Highly Soluble Dimethoxymethyl Tetrathiafulvalene with Excellent Stability for Non-Aqueous Redox Flow Batteries

Non-aqueous redox flow batteries (RFBs) are highly attractiveforgrid-scale energy storage applications because of their independentdesign of energy and power, high energy density and efficiency, easymaintenance, and potentially low cost. In order to develop activemolecules with large solubility, excellent electrochemical stability,and high redox potential for a non-aqueous RFB catholyte, herein,two flexible methoxymethyl groups had been attached to a famous redox-activetetrathiafulvalene (TTF) core. The strong intermolecular packing ofthe rigid TTF unit was effectively depressed, leading to a dramaticallyimproved solubility of up to 3.1 M in conventional carbonate solvents.The performance of the obtained dimethoxymethyl TTF (DMM-TTF) wasstudied in a semi-solid RFB system with Li foil as the counter electrode.When using porous Celgard as the separator, the hybrid RFB with 0.1M DMM-TTF had two high discharge plateaus at 3.20 and 3.52 V and alow capacity retention of 30.7% after 100 cycles at 5 mA cm(-2). Replacing Celgard with a permselective membrane, the capacity retentionwas increased to 85.4%. Further increasing the concentration of DMM-TTFto 1.0 M and current density to 20 mA cm(-2), thehybrid RFB exhibited a high volumetric discharge capacity of 48.5A h L-1 and an energy density of 154 W h L-1. The capacity was maintained at 72.2% after 100 cycles (10.7 days).The great redox stability of DMM-TTF was revealed by UV-visand H-1 NMR tests and verified by density functional theorycalculations. Therefore, the methoxymethyl group is an excellent groupto increase the solubility while maintaining the redox capabilityof TTF for high-performance non-aqueous RFBs.