The Study On The Synthesis And Characterization Of MoO₂ And MoS₂ Nanomaterials And Their Application For Energy Storage

The haze,green house caused by environmental pollution and more and more serious energy shortage are the problems that should be solved by the 21^stcentury.Considering the influence of energy crisis on countries worldwide,new materials and devices that have potential application in energy storage has attracted more and more attention from the scientists and researches worldwide.Among these materials and devices,electrochemical energy storage is considered to be one of the effective solutions that are capable of providing a clean energy and support.As one of the electrochemical energy storage devices,rechargeable lithium-ion battery is currently attracting more and more attentions from researchers.It can provide a relatively clean environment clean storage for portable devices and large-scale power grids.Therefore,the lithium-ion battery is considered to be an effective solution for energy storage.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.To solve these problems mentioned above,a new kind of economical low-dimensional layered structure has attracted many researchers'attentions.The transition metal molybdenum oxide,molybdenum sulfide could provide effective interstitial path for lithium atoms because of their unique layer structure,which could improve the lithium storage properties of the materials.However,the conventional molybdenum oxide and molybdenum sulfide have poor electrical conductivity,which decreases the diffusion speed of lithium ions,resulting in poor rate characteristics and cycle stability.According to the previous literatures,the carbon material?e.g.,amorphous carbon,graphene,carbon fiber,etc.?can significantly improve the conductivity of electrode material.In addition,these materials could be mixed with molybdenum oxide and sulfide complex,which can improve the electrochemical properties of the material.Therefore,this thesis studied the preparation of the molybdenum oxide,sulfide,and carbon composites of different material and did some research on the microstructure control,electrode design and lithium storage of the materials.The main contents of this paper are as follows:1.This charpter reports an experimental study on the synthesis of MoO₂nanoparticles dispersed on the graphene oxide for the first time through a facile and green hydrothermal method and thus their enhanced electrochemical performance for the application of lithium-ion battery anodes.Our experiment demonstrates that the optimum designed MoO₂-RGO composites can result in an enhanced specific capacity and cycling performance compared to commercical MoO₂.The MoO₂ nanoparticles are 5-15 nm in diameter.The electrode exhibits 817.6 mA h g^-1 at 100 mA g^-1 after 30th cycles,which is much higher that that of pure MoO₂ nanoparticles.It is anticipated that such improved lithium-ion storage performance would lead to a new advancement to the development of anode materials.2.Flower-like MoS₂ nanostructure composites with carbon coating were synthesized through a facile hydrothermal method and thus their enhanced electrochemical performance for the application of lithium-ion battery anodes.The expanded layer distance of the MoS₂ nanosheets up to 0.63 nm can facilitate a sufficient room for lithium-ions diffusion.Our experiment demonstrates that the optimum designed MoS₂nanostructure with appropriate carbon content can result in a high reversible specific capacity of 1419 m A h g^-1 at 0.1 A g^-1,after 50th cycles,it can retain 80%of the initial capacity.And the C@MoS₂?2:1?obtain 672 mA h g^-1with almost 100%Coulombic efficiency at 10 A g^-1.It is anticipated that such improved lithium-ion storage performance would lead to a new advancement to develop a high-performance anode material.3.A simple electrospinning method with a subsequent calcination has been demonstrated to fabricate corn-like MoS₂ nanocrystals successfully embedded into the mesoporous carbon nanofibers.The resultant nanofibers were 100-150 nm in diameter and constructed from MoS₂ nanocrystals of lateral diameter around 7 nm with specific surface areas of 135.9 m² g^-1.The thin amorphous carbon overcoat is the backbone of the as-synthesized nanofibers that not only guarantee the structure integrity but also afford a conductive framework thus then a contribution to the Li-ion storage capability.The better crystallinity and mesoporous microstructure of MoS₂@C nanofibers lead to an increased surface area,benefitting for the sufficient contact areas between electrodes and electrolyte,as well as facilitating diffusion rate within the electrode.Considering all these advantages,the mesoporous MoS₂@C nanofibers can deliver a remarkable reversible capacity of 1022 m A h g^-1with only 2.3%capacity loss at a current density of 100 mA g^-1after 165 cycles and retain an improved long cycling stability at a high current rate(1 A g^-1)when assembled as an anode material for Li-ion battery.According to these results,the extraordinary Li-ion storage capability of mesoporous MoS₂@C nanofibers can be attributed to the synergetic effect between ultrafine MoS₂ nanocrystals and carbon nanofibers with an increased surface area,better crystallinity and porosity features.4.MoS₂ nanocrystals embedded in mesoporous carbon nanofibers with graphene coating were synthesized by electrospinning followed with calcination.The graphene coating can lead to a high specific aurface area,good mechanical stability and improved conductivity,which ensure the fast diffusion rate of lithium-ions.The results showed that the nanofibers delivered an extraordinary capacity of 770 mA h g^-1 and improved cyclic stability with almost 100%coloumbic efficiency after 50 cycles at a current density of500 mA g^-1,as well as outstanding rate capability.The greatly improved kinetics and cycling stability of the MoS₂-C-G nanofibers could be attributed to the cross-linked conductive graphene-carbon nanofibers,the excellent mechanical stability of graphene,and better crystallinity of MoS₂.The increased capacity which is almost two times than the theoretical capacity can be attributed to the uncomplete reduction of the graphene.The uncompleted graphene can generate defects on the suface of the nanofibers,providing more open active rooms for lithium-ions,which may make a contribution to the increased capacity.It is believed that the integration of MoS₂ nanocrystals,carbon nanofibers and graphene coating may have a synergistic effect,which is a promising anode widening the applicability range into high performance and mass production for Li-ion battery market.