Preparation Of Molybdenum-based Compounds/nitrogen-doped Carbon Nanotube Composites And Their Application In Lithium-sulfur Batteries

Lithium-sulfur battery is currently one of the most promising secondary battery systems due to its high energy density,low raw material cost,and environmental friendliness.However,the commercialization of lithium-sulfur batteries still faces challenges.The current research on lithium-sulfur batteries mainly focuses on developing high-performance sulfur hosts to suppress the shuttle effect of lithium polysulfides and improve the electrochemical performance of lithium-sulfur batteries.Nanostructured molybdenum-based compounds combined with nitrogen-doped carbon nanotubes can be used as sulfur carriers for lithium-sulfur batteries to increase the redox reaction rate and suppress the shuttle effect of polysulfides.The hollow porous structure of nitrogen-doped carbon nanotubes can promote electron/ion transport,increase adsorption and catalytic activity sites for lithium polysulfides,alleviate the volume expansion effect of sulfur,and prevent the collapse of the cathode structure.Combining nitrogen-doped carbon nanotubes with polar molybdenum-based compound nanostructures as sulfur host is expected to significantly improve the performance of lithium-sulfur batteries.Based on the above analysis,we conduct the following research work in this thesis:1)A Mo₂C/NC nanotube composite was prepared using polypyrrole coated MoO₃nanorods as a template through heat-treatment carbonization/partial reduction,ammonia washing-etching,and high-temperature carbonization.When served as a sulfur carrier for lithium-sulfur batteries,its hollow tube-like nanostructure can provide buffer space for the volume expansion of sulfur during discharge.The polar Mo₂C nanoparticles encapsulated inside have strong adsorption capability and electrocatalytic activity for polysulfides.In addition,nitrogen-doped carbon nanotubes can shorten the transport path of lithium ions and fix the polysulfides through strong adsorption.Through DFT theoretical calculations,adsorption experiments,and XPS analysis,it was verified that Mo₂C/NC nanotubes can anchor lithium polysulfide through strong adsorption with lithium polysulfide,effectively alleviating the shuttle effect of lithium polysulfide.The symmetrical battery test results also confirmed that Mo₂C nanoparticles can promote the adsorption and catalytic conversion of lithium polysulfides.The novel nanotube encapsulated polar Mo₂C nanoparticle composite and the synthesis method developed in this study provide a new approach for the preparation of high-performance cathode materials for lithium-sulfur batteries;2)In order to enhance the adsorption capacity of carbon-based sulfur carrier materials towards the polysulfides,in this chapter,we still uses polypyrrole coated MoO₃ nanorods as templates,and prepares MoP nanoparticles encapsulated in the N-doped carbon nanotubes by heat-treatment carbonization/local reduction,ammonia washing-etching,and high-temperature phosphatization.When the MoP nanoparticles wrapped in N-doped carbon nanotubes served as sulfur carriers for lithium-sulfur batteries,they can slow down the volume expansion of sulfur,increase the contact area of the electrolyte,and thus promote the migration of lithium ions.In the cathode of lithium-sulfur batteries,MoP nanoparticles serve as electrocatalysts,which can significantly increase the redox reaction rate of polysulfides during the charge-discharge process,further improving the rate performance and cycling stability of lithium-sulfur batteries.Through the DFT theoretical calculations,adsorption experiments,and XPS analysis,it was verified that the MoP/S/NC nanotubes can anchor lithium polysulfides through their strong adsorption.3)In order to improve the electrocatalytic reaction rate of sulfur,in this chapter,MoS₂ nanosheets modified with nitrogen-doped carbon nanotubes with MoS₂nanoparticles encapsulated inside were prepared by heat-treatment carbonization/partial reduction,ammonia washing-etching,high-temperature sulfurization,and solvent thermal in-situ growth.The hollow structure and high specific surface area of nitrogen-doped carbon nanotubes are beneficial for increasing sulfur loading,increasing the contact area between electrodes and electrolytes,accelerating electron transport,and shortening the transport pathway of lithium ions.The MoS₂nanosheets on the outer layer of nitrogen-doped carbon nanotubes and the MoS₂nanoparticles wrapped inside can anchor the polysulfides through strong adsorption,slow down the shuttle effect,and enhance the electrochemical performance of lithium-sulfur batteries.The symmetrical battery test results also confirmed that the MoS₂nanostructures can promote the adsorption and catalytic conversion of lithium polysulfide,thereby accelerating the electrochemical reaction rate.Therefore,MoS₂/NC/MoS₂/S nanotube cathode exhibit excellent electrochemical performance,maintaining a reversible capacity of 498.9 m Ah g^-1 after 500 cycles at 1.0 A g^-1,and an average per-cycle capacity decay rate of approximately 0.061%.The design and synthesis of the MoS₂/NC/MoS₂/S nanotube electrode material is of great significance for the development of high-performance lithium-sulfur batteries;4)In order to further enhance the conductivity and reaction kinetics of the cathode of lithium-sulfur batteries,MoSe₂ nanosheets uniformly attached to the outer surface and inner wall of nitrogen-doped carbon nanotubes were prepared through heat-treatment carbonization,ammonia washing-etching,and solvent thermal in-situ growth.The hollow N-doped carbon nanotube structure,as the substrate for in-situ growth of MoSe₂ nanosheets,can not only inhibit the aggregation of MoSe₂ nanosheets,but also provide sufficient space for sulfur loading and effectively alleviate the volume expansion effect generated during sulfur lithiation.Nitrogen-doped carbon nanotubes have high conductivity and large specific surface area,which can accelerate electron transfer and shorten the migration pathways if lithium ion.Therefore,MoSe₂/NC/S nanotube cathode exhibit excellent electrochemical performance,maintaining a reversible capacity of 501.7 m Ah g^-1 after 500 cycles at 1.0 A g^-1,and an average per-cycle capacity decay rate of approximately 0.052%.The adsorption effect of MoSe₂/NC nanotubes rich in selenium vacancies on lithium polysulfides was investigated through the DFT theoretical calculations,adsorption experiments,and XPS analysis.