Mixed-Valent Redox-Equilibrated Polyoxovanadates for Symmetric Non-Aqueous Redox-Flow Batteries

Redox flow batteries (RFBs) are a key technology for decentralized, scalable electrochemical energy storage. One major challenge in RFB research is the design of redox-active electrolyte components that combine high solubility and stability with reversible multi-electron storage. Here, we report the use of the two-component polyoxovanadate co-crystal {MV13} (= (nBu4N)4[MV13O33Cl] (M= VIV=O2+ or MgOH+)) as a highly stable and highly redox-active nonaqueous RFB component. The mixed-valent (VIV/V) oxidation state of {MV13} enables a symmetric RFB configuration, while the organo-solubility of {MV13} allows deployment in acetonitrile. Previous electrochemical analyses have shown that {MV13} can reversibly store up to 16 electrons in the potential range of -2.2 to +1.4 V vs Fc+/Fc. Integration of {MV13} into a symmetric RFB results in a specific capacity of 24.5 mAh, a coulombic efficiency of 93% (at a current density j = 0.6 mA cm-2) and high stability with a capacity fading rate of 0.074% pre cycle. Operando UV-Vis spectroscopy provides initial mechanistic insights into the charge-discharge behaviour and stability of {MV13}. This study provides a blueprint for the design and integration of mixed-valent metal oxide clusters into non-aqueous electrochemical energy storage systems.