Article pubs.acs.org/JPCC
Adsorption of Dimethyl Sulfoxide on LiCoO2 Thin Films: Interface Formation Studied by Photoemission Spectroscopy Thomas Spaẗ h, Mathias Fingerle, Natalia Schulz, Wolfram Jaegermann, and René Hausbrand* Department of Material Science, Surface Science Division, Technische Universität Darmstadt, 64287 Darmstadt, Germany ABSTRACT: The formation of electrode−electrolyte interfaces is of key importance for the stability and performance of lithium-ion batteries. To increase efficiency and lifetime, a detailed understanding of the processes at these interfaces is necessary. Both chemical reactions resulting in the formation of interface layers as well as double layer formation, i.e., band bending, influence the batteries’ properties. In this contribution, we investigate the interface formation between a thin film LiCoO2 cathode material and a solvent adsorbate. Using a surface-science approach, dimethyl sulfoxide (DMSO, (CH3)2SO) was used as molecular probe and adsorbed stepwise onto LiCoO2 at low temperature. After each step, the interface was analyzed by synchrotron-based soft X-ray photoemission spectroscopy (SXPS). We established the energy level alignment of the LiCoO2−DMSO interface, showing a band bending downward in the cathode material attributed to Li-ion transport into the adsorbate phase. Lithium transport results in the formation of a reaction layer containing Li2CO3 and Li2O, constituting a solid electrolyte interphase layer (SEI).
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performed such model experiments on LiCoO2 thin film electrodes and different solvents such as diethyl carbonate (DEC)17 and water.18 In the case of DEC, a typical solvent used in Li-ion batteries,12 we find no indications for oxidative decomposition, but rather observe evidence for chemical decomposition or chemisorption. Here, we present the results of low-temperature adsorption of DMSO on a thin film LiCoO2 electrode, obtained by photoelectron spectroscopy using synchrotron radiation. Upon adsorption of DMSO we observe the formation of new species arising from a reaction with the substrate, which is discussed under consideration Li-ion and electron transfer properties of the interface.
INTRODUCTION The interfaces between electrodes and electrolyte play a key role for the performance and lifetime of lithium-based batteries. Next to Li-ion transfer, side reactions with the solvent and/or the supporting electrolyte occur at interfaces in Li-ion batteries resulting in the formation of surface, or solid electrolyte interface (SEI) layers.1,2 Such SEI layers increase the Li-ion transfer resistance and bind (mobile) lithium, leading to loss of rate capability and capacity, respectively.3 The formation of SEI layers depends to a high degree on electrode surface chemistry2,4 and electrode potential,5,6 i.e., on possible (dissociative) chemisorption, catalytic decomposition, and/or electron transfer. A detailed understanding of these processes and their effective control by additives7 and coatings8 is essential for the performance and stability of Li-ion electrodes. Dimethyl sulfoxide (DMSO) is a common aprotic solvent, used also for the development of batteries such as gel−polymer cells9 and in lithium−air cells.10 DMSO has an ionization potential of 9.1 eV,7,11 indicating a decent stability against oxidation,12,13 but is known to disproportionate into dimethyl sulfide (DMS) and dimethyl sulfone (DMSO2),14 indicating limited chemical stability. On positive electrodes, surface layers consisting of decomposition products of DMSO were found,10 but no details regarding composition or reaction mechanism were reported. The formation of surface layers on electrodes in contact with solvents can be investigated by different approaches, such as electrode emersion15 or low-temperature solvent adsorption.16 Low-temperature adsorption coupled with photoelectron spectroscopic analysis offers the advantage that surface layer formation can be tracked stepwise, which facilitates the identification of mechanisms. More recently, we have © 2016 American Chemical Society
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EXPERIMENTAL DETAILS The experiments were carried out at the beamline U49/2 of BESSY II at Helmholtz-Zentrum Berlin, using the experimental multichamber system “Solid/Liquid Analysis System” (SoLiAS) . The SoLiAS is equipped with a Specs Phoibos 150 electron analyzer. An UHV adsorption chamber is directly connected to the XPS analysis chamber (pressure: 10−10 mbar). Adsorption experiments were performed with a liquid nitrogen cooled manipulator at a temperature of around −160 °C. DMSO was stored and handled under Ar atmosphere, cleaned by molecular sieve, and filled into a Schlenk-type glass flask which is connected to the adsorption chamber, where the solvent was purified by vacuum distillation. Received: June 10, 2016 Revised: August 10, 2016 Published: August 15, 2016 20142
DOI: 10.1021/acs.jpcc.6b05881 J. Phys. Chem. C 2016, 120, 20142−20148
Article
The Journal of Physical Chemistry C
Figure 1. Evolution of Li 1s, Co 2p, O 1s, C 1s, S 2p and valence band spectra with increasing coverage of DMSO upon DMSO adsorption at −160 °C.
