Highlights-Electrochemical impedance spectra measured during the first and second lithiation.-SEI formation was studied at elevated and lower temperatures.

The solid electrolyte interphase (SEI) is a complex and fragile passivation layer with crucial importance for the functionality of lithium-ion batteries. Due to its fragility and reactivity, the use of in situ techniques is preferable for the determination of the SEI's true structure and morphology during its formation. In this study, we use in situ neutron reflectometry (NR) and in situ atomic force microscopy (AFM) to investigate the SEI formation on a carbon surface. It was found that a lithium-rich adsorption layer is already present at the open circuit voltage on the carbon sample surface and that the first decomposition products start to deposit close to this potential. During the negative potential sweep, the growth of the SEI can be observed in detail by AFM and NR. This allows precise monitoring of the morphology evolution and the resulting heterogeneities of individual SEI features. NR measurements show a maximum SEI thickness of 192 Å at the lower cutoff potential (0.02 V vs Li/Li), which slightly decreases during the positive potential scan. The scattering length density (SLD) obtained by NR provides additional information on the SEI's chemical nature and structural evolution.

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To gain new insights into the formation of the solid electrolyte interphase (SEI), as a basis for the safe and efficient use of new anode materials, we studied SEI formation on silicon and lithium titanate (LTO) anodes by electrochemical impedance spectroscopy (EIS) and ex situ X-ray photoelectron spectroscopy (XPS) measurements. While EIS measurements performed at equidistant voltage intervals provided insights into the SEI formation process, ex situ X-ray photoelectron spectroscopy (XPS) measurements supplied data on the chemical composition of the SEI layer. On silicon anodes we observed that resistance decreases in the second cycle which suggests the formation of a stable SEI with SiO 2 , Li 4 SiO 4 , LiF and different carbonates as its main components. On LTO anodes, however, resistance increases by a factor of two indicating incomplete SEI formation. Here LiF and different carbonates were identified as the SEI's main components.

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