Ab Initio Investigation of Li and Na Migration in Guest-Free, Type I Clathrates

Guest-free, type I clathrates with formula Tt(46) (Tt = Si, Ge, Sn) are comprised of open, cage-like frameworks with the potential for facile Li or Na conduction. Herein, ab initio density functional theory (DFT) is used to evaluate the ionic mobility of Li and Na through the clathrate crystal structures. The favorable Li and Na positions inside the clathrate structures are determined, and the migration pathways and barriers are evaluated using the nudged elastic band (NEB) method. The results show that it is energetically favorable for a Li atom to occupy the center position inside the small Tt cages while preferring the off-center positions in the larger Tt(24) cages. The lowest Li migration barriers are found to be 0.35, 0.13 and 0.37 eV for Si-46, Ge-46, and Sn-46, respectively, with the dominant diffusion pathway along channels of Tt(24) cages connected by hexagonal faces. Li accessibility to the Si-20 cage in Si-46 appears to be restricted in the dilute regime due to a high energy barrier (2.0 eV) except for the case in which Li atoms are present in adjacent cages; this lowers the migration barrier to 0.77 eV via a mechanism where a Si-Si bond is temporarily broken. In contrast, Na atoms show preference for the cage centers and display higher migration barriers than Li. Overall, the Tt(24) channel sizes in the guest-free, type I clathrates are ideal for fast Li diffusion, while Na is too large to migrate effectively between cages. The energy landscape for Li inside the type I clathrates is uniquely different than that in diamond cubic structures, leading to significantly lower energy barriers for Li migration. These results suggest that open frameworks of intermetallic elements may enable facile Li migration and have potential use as Li-ion battery anodes.