First-Principles Study of Lithiation of Type i Ba-Doped Silicon Clathrates

Silicon clathrate materials, previously known for their superconducting and thermoelectric characteristics, have also recently been investigated for their electrochemical properties as anodes for lithium-ion batteries due to their unique cage structure and ability to incorporate extrinsic guest atoms. To better understand the preferred structures for small degrees of lithiation, first-principles density functional theory (DFT) was used to investigate the type I clathrate compounds Si₄₆, Li_xBa_ySi₄₆ (0 ≤ x ≤ 8; y = 6, 8), and Li_xBa_yAl₆Si₄₀ (0 ≤ x ≤ 8; y = 6). The formation energies, electronic band structures, and density of states (DOS) were calculated. Lithium occupation in framework vacancies, empty and Ba-occupied cage cavities, and near the pentagonal and hexagonal faces of the clathrate polyhedra was considered. The data showed that Li insertion into framework or Ba vacancies could stabilize the clathrate structure. Silicon substitution by Al lowered the formation energies of the lithiated compounds and mitigated the calculated volume increase upon lithiation. The results also showed that it is energetically feasible for multiple guest atoms to be placed in the Si₂₄ cages. Changes in the clathrate atomic structure (e.g., bond lengths and angles) and electronic structure were highly dependent on the location of the Li and guest atom spacing within the clathrate framework. The results from this study can elucidate the preferred structural configurations for Li in type I, Ba-doped silicon clathrates and also be informative for efforts related to understanding the structures obtained after electrochemical insertion of lithium into silicon clathrates.