Insight into Electrochemical Properties and Reaction Mechanism of a Cobalt-Rich Prussian Blue Analogue Cathode in a NaSO₃CF₃ Electrolyte for Aqueous Sodium-Ion Batteries

Prussian-blue analogues attract significant interest as cathode materials for rechargeable aqueous sodium-ion batteries (SIBs) owing to their open-framework structure and good cycling stability in aqueous electrolytes, but they usually suffer from low practical specific capacities (∼70 mA h g^–1). Herein, the electrochemical properties of the nanostructured Na₂Co_0.8Ni_0.2[Fe­(CN)₆] compound in a high-concentration NaSO₃CF₃ electrolyte are systematically investigated by the cyclic voltammetry and galvanostatic technique. It is found that the material delivers a high reversible capacity of 116.4 mA h g^–1 at the current of 50 mA g^–1 and a working potential of 0.67 V (vs Ag/AgCl) on average, achieving a high theoretical specific energy of 171 W h kg^–1 in aqueous SIBs with a NaTi₂(PO₄)₃ anode. In particular, it exhibits good cycling performance with a capacity retention of 88% after continuously charging/discharging for 100 cycles at the current of 100 mA g^–1. Furthermore, the reaction mechanism is understood by combining ex situ X-ray diffraction, FTIR spectroscopy, and Raman spectroscopy. Experimental results reveal that the material undergoes an initial structural transformation from the rhombohedral phase to the cubic phase, and a subsequent solid-solution mechanism in a wide potential range, through reversible chemistry of Co³⁺/Co²⁺ and Fe³⁺/Fe²⁺ redox couples. The findings of this work open up more opportunities for designing high-energy aqueous SIBs.