Design, Synthesis, and Structure of Lithium Ion Conducting Materials for All-Solid-State Batteries

Abstract

Energy storage plays a pivotal role in enabling intermittent renewable energy sources, electrified transportation, and portable electronics. The rapidly growing energy demand in these sectors requires improvement to the commercial lithium-ion systems. All-solid-state batteries are candidates for next-generation batteries because of their potential to be paired with a lithium metal anode, which leads to significant energy density gains. Furthermore, solid electrolytes can dramatically improve the safety and longevity of battery technologies by replacing the flammable liquid organic electrolytes that are typically used. This thesis broadly focuses on two types of solid electrolytes: thiophosphates and thioborates.

Chapters 3 and 4 focus on thiophosphate type solid electrolytes. In chapter 3, an in-situ variable temperature neutron powder diffraction study on Li3PS4 was conducted to elucidate the crystal frameworks and lithium substructures of its respective polymorphs (gamma to beta to alpha). The lithium diffusion pathways of both the bulk beta and alpha polymorphs are evaluated using the maximum entropy method and bond valence site energy calculations, revealing that the structure of alpha-Li3PS4 favors facile 3D conduction. Building on these concepts, chapter 4 focuses on the experimental elucidation of lithium ion conductors in the Li1+2xZn1-xPS4 solid solution. Using neutron and synchrotron X-ray powder diffraction, their crystalline structures are resolved to show the nature of likely pathways for lithium ion conduction and this is correlated with the improved ion conductivity upon increasing the lithium concentration and inducing Li/Zn site disorder.

In chapter 5, new quaternary lithium oxythioborosilicate glasses (termed `LIBOSS') were synthesized that exhibit high ion conductivity up to 2x10-3 S/cm. Superionic conductivity can be achieved despite relatively high oxygen:sulfur ratios of more than 1:2, which also greatly reduces H2S evolution upon exposure to air. Stripping/plating onto lithium metal results in very low polarization at a current density of 0.1 mA/cm2 over repeated cycling. Evaluation of the optimal glass composition as an electrolyte in an all solid-state battery shows it exhibits excellent cycling stability and maintains near theoretical capacity for over 130 cycles at room temperature with Coulombic efficiency close to 99.9%, opening up new avenues of exploration for these quaternary compositions.

In chapter 6, a new class of lithium thioborate halides is reported. These materials adopt a so-called supertetrahedral adamantanoid structure that houses mobile lithium ions and halide anions within interconnected 3D structural channels. Investigation of the Li7.5B10S18X1.5 (X = Cl, Br, and I) structures using single-crystal XRD, neutron powder diffraction, and neutron PDF reveals significant lithium and halide anion disorder. These new superadamantanoid materials exhibit high ionic conductivities up to 1.4x10-3 S/cm. In chapter 7, a new fast-ion conducting lithium thioborate halide, Li6B7S13I is presented. Li6B7S13I exhibits a perovskite topology and an argyrodite-like lithium substructure that leads to superionic conduction with a theoretical Li-ion conductivity of 5.2x10-3 S/cm. Combined single-crystal XRD, neutron powder diffraction, and AIMD simulations elucidate the Li+ ion conduction pathways through three-dimensional intra and inter-cage connections, and Li-ion site disorder, which are all essential for high lithium mobility.