Direct Operando Observation of Double Layer Charging and Early Solid Electrolyte Interphase Formation in Li-Ion...

The solid electrolyte interphase (SEI) is the most critical yet least understood component to guarantee stable and safe operation of a Li-ion cell. Herein, the early stages of SEI formation in a typical LiPF₆ and organic carbonate-based Li-ion electrolyte are explored by operando surface-enhanced Raman spectroscopy, on-line electrochemical mass spectrometry, and electrochemical quartz crystal microbalance. The electric double layer is directly observed to charge as Li⁺ solvated by ethylene carbonate (EC) progressively accumulates at the negatively charged electrode surface. Further negative polarization triggers SEI formation, as evidenced by H₂ evolution and electrode mass deposition. Electrolyte impurities, HF and H₂O, are reduced early and contribute in a multistep (electro)chemical process to an inorganic SEI layer rich in LiF and Li₂CO₃. This study is a model example of how a combination of highly surface-sensitive operando characterization techniques offers a step forward to understand interfacial phenomena in Li-ion batteries.