LETTER pubs.acs.org/JPCL
Nanoscale Observation of the Electronic and Local Structures of LiCoO2 Thin Film Electrode by Depth-Resolved X-ray Absorption Spectroscopy Daiko Takamatsu,*,† Takayuki Nakatsutsumi,‡ Shinichiro Mori,‡ Yuki Orikasa,‡ Masato Mogi,† Hisao Yamashige,† Kenji Sato,† Takahiro Fujimoto,† Yu Takanashi,† Haruno Murayama,† Masatsugu Oishi,† Hajime Tanida,† Tomoya Uruga,§ Hajime Arai,† Yoshiharu Uchimoto,‡ and Zempachi Ogumi† †
Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan § JASRI/Spring-8, Kouto 1-1-1, Mikaduki-cho, Sayo-gun, Hyogo 679-5148, Japan ‡
bS Supporting Information ABSTRACT: The electronic and local structural changes during electrochemical delithiation processes occurring at the electrode/electrolyte interface of LiCoO2 thin film electrode prepared by pulsed laser deposition were clarified by using depth-resolved X-ray absorption spectroscopy technique. We successfully obtained detailed microstructural information around the Li1 xCoO2 electrode surface with a depth resolution of ca. 3 nm using a spectro-electrochemical cell. Our results revealed a remarkable increase in the local distortions at the Li1 xCoO2 surface after charging, and the distortions extended to the bulk of Li1 xCoO2. Such unprecedented local distortions might be attributable to the marked deterioration of the electrodes. SECTION: Energy Conversion and Storage
A
comprehensive understanding of the reactions occurring at the electrode/electrolyte interface in lithium ion batteries (LIBs) is crucially important in the quest to elucidate the dynamic chemical processes and designing electrode materials suitable for high power and long-life operation. Interfacial reactions, which are considered to occur on the nanometric scale, have indeed attracted prime attention in recent years.1,2 X-ray absorption spectroscopy (XAS), via which a nondestructive analysis of the electronic and local structures of a certain atom can be garnered,3 is, in principle, a potent and versatile technique to clarify the electronic structure of the Li-ion intercalated cathode materials.4 However, to the best of our knowledge, suitable XAS techniques capable of analyzing the surface structural changes of the electrode in electrochemical processes have not yet been established. Two important factors necessary to obtain a concise and meaningful surface data by XAS are worth mentioning. First is using a flat and well-defined electrode surface as the model interface. Interfacial reactions occurring in porous composite electrodes consisting of active materials, conductive additives, and binders are complex to analyze because of their rough morphology. For circumventing the complexities in analyzing the interfacial reactions, thin film model electrodes deposited by various methods such as the pulsed laser deposition (PLD),5 RF sputtering,6 and electrostatic spray deposition (ESD)7 have been utilized. Hirayama et al. have reported an X-ray reflectometry r 2011 American Chemical Society
(XRR) study of the LiCoO2 epitaxial thin films, which is one of the versatile methods for the identification of the interface.8 However, their oriented film is far from the actual LiCoO2 composite electrode due to the strong influence of the substrate. We herein utilize well-defined polycrystalline thin films as the model electrode to reflect the properties of the actual electrode. Another prerequisite is the utilization of an XAS technique capable of analyzing the solid electrode surface with high spatial resolution. With these factors in mind, a depth-resolved XAS (DRXAS) technique, which integrates the exit-angle dependence of the fluorescence-yield XAS for analyzing the interference effects of a Cr/ Au layered thin film, is deemed to be useful.9 This technique has been applied to clarify the chemical state of the heterointerface of all solid-state lithium batteries.10 Such considerations propelled us to apply DR-XAS to obtain detailed microstructural information at the solid electrode/electrolyte solution interface. In this Letter, we describe a DR-XAS technique capable of indepth analyses of the solid electrode/electrolyte solution interface. The electronic and local structural changes at the surface of Li1 xCoO2 electrode during charging were investigated by employing X-ray near-edge fine structure (XANES) and extended X-ray absorption fine structure (EXAFS) with nanoscale spatial resolution. Received: August 18, 2011 Accepted: September 20, 2011 Published: September 20, 2011 2511
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The Journal of Physical Chemistry Letters
LETTER
Figure 1. (a) XRD spectra of LiCoO2/Pt. *: Peaks originating from Pt substrate. (b) Cross-sectional TEM images of LiCoO2/Pt: (left) low-resolution image and (right) high-resolution image of a magnified square region in the left image. (c) CV of LiCoO2/Pt.
Figure 2. Schematic illustrations of the spectro-electrochemical cell for DR-XAS: configuration for (a) electrochemical and (b) DR-XAS measurement. (c) Schematic of DR-XAS principle and (d) LSV of LiCoO2/Pt performed in the configuration shown in panel a.
