Role of Oxygen in the Formation of the Solid-Electrolyte Interphase Evaluated by Online Electrochemical Mass Spectrometry and Electrochemical Atomic Force Microscopy

Solid-electrolyte interphase (SEI) is a passive film formed on the anode surface of Li-ion batteries by decomposing organic electrolyte solutions during the initial charging process. It plays an essential role in the stability and cyclability of Li-ion batteries. We found that a small amount of oxygen can significantly influence the SEI formation process and its properties on the carbon anode surface of Li-ion batteries in ethylene carbonate (EC)-based electrolyte solution. Herein, a quantitative evaluation of the effect of oxygen concentration was conducted by detecting volatile product evolution during SEI formation by online mass spectrometry. In addition, electrochemical atomic force microscopy was employed to observe the morphological features of the electrode surface during the SEI formation process in argon and oxygen atmospheres. Furthermore, attenuated total reflection Fourier transform infrared spectroscopy was used to investigate the chemical composition of the SEI. Our results show that the presence of oxygen strongly suppressed the generation of ethylene (C2H4) molecules during SEI formation. We propose a novel mechanism for SEI formation in the oxygen-containing EC electrolyte. In the presence of oxygen, the oxygen reduction reaction (ORR) generates superoxide ions which can induce EC decomposition and form a loose SEI film on the electrode surface. As the potential becomes more negative, EC molecules can be electrochemically reduced. The decomposition products can deposit on the defects and voids of the loose SEI film formed during the ORR stage, making the SEI film thicker and denser.