Application Of Crystalline Porous Framework Materials In Lithium-sulfur Batteries

Lithium-sulfur batteries are considered to be ideal secondary batteries for next-generation due to their advantages of high energy density,cheap raw materials and environmental friendliness.The sulfur electrode is reduced to soluble long-chain lithium polysulfide,and further to insoluble short-chain lithium polysulfide and final discharge product lithium sulfide in discharge process.During charging,the discharge product goes through the opposite process and is eventually oxidized to sulfur.As the solubility in electrolyte,the high-order lithium polysulfides can move across the battery separator and reach the lithium cathode,resulting in non-conductive lithium sulfide deposition.On the one hand,it causes the loss of active sulfur,on the other hand,it impedes the reactivity of the lithium cathode,which rapidly reduces the charging and discharging capacity of lithium-sulfur battery and shorts its service life.Therefore,finding suitable functional materials to inhibit the shuttle effect of lithium sulfide is an ideal strategy to improve the cycling stability of lithium-sulfur battery and realize its practical applications.Metal-organic frameworks?MOFs?have advantages of clear spatial structure,functionable modification and adjustable pore size.They are a promising cathode matrix material for lithium-sulfur batteries.However,as a positive electrode matrix,MOFs also have some limitations,such as poor conductivity and stability,which can be improved by loading conductive polymer.The non-conductivity of MOFs is utilized to construct a lithium ion transport channel by targeting functionalized channels,which can be used for separator coating to inhibit the shuttle effect of lithium polysulfides.In order to reduce the low mass specific capacity caused by the presence of metal centers,the light-weight covalent-organic frameworks?COFs?were used as cathode matrix material of Li-S batteries,and the cationic sites and functional groups were used as active sites for anchoring of polysulfides and improving the battery performance.Based on this,the research of this paper mainly includes the following four parts:1.A multi-core manganese-cluster-based MOF was ultilized as the cathode host of Li-S batteries for the first time.A large number of water molecules were coordinated with manganese sites and can be removed by thermal activation treatment to expose unsaturated coordination metal sites,meanwhile the framework can be well maintained.The open metal sites?OMSs?of MOF can effectively anchor lithium polysulfides so that inhibit their shuttle effect,and improve the cycling stability and rate performance of lithium-sulfur batteries.After the load of sulfur,S@MN-CCs can effectively improve the utilization of sulfur by reducing the loss of active sulfur.S@MN-CCs showed an initial specific capacity of 1460 m A h g^-1 and a residual specific capacity of 990 m A h g^-1 after 200 cycles at 0.2 C.The results of these experiments mentioned above present a wide range of possibilities for further development of new cathode matrix materials for Li–S batteries.2.In order to improve the conductivity and sulfur carrying capacity of MOF,a mesoporous MOF Ni₂?NDISA?-PDA loaded with polydopamine was selected as the cathode matrix material of li-sulfur battery.A mesoporous MOF Ni₂?NDISA?,filled with polydopamine to improve its conductivity and sulfur loading,was ultilized as the cathode host material.The existence of polydopamine not only improves the stability and the conductivity of mesoporous MOF,but also provides physical limiting of polysulfides.The N,N-dimethylformamide?DMF?coordinated with metal nodes can be removed by thermal activation treatment after solvent exchange to expose open metal sites,aming to anchor polysulfides and catalyze their redox reaction.S@Ni₂?NDISA?-PDA,as the cathode material of Li-S batteries,exhibits good cycling stability,rate performance,and low overpotential in charge-discharge process.It displayed an initial specific capacity of 412.6 m A h g^-1 even at a high current density of 5.0 C,and the capacity fading rate is only 0.036%per cycle for 600 cycles.3.A new MOF Eu₂?MISA?₃ with functional pore was constructed with mercapto ligand and used for the membrane coating of Li-S battery.The 1D channels functionalized with thiol provide a lithium ion transport pathway,and inhibit the shuttle of polysulfides by the charge repulsion between thiol and polysulfides,so as to improve the cycling stability of Li-S batteries.Battery with Eu₂?MISA?₃/PP separator showed an initial specific capacity of 1107.4 m A h g^-1,and a retained capacity of411.5 m A h g^-1 was achieved with a fading rate of 0.063%per cycle after 1000 cycles.4.In view of the low mass specific capacity of Li-S batteries caused by the presence of metal nodes in MOF,a cationic COF EB-COF-Br was ultilized as the cathode matrix material.The components of COF are C,H,N and other light elements,which are conducive to improving the mass specific capacity of the battery and the cationic sites can effectively absorb lithium polysulfides.Removing the counter anion Br^–by anion exchange,the cathode matrix EB-COF-PS could be obtained and ultilized as the cathode by being filled with sulfur.S@EB-COF-PS showed better rate performance and long-term cycling stability,especially at high current density.A capacity of 468 m A h g^-1 was retained after 300 cycles at 4.0 C.A reasonable conjecture can be made according to XPS and DFT calculations that the cationic sites have a tendency to deliver electrons to sulfur species as a bridge in discharge step to facilitate the polysulfides disintegration and gain electrons from polysulfides in charge process so that promote the polysulfides oxidation.