Crystal structures and electronic properties for the over-lithiated and Li–Ag substituted phases of Li9V3(P2O7)3(PO4)2 insertion electrode system

Abstract

For the Li9V3(P2O7)3(PO4)2 insertion electrode system with a multiple-electron reaction, the over-lithiated phase LixV3(P2O7)3(PO4)2 with 9<x≤10 and the Li–Ag substituted phase Li9−yAgyV3(P2O7)3(PO4)2 with 0<y≤1 are synthesized, and their crystal structures and electronic properties are explored through measurements of X-ray diffraction with a four-circle diffractometer, electrochemistry, electrical resistivity, and magnetization. Both the structures are isomorphous to that of Li9V3(P2O7)3(PO4)2 which has (VP22O7)3(P1O4)2 layers in the ab-plane built up of VO6 octahedra sharing corners with P1O4 tetrahedra and P22O7 groups. In LixV3(P2O7)3(PO4)2, the over-inserted Li4 ion surrounded by three oxygens in a planar triangular arrangement resides with a significantly large thermal parameter in the tunnels formed by the stacking of the layers and in between the octahedral Li1 sites, leading to a linear chain of Li ions with the distance 3.40Å parallel to the direction of diffusion pathways. In Li9−yAgyV3(P2O7)3(PO4)2, where the Li1 site is substituted by Ag, the reduction of the electron hopping and the electrostatic potential for V ions are revealed in comparison with those of unsubstituted parent, which may account for the increase of mean voltage in the Li ion battery.