Because of its high theoretical specific capacity, silicon is a promising candidate material to use as anodes in lithium-ion batteries. One major challenge for silicon in this context is its large volume expansion upon lithium insertion, up to 400%, which leads to anode pulverization and decreased performance. The use of various Si nanoarchitectures has been instrumental in improving performance due to increased ability to accommodate large volume changes. Though Si nanomaterial electrochemical performance has been studied extensively, mechanistic understanding of volume change upon lithium insertion in these materials is limited. S.W. Lee and colleagues at Stanford University report on anisotropic shape changes of silicon nanopillars induced by electrochemical lithiation as published in the June 9 online edition of Nano Letters (DOI: 10.1021/nl201787r).
The researchers used scanning electron microscopy (SEM) to study silicon nanopillars in varying states of lithiation. The nanopillars of distinct axial orientations are fabricated using deep reactive-ion etching on Si wafers with SiO2 nanospheres as an etch mask. This study includes nanopillars with axial orientations of <100>, <110>, and <111>. Lithiation is accomplished by using the nanopillars as the working electrode in electrochemical half cells with Li metal foil as the counter electrode.
Upon lithiation, surprising cross-sectional shape differences are observed between the different types of nanopillars. The circular cross sections of the pillars develop into “plus” sign shapes in the <100> pillars, ellipses in the <110> pillars, and rough hexagons in the <111> pillars. In the most extreme case, the <110> pillars expanded 245% along the long axis of the final ellipse and only 49% along the short axis.
These directions of increased expansion can be mapped to <110> directions in the Si crystal structure, a direction that provides large space between Si atoms allowing for ion diffusion to take place. The researchers propose fast lithium ion diffusion along the <110> directions and plastic deformation of the Li-Si alloy are responsible for the anisotropic growth behavior.
Unexpectedly, <100> and <111> pillars first exhibit a marked decrease in height, up to 9.5%, before ending within 1–2% of the initial height upon full lithiation. The <110> nanopillars increase in height throughout lithiation ending about 4% taller than they started. The researchers explain this height behavior by considering the relative effects of two processes: growth in <110> directions as just described versus decreased plane spacing between <111> planes due to broken Si–Si bonds where Li ions are inserted at tetrahedral sites.
Upon lithiation, the initially circular cross sections of crystalline Si nanopillars with <100>, <110>, and <111> axial orientations expand into cross, ellipse, and hexagonal shapes, respectively. A high-speed lithium ion diffusion channel along the <110> direction causes preferential volume expansion along this direction. Reproduced with permission from Nano Lett. (DOI: 10.1021/nl201787r). © 2011 American Chemical Society.
An understanding of the structural evolution of Si nanostructures during electrochemical lithiation is important for guiding development of higher performance Si anodes. According to the researchers, this work describes mechanistic insights into nanopillar expansion during lithiation that can be used as experimental handles to continue improving upon existing Si anode architectures.
