The Structure And Electrochemical Properties Of Two-Dimensional Transition Metal Nitrides And Borides

In the 21st century,energy technology development has been unable to meet the increasing demand for clean,efficient,and portable energy equipment.However,sodium(Na)ion batteries are widely concerned due to their low cost and good safety.Due to its unique layered structure and high surface area,two-dimensional(2D)materials make it possible to realize high ion conductivity and large-capacity metal-ion batteries.In this paper,the structural,physical,electronic,and adsorption properties of new 2D materials have been studied by first-principles based on density functional theory.Firstly,we designed allotropes composed of zirconium and nitrogen atoms in different proportions and studied their basic structural characteristics and stability.It was found that Zr₂N(MXene)was the most stable,and the adsorption characteristics of connected groups(-O,-OH,-F)at different positions were further studied.Secondly,the adsorption mechanism of Na-ions was studied qualitatively.By studying the adsorption process of different concentrations of Na-ions on Zr₂N,Zr₂NO₂,Zr₂N(OH)₂,and Zr₂NF₂,it is found that the average adsorption energy of Zr₂NF₂ is too low to be suitable as an electrode material.Finally,the diffusion barriers of Zr₂N,Zr₂NO₂,and Zr₂N(OH)₂ obtained from CI-NEB are 0.018 eV,0.08 eV,and 0.13 eV,respectively.In addition,according to the formula,the theoretical capacities are 545.9 mAh/g,469.4 mAh/g,and 413.1 mAh/g,respectively.Interestingly,Zr₂N without functional group adsorption has a higher theoretical capacity(545.9 mAh/g)and a very low diffusion barrier(0.018 eV),which has better performance than materials with functional groups.In addition,we further calculated the performance of V₂B₂(MBene)as the anode material for Na-ion batteries.Firstly,the dynamic stability,thermodynamic stability,and conductivity of V₂B₂ are studied.Secondly,the most stable Na ion adsorption sites were determined by the combination of adsorption energy and Bader charge,and the charge distribution of these different adsorption sites was visualized by the charge density difference.Secondly,the diffusion path of Na-ions and the corresponding diffusion barrier were studied by CI-NEB,and it was found that V₂B₂ has a very low Na ion diffusion barrier(0.011 eV),which indicates that Na-ions have an extremely high ion diffusion rate on V₂B₂ surface.Finally,the process of adsorption of a large number of Na-ions on the surface of V₂B₂ was studied.According to the adsorption energy of different concentrations of Na-ions,the average open-circuit voltage and the theoretical capacity of Na-ions were calculated.The results show that V₂B₂ has an average open-circuit voltage of 0.65 V and a high theoretical capacity of 814 mAh/g,which indicates that two-dimensional V₂B₂ has great potential as an anode material for Na-ion batteries.