Preparation Of MXene-based Modified Separators And Their Performance In Lithium-Sulfur Batteries

Currently,among various emerging energy storage systems,lithium-sulfur battery is expected to be one of the next-generation lithium secondary batteries with high efficiency.However,the practical application of lithium-sulfur batteries still faces many obstacles,the most serious of which are the“shuttle effect”and self-discharge caused by dissolved lithium polysulfides.Conventional lithium-sulfur battery separators cannot suppress the transmembrane diffusion of lithium polysulfides.However,functional modified separators can not only anchor lithium polysulfides in the cathode region,but also accelerate the conversion kinetics of lithium polysulfides.MXenes have the advantages of strong polarity,excellent electronic conductivity,unique layered structure,abundant termination groups,low Li₂S decomposition barrier,and low Li⁺diffusion barrier,etc.,which can be used as materials for modifying separators.However,the MXenes nanosheets are prone to self-stacking,resulting in the reduction of active sites.In this thesis,a series of Ti₃C₂T_x-based composites were fabricated,and the structure-activity relationship between the microstructure and electrochemical performance of the composites was emphatically discussed.To solve the shuttle effect of lithium polysulfides,ultrathin Ti₃C₂T_x nanosheets were prepared through the in-situ acid etching method and applied to separator modification to suppress the shuttle effect of lithium polysulfides.Ultrathin Ti₃C₂T_x nanosheets with enlarged interlayer spacings accelerateed migration of Li⁺.The abundant termination groups on the surface of Ti₃C₂T_x played the role of lithium polysulfide capture centers.When the mass loading of separator modification materials were set as 0.025 mg cm^-2,the as-prepared battery exhibited a reversible specific capacity as high as 780 m Ah g^-1 after 200 cycles at 0.2 C,and the single-cycle capacity decay rate was only 0.09%.Aiming at the poor rate performance and long-cycle stability of lithium-sulfur batteries,VS₄/Ti₃C₂T_x composites with a three-dimensional conductive network structures were synthesized in situ by a facile hydrothermal reaction,in which VS₄ was uniformly loaded on Ti₃C₂T_x nanosheets through V-C bonds.The self-stacking of Ti₃C₂T_x was suppressed.Under the synergistic effect of VS₄ and Ti₃C₂T_x,the shuttle of lithium polysulfides was effectively inhibited,and the interfacial charge transfer was enhanced.The kinetics of lithium polysulfide conversion was promoted,thereby improving the rate performance and cycling stability of the battery.The assembled battery could deliver a specific discharge capacity of 657 m Ah g^-1 after500 cycles at 1.0 C,with a high capacity retention rate of 71%.In view of the problems of slow redox reaction kinetics and low utilization rate of active materials in lithium-sulfur batteries,hollow Co_1-xS/3D-Ti₃C₂T_x composites with a three-dimensional multi-stage structure were prepared through various approaches such as hard template method,electrostatic adsorption,carbothermic reduction and thiourea method.Co_1-xS nanoparticles were anchored on hollow Ti₃C₂T_x microspheres.The hollow and porous hierarchical structure of Co_1-xS/3D-Ti₃C₂T_x established efficient electron and Li⁺transfer paths,thereby ensuring fast reaction kinetics and improving the utilization of sulfur,while easing the battery volume expansion during cycling,thus enhancing the stability of the structure.Benefiting from Co_1-xS/3D-Ti₃C₂T_x composites effectively controlling the dissolution and diffusion of lithium polysulfides,the battery achieved a reversible specific capacity as high as776 m Ah g^-1 after 500 cycles at 1.0 C,and the Coulombic efficiency was close to 100%,exhibiting excellent rate performance and cycle stability.Aiming at the problems of low reversible specific capacity and serious self-discharge in lithium-sulfur batteries,modified N-doped porous carbon nanosheets（PNCN）and Ti₃C₂T_xnanosheets were assembled by electrostatic interaction to synthesize PNCN/Ti₃C₂T_xcomposites with a three-dimensional porous interconnected conductive network,which retained the advantages of the two components.The electrostatic interaction between adjacent layers of the nanosheets overcame the effect of van der Waals forces and avoided the restacking of the nanosheets.The enlarged interlayer spacing was beneficial to enhance the transport rate of electrons and ions,and can buffer the volume change during the rapid charge/discharge of the battery.The unique conductive network of PNCN/Ti₃C₂T_x ensured the uniform deposition of Li₂S/Li₂S₂ and improved the interfacial state between PNCN/Ti₃C₂T_x and electrolyte,thereby enhancing the capacity of the battery.The battery released a specific discharge capacity of 861m Ah g^-1 after 500 cycles at 1.0 C.In addition,the PNCN/Ti₃C₂T_x modification significantly reduced the self-discharge rate of lithium-sulfur batteries,showing a capacity retention rate of99.9%in the 72-h self-discharge test.The research results of this thesis provided novel routes for the application of metal sulfides in the electrochemical reaction of lithium-sulfur batteries,and provided some theoretical guidance for the composite of MXenes and low-dimensional materials,and expanded the electrochemical application of MXene-based composites in energy storage.