| Currently, lithium ion batteries (LIBs) represent a key class of battery architecture for portable consumer electric products (such as tablet computers and laptops, smart-phones, digital cameras) and for hybrid electric vehicles or all electric vehicles (HEVs or EVs) and for home appliances, medical implant devices due to their security, environmental benignity, long cycle life and high energy density, etc.. Lithium ion batteries are mainly composed of positive electrode material, negative electrode material, separator and electrolyte, which codetermine the lithium storage and safety performance of lithium ion batteries. Since the cycle lifetime and energy density of batteries are mainly determined by the electrode materials, the development of high performance electrode materials has become the hot spot of the research.At present, graphite carbon anode material is used commonly. Although it has the advantages of high electronic conductivity, long cycle life, low cost, safety performance, however, the theoretical specific capacity is only 372 mA h g-1, it is difficult to meet the people’s demands of high performance and high capacity of lithium ion battery (LIBs), which limited its application. Among the various anode materials, nanomaterials of transition metal oxides (TMOs) have been intensively studied as anode materials for LIBs due to their huger theoretical capacities, environmental benignity, widespread availability and aiming at achieving higher specific capacities than graphite. However, their remarkable lithium-storage capabilities are often accompanied by an inherent poor electrical conductivity and large volume expansion upon cycling, resulting in the loss and pulverization of active materials, leading to a sharp attenuation of the capacity. Thus the cycle performance cannot be guaranteed and the electrochemical performance is affected. The previous studies show that the morphology and structure of the material have a very important influence on the performance. Rational design and facile synthesis of TMO-based electrode materials for highly reversible and high-rate lithium storage still remain a significant challenge. Presently, a lot of research works focus on two sides:on the one hand, designing the electrode materials with reasonable structure (such as hollow nanostructures) to relieve the volume expansion in the charge-discharge process; on the other hand, at the standpoint of preparing composite (such as TMO/carbon, graphene, etc.) to accommodate volume change and maintain the mechanical integrity of the composite. In this thesis, we focused mainly on the preparation and performances of TMO nanoparticles or nanocomposites with specific morphologies and structures on the purpose of solving the above problems, and improving the overall electrochemical performance of the TMO. Four kinds of electrode materials such as Fe2O3/ZnO, MoO2/RGO, hollow Co3O4 and Co3O4 nanoparticles were successfully prepared respectively. The synthesized samples were well characterized and their electrochemical performances were studied. The main work and achievements can be summarized as follows:1. Ferric chloride, sodium acetate and other reagents were taken as raw materials, by using one step hydrothermal method, Fe3O4 microspheres were successfully prepared. The average particle diameter was about 150 nm. With MAA-Fe3O4, zinc nitrate and trimesic acid as the raw materials, by using solvothermal MOFs templating method and subsequently an annealing process, the products with an average diameter of about 400 nm and porous rough surface were obtained. When used as an anode material for lithium-ion battery, thus-obtained composite showed excellent electrochemical performance, including good charge and discharge specific capacity, excellent rate capacity and a relatively stable cycle performance (ca.708 mA h g-1 at a current density of 100 mA g-1 after 60 cycles) than that of pure Fe2O3 (ca.390 mA h g-1) and ZnO nanoparticles (ca.361 mA h g-1). In addition, the coulomb efficiency of the Fe2O3/ZnO composite is more than 98%. The good performance can be attributed to the following aspects:1) it integrates two types of functional materials that can strengthen the electrochemical properties through the synergetic effect; 2) the composite can effectively prevent the aggregation of Fe2O3 nanoparticles; 3) the large internal voids increase the specific surface area, allow the expansion without compromising the integrity of the structure.2. We report a simple and cost-effective template assisted method and annealing process to synthesize Co3O4 hollow spheres (with the diameter of about 450 nm) composed of highly crystalline nanocrystals, in which carbonaceous (C) spheres with diameter of 100 nm are chosen as the removable template and Co-MOF as self-sacrificial agent. The high crystallinity, porous and hollow structure give rise to significant improvements in the cycling stability and rate performance of the Co3O4 hollow spheres. These hollow spheres exhibit exceptional lithium storage properties with ultrastable capacity retained at 1003 mA h g-1 after 100 cycles at a current density of 100 mA g-1. At the same time, we also synthesized Co3O4 nano-rods (rod length is about 300 nm and the length to diameter ratio is 4:1) with Co-MOFs as the precursor without adding carbon spheres. The specific capacity and cycle performance of the latter is far less than that of the former. Thanks to the use of carbon spheres as template and the porous structure is conductive to the transfer of lithium ion, also shorten the distance of electronic transport. The experiment process is controllable and can be large-scale production, and that it has potential application value in the development and use of lithium ion battery anode materials.3. A biomass carbon was prepared by pyrolysis a common lifewaste-green tea residue in nitrogen atmosphere. In addition, green tea residue were used as a template to fully absorb cobalt salt by combining high temperature calcination to remove the template to prepare Co3O4 nano material. The prepared carbon material and Co3O4 nano material were used as the active material to carry out the charge discharge performance test. The results show that the biomass carbon source has a certain specific capacity (stable at 360 mA h g-1). The Co3O4 nano materials prepared by the template of green tea residue still have the capacity of～690 mAh g-1 after 100 cycles under the current density of 100 mA g-1, and it also shows good rate performance and cycle performance. The experimental method is simple, has wide raw material sources, low cost, can perform large-scale production and has certain directive significance for processing of other types of biomass garbage and it will has potential application value in application of lithium ion batteries.4. Graphite oxide was synthesized from natural graphite powder (325 mesh) using a modified Hummers’ method. We adopted the freeze-drying method with thermal annealing in nitrogen atmosphere to prepare a hybrid containing molybdenum dioxide (MOO2) and reduced graphene oxide (RGO) from ammonium molybdate (NH4)6Mo7O244H2O and graphene oxide (GO). As a result, MoO2 nanoplates were uniformly anchored on RGO matrix, and when used as an anode material for lithium ion batteries, the hybrid delivered excellent electrochemical properties including high rate capability and stable cycle performance, while the reversible capacity remained at 750 mA h g-1 after 150 cycles at a current density of 100 mA g-1. The high capacity over prolonged cycling achieved could be attributed to the formation of two-dimensional electrical percolating networks, homogeneous dispersion and immobilization of MoO2 nanoparticles. |
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