Dynamic Structure and Chemistry of the Silicon Solid-Electrolyte Interphase Visualized by Cryogenic Electron Microscopy

The commercialization of the silicon (Si) anode has been hindered by the instability of its solid-electrolyte interphase (SEI), yet a comprehensive understanding of SEI properties remains underdeveloped owing to the challenge of characterizing this nanoscale passivation layer. In this work, we visualize the structure and chemistry of the SEI on silicon anodes using atomic-resolution cryogenic (scanning) transmission electron microscopy (cryo-(S)TEM) and electron energy loss spectroscopy (EELS), revealing its evolution over the first cycle. We discover the origin of the Si SEI instability in ethylene carbonate (EC) electrolytes, owing to the high reversibility of the SEI. The role of the critical electrolyte additive fluoroethylene carbonate is revealed, which extends the cyclability of the Si anode through deposition of an electrochemically irreversible polycarbonate layer on the anode surface. These findings provide a nuanced view into the Si anode instability in commercial EC-based electrolytes and the role of additives for SEI stabilization.