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Adsorption of Propylene Carbonate (PC) on the LiCoO2 Surface Investigated by Nonlinear Vibrational Spectroscopy Huijin Liu,† Yujin Tong,† Naoaki Kuwata,‡ Masatoshi Osawa,† Junichi Kawamura,‡ and Shen Ye*,†,§ Catalysis Research Center, Hokkaido UniVersity, Sapporo, Japan, Institute for Multidisciplinary Research of AdVanced Materials, Tohoku UniVersity, Sendai, Japan, and PRESTO, Japan Science and Technology Agency (JST), Japan ReceiVed: July 27, 2009; ReVised Manuscript ReceiVed: September 28, 2009
The adsorption of propylene carbonate (PC) on the surface of the LiCoO2 thin-film, one of the important cathode materials used in the lithium-ion (Li-ion) battery, has been investigated by in situ sum frequency generation vibrational spectroscopy. Two adsorption modes of PC molecules with opposite orientation geometries on the LiCoO2 surface have been observed. The adsorption structures of PC are affected by the solvation process of the Li+ ion. The lithium-ion (Li-ion) batteries have been regarded as one of the most important inventions in modern energy-storage technology.1,2 It has the highest energy density among all rechargeable batteries and is widely used as a power source for many applications such as personal computers, hybrid electric vehicles (HEV), and electric vehicles (EV). Reversibility and efficiency of the intercalation/deintercalation of the Li+ ion into/ from the electrodes in nonaqueous electrolyte solutions directly determine the performance of the Li-ion battery. As a solvent suitable for the Li-ion battery, one requires a long-term stability for the redox process in a wide potential region, a large dielectric constant, a high solubility for lithium salt, a low viscosity, as well as high safety. For this purpose, alkyl carbonates such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), or their mixtures are widely used as the solvents in the fundamental studies for the Li-ion batteries. It is known that these organic solvents also are involved in the intercalation and deintercalation processes of the Li+ ion as well as the formation of the solid electrolyte interface (SEI) on the electrode surfaces of the Li-ion battery. Many analytical methods such as Raman scattering,3-5 infrared (IR) spectroscopy,6-10 mass spectroscopy,11 and theoretical calculations12,13 have been employed to investigate the behaviors and structures of the solvents on the electrode surfaces in order to further improve the performance of the Li-ion battery. However, due to a lack of surface sensitivity, most of these earlier experimental investigations were limited to the solvent structures in solution phase rather than those on the electrode surface. As a second-order nonlinear optical technique, sum frequency generation (SFG) is attracting much attention in multidisciplinary research fields including surface science and electrochemistry,14-22 because of its intrinsically high surface selectiv* To whom correspondence should be addressed. E-mail: ye@ cat.hokudai.ac.jp. † Hokkaido University. ‡ Tohoku University. § PRESTO, JST.
10.1021/jp907146n CCC: $40.75
ity, sensitivity, and versatile applicability. By overlapping a visible laser beam (ωvis) and a tunable IR laser beam (ωIR) at an interface, the surface vibrational states can be probed by the SFG signal (ωSFG ) ωvis + ωIR) when ωIR is equal to the vibrational transition of the molecules at the interface. It is promising to quantitatively elucidate the solvent structures on the electrode surface of the Li-ion battery by this approach. In this letter, we will demonstrate an in situ SFG observation of the adsorption structure of the alkyl carbonate solvent on the surface of the LiCoO2, one of the most important cathode materials for Li-ion batteries. It is shown that the SFG spectral features for the solvent adsorption on the LiCoO2 surface are influenced by the addition of Li+ ion into the solution, which is partially attributed to the solvation process by Li+ ion. A LiCoO2 thin-film of ca. 50 nm was deposited on the flat surface of a hemicylindrical calcium fluoride (CaF2) prism (UV grade, d ) 12.5 mm, l ) 12.5 mm) by pulsed laser deposition (PLD) method23 (see the Supporting Information). To get the electric contact of the LiCoO2 