In Situ TEM Observations of Sn-Containing Silicon Nanowires Undergoing Reversible Pore Formation Due to Fast Lithiation/Delithiation Kinetics

In situ transmission electron microscopy (TEM) studies were carried out to observe directly in real time the lithiation and delithiation of silicon (Si) nanowires with significant amounts of tin (Sn). The incorporation of Sn significantly enhances the lithiation rate compared to typical Si nanowires: surface diffusion is enhanced by 2 orders of magnitude and the bulk lithiation rate by 1 order of magnitude, resulting in a sequential surface-then-core lithiation mechanism. Pore formation was observed in the nanowires during delithiation as a result of the fast lithiation/delithiation kinetics of the nanowires. Pore formation was found to be associated with nonlithiated crystalline domains in the nanowire, which prevent uniform structural changes of the nanowire, and the resulting pores increase in size after each cycle. When an amorphous Si shell was applied to the nano-wires, pore formation was not observed during the in situ TEM experiments. Ex situ TEM analysis of Sn-containing Si nanowires cycled in coin cell batteries also showed that the application of an amorphous Si shell slows pore formation in these nanowires, while fast lithiation/delithiation kinetics is retained.