Research On Thermodynamic And Electrochemical Properties Of TeO₂-V₂O₅ Glasses

TeO₂-V₂O₅?VT?glasses are considered to have high potential to be the anode materials for Li-ion Batteries?LIB?due to their open network structure,non-grain boundary,high electronic conductivity and the ability to insert lithium ions.This thesis mainly focuses on the study of the structural,thermodynamic and electrochemical properties of the VT glass anodes for LIBs.We proposed the disorder-order engineering concept by studying the mechanism for Nano-crystal formation induced by discharing/charging cycles.We also studied the effect of Li₂O on the electrochemical properties of 35TeO₂-65V₂O₅ glass anode.The thesis will be divided into four parts:?1?We firstly prepared a series of binary?100-x?TeO₂-xV₂O₅ glasses by melting-quenching method.The effect of the chemical composition on their structural and physical properties was studied by means of XRD,FT-IR and some calculations.The accurate contents of V⁴⁺ions in the studied glasses were determined by using the Electron Paramagnetic Resonance?EPR?technique.Moreover,the effect of chemical composition on the energy gaps of?100-x?TeO₂-xV₂O₅ glasses were studied by Ultraviolet Visible Diffuse Reflectance Spectrum?UV-vis?.We also investigated the effect of chemical composition on their thermodynamic properties by DSC measurements.Furthermore,the effect of chemical composition on the electrochemical properties of the studied glasses is mainly conducted by the Electrical impedance analysis?EIS?and the charging/discharging test.We found that the 70TeO₂-30V₂O₅?VT30?shows the highest reversible capacity and good cycling stability among all the studied glass anodes.?2?The 40TeO₂-60V₂O₅?VT60?glass was chosen as an object to study the disorder/order transition induced by discharging/charging process from the aspects of structural,morphological,thermodynamic and electrochemical properties.The results show that even the first cycle of discharging/charging can induce the disorder-order transition.The formed nano-crystals were randomly distributed in the glass matrix.With the increase of cycling,both the number and size of nano-crystals increase.The nano-crystals and disordered matrix exert a synergetic effect to facilitate the ionic and electronic transport,as well as to maintain the structure stable against discharging/charging.Consequently,both the capacity and the cycling stability were greatly enhanced.?3?We clarified the origins of both the nanocrystal formation and the enhancement of electrochemical performances of the VT60 glass anode in terms of the phase transition,redox reaction and structural heterogeneity.Through studying the enthalpy relaxation of the fast-quenched VT60 glass,we found that the VT60 glass has high degree of structural heterogeneity presented by a small stretching parameter?value describing the relaxation time distribution.In addition,the Hruby parameter K_H was determined to be only 0.15,implying a rather low glass stability of VT60 glass.It means that this glass is easily to crystallize.During the discharging/charging process,Li⁺ions gradually insert and diffuse into the VT60 glass anode through the electrolyte.Considering the high structural heterogeneity of the VT60 glass,the structural domains with higher potential energy are vulnerable to Li⁺ions.When these domains with higher energy are hit by the Li⁺ions during insertion,some of these ions will participate in the lattice of the high-energy domains in the glass network,and thereby lower their potential energy by forming ordered structures at nanoscale.Upon 5000discharging/charging cycles at a high current density of 1000 mA g^-1,the nanocrystals were identified to be?-Li₃VO₄.Owing to the metastable nature of the?-Li₃VO₄ phase,the glass anode becomes electrochemically active and highly ionic conductive.Simultaneously,the cycling stability is greatly enhanced by the nanostructured glass since the nanocrystal could suppress the propagation of micro-cracks generated by volume changes in glass matrix.?4?We studied the effect of Li₂O doping to 35TeO₂-65V₂O₅?VT-65?glass anode on the electrochemical properties.We found that the electrochemical properties of the Li₂O-TeO₂-V₂O₅ glass anode were highly enhanced compared to the binary VT-65 glass anode.The enhanced electrochemical performances can be attributed to:?a?the glass network becomes more open by introduction of network modifier oxide Li₂O,which is in favor of the transport of lithium-ions.?b?the introduction of Li₂O into the glass will increase the content of lithium-ions.This might increase the ionic conductivity of the glass anodes.We prepared the binary VT glasses by melting-quenching method and used them as the anode materials for LiBs.From the structural and thermodynamic point of view,we proposed a novel strategy,i.e.,the disorder/order engineering and clarified the origin of both the discharging/charging induced nanocrystal formation in VT60 glass anode and the resulting enhancement of the Li-ion battery performances.This study has opened a new way for developing high performance anodes for Li-ion batteries.