Preparation Of Iron And Cobalt Oxide Nanocomposite And Their Application As Anode Materials For (Flexible) Lithium Ion Batteries

Lithium ion batteries are rare and green secondary power source due to the following advantages, such as high energy density, small volume, light weight, long cycle life, no memory effect and environmental friendliness. Electrode materials (anode material and cathode material) play an important role in the lithium-ion battery. Transition metal oxide as the lithium-ion battery anode material has been extensively studied. Compared with the general graphite anode materials, these transition metal oxides posses many advantages, including high capacity, wide application, and good stability. Iron oxide and cobalt oxide possesses high capacity, low cost, eco-friendliness, and natural abundance and thus has attracted considerable attention. However, there is still a problem of rapid capacity fading because of poor conductivity and pulverization of iron and cobalt oxide during cycling which leads to the breakdown of electrical connection of anode materials from current collectors. Thus, preparing nanostructures with different morphology, designing unique configurations, controlling pore structures to improve their electrochemistry performance are hot issues in research of LIBs.Flexible electronics is an emerging and promising technology for the next generation optoelectronic devices which is receiving intensive research endeavor, aiming at powering a variety of novel class of electronics such as rollup displays, smart electronics and wearable devices. These devices demand high energy and power densities which make flexible lithium-ion batteries (LIBs) suitable for utilization. Flexible LIBs electrodes are usually made from various functional organic and/or inorganic materials built on flexible conductive membrane substrates without conductive additives and binders. In order to meet the flexible requirements of devices, various flexible electrode materials for LIBs have been prepared from carbonaceous and other membrane materials. For example, electrode materials based on CNTs, graphene, carbon cloth, conductive paper (cellulose), textiles, low-dimensional nanostructured materials and their composites have been reported. Carbon cloth, as a new kind of substrate, possess some advantageous properties over other materials, including low cost, high conductivity, high strength, excellent structural stability and good corrosion resistance, which make it commercially available and can be used as a lightweight, flexible current collector without any ancillary binders and conductive additives.1. Carbon-coated Fe3O4 quantum dots/graphene composite was prepared by a facile ultrasound-hydrothermal method. SEM and TEM characterization confirms that carbon-coated Fe3O4 nanoparticles with a size of around 7 nm are distributed uniformly on the surface of graphene nanosheets. As a result, the carbon shell can preserve the structure stabilization of Fe3O4 nanoparticles by avoiding the aggregation and buffering the volume expansion of Fe3O4 nanoparticles. In addition, the graphene nanosheets can form a three-dimensional network for the transportation of Li+ ions and electrons. This unique architecture can maintain the structural integrity of the electrode during the lithiation/delithiation processes. When evaluated as anode material for lithium-ion batteries, it shows improved electrochemical performance with high cycling stability and good rate capacity. This composite exhibits an initial discharge capacity of 1300 mAh g-1 and remains a reversible capacity of about 940 mAh g"’after 100 cycles at a current density of 150 mA g-1.2. Hierarchal three-dimensional (3D) CoO nanowire arrays are directly grown on flexible carbon cloth substrate via a hydrothermal method, which can be used as a binder-free flexible anode for lithium ion battery (LIBs). These porous CoO nanowire arrays give rise to large surface area and space for efficient buffering of the volume change during electrochemical lithiation. The integrated hierarchal electrodes possess many advantages by directly building 3D nanostructures on conductive substrate, such as short diffusion length, easy strain relaxation and fast electron transport. Thus, the CoO nanowire arrays show an improved lithium-storage performance with high cycling stability and good rate capacity. It exhibits an initial discharge capacity of 1730 mAh g-1 and retains a reversible gravimetric capacity of about 1300 mAh g-1 after 90 cycles at a current density of 100 mA g-1. Moreover, the integrated CoO nanowires/carbon cloth electrode shows high flexibility and high areal capacity (1.95 mAh cm-2), which can be directly used as the anode to build flexible LIBs.3. A three-dimensional CoFe2O4 nanowire array on carbon cloth was fabricated with a hydrothermal method together with a post-annealing treatment. As a binder-free and flexible anode material for LIBs, it showed an improved electrochemical performance with high cycling stability and excellent rate capability. It exhibited an initial discharge capacity of 1615 mAh g-1 and retained a reversible capacity of about 1204 mAh g-1 after 200 cycles at a specific current of 500 mA g-1. The high capacity, outstanding rate performance and cycling stability can be attributed to the special configuration of hierarchal porous CoFe2O4 nanowires on carbon cloth, which possess many advantages like short diffusion length, easy strain relaxation and fast electron transport. Moreover, the integrated CoFe2O4 nanowires/carbon cloth electrode shows high flexibility and high areal capacity (2.408 mAh cm-2), which makes it suitable for use as a binder-free anode to build flexible LIBs.