Transitioning from anhydrous Stanfieldite-type Na2Fe(SO4)2 Precursor to Alluaudite-type Na2+2sFe2-s(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 Na2+2sFe2-s(SO4)3, are crucial for future large-scale energy storage applications. This material is typically synthesized from the hydrated precursor Na2Fe(SO4)2 & sdot;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 Na2+2sFe2-s(SO4)3. For the first time, we synthesized a stable Stanfieldite-type Na2Fe(SO4)2 material using a welldesigned sol-gel method. This approach effectively mitigates the aforementioned risks by facilitating a unique transition from anhydrous Na2Fe(SO4)2 precursor to Na2+2sFe2-s(SO4)3 cathode. The enhanced crystallinity, controllable impurity fraction, and reduced migration barriers of the pristine Na2+2sFe2-s(SO4)3 cathode significantly improve electrochemical performance. Moreover, we constructed Na2+2sFe2-s(SO4)3/C composite cathodes to optimize their high-rate capacity and cycling retention. At 25 degrees 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 degrees C, 0 degrees C, and 60 degrees 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 Na2+2sFe2-s(SO4)3/C//HC full cells demonstrate superior rate capacity and long-term cycling stability, indicating their promising potential for commercial applications.