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

TL;DR

This study uses ab initio methods to compare lithium, sodium, and magnesium storage in pure and gallium-doped germanium, revealing the importance of both interstitial and substitutional sites and how doping affects insertion energetics.

Contribution

It provides the first detailed ab initio analysis of both interstitial and substitutional site preferences for Li, Na, and Mg in germanium, including effects of gallium doping.

Findings

Na and Mg are less thermodynamically favored than Li in Ge.

Gallium doping lowers defect formation energies for interstitial insertion.

Migration barriers vary significantly between different site types.

Abstract

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