thin film for the further electrochemical characterization, a gold thin-film of ca. 100 nm was evaporated on the CaF2 substrate surface prior to the PLD process, except a small area on the center of the substrate where the laser beams for the SFG observation will be focused on. As a typical example of the alkyl carbonate solvents, the structure of PC molecules on the LiCoO2 surface was mainly investigated in the work. PC solutions containing different concentrations of Li+ ion were prepared by diluting 1.0 M LiClO4/PC solution with the pure PC solvent. All chemicals are lithium battery grade (LBG) from Kishida Chemicals Co. Ltd. (Osaka, Japan). Details about our broadband SFG system were given elsewhere.24-26 Briefly, a femtosecond (fs) Ti:sapphire regenerative amplifier SpiteFire PRO (Spectra-Physics) was used to pump an optical parametric amplifier system TOPAS (Light Conversion) and a homemade Spectral Shaper to generate a broadband IR pulse (∼300 cm-1) tunable from 2.5 to 10 µm and a narrow-band pulse (∼10 cm-1) at 800 nm. The in situ 2009 American Chemical Society
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Letters
Figure 1. (a) sps- and ssp-SFG spectra (circles) of LiCoO2 surface in contact with PC. SFG spectra are offset for clarity. See text for details. (b) Two possible orientation structures for CdO group of PC observed at 1780 and 1830 cm-1. The y direction is the surface normal. (c) Schematic illustration of PC absorption on LiCoO2 surface. The simulated components for ssp-polarization are multiplied by a factor of 5 for clarity.
SFG measurements were carried out with the internal reflection geometry where both visible and IR pumping beams were incident from the backside of the substrate and overlapped on the same spot on the central area of the LiCoO2 surface in contact with the above solutions. The incident angles for the visible and IR lights are 70° and 50°, respectively. SFG spectra were recorded with polarization combinations of ssp (i.e., s-SFG/ s-visible/p-IR, same as below) and sps, which are sensitive to vibrational modes with IR dipole moments that contain a component perpendicular and parallel, respectively, to the surface. All SFG signals were normalized by a SFG spectrum from a gold thin-film evaporated on the flat surface of the CaF2 prism. The circle symbols in Figure 1a correspond to observed ssp(bottom) and sps-polarized SFG spectra (top), respectively, on the LiCoO2 surface in contact with a pure PC solvent in the IR frequency region between 1700 and 1900 cm-1. The spsspectrum shows a strong bipolar band while the ssp-spectrum exhibits a much weaker bipolar band reverse to that of the spsspectrum. These peaks were not observed when the PC solvent is absent, indicating that these SFG signals were associated with PC molecules. Since SFG does not occur in the homogeneous PC bulk solution with symmetry of inversion, the appearance of the SFG signals (Figure 1a) indicates that PC molecules on the LiCoO2 surface should have certain ordered structures. In order to quantitatively analyze the SFG spectra, SFG peak profiles (ISFG) were fitted by the following equation:14-17,24-26
ISFG ∝
|
A
∑ ωIR - ωνν + iΓν eiφ ν
ν
(2) + χNR
|
2
where Aν and Γν are the amplitude and the damping constant, respectively, of the vibrational mode ν at a frequency ων with (2) a phase angle (φν) with respect to nonresonant signal (χNR ). Assuming a single resonant component (ν ) 1) and a nonresonant background on the interface between the LiCoO2 and PC solvent, we were unable to fit the above SFG spectra by the equation. This suggests that adsorption of the PC molecules on the LiCoO2 surface should have at least two resonant modes (ν g 2). As shown by the pink solid traces in the Figure 1a, the best-fittings for the SFG observations were obtained by assuming
