Designing Alkylammonium Cations for Enhanced Solubility of Anionic Active Materials in Redox Flow Batteries: The Role of Bulk and Chain Length

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 (ΔGdis* ${({\rm \Delta }{G}_{dis}^{^{\ast}}}$ ), which includes sublimation ( ΔGsub* ${{\rm \Delta }{G}_{sub}^{^{\ast}}}$ ) and solvation ( ΔGsol* ${{\rm \Delta }{G}_{sol}^{^{\ast}}}$ ) 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 ΔGsub* ${{\rm \Delta }{G}_{sub}^{^{\ast}}}$ and ΔGsol* ${{\rm \Delta }{G}_{sol}^{^{\ast}}}$ . 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. We investigated the solubility of vanadium bis‐hydroxyiminodiacetate (VBH) crystals in acetonitrile, a key factor for their use as anionic catholytes in non‐aqueous redox flow batteries (NRFBs). By altering alkylammonium cation structures through modifications in chain length, bulky groups, and carbon spacers, the dissolution energy components – sublimation and solvation – are modulated to enhance solubility.