Chemical and Morphological Changes of Graphite Electrodes at Low Cathodic Potentials and Its Relevance to Li-Ion Batteries

Abstract

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

Bridging Electrochemistry and Ultrahigh Vacuum: “Unburying” the Electrode–Electrolyte Interface

“…To bridge electrochemistry to XPS/UPS and scanning tunneling microscopy (UHV-STM), the setup consists of two chambers composed of the analysis chambers kept at UHV and a dedicated EC chamber. 1,4,36 The intent of such a setup is to reduce unintended changes (i.e., contamination or otherwise) to the electrode surface when transferring between the two environments. The EC chamber contains a retractable EC cell, that can be pressurized with ultrapure Ar during electrochemical measurements, and then subsequently evacuated to enable sample transfer between the EC chamber and the analysis chamber.…”

“…On a similar thread, we have also examined the morphology changes of HOPG (highly oriented pyrolytic graphite) emersed from nonaqueous Li + electrolytes which is a representative model system as the graphite negative electrode in lithium-ion batteries. 4,57 Starting from the onset of ca. 2.4 V, the cathodic linear sweep voltammogram (LSV, Figure 6d and inset) in 1.1 M LiPF 6 in EC/DMC (ethylene and dimethyl carbonate) (3:7) is attributed to electrolyte reduction, forming the so-called solid−electrolyte interphase (SEI) consisting of both inorganic and organic decomposition products.…”