New Mesoporous Nanocomposites As Electrode Materials For Lithium-ion Batteries

Since the discovery of mesoporous materials in the early 1990s, mesoporous materials have been widely used in areas such as catalysis, adsorption, separation, biosensor and gas storage, as well as clean energy storage. Clean energy storage systems, including lithium-ion batteries, dye-sensitized solar cell and supercapacitors, are among the applications that can profit from mesoporous materials. With development of the electrode materials, it has been found that the hybrid carbon can effectively enhance the electrode conductivity and protect the electrode from direct contact with electrolyte, leading to enhanced cycle life. More recent, the ordered mesoporous carbon (OMC) has also been proven as a promising carbon matrix for fabricating electrode with high li-ion storage capability. The use of three-dimensional hybrid nanostructured materials as the electrode for LIBs can offer a high capacity, improved cycle stability and a better accommodation of the strain and volume changes during the charge-discharge process. These results encouraged us to extend our studies to the investigation of nanocomposites composed of mesoporous carbon and electrode material. In this dissertation, a series of new mesoporous nanocomposites have been prepared by using ordered mesoporous carbon as nano-reactor. Furthermore, the possible mechanisms for the formation of mesoporous nanocomposites, and the relationships between their intrinsic characteritics and electrochemical properties have also been investigated in detail.1. The nanocomposite of ordered mesoporous pure anatase TiO2-C was successfully synthesized for the first time by using the ordered mesoporous carbon as a nano-reactor and exhibited a short-range ordered mesoporous structure. It was found that carbon was coated on the surface of TiO2 nanoparticles to form a thin layer. Using this material as an anode in the rechargeable lithium-ion batteries, it displayed a large reversible capacity, high rate performance and excellent long-term cycling stability. For instance, a large reversible capacity of 166 mAh g-1 and an average Coulombic efficiency of 99.7% could be maintained even after 900 cycles at a current rate of 1 C. This can be attributed to the structure of the ordered mesoporous TiO2-C nanocomposite. Such nanostructure provides both electron and lithium-ion pathways which are essential for the rechargeable lithium-ion batteries with a large capacity and excellent long-term performance.2. The nanocomposite of MoO2-ordered mesoporous carbon (MoO2-OMC) was synthesized for the first time by using a carbon thermal reduction route and the mesoporous carbon as the nano-reactor. Furthermore, this nanocomposite was used as an anode material for Li-ion intercalation and exhibited large reversible capacity, high rate performance and good cycling stability. For instance, a high reversible capacity of 689 mAh g-1 can be remained after 50 cycles at a current density of 50 mA g-1. It is worth mentioning that the MoO2-OMC nanocomposite electrode can attain a high reversible capacity of 401 mAh g-1 at a current density as high as 2 A g-1. These results might be contributed to the intrinsic characteristics of nanocomposite and mesoporous structure, which offered a better accommodation of the strain and volume changes and a shorter path for Li-ion and electron transport, leading to the improved capacity and enhanced rate capability.3. The composites of V2O3-ordered mesoporous carbon (V2O3-OMC) and ZnVO4-OMC were synthesized and used as anode materials for Li-ion intercalation. These materials exhibited large reversible capacity, high rate performance and excellent cycling stability. The high electrochemical performance of the vanadium oxides-mesoporous carbon composites is attributed to the anchoring of nanoparticles on mesoporous carbon for improving the electrochemical active of vanadium oxides for energy storage applications in high performance lithium-ion batteries.4. The composites of Li3V2(PO4)3-C were synthesized for the first time by using carbothermal reduction route. The optimized Li3V2(PO4)3-C composites display high crystallinity and a homogeneous nanostructure, which provides a mechanically stable structure as well as a short diffusion path for Li-ion intercalation and extraction. The Li3V2(PO4)3-C composites have shown a high reversible capacity, excellent long term cycling stability, and promising rate capability for Li-ion battery applications. For instance, a large reversible capacity of 95 mAh g-1 could be maintained at a current rate of 20 C with 86% discharge capacity retention even after 3000 cycles. The outstanding performance of the Li3V2(PO4)3-C composites make them a promising candidate to construct a viable and low-cost Li-ion battery system for upcoming power and energy storage systems.5. Unique GeO2-Ge-ordered mesoporous carbon nanocomposites (GeO2-Ge-OMC) were successfully prepared through hydrogen reduction route by using mesoporous carbon as nanoreactor. Then these nanocomposites were characterized by means of X-ray diffraction (XRD), N2 adsorption-desorption, transmission electron microscopy (TEM). Furthermore, the nanocomposites were used as anode materials for Li-ion intercalation and exhibited large reversible capacity, high rate performance and cycling stability. These results might be contributed to the intrinsic characteristics of nanocomposite, which offered a better accommodation of the strain and volume changes and a shorter path for Li-ion and electron transport, leading to the improved capacity and enhanced rate capability.