Lithium Ferrocyanide Catholyte for High‐Energy and Low‐cost Aqueous Redox Flow Batteries**

Abstract

Aqueous redox flow batteries (ARFBs) are a promising technology for grid-scale energy storage, however, their commercial success relies on redox-active materials (RAM) with high electron storage capacity and cost competitiveness. Herein, a redox-active material lithium ferrocyanide (Li 4 [Fe(CN) 6 ]) is designed. Li + ions not only greatly boost the solubility of [Fe(CN) 6 ] 4À to 2.32 M at room temperature due to weak intermolecular interactions, but also improves the electrochemical performance of [Fe(CN) 6 ] 4À /3À . By coupling with Zn, ZIRFBs were built, and the capacity of the batteries was as high as 61.64 Ah L À 1 (pH-neutral) and 56.28 Ah L À 1 (alkaline) at a [Fe(CN) 6 ] 4À concentration of 2.30 M and 2.10 M. These represent unprecedentedly high [Fe-(CN) 6 ] 4À concentrations and battery energy densities reported to date. Moreover, benefiting from the low cost of Li 4 [Fe(CN) 6 ], the overall chemical cost of alkaline ZIRFB is as low as $11 per kWh, which is one-twentieth that of the state-of-the-art VFB ($211.54 per kWh). This work breaks through the limitations of traditional electrolyte composition optimization and will strongly promote the development of economical [Fe(CN) 6 ] 4À /3Àbased RFBs in the future.