Combined 7Li NMR, density functional theory and operando synchrotron X-ray powder diffraction to investigate a structural evolution of cathode material LiFeV2O7

Abstract

In our recent study, we demonstrated using ⁷Li solid-state Nuclear Magnetic Resonance (ssNMR) and single-crystal X-ray diffraction that the cathode LiFeV₂O₇ possesses a defect associated with the positioning of vanadium atoms. We proposed that this defect could be the source of extra signals detected in the ⁷Li spectra. In this context, we now apply density functional theory (DFT) calculations to assign the experimental signals observed in ⁷Li NMR spectra of the pristine sample. The calculation results are in strong agreement with the experimental observations. DFT calculations are a useful tool to interpret the observed paramagnetic shifts and understand how the presence of disorder affects the spectra behavior through the spin-density transfer processes. Furthermore, we conducted a detailed study of the lithiated phase combining operando synchrotron powder X-ray diffraction (SPXRD) and DFT calculations. A noticeable volume expansion is observed through the first discharge cycle which likely contributes to the enhanced lithium dynamics in the bulk material, as supported by previously published ssNMR data. DFT calculations are used to model the lithiated phase and demonstrate that both iron and vanadium participate in the redox process. The unusual electronic structure of the V⁴⁺ exhibits a single electron on the 3d_xy orbital perpendicular to the V–O–Li bond being a source of a negative Fermi contact shift observed in the ⁷Li NMR of the lithiated phase.