Thin Films: A Model Study of the Solid|Electrolyte I - ACS Publications

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Interaction between Li, Ultrathin Adsorbed Ionic Liquid Films and CoO(111) Thin Films: A Model Study of the Solid|Electrolyte Interphase Formation Florian Buchner, Katrin Forster-Tonigold, Jihyun Kim, Joachim Bansmann, Axel Gro#, and R. Jürgen Behm Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01253 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Chemistry of Materials

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Interaction between Li, Ultrathin Adsorbed Ionic Liquid Films and CoO(111) Thin Films: A Model Study of the Solid|Electrolyte Interphase Formation Florian Buchner, a Katrin Forster-Tonigold, b,c Jihyun Kim,a Joachim Bansmann,a Axel Groß,b,d and R. Jürgen Behm,*a,b [a]

Ulm University, Institute of Surface Chemistry and Catalysis, Albert-Einstein-Allee 47, D‒89081 Ulm, Germany

[b]

Helmholtz Institute Ulm Electrochemical Energy Storage (HIU), Helmholtzstraße 11, D‒89081 Ulm, Germany

[c]

Karlsruhe Institute of Technology (KIT), P.O. Box 3640, D-76021 Karlsruhe, Germany [d]

Ulm University, Institute of Theoretical Chemistry, Albert-Einstein-Allee 11, D‒89081 Ulm, Germany

Prof. Dr. R. J. Behm Universität Ulm Institut für Oberflächenchemie und Katalyse Albert‒Einstein‒Allee 47 D‒89069 Ulm, Germany Phone: +49 (0)731/50‒25451 Fax: +49 (0)731/50‒25452 E‒Mail: juergen.behm@uni‒ulm.de

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Abstract Aiming at a molecular level understanding of the processes at the electrode|electrolyte interface (EEI), we investigated the interaction between the battery-relevant Ionic Liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]+[TFSI]‒), Li and CoO(111) thin films on Ru(0001) as a model study of the solid|electrolyte interphase (SEI) in Li-ion batteries (LIBs). Employing mainly angle-dependent X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), in combination with dispersion-corrected density functional calculations (DFT-D) for characterization of the CoO(111) surface, we found that vapor deposition of metallic Li on CoO(111) at 300 K results in the conversion of Co2+ to Co0, together with the formation of Li2O and adsorbed surface Li2O2. The conversion starts in the near surface region (1-2 nm) and proceeds in the extended surface region (6-8 nm). If the surface is precovered by molecularly adsorbed [BMP][TFSI] species (solvent / electrolyte), stepwise postdeposition of small amounts of Li results in gradual decomposition of [TFSI] and [BMP] (= SEI formation ), forming products such as Li 3N, Li2S, LiF, LiwCxHyNz and other Li-bound fragments of the anion (e.g., LiNSO2CF3). For higher amounts of Li deposition, relative to the IL precoverage, IL decomposition is followed by conversion of CoO(111). Hence, the SEI resulting from IL decomposition is permeable for Li, which is essential for the storage of Li in the CoO(111) anode. This study demonstrates the potential of model studies for a molecular scale understanding of the initial stages of SEI formation at the EEI, and its role in Li storage in a CoO(111) model anode.


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1. Introduction The operation of Li-ion batteries (LIBs) involves two highly important processes, (i) the storage / extraction of Li-ions in / from the electrodes and (ii) the formation of the so-called solid-electrolyte interphase (SEI) at the electrode|electrolyte interface (EEI).1-2 In general, the reversible storage and extraction of Lithium (Li) usually occurs either by (de-)intercalation (e.g., for graphite anodes) or by (de-)insertion (e.g., for Li4Ti5O12 and LiCoO2). The Li storage capacity of these electrode materials is, however, limited by the available host sites in the electrodes. An alternative concept is the use of conversion materials with much higher Li storage capacities. For these materials, the storage / extraction of Li is accompanied by a phase transition of the electrode material.3 Cobalt oxides such as CoO and Co3O4 have been discussed as promising conversion materials for battery anodes in LIBs,3-7 for which (de)lithiation / (de-)conversion is manifested by the reversible reaction CoO + 2Li+ + 2e-  Co0 + 2 Li2O. Their main advantage is the high Li storage capacity, lying in the range of 700 – 1000 mAhg-1 3-7, which is significantly higher than that of other electrode materials (graphite: 372 mAhg-1

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; LiCoO2: 145 mAhg-1 3). Nevertheless, the cycling stability of metal oxides

has to be improved for their possible use in practical applications,8 which is likely limited by the massive structural changes of the electrode material during (dis-)charging. This in turn results in slow reaction kinetics and capacity losses.6 To resolve these issues, it is highly desirable to gain a detailed atomic/molecular scale understanding of the processes at the EEI. The second highly important process is the formation and chemical composition of the SEI, which is formed by decomposition of electrolyte at the interface.1 It not only protects the electrolyte from further decomposition, but, acting as Li+ ion conductor and e‒ isolator, also supports the ion transfer. This is basis for the long-term, stable performance of LIBs. Thus, a better understanding of the processes at the EEI during the SEI formation, on a molecular/atomic level, would be highly desirable. In real batteries, however, this is not feasible at present because there are no techniques available which could provide the ACS Paragon Plus Environment


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necessary information under operating conditions. Detailed mechanistic insights are also rudimentary due the complexity of the cell processes, including the transfer of Li+ ions, electrolyte reduction/oxidation and chemical reactions at the EEI, together with the fact that standard electrolytes usually consist of a mixture of several components and a Li salt. Therefore, in the last years we started to perform model studies on structurally well-defined electrode materials under idealized ultrahigh vacuum (UHV) conditions, using surface science techniques, with a focus on the surface chemistry during adsorption of individual components of the electrolyte (ionic liquids, carbonates, lithium) in the absence of an applied potential (at the open circuit potential (OCP)).9-16 Hence, these studies are expected to properly reproduce the chemical interactions between the different components, while effects induced by the potential and the ionic nature of the species, which are present in a real battery, cannot be accounted for. Here we report results of a combined experimental and computational study on the interaction

of

an

adsorbed

ionic

liquid

(IL)

1-butyl-1-methylpyrrolidinium

bis(trifluoromethylsulfonyl)imide ([BMP][TFSI], also referred to as [C4C1Pyrr]Tf2N, with Li on a CoO(111) thin film grown on Ru(0001) and on the conversion of CoO(111) upon deposition of metallic Li0 in the presence and absence of ultrathin IL films. This was investigated employing mainly angle-dependent X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), in combination with density functional (DFT) calculations for characterization of pristine CoO(111). To the best of our knowledge such model studies, exploring molecular/atomic level details on the SEI formation on cobalt oxides together with their lithiation / conversion, have not been reported so far. Ionic liquids (ILs),17-20 which are defined as room temperature (r.t.) molten organic salts with a melting point below 100°C, received considerable attention in the field of electrochemical energy storage as promising solvents in battery electrolytes, because of their attractive physical and chemical properties such as high ionic conductivity, electrochemical ACS Paragon Plus Environment

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stability and non-flammability.21-22 We note that beside their usage in LIBs, ILs are also discussed for the use in Li- and Zn- air batteries,23 where cobalt oxides are applied as air cathode (bifunctional catalyst)24 and hence the interaction of ILs with cobalt oxides is also of interest for these systems. Before presenting and discussing our results on the interaction of Li with CoO(111), of [BMP][TFSI] with CoO(111) and of postdeposited Li with adsorbed [BMP][TFSI] on CoO(111), we will briefly summarize previous findings relevant for this work. Upon vapor deposition of Li on polycrystalline CoO on a copper foil at 300 K under UHV conditions, Thorpe et al. found a partial conversion (formation of Co0 and Li2O).6 Luo et al.3 explored structural changes during the electrochemical (de-)lithiation of Co3O4 nanocubes (size of ~ 5nm) by in situ transmission electron microscopy, and detected that Co nanoparticles with a size of ~1 nm are formed during lithiation, which are surrounded by Li2O. XPS measurements on a CoO powder anode performed by Dedryvére et al. after (dis-)charge cycles in a standard coin cell revealed a potential-dependent CoO to Co0 conversion and the simultaneous formation of an organic layer.25 Furthermore, the interaction of [BMP][TFSI] with different model electrode surfaces,9,

