Open-framework aluminum hexacyanoferrate as a cathode material for high voltage aqueous zinc-ion batteries: effect of Al3+ cations on three-phase transition of AlFe(CN)6

Abstract

Aqueous zinc-ion batteries (AZIBs) are considered promising energy storage devices for future large-scale energy storage systems due to their high safety, low production cost, and non-combustibility. However, low operating voltage and energy output are still drawbacks in their practical applications. Herein, we develop a cubic aluminum hexacyanoferrate/graphene high-voltage cathode material for rechargeable AZIBs, and it exhibits an activation process and can deliver a high output voltage of 1.8 V (vs. Zn/Zn²⁺) with a specific energy density of 125.56 W h kg⁻¹. The voltage is increased by stretching the Fe–C bonds in the Fe–CN–Al covalent structure. The reversible phase transformation between cubic AlFe(CN)₆ and cubic–orthorhombic–monoclinic Zn_0.5AlFe(CN)₆ occurs with the intercalation/extraction of zinc ions, and the coordination environment of Al plays a decisive role in this key process. This work will provide a feasible reference for the structural design of high-voltage Prussian blue analog cathode materials.