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

Abstract

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 ] 4− to 2.32 M at room temperature due to weak intermolecular interactions, but also improves the electrochemical performance of [Fe(CN) 6 ] 4−/3− . By coupling with Zn, ZIRFBs were built, and the capacity of the batteries was as high as 61.64 Ah L −1 (pH‐neutral) and 56.28 Ah L −1 (alkaline) at a [Fe(CN) 6 ] 4− concentration of 2.30 M and 2.10 M. These represent unprecedentedly high [Fe(CN) 6 ] 4− 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 ] 4−/3− ‐based RFBs in the future.