Browsing by Author "Hofstetter, Kyle"

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    Studies on Chemical and Electrochemical Stability of Li-stuffed Garnets for Next Generation Lithium Batteries
    (2019-01-03) Hofstetter, Kyle; Thangadurai, V.; Sutherland, Todd C.; Welch, Gregory C.
    State-of-the-art lithium-ion batteries are responsible for the portable electronics revolution, enabling the use of cell phones, laptops, and other gadgets. However, even when fully developed, the energy density and safety are insufficient to meet the demands in electric vehicles and grid storage. Next generation batteries are projected to consist of a solid electrolyte, metallic Li anode and either O2 or S cathode. Specifically, aqueous Li-O2 batteries are very attractive due to their high theoretical energy density and increased safety. Among the solid Li-ion electrolytes, Li-stuffed garnets are deemed a very promising electrolyte due to their wide electrochemical window (> 6 V vs. Li/Li+), chemical stability against metallic Li, and high ionic conductivity (~ 1 mS cm-1) at room temperature. Even with these attributes, there is a lack of understanding towards the stability of Li-stuffed garnets with aqueous environment, CO2, and moisture. In this thesis, the chemical and electrochemical stability of various garnet-type ceramic electrolytes for next generation lithium batteries are reported. For next generation aqueous Li-O2 batteries, garnet-type Li6.5La2.5Ba0.5ZrTaO12 was treated with H2O, D2O, 1 M LiOH, 1 M LiCl, or brine (0.1 M LiCl, 0.1 M NaNO3, 0.1 M K2CO3) for up to 10 days to encourage a spontaneous H+ or D+/Li+ ion-exchange reaction. In all cases, an increase in ionic conductivity was observed after the first 24 hours of submersion, while the crystal structure was unaltered. Samples were characterized with a variety of methods including powder X-Ray diffraction (PXRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS). With a more in depth understanding of the aqueous stability of lithium-garnets, Li6.5La2.5Ba0.5ZrTaO12 was employed as a protective Li anode material in a Li-O2 cell, showing a promising and stable ~ 3 V. iii. Further gaseous stability of garnet-type Li7La2.75Ca0.25Zr1.75Nb0.25O12 with and without a thin Al2O3 layer under 3% H2O and CO2 rich atmospheres were investigated in regards to Li-ion conductivity. When exposed to CO2 rich atmospheres, samples without the coating show an ~ 8 % increase in resistance over 1000 h, whereas coated samples only show ~ 1 % increase over the same environmental and time duration. However, in the presence of 3% H2O, both coated and uncoated samples show large grain-boundary resistance, which is recoverable on removal of 3% H2O. Complete stability characterization was carried out using EIS, PXRD, SEM, atomic force microscopy (AFM) and cyclic voltammetry (CV).