Effect of silver co-sputtering on V2O5 thin films for lithium microbatteries

AgyV2O5-type thin films have been deposited by radiofrequency (rf) magnetron co-sputtering either in a pure argon or in a mixed argon/oxygen atmosphere with 14% oxygen partial pressure using a metallic Ag target and a V2O5 target. The silver content in the thin films has been controlled by simply varying the rf power ratio applied to the two targets. We have investigated the influence of the dopant on the thin film composition, the structure and the morphology as well as on their electrochemical performances compared to undoped V2O5 thin films prepared in the same sputtering conditions. The thin films have been analyzed by various characterization methods such as Rutherford Backscattering Spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy, X-ray diffraction, Scanning Electron Microscopy and electronic conductivity measurements. Electrochemical characterization has been carried out on two peculiar compositions: Ag0.32V2O4.6 (obtained in absence of oxygen) and Ag0.26V2O5 (prepared under PO2 = 14%). Furthermore, the redox processes occurring during lithium insertion and de-insertion have been investigated by using XPS. The Ag0.32V2O4.6 thin film is amorphous and dense with a smooth surface and shows a good cycling stability as well as discharge capacity values similar to thus obtained for an amorphous V2O5 thin film prepared in absence of oxygen. The Ag0.26V2O5 thin film is amorphous, porous and shows improved discharge capacities compared to crystallized V2O5 thin films, probably due to the additional participation of silver ions to the redox processes. Nevertheless, a continuous capacity fading is observed, as already mentioned for porous crystallized V2O5 thin films.