Highly concentrated electrolyte design for high-energy and high-power redox flow batteries

Date

2021-01-22

Authors

Zhang, Leyuan

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Abstract

Redox flow batteries (RFBs) have attracted immense research interests as one of the most promising energy storage devices for grid-scale energy storage. However, designing cost-effective systems with high energy and power density as well as long cycle life is still a big challenge for the development of next-generation RFBs. Eutectic electrolytes as a novel class of electrolytes have been recently explored to enhance the energy density of RFBs as they offer advantageous features such as low cost, ease of preparation, and high concentration of active materials. On the other hand, promising organic molecules with high solubility are also considered as potential candidates for next-generation RFBs due to the high tunability of redox potential, solubility, and stability. Furthermore, it is also necessary to design low-cost and high-performance membranes to realize the long-term stable cycling of RFBs. Here, the eutectic concept has been proposed as a new strategy to enable the design of highly concentrated electrolytes, thus boosting the energy density. The metal-based eutectic electrolytes are mainly formed by mixing anhydrous/hydrated metal halides with hydrogen bond donors (HBDs), such as urea or acetamide. Two metal-based eutectic electrolytes (Al & Fe) are mainly synthesized and studied and their redox reactions and physicochemical properties can be highly tuned. In the end, an all eutectic-based RFBs with high energy density is demonstrated. Besides, by incorporating the eutectic concept with the advantageous features of organic molecules, it becomes an alternative strategy to maximize the molar fraction of active species in organic-based eutectic electrolytes. Organic redox species can be further adopted to develop promising electrolytes with high concentration. By screening possible molecular structures integrated with molecular engineering and fundamental understanding, azobenzene- and organodisulfide-based molecules are found as promising redox species for high-energy and long-life RFBs. They show high solubility in supporting electrolytes and their electrochemical properties, stability, and redox chemistry are systematically studied via detailed electrochemical characterization and advanced calculation methods. Last but not least, we also explored the design of new membranes for RFBs. By utilizing the lamellar structure of stacked graphene oxide sheets or metal-organic frameworks, the designed membranes have the potential to provide high ionic conductivity and high selectivity for RFB applications. By integrating the highly concentrated electrolytes and new membrane design, we aim to provide an effective strategy to design the next-generation RFBs for grid-scale applications.

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