J. Phys. Chem. C 2009, 113, 15337–15342
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Anisotropic Surface Effect on Electronic Structures and Electrochemical Properties of LiCoO2 Masashi Okubo,† Jedeok Kim,‡ Tetsuichi Kudo,† Haoshen Zhou,† and Itaru Honma*,† National Institute of AdVanced Industrial Science and Technology (AIST), Umezono 1-1-1, Tsukuba, Ibaraki, 305-8578 Japan, National Institute for Materials Science, Tsukuba, Ibaraki, 305-0044 Japan ReceiVed: May 25, 2009; ReVised Manuscript ReceiVed: July 13, 2009
Increasing industrial needs for rechargeable batteries delivering higher power has encouraged advanced research on nanosized electrochemically active compounds. Although nanosized electrode materials are expected to possess much higher power output due to short diffusion length, the nanosize effects on electrodes have not been understood well to date. Here, we report that nanosized LiCoO2 as suggested as electrode material for Li-ion rechargeable batteries has anisotropic surface properties affecting electronic structures, which is evidenced by electron energy loss spectroscopy (EELS). EELS spectroscopy reveals the reduced valence state of the cobalt near the surface, especially along the stacking direction of CoO2 layers. The observed anisotropic surface property explains the nanosize effect on the electrochemical properties. Introduction Ionic diffusion in solids is an important fundamental issue in both science and technology.1 Faster ionic diffusion in electrode materials is favorable for higher power energy storage devices, sensitive electrochemical sensors, etc. With regard to energy storage devices, most portable consumer electronics are powered by Li-ion rechargeable batteries based on intercalating electrode materials that provide high voltage, portability, and excellent cyclability. However, a very strong industrial demand for market penetration of electric/hybrid vehicles requires rechargeable battery systems capable of higher charge and discharge rates. Commercial Li-ion rechargeable batteries usually consist of a lithiated carbon negative electrode (anode) and a LiCoO2 positive electrode (cathode).2 However, slow diffusion of Li ions in cathode materials is considered to be a main obstacle to improvement of high-charge/discharge-rate capability. Highcharge/discharge-rate capability depends on the Li-ion diffusion length, d, and the chemical Li-ion diffusion coefficient, DLi so that commercial cathodes with long d (ca. 1 µm) are not suitable for high-power batteries. Therefore, nanosized cathodes with short d have recently been intensively investigated.3-7 However, it is expected that the presence of large electrolyte-electrode hetero interface in nanoelectrodes might also alter the electrochemical properties. For example, the importance of the space charge zone in nanosized ionic or mixed conductors has been stressed recently.8,9 Furthermore, the poor charge/discharge cyclability has often been reported for the nanoelectrodes,5 where the hetero interface should also play an important role. In this study, we have explored the nanosize effect on the electronic structures and electrochemical properties of intercalation electrode materials and clarified that LiCoO2 has anisotropic surface properties affecting the electronic structures. The anisotropic surface properties in LiCoO2 explain the nanosize effect on the electrochemical properties. * To whom correspondence should be addressed. E-mail: i.homma@ aist.go.jp. † AIST. ‡ National Institute for Materials Science.
Figure 1. TEM images for nanocrystalline LiCoO2 of thicknesses (a) L ) 8 nm and (b) L ) 26 nm, synthesized through hydrothermal reaction. Nanoplatelet morphology is shown.
Experimental Section Nanocrystalline LiCoO2 with controlled nanoplate morphology and thickness was synthesized according to the procedure given in ref 5. Bulk LiCoO2 was purchased commercially from Honjo Chemical Corporation.
10.1021/jp904877d CCC: $40.75 2009 American Chemical Society Published on Web 07/30/2009
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Figure 2. Illustration of EELS line scan (a) along the c-direction and (b) along the ab-plane.
