Raman Microspectrometry Study of Electrochemical Lithium Intercalation into Sputtered Crystalline V₂O₅ Thin Films

Abstract

Raman microspectrometry has been used to investigate the local structural changes induced by the electrochemical lithium intercalation reaction in crystalline sputtered V 2 O 5 thin films in a liquid electrolyte. Contrary to usual composite electrodes made of a mixture of active material and conductive and binding agents, the use of a pure V 2 O 5 thin film allows a homogeneous Li insertion in the material and a high quality of Raman signatures to be obtained. The Raman spectra of Li x V 2 O 5 compounds for 0 < x < 1 are examined as a function of the lithium content and discussed in relation with the X-ray diffraction data pertinent to these h00 -oriented thin films and literature data. An assignment of all Raman bands is proposed, and the Raman fingerprint of the ϵ-type phase, whose interlayer distance continuously increases with x, is clearly evidenced all along the Li insertion process: lithium ions rapidly produce an orthorhombic ϵ phase characterized by a vanadyl stretching mode at 984 cm −1 for 0 < x < 0.5, and further Li accommodation induces a splitting into two stretching modes, the first one shifting from 984 to 975 cm −1, the second from x = 0.7 located at a fixed wavenumber of 957 cm −1 . Both modes are consistent with the local structure of the ϵ lithium-rich phase called ϵ′ and reflect the existence of two different lithium sites. This work illustrates that the structural changes, in terms of long-range order and local structure, are strongly dependent on the microstructure and morphology of the material.