Spatial Mapping of Solid Electrolyte Interphase via Synchrotron Infrared Microspectroscopy

Abstract The solid electrolyte interphase (SEI) governs lithium-ion transport and interfacial stability in lithium-ion batteries, yet its spatial heterogeneity remains poorly understood due to the limitations of conventional ensemble-averaged techniques such as X-ray photoelectron spectroscopy (XPS). Here, we employ synchrotron-based infrared (IR) microspectroscopy to spatially resolve SEI distribution on graphite anodes via characteristic vibrational signatures (C═O at 1645 cm–1 and C–O at 1198 cm–1). Our results reveal that the indirect SEI, located beyond the graphite surface, is significantly enriched in organic components compared to the direct SEI at the graphite particle surface. This previously overlooked heterogeneity reconciles long-standing discrepancies between XPS and cryogenic transmission electron microscopy (cryo-TEM) measurements and highlights the critical role of indirect SEI in governing interfacial chemistry. These findings establish spatially resolved IR microspectroscopy as a powerful platform for probing SEI heterogeneity and demonstrate that spatial heterogeneity in the SEI leads to non-uniform interfacial reactions, resulting in battery-to-battery capacity variations.