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

TLDR

Nonaqueous redox flow batteries promise higher energy densities than aqueous systems because of wider voltage windows, yet their performance is limited by the low solubility of redox species. This work reports a molecular engineering strategy that significantly increases the solubility of nonaqueous electrolytes. The authors designed and synthesized an ionic‑derivatized ferrocene with over 20‑fold higher solubility, characterized its solvation chemistry, and demonstrated its improved energy density in a hybrid lithium–organic flow battery. The resulting battery achieved a 3.49‑V cell voltage, about 50 Wh L⁻¹ energy density, and over 75 % efficiency, outperforming state‑of‑the‑art nonaqueous systems and validating a generic design route.

Abstract

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.