Intercalation and Deintercalation of Lithium at the Ionic Liquid

Nov 13, 2017 - *Institute of Surface Chemistry and Catalysis, Ulm University, Albert Einstein Allee 47, D-89069 Ulm, Germany. Phone: +49 (0)731/50-254...
0 downloads 0 Views 1MB Size
Download PDF
Letter pubs.acs.org/JPCL

Cite This: J. Phys. Chem. Lett. 2017, 8, 5804-5809

Intercalation and Deintercalation of Lithium at the Ionic Liquid− Graphite(0001) Interface Florian Buchner,†,‡ Jihyun Kim,§ Christiane Adler,§ Maral Bozorgchenani,§ Joachim Bansmann,§ and R. Jürgen Behm*,†,§ †

Helmholtz Institute Ulm Electrochemical Energy Storage (HIU), Helmholtzstraße 11, D-89081 Ulm, Germany Karlsruhe Institute of Technology (KIT), P.O. Box 3640, D-76021 Karlsruhe, Germany § Institute of Surface Chemistry and Catalysis, Ulm University, Albert-Einstein-Allee 47, D-89081 Ulm, Germany ‡

ABSTRACT: The intercalation and deintercalation of lithium (Li) into / out of graphite(0001), which is a highly important process in Li-ion batteries, was investigated under ultrahigh vacuum conditions as a function of temperature, employing X-ray and ultraviolet photoelectron spectroscopy. Both the up-shifts of the core-level binding energy and the lowering of the work function ΔΦ reveal that heating of a monolayer of the battery-relevant ionic liquid (IL) 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]+[TFSI]−) adsorbed on lithiated graphite at 80 K to >230 K facilitates an accumulation of partially charged Liδ+ atoms at the IL−graphite(0001) interface. This is accompanied by a partial IL decomposition, which is associated with the initial stages of the chemical formation of the solid−electrolyte interphase.

T

continues our previous studies on the initial stages of the chemical formation of the solid−electrolyte interphase (SEI),18−22 an interfacial film at the electrode, which is generated by the decomposition of the solvent/electrolyte at the interface, mostly upon interaction with the solid electrode, and plays a crucial role in the battery function.1,23 In our previous work, we have studied the adsorption behavior of the battery-relevant ionic liquid (IL) solvent 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]+[TFSI]−) on different surfaces18−22 in UHV, showing that [BMP]+[TFSI]− adsorbs intact on the basal plane of graphite(0001) at 300 K.20 It was found to decompose, however, into a variety of products like LiF, Li2S, Li3N, etc. when in contact with postdeposited Li, as recently demonstrated for Cu(111)21 and TiO2(110),22 where this occurs already at 80 K. In the present Letter we report on the intercalation of Li into graphite(0001) under UHV conditions and on how an ionic liquid adlayer is used as a probe to measure the deintercalation of Liδ+. Variable-temperature X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) were employed, which are sensitive to the surface chemistry on flat model surfaces. In a first step, Li was vapor deposited on pristine graphite(0001) at two different temperatures. After deposition of ∼1 ML of Li [we define 1 ML of Li by the density in the close-packed plane in a bcc Li crystal (density ∼12 Li atoms

he reversible intercalation and deintercalation of lithium (Li) into graphite, which is a well-established anode material in batteries, is a highly important process in Li-ion batteries.1 A multitude of studies have reported on the intercalation and diffusion of Li into/in graphite, which were carried out in situ by electrochemical methods,2−5 ex situ under ultrahigh vacuum (UHV)6−9 conditions, and by theory.10−16 For example, Senyshyn et al.17 performed in situ neutron powder diffraction and electrochemical measurements on Liion cells under operating conditions between 230 and 320 K, finding increasing discharge times at temperatures below 250 K. They attributed this to an energy barrier for the phase transition between different Li intercalation stages, but also freezing of electrolyte was assumed to play a role. In general, Li-ion batteries are highly complex systems, and their specific behavior is the result of the interplay between numerous components. The detailed understanding of the processes in a battery at the molecular level, which would be highly interesting for the improvement of future battery systems, is hampered by the complexity of the cell processes. In the present model study we reduced the complexity of the system significantly and studied the interaction of individual components of the electrolyte with a well-defined model electrode surface under UHV conditions, focusing on the surface chemistry in the absence of an applied potential between electrode and electrolyte. While Li intercalation into highly oriented pyrolytic graphite (HOPG) was investigated, for example, by Auger electron spectroscopy (AES) in the past,6 the deintercalation of Li has, to the best of our knowledge, not yet been explored on such systems under these conditions. This is the topic of the present work, which © XXXX American Chemical Society

