Adsorption of Diethyl Carbonate on LiCoO2 Thin Films: Formation of

Jan 7, 2014 - Gennady Cherkashinin , Markus Motzko , Natalia Schulz , Thomas Späth , and Wolfram Jaegermann. Chemistry of Materials 2015 27 (8), 2875...
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Adsorption of Diethyl Carbonate on LiCoO2 Thin Films: Formation of the Electrochemical Interface Dirk Becker, Gennady Cherkashinin, René Hausbrand, and Wolfram Jaegermann* Department of Materials Science, Darmstadt University of Technology, Jovanka-Bontschits-Straße 2, 64287 Darmstadt, Germany ABSTRACT: Despite numerous efforts to elucidate interface-related phenomena of Li ion battery cathodes, the exact nature of cathode/electrolyte interfaces is still not fully resolved. Key factors for the properties of semiconducting ionic electrodes are band bending and energy level alignment at the interface, which have not been given much attention in the past. In this contribution, we investigate the formation of the electrochemical interface for a LiCoO2 electrode in contact with a solvent adsorbate phase by a surface science approach. Diethyl carbonate (DEC) was adsorbed stepwise onto a LiCoO2 thin film electrode and the electrode surface analyzed with X-ray photoelectron spectroscopy (XPS) after each adsorption step. Adsorption results in the formation of a charged layer in the electrode, which we attribute to the transfer of lithium ions from the electrode to the adsorbed phase. The offset between the LiCoO2 valence band and HOMO of the adsorbed DEC is large (4 eV) under the experimental conditions, which renders solvent oxidation unlikely.

1. INTRODUCTION

processes. Results regarding chemical interaction were reported previously in ref 28.

The interfaces between electrodes and electrolyte strongly determine the performance and lifetime of Li ion intercalation batteries.1−9 Next to lithium ion transfer, processes known to occur at intercalation cathodes are electrolyte oxidation and chemical decomposition of electrolyte species. Such side reactions contribute to the formation of the solid electrolyte interface layer (SEI), which has more recently been studied in some detail.7,10−13 Fully lithiated lithium cobalt oxide (LiCoO2) is a semiconductor.14 Key elements of semiconductor electrodes are charge layers (band bending) and energy level alignment at the interface.15,16 However, until now, such properties have only been invoked or investigated for intercalation electrodes on exception (see, e.g., ref 17). To our knowledge, no experimentally based information on energy level diagrams or fundamental charge- transfer processes are available. Semiconductor electrode properties such as band bending and energy level alignment may be studied by surface science experiments such as low-temperature adsorption in vacuum conditions.18−20 Also, fundamental interaction, reaction, and charge transfer between adsorbed species and a solid surface can be investigated.21−27 Here, we employ low-temperature adsorption and photoelectron spectroscopy with synchrotron radiation (SXPS) to investigate the fundamental electrode properties and doublelayer formation of the LiCoO2 electrode. Diethyl carbonate (DEC), a common solvent in Li ion battery electrolytes, was adsorbed stepwise onto a LiCoO2 (LCO) thin film, and intermediate analysis was performed. Core level and valence band spectra as well as the work function were recorded for each adsorption step. From the data, we extracted the energy level diagram and deduced fundamental charge-transfer © 2014 American Chemical Society

2. EXPERIMENTAL SECTION Experiments were carried out at the undulator beamline U49/2 of BESSY II, using the experimental system “SoLiAS” (Solid/ Liquid Analysis System). The SoLiAS is equipped with an ultrahigh vacuum (UHV) adsorption chamber connected directly to the XPS analysis chamber (base pressure of 10−10 mbar). SXPS spectra were recorded at a kinetic energy of 120 eV. The work function was evaluated using the secondary electron cutoff. Adsorption experiments were performed on a manipulator cooled with liquid nitrogen, which transfers the sample between adsorption and the measuring chamber. DEC, stored and handled under an argon atmosphere and exposed to a molecular sieve, was filled into a Schlenk-type glass flask connected to the SoLiAS and purified by vacuum distillation. The substrate was a polycrystalline LiCoO2 film deposited on titanium foil at a substrate temperature of 550 °C by radio frequency (rf) magnetron sputtering in a oxygen/argon mixture (working pressure of 5 × 10−3 mbar, O2/Ar ratio 1:2). After preparation of the LCO thin film, the substrate lines were analyzed, and typical spectra of clean and stoichiometric LCO were found.29 DEC was dosed by a leak valve into the adsorption chamber in such a way that a pressure of 1 × 10 −7 mbar was reached and the sample was exposed stepwise for different times (exposure of 0.5−8 Langmuir (L), 1 L = 1 × 10−6 mbar s), and it was analyzed by SXPS after each step. After exposure to 8 L, desorption was performed by removing the sample from the cooled manipulator and heating the sample to Received: June 10, 2013 Revised: December 8, 2013 Published: January 7, 2014 962

