Comprehensive Study of Sodium Copper Hexacyanoferrate, as a Sodium-Rich Low-Cost Positive Electrode for Sodium-Ion Batteries

Prussian blue and its analogues are broadly recognized as positive electrodes for sodium-ion batteries, owing to their three-dimensional framework, low cost, and high capacity. However, they suffer from lower cell voltage and poor capacity utilization, which lead to low energy density. Herein, we report sodium-rich copper hexacyanoferrate (NaCuHCF) as a high potential cathode (∼3.25 V vs Na+|Na) with high purity and crystallinity, synthesized via the coprecipitation method. The cathode is used without any surface coating or modification. X-ray diffraction (XRD) along with Rietveld refinement confirms the cubic crystal lattice phase, which possesses the Fm3m space group. The NaCuHCF cathode exhibits an initial specific capacity of 64 mA h g–1 at a current rate of 0.05 A g–1 with the electrochemically active Fe2+/Fe3+ redox couple. It shows excellent cycling stability retaining 93% of the capacity over 100 cycles, with merely 0.07% capacity fading per cycle. Moreover, the cell (Na||NaCuHCF) delivers one of the highest energy density of 203 W h kgcathode–1 at a current rate of 0.05 A g–1 with excellent rate capability and minimum voltage hysteresis. Kinetic investigations reveal that the Na+ storage mechanism is controlled by surface capacitive behavior mainly at a high current rate. Additionally, the Na+ (de)intercalation mechanism is investigated by XRD, X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, and extended X-ray absorption fine structure analyses.