Received: September 25, 2017 Accepted: November 13, 2017 Published: November 13, 2017 5804

DOI: 10.1021/acs.jpclett.7b02530 J. Phys. Chem. Lett. 2017, 8, 5804−5809

Letter

The Journal of Physical Chemistry Letters nm−2)] on graphite(0001) at 300 K (Figure 1, bottom left panel), we could not detect any measurable XP signal in the

low coverages (θLi < 0.20 ± 0.05 ML) is assigned to adsorbed Liδ+ species in a dispersed two-dimensional (2D) gas phase, which induces a larger dipole moment, while the constant work function at higher coverages was explained by the increasing presence of a condensed phase with a smaller dipole moment per Liδ+, which coexists with the 2D gas phase, such that the overall dipole moment of the adlayer stays constant.26 We note that after the third Li exposure we observed a new feature with weak intensity at ∼13.8 eV, which is assigned to a contribution up-shifted from the main peak at 13.2 eV, most likely induced by charge transfer from Li to the uppermost graphene layers.32 In combination, the XPS and UPS measurements unambiguously prove that upon vapor deposition Li adsorbs as partially charged Liδ+ on the surface at 80 K, at least for low coverages, while it intercalates into graphite(0001) at RT. For the latter, Wang et al. proposed that virtually complete charge transfer to carbon leads to ionized, intercalated Li+ species.11 Hence, in the following, intercalated Li is assigned to Li+, while adsorbed Li is represented by Liδ+. The fact that ∼1 ML of intercalated Li+ could not be detected by XPS (Al Kα, grazing emission, 80° with respect to the surface normal; information depth of 1−3 nm) is indicative of the low density of Li+ in the LixC6 intercalation structures (0 < x < 1)9 and the broad vertical distribution of Li+ in the bulk. On the basis of temperaturedependent AES measurements on HOPG, Mandeltort et al.6 calculated a barrier of ∼0.16 eV (D ≈ 5 × 10−6 cm−2 s−1 at 300 K) for surface diffusion of Li along the (0001) plane and claimed that Li intercalation into the bulk most likely takes place at defects and step edge sites, which are reached via Liδ+ surface diffusion. In agreement with our observations they found that Li does not intercalate at ∼100 K, but Li intercalates completely upon annealing to around 250 K. Similarly, employing chronoamperometric response measurements (Devanathan−Stachurski methodology), Persson et al.2 showed that Li+ ions are highly mobile parallel to the basal planes of graphite(0001) (D ≈ 10−7−10−6 cm−2 s−1) but much less mobile perpendicular to the graphene layers (D ≈ 10−11 cm−2 s−1). They assumed that in the latter case diffusion mainly takes place along the grain boundaries. Density function theory based calculations with dispersion corrections (DFT-D calculations) by Hazrati et al. finally predicted an energy gain upon Li intercalation into graphite of −0.2 to −0.3 eV per Li atom,10 and Wang et al. calculated a formation enthalpy of LiC6 of −16.4 kJ mol−1.11 In the next step, we explored the chemical state of a [BMP]+[TFSI]− monolayer on lithiated graphite at different temperatures, after ∼1 ML of Li was intercalated into graphite(0001) at RT. After the lithiated graphite was cooled to 80 K, a [BMP]+[TFSI]− monolayer was vapor deposited on the surface. The XP N 1s spectrum in Figure 2 (top of the panel), which was acquired at 80 K, shows peaks at 402.4 and 399.3 eV, which are related to the N atoms in the cation (Ncation) and in the anion (Nanion), respectively, in the molecularly adsorbed [BMP]+[TFSI]− ion pairs. These peaks are essentially identical to peaks in spectra recorded on [BMP]+[TFSI]− covered pristine graphite(0001) at RT (cf. ref 20). The XP Li 1s spectrum (Figure 2, top of the panel) shows a featureless line, reflecting the absence of Li in the surface region. In the following, we recorded a series of N 1s and Li 1s spectra while slowly raising the temperature (1 K min−1) to RT. Up to 200 K, the spectra do not change at all. Reaching 230 K, the XP N 1s spectrum reveals a sudden upward shift of both the Ncation and Nanion peaks by about +0.35 eV (indicated by vertical

