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A mixed-valence V3+/V4+ composite material belonging to the Na3V2O2x(PO4)2F3-2x/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.
Serras et al. (Tue,) studied this question.