First-principles Study Of Anode Materials For Alkali Metal Ion Batteries Based On 2D BSi And Si₃N

The successful development of lithium-ion batteries has greatly contributed to the development of people’s lives and technology,which leads to the demand increasing of the higher performance of ion battery demand.High-performance ion batteries need high-performance electrode materials,but the current electrode materials due to poor electrical properties,mechanical properties and limited working environment and other shortcomings limit the further improvement of the performance of ion batteries,Therefore,it is particularly urgent to develop new electrode materials for the next generation.Since graphene has been successfully prepared,graphene has been used in alkali metal battery electrodes due to its excellent conductivity（zero band gap）,stable physical structure and other advantages.Then researchers began to conduct in-depth research on graphene-like two-dimensional material battery electrodes.Two-dimensional materials have unique structural characteristics,high specific area,directional diffusivity,excellent mechanical properties,thermal safety and other characteristics.These characteristics make two-dimensional materials have great competitiveness in the application of two-dimensional battery electrode materials.Based on these characteristics,researchers have studied many two-dimensional materials for alkali metal battery electrodes.At present,two-dimensional materials have achieved remarkable performance in battery electrodes.The work of this thesis is to investigate the mechanical properties,electronic structure characteristics,ion diffusion on the substrate,open circuit voltage and theoretical capacity of two battery electrode materials using a first-principles calculation method based on density flooding theory.The main studies are as follows:1)The geometrical structure,electrical and mechanical properties,alkali ion diffusion in single layer BSi,open circuit voltage（OCV）,theoretical capacity and other properties of silicon based BSi as electrode of alkali metal ion battery are studied.We predict the stable BSi structure by using the Particle Swarm Optimization（PSO）method,and the results show that BSi is a good semiconductor with a band gap of 0.4 e V.The results show that BSi has good conductivity.The calculation results of phonon dispersion spectrum and molecular dynamics show that BSi has good thermal stability and mechanical stability.We used charge local density analysis to determine the possible adsorption sites of alkali metal ions.The optimal adsorption energy of Na and K on BSi is-0.71 e V and-1.28 e V,respectively,calculated by the adsorption energy formula.On the basis of exploring the adsorption sites,we calculated the diffusion energy barriers of Na and K ions on monolayer BSi to be 0.38 and 0.22 e V by using NEB elastic band method.The low diffusion barrier indicates that BSi has a good charge and discharge rate when used as electrode material.The analysis of the low open circuit voltage（OCV）of BSi（Na:0.33 V,K:0.60 V）indicates the structure of BSi as a battery electrode can still keep stable in the charging and discharging process.The theoretical capacity of BSi is much higher than that of normal commercial anode materials（Na:3100 m Ah/g and K:2239 m Ah/g）.All the above calculation results show that BSi is a good candidate material for anode material of alkali metal battery,and the related calculation process and results have certain guiding significance for screening two-dimensional electrode materials of ion battery.2)The geometric and electronic structures of Si₃N on two-dimensional silicon substrates,ion diffusion on the substrate,OCV and theoretical capacity of adsorbed alkali metal ions were investigated.The stable Si₃N structure is predicted based on the Si-based characteristics and the properties exhibited by N-element-containing compounds.The results of energy band calculations show that the Si₃N material exhibits a metallic quality,indicating that Si₃N is well suited as an anode material for battery electrodes.By analyzing the charge local density map of Si₃N,we determined the adsorption sites of Na and K ions on the monolayer Si₃N substrate and calculated the optimal binding energies of-3.2 e V and-3.7 e V,respectively.and by using the NEB elastic energy band method,we calculated the diffusion energy barriers of Na and K ions in the substrate Si₃N as 0.3 e V/0.38 e V,respectively,predicting that Si₃N has as the negative electrode material has a good charge/discharge rate.In the process of investigating the theoretical number of adsorbed ions on the substrate,the average open-circuit voltages of Na and K adsorbed by Si₃N（Na:0.68 V and K:0.9 V）were calculated to confirm the structural stability of Si₃N during the charging and discharging process.Finally,the theoretical capacity of Si₃N in ion adsorption as an anode material for alkali metal batteries（Na:1230.67 m Ah/g and K:888.89 m Ah/g）was calculated.The above results indicate that two-dimensional Si₃N is a very promising candidate material for a new type of ion battery anode.