Co-based Composite Oxides Micro/Nanostructures: Controllable Preparation, Formation Mechanism And Lithium Storage Properties

In the 21st century, green energy, such as wind power, solar power, and rechargeable batteries, has attracted much attention due to the energy crisis and environmental problems. These problems are prominent with the passage of time and seriously affected on economic development of the country and normal life of people. The lithium-ion batteries (LIBs), which one of the green energy source have been successfully applied in portable electronic devices, such as mobile and portable computers and in future, researchers want to use these batteries in electric vehicles (EVs). However, the conventional bulk electrode materials are difficult to completely fulfill the requirements of electric vehicles (EVs)/hybrid electric vehicles (HEVs). In order to meet the requirements of applications, to search for electrode materials with high capacity and energy density is required. Transition metal oxides, as the representative of high capacity materials have been considered as the potential electrode in lithium-ion batteries. The synergetic effects of the different metal ions and complex composition of their oxides can cause to improve the performance. Unfortunately, transition metal oxides practical applications have been restricted because of their intrinsic characteristics like poor electrical conductivity and the volume effect. By designing and constructing transition metal oxides with nanostructures, the electrochemical performance can be greatly improved. At the meantime, the investigation of the formation mechanism of transition metal oxides can contribute to understand and design the electrode materials with favorable structure.In this thesis, electrode materials with micro-nano structures have been synthesized by hydrothermal or water bath method. The formation mechanism of crystalline materials and electrochemical performance of lithium-ion batteries have been explored and investigated. The main contents are as follows:(1) In this work, the as-prepared Co(OH)F wall arrays were assembled with ultrathin nanosheets on conductive nickel foam via hydrothermal route. In the reaction, Ammoniumfluoride (NH4F) is the key factor to organize single-crystalline nanosheets into wall arrays. By examining the intermediates collected at different reaction stages, a novel growth mechanism which is a gradual crystallization self-assembly process via Ostwald ripening has been proposed to understand the formation of wall arrays. Followed by the thermal decomposition of Co(OH)F precursor in air, mesoporous Co3O4 wall arrays are fabricated (denoted as Co3O4@Ni). Carbon-coated CoO wall arrays on nickel foam (denoted as CoO@Ci) have been obtained through the next step calcination of Co3O4@Ni at atmosphere of acetylene (C2H2). During the process, onion-like graphitic carbon can also be produced at relatively low temperatures (300-350℃) near the substrate because of the catalysis of nickel foam. The discharge capacities of CoO@C-Ni and Co3O4-Ni electrodes after 60 cycles are 804 and 697 mAh g-1, indicating 84% and 63% retention of the second discharge capacities, respectively. Benefitting from the unique structural features, the as-prepared hierarchical 3D cobalt oxide-based electrodes, especially CoO@C wall arrays, manifest excellent electrochemical performance with outstanding rate capability and good cycling stability for highly reversible lithium storage.(2) In this work, for the first time, through a facile water bath method, monoclinic Co2V2O7·nH2O monodisperse hexagonal nanoplatelets (MHNPs) have been synthesized successfully. The amount of hexamethylenetetramine (HMT) in the reaction was found to be pivotal to the formation of hexagonal nanoplatelets. Mesoporous Co2V2O7 MHNPs composed of numerous nanometer-sized subunits could be harvested though calcination of Co2V2O7·nH2O precursor in air. Benefiting from the unique structure and probably synergetic effects of different metal ions, the as-prepared Co2V2O7 MHNPs possess high specific capacity, long-term cycling stability, and good rate capability (a high reversible capacity of approximately 866 mAh g-1 after 150 cycles at a current density of 500 mAh g-1 and after rate test, the current density turns back to 2 A g-1, the reversible capacity can be retained at as high as 520 mAh g-1 after 580 cycles).(3) Most hollow structures of transition-metal oxides have been prepared by the template method. However, non-spherical metal oxide hollow hexagonal polyhedra have not been developed to date. In this work, a new phase of Co3V2O8·nH2O with morphology of hollow hexagonal prismatic pencils (HHPPs) has been synthesized by hydrothermal method without any hard template or organic surfactant. By varying the molar ratio of OH-/NH4- the hexagonal prismatic pencils gradually transform from solid into hollow structures, with sizes varying from 5 to 20μm. In our system, Co3V2O8·nH2O pencils could not overgrow and become hollow using the well-grown solid Co3V2O8·nH2O pencils as starting seeds even though an appropriate amount of NaOH was involved. Hence, the hollowing process is not due to the etching caused by the alkaline medium, in contrast to the previous reported results. As a consequence, NH4+can be reasonably concluded to be bound more strongly to{001} planes, resulting in its presence in the final product. The work has an important guiding role in the synthesis of other new structures. When used as a new anode material for LIBs, Co3V2O8·nH2O HHPPs display good electrochemical performance (after 255 cycles, a discharge capacity of 847 mAh g-1 at a current density of 500 mAh g-1) due to the unique structure of Co3V2O8·nH2O and likely the synergetic effects of the different. Especially, V-doped Co3O4 octahedron assembled hollow hexagonal prismatic pencils have been obtained by etching process in alkaline solution, which display excellent electrochemical performance.