Cu-substituted nickel hexacyanoferrate with tunable reaction potentials for superior ammonium ion storage

In this work, a variety of CuxNi2–xFe(CN)6 (x = 0, 0.4, 0.8, 1.2, 1.6, 2) cathodes for ammonium ion batteries are prepared and their electrochemical performances are investigated. During the introduction of copper in nickel hexacyanoferrate, the electrochemical performance varies without changing the structure of nickel hexacyanoferrate. The increase of Cu content in nickel hexacyanoferrate leads to the enhancement of reaction potential and capacity. Electrochemical results suggest that the substitution of Cu for Ni has a positive effect on improving the cycling stability and rate capacity of nickel hexacyanoferrate when x in CuxNi2–xFe(CN)6 is less than 0.4. Therefore, Cu0.4Ni1.6Fe(CN)6 exhibits the best cycling performance (capacity retention of 97.54% at 0.3 C) and the highest rate capacity (41.4 mAh g–1 at 10 C) in CuxNi2–xFe(CN)6. Additionally, the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) tests also reveal that the structural evolution of Cu0.4Ni1.6Fe(CN)6 is highly reversible upon NH4+ storage. Therefore, this work proposes a candidate material for ammonium-ion batteries and offers a novel avenue for adjusting the operating potential of the material.