Model Studies on the Formation of the Solid Electrolyte Interphase: Reaction of Li with Ultrathin Adsorbed Ionic-Liquid Films and Co3 O4 (111) Thin Films.

In this work we aim towards the molecular understanding of the solid electrolyte interphase (SEI) formation at the electrode electrolyte interface (EEI). Herein, we investigated the interaction between the battery-relevant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI), Li and a Co3 O4 (111) thin film model anode grown on Ir(100) as a model study of the SEI formation in Li-ion batteries (LIBs). We employed mostly X-ray photoelectron spectroscopy (XPS) in combination with dispersion-corrected density functional theory calculations (DFT-D3). If the surface is pre-covered by BMP-TFSI species (model electrolyte), post-deposition of Li (Li+ ion shuttle) reveals thermodynamically favorable TFSI decomposition products such as LiCN, Li2 NSO2 CF3 , LiF, Li2 S, Li2 O2 , Li2 O, but also kinetic products like Li2 NCH3 C4 H9 or LiNCH3 C4 H9 of BMP. Simultaneously, Li adsorption and/or lithiation of Co3 O4 (111) to Lin Co3 O4 takes place due to insertion via step edges or defects; a partial transformation to CoO cannot be excluded. Formation of Co0 could not be observed in the experiment indicating that surface reaction products and inserted/adsorbed Li at the step edges may inhibit or slow down further Li diffusion into the bulk. This study provides detailed insights of the SEI formation at the EEI, which might be crucial for the improvement of future batteries.

Adsorption of Ultrathin Ethylene Carbonate Films on Pristine and Lithiated Graphite and Their Interaction with Li.

Aiming at a better understanding of the solid-electrolyte interphase formation in Li-ion batteries, we have investigated the interaction of ultrathin films of ethylene carbonate (EC), which is a key solvent of battery electrolytes, with pristine and lithiated highly oriented pyrolytic graphite (HOPG) and with postdeposited Li. Employing X-ray and ultraviolet photoelectron spectroscopy as well as Fourier transform infrared spectroscopy under ultrahigh-vacuum conditions, in combination with density functional theory (DFT)-based calculations, we find that EC adsorbs molecularly intact on pristine HOPG in the entire temperature range between 80 K and desorption at 200 K. Features in the ultraviolet photoelectron spectra could be related to the molecular orbitals of EC obtained from DFT calculations, and a similar adsorption/desorption behavior is obtained also on lithiated HOPG. In contrast, stepwise postdeposition of ∼0.5 and one monolayer of Li0 on a preadsorbed EC adlayer leads not only to stabilization of Li+/Liδ+ at the surface, possibly as adsorbed Li+(EC) n species, but also to EC decomposition, forming products such as Li2CO3, ROCO2Li (CH2OCO2Li)2, and Li2O. Consequences on the electronic surface properties and on the stabilization of the resulting adlayer are discussed. Upon annealing up to room temperature, some residual Li-containing decomposition products remain on the surface, which is considered as the initial stage of the solid|electrolyte interphase formation at the electrode|electrolyte interface.

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

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.

The structure of water at a Pt(111) electrode and the potential of zero charge studied from first principles.

The structure of a liquid water layer on Pt(111) has been studied by ab initio molecular dynamics simulations based on periodic density functional theory calculations. First the reliability of the chosen exchange-correlation function has been validated by considering water clusters, bulk ice structures, and bulk liquid water, confirming that the dispersion corrected RPBE-D3/zero functional is a suitable choice. The simulations at room temperature yield that a water layer that is six layers thick is sufficient to yield liquid water properties in the interior of the water film. Performing a statistical average along the trajectory, a mean work function of 5.01 V is derived, giving a potential of zero charge of Pt(111) of 0.57 V vs. standard hydrogen electrode, in good agreement with experiments. Therefore we propose the RPBE-D3/zero functional as the appropriate choice for first-principles calculations addressing electrochemical aqueous electrolyte/metal electrode interfaces.

