Design Of MXene-Based Porous Materials And Their Application In Lithium–Sulfur Batteries

Two-dimensional nanomaterials have attracted much attention due to their unique structural advantages,among which MXene has been widely used in adsorption,energy storage,electromagnetic shielding and many other fields,especially in lithium–sulfur batteries（LSBs）,due to its good conductivity,adjustable layer spacing,low ionic diffusion barrier and other characteristics.As we all know,LSBs is considered to be a key technology for large-scale energy storage in the future due to its low cost of active substances and high energy density(2567 Wh kg^-1).However,its commercialization is hampered by poor cycle stability due to the insulation of sulfur and reaction products and the mobility of lithium polysulfide.In order to solve this series of problems,many researchers rely on MXene material,but this two-dimensional material is susceptible to van der Waals forces,resulting in lamellar restacking and aggregation,which hinders ion diffusion,reduces the availability of the material surface,and is not conducive to improving the electrochemical performance of lithium–sulfur batteries.In view of the above shortcomings,this thesis constructs two-dimensional MXene into three-dimensional porous interconnected network structure through structural regulation to prevent lamellar aggregation,increase the specific surface area of the material and improve ion diffusion.Specific work and conclusions are as follows:In this thesis,Ti₃C₂T_x MXene nanosheets were prepared from MAX phase materials by in situ synthesis HF etching and adjusting reaction parameters.Then,3D MXene/melamine-formaldehyde polymer aerogel（MMFA）with electrochemical energy storage application（lithium–sulfur battery）was prepared with MXene as the construction unit and melamine-formaldehyde polymer as the crosslinking agent.The material has a large specific surface area(221 m² g^-1),and as a sulfur carrier,it shows high sulfur utilization rate,high reversible specific capacity,excellent rate capability and long-term cycle stability.As a functional separator of lithium–sulfur battery,the specific discharge capacity of the battery reaches 1042 m A h g^-1 at 0.1 C current density.Compared to MXene/PP separator,MMFA/PP modified separator has higher capacity in LSBs.The pyrrole-formaldehyde polymer was introduced into MXene lamella by the sol-gel method,and the hydrogel was generated in situ,and then the MXene/pyrrole-formaldehyde polymer aerogel（MPF12）was prepared by freeze-drying.In order to further improve the performance of the material,inhibit the"shuttle effect"of polysulfide（Li PSs）in LSBs and alleviate the volume expansion of electrode material,the aerogel material was further activated to obtain nitrogen-doped MXene based porous material（aMPF12）material.The activation process increases the specific surface area of the material to 222 m² g^-1.Using aMPF12 as the main sulfur agent,the aMPF12@S composite electrode shows high rate performance,even after 200 cycles,the capacity retention rate is still 77.2%.The cycle performance of the battery can be achieved by further modifying the separator.Compared with the PP separator,the battery assembled by the aMPF12/PP separator had an initial specific capacity of 911 m A h g^-1 at 0.5 C and a capacity retention rate of 61.8%after 500cycles.In addition,by calculating the lithium ion diffusion rate,it is proved that the electronic conductivity and lithium ion transport capacity of separator-modified materials are crucial to improve the specific capacity and cycle stability of LSBs.