High specific capacity FeFe(CN)6 as the cathode material in aqueous rechargeable zinc–sodium hybrid batteries†

Aqueous zinc–sodium hybrid batteries with a Prussian blue cathode have been extensively studied in recent years. However, less research has been conducted on low-cost ferric ferricyanide (FeFe(CN)₆) cathode materials. Considering that both Zn²⁺ and Na⁺ can be reversibly embedded in FeFe(CN)₆ crystals, here we focus on mixed electrolytes with different concentrations of ZnSO₄ and Na₂SO₄ in deionized water to explore the preference of FeFe(CN)₆ towards Zn²⁺ and Na⁺. As a result, by using 0.1 M ZnSO₄ + 1 M Na₂SO₄ electrolyte, a superior battery performance is obtained, which reveals that the co-function of Zn²⁺ and Na⁺ in this electrolyte promotes Zn//FeFe(CN)₆ cells to exert a superior specific capacity. In this work, FeFe(CN)₆ is synthesized by a co-precipitation method and is analyzed by XRD, SEM, etc., and then used as the cathode material in Zn–Na hybrid batteries. Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) tests show that FeFe(CN)₆ in 0.1 M ZnSO₄ + 1 M Na₂SO₄ electrolyte delivers the highest discharge/charge capacities of 165.2/165.9 mA h g⁻¹ (theoretical specific capacity: 212.2 mA h g⁻¹) at a 0.1 C current density, with good capacity retention of 84% after 200 cycles at 15 C, outperforming many of the reported Zn–Na hybrid cells.