Computational Simulation Of Lithium Intercalation Behaviors And Structure Evolution Of Silicon Anode Materials

With the large-scale promotion of electric vehicles,large-capacity energy storage technology is crucial to the development of electric vehicles.At present,most of the negative electrodes of batteries are made of carbon materials,and their energy density is gradually unable to meet the development needs of large-capacity energy storage in the future.In recent years,due to its extremely high specific capacity,silicon has attracted extensive attention as the anode material for a new generation of lithium-ion batteries.However,during the charge-discharge cycle of the silicon anode material,there will be a huge volume change,which will cause the electrode powder to break up and fall off from the current collector,which will greatly reduce the service life of the battery and limit its development and application.At present,most of the research on the performance of lithium-ion battery anode materials is aimed at the existing silicon-lithium alloy or the lithium intercalation behavior at low concentrations.There are relatively few studies on the lithium intercalation behavior of silicon anode materials at high concentrations,and there is a lack of systematic and in-depth research.The research on the lithium intercalation behavior of silicon anode materials with large concentration is of great significance for improving the capacity and cycle performance of silicon-based batteries.Therefore,this paper studies the performance changes of silicon anode materials under the condition of large concentration of lithium intercalation,and explores the energy and structure changes during the lithium intercalation process,as well as the diffusion kinetics of lithium ions in silicon materials.In this paper,the structure model of silicon intercalation lithium intercalation unit cell under the condition of high concentration lithium intercalation(lithium intercalation concentration is 50%～81%)was first constructed,and the crystal lattice relaxation and structure optimization were carried out based on the first principles,and the large concentration The structural parameters,mechanical performance parameters and energy change rules of silicon-lithium unit cells under lithium intercalation conditions are aimed at revealing the performance changes and failure mechanisms of silicon negative electrode materials in the state of high-density lithiation.Then,based on the ab initio molecular dynamics calculation,the diffusion kinetic behavior of lithium ions inside the silicon anode material was calculated,and the influence of different lithium intercalation concentrations on the diffusion coefficient of lithium ions in the unit cell under the condition of large concentration lithium intercalation was analyzed,combined with temperature and The microstructure of silicon inside the unit cell was used to analyze the diffusion behavior of lithium ions,and the migration energy change of a single lithium ion inside the unit cell was calculated and analyzed.The main research contents are as follows:(1)The intercalation of large concentrations of lithium ions will lead to the destruction of the internal bonds of crystalline silicon,and the silicon will change from the original body-centered cubic structure to an isolated and dispersed small cluster structure,and as the concentration of lithium intercalation increases,the cluster size decreases,the number of clusters increases;the overall structure of silicon changes from the original crystalline structure to an amorphous structure,and the changes in the radial distribution function verify the above results.The normalized volume and density analysis shows that the silicon unit cell will experience a maximum volume expansion of about 300%,and the density will drop linearly with the increase of lithium intercalation concentration.(2)When lithium ions are intercalated into silicon,the overall formation energy has a minimum value at a lithium intercalation concentration of 76%,the overall difficulty of chemical combination is reduced,and the overall stability of the unit cell is better.The calculation and analysis of the mechanical properties of the silicon-lithium unit cell found that the overall shear resistance and deformation resistance of the silicon unit cell decreased after lithium intercalation,and the breakage of the silicon-silicon bond in the microstructure led to a decrease in mechanical properties.When lithium ions are embedded in silicon,the overall hybrid enthalpy occurs at the minimum value at 50%～70% of lithium intercalation concentration,the overall combination is reduced,and the trend of silicon lithium alloy with lithium ion and lithium ions.A large number of Li-Si ion bonds and Li-Li metal bonds inside the crystal cells will cause the intensity of the overall bond of silicon lithium crystal cells to weaken,which will lead to a decline in the mechanical properties of silicon crystal cells.The overall shear and deformation capacity of silicon crystal cells decreased after lithium embedded lithium,and the break of micro-structured silicon-silicon bonds led to a decrease in mechanical properties.(3)Analyze the diffusion behavior of lithium ions inside the silicon-lithium unit cell,and find that the diffusion coefficient of lithium ions will gradually increase with the increase of lithium intercalation concentration.When the lithium intercalation concentration increases from 50% to 81%,the lithium ion diffusion coefficient increased by two orders of magnitude,and the rising trend gradually flattened out with the increase of lithium intercalation concentration.The influence of temperature on the diffusion coefficient of lithium ions is also more obvious,but the influence on the diffusion coefficient is smaller than that of lithium intercalation concentration.The diffusion coefficient of silicon atoms is generally two orders of magnitude lower than that of lithium ions,and the diffusion coefficient of silicon atoms is less affected by the concentration of intercalated lithium.The number of existing forms of silicon atoms and the diffusion coefficient of lithium ions are generally inversely proportional.trend.(4)When a single lithium ion diffuses in the gap of the unit cell,the energy barrier required to cross is small,and the different paths of diffusion have little effect on the energy barrier,so when a single lithium ion diffuses in the gap inside the unit cell,The direction of its diffusion is random.When a single lithium ion diffuses in vacancies inside the unit cell,the energy barrier required to cross increases with the increase of the diffusion distance.The energy barrier for small-distance diffusion is similar to interstitial diffusion,and it is more difficult for large-distance diffusion to occur.