High-Capacity Organic Radical Mediated Phosphorus Anode for Sodium-Based Redox Flow Batteries

An innovative approach to improve the energy density of redox flow batteries (RFBs) through electrochemically mediated reactions is demonstrated. Soluble anion radical species (biphenyl and pyrene) mediate reversible sodium storage in a red phosphorus (P) anode located in an external packed bed reactor in the absence of binders or conductive additives. Because the anion radical species can be recycled several times through the cell stack during a single charge/discharge cycle, a mediated RFB effectively decouples the battery’s energy density from the redox species’ solubility in the electrolyte. The present study demonstrates the highest capacity reported to date for a mediated anode (up to 800 mAh/g P via reversible formation of Na x P). This approach represents a paradigm shift in the field and can conceivably achieve energy densities exceeding 200 Wh/kg, which is ∼10 times greater than that of conventional RFBs (e.g., 25 Wh/kg for an aqueous vanadium system).