Surface Analysis of Li-Ion Electrode-Electrolyte Interfaces: Insights By a Surface Science Approach

Abstract

Electrode-electrolyte interfaces play a key role for the performance and degradation of electrochemical devices such as lithium-ion cells. Side reactions at cathode-electrolyte interfaces result in the formation of a solid-electrolyte interface (SEI) layer, resulting in loss or performance due to lithium loss and increase in surface layer resistance. Over the past years, the cathodic SEI layer has been subject to extensive investigation, focused on the correlation between composition and battery cycling parameters. Yet, open questions remain with regard to formation mechanisms and properties. Most investigations on SEI layers have been performed by post mortem analysis of composite electrode surfaces by photoelectron spectroscopy (XPS). XPS gives information on the chemical structure by way of core level electron binding energies. Despite of the standard use of XPS for SEI analysis, unambiguous identification of the different phases is often not straightforward as binding energy shifts are encountered. Moreover, the post mortem analysis of composite materials is not ideally suited to investigate the SEI on the active material, as emissions from the other materials in the electrode (carbon, binder) add complexity. Nevertheless, such data on the SEI composition of composite electrodes are often used to propose reaction mechanisms on SEI-formation and side reactions (electrolyte oxidation) occurring at the surface of the active materials. Improved insights into SEI-composition and side reactions require experiments that address binding energies and interaction of single material surfaces with components of the electrolyte. We prepare model interfaces of LCO (LiCoO2) thin film electrodes with components of the SEI, artificial SEI-layers, as well as with solvents, and analyze them with surface analysis methods [1]. In such experiments, the SEI or electrolyte-related components are deposited as top phase either by thin film deposition or low temperature adsorption, and analysis is performed by photoelectron spectroscopy (XPS, UPS, SXPS) and high resolution electron energy loss spectroscopy (HREELS). Usually, the preparation is performed stepwise with intermediate analysis in order to investigate the interface formation in detail with respect to chemical structure, electronic structure and space charge effects. In this contribution, we present our results with respect to the deposition of several solid compounds such as lithium oxide and LiPON, as well as the exposure to several solvents (water, diethyl carbonate, etc.). In most cases, the formation of interface and/or gradient layers is observed. For the substrate-related binding energies we usually note a shift to higher values which is attributed to space charge layer formation in the LiCoO2 (band bending downwards). Also, we find indications that binding energies from nominally identical overlayer compounds and those related to the substrate are differently positioned against each other depending on the preparation method. Possible causes for this are charging effects, differences in the overlayer compound´s Fermi-level, or presence of interfacial dipoles. Mechanistic insights regarding the formation of cathode-electrolyte interfaces and of ionic interfaces in general are discussed [2][3]. [1] R. Hausbrand, D. Becker, and W. Jaegermann, Progress in Solid State Chemistry 42 (2014) [2] A. Schwöbel, W. Jaegermann, and R. Hausbrand, Solid State Ionics, in press. [3] D. Becker, et al., Journal of Physical Chemistry C, 118 (2014)