On the Way to Elucidate Reaction Mechanisms that Involve Metallic Lithium and Atmospheric Gases Using In Situ Ambient Pressure X-Ray Photoelectron Spectroscopy
Authors: Etxebarria Ane , Blum Monica , Yun Dong-Jin , Muñoz Márquez Miguel Ángel , Crumlin Ethan J 
Published: February 2020

Metallic lithium, which would be the ideal anode for Li-ion rechargeable batteries, has not reached yet the commercial stage because of safety issues related to instabilities arising on the Li surface during electrochemical cycling. When metallic lithium is in contact with the electrolyte, a surface layer called SEI (Solid Electrolyte Interface) is formed. In a conventional Li-ion battery with graphite-based anode, the SEI layer is a passivation interface that will help to stabilize the battery. However, when considering Li metal anodes, the SEI is unable to maintain a uniform plating and stripping of Li. During charge/discharge processes the SEI breaks, continuously exposing fresh lithium surface to the electrolyte, leading to an irreversible capacity loss and the formation of dendrites. The latter are nanostructured formations that can grow on the anode surface until reaching the cathode surface and originating the fatal failure of the cell [1]. There are many approaches that aim to improve the stabilization of the Li metal surface. The use of solid electrolytes can inhibit the growth of dendrites while electrolyte additives are used to form a more stable SEI. An alternatively approach is the direct modification of the metallic Li surface to form a designed passivation layer before being in contact with the electrolyte [2]. However, despite all these efforts, the interfacial chemistry of these surfaces and interfaces are currently still not well understood.

In order to find a suitable solution for the stabilization of the lithium surface, it is essential to have a full understanding of the nature and characteristics of its high reactivity. For this purpose, one of the preliminary steps is to analyze the effects that atmospheric gases, such as carbon dioxide gas, oxygen gas and water, have on the surface of Li metal. Although there are a lot of studies from the last decades analyzing these interactions using a wide range of tools of both theoretical and experimental methods that propose interesting reaction mechanisms [3], little has been done using in situ experiments. There is still a lack of knowledge in terms of understanding the mechanism behind the reaction of metallic lithium with atmospheric gases. Ambient Pressure X-Ray Photoelectron Spectroscopy (APXPS) is an ideal tool to explore this phenomenon. Being a chemically specific surface sensitive technique, it allows determining which components are present on the first nanometers of the surface of the metal. Furthermore, ambient pressure studies make it possible to follow the reactions in situ. The possibility to perform experiments using a synchrotron based facility will add crucial information about the distribution of the components on the surface by non-destructive deep profiling experiments.

APXPS from beamline 9.3.2 at the Advanced Light Source, Lawrence Berkeley National Laboratory (California, United States), fulfill all this criteria. Thanks to the use of soft X-rays (hv < 800 eV), it is an extremely surface sensitive technique that can operate from Ultra High Vacuum (UHV) conditions up to pressure < 1 Torr. Our recent studies done were focused on the effect that carbon dioxide gas, oxygen gas and water vapor produce on the surface of UHV cleaned metallic lithium. We clearly observe two regimens in the growth of the layer for the three gases, a reaction controlled regime and a diffusion controlled regime, in good agreement with recent studies [4]. These two regimens can be observed in the Figure 1 for CO2 gas at 10 mTorr and at 0.1 mTorr. These experiments are supplemented with previous ex-situ studies in conditions using a laboratory based XPS at CIC Energigune research center (Álava, Spain), where the effect of CO2, O2 and N2 gases were studied on a UHV cleaned lithium surface.

The various gas exposure conditions created different surface final states of metallic lithium, which will affect the composition of the SEI layer that will influence the stability of the interface between metallic lithium anode and electrolyte. APXPS technique is proving to be an essential tool to bring more insight into the mechanisms that lead to the formation of this specific surface layer.

[1] Langa J. et al., Energy Storage Materials
7, 115 (2017)

[2] Cheng X.-B et al., Journal of Materials Chemistry A
3, 7207 (2015)

[3] Zhuang et al., Surface Science, 418, 139 (1998)

[4] Li Y. et al, Nano Letters, 17, 5171 (2017)

Figure 1