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Lithium Insertion in an Oriented Nanoporous Oxide with a Tunnel Structure: Ti2Nb2O9 J.-F. Colin, V. Pralong,* M. Hervieu, V. Caignaert, and B. Raveau Laboratoire CRISMAT, UMR 6508 CNRS ENSICAEN, 6 bd Maréchal Juin, 14050 CAEN, France ReceiVed October 16, 2007. ReVised Manuscript ReceiVed NoVember 19, 2007
The electrochemical study of the empty tunnel structure Ti2Nb2O9 shows that it can intercalate lithium reversibly, through a solid-solution process, leading to the phase Li4Ti2Nb2O9. The high-resolution electron microscopy study of this phase shows that Ti2Nb2O9 exhibits an exceptional nanoporosity, involving the formation of aligned holes throughout the whole microcrystal, which may favor, together with the presence of tunnels, the diffusion of lithium during the insertion. The reversible capacity of this phase, of 125 mAh/g at 1.57 V vs Li/Li+, after 30 cycles, shows its great reversibility.
Introduction Lithium insertion in oxides is a very important topic in view of applications, especially for the realization of lithiumion batteries. Besides the well-known compounds that are now going to be optimized for such applications,1 there remains a vast field for the discovery of new materials that has not so far been investigated. The discovery of these oxides requires one to take into consideration mainly three criteria: the values of the redox potential of the involved transition-metal element, the open character of the crystallographic structure of the oxide, and the particle size of the material. Bearing in mind these considerations, titanium-based oxides have been extensively explored these last 15 years. It is the case of the spinel Li4Ti5O12 and of the anatase TiO2.2–7 It was indeed shown that three lithium atoms per formula can be reversibly inserted into Li4Ti5O12 at 1.5 V, corresponding to the maximum theoretical specific capacity of 175 mAh/g, whereas one lithium per formula can be inserted into TiO2 anatase for a theoretical specific capacity of 335 mAh/g. Importantly, it was shown some years ago that the particle sizes play a prominent role in the insertion and especially that nanosize particles strongly favor lithium insertion.8–11 In this way, oxides such as nanocrystalline rutile or brookite were shown to be able * Corresponding author. E-mail:
[email protected]. Fax: +33 2 31 95 16 00. Tel: +33 2 31 45 26 32.
(1) Whittingham, M. S. Chem. ReV. 2004, 104 (10), 4271–4302. (2) Ferg, E.; Gummow, R. J.; De Kock, A.; Thackeray, M. M. J. Electrochem. Soc. 1994, 141, L147. (3) Ohzuku, T.; Ueda, A.; Yamamoto, N. J. Electrochem. Soc. 1995, 142, 1431. (4) Nakahara, K.; Nakajima, R.; Matsushima, T.; Majima, H. J. Power Sources 2003, 117, 131. (5) Marchand, R.; Brohan, L.; Tournoux, M. Mater. Res. Bull. 1980, 15, 1129. (6) Armstrong, A. R.; Armstrong, G.; Canales, J.; Garcia, R.; Bruce, P. G. AdV. Mater. 2005, 17, 862. (7) Huang, S. Y.; Kavan, L.; Exnar, I.; Grätzel, M. J. Electrochem. Soc. 1995, 142, L142. (8) Sudant, G.; Baudrin, E.; Larcher, D.; Tarascon, J.-M. J. Mater. Chem. 2004, 15, 1263. (9) Hu, Y.-S.; Kienle, L.; Guo, Y.-G.; Maier, J. AdV. Mater. 2006, 18, 1421.
