Multiscale Simulation Of The Interaction Between Li And Dislocation In Silicon

In recent years, because Si has a very strong storage capcity for Lithium ion, Siand Si nanostructures have been regarded as the most popular electrode materials fordesigning the new generation Lithium ion battery with higher storage ability andlonger service life. But during alloying and dealloying with Li, the large volumevariation up to400%would happen to Si, and causes failture of the Si-electrdeLithium ion battery. During the big volume variation, variety of defects will beproduced to release the stress from volume varies. In this case, dislocation is one ofthe most important defect. Therefor the research about the interation betweendislocation and Li in silicon is very necessary, it will help us to realize the dynamiccharacteristics of Li in silicon with dislocations. The understanding of Li diffusiondynamics in Si is obviously necessary for designing a better Si-based electrodewhich would finally circumvent this pulverization and crack caused by volumechanges. In this thesis, the QM/MM multi-scale method wihich can couple the FirstPrinciple (DFT) and molecular dynamics (MD) was employed to study the stableconfiguration and dynamic characteristics of Li in silicon with or withoutdislocations.First, the stable configuration and diffusion process of Li atom in Bulk Si arestudied using multi-scale method. The most stable positon for Li atom to stay inBulk Si is at the center of tetrahedral lattice. Its diffusion pathway is periodiczig-zag path, the transition state position is located in the center of the hexagonatoms ring which is the one bottom of the tetrahedral lattice. Thest results areconsistent with the existing research using the first principle. The binding energyand diffusion barrier of Li atom in Bulk Si are also calculated. In constrast with theresults from regular DFT, the accuracy and convergence for energy calculation withthis multi-scale method are proved.Then, the interaction between Li atom and glide60°dislocation is investigatedby using the multi-scale method. When a Li atom enters into the glide60°dislocation core, it is found that its most stable position is in the open space betweentwo reconstructed Si–Si dimers. From the open space between the two dimers, the Liatom can diffuse into dislocation core with relatively lower barrier. During thediffusion of Li atom in core, the transition state position is in the area which consistsof two heptagonal atomic rings and one Si-Si dimmer. The two local transition statepositions are at the center of heptagonal atomic ring. By calculating, it is found thatthe diffusion of Li atom can be accelerated by glide60°dislocation which isconsistent with the existing research result (the phenomenon of pipe diffusion). Subsequently, the dynamic characteristics of Li in silicon affected by theshuffle60°dislocation are studied with the multi-scale method, including the stablepositions of Li atom in or around the shuffle60°dislocation core and the diffusionin the core. Li atom has two stable position in the shuffle60°dislocation core whichare in the two different kinds of areas that consist of heptagonal atomic rings.During the diffusion of Li atom in core, the transition state position is at the centerof heptagonal atomic ring. It can be concluded that it is more stable for Li atom stayin the core of shuffle60°dislocation than in glide60°dislocation, and the diffusionprocess can also be acclerated.Finally, the conditions of Li atom in30°particial dislocation and stacking faultare investigated. The stable configurations of Li atom in and around the dislocationcore are calculated. The diffusion paths of Li atom from outside into the core and inthe core are also calculated. Li atom has two stable positions in the core of30°particial dislocation: Oct-A and Oct-B, they are at the center of the octagon whichbelong to the projection of dislocation core onto the (111) plane, but they are not inthe same (111) plane. The binding energies of Li atom at Oct-A and Oct-B are bothlower than in Bulk Si. There are two different diffusion pathways for Li atomdiffusion in core with different basis set positions (Oct-A and Oct-B), which existwith noninterference. Different with the situations in shuffle and glide60°dislocations, the diffusion of Li atom is obviously decelerated by the30°particialdislocation in the core. In addition, the condition of Li atom with stacking fault (SF)is also simulated. The SF also decelerates the diffusion of Li atom. At the end of thispart, the stable positions and transition state positions of Li atom in two kinds of60°dislocation,30°dislocation and stacking fault are discussed.