Preparation Of High Performance Anode Materials For Lithium Ion Batteries Base On Nanotube Titanic Acid

Lithium ion battery has become research and development focus of power energy storage device due to its highlights, such as high voltage, high energy density, no memory effect, no pollution, low self-discharge rate and long cycle life. Electrode is the core and key material determining the performance of lithium ion batteries. Li4Ti5O12and TiO2have been demonstrated as the most promising anode material for high power lithium-ion batteries since they exhibit excellent Li-ion insertion/extraction reversibility with little structural change and a relatively higher operating voltage to ensure better safety of the battery by avoiding the trouble of lithium dendrites, which make them promising anode materials for high power lithium-ion batteries.Nanotube titanic acid (NTA, H2Ti2O5·H2O), prepared by treating TiO2powders in a superboiled (110-150℃) solution of concentrated NaOH (5-15M), has attracted considerable interest over the past10years because of it's one-dimensional hollow and layered structure, which owns large surface area, as well as stronger ion-exchange ability. A novel nano-TiO2can be prepared by the dehydration of nanotube titanic acid (NTA) at elevated temperature in air and nano-sized spinel lithium titanate (Li4Ti5O12) can be synthesized using nanotube titanic acid as a precursor via a facile solution ion-exchange method in association with subsequent calcination treatment at relatively low temperature.Despite these advantages, the rate capability of these two materials is relatively low because of large polarization at high charge-discharge rates due to the poor electrical conductivity and sluggish lithium-ion diffusion. In this thesis, we improved the electronic conductivity and ion conductance of electrode materials by using the combinative methods of the introduction of highly conductive phase and high-ion conductor, conductive surface coating and optimization of the electrode structure, and metal oxide composite and optimizing the electrode structure. The conductive network was constructed for effective linkage between particles. The major research content is as follows:(1) Li4Ti5O12/TiN nanocomposites are fabricated through high-energy ball-milling of the mixture of spinel lithium titanate and TiN powder with different mass ratios. All ball-milled samples exhibit markedly improved electrochemical properties than pristine Li4Ti5O12.Particularly, when the adding amount of TiN is2wt%, the electrode has a high capacity of130mAhg-1at a charge/discharge rate of20C and the capacity retention was85%after1000cycles at IOC, showing the best electrochemical performance and great potential as an anode material for high-rate lithium-ion batteries. The transmission electron microscopy. X-ray diffraction and X-ray photoelectron spectroscopy results indicate that the amorphous TiN can be generated on the surface of LTO, and more interestingly, Li3N, with high lithium ion conductivity, appears in the as-prepared samples after ball-milling process. The improved electrochemical performance may be attributed to the TiN and Li3N, which can significantly enhance the electronic conductivity and ionic conductivity of nanocomposites.(2) TiN and N doped carbon double layer conductive network coated Li4Ti5O12composites were prepared via a simple one-step chemical vapor deposition assisted solid-state route using ethylenediamine as Carbon source and N source. HRTEM results show that besides the uniform thin amorphous shells encapsulating the Li4Ti5O12nanoparticles, there was a large amount of amorphous structure wrapping and connecting the Li4Ti5O12nanoparticles, XPS prove that composite of the amorphous structure was N doped carbon and TiN. During their galvanostatic charge-discharge at varying rates, the double layer conductive network coated Li4Ti5O12composite exhibit markedly improved electrochemical properties than pristine Li4Ti5O12,show ultrafast charge-discharge rate capacity. There was no obviously decay and retention rate keep100%below3C, yielding capacity values166mAh/g and162mAh/g at10and20C discharge rates, respectively, and retains92%capacity at20C. The enhanced electrode-performance is ascribed to smaller grain size, the surface modification by the double layer conductive network, and a uniform distribution of conductive material.(3) N doped carbon conductive layer coated TiO2nanotube was prepared by the same method as section2. The results show that N doped carbon coated TiO2nanotube composite exhibit markedly improved electrochemical properties as compared with pristine TiO2sample. The incorporation of N doped carbon not only helps to retain the tubular morphology of the precursor but also can improve the electrochemical properties of the target products. The improved electrochemical properties are ascribe to tubular and layered structures and good electronic conductivity.(4) The NiO-TiO2and La2O3-TiO2nanotube composites were prepared by introducing the ethanol solution of metal oxide precursor into nanotube titanic acid (NTA) under vacuum and subsequent heat treatment. It was found that NiO or La2O3nanoparticles could prevent the nanotubular morphology from destruction during the dehydration of interlayered-OH groups of NTA and improve the electronic conductivity of TiO2nanotubes. Galvanostatic battery testing results showed that the composite exhibit excellent rate capability and good cycle performance. The enhanced performances can be attributed to its favorable tubular morphology and the better electrical contact between metal oxide nanopartices and TiO2nanotubes, which can improve the lithium-ion diffusion capacity and electronic conductivity of TiO2nanotubes respectively.(5) The novel TiO2was prepared by the dehydration of nanotube titanic acid (NTA) at elevated temperature in air. Such as-prepared novel TiO2possesses high concentration of intrinsic defects in bulk and stoichiometry on surface and long lifetime of conduction band electrons (43times compared with P25-TiO2). The novel TiO2shows the visible light response, but no photocatalytic activity for C3H6 oxidation, and the visible photocatalytic activity is successively realized by adding foreign electron traps (PtO2, PdO) on it. The possible mechanism of novel TiO2(A) having the visible light response but no visible light photocatalytic activity was discussed. This kind of TiO2is very different with the conventional TiO2(first generation) or doped TiO2(second generation), we called it the third generation TiO2.