to intercalate one lithium per formula at room temperature, in contrast to what had been observed previously for bulk materials. Among the various based titanium oxides that may be used for the generation of new lithium-intercalated materials, the oxides derived from the layered structure HTiNbO5 discovered almost 30 years ago12 are of great interest. Recently, a new layered oxide LiTiNbO5 with an original structure was indeed synthesized through a topotactic reaction, between this protonic oxide and the eutectic salt {LiNO3/LiOH}, and it was shown that the latter can intercalate 0.8 lithium ion reversibly at 1.6 V.13 The topotactic dehydration of HTiNbO5 into a tunnel structure, Ti2Nb2O9,12 represents an interesting direction for lithium insertion. We report herein on the electrochemical behavior of this phase and its structural study by transmission electron microscopy (TEM). We show that it can reversibly intercalate lithium, leading to the phase Li4Ti2Nb2O9, with a capacity of 125 mAh/g after 30 cycles. We emphasize the exceptional nanoporous character of Ti2Nb2O9, due to the topotactic dehydration, and we discuss its crucial role for lithium insertion. Experimental Section Powder X-ray diffraction (XRD) patterns were registered in the range 5–120° (2θ) using a Philips X’pert diffractometer with Bragg-Bretano geometry. The type of measurement was step scan, using Cu KR as the X-ray source and a step of 0.0167°, 2θ. The sample was put inside an Anton Paar TTK450 chamber under a flow of dry nitrogen to protect it from moisture. Thermogravimetric studies were carried out with a Setaram TG 92 instrument at a heating rate of 2 °C/min under flowing nitrogen gas. (10) Reddy, M. A.; Kishore, M. S.; Pralong, V.; Varadaraju, U. V.; Raveau, B. Electrochem. Solid State Lett. 2007, 10, A29. (11) Armstrong, G.; Armstrong, A. R.; Canales, J.; Bruce, P. G. Electrochem. Solid State Lett. 2006, 9, A139. (12) Rebbah, H.; Desgardin, G.; Raveau, B. Mater. Res. Bull. 1979, 14, 1125. (13) Colin, J. F.; Pralong, V.; Caignaert, V.; Hervieu, M.; Raveau, B. Inorg. Chem. 2006, 45 (18), 7217–7223.
10.1021/cm702978g CCC: $40.75 2008 American Chemical Society Published on Web 01/05/2008
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Figure 1. (a) Hypothetical topotactic mechanism of dehydration of HTiNbO5, leading to a 3D tunnel structure Ti2Nb2O9 (b) isotypic to KTiNb3O9 (c).
fiber sheet saturated with 1 M LiPF6 in 1:1 by weight ethylene carbonate/dimethyl carbonate (LP30, Aldrich) as the electrolyte. The composite positive electrode was prepared by mixing the active material with 30 wt % black carbon. Negative electrodes were lithium sheets (Aldrich, 1.5 mm thickness). The electrochemical reactivity was monitored with a VMP II potentiostat/galvanostat (Biologic SA, Claix, France). All of the voltage values in this paper are given versus Li/Li+.
Results and Discussion
Figure 2. XRD pattern of Ti2Nb2O9 and Bragg peaks positions for the KTi3NbO9 cell.
Ti2Nb2O9 Synthesis. The phase Ti2Nb2O9 is the metastable end member resulting from a series of topotactic reactions. First, the phase KTiNbO5 is prepared by a conventional solidstate reaction between stoichiometric amounts of K2CO3, Nb2O5, and TiO2. The initial reagents are first mixed and decarbonated at 650 °C and then heated at 1200 °C for 12 h in a platinum crucible. Then, KTiNbO5 is easily exchanged in an acidic medium (6 N HCl, 48 h), leading to the formation of the HTiNbO5 phase. The structure of this oxide (shown on Figure 1a), isotypic to KTiNbO5,14 consists of octahedral [TiNbO5]∞ layers, built up of 2 × 2 edge-sharing octahedra, forming infinite [TiNbO6]∞ ribbons connected to each other through their corners, with two successive layers being interconnected through short H-O · · · H hydrogen bonds. Finally, HTiNbO5 is dehydrated at 330 °C, resulting in the metastable phase Ti2Nb2O9 (Figure 1b) according to eq 1.15 2HTiNbO5 f Ti2Nb2O9 + H2O
Figure 3. Selected-area ED of Ti2Nb2O9 along (a) [010], (b) [001], (c) [1j20] patterns illustrating the presence of a 21 axis parallel to b c (indexed in the monoclinic double cell).
The electron diffraction (ED) study was carried out with JEOL 200CX and JEOL 2010 transmission electron microscopes equipped with energy-dispersive spectroscopy analyzers. The samples for electron microscopy were prepared by breaking the crystallites in n-butanol, and the small flakes in suspension were deposited on a holey carbon film, supported by a copper grid. The high-resolution electron microscopy (HREM) studies were carried with JEOL FEG extensively 2010 and TOPCON electron microscopes. The theoretical images were calculated, using Mac Tempas software, for different focus and crystal thickness values, on the basis of the positional parameters of the KTi3NbO9 phase,15 considering that the tunnels are empty. Ti2Nb2O9 was tested as a positive electrode in Swagelok-type cells assembled in an argon-filled glovebox with a borosilicate glass
(1)
Structural Characterization of Ti2Nb2O9. A topotactic mechanism was proposed more than 20 years ago15 for the dehydration of HTiNbO5, leading to a three-dimensional (3D) structure with empty tunnels and characterized by a framework similar to that of KTi3NbO9, synthesized by Wadsley.14 However, no structural resolution was made, and this still remains a hypothetical structure (Figure 1). To confirm the latter hypothesis, we have tried to refine the XRD pattern using the cell parameters of KTi3NbO9, i.e., a ≈ 6.4 Å, b ≈ 3.8 Å, and c ≈ 14.9 Å, in the Pnmm space group. Although we can index the peaks with these parameters (Figure 2), the structural resolution from these data remains impossible because of the broadness of the peaks and the poor crystallinity of this compound. In order to get (14) Wadsley, A. D. Acta Crystallogr. 1964, 17, 623. (15) Raveau, B. ReV. Chim. Miner. 1984, 21, 391.
