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

Abstract

Nonaqueous redox fl ow batteries are emerging fl ow-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 signifi cantly increased solubility. Its performance outweighs that of the state-of-the-art nonaqueous redox fl ow 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 modifi ed ferrocene compound. Electrochemical cycling testing in a hybrid lithium-organic redox fl ow battery using the assynthesized ionic-derivatized ferrocene as the catholyte active material demonstrates that the incorporation of the ionic-charged pendant signifi cantly improves the system energy density. When coupled with a lithium-graphite hybrid anode, the hybrid fl ow battery exhibits a cell voltage of 3.49 V, energy density about 50 Wh L −1 , and energy effi ciency 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.

Recent Developments and Trends in Redox Flow Batteries

“…However, ferrocene's low solubility (e.g., <0.1 M in carbonate-based solvent) restricts its use in RFBs for grid energy storage. Recently, Wei et al described a structural modification strategy to greatly enhance the solubility of ferrocene [79]. Figure 6a shows the structure of the ferrocene derivative (Fc1N112-TFSI).…”

A chemistry and material perspective on lithium redox flow batteries towards high-density electrical energy storage

“…127 Through the density functional theory calculation, they found that the preferential sites for the solvent molecules were near and around the ionic region due to its higher positive charge density around the functionalized ferrocene cation. The ionic pendant intensifies the interactions between the solvent molecules and the functionalized ferrocene, resulting in a greatly enhanced solubility up to 0. .…”

Towards Low Resistance Nonaqueous Redox Flow Batteries

“…[20][21][22][23][24][25][26] Even fewer studies have incorporated advanced flow cell designs to minimize area specific resistance (ASR) and increase area specific power density.…”

“…8 To date, most NAqRFB studies have focused on molecular discovery and electrolyte characterization at dilute active species concentrations, [13][14][15][16][17][18][19] with only a few published attempts to engineer higher performance systems with concentrated electrolytes. [20][21][22][23][24][25][26] Even fewer studies have incorporated advanced flow cell designs to minimize area specific resistance (ASR) and increase area specific power density. 21,[27][28][29] At present, reported NAqRFBs exhibit inadequate performance and durability, 8 but identifying performance limiting factors can be difficult because shortcomings could be attributed to active species degradation (e.g., instability, insolubility, chemical incompatibility), insufficient separator selectivity (e.g., crossover, low conductivity), or poor cell design (e.g., slow mass transfer, resistive electrodes, high pressure drop).…”