Regulating Framework Stability of Sodium Iron Sulfate Cathodes for Practical Sodium–Ion Batteries

Alluaudite–type sodium iron sulfates are attractive low–cost, high–voltage cathodes for sodium–ion batteries (SIBs), as the strong inductive effect of SO 4 2− lifts the Fe 2+ /Fe 3+ redox couple to ∼3.8 V. However, poor intrinsic framework stability hinders their practical deployment, causing Fe–Fe Coulombic repulsion–induced antisite defects, sluggish Na + diffusion, and moisture–triggered surface hydration. Herein, a cation–substitution strategy is developed to concurrently regulate the bulk and surface framework stability of Na 2+2x Fe 2−x (SO 4 ) 3 via partial Cu–for–Fe occupation. X–ray absorption spectroscopy, in situ X–ray diffraction, and density functional theory calculations reveal that Cu incorporation enlarges Fe–Fe separation, widens Na + migration channels, narrows the band gap from 3.27 to 2.23 eV, and suppresses surface H 2 O affinity. The optimized Na 2.6 Fe 1.6 Cu 0.1 (SO 4 ) 3 /C (NFCS/C) cathode delivers a reversible capacity of 109.6 mAh g −1 at 0.1C, retains 78.3% capacity over 10 000 cycles at 5C, sustains operation from −20°C to 80°C, and preserves its electrochemistry after 168 h ambient air exposure. The assembled HC//NFCS/C full cell with 5.1 mg cm −2 cathode loading achieves 89.6% capacity retention after 1300 cycles at 1C. This work establishes cation substitution as an effective handle for concurrent bulk–surface stabilization of alluaudite sulfates, advancing a viable route toward high–voltage, long–life SIBs.