Regulation of Coordinated Oxygen Species‐Dominated Mechanochemical Degradation to Stabilize Sodium Iron Hexacyanoferrate Cathode for Sodium‐Ion Batteries

Abstract The surface coordination environment of sodium iron hexacyanoferrate (FeHCF) cathode is crucial for ensuring its lifespan in sodium‐ion batteries (SIBs). This investigation delves into the impacts of surface coordinated oxygen species on the performance of FeHCF cathode. It is demonstrated that coordinated oxygen species alter the electron structure around Fe 2+ exposed at surface vacancies during cycling, resulting in spatial heterogeneity of reactive oxygen species concentration and promoting the disordered proliferation of the cathode electrolyte interface (CEI). Simultaneously, the electronic coupling between the coordinated oxygen species and high spin Fe 2+ (HS‐Fe 2+ ) weakens the strength of nearby chemical bonds and exacerbates the deformation of Fe─N bonds during Na⁺ ion migration, thereby increasing the fracture sensitivity under stress. This adverse synergistic interaction leads to the collapse of the surface structure for the FeHCF cathode prior to the degradation of its internal framework. Herein, Na 4 Fe(CN) 6 ∙10H 2 O electrolyte additive is used to regulate the surface coordination environment of FeHCF cathodes, achieving an initial capacity of 91.6 mAh g −1 with excellent capacity retention of 83.5% after 2000 cycles at 1 C. These research findings elucidate the predominance of surface coordinated oxygen species in the degradation mechanism of battery performance and provide critical insights for developing durable FeHCF cathodes for SIBs.