Research On Preparation Of Sulfur Based Composite Nanofibers For Cathode In Lithium-sulfur Batteries

In the new generation of energy storage and conversion devices, lithium-ion batteries have been widely used in our life. But for its limited specific capacity of below300 Wh kg-1, it is urgent to develop new battery with high capacity. Lithium sulfur batteries have high theoretical energy density（2600 Wh kg-1） and theoretical specific capacity（1675 m Ah g-1）. In addition, sulfur is abundant in nature, low cost, non-toxic and environmental friendly. So lithium sulfur batteries are of great potential in high-performance secondary battery. In order to improve the cycling stability and discharge capacity of cathodes, researchers adopted carbon materials, conductive polymers, nano metal oxides and so on to form composite materials with sulfur in the study of cathode materials, and the sulfur/carbon composite materials are most widely researched.In this thesis we prepared hollow silica nanospheres through micro emulsion method, and then obtained silica/polyacrylonitrile composite nanofibers through electrospinning technique. After carbonization at high temperature and sulfur loading,we got sulfur/silica/carbon composite nanofibers（S/SiO₂/CNFs）, in which the hollow SiO₂ nanospheres were used for storing sulfur and adsorbing sulfides, and the conductive carbon nanofibers were used for conductive medium. Cyclic voltammetry,AC impedance spectroscopy and constant current charge and discharge were used to test the electrochemical properties of the composite fibers. And the results showed that the addition of hollow SiO₂ nanospheres could improve the cycling stability and discharge capacity of the cathode.Subsequently, we studied the preparation and electrochemical properties of porous sulfur-based composite nanofibers, and the formation of porous structures could increase the specific surface area and inhibit volume expansion of the cathode materials.First, the activation of SiO₂/CNF composite nanofibers by KOH were carried out, and we used 1 M KOH solution to treat the SiO₂/CNFs composites. The prepared cathode delivered an initial discharge specific capacity of 1010 m Ah g-1 at 0.1 C, and after 50 cycles the specific capacity was 374 m Ah g-1. Contrasted with the S/SiO₂/CNF composites without KOH activation, its specific capacity decreased. The reason is that KOH or its decomposed components led to the destruction of the SiO₂/CNF structure,thus weakening the performance improvement by hollow SiO₂ nanospheres.Next, we investigated the effects of Zn（CH₃COO）₂ activation on S/CNF composite nanofibers. Different concentrations of Zn（CH₃COO）₂ solution we were adopted to activate CNFs. The S/CNF composites treated by 0.125 M Zn（CH₃COO）₂ solution showed best cycle stability in this work, and its initial discharge specific capacity was980 m Ah g-1, and after 50 cycles the specific capacity still had 689 m Ah g-1. As the concentration of Zn（CH₃COO）₂ decreased to a low level, the specific capacity decreased.Finally, we used 0.125 M Zn（CH₃COO）₂ to treat SiO₂/CNF composite nanofibers,and after carbonization and sulfur loading, porous S/SiO₂/CNF composite nanofibers were gotten. Compared with the S/PCNFs without SiO₂, the material showed a higher discharge specific capacity at low cycling rate and a lower discharge specific capacity at high cycling rate. The reason lied in: on one hand, the addition of hollow SiO₂ nanospheres could enlarge the surface area and absorption of polysulfides; on the other hand, the nonconductive SiO₂ spheres made the polarization voltage increase. At low rate the polarization voltage was not obvious, so the absorption effects standed out,while at high rate the polarization effects stand out, resulting in the decrease of discharge specific capacity of the cathode material.