Lithium Ion Migration In Li-Si Alloys: From First Principles Studies

Li ion diffusion behaviors in two typical Li-Si alloys Li1Si1and Li2Si1have beenstudied through first principles calculations. The Li vacancy migration and interstitialLi migration have been studied in the bulk phases and at the stable surfaces of the twoalloy phases. The Li migration pathways have been identified and optimized and thecorresponding energy barriers have been calculated.Results show that Li vacancy migration in the bulk phases have relatively lowenergy barrier, while the vacancy formation energy in the bulk phase is relatively high.This indicates that Li diffusion in the bulk phases of the Li-Si alloys can be fast ifthere are enough Li vacancies available in the bulk. Direct formation of the vacancy inthe bulk is not easy at room temperature since the vacancy formation energy is high,but vacancies can be formed indirectly through Li extraction at the surfaces or phaseboundary region and diffuses from the surface area to the bulk. At the surfaces, the Lican be extracted or removed through either vacancy assisted migration or interstitialLi migration, because the energy barrier of Li migration through those mechanismsare relatively low at the surfaces. Interstitial Li migration in the bulk phases is noteasy due to the relatively high energy barriers. In the Li2Si1phase, interstitial Limigration is one-dimensional because only one migration pathway is allowed and hasrelatively low energy barrier. Therefore, we can conclude that the Li diffusion in theLi-Si alloys have been dominated by the Li vacancy migration in the bulk phases,while the vacancy formation in the bulk is controlled by the surface Li diffusion,which can be assisted by either Li vacancy migration or interstitial Li migration.