Molecular Simulation And Quantum Mechanics Of Lithium Ion Diffusion In Electrode Materials

In recent years,with the development of smart devices such as new energy vehicles,the safety performance,charging rate and capacity density of lithium ion batteries(LIBs)have become the focus of research.The transfer diffusion properties of lithium ions play a significant role in the charge and discharge rate of batteries.The performance of the battery is closely related to the transfer properties of lithium ions.The structure and conductivity of electrode materials and the solid electrolyte interface(SEI)films are the critical factors affecting lithium ion transfer.For these factors,molecular dynamics and quantum mechanics can be calculated fairly accurately.As a result,molecular dynamics simulation and density functional theory will be used to explore.In this article,we used molecular dynamics(MD)simulation to mimic lithium ions migration behavior in graphite,MoS2 and G/MoS2 composites during charging process.We investigated the relationship between the diffusion time,diffusion coefficient and adsorption energy of lithium ions,and examined the influence of graphite,MoS2 layer spacing and edge structure on lithium ions diffusion.It has been shown that transport diffusivity varies from the self-diffusivity by 5?9 orders of magnitudes.And the structure of electrode material has a significant influence on lithium ion migration.For G/MoS2 composites,we have found that the more homogeneous the composites are,the faster the diffusion will be.The transport properties of the SEI films play an important role in the charging and discharging process of LIBs.We used non-equilibrium molecular dynamics(NEMD)simulation to mimic the lithium permeability of SEI films with different chemical compositions,thicknesses and densities.We found the order of permeability to be Li2CO3>LiF>compound>Li2O>LiOH.Permeability is likely more affected by the binding energy between the lithium ions and the SEI atoms than by structural qualities such as surface area and porosity.The density profiles indicate lithium ions tend to accumulate on the SEI-vacuum interface when the film is less permeable.The transport process does not satisfy Einstein's diffusion equation,thereby revealing the non-equilibrium nature.The relationship between the chemical compositions,densities,thicknesses and permeabilities of each film is expressed by an empirical equation,which can be used to quickly predict the transport properties of SEI films.Finally,We used quantum mechanical simulation(DFT)to calculate the adsorption energy,energy storage density and conductivity of 46 materials to lithium ions.The results show that MnS,VO2 and MoO2 are ideal electrode materials,which have higher adsorption energy and energy storage density for lithium ions.In the composite material,it is found that the uniformity and mass fraction of graphite and MoS2 have a significant influence on the conductivity of G/MoS2 composites.We predict that when the mass fraction of graphite is less than 5%,the properties of G/MoS2 composites will approach that of pure MoS2 materials,which provides a new basis for the design of LIBs electrode materials.