Lithiation and Characterisation by Ion Beam Techniques of V2O5 Thin Films

Abstract

In this work, studies on the mechanism of lithium ion intercalation (Li+) in vanadium pentoxide thin films (V2O5) were performed. The intercalation was induced by electrochemical experiments and the conditions and parameters used were optimised to allow structural, optical and electrical characterisation. Rutherford Backscattering Spectrometry (RBS) and Nuclear Reaction Analysis (NRA) ion beam techniques, performed at the Centre for Nuclear Technologies (CTN/IST), were used to locate and quantify the presence of Li+ ions in the crystalline structure of the material. In addition, the effect of pre-lithiation by ion implantation on V2O5 thin films was studied. V2O5 thin films were deposited by electron beam assisted evaporation (EBPVD) on glass substrates with an ITO layer. The samples were subjected to cyclic voltammetry (CV) and chronoamperometry (CA) as a procedure for intercalating Li+ ions in the V2O5 structure. Optical characterization by visible spectroscopy revealed a recovery of the film’s initial transmittance state after the sample was subjected to five cycles of CV. The corresponding I(V) curves displayed a comparable symmetry of peak anodic and cathodic currents indicating reversibility of Li+ intercalation. The X-Ray Diffractograms (XRD) of these samples showed the presence of an orthorhombic structured V2O5 with a preferential orientation in the (0 0 1) plane. The intercalation resulted in an increase in interplanar spacing of 3% along the lattices’ c-axis, which varied proportionally with the applied voltage. RBS and NRA spectra revealed distinct peaks, characteristic of 7Li+ ions and it was possible to quantify Li as an atomic percentage of the samples’ composition along its depth. It was also observed that the depth profile for Li extended beyond the ITO layer. The electro-optical characterisation of Li+ ion implantation found a reduction in the samples’ intercalation reversibility, with depth profiles suggesting Li entrapment. It was possible to quantify and detect the presence of Li+ ions and correlate these results with the structural expansion induced by the intercalation.