Regulation of Coordination Chemistry To Accelerate Na+ Transfer Kinetics of Polyanionic Cathodes for Ultrastable Sodium-Ion Batteries

Alluaudite-type iron-based sulfate structure (Na_2+2xFe_2-x(SO₄)₃) has attracted wide attention due to its high working voltage and low cost. However, their practical application is hindered by challenges such as limited reversible capacity and sluggish transfer kinetics. Herein, we proposed an anion substitution strategy to optimize iron-based sulfate cathode materials. The electrochemical characterization and theoretical calculations confirmed a reduction in the migration barrier of Na⁺ ions in various pathways. Besides, fluorine weakened the electron density of the crystal plane, thereby impeding the continuous side reaction of the electrolyte. As expected, the NFSF cathode can exhibit a capacity of 121.5 mAh g^-1 at 12 mA g^-1 and keep a high retention of 78.8% after 1000 cycles at 600 mA g^-1. In addition, the NFSF-based cathode and hard carbon (HC)-based anode were assembled into a laboratory-scale pouch cell to demonstrate the electrochemical performance and practical applications.