Cobalt Hexacyanoferrate Cathode with Stable Structure and Fast Kinetics for Aqueous Zinc-Ion Batteries.

Prussian blue analogues (PBAs) show great promise as cathode candidates for aqueous zinc-ion batteries thanks to their high operating voltage, open-framework structure, and low cost. However, suffering from numerous vacancies and crystal water, the electrochemical performance of PBAs remains unsatisfactory, with limited capacity and poor cycle life. Here, a simple coprecipitation method is shown to synthesize well-crystallized cobalt hexacyanoferrate (CoHCF) with a small amount of water and high specific surface area. Benefitting from two redox-active sites, CoHCF could deliver 104.6 mA h g at 0.02 A g and 72 mA h g at 1 A g, with good capacity retention of 92.4% after 300 cycles at 0.5 A g. Several electrochemical kinetic tests indicate that the reaction is dominated by capacitive behavior and that the diffusion coefficient of Zn ions is approximately 10 cm s. Furthermore, ex-situ XRD indicated a reversible insertion/extraction of Zn ions without any phase transition.