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Figure 5. Local formation of the monoclinic Ti2Nb2O9 phase.
Figure 4. BF images of a typical Ti2Nb2O9 crystallite: (a) general view; (b) enlargement showing the alignment of the holes along the a axis; (c) enlarged [010] image exhibiting a chevron-like contrast.
more information on the real structure of this compound, a detailed investigation was carried out by TEM. The first result deals with the lamellar habitus of the grains, (001)-oriented, in spite of the care taken in the course of the sample preparation. The reconstruction of the reciprocal space was carried out by tilting around the crystallographic axes. Typical [010] and [001] patterns are given in Figure
3. In both patterns, the system of intense reflections corresponds to the one of the pseudo-orthorhombic subcell with as ≈ 6.4 Å, bs ≈ 3.8 Å, and cc ≈ 14.9 Å (with the suffix s referring to the subcell); this subcell is consistent with the hypothesis of a tunnel structure isotypic with KTi3NbO9. However, a system of additional spots is observed, indicated by white arrows in Figure 3a,b, which involves a doubling of the as parameter (a ) 2as ≈ 12.8 Å). Diffuse streaks are observed in both patterns, along b a* (Figure 3b) and along b c* (Figure 3a), revealing that disorder phenomena exist along these two axes. A monoclinic distortion has been observed in the [001] pattern of numerous crystallites, with the γ angles varying between 90° and 89°. The ED study allowed us to conclude that the title compound Ti2Nb2O9 exhibits a monoclinic cell with a ≈ 12.8 Å, b ≈ 3.8 Å, c ≈ 14.9 Å, and γ ≈ 90°. There is no condition limiting the hkl reflections; the only condition is 00l, where l ) 2n, as illustrated in Figure 3c by the ED pattern recorded in the course of the rotation around b c* (the [1j20] pattern is indexed in the double monoclinic cell). These observations are c, consistent with the presence of a 21 axis parallel to b implying P21 and P21/m as possible space groups. The bright-field (BF) images evidence an unusual appearance of the grains, in the form of striped holey lamella, as illustrated in Figure 4a. A typical enlarged [010] image presents the main features at the origin of the contrast and of the disorder which could be correlated to the presence of the diffuse streaks along b c* (Figure 4b). The crystallites can be described as a succession of different slices stacked along b c, whose thickness is on the order of a few nanometers. The layered microstructure of the BF images is mainly generated by the presence of defective slices running along b a, throughout the crystallites. Finally, the brighter zones in Figure 4a,b (labeled H) are associated with a loss of matter, which involves the formation of “holes” crossing the whole
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Figure 6. (a) [010] image of a defect (white arrows) and holes (H) containing a domain. (b) Enlargements focused on the centers of holes.
