Thermodynamically stable lithium silicides and germanides from density functional theory calculations

High-throughput density functional theory (DFT) calculations have been performed on the Li-Si and Li-Ge systems. Lithiated Si and Ge, including their metastable phases, play an important technological role as Li-ion battery (LIB) anodes. The calculations comprise structural optimizations on crystal structures obtained by swapping atomic species to Li-Si and Li-Ge from the
X − Y structures in the International Crystal Structure Database, where
X = {Li, Na, K, Rb, Cs} and Y = {Si, Ge, Sn, Pb}. To complement this at various Li-Si and Li-Ge stoichiometries, ab initio random structure searching (AIRSS) was also performed. Between the ground-state stoichiometries, including the recently found Li₁₇Si₄ phase, the average voltages were calculated, indicating that germanium may be a safer alternative to silicon anodes in LIB due to its higher lithium insertion voltage. Calculations predict high-density Li₁ Si₁ and
Li₁ Ge₁ P4/mmm layered phases which become the ground states above 2.5 and 5 GPa, respectively, and reveal silicon and germanium's propensity to form dumbbells in the Li_x Si, x = 2.33–3.25, stoichiometry range. DFT predicts the stability of the Li₁₁ Ge₆ Cmmm, Li₁₂ Ge₇ Pnma, and Li₇ Ge₃ P32₁2 phases and several new Li-Ge compounds, with stoichiometries Li₅ Ge₂, Li₁₃ Ge₅,
Li₈ Ge₃ and Li₁₃ Ge₄.