Metal Oxide-Based Nanostructures: Self-Assembly Synthesis And Electrochemical Performance

Self-assembly synthesis is a novel approach to prepare the hierarchical micro-nanometer and/or composite materials by regulating the self-assembly behavior of nanoparticles, which mainly includes the two-step assembly and in situ carbonization etc. The self-assembly structures not only have advantages of micro-nano scale structures and/or composites materials, but also show more excellent properties due to the production of synergistic effect. So far, the self-assembly synthesis has been widely used in many fields, such as biomedical, optoelectronic and photovoltaic. Especially in the field of lithium-ion batteries, the electrochemical performance of electrode materials with self-assembled structure synthesized by this technology can be significantly improved. Metal oxides are regarded as promising anode materials because of many advantages, such as high theoretical specific capacity, abundance and environment friendly. The lithium storage mechanism of metal oxide is mainly divided into three categories; the first mechanism is intercalation-deintercalation reaction, which minor modifications of the crystal structure do occur and the electrodes have a good stability during charge-discharge. However, most oxides based on this mechanism have a relative low capacity. The other two lithium storage mechanism is alloying-dealloying and conversion reaction. Metal oxides based on those two mechanisms have a high theoretical capacity, but dramatic volume change occurred during cycling process hinder their practical applications.In this thesis, we studied the self-assembly synthesis of TiO2, ZnO and iron oxide, which are the typical representatives of different lithium storage mechanisms. To overcome the shortcoming of different electrodes, we have designed and prepared various hierarchical micro-nano structures and composite materials. The specific capacity, cyclic stability and rate performance of electrodes are significantly improved. The main research contents and results are as follows:1. We develop a simple bottom-up method to prepare mesoporous TiO2 sub-microspheres constructed by TiO2 nanorods. The pore structure has pseudocapacitance effect in electrochemical process, which can improve the specific capacity of TiO2 electrode. The self-assembly behavior, structures and properties of TiO2 nanorods in octadecene and cyclohexane solution are studied. In octadecene solution:Primary self-assembly occurred in synthesis process and formed two structures, in the morphology of ribbon and honeycomb. Secondary-assembly took place when the products were placed at lower temperature, where the ribbon and honeycomb structure were used as building units. Various structures are obtained with the increase of TiO2 concentration and show nematic, spherulites, and lamellar phases respectively. In cyclohexane solution:The assembly process is achieved by dissolving the oleic acid-coated NRs into cyclohexane solution and refluxing without any other surfactants or complex steps. The TiO2-C sub-mcrospheres are synthesized by carbonization and the amorphous carbon layer is formed in situ on the surface of the nanorods. As a Li-ion battery anode material, the TiO2-C sub-microspheres exhibit better electrochemical performance with enhanced capacity, greater cyclic stability compared to those of unassembled nanorods. The composite electrode deliever a capacity of 151 mAhg-1 at 1C and 90.1 mAhg-1 at a high current density of 10C.2. A novel architecture of three-dimension carbon framework to encapsulate tetrahedron ZnO nanocrystals was prepared by an in situ carbonization self-assembly approach and exhibited the superior lithium storage performance. Experiments show that the carbonization temperature is a key facor to influence the structure and properties of the products. The ZnO crystal calcined at 500℃ (ZnO@C-5) possesses regular tetrahedron shape with a side length of 150-200 nm and all of them are uniformly anchored among the network of amorphous carbon, which is bettern than that of products carbonized at 400 or 700℃. The ZnO/C-5 electrode exhibits the discharge capacity of 518 mAhg-1 after 300 cycles. A value of 375.7 mAhg-1 can still be retained even at the current density of 1000 mAg-1. The reasons for the improved cycling performance of the ZnO@C-5 can be ascribed to three features. Firstly, the special structure and uniform size distribution of the ZnO@C-5 provide it an excellent electrochemical performance, due to the fact that regular tetrahedron structure may possess more reversible lithium storage site, which suggests that it has more interfacial charge storage and/or reversible side reactions. Secondly, the ZnO crystals are uniformly anchored among the network of amorphous carbon, which inhibits pulverization and maintains the integrity of ZnO tetrahedron to allow extended charge-discharge cycles.3. The iron-containing precursors are prepared by an internal-refulx, and various Fe2O3-based composite materials are obtained after carbonization. The synthetic procedure is simple, enviormental friendly and applicable for large-scale production. Experiments show that the y-Fe2O3/carbon composites are obtained when the carbonization temperature is 500 or 600℃. With the temperature increasing, parts of Fe3+ will be reduced to metallic Fe and the y-Fe2O3/Fe/carbon composites are obtained. When was used as anode materials for lithium-ion batteries, the composites preserves a capacity as high as 1198.2 mAhg-1 after 215 cycles at a current density of 200 mAg-1 and provide a capacity of 1404.3 mAhg-1 after 500 cycles at a current density of 500 mAg-1. Even at a very high current density of 4000 mAg-1, the γ-Fe2O3/Fe/carbon composites still maintain a capacity of 640 mAhg-1. The metallic Fe may play an important role in improving the electrochemical properties, which it can not only enhance the conductivity of electrode, but also buffer volume expansion like the carbon matrix.4. We have demonstrated the coordination-driven of self-assembly between Fe3O4 and graphene sheets under hydrothermal condition for simple in situ synthesis of a 3D Fe3O4-graphene hybrid architecture. By using ethylene glycol-water mixed solution and the introduction of graphene oxide, the self-assembly of Fe3O4 particles and graphene oxide occurred simultaneously. The two assembly process reinforced and interacted with each other, namely, the coordination-driven of self-assembly. The threshold of GO assembly decreases dramatically due to the presence of coordination-driven of self-assembly. Even if the concentration of GO was as low as 0.13 mg/mL, Fe3O4/G hybrid-gel could still be obtained. The Fe3O4-graphene composites deliver a discharge of 1164 mAhg-1 after 500 cycles at a current density of 500 mAg-1, with high capacity retention of 99.4%. Moreover, the Fe3O4/G electrode exhibits exceptional rate performance, which deliever a capacity of 510 mAhg-1 at a high current density of 2000 mAg-1.