Influence Of Li Intercalation And Oxygen Defects On Electronic Structure Of V₂O₅

Because it is a promising material in many technological applications,vanadium pentoxide(V₂O₅) has been the subject of intense study.Especially,the applications of V₂O₅ in electrochemical charge storage and electrochromic devices have been paid the most attention.However,several critical factors have restricted the development of its applications:1) the mechanism which changes the electron properties of V₂O₅ due to the intercalation/de-intercalation of Li is still not clear;2) how does the oxygen vacancy affect the properties of V₂O₅ are still controversial.In this thesis,both theoretical calculations and experimental measurements are applied for studying these factors.For theoretical calculations,the influences on the electron properties of V₂O₅ by intercalation of Li and oxygen vacancy were investigated by using first principle method based on the density functional theory.For experimental studies,two kinds of V₂O_5-x have been prepared by physical vapor deposition and radio frequency(rf) magnetron sputtering.Scanning Electron Microscope(SEM),X-ray Photoelectron Spectroscopy(XPS),Ultraviolet Photoelectron Spectroscopy(UPS) were employed to detect the properties of film samples.At the same time,some useful conclusions have been obtained by comparing experimental data with theoretical calculations.On the aspect of Li intercalation,different intercalation sites of Li in the V₂O₅ lattices have been considered.The total energies are similar for different Li intercalation sites when the chemical composition is Li_0.5V₂O₅,which indicate there is a flat potential energy surface and Li ions can diffuse along the b direction.When it is LiV₂O₅,there is a lowest one when the intercalated Li ions only locate on the c axis between two bridging oxygen ions at sequential V₂O₅ layers.The intercalation of Li into V₂O₅ does not change the electron transition property of V₂O₅,which is indirect band gap semiconductor.The Fermi lever of Li_xV₂O₅ increases with the amount of Li, which maybe the main reason why blue shift has been found on the absorption of Li_xV₂O₅.On the aspect of oxygen(O) vacancies,different vacancy concentration samples were defined by XPS,and the vanadium ions were gradually reduced to lower oxidation states with the increase of O vacancy density.Simultaneously,the formation of O vacancy leads to decrease the work function of V₂O_5-x,which was confirmed by UPS.At the same time the absorption edge moved to higher energy due to the formation of O vacancies.Theoretical calculations further proved that the formation of O vacancies would cause the reduction of V ions and the decrease of work function. The electronic structures of V ions are strongly alerted by the removal of O ions nearby due the electron transfer to the V 3d orbitals.Additionally,the optical band-gap was broadened with the increase of O vacancies density.In summary,we prove the theoretical results on the Li intercalation and O vacancies on the effects of V₂O₅ electronic structures.The results will help us to better understand the intercalation reactions and prepare high quality V₂O₅ thin films.