Boosting Charge Storage in Ferri/Ferrocyanide-Based Alkaline Redox Flow Batteries

The ferri/ferrocyanide redox couple has long served as a benchmark catholyte in aqueous redox flow batteries (RBFs), particularly under highly alkaline conditions. However, the limited alkaline solubilities of commercially available Na⁺ and K⁺ forms constrain the attainable energy density of these systems. Here, we demonstrate that Li⁺ substitution substantially boosts the alkaline solubility of ferri/ferrocyanide, offering a promising route to overcome the long-standing energy density limitation. Complementary spectroscopic and computational analyses reveal a distinct solubilization mechanism in which Li⁺ promotes mixed-solvation and aggregate-rich solution structures that sustain highly concentrated, fully dissolved electrolytes beyond the saturation behavior of Na⁺ and K⁺ analogs. When Li_3/4Fe(CN)₆ is paired with polysulfide or organic anolytes, the resulting flow cells demonstrate exceptional cycling stability and energy density in alkaline environments. Using symmetric flow cell configurations, we further elucidate the stability and decomposition mechanisms of the corresponding redox materials, revealing contrasting effects of electrolyte alkalinity: it compromises ferri/ferrocyanide stability while substantially enhancing polysulfide stability. These findings establish complementary molecular insights into redox materials solubilization and stabilization, providing a mechanistic foundation for developing energy-dense and durable alkaline RBFs.