Structural And Morphology Regulation Of Ti₃C₂T_x MXene For Electrochemical Energy Storage

Transition metal carbide,nitride and carbonitride（MXene）materials present great potentials in the field of electrochemical energy storage because of their large specific surface area,excellent electrical conductivity,controllable interlayer spacing and surface functional groups.However,MXene prepared by fluorine-containing etchants possesses a large number of undesirable functional groups such as-F and-OH,which are detrimental to its electrochemical performance.In addition,the re-stacking issue of MXene nanosheets adversely affects its large-scale application.Herein,in this paper,Ti₃Al C₂ MAX powders were prepared by microwave-assisted molten salt method,and the formation mechanism of Ti₃Al C₂in molten salt was systematically investigated.Then,Ti₃C₂T_x MXene was prepared by selective etching the Ti₃Al C₂ powders.On this basis,the interlayer spacing and surface functional groups of Ti₃C₂T_x MXene were regulated by molten salt method.In addition,the S-doped Ti₃C₂T_x/bacterial cellulose（BC）derived carbon fiber（S-Ti₃C₂T_x/BCCNFs）composite,hollow Ti₃C₂T_x MXene tubes（HTCTs）,and hollow Ti₃C₂T_x tubes/Co/CNTs（HTCTs/Co/CNTs）composite with unique structure were constructed by the composite modification and structural optimization of Ti₃C₂T_xMXene via interfacial assembly and sacrificial template method.Then,the obtained samples with well-designed structure were respectively applied to lithium ion batteries（LIBs）,potassium ion batteries（PIBs）and lithium sulfur batteries（LSBs）.The structure-activity relationship between the structural design and the electrochemical performances were comprehensively studied.The main contents and results are as follows:（1）Ti₃Al C₂ powders were prepared by microwave-assisted molten salt method and the effect of different reactant types,raw materials ratio,and heating temperatures on the purity of Ti₃Al C₂ powders were systematically investigated.The results indicate that Ti₃Al C₂ powders with a high purity of 98.5%could be successfully prepared using Ti H₂/Al/1.8Ti C as raw materials,and Na Cl-KCl as molten salt medium at 1050 ^oC for 30 min.Owing to the heating effect of microwave and the liquid-phase reaction environment provided by molten salt,both the reaction temperature and holding time of this method were much milder than those of most reported synthetic methods.（2）The pristine multilayer Ti₃C₂T_x MXene was thermally treated at different temperatures using Li Cl/KCl eutectic salts as salt medium.The results indicated that the Ti₃C₂T_x obtained at 450℃（LK-Ti₃C₂T_x-450）presented the enlarged interlayer spacing,the decreased content of-F and-OH functional groups and the increased content of-O and-Cl functional groups.DFT calculations showed that O-terminated Ti₃C₂T_x（Ti₃C₂O₂）and Cl-terminated Ti₃C₂T_x（Ti₃C₂Cl₂）MXene possessed higher adsorption ability for Li atom than those of OH-terminated Ti₃C₂T_x（Ti₃C₂（OH）₂）and F-terminated Ti₃C₂T_x（Ti₃C₂F₂）MXene.As the anode for LIBs,LK-Ti₃C₂T_x-450 exhibited discharge specific capacities of 349.4 m Ah g^-1 at0.1 A g^-1 after 100 cycles and 233.3 m Ah g^-1 at 1.0 A g^-1 after 500 cycles,which is far higher than that of pristine Ti₃C₂T_x.（3）S-Ti₃C₂T_x/BCCNFs composite were successfully prepared via the formation of hydrogen bonds between BC and few-layer Ti₃C₂T_x,and combined with the subsequent annealing treatment and melt-diffusion method.The three-dimensional conductive networks of BC derived carbon fiber could promote the uniform dispersion of MXene nanosheets,and then effectively alleviate their re-stacking.In addition,the introduction of-S functional groups and the decrease of the content of-F functional groups were beneficial to providing more electrochemical active sites,and thereby improving the rate performance and cycle stability.As the anode for LIBs,the specific capacity of S-Ti₃C₂T_x/BCCNFs could be remained at 687.9 m Ah g^-1 after 100 cycles at 0.5 A g^-1 and 588.2 m Ah g^-1 after500 cycles at 1.0 A g^-1.（4）Poly（methyl methacrylate）fibers were firstly prepared by electrospinning,and then assembled with Ti₃C₂T_x nanosheets,and finally HTCTs were successfully prepared by heat treatment to remove template.This unique hollow structure effectively alleviated the re-stacking of MXene nanosheets,and reduced the content of-F and-OH functional groups and increased the content of-O functional groups on the surface of Ti₃C₂T_x MXene.DFT calculations indicated that Ti₃C₂O₂MXene possessed higher adsorption ability and lower diffusion barrier for K atom than those of Ti₃C₂（OH）₂ and Ti₃C₂F₂ MXene,which was beneficial to improving the storage capability and the diffusion rate of K ions.Thus,the obtained HTCTs presented outstanding rate performance and excellent cycle stability as the anode for PIBs.The specific capacity could be remained at 199.2 m Ah g^-1 after 300cycles at 0.1 A g^-1 and 122.0 m Ah g^-1 after 350 cycles at 0.2 A g^-1.（5）Based on the preparation of HTCTs,HTCTs/Co/CNTs composite were prepared by heat treatment combined with chemical vapor deposition method using cobalt acetylacetone as catalyst precursor and polystyrene as carbon source.The hierarchically porous structure of HTCTs/Co/CNTs could not only increase the sulfur loading and alleviate the volume expansion of active materials,but also effectively suppress the shuttling effect of polysulfide through the synergistic effect of the physical confinement and chemisorption.Additionally,the combination of Ti₃C₂T_x MXene and Co nanoparticles is conducive to accelerate the catalytic conversion of polysulfide,and thereby improving the utilization and the cycling stability of sulfur.When used as host materials for LSBs,the specific capacity of HTCTs/Co/CNTs/S electrode could be remained at 901.9 m Ah g^-1 after 100 cycles at 0.2 C and 725.9 m Ah g^-1 after 500 cycles at 1.0 C.