Research On Hybrid Nanostructure Design And N Doping Modification In The Carbon Matrix Of Carbon-sulfur Materials For Lithium-sulfur Batteries

Abstract:Recently, lithium-sulfur batteries as the promising of next-generation high-energy rechargeable batteries have drawn more and more attention in the world. Among the cathode materials for lithium-sulfur batteries, carbon-sulfur materials are the most attractive, in which great achievement has been made. While, before full utilization of carbon-sulfur materials as the partical cathode materials, there are still several issues needed to be addressed, such as the high solubility of polysulfides intermediates in the electrolyte, the less-than-ideal electrochemical performance under high-rate conditions, and the low sulfur content in the composites. Aiming at those problems above, supported by the National Nature Science Foundation of China (51274240and51204209), we carried out some research works in terms of the carbon-sulfur materials through hybrid nanostructure design and N doping modification in the carbon matrix, and some progress has been obtained. The main works and results are summarized as follows:(1) To address the dissolution of polysulfides in porous carbon-sulfur composites, the carbon matrix with hybrid nanostructure formed by graphene (RGO) and mesoporous carbon (CMK-3) was designed, and the RGO@CMK-3/S composite was successfully synthesized. The RGO skin coated on the surface of CMK-3/S can not only improve conductive of the composite, but also can implant chemical and physical barriers in cathode for absorbing polysulfides. As the tests showed, the RGO@CMK-3/S composite displays a reversible discharge capacity of about734mAh g-1after100cycles at0.5C.(2) To improve the high-rate cycling performance of carbon-sulfur materials, the carbon matrix with three-dimensional hybrid nanostructure composed by MWCNTs webs (MWCNTs-W) and RGO, and the RGO@MWCNTs-W/S composite was successfully synthesized. After activation with KOH under high temperature, the surface of MWCNTs became rough. Moreover, some oxygen-containing function groups were created on the surface of MWCNTs, which would be helpful for the fine distribution of sulfur in the MWCNTs-W matrix. As both MWCNTs-W and RGO can provide fast Li+and electrical diffusion pathways, the RGO@MWCNTs-W/S composite favors fast charge-transfer response. At an ultra-high rate (5C), a discharge capacity as high as620mAh g-1was retained for the RGO@MWCNTs-W/S composite after200cycles.(3) The functionalized N-doped porous carbon nanfiber webs (N-PCNF) were synthesized by a facile approach, wich consisted of the chemical-reduction process, the pyrolyzation process, etc. And then, we prepared the N-PCNF/sulfur composites (N-PCNF/S) as cathode materials for advanced lithium-sulfur batteries. The N heteroatoms in the N-PCNF/S composites can not only enhance the electronic conductivity, but also assist N-PCNF to suppress the diffusion of polusulfides into the electrolyte. As the tests showed, the N-PCNF/S composite with77.01wt%sulfur content displays a discharge capacity of666.0mAh g-1after200cycles at1C. Thus, combining the favorable aspects of N doping modification and one-dimensional nanostructure in the carbon matrix design is an effective way to improve electrochemical performance of the carbon-sulfur materials.