Fabrication And Characterization Of Cobalt-and Vanadium-Based Nanostructured Electrode Materials For Lithium-ion Batteries

Lithium ion batteries have been widely applied in portable devices due to the high energy density. Nevertheless, as the modern portable electronic devices such as smart phones, laptops and digital cameras become more delicate and light, there is a demand for lithium ion batteries with higher energy density and smaller volume. Meanwhile, the promising applications for lithium ion batteries are in electric vehicles and the large energy storage systems in power stations. In order to address the needs of longer driving distances and better ability of acceleration for electric vehicles, the batteries should be able to be discharged at high current densities while maintaining high capacity and high power. On the other hand, batteries for energy storage power station should have better safety, lower cost and longer cycle and shelf life. The traditional lithium ion batteries with LiCoO2as the cathode and graphite as the anode cannot satisfy these new requirements. Thus, the exploration of new electrode materials with longer-life, higher capacity and better capability of fast charge/discharge become a research hotspot for lithium ion batteries nowadays.In this thesis, a series of cobalt-and vanadium-based thin films or nanofibers as negative/positive electrode materials are fabricated, which include CO3O4, CoO/Co, V2O5thin films and Li3V2(PO4)3/C nanofibers. In addition, we have also studied the synthesis and measurement of free-standing VO2/graphene electrode.In Chapter1, a general introduction is given on the following aspects:the configurations, charge-discharge mechanism and the regular electrode materials. We also introduce the current development directions of lithium ion batteries. The background of this thesis is presented.In Chapter2, the raw materials used in the project of this thesis and the mechanism of electrostatic spray deposition (ESD) and electro spinning are briefly introduced. Then, the assembling process of coin-type cells, some common methods of structural and electrochemical characterizations are also introduced.In Chapter3, we mainly synthesize several suspensions consisting of cobalt, vanadium or manganese alkoxide particles with some alcohols as the solvent. In order to study the influence to the morphology of cobalt alkoxide particles, the factors of reactant concentration, reaction time and the composition of solvents are investigated. In addition, the hollow spherical cobalt alkoxide particles is characterized and the formation mechanism of this structure is discussed. Meanwhile, we synthesize the LiCoO2from some selected spherical cobalt alkoxide particles with different particle size and investigate their electrochemical performances.Based on the stable suspension of cobalt alkoxide that is synthesized in Chapter3, we fabricate CO3O4thin films via ESD technique in Chapter4. The thin films are composed of interconnecting hollow spherical particles. Because the volume changes during the electrochemical cycling are accommodated by the void interior space of particles, the thin films electrode show high reversible capacity and stable cycling performance. Then, when the deposition temperature is lowered and an annealing process under nitrogen atmosphere is introduced, CoO/Co composite films are achieved. The XRD and XPS analyses indicate that the ratio of metallic cobalt is increased with increasing the annealing temperature. The electron energy loss spectroscopy (EELS) is applied to clarify the distribution of metallic cobalt in the composite films. Such CoO/Co composite films display outstanding rate capability and cycling performance due to the increased electronic conductivity by the in-situ metallic cobalt. The film electrodes annealed at400℃and500℃show the capacity retention at5C of71%and83%, respectively, which are higher than that of above CO3O4film.In Chapter5, vanadium oxide films composed of porous walnut-like particles are fabricated via ESD technique with an oil-bathed suspension precursor of vanadium alkoxide. The micron-sized walnut-like particles are constructed from vanadium oxide nanocrystals of size around50-100nm and the valence of vanadium is determined by EELS. As a cathode material for rechargeable lithium batteries, it exhibits high reversible specific capacity in the voltage range of2.1-4.0V. It delivers a specific discharge capacity of254mAh g"1in the first cycle, and still maintains200mAh g"1after100cycles. When the cell is cycled between2.5and4.0V, it displays ultra-high rate capability. Its discharge capacity reaches103mAh g-1at50C with a capacity retention of88%after100cycles at10C charge/discharge. Meanwhile, the electrode also exhibits good electrochemical performance under low temperature conditions. The porous structure of the film is believed to be beneficial to achieve a large contact area between the electrolyte solution and the electrodes. It can also decrease the diffusion distance for lithium ions so that the electrochemical reaction can proceed efficiently at a lager current density or under low temperature conditions. Based on the CV curve at the different scan rates, we calculate the diffusion coefficient of lithium ions in vanadium oxide film at25℃,0℃and-10℃. Besides, the activation energy is obtained.For traditional Li-ion battery electrodes, polymer binders are added to improve the binding of active materials to aluminum or copper current collectors. Nevertheless, the binder causes an undesirable decrease of the electrical conductivity and the current collectors lead to the decrease in the energy density. In Chapter6, we synthesize the V2O5?O.86H2O/RGO nanoribbons via a hydrothermal method. The process of the ribbon growth is investigated by TEM analysis. Then, after the filter process followed by a heat treatment step, a free-standing binder-free VO2/RGO composite film is obtained. XPS analysis indicates the presence of a little amount of V5+in the film. The formula of the composite can be determined as VO2.06and9wt%carbon based on the TGA curves. The free-standing VO2.06/RGO film exhibits better electrochemical performance than RGO-free electrode. That should be attributed to the intimate contact between the vanadium oxide ribbons and the fast pathways for lithium ions and electrons.In Chapter7, we synthesize LiaV2(PO4)3/C nanofibers via an electrospinning technique. The effects of sintering temperature for the structure and morphology of fibers are investigated by XRD, SEM and TEM analyses. Based on the electrochemical test in different voltage range, it is found that the sample prepared at750℃shows better electrochemical properties.Finally, a brief summary of the achievements and deficiency of this thesis is presented in Chapter8. We also point out the possible research direction and aims of the improvement in the future.