(Invited) 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. Na+/Na for V3+/V4+ in Na3V2(PO4)2F3) could allow the achievement of 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.During this talk, through the review of redox mechanisms and structural evolutions occuring upon electrochemical operation of selected widely studied materials, we will 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).1-4 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. Acknowledgements This study is performed in the frame of the French RS2E and European Alistore-ERI networks on battery research. The authors thank Région Nouvelle Aquitaine, the French National Research Agency (STORE-EX Labex Project ANR-10-LABX-76-01) and the European Union’s Horizon 2020 research and innovation program under grant agreement No875629 (NAIMA) for their financial support.