A novel Na8Fe5(SO4)9@rGO cathode material with high rate capability and ultra-long lifespan for low-cost sodium-ion batteries

Sodium-ion batteries (SIBs) are regarded as the most promising technology for large-scale energy storage systems. However, the practical application of SIBs is still hindered by the lack of applicable cathode materials. Herein, a novel phase-pure polyanionic Na₈Fe₅(SO₄)₉ is designed and employed as a cathode material for SIBs for the first time. The Na₈Fe₅(SO₄)₉ has an alluaudite-type sulfate framework and small Na⁺ ion diffusion barriers. As expected, the as-synthesized Na₈Fe₅(SO₄)₉@rGO exhibits a high working potential of 3.8 ​V (versus Na/Na⁺), a superior reversible capacity of 100.2 mAh g⁻¹ at 0.2 ​C, excellent rate performance (∼80 mAh g⁻¹ at 10 ​C, ∼63 mAh g⁻¹ at 50 ​C), and an ultra-long cycling life (91.9% capacity retention after 10,000 cycles at 10 ​C, 81% capacity retention after 20,000 cycles at 50 ​C). We use various techniques and computational methods to comprehensively investigate the electrochemical reaction mechanisms of Na₈Fe₅(SO₄)₉@rGO.