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Lu Zhang1 2 Jingjing Zhang2 1 Xiaoliang Wei4 Fikile Brushett2 3

1, Argonne National Laboratory, Lemont, Illinois, United States
2, Joint Center for Energy Storage Research, Lemont, Illinois, United States
4, Indiana University-Purdue University Indianapolis, Indianapolis, Indiana, United States
3, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

Redox-active organic materials (ROMs) have shown great promise for redox flow battery applications but generally encounter limited cycling efficiency and stability at relevant redox material concentrations in nonaqueous systems. In this talk, a new family of heterocyclic organic anolyte molecules, 2,1,3-benzothiadiazole (BZNSN) and derivatives,1-2 will be discussed, which demonstrated high solubility, low redox potential, and fast electrochemical kinetics. By coupling BZNSN with a benchmark catholyte ROM (DBMMB),3 the nonaqueous organic flow battery delivered significant improvement in cyclable redox material concentrations and cell efficiencies compared to the state-of-the-art nonaqueous systems. In order to obtain insightful understanding of this family of molecules, a series of derivatives have been developed by varying the 5-position substitutions with various electron accepting/withdrawing ability. The substituent effects on their properties of interest have been examed, including redox potentials, calendar lives of charged ROMs in electrolyte, and the flow cell cycling performance. While the calendar life of energized fluids can be tuned in a predictable fashion over a wide range, the improvements in the calendar life do not directly translate into the enhanced cycling performance, indicating that in addition to the slow reactions of charges species in the solvent bulk, there are other parasitic reactions that occur only during electrochemical cycling of the cell and can dramatically affect the cycling lifetime.
1. Duan, W.; Huang, J.; Kowalski, J. A.; Shkrob, I. A.; Vijayakumar, M.; Walter, E.; Pan, B.; Yang, Z.; Milshtein, J. D.; Li, B.; Liao, C.; Zhang, Z.; Wang, W.; Liu, J.; Moore, J. S.; Brushett, F. R.; Zhang, L.; Wei, X., “Wine-Dark Sea” in an Organic Flow Battery: Storing Negative Charge in 2,1,3-Benzothiadiazole Radicals Leads to Improved Cyclability. ACS Energy Letters 2017, 2 (5), 1156-1161.
2. Huang, J.; Duan, W.; Zhang, J.; Shkrob, I. A.; Assary, R. S.; Pan, B.; Liao, C.; Zhang, Z.; Wei, X.; Zhang, L., Substituted thiadiazoles as energy-rich anolytes for nonaqueous redox flow cells. Journal of Materials Chemistry A 2018, 6 (15), 6251-6254.
3. Huang, J.; Cheng, L.; Assary, R. S.; Wang, P.; Xue, Z.; Burrell, A. K.; Curtiss, L. A.; Zhang, L., Liquid Catholyte Molecules for Nonaqueous Redox Flow Batteries. Advanced Energy Materials 2015, 5 (6), 1401782.

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