Ethylene Glycol‐Functionalization of Naphthalene Diimides in Nonaqueous Redox Flow Batteries: Balance Between Solubility, Diffusivity, and Crossover

Redox flow batteries (RFBs) offer a scalable and safe solution for storing energy from intermittent renewable sources. Naphthalene diimide (NDI) with its π‐conjugated core enables stable two‐electron redox reactions, making it a promising redox‐active material. However, its limited solubility, diffusivity, and crossover in nonaqueous solvents restrict practical applications. Here, NDI is functionalized with ethylene glycol (EG) chains of varying lengths via nucleophilic substitution. Increasing EG chain length enhances solubility in acetonitrile and reduces membrane crossover, due to increased polarity and molecular size, respectively. However, excessive EG‐functionalization reduces molecular diffusion, hindering energy efficiency. Balancing these trade‐offs, the length of EG chains is tuned to achieve high RFB performance. When employed as a negolyte in a full RFB cell paired with a ferrocene‐based posolyte, the cell retains capacity over 1000 cycles with a low fading rate of 0.00516% per cycle and 0.0060% per hour.