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. Li3V2(PO4)3, Na3V2(PO4)3 analysed during charge-discharge cycles shows spontaneous surface lithiation is observed for Li3V2(PO4)3 after electrolyte exposure. Irreversible V5+ states are found in TEY spectra for LixNa3-xV2(PO4)3, suggesting increased structural distortion of VO6 octahedra at the surface on discharge. [Display omitted] •Study shows electronic changes of Li3V2(PO4)3, Na3V2(PO4)3 during cycling vs Li.•Li electrolyte causes surface lithiation in Li3V2(PO4)3, Na3V2(PO4)3 pre-cycling.•Na3V2(PO4)3 exhibits VO6 tetrahedral distortion and V5+ state on discharge vs Li.•Capacity loss during cycling vs Li ascribed to Li/V anti-site mixing in Na3V2(PO4)3.