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Nanosized Effect on Electronic/Local Structures and Specific Lithium-Ion Insertion Property in TiO2B Nanowires Analyzed by X-ray Absorption Spectroscopy Toyoki Okumura,† Tomokazu Fukutsuka,‡ Asuki Yanagihara,† Yuki Orikasa,† Hajime Arai,*,§ Zempachi Ogumi,§ and Yoshiharu Uchimoto† †
Department of Interdisciplinary Environment, Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan ‡ Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan § Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji 611-0011, Japan
bS Supporting Information ABSTRACT: TiO2B nanowires were prepared by hydrothermal reaction, and the nanosized effect on lithium-ion insertion was investigated by using X-ray absorption spectroscopy (XAS). On the basis of the results of O K-edge X-ray absorption near-edge structure (XANES) of TiO2B with various particle sizes, it was suggested that the surface of TiO2B has local and electronic structures being different from the bulk, and the band energy of surface of TiO2B was lower than that of the bulk. The band vended structure, which is called the space charge layer (SCL), makes lithium-ion insertion in TiO2B smooth because the local structure of the SCL is maintained during the lithium-ion insertion, which is shown by Ti K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. It is suggested that control of the SCL, that is the nanosized effect, is a new design concept for achieving higher rate capability. KEYWORDS: lithium-ion battery, nanosized effect, electrode/electrolyte interface, X-ray absorption spectroscopy
’ INTRODUCTION Nanostructured TiO2B materials such as nanoparticles, nanotubes, and nanowire structures have been the focus of potential negative electrode materials for lithium-ion batteries (LIBs).15 The relatively positive lithium insertion/extraction potential (1.5 V vs Li+/Li) is beneficial for improving the safety and durability of the battery because, unlike graphite with negative lithiation potential (below 0.2 V vs Li/Li+), there is little possibility of having metallic lithium dendrite formation in contingent overcharge states and lithium-consuming solid electrolyte interphase (SEI)6,7 formation. The high capacity (theoretically 335 mA h g1), good cycleability, and high rate performance is also attractive.15 Understanding the lithium insertion/extraction behavior of nanostructured TiO2B is vital for applying this material for the LIBs. In the quest to understand the relationship between intrinsic lithium-ion insertion behavior and specific structural changes of nanostructured TiO2B, we should separate two important factors affecting the lithium-ion insertion behavior and discuss each of them in order: (1) the way TiO6 octahedrals stack in the TiO2B bulk region and (2) the surface structure caused by nanosized particles. In previous work, we have studied the lithium insertion behavior of submicrometer-sized TiO2B (bulk TiO2B) and its local structures around the Ti atoms measured by extended X-ray absorption fine structure (EXAFS) r 2011 American Chemical Society
spectroscopy, where the specific structural changes with the surface effect (the latter factor mentioned above) can be disregarded.8 Figure 1 shows possible lithium-ion sites in TiO2B that have been estimated from the first principles calculation by Arrouvel et al.9 From this estimation and our EXAFS results, it is proposed that the favorable lithium-ion sites are the 5-fold coordinated sites and/or the distorted octahedral sites distributed at the vicinity of O layers parallel to the ab plane for 0 e x e 0.5 in LixTiO2B.8 The lithium ion can hardly be inserted into the later sites for x > 0.5 because of the large distortion of TiO6 octahedrons caused by the reduction of Ti4+ to Ti3+ and the interaction between the lithium ion and the TiO6 octahedron. Thus, the lithium ion is considered to occupy the 5-fold coordinated sites distributed at the vicinity of TiO2 layers parallel to the ab plane for x > 0.5. Armstrong et al. have reported5 that the capacity of TiO2B nanotubes is 338 mA h g1, which is much larger than 224 mA h g1 for bulk TiO2B in our study. It seems that the difference comes from the short lithium-ion diffusion length for the TiO2B nanotubes and the low current density owing to their large surface area. Moreover, the local structural characteristics of the TiO2B nanotubes can be different from that of the bulk TiO2B. That is, we can expect the Received: March 30, 2011 Revised: July 3, 2011 Published: July 26, 2011 3636
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Figure 1. Schematic description of TiO2B crystal structure, in which ReO3 type blocks are enclosed. The ReO3 type block in TiO2 structure is composed of the stacking of O layers and TiO2 layers. Two different possible lithium-ion inserted sites are shown.