12-13, 26-27

and, to mimic the situation in an electrolyte, effects

induced by postdeposition of Li were investigated in recent years in several studies. This included studies on a variety of different model electrode and electrode materials such as Cu(111),26 a copper foil,28-29 highly oriented pyrolytic graphite (HOPG),9, 11 TiO2(110)27 and Si(111).30 In all cases the IL was found to decompose upon contact with Li. Nevertheless, the results demonstrate that parameters like the film thickness and the nature of the model electrode are crucial, that is, either the cations or the anions or both ions react differently with Li. For example, for an adsorbed [BMP][TFSI] monolayer on HOPG, we recently identified in a combined experimental and computational study a charge transferred from Li0 to the surface and the redistribution of charge to the LUMO of the [TFSI] anion, resulting mainly in anion decomposition.9 In contrast, Olschewski et al.28-29 reported predominantly [BMP] ACS Paragon Plus Environment


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decomposition upon deposition of metallic Li0 on thick IL films. Hence, one cannot easily predict results of the interaction of [BMP][TFSI] and Li on different surfaces. This would be highly interesting in particular for materials relevant for applications such as cobalt oxides, where one would want to be able to identify factors that influence IL decomposition and hence the SEI formation. Also computational studies have been performed on related topics. Studying the electrochemical reactions of a [TFSI]-based IL by ab initio molecular dynamic simulations, where they included also an electric field, Ando et al.31 predicted that [TFSI]‒ interacts with a Li electrode via the oxygen atoms, resulting in S‒C and C‒F bond cleavage, and the formation of LiF. Likewise, Yildirim et al. concluded from ab initio molecular dynamics simulations that interaction of [TFSI]‒ with Li(100) leads to a charge transfer from Li and to the break-up of the C-S and / or the S-N bond, while the [BMP]+ cation was not affected by the interaction with Li.32 Following a brief description of the experimental and theoretical techniques and procedures, will first report on the CoO(111) thin films (thickness ca. 6 - 9 nm), which were grown on Ru(0001) and which were carefully characterized by XPS and STM in combination with DFT calculations. Then we describe results by angle-dependent XPS measurements on (i) the interaction of the CoO(111) films with metallic Li0 , on (ii) the interaction of CoO(111) with [BMP][TFSI] as well as on (iii) the interaction of ultrathin [BMP][TFSI] films (mono- and multilayers) with Li on a CoO(111) film substrate and on the resulting changes of the chemical state of the surface. In our opinion, such kind of molecular scale information on the surface chemistry going on at the electrode – electrolyte interface is crucial for generating better SEIs, as an important step to improving the performance of future batteries.

2. Methods 2.1 Experiments


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The experiments were carried out in a commercial UHV system (SPECS) with a base pressure of 2  10‒10 mbar (partial pressures: H2 (< 5 10‒11 mbar), H2O (< 4 10‒11 mbar), CO/N2 (< 4 10‒11 mbar), O2 (< 1 10‒11 mbar) and CO2 (< 3 10‒11 mbar). It consists of two chambers, one containing an Aarhus-type STM/AFM system (SPECS Aarhus SPM150 with a Colibri sensor), the other one is equipped with an X-ray source (SPECS XR50, Al-Kα and Mg-Kα), a He lamp (SPECS UVS 300) and a hemispherical analyzer (SPECS, DLSEGDPhoibos-Has3500) for XPS and UPS measurements. CoO(111) thin films were prepared on a Ru(0001) substrate from MaTecK (purity 99.99%, surface roughness < 0.01 µm, orientation accuracy < 0.1°) following procedures described for the preparation of cobalt oxides (CoO and Co3O4) on Ir(100) in in literature.33-36 First, the Ru(0001) was cleaned by repeated cycles of Ar+ sputtering (0.7 keV, r.t.), followed by cycles of flash annealing to 1670 K and O2 adsorption at temperatures below 500 K (see ref.37) and a final flash annealing to the same temperature at the end. Afterwards, CoO(111) thin films were grown on (2×2)−O/Ru(0001) by vapor deposition of Co0 (Tectra twin pocket dual mini e–beam evaporator, equipped with a 2 mm Co rod from Alfa Aesar 99,995%, Iflux ~ 20 nA, Iemission ~ 25 mA, U ~ 1375 V) and in a background atmosphere of O2 (oxygen 6.0, AIR LIQUIDE, 1 – 3  10-6 mbar) at RT. The evaporation rate of Co was around 4 – 6 Å/ min. After deposition of 6 – 9 nm, the resulting film was annealed in an O2 atmosphere (1 – 3  106

mbar) at ~ 520 K for 2 min and afterwards in UHV at around 670 – 700 K for 5 min. XPS

measurements revealed that annealing temperatures slightly higher than this resulted in the onset of Co2+ reduction, as evidenced by the appearance of a Co0 state in the Co 2p regime in 60, 70, 80° emission (information depth (ID) 1 – 4 nm). This must be due to surface reduction, since in normal emission (ID 6 – 8 nm) the Co 2p spectrum still revealed a pure Co2+ state. The preparation resulted in clean CoO(111) films as determined by angle resolved XPS measurements of the Co 2p, O 1s and C 1s regions. An example of a slightly reduced



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CoO(111) film is shown in Figure 10b (right panel, black solid line); the onset of a weak Co0 state is marked with a red arrow. Its thickness of around 6 – 9 nm was derived from the almost complete attenuation of the Ru 3d intensity at normal emission. The ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]+[TFSI]‒, also referred to as [C4C1Pyrr]Tf2N was purchased from Merck (ultrapure) filled into a quartz crucible, which was mounted in a Knudsen effusion cell (Ventiotec, OVD3). Prior to its use, the IL was carefully degassed in UHV at around 400 K for 24 h to generate pure, water-free IL. Therefore, and considering also the low partial pressure of water in UHV (pH2O < 4 10‒11 mbar), we think that water effects are negligible in the present work. To generate IL adlayers on CoO(111), we evaporated the IL at a temperature of the IL source of 450 K. Under these conditions the deposition rate was ~ 0.1 ML min-1, as determined by XPS and STM measurements in our previous publications,9-14,26 with 1 monolayer (ML) defined as a full layer at saturation coverage. Lithium metal was deposited from an alkali getter source (SAES Getters), by resistively heating the source (7.1 A, 1.1 V) in line-of-sight of the sample at a distance of around 6 cm. During Lithium deposition the pressure in the UHV system usually remained in the 10-10 mbar range, but never exceeded 1 - 2  10‒9 mbar. The Li deposition rate was calibrated on a highly oriented pyrolytic graphite (HOPG) substrate using Al-KeV) radiation. Deposition rates in monolayer equivalents (MLE) of approximately 0.04 ‒ 0.05 MLE min-1 were calculated from the damping of the C 1s substrate peak after successive vapor deposition of Li on the HOPG substrate at temperatures where Li adsorbs on the surface (~80 K) (cf. ref.11,12). For the evaluation we assume that one monolayer equivalent (MLE) of Li has a thickness d of 2.48 Å, equivalent to the interplanar distance between the close-packed (110) planes in a body centered cubic lattice (the most stable configuration of a Li metal at r.t.). The layer thickness d


