Relationship between Electrode Performance and Chemical Bonding

J. Phys. Chem. C 2009, 113, 21941–21948

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Relationship between Electrode Performance and Chemical Bonding Nature in Mesoporous Metal Oxide-Layered Titanate Nanohybrids Hyung-Wook Ha, Tae Woo Kim, Jin-Ho Choy, and Seong-Ju Hwang* Center for Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nano Sciences, College of Natural Sciences, Ewha Womans UniVersity, Seoul 120-750, Korea ReceiVed: February 28, 2009

We investigated the applicability of mesoporous MOx-layered titanate (M ) Fe or Ni) nanohybrids as a lithium intercalation electrode to elucidate the relationship between electrode activity and chemical bonding nature in these materials. Electrochemical measurements clearly demonstrate that the mesoporous nanohybrids show better performance of lithium cation intercalation electrodes, compared with the pristine titanate. This finding underscores that hybridization with metal oxide nanocrystals is quite effective in improving the electrochemical property of titanium oxide. The anode performance of the NiOx-layered titanate nanohybrid can be further improved by a postcalcination at 300 °C whereas there is less significant increase in the discharge capacity of the iron oxide homologue after the heat-treatment at the same temperature. Such different effects of the postcalcination can be understood in terms of local structural evolution of guest species; the nickel species in the as-prepared NiOx-layered titanate nanohybrid exist in the form of nickel hydroxide and the postcalcination at g300 °C induces a structural transformation to rocksalt-structured nickel oxide. On the contrary, the local structure around iron ions in the FeOx-layered titanate nanohybrid does not experience any notable modifications upon the postcalcination. According to X-ray absorption spectroscopic analyses for the chemically lithiated nanohybrids, the lithium insertion via n-BuLi treatment decreases the oxidation states of iron and nickel ions with negligible change in Ti valency. This result strongly suggests that the insertion of Li+ ions into the present nanohybrids can be achieved by a redox process of hybridized iron oxide or nickel oxide, and hence this functionality is strongly dependent on the chemical bonding nature of hybridized guest species. Introduction Most of lithium rechargeable batteries have adopted microcrystalline LiCoO2 as a cathode and carbon-based material as an anode. During the last several decades, intense research has focused on the exploration of new efficient electrode materials for lithium secondary batteries.1,2 Nanocrystalline metal oxides have attracted intense research interest as alternative electrode materials for lithium secondary batteries.3–7 A small particle size of the nanocrystals provides a short diffusion path of lithium ion during charge-discharge process, resulting in excellent performance under high current density. The expanded surface area of the nanocrystals can provide many surface sites for the grafting of Li+ ions. Since the surface grafting of lithium ions occurs at relatively low potential, nanocrystalline metal oxides with large surface area would be suitable for anode application.8–11 In terms of large surface area and short Li+ diffusion path, not only nanocrystalline metal oxides but also mesoporous ones are expected to be applicable as anode materials. The mesoporous metal oxides, however, were very rarely applied as lithium intercalation electrode.12,13 This would result from difficulty in preparing mesoporous compounds containing a high concentration of electrochemically active transition metal ions. The use of organic surfactant molecules as soft template is known to be very effective in synthesizing mesoporous silicates like MCM41 and MCM-48.14 But this well-known method is not readily applicable for the synthesis of mesoporous transition metal oxides. Since the transition metal ions incorporated into the * To whom all correspondences should be addressed. Tel: +82-2-32774370. Fax: +82-2-3277-3419. E-mail: [email protected].

tetrahedral sites of mesoporous materials prefer to higher coordination sites, the obtained mesoporous transition metal oxide tends to experience a phase transformation into nonporous transition metal oxides with octahedral metal sites upon the removal of organic templates at elevated temperature.15 An intercalative hybridization between 2D nanosheets of exfoliated titanate and 0D nanoclusters of metal hydroxide can provide an alternative route to mesoporous transition metal oxides such as MOx-layered titanate nanohybrids (M ) Ni, Fe, Cr, and Zn).16,17,19,20 Considering the porous structure of these materials and high content of electrochemically active transition metal ions,3,21–24 we expected mesoporous FeOx-layered titanate or NiOx-layered titanate nanohybrids to possess potential functionality as lithium intercalation electrodes, as well. In the present study, we investigated the electrochemical activity of mesoporous MOx-layered titanate (M ) Fe or Ni) nanohybrids synthesized by a reassembling reaction between layered titanate nanosheets and transition metal hydroxide nanoclusters. The relationship between electrode performance and chemical bonding nature in these compounds was examined with X-ray absorption spectroscopy (XAS) and electrochemical measurement. In addition, evolutions of the electronic structures of these nanohybrids upon lithiation process were probed to elucidate the origin of their electrochemical activity. Experimental Section Sample Preparation. The pristine cesium titanium oxide Cs0.67Ti1.83O4 was prepared by the solid-state reaction with stoichiometric mixture of Cs2CO3 and TiO2, and its protonated