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site. The reaction may either be chemisorptions or decomposition (dissociative adsorption). The intensity of the additional signals and their ratio change with exposure time, indicating an interface interaction or ion solvation with a complex reaction sequence during the experiment (see ref 28 for details). In any case, it may be interpreted as a formation of a partial solid−electrolyte−interface layer. The peak positions indicate the presence of reaction products with two-fold oxygen-coordinated carbons, like in a carbonyl group or in a polyether.30 In Figure 2, the Co 2p core level spectra during the adsorption sequence are shown. The signal intensity decreases

room temperature (RT). Intensities were normalized to the electron storage ring current. For calibration of the energy scale, a sputter-cleaned gold sample was used.

3. RESULTS AND DISCUSSION 3.1. Results. After each adsorption step, the core level spectra of carbon, oxygen, cobalt, and lithium as well as valence band spectra and the work function were recorded. We first present the carbon spectra (exclusively related to the adsorbate) and then the cobalt spectra (related to LCO). The C 1s spectra were already reported in ref 28 and are included here for the sake of completeness. Consecutively, the lithium, oxygen, and valence band spectra are presented. Lithium ions may be mobile across the interface, and oxygen and valence spectra contain features from both the substrate as well as adsorbate layer. The carbon 1s spectra at different exposure times are shown in Figure 1a. The freshly prepared LCO film is free of carbon

Figure 2. XPS Co 2p spectra of a LiCoO2 thin film after 0.5, 1, 2, 4, and 8 L DEC exposure.

with exposure time due to attenuation in the adsorbate layer of increasing thickness. Features of the Co 2p signal are a splitting due to spin−orbit coupling (Co 2p 3/2 signal at 778.5 eV and the Co 2p 1/2 signal at 793.5 eV) as well as correlated satellites originating from different final states (transition metal d orbital−oxygen 2p orbital interaction). With exposure time, the whole spectrum shifts to higher binding energies, which is attributed to band bending and will be discussed later, but we do not find any significant change in signal shape. The maximum value of this shift is just over 0.8 eV. In Figure 3, the photoemission spectra of lithium Li 1s and Co 3p are shown. In agreement with the Co 2p signal, the

Figure 1. (a) XPS C 1s spectra of a LiCoO2 thin film after 0, 2, 4, and 8 L DEC exposure. (b) XPS C 1s spectrum after 2 L DEC exposure fit with five components.

(spectrum denoted). Exposure to DEC leads to the emergence of C 1s lines representing the three different environments of carbon in the DEC molecule. The methyl carbon is detected at about 285.4 eV, the ethoxy carbon at 287.1 eV, and the carbonate carbon at 290.9 eV. The assignment of the main lines is also given in Figure 1a. It is evident from the intensity ratio of the main lines that DEC is mostly physisorbed as intact molecules onto the LCO surface. The intensity increases with the exposure time, as expected for an increasing amount of physically adsorbed molecules. However, a more detailed analysis of the C 1s spectra Figure 1a reveals the presence of two more signals, showing that carbon is also present in two additional chemical environments, indicating chemical interaction/reaction.28 Figure 1b shows a deconvoluted spectrum (2 L exposure). The two additional types of carbon occur mostly at the expense of the carbonate carbon, indicating a surface reaction with involvement of this

Figure 3. XPS Li 1s spectra of a LiCoO2 thin film after 0.5, 1, 2, 4, and 8 L DEC exposure. The Co 3p spectra are also shown.

intensity of the Li 1s signal decreases and shifts with exposure time. For the freshly prepared sample, we find a signal with a slightly asymmetric shape and a maximum at 53.5 eV. The XPS signal of the Co 3p core level is detected at 60.2 eV. The satellite structure is visible in the range of 68.0−73.0 eV. The changes in Li 1s signal shape upon adsorption are minor, but significant. Figure 4 shows the Li 1s 8−0 L difference spectrum, indicating an additional component in the Li 1s 963

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With exposure time, the position of the lattice oxygen shifts, and its intensity decreases. Again, the shift is attributed to band bending (in the case of band bending, all peaks shift), and the intensity decrease is attributed to damping of the substrate signal due to increased thickness of the adsorption layer. The oxygen detected in the range of 531.0−535.0 eV can be related to the adsorption of DEC. Individual signals can hardly be distinguished in this region, indicating the presence of multiple oxygen environments. We attribute the signals at 531.8 and 534.5 eV, best outlined in the spectra of high exposure times where physical adsorption dominates, to the ethoxy and carbonate bonding configuration, respectively. The remaining signal intensity is attributed to several different oxygen environments encountered by reaction products, in line with the findings related to the C1s signal. Due to the high complexity of the oxygen signal, we have refrained from any deconvolution. Figure 5b shows the O1s spectra taken before and after desorption of DEC at the end of the experiment. This spectrum contains the fingerprint of the oxygen within the reaction products but also the signals from oxygen related to the LCO substrate. The most pronounced signal related to reaction products is found at about 530.9 eV and is attributed to the presence of the same groups discussed for the C 1s signal at 289.5 eV. Figure 6 presents the evolution of the valence band spectra during the experiment. The spectrum of the clean LCO surface

