Nonaqueous redox flow batteries (RFBs) represent a promising technology for grid-scale energy storage. A key challenge for the field is identifying molecules that undergo reversible redox reactions at the extreme potentials required to leverage the large potential window of organic solvents. In this Article, we use a combination of computations, chemical synthesis, and mechanistic analysis to develop thioether-substituted cyclopropenium derivatives as high potential electrolytes for nonaqueous RFBs. These molecules exhibit redox potentials that are 470−500 mV higher than those of known electrolytes. Strategic variation of the alkyl substituent on sulfur afforded a derivative that undergoes charge−discharge cycling at +1.33 V vs ferrocene/ferrocenium in acetonitrile/tetrabutylammonium hexafluorophosphate. This electrolyte was paired with a phthalimide derivative to achieve a proof-of-principle 3.2 V all-organic RFB.

The implementation of redox active organics in nonaqueous redox flow batteries requires the design of molecules that exhibit high solubility (>1 M) in all battery-relevant redox states. Methods for forecasting nonaqueous solubility would be valuable for streamlining the identification of promising structures. Herein we report the development of a workflow to parametrize and predict the solubility of conformationally flexible tris-(dialkylamino)cyclopropenium (CP) radical dications. A statistical model is developed through training on monomer species. Ultimately, this model is used to predict new monomeric and dimeric CP derivatives with solubilities of >1 M in acetonitrile in all oxidation states. The most soluble CP monomer exhibits high stability to electrochemical cycling at 1 M in acetonitrile without a supporting electrolyte in a symmetrical flow cell.

This Article describes the development of 1,2-bis­(diisopropyl­amino)-3-cyclo­propenylium-functionalized (DAC-functionalized) benzene derivatives as high-potential catholytes for non-aqueous redox flow batteries. Density functional theory (DFT) calculations predict that the oxidation potentials (in CH3CN) of various DAC-benzene derivatives will range from +0.96 to +1.64 V vs Fc+/0, depending upon the substituents on the benzene ring. To test these predictions, a set of eight DAC-arene derivatives were synthesized and evaluated electrochemically. The molecule 1-DAC-4-tert-butyl-2-methoxy-5-pentafluoro­propoxybenzene was found to offer the optimal balance of high redox potential (E 1/2 = +1.19 V vs Fc+/0) and charge–discharge cycling stability (with 92% capacity retention over 116 h of cycling at 0.3 M concentration in a symmetrical flow cell). This optimal derivative was successfully deployed as a catholyte in a non-aqueous redox flow cell with butyl viologen as the anolyte to yield a 2.0 V battery.

This report describes the design of diaminocyclopropenium-phenothiazine hybrid catholytes for non-aqueous redox flowb atteries. The molecules are synthesized in ar apid and modular fashion by appending ad iaminocyclopropenium (DAC)s ubstituent to the nitrogen of the phenothiazine.C ombining av ersatile C-N coupling protocol (which provides access to diverse derivatives) with computation and structure-property analysis enabled the identification of ac atholyte that displays stable two-electron cycling at potentials of 0.64 and 1.00 Vvs. Fc/Fc + as well as high solubility in all oxidation states (! 0.45 Mi nT BAPF 6 /MeCN). This catholyte was deployed in ah igh energy density two-electron RFB,e xhibiting > 90 %c apacity retention over 266 hours of flowc ell cycling at > 0.5 Me lectron concentration.