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

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.