Cu+ optimizes the FeHCF coordination environment to enhance high-voltage energy storage

Prussian blue analogs (PBAs) are considered one of the excellent cathode materials for sodium-ion batteries due to their low cost and high theoretical specific capacity, especially sodium-rich iron-based PBAs (FeHCF) can provide higher energy density. FeHCF has a poor charge/discharge platform stability at high voltages (FeC₆ moiety), which is mainly affected by its coordination environment. In this research, Cu⁺ (six-coordinated), which is close to the ionic radius of Fe²⁺, was used for substitution, the FeC₆ vacancies of FeHCF were reduced, and the coordination environment was optimized. The low Cu⁺-substituted FeHCF (Cu⁺0.625) has an optimal electrochemical performance at 8.5 mA/g with a reversible specific capacity of 142 mA h/g and FeC₆ moiety contribution of more than 68 mA h/g, which is superior to that of unmodified and other Cu²⁺-substituted FeHCFs. In situ tests demonstrate the reversible structural stability of the Cu⁺0.625, supporting the stability of their high-voltage platform capacity. This Cu⁺ substitution strategy further enriches the approach to optimize the coordination environment of sodium-rich FeHCF.