Theoretical Study On The Application Of MXene-based Two-dimensional Heterostructures As Anode Materials For Li-ion Batteries

Lithium-ion batteries(LIBs)play an important role in mobile power,new energy vehicles and other fields due to their high energy density,high working voltage,and no memory effect.Nevertheless,lithium-ion batteries also have many problems,such as low capacity and poor safety.In order to cope with these problems,the development of anode materials with large capacity,low barrier for Li⁺diffusion,and stable structure has become the biggest challenge for lithium-ion batteries research.Nowadays,one of the development directions of lithium battery anode materials is 2D materials(MXene,boronene,etc.).MXene is a new type of metal carbide or metal nitride with a two-dimensional layered structure.It shows advantages(high conductivity and low diffusion barrier)in the application of anode materials for lithium-ion batteries.However,MXene faces some difficulties.Low capacity and large volume changes resulting from functionalization and overlap.In order to solve these problems,an effective strategy is to form a heterostructure with two different two-dimensional materials,which can complement each other or alternatively enhance the stability of the material.In addition,some studies have shown that heterostructure can produce new properties.Research has shown that the heterostructure formed by MXene and other two-dimensional materials has enhanced Li⁺storage performance.Nevertheless,its enhancement mechanism has not yet been determined and further investigation is needed.This research explores the effects of different functional groups,doping,and heterostructures on the performance of Li⁺adsorption and diffusion through theoretical simulations,and analyzes the reasons for their effects.This will enrich the study of two-dimensional materials as the anode materials of lithium-ion batteries,and provide a certain theoretical basis for expanding the application of this type of system in batteries,supercapacitors,and biosensors.First,we used first-principles density functional theory to study the geometric and electronic properties of the heterostructure composed of Ti₂CT₂(T=O,F,OH)and N-doped graphene(NDG),and adsorption and diffusion of Li⁺in heterostructures.The results show that the direction and size of electron in N-doped graphene vary with the functional group of MXene and the type of N doping,which further affects the adsorption and diffusion of Li⁺.The NDG/MXene heterostructure is very stable,which can effectively avoid the deformation of the electrode structure during the Li⁺adsorption/desorption process,greatly improve the cycle stability,and significantly enhance the conductivity of the electrode and the adsorption and diffusion performance of Li⁺.It will be of great significance to the improvement of lithium battery performance.Besides,we have studied three different types of MXene(Sc₂CO₂,Ti₂CO₂,V₂CO₂)and two types of B-doped graphene(BDG),built six different heterostructures,and studied their effects on Li⁺adsorption and diffusion.Results show that the influence of heterostructures on Li⁺diffusion is limited to the heterostructure internal area,making the diffusion energy barrier slightly larger.The B doping in graphene can lower the diffusion barrier of Li⁺in neighboring position.Comparing these several heterostructures,the heterostructure composed of Sc₂CO₂ and BDG has the highest theoretical capacitance,indicating that it can be used as an efficient anode electrode material.Finally,we studied the twenty four heterostructures builded by three different types of MXene(Sc₂CO₂,Ti₂CO₂,V₂CO₂)and graphene with different numbers of epoxy groups.We studied the effect of epoxy functional group coverage on the geometric structure and electronic properties of the heterostructure,as well as the effect of the heterostructure on the adsorption performance of Li⁺.The research results show that the coverage of epoxy functional group and its position affect the geometry and electronic structure of the heterostructure,and the adsorption performance of the heterostructure on Li⁺increases with the coverage of epoxy functional group.