Application Of Mxene-Based Composite In Lithium-Sulfur Batteries

Lithium-sulfur（Li-S）batteries have an high theoretical energy density(2600 Wh kg^-1).Moreover,as the cathode material,sulfur is low-cost and environment-friendly.Thus,Li-S batteries have been regarded as one of the most promising energy storage technologies.However,Li-S batteries suffer from the problems such as the poor conductivity of sulfur and Li₂S,and the“shuttle effect”of lithium polysulfides,which make the Li-S batteries cannot reach their full potential,limiting their large-scale application.2D transition metal nitrides and carbides（MXene）used in Li-S batteries can enhance the conductivity of sulfur,and provide chemical adsorption to lithium polysulfides,limiting the“shuttle effect”.In this dissertation,MXene-based materials are used to improve the electrochemical performance of sulfur cathode.The strategies of preparing three-dimensional（3D）MXene/reduced graphene oxide（RGO）matrix to load sulfur,establishing free-standing MXene/RGO-C₃N₄composite aerogel as matrix to load sulfur,and modifying separator with MoO₃/MXene interlayer,are proposed to enhance the cycling and rate performance of Li-S batteries.The details are as follows:（1）3D porous MXene/RGO aerogels were prepared by low-temperature hydrothermal method,which were then used as matrix to load sulfur to prepare cathode materials for Li-S batteries.The 3D structure of matrix has developed porosity,endowing the cathode with high sulfur loading.The porous structure of the matrix is also beneficial for rapid electrolyte penetration and ion transport.The MXene/RGO framework in the matrix has excellent electrical conductivity,in favor of improving the rate capability of the cathode.In addition,MXene has strong adsorption capacity for lithium polysulfides,which can effectively inhibit the shuttle effect.As a result,the prepared MXene/RGO@S（mass ratio of MXene/RGO is 1:1）cathode with a sulfur loading of 85.4 wt%exhibits high capacity and good cycling and rate performance.It has an initial specific capacity of 1253.1 m Ah g^-1 at 0.2 C,and remains at 792.8 m Ah g^-1 after 100 cycles.Meanwhile,the capacity is up to782.7 m Ah g^-1 at 2 C.（2）Using MXene/RGO as the conductive matrix,the MXene/RGO-C₃N₄composite aerogel was prepared by solution impregnation and high-temperature pyrolysis.After cut,the composite aerogel is used to load the Li₂S₆ solution,and then it can be directly used as free-standing cathode for Li-S batteries.The MXene/RGO-C₃N₄ aerogel has an appropriate hierarchical porous structure,which benefits fast ion transport and high sulfur loading.The MXene/RGO conductive matrix provides a continuous conductive network,and the introduction of Graphitic carbon nitride（C₃N₄）enhances the adsorption of polysulfides and the catalytic effect on kinetic conversion of sulfur.As a result,the Li₂S₆@MXene/RGO-C₃N₄ cathode exhibits excellent cycling and rate performance.It has an initial specific capacity of 1315.6 m Ah g^-1 at 0.2 C,a high capacity retention of 97.5%after 100 cycles,and a high capacity of 1167.4 m Ah g^-1 at 2 C.When the areal sulfur loading of the cathode reaches up to 4.92 mg cm^-2,it still exhibits excellent performance with a capacity of 940.3 m Ah g^-1(4.62 m Ah cm^-2)after 200 cycles at 0.1 C.（3）MoO₃ nanobelts were prepared by hydrothermal method,which were then mixed with MXene nanosheets and vacuum filtrated through polypropylene（PP）separator to obtain a MoO₃/MXene modified interlayer on the PP separator.In the interlayer,MoO₃ nanobelts evenly distributed between MXene layers,acting as spacers to effectively avoid the re-stacking of MXene nanosheets,which is conducive to the rapid electrolyte penetration and fast ion transport.The MXene conductive network can improve the electrical conductivity of the interlayer.Both MoO₃ and MXene can effectively adsorb lithium polysulfides,inhibiting the shuttle effect and improving the cycle stability of the battery.In addition,the catalytic property of MoO₃ can accelerate the polysulfide redox process.Thanks to the synergistic effect of MXene and MoO₃,the Li-S battery using the MoO₃/MXene（mass ratio is 7:3）interlayer achieves an initial capacity of 1111 m Ah g^-1 at 0.2 C and the capacity remains at 837.2 m Ah g^-1 after 100 cycles,with a capacity retention of 73.35%.Also,it has a capacity of 651.2 m Ah g^-1 at 2 C.