Synergistic ionic modification strategy enhances the stability of naphthalene diimide zwitterions for cost-effective aqueous organic redox flow batteries

Aqueous organic redox flow batteries (AORFBs) hold significant promise for energy storage due to their unique advantages and characteristics. However, their development is hindered by the lack of decomposition resistance and cycle stability over long periods. In this study, we synthesized naphthalene diimide (NDI) derivatives with zwitterions in their side chains via the atmospheric pressure method, namely (CBu)₂NDI and (SPr)₂NDI. The electrostatic repulsion between (CBu)₂NDI precisely regulates π-π stacking into a parallel-staggered pattern. The synergistic zwitterions strategy effectively mitigates the positive charge of N⁺ in (CBu)₂NDI compared with (NPr)₂NDI and dex-NDI; this not only enhances the aromaticity of the naphthalene diimide core but also inhibits the side chain decomposition caused by the S_N2 nucleophilic attack of hydroxyl ions (OH^-) on the C=O. The calculation of the single point energy proves that during the charging processes of (CBu)₂NDI, the K⁺ will be close to the naphthalene core to form dimers or monomers with lower energy configurations under electrostatic attraction. (CBu)₂NDI achieved a water solubility up to 1.49 M, which can be paired with K₄Fe(CN)₆ under two-electron transfer with total electrolyte costs as low as $6.58 Ah^-1. The 0.1 M battery maintains full capacity after 5070 cycles. Furthermore, the battery delivers an impressive 100% capacity retention under 2 M e^- during 220 cycles.