Figure 4. XPS Li 1s difference spectrum of core levels measured after 8 L exposure and before exposure (0 L). Before subtraction, the spectrum taken at 8 L was calibrated to the Co 3p peak of the spectrum taken at 0 L with regard to intensity and binding energy.

signal at 8 L. We attribute this component to lower binding energies to lithium that has moved into the adsorbate phase and experiences a different chemical environment than that in the electrode. The detection of this component is possible due to the high surface sensitivity of SXPS. This view is supported by earlier findings concerning lithium ions adsorbed on an intercalation phase.31 In Figure 5a, the O 1s core level spectra of freshly prepared LCO and after different exposure times are shown. The

Figure 6. Valence band spectra of a LiCoO2 thin film after 0.5, 1, 2, 4, and 8 L DEC exposure.

consists of a prominent signal at 1.5 eV, related to the Co 3d states, and features between 3.0 and 8.0 eV, assigned to mixed Co 3d/O 2p states and O 2p states.29 The Fermi level (BE = 0 eV) is located at the low binding energy edge of the Co 3d states (valence band edge) indicating p-doping of the LCO thin film after synthesis. Exposure to DEC leads to an intensity decrease and a binding energy shift (shift of the Fermi level away from the valence band edge, downward band bending), as already observed for the core level spectra. In addition, the shape of the spectrum changes in the region of the O 2p and O 2p hybride states. Additional states appear and increase in intensity with exposure time and are assigned to the adsorbate layer. In order to see the additional states more clearly, the spectrum taken after 8 L exposure was subtracted from the spectrum of the clean LCO surface. The result is shown in Figure 7. The adsorbate states are clearly seen in the energy region of 4−10 eV. Notably, these states are dominated by physisorbed species, but also contributions from chemisorbed (reacted) species are present. A distinction between the different species proved difficult due to, for example, the low amount of chemisorbed (reacted) species.

Figure 5. (a) XPS O 1s spectra of a LiCoO2 thin film after 0, 0.5, 1, 2, 4, and 8 L DEC exposure. (b) XPS O 1s spectra of a LiCoO2 thin film before DEC exposure (0 L) and after desorption at RT (corrected for the energy shift).

spectrum taken without exposure is typical of a freshly prepared thin film LCO,29 with lattice oxygen at 528.5 eV and an additional component to higher binding energies, which is attributed to surface oxygen remaining from the sputter gas (Ar/O2 mixture) during deposition, similar to other sputterprepared oxides such as InO or In2O3.32 964

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and charge transfer. Energy level alignment and the space charge layer in the LCO can be extracted unambigously from the data. It is clear that DEC does not lead to a strong decomposition reaction of the substrate as no chemically shifted components are observed, as would be evident for chemically modified LCO. 3.2. Energy Level Alignment and Double Layer. The electron energy level diagram of the LCO−DEC interface after exposure to 8L is shown in Figure 9a. Key parameters in such a diagram are the extent of band bending as well as the offset between the valence band maximum and occupied adsorbate states. In the present case, the coordinated energy shift of all substrate signals with exposure time clearly indicates band bending in the semiconducting p-doped LCO substrate. Previous to exposure, the Fermi level of the LCO is located at the top of the valence band, indicating a flat band situation and the absence of any donor states located in the band gap,

Figure 7. Difference spectrum of the valence band, taken after 8 L exposure and before adsorption (0 L). Before subtraction, the spectrum taken at 8 L was calibrated to the Co 3d peak of the spectrum taken at 0 L with regard to intensity and binding energy.

We do not relate the features appearing in the energy region of 0−3 eV to a change in electronic states in this region but rather to peak broadening. This strong rather symmetric broadening of the Co 3d related states at the valence band edge gives rise to the double-peak structure between 2 and 3 eV of binding energy. We attribute this effect to a distribution of peak positions across a narrow space charge layer. Figure 8 shows the evolution of band bending and work function during the experiment. Band bending is demonstrated

Figure 8. Energy shift of the Co 2p 3/2 core level and change of the work function of LiCoO2 thin films after 0.5, 1, 2, 4, and 8 L DEC exposure.

