Modified Separators Based On Porous Metal-organic Framework Materials For Long-life Lithium-sulfur Batteries

Lithium-sulfur batteries have received widespread attention for its excellent high theoretical specific capacity(1675 m A h g^-1)and energy density(2600 W h kg^-1),as well as the low price and natural abundance of sulfur.However,lithium-sulfur batteries suffer from serious polysulfide shuttle effect,low utilisation of active sulfur,short cycle life and low energy density,which seriously restrict their commercialisation.Metal organic frameworks（MOFs）are widely used in energy storage and other research fields due to their highly tunable pore structure,high porosity,large specific surface area and abundant functional sites on the surface.It is difficult to further deepen the understanding of the relationship between the performance of the separator and the structure of MOFs at the pore structure level by simply selecting the existing single MOFs to prepare the modified separators.Neglecting MOFs’flexible and tunable structure can provide great advantages for regulating efficient lithium-ion transport and suppressing polysulfide shuttling.Moreover,the prepared MOFs-based modified separators generally have problems such as poor membrane coverage and compactness,which cannot maximize the inherent pore structure advantages of MOFs materials.Therefore,this thesis focuses on the key issues of MOFs materials for lithium-sulfur battery separator modification,develops a simple and effective strategy to design MOFs materials with precisely tuned pore size and novel pore size environment,and seeks suitable preparation methods for Dense modified separators are obtained to realize the functionalized advantages of simultaneously alleviating polysulfide shuttling and enhancing Li⁺ion transport,resulting in long-cycle-life Li-S batteries.The specific research contents are as follows:（1）An elaborately modified separator capable of simultaneously blocking polysulfide shuttle and promoting Li⁺ion transport is crucial for developing high-energy-density lithium-sulfur（Li-S）batteries with outstanding rate performance and long cycling stability.Herein,we demonstrate an intriguing pore-space-partition strategy to reconstruct the pore network and pore chemical environment of a prototype metal-organic framework（MOFs）,empowering it with remarkably enhanced performance as a separator coating in Li-S batteries.The finely tailored MOFs has a rational pore size,large specific surface area and abundant catalytic sites,which synergistically facilitate Li⁺ion conduction,inhibit polysulfide shuttle,and boost catalytic polysulfide conversion,thereby realizing both outstanding rate capability and cycling stability in high sulfur loading cathodes.The as-developed cell exhibits superb cyclability with a capacity retention of 79.0%over 500cycles.More impressively,steady cyclability and a high areal capacity of 3.96 m A h cm^-2are achieved at a current density of up to 3.1 m A cm^-2 with 78 wt%high sulfur content.（2）A uniform and dense Mg₂（dobdc）modified layer was obtained on the surface of the PP separator by a direct solvothermal in-situ growth method.The internal pore structure of Mg₂（dobdc）can adsorb anions in the electrolyte and promote the migration of lithium ions.The flower-like structure of the Mg₂（dobdc）layer is conducive to obtaining a well-distributed and dense film modification layer,which can better inhibit the transport of polysulfide.When the MOFs particles are densely packed,the Mg₂（dobdc）modified layer can act as an ion sieve to inhibit polysulphides,catalyse the conversion of lithium polysulphide,promote the migration of lithium ions and accelerate the redox kinetics of electrochemical reactions due to its 1D microporous channels with suitable pore size and internal pore environment.Also,comparing the differences in pore sizes within different MOFs materials,it was concluded that cells with Mg₂（dobdc）/PP modified separator of moderate pore size exhibited excellent cycling performance.