Ni-Containing Electrolytes for Superior Zinc-Ion Aqueous Batteries with Zinc Hexacyanoferrate Cathodes

A one-step coprecipitation process is designed to synthesize zinc hexacyanoferrate (ZnHCF) cathodes in aqueous zinc-ion batteries (ZIBs). The morphology of the cathode is influenced by the concentration of the precursor solution and valence of iron ions. The rhombohedral ZnHCF sample exhibits high crystallinity on the microscale in the cut-angle cubic structure, whereas Na-rich NaZnHCF contains many interstitial water molecules in the rhombic nanoplates. Both samples show effective insertion of Zn ions in the aqueous ZnSO₄ solution. ZnHCF shows a specific capacity of 66.7 mA h g^-1, a redox voltage of 1.73 V, and fast decline in a few cycles. On the other hand, NaZnHCF has a lower specific capacity of 48.2 mA h g^-1, showing two voltage platforms and robust cycling stability. However, owing to serious side reactions, both samples have low Columbic efficiency. To improve the properties such as Coulombic efficiency, specific capacity, and cycling stability, Ni ions are introduced by adding 10 wt % NiSO₄ to the ZnSO₄ electrolyte. The ZnHCF cathode in the Ni-containing electrolyte has the best properties such as a high specific capacity of 71.2 mA h g^-1 at a current density of 100 mA g^-1, 93% retention of the Coulombic efficiency, and a good rate performance manifested by a reversible capacity of 58.2 mA h g^-1 at 1 A g^-1. The results reveal a strategy to improve the electrochemical properties of aqueous ZIBs by modifying the electrolytes.