Direct Operando Observation of Double Layer Charging and Early SEI Formation in Li-Ion Battery Electrolytes

The solid electrolyte interphase (SEI) is one of the most critical, yet least understood, components to guarantee a
stable, long-lived and safe operation of the Li-ion cell. Herein, the early stages of SEI formation in a typical commercially-available
LiPF6 and organic carbonate based Li-ion electrolyte are explored by operando surface enhanced Raman spectroscopy (SERS),
online electrochemical mass spectrometry (OEMS), and electrochemical quartz crystal microbalance (EQCM). The electric double-
layer is directly observed to charge as Li+ solvated by EC progressively accumulates at the negatively charged electrode surface.
Further negative polarization triggers SEI formation as evidenced by H2 evolution, electrode mass deposition, and expulsion of the
electrolyte from the electrode surface. Electrolyte impurities, such as HF and H2O, are reduced early and contribute in a multistep
electro-/chemical process to an inorganic SEI layer rich in LiF and Li2CO3. These results underline the strong influence of trace
impurities on the buildup of the SEI layer, and give new insight into the formation mechanism of the multi-layered SEI. The presented
study is a model example of how a combination of complementary and highly surface-sensitive operando characterization techniques
offer a step forward to understand interfacial phenomenon and SEI formation mechanisms in future Li-ion batteries