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was calculated by Id = I0 exp (-d / λ cos ), with an electron inelastic mean free path (IMFP) λ for Li of 46 Å38 at the position of the graphitic carbon peak at kinetic energies of ~ 1200 eV. STM measurements were performed in the constant current mode with currents between 15 and 30 pA and bias voltages between -0.8 and -3.9 V (applied to the sample). The wurtzite surface termination of CoO(111), shows no band gap except for a reduced density of states around the Fermi level, as demonstrated by scanning tunneling spectroscopy measurements by Heinz et al.,39 enabling STM imaging of these films. For the XPS measurements we used an Mg-Kα X-ray source (1253.6 eV) operated at a power of 250 W (U = 14 kV, I = 17.8 mA). The correct BE calibration was confirmed by the position of the Ru 3d5/2 peak at 280.0 eV.40 XP spectra were recorded at a pass energy Epass of 100 eV at normal and grazing emission (0 and 80° to the surface normal). To minimize beam damage during X-ray exposure, we reduced the number of scans for all detail spectra to one or a few scans. In addition, the XPS measurements upon stepwise postdeposition of Li to an adsorbed IL adlayer were performed once in an experiment where changes after each deposition step were followed only in the N 1s spectral range, while spectra of the F 1s, O 1s, S 2p and Li 1s spectral ranges were only acquired after the last Li deposition step. In further experiments, spectra in the F 1s, O 1s, S 2p and Li 1s ranges were acquired after every second Li deposition step, in addition to the N 1s spectra recorded after each deposition step. The XP spectra of both sequences showed the same result after the last Li deposition step. Therefore we consider these data as free from beam damage effects. For fitting of the XP spectra we used the Igor Pro 8.0.2.1 software, which includes a simultaneous fit of background (Shirley + slope) and signal, applying an asymmetric pseudo-Voigt-type function.

2.2

Computations

Periodic density functional theory calculations were performed using the Vienna ab initio simulation package (VASP 5.4.1).41-42 Ion cores were represented by means of the projected ACS Paragon Plus Environment


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augmented wave method.43-44 The electronic wave functions were expanded in a plane wave basis set up to a cutoff energy of 520 eV. The exchange-correlation energy was approximated within the GGA scheme, employing the revised PBE functional of Hammer and Nørskov (RPBE).45 Dispersion effects are known to be crucial to appropriately describe the adsorption of ionic liquids surfaces.12 They were included by means of the semi-empirical correction scheme D3 of Grimme

46

in combination with a damping function proposed by Chai and

Head-Gordon (“zero-damping”).47 To account for on-site Coulomb interactions a Hubbard like term (+U) was added in the way proposed by Dudarev.48 It has been shown recently that the combination of RPBE-D3+U can improve the description of polymorph stabilities of a metal oxide, while keeping the U-correction at low values and thus retaining the agreement of structural parameters with experiment.49 We used an Ueff (=U-J) value of 4 eV for the delectrons of Co, as it both is in the range of U-values that well predict the oxidation energy of CoO to Co3O4

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and leads to a good description of the CoO rock salt structure that is

considered in here. The magnetic configuration of CoO was set according to the type II antiferromagnetism, i.e., the spins of Co are altered along the direction. Using these parameters, the lattice vectors of the magnetic unit cell of the CoO rock salt crystal are a=5.295 Å, b=3.042 Å, c=3.027 Å, =125.61° and are thus comparable to the experimentally determined values a=5.182 Å, b=3.018 Å, c=3.019 Å, =125.58° at 10 K.51 The calculated lattice constant was used in the calculations of the oxygen terminated CoO(111) surface. Symmetric slabs of 13 atomic layers were employed, which are separated by a vacuum region of 15 Å. During geometry optimization the outer 4 atomic layers on both sides of the slab were allowed to relax, while the inner 5 layers were kept fixed. In order to allow for reconstruction effects, a (√3√3) 𝑅30° superstructure of the CoO(111) surface has been modeled. For the integration over the first Brillouin zone we employed a 991 k-point mesh with a Gaussian smearing of 0.05 eV.


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The simulation of the STM image was based on the approximation that the tunneling current is proportional to the local density of states (LDOS) close to the Fermi energy at the position of the center of the tip apex (Tersoff-Hamann).52 Constant-current images were simulated by an isosurface (isosurface value: 510-7 e-/Å3) of the LDOS integrated between the Fermi energy and the sample bias (-2.3 eV).

3. Results 3.1 Structure and composition of CoO(111) films on Ru(0001) To begin with, we characterized CoO(111) thin films on Ru(0001) (d ≈ 6 – 9 nm) by XPS and STM, in combination with DFT-D calculations, before we concentrate on the results of the interaction of Li with CoO(111), the interaction of [BMP][TFSI] with CoO(111) and on the interaction of Li with a CoO(111) surface precovered with an ultrathin [BMP][TFSI] film. The angle-dependent XPS measurements were performed at 0° and 80° with respect to the surface normal, respectively, which in the following is termed as normal emission (‘bulk’ sensitive, extended surface region, information depth (ID) around 6 – 8 nm) and grazing emission (‘surface’ sensitive, near surface region, ID 1 – 2 nm). Adsorbate-related spectra were generally measured at an emission angle of 80°. The normal emission Co 2p core level spectra in Figure 1a show the spectrum of a metallic Co0 film, which is included as reference, and of a CoO(111) film (top of the panel). The CoO film was generated by vapor deposition of Co0 in an O2 atmosphere (for details see experimental section). In the first case, after vapor deposition of metallic Co0 under UHV conditions on clean Ru(0001) at RT, the Co 2p range shows a Co0 doublet with peaks at binding energies (BEs) of 778.5 (Co 2p3/2) and 793.5 eV (Co 2p1/2) (filled yellow), in agreement with the BEs reported for metallic Co0 in the literature.53 In contrast, the Co 2p spectrum of the thin CoO(111) film shows typical features at rather different BEs, which are assigned to a Co2+ doublet at 780.1 (Co 2p3/2) and 796.0 eV (Co 2p1/2) (filled blue), together ACS Paragon Plus Environment


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with two pronounced satellites for each peak.53 According to the literature, these satellites are typical for late 3d transition metal monoxides with partially filled eg character.54 Typical STM images recorded on a CoO(111) film are shown in Figure 1b – g. The largescale STM images in Figure 1b and c reveal extended flat island structures with oval shapes, with their long sides ranging between around 10 and 40 nm. Their apparent height with respect to local holes (Figure 1c, marked with a white arrow), is around 3.5 nm, which indicates that the islands are at least several monolayers in height. Such holes might have formed during annealing of these films due to strain relaxation. The islands are often separated by trenches or ‘cracks’ with an apparent depth in the range of 1 – 2 nm. In contrast, the height

Figure 1. (a) Co 2p core level spectra of a metallic Co0 film (reference spectrum) and of a thin CoO(111) film (d ~ 6 nm) grown on Ru(0001). (b-f) Large-scale STM images (b,c) and STM images with atomic resolution (d-f) of the CoO(111) film. The white arrow in image (c) indicates a local hole (see description in the text). (g) Simulated STM image of the reconstructed wurtzite type terminated CoO(111) surface. Top view of the surface slab superimposed on the STM image (oxygen atoms: red; cobalt atoms: blue), a side view is shown below. (Tunnel parameters: (b) 20 pA, -2.3 V, (c) 20 pA, 2.3 V, (d) 30 pA, -1.0 V, (e) 30 pA, -0.8 V, (f) 20 pA, -2.3 V, (g) isosurface value: 5x10-7 e-/Å3, -2.3 eV).


