Graphite Electrodes Immersed in Nonaqueous Li⁺ Electrolytes Studied with a Combined Ultrahigh Vacuum–Electrochemistry Approach

Abstract

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.

In situ and ex situ approaches for molecular scale understanding of electrochemical interfaces

“…The 1.6 eV peak in the pristine sample has been attributed to the HOMO states of ferrocene spatially localized on Fe 2+ ion. [53][54][55][56] This peak disappears when the electrode is emersed at the higher potential (the Fe 3+ state). This means that as the molecule is oxidized, the ferrocene becomes positively charged and the entire orbitals shift toward the higher binding energy side.…”

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

“…Figure and Figure S1 provide a high-level overview of the UHV-EC experimental workflow employed in our laboratory. 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. ,, 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.…”

“…(f) After emersion at 1.75 V (V vs Li/Li + ) ( V = 0.5 V, I = 0.2 nA, 200 × 200 nm 2 ). UHV-STM imaging was performed at ∼5 K. Reproduced from ref . Copyright 2021 American Chemical Society.…”

“…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. , Starting from the onset of ca . 2.4 V, the cathodic linear sweep voltammogram (LSV, Figure d 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. , At further cathodic potentials following SEI formation, Li + intercalation occurs predominantly at cathodic potentials <0.27 V.…”

“…There is a shared consensus on the need to obtain a more comprehensive microscopic (molecular-level) understanding of electrode–electrolyte interfaces using model systems. − This is because it is recognized as a prerequisite and co-requisite to create materials and interfaces with controllable functionality and behavior. This encompasses, for example, energy storage/conversion via batteries and electrocatalysis for our transition to carbon neutrality − and important contributions to healthcare and environmental diagnostics through electroactive materials and bio­(chemical) sensing. , A further nonexhaustive list includes corrosion, metallurgy, and organic synthesis, thus exemplying the growing influence of electrochemistry in a broad range of fields .…”