Construction Of Transition Metal Oxide Nanostructures And Their Lithium Storage Properties

One of the most important challenges academia and industries are facing today is to provide high efficient, low cost and environmental friendly energy storage devices to address the problems of environmental pollution, the impending exhaustion of fossil fuels and to support the sustainable usage of green and clean energy sources likenuclear power,solar, etc. Since the practical applications of LIBs highly rely on the performance of electrodes, how to design and achieve high-efficient, low-cost and safe electrode materials turns into a great challenge. Transition metal oxides are promising electrode materials because of their high specific capacity, typically2-3times higher than that of carbon/graphite based materials. However, their cycling stability and rate performance still can not meet the requirements of practical applications due to their poor electronic conductivity and large volume expansion during cycling. 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 rationally designed nanostructured electrode materials withexcellent electrochemical performance for the LIBs. The main results are as follows:(1) Hierarchical nanocomposites rationally designed in component and structure, are highly desirable for the development of lithium ion batteries, because they can take full advantages of different components and various structures to achieve the superior electro-chemical properties. Here, the branched nanocomposite with β-MnO2nanorods as the back-bone and porous α-Fe2O3nanorods as the branches are synthesized by a high-temperature annealing of FeOOH epitaxially grown on the β-MnO2nanorods. Since the β-MnO2nanorods grow along the four-fold axis, the as-produced branches of FeOOH and a-Fe2O3are aligned on their side in a nearly four-fold symmetry. This synthetic process for the branched nanorods built by β-Mn02/a-Fe2O3is characterized by XRD patterns, SEM, TEM and HRTEM images. The branched nanorods of β-MnO2/a-Fe2O3present an excellent lithium-storage performance. They exhibit a reversible specific capacity of1028mAh g-1at a current density of1000mA g-1up to200cycles, much higher than the building blocks alone. Even at4000mA g-1, the reversible capacity of the branched nanorods could be kept at881mAh g-1. The outstanding performances of the branched nanorods are attributed to the synergistic effect of different components and the hierarchical structure of the composite. The disclosure of the correlation between the electrochemical properties and the structure/component of the nanocomposites, would greatly benefit the rational design of the high-performance nanocomposites for lithium ion batteries in the future.(2) High-performance anode materials in lithium ion batteries greatly lie on the elaborate controls on their size, shape, structure and surface. However, it is difficult to assemble all the controls within one particle, due to difficulties in synthesis. Here, hierarchical carbon-coated a-Fe2O3nanotubes were prepared by a facile hydrothermal reaction between branched MnO2/Fe2O3nanorods and glucose. The resulting nanotubes realize all these controls in one particle in terms of nanoscale size, one-dimensional shape, hollow structure, hierarchical surface and carbon coating. Meanwhile, the thickness of the carbon layer could be easily controlled by the ratio between different reactants. Electrochemical measurements show that the core-shell nanotubes with a thinnest carbon layer give the best cycling and rate performances. They deliver a specific capacity of1173mAh g-1after100cycles at a current density of0.2A g-1,or1012mAh g-1after300cycles at1A g-1. Even after1000cycles at a current density of4A g-1, the specific capacity could be still kept at482mAh g-1. The excellent lithium-storage performances could be attributed to the well-designed controls in this nanocomposite and a thin carbon layer that increase the electron conductivity of the electrode and keep the carbon content lower simultaneously.(3) MnOOH@PPy coaxial nanorods were firstly prepared by the polymerization reaction of pyrrole in the present of MnOOH nanorods. Then, MnOOH@PPy core-shell nanorods tansformed to coaxial MnO@C-N nanorods after treated in Ar/H2at700℃o The Li/MnO@C-N electrode demonstrates a specific capacity of982mAh g"1after100cycles at500mA g-1, higher than that of Li/MnO electrode. Even after 900cycles at1000mA g-1,Li/MnO@C-N electrode can still display679mAh g-1. The excellent electrochemical properties are related to the rationally designed nanostructures, such as, one dimensional, carbon coating and N-doping.