Electrochemical Na Extraction/Insertion of Na₃V₂O_2<i>x</i>(PO₄)₂F_{3–2<i>x</i>}

Abstract

A mixed-valence\nV³⁺/V⁴⁺ composite material\nbelonging to the Na₃V₂O_2<i>x</i>(PO₄)­2F_{3–2<i>x</i>}/C\nfamily is synthesized and the electrochemical Na extraction/insertion\nmechanism is determined using a combination of high-resolution synchrotron\nX-ray diffraction (XRD) data, X-ray absorption spectroscopy (XAS), ²³Na and ¹⁹F solid state nuclear magnetic resonance\n(NMR), double titration (for the elucidation of the vanadium oxidation\nstate), and electrochemical measurements. The vanadium oxidation state\nis found to be +3.8 for the as-prepared sample. Detailed analysis\nof the cathode structural evolution illustrated that the V⁴⁺/V⁵⁺ couple is active in this compound during electrochemical\ncycling between 2.8 V and 4.3 V. This study demonstrates how the sodium-ion\nextraction and insertion pathways in cathode materials can be followed\n(and verified) using several experimental techniques, especially when\nmultiple potential oxidation states are present in the parent compound.