Transitioning from anhydrous Stanfieldite-type Na2Fe(SO4)2 Precursor to Alluaudite-type Na2+2δFe2-δ(SO4)3/C composite cathode: A pathway to cost-effective and all-climate sodium-ion batteries

Low-cost and long-life cathode materials, such as the polyanionic iron-based Alluaudite-type Na_2+2δFe_2-δ(SO₄)₃, are crucial for future large-scale energy storage applications. This material is typically synthesized from the hydrated precursor Na₂Fe(SO₄)₂·4H₂O. However, the vapor released during the heating of crystal water can lead to reduced crystallinity, increased fraction of Na₆Fe(SO₄)₄ impurities, and potential structural damage to Na_2+2δFe_2-δ(SO₄)₃. For the first time, we synthesized a stable Stanfieldite-type Na₂Fe(SO₄)₂ material using a well-designed sol-gel method. This approach effectively mitigates the aforementioned risks by facilitating a unique transition from anhydrous Na₂Fe(SO₄)₂ precursor to Na_2+2δFe_2-δ(SO₄)₃ cathode. The enhanced crystallinity, controllable impurity fraction, and reduced migration barriers of the pristine Na_2+2δFe_2-δ(SO₄)₃ cathode significantly improve electrochemical performance. Moreover, we constructed Na_2+2δFe_2-δ(SO₄)₃/C composite cathodes to optimize their high-rate capacity and cycling retention. At 25 °C, these composites exhibit remarkable high-rate capacity and maintain an impressive 92.2 % capacity retention after 1000 cycles at 10 C. In tests under extreme conditions at −25 °C, 0 °C, and 60 °C, they sustained over 90 % capacity retention after 100 cycles at 1 C or exceeded 95 % after 200 cycles at 2 C. Furthermore, the assembled Na_2+2δFe_2-δ(SO₄)₃/C//HC full cells demonstrate superior rate capacity and long-term cycling stability, indicating their promising potential for commercial applications.