Controlled Fabrication And Lithium Storage Properties Of Nanostructured Zn-based Composite Metal Oxides

The power consumption of mobile devices such as smart phones, laptops and electronic readers etc. increases with the rapid development of electronic technology. The requirement of high capacity, long calendar life and fast charge ability becomes more stronger. The storage of new enegy sources like wind and solar power turns out to be more important with the exhaustion of traditional fossil fuel. Lithium ion batteries are considered to be one of the idea candidate. However, the electrode materials lithium cobalt and the graphite cannot meet the demand nowadays. Consequently, it is urgent to explore new electrode materials with high capacity, excellent rate capability, long cycle life, low cost and environmental benignity.The strategies of nanostructuring and carbon hybridization are involved in this dissertation for preparing hybrid nanostructures of zinc-based metal oxides. Their electrochemical performances are also investigated in detail.Zn2GeO4was prepared by a solvothermal method using zinc acetate and germanium dioxide as the reactants, water and tert-butylamine as the solvent. Urchin-like heterosturcture can be obtained by adjusting the concentration of reactants and the ratios of water to amine. The product comprises nanorods with an average diameter of20nm and length of500nm. Such a one-dimentional structure can facilitate the charge transfer and improve the effeciency. In addition, the void space between the nanorods can accommodate the volume expansion during the lithium storage process. It is shown that the heterostructure possesses a high capacity and good rate capability. The growth mechanism of this unique nanostructure was also investigated and the results can be extended to prepare other metal oxides with similar nanostructure.A Zn4GeO4/nitrogen-doped graphene nanocomposite has been successfully synthesized via a microwave-assisted hydrothermal process. Zn2GeO4-ethylenediamine hybrids were used as the precursor, after they are dispersed in the graphene oxide suspension. The final products were achieved in a short time (15min) under microwave irradiation. The chemical reaction can be accelerated by high frequency friction between molecules. Ethylenediamine that is dissociated from the hybrids can reduce the graphene oxide into graphene with nitrogen doping. Zn2GeO4nanorods are well encapsulated with the N-doped graphene layers. When evaluated as anode materials for lithium ion batteries, the hybrid exhibits much enhanced electrochemical performance than those without using graphene. At a current density of100mA g-1, the Zn2GeO4/N-doped graphene nanocomposite can maintain a reversible capacity of1044mAh g-1after100cycles. Remarkably, it can still deliver a capacity of531mAh g-1, which is much higher than graphite, at a current density of3.2A g-1.A facile, low cost and environmentaly benign approach has been developed to prepare Zn2GeO4/graphene oxide nanocomposite with a sandwich structure via an ion exchange reaction at room temperature. Zn2GeO4nanorods are uniformly dispersed into the layers of graphene oxide, thus the volume expansion during the lithium-cycling process can be buffered by the elastic graphene oxide and the structure can be maintained. Thus, Li2O arising from the initial discharge process, can be reversibly turned into lithium ions and metal oxides in the following charge cycles. The reversibility can be calculated to be much as64%, as a result, the capacity can be largely improved. Also, the resulting hybrid material exhibits exciting rate capability and long cycle life. The stacking of graphene oxide can be also prevented by the Zn2GeO4nanorods, thus the availability of graphene oxide can be improved, leading to the reduced carbon content and the enhanced volume energy density. It shows a possibility for the large scale application.A mesoporous ZnO/ZnFe2O4/C hollow octahedral structure can be achieved by thermal treatment of a metal organic framework (MOF) precursor. Relying on the special structure of MOF, metal oxides arise from metal elements can be uniformly coated by carbon arising from the organic ligands during the carbonization process. The particle size of metal oxides can be confined into about5nm by the carbon layers. The wall thickness of hollow octahedron is only10nm and the major pore size is7.5nm. It shows an extrodinary performance when using as an anode material for lithium ion batteries. A reversible capacity of1047mAh g-1can be achieved in the initial cycle with a high coulombic efficiency of75.6%, which is higher than other similar metal oxides. The capacity increases to1390mAh g-1after100discharge-charge cycles. When the current density increases to10A g-1, a reversible capacity of762mAh g-1can be delivered. Such an outstanding performance can be attributed to the unique structure. The volume expansion can be accommodated effectively by the carbon layers combined with ultrafine (5nm) metal oxides nanoparticle, the accessibility of the electrolyte can be facilitated by the porous and hollow structure, the contact area between the electrode and the electrolyte can be enlarged and therefore the kinetics of electrochemical reactions can be improved. Moreover, the lithium diffusion path can be effectively shortened by the thin wall of octahedra and the rate performance can be improved.