Detailed spectra were measured of the O 1s, Co 2p, Li 1s, C 1s, and S 2p and the valence band region. The excitation energy was varied to obtain a constant kinetic energy excess of 120 eV according to the respective main peak component. The secondary electron cutoff was measured by applying a bias current to obtain the work function of the layer system. To make sure that changes due to the radiation are negligible during the experiment, we checked the S 2p and C 1s signal regularly, observing no appearance of new peaks and only minor changes in peak intensities. All spectra were background subtracted using the Shirley method.19 For calibration of the energy scale, a sputter-cleaned gold sample was used. For analysis and presentation all spectra were energetically shifted to the binding energy values of a pure HT-LiCoO2 thin film as measured at our home XPS source.
A homemade radio-frequency magnetron sputtering chamber was brought to BESSY and connected directly to die SoLiAS station. This allows in-situ measurements of freshly prepared samples. HT-LiCoO2 polycrystalline samples with an (003) oriented texture were prepared at a working pressure of 8 × 10−3 mbar in a 1:1 Ar/O2 gas mixture at a temperature of 550 °C. For our experiments we used n-doped Si(111) as substrate. DMSO was dosed using a leak valve until a pressure of 1 × 10−7 mbara had been reached. The time was varied in a way that the desired nominal exposition rate was reached. Nominal expositions between 0.5 and 8 langmuirs (1 langmuir = 1.33 × 10−6 mbar·s) have been used for the experiment. After the last adsorption step, the sample was heated up to room temperature by removing the sample from the cooled manipulator for around 10 min and then measured again. To identify the physisorbed species, another LiCoO2 thin film sample was directly exposed to 100 langmuirs of DMSO. Additionally, 12 langmuirs of DMSO was adsorbed on a sputter-cleaned gold foil for comparison.
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RESULTS For an overview, Figure 1 presents the evolution of the Li 1s (including the Co 3p peak), Co 2p, O 1s, C 1s, and the S 2p 20143
DOI: 10.1021/acs.jpcc.6b05881 J. Phys. Chem. C 2016, 120, 20142−20148
Article
The Journal of Physical Chemistry C spectra during the experiment. The pristine LiCoO2 thin film shows the typical features observed for highly surface sensitive analysis, free of any surface contamination: The main components are observed at 53.5 eV for Li, 528.5 eV for O, and 778.5 eV for Co. The Li 1s and O 1s spectra have high binding energy (BE) components at 54.4 and 530.2 eV attributed to surface components.20,21 These surface components arise from atoms with an different charge, as described in earlier publications.22 The Co 2p peak exhibits in total four features: Due to spin−orbit splitting, two main features of Co 2p3/2 and Co 2p1/2 can be observed. In addition, each of these features shows a satellite due to final state effects. The signature of the Co spectra with a BE difference between main feature and satellites of 10 eV is representative of cobalt in the 3+ oxidation state.23 Exposure to DMSO results in the appearance of carbon and sulfur, mostly related to physisorbed species as demonstrated by their decrease upon heating to room temperature and to the attenuation of the substrate-related features. On the basis of the damping of the substrate-related features, we obtain film thicknesses up to 0.61 nm after adsorption of DMSO.b The large number of components observed in the C 1s, S 2p, and O 1s spectra indicates the presence of more than one physisorbed species and in addition to chemically modified species. With increasing exposure, for all elements a common peak shift (0.4 eV) to higher binding energies is observed, which is attributed to band bending. In the following, we present the evolution of the different spectra in detail and assign the components. Core Level Spectra. The evolution of the C 1s and S 2p peak is shown in Figure 2. All spectra were corrected for band bending. As sulfur shows a spin−orbit splitting, all components are fitted with two peaks. The distance between S 2p3/2 and S 2p1/2 was kept constant (1.2 eV) as well as the peak area ratio and the peak width. In total, three different components are