Figure 1. XP Li 1s core-level spectra after vapor deposition of ∼1 ML of Li on graphite(0001) at RT (bottom left panel) and at 80 K (top left panel), respectively. UP spectra after vapor deposition of ∼1 ML of Li on pristine graphite(0001) at 300 K (bottom right panel) and after stepwise vapor deposition of Li (ΔΦmax = −1.7 ± 0.1 eV at θ = 0.20 ± 0.05 ML) on pristine graphite(0001) at 80 K (top right panel).

region of the Li 1s peak. When the same experiment is performed with a pristine sample cooled to 80 K, in contrast, the situation changes completely. Now the Li 1s region displays a slightly asymmetric peak at a binding energy (BE) of 55.4 eV, which is indicative of metallic adsorbed Li (Figure 1, top left panel).21 Note that because of the low photoionization cross section of Li the intensity of the Li 1s peak is very low for Li coverages of around θLi ≤ 0.3 ML in our experimental setup. Considering much lower photon energies in the ultraviolet range (He I line), the spectrum of pristine graphite(0001) (Figure 1, bottom right panel) (black solid line) displays a characteristic peak at 13.2 eV, which had been assigned to electrons that are inelastically scattered into unoccupied states in the conduction band.24 The cutoff energy Ecutoff of the secondary electrons was at ∼16.4 eV, which corresponds to a work function Φ of graphite(0001) of 4.8 ± 0.1 eV [subtraction of Ecutoff from hν (He I: 21.21 eV)], in good agreement with previous findings.24 A very similar UP spectrum was obtained also after Li deposition at room temperature (RT) (red solid line). The UP spectra are distinctly different for Li adsorption at 80 K (top right panel). In this case, stepwise vapor deposition of metallic Li results in a successive upward shift of the cutoff energy to higher BEs, reflecting a rapid lowering of the work function Φ. This reaches a minimum of ∼3.1 eV, corresponding to a maximum change in work function of ΔΦmax = −1.7 ± 0.1 eV at a Li coverage of around θLi ∼ 0.20 ± 0.05 ML. For higher coverages, no further shifts are observed. Hence, the work function Φ decreases rapidly for very low Li coverages and then remains constant, in good agreement with the coverage-dependent trends in ΔΦ for other adsorbed alkali metals on graphite, e.g., Na/graphite,25 K/graphite,26 and Cs/ graphite.27 The shift of the work function was attributed to a modification of the surface dipole layer caused by the donation of the Li 2s electron to the substrate.28 More recently, this picture was slightly modified, assuming that adsorbed Li is only partly ionic (Liδ+).29−31 The decrease of the work function at 5805

DOI: 10.1021/acs.jpclett.7b02530 J. Phys. Chem. Lett. 2017, 8, 5804−5809

Letter

The Journal of Physical Chemistry Letters

Figure 2. N 1s and Li 1s XP core-level spectra after vapor deposition of a (sub-) monolayer of [BMP]+[TFSI]− on lithiated graphite(0001) at 80 K (top of each panel) and during slow annealing (1 K min−1) up to 300 K. A molecular representation of [BMP]+[TFSI]− is inserted above the panels: nitrogen (blue), fluorine (green), sulfur (gold), oxygen (red), and carbon (black). The experimental steps are drawn as sketches on the right side of the figure: (1) Li intercalation at 300 K, (2) adsorption of [BMP]+[TFSI]− on lithiated graphite at 80 K, and (3) subsequent heating.

