XPS and SIMS Analysis of Solid Electrolyte Interphases on Lithium Formed by Ether-Based Electrolytes

The chemical composition and microstructure of the solid electrolyte interphase (SEI) on lithium metal in ether-based electrolytes are investigated using X-ray photoelectron spectroscopy (XPS), time of flight secondary ion mass spectrometry (ToF-SIMS) and scanning electron microscopy (SEM). Electrolytes based on 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) solvents with lithium bis(trifluoromethane)sulfonamide (LiTFSI) conducting salt are employed to passivate lithium surfaces for different periods of time. Two types of model experiments (immersion-type and galvanostatic current load) are performed to generate self- and current-induced SEIs. Reference measurements of predicted SEI components and an advanced XPS signal coupling method facilitate the deconvolution of the spectroscopic and spectrometric data. Both ether solvents showed specific SEI formation for each experimental setup. DME appears to decompose immediately on the lithium surface and forms a thin passivation film of various salts in a lithium-alkoxide framework. DOL shows a slow SEI formation of organic and inorganic salts revealing problems in the lithium surface protection. Under current load the reaction rate is increased in DME maintaining the three-dimensional microstructure. For DOL the current load leads to a multi-layer structure of organic host materials comprising LiTFSI and lithium fluoride. Finally, two-dimensional schematic pictures of the SEI microstructure are developed.