Direct Spectroscopic Observation of the Reversible Redox Mechanism in A3V2(PO4)3(A=Li,Na) Cathode Materials for Li-ion Batteries

Alkali Vanadium (III) phosphates are a promising class of positive electrode materials for Li-, Na-and K-ion battery applications. Herein, the reversible redox mechanism of Li3V2(PO4)3 and Na3V2(PO4)3 cathodes during charge/discharge vs. Li+/Li0 is directly observed by Soft X-ray Absorption Spectroscopy. For both Li3V2(PO4)3 and Na3V2(PO4)3 spontaneous surface Li + insertion is observed after exposure to Li + electrolyte. Furthermore, for Li3V2(PO4)3 a reversible charge-discharge process is observed from 3.0 V-4.8 V with only slight increase in O contributions to O 2p -V 3d unoccupied hybridised states after charge/discharge vs. Li+/Li0. For Na3V2(PO4)3, persistent VO6 tetrahedral distortion and an accompanying V5+ state is observed, particularly at the surface, after discharge vs. Li+/Li. These observations further elucidate the particular atomic and electronic structure changes and charge-discharge mechanisms of high voltage NASICON and anti-NASICON cathode materials for Li-ion battery applications.