Figure 1 summarizes the structural and electrochemical properties of the LiCoO2 thin film electrode prepared by PLD on flat Pt substrates. Figure 1a shows typical X-ray diffraction (XRD) patterns of LiCoO2/Pt. A conspicuous diffraction peak at around 2θ = 18.9° arising from the hexagonal LiCoO2 (003) was apparent, and no other diffraction peaks could be detected except for the peaks originating from the Pt substrate. This unambiguously suggests that the LiCoO2 film is a single phase with a c-axis orientation and no other impurity phases are formed. The calculated lattice distance of the c axis of LiCoO2 was 1.408 nm, which is in excellent accord with the reported value of LiCoO2 bulk electrodes.11 Figure 1b shows the transmission electron microscope (TEM) images of LiCoO2/Pt. The film thickness was estimated to be ca. 50 nm, and the c axis of LiCoO2 film was inclined perpendicular to the Pt surface. From the observed interval streaks of ca. 0.47 nm, the lattice distance of the c axis is 1.41 nm, which coincides very well with the XRD results. Atomic force microscopy (AFM) (not shown) further revealed the flat surface of the LiCoO2 with an arithmetic mean roughness value (Ra) ∼2 nm over an area of 1 1 μm2. Figure 1c shows the cyclic voltammogram (CV) of the LiCoO2 film electrode, which was conducted using a three-electrode cell with lithium metal as the
counter and reference electrodes. We used 1 M LiClO4 in a propylene carbonate (PC) as the electrolyte. A conspicuous sharp peak indicating Li (de)intercalation at ∼3.9 V versus Li/ Li+ is clearly evident, which can be attributed to the coexistence of both Li-poor and Li-rich phases. The small inconspicuous peaks at approximately 4.06 and 4.18 V can be ascribed to the phase transitions between ordered and disordered Li ion arrangements.12 On the basis of these results, we confirmed the obtention of a LiCoO2 thin film that is flat on nanometric scale and has excellent electrochemical properties. Therefore, we are reasonably confident that the prepared thin film is indeed suitable as a model electrode for the following nanoscale DR-XAS analysis. Co K-edge DR-XAS was performed at BL37XU at SPring-8, JASRI, Japan. Contrary to solid/solid interface,10 it is difficult to obtain concise and meaningful surface data by DR-XAS in the case of solid electrode/electrolyte solution interface due to the existence of organic solvents. Therefore, we designed a unique spectro-electrochemical cell for the DR-XAS, as shown in Figure 2a,b. Figure 2a shows a two-electrode cell composed of a LiCoO2 film on Pt substrate (working)/1 M LiClO4 in a 1:1 mixture of ethylene carbonate and diethyl carbonate (EC/DEC)/ Li metal (counter). Electrochemical Li (de)intercalation was 2512
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The Journal of Physical Chemistry Letters performed by linear sweep voltammetry (LSV) using a potentiostat (PS) in the configuration shown in Figure 2a. The cell was completely sealed, and the free space of the cell was filled with helium to suppress the deterioration of the electrode and electrolyte. After stabilizing the cell potential using LSV, the Li1 xCoO2 film was taken out from the electrolyte solvent. Most of the residual solvent on the surface film was thereafter removed so that X-ray can penetrate through the solvent. After ensuring that the surface remained unchanged, DR-XAS was performed in the configuration shown in Figure 2b. Because X-ray can penetrate through the kapton film and helium gas environment, this configuration has the advantage to prevent X-ray attenuation. After the DR-XAS measurements, the LiCoO2 film was again immersed into the electrolyte and the cell potential changed until it attained a fixed value for the next potentiostatic measurement. Figure 2c shows a schematic of the DR-XAS principle. When the incident X-ray enters into the thin film sample, the fluorescence X-rays are emitted. The emitted fluorescence signals are detected by a 2D pixel array detector (PILATUS-100K),13 which is positioned in the direction parallel to the sample plane, as shown in Figure 2c. The lower channel number of PILATUS consists of signals only from the surface of LiCoO2 film, whereas the higher channel number comprises signals from both the surface and bulk of the film. From our DR-XAS experimental configuration, the depth resolution from one pixel array can be estimated to be ca. 3 nm. (See the Supporting Information.) Figure 2d shows the LSV results measured by the spectro-electrochemical cell, indicating that the charging process of LiCoO2 was successfully performed.
Figure 3. Co K-edge DR-XAS spectra of the LiCoO2 film at each channel of PILATUS detecting around the interface before charging (3.2 V versus Li/Li+): (a) XANES and (b) FTs of the k3 -weighted EXAFS oscillations.
