The Design And Preparation Of MOFs Derivatives As Sulfur Host Materials For Lithium Sulfur Battery And The Research Of Electrochemical Performance

Lithium-sulfur?Li-S?batteries with high energy density(2600 Wh kg^-1)and theroretical specific capacity(1675 m Ah g^-1)have attracted intensive attention in recent years due to its favorable prospect in next-generation batteries.Although the sulfur holds the advantages of high abundance,low-cost and environment friendliness,however,the inherent problems of the sulfur cathode have greatly limited the pratical applications of Li-S batteries.To address thses issues,this thesis focuses on the high-performance sulfur host materials based on metal organic frameworks?MOFs?derivatives.By properly designing the experiment details,the pore distribution and composition ratio of the MOFs derivatives can be effectively controlled.The nanopolyhedras with good morphology,uniform size,cobalt and nitrogen co-doped yolk-shell structure were successfully prepared.When used as Li-S cathodes,it delivers excellent cycle performance and rate capability.In addition,the chemisorption ability for polysulfides is enhanced by in-situ form a layer of TiO₂ outside of MOFs,resulting in a better electrochemical performance.The specific research works are as follows:1.The ZIF-67/CNTs nanomaterials were successfully fabricated via a facile and scalable insitu synthesis through electrostatic adsorption.Then,the CNTs-connected Co-N-doped carbon nanopolyhedra?S@Co-N-C/CNTs?with yolk-shell structure and high sulfur content was prepared by melt diffusion methods.When used as sulfur cathode,the S@CoN-C/CNTs composites exhibited excellent electrochemical performance due to the synergistic effect of the high conductivity,hollow structure and the Co-N co-doped induced strong chemisorption for lithium polysulfides.The cathode showed a high specific capacity of 712.2 m Ah g^-1 after 300 cycles at the current density of 1675 m A g^-1?1 C?.2.In order to enhance the chemisorption ability of the sulfur host materials,a amorphous layer of Ti?OH?x with appropriate thickness coated on the ZIF-67 surface through the electrostatic adsorption.After carbonization,TiO₂-coated carbon polyhedrals were abtained.The introduction of TiO₂ not only enhanced the adsorption to polysulfide,but also prevented the loss of internal sulfur.This material was used as host for sulfur cathode,showing a good electrochemical performance and delivering a high specific capacity of 384 m Ah g^-1 after 300 cycles at the current density of 3350 m A g^-1?2 C?.