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Towards reversible high-voltage multi-electron reactions in alkali-ion batteries using vanadium phosphate positive electrode materials

Abstract : Vanadium phosphate positive electrode materials attract great interest in the field of Alkali-ion (Li, Na and K-ion) batteries due to their ability to store several electrons per transition metal. These multi-electron reactions (from V2+ to V5+) combined with the high voltage of corresponding redox couples (e.g., 4.0 V vs. for V3+/V4+ in Na3V2(PO4)2F3) could allow the achievement the 1 kWh/kg milestone at the positive electrode level in Alkali-ion batteries. However, a massive divergence in the voltage reported for the V3+/V4+ and V4+/V5+ redox couples as a function of crystal structure is noticed. Moreover, vanadium phosphates that operate at high V3+/V4+ voltages are usually unable to reversibly exchange several electrons in a narrow enough voltage range. Here, through the review of redox mechanisms and structural evolutions upon electrochemical operation of selected widely studied materials, we identify the crystallographic origin of this trend: the distribution of PO4 groups around vanadium octahedra, that allows or prevents the formation of the vanadyl distortion (O…V4+=O or O…V5+=O). While the vanadyl entity massively lowers the voltage of the V3+/V4+ and V4+/V5+ couples, it considerably improves the reversibility of these redox reactions. Therefore, anionic substitutions, mainly O2− by F−, have been identified as a strategy allowing for combining the beneficial effect of the vanadyl distortion on the reversibility with the high voltage of vanadium redox couples in fluorine rich environments.
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https://hal.archives-ouvertes.fr/hal-03196199
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Soumis le : lundi 12 avril 2021 - 15:43:06
Dernière modification le : mercredi 1 décembre 2021 - 15:00:09
Archivage à long terme le : : mardi 13 juillet 2021 - 18:59:36

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2021-056.pdf
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Edouard Boivin, Jean-Noël Chotard, Christian Masquelier, Laurence Croguennec. Towards reversible high-voltage multi-electron reactions in alkali-ion batteries using vanadium phosphate positive electrode materials. Molecules, MDPI, 2021, 26 (5), 1428 (22 p.). ⟨10.3390/molecules26051428⟩. ⟨hal-03196199⟩

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