Preparation Of Novel Metal Oxide/metal Sulfide And Their Energy Storage Properties

With the heavy consumption of traditional energy sources like petroleum,the non-renewable energy will eventually be depleted.As a result,over-utilization of fuels has also brought about increasingly serious environmental pollution.Lithium-ion secondary batteries are considered as the most promising energy storage systems because of their long cycle life,high energy density,high output voltage,fast charge-discharge process and memory-less effects and are widely used in notebooks,digital cameras and mobile phones and other portable electronic products.They are important power sources of hybrid electric vehicles,electric vehicles and gradually to the field of high-power system equipment development.Currently,the most widely used lithium-ion battery cathode materials are carbon materials,but its low theoretical capacity and safety problem limit its further application.Transition metal oxides have received extensive attention in view of their high theoretical specific capacity.However,their low electron conductivity and huge volume expansion-contraction resulting in a large irreversible capacity and poor cycle stability.The electrochemical performance of transition metal oxides can be improved by synthesis transition metal oxides with different morphologies and sizes,then,decorated with other nano-sized materials.In this paper,with transition metal oxide as the main research object,Cu₂O nanocubes,MoS₂/Cu₂O nanocomposites,Co₃O₄ nanosheets and MoS₂/Co₃O₄nanocomposites were prepared.X-ray powder diffraction?XRD?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?and other characterization methods were used to study the morphology,size and crystal structure of the samples.The electrochemical performances of materials were explored by Electrochemical Impedance Spectroscopy?EIS?,Cyclic Voltammetry?CV?,galvanostatic and high rate charge/discharge test.The main work includes the following two parts:1.According to the literature,the ultra-thin MoS₂ nanosheets were prepared by simple liquid-phase exfoliation and ultrasonic exfoliation.Cu₂O cubes with uniform size and smaller volume were prepared by aqueous solvent method.The two materials were mixed to obtain MoS₂/Cu₂O nanocomposite.The results of XRD and TEM show that the resultant Cu₂O cubes have a uniform size,but easily aggregate.The prepared MoS₂/Cu₂O nanocomposite effectively improved the agglomeration of the Cu₂O cube,the Cu₂O cube uniformly loading on the surface of thin MoS₂ nanosheet.The electrochemical tests results showed that the MoS₂/Cu₂O composites exhibited much better electrochemical performance than the Cu₂O cubes.At a current density of 200mA g^-1,the MoS₂/Cu₂O composites exhibited a high initial discharge specific capacity of 497.52 mAh g^-1 and can maintain a capacity above 519.2 mAh g^-1 after 50 cycles,after 180 cycles,the capacity of the MoS₂/Cu₂O composites still above 524.9 mAh g^-1,the corresponding CE maintained at more than 98%.Therefore,it is concluded that the synthesized MoS₂/Cu₂O nanocomposites can not only improve the electrochemical activity of MoS₂ and Cu₂O but also have a synergistic effect between the two active materials thus alleviating the volume and structure change of the electrode material and providing the lithium ion larger storage space.2.MoS₂ powder was dispersed in N-methylpyrrolidone?NMP?to obtained ultra-thin MoS₂ nanosheets,then dispersed MoS₂ nanosheet in a solution of water and ethanol by ultrasonication.Cobalt chloride hexahydrate?CoCl₂.6H₂O?and hexamethylenetetramine?HMT?were added into the MoS₂ dispersion and reacted at90?.The obtained product was calcined in a muffle furnace to obtain a MoS₂/Co₃O₄composite.The SEM and TEM characterization results show that the resultant Co₃O₄hexagonal flakes have uniform size and regular morphology,after compounded with MoS₂ nanosheets,the surface of Co₃O₄ become rough.The results of electrochemical tests show that the MoS₂/Co₃O₄ composites exhibited much better electrochemical properties than Co₃O₄ flakes.The initial discharge specific capacity of MoS₂/Co₃O₄composites was 1130.7 mAh g^-1 with a constant current density of 200 mA g^-1,the specific capacity of the MoS₂/Co₃O₄ composites also remained 1013.3 mAh g^-1 after55 cycles.When the current density increased step by step from 100 to 200,400 and800 mA g^-1,the specific capacities were 1301.6,1099.7,960.5 and 763.5 mAh g^-1,respectively.The above results verified that Co₃O₄ hexagonal flakes grew on MoS₂flexible substrate formed the composite structure can provide more active sites for Li⁺,effectively buffer the volume change and protect the structure of the electrode material during charging and discharging process and improve the cycle stability of the material.