| Since1990, lithium ion battery has been widely used in portable electronic devices, electric vehicles (EVs) and uninterruptible power system (UPS). Nanostructured electrode materials can remarkably promote charge/discharge rate due to their reduced dimensions, but it could encounter some problems in practical applications, such as side reactions, low tap density, etc. Micro/nano hierarchical electrode materials, as special nanomaterials, can take advantages of both nanomaterials and micromaterials. The ultraflne dimension of its building blocks can effectively shorten the lithium ion diffusion path and promote the rate performance during the charge-discharge process, while the microscale assemblies with relatively smaller specific surface area and better structure stability compared with unitary nanostructured materials could weaken the side reactions and avoid the active materials aggregation, which can enhance the cycling performance. In this dissertation, micro/nano hierarchical LiMn2O4microspheres, LiFePO4microflowers and CuO microcog films were successfully prepared and the electrochemical performances related to their microstructures were investigated. Besides, a novel route based on hydrothermal method was developed for preparing Cu2+doped LiFePO4/C cathode materials.In chapter1, a general introduction was given to the history and working principles of lithium ion batteries, including three important cathode materials (LiCoO2, LiMn2O4and LiFePO4) and four kinds of anode materials (carbon, Li-alloy, Li4Ti5O12and transitional metal oxides). Particularly, the developments of nanostructured electrode materials for lithium ion batteries were reviewed.In chapter2, hierarchical LiFePO4microflowers have been successfully synthesized via a solvothermal reaction in ethanol solvent with the self-prepared ammonium iron phosphate rectangular nanoplates as precursors, which were obtained by a simple water evaporation method beforehand. The hierarchical LiFePO4microflowers are self-assemblies of a number of stacked rectangular nanoplates with length of6-8μm, width of1-2μm and thickness of around50nm. When ethanol was replaced with the water-ethanol mixed solvent in the solvothermal reaction, LiFePO4micro-octahedrons instead of hierarchical microflowers were prepared. Then both of them were modified respectively with carbon coating through a post-heat treatment and their morphologies were retained. As cathode materials for rechargeable lithium ion batteries, the carbon-coated hierarchical LiFePO4microflower sample delivers high initial discharge capacity (162mAh g-1at0.1C), excellent high-rate discharge capability (101mAh·g-1at10C), and cycling stability. It exhibits better electrochemical performances than the carbon-coated LiFePO4micro-octahedron sample. The enhanced electrochemical properties can be attributed to the micro/nano hierarchical structure of the electrode material. It can take advantage of structure stability of micromaterials for long-term cycling. Furthermore, the rectangular nanoplates as the building blocks can improve the electrochemical reaction kinetics and finally promote the rate performance.In chapter3, Cu2+-doped LiFePO4/C nanoparticles with the particle size of400-500nm were successfully prepared by Cu2+doping and carbon coating pure phase LiFePO4nanoparticles from hydrothermal synthesis. Compared with undoped LiFePO4/C sample, the Cu2+-doped LiFePO4/C sample exhibit remarkably-improved electrochemical performance at high charge/discharge rate. It can deliver discharge capacities up to154mAh g-1,148mAh g-1,143mAh g-1,111mAh g-1and86mAh g-1at0.1C,1C,2C,10C and20C. Meanwhile, the Cu2+-doped LiFePO4/C cathode material also gives an excellent cycling performance, with no obvious capacity loss over50cycles. In addition, this Cu2+-doped LiFePO4/C composites present an outstanding low-temperature electrochemical performance, when the environmental temperature decreases to-30℃, it can still deliver a discharge capacity of102mAh g-1at0.1C rate. The electrochemical performance can meet the need of power batteries. In comparison with the solid state reaction method, this preparation process can fabricate LiFePO4cathode materials with higher electrochemical performance at lower cost.In chapter4, micro/nano hierarchical LiMn2O4microspheres have been successfully prepared through calcinating the carbonate precursor from co-precipitation. The results indicate that the samples are pure-phase spinel LiMn2O4and exist in microspheres with the diameter of600-900nm, which are assembled by nanoparticles of about30-40nm. The electrochemical performance of the as-prepared LiMn2O4microspheres has been investigated by galvanostatic charge-discharge test. The results show that the sample prepared at750℃for9h possesses excellent electrochemical performance. Its discharge capacities are124.71mAh g-1at0.1C and122.99mAh g-1at1C, and after50times cycling, the specific discharge capacity is still as high as113.98mAh g-1. Such excellent rate and cycling performance can be attributed on one hand to its micro/nano hierarchical structure with ultrafine nanoparticles as the building blocks, which can shorten the lithium ion diffusion path, and on the other hand to its micro-sized assembly with relatively smaller specific surface area compared to nanoparticles, which can ease the dissolution of Mn3+in the electrolyte. In addition, these micro/nano hiearchical LiMn2O4microspheres present excellent rate performance in2M Li2SO4aqueous solution. When the current density increases to1A g-1,1.5A g-1,2A g-1and2.5A g-1, the specific discharge capacity are all about110mAh g-1. Even at5A g-1, the specific discharge capacity can still retain71mAh g-1.In chapter5, CuO nanorod film with diameter of300-500nm and CuO hierarchical microcog film with diameter of4-6μm which are assembled by ultrafine nanofibers with the diameter of20nm were respectively prepared by aqueous solution method and micro-emulsion method. These two kinds of films and CuO microparticles were characterized by electrochemical tests. The electrochemical performance shows that the first discharge and charge capacity of this hierarchical CuO microcog film are1057mAh g-1and779mAh g-1, respectively. Moreover, it shows excellent rate performance and cycling performance. The discharge capacity of the microcog film hardly decays during the50times cycling process and has a discharge capacity of579mAh g-1at3C rate. Compared with CuO microparticles and CuO nanorod film, this microcog film shows smaller irreversible capacity, better cycling and rate performance. Such better performance can be ascribed to the small dimension of its building blocks which can improve the electrochemical reaction extent during the charge process, shorten the lithium ion diffusion path and accommodate the volume expansion during the charge-discharge process. Meanwhile, the assemblies with relatively smaller specific surface area could confine the excessive SEI formation, thus reduce its initial capacity loss.In chapter6, we assembled full batteries by using the LiFePO4microflowers as the cathode materials to combine with three kinds of anode materials (graphite, Li4Ti5O12and CuO microcog film) respectively. The LiFePO4/CuO full battery presents the best electrochemical performance compared with other kinds of full batteries. The specific discharge capacity is140mAh g-1at1C rate (based on LiFePO4) and after100cycles, the specific discharge capacity is still as high as134mAhg-1.Finally, in chapter7, an overview on the research achievements is summarized and some prospects of the future research are pointed out. |
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