Structure and Electrochemical Properties of Cathode Materials (Na3V2 ‒ xScx(PO4)3) for Sodium-Ion Batteries

Solid solutions Na3V2 − xScx(PO4)3 (0 < x < 2) are synthesized by the sol-gel method followed by annealing in inert atmosphere. The structure of Na3V2 − xScx(PO4)3 (x = 0.5, 1.2) compounds is studied by the method of powder X-ray diffraction. As the degree of substitution increases, the unit cell parameters and volume tend to increase on retention of the NASICON-type structure. The electrochemical properties of Na3V2 − xScx(PO4)3/C materials as the cathodes for sodium-ion batteries are studied in sodium half-cells in different potential intervals: 2.5−3.8, 2.5−4.5, and 1.0−4.5 V vs. Na/Na+. The charging curves of all materials demonstrate two plateaus: at ≈3.5 and ≈4 V vs. Na/Na+, corresponding to the successive transitions V3+/V4+ and V4+/V5+. However, the high-voltage plateau is reversible at the subsequent discharge only for the Na3V1.5Sc0.5(PO4)3/C material. This allows one to carry out the stable reversible cycling of this material in the potential interval of 1.0−4.5 V vs. Na/Na+ with the capacity higher than 170 mA h g−1, which corresponds to (de)intercalation of three Na+ per formula unit.