LETTER
Figure 3a,b shows the normalized Co K-edge DR-XAS ((a) XANES, (b) EXAFS) at each channel of PILATUS detecting the interface at 3.2 V versus Li/Li+ (before charging), respectively. As the channel number decreases, the energy level shifts toward lower energy in the XANES spectra of Figure 3a, attributable to the Co ions at the very surface of LiCoO2 electrode being reduced during the electrolyte soaking process. In this study, the Fourier transformations (FTs) of the k3-weighted Co K-edge EXAFS oscillations were calculated from k = 3.7 to 12 Å 1 range. The pseudoradial structure functions (RSFs) of the local atomic environments around Co atoms reveal two peaks at 1.5 Å (corresponds to the six-coordinated Co O bonds in CoO6 octahedra) and 2.6 Å (corresponds to the six-coordinated Co Co interactions along CoO2 layer) in Figure 3b.4 Owing to the fact that the change in the valence state of Co directly influences the nearest-neighbor oxygen atoms, we focused on the first peak at 1.5 Å corresponding to the first neighboring atoms of Co O bonds. The peak intensity of Co O bonds decreases with decreasing channel number, whereas the distance of Co O bonds remains constant in the EXAFS spectra of Figure 3b. The decrease in the amplitude of Co O peak is a manifest of an increase in the DW factor because we tacitly assumed that the coordination numbers remain unperturbed in the calculation (see the Supporting Information), unequivocally indicating that the local environment of Co ions near the interface becomes more distorted as compared with the bulk of the LiCoO2 film. On the basis of these results, we confirmed that the structural changes occurring at the solid electrode/electrolyte interface can indeed be detected in the nanoscale spatial resolution by using DR-XAS technique. Figure 4 summarizes the channel dependencies on the energy levels, the Co O distances, and the DW factors obtained at 3.2 V versus Li/Li+ (before charging), 4.2 V (after charging), and 4.4 V (after overcharging), respectively. Figure 4a shows the absorption energy at the normalized intensity of 0.5 in XANES spectra as a function of the channel number. Prior to charging, the energy level shifts to lower values as the channel number decreases (i.e., toward the interface), implying that Co3+ is unstable in the reducing atmosphere of the organic electrolyte. After charging to 4.2 V, the energy level shifts higher energy, suggesting that the Co ions are oxidized by the deintercalation of Li ions from LiCoO2.14 Figure 4b,c shows the calculated Co O distances and DW factors as a function of the channel number. An increase in the DW factors at the interface was observed prior to charging.
Figure 4. Dependence of PILATUS channel number of (a) absorption energy levels, (b) Co O interatomic distance, and (c) Co O DW factors before charging (squares), after charging at 4.2 V (circles), and overcharging at 4.4 V (triangles). 2513
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The Journal of Physical Chemistry Letters The Co O distance decreased upon charging to 4.2 V, which signifies the oxidation of Co ions. Upon charging to 4.2 V, a remarkable increase in the DW factor toward the surface is apparent, suggesting that the Li extraction, that is, the oxidation of Co ions, predominantly proceeded at the interface. On the basis of our results, the reason behind the almost constant Co O distance observed upon charging to 4.2 V can be attributed to structural relaxation. When overcharged to 4.4 V, the Co O distance is relatively smaller at the bulk than at the interface, a clear indication that the oxidation of Co ions occurs only at the bulk. This would imply that the Co atoms at the surface of LiCoO2 become inactive and that the electrolyte decomposition might occur at the surface instead of lithium extraction. The DW factor obtained during overcharging to 4.4 V increased considerably not only at the interface but also in the bulk, whereas upon charging to 4.2 V the DW factor increase in the bulk was not that significant. It is known that the reversible charge/discharge process in LiCoO2 electrode is limited by an upper potential of 4.2 V versus Li/Li+, which is regarded to lie in the nominal range 0 e x e 0.5 in Li1 xCoO2.15 The tremendous distortions in the bulk evident upon overcharging to 4.4 V can be attributed to the well-known irreversibility for the Li1 xCoO2 composite electrode at high delithiated states (i.e., x > 0.5). Our results suggest that the local distortions occurring at the surface of the Li1 xCoO2 electrode extends to the bulk during the overcharge process, which might be the principle cause of the marked deterioration mechanism of the electrodes.
’ EXPERIMENTAL METHODS LiCoO2 thin films were deposited on flat Pt substrates by PLD method. The Pt substrates were mechanically polished platinum polycrystals. A nonstoichiometric LiCoO2 target with 15 wt % excess Li2O was used to compensate for Li loss during the deposition. The target and substrate were placed inside a vacuum chamber of the PLD that had a pressure of
