We report on a combined ultrahigh vacuum–electrochemistry (UHV-EC) approach used to probe the chemical and morphological changes of highly oriented pyrolytic graphite (HOPG) electrodes immersed in Li+-containing nonaqueous electrolytes. UHV-EC provides a “snapshot-like” analysis of electrode surfaces without perturbation from washing or rinsing. This is accomplished by performing electrochemistry, followed by clean transfer into vacuum for X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (UHV-STM). Employing this approach together with HOPG serving as a model system for the graphite negative electrode in lithium-ion batteries, we focus on the changes to the electrode surface induced by cathodic polarization. XPS following polarization at ca. 1.75 V vs Li/Li+ identifies a low amount of the residual electrolyte and products from the decomposition of the Li salt (LiPF6) including LiF. Meanwhile, high-resolution UHV-STM imaging shows the occurrence of graphite exfoliation, and at higher magnifications, the edge planes show aggregated structures while also suggesting the presence of residual electrolyte and decomposition products. Our work shows that notable changes to the graphite electrode surface are already present prior to the potentials, where significant surface film (solid electrolyte interphase) formation occurs.

Lithium-ion (Li-ion) batteries have become ubiquitous to modern-day electronics and electric vehicles. A detailed understanding regarding the reversible and irreversible nature of Li-ion electrode processes is a prerequisite to rationally delineate the factors affecting device performance and clarify degradation mechanisms. This includes, for instance, the role of electrode cracking and solid solid-electrolyte-interphase (SEI) formation which can contribute toward irreversible capacity loss. In terms of experimental approaches, aside from the high surface area electrodes used in practical systems, one powerful approach is to incorporate the study of well-defined electrodes and surface science techniques.1 Herein, we employ highly oriented pyrolytic graphite (HOPG) as a well-defined model anode in Li-ion batteries and reveal the chemical and morphological changes at low cathodic potentials prior to the potentials of bulk SEI formation.2 To achieve this, we employ the use of X-ray photoelectron spectroscopy (XPS) and ultrahigh vacuum scanning tunneling microscopy (UHV-STM). We utilize a so-called UHV-EC setup to provide a snapshot-like analysis of the electrode surface without the involvement of electrode washing or exposure to air. Upon cathodic polarization at ca. 1.75V (Figure 1), STM imaging shows the onset and occurrence of graphite exfoliation which can originate from the intercalation of solvated Li+ ions. Meanwhile, XPS shows a minimal amount of residual electrolyte along with the observation of LiF indicating the decomposition of the Li salt (LiPF6). The future extensions of our methodology and additional details will be provided at the presentation. Figure 1. (a) Cathodic linear sweep voltammogram in 1.1 M LiPF6 EC/DMC. Following polarization at ca. 1.75 V (b) STM imaging, and (c) XPS F 1s and Li 1s spectra. References [1] T. Minato, T. Abe, Prog. Surf. Sci. 92, 240-280 (2017). [2] R. A. Wong, Y. Yokota, E. Kazuma, M. Oniki, Y. Kim, J. Phys. Chem. C 125, 21093-21100 (2021). Figure 1

No abstract