Towards High‐Performance Nonaqueous Redox Flow Electrolyte Via Ionic Modification of Active Species

Nonaqueous redox flow batteries are emerging flow‐based energy storage technologies that have the potential for higher energy densities than their aqueous counterparts because of their wider voltage windows. However, their performance has lagged far behind their inherent capability due to one major limitation of low solubility of the redox species. Here, a molecular structure engineering strategy towards high performance nonaqueous electrolyte is reported with significantly increased solubility. Its performance outweighs that of the state‐of‐the‐art nonaqueous redox flow batteries. In particular, an ionic‐derivatized ferrocene compound is designed and synthesized that has more than 20 times increased solubility in the supporting electrolyte. The solvation chemistry of the modified ferrocene compound. Electrochemical cycling testing in a hybrid lithium–organic redox flow battery using the as‐synthesized ionic‐derivatized ferrocene as the catholyte active material demonstrates that the incorporation of the ionic‐charged pendant significantly improves the system energy density. When coupled with a lithium‐graphite hybrid anode, the hybrid flow battery exhibits a cell voltage of 3.49 V, energy density about 50 Wh L^?1, and energy efficiency over 75%. These results reveal a generic design route towards high performance nonaqueous electrolyte by rational functionalization of the organic redox species with selective ligand.