Synthesis Of Cathode Material For Lithium Sulfur Batteries And Its Electrochemical Properties

As a new type of cathode materials for lithium-ion battery, sulfur has advantages of high theoretical specific capacity, low cost, natural abundance and environmental friendliness. So lithium-sulfur batteries are promising. While pure sulfur is insulating and polysulfide (Li2Sx, x=4,6,8) are soluble in the organic electrolyte during the charge and discharge process and results in low utilization of sulfur and poor cycle performance, which limit the application of lithium sulfur batteries in commercial production. Hollow and porous carbon nanofibers (HPCNFs) were prepared by coaxial electrospinning with polyacrylonitrile (PAN) and polymethylmethacrylate (PMMA) as starting materials after carbonization, and we study the effect of structure at different carbonization temperature. Li2S was synthesised using LiH and S as the precursor by ball-milling. Then PVDF was used as the carbon source to cover the Li2S to improve its electrochemical performance. A series of modern material analyzing and testing methodsd including X-ray diffraction, field emission scanning electron microscope, transmission electron microscopyand, thermo gravimetry, specific surface area test, Raman test, etc. and electrochemical testing of galvanostatic charge-discharge, cyclic voltammograms, electrochemical impedance to investigate the fabrication technique and its parameters on the structure and electrochemical properties.The results showed that hollow and porous carbon nanofibers (HPCNFs) were prepared after carbonization with8wt.%PAN solution and16wt.%PMMA solution used as outer and inner solution, respectively. And then we synthesize the cathode by mixing carbon nanofibers and sulfur under155℃. The HPCNFs provide not only the mesoporous carbon for sulfur loading, which could prevent the dissolution of active sulfur. But also conductive carbon fiber webs, which could improve lithium ion transfer to the cathode for improved performance during electrochemical test.As the carbonization temperature increased, carbon nanofibers had higher degree of graphitization and better conductivity. While its Specific surface area declined, result in the decrease of adsorption of polysulfide. So the batteries showed high initial capacity, while the capacity fades gradually in the subsequent cycles. And the experiment result also showed the battery could get relative high capacity and cycling stability with60wt.%sulfur. Li2S was synthesized by ball-milling LiH and S. It showed better electrochemistry after coating Li2S with carbon by carbonizing PVDF.