二维六方Mo_(2)B_(2)作为金属离子电池负极材料的第一性原理研究

First Principles Study of Two-Dimensional h-Mo_(2)B_(2)as a Negative Electrode for Metal-Ion Batteries

可充电金属离子电池(RMBs)迫切需要开发新型高比容量的负极材料。采用第一性原理计算方法研究了六方h-Mo_(2)B_(2)MBene作为RMBs(Li、Na、Mg和K离子)负极材料的潜力。计算结果表明,h-Mo_(2)B_(2)MBene结构稳定且具有良好的导电性。作为RMBs负极材料组装成Li离子电池、Na离子电池、K离子电池和Mg离子电池时,其理论比容量分别为:753、314、125 mA·h·g^(–1)和1506 mA·h·g^(–1)。与传统石墨负极相比,h-Mo_(2)B_(2)在锂离子电池中具有更高的比容量,而Mg离子在h-Mo_(2)B_(2)上的超大理论容量得益于Mg离子可以携带更多的电荷。Li、Na、Mg和K离子在h-Mo_(2)B_(2)上具有较低的扩散势垒,分别为39、10、37 meV和7 meV。Li、Na、Mg和K离子在h-Mo_(2)B_(2)中的平均开路电压分别为:0.36、0.47、0.63 V和0.63 V。这些优异的性能表明,h-Mo_(2)B_(2)MBene可以作为锂离子、钠离子以及镁离子电池等领域中一种非常有前景的负极材料。