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variation on top of these islands structures is much lower, in the sub-nm range. Obviously, the Ru(0001) surface is covered by a quasi-continuous CoO(111) film, which exhibits distinct ‘cracks’. The higher resolution STM images in Figure 1d and 1e reveal small amounts of protrusions located on top of the CoO(111) surface. According to Fester et al., who observed very similar protrusions on CoO nanoislands on Au(111), these protrusions may be related to hydroxyl groups55 generated upon dissociation of residual water in the UHV on the surface. Another possibility would be small amounts of Co0 on the surface, which may be generated during annealing in UHV due to the onset of a loss of oxygen (see experimental part). Finally, in the atomic resolution STM images in Figure 1f, we clearly resolve an atomic lattice. For better identification we included the unit cell. The dimension of the unit cell is ⃗ | = |B ⃗ | = 0.50 ± 0.03 nm with an angle of 120 ± 5°in between. |A To elucidate the origin of the atomic structure in our experimental STM images, we performed DFT calculations. Note that the surface energy of compound materials in equilibrium may depend on the environmental conditions expressed through the chemical potentials of the constituents. This concept, however, is not applicable to surfaces in UHV that do not necessarily correspond to equilibrium structures. Furthermore, for some of the surfaces considered here no symmetric slab models can be created, so that only mean effective surface energies corresponding to an average over the upper and lower surface of the slab can be derived. Therefore here we base our analysis of the observed surfaces not on stability arguments, but rather on the agreement between measured and simulated DFT images. In the DFT calculations of the oxygen terminated CoO(111) surface we considered not only the bulk or rock salt type termination (Co in octahedral sites) of the crystal, but also a wurtzite-type surface termination with Co in tetrahedral sites. Such a kind of surface structure has been reported before.56-57 Furthermore, we considered a reconstruction of the wurtzitetype oxygen terminated CoO(111) surface, in which every third surface O atom is slightly lifted, as it was proposed by Meyer et al.58 Based on the LEED patterns of CoO films on ACS Paragon Plus Environment


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Ir(100)-(11), Meyer et al. derived a unit cell close to that of bulk rock-salt CoO(111), where the latter has a hexagonal structure with lattice vectors of 3.01 Å.56 For the wurtzite-type type film (thickness ~130 Å) they obtained (√3√3)R30° superstructure with respect to the CoO(111) bulk unit cell.58 The structural models of the rock salt type terminated and reconstructed wurtzite-type terminated CoO(111) surface that have been used for the calculations are shown in Figure 2.

Figure 2. Side view of structural models (a) of the rock salt type O-terminated and (b) of the reconstructed wurtzite type O-terminated CoO(111) surface; (red: oxygen atoms; blue: cobalt atoms).

The structural transition and the reconstruction into a (√3√3) 𝑅30° superstructure result in an energy gain of 23 meV/Å2 relative to the rock salt type oxygen terminated CoO(111) surface. The calculated interlayer distances of the outermost layers of the reconstructed wurtzite type terminated CoO(111) surface obviously differ significantly from those of the rock salt type terminated surface, but agree well with previous experimental values, which were determined from low energy electron diffraction (LEED) I-V analysis (see Table 1).58 Table 1: Interlayer distances (in Å) of different oxygen terminated CoO(111) surfaces. The interlayer distances are defined in Figure 1. Experimental values are taken from Ref.58


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Rock salt type terminated

d12 d23 d34 d45 b

0.78 1.39 1.23 1.25 -

Wurtzite type terminated – reconstructed 0.68 1.81 1.09 1.26 0.31

Exp. 0.61 1.91 0.94 1.23 0.11

Simulations of an STM image of the reconstructed wurtzite-type oxygen terminated CoO(111) surface (Figure 1g) reveal that the elevated O atoms of the superstructure (marked with ) appear as bright features, while the other two O atoms of the supercell (marked with ) are hardly visible (Figure 1g). Thus, the hexagonal unit cell deduced from the experimental atomic resolution STM images (unit cell vector length: 5.0 ± 0.3 Å, see Figure 1 f) agrees very well with the proposed (√3√3) 𝑅30° superstructure (calculated unit cell vector length: 5.2 Å) with an elevated O atom, and differs distinctly from the rock salt surface structure.

3.2 Interaction of CoO(111) with Li Second, STM images of the pristine CoO(111) film and after deposition of around 0.3 and 1 MLE of Li (for definition of monolayer equivalent (MLE) see experimental part), respectively are shown in Figure 3 (deposition at RT). Before Li deposition the atomic resolution STM image of CoO(111) (Figure 3a) reveals flat terraces, which are basically free of undesired adspecies, except of a few protrusions, which we already discussed before (see Figure 1). Note that the pristine CoO(111) film shown here exhibits a lower density of these protrusions than the image presented in Figure 1, which reflects the variations caused by subtle differences in the preparation procedure or the final annealing temperature of these films in UHV (670 – 700 K). The first Li deposition step (0.3 MLE) (Figure 3b and c), leads to the formation of a few nanoclusters (small aggregates) on the terraces. These clusters exhibit a full width at half maximum of around 0.6 nm in average. The STM images in Figure



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3b and c also show some noisy regions (marked by white arrows), which had not been observed on pristine CoO(111) before.

Figure 3. STM images a) of pristine CoO(111) (left panel), b, c) after deposition of 0.3 MLE of Li (the white arrows indicate regions where the surface structure could eventually start to convert – see text), and d) after deposition of 1 MLE Li. ((a) 30 pA, -0.8 V, (b) 20 pA, -3.9 V, (c) 20 pA, -3.9 V (d) 30 pA, -2.4 V).

We speculate that these may be regions where the surface structure starts to convert into metallic Co and Li2O, which are identified as final products by XPS (see Figure 4). The atomic resolution STM image in Figure 3c also resolves that next to these nanoclusters the surface is still well-ordered after Li deposition (the different color code improves the visibility of the atoms), proving that at the given Li dose the surface is still dominated by crystalline CoO(111). After the second Li deposition step (1 MLE), the terraces are homogeneously covered with nanoclusters with a density of ~ 0.6 – 0.7  1014 cm-2, which is better resolved in the inserted STM image with enhanced contrast (Figure 3d). These nanoclusters are most likely formed upon reaction of the pristine CoO(111) with Li0 to form Co0 and Li2O on the surface. XPS measurements performed after Li deposition, which are displayed in the Supporting Information Figure S1, confirm the formation of metallic Co0 in the Co 2p range. ACS Paragon Plus Environment

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Hence, the nanoclusters are most likely composed of Co0, Li2O, or a combination thereof, which cannot be resolved by STM. Next we explored the interaction of metallic Li0 with the pristine CoO(111) film during stepwise deposition of 1.0 – 6.0 MLE of Li at room temperature (RT) using angle-dependent XPS. Grazing emission XP spectra (ID 1 – 2 nm) of the Co 2p, O 1s and Li 1s spectral ranges are shown in Figure 4. For better visibility of the changes between these spectra they are shown both as waterfall plots (raw data) (Figure 4, top of each panel) and as individual peak deconvoluted spectra (Figure 4, bottom of each panel). In the latter case, the Co 2p and O 1s spectra were normalized to a constant peak area to better illustrate the extent of the changes. Furthermore, we present a quantitative analysis of XP spectra recorded at different emission angles (0°, 80°) after the last Li deposition step (Figure 5). Note that similar XPS measurements were performed by Thorpe et al. upon successive vapor deposition of Li on a CoO film on a copper foil. Interestingly, those authors also observed the evolution of a Co0 peak upon Li deposition, indicating a partial conversion of the surface layer.6 The waterfall plot of the Co 2p range in Figure 4 demonstrates that with increasing Co deposition first a shoulder and then a new peak gradually arises at the low BE side of each of the two Co2+ peaks, while the latter successively decrease in intensity. This is also clear from the peak deconvoluted XP spectra recorded during Li deposition (1.0 - 6.0 ML), which reveal that besides the two Co2+ peaks at 780.1 and 796. eV (filled blue), respectively, two new Co0 peaks (filled yellow) at 778.5 and 793.5 eV, respectively, emerge and grow in intensity. Obviously, there is some conversion of CoO to Co0, which can only be explained by the formation of Li2O according to the reaction CoO + 2Li+ + 2e-  Co0 + Li2O. Correspondingly, we find a weak peak in the Li 1s range, whose intensity gradually increases during Li deposition. Note that the steeply increasing intensity at slightly higher BE next to the Li 1s peak is due to a Co 3p emission. The Li 1s peak reflects the increasing presence of Li species, most likely of Li2O (Figure 3, right panel). The low intensity of this peak is related ACS Paragon Plus Environment


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to the low cross-section of this state for photoionization using Mg-K radiation. The chemical nature of the Li species will be discussed in more detail below.

