Construction Of Transition Metal Oxide Micro/Nanostructures And Their Electrochemical Properties

One of the major challenges researchers are facing today is to provide highly efficient, low cost, and environmentally benign electrical energy storage devices to address the problems of climate, the impending exhaustion of fossil fuels and the need for efficient storage of energy produced by solar and wind power. Lithium ion batteries (LIBs) and supercapacitors are at the frontier of this research effort, as they play important roles in our daily life by powering portable consumer electronic devices and even electric vehicles. Transition metal oxide materials are promising electrode materials for lithium ion batteries and supercapacitors because of their high specific capacity/capacitance, typically2-3times higher than that of the carbon/graphite based materials. However, their cycling stability and rate performance still can not meet the requirements of practical applications. It is therefore urgent to improve their overall performance, which depends on the development of the advanced electrode architectures. Metal oxide materials have long been studied as potential electrode materials for LIBs and psedocapacitors due to the ease of large-scale fabrication and high electrochemical activity. However, metal oxide electrodes also have some drawbacks, such as poor electronic conductivity, easy aggregation during cycling, side reaction with the electrolyte, etc. It is reported that nanoscale materials can greatly increase the performance of the metal oxide electrodes. Therefore, numerous efforts are urgently required to design advanced electrode nanostructures with high performance in the application of electrochemical energy storage devices.In this paper, we aim at utlizing simple methods to design nanostructured electrode materials with high performance for the LIBs and psedocapacitors. The main results are as follows:(1) We use a novel hydrothermal route to synthesize α-Ni(OH)2, in which urea has been utilized not only to produce hydroxyl anions, but also to organize ultrathin nanowires/nanosheets into network-like hierarchical assemblage. The morphological evolution process of this organized product has been investigated by examining different reaction intermediates during the synthesis. The growth and thus final assemblage of a-Ni(OH)2can be finely tuned by selecting preparative parameters, such as the molar ratio of starting chemicals. Based on the toptactic transformation from α-Ni(OH)2, various mesoporous NiO hierarchical microspheres by ultrathin nanowires/nanosheets self-assembly have been prepared via thermal decomposition in air atmosphere. The electrochemical performances of the typical nickel oxide products are evaluated. It is demonstrated that tuning of the surface texture and the pore size of the NiO products is very significant in electrochemical capacitor. Mesoporous NiO network-like hierarchical microspheres exhibit excellent cyclic performance with a97%capacity retention at current density of10A/g in a testing range of2000cycles.(2) Porous NiO nanoflowers with uniform morphology and high surface area have been obtained by annealing precursor synthesized by a facile solvothermal method. The results show that the ratio of ethylene glycol and water has an important impact on the morphology of the precursor. After heat treatment, the as-prepared NiO nanoflowers are applied as the electrode material for supercapaciors, and the sample exhibits superior performance with a high specific capacitance of335F/g and91%capacity retention at current density of10A/g after1500cycles.(3) A facile method is designed for large-scale preparation of joint-like mesocrystalline MnO@carbon core-shell nanowires for the first time based on rational constructed functional systems. The nanostructures present the unique feature of the highly oriented-interconnected of MnO nanorods encapsulated inside and graphitized carbon layers coating outside. Through comparing and analyzing the MnOOH, Mn2O3and MnO@C crystal structures, the sequential topotactic transformation of the corresponding precursors to targets is proposed here. Li-ion battery testing is given to demonstrate that MnO@carbon core-shell nanowires show excellent capacity retention, superior cycling performance and high rate capability. Specifically, the MnO@carbon core-shell nanostructure could deliver reversible capacity as high as801mAh g-1at a high current density of500mA g-1with excellent electrochemical stability after200cycles testing. The remarkable electrochemical performance is mainly attributed to the highly uniform carbon layer around the MnO nanowires which can not only effectively buffer the structural strain and volume variations of anodes during the repeated electrochemical reactions, but also greatly enhance the conductivity of electrode materials.(4) A simple microemulsion based method has been developed to synthesize ZnCo2O4hierarchical superstructure stacked by nanoplates, which was transformed from corresponding precursor by annealing process. The nanoplates shown a thickness of10-20nm were connected together to form well assembled three-dimension structure. When applied as anode material of lithium ion battery, ZnCo2O4hierarchical superstructure demonstrated high reversible capacity and good rcyclability. The ZnCo2O4hierarchical superstructure could deliver reversible capacity as high as930mAh g-1at a current density of100mA g-1with excellent electrochemical stability after100cycles testing. The improved electrochemical performance of the as-synthesized ZnCo2O4nanoplates might benefit from its unique hierarchical superstructure, which provides a short path for Li+ion diffusion, and enough free space to buffer the large volume changed during cycling.(5) Urchin-like hierarchical NiCo2O4nanostructure has been synthesized by a simple hydrothermal method free of any template. As-synthesized NiCo2O4has a uniform diameter of5μm with numerous small nanorods radially grown from the center. Typical nanorods have a diameter of100-200nm. The NiCo2O4+was first investigated on the application of nonenzymatic glucose detection, demostrating a rather low detection limit, a fast responsible time less than1second and a wide detection range for the electrochemical detection of glucose. It is expected to have potential application in real sample analysis.