| Energy and environment are two related systems. Environment pollution companies with energy consumption and the shortage of energy will prohibit the development of social economy. With growing concerns over increased oil price and environmental issues, the demand for developing green energy sources has attracted considerable attention in today’s energy-based society. In order to make full use of green energy such as wind and solar, lead-acid batteries are commonly used in the energy storage system. However, the recovery of the lead in these batteries by crude methods has resulted in the release of millions of tons of lead into the environment, which is harmful for the health of human beings. As an environmental power source, lithium-ion battery can used in electric vehicles and energy storage system, which is beneficial for energy conversation and reducing the global warming effect coming from the production of carbon dioxide. Compared with other chemical power sources, Lithium-ion batteries have become attractive for portable electric devices due to their higher energy density and long cycling life.Li4Ti5O12 (LTO) is considered to replace the currently used carbon anodes in lithium-ion batteries. LTO is a technically important anode material for new-generation power lithium-ion battery applications because of its abundant titanium dioxide raw materials, excellent cycle reversibility and stability, relatively high capacity (175 mAh·g-1), and safety characteristics due to its zero-strain volume during charge-discharge processes and high lithiation voltage plateau at 1.55 V vs. Li/Li+, which sufficiently avoid the formation of metallic lithium and thus improve the safety of lithium-ion batteries. However, the commercial use of LTO anode material in power lithium-ion batteries of electric vehicles or energy storage system has been hindered by its poor electronic conductivity. As doping has been proved to be an effective way to increase electronic conductivity of LTO, in this thesis, we improved the rate ability of LTO anode by doping some metallic ions such as Ca2+, W6+, Gd3+ and Nd3+.Firstly, Ca2+ was chosen as a doping ion to increase the conductivity of LTO. Ca-doped LTO with the formula of Li4-xCaxTi5O12 (x=0,0.05,0.1,0.15,0.2) were synthesized as anode materials by a simple solid-state reaction. XRD reveals that the Ca-doping caused no change on the phase structure and highly crystalline Li4-xCaxTi5O12 (0≤x≤0.2) powders without any impurity were obtained. SEM images showed that the as-prepared powders have similar particulate morphologies and the particle size distribution was in the range of 1-2μm. Among all prepared samples, the Li3.9Ca0.1Ti5O12 exhibited a higher specific capacity, better cycling performance and rate capability than other electrodes. The Li3.9Ca0.1Ti5O12 material showed discharge capacities of 162.4 mAh·g-1,148.8 mAh·g-1 and 138.7 mAh·g-1 after 100 cycles at 1 C, 5 C and 10 C charge-discharge rates, respectively.To improve the energy density of Li3.9Ca0.1Ti5O12 (LCTO), the cell was discharged to 0 V cut-off voltages. LTO and LCTO were synthesized by a solid-state reaction. XRD reveals that highly crystalline LCTO is obtained without any impurity. LCTO presents a higher discharge specific capacity and better cycling stability than Li4Ti5O12 (LTO). The LCTO exhibits a capacity as high as 240mAh·g-1 after 200 cycles at 1 C when discharged to 0 V, which is much higher than the LTO (only 127.3 mAh·g-1). The electrochemical performances of LCTO and LTO in the voltage range from 0 V to 2.5 V are investigated. The effect of Ca-doping on the improving the energy density of LTO discharged to 0 V was also discussed.Then, we chose W6+ as a doping ion to improve the rate capability of LTO. W-doped LTO in the form of Li4Ti5-xWxO12 (x=0.05,0.1,0.15 and 0.2) were prepared by sol-gel method and following two-step calcinations in the air and argon atmosphere, respectively. It is observed that the lattice parameters of the W-doped LTO samples are slightly bigger than that of the pure LTO sample, and W-doping does not change the structure of the cubic spinel LTO. W-doped LTO samples employed as the anode materials of lithium-ion batteries deliver excellent electrochemical performances, and sample Li4Ti4.9W0.1O12 exhibits the best rate capability and cycling stability, when the charge-discharge rate of 1 C,5 C and 10 C, its discharge capacities are 162.5 mAh·g-1,145 mAh·g-1 and 128.1 mAh·g-1, respectively, at 100th cycle.The Gd dopant for cathode materials can significantly improve the rate capability. However, full details of the effects of Gd-doping in spinel LTO anode materials have not been reported until now. Li4Ti5-xGdxO12 (x=0.05,0.10 and 0.15) were prepared by a simple solid-state reaction in air. XRD revealed that only a small amount of the dopant can enter the lattice structure of LTO, and excessive part existed as the impurity of Gd2O3. The Gd doping did not change the spinel structure and electrochemical reaction process of LTO. The particle size of as-prepared samples ranged between 0.5-1.5 μm. The Gd-doped materials showed much improved rate capability and specific capacity compared with undoped LTO. Especially, the Li4Ti4.95Gd0.05O12 exhibited the best rate capability and cycling stability among all samples. However, too much impurity of Gd2O3 in the LTO was adverse to the electrochemical performance.Additionally, doping with lower valence Nd3+ in LiMn2O4 can yield oxygen ion vacancies, which behave as ionic charge carriers to greatly enhance the electronic conductivity of LiMmO4. Inspired by this work, Nd doped LTO was synthesized by a sol-gel method. The structure and electrochemical properties of the as-prepared powders were systematically investigated. Li4Ti4.98Nd0.02O12 exhibits excellent rate capabilities and cycling stability even at a high current rate of 10 C.The novel metallic ions doped LTO by Ca2+, W6+, Gd3+, and Nd3+ stand as promising potentially high-rate anode materials for lithium-ion batteries to be used in electric vehicles or electricity storage system of wind and solar energy, which are beneficial for environmental protection, energy saving and emission reduction.We also recycled the waste lithium-ion batteries which have been used for tests above by an organic solvent immersion method. The structure, morphology and electrochemical performance of the recycled LTO as anode material for lithium-ion battery were characterized. The results indicate that the final recycled LTO electrode material exhibits excellent cyclic stability and reversibility, which can be reused in lithium-ion battery. |
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