Preparation Of Novel Metal Oxide-based Nanocomposite Materials And Their Energy Storage Properties

With the depletion of traditional and non-renewable energy sources and the aggravation of environmental pollution,an intensified attention has been paid to clean,efficient and environmentally friendly energy conversion and storage equipment.In recent years,as a new generation energy supply device,rechargeable lithium ion batteries have been widely used in small portable devices such as mobile phones and computers,and gradually to the development of the electric vehicle/high power hybrid vehicle owing to their superior properties such as high energy density,long cycle life,low self-discharge rate and environmental benignity.Compared with traditional carbon materials,transition metal oxides have attracted more interests in view of its high theoretical capacity.Nevertheless,their huge volume expansion-contraction lead to a large irreversible capacity fading,poor conductivity and rate performance during numerous lithiation-delithiation process.Recently,the electrochemical properties of the transition metal oxides can be improved by building nanomaterials with different morphologies and sizes,or decorated with other materials.Herein,Co3O4 nanoparticles,MoS2/Co3O4 nanocomposites,NiCoO2 nanosheets and RGO/NiCoO2 nanocomposites were prepared.X-Ray Diffraction(XRD),Scanning Electron Microscopy(SEM),Transmission Electron Microscopy(TEM),nitrogen absorption apparatus and Raman were used to research the crystal structure,morphology,particle size and specific surface area of the samples.The electrochemical properties of the samples were investigated by Cyclic Voltammetry(CV),Electrochemical Impedance Spectroscopy(EIS),galvanostatic and high rate charge/discharge test.The main research work includes the following two parts:1.The ultrasmall Co3O4 NPs were synthesized according to previously reported process.Bulk MoS2 powder was dispersed in N-methyl pyrrolidone(NMP),unthin MoS2 nanosheets can be obtained by liquid-phase route.The MoS2/Co3O4 nanocomposites were successfully synthesized by the combination of Co3O4 NPs and MoS2 nanosheets.The results of XRD and TEM showed that the resultant Co3O4 NPs have a significant tendency to aggregate due to the absence of MoS2 nanosheets,and the average size of Co3O4 NPs is about 12.5 nm.The aggregation of Co3O4 NPs was effectively improved by synthesizing the MoS2/Co3O4 nanocomposites.The Co3O4 NPs are uniformly and densely distributed on the thin MoS2 nanosheets.Both of the MoS2/Co3O4 nanocomposites and the pristine Co3O4 NPs displayed high the specific surface area of～104.3 m2 g-1 and 88.6 m2 g-1 Electrochemical test results demonstrated that the MoS2/Co3O4 nanocomposites electrode exhibits much better electrochemical performance compared with the pristine Co3O4 NPs.At a constant current density of 200 mA g-1,the nanocomposites electrode showed a high initial discharge capacity of 1926 mAh g-1 and can sustain a high capacity above 1100 mAh g-1 after 50 cycles,and still above 920 mAh g-1 after 100 cycles.Moreover,at a high current density of 500 mA g-1,the MoS2/Co3O4 anode can maintain a reversible capacity of 947 mAh g-1 after 50 cycle,and the corresponding CE almost remains above 98%.These results illustrated that the designed structure of the nanocomposites can maximally utilized electrochemical active MoS2 nanosheets and Co3O4 NPs for lithium ion batteries.Moreover,this composite structure may provide more extra spaces for the storage of lithium ions and help to accommodate the volume change through a synergistic interaction between the two active materials.2.We reported the synthesis of the PS-NiCo precursor by a facile solution-phase process.The NiCoO2 nanosheets were obtained after annealing at 350℃ for 2 h in air.The preparation of Graphene oxide(GO)was adopting modified Hummers method.The GO was partially reduced to reduced graphene oxide(RGO)after annealing in N2.The RGO/NiCoO2 nanocomposites were synthesized by a facile solution method.The SEM and TEM measurements verified that the resultant ultrathin NiCoO2 nanosheets with regular morphology and uniform size due to the existence of the polystyrene microsphere.The obtained RGO/NiCoO2 nanocomposite was composed of NiCoO2 nanosheets anchored on RGO nanosheets.Electrochemical test results demonstrated that the RGO/NiCoO2 nanocomposite electrode exhibits excellent electrochemical performance.The nanocomposite electrode showed a high initial discharge capacity of 1646 mAh g-1 and can maintain a high capacity above 1200.0 mAh g-1 after 50 cycles at a constant current density of 200 mA g-1.With the increase of the current densities step by step from 100 to 200,500,and 1000 mA g-1,the capacities of RGO/NiCoO2 nanocomposite are 1189,1120,1105 and 828 mAh g-1,respectively,indicating the admirable rate performance.The superior electrochemical performance of RGO/NiCoO2 nanocomposite demonstrated that the electrode structure of materials can not be seriously damaged due to RGO can act as an elastic substrate to buffer the large volume change,thus ensuring the electrochemical stability.