Design, Synthesis And Electrochemical Properties Of Carbon/Sulfur Cathode For Lithium Sulfur Batteries

With the advantages of high theoretical specific capacity (1675 mAh g-1), natural abundance, low cost and environmental friendliness, sulfur, a promising cathode material, makes lithium-sulfur battery show great potential for one of the most promising rechargeable battery systems. However, the insulating nature of sulfur and the dissolution of polysulfides result in the low utilization of active material and poor cycle life, which limit the application of lithium sulfur batteries in commercial production. This research work focuses on the preparation of novel structured carbon material as the substrate for sulfur to improve the electrochemical performance of sulfur cathode. The main research contents and results are as follows:(1) Sandwich-type hierarchical porous carbon nanosheets (SHPC) are prepared through hydrothermal and KOH-assisted carbonization method using graphene oxide as the shape-directing agent. The porous carbon layer covering on the surface of graphene ensures the high sulfur loading in uniform distribution, while the inner graphene ensures the high electronic conductivity of the composite. The SHPC/S composite is fabricated via the melting-diffusion method. The composite exhibits a high initial discharge capacity of 1291.5 mAh g-1 and maintains over 650 mAh g-1 after 100 cycles at 0.5 C.(2) A self-assembly approach is used to fabricate a graphene-coated mesoporous carbon/sulfur composite (rGO@CMK-3/sulfur). The graphene coating on the surface of the composite is 15-20 nm in thickness, and sulfur is well dispersed in the pores of mesoporous carbon and the interfaces between the mesoporous carbon and graphene sheets after a heat treatment at 300℃. The graphene coating helps to increase the sulfur content and retard the diffusion of polysulfides. The synergistic effect of graphene coating and mesoporous structure can improve the electrochemical performance of the composite. The reversible specific capacities of rGO@CMK-3/sulfur composite electrode after 100 cycles are 650 mAh g-1 at 0.1 C and 550 mAh g-1 at 1 C, with the coulombic efficiency maintaining over 97%.