The Synthesis And Characteristion Of Co-based Compounds Hybridized With Carbon Materials In Lithium-Sulfur Batteries

The growing concern of traditional energy crisis and serious environmental pollution have given impetus to the worldwide research in highly efficient energy storage system.Lithium-Sulfur batteries（LSBs）have emerged as one of the promising next generation energy storage system,owing to its prominent theoretical energy density(2600 Wh kg^-1 and 2800 Wh L^-1),low cost and environmentally friendly.However,several critical drawbacks have impeded the commercialization of LSBs,involving:（1）the poor conductivity of sulfur active material.（2）the so-called“shuttle effect”caused by the soluble lithium polysulfides（LiPSs）（Li₂S_x,4≤x≤8）.（3）the sluggish redox kinetics stemming from the electrochemically inert of the discharged products（Li₂S/Li₂S₂）.In order to address these problems of lithium sulfur batteries,different Co-based compounds hybridized with carbon material were designed and synthesized for modifying the cathode of lithium sulfur batteries in this thesis.These composites have many unique advantages:（1）they have excellent electron transfer capability,significantly improving the problem of poor conductivity of sulfur electrode.（2）polar cobalt-based compounds can chemically anchor LiPSs,to a certain extent,inhibiting the occurrence of“shuttle effect”.（3）by virtue of the excellet electrocatalytic properties,these composites can remarkablely reduce the electrochemical polarization,and promote the catalytic conversion of polysulfides,improving the utilization of active material.The main research topic of this paper is as follows:（1）We report a facile strategy by one step pyrolysis process for synthesizing a bimetallic NiCoP/CNTs.S/NiCoP/CNTs was prepared by the melt-diffusion method.And then,the electrochemical performance of S/NiCoP/CNTs was investigated.When cycled at 0.5 C,S/NiCoP/CNTs delivers a high initial discharge capacity of 1039 m Ah g^-1.After 200 cycles,the S/NiCoP/CNTs cathode also delivers a capacity of 580 m Ah g^-1,and a low capacity decay rate of only 0.25%per cycle.NiCoP/CNTs not only exhibits a stronger adsorption affinity for LiPSs than the single metal phosphide,but also can accelerate the catalytic conversion kinetics of LiPSs.It is precisely owing to the synergistic effect between Ni and Co.（2）In order to improve the poor conductivity and the redox reaction kinetics of sulfur,we fabricated a“dandelion-like”carbon sphere decorated with nanosize Co₉S₈ particles,which was intertwined by CNTs（Co₉S₈-C/CNTs）.And then,the electrochemical performance of S/Co₉S₈-C/CNTs was investigated.At 0.2 C,with an E/S ratio of 7.7μL mg^-1,S/Co₉S₈-C/CNTs has the initial discharge capacity of 1129 m Ah g^-1,and the capacity can be maintained at 768 m Ah g^-1 even after 100 cycles.Rich pore peculiarities of Co₉S₈-C/CNTs,not only can physically confine the LiPSs,but also contribute to good permeation of the electrolyte.It was noted that small size Co₉S₈ nanoparticles could create more active sites,which can accelerate the redox reaction kinetics of sulfur.（3）We chose a popcorn-derived carbon material as the substrate.CoS_x/QKC was obtained by Na BH₄ wet chemical reduction and the vulcanization method.We have demonstrated that the S/CoS_x/QKC cathode exhibits excellent cycle stability.Owing to ultrahigh specific surface area,CoS_x/QKC can ensure uniform dispersion of the active material.At the same time,this composite has a wealth of microporous and mesoporous structure to achieve the physical confinement of LiPSs.These good electrochemical performances of S/CoS_x/QKC were ascribed to CoS_x nanoparticles,which can accelerate the redox reaction kinetics of LiPSs.Therefore,CoS_x/QKC plays an important role in chemical adsorption and catalytic conversion of polysulfide.