NaO₆ Octahedron-Engineered Sodium Iron Sulfate Cathodes for High-Rate and Sustainable Sodium-Ion Batteries

The alluaudite-type sulfate Na₂Fe₂(SO₄)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 Fe³⁺ migration. Herein, NaO₆ units have been incorporating into NFS to form a sodium-rich site-type sodium iron sulfate (Na_6.4[Fe_5.5Na_0.6](SO₄)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 [Fe₂O₁₀] dimers mitigates repulsion-driven Fe migration, synergistically enhancing lattice stability. Furthermore, enhanced oxygen ionicity in NaO₆ units elongates the rate-determining Na─O bond, thus enhancing the Na⁺ migration kinetics. As a result, the Na_6.4[Fe_5.5Na_0.6](SO₄)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.