High-Efficiency Na-Storage Performance of a Nickel-Based Ferricyanide Cathode in High-Concentration Electrolytes for Aqueous Sodium-Ion Batteries

Nickel-based ferricyanides have attracted much attention as cathode materials for aqueous sodium-ion batteries, owing to their room-temperature synthesis and open structural framework. However, factors affecting their electrochemical performance are still not clear. Herein, the effect of electrolyte concentrations on electrochemical properties of the ferricyanide cathode has been investigated by combining cyclic voltammetry, charge/discharge tests, and electrochemical impedance microscopy. It is found that high-concentration electrolyte can not only raise the working potential, owing to increased activity of Na+ ions, but also increase the initial coulombic efficiency due to suppression of side reactions. The optimized electrolyte enables it cycling with an initial coulombic efficiency of 99.3%, excellent high-rate capability (68.1 mAhg(-1) at 0.5 C and 63.1 mAhg(-1) at 10 C), and outstanding cycling stability (96.3% capacity retention after 1000 cycles at 10 C). The finding indicates that designing high-concentration electrolytes is an effective strategy to improve electrochemical performance of ferricyanide cathodes for aqueous sodium-ion batteries.