The Study On The Synthesis Of Three-dimensional Molydeum And Tungsten Oxide Nanomaterials And Their Application For Lithium Ion Storage

The rechargeable lithium-ion battery as a clean energy and support is considered to be one of the effective solutions to solve the haze,green house caused by environmental pollution and more and more serious energy shortage.Currently,most lithium-ion battery electrode materials have the disadvantages of poor stability,weak safety,under high magnification capacity fading fast and so on,which limit the development and application of the next-generation lithium-ion battery.As a result,it is highly urgent to develop high performance anode materials.As ones of the most promising alternative anode,molybdenum oxide and tungsten oxide have attracted much attention owing to its high capacity and structural stability.However,the conventional molybdenum oxide and tungsten suffer from poor rate performance and cycling stability causing by the change of volume and decrease of conductivity during the charge-discharge process.According to the literature,it is reported that the three-dimensional（3D）structure can effectively improve the structural stability during the cycling,which will lead to an improvement of the electrochemical performance.Therefore,this thesis focuses on the design and synthesis of 3D structure of the molybdenum oxide and tungsten oxide and has done some research on the microstructure control,electrode design and lithium storage of the materials.Using the reduced graphene oxide（rGO）as the 3D skeleton,a disordered 3D framework composited of rGO and MoO₂ has been constructed by a simple two-step hydrothermal method.The 3D structure can be controlled by the change of ratio between rGO and MoO₂ nanoparticles.When the proportion is appropriate,the rGO and MoO₂ nanoparticles in the 3D structure can play a supporting role to each other,so that the electronic conductivity and the ion diffusion can be improved.When the current density is 0.1 mA cm^-2,the discharge capacity of the 3D structure is as high as1269 mAh g^-1,while that of pure MoO₂ nanoparticles is only 256 mAh g^-1.A disordered self-supported 3D framework,which is consisted of WO₃nanowires,has been constructed on the heterogeneous Ti substrate by hydrothermal method.It realizes the rapid electronic conduction between framework active material and the substrate.At the same time,the mutually-supporting system can prevent the nanowires from the damage of structure during the charge-discharge process and the cycling stability is improved.The initial discharge capacity of this disordered self-supported 3D framework is 628 mAh g^-1,it retains to 453 mAh g^-1 after 60 cycles.Furthermore,Mo-doped can further increase the electronic conductivity of the WO₃3D framework,while the polarization will be reduced and the rate capacity will increase.When the current density is 0.2,0.5,1 and 2 C,the discharge capacity is 958,788,684 and 340 mAh g^-1,respectively.After the current density back to 0.2 C,the capacity also recover the 84.13%of the initial one.The pre-lithiation can increase the lithium ion diffusion effectively.The initial discharge capacity of the pre-lithiation anode can be improved to 785 mAh g^-1,it retains to 536 mAh g^-1 after 150 cycles.Through the anodic oxidation method,the homogeneous substrate W foil has been oxided to construct the ordered 3D self-supported electrode material,which is consisted of the interconnected WO₃nanosheets.This self-supported framework is hard to drop from the current collector.Consequently,the cycling stability is ensured.Moreover,loading Ag nanopaticles on the nanosheets can further improve the electronic conductivity and induce to form uniform and thin SEI film,which will lead to the little change of electronic conductivity before and after cycling and reduce the polarization effecively.With the optimized Ag NPs,the initial capacity and that after150 cycles can be improved from the 685 mAh g^-1 and 359 mAh g^-1 for the pure WO₃to 917 mAh g^-1 and 681 mAh g^-1,respectively.