10.1021/jp910091y  2009 American Chemical Society Published on Web 12/07/2009

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Figure 1. Schematic diagram of exfoliation-reassembling route to mesoporous MOx-layered titanate nanohybrids (M ) Fe or Ni). The photograph at the right-bottom side represents the picture of colloidal suspension of exfoliated titanate. The observation of Tyndall phenomenon clearly demonstrates the formation of colloidal suspension by exfoliation process.

derivative was obtained by 1 M HCl treatment for the pristine cesium titanate at room temperature for 3 days.18 The exfoliation of layered titanate was achieved by reacting the protonated titanate with tetrabutylammonium hydroxide (TBA · OH) at room temperature.18 The hybridization between layered titanate nanosheets and iron oxide or nickel oxide was done by the dropwise addition of the 0.25 M aqueous solution of iron(II) acetate or nickel(II) acetate into the colloidal suspension of exfoliated titanium oxide nanosheets (0.1 g L-1) under vigorous stirring. The pH of the aqueous solution of metal acetate was controlled at 1.5 for M ) Fe and 10 for M ) Ni, since the polynuclear metal hydroxide nanoclusters are known to exist in aqueous solution at given pH region.25 The hybridization reaction proceeded at 80 °C for 3 days, as reported previously.16,17 The obtained powders were separated by centrifuging, washed thoroughly with distilled water, and dried at 60 °C in oven. The formation of layer-by-layer heterostructured materials was achieved by an electrostatic attraction between negatively charged titanate nanosheets and positively charged metal hydroxide nanocluster, as illustrated in Figure 1. To probe the effects of the postcalcination on the local structure and electrode functionality of the nanohybrids, the as-prepared FeOx-layered titanate and NiOx-layered titanate nanohybrids were heat-treated at 200, 300, and 400 °C for 2 h under nitrogen and oxygen atmosphere, respectively. Also, the lithiated derivatives were prepared by reacting the MOx-layered titanate nanohybrids with excess 1.6 M n-BuLi in hexane for 48 h to investigate the effect of Li insertion on the electronic structure of the nanohybrids. After the lithiation reaction, the lithiated samples were thoroughly washed with hexane and ethanol and dried in vacuum. Sample Characterization. The crystal structure of the present nanohybrid samples was examined using high-resolution transmission electron microscopy, HR-TEM, (Philips-CM200 microscope) and powder X-ray diffraction, XRD, (Ni-filtered Cu KR radiation with a graphite diffracted beam monochromator, 40 kV, 30 mA). The cross-sectional HR-TEM images of the nanohybrids were obtained from the sliced samples prepared with the ultramicrotome. The chemical composition of the nanohybrids and their lithiated derivatives were determined with inductively coupled plasma spectrometry, ICP, and energy dispersive spectrometry, EDS. XAS experiments were carried out at Ti K-, Fe K-, and Ni K-edges with extended X-ray absorption fine structure, EXAFS, facility installed at the beamline 7C at the Pohang Accelerator Light Sources (PAL, Pohang, Korea) in Korea. XAS data were collected at room temperature in a transmission mode using gas-ionization detec-

