Achieving High-Voltage and High-Capacity Aqueous Rechargeable Zinc Ion Battery by Incorporating Two-Species Redox Reaction

Herein, a two-species redox reaction of Co(II)/Co(III) and Fe(II)/Fe(III) incorporated in cobalt hexacyanoferrate (CoFe(CN)₆) is proposed as a breakthrough to achieve jointly high-capacity and high-voltage aqueous Zn-ion battery. The Zn/CoFe(CN)₆ battery provides a highly operational voltage plateau of 1.75 V (vs metallic Zn) and a high capacity of 173.4 mAh g^-1 at current density of 0.3 A g^-1, taking advantage of the two-species redox reaction of Co(II)/Co(III) and Fe(II)/Fe(III) couples. Even under extremely fast charge/discharge rate of 6 A g^-1, the battery delivers a sufficiently high discharge capacity of 109.5 mAh g^-1 with its 3D opened structure framework. This is the highest capacity delivered among all the batteries using Prussian blue analogs (PBAs) cathode up to now. Furthermore, Zn/CoFe(CN)₆ battery achieves an excellent cycling performance of 2200 cycles without any capacity decay at coulombic efficiency of nearly 100%. One further step, a sol-gel transition strategy for hydrogel electrolyte is developed to construct high-performance flexible cable-type battery. With the strategy, the active materials can adequately contact with electrolyte, resulting in improved electrochemical performance (≈ 18.73% capacity increase) and mechanical robustness of the solid-state device. It is believed that this study optimizes electrodes by incorporating multi redox reaction species for high-voltage and high-capacity batteries.