Study Of The Energy Storage Properties Of Two-dimensional Layered Fe₂C And VS₂/WS₂ Heterojunctions

At present,smart phones and electric vehicles are booming,and users' needs for device endurance have become extremely urgent.Many scientific researchers have spent a lot of energy researching and developing high-density electrical energy storage devices.As an important part of electric energy storage equipment,rechargeable batteries include three important parts: positive and negative electrodes and electrolyte,among which the electrode material has a very important influence on its energy storage properties.Researchers actively strive to find electrode materials with high energy density,high stability and fast chargedischarge performance.As a new class of materials,two-dimensional materials show excellent electrical conductivity,higher theoretical capacity,and stable voltage platform when used as electrode materials.In recent years of research,two-dimensional materials have broad application prospects in the field of energy storage.This paper focuses on the application and the related issues of two-dimensional layered material in the electrode,such as low open circuit voltage and poor cycling stability.The first-principles method was used to calculate and simulate the transition metal carbide(Fe2C)and transition metal sulfide(VS₂)electrode materials,showing excellent battery performance.The calculation results of the battery are as follows:1.Theoretical research on Fe₂C as electrode material.Fe₂C MXene is a material with a layered structure,this structure contributes to ion transport in the insertion and extraction of material.At the same time,its own ferromagnetism can provide a higher open circuit voltage and contribute to its energy storage properties.Firstly,by analyzing the formation energy and phonon dispersion curves,in the AA configuration,only F,O,and Se are used as functionalized terminal groups to achieve a more stable structure.Secondly,to assess the electrical conductivity at different functionalized terminal groups,the electronic structure of Fe₂C at different functionalized groups was calculated.The results show that all three functionalized Fe₂C exhibit conductor properties.This conductivity is important for electrode materials.Next,a series of cations M(M= Li⁺,Na⁺,K⁺,Mg²⁺,Ca²⁺,Zn²⁺,NH⁴⁺)are intercalated calculations,and cations with negative intercalation energy are selected as the transport ions of the secondary battery.Consider the diffusion barrier and open circuit voltage of transport cations in different functionalized Fe₂C.According to the calculations,some combinations of charge carriers and electrode materials with fast ion diffusion are identified.The diffusion potential of potassium ions is the lowest among all considered cations,which means that all three functionalised Fe2CT2(T=O,F,Se)are fast potassium ion conductors.O-functionalized Fe₂C has a higher open circuit voltage and a larger theoretical capacity.From the point of view of our theoretical results,Fe2CSe2 can do calcium-ion battery cathode materials.Fe2CO2 and Fe2CF2 can be used as cathode materials for ammonium ion batteries.2.Theoretical study of VS₂\WS₂ van der Waals heterojunction as electrode material.Studies have shown that combining transitional metal dichalcogenides with other layered materials can effectively slow down the deformation during cycling and improve the structural stability of the two-dimensional electrode.Therefore,this paper combines VS₂ and WS₂ into a van der Waals heterojunction.Based on the first principles,the formation energy of the heterojunction is calculated.It is determined that the formation energy of AB-stacked heterojunction is the lowest negative value.Calculate the phonon dispersion curve of the VS₂\WS₂ van der Waals heterojunction to prove that the structure is stable.In addition,the results of molecular dynamics calculations show that the van der Waals heterojunction has good stability in thermodynamics.By calculating the adsorption energy of lithium ions at different positions in the van der Waals heterojunction,the most stable adsorption sites of lithium ions in the heterojunction are obtained.It was found that the diffusion potential barrier of lithium ions in the heterojunction was 0.18 e V on the VS₂ side,0.22 e V on the WS₂ side and 0.34 e V between the heterojunctions.At the same time,the open circuit voltage and theoretical capacity are obtained.In addition,the torsion angle effect of VS₂\WS₂ van der Waals heterojunction is considered.It is found that when the torsion angle is 13.1°,the optimal adsorption site for lithium ions changes.Here,based on the method of scanning the potential energy surface,we obtained the energy of lithium ions at different sites in the heterojunction,and determined the adsorption site with the lowest energy.The NEB method is used to calculate the halfway diffusion barrier,and the string method is used to calculate the full-length diffusion barrier.The two methods show that the diffusion barrier of lithium ions in the heterojunction with twist angle is smaller.It shows that VS₂\WS₂ heterojunction can be used as the electrode material of lithium battery.In addition,we selected three different WS₂ structures to obtain the electronic structure and optical properties.The related properties of the single-layer structure after oxygen doping are calculated.Finally,it was found that WS₂ changed the structure and added oxygen atoms to control the electronic structure and optical properties.