Molecular engineering the naphthalimide compounds as High-Capacity anolyte for nonaqueous redox flow batteries

Nonaqueous redox flow batteries (NARFBs) hold great promise to offer high energy density due to the broader electrochemical window. 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 can be employed as stable anolyte material for NARFBs. 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 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 16.3 Ah L^-1 and energy density of 35.6 Wh L⁻¹ can be achieved at 1.0 M, which represent the benchmark of high-energy–density NARFBs.