effect of the structural changes at the surface area (nanosized effect10) on the specific electrochemical properties of TiO2B nanoparticles. Recently, nanoionic phenomena have been focused on nanosized materials.1015 At the heterointerface between a mixed ionic conductor and an ionic conductor, a space charge layer (SCL) is formed by the relaxation of not only electrons but also ions, and accordingly, the charge (ion) density in the SCL is different from that in the bulk.12 It has been shown that this unique charge (ion) density leads to higher ionic conductivity in a lithium-ion conductor containing nanoporous alumina and in a fluoride-ion conductor with CaF2/BaF2 heterostructures.16,17 The electrode/electrolyte interface area in the nanosized systems are at least tens of times greater than that in microsized systems with the same volume; therefore, the nanoionic effects would be enhanced in such nanosized grains.12 Okubo et al. have reported nanosized effects on the rate capability of the nanocrystalline LiCoO2 electrode.18,19 In their paper, it has been reported that the lattice expansion and the change of site energy at the surface in nanocrystalline LiCoO2 electrodes lead to higher rate capability. This result is important as an example of nanoionic phenomena in LIBs and suggests that the designing concept of LIB with high rate capability lies not only in increasing the reaction area and decreasing the diffusion length using nanosized materials but also utilizing the SCL at the interface, which is enhanced with the nanosized materials. The SCL at the interface would change the electronic and local structures at the surface of the particles. X-ray absorption fine structure (XAFS) techniques are useful to clarify the electronic and local structures; however, the electronic and local structures of nanosized materials of LIBs have not been reported from the view of nanoionic effect. In this study, we compare the electronic and local structures and the electrochemical behavior of LixTiO2B nanoparticles at different particle sizes to clarify the effect of the SCL at the interface on the electrochemistry of nanostructured materials.
’ EXPERIMENTAL SECTION TiO2B nanowires were prepared by the hydrothermal reaction.20 In this method, the sizes of samples can be controlled by the concentration of NaOH aqueous solutions, reaction time, and reaction temperatures. The TiO2B nanowires were obtained as follows. Aqueous solution of 10 or 15 mol dm3 NaOH was added to 7.5 g of anatase TiO2 (Wako
Figure 2. TEM images of resulting TiO2B treated with (a) 10 mol dm3 NaOH solution before heating at 170 °C for 72 h, (b) 15 mol dm3 NaOH solution before heating at 170 °C for 72 h, and (c) 15 mol dm3 NaOH solution before heating at 170 °C for 144 h.
pure chemical: 99.9%). After stirring for 1 h, the mixtures were heated at 170 or 200 °C for 24, 72, or 144 h in Teflon-lined autoclaves. After the ion exchange process using 0.05 mol dm3 HCl, the products were dried at 80 °C for 15 h and annealed at 400 °C for 4 h in air. The TiO2B samples with submicrometer particle sizes, called bulk TiO2B, were also prepared by the solid-state reaction as has been previously reported.8,21 The crystal structure of each sample was identified by powder X-ray diffraction (XRD) analysis (RINT-2200 V, Cu KR). Transmission electron microscope (TEM) analysis was carried out with Hitachi H-3000 (300 kV). Electrochemical measurements were carried out by using a threeelectrode cell. A working electrode was the mixture of 60 wt % active material, 30 wt % vapor grown carbon fiber (VGCF), and 10 wt % polyvinylidene difluoride (PVDF). Counter and reference electrodes of metallic lithium foil were used in the three-electrode cell together with 3637
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Figure 3. Powder XRD patterns of various particle sizes of TiO2B nanowire. The particle sizes are (a) 64 nm, (b) 41 nm, and (c) 27 nm, respectively. Simulated pattern of TiO2B is shown in (d) for comparison. the working electrode. For the electrolyte solution, 1 mol dm3 LiClO4/ PC (propylene carbonate) was used. Electrochemical discharge profile measurements were carried out at a constant current density between 2.5 and 1.0 V vs Li/Li+ at room temperature. Lithium-ion inserted LixTiO2B samples for ex-situ XAFS measurements were prepared by the electrochemical method using the three-electrode cell mentioned above, except that the mixture of 60 wt % active material, 30 wt % VGCF, and 10 wt % poly tetrafluoroethylene (PTFE) pressed into a pellet and