observed, marked in red (at 166.0 and 167.2 eV), blue (at 164.0 and 165.2 eV), and green (at 168.5 and 169.7 eV). The red and blue components are already present at low exposure time, while the green component emerges only at high exposure time. We assign the red component to sulfur in DMSO. This assignment is also in agreement with sulfur in physisorbed DMSO found in other experiments.25 The component to lower binding energy (blue) demonstrates that sulfur is present also in a reduced form. We assume that this is sulfur in DMS, which is present as a reaction or decomposition product in a physisorbed state. The green component is attributed to further oxidized sulfur, i.e.,−SO2 or −SO4 groups, forming Li2SO426 or DMSO2, respectively. The presence of the different species suggests that DMSO reacts with the substrate to form DMS and inorganic compounds containing lithium, as is discussed in more detail in the next section. The C 1s spectra reflect largely the S 2p spectra and confirm the interpretation of the S 2p data. The red component in Figure 2b can be assigned to DMSO (287.7 eV) and the smaller component at 287.1 eV (blue) to DMS. The other two components (green and yellow) are only visible in the 8 langmuir spectrum and below, presumably due to the attenuation of the overlying physisorbed species. Therefore, we conclude that these components are only present at the surface and belong to chemisorbed and/or reacted species. The green component at 291.0 eV is assigned to carbonate carbon, and the yellow component at 285.2 eV is assigned to (traces of) methyl carbon, an organic component which may have formed by decomposition of DMSO. These binding energy values are in good agreement with our previous results.17,27 Looking at the O 1s spectra (Figure 3), it can be observed that several additional components emerge in the adsorption
Figure 2. (a) Detailed spectra of the S 2p peak after adsorption of DMSO at −160 °C. (b) Detailed spectra of the C 1s peak after adsorption of DMSO at −160 °C. In both figures all spectra are corrected for bend bending. Formation of Li2CO3, DMS, and a −SOx (x ≥ 2) component is observed with increasing coverage.
Figure 3. Detailed spectra of the O 1s peak upon adsorption of DMSO at −160 °C: At low coverage a formation of Li2O, Li2CO3 building, and a −SOx (x ≥ 2) component are observed. For higher coverage physisorption of DMSO is the main process going on. All spectra are corrected for bend bending. 20144
DOI: 10.1021/acs.jpcc.6b05881 J. Phys. Chem. C 2016, 120, 20142−20148
Article
The Journal of Physical Chemistry C
intensities of the LiCoO2 thin film and the reaction compounds are increased, showing a reduction of the adsorbed layer thickness from 0.61 nm (reaction layer + physisorbed layer) to 0.17 nm (reaction layer). Summarizing the previous findings, we conclude that the exposure of LiCoO2 to DMSO leads to the formation of a reaction layer as indicated by new components emerging in the Li 1s, C 1s, S 2p, and O 1s spectra. Binding energies as well as the general trend of peak intensities demonstrate that the reaction layer consists of Li2CO3, Li2O and a −SO2 or −SO4 component as well as a small amount of organic compounds. The surface reaction layer is covered by a physisorbed layer, consisting of DMSO and another compound identified as DMS. Assuming a homogeneous reaction layer, the thickness of the layercalculated from the attenuation of the Co 3p peak is very thin (
