Tailoring metal-ligand coordination in Prussian blue cathodes for long-cycling and high-rate sodium-ion batteries

Prussian blue analogs (PBAs) are widely recognized as ideal cathode materials for sodium-ion batteries (SIBs) due to their numerous advantages. In this study, we propose a novel balanced coordination environment strategy by modulating the chelating ability of a strong chelator (Na₃-Cit) with ascorbic acid (VC). This approach reduced [Fe(CN)₆]^4- vacancies and interstitial water while successfully activating the low-spin (LS) iron centers in PBAs, which exhibited an extended high-voltage plateau when used as cathode materials for SIBs. Ex situ XRD and ex situ X-ray Photoelectron Spectroscopy characterization confirmed the material's enhanced electrochemical stability and sodium storage capacity. Consequently, the FeMn-2VC samples demonstrated exceptional rate capability and cycling stability, including an ultra-long cycle life (71.88% capacity retention after 700 cycles at 0.5 A g^-1) and outstanding rate performance (105.98 mAh g^-1 at 0.5 A g^-1). Furthermore, when paired with commercial hard carbon, the PBA-based SIBs delivered a high energy density (117.8 mAh g^-1 at 10 mA g^-1) and excellent capacity retention (57.9% after 500 cycles at 0.5 A g^-1), underscoring their potential for large-scale energy storage applications.