| Lithium-sulfur batteries have been considered as one of the most promising next generation high-performance secondary battery systems for its overwhelming merits of ultrahigh theoretical energy density,low cost of raw materials,and environmental friendliness,etc.However,the commercial application of lithium-sulfur batteries still encounters critical drawbacks including low Coulombic efficiency,short cycling life,and poor rate performance,etc.Designing and manufacturing multifunctional sulfur cathodes is an effective method to improve the performance of lithium-sulfur batteries,including designing electron/ion conductive channels,constructing buffer space for volume change,improving polysulfides adsorption ability,and promoting polysulfide conversion and so on.Metal organic frameworks（MOFs）derived materials have the advantages of abundant hierarchical pore structures and tunable nano-metal-based active sites,which can meet the above-mentioned requirements of sulfur cathode materials.However,MOFs-based precursors tend to have large particle sizes,resulting in the inability of the active sites inside to fully function during electrochemical processes.In addition,MOFs-derived carbon materials lack mesoporous and macroporous structures,which are unfavorable for the improvement of sulfur loading.To address these issues,we prepared ZIF-67-derived cobalt-based hollow carbon materials with different structures based on the idea of"nano-confinement".Taking the cavity of hollow carbon spheres and the continuous pores of honeycomb-like carbon as nano-reactors,proving nano-confined growth environment for MOFs,we have synthesized yolk-shell Co-NC@N-HCS and honeycomb-like Co Se2-NC@HC and applied then in sulfur cathodes.The growth mechanism of composites with different structures was systematically studied,and the configurations between the structure of cobalt-based hollow carbon materials and their corresponding electrochemical performance were explored,with a focus on the adsorption and catalytic effects of cobalt-based hollow carbon materials on polysulfides.The main research conclusions are as follows:（1）Based on the idea of nano-confined synthesis,closed hollow carbon spheres and semi-closed honeycomb-like carbon materials were used as nano-reactors,and the size of ZIF-67 particles was limited to about 50 nm since the spatial confinement effect inside the nano-reactors.Cobalt-based hollow carbon materials with two different structures were prepared,including yolk-shell ZIF-67@N-HCS and three-dimensional porous honeycomb-like ZIF-67@HC.The influence mechanism of reactant ratio and concentration on the growth of ZIF-67 in nano-confined environment was studied:the appropriate reactant ratio and concentration benefit the"nucleate-growth"of ZIF-67 in the closed hollow carbon spheres.Both closed and semi-closed nano-reactors can provide preferential nucleation sites for ZIF-67,and the steric hindrance effect of nano-reactors can effectively inhibit the free growth of ZIF-67.（2）Taking ZIF-67@N-HCS as precursors,yolk-shell Co-NC@N-HCS with high specific surface area and pore volume was synthesized after calcination and acid etching.The S/Co-NC@N-HCS cathode exhibited a specific capacity of 1173.5 m Ah·g-1 at 0.1 C,and a specific capacity of 933.5 m Ah·g-1 at 0.5 C.After 150 cycles,the decay rate per cycle reached 0.18%.The S/Co-NC@N-HCS cathode displayed lower polarization potentials（0.249-0.575 V）,a smaller interfacial impedance（22.06Ω）,and stronger adsorption ability towards polysulfides.During the charge-discharge process,the uniformly distributed nano-sized Co-NC cores inside the carbon sphere cavity can significantly enhance the ion/electron transport ability.On the other hand,the combination of physical confinement of nitrogen-doped carbon shells and chemical adsorption of Co-NC cores can effectively suppress the shuttle effect of polysulfides.Meanwhile,the catalytic effect of Co-NC cores can accelerate the redox reaction kinetics of lithium polysulfides obviously.（3）Taking ZIF-67@HC as precursors,honeycomb-like S/Co Se2-NC@HC composite sulfur cathodes were obtained after in-situ selenization and molten sulfur loading treatment.The initial discharge specific capacities of S/Co Se2-NC@HC cathodes at 0.1 C and 1 C reached 1180.7 and 1066.0 m Ah·g-1,respectively,and the specific capacity decay rate per cycle after 600 cycles at 1 C was 0.12%.The performance was significantly better than that of S/HC cathodes.It was revealed that the nano-Co Se2-NC particles cooperate with honeycomb-like carbon to form a three-dimensional conductive network structure,which further enhances the electron/ion transport capacity of sulfur cathodes.Moreover,the strong adsorption and catalysis of Co Se2-NC can promote the redox kinetics of lithium polysulfides effectively,leading to much-enhanced cycling performance and rate capability. |
