| Lithium-sulfur batteries(LSBs)have been regarded as one of the most wonderful prospect for energy storage technologies.High theoretical specific capacity(1675 mAh g-1),high energy density(2600 Wh kg*1),rather low price and nontoxicity of sulfur active material make Li-S batteries more attractive.However,there are also some problems,such as the shuttling of soluble lithium polysulfides,volume expansion and low conductivity,etc.,which inhibit the practical application of LSBs severiously.This work focused on the fabrication of carbon nanocomposites from metal-organic frameworks(MOFs).Both the structures and compositions of the nanocomposites have been elaborately tuned for the development of Li-S batteries with high performance.Main research results are shown as below:(1)Preparation of nitrogen-doped hierarchical porous carbon(NHPC).3D metal-organic aerogels consisting of rod-like building blocks were created with the assistance of crosslinks.After carbonization,metal etching and melting diffusion of sulfur,sulfur/nitrogen-doped hierarchical porous carbon(S/NHPC)composites were successfully fabricated.Benefiting from the nitrogen doping and rod-like and hierarchical porous structures,the S/NHPC showed excellent rate capability and stable cycling performance.High capacity of 697 mAh g-1 was achieved at 0.5 C when S/NHPC was used as cathode material for LSBs.Stable cycling performance could be kept up to 500 cycles with an ultralow decay ratio of 0.083%in per cycle.(2)Double-solvent assisted and nanopore-confined preparation of graphitic carbon nitride(g-C3N4)nanodots in the nanopores of MOFs.Double-solvent strategy was used to fill ammonium thiocyanate into the nanopores of NH2-MIL-101(Al).Followed with pyrolysis,we had successfully obtained g-C3N4 nanodots based on a pore confinement eflfect,which were embedded in a MOF-derived N,S co-doped hollow porous carbon shell(CN@NSHPC).Benefiting from the physical confinement and chemical adsorption of the N,S co-doping hollow carbon and g-C3N4 nanodots,S/CN@NSHPC composites cathode promoted the electron transfer and inhibited the shuttle effect of polysulfides.The S/CN@NSHPC cathode delivered excellent specific capacity of 1447 mAh g-1 at 0.2 C and outstanding cycling stability,with only 0.048%decay per cycle at 1.0 C over 500 cycles.(3)Preparation of free-standing Co4N-porous carbon nanosheet arrays on carbon cloth.ZIF-67 sheet arrays were grown on the surface of carbon cloth,after subsequent carbonization,oxidation and nitridation,free-standing materials consisting of carbon cloth grown with porous carbon nanosheet arrays embedded by Co4N nanoparticles(note as CC@Co4N-PCNA)are prepared.Both experimental results and density functional theory(DFT)calculations demonstrated Co4N could strongly interact with polysulfides.Co4N facilitated electron transfer and accelerates the reaction kinetics of LSBs due to its excellent cayalytic activity and electrical conductivity.Benefiting from the strong adsorption,high catalytic transformation of polysulfides,and the uniform nanostructure,S/CC@Co4N-PCNA cathode showed excellent high-rate capability at 5.0 C,and good long-life cyclability over 500 cycles with ultralow 0.035%decay per cycle(598 mA h g-1 at 5.0 C).(4)The preparation of p-n typed Co9S8/Co4N heterojuction composites.By using ZIF-67 arrays grown on the surface of carbon cloth as the starting materials,a p-n typed sulfide/nitride heterojunction hollow nanosheaths were obtained(CC@Co9S8-Co4N)via sulfidation and in-situ nitridation.When used as the LSBs host materials,benefiting from the abundant active sites provided by p-n heterojuction,the strong polarity of Co9S8-Co4N,as well as effective control of the outward diffusion of polysufides in electrolytes,the S/CC@Co9S8-Co4N electrodes achieved a high reversible capacity of 1221 mAh g-1,an ultralow decay rate of 0.027% per cycle over 1000 cycles at 5.0 C... |
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