Structural and electrochemical behaviour of sputtered vanadium oxide films: oxygen non-stoichiometry and lithium ion sequestration

Abstract

Structural and electrochemical aspects of vanadium oxide films recently reported from ICMCB/ ENSCPB have been examined using appropriate structural models. It is shown that amorphous films are nonstoichiometric as a result of pre-deposition decomposition of V₂O₅. It is proposed that the structure of amorphous films corresponds to a nanotextured mosaic of V₂O₅ and V₂O₄ regions. Lithium intercalation into these regions is considered to occur sequentially and determined by differences in group electronegativities. Open circuit voltages (OCV) have been calculated for various stoichiometric levels of lithiation using available thermodynamic data with approximate corrections. Sequestration of lithium observed in experiments is shown to be an interfacial phenomenon. X-ray photoelectron spectroscopic observation of the formation of V³⁺ even when V⁵⁺ has not been completely reduced to V⁴⁺ is shown to be entirely consistent with the proposed structural model and a consequence of initial oxygen nonstoichiometry. Based on the structural data available on V₂O₅ and its lithiated products, it is argued that the geometry of VO_n polyhedron changes from square pyramid to trigonal bipyramid to octahedron with increase of lithiation. A molecular orbital based energy band diagram is presented which suggests that lithiated vanadium oxides, Li_xV₂O₅, become metallic for high values ofx.