Thermodynamically stable lithium silicides and germanides from density functional theory calculations

• Published in: Physical review. B, Condensed matter and materials physics Condensed matter and materials physics, 2014-08, Vol.90 (5), Article 054111
• Main Authors: Morris, Andrew J., Grey, C. P., Pickard, Chris J.
• Format: Article
• Language: English
• Subjects: Atomic structure; Crystal structure; Electric potential; Lithium; Mathematical analysis; Silicon; Stoichiometry; Voltage
• ISSN: 1098-0121; 1550-235X
• DOI: 10.1103/PhysRevB.90.054111

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 Li sub(17) Si sub(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 sub(1) Si sub(1) and Li sub(1) Ge sub(1) 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 sub(x) Si, x = 2.33-3.25, stoichiometry range. DFT predicts the stability of the Li sub(11) Ge sub(6) Cmmm, Li sub(12) Ge sub(7) Pnma, and Li sub(7) Ge sub(3) P32 sub(1)2 phases and several new Li-Ge compounds, with stoichiometries Li sub(5) Ge sub(2), Li sub(13) Ge sub(5), Li sub(8) Ge sub(3) and Li sub(13) Ge sub(4).