Study On The Preparation And Electrochemical Properties Of MXene-based Self-supported Electrodes For Sodium-ion Storage

With the development of the modern society and technology,people have more and more diversified requirements for energy storage devices.As the next generation of electrochemical energy storage device that may be popularized on a large scale,sodium-ion batteries（SIBs）have a working principle similar to lithium-ion batteries（LIBs）.In addition to the cost-effectiveness,SIBs can absorb the most cutting-edge achievements of LIBs research in recent years.However,due to the large radius of sodium ion,it can not reversibly insert into the current commercial negative electrode materials like graphite.It is thus required to develop novel negative electrode materials for SIBs.In addition,flexible or portable electronic devices are developing rapidly,which puts forward higher requirements for batteries.For one thing,the current method of coating active materials mixed with binder and conductive agent on the electrode complicates the whole processes and increases the cost.On the other hand,a large number of substances without capacity contribution but occupying volume space are introduced,which limits the space utilization of the battery.At the same time,when the device is bent,the powder coated electrode is prone to powder desorption,resulting in battery failure.As the most studied member of transition metal carbide and/or nitride（known as MXenes）,Ti₃C₂T_x has good mechanical properties,conductivity and adjustable layer spacing.Ti₃C₂T_x MXene-based self-supported electrodes are promising for SIBs.This thesis focuses on the preparation of self-supported anode materials for SIBs by different film-forming methods.The electrochemical performances of the electrode have been further enhanced by adjusting the structure of Ti₃C₂T_x and introducing other high-capacity active materials.Meanwhile,several means of physical and chemical characterization are used to reveal the structure-performance relationship of Ti₃C₂T_x-based films.（1）Through the in-situ sulfur template method,a sulfur doped mesoporous Ti₃C₂T_x film is designed in this paper,which realizes high volumetric capacity and good mechanical properties.Specifically,after sulfur nanoparticles are deposited in-situ on Ti₃C₂T_x nanosheets through the disproportionation reaction of sodium persulfide,they are filtrated to form a film through the vacuum-assisted filtration,and then calcined to remove sulfur nanoparticles,leaving a mesoporous structure.This unique mesoporous structure can not only effectively solve the stacking problem of Ti₃C₂T_x,but also provide a large number of active sites and shorten the ion transportation distance.In addition,it can also maintain the high density of Ti₃C₂T_x film,which is vital to the realization of high volumetric capacity.Secondly,the introduction of sulfur doping after sulfur removal can improve the conductivity and wettability of Ti₃C₂T_x film to electrolyte,and further enhance the electrochemical properties of Ti₃C₂T_x film.The results show that the sulfur doped mesoporous Ti₃C₂T_x film,as a self-supported anode material for sodium-ion storage,achieves an ultra-high volumetric capacity of 625.6 mAh cm^-3 at a current density of 0.1 A g^-1 and is stable after 5000 cycles without capacity attenuation at a current density of 1 A g^-1.（2）SnS has a stable layered structure and high capacity,so it is an important sodium-ion storage anode material.In this paper,a simple and efficient method is designed to prepare SnS/Ti₃C₂T_x three-dimensional porous composite films.In this method,the composite porous powder is first treated by rapid freeze-drying.After that,the film is formed by cold pressing,and then calcined to obtain the final SnS/Ti₃C₂T_x composited film.Even when the electrode thickness is 216.4 μm,the specific capacity of 335.4 mAh g^-1 can be achieved.The three-dimensional porous structure of Ti₃C₂T_x constructed by rapid freeze-drying can also well limit the volume expansion of SnS and achieve good cycle stability.This work is expected to be the inspiration for the hybridization of high-capacity materials and Ti₃C₂T_x to form self-supported film electrodes for SIBs.