Determination of solid electrolyte interphase formation mechanism on negative electrode surface in Li-O2 battery electrolyte by operando electrochemical atomic force microscopy observation

Abstract

Solid electrolyte interphase (SEI) on the negative electrode surface affect the electrochemical operation and cycle performance of the Li-O2 batteries. Two-type of SEI formation processes involving the O2 molecule on negative electrode surface have been suggested: (i) direct electrochemical reaction and (ii) indirect multi-step chemical reaction. In this research, the operando electrochemical atomic force microscopy observation was conducted to determine the proper model. Well-polished cupper substrate was used as model working electrode and was observed during the cyclic-voltammetry operation in the tetraglyme (G4) based electrolyte with and without dissolved O2. In the pristine electrolyte, in which O2 is not dissolved, no solid formation was observed. In the O2-dissolved electrolyte, the solid formation was suddenly observed from 1.3 V vs. Li counter electrode (LiCE) in the negative scan, which voltage is remarkably lower than the onset voltage of reduction current at 1.8 V vs. LiCE. This significant delay of solid formation against the electron transfer reaction strongly supported the multi-step solid formation model. The direct observation of electrode surface coupled with electrochemical operation determine the SEI formation mechanism in the Li-O2 battery.