Reactivity and Evolution of Ionic Phases in the Lithium Solid–Electrolyte Interphase

The unstable solid–electrolyte interphase (SEI) on Li anodes is the origin of major performance challenges in Li batteries, namely, insufficient Coulombic efficiency (CE) and cycle life. While it is known that the SEI participates in aging processes, pinpointing the chemical origins by tracing them to specific SEI phases has been experimentally challenging. Here, we formed single-phase, thin (<50 nm) interfaces of Li2O or LiF—the two most common ionic SEI phases—on Li and investigated their stability upon immersion in ether- or carbonate-based electrolytes. Contrary to some conventional wisdom that ionic phases are stable, we find by electrochemical impedance and X-ray spectroscopy that ionic SEI|electrolyte interfaces can undergo significant chemical evolution. While DOL/DME electrolytes impart minimal changes, organic/F-rich layers evolve at interfaces between Li2O or LiF and carbonate electrolyte containing LiPF6 salt, exacerbating subsequent plating overpotentials. The results suggest that electrolyte selection is important to improve transport in ionic-rich Li interfaces.