by the binding energy shift of the whole photoemssion spectrum (coordinated binding energy shift) and is shown here by means of the shift of the Co 2p core level binding energy. The direction of the band bending is downward, as concluded from the shift to higher binding energies. It increases with exposure until a value of 0.8 eV at 8 L and decreases again after the desorption step at the end of the experiment. The work function decreases with exposure and is, after exposure to 8 L, more than 0.5 eV lower than that for the clean substrate. Changes in work function (ΔΦ) are both related to bend bending (Vbb) and changes in the surface (or interface) dipole potential (Δχ), according to ΔΦ = eVbb + eΔχ

Bend bending downward contributes to a decrease of the work function. As band bending exceeds the change in work function, we derive the presence of a dipole contribution that opposes the contribution of band bending (positive dipole). In the next section, we will discuss the results with regard to energy level alignment at the interface, double-layer formation,

Figure 9. (a) Electron energy level diagram of the LCO−DEC interface after exposure to 8 L. See the text for explanation. (b) Illustration of the Li ion energy levels and Li ion transfer. 965

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4. CONCLUSIONS We investigated the interface formation between LiCoO2 and DEC by a surface science approach consisting of an adsorption sequence and intermediate analysis by photoelectron spectroscopy. We established the energy level diagram for an intercalation electrode in contact with a solvent and investigated fundamental charge-transfer processes. The presented surface science approach appears to be a useful tool to study intercalation electrodes. The results demonstrate that LiCoO2 behaves like a semiconducting electrode and exhibits band bending. In the present experiment, band bending was in the downward direction and is attributed to transfer of Li ions from the electrode to adsorbate phase, driven by the difference in the lithium ion chemical potential between the electrode and adsorpbate phase. The offset between the LiCoO2 valence band and HOMO of the adsorbed DEC is large (4 eV) under the experimental conditions, which renders solvent oxidation unlikely.

respectively. Thus, we identify the energy shift of the Co 2p signal (0.8 eV) with the formation of a space charge layer (band bending) within the LCO. As the energy shift is to higher binding energies, the band bending is downward from the bulk to the surface, indicating transfer of positive charge from the LCO bulk to the surface layer. Such a binding energy shift related to band bending is a well-known phenomenon for semiconductor/adsorbate interphases and has already been detected for alkali intercalation into layered semiconductors.31 The energetic offset between the valence band maximum and occupied adsorbate states can be directly extracted from the valence band spectra and amounts to approximately 4 eV, as stated in the last section. Indications about the interfacial dipole formation and surface potential of the adsorbate can be deduced from the work function change. Comparing the work function evolution with the evolution of the adsorbate binding energy (band bending), we find a significantly lower decrease of the work function compared to the binding energy increase of the substrate and valence band signals between 0.5 and 8 L exposure, which is related to a surface dipole with the negative charge orientated toward vacuum. 3.3. Charge Transfer and Reaction Mechanism. The band bending in LCO shows that charge transfer between LCO and the adsorbate layer takes place. Negative charge resides in the near-surface region of the LCO, and positive charges are transferred across the interface on the side of the adsorbate layer. Principally, electrons and lithium ions are mobile across the interface, and based on the sign of band bending, there are two possibilities for charge transfer, transfer of lithium ions from the LCO to the adsorbate layer or transfer of electrons from the adsorbate to the LCO (the latter is usually formulated as hole transfer for valence band processes). Given the band level alignment shown in Figure 9a, with the (occupied) adsorbate states much lower than the valence band of LCO, hole transfer is unlikely. In any case, an efficient hole/ electron transfer at the interface requires the presence of DECrelated donor states in the band gap of LCO, for example, originating from interaction of LCO and DEC states, which we have not observed. In the present case, we favor the transfer of lithium ions as the reason for band bending, which is supported by the presence of an additional lithium component after 8 L adsorption. When lithium ions transfer from LCO to the adsorbate layer, they leave behind Co3+ ions or negatively charged vacancies. Lithium ions may transfer due to entropy effects or solvation energy, that is, in the direction of a low lithium ion chemical potential in the adsorbate layer. Figure 9b illustrates Li ion energy levels and Li ion transfer. Lithium ions are depicted as positive charges at the adsorbate side of the interface and are assumed to be partially or fully solvated. Below the electron energy levels, lithium ion energy levels are indicated. The lithium ion chemical potential is assumed to be constant at the interface after Li ion transfer from LCO to the Li ion solvating adsorbate layer. Similar to the electron energy bands, the Li ion energy levels bend due to the presence of the charge layer. The effect of equilibration of the chemical potential of mobile ions at interfaces originally containing different concentrations/ activities of the ions and thus having different chemical potentials has already been discussed for many interfaces from theoretical considerations.27

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. +49 6151 16-6304. Fax +49 6151 16-6308. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG) and the Federal Ministry of Education and Research (BMBF, Support Code 03KP801). We are deeply grateful to the Helmholtz Zentrum Berlin (HZB) BESSY II for measuring periods at the SoLiAS workstation at the U 49 II/PGM2 beam line.

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