Ha et al. tors. All the present spectra were calibrated by simultaneously measuring the spectrum of Ti, Fe, or Ni metal foil. The data analysis for the experimental spectra was done by the standard procedure reported previously.26 The electrochemical measurements were performed with the cell of Li/1 M LiPF6 in ethylene carbonate/diethyl carbonate (50:50 v/v) composite electrode, which was assembled in a drybox. The composite electrode was prepared by mixing thoroughly the active material (70%) with 20% of acetylene black and 10% of PTFE (polytetrafluoroethylene). Coin-type cells (2032) were assembled in a drybox. The 20 mg of active electrode material was mixed with 12 mg of conductive binder (8 mg of teflonized acethylene black and 4 mg of graphite) and a pure lithium foil was used as cathode electrode. The electrodes were dried at 120 °C overnight before use. A 1 M solution of LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 v/v) was used as the electrolyte. The geometrical area of the electrochemical cell is 2.54 cm2 and the anode and the cathode are separated by a separator membrane with the thickness of 25 µm. All the experiments were carried out in a galvanostatic mode with a Maccor multichannel galvanostat/potentiostat in the voltage range of 0.01-3.0 V at a constant current density 20 mA/g. Results and Discussion Structure Determination. The formation of the intercalative heterostructure of the MOx-layered titanate nanohybrids (M ) Fe or Ni) was evidenced by HR-TEM analysis. Figure 2 illustrates cross-sectional HR-TEM images of both the nanohybrids. These materials demonstrate an assembly of parallel dark lines representing the titanate layers, indicating the layerby-layer ordering of the layered titanate. Although the presence of guest species in-between the titanate layers is not clearly observed in the present HR-TEM images, the observed interstratification of layered titanate is surely caused by the electrostatic attractions between negatively charged titanate and positively charged metal hydroxide nanoclusters (Figure 1). From the distances between parallel lines, the basal spacing was estimated as 24.0 Å for NiOx-layered titanate and 13.3 Å for FeOx-layered titanate. The HR-TEM results are very consistent with the previously reported XRD results showing the formation of intercalation compounds with the basal spacings of 24.01 Å for NiOx-layered titanate and 13.33 Å for FeOx-layered titanate. The basal spacings of these nanohybrids are much larger than that of the protonated titanate (9.09 Å).16,17 The ICP analysis revealed that the present hybrid materials contain a significant amount of electrochemically active transition metal ions with the ratio of Ni/Ti ) 1.2 and Fe/Ti ) 0.3.16,17 The higher content of nickel ions than iron ions would be related to the lower charge of the former ions. According to the previouslyreportedN2 adsorption-desorptionisothermanalysis,16,17 the present MOx-layered titanate nanohybrids show remarkably expanded surface areas of ∼97-190 m2/g for M ) Ni and ∼103-230 m2/g for M ) Fe. These values are much larger than that of the pristine cesium titanate (∼1 m2/g), underscoring the usefulness of hybridization in increasing the surface area. The porous structure of these materials consists of micropores originated from the intercalation structure and mesopores formed by the house-of-cards type stacking of the layered crystallites.16,17 Ni K-edge and Fe K-edge EXAFS Analysis. We investigated the local structure of the guest nickel species in the NiOxlayered titanate nanohybrid and its derivatives calcined at 200-400 °C using Ni K-edge EXAFS technique. Figure 3 represents the k3-weighted Ni K-edge EXAFS spectra for the as-prepared NiOx-layered titanate nanohybrid and its calcined

Mesoporous Metal Oxide-Layered Titanate Nanohybrids

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Figure 2. Cross-sectional HR-TEM images of (a) NiOx-layered titanate nanohybrid and (b) FeOx-layered titanate nanohybrid.

Figure 3. (left) k3-weighted Ni K-edge EXAFS spectra and (right) Fourier-transformed spectra for (a) NiOx-layered titanate nanohybrid and its derivatives calcined at (b) 200, (c) 300, and (d) 400 °C, (e) Ni(OH)2, and (f) NiO. The empty circles represent the experimental data and the solid lines the calculated data.

derivatives, and the references of Ni(OH)2 and NiO. Despite a slight difference in amplitude, there are close similarities in the overall features of EXAFS oscillations of the as-prepared nanohybrid and the reference Ni(OH)2, strongly suggesting the stabilization of nickel hydroxide nanoparticles in the as-prepared nanohybrid. While the nanohybrid calcined at 200 °C shows nearly identical spectrum to the as-prepared sample, the heattreatment at higher than 300 °C causes marked spectral changes to NiO-like EXAFS oscillation. This spectral variation indicates a structural transformation from nickel hydroxide to nickel oxide upon the postcalcination process at g300 °C. The present EXAFS data were transformed into R-space to obtain information about the coordination environment of nickel ion (see the right panel of Figure 3). All of the present compounds display two intense FT peaks at ∼1.0-3.5 Å, which are attributed to the (Ni-O) shell and the (Ni-Ni) shell, respectively. Compared with the reference Ni(OH)2, the reference NiO shows higher intensity and shorter bond distance for the second (Ni-Ni) peak. This is in good agreement with their crystallographic data, in which the reference NiO possesses twelve (Ni-Ni) bonds at 2.95 Å whereas there are six (Ni-Ni) bonds at longer bond distance of 3.13 Å for the reference Ni(OH)2.27 The as-prepared nanohybrid demonstrates almost the same FT spectrum as the reference nickel hydroxide. As the calcination temperature increases, the second peak corresponding to (Ni-Ni) bond is displaced toward low R region with an

