Synthesis And Electrochemical Performance Of High Performance Porous Sulfur Cathode Material

Lithium-sulfur batteries are attractive in this regard due to their high energy density and the abundance of sulfur, but several hurdles such as poor cycle life and inferior sulfur utilization need to be overcome for them to be commercially viable. One of the most popular approaches to resolve these issues has been encapsulation of sulfur by carbon materials and conducting polymers, which can mitigate the shuttling effect. Based on the application of carbon materials, conducting polymers and metal oxides in lithium sulfur batteries, this thesis mainly focused on the control the structrues of the cathode materials. The main results and new findings in this work are summarized as follows:(1) Preparation of ordered mesoporous carbon/sulfur composite and wrapped with graphene for Li-S batteries:Mesoporous carbon/sulfur (CMK-3/S) composites, controllably coated with graphene sheets via the functionalization of the CMK-3 surface with the assistance of a bridging agent (L-lysine and PEG), are presented here. In this unique "double-carbon" structure, the abundant pore structure of the CMK-3 and graphene coating layers serve to preventing the dissolution of polysulfide intermediates from the cathode and improving the sulfur nanoparticles'electrical conductivity. Galvanostatic charge-discharge tests indicated that the inhibiting effect is limited of the CMK-3 for polysulfide, can't effectively prevent the shuttling effect. Graphene coating can improve the electrochemical performance of the composite, and with the improvement of coating effect, the better electrochemical performance can be obtained. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrated that the graphene could be coated uniformly and tightly on the surface of mesoporous carbon/sulfur (RGO@CMK-3/S) particles in the presence of L-lysine. Therefore, the RGO@CMK-3/S composite prepared using L-lysine exhibited high and stable specific capacities of up to 720 mA h g-1 over more than 100 cycles at 0.5 C, and an excellent rate capability of 660 mA h g-1 at a rate of 2 C.(2) Porous Polyaniline/Sulfur Cathode for Lithium-Sulfur Batteries:Mesoporous polyaniline conducting matrixs have been successfully prepared via SBA-15 hard template to house sulfur as a high performance cathode for lithium-sulfur batteries. In this composite, the M-PANI with good lithium ion conductivity can effectively prevent the shuttling effect of polysulfide by a strong chemical interaction and physical adsorption effect, thus markedly improving the electrochemical performance. Galvanostatic charge-discharge tests indicated that the M-PANI/S composite exhibited high and stable specific capacities of up to 517.3 mA h g-1 over 400 cycles with capacities of 928 mAh g-1 for the first cycle at 1 C, and an excellent rate capability of 600 mA h g-1 at a rate of 2 C.(3) Synthesis of TiO2/S composite and coated by graphene as Lithium-Sulfur batteries cathode material:Hollow TiO2 nanospheres have been prepared via CaCO3 hard template to encapsulated sulfur. Subsequently, the effect of sintering temperature on the electrochemical properties of the cathode materials was investigated. Galvanostatic charge-discharge tests indicated that the composite sintered at 500? exhibited high and stable specific capacities of up to 399.6 mA h g-1 over 100 cycles with capacities of 852.6 mAh g-1 for the first cycle at 1 C, and an excellent rate capability of 550 mA h g-1 at a rate of 2 C. Then, encapsulating sulfur nanoparticles in hollow TiO2 spheres sintered at 500? followed by graphene coating, thus could further improve the electrochemical performance of lithium sulfur batteries. The G-TiO2@S electrode exhibits a high discharge capacity of 872.8 mAh g-1 at 1 C and retains a capacity of up to 555.8 mAh g-1 over 200 cycles.