Introduction With the continuous growth in energy storage demands for portable electronic devices,electric vehicles,and grid energy storage,rechargeable metal-ion batteries have found extensive applications in energy supply and storage due to their advantages of low self-discharge,high energy density,and environmental friendliness.One of the essential components of metal-ion batteries is the negative electrode material,and its physical and chemical properties are crucial for battery performance.However,in practical applications,there is still a shortage of high-performance negative electrode materials for metal-ion batteries.Traditional three-dimensional electrode materials suffer from limited storage capacity and less than ideal charge-discharge rates,primarily because of the limited number of lattice vacancies in their structure.This limitation hinders their ability to meet market demands,particularly in scenarios where faster charge-discharge rates are required,such as electric vehicles and grid energy storage.In contrast,two-dimensional materials offer advantages such as a larger specific surface area and enhanced metal ion diffusion,making them suitable for energy storage in batteries.Among two-dimensional materials,the emerging class of two-dimensional transition metal borides(MBenes)exhibits excellent electrical conductivity,structural stability,and high specific capacity.As a result,an increasing amount of research work is considering them as electrode materials for energy storage systems.Compared to traditional experimental research methods,first-principles computational techniques can better assist in designing novel high-performance electrode materials at the atomic and electronic scale.In this paper,we aim to explore the potential of h-Mo_(2)B_(2) MBene as a negative electrode material for metal-ion batteries using first-principles calculation methods.We systematically investigate its structural stability,electronic structure,and electrochemical properties.The studies suggest that h-Mo_(2)B_(2) holds promise as a prospective negative electrode material for application in metal-ion batteries.Methods In this paper the calculations are based on Density Functional Theory(DFT)first-principles methods,implemented using the Vienna Ab initio Simulation Package(VASP).The Projector Augmented Wave(PAW)pseudopotential approach is utilized,with a plane wave cutoff energy of 500 eV.The Perdew-Burke-Ernzerhof(PBE)generalized gradient approximation(GGA)is employed for the exchange-correlation functional.To account for the interaction between metal cations and 2D materials,van der Waals interactions are considered in the calculations.During the geometric structure optimization,energy and force convergence criteria are set to 10^(–5)eV/atom and 0.01 eV/A,respectively.The K-point grids used for h-Mo_(2)B_(2)unit cell and 2×2×1 supercell calculations are20×20×1 and 5×5×1,respectively.A vacuum layer with a thickness of 20A is included to eliminate the spurious interaction.Phonon spectra calculations are performed using density functional perturbation theory.Differential charge calculations are employed to study charge redistribution and transfer between adsorbed metal atoms and 2D materials.Bader charge analysis is utilized to assess the amount of charge transfer between the metal ions and the 2D material.The Climbing Image Nudged Elastic Band(CI-NEB)method is used to calculate the migration energy barriers and migration pathways of metal ions on h-Mo_(2)B_(2).Results and discussion The 2D h-Mo_(2)B_(2)studied in this paper belongs to the P6/mmm space group within the hexagonal crystal system.It comprises three atomic layers stacked in a Mo-B-Mo sequence,with hexagonal B atomic layers situated between the upper and lower Mo atomic planes.To evaluate the dynamical stability of h-Mo_(2)B_(2),phonon spectrum calculations were conducted,and no imaginary frequencies were observed throughout the entire Brillouin zone.This indicates that h-Mo_(2)B_(2)exhibits dynamical stability.The band structure of h-Mo_(2)B_(2)reveals numerous bands crossing the Fermi level,confirming its metallic nature.This exceptional electrical conductivity of h-Mo_(2)B_(2)can significantly enhance the rate performance of electrodes.Adsorption energy is a fundamental criterion for assessing whether a material can be utilized as a negative electrode.The adsorption energies of Li,Na,Mg,and K on the h-Mo_(2)B_(2)surface were calculated,and all exhibited negative values,indicating effective adsorption of all metal atoms on a monolayer of h-Mo_(2)B_(2).Rapid charge-discharge rates are crucial for secondary batteries,and the migration energy barrier of metal ions is a key factor determining the charge-discharge rate.The migration energy barriers for the four metal atoms on the h-Mo_(2)B_(2)surface are ranked as K(7 meV)<Na(10 meV)<Mg(37 meV)<Li(39 meV).The extremely low migration energy barriers suggest that Li,Na,Mg,and K can easily diffuse on h-Mo_(2)B_(2),making it a promising high-rate electrode material.As a negative electrode material for lithium-ion batteries,h-Mo_(2)B_(2) exhibits a theoretical specific capacity of 735 mA.hg,significantly surpassing graphite's 372 mAhrg.For sodium ion,h-Mo_(2)B_(2) boasts a theoretical specific capacity of 314 mAhrg,while for magnesium ion,due to multilayer adsorption and divalent ions carrying more charge,it reaches I 506 mAhg,far exceeding the capacity for Li and Na.Theoretical calculations also indicate that h-Mo_(2)B_(2) possesses low average open-circuit voltages of 0.36 V(Li),0.47 V(Na),and 0.63 V(Mg),all within the0-1 V range.This lowers the likelihood of dendrite formation in the negative electrode,thereby enhancing the safety and stability of metal-ion batteries.This paper,from a theoretical standpoint,elucidates and confirms the potential of h-Mo_(2)B_(2) as a negative electrode material for rechargeable metal-ion batteries.It paves the way and provides valuable insights for the design of high-performance electrode materials.Conclusions Based on first principles calculations,we systematically investigated the physical properties and electrochemical performance of 2D h-Mo_(2)B_(2) MBene.It was found that h-Mo_(2)B_(2) is dynamically stable and exhibits metallic conductivity.The diffusion barriers for metal atoms(M=Li,Na,Mg,K)on h-Mo:B2 are remarkably low,all being less than 0.04 eV,indicating that h-Mo_(2)B_(2) can facilitate rapid charge and discharge when employed as a negative electrode material in metal-ion batteries.The theoretical speific capacities of h-Mo_(2)B_(2) for Li,Na,and Mg are 753,314,and 1506 mA·h·g^(-1),respectively.Furthermore,the average open-circuit voltages(OCV)for h-Mo_(2)B_(2) with Li,Na,and Mg are calculated to be 0.36,0.47,and 0.63 V,respectively.All these findings collectively support that 2D h-Mo_(2)B_(2) MBene can serve as a high-performance negative electrode material for lithium,sodium,and magnesium ion batteries.