CNT: The Protector of Fe²⁺ and Accelerator of Heat in the Calcination of Na_2.65Fe_1.675(SO₄)₃ Cathode

Na_2+2xFe_2-x(SO₄)₃ (NFS) is considered as a potential cathode material of sodium-ion batteries (SIBs) due to abundant and extensive distribution of raw materials, high operating voltage, and extremely low cost. Composite with carbon materials has been proved an effective way to optimize poor rate capability and low capacity caused by the intrinsic low electrical conductivity of NFS. However, the function of carbon materials during the calcination process has not been studied yet. Herein, we compare the calcination process of NFS precursors with and without CNT synthesized by the spray drying method. It is shown that CNT can inhibit the oxidation of Fe²⁺, enhance heat transfer and uniformity of reaction. Moreover, the Mössbauer spectroscopy shows that the addition of CNT not only protects the NFS from oxidation, but also inhibits the distortion of FeO₆ octahedra. The best sample NFS-C-390 shows excellent cycle stability, which maintains 84.37% capacity retention after 5000 cycles at 5 C in pouch cells with hard carbon as anode. This work reveals the mechanism of carbon materials during calcination process and provides theoretical support for carbon addition in the production of Na_2+2xFe_2-x(SO₄)₃ based material.