Imide-Based Multielectron Anolytes as High-Performance Materials in Nonaqueous Redox Flow Batteries

Recent developments toward high-energy-density all-organic redox flow batteries suggest the advantageous use of molecules exhibiting multielectron redox events. Following this approach, organic anolytes are developed that feature multiple consecutive one-electron reductions. These anolytes are based on N-methylphthalimide, which exhibits a single reversible reduction at a low potential with good cycling stability. Derivatives with two or three imide groups were synthesized to enable multielectron reduction events. By incorporating suitably designed side chains, a volumetric capacity of 65 Ah/L is achieved in electrolyte solutions. Bulk-electrolysis experiments and UV-vis-NIR absorption spectroscopy revealed good cycling stability for the first and second reduction of monoamides and diimides, respectively, but a loss of stability for the third reduction of triimides. We identify N-2-pentyl-N′-2-(2-(2-methoxyethoxy)ethoxy)ethylaminepyromellitic diimide as a very promising multielectron anolyte with an excellent volumetric capacity and superior cycling and shelf-life stability compared to monoimides and triimides. The outstanding performance of this anolyte was demonstrated in proof-of-principle redox flow batteries that reach an energy density of 24.1 Wh/L.