Journal
CHEMISTRY OF MATERIALS
Volume 28, Issue 21, Pages 7683-7692Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.chemmater.6b02659
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Funding
- RS2E
- Region Aquitaine
- French National Research Agency [ANR-10-LABX-76-01, ANR-13-RESC-0001-02]
- European Union [646433-NAIADES]
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Polyanionic materials such as Na3V2(PO4)(2)F3-yOy (0 <= y <= 2) are of high interest as positive electrode for Na-ion batteries since they offer competitive electrochemical performances compared to sodiated transition metal oxides. The composition Na3V2(PO4)(2)F-3 (y = 0) has the highest theoretical energy density among the series, but surprisingly a lot of discrepancies are reported throughout the literature considering its structure and its electrochemical properties. We will show that most of the compounds reported as being Na3V2III(PO4)(2)F-3 are in fact slightly oxidized due to synthesis conditions resulting in a partial oxygen substitution for fluorine. To get an in-depth understanding of this system, a series of compositions Na3V2(PO4)(2)F3-yOy (0 <= y <= 0.5, i.e., near the fluorine-rich composition) was synthesized and characterized combining synchrotron X-ray diffraction, X-ray absorption spectroscopy, solid state nuclear magnetic resonance spectroscopy, and galvanostatic electrochemical tests. The structural features and electrochemical signatures of these oxidized compounds will be carefully compared to those recently obtained for Na3V2III(PO4)(2)F-3 by Bianchini et al.
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