An electron energy loss spectroscopy (EELS) line scan was performed with GATAN GIF Tridiem electron energy loss spectrometer on a JEOL JEM2100F high-resolution transmission electron microscope. The attainable energy resolution was about 1.0 eV. The probing electron beam size was 0.7 nm diameter. For the electrochemical measurements, each sample (50 mg) was ground into a paste with acetylene black (45 mg) and Teflon (5 mg). Lithium metal was used for the reference and counter electrodes. A 1 M LiClO4 ethylene carbonate/diethylcarbonate solution was used as the electrolyte. Generally speaking, normal electrodes in commercial Li-ion rechargeable batteries contain only 5% or less of conductive materials. However, our scope of this paper was focused on the estimation of the electrochemical properties of nanomaterials. From this viewpoint, we used 45 wt % of the conductive material to improve the conductivity of the electrodes. Results and Discussion Nanocrystalline LiCoO2 was prepared by hydrothermal reaction.5 As reported in ref 5, the hydrothermally synthesized LiCoO2 had nanoplatelet morphology with a well-controlled crystallite size. Parts (a) and (b) of Figure 1 show the representative transmission electron microscope (TEM) images for the nanocrystalline LiCoO2 with thicknesses (L) of 8 and 26 nm, respectively. Note that the thickness of the nanocrystalline LiCoO2, corresponding to the stacking direction of the CoO2 layers, was estimated from the (003) peak in the X-ray diffraction pattern. As reported in ref 5, L from the (003) peak agrees well with the crystallite size along the stacking direction of CoO2 layers in TEM images. To investigate the surface effect on the electronic structures in LiCoO2, an EELS line scan for the nanocrystalline LiCoO2 was carried out. The EELS spectra of the cobalt L2,3-edges,
which correspond to excitations of the 2p electrons to 3d orbital, are sensitive to the valence state of the cobalt.10-13 First, we carried out the line scan along the stacking direction of the CoO2 layer, i.e., the c-direction. Figure 2 shows the EELS line scan in the TEM image for the nanocrystalline LiCoO2 (L ) 15 nm). The EELS scan was performed from one surface to another surface along a line indicated in Figure 2a. Note that the line scan is along the c-direction since the CoO2 layer (ab plane) is imaged perpendicular to the line scan. Figure 3a shows the selected EELS Co L2,3-edge spectra. While both the L3- and L2-edges were observed regardless of the distance from the surface, the top peak of both edges shows the evident red shift near the surface. Figure 3b depicts the scan position dependence of the top peak values. The red shift of the energy loss near the surface is clearly shown. The red shift of the energy loss generally suggests a reduction of valence state since screening of the added electrons results in the higher energy level of the 2p orbital.11 The reduction of cobalt near the surface is also confirmed by the white line intensity ratio between L3- and L2-edges. The white line intensity ratio is known to be stable to neither the specimen thickness nor the noise level in the spectrum, but sensitive to the valence state. As reported in the literature,11,12 the white line intensity ratio increases with reducing cobalt since their intensities are related to the unoccupied states in the 3d orbital. Figure 3b shows the scan position dependence of the white line intensity ratio. The white line intensity obtained at the inner core position was about 2.5, which corresponds to Co3+.12 However, as expected, the white line intensity ratio increases near the surface, which supports the reduction of the cobalt near the surface. The valence state of the cobalt deduced
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Figure 3. (a) Selected EELS Co-L2,3 edge recorded along the c-direction. (b) Spatial resolution of L3-edge top peak value, L2-edge top peak value, and the white line intensity ratio between L3- and L2- edges.
Figure 4. (a) Selected EELS Co L2,3-edge recorded along the ab-plane. (b) Spatial resolution of L3-edge top peak value, L2-edge top peak value, and the white line intensity ratio between L3- and L2-edges.
from the white line intensity ratio at the surface is about +2.7,12 and the extent of the surface effect reaches 3 nm along the c-direction. In contrast to the relatively large surface effect along the c-direction, the EELS line scan revealed that the EELS Co L2,3edge spectrum did not strongly depend on the scan position in the ab-plane. Figure 2b shows the EELS line scan in the TEM image for the nanocrystalline LiCoO2 with the thickness of L ) 15 nm. The EELS scan was performed along a line indicated in Figure 2b. Figure 4a shows the selected EELS Co L2,3-edge spectra. Both the L3- and L2-edge were observed regardless of
the distance from the surface. Figure 4b depicts the scan position dependence of the top peak values, where any peak shift of the energy loss or the intensity ratio change near the surface could not be observed. This result suggests that the surface effect within the ab-plane could be neglected. Regarding the origin of this anisotropic surface property, we cannot arrive at a single cause. When the surface can have a drastic influence on the properties of a material, there generally does not exist a single simple factor, but several simultaneous actors such as the confinement effect, structural defects, and elastic constraints. At least, based on the magnetic measurements
