First-Principles Study of Lithiation of Type I Ba-Doped Silicon and Germanium Clathrates

Si and Ge clathrate materials have recently been investigated for their electrochemical properties as anodes for Li-ion batteries due to their unique cage structures and ability to incorporate extrinsic guest atoms. First-principles calculations were used to investigate the type I clathrate compounds Si₄₆, Li_xBa_ySi₄₆, Li_xBa_yA_l6Si₄₀ and Li_xBa₈Ge₄₃, for understanding the preferred structures of small degrees of lithiation and Li diffusion paths. The results showed that Li insertion into framework or Ba vacancies could stabilize the clathrate structure. Si substitution by Al lowered the formation energies of the lithiated compounds. For Ba-doped Ge clathrates, it was found that Li insertion into the three framework vacancies in Ba₈Ge₄₃ is energetically favorable. However, the high energy barrier (1.6 eV) for Li diffusion between vacancies and around Ba guest atoms suggests that framework vacancies are unlikely to significantly contribute to lithiation processes unless the Ba guest atoms are absent. The study elucidated the preferred structural configurations for Li in type I, Ba-doped Si/Ge clathrates and also be informative for efforts related to understanding the structures obtained after electrochemical insertion of lithium into the clathrates.