two vibrational modes (ν ) 2) at 1830 and 1780 cm-1. In order to show the results clearly, the two single vibrational modes at 1830 and 1780 cm-1 are plotted by blue and red dotted traces, respectively, together with respective sps- and ssp-polarized SFG spectra (Figure 1a). The vibrational component at 1830 cm-1 (red dotted traces) always exhibits opposite peak direction to that at 1780 cm-1 (blue dotted traces) in the same SFG spectrum. On the other hand, it is interesting to note that the same component in the sps-spectrum displays opposite sign in the ssp-spectrum if one assumes that the phase angle φν for the two spectra are similar (Figure 1a). It has been reported previously that the CdO stretching (νCdO) mode for PC molecules was observed around 1789 cm-1 in the bulk solution but 1822 cm-1 in a CCl4 solution.27 Furthermore, it is known that the νCdO from alkyl carbonates shifts to lower frequency due to strong interaction and solvation process with Li+ ion.3-10 Matsui et al. reported an IR peak at 1830 cm-1 for νCdO of PC on a LiCoO2 electrode surface by the polarization modulation FTIR (PM-FTIR) measurements.6 They found that the peak position of PC was shifted to higher frequency in comparison with that observed in PC bulk solution (1790 cm-1). Ikezawa et al. observed a red-shift for the νCdO to 1765 cm-1 with addition of Li+ ion into PC solvent by ATR-IR measurement.9 Li et al. also confirmed similar red-shifts for νCdO of EC and DMC solvents with Li+ ion addition by in situ microscope FTIR measurements.10 Based on the information, the two peaks observed in the SFG spectra (Figure 1a) are attributed to νCdO of PC molecules organized on the LiCoO2 surface with different adsorption geometries. The peak at 1780 cm-1 was tentatively assigned to νCdO of PC adsorbed on the LiCoO2 surface by pointing its CdO group to the substrate. In contrast, the SFG peak at 1830 cm-1 was assigned to νCdO of PC adsorbed on the LiCoO2 surface through its five-membered ring with its CdO group pointing away from the surface. The latter adsorption geometry was expected to have a weaker interaction with the LiCoO2 substrate and, thus, appeared in the higher frequency region. From the relative peak intensities and phases obtained from the fitting results for the sps- and ssppolarized SFG spectra (Figure 1a), the νCdO mode at 1830 cm-1 was estimated to tilt ca. 48 ( 2° from the surface normal with a δ-distribution (Figure 1b).28,29 In contrast, the tilt angle for the νCdO at 1780 cm-1 was ca. 202 ( 2° from the surface
Letters
Figure 2. (a) ssp-SFG spectra of LiCoO2 surface in PC with different Li+ ion concentrations. Circles represent observation results. Solid traces represent fitting results. (b) Amplitude (Aν) of νCdO determined from the fitting process are shown as a function of concentration of Li+ ion for the peaks at 1830 (black circles) in a log-log plot. All SFG spectra are offset for clarity.
normal, i.e., approximately opposite to that at 1830 cm-1 (see the Supporting Information). These results imply that the molecular planes of both PC adsorption geometries are inclined to the LiCoO2 surface. The calculation shows that the molecules numbers of PC assigned for the mode at 1830 cm-1 are ca. four times higher than those for 1780 cm-1 (see the Supporting Information). Figure 1c schematically shows two kinds of adsorbed PC on the LiCoO2 surface in contact with PC solvent. It is worth noting that this is a great advantage for SFG spectroscopy which can exactly probe the different geometries of PC adsorption on the LiCoO2 surface at a molecular level.14-22 The SFG spectra observed on the LiCoO2/PC interface (Figure 1a) are stable with time under the SFG observation condition. However, when a small amount of water molecules were introduced into the cell, the SFG signals quickly decrease. It is expected that the adsorption of the water molecules may take place on the LiCoO2 to replace the PC adlayer there. On the other hand, the decomposition of the PC solvent molecules on the LiCoO2 surface may also occur under the presence of water molecules. Detailed experiments are still in process. In order to elucidate the influence of the Li+ ion on the structures of solvent adsorption on the LiCoO2 cathode surface, SFG measurements were carried out in PC with different concentrations of LiClO4 (0-1.0 M). Figure 2a shows spspolarized SFG spectra of PC on the LiCoO2 surface in contact with PC solutions containing various concentrations of Li+ ion. The SFG peak intensities largely decrease with addition of Li+ ion. Two similar vibrational components as those in the pure PC solvent (Figure 1a) were obtained. For example, the amplitude (Aν) of the adsorbed PC at 1830 cm-1 decreases to approximately one forth in 1.0 M LiClO4/PC solution to that in pure PC solvent, whereas that at 1780 cm-1 becomes very weak in 0.1 M LiClO4/PC solution. The surface structures of the PC solvent on the LiCoO2 surface seem to be affected by the presence of Li+ ion. As reported previously,11 Li+ ion is known to be solvated to species such as [Li(PC)3]+ and [Li(PC)2]+. These solvated species with positive charge can interact and specifically adsorb on the LiCoO2 surface to replace the ordered PC adlayer from the pure solvent. From the observed SFG spectra in PC solutions with different concentrations of Li+ ion,
