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 Na 2.9 Fe 1.7 (SO 4 ) 2.7 (PO 4 ) 0.3 cathode material with higher air stability and faster ion/electron transport kinetics compared to pristine Na 2.6 Fe 1.7 (SO 4 ) 3 cathode as evidenced by first-principles calculations. The obtained Na 2.9 Fe 1.7 (SO 4 ) 2.7 (PO 4 ) 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, Na 2.9 Fe 1.7 (SO 4 ) 2.7 (PO 4 ) 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 Na 2.9 Fe 1.7 (SO 4 ) 2.7 (PO 4 ) 0.3 could be a competitive cathode candidate for low-cost sodium-ion batteries.