NaO 6 Octahedron‐Engineered Sodium Iron Sulfate Cathodes for High‐Rate and Sustainable Sodium‐Ion Batteries

ABSTRACT The alluaudite‐type sulfate Na 2 Fe 2 (SO 4 ) 3 (NFS) has attracted considerable interest as the cathodes for sodium‐ion batteries (SIBs) due to its low cost and high operating voltage. However, it is plagued by sluggish Na + transport kinetics and irreversible lattice distortion arising from Fe 3+ migration. Herein, NaO 6 units have been incorporating into NFS to form a sodium‐rich site‐type sodium iron sulfate (Na 6.4 [Fe 5.5 Na 0.6 ](SO 4 ) 9 ), in which Na + substitution at Fe sites induces intrinsic Na + occupation of structurally unstable Fe sites, effectively suppressing Fe migration. Meanwhile, the enlarged Fe–Fe spacing within [Fe 2 O 10 ] dimers mitigates repulsion‐driven Fe migration, synergistically enhancing lattice stability. Furthermore, enhanced oxygen ionicity in NaO 6 units elongates the rate‐determining Na─O bond, thus enhancing the Na + migration kinetics. As a result, the Na 6.4 [Fe 5.5 Na 0.6 ](SO 4 ) 9 cathode achieves an impressive rate performance (100.2 and 75.2 mA h g −1 at 0.1 and 20C, respectively), with 98.2% capacity retention after 2000 cycles at 20 C. Moreover, the corresponding pouch cells stably operate for 500 cycles with 80.9% capacity retention. Rather than simply increasing Na occupancy at pre‐existing Na sites, this sodium‐rich site strategy introduces new Na sites at specific lattice positions, providing a viable design paradigm for advancing low‐cost polyanionic Na‐storage materials.