Figure 3. XP F 1s, S 2p, O 1s, C 1s, and Li 1s core-level spectra (a−e) after vapor deposition of a (sub-) monolayer of [BMP]+[TFSI]− on lithiated graphite(0001) at 80 K (blue) and during stepwise annealing (1 K min−1) up to 300 K (red). (f) Cutoff of the secondary electron emission in the UP spectra of lithiated graphite (black), after adsorption of a [BMP]+[TFSI]− (sub-) monolayer at 80 K (blue) and during stepwise annealing to 300 K (violet to red).

dashed lines) to 402.8 and 399.6 eV, respectively. At 250 K, the Ncation and Nanion peaks shifted further to 403.1 and 399.8 eV, respectively, and now also the Li 1s region exhibits a very low intensity feature. Upon further increasing the temperature to 270 K, the Ncation and Nanion peaks continue to upshift to 403.4 and 400.1 eV, respectively, and additionally a shoulder with low intensity appears at the low BE side at 398.5 eV (filled with yellow), which has grown at the expense of the Ncation and Nanion peaks. This process is accompanied by an increase of the Li 1s peak at 56.5 eV, which is located at a higher BE than metallic Li (cf. with Figure 1) as a consequence of a chemical bond with the IL. Finally, at 300 K, the Ncation and Nanion peaks (404.0 and 400.6 eV) are up-shifted by 1.6 and 1.3 eV in total, respectively, compared to the situation at 80 K. Furthermore, both peak areas decreased moderately, while simultaneously the peak at 398.5 eV increased. On the basis of its BE, we tentatively assign the new peak in the N 1s range to Li3N, which is formed upon a partial decomposition of [BMP]+[TFSI]−. In total, the gradual shift of the Ncation and Nanion peaks, the emergence of a peak in the Li 1s range, and the growth of a new Li3N peak upon annealing from 80 to 300 K indicate that part of the intercalated Li+ has diffused back from the bulk to the surface at >230 K, reacting with and partially decomposing the [BMP]+[TFSI]− adlayer. Only for low-temperature deposition is it possible to adsorb the ionic liquid without decomposition, because under these conditions the concentration of Liδ+ on the surface is negligible. Next we concentrate on a similar type of temperaturedependent XPS measurements in the F 1s, S 2p, O 1s, C 1s, and Li 1s spectral ranges (Figure 3a−e) and on the work function changes measured by UPS (Figure 3f). Adsorption of a monolayer of [BMP]+[TFSI]− on lithiated graphite(0001) at 80 K leads to the following adsorbate-related

XPS peaks, which were derived by peak deconvolution (details about the fitting are shown in Experimental Methods). Within the limits of accuracy, they reflect the nominal ratio of atoms with different chemical environments in [BMP]+[TFSI]− (C11H20N2F6S2O4) (Table 1) and the carbon substrate peak Table 1. XPS Peaks after Vapor Deposition of a (Sub-) Monolayer of [BMP]+[TFSI]− on Lithiated Graphite(0001) at 80 K name F 1s S 2p3/2 O 1s C 1s C 1s C 1s C 1s

Fanion Sanion Oanion Canion Chetero Calkyl Cg

group

BE (eV)

nominal amount

−CF3 −SO2 −SO2 −CF3 −CN− −C−C−

688.9 168.9 532.6 292.8 286.7 285.7 284.6

6 2 4 2 4 5

Cg. Similar to that for the XP N 1s signal, the peaks in the other spectral ranges also revealed a constant BE at least up to 200 K, while at higher temperatures (≥230 K) all XP peaks (Fanion, Sanion, Oanion, Canion, Chetero, and Calkyl) gradually shifted to higher BEs. This upshift reached a maximum of +1.15 ± 0.1 eV at 300 K. The upshift is somewhat less than the shifts observed for the N 1s peaks. On the other hand, for the asymmetric graphite substrate peak Cg, the upshift is significantly smaller, with around +0.3 eV. This peak shift will be discussed below. In addition to the shifted peaks, the deconvoluted XP spectra at 300 K revealed additional low-intensity peaks, which are 5806

