Research On The Cathode Host Of Lithium Sulfur Batteries Based On Modified Carbon Nanotubes

Recently,Lithium sulfur batteries,as one promising candidate in energy conversion and stortage system,have drawn much interest due to the high theoretical energy density.However,several issues make it difficult to apply.Firstly,the intrinsic insulating nature of the sulfur and the discharge product make slow electron transfer process.Then,the dissolution of the polysulfide intermediates in the organic electrolyte result in shuttle effect.Lastly,volume expansion in redox reaction lead to structure collapse of matrix material.Many efforts have been dedicated to addressing these problems.One effective approach is to select a suitable conductive matrix to load and disperse sulfur particles.At present,carbon nanomaterials have been most widely investigated.In this thesis,carbon nanotubes have been exploited as cathode materials,which was modified by serval methods to improve the elelctrochemical performance.The main contents are presented as follows:?1?The CNTs are chemically tailored into OSSCNTs?oxide super short carbon nanotubes?with 200-500 nm lengths by a simple ultrasonic oxidation-cut method.Then,the OSSCNTs was mixed with sulfur with melt diffusion processs.Finally,a three-dimensional multilayer Gr-OSSCNTs/S architecture is prepared by hydrothermal reduction route.Characterization results showed that the unique hybrid3D framework not only prevented the restacking of rGO to exhibited considerable electrical conductivity,but also displayed the double shell to alleviate the shuttle effect.The result donmonstrated that Gr-OSSCNTs/S cathode show significantly enhanced elelctrochemical performance.?2?A boron-doped unzipped carbon nanotubes/sulfur?BUCNTs/S-1?composite was first prepared via a one-pot facile hydrothermal process by using oxidized unzipped carbon nanotubes?O-UCNTs?,boron acid and element sulfur as the carbon precursor,B and S sources,respectively.This hydrothermal process simultaneously achieved successful B doping and highly homogeneous distribution of sulfur particles on the carbon matrix.The as-obtained BUCNTs/S-1 composite was characterized by XRD,SEM,Raman,XPS and so forth.The resulted showed that the BUCNTs/S-1composite exhibit enhanced conductivity,promoted sulfur dispersibility and strengthened adsorbability for the sulfur guest.Benefitting from all these advantages,the as-prepared BUCNTs/S-1 cathode possessed much better rate capability and cycling stability.?3?Nitrogen doped carbonnanotube?N-CNTs?was prepared by hydrothermal process.Then the engineered MWCNTs with thiol groups?SH-N-CNTs?were prepared by the diazonium chemistry method using the N-CNTs as precursor.The result of mporphology,structure and composite indicated that the SH-N-CNTs exhibited improved conductivity and good adsorption for S^2-due to the introduction of nitrogen atoms and thiol groups.When tested as cathode in lithium sulfur batteries,SH-N-CNTs composite delivers excellent cyclic performance.