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. (© 2023 Wiley‐VCH GmbH)