Stable Bifunctional Perylene Imide Radicals for High-Performance Organic-Lithium Redox-Flow Batteries

The search for high-performance organic redox-active materials for non-aqueous redox-flow batteries remains a key challenge. Organic radicals and aromatic imides are two promising classes of redox-active materials with complementary advantages, such as the specific capacity, operating voltage, and stability, etc. Herein, this work reports two stable bifunctional radicals synthesized by the C-C coupling of redox-active phenoxyl radicals and perylene diimides (PDIs, 1^. ) or benzo[ghi]perylene triimides (BPTIs, 2^. ). The incorporation of electron-deficient PDIs or BPTIs into phenoxyl radicals is desired, to not only increase the number of redox-active groups per molecule and, thus, improve their specific capacities, but also to increase the redox potential and the stability of the phenoxyl radicals and, thus, enhances their battery voltages and cycle lives. When serving as the redox-active species in the catholyte of a non-aqueous static redox-flow battery, both radicals 1^. and 2^. exhibited a cooperatively enhanced performance with an unprecedented initial discharge voltage up to 3.12 V versus Li⁺ /Li, which is the hitherto most presentable potential for imide- and radical-based energy storage materials in redox-flow batteries.