wrapped with a Ni mesh was used as the working electrode. Electrochemical insertion of the lithium ion was carried out at a constant current of 10 mA g1. The amount of x in LixTiO2B was calculated assuming that all the electric current was utilized for the lithium-ion insertion process. XAFS measurements at Ti K- and Ti L- or O K-edge were respectively performed in BL-7C and BL-11A at Photon Factory, High Energy Accelerator Research Organization, Tsukuba, Japan. The spectra were recorded by the transmission mode for Ti K-edge, while they were recorded by the total-electron-yield mode for Ti L- and O K-edges. Fourier transformations were performed using k3 weighting. The structural parameters were determined by curve-fitting procedures using Rigaku REX2000 data analysis software.22 The effective backscattering amplitude Feff, phase correction γ(k), and total central atom phase shift ξ were calculated with the multiple-scattering theoretical calculation program, FEFF8.20.23 The model of TiO2B was selected to input the starting parameters for the theoretical calculation. The χ(k) function was fitted according to the equation χðkÞ ¼
∑i
NS0 2 jf ðk, πÞjexpð 2σ i 2 k2 Þexpð 2Ri =λi Þsin½2kRi þ ϕi ðkÞ kRi 2
ð1Þ where N is the number of neighboring atoms, S02 is the amplitude of χ(k), R is the atomic distance to the neighboring atom, σ2 is the DebyeWaller (DW) factor, λ is the mean free path, and ϕ is the total phase shift.
’ RESULTS AND DISCUSSION Characterization of TiO2B Nanowires. Figure 2 shows the TEM images of three TiO2B nanowire samples. It was found that the TiO2B nanowire samples were needlelike, and the size
Figure 4. (a) Initial discharge profile of TiO2B with various sizes. Current density is 50 mAg1 (1/5 C). Solid line represents 1.45 V vs Li/ Li+, and CA and CB mean the capacities below 1.45 V vs Li/Li+ and the capacities under 1.45 V vs Li/Li+, respectively. (b) Ratio of capacities under 1.45 V vs Li/Li+ to total capacities of TiO2B nanowires.
depends on the synthesis conditions. The average particle sizes of 27, 41, and 64 nm were obtained under the synthesis conditions with a 10 mol dm3 NaOH solution followed by heating at 170 °C for 72 h, a 15 mol dm3 NaOH solution followed by 170 °C for 72 h, and a 15 mol dm3 NaOH solution followed by heating at 170 °C for 144 h, respectively. Hereafter, the explanatory names of the samples are referred as their average particle sizes (27, 41, and 64 nm). The TEM images indicated that these three nanowire samples show the deviation of a few nanometers in their sizes and partially contain some nanosheets, but the nature of the nanowire samples can be mostly represented by the average particle size, as shown below. The XRD patterns of the TiO2B nanowires are shown in Figure 3. The peaks are in agreement with the simulation result,24 and there was no peak of impurity such as anatase TiO2, which has been reported as a major impurity.5 Singlephase formation of TiO2B with a space grope of C2/m was found for all the samples, irrespective of the particle size. We also tried to prepare the TiO2B nanowires with smaller particle sizes because the smaller particle has an advantage for researching the surface specific properties on electrochemistry. For preparing smaller particles of TiO2B, the preferred hydrothermal reaction conditions are low acid concentration, low reaction temperature, and short reaction time. However, no single phase was obtained under these modified conditions because of the formation of anatase TiO2 as an impurity phase. Accordingly, the detailed characteristics of the TiO2B nanowires with smaller particle sizes were not examined. Clarification of Specific Surface Properties of TiO2B Nanowire with Electrochemical Measurements. Panel (a) of Figure 4 shows the first discharge profile of TiO2B nanowires and bulk TiO2B. The electrochemical lithium-ion insertion occurred for all the TiO2B samples. The lithium-ion insertion capacities of the TiO2B nanowires were larger than that of the bulk TiO2B, and the specific capacity was increased 3638
