| Lithium-selenium battery is a new type of high energy density battery system after lithium-sulfur battery,which has great development prospect.Faced with the same situation as the lithium-sulfur battery,the fast decay of capacity and low Coulombic efficiency are also key issues hindering the industrialization of lithium-selenium battery.Thus,the emphasis and focus of research lie in exploring porous carbon skeletons with high conductivity,high specific surface area,suitable pore structure and pore size,which combined with selenium to obtain high energy and good cycle performance of the selenium-based cathode material.Further improving the electrochemical activity and cycle stability of the positive electrode,so that the loading of selenium and utilization can effect in cooperation to the maximum extent,which is of great theoretical and practical significance for the development of high energy density lithium-selenium battery.In this paper,three kinds of porous metal organic frameworks with large specific surface area and suitable pore size were prepared by using metal organic framework as the precursor of porous carbon skeleton.Ni2O(BDC)2,Ni3(BTC)2 and Zn2ONi2O(BDC)2(BDC= terephthalic acid,BTC= isophthaloate),three kinds of porous carbon skeletons with excellent electrochemical performance were prepared by high temperature carbonization,and then porous carbon was realized by melt-diffusion process The mesoporous carbon/selenium composite cathode material,MOF-derived hollow hierarchical porous carbon spheres(MHPCS)/selenium composite cathode and hierarchically porous carbon microcubes(CMCs)/selenium composite electrode were synthesized,and their physical properties as well as electrochemical propertieswere were studied,the results are as follows:1.Meso-C@Se composite: With MOF-Ni as precursor,a matrix with mesoporous carbon properties was obtained by one-step carbonization method,which was used for melt-impregnation of selenium as cathode material for Li-Se battery.The thermogravimetric analysis shows that the Se was evenly distributed in the composite material with a mass percentage of up to 48%.The electrochemical performance test finds that the initial discharge specific capacity of meso-C@Se is 599.7 mAh g-1,and remains 417 mAh g-1 after 100 cycles at a high current density of 3 C,and the Coulombic efficiency is as high as 99.9%.Its excellent electrochemical properties are attributed to the enhancement of the electrical properties and the pore channel structure of the carbon matrix derived from MOF-Ni.2.MHPCS/Se composite: A kind of hollow hierarchical porous carbon microspheres(MHPCS)was successfully prepared by solvothermal method and high temperature carbonization.The results show that MHPCS has mesoporous structure as well as abundant microporous structure,and each MHPCS is made up of nano-carbon hollow bubbles interconnected to each other,whose radius is about 20 nm and thickness of shell is about 5 nm.The MHPCS/Se composite,as the positive electrode of the Li-Se battery,providing an initial specific capacity up to 588.2 mAh g-1 at a current density of 0.5 C,exhibiting outstanding cycling stability during 500 cycles with the rate of decay as low as 0.08%.Under 1 C test conditions,the specific capacity is maintained greater than 200 mAh g-1 even after 1000 cycles.The improved electrochemical performance should be attributed to the hierarchical porous structure of MHPCS,mesopores facilitate the immersion of Se and ion transportation,while the micropores are used to immobilize Se from dissolving due to the volume expansion during charging and discharging.3.Se/CMCs composite: The Zn/Ni-MOF-2 cube was obtained by thermal reaction of bimetallic ions(Ni2+,Zn2+)as the center,and the cubic morphology of CMCs is highly maintained after calcination at high temperature in Ar atmosphere.CMCs possess a hierarchical porous structure with a specific surface area of 1013.6 m2 g-1,resulting in approximately 50 wt% of Se loading.In the carbonate-based electrolyte,the Se/CMCs composite exhibits ultra-high initial discharge specific capacity and reversible capacity.In particular,the CMCs porous structure effectively relieves the volume expansion effect,because its excellent conductivity can effectively reduce the charge transfer resistance and inhibit the dissolution of the polyselenide in the electrolyte,thus significantly improving the electrochemical properties of Se-based cathode.In summary,we have built several hierarchical porous carbon skeletons with metal organic framework materials as precursors,which have good loading and fixation effect on the active selenium,thus,they can be used to modify the "shuttle effect" and volume expansion problem of lithium-selenium battery,which provides a new idea for the preparation of high-capacity cathode composite material for lithium-selenium battery. |
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