Na3V2(PO4)3 as the Sole Solid Energy Storage Material for Redox Flow Sodium-Ion Battery

Redox flow batteries have considerable advantages of system scalability and operation flexibility over other battery technologies, which makes them promising for large-scale energy storage application. However, they suffer from low energy density and consequently relatively high cost for a nominal energy output. Redox targeting-based flow batteries are employed by incorporating solid energy storage materials in the tank and present energy density far beyond the solubility limit of the electrolytes. The success of this concept relies on paring suitable redox mediators with solid materials for facilitated reaction kinetics and lean electrolyte composition. Here, a redox targeting-based flow battery system using the NASICON-type Na3V2(PO4)(3) as a capacity booster for both the catholyte and anolyte is reported. With 10-methylphenothiazine as the cathodic redox mediator and 9-fluorenone as anodic redox mediator, an all-organic single molecule redox targeting-based flow battery is developed. The anodic and cathodic capacity are 3 and 17 times higher than the solubility limit of respective electrolyte, with which a full cell can achieve an energy density up to 88 Wh L-1. The reaction mechanism is scrutinized by operando and in-situ X-ray and UV-vis absorption spectroscopy. The reaction kinetics are analysed in terms of Butler-Volmer formalism.