Redox Behavior of vanadium oxide nanotubes as studied by X-ray photoelectron spectroscopy and soft X-ray absorption spectroscopy

Ex situ soft X-ray absorption spectroscopy (SXAS) and X-ray photoelectron spectroscopy (XPS) were used to study the redox behavior of sodium-containing vanadium oxide (Na-VOx) nanotubes in a rechargeable Li battery context. This nanotubular material consists of scroll-like layers of vanadium oxide separated by structure-directing agents, in this case Na+ ions. As lithium ions are inserted into the Na-VOx nanotubes, the V 2p peak weight shifts toward lower energies. This effect can be seen in both the SXAS and XPS results. Average oxidation states could be obtained from the XPS results, and these agree well with those estimated from electrochemical measurements. At potentials below 2.0 V, there is a coexistence of V(V), V(IV), and V(III), which suggests that some of the vanadium is inaccessible to reduction during the first cycles.