Investigations On Metal Ions Doping,Perpromance And Mechanism Of V₂O₅ As Cathode Materials For Li-ion Batteries

The rapid development of mobile electronic devices and electric vehicles has put forward higher requirements for the energy density and power density of lithium-ion batteries（LIBs）.As a potential cathode mateirals for LIBs,V₂O₅ has a large theoretical capacity of 441 m Ah·g^-1,which is far higher than the commonly used Li Co O₂ cathode(140 m Ah·g^-1)and Li Fe O₄ cathode(170 m Ah·g^-1);In addition,V₂O₅has the advantages of abundant sources,relatively low price,and easy preparation.Therefore,V₂O₅ has attracted great attention in recent years.However,the poor cycling stability and sluggish reaction kinetics of V₂O₅ greatly limit its practical application and development as a cathode material for LIBs.Aming at the above problems,this thesis performed experimental and theoretical calculation studies from the aspects of construcing nanostructures and and metal ions(Cu²⁺,Al³⁺,and Sn⁴⁺)doping.The main research contents are as follows:1.Vanadium metal-organic frameworks（MOFs,MIL-88B（V））was prepared by solvothermal method.Further,porous V₂O₅ nanomaterials with micro/nano hierarchical structure were obtained by heat treatment of MIL-88B（V）.The effect of heat treatment temperatures（400,500,600,700 and 800℃）on the structure and lithium storage properties of the porous V₂O₅nanomaterials were systemically investigated.The results show that the porous V₂O₅nanomaterial prepared at the heat treatment of 600℃has the relatively good lithium storage performance.Within the potential window of 2.0-4.0 V（vs.Li/Li⁺）,the porous V₂O₅nanomaterial exhibits the highest specific discharge capacity of 214.8 m Ah·g^-1 at the current density of 0.2A·g^-1;after 200 cycles,the specific discharge capacity decreases to 114.8 m Ah·g^-1.At a high current density of 2 A·g^-1,the reversible specific capacity remains at 65.0m Ah·g^-1.CV and EIS analysis desmostrate that the porous V₂O₅nanomaterial prepared at the heat treatment of 600℃possesses the highest lithium ion diffusion coefficient and the lowest electrochemical reaction impedance.2.Cu-doped MIL-88B（V）was prepared by solvothermal method.Then,Cu-doped V₂O₅ nanomaterials was obtained by heat treatment Cu-doped MIL-88B（V）at 600 ^oC.The results show that the Cu doped V₂O₅ material prepared at the n（Cu）/n（V）of 3%has superior lithium storage performance.In particular,Cu doping effectively improves the rate capability and lithium storage activity of V₂O₅(the reversible specific capacity of Cu the V₂O₅ is up to 112.9 m Ah·g^-1 at the high current density of 2 A·g^-1),enhances the lithium ions diffusivity,and reduce the electrochemical reaction impedance.Through first-principles DFT calculation,it is found that the stable doping site of Cu is in the interlayer of V₂O₅;Cu doping introduces impurity energy levels between the valence band and the conduction band of V₂O₅,which is beneficial to improve the conductivity and rate performance of the material.3.Al-doped MIL-88B（V）was prepared by solvothermal method,and the Al-doped V₂O₅ nanomaterials were obtained by heat treatment of the Al-doped MIL-88B（V）at 600℃.It was found that the Al-doped V₂O₅ prepared at the n（Al）/n（V）of 3%has the best lithium storage performance.Al doping can greatly improve the lithium storage activity of V₂O₅(the reversible specific capacity of Al-doped V₂O₅is up to 298.1 m Ah·g^-1 at the current density of 0.2 A·g^-1)and reduce the electrochemical reaction impedance of the material.Through first-principles DFT calculation,it is found that the stable doping site of Al in V₂O₅ is to enter the interlayer;however,the formation energy of Al doping is lower than that of Cu doping;in addition,Al doping slightly widens the band gap of V₂O₅,which is not helpful for improving the conductivity and rate performance of the material.4.Sn-doped MIL-88B（V）was prepared by solvothermal method.Then,Sn-doped V₂O₅ nanomaterials were obtained by heat treatment of the Sn-doped MIL-88B（V）at 600℃.Compared with Cu and Al doping,Sn doping has limited effect on improving the lithium storage performance of V₂O₅ in the potential window of 2.0-4.0 V（vs.Li/Li⁺）.Furtherly,Sn-doped V₂O₅ materials with 2D structure were prepared by sol-gel method.It is found that the Sn-doped V₂O₅ with n（Sn）/n（V）of 5%exhibit superior lithium storage activity and rate capability in the potential window of1.5-4.0 V（vs.Li/Li⁺）.After Sn-doping,the electrochemical reaction impedance of V₂O₅ was decreased and the lithium ions diffusivity was strengthened.Through first-principles DFT calculation,it is found that the stable doping site of Sn in V₂O₅ is the interlayer.Sn doping Narrows the band gap of V₂O₅ slightly,and introduces impurity energy level in the band gap,which is beneficial to improve the conductivity of the material to a certain extent.