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and inductively coupled plasma atomic emission spectroscopy (ICP-AES), the existence of CoII ions with excess Li ions was suggested in ref 5, which agrees well with the above EELS results. Therefore, it is most likely that the excess Li ions are adsorbed at the surface or the defects near the surface in the form of Li1+δCoIIδCoIII1-δO2. However, it should be mentioned that the finite lattice of nanocrystals should affect the Madelung potential near the surface, which also largely alters the electronic structures. It should also be considered that elastic constraints such as surface tension, the oxygen deficiency, or the hydroxyl ions can also affect the electronic structures. Furthermore, charge storage in the oxygen p orbital like Co+3-δ-O-2+δ due to the strong hybridization between the oxygen 2p orbital and cobalt 3d orbital could also be a possible explanation for Co2+ near the surface. Another possible explanation for this anisotropic surface property affecting the electronic structures in LiCoO2 is the space charge effect due to the positively charged surface. In such space charge region, the concentration of negatively charged species is increased, while the concentration of positively charged species is decreased.8,9 If the electron is assumed as the important negatively charged species localized on the cobalt ion in the form of Co2+, the reduction of the cobalt near the surface could be explained. However, the explanation by the space charge effect is rather speculative, and to prove the space charge effect in LiCoO2 clearly, the concentration gradient of Li ions within nanocrystals should be determined, although it is difficult to be determined experimentally even by the EELS line analysis. Nevertheless, the space charge region must be considered in any description, whenever the hetero interface exists. In the above discussion, we clarified the anisotropic surface property affecting the electronic structures in LiCoO2 along the c-direction. However, the most important point is whether the effect on the electrochemical properties in LiCoO2 exists or not. The Li intercalation/extraction voltage E for the electrode materials in the equilibrium state is directly determined by Li chemical potential, µLi. Since µLi ) dH - T dS and dH ) dU + p dV, E is mainly dominated by the internal energy change during the reaction, dU.14 If one Li is intercalated into the space coordinate r within the crystal, the corresponding dU(r) is expressed as15,16
dU(r) )
e2 4πε0
z
(
∑ |r -j Rj| - λ ∑ exp j
j
|r - Rj | F
)
where the first term is the Madelung energy from the jth ion at the space coordinate Rj with the charge number zj and the second term is the repulsion energy with the empirical parameters of λ and F. The summation is carried out over the entire volume of the material. Therefore, as expected in the literature,17 the finite lattice of nanocrystals should affect the Li intercalation voltage, especially for the intercalation near the surface. Furthermore, since the reduction of the cobalt near the surface was observed in the EELS experiments, the Li intercalation voltage near the surface should further be affected. Likewise, Jamnik and Maier previously expected that, for small particles less than 100 nm, µLi is also affected by the surface tension.18 To reveal the surface effect on the electrochemical properties, we carried out the theoretical analysis on the electrochemical properties of the nanocrystalline LiCoO2. Since the measured electrochemical signals for the nanoelectrode materials are frequently affected by the microstructure, e.g., poor electronic
wiring, we mixed the nanocrystalline LiCoO2 with 45 wt % of acetylene black to be free from such effects. To analyze the observed electrochemical properties of the nanocrystalline LiCoO2, we employed the lattice-gas model. The black dotted lines in Figure 5a show the discharge curves for nanocrystalline LiCoO2 (L ) 8, 13, 18, and 26 nm) at the constant discharge current of 68 mA/g. As reported previously,5 with decreasing L, the capacity above 3.85 V drastically decreases while that below 3.85 V increases. Since the charge needed for the electric double layer is small enough to be neglected (∼5 mA · h/g at most), this behavior can be explained by the energy distribution of dH, which is the so-called Li-site energy µLi0 in the lattice-gas model. µLi0 is defined as the energy required to put a Li ion and an electron into the lattice. It should be emphasized here that µLi must be spatially uniform and depend only on the composition R in Li1-RCoO2. In contrast, the energy distribution of the Li-site energy near the surface requires the spatial distribution of Li-site energy, µLi0(r). Since the lattice-gas model ignores the composition dependence of µLi0 (∼dU) in the first approximation, the relationship between µLi and µLi0 can be described as
µLi(R) ) µLi0(r) + kT ln aLi(r, R) where aLi(r,R) is the activity of Li as a function of r and R. Since the activity of Li is the product of Li concentration and activity coefficient, the spatial equality of µLi should be maintained by compensating for the spatially distributed µLi0 with the gradient of Li concentration, when the activity coefficient is constant. Let us consider the lattice-gas model where each site in a lattice has two states, i.e., “full” or “empty”. If the number of sites with the energy ε is N(ε) and the occupancy (f(ε)) of the Li-ion site at the voltage E [V vs Li/Li+] obeys the Fermi distribution, the number of occupying Li ions n(ε) can be expressed as
n(ε) )
N(ε) ε + eE 1 + exp kT
(
)
Integration of n(ε) over the entire energy range must equal the number of intercalated Li ions in the host material, n. Thus,
n)
∫-∞+∞
N(ε) dε ε + eE 1 + exp kT
(
)
If we approximated the distribution of the site energy g() as
g(ε) )
Ni for εi < ε < εi+1, and i ) 0, 1, ..., n εi+1 - εi
the integration can be calculated as
n
{
NikT n)N ln ε - εi i)0 i+1
∑
(
)
εi + eE kT εi+1 + eE 1 + exp kT 1 + exp
(
)
}
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Figure 5. (a) Discharge curves at 68 mA/g (black dotted lines) and simulated discharge curves (red solid lines) for the nanocrystalline LiCoO2 (L ) 8, 13, 18, and 26 nm). (b) Distribution of the Li-site energy µLi0 obtained from the discharge curve simulation. (c) The crystallite size dependence of the normalized density of state above -3.85 eV (blue triangles) and below -3.85 eV (red triangles). The dotted lines are the calculated results assuming the surface layer of 3 nm thick along the stacking direction (see text). (d) Schematic description of the spatial distribution of µLi0 in the nanocrystalline LiCoO2 along the c-direction.
