A First-Principles Comparative Study of Lithium, Sodium, and Magnesium Storage in Pure and Gallium-Doped Germanium: Competition between Interstitial and Substitutional Sites

摘要

Thermodynamics and kinetics of Li, Na, and Mg storage in Ge is studied abinitio. The most stable configurations can consist of tetrahedral,substitutional, or a combination of the two types of sites. In the dilutelimit, Li, Na prefer interstitial, while Mg prefers substitutional sites. Athigher concentrations of Li, Na, and Mg, there is a combination of interstitialand substitutional sites. This is an important finding, as previous ab initiostudies of alloying type electrode materials ignored substitutional sites.Insertion energies computed at dilute concentration (x = 1/64) show that Na andMg insertion are not thermodynamically favored in Ge vs formation of bulk Naand Mg, as opposed to Li insertion which is favored. We investigate the effectof p-doping of Ge (with Ga) on the thermodynamics and find that it considerablylowers the defect formation energies associated with the insertion of Li/Na/Mgat tetrahedral sites. On the other hand, the energetics associated withLi/Na/Mg insertion at substitutional sites are not significantly affected. Inaddition, we compute the migration energy barriers for Li/Na/Mg diffusionbetween two tetrahedral sites (0.38/0.79/0.66 eV), between two substitutionalsites (0.77/0.93/1.83 eV), and between two sites of different type(2.15/1.75/0.85 eV).