Nickel-Substituted Copper Hexacyanoferrate as a Superior Cathode for Aqueous Sodium-Ion Batteries

Copper-based hexacyanoferrates have received important interest as cathode candidates for aqueous sodium-ion batteries (SIBs), owing to the room-temperature synthesis route, fast reaction kinetics, and high working potential. However, they suffer from insufficient cycling performance in medium aqueous electrolytes. Herein, a nickel-substituted copper hexacyanoferrate (Na2Cu0.6Ni0.4[Fe(CN)(6)]) is reported as a superior cathode for aqueous SIBs, which is developed by studying the effect of Ni substitution on the electrochemical properties of Na2Cu1-xNix[Fe(CN)(6)] (0x1) series. It exhibits a reversible capacity of 62mAhg(-1) and an average working potential of 0.62V (vs. Ag/AgCl) at a current rate of 0.5 C. Even though being cycled at a high current rate of 10 C, it can achieve a discharge capacity of 56mAhg(-1) and render a capacity retention of 96% after 1000 cycles. Relative to most previously reported cathode materials, the material shows superior overall performance including improved specific energy, outstanding high-rate capability, and excellent cycling performance. This indicates that Na2Cu0.6Ni0.4[Fe(CN)(6)] is a promising cathode candidate for aqueous SIBs.