Nickel ferrocyanides for aqueous ammonium ion batteries

Abstract

In this report, we design a structural optimization method for Ni₂Fe(CN)₆ through a partial substitution of nickel with sodium, and investigate the electrochemical performance of a series of Na_2xNi_2−xFe(CN)₆ (x = 0, 0.25, 0.5, 0.75, and 1) materials. During the replacement of nickel with sodium, the structure has turned from a cubic phase to a rhombohedral phase, which results in the changes of electrochemical performance. The electrochemical results reveal that a partial substitution of nickel with sodium can enhance the operating potential, and Na_1.5Ni_1.25Fe(CN)₆ has a better cycling performance than the other four samples. It can provide a reversible capacity of 56.1 mA h g⁻¹ with 93.2% capacity retention after 150 cycles. Interestingly, the sodium content in Na_1.5Ni_1.25Fe(CN)₆ hinders its rate performance. Additionally, we investigate its ammonium-storage mechanism by ex situ XRD and FTIR, which suggests that Na_1.5Ni_1.25Fe(CN)₆ undergoes a phase transition when it is soaked into the electrolyte and exhibits good reversibility and a stable structure during cycling.