High‐Voltage Catholyte for High‐Energy‐Density Nonaqueous Redox Flow Battery
Jack McGrath, Rajeev K. Gautam, Xiao Wang, Jianbing Jiang
Abstract
Abstract Redox flow batteries (RFBs) with high energy densities are essential for efficient and sustainable long‐term energy storage on a grid scale. To advance the development of nonaqueous RFBs with high energy densities, a new organic RFB system employing a molecularly engineered tetrathiafulvalene derivative ( (PEG3/PerF)‐TTF ) as a high energy density catholyte was developed. A synergistic approach to the molecular design of tetrathiafulvalene ( TTF ) was applied, with the incorporation of polyethylene glycol ( PEG ) chains, which enhance its solubility in organic carbonate electrolytes, and a perfluoro ( PerF ) group to increase its redox potential. When paired with a lithium metal anode, the two‐electron‐active (PEG3/PerF)‐TTF catholyte produced a cell voltage of 3.56 V for the first redox process and 3.92 V for the second redox process. In cyclic voltammetry and flow cell tests, the redox chemistry exhibited excellent cycling stability. The Li| (PEG3/PerF)‐TTF batteries, with concentrations of 0.1 M and 0.5 M, demonstrated capacity retention rates of ~94 % (99.87 % per cycle, 97.52 % per day) and 90 % (99.93 % per cycle, 99.16 % per day), and the average Coulombic efficiencies of 99.38 % and 98.35 %, respectively. The flow cell achieved a high power density of 129 mW/cm 2 . Furthermore, owing to the high redox potential and solubility of (PEG3/PerF)‐TTF , the flow cell attained a high operational energy density of 72 Wh/L (100 Wh/L theoretical). A 0.75 M flow cell exhibited an even higher operational energy density of 96 Wh/L (150 Wh/L theoretical).