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with the decrement of the particle size. Moreover, it is noticed that the difference in lithium-ion insertion capacity between the TiO2B nanowires with various sizes and the bulk TiO2B mainly comes from the capacity below 1.45 V vs Li/Li+ (CB region in Figure 4a). Panel (b) of Figure 4 shows the plot of CB (discharge capacity below 1.45 V vs Li/Li+)/(CA + CB) (total discharge capacity) against the average particle size of the TiO2B nanowires. In this plot, the CB/(CA + CB) value is increased monotonically with the decrease in average particle size. Because the discharge capacity above 1.45 V vs Li/Li+ is almost the same for these three samples, the difference below 1.45 V vs Li/Li+ can be attributed to the difference of the sample particle size. Accordingly, the contribution of CB to the total capacity can be attributed to the specific electrochemical property at the surface of TiO2B, that is, the nanosized effect. The influence of the surface of TiO2B on the lithium-ion insertion behavior was further analyzed as shown below. The relationship between the shape of discharge curves and the site energy of the surface layer was examined on the basis of the latticegas model.18,25 From the latticegas model, the lithiumion site occupancy, x, is represented by the equation εi þ eE 1 þ exp n N n kT kT i ¼ 1 x¼ ln εiþ1 þ eE N i ¼ 0 N εiþ1 εi 1 þ exp kT
∑
ð2Þ where E is electrode potential, Ni is the number of the lithium-ion sites where the site energy is εi, N is the number of total lithiumion sites, n is the unoccupied lithium-ion site, k is Boltzmann constant, and e is an elementary electric charge. Because the site energy of the surface layer is considered to be different from that of the bulk, the second term in eq 2 should be divided into two terms. Assuming that the site energy changes stepwise from εa to εb at the surface layer and εb to εc in the bulk region, eq 2 can be expressed in a reduced form as eq 3 εa þ eE 1 þ exp Na kT kT x ¼ 1 ln εb þ eE N εb εa 1 þ exp kT εb þ eE 1 þ exp Nb kT kT ln ð3Þ εc þ eE N εc εb 1 þ exp kT where Na/N is the ratio of the number of lithium-ion sites at the surface layer and Nb/N is the ratio of the lithium-ion sites in the bulk region. By using eq 3, the discharge curves were fitted. The Na/N value was assumed to be the volume ratio of the surface layer, which was calculated as follows. The thickness of the surface layer which has specific electrochemical potential was assumed to be 2 nm from the surface edge, and the shape of the TiO2B particle was assumed to be a cylinder with a diameter of the average particle size. Figure 5 shows the experimentally obtained first discharge curves and the calculated discharge curves for the three samples. The calculated values agree well with the experimentally observed ones, indicating that the assumptions, including the nanosize effect at the surface with 2 nm in depth, were appropriate. The good fits
Figure 5. Calculated and observed discharge curves at 50 mA/g. (a) 27 nm, (b) 41 nm, and (c) 64 nm.
also indicate that the effect of the size and shape deviations on the electrochemical behavior was minor, and the nanowires with the assumed average sizes dominantly represent the nature of the obtained samples. The fitting refinement results show that the εa, εb, and εc values for the site energies at the surface edge, between the surface and the bulk region and the bulk region are respectively 1.0 eV, 1.45 eV, and 1.85 eV. There are some discrepancies between the calculated and experimentally observed values, mainly in the region before the specific capacity reached 80 mA h g1. This would come from the site energy change in this region; however, we believe that these discrepancies have little influence on the discussion of the nanosize effect as this region comes from the reaction in the bulk. It is therefore summarized that the specific discharge capacities below 1.45 V vs Li/Li+ (CB region) in TiO2B nanowires come from the nanosized effect at the surface layer. Focusing on the rate capability, the discharge profiles of TiO2B with different particle sizes were measured at various current densities. Figure 6 shows the plots of the discharge capacity over 1.45 V, the discharge capacity below 1.45 V, and the total discharge capacity against various current densities estimated from the discharge profiles. The discharge capacities over 1.45 V were decreased with the increase in the current density. For the large particle TiO2B such as the bulk TiO2B, the capacity fade with increasing the current density was especially large. This result indicates that the low lithium-ion diffusion coefficient at the bulk region affects the rate properties of the 3639
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Figure 6. Discharge capacities of TiO2B at different current densities: 9, total capacity; b, capacity over 1.45 V; and 2, capacity below 1.45 V. (a) 27 nm, (b) 41 nm, (c) 64 nm, and (d) bulk TiO2B.
Figure 7. Ti L3,2-edge XANES spectra of TiO2B with various particle sizes.
