Revealing the structural chemistry in Na6-2xFex(SO4)3 (1.5 < x < 2.0) for low-cost and high-performance sodium-ion batteries

Fe-based polyanionic sulfate materials are one of the most promising candidates for large-scale applications in sodium-ion batteries due to their low cost and excellent electrochemical performance. Although great achievements have been gained on a series of Na6-2xFex(SO4)(3) (NFSO-x, 1.5 <= x <= 2.0) materials such as Na2Fe2(SO4)(3), Na2Fe1.5(SO4)(3), and Na2.4Fe1.8(SO4)(3) for sodium storage, the phase and structure characteristics on these NFSO-x are still controversial, making it difficult to achieve phase-pure materials with optimal electrochemical properties. Herein, six NFSO-x samples with varied x are investigated via both experimental methods and density functional theory calculations to analyze the phase and structure properties. It reveals that a pure phase exists in the 1.6 <= x <= 1.7 region of the NFSO-x, and part of Na ions tend to occupy Fe sites to form more stable frameworks. The NFSO-1.7 exhibits the best electrochemical performance among the NFSO-x samples, delivering a high discharge capacity (104.5 mAh g(-1) at 0.1 C, close to its theoretical capacity of 105 mAh g(-1)), excellent rate performance (81.5 mAh g(-1) at 30 C), and remarkable cycle stability over 10,000 cycles with high-capacity retention of 72.4%. We believe that the results are useful to clarify the phase and structure characteristics of polyanionic materials to promote their application for large-scale energy storage.