Lithium intercalation behaviors in Ge and Sn crystalline surfaces.

Fast Li transport inside the Si electrode of Li-ion batteries is retarded at the electrode surface. We investigated the intercalation of a Li atom into the surface and subsurface layers of Ge and Sn using density functional calculations. In the Ge and Sn surfaces, the Li atom diffuses about four and twelve times faster, respectively, along the <100> direction than along the <111> direction. The retardation of Li transport is negligible in Ge surfaces, but some retardation occurs in Sn surfaces. The rate-limiting step resulting in retardation of Li transport in Sn is the subsurface diffusion in both the Sn(100) and Sn(111) surfaces. Our calculations suggest that while the Li transport efficiency in the Ge electrode remains unchanged regardless of the electrode size, transport in the Sn electrode could deteriorate due to the effect of surface retardation as the surface-to-volume ratio of the electrode increases.