Impact of the F<sup>–</sup> for O<sup>2–</sup> Substitution in Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>3–<i>y</i></sub>O<sub><i>y</i></sub> on Their Transport Properties and Electrochemical Performance
Runhe Fang, Jacob Olchowka, Chloé Pablos, Rafael Bianchini Nuernberg, Laurence Croguennec, Sophie Cassaignon
Abstract
The series of polyanionic compounds Na3V3+2–yV4+y(PO4)2F3–yOy (0 ≤ y ≤ 2) attracts much attention as positive electrode material for Na-ion batteries because of its high operating potential and stable cycling performance. A series of nanospherical Na3V3+2–yV4+y(PO4)2F3–yOy (NVPFOy) materials with y = 0.8, 1.35, 1.6, and 2 were synthesized using a solvothermal reaction, and changes in the vanadium average oxidation state were fully characterized by combining analyses of Raman and infrared spectroscopies and X-ray diffraction. Raman spectroscopy, beyond checking for the absence of a carbon coating, was in fact used for its sensitivity to the vanadium environment and turned out to be an efficient characterization technique to estimate the oxygen content within the Na3V3+2–yV4+y(PO4)2F3–yOy family. The impact of the oxygen content on the transport properties was evaluated by electrochemical impedance spectroscopy. The material with y = 1.35 demonstrates the smallest electrical resistivity in the series as well as the best rate capability and cyclability upon long-term cycling, despite no carbon coating and a high mass loading positive electrode.