GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Tuesday, March 10, 2020

4:00 PM
in Love 109

will be held the

DISSERTATION DEFENSE

for

Ah-Young Song

"Understanding Lithium Ion Dynamics in Lithium Hydroxide Chloride and Related Solid Electrolytes"

Committee Members:

Prof. Gleb Yushin, Advisor, MSE

Prof. Angus Wilkinson, CHEM

Prof. Johannes Leisen, CHEM

Prof. Josh Kacher, MSE

Prof. Matthew McDowell, MSE/ME

Abstract:

Solid state electrolytes (SSEs) promise to greatly enhance properties attainable in the next generation rechargeable Li or Li-ion batteries (LIBs) for a wide range of applications from portable electronic equipment to battery-powered electric vehicles (EVs) because many SSEs are non-flammable, less toxic, more stable in higher thermal environments, and potentially more compatible with high capacity Li or Li alloy anodes and high-voltage cathodes. Most battery applications would benefit greatly from high power-to-weight ratio, low cost, and affordable large-scale production of solid state LIBs.

Li-rich antiperovskite (LiRAP) has emerged as a promising SSE due to low-cost and broad availability of starting materials, LiRAP low density (light weight) and a low melting point for inexpensive and mass production. Unfortunately, there was a lack of clear understanding of the LiRAP chemical composition produced due to a difficulty in identifying lithium and hydrogen by using  conventional tools. To demonstrate the impact of proton on total and Li-ion conductivities, I used two complementary approaches: (1) investigating the change in ionic conductivity of Li2OHCl with controlled amount of H replaced by Li in the material, and (2) probing Li+ and H+ dynamics at different temperatures through solid-state nuclear magnetic resonance (NMR) spectroscopy.

Reducing amount of hydrogen was found to decrease Li ionic conductivity, which suggested that the hydrogen assists Li+ hopping. A rotation of a short OH-group opens lower-energy pathways for Li+ jumps, such a Li+ conduction mechanism was confirmed by magic-angle spinning (MAS) 7Li NMR, static 7Li and 1H NMR and spin-lattice T1(7Li)/T1(1H) relaxation experiments. By measuring both proton and lithium at various temperatures, I have unambiguously determined that protons (H+) do not contribute to long-range diffusion and H+ dynamics is constrained to mostly rotation in the absence of water. Instead, the rearrangement of the OH group controls Li+ and H+ ion dynamics.

I expect the findings reported in my thesis to show new avenues for enhancing Li-ion conductivities in SSEs and contribute to the development of safer and more energy-dense solid-state Li and Li-ion batteries.