The composition, morphology and thickness of solid electrolyte interphase (SEI) on silicon anodes are dictated by multiple factors, ranging from the chemistry of silicon surfaces, electrolyte solution structure and chemical passivation on Si surface. In addition, the SEI constantly evolves, as it is electrochemically lithiated (delithiated) due to the enormous volume changes. SEI on silicon typically is reported to be in the range of tens of nanometers and can be compositionally heterogeneous. Normal vibrational spectroscopy techniques such as confocal micro-Raman and FT-IR are useful, but they only provide chemical information at a relatively bulk scale, typically in micron-distance from the solid surface. This makes it extremely hard for getting information from the real SEI, which can be only on the order of 10 nm. Tip Enhanced Raman Spectroscopy (TERS) combines surface plasmon-enhanced Raman spectroscopy with atomic force microscopy, capable of providing the chemical vibrational information and topography of the sample in the nanometer spatial resolution simultaneously. In this report we demonstrate the first TERS study of SEI on cycled amorphous silicon. Amorphous silicon anode (20 nm in thickness on copper current collector) was cycled in a commercial lithium-ion battery electrolyte (1M LiPF₆ in EC:DEC, 1:1 vol). TERS analysis on cycled amorphous silicon sample indicates that the nanometer scale SEI “islands” are unevenly distributed on the Si anode surface. Even for the same SEI “island”, the composition is different from point to point with inter-point distance smaller than 2 nm.

Acknowledgment

This work is funded by Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy