- Email: [email protected]
Electrochemical lithium and sodium intercalation into the tantalum-rich layered chalcogenides Ta2Se and Ta2Te3
Journal of Alloys and Compounds 282 (1999) 93–100
L
Electrochemical lithium and sodium intercalation into the tantalum-rich layered chalcogenides Ta 2 Se and Ta 2 Te 3 a b b b a, P. Lavela , M. Conrad , A. Mrotzek , B. Harbrecht , J.L. Tirado * a
´ ´ ´ ´ , Facultad de Ciencias, Universidad de Cordoba , Avda. San Alberto Magno s /n, 14004 Cordoba , Spain Laboratorio de Quımica Inorganica b ¨ Marburg, Fachbereich Chemie, Hans-Meerwein-Strasse 1, D-35043 Marburg, Germany Philipps-Universitat Received 19 August 1998
Abstract Two-layered tantalum chalcogenides are evaluated as alkali metal intercalation hosts in lithium and sodium electrochemical cells. The metal-rich pseudo-two-dimensional solid Ta 2 Se shows a poor intercalation behaviour. Lithium reacts with the selenide by deintercalating selenium from the blocks of Ta-related b.c.c. structure leading to a collapse of the structure and the formation of tantalum metal. Sodium is reversibly intercalated to a limited extent leading to complex structural changes in the selenide, as revealed by electron diffraction. The two-dimensional telluride Ta 2 Te 3 allows a topotactic intercalation of lithium below 1 F / mol, while a more extended reaction leads to sample amorphization. The better intercalation behaviour of this solid can be related with the one-atom thick metal layer and the van der Waals gap separating tellurium atoms of successive layers. Sodium can be reversibly intercalated into Ta 2 Te 3 in sodium cells which show a good cycling behaviour. Exposure of the intercalated solid to water vapour allows the preparation of hydrated products with a monolayer or a bilayer of water molecules solvating sodium in the interlayer space. 1999 Elsevier Science S.A. All rights reserved. Keywords: Metal chalcogenides; Tantalum selenide; Tantalum telluride; Alkali metal intercalation
1. Introduction A remarkable progress in the understanding of the structure and chemistry of metal chalcogenides has been achieved during the last decade. The stoichiometry of these compounds span from chalcogen-rich solids, particularly of highly electropositive alkali metals such as Cs 3 Te 22 [1], to metal-rich chalcogenides of the transition elements (e.g. Ta 6 S [2], Ti 9 Se 2 [3], Zr 3 Te [4], Ti 11 Se 4 [5], Sc 8 Te 3 [6], Ta 5 Q 2 [7], M 2 Q [8,9], M 9 Q 5 [10], Ta 5 Q 3 [11] and Ta 3 Q 2 [12]), through different intermediate compositions (e.g. Ta 2 Te 3 [13] and TaTe 2 [14] in the Ta–Te system). Among transition metal chalcogenides, low-dimensional structures and metal clustering are frequently found for different stoichiometries. A relevant example of a layered metal-rich material is provided by Ta 2 Se [8] and the structurally similar ternary compounds Ta 2 (Se,Te) and Ta 2 (S,Se) [9]. These are quasi-two-dimensional solids and their structures depart largely from the sandwich structures of layered MQ 2 solids. Remarkably, the layers of metal and chalcogen atoms are arranged in the same way as in *Corresponding author. Tel.: 134-957-218637; fax: 134-957-218606; e-mail: [email protected]
the structure of b.c.c. tantalum. Alternatively, these solids can be formally seen as the result of an oxidative insertion of Q into b.c.c. tantalum. A different structural arrangement is provided by Ta 2 Te 3 [14]. In this compound, Ta atoms are arranged in corrugated single layers, sandwiched by puckered, closepacked layers of Te atoms. The presence of short Ta–Ta intralayer distances (297.1–309.7 pm) allows a denser packing of metal atoms in the layers as compared to the CdI 2 - type-related solid TaTe 2 . However, the distances between tellurium atoms belonging to consecutive layers are sufficiently long (.372.9 pm) to consider this solid as a quasi-two-dimensional solid with van der Waals interlayer interactions. