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

Abstract

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 (Na6.4[Fe5.5Na0.6](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 Na6.4[Fe5.5Na0.6](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.