Synthesis of organic cathode materials and electrolyte additives for lithium-based rechargeable batteries

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Synthesis of organic cathode materials and electrolyte additives for lithium-based rechargeable batteries
Shin, Dong-Seon
Hong, Sung You
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Graduate School of UNIST
Redox active organic electrode materials are considered as promising candidates for the next-generation battery electrodes. These compounds attracted attention owing to their unique advantages, such as low energy requirement during synthetic process, environmentally benign, and easy to tune their molecular structures. Among them, conjugated carbonyl compounds were studied extensively for the promising electrode materials in several rechargeable battery systems. Especially, as the solution state of redox active materials in the redox flow battery system, the arylated quinone compounds were extensively studied due to their superior electrochemical performances. However, since the research is still in the infant stage, most studies were conducted with commercial sources or restricted addition of functional group, and detailed studies about molecular modification was rarely discussed. The molecular structure modification strategy can effectively improve the performance of a batteries by changing the electrochemical performance or physical/chemical properties of the molecule. The lithium ion battery electrolyte forms a thin film on each electrode by decomposition reaction, during the cycling. In the case of a well-formed layer, it helped to prevent additional decomposition of the electrolyte solution without any hindrance of lithium ion transports, but conversely, in the case of the unstable layer, it caused severe problems such as poor battery performance, due to continuous electrolyte and Li ion consumption. Therefore, to form the stable interfacial layer, several additives are used by dissolving small amounts of organic materials into the electrolyte. Also, the safety issues, such as flame and explosion caused by overcharging, short-circuit, and other external factors can be solved by small portion of additives. In the first chapter of this dissertation, I describe the structural dependent electrochemical performances of two positional isomer. The electrochemical studies with lithium coin half-cell reveals that asymmetric quinone isomer deliver superior electrochemical performances, and the origin of performance differences were analyzed via experimental and computational methods. Furthermore, we applied quinone catholyte to novel tube type flexible lithium battery for investigating electrochemical performance in various conditions. In addition, I devised molecular modification strategies of the redox active molecules by cross-coupling reactions. The addition of aromatic functional groups enhances electrochemical stability of quinone derivatives, and electron density affecting functional groups changes redox properties of quinone moieties. Also, the enhanced solubility of active materials enables fluent flow battery operation. In the last chapter, I report organofluoro electrolyte additives for lithium ion batteries. Synthesized materials contribute to improvement of thermal stability and battery performances during the cycling, and their affections were investigated with instrumental analysis.
Department of Chemical Engineering
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