Understanding structure-property relations is essential for designing energy-rich redox active materials (ROMs) for all-organic redox flow batteries.^1-2 Herein we examine thiadiazole ROMs for storage of negative charge in the flow cells.³ These versatile molecules have excellent solubility and low redox potentials, allowing to achieve high energy density. By systematically incorporating groups with varying electron accepting/withdrawing ability, we have examined substituent effects on their properties of interest, 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 automatically 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. This finding strongly indicates the significant role of structural engineering in achieving improvement of radical ion stability, suggesting a generic guidance for developing long-cycling ROM-based flow cells. With these molecules we are on the very edge of possibility, reaching the compromise between the highest energy density and the stability of the radical anion that can be achieved in acetonitrile. We believe that setting these limits is more important for the energy storage community than surveying of different classes of the anolytes.

Figure 1 Structures and cyclic voltammogram of benzothiadiazole derivatives and the correlation of the half-decay lifetime of the radical anions with the redox potentials. Mind the logarithmic vertical scale.

1. Zhang, J.; Yang, Z.; Shkrob, I. A.; Assary, R. S.; Tung, S. o.; Silcox, B.; Duan, W.; Zhang, J.; Su, C. C.; Hu, B.; Pan, B.; Liao, C.; Zhang, Z.; Wang, W.; Curtiss, L. A.; Thompson, L. T.; Wei, X.; Zhang, L., Annulated Dialkoxybenzenes as Catholyte Materials for Non-aqueous Redox Flow Batteries: Achieving High Chemical Stability through Bicyclic Substitution. Advanced Energy Materials 2017, 7 (21), 1701272-n/a.
2. Zhang, L.; Zhang, Z.; Redfern, P. C.; Curtiss, L. A.; Amine, K., Molecular engineering towards safer lithium-ion batteries: a highly stable and compatible redox shuttle for overcharge protection. Energy & Environmental Science 2012, 5 (8), 8204-8207.
3. 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.