Unraveling the Nanoscale Heterogeneity of Solid Electrolyte Interphase Using Tip-Enhanced Raman Spectroscopy

We employ tip-enhanced Raman spectroscopy (TERS) to study model amorphous silicon (a-Si) thin film anodes galvanostatically cycled for different numbers. For the 1× cycled a-Si, TERS shows good correlation between solid electrolyte interphase (SEI) topography and chemical mapping, corresponding to distribution of lithium ethylene dicarbonate (LEDC) and poly (ethylene oxide) (PEO)-like oligomer species. Subsequent electrochemical cycling makes the SEI relatively thick and rough with the chemical composition heavily dominated by LEDC monomer-dimer for 5× cycled a-Si. For 20× cycled a-Si, the TERS signal is dominated by carboxylate (RCO2Li) compounds of various conformations and fluorinated species (LixPOyFz). A nanomosaic-multilayer hybrid SEI model on top of the a-Si anode is proposed. The significance of this work is applicable not only to silicon, where SEI plays a dominant role in determining the cycle life performance and reversibility, but also for a number of other relevant battery chemistries such as Na-ion and multivalent redox systems. [Display omitted] •TERS measures the topography and chemical heterogeneity of SEI on a-Si at nanoscale•SEI composition of SEI constantly evolves with progressive galvanostatic cycling•TERS can provide molecular level conformational changes•A nanomosaic-multilayer model for SEI on a-Si is proposed Solid electrolyte interphase (SEI) is considered the key component responsible for the safety, performance, and life for most secondary batteries. Despite this, a unified picture encompassing its chemical composition, distribution of phases, and topography at nanoscale is still lacking. We report the first observation of nanoscale chemical and topographical heterogeneity of a SEI formed on amorphous silicon (a-Si) using tip-enhanced Raman spectroscopy (TERS). Unlike ensemble averaged and relatively bulk sensitive vibrational spectroscopic techniques such as Raman and Fourier-transform infrared (FTIR), TERS is sensitive to dynamical evolution of the SEI at a lateral resolution of