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.

Automated summary

The study unveils the mechanism behind the unusually high initial Coulombic efficiency of the prepared Ni2Fe(CN)(6) electrode, which is attributed to the oxidation of ferricyanide ions. A simple approach is designed to solve this issue, resulting in improved electrochemical performance of the electrode.