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

A mixed-valence V³⁺/V⁴⁺ composite material belonging to the Na₃V₂O_2<i>x</i>(PO₄)­2F_{3–2<i>x</i>}/C family is synthesized and the electrochemical Na extraction/insertion mechanism is determined using a combination of high-resolution synchrotron X-ray diffraction (XRD) data, X-ray absorption spectroscopy (XAS), ²³Na and ¹⁹F solid state nuclear magnetic resonance (NMR), double titration (for the elucidation of the vanadium oxidation state), and electrochemical measurements. The vanadium oxidation state is found to be +3.8 for the as-prepared sample. Detailed analysis of the cathode structural evolution illustrated that the V⁴⁺/V⁵⁺ couple is active in this compound during electrochemical cycling between 2.8 V and 4.3 V. This study demonstrates how the sodium-ion extraction and insertion pathways in cathode materials can be followed (and verified) using several experimental techniques, especially when multiple potential oxidation states are present in the parent compound