Electrochemical Properties and Redox Mechanism of Na₂Ni0.4Co0.6[Fe(CN)₆] Nanocrystallites as High-Capacity Cathode for Aqueous Sodium-Ion Batteries

Nickel-based ferrocyanides have attracted wide interest as cathode for aqueous sodium-ion batteries in fields of sustainable energies such as wind and solar, owing to excellent cycling stability and high-rate capability. However, they suffer from low specific capacities (∼60 mAh g –1 ). Herein, Na 2 Ni 0.4 Co 0.6 [Fe(CN) 6 ] nanocrystallites are reported for the first time as high-capacity cathode for aqueous sodium-ion batteries. Its electrochemical properties and redox mechanism have been understood by combining the X-ray diffraction technique, Fourier-transform infrared spectroscopy, cyclic voltammetry, electrochemical impedance microscopy, and charge/discharge measurements. It is revealed that the material undergoes a reversible three-step single-phase reaction mechanism during Na extraction through sequential electrochemical oxidation of nitrogen-coordinated Co 2+ ions and carbon-coordinated Fe 2+ ions and achieves superior electrochemical performance with a high reversible capacity of 85 mAh g –1 at 0.5 C, an average operating potential of 0.62 V (vs Ag/AgCl), and a high capacity retention of 90% after 100 cycles. The combination of high specific energy and good cycling performance enables the Na 2 Ni 0.4 Co 0.6 [Fe(CN) 6 ] material exhibiting promising application for high-performance aqueous sodium-ion batteries.