In lithium batteries, the solid electrolyte interphase (SEI) at the negative electrode | electrolyte interface 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.
With a typical information depth of 5 – 10 nm, X-ray photoelectron spectroscopy (XPS) allows for an investigation of the SEI which is typically as thin as a few nanometers. Determination of SEI composition and thickness has been the subject of intensive research. Several aging mechanisms, such as loss of lithium inventory (LLI), loss of active material (LAM) and increase of internal resistance (LRI), can result in capacity fading – especially at elevated temperature. It is not known, however, whether XPS can be used to accurately quantify LLI for cells, which have been aged under harsh conditions, i.e. if the surface composition changes towards Li-rich components and the SEI gradually grows thicker. Due to usually high standard deviation in the SEI composition, an XPS-based methodology for quantitative SEI homogeneity determination was developed, which is now applied to cells after cycling at 60 °C.
Several characteristic aging phenomena play a role in the strong capacity fading at 60 °C. LRI / capacity trapping and weight loss of the positive electrode are observed. XPS shows a high concentration of fluorophosphate-type species, indicating continuous partial SEI dissolution. At the same time, the specific surface area of the negative electrodes increases strongly. Thus, the necessary SEI re-formation leads to ever-increasing LLI. This LLI can, however, not directly be quantified using XPS for cells after 60 °C cycling. So far, only the thickness of the currently “active” part of the SEI, but not the thickness of the partially dissolved SEI can be assessed. If a reliable substrate signal can be determined for this, XPS could be of great help quantifying LLI based on the thickness and composition found within the layer of deactivated, partially dissolved SEI.
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