Credibility Evaluation Of First-principles Calculation Method In Lithium Ion Battery Materials Applications

To test of material under service condition is more and more difficult for the complex application environment.It is still a great challenge to understand the properties of materials.The rapid development of modern new materials science induced the research scale down to nano meter,even to atomic scale.Obviously,the experiment technical is far away this requirement to date.While the improvement of computer computing power and the development of calculation methods,theoretical calculation has solved the problems faced by materials science to a certain extent-that is,understanding the thermodynamics and dynamics of materials from the atomic scale.The research of lithium ion battery materials is one of them.Density Functional Theory?DFT?has proven to be one of the most practical method in computational simulation,and can well explain the thermodynamic and dynamic problems of lithium-ion battery materials,however,compared with the actual situation,the calculation results are still some discrepancies.For example,in the DFT calculation process,which is based on the Bonn-Oppenheimer?B.O.?approximation,only the interaction between the electron and the nucleus is considered,while the interaction between the nucleus is ignored,so it cannot be well described the strong molecules interaction.In this paper,via DFT-vdW we studied the error problems related to the calculation of lithium ion battery materials,and selected van der Waals?vdW?effect and the lattice dynamics and thermodynamics of solid material as objects,we values reliability of first-principles calculation in lithium ion battery materials applications.First,we calculate the phonon dispersion curve of the one-dimensional atom chain and bulk metal lithium in primary cells.Due to the influence of boundary conditions,the calculation of the phonon frequency will produce a certain error at the boundary of the Brillouin zone.In this work,by comparing the DFT calculation of the one-dimensional atomic chain under the simple harmonic approximation and the neutron inelastic scattering experiment results,we evaluated the phonon dispersion curve and thermodynamic parameters calculated by the principle method.The research results show that the lattice dynamics calculated based on the density functional theory has a high reliability.Second,by applying ten different vdW functions to Li_xCoO₂?x=1,8/9,7/9…1/9,0?,Li₁Si₁,Li₂Si₁,Li₇Si₂,lithium/graphene,lithium/silene,Li_x C₆?x=1,0.5,0.33,0.2?and other systems to consider the van der Waals force,so that we can get a systematically studied the size of lithium vdW interaction in electrode materials and the effect of different vdW functions on the calculation results.It was found that the lithium in these electrode materials exists in the form of lithium ions,which has nothing to do with the consideration of vdW.On the other hand,the more accurately the vdW function describes the vdW interaction of lithium ions,the calculated results can better match the experimental values.The calculation result of the D2 function will overestimate the delithiation voltage compared with the experimental value.This is because the D2 function overestimates the vdW interaction coefficient of lithium in the calculation,and the three functions TS,TSHI,TSSCS will seriously underestimate the calculation of lithium metal The lattice constant of lithium and the free energy of lithium,the calculation result of the TSHI function is better than that of the TS function.Since the TSHI function can modify the C₆ value of lithium according to the system,the results calculated by TSHI and TSHI_no-LivdW are basically the same.For the TSSCS function,the performance of the Li_xCoO₂,Li₁Si₁,Li₂Si₁,Li₇Si₂ system is very poor.For the three functions D3,D3bj,and dDsC,their C₆ values can be automatically corrected according to the local environment in which the atoms are located.The calculated results are in good agreement with the experimental values.