Density Functional Theory Study Of Two-Dimensional AB₂-Type Lithium-Ion Electrode Materials

With the rapid development of science and technology,the demand for energy grows constantly.The conventional non-renewable energy sources such as traditional oil and natural gas can no longer meet the new needs.Further,the greenhouse gases such as carbon dioxide produced during the combustion process of traditional fossil energy sources have also brought incalculable damage to the environment.Therefore,it is especially urgent to find clean and reliable renewable energy sources.Electricity is of great interest to researchers as an efficient renewable energy source.For example,the batteries for electric vehicles and portable electronics are mainly lithium-ion batteries（LIBs）,while the other metal-ion batteries have also received extensive attention as alternatives to LIBs.The current productions of the applicable metal ion batteries mainly include LIBs/NIBs（Li ion batteries/Na ion batteries）.The common problems of these batteries are lower energy density,lower power,lower recyclability and lower safety,which is attributable to the insufficient performances of the electrode materials that make up the metal ion batteries.It is generally believed that two-dimensional materials would have high potential as electrode materials for metal ion batteries due to their large specific surface area.Therefore,this thesis mainly focuses on the structures and electrode properties of some AB₂ type two-dimensional electrode materials,including the following aspects.Based on the literatures available,it is concluded that new two-dimensional materials contain the second to sixth main group elements with hexagonal graphene-like structures,which may provide the primary choice of the AB₂ type two-dimensional electrode materials for metal ion batteries.In the design for the AB₂ type two-dimensional materials of this work A and B are assigned as the IIA and VA main group elements,i.e.,beryllium in IIA and nitrogen/phosphorus in VA groups,so as to form two two-dimensional materials,1H-BeN₂ and 1H-BeP₂,with the 1:2 stoichiometric ratio for graphene-like 1H space group.Their relevant geometric structures as well as electronic properties and adsorption behaviors of metal ions for both AB₂ type two-dimensional materials are calculated by using density functional theory（DFT）calculations.The feasibility of the two materials as two-dimensional electrode materials for metal ion batteries is also evaluated.The results indicate that both AB₂ type two-dimensional materials have the potential to act as the electrode materials for metal ion batteries based on the following points.（1）Both AB₂-type materials exhibit good metallicity and stability.（2）Both AB₂-type materials show the extraordinarily high theoretical capacities of 1448m Ah·g^-1/1448 m Ah·g^-1/724 m Ah·g^-1 of Li/Na/K for 1H-BeN₂,and 1510 m Ah·g^-1/1510 m Ah·g^-1/377.5 m Ah·g^-1 of Li/Na/K for 1H-BeP₂,respectively.（3）Both AB₂-type materials exhibit negative adsorption energies for different concentrations of metal ions（Li/Na/K）.（4）Metal ions exhibit low isotropic diffusion energy barriers on the surface of both AB₂-type materials.In summary,the two AB₂ materials adopted as electrode materials for metal ion batteries demonstrate excellent electrical conductivity,high theoretical capacity and fast charge/discharge rates,further reflecting their potential as metal ion batteries.