Figure 4. Co 2p, O 1s and Li 1s core level spectra (top of the panels) recorded upon stepwise vapor deposition of metallic Li0 on a Co(111) thin film (80° with respect to the surface normal) at RT; waterfall spectra (upper panel) and peak deconvoluted spectra (bottom panel).

To gain more information on the expected formation of Li oxides during the conversion reaction, we also show the O 1s spectra (Figure 4). For pristine CoO(111) (topmost spectrum, bottom panel, middle), the O 1s spectra is dominated by a peak centered around 530.0 eV, which is representative for lattice oxygen in CoO (OCoO) (cf. ref.6). In addition, we find a shoulder with very low intensity at the high BE side of the peak, at ~532 eV, which could result from OH groups55 (Oa) that are formed on the surface via dissociation of residual water. Following the O 1s region during the successive steps of Li vapor deposition, we expect the development of a new Li2O component along with the conversion reaction. Unfortunately, the O 1s peaks of Li2O and CoO are located at rather similar BEs (530.0 eV). This is demonstrated by the Li2O reference spectrum in the O 1s regime (top panel, dashed line), ACS Paragon Plus Environment

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which was recorded after deposition of metallic Li0 on HOPG at 80 K and leaving the sample for more than one hour in UHV. During this time interaction with residual water traces in the UHV partially transforms metallic Li0 into Li2O. Previously, an O 1s peak at a rather similar BE of 530.5 eV, which developed upon deposition of O2 on a Li film, was assigned to Li2O species by Qiu et al. (for higher O2 deposits they observed an additional Li2O2 peak (532.5 eV)).59 On the other hand, also signals with significantly lower BEs (528.6 eV) were associated with Li2O.60 These different results are most likely due to the specific sample condition, resulting from different Li2O amounts, film thicknesses, etc. Due to the essentially identical O 1s peak positions of CoO and of the Li2O reference peak we can hardly discriminate between CoO and Li2O in the O 1s spectra (Figure 4). In addition, however, we find a new peak growing in at 532.5 eV, which is clearly visible in the spectra, both in the waterfall plot (top panel) and in the peak deconvoluted spectra (bottom panels), at least for the third and fourth Li deposition step (4.0 and 6.0 MLE). This peak we tentatively assign to Li2O2, considering that similar BEs were reported previously for Li 2O2 formed upon successive deposition of Li on a polycrystalline CoO film. In that work the O 1s BEs varied between 532.0 – 532.4 eV as a function of the Li amount (arbitrary units).6 Another study reported an O 1s peak upon deposition of O2 on a Li film (532.5 eV), which was also assigned to Li2O2.59 Here we assume that the Li2O2 species formed on the CoO(111) thin film resulted from interaction of unreacted, metallic Li with residual gas components in the UHV. For comparison, upon successive deposition of Li0 onto a TiO2(110) surface27we observed a partial reduction from Ti4+ to Ti3+ states (electron transfer from Li to Ti4+) (Ti 2p range) and the formation of a new oxygen species (O 1s region) at the high BE side of the main TiO2 peak, which we had interpreted as a LixO species. Hence, both Li2O and Li2O2 species are expected to contribute to the low-intensity peak in the Li 1s range (~55.5 – 56.0 eV) in Figure 4, however, due to the low cross-section these compounds cannot be resolved. Thorpe et al. speculated that the formation of a Li2O2 overlayer during deposition of Li0 on a ACS Paragon Plus Environment


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polycrystalline CoO film on a copper foil could suppress the reaction of the Li species with the CoO substrate by hindering the Li diffusion.6 Unfortunately, the error range in the ratio of the Co2+ (yellow) and Co0 (yellow) species in the peak deconvoluted XP spectra in Figure 4 is rather large, hindering a quantitative evaluation of the amount of metallic Co0 formed upon Li deposition. Already a slight variation of their peak positions and of their full width at half maximum resulted in pronounced changes of the peak areas of the related states. For the quantitative analysis we therefore used an alternative fitting procedure to determine the amount of the different species (Co2+, Co0 and Li2O) in the near and extended surface regime (Figure 5), applying a linear combination of the Co 2p XP spectra of CoO and Co0 as shown in Figure 1a (black solid lines), after subtraction of a Shirley background. In the Co 2p range, both in the near (1 – 2 nm) and extended (6 – 8 nm) surface region (bottom and top panel), the spectrum resulting from the linear combination (green spectra) almost perfectly agrees with the experimental Co 2p spectra recorded after vapor deposition of 6.0 ML of Li. From this linear combination we derive a CoO : Co 0 ratio of 63 : 37 in the near surface regime (ID of 1 – 2 nm) and of 88 : 12 in the extended surface regime (ID of 6 – 8 nm). This discrepancy underlines a significantly higher conversion near the surface than in deeper regions. With this quantitative information, it is now possible to fit the O 1s range with the CoO and Li2O peaks, respectively, using the CoO : Co0 ratio determined before from the Co 2p range. According to the conversion equation: CoO + 2 Li + + 2 e-  Co0 + Li2O, this also defines the CoO : Li2O ratio in the O 1s signal, which cannot be derived directly from the O 1s signal. The resulting O 1s peaks of CoO (blue) and Li2O (violet) are shown in the right panel in Figure 5. In addition, the more pronounced O 1s signal of the Li2O2 species in grazing emission, in the ‘surface’ sensitive mode, than in normal emission (‘bulk’ sensitive) obtained after the last Li deposition step (Figure 5, right upper panel) indicates that the Li2O2 is predominantly formed at the surface (see also ref.6). Most likely it forms an overlayer, which in turn could act as a barrier for Li diffusion into the ACS Paragon Plus Environment

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subsurface region as already discussed before (we note that the low-intensity peak Oa might still contribute to the spectrum, but is covered by the much larger O 1s peak of the Li 2O2, which appears at about similar BE). Finally, our angle dependent XPS data clearly indicate that Li2O2 is predominantly formed in the direct surface region, in the topmost 1 – 2 nm, while in deeper regions Li2O is more dominant. The physical reasons for this unexpected result, however, are not yet clear.

Figure 5. Angle-resolved Co 2p and O 1s core level spectra (0° and 80° with respect to the surface normal) of a thin CoO film on Ru(0001) and after subsequent vapor deposition of 6 MLE of Li at RT. A linear combination of the Co 2p XP spectra of CoO (filled blue) and Co0 (filled orange) was used to determine the amount of the different species (Co2+, Co0 and Li2O) in the near and extended surface regime ( and  are the linear factors).

