Ligand Engineering for High-Voltage Aqueous Flow Battery Electrolytes

Flow batteries represent a promising opportunity for large-scale energy storage, particularly when built with sustainably-sourced aqueous electrolytes. Central to advancing this technology lies the rational design of electrolytes, where molecular control over coordination environment around the redox carrier is essential for performance and stability. We will present a series of studies in electrolyte engineering that leverage coordination chemistry to address fundamental challenges in redox stability, solubility, and light-induced degradation. Anionic chelating ligands stabilize high oxidation states, enable the generation of strongly reducing negolytes, and manifest high solubilities. These properties help circumvent the most common pitfalls in electrolyte design - self discharge via precipition, electrolyte decomposition, and reactivity with solvent. We have also identified and addressed a critical yet underappreciated issue: light-mediated electrolyte decomposition. Through reinvestigation of a previously reported Ce(III) complex, we demonstrate that reversible charging and stable battery performance are possible, provided that light exposure is tightly controlled. Rapid degradation arises from photo-oxidation, revealing a fundamental limitation that likely extends to all strongly oxidizing electrolytes.