Tetrathiafulvalene (TTF) derivatives as catholytes for dual-type redox flow batteries: molecular engineering enables high energy density and cyclability

Abstract

Redox flow batteries (RFBs) have received increasing attention on large-scale energy storage owing to their ability to decouple energy and power. Despite remarkable progress, the application of RFBs is greatly restricted by their limited energy density, resulting from the limited solubility of the redox species. It is still challenging to develop RFBs with high energy density and cyclability owing to the inherent instability of redox materials and high-concentration-induced parasitic reactions. Herein, we demonstrate the viability of a new family of redox compounds, tetrathiafulvalenes (TTFs), in both slurry- and solution-based RFBs using aqueous and nonaqueous electrolytes, respectively. In former batteries, pristine TTF was derivatized with four cyanoethyl chains (CN-TTF) to suppress the solubility and obtain suspended CN-TTF in an aqueous electrolyte as a slurry. The incorporation of the hydrophobic chains into TTF enhanced the redox stability by suppressing the dimerization of TTF while preserving the highly stable redox properties of TTF, resulting in a capacity retention of 75.8% after 1000 cycles (99.97% per cycle) (226 h, 9.4 d). Furthermore, TTF was derivatized with four poly(ethylene glycol) chains (PEGn-TTF, n = 1 and 3) to obtain a concentration of 0.5 M in a carbonate electrolyte, corresponding to an electron concentration of 1.0 M. When paired with Li as the anode, the solution-based battery exhibited a cell voltage of 3.64 V and a capacity retention of 82.9% after 18.5 d and a high energy density of 88 W h L⁻¹. This study introduces a new family of organic compounds for the dual-type flow battery applications and provides molecular design principles for the development of robust organic materials with desirable properties for large-scale energy storage.