Electrochemical Energy Storage Performance Of Mxene （Ti₃C₂） And Its Composite Material

The continuous consumption of traditional energy has led to the development and utilization of intermittent sustainable energy.However,sustainable energy based on wind energy,tidal energy,etc.is volatile and indirect and cannot be used directly.Hence,the key is to develop cheap and efficient energy storage technologies.Both sodium ion batteries and lithium sulfur batteries are expected to be the key technologies for large-scale energy storage in the future,and have attracted extensive attention of researchers.Recently,a new group of two-dimensional（2D）transition metal carbides,nitrides,carbonitrides,MXene,has been widely used as electrode materials in the field of energy storage due to its advantages of good electrical conductivity,unique two-dimensional structure,adjustable layer spacing and low ion diffusion barrier.Studies have shown that MXene electrodes have good rate performance and cycling stability when used as anode material for sodium ion batteries,but the specific capacity is low,so it is of great research significance to increase its capacity.In addition,the hydrophilic functional groups on MXene surface and the transition metal atom exposed to the surface of MXene have strong adsorption to polysulfides,so they can also be used as sulfur host for lithium sulfur batteries.However,the small specific surface area of MXene leads to less sulfur loading,which is not conducive to improving the energy density of lithium sulfur batteries.In the light of the problems,based on the most widely studied Ti₃C₂T_x MXene,some research works were carried out:SnS/Ti₃C₂T_x,CoS/Ti₃C₂T_x and N-PC/Ti₃C₂T_x composites were prepared and used as electrode materials for electrochemical energy storage.The specific research works are as follows:1.SnS/Ti₃C₂T_x composite was prepared by multi-layer Ti₃C₂T_x as conductive substrate by hydrothermal and annealing methodes.XRD,XPS and FESEM results show that SnS particles successfully grow in situ on the surface and interlayer of Ti₃C₂T_x.The introduction of SnS also enlarges the interlayer spacing of Ti₃C₂T_x,which could be conducive to the deintercalation of sodium ions.In addition,it is also demonstrated that the Ti₃C₂T_x substrate can not only provides a conductive network,but also prevents agglomeration of SnS particles.SnS/Ti₃C₂T_x electrode has 412 mA h g^-1 capacity at the current density of 0.1 A g^-1 when used as anode material of sodium ion batteries.2.Single layer of Ti₃C₂T_x with better conductivity and more active sites was obtained by mild etching method.After pre-intercalation of cobalt hydroxide,the CoS/Ti₃C₂T_x composite was obtained by in-situ sulfuration.In the composite,the particle size of cobalt sulfide is only 6 nm and uniformly distributed on the Ti₃C₂T_x layer.The experimental results show that Ti₃C₂T_x acts as a conductive network in the CoS/Ti₃C₂T_x electrode,and it helps to stabilize the structure and reduce the particle size of CoS,while CoS provides most of the capacity and prevents Ti₃C₂T_x from restacking.When used as anode material for sodium ion batteries,it shows a capacity of 508 mA h g^-1 at the current density of 0.1 A g^-1.In addition,it delivers a high reversible capacity of 267 mA h g^-1 after 1700 cycles at the current density of 2 A g^-1with an extreme low capacity decay rate of only～0.0072%per cycle.3.Foam-like ZIF-67 derived nitrogen-doped porous carbon and Ti₃C₂T_x composite was prepared by simple solution mixing and high temperature carbonization.ZIF-67 derived carbon can effectively increase the specific surface area of Ti₃C₂T_x,thereby increasing the sulfur loading of the composite.BET tests show that the specific surface area of the composite is about 8 times that of Ti₃C₂T_x,which undoubtedly proves that the composite with carbon material can greatly increase its specific surface area.TGA test shows that the sulfur loading has reached up to 80%.The adsorption experiments of lithium polysulfide confirmed that the composite has a strong chemical adsorption for lithium polysulfide,which can suppress the shuttle effect of lithium polysulfide and prolong the life of lithium sulfur batteries.The electrode shows the capacity of 386 mA h g^-1 after 1500 cycles at the current density of 1 C,and the decay rate is only 0.03%per cycle.Owing to the unique advantages of MXene-based composites when used as anode materials for sodium ion batteries or as sulfur hosts for lithium sulfur batteries,three kinds of MXene composite materials are synthesized and applied to the field of energy storage.This work will provide some insights to the design of MXene-based electrode for sodium ion batteries and lithium sulfur batteries or other energy storage devices.