TiO2B. On the other hand, the discharge capacities below 1.45 V were independent of the current density in all particle sizes of the TiO2B. Hence, it is considered that lithium-ion insertion at below 1.45 V is a fast reaction and occurs at the surface region, suggesting that the local and electronic structures at the surface layer is different from those in the bulk region in the TiO2B. Clarification of Electronic Structures by the Comparison of XANES for Various Sizes of TiO2B Nanowires. We measured the XANES spectra of the TiO2B nanowires to consider the electronic structure at the surface layer. The Ti L-edge XANES spectra of the TiO2B with various particle sizes are shown in Figure 7. These spectra consisted of two sets of peaks, L3 (a, b) and L2 (c, d), separated by about 56 eV. They correspond to the electronic transitions from the 2p3/2 and 2p1/2 sites to an unoccupied 3d state.2630 The two L2-edge peaks (c, d) are more broadened than those of the L3-edge (a, b). This difference has been explained by CosterKronig Auger decay,26 which has been
Figure 8. O K-edge XANES spectra of TiO2B with various particle sizes.
previously reported for the bulk TiO2B.22 There are two peaks in the L3- or L2-edges because the Ti 3d orbital splits to t2g (a, c) and eg (b, d) states by the ligand field theory under octahedral (Oh) symmetry.26 Each absorption edge energy was unchanged with particle size. This result indicates that valence of the titanium ion is the same for all the tested samples. The O K-edge XANES spectra of the TiO2B with various particle sizes are shown in Figure 8. The spectra show the transitions from the O 1s core orbital to the unoccupied O 2p orbital with the dipole transition selection rules. It is suggested in the TiO2B that the O 2p orbital hybridized with the Ti 3d and 4sp orbital and that the absorption peaks in the O K-edge XANES spectra reflect the ratio of the unoccupied O 2p orbital hybridized with a Ti 3d and 4sp orbital character.31 The peaks a and b can be assigned to the transitions from the O 1s orbital to the hybridized orbital between the O 2p orbital and the Ti 3d t2g, or Ti3d eg orbital, respectively.8 In the TiO2B nanowires, some 3640
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Figure 9. Ti K-edge XANES spectra of LixTiO2B with various particle sizes.
crests are observed between peaks a and b. This would come from the deformation of hybridization with the distortion of octahedral crystal field.31 The broad peaks (c and d) can be attributed to the transition from the O 1s orbital to the hybridized orbital between the O 2p and Ti 4sp orbital.32 The absorption edge energy shifts to lower energy levels with the decrease in average particle size. Assuming that the sample with a smaller particle size has more surface area, this result shows that the band energy of the TiO hybridized bond decreased at the surface region compared with the bulk. It has been suggested from our recent results8 that the electronic structure of the TiO2B is not a MottHubbard type like other early transition metal oxides but rather a significant chargetransfer type or an intermediate between the chargetransfer regime and the MottHubbard regime. The exchange of electron or hole occurs at the O 2p orbital hybridized with Ti 3d orbital. Summarizing the XANES results, the absorption edge energy of O K-edge XANES spectra shifts to lower energy levels with the decrease in average particle size, although that of Ti L-edge spectra is maintained. These results represent that the partial density state of the O 2p orbital in the hybridized state of the O 2p and Ti 3d orbitals is decreased with the decrease in particle size. Therefore, it is clarified that the electronic structure at the surface layer of the TiO2B is different from that at the bulk region, which would be caused by the nanosized effect. Effect of Surface-Specific Electronic and Local Structures on Electrochemical Property with Comparison of XANES and EXAFS Spectra in Lithiated LixTiO2B Nanowires. The XAFS measurements were performed with the lithium-ion inserted LixTiO2B samples prepared by the electrochemical method. The Ti K-edge XANES spectra of LixTiO2B with various particle sizes are shown in Figure 9. The K-edge XANES
