| Lithium ion batteries (LIBs) have been considered as the most ideal green energy for the next generation due to their high energy density, long cycle life and environmentally friendly. Sodium has similar physical and chemical properties as lithium, in addition its abundance, which would effectively solve the high cost of LIBs. Therefore, the sodium ion batteries (SIBs) have the potential to use as a new power source. Among the anode materials for LIBs and SIBs, TiO2 has been considered as a promising candidate material due to its inherent structural stability, low cost, minimal toxicity and high specific capacity. However, the low ion diffusivity and poor conductivity of TiO2 limit its application. The aim of this thesis is to improve the electrochemical performance of TiO2 through the synthesis of porous TiO2/C and ion doping.In this work, we prepared mesoporous anatase TiO2/C by precipitation method using titanium (Ⅳ) sulfate and ammonia as precursor and precipitator, rape pollen and yeast as biological template due to their low-cost and environmentally friendly. The structure, morphology and performance of the prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), Fourier transform infrared spectroscopy (FT-IR), electrochemical impedance spectroscopy (EIS), cyclic voltammograms (CV) and galvanostatic charge/discharge tests. The results show that an aggregated mesoporous material of anatase TiO2/C composed of ～15 nm primary particles and 14.0 nm mesoporous channels with a specific surface area of 69.37 m2·g-1 was prepared by rape pollen. And that mesoporous microspheres anatase TiO2/C composed of ～15 nm primary particles and 15.1 nm mesoporous channels with a specific surface area of 72.05 m2·g-1 was prepared by yeast cells. The amorphous carbon from the thermal decomposition of rape pollen and yeast cells is uniformly dispersed between TiO2 nanocrystallines, and the amount of carbon is 1.6 wt% and 1.5 wt% for the sample prepared by rape pollen and yeast cells, respectively. As an anode material, mesoporous TiO2/C composites prepared by rape pollen and yeast cells deliver a discharge capacitiy of 118.1 mAh·g-1 and 119.9 mAh·g-1 at 5 C for SIBs, and that give a discharge capacitiy of 100.6 mAh·g-1 and 106.6 mAh·g-1 at 20 C for LIBs, respectively. The mesoporous TiO2/C composites exhibiting good rate performance owing to a high electronic conductivity and quick Li+ or Na+ permeation in the unique geometrical structures of porous TiO2/C electrodes.Secondly, mesoporous TiO2/C micro-spheres prepared by yeast cells were doped with Nb5+, Zn2+ or Mg2+ and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The results of XRD, XPS and EPR indicate that Nb5+, Zn2+ or Mg2+ is successful doped into the TiO2 lattice to form a solid solution. The cation doped at Ti site can generate oxygen vacancy or Ti3+ and change the electron density distribution, leading to an improvement of the electrochemical performance. Among the ion-doped TiO2/C composites, Ti0.94Nb0.06O2, Ti0.94Zn0.06O2 and Ti0.95Mg0.05O2 deliver a capacity of 163.5,148.7 and 142.1 mAh·g-1 after 100 cycles at 5 C for SIBs and 167.8,163.8 and 161.4 mAh·g-1 for LIBs, respectively, showing an improved electrochemical performance. As used in SIBs and LIBs, the discharge capacity of Nb-doped TiO2/C at 5 C is 43.6 and 23 mAh·g-1 larger than that of undoped TiO2/C, respectively, exhibiting the best performance.The N-doped TiO2/C particles were synthesized through treating the mixture of TiO2/yeast by microwave-assisted hydrothermal method using NH4F or CO(NH2)2 as nitrogen source. NJ3+ doped into the oxygen site of TiO2 can generate more oxygen defects and Ti3+, which can further improve the electronic conductivity of the material. On the other hand, the gas from the decomposition of urea and ammonium fluoride under the hydrothermal conditions can further inhibit the growth of TiO2 particles, resulting in the reduction of particle size and the increase of surface area. As an anode material for SIBs and LIBs, the N-doped TiO2 prepared by NH4F and CO(NH2)2 with a N:Ti molar ratio of 1.0 gives a discharge capacity of 150.1,165.2 mAh·g-1 and 140.2,162.5 mAh·g-1 after 100 cycles at 5C, respectively. The discharge capacity of N-doped TiO2/C prepared by NH4F and CO(NH2)2 is 40.1,25.7 mAh·g-1 and 30.2,23.0 mAh·g-1 improvement compared with the undoped TiO2/C, respectively. The enhanced performance of N-doped TiO2/C can be ascribed to the improvement of the electron conductivity and electrochemical kinetics.Compared with the methods used to improve the performance of TiO2, bio-template can prepare unique porous TiO2/C electrode material with good performance, and ion doping can effectively adjust the concentration of Ti3+and Oxygen defects to improve the electrochemical performance of TiO2. The research provided in here can be considered as the basis for developing the effective route to prepare high performance TiO2/C for LIBs and SIBs. |
PDF Full Text Request