On the Mechanism of the Improved Operation Voltage of Rhombohedral Nickel Hexacyanoferrate as Cathodes for Sodium-Ion Batteries

We reported a rhombohedral Na-rich nickel hexa­cyano­ferrate (r-NiHCF) with high discharge voltage, which also possesses long cycle stability and excellent rate capability when serving as the cathode material of Na-ion batteries. First-principles calculations suggest that the high working voltage of r-NiHCF is correlated to the asymmetric residence of Na⁺ ions in the rhombohedral framework in parallel with the low charge density at the Fe²⁺ ions. In both aqueous and ether-based electrolytes, r-NiHCF exhibits higher voltage than that of cubic NiHCF. Rate and cycle experiments indicate that r-NiHCF delivers a specific capacity of 66.8 mAh g^–1 at the current density of 80 mA g^–1, which is approximate to the theoretical capacity of r-NiHCF. A capacity retention of 96% can be achieved after 200 cycles. The excellent stability of r-NiHCF can be assigned to the absence of rhombohedral–cubic phase transition and negligible volume variation during electrochemical redox, as proven by the ex situ XRD patterns at different depths of charge/​discharge and the DFT calculations, respectively.