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, achieved utilizing a meta-substitution approach based on the pristine viologen derivatives ([(NPr)₂V]Cl₄). Compared to other ortho- and para-substitution methods, this strategy features a locked plane configuration (0.14°), effectively regulating π-π 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 significantly improves water solubility (2.39 M), enhances aromaticity, and extends radical lifetime compared with para-substituted [(NPr)₂TV]Cl₄. Consequently, the cycling stability of the [(NPr)₂SV]Cl₄-based AORFB over 2500 cycles is 4.1 times higher than that of [(NPr)₂TV]Cl₄. 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.