High-energy nanostructured Na₃V₂(PO₄)₂O_1.6F_1.4 cathodes for sodium-ion batteries and a new insight into their redox chemistry

A microwave-assisted solvothermal procedure is employed for the rapid fabrication of nanostructured Na₃V₂(PO₄)₂O_1.6F_1.4 with a space group of I4/mmm. This Na₃V₂(PO₄)₂O_1.6F_1.4 exhibits a high energy density (518 W h kg^-1) and impressive power performance (11127 W kg^-1 while retaining an energy density of 306 W h kg^-1) in Na-ion coin cells. Furthermore, the redox chemistry of Na₃V₂(PO₄)₂O_1.6F_1.4 was investigated by combination of electron paramagnetic resonance and X-ray diffraction techniques. The results substantiate that: (i) both V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ couples are active in this Na₃V₂(PO₄)₂O_1.6F_1.4, corresponding to a total amount of 2 electrons transferred per formula unit; (ii) the Na extraction/intercalation process is mainly a solid-solution mechanism.