Halide adsorption on close-packed metal electrodes.

Two mechanisms have been cited as the reason for unexpected work function decrease upon adsorption of electronegative adatoms: electron spillout depletion [Michaelides et al., Phys. Rev. Lett., 2003, 90, 246103] and polarization of the adatom [Roman et al., Phys. Rev. Lett., 2013, 110, 156804]. We attempt to bridge the two pictures in this work. Work function changes due to the adsorption of halides on (111) surfaces of fcc metals (Ca, Sr, Ni, Pd, Pt, Cu, Ag, Au, Al and Pb) were studied using periodic density functional theory. The two mechanisms were found to be clearly independent of each other because of the opposite factors that lead to the work function decrease, and are therefore easy to distinguish. A more general picture of interpreting bond ionicity based on observed work function changes is discussed.

Dispersion corrected RPBE studies of liquid water.

The structure of liquid water has been addressed by ab initio molecular dynamics simulations based on density functional theory. Exchange-correlation effects have been described by the popular PBE and RPBE functionals within the generalized gradient approximation as these functionals also yield satisfactory results for metals which is important to model electrochemical interfaces from first principles. In addition, dispersive interactions are included by using dispersion-corrected schemes. It turns out that the dispersion-corrected RPBE functional reproduces liquid water properties quite well in contrast to the PBE functional. This is caused by the replacement of the over-estimated directional hydrogen-bonding in the PBE functional by non-directional dispersive interactions.

Temperature effects in the vibrational spectra of self-assembled monolayers.

The vibrational spectrum of a thiolate-based self-assembled monolayer fabricated by the adsorption of benzylmercaptan on a Au(111) substrate is studied using a combined experimental and theoretical approach employing infrared reflection absorption spectroscopy and density functional theory. The vibrational spectra are derived both using a finite differences approach and from ab initio molecular dynamics simulations at various temperatures. In addition, the possibility of adsorbate-induced reconstructions of the Au(111) substrate is taken into account. It turns out that the measured spectra can only be understood by taking finite temperatures into account.

Dispersive interactions in water bilayers at metallic surfaces: a comparison of the PBE and RPBE functional including semiempirical dispersion corrections.

The accuracy and reliability of the density functional theory (DFT)-D approach to account for dispersion effects in first-principles studies of water-metal interfaces has been addressed by studying several water-metal systems. In addition to performing periodic DFT calculations for semi-infinite substrates using the popular PBE and RPBE functionals, the water dimer and water-metal atom systems have also been treated by coupled-cluster calculations. We show that indeed semiempirical dispersion correction schemes can be used to yield thermodynamically stable water bilayers at surfaces. However, the actual density functional needs to be chosen carefully. Whereas the dispersion-corrected RPBE functional yields a good description of both the water-water and the water-metal interaction, the dispersion-corrected PBE functional overestimates the energies of both systems. In contrast thereto, the adsorption distances predicted by the PBE functional is hardly changed due to the additional dispersion interaction, explaining the good performance of previous DFT-PBE studies of water-metal systems.

The role of surface defects in large organic molecule adsorption: substrate configuration effects.

The role of the configuration of metal surface atoms in the interaction between individual large, planar organic molecules and a metal substrate was investigated by low-temperature scanning tunneling microscopy and density functional theory calculations, including a semi-empirical correction scheme to account for dispersion effects. As test case, we used the adsorption of the oligopyridine derivative 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(pyridine-4-yl)pyridine-2-yl)pyrimidine (2,4'-BTP) on a stepped Ag(100) surface. Both experiment, via statistical evaluation of the adsorption site and orientation of 2,4'-BTP admolecules, and theory indicate distinct structural effects. The results are compared with the adsorption behavior of pyridine derivatives and benzene on metal surfaces. Consequences on the understanding of the interaction between heteroatoms or functional groups in large organic adsorbates and metal atoms in typical nano-scaled surface defects and hence of the interaction with more realistic metal surfaces are discussed.

