The solid electrolyte interphase (SEI) is a passivating layer of electrolytic decomposition products at the negative electrode | electrolyte interface in lithium batteries [1,2]. The SEI must passivate the electrode surface from ongoing electrolyte decomposition. It must not, however, decelerate interface kinetics to a major extent. In that regard, an effective SEI governs cell performance if it possesses the optimal set of electrical and mechanical properties.
X-ray photoelectron spectroscopy (XPS) is a well-established surface analytical technique. With a typical information depth of 5 – 10 nm, XPS allows for an investigation of the SEI which is typically as thin as a few nanometers. The composition of the SEI has been the subject of intensive research. Typical SEI constituents comprise inorganic species, such as lithium fluoride and lithium carbonate, as well as organic species, such as polymers and organic carbonates. 
In a previous study, an XPS-based methodology for quantitative SEI homogeneity determination of electrochemically formatted cells was introduced . Here, this method is applied to cells after electrochemical cycling at 60 °C. The homogeneity of the surface composition is assessed within differently sized sampling areas and between different cells. It is found, that temperature-induced SEI growth diminishing the Li+ inventory is not necessarily the main reason for less-than-ideal cycling performance. In contrast, a homogenization of the surface composition is observed.
The authors acknowledge the BMBF for funding the projects “HiT-Cell” (03XP0113) and “KlemA” (03XP0190D).
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