where N is the total number of Li sites (N ) N0 + N1 +... + Nn). Therefore, the discharge curve for Li1-RCoO2 (0 < R < 1) can be expressed as
n
{
Ni kT 1 ln R) 2 i)0 N εi+1 - εi
∑
(
)
εi + eE kT εi+1 + eE 1 + exp kT 1 + exp
(
)
}
The red solid lines in Figure 5a are the simulated discharge curves based on the above expression, and Figure 5b shows the obtained distribution of µLi0. As expected, the density of states above -3.85 eV increases with decreasing L since the decrease in L drastically increases the number of the Li-ion sites near the surface. Figure 5c shows the crystallite size dependence of the normalized number of Li-ion sites (N(ε)) above -3.85 eV (blue triangles) and below -3.85 eV (red triangles). The dotted lines are the calculated results assuming that the surface layers that are 3 nm thick have the Li-ion site energy above -3.85 eV, which agrees well with the experimental results.
Therefore, if we introduce the space coordinate along the c-axis, x (0 < x < L), the spatial distribution of µLi0(x) could be schematically described as Figure 5d. The spatial distribution of µLi0(x) at the surface could be understood as the electrostatic interaction change due to the finite lattice, while the previously reported lattice expansion in the nanocrystalline LiCoO2,5 the reduction of the Co ion near the surface, and the structural disorder near the surface such as the cation mixing between Co ion and Li ion19 or the oxygen deficiency could also be responsible. It should be noted that the lattice-gas model does not consider the polarization effect in the discharge process and the decrease in the discharge capacity due to the cation mixing between cobalt and Li ion. Presumably, the difference in the highest Li-site energy of each compound (for example, -1.5 eV for L ) 18 nm, but -2.1 eV for L ) 8 nm) could be ascribed to such contributions and also the surface state for each compound. Nevertheless, the obtained distribution of µLi0 agrees well with the EELS results. This result supports the theory that the Lisite energy distribution plays a crucial role rather than such contributions. Furthermore, in the above discussion, we con-
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sidered the intercalation reaction under the assumption that a Li+-electron pair behaves as one particle. However, as mentioned in the EELS section, the explanation by the space charge effect is also possible. In the space charge region, the Li ion and electron separate from each other and redistribute themselves individually so as to achieve minimum free energy. To understand the electronic structures and electrochemical properties of nanosized electrode materials, further study is indispensable.
Okubo et al. f(ε) n(ε) n g(ε) N R x C r
occupancy of Li sites with energy ε number of Li occupying Li sites with energy ε number of Li intercalated into host distribution of Li-site energy (density of states) number of Li sites in the host stoichiometry of Li1-RCoO2 space coordinate along the c-axis Li-ion concentration cylindrical coordinate
Conclusion We have shown the anisotropic surface properties affecting the electronic properties of LiCoO2 by the EELS, and the electrochemical properties by the theoretical analysis of the discharge curves. As the nanocrystalline thickness decreases, the spatial distribution of the Li-site energy due to the reduction of Co ions near the surface plays an important role in the electrochemical properties. Acknowledgment. The authors would thank Mr. Toshio Sasaki (National Institute for Materials Science) for performing the TEM measurements. This work was supported by the New Energy and Industrial Technology Development Organization Japan, under a grant for Research and Development of Nanodevices for Practical Utilization of Nanotechnology (Nanotech Challenge Project). One of the authors (M.O.) was financially supported from Grant-in-aid for Scientific research from the Ministry of Education, Culture, Sports, Science and Technology, Japan. M.O. was also financially supported from Tokuyama Science Foundation and Mitsubishi Chemical Corporation Fund. Glossary List of Symbols d D L zi µLi0 E λ N(ε)
Li-ion diffusion length chemical Li-ion diffusion coefficient thickness of the nanoplate LiCoO2 charge number of ion i Li-site energy voltage [V vs Li/Li+] F empirical parameters for repulsion energy number of Li sites with energy ε
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