Role of Oxygen in Surface Structures of the Solid-Electrolyte Interphase Investigated by Sum Frequency Generation Vibrational Spectroscopy

The solid-electrolyte interphase (SEI) layer on the anode surface is crucial for the operation of lithium-ion batteries. It remains unclear how oxygen in electrolyte influences the SEI formation. We now report that the surface structures of the SEI layers are significantly affected by the oxygen in the electrolyte, by studying the SEI formation on a single-layer graphene electrode with an ethylene carbonate (EC)-based electrolyte as a model system. Surface-specific sum frequency generation spectroscopy measurements revealed that the presence of oxygen may significantly increase the polymeric species on the SEI surface. The superoxide radical anion, generated from the oxygen reduction reaction, is proposed to induce radical-initiated polymerization of the EC and form a large amount of polymeric species on the electrode surface.