Nonaqueous redox flow batteries (NARFBs) are a potential candidate for high-energy-density storage systems because of their wider electrochemical windows than that of the aqueous systems. However, the achieved energy density is relatively low due to the poor stability and low solubility of redox active materials. Herein, we demonstrate that N-substituted naphthalimides, which used to be reported as the receptor in fluorescent probes on PET, exhibit electrochemical reversibility and stability. N-(naphthalimidoethyl)-N,N-dimethyl-N-ethylammonium bis(trifluoromethane-sulfonyl)imide (NI-TFSI) was designed via an ionic modification strategy, resulting in enhanced solubility from 0.31 M to 1.22 M. The NARFB based on NI-TFSI exhibits stable cycling with 90.3% discharge capacity retention over 100 cycles (99.903% capacity retention per cycle) and high-rate capability with the average discharge capacity of 1.27 Ah L-1 even at 120 mA cm−2. Benefiting from the high solubility of the redox active materials, capacity of 16.0 Ah L-1 and energy density of 35.5 Wh L−1 can be achieved at 1.0 M, which represent the benchmark of high-energy-density NARFBs.

Molecular engineering the naphthalimide compounds as high-capacity anolyte for nonaqueous redox flow batteries

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