Preparation And Electrochemical Properties Of Sulfur/Porous Carbon Composite As Cathode Materials

The lithium-sulfur (Li-S) battery is considered as a most promising next-generation energy storage system due to its high theoretical energy density, low cost and environmental benign. However, Li-S batteries still face several challenges, including the insulating nature of sulfur, the shuttling behavior of soluble lithium polysulfides, and the large volume change of sulfur during cycling. To overcome these problems, carbon materials with different structures have been used as the sulfur hosts to prepare various sulfur/carbon cathodes in this thesis. sulfur/carbon cathodes with good electrochemical performances were obtained by the structure design, process optimization, and material modification of cathode. It will provide an significant reference for the applied research of Li-S batteries. The details are as follow:(1) First, through the electrochemical performance research of three carbon materials with different specific surface areas and pore structures, It is found that the high specific surface area is beneficial for the distribution of sulfur to improve the sulfur utilization and the carbon material with more micropores can adsorb more polysulfides to improve the cycle performance. Second, compared with the conventional Al foil collector, the carbon paper collector with the interconnected structure not only increases the sulfur loading (3 mg cm-2), but also adsorbs the electrolyte containing polysulfides for secondary fixation of sulfur. Last, the electrochemical performance of sulfur cathode is appreciably affected by the content of electrolyte. Added too much electrolyte, a large amount of sulfur is dissolved in the electrolyte, which results in serious shuttle effect to decrease the capacity and coulomb efficiency of sulfur cathode. However, the electrolyte is too little to wet the cathode, leading to the low utilization of sulfur. Thus, S/BP coated on carbon paper shows a optimal electrochemical performance with 60 wt% sulfur content and 30 ul mg-1 electrolyte (based on sulfur content), which delivers a initial specific capacity of 1136.0 mA h g-1 and a reversible capacity of 775.3 mAh·g-1 after 100 cycles at 0.2 C.To further improve the electrochemical performance of S/BP cathode, a conductive polymer PEDOT-PSS layer was coated on the surface of S/BP by a facile solution mixing method, which enhance the conductivity of S/BP and impede the migration of polysulfides as physical barriers. Thus, the utilization of sulfur and cycle performance are improved. The PEDOT-PSS@S/BP cathode with 56 wt% sulfur content yields a high initial capacity of 1218.5 mA h g-1 and a reversible capacity of 861.6 mA h g-1 after 100 cycles at 0.2 C. The capacity retention and capacity decay are 82.7% and 0.173 per cycle, respectively (based on the specific capacity at second cycle).(2) Monodisperse microporous carbon spheres (A-MRF) with a high specific surface area and a narrow pore size distribution were synthesized by hydrothermal route. Sulfur exists as chainlike smaller sulfur molecules S2-4 in the ultrafine micropores of carbon spheres. During discharge process, S2-4 directly converts to L12S2 or Li2S, avoiding the formation of polysulfides for the stable cycle performance of S/A-MRF cathode. When the sulfur content is 51 wt%, the S/A-MRF cathode exhibits a high initial capacity of 847.1 mA h g-1 and a reversible capacity of 734.3 mA h g-1 after 100 cycles at 0.2 C. The capacity retention and capacity decay are 86.7% and 0.133% per cycle, respectively.(3) A hierarchical micro/mesoporous carbon fiber (HPCF) with a ultra-high surface area and a large pore volume was prepared via electrospinning. Sulfur exists as ringlike molecules S8 and chainlike smaller sulfur molecules S2-4 in HPCF. Due to the loose network structure of HPCF, the S/HPCF cathode increases the conductivity and alleviates the adverse effect of sulfur volumetric expansion. Moreover, the hierarchical micro/mesoporous structure of HPCF can promise the fast transport of lithium ions and effectively suppress the diffusion of polysulfides. When the sulfur content is 66 wt%, the S/HPCF cathode shows an excellent electrochemical performance. The S/HPCF cathode delivers a high initial capacity of 1070.6 mA hg-1 and a reversible capacity of 946.2 mA h g-1 after 100 cycles at 0.5 C. The capacity retention and capacity decay are 88.4% and 0.116% per cycle, respectively.(4) The nitrogen-riched porous carbon materials (NPC) were prepared by carbonizing the melamine formaldehyde resin. The nitrogen content is as high as 23.87 wt%, and 85 percent of the nitrogen exists as pyridinic and pyrrolic nitrogen, which can anchor the polysulfides by chemical adsorption. Thus, S/NPC cathode exhibits a long cycle life, which delivers a reversible capacity of 610.3 mAh g-1 after 400 cycles at 0.2 C. To further improve the utilization of sulfur, a reduced graphene oxide layer (RGO) was coated on the surface of NPC by the electrostatic force. The RGO layer not only enhances the conductivity, but also impedes the migration of polysulfides and alleviates the adverse effect of sulfur volumetric expansion. When the sulfur content is 70 wt%, the S/RGO@NPC cathode exhibits a high initial capacity of 880.8 mA h g-1 and a reversible capacity of 600.1 mA h g-1 after 400 cycles at 0.5 C. The capacity retention and capacity decay are 68.1% and 0.0874% per cycle, respectively.