Spherical Shell with CNTs Network Structuring Fe-Based Alluaudite Na_2+2δFe_2−δ(SO₄)₃ Cathode and Novel Phase Transition Mechanism for Sodium-Ion Battery

Iron-based sulfate cathodes of alluaudite Na_2+2δFe_2−δ(SO₄)₃ (NFS) in sodium-ion batteries with low cost, steady cycling performance, and high voltage are promising for grid-scale energy storage systems. However, the poor electronic conductivity and the limited understanding of the phase-evolution of precursors hinder obtaining high-rate capacity and the pure phase. Distinctive NFS@C@n%CNTs (n = 1, 2, 5, 10) sphere-shell conductive networks composite cathode materials are constructed creatively, which exhibit superior reversible capacity and rate performance. In detail, the designed NFS@C@2%CNTs cathode delivers an initial discharge capacity of 95.9 mAh g⁻¹ at 0.05 C and up to 60 mAh g⁻¹ at a high rate of 10 C. The full NFS@C@2%CNTs//HC cell delivers a practical operating voltage of 3.5 V and mass-energy density of 140 Wh kg⁻¹ at 0.1 C, and it can also retain 67.37 mAh g⁻¹ with a capacity retention rate of 96.4% after 200 cycles at 2 C. On the other hand, a novel combination reaction mechanism is first revealed for forming NFS from the mixtures of Na₂Fe(SO₄)₂·nH₂O (n = 2, 4) and FeSO₄·H₂O during the sintering process. The inspiring results would provide a novel perspective to synthesize high-performance alluaudite sulfate and analogs by aqueous methods.