Abstract
Advancing grid-scale energy storage technologies is crucial for realizing a fully renewable energy landscape, with non-aqueous redox flow batteries (NRFBs) presenting a promising solution. One of the current challenges in NRFBs stems from the low energy density of redox active materials, primarily due to their limited solubility in non-aqueous solvents. Herein, this study explores the solubility of vanadium(IV/V) bis-hydroxyiminodiacetate (VBH) crystals in acetonitrile, aiming to use them as anionic catholytes in NRFBs. We focused on enhancing VBH solubility by modifying the structure of the alkylammonium cation. Employing periodic density functional theory and a solvation model, we calculated the dissolution free energy (Delta Gdis* ${({\rm \Delta }{G}_{dis}<^>{<^>{\ast}}}$ ), which includes sublimation ( Delta Gsub* ${{\rm \Delta }{G}_{sub}<^>{<^>{\ast}}}$ ) and solvation ( Delta Gsol* ${{\rm \Delta }{G}_{sol}<^>{<^>{\ast}}}$ ) energies. Our results indicate that neither elongating straight-chain alkyl groups beyond a tetrabutylammonium baseline nor introducing bulky substituents at the nitrogen center significantly enhances solubility. However, the introduction of carbon spacers combined with terminal bulky substituents markedly improves solubility by favorably altering both Delta Gsub* ${{\rm \Delta }{G}_{sub}<^>{<^>{\ast}}}$ and Delta Gsol* ${{\rm \Delta }{G}_{sol}<^>{<^>{\ast}}}$ . These findings underline the nuanced impact of cation structure on solubility and suggest a viable approach to optimize VBH-based anionic catholytes. This advancement promises to enhance NRFB efficiency and sustainability, marking a significant step forward in energy storage technology.