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

Abstract 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 (Na 2 Cu 0.6 Ni 0.4 [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 Na 2 Cu 1‐ x Ni x [Fe(CN) 6 ] (0≤ x ≤1) series. It exhibits a reversible capacity of 62 mAh g −1 and an average working potential of 0.62 V (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 56 mAh g −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 Na 2 Cu 0.6 Ni 0.4 [Fe(CN) 6 ] is a promising cathode candidate for aqueous SIBs.