increase in peak intensity. The present finding supports the structural modification of the guest species from Ni(OH)2 to NiO. For the quantitative determination of structural parameters, the first and second FT peaks were isolated by inverse Fourier transformation (i.e., Fourier-filtering) to k space. The FT spectra and Fourier-filtered spectra of the nanohybrids, Ni(OH)2, and NiO are represented in Figures 3 (right panel) and 4, respectively, together with their best fits. The EXAFS spectra of the reference Ni(OH)2 and NiO were well-fitted with the CdI2-type and rocksalt-type structures, respectively. As listed in Table 1, the best-fit bond distances of (Ni-O) and (Ni-Ni) shells are in good agreement with the corresponding crystallographic values, verifying the reliability of the present fitting analysis. Like the cases of experimental EXAFS oscillations, there are close similarity among the Fourier-filtered spectra of Ni(OH)2 and the as-prepared nanohybrid and its derivative calcined at 200 °C. In contrast, the Fourier-filtered spectra of the nanohybrids calcined at 300-400 °C appear nearly identical to that of the reference NiO. As can be seen clearly from Figure 4, the Fourier-filtered EXAFS spectra of the as-prepared nanohybrid and the calcined derivative at 200 °C were well-reproduced with Ni(OH)2 structure. This result highlights that the guest species are intercalated into the layered titanate in the form of nickel hydroxide nanocluster. In the cases of the nanohybrids calcined at 300-400 °C, good fits were obtained with the rocksalt NiO

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Figure 4. Fourier-filtered Ni K-edge EXAFS spectra for (a) NiOxlayered titanate nanohybrid and its derivatives calcined at (b) 200, (c) 300, and (d) 400 °C, (e) Ni(OH)2, and (f) NiO. The empty circles represent the experimental data and the solid lines the calculated data.

structure, obviously confirming the structural modification from nickel hydroxide to nickel oxide upon the postcalcination. As listed in Table 1, the (Ni-Ni) bond distance of the as-prepared nanohybrid decreases toward the value of the reference NiO as the calcination temperature increases. The coordination number (CN) of the (Ni-Ni) shell was estimated to be ∼6.9-7.3 for the as-prepared nanohybrid and its derivative calcined at 200 °C, which is rather similar to that of the reference Ni(OH)2. After the calcination at 300-400 °C, the corresponding CN becomes larger to ∼10.1, which is compatible with that of NiO. Also the as-prepared nanohybrid and its derivative calcined at 200 °C show smaller amplitude reduction factors, (S02 ) 0.55 and 0.63) compared with those of the microcrystalline NiO and Ni(OH)2 references (S02 ) 0.90 and 0.95). Such a decrease in the amplitude reduction factors implies the decrease in EXAFS amplitude, reflecting the nanocrystalline nature of the hybridized guest species in these nanohybrids having a large number of surface metal ions with imperfect coordination and increased structural disorder.28 The obtained variations in the bond distance and CN of the (Ni-Ni) shell support a structural transformation from Ni(OH)2-type to NiO-type atomic arrangement upon the postcalcination, as illustrated in the top panel of Figure 5. As listed in Table 1, all the fitting analyses yielded the acceptable values of energy shift (∆E), confirming the reliability of the

present fits. In addition, the nanohybrids show larger DebyeWaller (σ2) factors for the distant (Ni-Ni) shell than the reference NiO and Ni(OH)2. Since this factor reflects the degree of structural disorder around absorber ions,28 the observed large Debye-Waller factors in the nanohybrids underscore that nickel ions in the nanohybrids have a higher degree of structural disorder and a short structural coherence unit than those in the microcrystalline NiO and Ni(OH)2. The degrading of local structural order and the decrease in structural coherence unit are typical of nanocrystalline compounds.17,18 In the case of FeOx-layered titanate, we previously reported the results of Fe K-edge EXAFS analysis.17 According to the quantitative curve fitting analysis, the hybridized iron oxide crystallizes with loosely packed network of three edge-shared FeO6 octahedral units, as illustrated in the bottom panel of Figure 5. Of special interest is that the atomic arrangement around iron ions remains nearly unchanged after the calcination at e400 °C.17 This observation shows an excellent thermal stability of the hybridized iron oxide species, which surely contrasts with a distinct phase transition of nickel species in the NiOx-layered titanate caused by the postcalcination process. Electrochemical Measurements. It was reported that the intercalation structure of the present nanohybrids can be maintained up to 400 °C for FeOx-layered titanate and 300 °C for NiOx-layered titanate.16,17 Since the as-prepared samples must suffer from the detrimental effect of surface-adsorbed water on their electrode activity, the as-prepared nanohybrids are considered not to be suitable as lithium intercalation electrode. Hence, we examined the electrode performances of the MOxlayered titanate nanohybrids after the postcalcination not only to appropriately evaluate the applicability of these mesoporous nanohybrids as anode materials but also to investigate the relationship between electrode activity and the local structural evolution of guest species caused by the calcination. We selected the calcination temperatures of 200 and 300 °C, because the heat-treatment at 400 °C induces the collapse of the intercalation structure of NiOx-layered titanate and the decrease in surface area as well, and moreover the local structural transition of the hybridized nickel species occurs between 200 and 300 °C. As plotted in Figure 6, all the present MOx-layered titanate nanohybrids can deliver much larger discharge capacity than the pristine titanate, indicating the positive effect of hybridization in improving the capacity of the titanate. The reported capacity of the present nanohybrids is slightly larger than that of the

