Unlocking Phase Purity of Sodium Iron Sulfate for Low-Cost and High-Performance Sodium-Ion Batteries

The alluaudite-type sulfate Na₂Fe₂(SO₄)₃ has gained significant attention as a promising cathode material for sodium-ion batteries (SIBs). However, the inevitable formation of impurities during synthesis and the irreversible structural distortion caused by Fe–Na exchange during electrochemical reactions severely hinder its electrochemical performance. Herein, we tackle these challenges by engineering an enlarged Fe–Fe distance in the lattice through partial PO₄^3– substitution. This strategic modification significantly alleviates the Coulombic repulsion between Fe ions and effectively prevents Fe-migration during the electrochemical reaction. Moreover, the unique ion state within the structure ensures enhanced ion/electron transport kinetics, minimal volume change, and a stable framework conducive to long cycling life. Notably, the novel Fe-fully occupied phase-pure Na_2.5Fe₂(SO₄)_2.5(PO₄)_0.5 [also denoted as Na₅Fe₄(SO₄)₅(PO₄)] electrode delivers a record-high discharge capacity of 112 mA h g^–1 at 0.2C, coupled with exceptional cycling stability with 88.8% capacity retention over 10,000 cycles at 10C. Additionally, the enhanced adsorption energy of Na_2.5Fe₂(SO₄)_2.5(PO₄)_0.5 cathode toward H₂O contributes to its outstanding air stability in humid atmosphere. This finding offers valuable insights for the development of advanced, low-cost materials for SIBs.