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

Abstract

<div>The solid electrolyte interphase (SEI) is one of the most critical, yet least understood, components to guarantee a </div><div>stable, long-lived and safe operation of the Li-ion cell. Herein, the early stages of SEI formation in a typical commercially-available </div><div>LiPF<sub>6</sub> and organic carbonate based Li-ion electrolyte are explored by <i>operando</i> surface enhanced Raman spectroscopy (SERS), </div><div>online electrochemical mass spectrometry (OEMS), and electrochemical quartz crystal microbalance (EQCM). The electric double-</div><div>layer is directly observed to charge as Li<sup>+</sup> solvated by EC progressively accumulates at the negatively charged electrode surface. </div><div>Further negative polarization triggers SEI formation as evidenced by H<sub>2</sub> evolution, electrode mass deposition, and expulsion of the </div><div>electrolyte from the electrode surface. Electrolyte impurities, such as HF and H<sub>2</sub>O, are reduced early and contribute in a multistep </div><div>electro-/chemical process to an inorganic SEI layer rich in LiF and Li<sub>2</sub>CO<sub>3</sub>. These results underline the strong influence of trace </div><div>impurities on the buildup of the SEI layer, and give new insight into the formation mechanism of the multi-layered SEI. The presented </div><div>study is a model example of how a combination of complementary and highly surface-sensitive operando characterization techniques </div><div>offer a step forward to understand interfacial phenomenon and SEI formation mechanisms in future Li-ion batteries</div>