TABLE 1: Results of Nonlinear Least Square Curve Fittings for the Ni K-Edge EXAFS Spectra of the As-Prepared and Calcined NiOx-Layered Titanate Nanohybrids, Ni(OH)2, and NiO sample NiOx-titanate-RTa NiOx-titanate-200 °Cb NiOx-titanate-300 °Cc NiOx-titanate-400 °Cd Ni(OH)2e NiOf

bond

CN

R (Å)

∆E (eV)

σ2 (10-3 × Å2)

(Ni-O) (Ni-Ni) (Ni-O) (Ni-Ni) (Ni-O) (Ni-Ni) (Ni-O) (Ni-Ni) (Ni-O) (Ni-Ni) (Ni-O) (Ni-Ni)

6.0 6.9 6.0 7.3 6.0 10.1 6.0 10.1 6.0 6.0 6.0 12.0

2.05 3.09 2.04 3.09 2.02 3.00 2.05 2.96 2.06 3.13 2.08 2.94

-7.32 -6.29 -8.70 -6.17 -3.71 1.75 -1.96 -1.65 -4.89 -2.38 -1.17 -2.56

3.67 9.48 6.05 10.82 10.17 14.42 8.28 12.57 6.37 6.57 6.53 6.89

a The curve fitting analysis was performed for the range of 1.227-R-3.160 Å and 3.10-k-11.15 Å-1. b The curve fitting analysis was performed for the range of 1.197-R-3.160 Å and 3.10-k-11.10 Å-1. c The curve fitting analysis was performed for the range of 1.135-R-3.068 Å and 3.45-k-10.85 Å-1. d The curve fitting analysis was performed for the range of 1.104-R-3.037 Å and 3.45-k-10.85 Å-1. e The curve fitting analysis was performed for the range of 1.166-R-3.283 Å and 3.15-k-11.75 Å-1. f The curve fitting analysis was performed for the range of 1.166-R-2.976 Å and 3.45-k-11.55 Å-1.



Figure 5. Structural evolution of MOx-layered titanate nanohybrids (M ) Ni or Fe) upon the postcalcination.

Figure 6. Potential profiles of NiOx-layered titanate nanohybrids calcined at (a) 200 and (b) 300 °C, the FeOx-layered titanate nanohybrids calcined at (c) 200 and (d) 300 °C, and (e) the pristine cesium titanate for the 2nd (solid lines), 5th (dotted lines), 10th (dashed lines), and 30th cycles (solid lines). The applied current condition is 20 mA/g.

spinel lithium titanate Li4Ti5O12 (160 mAh/g)29 but smaller than those of bare nickel oxide or iron oxide.3 More importantly, the present results suggest that the coupling with nanocrystalline nickel oxide or iron oxide is effective in enhancing the electrode performance of titanium oxide. This finding is considered to be quite valuable taking into account the fact that spinel lithium

titanate Li4Ti5O12 has attracted intense interest as new promising electrode materials applicable for electrical vehicle.29 The coupling of Li4Ti5O12 with FeOx or NiOx nanocrystals is expected to induce further improvement of the electrode performance of this titanate phase. Between both the nanohybrids, the FeOx-layered titanates show better capacity retention than the NiOx-layered titanates, strongly suggesting that the former compounds are more suitable as promising anode materials having large capacity and high cyclability. The discharge capacities of the present nanohybrid materials are plotted in Figure 7 as a function of cycle number. The initial discharge capacity delivered by the nanohybrids corresponds to ∼1100 mAh/g, whereas it drops significantly to ∼300 mAh/g after the early cycles. The origin of excess capacity delivered during the first cycle was extensively investigated by Tarascon et al., which is believed to result from the formation of an unusually thick SEI layer on the electrode surface, possibly due to the catalytic activity of the displaced metal with the electrolyte.3,30,31 Since theoretical charge capacity corresponding to the redox process of iron or nickel ions in the nanohybrids was calculated to be much less than the observed initial capacity, very high capacity of these materials for the first cycle cannot be ascribed to the irreversible oxidation of iron or nickel ions. In addition, to check out the possibility of metal dissolution during electrochemical cycling, we monitored the variation of metal content before and after cycling using elemental analysis. There occurs no marked dissolution of Fe or Ni, indicating negligible contribution of metal dissolution to the observed capacity fading. After several initial cycles, the discharge capacity of the nanohybrids becomes stable. For the entire cycle, the nanohybrids show much greater capacity than the pristine layered titanate, confirming the positive effect of metal oxide hybridization. As shown in Figure 7, the NiOx-layered titanate nanohybrid calcined at 300 °C delivers a much larger discharge capacity and better cyclability compared with the homologue calcined at 200 °C. This phenomenon can be correlated with the local structural variation of hybridized nickel species upon the post calcination (Figures 3 and 4); nickel species in the nanohybrid calcined at 200 °C exist in the form of nickel hydroxide whereas the postcalcination at above 300 °C leads to local structural variation to nickel oxide. In general, metal hydroxide can react more easily with electrolyte LiPF6 to produce corrosive HF

