Preparation And Electrochemical Properties Of Anode Materials（Nb₂O₅ And FeNbO₄） For Lithium Ion Batteries

In recent years,lithium-ion batteries have been successfully used in electric vehicles（EVs）and hybrid electric vehicles（HEVs）and other high-power means of transport,therefore,the development of power battery materials have gradually aroused considerable interest.Among them,the energy density,power density and safety performance and other factors determine the pace of development of electric vehicles and hybrid electric vehicles.Compared with Li₄Ti₅O₁₂,Nb₂O₅ exhibit similar lithium-insertion potential（1.2-1.6 V）,higher specific capacity(200 mAh×g^-1),and exhibit excellent cycling stability at high temperature.However,its conductivity is low(σ ≈ 3×10^-6 S·cm^-1).The work of this paper is divided into the following three parts:（1）The preparation of Nb₂O₅@carbon composite by sol-gel method using sucrose and egg white as carbon sources,respectively;（2）Nb₂O₅ doped with Fe（Ⅲ）was synthesized by hydrothermal method;（3）The electrochemical properties of FeNbO₄ and FeNbO₄/rGO composites were investigated.The details are as follows:（1）The effects of different calcination temperatures on the morphology and structure of Nb₂O₅ nanomaterials which were prepared by sol-gel method were investigated,and the differences of their electrochemical properties were discussed.The results showed that orthorhombic Nb₂O₅ possessed good rate performance and excellent cyclic stability.In order to improve its rate performance,Nb₂O₅@C and Nb₂O₅@NC were synthesized by coating with sucrose and egg white,respectively,and the carbon layer inhibited the growth of the particles and ensured the rapid deintercalation of Li+.The results showed that the discharge capacity of Nb₂O₅@NC can be up to 328 mAh×g^-1at a current density of 100 mAh×g^-1.At a current density of 2 A×g^-1,the discharge capacity is at 193.7 mAh×g^-1 and after 1000 cycles the capacity retention rate is still maintained at 93.5%.（2）The effects of Fe（Ⅲ）doping Nb₂O₅ on crystal morphology and electrochemical properties were investigated,the orthorhombic Fe_0.4Nb₂O₅ which was prepared by hydrothermal method was self-assembled into micro-nano-sphere structure,improved the utilization of its active material and thus improve its capacity.The initial discharge specific capacity was at 325 mAh×g^-1 at the rate of 50 mA×g^-1,it still could be remained at 108.4 mAh×g^-1,even a large current density was at 5 A×g^-1.The structure has a rough surface and a rich porous structure,conduciving to increasing the contact area of electrode material and electrolyte.When the heat treatment temperature increased（950 oC）,it changed into a tetragonal crystal.At the current density of 50 mA×g^-1,the initial discharge specific capacity was at 365 mAh×g^-1,and the capacity can be maintained at 88.4 mAh×g^-1at 5 A×g^-1current density.The cycling stability and rate performance of Fe（Ⅲ）doped Nb₂O₅ were improved obviously,which indicated that Fe（Ⅲ）doped Nb₂O₅ could be used as a negative electrode material with promising high rate performance.（3）The FeNbO₄/rGO composite was prepared by hydrothermal method using ammonia as pH regulator and nitrogen source.The hydrothermal reduction and nitrogen doping process of the graphene oxide were accompanied by the growth of FeNbO₄ nanophase particles at the same time.The electrochemical properties of FeNbO₄/rGO composites,FeNbO₄ and bulk FeNbO₄ synthesized by solid-state reaction at high temperature were discussed in detail.The discharge capacity of FeNbO₄/rGO composites gradually stabilized at 377.8 mAh×g^-1 at the rate of 300 mA×g^-1 and the acapacity fading of FeNbO₄/rGO composites is only 7% after 1000 cycles.