A highly ordered, aromatic bidentate self-assembled monolayer on Au(111): a combined experimental and theoretical study.

Self assembled monolayers (SAMs) made from an aromatic organodithiol, 2-mercaptomethylbenzenethiol (C(6)H(4)SHCH(2)SH) on Au(111) have been investigated in the context of a combined experimental and theoretical approach. The SAMs prepared by immersion of an Au-substrate in corresponding ethanolic solutions were characterized using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS) and thermal desorption spectroscopy (TDS). Adapted from the experimentally obtained unit cell of the SAM, density functional theory (DFT) calculations were applied to get a deeper insight into the structure of these dithiolate based SAMs. On the basis of the experimental and theoretical findings we provide a detailed structural model for this aromatic SAM consisting of the phenyl-group rigidly anchored to the substrate by two thiolate-bonds.

Adsorption of small aromatic molecules on the (111) surfaces of noble metals: A density functional theory study with semiempirical corrections for dispersion effects.

The adsorption of benzene, thiophene, and pyridine on the (111) surface of gold and copper have been studied using density functional theory (DFT). Adsorption geometries and energies as well as the nature of bonding have been analyzed and compared to experimental results. Dispersion effects between neighboring molecules and between molecules and the surface have been taken into account via a semiempirical C(6)R(-6) approach. The C(6) coefficients for metal atoms have been deduced using both atomic properties and a hybrid QM:QM approach. Whereas the pure DFT calculations underestimate the adsorption energies significantly, a good agreement with experimental results is obtained using the DFT-D method based on the QM:QM hybrid approach.

Improved DFT Adsorption Energies with Semiempirical Dispersion Corrections.

Over the past years, density functional theory (DFT) became a widely approved and successful method for calculating properties of various materials and molecules. Especially suited for systems with delocalized electrons like metals, the efficient treatment of the van der Waals interaction remained a problem for DFT functionals within the generalized gradient approximation (GGA). Combining Grimme's D3 correction with the RPBE functional and using a previously published data set, we show that this yields a functional that is well-suited for an accurate and balanced description of adsorption energies. The RPBE-D3 approach performs comparably to higher-level methods such as the BEEF-vdW and the SW-R88 method. Even for oxide systems, which traditionally are not well-described by GGA functionals, RPBE-D3 leads to satisfactorily results when combined with the +U approach, as demonstrated with respect to the energetic ordering of the three TiO2 polymorphs rutile, anatase, and brookite.

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

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.

Change of the work function of platinum electrodes induced by halide adsorption.

The properties of a halogen-covered platinum(111) surface have been studied by using density functional theory (DFT), because halides are often present at electrochemical electrode/electrolyte interfaces. We focused in particular on the halogen-induced work function change as a function of the coverage of fluorine, chlorine, bromine and iodine. For electronegative adsorbates, an adsorption-induced increase of the work function is usually expected, yet we find a decrease of the work function for Cl, Br and I, which is most prominent at a coverage of approximately 0.25 ML. This coverage-dependent behavior can be explained by assuming a combination of charge transfer and polarization effects on the adsorbate layer. The results are contrasted to the adsorption of fluorine on calcium, a system in which a decrease in the work function is also observed despite a large charge transfer to the halogen adatom.

Toward the microscopic identification of anions and cations at the ionic liquid|Ag(111) interface: a combined experimental and theoretical investigation.

