Facile synthesis of Na2FexFe1 − x(SO4)2(OH)x material as a cathode for sodium-ion batteries

In the present work, a new cathode material of Na2FexFe1 − x(SO4)2(OH)x is synthesized for sodium-ion batteries, and its morphologies and structures are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). SEM results show that the morphology of Na2FexFe1 − x(SO4)2(OH)x is dumbbell-shaped nanorods with the length of about 1.3 μm and the width of 0.3–0.4 μm. The obtained material exhibits the initial reversible capacity that is 106 mAh g−1 and 154 mAh g−1 in the potential range 2.0~4.2 V and 1.5~4.5 V, respectively. After 65 cycles, the reversible capacity decreases to 74 mAh g−1 and 115 mAh g−1 with capacity retention of 70% and 74%, respectively, displaying an excellent cycle performance. Furthermore, the first charge process of Na2FexFe1 − x(SO4)2(OH)x electrode is investigated by electrochemical impedance spectroscopy (EIS) at different potentials, and the spectra exhibit four semicircles and a slightly inclined line that appear successively as the frequency decreases, representing the Na-ion migration in solid electrolyte interface film (SEI film), the electronic properties of materials, charge transfer, solid state diffusion, and phase transition. According to the results of equivalent circuit analysis, the change of kinetic parameters for desodiation process of Na2FexFe1 − x(SO4)2(OH)x electrode as a function of potential is discussed in detail.