Dynamic iron migration triggers single-to-dual electron redox conversion in hexacyanoferrates for stable aqueous potassium ion batteries

Abstract

Metal hexacyanoferrate-based cathodes (HCFs) with tunable redox potentials, low cost, and high capacity are ideal for aqueous potassium ion batteries (AKIBs). However, vacancies and the high diffusivity of water in aqueous electrolytes drain metal ions in HCFs, decreasing the discharge capacity and structure integrity during cycling. To address these problems, an electrochemically induced rectification strategy is applied through a Fe ion electrolyte additive. Nickel hexacyanoferrate is selected as a model cathode. Pre- and post-cycling in the modified electrolyte (ME) revealed that the surface vacancies of the K_1.80Ni[Fe(CN)₆]_0.7498·2·51 H₂O (KNiHCF) are in-situ replenished with electroactive Fe ions and form a double-redox HCFs, resulting in enhanced conductivity and discharge capacity. Moreover, owing to the surface vacancy replenishment and gradient formation, the dissolution of KNiHCF in ME is decelerated. The resultant KNiHCF-ME cathode exhibits excellent rate performance (7000 mA g^-1) and cycling stability for both half (98.50 % after 1000 cycles at 1000 mA g^-1) and full (80.08 % after 5000 cycles at 2000 mA g^-1) cells. Moreover, similar findings have been observed for the KCuHCF and KCoHCF electrodes, demonstrating the universality of this strategy. This study contributes to the possible solution of defective HCFs with surface vacancies, which may be extended to other cathodes for AKIBs.