Advanced Interface Characterization Using Surface Science Methods: Recent Insights for Interfaces in Liquid and Solid Electrolyte Based Cells

摘要

The analysis of interface (SEI) layers in Li-ion batteries is a challenge both in the case of liquid- and solid electrolyte based cells. Their buried nature and high reactivity towards ambient atmosphere require strategies to access the layers with surface sensitive characterization methods and to analyze them in an unaltered state. An additional issue is the high compositional complexity of composite electrodes and their surface layers, which impairs the identification of the different phases and renders the deduction of reaction mechanisms difficult. In this contribution, we present our view on surface and interface layer formation in liquid and solid electrolyte cells as obtained from dedicated analysis by photoelectron spectroscopy. In our approach, we use reference spectra and analysis of thin film model electrodes in order to support the identification of the different phases and possible reaction schemes. Specifically, we present recent insights on the surface layer formation in liquid electrolyte composite cathodes as well as on the formation of solid electrolyte interfaces. Our studies on liquid electrolyte based LCO cathodes demonstrate that hydroxides may be formed in the near-surface region of cathodes, which we attribute to the presence of trace water in the electrolyte. Furthermore, our studies highlight the relevance of solvent reduction and catalytic processes for solvent decomposition at liquid electrolyte based cathodes. For solid electrolyte interfaces, we find reactive or non-reactive interfaces, depending on the material combination. Often, space charge layer formation is observed. Both for liquid and solid electrolyte systems, reactivity is strongly related to the presence of electronic interface states and/or the transfer of oxygen species, which must be eliminated to obtain stable interfaces.