Structural Investigation of Silver Vanadium Phosphorus Oxide (Ag₂VO₂PO₄) and Its Reduction Products

A combination of powder XRD, solid-state NMR, and electrochemical impedance spectroscopy (EIS) is utilized to study the high capacity primary lithium battery cathode material (Ag2VO2PO4), as well as its electrochemically reduced counterparts. EIS provides information about the reduction process, and powder XRD is useful in determining the structure of pristine SVPO and the 1 electron equivalent (1ee) reduced materials, which are crystalline and contain edge-sharing VO6 units. However, SVPO becomes increasingly paramagnetic and amorphous as reduction proceeds from 1ee up to 3ee. Here we probe the VO6 coordination environment in the parent SVPO material as well as the reduced materials using both static and magic-angle spinning solid-state 51V NMR spectroscopy. The chemical shielding anisotropy (CSA) and electric field gradient (EFG) NMR tensors were determined through fitting of the spinning side bands or via density functional theory (CASTEP), or both. These tensors provide useful insight into the structure of the reduced materials. All materials exhibit similar isotropic chemical shifts for 51V; however, other line-shape elements vary as the coordination environment changes upon reduction. We find that the 51V NMR tensor parameters (δCS and CQ) are better at indicating changes to the local environment than the isotropic chemical shift.