Reaction Front Evolution during Electrochemical Lithiation of Crystalline Silicon Nanopillars

The high theoretical specific capacity of Si as an anode material is attractive in lithium-ion batteries, although the issues caused by large volume changes during cycling have been a major challenge. Efforts have been devoted to understanding how diffusion-induced stresses cause fracture, but recent observations of anisotropic volume expansion in single-crystalline Si nanostructures require new theoretical considerations of expansion behavior during lithiation. Further experimental investigation is also necessary to better understand the anisotropy of the lithiation process. Here, we present a method to reveal the crystalline core of partially lithiated Si nanopillars with three different crystallographic orientations by using methanol to dissolve the Li atoms from the amorphous Li-Si alloy. The exposed crystalline cores have flat {110} surfaces at the pillar sidewalls; these surfaces represent the position of the reaction front between the crystalline core and the amorphous Li-Si alloy. It was also found that an amorphous Si structure remained on the flat surfaces of the crystalline core after dissolution of the Li, which was presumed to be caused by the accumulation of Si atoms left over from the removal of Li from the Li-Si alloy.