Study Of Surface-Modified Titanium-Based MXenes As Anode Of Alkali Metal Ion Batteries

As a rechargeable battery,alkali metal ion battery is one of the most important energy storage devices at present.Electrode material is one of the important factors affecting the electrochemical properties of alkali metal ion batteries（AMIBs）.Therefore,the search for electrode materials with high specific capacities and stable structures is the key factor to promote the development of AMIBs.MXenes have been widely studied due to the excellent specific surface area,high conductivity and component tunability,and have the enormously developmental potential in the field of electrode materials for high-efficiency AMIBs.However,the limited storage capacity and the dramatic lattice expansion caused by ion diffusion limit the application of MXenes as an electrode material.In this paper,using first-principles calculations,the results revealed that Ti₃C₂and Ti₂C MXenes are promising anode materials for AMIBs such as lithium-ion batteries or sodium-ion batteries.The main research contents are as follows:Firstly,a representative Ti₃C₂MXene material was selected.The results revealed that Ti₃C₂T₂（T=F,Cl or Br）MXenes with surface halogenation exhibit the excellent metallic conductivity and structural stability.Monolayer Ti₃C₂Cl₂can exhibit the larger elastic modulus（321.70 N/m and 329.43 N/m）,lower lithium ion diffusion barrier（0.27 e V）,lower open circuit voltage（0.54 e V）and higher storage capacity（674.21 m A·h/g）.Moreover,surface chlorination yields the expanded interlayer spacing in double-layer Ti₃C₂Cl₂,which can improve Li-ion accessibility and fast charge-discharge rate.Secondly,the configurations of Ti₂CT₂（T=F,Cl,Br,O,S or Se）MXenes were constructed and the lithium storage performance was studied.The results revealed that Ti₂CT₂MXenes can exhibit the excellent metallic conductivity and structural stability.Compared with other monolayer Ti₂CT₂,monolayer Ti₂CS₂can display lower lithium ion diffusion barrier（0.25 e V）,larger elastic modulus（362.72 N/m and 362.37 N/m）,lower open circuit voltage（0.90 V）and higher storage capacity（935.52 m A·h/g）.Furthermore,surface sulfuration expands the layer spacing of Ti₂CT₂MXenes,effectively improving the accessibility of Li-ion in Ti₂CS₂.Finally,the sodium storage performance of Ti₂CT₂MXenes as electrode materials was studied.Compared with other monolayer Ti₂CT₂（T=F,Cl,Br,O and Se）,monolayer Ti₂CS₂displays lower sodium ion diffusion barrier（0.15 e V）,lower open circuit voltage（0.36 V）and higher theoretical sodium ion storage capacity（935.52 m A·h/g）.Thus,Ti₂CS₂has the excellent rate performance as electrode material for sodium-ion batteries.Moreover,the migration of sodium ions in double-layer Ti₂CS₂was studied.The results showed that Ti₂CS₂MXene has the large interlayer spacing,thereby improving the accessibility of sodium ions and the rapid charge and discharge rate.This paper can confirm that surface modifications can effectively tune the properties of Ti₃C₂and Ti₂C MXenes,provide novel design ideas for expanding the layer spacing and improving their electrochemical properties.The results can offer reliable reference data for the application of MXenes materials as electrode materials in alkali metal ion battery,and provide importantly theoretical guidance for further experimental research.