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

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.