3.3 Interaction of CoO(111) with BMP-TFSI Next, we adsorbed a monolayer of [BMP][TFSI] on the CoO(111) thin film at RT (Figures 6 and 9) by XPS. In the resulting spectra, IL-related peaks appear in the F 1s, O 1s, N 1s, C 1s and S 2p ranges (FTFSI, OTFSI, NTFSI, CTFSI and STFSI), which are typical for adsorbed [BMP][TFSI], as has been described previously.12, 61 First, we will focus on the C 1s and N 1s ranges (Figure 6), which contain information on both anions and cations, before presenting the F 1s , O 1s, C 1s and S 2p spectra later (Section 3.4, Figure 9). The ‘surface sensitive’ ACS Paragon Plus Environment


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grazing emission C 1s spectra reveal three adsorbate-related peaks at 293.3, 286.9 and 285.8 eV, which were previously assigned to the CTFSI (-C–F-) (cyan), Chetero (-C–N-) (light red) and Calkyl (-C–C-) (red) carbon atoms, respectively.12,

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Furthermore, very weak contributions

from the Ru 3d states (Ru 3d5/2, 280.0 eV) of the substrate also appear in this range, even though we expect that the 6 – 9 nm thick CoO(111) film largely damps the Ru signal. These possibly arise from local holes in the CoO film (see also Figure 1 and related discussion). These contributions are clearly visible in the XP background spectra in Figure S4 before IL deposition. The grazing emission N 1s spectrum (Figure 6) reveals two peaks at 402.9 and 399.8eV, which are related to the nitrogen atoms in the [BMP] cation (NBMP (-C–N-)) and in the [TFSI] anion (NTFSI, (-S–N–S-)), respectively. We note that the background in the N 1s regions it not as flat as expected, which results from a weak contribution from a Ta 4p3/2 peak caused by the the Ta sample holder (Ta2O5) The relative intensities of the adsorbate-related peaks in the C 1 and N 1s ranges determined experimentally agree with the nominal ratios of the CTFSI : Chetero : Calkyl peaks and of the NBMP : NTFSI peaks of 2 : 4 : 5 and 1 : 1, respectively, within the limits of accuracy. This clearly points to the presence of intactly adsorbed molecule ions,12 indicating that the [BMP] and [TFSI] ions did not decompose upon adsorption. Furthermore, previous calculations had shown that the adsorbed molecule ions alternate, as expected from electrostatic reasons (cf. ref.

12,61

). Almost identical IL-related XP peaks at

similar BEs have been observed also on another oxide surface previously, after vapor deposition of [BMP][TFSI] on TiO2(110) at RT.27


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Figure 6. C 1s and N 1s core level spectra of an adsorbed [BMP][TFSI] monolayer on a CoO(111) thin film on Ru(0001) deposited / recorded at RT. Spectra recorded before IL deposition are shown at the bottom of each panel. Low-intensity Ru 3d doublets are visible in the C 1s range before and after IL deposition. Molecular representations of [BMP][TFSI] (also referred to as [C4C1Pyrr]Tf2N) are shown above the panels (F (green), O (red). N (blue), F (black), S (gold)).

3.4 Interaction BMP-TFSI with postdeposited Li on CoO(111) In this last section, we describe results of experiments where we tried to mimic the Li + ion transfer from the electrolytes into the electrode by postdeposition of Li0 on a CoO(111) film surface covered by a preadsorbed IL monolayer. STM images recorded at RT on a CoO(111) thin film after postdeposition of ~ 3 MLE of Li onto a preadsorbed IL monolayer (Figure 7) show a surface appearance which is very different from that of pristine CoO(111) (cf. with Figure 1). The images in Figure 7 reveal small agglomerates, which are evenly distributed over the surface and which fully cover it. The large-scale STM image (top and left) (300 nm  300 nm, 100 nm  100 nm, respectively) demonstrates that the terrace / island morphology characteristic for the pristine CoO(111) film surface (see Figure 1) cannot be resolved any more. This can either result from local restructuring of the surface, as a first step of a phase transition from CoO and Li to Co0 and Li2O during the conversion reaction or by the formation of local aggregates of IL decomposition products. Both can result in disordered local structures, which deteriorate the ACS Paragon Plus Environment


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resolution during STM imaging. The smaller scale STM image (right) (50 nm  50 nm) reveal also smaller features caused by agglomerates of different sizes. These agglomerates and smaller clusters could include a variety of different possible components, such as IL decomposition products, Li2O, Li2O2 or Co0, which can neither be identified nor be distinguished from these STM images.

Figure 7. Different scale STM images recorded after post-deposition of approximately 3 MLE of Li on a preadsorbed [BMP][TFSI] monolayer on a CoO(111) film on Ru(0001) (deposition at RT). (Tunnel parameters: left :: 15 pA, -3.9 V, V, right: 20 pA, -3.8 V).

More information on the chemical composition of the surface region comes from a series of XP spectra, which were recorded during stepwise postdeposition of Li (approximately 0.2 – 1 MLE) on a surface covered by a preadsorbed IL monolayer (Figures 8, 9) and during postdeposition of higher Li amounts on preadsorbed IL mono- and multilayer film in Figure 10. To begin with, we show the resulting N 1s spectra for a preadorbed [BMP][TFSI] monolayer together with a plot of the different N 1s peak areas (Ntotal, Ncation, Nanion, Nproduct) as a function of Li coverage in Figure 8. The N 1s spectrum at the bottom of the panel is typical for adsorbed anion-cations pairs on CoO(111) (Nanion and Ncation) (see also Figure 6


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and description), except of a low-intensity peak (Nadv). The latter may be caused by small amounts of IL decomposition on defects in the oxide film, which result from incomplete oxidation of the Co species. Upon deposition of 0.2 – 1.0 ML of Li the Ncation (light red) and Nanion (cyan) peaks similarly decrease in intensity and a new peak (Nnew) arises at the low BE side (yellow), which we assign to possible decomposition products such as Li3N or LiwCxHyNz (cf. refs.9, 11-12, 26). In contrast to Nadv, the species related to Nnew is formed during Li deposition on the IL precovered surface, which makes it likely that this refers to another Ncontaining species, albeit with rather similar BE. Note that the formation of Li3N or of LiwCxHyNz (filled yellow) requires C–N bond breaking of the pyrrolidinium ring of the cation. Li3N could, however, also be generated by cleavage of the S–N bonds of the anion, as had been indicated in previous computational studies on the decomposition of ILs.9,

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Interestingly, for Li doses >0.7 MLE the Ncation peak (light red) decreases stronger in intensity than the Nanion peak (cyan), i.e., the intensity ratio of these deviates from the 1 : 1 ratio expected for molecularly adsorbed intact [BMP][TFSI] species. Most likely, at least part of the remaining peak formerly associated with Nanion species (cyan) is now related to other Li bound fragments of the anion (e.g., LiNSO2CF3) or to Li(CF3SO2SN)2 with rather similar BE, which cannot be separated in these measurements. We assume that such Li fragments preferentially form after the dissociation of the S–N bond and subsequent bonding of the resulting fragment with Li, based on results of our previous study on the interaction of Li with [BMP][TFSI] on highly oriented pyrolytic graphite (HOPG).9 Calculations in that study showed that charge transfer from HOPG into the LUMO of the anion results in cleavage of the S–N bond of the anion and subsequent bonding to Li.9 We note that during postdeposition of Li, we observed BE up-shifts of the NBMP and NTFSI peaks. We assume that they arise from a similar effect as reported in our previous publication,9 where an up-shift of the NBMP and NTFSI peaks, accompanied by a lowering of the work function, was assigned to vacuum level pinning (see ref. 9). ACS Paragon Plus Environment


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Figure 8. N 1s and Co 2p core level spectra (80° with respect to the surface normal) of an adsorbed [BMP][TFSI] monolayer and after stepwise post-deposition of 0.2 to 1 MLE of Li (left panel). A plot of the N 1s peak areas vs. coverage is also shown.

