Computational Study Of Effect Of Defect On The Diffusion Of Ions In Solid Electrolyte

As an important component of solid state batteries,solid electrolytes have many advantages compared with liquid electrolytes,so it is significant to study them.Defect is a common phenomenon in solid material preparation,and it has an important influence on ionic conductivity of solid electrolyte.The effect of the defect on the ionic conductivity of solid electrolyte needs to be studied.The ionic conductivity of solid electrolyte is related to the diffusion of ions.In this paper,we study the effect of defects on the conductivity of defects by studying the effect of defects on their ionic diffusion.The influence of interstitial atom defects on the diffusion of sodium ions in sodium ionic solid electrolyte（c-Na₃SbS₄）,is studied by first principles.The results show that the sodium ion interstitial atom can effectively increase the diffusion coefficient of sodium ions,and increase the room temperature conductivity of sodium ions by about two orders of magnitude.This paper also studies interstitial defect structures by doping tetravalent ions(Si⁴⁺,Ge⁴⁺,and Sn⁴⁺).Through the calculation of the formation energy of doping,it is theoretically indicated that the doped structures can be synthesized in practical experiments.The results show that doping can accelerate the diffusion of sodium ions,thus improving the conductivity of sodium ions.The effect of different doping elements is different.When the doping concentration is 0.0625,the effect of Sn⁴⁺doping is the most,and the electrical conductivity at room temperature can reach 45 mS/cm.Meanwhile,by studying the effect of different Si⁴⁺doping concentration,it is found that the higher the doping concentration is,the greater the ionic conductivity at room temperature.The influence of vacancy defects on the diffusion of sodium ions in c-Na₃SbS₄ is studied by first principles calculations.The sodium ion vacancy is beneficial to the diffusion of sodium ions.The higher the vacancy concentration is,the more beneficial to the Na-ion diffusion.By studying the structure of sodium ion vacancy caused by the doping of halogen ions（F^-,Cl^-,Br^-,I^-）,it is found that the doping of halogen ions is beneficial to the diffusion of sodium ions,in which the Cl^-doping has the greatest influence.By calculating the formation energy of halogen doped structure,it is found that the formation energy of halogen doped structure is relatively low,and it can be synthesized in experiment.The effect of grain boundary on oxygen ion diffusion in the oxygen ion solid electrolyte（CeO₂）is also simulated by molecular dynamics method.The calculated results show that the diffusion coefficient of oxygen ions at grain boundaries is two orders of magnitude higher than that in perfect crystals,which means that grain boundaries in CeO₂can be used as oxygen ion diffusion channel.The diffusion of oxygen ions in CeO₂polycrystals is also studied.It is found that the diffusion coefficient of oxygen ions in polycrystalline is much higher than that in the perfect crystal.And the smaller the grain size is,the greater the diffusion coefficient of oxygen ions.