Understanding Ionic Conduction in Solid Electrolytes for Lithium and Sodium Ion Batteries

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Xiong, Shan
Chen, Hailong
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Rechargeable batteries are at the heart of technological innovations ranging from consumer electronics and electric vehicles to large-scale energy storage for intermittent renewable sources, such as wind and solar energy. All-solid-state batteries (ASBs) show great promises as next-generation energy storage architectures because of their enhanced safety and higher energy density compared to conventional batteries with flammable liquid electrolytes. Solid electrolyte (SE) materials with good ionic conductivity and electrochemical stability are necessary for ASBs to be viable. Such SEs can be developed from rationally designed candidate materials based on an in-depth understanding of ionic conduction behavior in solids and the underlying material structural parameters. In this dissertation, I present design strategies for several recently developed lithium and sodium SE materials, discuss the characteristics and performance of these materials, and demonstrate the establishment of structure-property relationships in various material groups. First, a group of new Li ion conductors are designed through a joint computational and experimental study. Aided by first principles computation and in situ experimental methods, a group of oxide-based Li SEs with sphene structure and greatly improved ionic conductivities was identified and synthesized for the first time. Next, a series of sulfide-based Na SE materials is investigated. These materials originate from Na3SbS4 compound by iso-valent substitution and bill-mill processing. High resolution X-ray diffraction coupled with pair distribution function analysis shed light on their structural evolution during composition variation and synthesis. Lastly, a systematic investigation on Na4SnS4-based SE materials through alio-valent substitution is presented. The key underlying structural contributors for significantly improved conductivities are identified and discussed. In summary, this dissertation highlights the importance of understanding the structure-property relationship of SE materials to rationally tune material structure and composition for enhanced ionic conductivities. The material design methodologies in this work provide guidance for fast ion conductor development, which is applicable not only for the design of ASBs, but also for fuel cells, gas sensors, and other potential applications.
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