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^-1 at 0.02 A g^-1 and 72 mA h g^-1 at 1 A g^-1, with good capacity retention of 92.4% after 300 cycles at 0.5 A g^-1. Several electrochemical kinetic tests indicate that the reaction is dominated by capacitive behavior and that the diffusion coefficient of Zn²⁺ ions is approximately 10^-9 cm² s^-1. Furthermore, ex-situ XRD indicated a reversible insertion/extraction of Zn²⁺ ions without any phase transition.