DOI: 10.1021/acs.jpclett.7b02530 J. Phys. Chem. Lett. 2017, 8, 5804−5809

Letter

The Journal of Physical Chemistry Letters

different shift of the adsorbate-related XP peaks to higher BEs is most likely related to an initial state effect, induced by changes of the chemical environment of the adsorbed ionic liquid by the positive electrostatic potential of Liδ+ on the surface. The moderate shift of the graphite substrate peak Cg by +0.3 eV is assigned to changes in the surface core level by the charge transfer from Li to the uppermost graphene layers. Similar effects have been observed for K on HOPG.34 In total, the variable-temperature XPS and UPS measurements, performed upon adsorption of [BMP]+[TFSI]− on lithiated graphite(0001) at 80 K and subsequent heating to 300 K, demonstrate that Li+ deintercalates and accumulates as Liδ+ in the adsorbate phase. Some of these Liδ+ atoms are involved in the partial decomposition of a [BMP]+[TFSI]− adlayer at temperatures ≥230 K. The whole deintercalation process starts close to the temperature of 250 K, where Li intercalation on graphite(0001) was reported by Mandeltort et al.6 Hence, as intuitively expected, the energy barriers for intercalation and deintercalation seem to be identical. The accumulation or enrichment of Liδ+ on the HOPG surface (at the interface) can most easily be explained by a dynamic equilibrium between Li+ in the bulk and Liδ+ on the surface, where the surface species are stabilized by the reactive or nonreactive interaction with the [BMP]+[TFSI]− adlayer. As stated before, Liδ+ surface species are not observed for clean lithiated graphite surfaces in the absence of kinetic limitations. Stabilization is caused by the formation of stable Li surface compounds such as Li3N, due to reactive decomposition of the IL, and by electrostatic interactions with the adsorbed ions of the IL. Considering that we find ∼0.7 ML of Li+ reappearing at the surface, the majority of the initially intercalated Li+ (deposition of ∼1 ML at RT) has deintercalated. From the present data we cannot derive, at least not directly, whether Liδ+ (de)intercalation proceeds via penetration of the graphene layers or via defect sites (vacancies, step edges, grain boundaries) of graphite(0001). For deposition at 300 K, the surface mobility of adsorbed Liδ+ species is certainly sufficiently high to reach such defects even on the highly perfect HOPG surfaces. For deintercalation on the IL-covered surface, in contrast, the situation is less clear, because we expect that the mobility of adsorbed Liδ+ species in the IL adlayer on the graphite(0001) surface is rather low. On this basis, we tend to favor a diffusion pathway of the Li+ ion through the (ILcovered) graphene surface layer. Finally, we emphasize that the present observations on the deintercalation of Li+ out of graphite(0001) and the reaction with adsorbed [BMP]+[TFSI]− are highly relevant for batteries because they indicate that Li+ can easily (de)intercalate at RT already at the open-circuit potential, without an applied potential. The resulting decomposition products can be considered as the initial stage of the chemical formation of the SEI. Overall, this work clearly demonstrates the potential of this kind of model studies for gaining improved insight into interfacial processes between battery electrodes and electrolyte at a molecular scale, which is the basis for a systematic approach toward SEIs with improved properties. In summary, we have shown in a model study, performed under UHV conditions and using a well-defined HOPG model electrode, that intercalation and deintercalation of vapordeposited Li into/out of HOPG is a reversible process, which is kinetically inhibited at T < 200 K but increasingly facile at T > 230 K. For the pristine HOPG sample, bulk dissolved

