Preparation And Electrochemical Properties Of TiO₂Anode Materials For Lithium-ion Battery

Nanostructured titanium oxide-based materials have attracted enormous research interest asgood alternatives for potential application in high power rechargeable lithium-ion batteries.Based on the controllable preparation, establishing the microstructure-property relationship ofthe target materials are of great significance for the enhancement of the potential applications.In this thesis, in the light of the improvement of safety, stability, high rate capability, andenhancement of charge transfer and ionic diffusion, we have engineered various TiO₂nanostructures with different morphology, such as single crystalline TiO₂nanosheets withdominantly exposed {001} crystal planes, hollow single crystalline TiO₂nanocages,hierarchical rutile TiO₂microspheres assembled by porous nanorods and so on. In addition, inorder to enhance the charge transfer process，anatase TiO₂@MWNTs core/shell coaxialnanocables and anatase TiO₂anchored on graphene sheets are also in-situ synthesizedsuccessfully in this paper. The formation mechanism and the electrochemical performancerelated to these materials are studied systematically. The main conclusions have beensummarized as following:1. Since more Li+ion insertion/extraction can occur through the less thermodynamicallyfavored {001} surface, which agrees well with the theoretical predictions of a low barrier forsurface transmission of Li+ions connecting the octahedral vacant sites in the anatase TiO₂framework. So we have fabricated anatase TiO₂nanosheets with dominant {001} facets underhydrothermal conditions on account of controllable preparation of anatase TiO₂nanomaterialsexposed {001} facets. Both isopropanol and hydrofluoric acid serve as capping agents inthis reaction. The initial irreversible loss of anatase TiO₂nanosheets with dominant {001}facets is just17.4%, indicating excellent stability performance. Single crystalline TiOF₂nanocubes have been synthesized under above hydrothermal conditions without addingisopropanol. The corresponding hollow TiO₂nanocages with dominantly exposed {001}facets can be easily obtained by calcinating TiOF₂nanocubes under air atmosphere. The percentage of （001） facets in the sheets is surprisingly as high as～83%. It is also found thatthe presence of acetic acid gives rise to uniform hollow TiO₂nanocages, which play thecrucial role in hollow formation. The TiO₂electrode presented here exhibits much excellentelectrochemical performance: it delivers a capacity of312mAh·g^-1in the first discharge cycleat0.1C, the capacity can still remained at156.3mAh·g^-1at2C after100cycles, thecoulombic efficiency in100long cycles period keeps almost constant at100%.2. In order to enhance the charge transfer process of anatase TiO₂, the TiO₂@MWNTsnanocomposites have been prepared by a simple one-step solvothermal method usinglow-cost TiOSO4as the raw material in the presence of MWNTs. The nitrogenadsorption–desorption curve and pore size distribution verified that the TiO₂@MWNTsnanocomposites had rich hierarchical pores and large specific surface area. Compared withpristine TiO₂, these TiO₂@MWNTs nanocomposites exhibit much higher rate capability andeven better capacity retention. The reversible capacity of the nanocomposites electrode is upto200mAh·g^-1and remain at182mAh·g^-1at1C after100cycles. The reversible capacity isabout100mAh·g^-1even at the high rate of10C. Besides, we demonstrated aenvironment-friendly solvothermal approach for creating well-organized TiO₂-graphenenanocomposites, using titanium （IV） isopropoxide （TIP） as titanium source, N-Methylpyrrolidone （NMP） as sovlent. It is also worth noting that the presence of NMP not onlybeing chemical reduct agent of graphene oxide （GO） in one step, but is also crucial for theformation of uniform TiO₂nanoparticles supported on graphene nanosheets. In comparisonwith prisine TiO₂anode, the TiO₂-GNS composite anodes possess enhanced rate performance.3. Due to the limitation of kinetically restricted structure of rutile TiO₂, we try to improveits Li+sluggish chemical reactions through controlling morphology. The hierarchicalmicrospheres assembled by rodlike TiO₂mesocrystals can be synthesized under lowtemperature solution method. High-density nanocavities inside rutile TiO₂superstructures canbe obtained by simply heating the hierarchical rutile TiO₂mesocrystals in air. The diameter of microsphere is1-2μm. The rutile TiO₂microspheres are actually constructed from nanorodswith50nm in diameter,500-600nm in lenghth. On the basis of the morphologyaccompanied with crystal structure, a main formation process involved the following foursteps:（i） the initial nanowires nucleating process;（ii） the uniform nanorods assembled by thebuilding blocks （nanowires）;（iii） the nanorods transform into spherical superstructures;（iv）the formation of nanocavities in spherical superstructures. Compared to hierarchical rutileTiO₂mesocrystals, the annealed rutile TiO₂microspheres indicates higher reversible capacityand rate capability. The reversible capacity can still remained at160mAh g1at1C after200cycles and130mAh g1at2C. The3D network of annealed rutile TiO₂microspheres incombination with the nanocavities structure benefit the transportation for both lithium ionsand electrons during charge and discharge process, and the single-crystalline nature keep thestability of rutile TiO₂during cycling.