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Figure 7. Dependence of discharge capacity on cycle numbers for the pristine cesium titanate (closed triangles), NiOx-layered titanate nanohybrids calcined at 200 (closed circles) and 300 °C (open circles), and FeOx-layered titanate nanohybrids calcined at 200 (closed diamonds) and 300 °C (open diamonds). The applied current condition is 20 mA/g.

Figure 8. Ti K-edge XANES spectra for MOx-layered titanate nanohybrids calcined at (a) 200 and (b) 300 °C, and (c,d) their lithiated derivatives, (e) cesium titanate, (f) rutile TiO2, (g) anatase TiO2, and (h) Ti2O3. The spectra (a-d) correspond to the data of the NiOx-layered titanate in the left panel and those of the FeOx-layered titanate in the right panel.

compared with metal oxide, and hence it is less advantageous for the electrode performance. In comparison, metal oxide is not so reactive against the electrolyte, leading to negligible formation of HF. Since the nickel species in the nanohybrids calcined at 200 and 300 °C are commonly stabilized in the interlayer space of the layered titanate with similar gallery heights, the variation of their particle size is not responsible for the observed change of electrode performance upon the postcalcination. In the case of the FeOx-layered titanate nanohybrids, there is only a weak dependence of their discharge capacities on the calcination temperature, clarifying insignificant influence of the heat-treatment on their electrode performance. This finding could be understood from the Fe K-edge EXAFS results showing negligible change in the local structure of hybridized iron species upon the heat-treatment below 400 °C (Figure 5).17 Summarizing the present electrochemical measurements, it is concluded that the hybridization with nanocrystalline nickel oxide or iron oxide is effective in increasing the discharge capacity of titanium oxide. Although the present nanohybrids show marked capacity decay in early cycles (Figure 7), the control of porosity in the future work would make it possible to minimize the initial capacity loss of these materials. The observed variations in the electrode performance of mesoporous MOx-layered titanates upon the postcalcination are well correlated with the accompanying structural change of hybridized guest species. Variations in the Electronic Structures of Nanohybrids upon Lithiation. To account for the origin of the electrode activity of the present nanohybrids, we studied the influences of Li+ insertion on the electronic structure of component metal ions using XANES analyses at Ti K-, Fe K-, and Ni K-edges. According to the ICP analyses, the amount of inserted lithium ions was determined to be 0.43 for the unit formula of NiOxlayered titanate nanohybrid and 0.51 for that of the FeOx-layered titanate nanohybrid, confirming the insertion of Li+ ions via chemical lithiation process. The lithium ions can be chemically inserted into the nanohybrid without electrochemical charging process, since the as-prepared nanohybrids have redoxable transition metal ions with high oxidation state and have no lithium ions to be extracted. It should be mentioned that, although lithium ions can be inserted by n-BuLi treatment, the chemically lithiated materials are not exactly the same as the electrochemically discharged materials, since no additional

