Electrode Materials For High Performance Lithium Ion Batteries Prepared Via Electrostatic Spray Deposition And Electrospinning

Energy shortage is one of the biggest challenges on the world, and the demands of energy storage technology keep growing while a lot of efforts have been devoted to developing new energy source. Lithium ion batteries have dominated the portable electronic device market since they were firstly commercialized in 1990s. Meanwhile, as the portable electronic devices such as mobile phones, digital cameras and laptops become smaller and smaller, there is an urgent demand for new power sources with smaller volume and higher energy density. Therefore, the exploitation of new electrode materials for lithium ion batteries with high energy, longer-life and capability of fast charge/discharge becomes a research hotspot in materials science nowadays.The study in this Ph.D thesis mainly focuses on the syntheses of a series of carbon composite thin films or nanofibers as negative/positive electrode materials (C/Co,C/Fe3O4, C/Si, LiFePO4/C and Li3V2(PO4)3/C). Meanwhile, a porous metal oxide thin film electrode with a bimodal pore size distribution is also presented. The effects of nanostructure and carbon composite have been investigated in this thesis.In Chapter 1, a general introduction is given as following aspects:the advantages of lithium ion batteries compared with other batteries; the working principle and research status of some regular electrode materials. At the end of this chapter, a brief introduction of carbon composite electrode materials is presented.As effective synthesis techniques for thin film and nanofibers, electrostatic spray deposition (ESD) and electrospinning are widely employed in many research areas. In Chapter 2, there is a set up description of these two techniques and an introduction of the experimental equipments and methods used in the project of this thesis.In Chapter 3, homogenous polymer nanofibers are synthesized via electrospining, and a disordered carbon fiber with higher capacity than graphite has is obtained from subsequent thermal treatment. After an inorganic cobalt salt is added into the precursor, a C/Co composite nanofiber consists of cobalt nanoparticles is obtained. The interfacial lithium storage is enhanced by introducing cobalt nanoparticles and the A.C. impedance test reveals that the C/Co nanofiber has superior conductivity compared with the carbon nanofiber obtained at the same carbonization temperature, so higher revisable capacity and more stable cycling performance have been achieved.In Chapter 4, we synthesize a C/Fe3O4 composite nanofiber based on the work of Chaper 3, and the cobalt which is electrochemical inactive is replaced by Fe3O4 that can react with lithium. According to the calculation from XRD pattern and TEM characterization,600℃derived C/Fe3O4 composited nanofiber contains some well dispersed Fe3O4 nanoparticles (20 nm in diameter) inside the disordered carbon fiber. According to the convert reaction mechnasim, lithium ions can not only be stored in the carbon nanofiber, but also reduce Fe3O4 into Fe. The as derived Fe could enhance the electronic conductivity of the electrode during discharge, which can play the same role as cobalt does in C/Co composite nanofiber. Ex-situ SEM measurement reveals that powderization phenomenon will occur in C/Fe3O4 composite nanofbier electrode after several cycles, however, such a structural change in electrode does not cause any capacity decline. On contrary, the contact area becomes larger and the cell resistance because of the powderization, so a capacity increase is observed during the long-time cycling measurement. In general, such a C/Fe3O4 composite nanofiber not only can be employed as anode material for high performance lithium ion batteries, but also provides us a new straightforward method to prepare carbon/metal oxide nano materials.In Chapter 5, we prepare a-Fe2O3 thin film electrode at different temperatures by ESD. The 3D porous structure can effectively prevent the active material breaking off from the current collector, so the thin film exhibits same morphology as it before even after 15 cycles and stable capacity retention has been achieved. The a-Fe2O3 thin film deposited at 200℃exhibits an initial capacity loss less than 20%, while the energy conversation efficiency calculated from voltage profiles is 65.8%, which is much higher than NiO or CoO that usually has a value between 55% and 60%. We introduce a novel parameter called energy average voltage (Eav) to evaluate the working potential of metal oxides. Calculation results lead us to a conclusion that the ESD prepared a-Fe2O3 thin film has higher capacity (500 mAh g-1 under 5 C) while possessing a similar working potential to Li4Ti5O12, which is a promising anode material for high power batteries, Meanwhile, the porousα-Fe2O3 thin film can work in a wide temperature window according to the high-low temperature condition tests. In the last part of this chapter, Li2O is introduced into the film and effects on film morphology and electrochemical properties are investigated. When Li2O is introduced, the film becomes denser and the production of iron nitride decomposition changes into Fe3O4 instead ofα-Fe2O3. Nevertheless,Li2O does not oxidize Fe2+ to higher oxidation state. Silicon may have attractive prospect of applications because its theoretical capacity is much higher than those of other anode materials, and carbon composite silicon based materials have been considered to be able to effectively improve the cycling performance of silicon. In Chapter 6, we synthesize a C/Si composite nanofiber with 23 wt% silicon loaded. The composite nanofiber delivered a high capacity as 1100 mAh g-1 after the introducing of silicon. Thanks to the fibrous morphology, which provides sufficient free space, the electrode can survive from the large volume expansion caused by lithium intercalation. On the other side, in the purpose of improving its volumetric capacity density of carbon silicon composite material, we firstly synthesize C/Si composite thin film by employing the solvent as carbon precursor. Silicon thin film consists of polyglycerol which is originated from the polymerization of glycerin during ESD process is obtained, and the film exhibits a capacity retention reaches to 80% after 50 cycles due to the well dispersed poly glycerin. Due to the carbonization of poly glycerin, the revisable capacity can be improved when the film is treated under Ar atmosphere at high temperature. This carbon silicon composite with organic solvent as carbon precursor shows a new approach to synthesize carbon coated materials, and could be extensively applied in synthesis of other electrode materials.In Chapter 7, we synthesize two kinds of carbon composite cathode thin film electrode (Li3V2(PO4)3/C and LiFePO4/C). It is observed from the TEM pictures that there are lots of Li3V2(PO4)3 nanoparticles well dispersed in a carbon matrix generated from glucose decomposition in the walnut-like Li3V2(PO4)3/C thin film, and parts of the particles are coated by the carbon. Such a carbon composite nanastructured Li3V2(PO4)3/C thin film exhibits excellent rate capability, and all of the discharge capacity is above 3.0 V even under a current density of 24 C. A.C. impedance spectroscopy studies reveal that there is no obvious resistance change during charge/discharge process of Li3V2(PO4)3/C thin film, therefore, most of its capacity can be delivered under high current densities. The morphology of the thin film can be controllable tuned by changing the ration of solvent with different boiling point in the precursor, so that the porosity and electrochemical performance of the film can be adjusted. In the last part of this chapter, characterization and electrochemical measurements are conducted on a porous LiFePO4/C thin film with cage/sponge-like morphology. The results indicate that the nanoparticles consisted in the film could deliver their capacity well while the particles in micrometer size show rapidly capacity decline as the current density increases. The results lead to a conclusion that the nanoparticles with narrow particle distribution should be the key factor to improve the rate capability of LiFePO4 based materials.Finally, an overview on the achievements and deficiency of this thesis is presented at the end of the thesis (Chapter 8). Some prospects and suggestions in improvement and possible research directions are also pointed out.