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

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.