The interaction between an adsorbed 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [BMP][TFSA], ionic liquid (IL) layer and a Ag(111) substrate, under ultrahigh-vacuum conditions, was investigated in a combined experimental and theoretical approach, by high-resolution scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and dispersion-corrected density functional theory calculations (DFT-D). Most importantly, we succeeded in unambiguously identifying cations and anions in the adlayer by comparing experimental images with submolecular resolution and simulated STM images based on DFT calculations, and these findings are in perfect agreement with the 1:1 ratio of anions and cations adsorbed on the metal derived from XPS measurements. Different adlayer phases include a mobile 2D liquid phase at room temperature and two 2D solid phases at around 100 K, i.e., a 2D glass phase with short-range order and some residual, but very limited mobility and a long-range ordered 2D crystalline phase. The mobility in the different adlayer phases, including melting of the 2D crystalline phase, was evaluated by dynamic STM imaging. The DFT-D calculations show that the interaction with the substrate is composed of mainly van der Waals and weak electrostatic (dipole-induced dipole) interactions and that upon adsorption most of the charge remains at the IL, leading to attractive electrostatic interactions between the adsorbed species.

Effects of aggregates on mixed adsorption layers of poly(ethylene imine) and sodium dodecyl sulfate at the air/liquid interface.

We have exploited the spatial and kinetic resolution of ellipsometry to monitor the lateral movement of inhomogeneous patches of material in mixed adsorption layers of poly(ethylene imine) and sodium dodecyl sulfate at the air/liquid interface. We show that the choice of sample preparation methods can have a profound effect on the state of the interface for chemically equivalent samples. The extent of aggregation in the bulk solution on relevant time scales is affected by specific details of the polymer/surfactant mixing process, which produces varying numbers of aggregates that can become trapped in the interfacial layer, resulting in an enhanced and fluctuating ellipsometry signal. It can be beneficial to apply the surface-cleaning method of aspiration prior to physical measurements to remove trapped aggregates through the creation of a fresh interface. At low pH, the ellipsometry signal of samples prepared with surface cleaning is remarkably constant over a factor of >500 in the bulk composition below charge equivalence, which is discussed in terms of possible adsorption mechanisms. At high pH, through observing temporal fluctuations in the ellipsometry signal of samples prepared with surface cleaning, we reveal two important processes: there is the spontaneous adsorption of aggregates > 0.2 microm in diameter into the interfacial layer, and with time there is the fusion of smaller aggregates to generate new large surface aggregates. We attribute the favorability of the adsorption and fusion processes at high pH to reduced electrostatic barriers resulting from the low surface charge density of the aggregates. It is inappropriate in this case to consider the interface to comprise a homogeneous adsorption layer that is in dynamic equilibrium with the bulk solution. Our work shows that it can be helpful to consider whether there are macroscopic particles embedded in molecular layers at the air/liquid interface for systems where there is prior knowledge of aggregation in the bulk phase.

Structure formation in bis(terpyridine) derivative adlayers: molecule-substrate versus molecule-molecule interactions.

The influence of the substrate and the deposition conditions-vapor deposition versus deposition from solution-on the structures formed upon self-assembly of deposited bis(terpyridine) derivative (2,4'-BTP) monolayers on different hexagonal substrates, including highly oriented pyrolytic graphite (HOPG), Au(111), and (111)-oriented Ag thin films, was investigated by high-resolution scanning tunneling microscopy and by model calculations of the intermolecular energies and the lateral corrugation of the substrate-adsorbate interaction. Similar quasi-quadratic network structures with almost the same lattice constants obtained on all substrates are essentially identical to the optimum configuration expected from an optimization of the adlayer structure with C-H...N-type bridging bonds as a structure-determining factor, which underlines a key role of the intermolecular interactions in adlayer order. Slight distortions from the optimum values to form commensurate adlayer structures on the metal substrates and the preferential orientation of the adlayer with respect to the substrate are attributed to the substrate-adsorbate interactions, specifically, the lateral corrugation in the substrate-adsorbate interaction upon lateral displacement and rotation of the adsorbed BTP molecules. The fact that similar adlayer structures are obtained on HOPG under ultrahigh vacuum conditions (solid|gas interface) and on HOPG in trichlorobenzene (solid|liquid interface) indicates that the intermolecular interactions are not severely affected by the solvent.