Interior-Confined Vacancy in Potassium Manganese Hexacyanoferrate for Ultra-Stable Potassium-Ion Batteries

Metal hexacyanoferrates (HCFs) are viewed as promising cathode materials for potassium-ion batteries (PIBs) because of their high theoretical capacities and redox potentials. However, the development of an HCF cathode with high cycling stability and voltage retention is still impeded by the unavoidable Fe(CN)₆ vacancies (V_FeCN) and H₂O in the materials. Here, a repair method is proposed that significantly reduces the V_FeCN content in potassium manganese hexacyanoferrate (KMHCF) enabled by the reducibility of sodium citrate and removal of ligand H₂O at high temperature (KMHCF-H). The KMHCF-H obtained at 90 °C contains only 2% V_FeCN, and the V_FeCN is concentrated in the lattice interior. Such an integrated Fe–CN–Mn surface structure of the KMHCF-H cathode with repaired surface V_FeCN allows preferential decomposition of potassium bis(fluorosulfonyl)imide (KFSI) in the electrolyte, which constitutes a dense anion-dominated cathode electrolyte interphase (CEI), inhibiting effectively Mn dissolution into the electrolyte. Consequently, the KMHCF-H cathode exhibits excellent cycling performance for both half-cell (95.2 % at 0.2 Ag⁻¹ after 2000 cycles) and full-cell (99.4 % at 0.1 Ag⁻¹ after 200 cycles). This thermal repair method enables scalable preparation of KMHCF with a low content of vacancies, holding substantial promise for practical applications of PIBs.