In-situ EC-AFM and ex-situ XPS characterization to investigate the mechanism of SEI formation in highly concentrated aqueous electrolyte for Li-ion batteries

Abstract

The structure and chemical composition of solid electrolyte interphase (SEI) play a critical role in the performance of Li-ion batteries. However, the complexity of SEI layer and lack of relevant and efficient characterization techniques limit understanding of SEI formation mechanism and associated properties. Moreover, compared with organic electrolyte-based SEI, few studies have focused on aqueous electrolyte-based solid-electrolyte interphase (A-SEI). Herein, we present in-situ AFM images to demonstrate the morphological evolution of SEI in the presence of 21 M lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) aqueous electrolyte. Moreover, a combination of electrochemical-controlled atomic force microscopy (EC-AFM) and ex-situ X-ray photoelectron spectroscopy (XPS) is utilized to unveil the mechanism of SEI formation and identify its components. The results reveal that A-SEI layer is unevenly distributed and the inner and outer layers are mainly composed of Li2CO3 and LiF, respectively. The Young’s modulus of 4–6 nm thick SEI layer is found to be 30 ± 10 GPa. The current study provides novel insights into the formation of A-SEI by using in-situ AFM and provides a baseline for the future development of aqueous electrolytes for Li-ion batteries.