spectra of 3d transition metals have been described with the combination of the selection rule, coordination number, number of d electrons, and symmetry of the coordination sphere,33 which makes the spectra interpretation complex.3335 The peak crests of the Ti K-edge XANES (features C1 and C2 in Figure 9) simply represents the electric dipole transition from the core Ti 1s orbital to unoccupied Ti 4p orbital due to the dipole selection rule. The features of the two crests (C1 and C2) probably indicate the signature of the interaction between O and Ti positioning in a long distance.36 The main peak in the transition metal K-edge XANES spectra gives information on the oxidation state of the absorbing atoms and their local geometric structure. As shown in Figure 9, the normalized Ti K-edge XAFS spectrum monotonically shifts to lower energy levels as the amount of the electrochemically inserted lithium ions increases, which indicates that the average oxidation state of the titanium ions is reduced upon the electrochemical lithium-ion insertion. In the case of the bulk TiO2B, the charge variation by the lithium-ion insertion was compensated by the redox of Ti3+/Ti4+ couple.8 The drastic spectrum change in the region between 0.40 < x < 0.53 in the bulk LixTiO2B indicates that the local environment around the Ti atom critical changes upon lithium insertion. On the other hand, the Ti K-edge XAFS spectra of LixTiO2B nanowires was nearly invariant with x. This result represents that the electronic compensation with the lithium-ion insertion in the LixTiO2B nanowires is not caused by the redox of Ti3+/Ti4+ couple. The local structural changes around the Ti atom upon lithium insertion are limited with the nanoparticles because little change in two crests (C1 and C2) was observed in LixTiO2B nanowires. Next, the pre-edge of the Ti K-edge XANES spectra of LixTiO2B is focused. Three pre-edge features A1A3 are observed in each particle size of LixTiO2B as shown in Figure 9. 3641
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Chemistry of Materials The insets in Figure 9 show that the intensity of pre-edges A1A3 is decreased with the lithium insertion into LixTiO2B, namely, with the injection of electrons into the Ti 3d orbital. It is difficult to explain the change of these pre-edge features because many explanations for these pre-edges have been reported.8 Nevertheless, it is deduced that the change in the hybridization between Ti and oxide ions accompanied with the structural change of LixTiO2B effects on the spectrum changes as well as the electronic structure changes because the dipolar and quadrupolar transition are sensitive to the local structural change. Moreover, pre-edge peaks A1 and A3 perfectly disappeared upon
Figure 10. O K-edge XANES spectra of LixTiO2B with various particle sizes. Solid lines represents the spectra of the pristine samples and dashed lines represent the spectra of the samples lithiated until 1.0 V vs Li/Li+.
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x > 0.53 in the bulk LixTiO2B.8 This phenomenon implied that the lithium-ion insertion sites for x > 0.53 in LixTiO2B differ from those for 0.40 < x. On the other hand, such disappearance of two pre-edges (A1 and A3) was not observed in the LixTiO2B nanowires, suggesting that the local structural change around the Ti atom upon lithium insertion is small in the LixTiO2B nanowires. This corresponds to the behavior observed in the Ti K-edge XANES spectra. The O K-edge XANES spectra of LixTiO2B at various lithium contents are shown in Figure 10. The peaks around 528535 eV, which are assigned to the transitions from the O 1s orbital to the hybridized orbital between the O 2p and Ti 3d orbital, decrease with the lithium-ion insertion, indicating that the transition probability to the unoccupied hybridization orbital decreases with the lithium-ion insertion in LixTiO2B. The partial density state of the O 2p orbital in the hybridized O 2pTi 3d orbital becomes lower at the surface layer of LixTiO2B compared with that in the bulk region, which suggests the electron injection into the unoccupied O 2p orbital in the O 2p Ti 3d hybridized orbital. The Ti K-edge EXAFS spectra were measured for investigating the effect of the surface layer on the local structure around the Ti atom in LixTiO2B. Figure 11 shows the Fourier transforms (FTs) of the EXAFS oscillations in LixTiO2B with various particle sizes, which are the pseudoradial structure functions (RSFs) of the local atomic environments around the Ti atoms. The FTs of the k3-weighted Ti K-edge EXAFS oscillations for the each sample were calculated within k = 2.7 11.0 Å1 (this range was chosen to minimize noise). The RSFs of the Ti K-edge EXAFS spectra show three peaks at about 1.4, 2.4, and 3.2 Å. The first peak corresponds to the six coordinated TiO bonds in TiO6 octahedrons. The second and third peaks correspond to the
Figure 11. Fourier Transforms for LixTiO2B with various particle sizes around Ti atom from EXAFS oscillations. (a) 27 nm, (b) 41 nm, (c) 64 nm, and (d) bulk TiO2B. 3642
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Figure 12. Plots of atomic distance (a, b, c) and DebyeWaller (DW) factor (d, e, f) obtained by EXAFS analysis of Ti K-edge spectra for LixTiO2B with various average particle sizes. (a) and (d) are TiO interactions, (b) and (e) are TiTi(edge) interactions, and (c) and (f) are TiTi(corner) interactions.
