Controlling π–π Interactions of Highly Soluble Naphthalene Diimide Derivatives for Neutral pH Aqueous Redox Flow Batteries

Abstract Organic redox‐active molecules are a promising platform for designing sustainable, cheap, and safe charge carriers for redox flow batteries. However, radical formation during the electron‐transfer process causes severe side reactions and reduces cyclability. This problem is mitigated by using naphthalene diimide (NDI) molecules and regulating their π–π interactions. The long‐range π‐stacking of NDI molecules, which leads to precipitation, is disrupted by tethering four ammonium functionalities, and the solubility approaches 1.5 m in water. The gentle π–π interactions induce clustering and disassembling of the NDI molecules during the two‐electron transfer processes. When the radical anion forms, the antiferromagnetic coupling develops tetramer and dimer and nullifies the radical character. In addition, short‐range‐order NDI clusters at 1 m concentration are not precipitated but inhibit crossover. They are disassembled in the subsequent electron‐transfer process, and the negatively charged NDI core strongly interacts with ammonium groups. These behaviors afford excellent RFB performance, demonstrating 98% capacity retention for 500 cycles at 25 mA cm^‐2 and 99.5% Coulombic efficiency with 2 m electron storage capacity.