| Lithium-sulfur batteries are considered to be one of the new generation of the most potential energy storage systems due to their high theoretical specific energy and cost advantages.However,the inherent insulating properties of elemental sulfur and its discharge products,the volume effect during charge and discharge,the"shuttle effect"and the slow redox kinetics of intermediate polysulfides all seriously hinder the practical application of lithium-sulfur batteries.In view of the above problems,this paper combines the advantages of non-polar carbon materials and polar compounds and hollow porous structures to design an excellent sulfur cathode host material to improve the electrochemical performance of lithium-sulfur batteries.The specific research contents and results are as follows:(1)The structural design and the study on electrochemical properties of 3D hybrid Co9S8@N-CHS hollow spheres.We prepared 3D hybrid Co9S8@N-CHS hollow spheres through in situ thermal reduction and vulcanization.The self-made SiO2 microspheres as template,Co-silicate was firstly coated on the surface of SiO2 by hydrothermal method and then the ratio of polybenzoxazine and SiO2 was controlled to make SiO2@Co-silicate@resorcinol microspheres.After in-situ thermal reduction and vulcanization,the 3D hybrid material Co9S8@N-CHS hollow spheres were obtained.Co9S8 nanoparticles embedded in carbon hollow spheres synergizing with the rich hierarchical porous structure can form an effective confinement space,and the enhancement mechanism of Co9S8@N-CHS composites for lithium-sulfur batteries cathodes were also clarfied.Compared with the S/AC and S/N-CHS electrodes,the S/Co9S8@N-CHS electrode exhibits more excellent rate characteristics and cycle performance.At a current density of 0.2 C(1 C=1675 mA·g-1),the initial discharge specific capacity of Co9S8@N-CHS is as high as 1363 mAh·g-1;when the current density is increased to 1 C,there is still 660 mAh g-1 after 1000 cycles with with a low capacity decay of 0.041%per cycle and the Coulomb efficiency is above 99%.The S/Co9S8@N-CHS-CuO-Li full battery was assembled by matching with the electrochemical pre-lithiated CuO anode and has a good cycle stability.After 100 cycles at 0.5 C,the specific capacity is still able to maintain 460 mAh g-1 and the coulomb efficiency is about 97%..(2)The structural design and the study on electrochemical properties of Co9S8-NSC@MOS2 nanobox with heterogeneous structure.Firstiy,Co-Mo precursor was obtained by one-pot method which using ZIF-67 as precursor,glucose as carbon source and sodium molybdate as molybdenum source and then further heat-treated to prepared Co9S8-NSC@MOS2 nanobox with heterostructure.The effects of different heat treatment temperatures on the structure and morphology of Co9S8-NSC@MoS2 composite were investigated.When heated at 600?,the structure and morphology of Co9S8-NSC@MoS2 were the best and kept the original MOF structure.The nanosheets in the surface of Co9S8-NSC@MoS2 were clearly visible.When used as a sulfur host material,the S/Co9S8-NSC@MoS2 electrode exhibits a higher discharge specific capacity and much more excellent cycle life and rate characteristics.After 200 cycles at 0.2 C,the specific capacity was maintained at 567 mAh g-1;after 1000 cycles at 1 C,the specific capacity of 360 mAh g-1 was also kept and the charge and discharge efficiency was above 98%.More importantly,When the sulfur content is up to 2.5 mg·cm-2,the S/Co9S8-NSC@MOS2 electrode can still maintain a good specific capacity and cycle performance.At the current density of 0.5 C,the initial discharge specific capacity is 536 mAh g-1;when the current density increases to 1 C,the capacity also can maintained at 567 mAh·g-1 after 200 cycles,with an average attenuation of 0.033%per cycle,which helps to promote the practical application of lithium-sulfur batteries in the field of smart grid and electric vehicles and so on. |
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