Synthesis And Electrochemical Properties Of Ti-Based Oxides As Anode Materials For Lithium-Ion Batteries

With the increasing concerns over increased energy consumption and environmental issues, the demand for developing renewable and clean energy technologies is becoming more and more critical. Rechargeable lithium-ion batteries (LIBs), recognized to be the most important energy storage and conversion equipment with advantages of high energy density, long cycle life and no "memory effect", have the potential for large-scale applications in communication facilities, portable electronics, stationary energy storage systems and enormous markets of electric vehicles.The battery performance of LIBs strongly depends on the electrode properties, and thus a strict choice of electrode materials including both the cathodes and anodes plays a great role. In general, graphitic carbon is the most common anode material for LIBs, because of its low cost, high abundance and outstanding kinetics. However, its operating voltage is below0.2V versus Li/Li+. This voltage is close to the lithium electroplating potential, especially at high rates, which may cause a safety issue. In addition, a layer of electronically insulating solid-electrolyte interphase (SEI) is inevitably formed on the surface of graphite below1.0V versus Li/Li+. A fundamental solution to solve this issue is to find excellent alternatives to graphite with better cycling stability and safety. Ti-based oxides such as Li4Ti5O12and TiO2have been considered as potential alternative materials, due to their good reversibility and a higher working voltage assuring a better safety of LIBs. Nevertheless, the main disadvantages of Li4TisO12and TiO2lie in the low electrical conductivity, poor electron transport and aggregation tendency of nanoparticles, resulting in the deterioration of reversible capacity and rate capability, thus inhibiting their applications in LIBs. In this dissertation, the electrochemical performances of Ti-based oxides were improved by new synthetic and surface-modification methods. The main contents and results are summarized as follows:The availability of high-quality nanocrystals underpins a diverse range of applications and investigations into size-dependent physical and chemical properties. Effective synthetic methods that yield uniform nanocrystals are critically important. Here we demonstrate a fast and economical microwave-assisted solid-state method to prepare spinel Li4Ti5O12 nanocrystallites in large quantities using cost-effective commercial TiO2as a raw material of titanium. This method easily programs the synthetic conditions including temperature and time, and significantly shortens the synthesis time to minutes. The as-formed LiTi5O12nanocrystals prepared by a microwave-assisted solid-state method exhibit a distinctively narrower particle size distribution without agglomeration, and the particle size ranges from100to350nm with an average size of180nm. When evaluated as an anode material for lithium-ion batteries, they exhibit greatly enhanced electrochemical lithium-storage performances, including not only high rate capabilities but also a highly reversible capability of～160mA h g-1over500cycles at1C.A fast and economical route based on an efficient microwave-induced solid-state process has been developed to synthesize metastable TiO2(B) nanobelts on a large scale. This new method reduces the synthesis time for the preparation of TiO2(B) nanobelts to less than half an hour, allowing the screening of a wide range of reaction conditions for optimizing and scaling up the production and facilitating the formation of metastable-phase TiO2(B). The as-formed TiO2(B) nanobelts with widths of30-100nm and lengths up to a few micrometers exhibit enhanced lithium-storage performances, compared with the TiO2(B) product obtained by the conventional heating. This work provides a new way for large-scale industrial production of high-quality metastable TiO2(B) nanostructures.Nanoporous TiO2spheres coated with N-doped carbon (NC) have been successfully synthesized via a facile solution-phase process and subsequent heat treatment. Compared with nanoporous TiO2spheres, the as-formed TiO2@NC nanocomposites shown a significantly higher specific capacity and rate capability, which can be ascribed to the synergistic effect of the porous structural configuration and the uniform NC layer with high conductivity. The TiO2@NC nanohybrids not only delivered a high capacity of～170mA h g-1at a current density of0.1A g-1, but also maintained an excellent performance as the current density increased to2.0A g-1, demonstrating that the NC coating is a promising approach for preparation of high-performance electrode materials for lithium ion batteries.We demonstrate a facile but versatile in-situ photochemical strategy for grafting inorganic conductive molybdenum oxysulfide (MoOxSy) clusters in the framework of nanoporous TiO2spheres.The as-obtained TiO2@MoOxSy, nanohybrids as an anode material for lithium-ion batteries exhibit high lithium storage performances with high specific capacity and remarkable rate capability. Results show that the conductive MoOxSy. clusters are uniformly distributed on the surface of nanoporous TiO2spheres. The synergistic effect of the nanoporous structure and the amorphous conductive layer of MoOxSy clusters may contribute to the enhanced lithium-storage capability of TiO2@MoOxSy nanohybrids. This work presents a new powerful strategy toward preparing high-performance Ti-based anode materials for next-generation LIBs with high power and energy densities. It is expected that the present photochemical grafting method can be extended to design other high-performance and multifunctional TiO2@shell nanostructures, and more importantly, provide insight into the design of advanced conformal porous nanohybrids.