first neighbor five-coordinated TiTi(edge) interactions and the third neighbor four-coordinated TiTi(corner) interactions, respectively. Here the edge and the corner mean two TiO2 octahedra connected with OO edges of each other (featured as orange lines in Figure 1) and those connected with corner of each other (shown as light blue lines in Figure 1). The RSFs around the Ti atoms were inversely Fourier-filtered over the three peaks (R = 1.0 3.9 Å) in the transformation and fitted with eq 1. The structural parameters were determined using curve-fitting, and the atomic distances and DW factors were compared with the x values (Figure 12 and see the Supporting Information for quantity values of each parameter). In the bulk LixTiO2B, the bond length of TiO is increased with increasing lithium-ion content because of the reduction of Ti from Ti4+ to Ti3+, leading to the TiO6 octahedron expansion.8 Additionally, the atomic lengths of TiTi(edge) and TiTi(corner) also increase with this expansion of the TiO6 octahedrons. The DW factors of each interaction [TiO, TiTi(edge) and TiTi(corner)] are increased with each increment of the lithium-ion content, indicating that the distortion of the TiO6 octahedrons is increased with a low symmetric property of 3d orbital in Ti3+. Focusing on the degree of local structural changes with increases in the x values in the bulk LixTiO2B, the atomic distance and the distortion of each interaction has the inflection points around x = 0.5. The lithium ions occupy the 5-fold coordinated sites and/or distorted octahedral sites distributed at the vicinity of O layers parallel to the ab plane until x e 0.5 (Figure 1). In the latter stage of the discharge process, the 5-fold coordinated sites distributed at the vicinity of TiO2 layers parallel to the ab plane are occupied (Figure 1). The drastic increment of the atomic distance and the distortion of each interaction for x > 0.5 in the bulk LixTiO2B suggest that the TiO6 octahedrons expand and distort with the insertion of lithium ions into sites that are close to TiO6 octahedron layer. On the other hand, such
inflection points around x = 0.5 cannot be observed in LixTiO2B nanowires, and the degree of local distortion around the Ti atoms with lithium-ion insertion is decreased for LixTiO2B samples with a smaller particle size. This result indicates that the structure at the surface layer in LixTiO2B nanowires is nearly unchanged with the lithium-ion insertion into the 5-fold coordinated site distributed at the vicinity of TiO2 layers parallel to the ab plane. It is clarified that the surface region of the nanosized TiO2B has specific electronic and local structures and molecular orbital energy levels, which are different from those in the bulk region. Assuming that the change of molecular orbital energy level is continuous from the surface area to the bulk region,18 it is expected that the SCL formed on the surface of TiO2B has a structure with its band vending. The SCL is expected to reduce the local distortion of TiO2B during lithium-ion insertion and to cause higher rate capability. It is also thought that the SCL influences the lithium-ion diffusion in the bulk and makes fast lithium-ion transfer possible.
’ CONCLUSIONS In order to clarify the nanosized effect of the lithium-ion insertion behavior, nanosized TiO2B materials with various particle sizes were prepared. The average particle size of TiO2B could be controlled by selecting suitable synthetic conditions. In all TiO2B samples, discharge capacities below 1.45 V were independent of the current densities while those above 1.45 V were decreased with an increase in current densities. It is considered that lithium-ion insertion at below 1.45 V is fast and occurs at the surface region, that is, in the space charge layer (SCL). The XAS measurements and the discharge curve fitting in this study indicate that the SCL has a different structure from the bulk with its band vending. The rate capability of TiO2B was enhanced with the increase in the SCL region. It is suggested that 3643
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Chemistry of Materials this phenomenon is an example of the nanoionic effect, and a new design concept is required to use nanosized compounds as active materials for lithium-ion batteries with high rate capability.
’ ASSOCIATED CONTENT
bS
Supporting Information. Curve-fitting results for inverse FT spectra of LixTiO2B and calculated parameters estimated from Ti K-edge EXAFS spectra for LixTiO2B. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +81-774-38-4974. Fax: +81-774-38-4993. Email: h-arai@ saci.kyoto-u.ac.jp.
’ ACKNOWLEDGMENT A part of this work was supported by Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING) project from New Energy and Industrial Technology Department Organization (NEDO) in Japan. The authors greatly acknowledge it.
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dx.doi.org/10.1021/cm200902r |Chem. Mater. 2011, 23, 3636–3644
