On the origin of anisotropic lithiation in crystalline silicon over germanium: A first principles study

• Published in: Applied surface science Vol. 323; pp. 78 - 81
• Main Authors: Chou, Chia-Yun, Hwang, Gyeong S.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30.12.2014
• Subjects: Crystal structure; Crystallographic orientation dependent lithiation; Density functional theory; Dynamical systems; Dynamics; Germanium; Lattices; Lithium ion battery anode; Molecular dynamics; Molecular structure; Silicon; Germanium; Density functional theory; Silicon; Lithium ion battery anode; Crystallographic orientation dependent lithiation
• ISSN: 0169-4332
• DOI: 10.1016/j.apsusc.2014.08.134

•We examine the underlying reasons for the anisotropic lithiation of Si over Ge in the crystalline phase.•Crystalline Si is lithiated in a layer-by-layer fashion, yielding a sharp amorphous–crystalline interface.•Lithiated c-Ge exhibits a graded lithiation front, which proceeds much faster than that in c-Si.•Lithiation behavior tends to be subject to the stiffness and dynamics of the host matrix.•We reveal the origin and extended impacts of the anisotropic Si vs. isotropic Ge lithiation. Silicon (Si) and germanium (Ge) are both recognized as a promising anode material for high-energy lithium-ion batteries. Si is abundant and best known for its superior gravimetric energy storage capacity, while Ge exhibits faster charge/discharge rates and better capacity retention. Recently, it was discovered that Si lithiation exhibits strong orientation dependence while Ge lithiation proceeds isotropically, although they have the same crystalline structure. To better understand the underlying reasons behind these distinctive differences, we examine and compare the lithiation behaviors at the Li4Si/c-Si(110) and Li4Ge/c-Ge(110) model systems using ab initio molecular dynamics simulations. In comparison to lithiated c-Si, where a sharp amorphous–crystalline interface remains and advances rather slowly, lithiated c-Ge tends to loose its crystallinity rapidly, resulting in a graded lithiation front of fast propagation speed. Analysis of the elastic responses and dynamics of the host Si and Ge lattices clearly demonstrate that from the beginning of the lithiation process, Ge lattice responds with more significant weakening as compared to the rigid Si lattice. Moreover, the more flexible Ge lattice is found to undergo facile atomic rearrangements during lithiation, overshadowing the original crystallographic characteristic. These unique properties of Ge thereby contribute synergistically to the rapid and isotropic lithiation.