Ion-Coupled Electron Transfer of Naphthalene Diimide for Organic Redox Flow Batteries

Organic redox flow batteries (RFBs) have been investigated for future energy storage systems (ESSs). Tailoring organic redox-active molecules tunes solubility, redox potential, and chemical stability, which gives the promise to enhance energy density, cyclability, and calendar life in RFBs. Numerous studies focused on increasing molecular solubility to satisfy Econo-technical levels. In addition, introducing electron-donating and withdrawing groups to the redox-active core modulated redox potentials to negative and positive shifts, respectively. The most effective way to multiply raising energy density is to find a redox-active core undergoing multiple electron-transfer processes. Quinone is the representative one providing a single two electrons transfer in an aqueous electrolyte solution. Fast redox kinetics and chemical stability are achieved by H⁺ coupling; The reduced form, hydroquinone, promoted the following electron transfer in the acidic solution. A similar concept is applied for non-aqueous electrolyte solutions and using cations of supporting electrolytes. However, such an ion-coupled electron transfer was rarely studied to date.<br/>Here, we demonstrated naphthalene diimide (NDI) and Li⁺ coupled electron transfer in acetonitrile (MeCN). The NDI is an excellent model for the stepwise two-electron transfer process. Its low solubility by the strong p-stacking interaction was surmounted by introducing ammonium cationic substituents to the NDI via simple condensation and <i>N</i>-alkylation. As a result, two ammonium-tethered NDI and bistriflimide (TFSI^-) as the counter anion showed 0.9 M solubility in MeCN. Two cathodic events made an anionic radical and dianionic NDI core, respectively. The received electron is delocalized over the NDI core and also stabilized by pairing it with the cation. Cyclic voltammograms showed ~120 mV of potential difference from two cathodic waves with Li⁺ of LiTFSI electrolyte in MeCN, which was narrower than ~370 mV with K⁺ of KTFSI. It suggested that the Li⁺ possessing high charge density was closely coupled with the anionic radical NDI, expediting the second electron-transfer process. The process was also dependent on non-aqueous solvents. High donor-number (DN) solvents widened the potential difference with Li⁺ because a thick solvation shell of Li⁺ weakened the ion coupling. We applied this system for non-aqueous RFBs. Unlike two distinct galvanostatic plateaus appearing with K⁺, the ammonium-introduced NDI with Li⁺ showed almost a single overlaid plateau caused by the rapid reduction of the anionic radical. The ammonium-tethered NDI/Li⁺ in MeCN cells performed high cyclability and low crossover through an anion exchange membrane, showing a capacity fading rate of 0.0089% for 1000 cycles in RFBs.