Synthesis And Electrochemical Properties Of TiO₂ Composite As Anode Material For Lithium Ion Batteries

Li-ion batteries as a kind of rechargeable device have been widely used owing to their large specific capacity,long cycle life,and low self-discharge rate and so on.Titanium oxide（TiO₂）was a potential candidate as anode materials for lithium ion batteries,due to its low toxicity,low pollution,high discharge potential,stable chemical properties and low cost.In this paper,TiO₂ nanofibers were prepared by electrospinning and introducing the other elements.The microstructure and morphology of mesoporous nanofibers were confirmed by X-ray diffraction,scanning electron microscopy,transmission electron microscopy,X-ray photoelectron spectroscope and nitrogen adsorption-desorption.The corresponding electrochemical performances are measured by galvanostatic charge-discharge,cycle stability,rate performance,cycle voltammetry and electrochemical impedance spectroscopy measurements.1.Preparation and research of Ag@TiO₂ nanofibers anode materials:In this project,the electrospinning technique followed by the annealing was applied to synthesize mesoporous Ag@TiO₂ nanofibers as LIB anodes,with pristine TiO₂nanofibers as the reference.AgNO₃ was used as Agdopant source,and diisopropyl azodiformate（DIPA）was added into the precursor（tetra-n-butyl titanate and polyvinylpyrrolidone）to create a porous structure.Detailed microstructure of the nanofibers with and without the introduction of Agwas examined by FE-SEM and TEM.All of the nanofibers represents the continuous 1D structure with an average diameter of 100±20 nm.Agnanoparticles were uniformly distributed throughout TiO₂nanofibers.The introduction of Aggreatly reduces Brunner-Emmet-Teller（BET）surface area of TiO₂ nanofibers,which should be attributed to partial pores filled with Agnanoparticles in Ag@TiO₂ nanofibers.The results indicated that the coulombic efficiency of the Ag@TiO₂ nanofibers electrode at the first three cycles was about 56%,93%and 96%,which was higher than that without Ag（48%,66%and 79%）.The Ag@TiO₂ nanofibers electrode exhibited a higher specific discharge capacity of about 128.23 m Ah×g^-1 when compared with that without Ag(72.76 m Ah×g^-1)after 100 cycles at 100 m A×g^-1.The Ag@TiO₂ nanofibers electrode exhibited a higher specific discharge capacity of about 128.23 m Ah×g^-1 when compared with that without Ag(72.76 m Ah×g^-1)after 100 cycles at 100 m A×g^-1.With the current density sharply increased from 40 m A×g^-1 to 1000 m A×g^-1,the higher average discharge capacity of 56.35 m Ah×g^-1 was remained in the electrode with Ag,when compared with the electrode without Ag(average discharge capacity of about 12.14m Ah×g^-1).The introduction of Agnanoparticles into TiO₂ nanofibers significantly improved the diffusion coefficient of Li ions and the electronic conductivity of TiO₂,based on which the comprehensive electrochemical performances were greatly enhanced:2.Preparation and research of Graphene/Ag/TiO₂ nanofibers anode materials:The Graphene/Ag/TiO₂ nanofibers were prepared by electrospinning followed by the annealing.Silver nitrate（AgNO₃）was added to the precursor solution as the silver source,diisopropanolamine（DIPA）as the foaming agent,and graphene with different content（1 mg,2 mg,3 mg,4 mg）was added to the precursor solution to prepare G/Ag/TiO₂ nanofibers.We can confirm the existence of graphene by XRD,XPS and Raman test.We identify that graphene and silver nanoparticles are distributed in TiO₂nanofibers through SEM and TEM morphology characterization.The introduction of graphene increases the Brunner-Emmet-Teller（BET）surface area of Ag@TiO₂ nanofibers.Through a series of electrochemical performance tests,the best electrochemical performance is that the Ag@TiO₂ nanofibers doped with 3 mg graphene electrode（GAT-NFs-3 nanofibers electrode）has the best discharge/charge specific capacity(490.60/287.87 m Ah×g^-1)at the current density of 100 m A×g^-1,and the reversible capacity can still be maintained at 202 m Ah×g^-1 after 100 cycles.In addition,GAT-NFs-3 nanofibers electrode also show great rate performance.With the current density from 40,100,200,400to 1000 m A×g^-1,at the maximum current density,the average discharge specific capacity of the GAT-NFs-3 nanofiber electrode can be maintained at 97.63 m Ah×g^-1 compared with the Ag@TiO₂ nanofiber electrode(the average discharge capacity is about 56.35 m Ah×g^-1);when the current density recover to 40 m A×g^-1,the capacity of GAT-NFs-3 nanofibers electrode was 84.99%of the initial value(224.76 m Ah×g^-1).To sum up,compared with Ag@TiO₂ nanofibers which are not compounded with graphene,the composite products can significantly improve the electrochemical performance.Graphene has good conductivity and large surface,which can shorten the length of the transmission path and reduce the internal resistance of lithium-ion batteries.Moreover,graphene and Ag@TiO₂ nanofibers have synergistic effects:the Ag@TiO₂ nanofibers can be used as the skeleton of graphene to prevent the agglomeration of graphene and keeps the activity of graphene.