Anion-doped Na2.9Fe1.7(SO4)2.7(PO4)0.3 cathode with improved cyclability and air stability for low-cost sodium-ion batteries

Polyanion-type Fe-based sulfates are one of the most promising cathode candidates for sodium-ion batteries (SIBs) due to the low cost and high voltage. However, their practical application is still hindered by the poor air stability. Herein, we report a novel anion-doped Na2.9Fe1.7(SO4)2.7(PO4)0.3 cathode material with higher air stability and faster ion/electron transport kinetics compared to pristine Na2.6Fe1.7(SO4)3 cathode as evidenced by first-principles calculations. The obtained Na2.9Fe1.7(SO4)2.7(PO4)0.3 not only manifests a high discharge capacity of 100.4 mAh g-1 with an operating voltage of ~3.57 V (Na+/Na), but also exhibits excellent rate performance (~86.7 mAh g-1 at 30 C) and cycle stability over 6000 cycles (capacity retention of 76.3%). Particularly, Na2.9Fe1.7(SO4)2.7(PO4)0.3 cathode still maintain its original crystal structural and sodium-storage property after exposure to air for one week. The high-voltage, long-life, and air-stable Na2.9Fe1.7(SO4)2.7(PO4)0.3 could be a competitive cathode candidate for low-cost sodium-ion batteries.