Anion electrostatic insertion boosts efficient zinc ferrocyanide cathode for aqueous dual-ion battery

Prussian blue analogs (PBAs) have been considered as a kind of promising cathode materials, but its poor cycle performance severely hinders their industrialization. Herein, by combining a zinc ferrocyanide (ZnHCF) cathode with aqueous ZnSO4/LiTFSI electrolyte and zinc metal anode, we achieved a stable anion insertion-type aqueous dual-ion battery (DIB) which displays a discharge specific capacity of 75.9 mAh g-1, an energy density of 124 Wh kg-1, and a discharge plateau of 1.8 V at 5 A g-1. Its specific capacity can still reach 57.4 mAh g-1 after 1200 cycles with a Coulombic efficiency of 97 %. A reversible anion insertion mechanism of ZnHCF cathode based on the changes of the valency of iron and the insertion of TFSI- ion on crystal surface caused by electrostatic interaction is proposed and confirmed with a series of characterizations and density functional theory calculations (DFT). This work proposes the study of anion energy storage mechanism of PBAs and brings more opportunities for their large-scale applications in the future.