| While car companies race to develop electric and hybrid electric vehicles (HEVs), one of the biggest challenges they face is finding a suitable energy storage system. Lithium-ion batteries (LIBs), which currently power a variety of smaller consumer electronics devices, could ideally fill this role. But at the moment, they require further improvements in terms of energy density and power density in order to be used effectively in electric vehicles. The key to the high performance lies in the battery’s electrode materials. The search for better electrode materials has been the most dynamic motivation for the R&D of LIBs. To obtain a noticeable improvement in the specific capacity of LIBs, it is essential to develop electrode materials with higher reversible capacity. Currently, the most dominant anode material of LIBs is traditional graphite. Yet, graphite does not still satisfy practical applications of the next generation large scale LIBs, because of its lower theoretical capacity (372mAh/g). This thesis focuses on several kinds of alternative anode materials, such as amorphous carbon materials, Li4Ti5O12and silicon. Their synthetic methods and electrochemical properties are discussed. In addition, since low cost and widespread geological distribution of sodium, room-temperature sodium ion batteries (NIBs) are promising alternatives to LIBs. We also investigate some promising anode materials for NIBs, such as Li4TisOi2anode.In Chapter1, the author briefly introduces the working principle of LIBs and NIBs. Special attention is given to anode materials for both LIBs and NIBs. In addition, the research background and motivation of this thesis including the research methods are presented in the last part of1st chapter.In Chapter2, we list the synthesis strategy, reagent, instruments, and analysis method in this thesis. Among them, we mainly introduce the electrospinning technology. Its principle, structure and application in the field of battery electrode materials are briefly described.In Chapter3, we use a kind of biomass waste (rice husk) and low-cost triphenylphosphine (TPP) as raw material and succeed in synthesizing porous carbon materials with phosphorus doping. Firstly, the refluxing time was optimized to totally dissolve the cellulose and lignin, which form the main body of the husk, thus a more porous carbonous3D nanostructure could be obtained. The optimized porous carbon electrode (refluxing time is6hours) could deliver a reversible capacity of525mAh/g after100cycles. Secondly, to enhance its performance, the dopant phosphorus was covalently bonded to the porous carbon framework. After doping with phosphorus, the electrode delivers outstanding lithium storage properties. At a current density of100mA/g, it could deliver a reversible capacity of757mAh/g after100cycles. Compared with other carbon materials, it combines a variety of advantages:easy access of electrolyte, short transport path of Li+, and high conductivity transport of electrons through the porous carbon. The well-dispersed phosphorus atoms doped within the carbon framework also introduce more edges and topological defects to generate a more disordered structure. This work provides a simple, low-cost preparation method to prepare anode materials for high performance LIBs.In Chapter4, we use conventional solid state method to prepare Li4Ti5O12in micron size. To enhance its electrochemical performance, we first doping Br-into the O site. After Br doping, Li4Ti5O12shows enhanced lattice parameter slightly due to larger ion radius, which benefits the Li+diffusion in micron-sized Li4Ti5O12. In order to further improve its rate performance, we apply thermal nitridation with the optimized Br-doping Li4Ti5O12(Li4Ti5O11.7Br0.3) anode. After treated by NH3, there is a layer of TiN forming on the surface of the crystal particle, which has been demonstrated by XPS, EDS mapping and HRTEM images. The existence of Ti3+can increase the conductivity. Benefiting from Br-doping and thermal nitridation, Li4Ti5O12shows much better electrochemical performance in terms of specific capacity, cycling and rate performance than pristine Li4Ti5O12. When cycled at a high current density of20C, its reversible capacity could be still remain above100mAh/g. This work indicates the feasibility of improving electronically conductivity of micron-sized Li4Ti5O12through the synergistic effect of surface termination and bulk doping procedures, without give up tap density which is very important for Industrial applications.In Chapter5, we use the electrospinning method to fabricate nanofibers with α-MoO3particles wrapped in the carbon nanofiber matrix. α-MoO3has a high theoretical capacity of1117mAh/g but poor conductivity. The carbon nanofiber can provide a conductive channel for α-MoO3and suppress grains growth during phase formation process. The products after carbonization are mixture of α-MoO3and MoO2. To remove the impurity phase (MoO2), we carry out a post-thermal treatment process. What’s more, since the fibers after carbonization cross-link to form a network and show good mechanical strength, the obtained membrane could be directly used as free-standing electrodes. Such design avoids using current collector, adhesives and other additives, thus improving the energy density.In Chapter6, we design and prepare the free-standing CNT-loaded electrospun Li4Ti5O12/Carbon composite nanofibers by using electrospinning technology for NIBs. Homogeneously dispersed CNTs are incorporated into the composite nanofibers to offer mechanical strength and well conducting three-dimensional (3D) network between scattered Li4Ti5O12particles. The free-standing composite can achieve improved cyclability and rate capability when used as materials for NIBs, which could attribute to the special double carbon decorated structure providing special advantages. Besides, similar with the work in Chapter5, the energy density is also improved.Silicon shows the highest theoretical capacity of4200mAh/g based on the alloying/de-alloying mechanism with Li+. But the volume change during charging-discharging processes makes its capacity fades fast because of pulverization and losing contact with the current collector. In Chapter7, we synthesize a favorable Si (core)-hollow carbon nanofiber (sheath) nanocomposite by a coaxial electrospinning technique. TEM images clearly indicate that the silicon nanoparticles is encapsulated in the hollow carbon nanofibers. The outer wall thickness of the carbon sheath is around200nm which can effectively alleviate the volume change of silicon during charging-discharging processes. While the carbon hollow fibers offer adequate void space which acts as a "buffer-zone" to accommodate the large volume. Besides, the silicon nanoparticles encapsulated by carbon could be kept from contact with the electrolyte, forming a more stable SEI layer. Thus the resulting materials deliver a significant improvement of the electrochemical characteristics, specifically for both highly reversible lithium-storage capacity and excellent cycling.In Chapter8, we give a short summary of the series work, both achievements and defects. And we also point out the probable research directions in the future. |
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