Understanding Lithium Ion Dynamics in Lithium Hydroxide Chloride and Related Solid Electrolytes
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Solid state electrolytes (SSEs) promise to greatly enhance properties attainable in the next generation Li-ion batteries (LIBs) because of non-flammable, less toxic, high thermal stability, and more compatible with electrodes. Li-rich antiperovskite (LiRAP) has emerged as a promising SSE due to low-cost and easy availability of starting materials, 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 and its impact on the conductivity. To demonstrate the impact of proton on total and Li-ion conductivities, I used two complementary approaches: (1) investigating the change in the conductivity of Li2OHCl with controlled amount of H in the material, and (2) probing Li+ and H+ dynamics through solid-state nuclear magnetic resonance (NMR) spectroscopy. Reducing amount of proton (H+) was found to decrease the measured conductivity, which suggested that the H+ assists Li+ hopping. A rotation of OH-group opens lower-energy pathways for Li+ jumps, such a Li+ conduction mechanism was confirmed by line shape and spin-lattice T1 relaxation experiments of 7Li and 1H NMR. I have unambiguously determined that H+ are constrained to mostly rotation, which do not contribute to long-range diffusion. Instead, the rotation 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 LIBs.