The First-principles Calculations Of Lithium Titanates' Electrical And Optical Properties

As a new type of carriers for environmental friendly energy,Li-ions batteries have attracted intensive attentions and wide investigations due to their tremendous application potentials.Thus many scientists have taken Li-ions batteries as an effective way to deal with the global energy crisis.Actually the storage capacity of Li-ion batteries is related to its anodes and cathodes,while LiyLi1+xTi2-xO4(0?x?1/3;0?y?1)has been considered as one of the most prospective Li-ions battery anode materials because of its excellent electrical properties.In recent years,numerous studies focus on improving LiyLi-xTi2-xO4's electrical properties and exploring its industrial applications in other areas.In this thesis,based on the density functional theory(DFT),we use the first-principles method to investigate the electrical and optical properties of LiTi2O4,Li4Ti5O12,Li2Ti2O4 and Li7Ti5O12.Moreover,we study the effect of point defects and transition metal elements on LiTi2O4's and Li2Ti2O4's electrical and optical properties,so we pave a new way to effectively improve LiyLi1+xTi2-xO4's electrical properties.Meanwhile,we also broaden LiyLi1+xTi2-xO4's industrial applications in optical materials.Firstly,we use the first-principles method to study the electrical and optical properties of LiTi2O4,Li4Ti5O12,Li2Ti2O4 and Li7Ti5O12,respectively.These optical properties include the complex dielectric function ?(?),the absorption coefficient ?(?),the refractivity n(?)and the extinction coefficient k(?).According to band structures,density of states and electron density contour plots,the electronic transitions between O-2p and Ti-3d states dominate lithium titanates' electrical and optical properties,but this electronic transition can be influenced by the intensity of Ti-O hybridization.To be specific,the Ti-O hybridization is stronger,the electronic transition is easier.As a result,compared with Li4Ti5O12,other three compounds' average voltages become smaller and their the complex dielectric function ?(?),absorption coefficeint ?(?)and extinction coefficient k(?))all move to the higher energy region,which is caused by the weaker Ti-O hybridization.These results reveal the fundamental origin of electrical and optical properties,and they also provide practical guidance on regulating their working wave length when they used as optical materials.On this basis,we theoretically investigate the effect of point defects on the electrical and optical properties of LiTi2O4 and Li2Ti2O4.The single Li vacancy(VLi),Ti vacancy(VTi),O vacancy(VO),Li antisite(LiTi)and Ti antisite(TiLi)are inserted into LiTi2O4 and Li2Ti2O4,respectively.Based on electronic structures,p-type defects VLi,VTi and LiTi enhance the intensity of Ti-O hybridization,but n-type defecsts VO and TiLi have an adverse effect.Because the Ti-O hybridization plays a major role in the electrical and optical properties,so VLi,VTi and LiTi increase the average voltages and introduce a red shift to complex dielectric function ?(?),while VO and TiLi decrease the average voltages and introduce a blue shift to ?(?).Therefore,when LiTi2O4 and Li2Ti2O4 used as Li-ions batteries' anode,we should avoid inserting n-type defects.Moreover,we can change LiTi2O4's and Li2Ti2O4's working wave length by inserting different types of point defects.In the end,we explore the effect of transition metal elements on LiTi2O4's and Li2Ti2O4's electrical and optical properties.Single Ti ion is substituted by Cr,Mn,Fe or Ni ions.Considering band structures and density of states,transition metal elements introduce a new MT-O hybridization(MT is Cr,Mn,Fe or Ni),and they also enhance the intensity of MT-O and Ti-O hybridization.Therefore,inserting transition metal elements not only improves LiTi2O4's and Li2Ti2O4,s average voltages but also introduces a red shift to their optical spectrums.