| Summary: | 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 V<sup>2+</sup> to V<sup>5+</sup>) combined with the high voltage of corresponding redox couples (e.g., 4.0 V vs. for V<sup>3+</sup>/V<sup>4+</sup> in Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>3</sub>) 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 V<sup>3+</sup>/V<sup>4+</sup> and V<sup>4+</sup>/V<sup>5+</sup> redox couples as a function of crystal structure is noticed. Moreover, vanadium phosphates that operate at high V<sup>3+</sup>/V<sup>4+</sup> 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 PO<sub>4</sub> groups around vanadium octahedra, that allows or prevents the formation of the vanadyl distortion (O<b><sup>…</sup></b>V<sup>4+</sup>=O or O<b><sup>…</sup></b>V<sup>5+</sup>=O). While the vanadyl entity massively lowers the voltage of the V<sup>3+</sup>/V<sup>4+</sup> and V<sup>4+</sup>/V<sup>5+</sup> couples, it considerably improves the reversibility of these redox reactions. Therefore, anionic substitutions, mainly O<sup>2−</sup> by F<sup>−</sup>, 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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