Lithiation of Single-Crystalline Ge(111) and Si(111) Investigated by X-ray Photoelectron Spectroscopy

Silicon and germanium are considered as very promising anode materials for lithium-ion batteries due to their high capacities. Understanding the lithiation mechanisms of Si and Ge is important to overcome problems, such as volume expansion, with such materials. In this paper, the changes in the chemical state and the local structure of single-crystalline Si(111) and Ge(111) during the lithiation process on an atomic scale were studied by X-ray photoelectron spectroscopy (XPS). Remarkable differences in the lithiation process between Si(111) and Ge(111) were observed. Two Si 2p peaks in the XPS were observed during the lithiation of Si(111), which can be assigned to the elemental Si 2p and LixSi alloying Si 2p. In contrast, Ge(111) shows one peak in the Ge 3d regime and the peak gradually reduces in intensity and shifts to lower binding energy upon lithiation. Our results also indicate that ∼SiLi3.0 was formed at the surface and metallic Li was more likely to accumulate on the surface of Si(111), even though elemental Si was still available. Nevertheless, Ge(111) shows a homogeneous phase upon lithiation, and different intermediate phases, such as GeLi0.9, GeLi1.5, GeLi3.6, are present with the increase in Li. We expect that our understanding of the lithiation process could contribute to the development of the next-generation high-performance Si- and Ge-based anodes.