Theoretical Study Of Nb₂C/TiNbC MXenes As Anode For Na-ion Batteries

In recent years,with the development of clean energy,the rapid development of power grid energy storage and electric vehicles driven by the policies of various governments,the demand for lithium-ion batteries will show an explosive growth.These social changes will bring huge pressure on the supply of global lithium resources,and it is urgent to find a new type of energy storage device to relieve the pressure on the supply of global lithium resources.Na-ion batteries have attracted widespread attention due to the abundant reserves of Na in the earth’s crust and their similar working principle as that of lithium-ion batteries.However,because the ionic radius of Na is larger than that of Li,none of the cathode materials of Na-ion batteries can be comparable to the graphite in lithium-ion batteries.it became hinder the further development of sodium ion batteries.An ideal anode material for sodium-ion batteries requires large layer spacing,large specific surface area,good conductivity,high ion migration rate and high energy density.The MXenes synthesized in 2011 are a typical kind two-dimensional materials of transition metal carbides/nitrides,which have the properties required for the anode of sodium-ion batteries.Moreover,different functional groups are generated during the synthesis of MXenes,which can adjust the physical and chemical properties of MXenes materials and endow MXenes with abundant surface chemical activities.Therefore,MXenes materials are suitable as anode materials for Na-ion batteries.This dissertation focuses on the geometric structure and electronic properties of Nb₂C and TiNbC MXenes with and without O/F functional groups,as well as their performance as anode materials for Na-ion batteries.A comparative analysis of the synergistic effect between the dual transition metals in double transition metals in double transition metal carbide TiNbC.Finally,it is concluded that single and double transition metal carbides Nb₂C and TiNbC MXenes have different performances as anode materials for Na-ion batteries,and it is expected to give experimental suggestions.In this paper,the following two aspects were systematically studied through the first-principles method based on density functional theory:1.Geometrical structure and electronic properties of single transition metal Nb₂C MXenes with or without O/F functional group modification and their performance as anode material for Na-ion batteries.By studying the adsorption of Na atoms on 2D Nb₂C,Nb₂CF₂,and Nb₂CO₂ MXenes,we found that all three MXenes exhibited metallic properties after Na atom adsorption.The adsorption and diffusion of Na atoms on Nb₂C,Nb₂CF₂ and Nb₂CO₂ were then analyzed.It is concluded that Nb₂CF₂ is not suitable as a negative electrode for Na-ion batteries due to its unstable structure when adsorbing multiple layers of Na atoms.The diffusion barriers of Na atoms on Nb₂C and Nb₂CO₂ are 0.028 eV and 0.227 eV,the theoretical sodium storage capacity and open circuit voltage of Nb₂C and Nb₂CO₂ are 542 mAh/g,0.59 V and 466 mAh/g,0.77 V,respectively.2.Geometric structure and electronic properties of double transition metal TiNbC MXenes with and without O/F functional group modification and their performance as anode materials for Na-ion batteries.By studying the adsorption of Na atoms on 2D TiNbC,TiNbCF₂,and TiNbCO₂ MXenes,we found that all three MXenes exhibited metallic properties after Na atom adsorption.TiNbCF₂ is not suitable as a negative electrode for Na-ion batteries due to its small capacity of only 170 mAh/g.The diffusion barrier,theoretical capacity and open circuit voltage of Na on the surface of TiNbC and TiNbCO₂ monolayers are:TiNbC:0.015 eV,702 mAh/g,0.49 V;TiNbCO₂:0.19 eV,580 mAh/g,0.89 V.Through the research and analysis of the above two systems,the following conclusions are drawn:Nb₂C,Nb₂CO₂,TiNbC,TiNbCO₂ have good electrical conductivity,low diffusion barrier,low open circuit voltage and high theoretical capacity and other properties,so they can be used as ideal anode materials for sodium-ion batteries.Among them,the performance of TiNbC and Nb₂C is higher than that of TiNbCO₂ and Nb₂CO₂,respectively,because there is a layer of negative electronic cloud on the surface of pure MXenes,which can block the interaction between Na and Nb or Ti,so its diffusion is faster.The MXenes with O functional groups have a lower capacity than pure MXenes because their molar mass is higher than that of pure MXenes when the number of adsorbed Na atoms is the same as that of pure MXenes.The performance of double-transition metal MXenes TiNbC or TiNbCO₂ is higher than that of single-transition metal MXenes Nb₂C and Nb₂CO₂,respectively.This is because the synergistic effect between Tiand Nb leads to the enhanced adsorption of Na on the substrate,and the replacement of Nb by Tiwill lead to the redistribution of charges and thus the potential energy surface of the surface will be changed,resulting in the enhanced diffusion of Na on the double transition metals.Similarly,since the molar mass of Tiis smaller than that of Nb,the capacity of TiNbC is higher than that of Nb₂C when adsorbing the same number of Na atoms.Our results reveal that the synergistic effect of double transition metals can tune the adsorption energy and diffusion rate of Na atoms on the substrate as well as the theoretical capacity,which will be helpful for the design of other MXenes as anode materials for Na-ion batteries.