During the stepwise Li postdeposition we also monitored the Co 2p regime (Figure 8). Different from the Li deposition on the pristine CoO(111) film, we do not find any evidence for the formation of metallic Co, a new Co0 peak is not observed. Thus, the conversion reaction observed for Li deposition on pristine CoO(111) is inhibited on the IL precovered surface for the deposited Li amounts of approximately 0.2 – 1 MLE. Obviously, under these conditions most of the Li interacts with the IL adlayer, resulting in its decomposition. This decomposition process can be associated with the initial stages of the chemical SEI formation, in the absence of an applied potential, i.e., at the open circuit potential (OCP). The very efficient IL decomposition already at RT indicates a rather low reaction barrier for IL decomposition. This result agrees very well with recent findings for the reaction of postdeposited Li with a preadsorbed IL monolayer on Cu(111) at 80 K.26 In principle, the absence of CoO conversion could also be explained by inherently slow kinetics of this process at RT. Considering, however, that Li deposition on pristine CoO(111) (see Figure 2) caused conversion already for Li doses at around 1 MLE (see Figure 3), this can be ruled out, and IL ACS Paragon Plus Environment

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decomposition is simply more facile than CoO conversion, at least for small amounts of Li deposition, even though we cannot completely exclude that part of the highly reactive Li0 simply first reacts with the IL covering the surface. Spectra of the F 1s, O 1s, C 1s and S 2p spectral ranges recorded before and after Li deposition in the same experiment as shown in Figure 8, are presented in Figure 9. Before Li deposition (bottom curves) they show the anion- and cation-related FTFSI, OTFSI, CTFSI Chetero, Calkyl and STFSI features expected for molecularly intact adsorbed [BMP][TFSI] species (cf. Figure 6 and with ref.12). The low-intensity doublet at the low BE side of the STFSI peak (S 2p3/2: 167.5 eV) is likely to result from small amounts of IL decomposition on defect sites, even though the nature of this decomposition product is unknown, as discussed for the N 1s peak in Figure 8 before. After postdeposition of approximately 1 MLE of Li, i.e., after the last deposition step, all anion-related peaks lost more than half of their original intensity (upper curves in each panel in Figure 9), reflecting the decomposition of the anions. In addition, we also show the Li 1s spectrum as inset in the F 1s spectrum to demonstrate the presence of the deposited Li. The cation-related Ncation and Chetero peaks (see Figures 8 and 9) reveal an even more pronounced loss of intensity (-70 %). The loss of intensity of the anion- and cationrelated peaks in the XP spectra is accompanied by the formation of new peaks (filled yellow), reflecting the formation of various decomposition products. Based on their BEs and considering also previous proposals,63,26-29 possible products are Li3N or LiF (F 1s, 685.7 eV), Li2O2 (O 1s, 532.3 eV), Li2S (S 2p3/2, 161.8 eV) or other Li bonded fragments of the cation such as LiwCxHyNz (N 1s, 398.5 eV) as well as other Li bonded fragments of the anion (e.g., LiNSO2CF3), which exhibit the same BE in the N 1s peak as Nanion peak (400.9 eV). We note that also other products (e.g. Li2O, etc.) are possible (cf. possible decomposition pattern by Aurbach et al.63). Overall, we interpret the formation of these Li-bonded fragmentation products as the initial stages of the chemical SEI formation. Interestingly, we found very similar new peaks and thus decomposition products recently upon postdeposition of Li on a ACS Paragon Plus Environment


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preadsorbed [BMP][TFSI] monolayer on HOPG.9,

12

The close similarity seems to indicate

that under these conditions, for deposition of submonolayer amounts of Li on a surface covered by a monolayer IL film, the nature of the substrate does not seem very important. In addition to the formation of adsorbed decomposition products, the interaction with Li must also result in the formation of volatile species, which desorb upon formation. This is evidenced by the decrease of the total peak area in the N 1s range, which is indicated by the black solid line in the plot of the peak areas vs. Li coverage in the Figure 8. The same is true also for the C 1s signal (Figure 9), where the decay of the total intensity is clearly visible by eye. Possible candidates may be CF3 or CN containing volatile species ref.64

Figure 9. F 1s, O 1s, C 1s, S 2p and Li 1s core level spectra (80° with respect to the surface normal) of an adsorbed [BMP][TFSI] monolayer (bottom of each panel) and after post-deposition of 1 MLE of Li (top of each panel).

To test whether the conversion reaction with the CoO(111) substrate can take place even in the presence of adsorbed [BMP][TFSI] mono- and multilayers (1 and 3 ML) (Figure 10a and c, respectively), if larger amounts of Li (> 1 MLE) are deposited, we performed a set of similar experiments as before, but with larger [BMP][TFSI] and Li depositions (Figure 10a-


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d). In the first experiment we started by depositing a [BMP][TFSI] monolayer film, as indicated by the Ncation (light red) and Nanion (cyan) peak intensities (Figure 10a), followed by stepwise postdeposition of Li, reaching up to 3 MLE in total. This results in the same trend of IL decomposition as described already before in Figure 8 (see Figure S2). After deposition of the last Li dose (~ 3 MLE) the N 1s spectrum in Figure 10a (top spectrum) shows that the Ncation peak (light red) has almost completely disappeared, while the Nanion peak (cyan) is still visible, together with a dominant new peak (yellow), which must be related to a decomposition product as discussed in detail before. As also shown before (see Figure 8 and explanation in the text), we observed BE upshifts of the NBMP and NTFSI peaks. In general, for Li exposures of > 1 MLE the N 1s regime changes only little (compare topmost N 1s spectra in Figure 8 and in Figure 10), indicating that the decomposition reaction is almost complete at around 1 MLE of Li. Most important, deposition of larger amounts of Li also leads to changes in the Co 2p region (Figure 10b, bottom spectrum in the lower panel, 60° emission, ID of ~ 3 – 4 nm). In this case, the initial pristine CoO(111) surface (black solid line) was slightly reduced, reflecting incomplete oxidation during preparation. This is indicated by low-intensity peaks in the Co 2p regime at the BEs of the Co0 states, which are marked by arrows in Figure 10a (black solid line). Upon stepwise postdeposition of approximately 1.5, 2 and 3 MLE of Li in total (brown, orange and blue solid lines, respectively), the Co0 peaks became more and more pronounced. For better visualization of these changes, we show XP difference spectra between the Co 2p spectra, before and after Li deposition in Figure 10b (top panel). They all clearly reveal a Co0 state (positive amplitudes at 778.5 and 793.5 eV). Hence, for larger Li deposits of 1.5, 2 and 3 MLE the reactive conversion of the CoO(111) surface takes place also in the presence of a preadsorbed IL monolayer. The XP difference spectra also reveal peaks with negative amplitudes after deposition of 2 and 3 MLE of Li at the position where the Co 2+ states of CoO(111) and their satellites are located. This is caused by two effects, the loss of Co2+ due to CoO conversion and damping of the substrate signal by adsorbed Li-containing ACS Paragon Plus Environment


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adspecies. The latter damping will of course affect also the Co0 signal, and indeed, the Co0 peak intensities are slightly decreasing during Li deposition, as visible from the Co0 2p3/2 difference peaks after the last two Li deposits in Figure 10b (top of the right panel, orange and blue solid lines, respectively).

Figure 10. (a) N 1s core levels spectra (80° emission angle) of an adsorbed [BMP][TFSI] monolayer on a CoO(111) film on Ru(0001) before and after vapor deposition of 3 MLE of Li (indicated by the blue arrow). The Li 1s spectrum is inserted in the N 1s panel. b) Lower panel: Co 2p spectra of pristine CoO(111) (black solid line) (slightly reduced as indicated by the red arrow) and after deposition of 1.5, 2 and 3 MLE of Li, respectively (60° emission angle). Upper panel: XP difference spectra of the spectra presented in the bottom panel revealing the evolution of a Co0 state. (c) N 1s core levels spectra (80° emission angle) of an adsorbed [BMP][TFSI] multilayer (3 ML) on a CoO(111) film on Ru(0001) and after vapor deposition of 4 MLE of Li (indicated by the orange arrow). The Li 1s emission is inserted in the N 1s panel. d) Angle resolved XP difference spectra of the Co 2p spectra recorded during Li deposition (0.2 – 6 MLE of Li) resolving the emergence of a Co0 state. The dashed lines in the right panels denote the BEs of the Co 2p3/2 and Co 2p1/2 peaks for Co0 and Co2+ species.

