Enhanced Synthesis and Electrochemical Performance of Cobalt Hexacyanoferrate via Protonation-Regulated...

Metal hexacyanoferrates (MHCFs) are promising cathode materials for aqueous batteries owing to their high capacity and operating voltage. However, their electrochemical performance is often compromised by the presence of structural vacancies in the hexacyanoferrate framework. Conventional strategies to mitigate these vacancy defects typically rely on slowing the synthesis kinetics, which significantly reduces production throughput and hinders large-scale applications. Herein, we introduce a protonation-regulated coprecipitation (PRC) method that leverages the protonation equilibria of hexacyanoferrate (II/III) in an acidic medium, enabling the rapid and scalable synthesis of cobalt hexacyanoferrate (CoHCF). This approach achieves a high yield of approximately 90% within 15 min while maintaining low defect density. Adjusting the initial molar ratio of hexacyanoferrates enables effective modulation of the nucleation behavior of CoHCF and fine control over the particle geometry. The CoHCF with optimal geometry exhibits a high capacity of 159.4 mAh g^-1 at a current density of 100 mA g^-1 during sodium ion insertion-extraction. A corresponding H-type cell employing a Zn metal anode delivers an outstanding energy density of 236.6 Wh kg^-1 with an energy efficiency of 93.0%. These results demonstrate the effectiveness of the PRC process in producing high-performance CoHCF, providing a scalable strategy for grid-scale aqueous energy storage technologies.