Thermodynamically stable lithium silicides and germanides from density-functional theory calculations

Andrew J. Morris; C. P. Grey; C. J Pickard

doi:10.1103/PhysRevB.90.054111

paper

Thermodynamically stable lithium silicides and germanides from density-functional theory calculations

arXiv:1402.6233 · doi:10.1103/PhysRevB.90.054111

Abstract

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 rôle as Li-ion battery (LIB) anodes. The calculations comprise structural optimisations 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 Li $_{17}$ Si $_{4}$ 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 Li $_{1}$ Si $_{1}$ and Li $_{1}$ Ge $_{1}$ $P 4/ mmm$ layered phases which become the ground state 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 Li $_{11}$ Ge $_{6}$ $C mmm$ , Li $_{12}$ Ge $_{7}$ $P nma$ and Li $_{7}$ Ge $_{3}$ $P 3 2_{1} 2$ phases and several new Li-Ge compounds, with stoichiometries Li $_{5}$ Ge $_{2}$ , Li $_{13}$ Ge $_{5}$ , Li $_{8}$ Ge $_{3}$ and Li $_{13}$ Ge $_{4}$ .

10 pages, 5 figures

Thermodynamically stable lithium silicides and germanides from density-functional theory calculations

Abstract

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