Reaction Front Evolution during Electrochemical Lithiation of Crystalline Silicon Nanopillars

Silicon is one of the most promising anode materials for use in rechargeable lithium-ion batteries due to its high theoretical specific capacity of 4200 mAhg 1 and low cost. However, this high lithium storage capacity results in enormous volume expansion and contraction during electrochemical lithiation and delithiation, which can induce mechanical fracture and severe capacity fading. Recently, studies have utilized Si nanostructures, such as nanowires, nanotubes, and nanoparticles, to improve cycling performance; these nanostructures have greater fracture resistance than larger particles because lower stresses are created in the nanostructures during the volume changes. Despite an abundance of research on electrochemical insertion/extraction of Li into/from Si nanostructures, fracture of these nanostructures is not yet completely understood. Further research on the Li-Si alloying process is necessary to understand how nanostructure dimensions, morphology, and crystalline orientation affect the fracture of these structures during lithiation and delithiation. In order to understand the fracture of Si electrodes during electrochemical Li insertion, various theoretical models have been proposed. Most models describe fracture as resulting from diffusion-induced stresses and assume that the process of lithiation is isotropic. These theories do not take into account the experimentally observed anisotropic volume expansion behavior along <110> directions or the two-phase reaction boundary separating crystalline Si from amorphous Li-Si during lithiation. Furthermore, recent studies have shown that single-crystalline Si nanostructures have a tendency to fracture between neighboring {110} surfaces during lithiation because of intensified tensile hoop stresses induced by anisotropic volume expansion. To approach these issues, Zhao et al. have developed a model of concurrent reaction and plasticity considering a sharp phase boundary between the crystalline Si and the lithiated Si. They explained how tensile hoop stresses develop at the particle surface and also estimated the shape of the crystalline core and the extent of anisotropic volume expansion during partial lithiation. A different in situ transmission electron microscopy study of the lithiation of crystalline Si nanoparticles showed that the crystalline core becomes noticeably faceted as it shrinks due to anisotropic volume expansion. To build on this and to develop a better understanding of the lithiation process, it would