Solid electrolytes have attracted growing research interest for their promise to offer the safety and energy density necessary for future battery systems. Not only being the primary component in all-solid-state batteries, solid electrolyte materials also demonstrate their importance as a protector for lithium or sodium metal anodes in novel battery configurations, such as Li-S, Li-air, and flow batteries. The impedance at interfaces associated with solid electrolytes, i.e. internal grain and phase boundaries, and their interfacial stability with electrodes, are currently two key factors limiting the performance of solid electrolyte in batteries. Only will a mechanistic understanding of the root origin of these interfacial resistance and potential instability pave the way to the design of high-performance solid electrolyte-based batteries. In this Dissertation, I start with an introduction to the fundamentals of solid electrolytes and challenges associated with tuning their physical properties and their interfaces. I also discuss techniques that allow for an atomic-scale understanding of ion transport and stability in solid electrolytes and at their interfaces. I have selected representative examples from current literature that exemplify recent fundamental insights gained through advanced characterization techniques and high-throughput theoretical methods. Different strategies for improving ion conduction and stability in solid electrolytes and interfaces are discussed. In the following chapters, several solid electrolytes are introduced and discussed in detail, including b-Li3PS4, Li4P2S6, Li2OHCl, and Na4P2S6. Different synthetic and processing methods were employed to prepare these new solid electrolytes and understand their electrochemical performance with metallic lithium or sodium anode. In the case of b-Li3PS4, I describe a strategy for improving ion conduction in nanostructured b-Li3PS4 through the formation of nanocomposites with ion conducting and non-conducting oxide-based fillers. The work related to b-Li3PS4 is further extended to the fabrication of thin membranes (<50 um) via tiled assembly of shape-controlled, nanoscale building blocks. This method is based on facile and low-cost solution-based soft chemistry to create membranes with tunable thicknesses. Next, I discuss Li4P2S6, a largely overlooked material that appears as a decomposition product in the Li-P-S system, on the basis of combined experimental and theoretical investigations. I also discuss the LiOH-LiCl system of electrolytes and their compatibility with metallic lithium anode; I show that Li2OHCl solid electrolyte forms a stable solid electrolyte interphase layer with a metallic lithium anode, even past the melting point of lithium metal. I subsequently discuss a new sulfide-based sodium conductor, Na4P2S6. The design of the solid electrolyte Na4P2S6 is described, realizing excellent air stability and an economic soft chemistry synthetic approach in the presence of water. This Dissertation concludes by highlighting opportunities and perspectives for future research that will achieve an enhanced understanding of solid electrolytes and bridge the gap between the mechanistic understanding of solid electrolytes and their electrochemical performance in different battery configurations.
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