In Situ Transmission Electron Microsopy of Oxide Shell-Induced Pore Formation in (De)lithiated Silicon Nanowires

Silicon (Si) nanowires with a silicon oxide (SiOx) shell undergoing lithiation and delithiation were examined by in situ transmission electron microscopy (TEM). Large pores formed in the nanowires during the delithiation cycle. We found that the oxide shell constrains the expansion of the Si nanowires during lithitation and then induces pore formation in the nanowires. We propose that the SiOx shell prevents the vacancies that result from the loss of lithium from escaping the Si core, leading to pore nucleation and growth. It is also possible that the difference in mechanical properties of the expanding and contracting Si nanowire and SiOx shell contribute to the observed pore formation. This in situ study reaffirms the need to directly observe structural changes that occur during cycling in battery materials, especially when modified by coatings.