Preparation Of MXene-Based Composites And Their Performance In Lithium-Ion Batteries As Anodes

With the rapid development of portable electronic equipment,new energy vehicles,large-scale energy storage power stations,and other industries,the demand for energy density and the long cycle life of lithium-ion batteries is increasing.High-capacity electrode materials are vital for developing lithium-ion batteries（LIBs）.Among many negative electrode materials,MXene,a new two-dimensional layered material,has shown great application potential in lithium-ion batteries with its unique advantages,such as high conductivity,large specific surface area,abundant surface functional groups,and adjustable layer structure.However,MXene material is easily oxidized,and the intermediate structure is easy to accumulate in the synthesis process,which reduces the surface area,hinders ion transport,and dramatically reduces its capacity,thus inhibiting its practical application in LIBs.Expanding layer spacing and designing composite materials are effective strategies to solve the above problems.On the one hand,expanding MXene layer spacing can increase the energy storage space between layers and shorten the ion diffusion path,to improve its lithium storage performance.On the other hand,the electrode prepared by a single MXene has congenital disabilities and cannot meet specific practical application requirements.Therefore,the rational design of MXene nanocomposites can fully use the structural advantages of MXene and improve its volume capacity.Based on this,this paper synthesizes Ti₃C₂MXene matrix composites with different structures,aiming to solve its application difficulties and improve its lithium performance by expanding the layer spacing and utilizing the synergic advantages of the composites.The research content of this paper includes the following parts:（1）Preparation of urchin-like Ti₃C₂/CNT composites and study on electrochemical performance.In this chapter,a kind of urchin-like Ti₃C₂/CNT composite material is designed and prepared to suppress the self-restacking of the two-dimensional layers and meet the requirements of efficient and fast-charging LIBs materials.The three-dimensional hollow Ti₃C₂can be obtained by mixing two-dimensional Ti₃C₂T_xand melamine-formaldehyde（MF）microspheres evenly based on the electrostatic self-assembly and templating mechanisms and further combining with high-temperature pyrolysis to remove the template,and three-dimensional hollow Ti₃C₂can be obtained.Bamboo carbon nanotubes and three-dimensional hollow Ti₃C₂composite（Ti₃C₂/CNT）were prepared by in situ metal-catalyzed tip-growth mechanism to construct a three-dimensional conductive network.The closely connected carbon nanotubes were uniformly grown on the three-dimensional hollow surface of Ti₃C₂,forming a sea urchin-like morphology.This kind of three-dimensional urchin-like network can not only significantly improve the conductivity of composite materials and the transmission dynamics but also make the electrode materials have good mechanical stability and better withstand the problem of material pulverization in the electrochemical process.The electrochemical performance of the Ti₃C₂/CNT composite is satisfactory（the specific capacity of 3D Ti₃C₂T_x/CNT is 286.2 m A h/g in 2000 cycles at 1 A/g current density）.（2）Preparation of honeycomb Ti₃C₂/Ag composites and lithium electric properties.To further expand the application of three-dimensional hollow Ti₃C₂in a lithium-ion battery field,the honeycomb Ti₃C₂/Ag composite was prepared by mixing three-dimensional hollow Ti₃C₂with Ag NO₃solution by in-situ redox mechanism（Ag⁺reduced by Ti on MXene surface）.In the design of the composite material,the metal Ag nanoparticles as spacers can expand the layer spacing and inhibit the stacking collapse of active materials.And metals can significantly improve their electrical conductivity,increasing the rate at which electrons travel.Therefore,compared with the conventional two-dimensional Ti₃C₂multilayer structure,the three-dimensional honeycomb Ti₃C₂/Ag composite shows a significantly improved reversible capacity and good cyclic stability（the reversible capacity can be maintained at 310 m A h/g after 2000 cycles at the current density of 3 A/g）.This work is expected to provide a new research idea for the design of cathode materials that can be used in efficient and fast-charging LIBs.（3）Preparation of three-dimensional porous Ti₃C₂/SnO composites and study on lithium electric properties.In order to improve the inherent defects of the electrode material prepared by single MXene,we designed a composite material;SnO nanoparticles were grown on a three-dimensional honeycomb Ti₃C₂by one-step hydrothermal synthesis.In the composite structure,SnO nanoparticles were embedded into the 3D Ti₃C₂MXene frame as a filler,which could inhibit the stacking of MXene itself.At the same time,Ti₃C₂MXene can be used as a good conductive substrate to effectively reduce the volume expansion of SnO nanoparticles during charging and discharging.This synergistic and complementary effect of the composites can greatly improve the lithium electric performance of the active material（the reversible capacity can be maintained at 497 m A h/g after 100 cycles at 0.1 A/g current density）.