Preparation And Electrochemical Performance Of VO_x-TiO₂/C Anode Materials

In this investigation,the Ti_m（OCH₂CH₂O）_2m/CNTs composite was synthesized by the polymerization of isopropyl titanate and ethylene glycol.And the composite was further reacted with V₂O₅ to synthesize the vanadium containing organometallic polymer Ti_hV_jO_k（OCH₂CH₂O）_g/CNTs,then the Ti_hV_jO_k（OCH₂CH₂O）_g/CNTs was thermally decomposed at high temperature in nitrogen atmosphere to obtain VO_x-Ti O₂/CNTs.The as synthesized VO_x-Ti O₂/CNTs was used as anode material for lithium-ion batteries.The electrochemical behavior of the material was studied.Varieties of physical and chemical characterizations of the material were conducted.The investigation results of the third chapter show that large number of nano-scale pores are formed in VO_x-Ti O₂/CNTs.These nano pores could be formed from the thermal decomposition process of the material.The investigation also show that the introduction of Vⁿ⁺into Ti O₂ phase could stabilize the pore structure of the material.The Vⁿ⁺species exist in Ti O₂ phase in the forms of V⁵⁺,V⁴⁺and V³⁺valence states.There are part of Ti⁴⁺in Ti O₂ replaced by V⁵⁺.In order to maintain electrical neutrality,one Ti³⁺must be formed around each V⁵⁺.The investigation reveals that there is up to60.2%of the Ti⁴⁺ions are reduced to Ti³⁺ions.At the same time,some of Ti⁴⁺ions could be replaced by Ti³⁺ions or V³⁺ions to form large number of oxygen vacancies in Ti O₂ phase.The abundant pore structure and the high concentration of oxygen vacancies in the material provide large number of diffusion channels for lithium ions.The diffusion distances of Li⁺in Ti O₂ phase could be shorten.The formation of oxygen vacancies,the reduction of Ti⁴⁺ions to Ti³⁺ions,and the reduction of V⁵⁺ions to V³⁺ions could enhance the electronic conductivity of the material.The existence of rich lithium ion diffusion channels and the excellent electronic conductivity rend the material having higher rate capability and open up more active sites for Li⁺ion storage.Hence,the as synthesized material has high specific capacity at high charge/discharge rates.Specific capacities 310,240,and 210 m Ah/g are reached at current densities of0.1 A/g,0.5 A/g,and 1.0 A/g,respectively.Except the high rate capability of the material,the cycling life-time test shows that the material has also good stability.At the current density of 50 m A/g,specific capacity of 345 m Ah/g has been reached and after 100 cycles of charge/discharge,the capacity retention rate is about 83%.The investigation also shows that the addition of appropriate amount of Li OH into the VO_x-Ti O₂/CNTs material could inhibit the reduction of Ti⁴⁺to Ti³⁺to a certain extent,therefore preventing the formation of excessive amount of oxygen anion vacancies and defects in the Ti O₂ phase and therefore,slightly stablizes the structure of the material.The Li OH addition further improves the structural stability and rate capability of the material.Specific capacities 320,270,and 222 m Ah/g are obtained at current densities of 0.1 A/g,0.5 A/g,and 1.0 A/g,,respectively.The investigation results of the fourth chapter show that the anode materials synthesized using CNTs slurry have nano-scale particle size and good dispersibility,while the particle size of anode materials synthesized using CNTs powder,acetylene black and graphite all reach micron scale.The unique advantages of CNTs slurry make it more beneficial to improve the morphology and structure of the materials and improve the electrochemical performance of the materials.