electron can flow from counter electrode to compensate the charge change. The Ti K-edge XANES spectra of the pristine and lithiated MOx-layered titanate nanohybrids (M ) Fe or Ni) are illustrated in Figure 8, together with those of several Ti-containing references. All the pristine nanohybrids show similar edge positions to the reference TiO2 and cesium titanate, indicative of the tetravalent oxidation state of titanium ions. After the lithiation process, there is only a negligible displacement of edge jump of the nanohybrids, reflecting little change in Ti valency. This observation is not consistent with the previous study on the spinel lithium titanate Li4Ti5O12 showing the reduction of Ti ions during electrochemical cycling. Such discrepancy might be related to the difference in the method of lithiation (electrochemical charging vs chemical lithiation) and/or the dissimilarity in the chemical and structural properties of spinel lithium titanate and the present nanohybrids.32 As shown in Figure 8, three pre-edge peaks P1, P2, and P3 corresponding to the transitions from core 1s level to unoccupied 3d states are observable for all the nanohybrids and the refererences.33 A closer inspection on the pre-edge region demonstrates that the shoulder peak P2′ is suppressed upon the lithiation process. Considering the fact that this shoulder peak reflects the structural disorder around titanium ions, the octahedral environment around Ti ions becomes less distorted after the Li insertion. In main-edge region, there are several peaks A, B, and C corresponding to the dipole-allowed 1s f 4p transitions.33 Among them, the main-edge feature A would originate from the 1s f 4pz transition with shakedown process, whereas the peaks B and C can be attributed to the transitions to out-of-plane 4pz and in-plane 4px,y orbitals, respectively. Therefore, the observation of the two peaks B and C is a result of structural distortion around titanium ions, leading to the energy splitting of 4px,y and 4pz orbitals. Upon the lithiation process, these peaks B and C are almost merged into a single feature, indicating the depression of structural distortion around titanium ions. This spectral variation in the main-edge region is very consistent with the accompanying depression of preedge feature P2′. We also found that such a spectral variation upon the lithiation is more prominent for the NiOx-layered titanate than for the FeOx-layered titanate, suggesting stronger evolution of the Ti local structure in the former. Figure 9 represents the Fe K-edge XANES spectra for the FeOx-layered titanate nanohybrids calcined at 200 and 300 °C,


Figure 9. Fe K-edge XANES spectra for the FeOx-layered titanate nanohybrids calcined at (a) 200 and (b) 300 °C, and (c) reference FeO (dot-dashed lines), Fe3O4 (dashed lines), and Fe2O3 (solid lines). In (a) and (b), the solid lines represent the spectra of the pristine nanohybrids and the dashed lines those of the lithiated derivatives.

and their lithiated derivatives, together with those for the references of FeO, Fe3O4, and Fe2O3. In terms of edge position, both the calcined nanohybrids are nearly identical to the reference Fe2O3, underscoring the trivalent oxidation state of iron ions in this compound. As can be seen clearly from the reference spectra of Fe3O4 and Fe2O3, a partial reduction of the trivalent iron ions leads to a red-shift of the spectrum around the peaks A and B, reflecting the formation of divalent iron ions. Similarly, the lithiation process of the nanohybrids causes the low-energy shift of the spectrum in the same region, indicative of a decrease in the Fe oxidation state. This result strongly suggests that the redox process of iron ions contributes to the electrode activity of the present nanohybrids. In the preedge region, all the present materials exhibit a pre-edge peak P corresponding to the 1s f 3d transition.34 Since this transition is not allowed for centrosymmetric octahedral geometry, the intensity of this pre-edge feature can provide an indicator for the deformation of iron local symmetry from regular octahedra. Among the present references, Fe3O4 shows the strongest intensity of this peak with the normalized height of 0.12. This observation results from the fact that Fe3O4 crystallizes with the spinel structure in which one-third of iron ions exist in noncentrosymmetric tetrahedral geometry. Conversely, only a weak pre-edge peak P with the normalized height of 0.06 appears commonly for the FeOx-layered titanate nanohybrids calcined at 200 and 300 °C, which is compatible with the peak height of the reference Fe2O3 (0.07) having trivalent iron ions in octahedral symmetry.24 This finding indicates the octahedral symmetry of iron ion in the nanohybrid materials. After the lithiation process, the spectral weight of the peak P becomes slightly increased, indicating an enhancement of the local disorder around iron ions. In the main-edge region, all the present materials demonstrate several peaks A, B, and C corresponding to dipole-allowed 1s f 4p transitions. These nanohybrids display weaker intensity and broad shape of these main-edge features compared with the reference iron oxides, reflecting the nanocrystalline nature of the hybridized guest species.17 The Ni K-edge XANES spectra for the NiOx-layered titanate nanohybrids calcined at 200 and 300 °C and their lithiated derivatives are illustrated in Figure 10, in comparison with the reference NiO and LiNiO2. The nanohybrids exhibit nearly the same edge energy as NiO, indicative of the divalent oxidation state of nickel in these compounds. The lithiation process causes


Figure 10. Ni K-edge XANES spectra for NiOx-layered titanate nanohybrids calcined at (a) 200 and (b) 300 °C, (c) NiO, and (d) LiNiO2. In (a) and (b), the solid lines represent the spectra of the pristine nanohybrids and the dashed lines those of the lithiated derivatives.

