinkman, Committee Chair

Committee Member

Dr. O. Thompson Mefford

Committee Member

Dr. Fei Peng

Abstract

Garnet-type Li7La3Zr2O12 (LLZO) has been demonstrated as a promising solid-state electrolyte material for lithium ion batteries. Solid-state electrolytes with room temperature conductivities in excess of 10-4 S/cm are being considered as a substitute for the current liquid electrolyte and polymer based separators which would result in the enhanced safety. In the present work, the phase formation, sinterability and electrochemical properties of LLZO as a function of Ga dopant composition and synthesis route have been investigated. X-ray diffraction, combined with differential thermal analysis (DTA), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) characterization methods have been utilized to investigate the impact of Ga as a dopant. Tetragonal-LLZO has been synthesized by conventional solid-state reaction process and conventional sintering method with a total conductivity ~10-7 S/cm at 21℃. LLZO pellets with single cubic phase have been attained by quenching the t-LLZO in water at 600 ℃exhibiting a conductivity of ~10-4 S/cm at 21℃. The phase transition from tetragonal to cubic phase occurs when 0.1 mole of Ga was added to 1 mole LLZO powder (Li6.7La3Zr2Ga0.1O12), however, the resulting phase assemblage is still a mixture of tetragonal and cubic phases. Calcined powders with a composition of 0.5Ga-LLZO exhibited a single cubic phase as verified by XRD, further proving the supposition that higher content of Ga is beneficial for cubic phase formation. At higher doping levels, an excess of Ga in sample 1.0Ga-LLZO has been found as LiGaO2 in the powder, which works as a sintering aid and accumulates in the grain boundaries. Compared with conventional sintering process, pellets densified via spark plasma sintering displayed a higher relative density at a lower sintering temperature. The highest total ionic conductivity at room temperature in the range of 5.81×10-5 S/cm for sol-gel synthesized Li5.5La3Zr2Ga0.5O12 and sintered by conventional sintering method in the air. Also, a total lithium ionic conductivity in the range of 4.37×10-6 S/cm at room temperature for SSR synthesized Li5.5La3Zr2Ga0.5O12 and sintered by conventional sintering method in the air. Grain growth investigation on 0.5Ga-LLZO sintered by SPS and following by conventional sintering has verified the suggestion that larger grains are beneficial to the total ionic conductivity. In addition to room temperature Li-ion conductors, Ga doping in perovskite Yttrium-doped Barium Zirconate (BZY) serving as a high temperature proton conducting electrolyte material has been investigated. The sintering temperature was effectively decreased when Ga was doped into BZY structure, however, there was a concomitant decrease in the total conductivity.

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