Structure formation and surface chemistry of ionic liquids on model electrode surfaces—Model studies for the electrode | electrolyte interface in Li-ion batteries

Abstract

Ionic liquids (ILs) are considered as attractive electrolyte solvents in modern battery concepts such as Li-ion batteries. Here we present a comprehensive review of the results of previous model studies on the interaction of the battery relevant IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]+[TFSI]−) with a series of structurally and chemically well-defined model electrode surfaces, which are increasingly complex and relevant for battery applications [Ag(111), Au(111), Cu(111), pristine and lithiated highly oriented pyrolytic graphite (HOPG), and rutile TiO2(110)]. Combining surface science techniques such as high resolution scanning tunneling microscopy and X-ray photoelectron spectroscopy for characterizing surface structure and chemical composition in deposited (sub-)monolayer adlayers with dispersion corrected density functional theory based calculations, this work aims at a molecular scale understanding of the fundamental processes at the electrode | electrolyte interface, which are crucial for the development of the so-called solid electrolyte interphase (SEI) layer in batteries. Performed under idealized conditions, in an ultrahigh vacuum environment, these model studies provide detailed insights on the structure formation in the adlayer, the substrate–adsorbate and adsorbate–adsorbate interactions responsible for this, and the tendency for chemically induced decomposition of the IL. To mimic the situation in an electrolyte, we also investigated the interaction of adsorbed IL (sub-)monolayers with coadsorbed lithium. Even at 80 K, postdeposited Li is found to react with the IL, leading to decomposition products such as LiF, Li3N, Li2S, LixSOy, and Li2O. In the absence of a [BMP]+[TFSI]− adlayer, it tends to adsorb, dissolve, or intercalate into the substrate (metals, HOPG) or to react with the substrate (TiO2) above a critical temperature, forming LiOx and Ti3+ species in the latter case. Finally, the formation of stable decomposition products was found to sensitively change the equilibrium between surface Li and Li+ intercalated in the bulk, leading to a deintercalation from lithiated HOPG in the presence of an adsorbed IL adlayer at >230 K. Overall, these results provide detailed insights into the surface chemistry at the solid | electrolyte interface and the initial stages of SEI formation at electrode surfaces in the absence of an applied potential, which is essential for the further improvement of future Li-ion batteries.

Interaction of Ultrathin Films of Ethylene Carbonate with Oxidized and Reduced Lithium Cobalt Oxide—A Model Study of the Cathode|Electrolyte Interface in Li‐Ion Batteries

“…On lithiated HOPG, this was explained by the complete desorption of EC at low temperatures (≈200 K), where the mobility of Li + in the bulk of lithiated HOPG is too low to segregate toward the surface and react with the adsorbate phase . We recently demonstrated that Li can in fact easily deintercalate from the bulk of lithiated HOPG, but only at temperatures higher than 230 K and only in the presence of a strongly adsorbed species such as an ionic liquid adlayer (desorption temperature >400 K), which chemically stabilizes the surface Li …”

“…One possible way to determine the initial stages of the SEI formation at the EEI at the atomic/molecular level involves the use of surface science techniques, studying the interaction of individual components of electrolytes, such as the typical key component EC (or other electrolyte components like ionic liquids), with well‐defined model electrodes under idealized ultrahigh‐vacuum (UHV) conditions, which is focus of the ongoing work in our laboratory. Following a previous study on the interaction of EC with Li‐free and lithiated highly oriented pyrolytic graphite (HOPG) as model for the anode, we here report the results of a similar type of study on the interaction of EC with well‐defined LiCoO 2 electrode surfaces, both fully oxidized LiCoO 2 and partly reduced LiCoO 2− δ surfaces, as models for the cathode.…”

Model Studies on the Formation of the Solid Electrolyte Interphase: Reaction of Li with Ultrathin Adsorbed Ionic‐Liquid Films and Co3O4(111) Thin Films

“…A highly oriented pyrolytic graphite test substrate cooled to 80 K (no intercalation) was used for calibration of the Li deposition rate. [14] Deposition rates in MLE of approximately 0.04-0.05 MLE min À 1 were calculated from the damping of the C 1s substrate) peak after successive vapor deposition of Li at temperatures where Li adsorbs on the surface. For the evaluation we assume that 1 MLE of Li has a thickness d of 2.48 Å, equivalent to the (110) interplanar distance in a body centered cubic lattice (the most stable configuration of a Li metal at r.t.).…”

“…Therefore, in our model approach, we reduced the complexity significantly, and prepare well-defined model electrodes like Co 3 O 4 (111) thin film electrodes, under idealized ultrahigh vacuum (UHV) conditions, and explore the interaction of individual components of the battery electrolyte (e. g., ionic liquids, carbonates, lithium) using surface science techniques. [11][12][13][14][15][16][17][18] In particular ionic liquids (ILs), [19][20][21][22] that are organic salts with a melting point below 100°C, got into the focus as promising solvents in battery electrolytes, [23][24] as they are, for example, not flammable, such reducing the hazard of battery fires. [25] Consequently, they represent an important class of compounds whose interactions with electrodes need to be studied.…”

“…Another first principle calculation by Yao et al [28] identified a stable Li n Co 3 O 4 (n � 3) insertion phase, which preserves a rigid oxygen framework during lithiation and which is proposed to have a constrained volume expansion and enhanced cycling stability at a capacity of~334 mAh g À 1 , before a conversion-type reaction starts above 3 mol of Li per formular unit of Co 3 O 4 . In addition, several model studies were conducted on the interaction of BMP-TFSI with different model electrode surfaces, and after Li exposure, e. g., on Cu(111), [29] a copper foil, [30][31] highly oriented pyrolytic graphite (HOPG), [11,[13][14] TiO 2 (110), [32] Si(111) [33] and very recently on CoO(111). [26] On CoO(111) stepwise post-deposition of small amounts of metallic Li (< one monolayer equivalent (MLE)) to a pre-adsorbed BMP-TFSI adlayer results in the gradual decomposition of BMP-TFSI into several decomposition products such as Li 2 S, LiF, LiC x H y N z Li 2 NSO 2 CF 3 , SO 2 CF 3 .…”