Finally, we performed a similar experiment with a [BMP][TFSI] multilayer (3 ML) film predeposited, followed by stepwise Li postdeposition of 0.2 – 6 MLE (Figure 10c and d). The coverage of the [BMP][TFSI] film was estimated from the Ncation (light red) and Nanion (cyan)


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peaks (Figure 10c). The stepwise changes of the N 1s spectra are at least qualitatively very similar to those described before; they are shown in Figure S3. After deposition of about 4 MLE of Li, the Ncation peak (light red) has almost disappeared, while the Nanion peak (cyan) is still present and a new peak (yellow) has emerged at the low BE side (discussed in detail before). Again, we observed BE upshifts of the NBMP and NTFSI peaks, which were already discussed before (see Figure 8 and explanation in the text). Note that the peak formerly designated as Nanion peak (cyan) (Figure 10c) is more pronounced than after Li exposure to the IL monolayer (see Figure 10a). This can be explained by a higher amount of Li-bound fragments of the anion, which as discussed before appear with a similar N 1s BE as the N anion signal. At the same time the Co 2p difference spectra (Figure 10d) show no indication of a Co0 state after the first and second deposition step (~0.2 and 1 MLE). For a total Li deposit of 2 MLE the Co0 state is clearly visible. It further increases in intensity after the total Li doses of ~4 and 6 MLE, respectively. Furthermore, as already described before, the XP difference spectra reveal peaks with negative amplitudes after deposition of 4 and 6 MLE of Li at the position where the Co2+ states of CoO(111) and their satellites are located, which are due to a loss of Co2+ during CoO conversion or to damping of the substrate signal by adsorbed Licontaining adspecies, respectively. In this case they clearly confirm the onset of a measurable conversion of the CoO(111) surface upon postdeposition of about 2 MLE of Li in the presence of the 3 ML IL film. This result can be compared with our previous observation that postdeposition of Li on a preadsorbed [BMP][TFSI] monolayer on TiO(110)27 results in IL decomposition together with the partial transformation of Ti4+ into Ti3+ species. Also in that case part of the Li deposit can diffuse through the IL adayer and its decomposition products to the TiO2(110) surface and reacts with that. Different from the present case, however, the reduction of Ti4+ stops at the Ti3+ level rather than proceeding up to the metallic state, as observed for Co2+ / Co0. Overall, it seems to be a general trend that in the presence of an IL adlayer, postdeposited Li initially reacts with the IL adlayer, while for increasing deposition ACS Paragon Plus Environment


32

Li passes through the adlayer and reacts with the oxide substrate, which is also essential for the storage of Li in the anode of LIBs. Overall, our measurements reveal that stepwise postdeposition of Li on a CoO(111) thin film on Ru(0001) covered with a preadsorbed [BMP][TFSI] adlayer at RT initially results in the gradual decomposition of the IL, which can be considered as the initial stages of the chemical SEI formation at the EEI in the absence of an electric field, i.e., at the open circuit potential (OCP). Initially both the anions and cations decompose, before at higher Li doses cation decomposition is more pronounced. The efficient SEI formation at RT reflects a rather low barrier for IL decomposition upon interaction with Li. For higher Li doses, CoO(111) lithiation / conversion is initiated in the near surface region, which is essential for the function of a battery. Higher Li exposures do not cause significant changes of the chemical composition of the SEI, most likely because all available electrolyte (IL) has already reacted, indicating that a stable SEI is established, which is permeable for Li. In this case postdeposited Li reaches the oxide surface and reacts with that in the surface near region, similar to the situation experienced upon deposition of Li on a pristine CoO film. Finally, we note that the chemical decomposition reaction of the IL adlayer on CoO(111) in our model study occurs by interaction of the IL with metallic Li, while under battery conditions, Li + ions would arrive at a polarized interface due to the applied electrical field. Nevertheless, in our previous model studies9,12 we observed a similar decomposition of an adsorbed [BMP][TFSI] monolayer on HOPG upon interaction with Li. There was no difference, regardless if Li+ was deintercalated from the bulk of lithiated HOPG or if Li0 was post-deposited from the vacuum side. Hence, the interaction of Li+ and Li0 with an IL monolayer seems to be quite similar. In addition, it should also be mentioned that in our study we reproduced the first lithiation step, while the process might be much more complex in a real battery. For closer similarity to real electrolytes we plan experiments with mixtures of [BMP][TFSI] and Li[TFSI] for the future.


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4. Conclusions Aiming at a detailed molecular/atomic scale understanding of the processes at the IL|solid interface as a model for the solid|electrolyte interphase developing at the electrode|electrolyte interface in LIBs, we have investigated the interaction of ultrathin films of the ionic liquid [BMP][TFSI] (solvent / electrolyte) with Li (effect of the Li+ shuttle) on a CoO(111) thin film on Ru(0001) with a thickness of around 6 – 9 nm and also the conversion / lithiation of the CoO(111) films. We arrive at the following main results and conclusions: (1) CoO(111) thin films prepared by simultaneous dosing of Co0 and O2 on Ru(0001) at RT exhibit the expected pure Co2+ state in the Co 2p core level spectra. STM imaging reveals extended flat island structures, which have coalesced and homogeneously cover the surface. The DFT calculations reveal an oxygen terminated (√3√3) 𝑅30° superstructure, which agrees excellently with the atomic resolution STM images. (2) Based on the XPS data mono- and multilayers of the IL [BMP][TFSI] adsorb as molecularly intact species on CoO(111) at RT. (3) Exposure of pristine CoO(111) to stepwise increasing amounts of Li (around ~1 – 6 MLE) leads to the increasing conversion from Co2+ to Co0 and formation of Li2O in the near surface regions (ID of 1 – 2 nm), which is faster than that in the extended surface regime (ID of 6 – 8 nm). In addition, Li2O2 is formed at the surface, most likely by competing interaction of unreacted Li with residual gases. (4) Stepwise postdeposition of small amounts of Li (0.2 – 1 MLE) on a CoO(111) surface precovered with a [BMP][TFSI] monolayer film initially results in anion and cation decomposition, while at higher Li doses cation decomposition becomes dominant. This process, where CoO conversion is absent, is considered as the initial stage of the chemical SEI formation, with a low reaction barrier for IL decomposition upon interaction with metallic Li. XPS measurements reveal IL decomposition products such



34

as Li3N, LiwCxHyNz (Li-bound fragment of the cation), LiF, Li2O2, Li2S, together with other Li-bound fragments of the anion (e.g., LiNSO2CF3), but also other products like Li2O, etc., are possible. (5) For higher amounts of Li deposition, relative to the amount of preadsorbed [BMP][TFSI], IL decomposition is followed by conversion of CoO(111), indicated by the formation of a Co0 state in the Co 2p core level spectra. This reaction, which occurs both for mono-and multilayer IL films at excess Li deposits, demonstrates that the SEI layer formed by the reaction of the IL with Li, is permeable for Li, but does not yet provide proof that this is possible also for Li+ ions. Overall, the present work clearly illustrates molecular scale details that can be obtained in such a model study, and their potential for an improved understanding of the processes taking place at the interface between (model) electrodes and battery electrolytes, which is essential for the development of improved future batteries.


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ASSOCIATED CONTENT

Supporting Information. XP Co 2p, O 1s and Li 1s core level spectra after deposition of 1 MLE of Li on a CoO(111) thin film on Ru(0001) (Figure S1). XP N 1s core level spectra during stepwise postdeposition of Li to adsorbed [BMP][TFSI] mono- and multilayers on a CoO(111) thin film (Figure S2 and S3). C 1s and N 1s core level spectra of an adsorbed [BMP][TFSI] monolayer on a CoO(111) thin film before and after IL deposition (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *Phone: +49 (0)731/50-25451. Fax: +49 (0)731/50-25452. Email: juergen.behm@uni‒ulm.de Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the German Federal Ministry of Education and Research in the project

LiEcoSafe

under

contract

number

03X4636C

and

by

the

Deutsche

Forschungsgemeinschaft (DFG) via project BE 1201/22-1 (Zn-air batteries) and via the project ID 422053626 (Cluster of Excellence “Post-Li Storage”). The work contributes to the research performed at CELEST (Center for Electrochemical Energy Storage Ulm-Karlsruhe).



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