Figure 7. Potential versus capacity curve at C/10 in the 3–1 V potential window. Inset: corresponding incremental capacity dx/dV versus potential curve of the second cycle. Table 1. Evolution of the Reversible Capacity against the C Rate rate
reversible capacity (number of lithium atoms)
reversible capacity (mAh/g)
C/5 C/10 C/20 C/60 C/100
1.8 2.3 2.6 2.7 2.6
114 144 161 167 161
crystallite or forming voids. The latter ones are all aligned along b a, giving a lacy habitus to the crystallites. The HREM images confirm that the majority of the slices exhibit the expected tunnel structure. The doubling of the a parameter, characteristic of the Ti2Nb2O9 phase, is clearly visible throughout the whole matrix as well in the overview
(Figure 4b), as in Figure 5 (see am ) 2as between the two horizontal right arrows). The enlarged image (Figure 4c) presents a chevron contrast, consistent with the arrangement of the building blocks of four edge-sharing octahedra (in the ovals) as in the ideal structure drawn at the same scale. To get more accurate information, the experimental throughfocus series were compared to theoretical ones, obtained by considering the atomic positions of KTi3NbO9, with empty tunnels. The experimental images present similarities with the calculated ones. Although these images have only qualitative values, because of the difference between KTi3NbO9 and Ti2Nb2O9 (different symmetry, cell parameters, and Ti/Nb ratios), on the one hand, and the strain effects especially associated with the disorder phenomena, on the other hand, they are consistent with the average structure proposed for Ti2Nb2O9. The HREM images show also that the defects, which separate the monoclinic slices, are generally extended between two and four [TiNbO5]∞ octahedral layers. The larger defective slabs, such as the one indicated by a white arrow in Figure 4b, are made of less ordered matter. However, some of them, such as the domain outlined by a vertical arrow in Figure 5, show a regular contrast different from the one observed in the monoclinic domain and different from the one expected for a KTi3NbO9-type domain. In the present image (estimated for a focus value close to -50 Å and a crystal thickness close to 30 Å), the brighter dots are associated with the Ti and Nb positions and the darker contrast is associated with the light electron density zones. The doubling of the parameter along b a is no longer observed, and the periodicity along b c varies: more especially, the vertical lines of darker contrast (indicated by black
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Figure 8. XRD pattern of (a) pristine Ti2Nb2O9, (b) the discharged phase at 1 V, and (c) the charged phase at 3 V.
Figure 9. Capacity versus cycle number for the Ti2Nb2O9 electrode containing 20 wt % acetylene black. The potential window is 3-1 V.
triangles), which are associated with the rows of bridging oxygens and empty tunnels running along b a, are not visible anymore at the level of the defective slab. The local formation of this nanophase could be explained by a collapsing or shearing mechanism of a part of the structure in the course of the topotactic reaction. The presence of these large defective slices is at the origin of the periodicity variation along b c and of superdislocations, visible by viewing at grazing incidence (white arrows in Figure 6a). These phenomena lead to strong strain effects that are released on the weaker points of the structure, i.e., at the oxygen atoms bridging two blocks of octahedra, in the tunnel rows. They appear to be associated with the void formation, which would be caused by small flakes leaving the crystallite to release the stress. From the overview BF images, the maximum length, width, and thickness (Figure
4a) of the flakes are on the order of a few nanometers. The latter point is illustrated by the two enlarged images picked up in two zones denoted “H” (Figure 6b). In the upper one, the through-focus series confirmed that a real hole has been created, but in the thicker parts of the crystallites, more or less organized matter is still observed. This study explains the difficulty we faced to get a proper structure from XRD. However, the TEM images also show that the previously proposed hypothetical structure15 is consistent with the results and can be considered as the mean structure of this compound. Another important point deals with the lacelike morphology of the crystallites, which exhibit a nonordered but oriented nanoporosity. This nanoporosity combined to the empty tunnels of the structure could be a great advantage for the insertion of lithium in this material. Moreover, we recently have evidenced lithium insertion in another titanoniobate derived from HTiNbO5. The lamellar oxide LiTiNbO5 exhibits indeed a reversible insertion of 0.8 lithium per formula, leading to a 92 mAh/g capacity through a first-order transformation.13 We have thus investigated the electrochemical behavior of Ti2Nb2O9 against lithium. Electrochemical Behavior versus Lithium. The theoretical specific capacity for Ti2Nb2O9 is 252 mAh/g if four lithium atoms per formula could be inserted into the structure. Indeed, we expect to play with two redox couples, Ti4+/ Ti3+ and Nb5+/Nb4+, because these two redox couples are very close to each other at 1.6 V vs Li/Li+, according to the (16) Cava, R. J.; Murphy, D. W.; Zahurak, S. M. J. Electrochem. Soc. 1983, 130, 2345. (17) Patoux, S.; Dolle, M.; Rousse, G.; Masquelier, C. J. Electrochem. Soc. 2002, 149, A391.
