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.