Designed Syntheses Of M_xO_y/C Nanocomposites And Their Li~+Storage Properties

ABSTRACT:In this thesis, three dimensional (3D) nanostructured composite materials based on transition metal oxides (MxOy=Fe2O3, Fe3O4and TiO2) and nano-carbon materials (graphene and multi-walled carbon nanotubes) were designed and synthesized via hydrothermal/hydrolysis approaches. The as-synthesized composites were characterized using XRD, FTIR, Raman, TGA, ICP, EDX, XPS, SEM, TEM, HRTEM and STEM. Their Li+storage properties were further investigated by CV, galvanostatic discharge/charge and EIS tests. The main results are presented as follows.A3D Fe2O3/rGO/MCNTs ternary composite was fabricated by a facile, green and economical one-step urea-assisted hydrothermal approach. Hematite Fe2O3nanoparticles with a diameter of～35nm anchor on/among the3D conducting network constructed by rGO and MCNTs. The as-fabricated Fe2O3/rGO/MCNTs exhibits highly enhanced specific capacity, cycling performance and rate capability:initial discharge and charge capacities of1692and1322mAh g-1at100mA g-1, respectively,1118mAh g-1after50cycles at100mA g-1and785mAh g-1at1000mA g-1.Nano/micro-structured Fe3O4/rGO/MCNTs ternary composite was also synthesized using a facile, self-assembled and one-pot hydrothermal approach. Highly pure magnetite Fe3O4microspheres with0.2-0.5μm in diameter disperse in the hierarchical architecture of rGO and MCNTs. The as-obtained Fe3O4/rGO/MCNTs exhibits superior Li+storage performances:initial discharge and charge capacities of1452and1036mAh g-1at100mA g-1, respectively, reversible capacity of208mAh g-1at10A g-1, excellent capacity retention up to776mAh g-1after200cycles with average coulombic efficiency as high as98%.Anatase-TiO2/MCNTs nanocomposite was prepared via a facile two-step synthesis by ammonia water assisted hydrolysis and subsequent calcination. Pure anatase TiO2nanoparticles with a diameter of～10nm are uniformly distributed on/among the MCNTs conducting network. The as-synthesized nanocomposite exhibits remarkably improved electrochemical performances, especially super-high rate capability and cycling stability. A reversible capacity as high as92mAh g-1is achieved even at a current density of10A g-1(60C). After100cycles at0.1Ag-1, it shows good capacity retention of185mAh g-1with an outstanding coulombic efficiency up to99%.Based on the above researches, TiO2/FexOy/MCNTs was designed and synthesized by a simple and inexpensive bottom-up assembly approach. In such a hierarchical nanostructure, TiO2-modified FexOy nanoparticles grow on/among MCNTs framework. With the introduced TiO2as a buffer material, and the MCNTs framework as a porous3D conducting/buffering network and an effective substrate for anchoring FexOy nanoparticles, as compared to the existing analogues ever reported, the as-designed TiO2/FexOy/MCNTs display overwhelmingly superior Li+storage properties:good capacity retention with922mAh g-1at500mA g-1and1089mAh g-1at200mA g-1, excellent rate capability with current densities varying from50mA g-1to10A g-1and remarkable cyclic performance with the capacity increased from584to922mAh g-1at500mA g-1over450cycles (remaining stable around920mAh g-1after the420th cycle) after the rate tests for the same tested cell. The fascinating TiOi/FexOy/MCNTs as well as other hybrids with similar structures show great potential in applications of high-performance lithium-ion batteries.