assigned to decomposition products such as LiF (686.0 eV), Li2SO3 or Li2SO4 (168.4 eV), and LiOH (532.5 eV) (cf. refs 21 and 33). Because of the low cross section, these compounds cannot be resolved in the Li 1s region in Figures 2 and 3. The formation of these compounds, together with Li3N, reflects the initial stages of the chemical formation of an interfacial layer, comparable to the electrochemical formation of this layer at the electrode−electrolyte interface during battery operation. In analogous UPS experiments, we monitored changes in the work function Φ as a function of temperature (Figure 3f). After the lithiated graphite(0001) sample is cooled to 80 K (identical work function as pristine and lithated graphite at RT, i.e., no surface Li δ+ ) and subsequent vapor deposition of a [BMP]+[TFSI]− monolayer, the work function Φ increases little (the electron cutoff shifts by ∼ −0.2 eV), indicating rather weak modifications of the surface dipole layer upon deposition. Upon annealing to 210 K, we observe a reverse shift back to the initial work function value of lithiated graphite. Considering that based on the XPS observations there is no change in the chemical composition of the adlayer, we tentatively assign this to a temperature-induced rearrangement or configurational change of the IL ion pairs in the adlayer, where molecular scale details are not yet known. Such effects were indicated in scanning tunneling microscopy measurements on Ag(111), where we demonstrated that the well-ordered [BMP]+[TFSI]− (2D) crystalline phase (100 K) melts in a comparable temperature range (>180 K), leading to a mobile (2D) gas phase and possibly a modified adsorption geometry.18 After the sample is heated to 230 K, the work function starts to decrease and stepwise decays up to 300 K. Finally, it is ∼1.2 eV lower compared to that of the lithiated graphite(0001) sample (ΔΦ = −1.2 ± 0.1 eV). The gradual lowering of the work function at >230 K is assigned to the increasing presence of partially charged Liδ+ atoms at the surface and their reaction with the [BMP]+[TFSI]− adlayer. On the basis of these results, we conclude that upon annealing to >230 K, Li+ segregates back to the surface (θLi ∼ 0.3, 0.4, and 0.7 ML at 270, 285, and 300 K, respectively, as derived from the peak area of the Li 1s peak) and that part of the resulting Liδ+ species react with the IL adlayer, forming a variety of decomposition products, which dominate the peak at 56.5 eV in Figures 2 and 3. Another part of the Li species is stabilized as Liδ+ at the surface by nonreactive interaction with the IL adlayer. The small amount of trapped Liδ+, which will contribute to the peak at 56.5 eV, is considered as the main reason for the significant decrease of the work function Φ in UPS (Figure 3). This is based on the fact that for small amounts of surface Li we see already a massive change in work function (see Figure 3). In the absence of resolvable differences in the Li 1s peak and considering also the unknown surface dipoles associated with these species (unknown contribution to the work function), a more quantitative determination of the ratio of reacted surface Li and trapped surface Liδ+ is hardly possible. The variable-temperature XPS and UPS experiments reveal a collective, but slightly different energy shift of all adsorbaterelated XP peaks, by +1.1−1.2 eV in the F 1s, S 2p, O 1s, and C 1s region and by +1.3 and +1.6 eV for the Nanion and Ncation peaks, respectively, to higher BEs and, simultaneously, a lowering of the work function by a similar value (ΔΦ ∼ −1.2 eV) for temperatures higher than 230 K. The lowering of the work function Φ is at least partly related to the increasing presence of partially charged Liδ+ atoms and the subsequent change of the surface dipole. In contrast, the simultaneous, but 5807

DOI: 10.1021/acs.jpclett.7b02530 J. Phys. Chem. Lett. 2017, 8, 5804−5809

Letter

The Journal of Physical Chemistry Letters inserted Li+ is thermodynamically stable but in dynamic equilibrium with adsorbed Liδ+ under these conditions. This is different for a surface covered by a monolayer of the ionic liquid [BMP]+[TFSI]−, where interaction of surface Liδ+ with the ionic liquid adlayer, either by reactive decomposition of the ionic liquid and formation of stable surface Li compounds such as Li3N etc. and/or by nonreactive electrostatic interaction, stabilizes Liδ+ at the surface. The reaction between Liδ+ and an ionic liquid adlayer can be considered as a first step toward the chemical formation of a solid−electrolyte interphase at the electrode−electrolyte interface.

1s range after each annealing step, respectively. In the latter case the missing spectral ranges were acquired after the last heating step, however, they all show the same result as in the former case. Therefore, we consider these data as free from beam damage effects. For the fitting of the XP spectra, a simultaneous fit of background (Shirley + slope) and signal was used, applying an asymmetric pseudo-Voigt-type function. For the fitting of the S 2p range we used a pseudo-Voigt-doublet for the S 2p1/2 and S 2p3/2 peaks with a spin−orbit split of 1.20 ± 0.05 eV and a peak area ratio of 1:2.

■

■

EXPERIMENTAL METHODS The experiments were carried out in a commercial UHV system (SPECS) with a base pressure of 2 × 10−10 mbar. One chamber 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, DLSEGD-Phoibos-Has3500) for XPS and UPS measurements. The highly oriented pyrolytic graphite(0001) (HOPG) single crystal was purchased from MaTeck (ZYA, mosaic spread 0.4° ± 0.1°), exhibiting a cuboid shape with a size of 10 mm × 10 mm ×