a slight red shift of the edge energy of the nanohybrids, indicating a contribution of nickel ions to the electrochemical activity of the nanohybrids. Like the Fe K-edge data, only a weak pre-edge peak P corresponding to the 1s f 3d transition appears in the spectra of the NiOx-layered titanate nanohybrids,27 indicating the octahedral symmetry of nickel ions in these materials. Upon the lithiation process, the pre-edge peak P shows slight decrease in its intensity especially for the sample calcined at 300 °C, indicating the improvement of structural order around nickel ions upon the Li+ insertion. This would be related to the tendency of divalent nickel ions that prefer to the square planar local geometry over the octahedral one.35 Although the Ni2+ ions in the inner part of guest species have octahedral geometry, those on the surface with lower CN would have significant deformation from the octahedral symmetry to square planar one. This tendency of nickel ions toward local disorder is depressed by the reduction of Ni2+ to Ni+. All the present materials show several peaks A, B, and C, which are assigned as dipole-allowed 1s f 4p transitions. Like the Fe K-edge data, the nanohybrids exhibit weaker intensity of the main-edge features than the reference nickel oxides, suggesting the nanocrystalline nature of hybridized nickel species. Such a nanocrystalline character of the guest species was evidenced by the Ni K-edge EXAFS results, in which the decrease in CN and the increase in Debye-Waller factor for the distant (Ni-Ni) shell reflect a smaller structural coherence unit of the hybridized nickel species. The present XANES results suggest that the redox process ofthepresentnanohybridsismainlyrelatedtothereduction-oxidation of hybridized iron or nickel oxides. As plotted in Figure 7, the NiOx-layered titanate nanohybrids show larger discharge capacities for initial cycles than the FeOx-layered titanate nanohybrids, which is in good agreement with the higher Ni content of the formers than the Fe content of the latter. However, after several cycles, the NiOx-layered titanate nanohybrids experience more severe capacity fading than the FeOx-layered titanate ones. This phenomenon would be related to more prominent influence of the lithiation on the Ti K-edge XANES spectra of the formers than those of the latters (Figure 8), indicating the weak structural stability of the FeOx-layered titanate nanohybrids. On the other hand, it was reported that bare nickel oxide or iron oxide nanocrystal shows excellent anode performance through a reversible phase transformation between neutral metal and metal oxide.3 This contrasts with the present results in which the

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electrochemical discharge process for the metal oxide-layered titanate nanohybrids leads to a partial reduction of metal oxide to lower valent metal oxide. Such a limited reduction of the hybridized metal oxides is responsible for their smaller discharge capacity compared with bare metal oxide. The smaller capacity of the nanohybrids than the bare guest metal oxide seems to cast doubt about the usefulness of hybridization technique in developing new efficient electrode materials. However, the present results show that the hybridization with nanocrystalline nickel oxide or iron oxide is effective in enhancing the electrode performance of titanium oxide. Recently spinel lithium titanate Li4Ti5O12 has attracted intense interest as promising electrode materials for electrical vehicle.29 Thus, the hybridization of Li4Ti5O12 with FeOx or NiOx nanocrystals makes possible the optimization of the electrode performance of this lithium titanate phase. Also, the stability of the iron oxide against chemical corrosion becomes markedly enhanced by the hybridization with layered titanate.,17,20 This would minimize the reaction between iron oxide nanocrystals and electrolyte, which makes a partial contribution to good cyclability of the present nanohybrids. Conclusion In the present study, we clearly demonstrate that the hybridization with nanocrystalline iron or nickel oxide is very effective in improving the electrochemical activity of titanium oxide. The electrode performance of NiOx-layered titanate nanohybrid can be optimized by the postcalcination at 300 °C. This phenomenon can be understood on the basis of the Ni K-edge EXAFS results showing that the postcalcination at above 300 °C gives rise to the local structural change from nickel hydroxide to nickel oxide. Such a positive effect of the postcalcination is negligible for the FeOx-layered titanate nanohybrid in which the postcalcination induces neither a significant improvement of anode performance nor a marked change in the local structure of hybridized iron oxide. On the basis of the XAS study for the lithiated nanohybrids, we conclude that the redox process of hybridized guest species is mainly responsible for the electrochemical activity of the nanohybrids and their chemical bonding nature affects significantly the electrode performance of the MOx-layered titanate nanohybrids. This study clearly shows that the hybridization with transition metal oxide nanocrystals is an efficient route to improve the electrode performance of titanium oxide-based materials. Our group is investigating the hybridization between nanostructured Li4Ti5O12 and nanocrystalline iron oxide or nickel oxide. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (Grant 2008-0061493), by the General R/D Program of the Daegu Gyeongbuk Institute of Science and Technology (DGIST), and by National Research Foundation of Korea Grant funded by the Korean Government (20090063005). The experiments at Pohang Accelerator Laboratory (PAL) were supported in part by MOST and POSTECH. References and Notes (1) Kang, K.; Meng, Y. S.; Bre´ger, J.; Grey, C. P.; Geder, G. Science 2006, 311, 977.

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