Understanding the electrochemistry and reaction mechanisms of solid-state sulfides with application to the lithium-sulfur battery system
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The lithium-sulfur (Li-S) battery is a highly promising technology for next-generation high energy density storage. This high energy density has its roots in the conversion chemistry of the Li-S system, which also imparts numerous challenges to the realization of practically viable cells. This dissertation focuses on improving the performance and understanding of insulating solid-state lithium sulfides, which are the source of many of the challenges inherent to Li-S batteries. First, a facile strategy is presented to generate a manganese sulfide surface layer on Li₂S particles, which dramatically improves cycling performance. Analysis of this reaction mechanism demonstrates how surface layers with limited conductivity but high electrochemical stability and facile charge transfer can profoundly improve the solid Li₂S charge mechanism. The role of solid sulfur-sulfur bonding in the cycling mechanism was then analyzed by direct chemical synthesis and isolation of insoluble sulfur-sulfur bonded species (i.e., Li₂S₂-type species). While these syntheses are shown not to generate Li₂S₂ separate from Li₂S, the insoluble polysulfide species were isolated from the soluble polysulfides. These isolated insoluble sulfides are used to demonstrate that solid-state sulfur-sulfur bonds can be reduced in the absence of soluble polysulfides, and the formation of Li₂S₂ is thus not inherently limiting to the capacity of Li-S batteries. To further clarify the fundamental limitation of Li₂S thickness on Li-S battery rate performance, a system was built to sputter-deposit air-sensitive lithium sulfide films of arbitrary thickness. It is shown that while the deposition initially generates a novel sulfide structure containing polymer-like Li₂S units, highly pure crystalline films of Li₂S can be generated with annealing. These Li₂S films are used to systematically determine the maximum thickness of Li₂S that can be charged at a practical rate is approximately 40 nm at a local charge density of 1 μA cm-2. This systematic approach additionally identified the appearance of the activation overpotential when charging Li₂S to be associated with the generation of soluble polysulfide species. Finally, these results are used to develop a model for the rational design of Li-S cathodes by tailoring the conductive pore structure around the local charge density and total sulfur content.