The electronic structure and lithiation of electrodes based on vanadium–oxide nanotubes

Abstract

The effect of lithium insertion (lithiation) in electrodes of vanadium oxide nanotubes has been studied using resonant soft-x-ray emission spectroscopy (RSXES) and soft x-ray absorption spectroscopy (SXAS). Spectra at the V $L$ edges were recorded ex-situ on a series of cycled electrodes that had been discharged to different potentials (3.0, 2.5, 2.0, and 1.8 V). Different chemical states upon discharge effectively correspond to different doping levels: the lower the cell potential, the higher the lithiation. The RSXES and SXAS spectra reflect the occupied and unoccupied electronic states, respectively. Spectral changes are observed as a function of the reduction process of vanadium by the electrochemical process. Lithiation induces a gradual enhancement of features attributed to occupied $\mathrm{V}\text{\hspace{0.05em}3d}$ states in the RSXES spectra, indicating that the intercalated charges occupy bands of $\mathrm{V}\text{\hspace{0.05em}3d.}$ Both SXAS and RSXES results suggest a partial reduction to ${\mathrm{V}}^{\text{3+}}$ at potentials below 2.0 V and the presence of a mixture of three different oxidation states. RSXES spectra also reveal the low energy-loss feature of 1 eV, suggesting strong correlation of the $\mathrm{V}\text{\hspace{0.05em}3d}$ orbitals.