J. Phys. Chem. C, Vol. 113, No. 48, 2009 20533 it is expected that the structures of these charged solvated species on the LiCoO2 surface are disordered and thus give weaker SFG signals in Li+ ion contained PC solution. On the other hand, as Li+ ion is added into PC, an electric double layer is expected to be constructed near the LiCoO2 surface induced by the interaction with PC-solvated Li+ species.30 As reported previously, the molecules in the electric double layer are forced to be more aligned by the electric field, and SFG signals will be enhanced with the field intensity and thickness of the double layer.31-33 In the present work, however, it was found that the SFG signals largely decrease after a small amount of Li+ ions is added into the PC solution. Furthermore, based on the Gouy-Chapman theory, the thickness of the electric double layer is proportional to (ionic strength)-1/2, i.e., (Li+ concentration)-1/2, of the electrolyte solution.30-33 Figure 2b shows Aν for the peak at 1830 cm-1 (circle symbols) as a function of concentration (C) in a log-log style. If the numbers of PC molecules observed by SFG are proportional to the thickness of the electric double layer, a linear relation should be observed between the log(Aν) and log(C).12,13 The linear relation is not found here (Figure 2b). As mentioned above, although both effects can give contribution to SFG intensity changes for PC adsorption on the LiCoO2 surface with the change of Li+ ion concentration, we believe that the specific adsorption of the PCsolvated Li+ ion species should play more important roles here. However, it should be mentioned that the SFG spectral features are also affected by the surface state, film thickness as well as nonresonance background of the LiCoO2 thin film on the substrate, detailed studies are still in progress. In summary, we successfully evaluated the adsorption structure of PC solvent on the LiCoO2 surface by in situ SFG vibrational spectroscopy. Two kinds of adsorption species with opposite orientations are observed. The adlayer structures of PC on the LiCoO2 surface are also affected by addition of Li+ ion in solution. The present work is expected to help understanding of the fundamental processes on the electrode surface and improve the performance and safety of Li-ion batteries. Further experiments of the intercalation and deintercalation processes of Li+ ion under the electrochemical controls are in progress and will be reported elsewhere soon. Acknowledgment. The work is supported by the New Energy and Industrial Technology Development Organization (NEDO), and PRESTO, Japan Science and Technology Agency (JST) and a Grant-in-Aid for Exploratory Research 21655074 from MEXT. Supporting Information Available: Details about the sample preparation procedures, SFG system, and calculation are given. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Balbuena, P.; Wang, Y. Lithium-ion Batteries; Imperial College Press: London, 2007. (2) Xu, K. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chem. ReV. 2004, 104, 4303–4417. (3) Hyodo, S.-A.; Okabayashi, K. Raman intensity study of local structure in non-aqueous electrolyte solutions--I. Cation-solvent interaction in LiClO4/ethylene carbonate. Electrochim. Acta 1989, 34, 1551–1556. (4) Klassen, B.; Aroca, R.; Nazri, M.; Nazri, G. A. Raman Spectra and Transport Properties of Lithium Perchlorate in Ethylene Carbonate Based Binary Solvent Systems for Lithium Batteries. J. Phys. Chem. B 1998, 102, 4795–4801. (5) Morita, M.; Asai, Y.; Yoshimoto, N.; Ishikawa, M. A Raman spectroscopic study of organic electrolyte solutions based on binary solvent
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