Density Functional Theory Studies On The Influence Of Surface Functions And Early Transition Metal Doping On The Lithium Storage Performance Of Ti₃C₂ Mxene Materials

With the development of new energy and portable devices,enterprises and consumers urgently need a safe,efficient,and lightweight energy storage device.As the basis of common energy storage devices such as secondary batteries and supercapacitors,electrode materials are the focus of current researches.Among a wide variety of electrode materials,MXenes,as a new type of two-dimensional nano-material,with high specific surface,good conductivity,excellent mechanical properties,and replaceable structure is one of the most potential alternative electrode materials at present.However,the performance of MXene material is highly dependent on its surface structure.Due to the HF etching synthesis process,its surface chemical environment is complex,and its physical and chemical properties and energy storage mechanism need to be further explored.In order to further understand the surface structure of MXene,explore the storage mechanism for lithium,and demonstrate the influence of functions modification and early transition metal doping on the properties of MXene,this paper uses density functional theory to construct a series of models with different functions and doping early transition metals,systematically studied the effect of surface structure on the energy storage performance of Ti₃C₂MXene from the aspects of the stability of adsorption,the barrier of migration and the theoretical lithium storage capacity.What's more,conductor-like screening model was introduced to explore the effect of solvent on the adsorption and migration of lithium atoms on the surface of MXene.This paper provided a theoretical basis and reference for the experimental research and modification of MXene materials.Some substantive research results have been obtained as follows:?1?Ti₃C₂ and 6 kinds of uniform Ti₃C₂T₂?T=C,N,O,F,S or OH?models were constructed and systematically studied those materials in terms of lithium atom adsorption,migration,and lithium storage capacity.Among those materials,Ti₃C₂O₂ and Ti₃C₂S₂ have low migration barriers and high theorical capacities of 471 and 413 m Ah g^-1,respectively,which make them the most potential electrode material candidates of the Ti₃C₂T₂.?2?Based on Ti₃C₂O₂ and Ti₃C₂S₂,a series of bifunctionalize Ti₃C₂ MXenes with different coverages of functions were constructed and systematically studied from the perspective of lithium atom adsorption,migration and lithium storage capacity.When the functions are evenly distributed and O:S=2:1,the barrier of lithium atoms on the surface of the material is relatively lower.As the increases of adsorbed lithium atoms on both sides of the material,S-functions will intercalate into the lithium atomic layers to form an adsorption center,which enhance adsorption stability.?3?Using conductor-like screening model,the effect of solvent on the adsorption and migration of lithium atoms on the surface of Ti₃C₂T₂ and O,S bifunctionalized materials was systematically studied.Ti₃C₂ MXene materials,except Ti₃C₂?OH?₂,usually have better performance in solvents with a dielectric constant higher than 20.?4?Based on Ti₃C₂,Ti₃C₂O₂ and Ti₃C₂F₂,three most common MXene materials in the experiment,the effect on lithium adsorption of early transition metals?Sc,V,Cr,Mn,Y,Zr,Nb and Mo?doping were systematically studied in vacuum and in different solvents.From the perspective of improving the adsorption stability of lithium atoms,for Ti₃C₂ and Ti₃C₂O₂,the best doping element is Cr.And for Ti₃C₂F₂,the best doping element is Mn.Among those materials,the doping effect of Mn on Ti₃C₂F₂ is most obvious.The adsorption energy of lithium atoms near Mn-substituted atoms is 1.61 e V,which is almost as twice as the adsorption energy of lithium atoms on the surface of pristine Ti₃C₂F₂?0.82 eV?.