Insight into Electrochemical Properties and Reaction Mechanism of a Cobalt-Rich Prussian Blue Analogue Cathode in a NaSO<sub>3</sub>CF<sub>3</sub> Electrolyte for Aqueous Sodium-Ion Batteries
Dongxue Luo, Ping Lei, Guorong Tian, Yunxuan Huang, Xuefei Ren, Xingde Xiang
Abstract
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 Na2Co0.8Ni0.2[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.