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.