The Nature of the Ultrahigh Initial Coulombic Efficiency of Ni₂Fe(CN)₆ in Aqueous Ammonium-Ion Batteries

As a member of Prussian blue analogs, insoluble ferrocyanides encounter a problem wherein their initial Coulombic efficiencies are frequently higher than 100% when those values should be lower than 100% under normal circumstances. Herein, we study the topochemistry of NH₄⁺ in Ni₂Fe(CN)₆ during the electrochemical reaction and meet the same problem. The initial Coulombic efficiency of Ni₂Fe(CN)₆ is as high as 122.9%. Our investigation from ex situ X-ray photoelectron spectroscopy analysis suggests that such case is due to the oxidization of ferricyanide ions. We thereby design a simple approach to solve this problem. After the treatment, the weird initial Coulombic efficiency of the as-prepared Ni₂Fe(CN)₆ electrode has changed to 95.9%, and even its electrochemical performance is improved. Briefly, its capacity retention is modified from 81.4 to 87.3% after 85 cycles, and the rate capacities of Ni₂Fe(CN)₆-R are higher than those of Ni₂Fe(CN)₆. Our work reveals the mechanism about the weird initial Coulombic efficiency of the as-prepared Ni₂Fe(CN)₆ electrode and provides a way to solve this problem.