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mechanism. This characteristic is confirmed by the XRD pattern of an electrode at the end of the first discharge and charge, shown in Figure 8. Upon cycling, very few changes are observed: a slight amorphization happens and some intensity varies as a result of the difference valence of the transition metals, but no new peak or significant shift is observed. This difference of the mechanism for the insertion of lithium between the 2D LiTiNbO5 and the 3D Ti2Nb2O9 can be explained by the rigidity of the 3D structure compared to the lamellar structure, preventing important structural change. Moreover, upon further cycling, a stable reversible capacity of 125 mAh/g is obtained even after 30 cycles (Figure 9). Thus, the empty space offered by the tunnels in Ti2Nb2O9 allows lithium to be accommodated without any structural change. However, the crystallite habitus showing a lace-type morphology characterized by a “nanoporosity” should enhance the lithium diffusion toward the material. In order to access to the diffusion coefficient, we engaged a potentiodynamic titration study. The potentiodynamic protocol used for the study was as follows: we performed a stepwise scanning of the potential, with 10 mV steps, keeping the potential levels for a defined duration of 1 h (Figure 10a) or until the current has decayed to a preset minimum limit that we express in terms of the equivalent galvanostatic rate C/30 (Figure 10b).18 Recording the chronoamperometric responses of the system during every potential level gives access to the evolution of the kinetics with the redox level. The exponential current decay for each potential step (Figure 10c), concomitantly with the S-shaped potential curve, evidence a diffusion-limited process. In the long time approximation t . L2⁄D, where L is the diffusion length (0.5 µm, half of the particle size), the chronoamperograph follows the equation 2FS(Cs - C0)D π2Dt I(t) ) exp L 4L2 where F is the Faraday constant, S is the electrochemically active surface area, and (Cs - C0) is the concentration difference of the lithium at the interface.18 Thus, we found the diffusion value to be equal to 10-12 cm2/s. Interestingly, this value is higher than the diffusion coefficient given for LiFePO4.19 The large tunnels and the nanoporosity due to the “holes” seen in the microscopic study may promote a high ionic diffusion by giving to lithium a lot of access points to the structure and easy pathways in the material. To summarize, from the electrochemical study, we find that Ti2Nb2O9 can reversibly intercalate 2.3 lithium atoms through a solid-solution insertion at 1.57 V. No clear evidence on the derivative curve (Figure 8) is observed to determine which redox couple is involved in the reversible phenomena. In fact, the random cationic distribution between Ti4+ and Nb5+ and their similar redox potentials prevent us from gaining more detailed insight into this mechanism. It is therefore worth noting that no insertion is possible in the isotypic phase KTi3NbO9 (Figure 1c) because of the absence of available sites for the lithium ions. The good lithium
[
Figure 10. PITT measurements during the first discharge of Ti2Nb2O9: (a) limitation of the potential step in duration of 1 h; (b) limitation of the potential step by an equivalent galvanic current Ilimit ) IC/30; (c) enlargement of the current response showing the exponential decay.
literature.16,17 As reported in Figure 7, three lithium atoms are intercalated during the first discharge at a C/10 rate. This content, lower than the theoretical one, is mainly due to the rate dependence of the insertion. Indeed, for a C/100 rate, we observe the insertion of four lithium atoms, as expected (not shown). This rate dependence also affects the reversible capacity, as shown in Table 1. We clearly observe an increase of the capacity from 1.6 lithium atoms for a C/5 rate to 2.6 lithium atoms for a C/20 rate, but then no significant increase is seen for a slower rate than C/60. Interestingly, the potential-composition curve does not show a biphasic insertion process as observed on LiTiNbO5 but an S-shaped voltage profile with a half-discharge potential at about 1.57 V vs Li/Li+. In addition, the large full width at half-maximum on the derivative curve (inset of Figure 7) confirms that the insertion proceeds through a solid-solution
]
(18) Thompson, A. H. J. Electrochem. Soc. 1979, 126, 609. (19) Prosini, P. P.; Lisi, M.; Zane, D.; Pasquali, M. Solid State Ionics 2002, 148, 45.
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diffusion coefficient is explained by the special morphology of the crystallites at the nanoscale. Conclusion In conclusion, this study of the oxide Ti2Nb2O9 with a tunnel structure shows that it can intercalate reversibly lithium via a solid-solution mechanism, leading to the phase Li4Ti2Nb2O9, whereas the parent isotypic phase KTi3NbO9 can only contain one inserted cation per formula. Remarkably, the reversibility of the insertion/extraction of this phase is good, leading to a capacity of 125 mAh/g after three cycles.
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The TEM study of this oxide shows that, besides the tunnel character of the structure, another factor, its nanoporosity, resulting from the topotactic dehydration of HTiNbO5, may play a crucial role in the reversible insertion of lithium in this compound. Acknowledgment. We gratefully acknowledge the CNRS and the Ministry of Education and Research for financial support through their Research, Strategic, and Scholarship programs and the European Union for support through the network of excellence FAME. CM702978G
