Preparation Of Nanocomposite Materials Based On MXene And Their Applications In Electrochemical Energy Storage

A new two-dimensional material called MXene has attracted wide attention since professor Gogotsi from Drexel University successfully etched Ti₃AlC₂ with HF to obtain Ti₃C₂T_x?T=-OH,-F,-O,etc.?in 2011.MXene possesses excellent conductivity owing to its band structure similar to metal.MXene has a large number of hydrophilic surface functional groups such as-OH,-F,-O.The high density of MXene makes it excellent volumetric performance.Because of these advantages,MXene has been used in many fields such as lithium ion battery,micro-supercapacitor and so on.However,the self-stacking and low gravimetric specific capacity of MXene limits its further applications.The main strategy to solve this problem is that combining MXene with other materials.The key to compounding is the selection of components and the design of structures.Different strategies will be adopted for different application fields.When used as anode materials for lithium ion battery,MXene possesses excellent volumetric performance,but its low gravimetric specific capacity limits its further applications.Therefore,researchers combine MXene with other materials with high theoretical gravimetric specific capacity such as transition metal oxides.However,the main issue of this strategy is that most of the studies adopt physical composite methods,such as mechanical mixing,which results in inhomogeneous composite structure.To overcome this issue,we prepared Fe₂O₃@Ti₃C₂ hybrids by in situ chemical method.On one hand,Fe₂O₃ can prevent the restacking of Ti₃C₂ flakes,which is beneficial to fully utilize the advantages of Ti₃C₂.On the other hand,Ti₃C₂contributes excellent conductivity to the composite material and can accommodate volume change of Fe₂O₃ during lithiation/delithiation process.When used as electrode materials for supercapacitor,especially for micro-supercapacitor,MXene plays an important role due to its excellent volumetric performance.MXene is widely used in the research of electrode materials for micro-supercapacitors with high area capacitance,but the self-stacking of MXene limits its further applications.So,researchers use etched MXene nanoparticles as electrode materials directly to solve the self-stacking of MXene flakes fundamentally.However,the main issue of this strategy is that the dense structure of MXene nanoparticles leads to inadequate contact between MXene nanoparticles and electrolyte,which results in the blockage of ion diffusion channels.Moreover,surface defects result in the high contact resistance between MXene nanoparticles.To overcome this issue,we combine the etched MXene nanoparticles with graphene.On one hand,graphene provides excellent electrical conductivity,and constructs three-dimensional porous structure to provide diffusion channels for ions.On the other hand,MXene can contribute high area capacitance,and prevent graphene from self-stacking in some degree.The detailed research contents and conclusions are as follows:?1?Preparation of MXene/Fe₂O₃ nanocomposites and their application in lithium ion battery.Fe₂O₃ is chosen to be combined with MXene due to its prominent advantages of high theoretical gravimetric specific capacity?1007 mAh/g?,inexpensiveness,abundant reserves,and nontoxic nature.We prepared Fe₂O₃@Ti₃C₂hybrids by liquid-phase synthesis and thermal annealing process.The structures,properties and electrochemical performance of hybrids were analyzed.As seen from SEM and TEM images,Fe₂O₃ nanoparticles are uniformly distributed on Ti₃C₂nanosheets.This new Ti₃C₂ MXene-based hybrid shows excellent electrochemical performance as an anode material for lithium ion battery.A high reversible capacity of 264.3 mAh/g at 1 A/g after 1000 cycles was achieved for Fe₂O₃@Ti₃C₂-2:3 hybrid.In addition,Fe₂O₃@Ti₃C₂-2:3 hybrid displays good rate capability with capacity retention of 29.6%at high current density of 4.0 A/g.?2?Preparation of MXene/graphene nanocomposites and their application in micro-supercapacitor.The etched MXene was directly combined with graphene to obtain Gr-MXene composite film through ultrasonic mixing and vacuum filtration.The structures,properties and electrochemical performance of composite film were analyzed.As seen from SEM and TEM images,MXene nanoparticles are covered and connected by graphene.The Gr-MXene composite film electrode shows excellent electrochemical performance as electrode for micro-supercapacitor.A high capacitance retention of 91.3%at high current density of 1.0mA/cm² after 10000cycles was achieved for Gr-MXene composite film electrode.After bending,the CV curves and GCD curves of Gr-MXene composite film electrode all remain unchanged.