Preparation Of Vanadium-based Oxide Nanostructures And Their Electrochemical Properties

As a clean and reliable energy storage device, lithium-ion batteries (LIBs) have been widely applied in many fields. In 2013, five billion LIBs were used to supply power-hungry laptops, cameras, mobile phones, pure electric or hybrid electric vehicles. These applications not only greatly stimulate the development of LIBs, but also propose a high expectation on electrodes with high energy density and strong power density. Owing to the special electric structure, vanadium oxides have different elemental composition forms, abundant oxidation states and various coordination polyhedra, which entail the controlling of structural parameters to further improve their structure-directed properties. In this regard, vanadium oxides have received particular interest for efficient energy utilization such as LIBs and superpacitors, and the search for new kinds of vanadium oxides in the applications of the energy issues has been highlighted in recent years.The goal of this dissertation is to explore the controllable synthesis of vanadium-based oxides micro-nanostructures via simple solid or solution method and their application on LIBs. On the basis of the unique micro-nanostructures and surface structures of these vanadium-based oxides, the corresponding structure-property relationships have been also investigated in this dissertation. The details are summarized briefly as follows:(1) Hierarchical V2O5 microflowers composed of thin nanosheets have been achieved by a solvothermal reaction first and then a low-temperature calcination process. The nanoscale size and sheet-like structure of the building blocks in V2O5 microflowers make them a promising cathode material for lithium ion batteries. After 1500 cycles at a current density of 1 A g-1, the reversible capacity of V2O5 micro-flowers is kept at 104 mAh g-1. Even at a rate of 2 A g-1,the reversible capacity is still above 80 mAh g-1 after 3000 cycles. To the best of our knowledge, these V2O5 microflowers present not only the longest cycle life, but also the ultralow fading rate per cycle. The cyclic voltammetry, together with the electrochemical impedance spectra, reveals the considerable capacitive components in the specific capacity, which improves the cycling stability and rate capability of V2O5 microflowers.(2) Oxygen-deficient Li3VO4 (Li3VO4-δ), synthesized by a simple annealing of Li3VO4 in vacuum, is made of a crystalline core and an amorphous surface rich of V4+ ions/oxygen vacancies. Compared with the case of Li3VO4, the presence of this amorphous surface greatly enhances the electrochemical performances of Li3VO4-δ in both reversible capacity and columbic efficiency for the first discharge/charge. These improvements could be ascribed to enhanced charge transfer kinetics of Li3VO4-δ, where its unique amorphous surface rich of structure defects is vital by the comparison to Li3VO4. The charming aspects of this method lie in that it realizes the performance improvements of electrode materials via another strategy totally different from nanostructure engineering and surface coating with carbon, effectively avoiding their negative consequences. Most important, this synthesis is convenient and cost-effective, then particularly suitable for the mass production of high-performance electrode materials.(3) Mesoporous Li3VO4 nanoparticles have been obtained via a solvothermal reaction with further annealing at 550℃. The Li-storage behavior of the nano-sized mesoporous Li3VO4 have been investigated as a promising anode material. After 800 cycles at a current density of 1 A g-1, the reversible capacity of mesoporous Li3VO4 nanoparticles is kept at 302 mAh g-1. Even at a rate of 2 A g-1, the reversible capacity is still above 209 mAh g-1 after 1000 cycles. We determine the capacitive contribution by using the voltammetric sweep rate dependence from the cyclic voltammetry. The achieved results demonstrate the pseudocapacitive characteristic of Li-storage in the mesoporous Li3VO4 nanoparticles. These insights will be of benefits in the future study of Li3VO4 materials.