Preparation And Electrochemical Performance Of Molten Salt-MXene

Two-dimensional transition metal carbides(MXene)have shown wide application prospects in the field of energy storage due to their unique twodimensional structure,metal-like conductivity and excellent mechanical properties.However,the etching method based on fluoride limits the large-scale practical application of MXenes.It is urgent to develop an efficient,safe and universal etching method for the preparation of MXene materials.The Lewis acid molten salt etching method can replace the A-layer atoms in the MAX phase by molten salt cations in the high-temperature molten salt environment,which realize the etching of the MAX phase and obtain multi-layer MXenes.This method avoids the use of hydrofluoric acid etching agent and can realize the preparation of a variety of MXenes.Therefore,it has the characteristics of safety,efficiency and universality,which attracted wide attention of researchers once found.However,the molten salt etching method still faces a series of problems to be solved,such as the optimization of preparation conditions,the delamination of multi-layer molten salt-MXene and the construction of highperformance molten salt-MXene-based electrode materials.In view of the above problems,this paper explored the influence of etching conditions on the morphology,structure and lithium storage performance of molten salt-MXene.Based on the electrochemical performance,the etching conditions for preparing multi-layer molten salt-MXene were optimized,and the delamination of multilayer molten salt-MXene was studied.In addition,TiO2/MXene composites with high performance were prepared by in-situ oxidation method based on multi-layer molten salt-MXenes.(1)The effects of etching time,copper removal process,MAX phase type on the morphology,structure and electrochemical performance of molten saltMXene were studied by adjusting the etching process parameters.The structure of multi-layer molten salt-MXene is more open with the increase of etching time,and the specific surface area increases gradually.However,too long or too short etching time will reduce the lithium storage capacity and rate performance of MXene.Compared with the ammonia-ammonium chloride mixed solution,the removal of copper by ammonium persulfate with strong oxidizing property will make the molten salt-MXene surface rich in-O functional groups,thus significantly improving the lithium storage capacity.In summary,the MXene prepared with 8 h etching time and used ammonium persulfate for copper removal maintained a reversible capacity of 201.8 mAh g-1 after 100 cycles at a current density of 50 mA g-1,with excellent cycle stability.When the current density is increased to 1 A g-1,the capacity retention is 67%,showing excellent rate performance.In addition,LiCl,TBAOH and TMAOH were used as intercalation agents to study the delamination effect of multi-layer molten saltMXene.Among them,TMAOH showed the best delamination effect.(2)High-performance TiO2/MXene composites were prepared by in-situ oxidation of multi-layer molten salt-MXene by high temperature calcination in inert atmosphere.As the "matrix" of composites,MXene can significantly improve the conductivity of composites and inhibit the aggregation of TiO2 nanoparticles,which makes TiO2/MXene composites exhibit excellent lithium storage performance.The Ti3C2/TiO2 and Ti2C/TiO2 composites achieved reversible specific capacities of 290 and 416.2 mAh g-1 at the current densities of 50 mA g-1 and 100 mA g-1,respectively,and exhibited excellent long cycle life and rate performance.