Preparation Of Composites Derived From Layered MOF-71 And Study On The Performance Of Lithium-Sulfur Cathodes

After decades of development,rechargeable lithium-ion batteries（LIBs）using lithium cobalt oxide,lithium iron phosphate,etc.as the positive electrode and graphite material as the negative electrode have been unable to meet people’s needs for high specific capacity and high power energy storage devices.Lithium-sulfur battery is expected to become a new generation of power source due to its high theoretical specific capacity(1675 m Ah·g^-1)and energy density(2600 Wh·kg^-1).However,due to the poor conductivity of the sulfur positive electrode active material of lithium-sulfur batteries and the shuttle of polysulfide,the intermediate product of the reaction,between the positive and negative electrodes,the capacity of the battery is rapidly decayed,which limits its large-scale commercial application.In order to increase the utilization rate of active material sulfur and effectively improve the cycle stability of lithium-sulfur batteries,the layered metal-organic framework MOF-71synthesized by solvothermal method was used as the precursor.The precursor through different heat treatments such as carbonization,oxidation,phosphating,etc.,the carbon-containing cobalt oxide（Co₃O₄/C）and phosphide（Co-Co P/C）derived from MOF-71were prepared,and then two materials were used as the positive electrode matrix to combined with the active material sulfur to assemble lithium-sulfur batteries for electrochemical performance test.Combining the XRD,SEM,XPS,BET and other physical characterization results of the two materials,the influence of the cobalt-based compounds derived from MOF-71 on the battery performance was analyzed and discussed.The specific research content and experimental conclusions of this paper are as follows:（1）Co₃O₄/C composite material was prepared by carbonization and oxidation treatment with using layered MOF-71 as the precursor,and combined with active material sulfur to obtain a Co₃O₄/C@S composite electrode.On the one hand,the layered Co₃O₄/C material and the conductive network derived from carbon help to shorten the charge transfer;on the other hand,the polar Co₃O₄ has a strong interaction with polysulfides,which can effectively alleviate the"shuttle effect"and increase the active material utilization rate.The prepared Co₃O₄/C@S assembled battery showed good electrochemical performance:at a discharge rate of 0.2 C,the first-cycle discharge specific capacity was 1142.2 m Ah·g^-1,and the Coulombic efficiency remained above 99%after 100 cycles.In addition,the battery maintains a Coulombic efficiency of more than 98%after 300 cycles of charging and discharging at 0.5 C.（2）The layered MOF-71 precursor was carbonized and then phosphatized to prepare Co-Co P/C material,and combined with active substance sulfur to obtain a Co-Co P/C@S composite electrode.The physical characterization results indicate that Co-Co P/C is a layered porous composite material with a specific surface area of 113 m²·g^-1,with more micropores and mesopores,which is beneficial to the transmission of ions and electrons.The lithium-sulfur battery assembled with Co-Co P/C@S as the positive electrode showed excellent cycle stability in the electrochemical performance test:tested at a rate of 0.2 C,the high initial discharge specific capacity of the battery reached 1441.9 m Ah·g^-1.The utilization rate of active material is 86.08%.In addition,the long-cycle stability performance test was performed at a rate of 0.5 C,and the battery still maintained a reversible specific capacity above 600 m Ah·g^-1 after 300 cycles of charge and discharge.The excellent cycle stability benefits from MOF-derived metal phosphide Co-Co P/C layered porous structure,its strong polarity can not only be used as an active site to have a strong ability to adsorb polysulfides,but also cobalt phosphide can also promote the catalytic conversion of polysulfides,thereby helping to accelerate polysulfides reaction kinetics of lithium-sulfur batteries.