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.