Design Of Metal-Organic Frameworks And Their Derivatives Adsorption And Catalytic Site And Their Application In Lithium Sulfur Batteries

In recent years,the continuous consumption of non-renewable fossil resources has caused serious environmental crisis and energy crisis.And the renewable clean energy resources have the problem of unstable output,so the secondary battery is particularly important as a new energy storage device.Among the state-of-the-art energy storage devices,lithium-sulfur batteries(LSBs)have attracted much attention due to their high theoretical energy density.However,there are still many challenges to overcome in its commercial application,such as low sulfur utilization,large volume variation(80%),sluggish redox reaction kinetics,and the shuttle effect of soluble polysulfides(Li PSs).Large amounts of instructive research show that designing polar catalyst materials to adsorb Li PSs and promote the conversion of Li PSs to Li₂S₂/Li₂S is an effective way to solve the above problems.Metal-organic framework materials(MOFs)exhibit the great potential to regulate the adsorption and catalytic conversion of Li PSs in LSBs by virtue of their sufficient active sites and structural tenability.In this thesis,the adsorption and catalytic conversion of Li PSs were successfully realized through the specific chemical design of the adsorption and catalytic sites of MOFs.A variety of separator-modified interlayer and sulfur hosts were prepared based on MOFs,respectively,which suppressed the shuttle effect during the charging/discharging processes and greatly improved the capacity and cycle life of LSBs.The relationship between the structural features of MOFs,such as metal centers and ligand functional groups,and the adsorption and catalytic activity of Li PSs was systematically investigated.The specific research contents are as follows:(1)To solve the problem that the catalytic metal centers of MOFs are usually fully coordinated with ligands and inactivated,three task-specific Bi-MOFs(Bi-MOF-1,Bi-MOF-2,and Bi-MOF-3)were designed and synthesized to aim at the artificial regulation of the Li PSs adsorption and catalytic effect of MOFs via coordination engineering.The catalytic activity of Bi-MOFs can be activated by the exposed open metal sites(Bi-MOF-1),but can also be switched off by either naturally or artificially coordinated small molecules onto the active sites in Bi-MOF-2 and Bi-MOF-3,respectively.The open metal sites on Bi³⁺clusters onto the active sites in Bi-MOF-2 and Bi-MOF-3,respectively.The open metal sites on Bi³⁺clusters and Bi³⁺-S interaction within Bi-MOF-1 are used for adsorbing and catalyzing Li PSs.Moreover,Bi-MOF-1 can improve the specific capacity of LSBs by 50%and decrease the decay rate by 80%after 1000 cycles at 1 C,compared with the LSBs without catalytic interlayer,showing the great potential of catalytic MOFs for high-performance LSBs.(2)To optimize the problem that the MOFs-based separator-modified interlayer usually needs to add a large number of conductive materials to promote its electron transfer,which leads to the low overall energy density of the battery.The ultra-thin(?0.08 mg cm^-2,1.8?m)and conductive Ni-TABQ membrane with dual catalytic sites(Ni-N₄ and quinone chemical group)was in-situ grown on plasma-treated polypropylene(PP)separator,enabling the adsorption and multi-step catalysis conversion of Li PSs.Benefiting from these advantages,the LSBs assembled with Ni-TABQ interlayer deliver a high capacity and ultra-low decay rate of 0.198‰after 1000 cycles at 1 C.Even under high sulfur loading(6.8 mg cm^-2)and lean electrolyte(E/S ratio of 5.0?L mg^-1_sulfur),a capacity up to 5.59 m Ah cm^-2 can be achieved after 100 cycles.(3)In view of the problem that the low energy density of the battery with the addition of the interlayer,it is particularly important to design a new cathode host material.However,the conductivity of MOFs is usually insufficient to meet the requirements of the cathode in LSBs.Herein,a Bi-MOF/Graphene derived hierarchical sulfur host with Bi/Bi₂O₃heterostructures enriched at the outer layer of carbonized MOF(denoted as Bi/Bi₂O₃@C@G).Among them,Bi₂O₃ has strong adsorption for Li PSs and Bi provides better conductivity and higher redox activity,promoting the electron transfer during Li PSs conversion.Thus,the assembled LSBs constructed with Bi/Bi₂O₃@C@G-S electrode effectively block the Li PSs shuttling and improve the utilization of sulfur.Such LSBs enable an ultra-low decay rate of 0.022%per cycle over 1000 cycles at 1 C.Moreover,even with a high sulfur loading of 8.1 mg cm^-2,a high areal capacity of 6.6 m Ah cm^-2 can be maintained for over 100 cycles.(4)To optimize the problems that the agglomeration and uneven distribution of active(CNTs)to obtain Cu-TABQ@CNTs composite cathode host material.The electron transfer ability of Cu-TABQ was improved by CNTs conductive network.And the assembled two-dimensional Cu-TABQ nanosheets can increase the number of active sites(Cu-N₄ and quinone),realize uniform dispersion and full exposure of active sites to efficiently adsorb Li PSs and catalyze their conversion.After 1000 cycles at 1 C,the discharge capacity of the LSBs with Cu-TABQ@CNTs host is stable at 737 m Ah g^-1,and the capacity decay rate is0.047%per cycle.