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 LiPF 6 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 H 2 evolution, electrode mass deposition, and expulsion of the electrolyte from the electrode surface. Electrolyte impurities, such as HF and H 2 O, are reduced early and contribute in a multistep electro-/chemical process to an inorganic SEI layer rich in LiF and Li 2 CO 3 . 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