Meta-substituted thienoviologen with enhanced radical stability via π-π interaction modulation for neutral aqueous organic flow batteries

The aqueous organic redox flow batteries (AORFBs) are recognized as the most promising large-scale storage technology for long-duration energy storage (LDES). Although viologen derivatives are widely used as anolyte materials for AORFBs, their practical application is strongly limited by weak conjugation and unstable radicals. Here, we present a novel thienoviologen derivative, [(NPr)2SV]Cl4, 2 SV]Cl 4 , achieved utilizing a meta-substitution approach based on the pristine viologen derivatives ( [(NPr) 2 V]Cl 4 ). Compared to other ortho-- and para-sub- stitution methods, this strategy features a locked plane configuration (0.14 degrees), degrees ), effectively regulating pi-pi interactions and suppressing reactivity between radical and oxygen, which is confirmed via X-ray single-crystal structural analyses, spectroscopy techniques, molecular dynamics simulation accompanied by linear ion trap mass spectrometry experiments. Additionally, the meta-substituted [(NPr)2SV]Cl4 2 SV]Cl 4 significantly improves water solubility (2.39 M), enhances aromaticity, and extends radical lifetime compared with para-substituted [(NPr)2TV]Cl4. 2 TV]Cl 4 . Consequently, the cycling stability of the [(NPr)2SV]Cl4-based 2 SV]Cl 4-based AORFB over 2500 cycles is 4.1 times higher than that of [(NPr)2TV]Cl4. 2 TV]Cl 4 . Furthermore, the 0.5 M battery delivers an impressive 99.83 % capacity retention during 200 cycles and a power density of 147 mW cm-2 .