Interaction of a Self-Assembled Ionic Liquid Layer with Graphite(0001): A Combined Experimental and Theoretical Study

Abstract

The interaction between (sub)monolayers of the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide [BMP](+)[TFSA](-) and graphite(0001), which serves as a model for the anode|electolyte interface in Li-ion batteries, was investigated under ultrahigh vacuum conditions in a combined experimental and theoretical approach. High-resolution scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and dispersion-corrected density functional theory (DFT-D) calculations were employed. After vapor deposition at 300 K, XPS indicates molecular adsorbates with a 1:1 ratio of cations/anions. Cool down to ∼100 K leads to the formation of an ordered (2D) crystalline phase, which coexists with a mobile (2D) liquid. DFT-D calculations reveal that adsorbed [BMP](+) and [TFSA](-) species are arranged alternately in a row-like adsorption structure (cation-anion-cation-anion) and that adsorption is dominated by dispersion interactions between adlayer and substrate, on the one hand, and electrostatic interactions between the ions in a row, on the other hand. Simulated STM images of that structure closely resemble the experimental molecular resolved STM images and show that the resolved features mostly stem from the cations.

Surface Science and Electrochemical Model Studies on the Interaction of Graphite and Li‐Containing Ionic Liquids

“…In the S 2p region, finally, a doublet for the −SO 2 CF 3 species (S TFSI ) appears at 169.6 (S 2p 3/2 ) and 170.8 eV (S 2p 1/2 ). Our assignment is in agreement with those reported previously for [BMP][TFSI] on metal (oxide) and HOPG substrates, as well as for similar ionic liquids containing [TFSI] − anions on Au(1 1 1) …”

“…Owing to its high decomposition temperature, very low vapor pressure, and a large stability window ranging from −2.5 to 3.0 V versus Ag/AgCl (about 0–5 V vs. Li/Li + ), [BMP][TFSI] is a very promising candidate for LIBs. Special interest was placed on the interactions at the substrate|IL interface under UHV conditions by using several model substrates, for example, single‐crystalline metal surfaces, oxide surfaces, and highly oriented pyrolytic graphite . The above studies, which were conducted in the absence of an applied potential and which focused on the structure formation and the decomposition of the ionic liquid, clearly demonstrated that the chemical interaction with the substrate surface and/or with the added lithium is sufficient to cause decomposition of the IL.…”

Accurate chemical analysis of oxygenated graphene-based materials using X-ray photoelectron spectroscopy

“…This allows the quantification of elemental composition in the parts-per-thousand range, as well as the nature of the chemical bonds. XPS can thus provide a measure of the number of defects through the O/C ratio, quantify the different types of carbon functionalities present, indicate the formation of chemical bonds, and evaluate the physisorption of molecules [11,12]. The importance of this quantification is crucial in order to correlate chemical properties of GRMs with their performance, for example, in permeability [5], water purification [13] or bio-sensing [14].…”

Fundamental aspects of electric double layer force-distance measurements at liquid-solid interfaces using atomic force microscopy

“…In recent years numerous efforts have gone toward understanding the complex structure of ILs at the solid-liquid interface using both theoretical approaches6789101112 and experimental methods such as scattering techniques131415, sum-frequency generation161718, surface-force apparatus19202122232425, and scanning probe techniques. For the latter, scanning tunneling microscopy (STM)2627282930313233343536 and dynamic3738394041 and static42434445464748495051 atomic force microscopy (AFM) approaches have made large progress towards imaging the ion layers in two and three dimensions at neutral and charged surfaces such as mica, silica, gold, and highly-oriented pyrolytic graphite (HOPG). Scanning probe microscopy based techniques offer the advantage of high spatial resolution in three dimensions compared to other techniques, allowing for the ion structure to be visualized in a 3D manner, as opposed scattering techniques and SFA where the response is averaged over large areas.…”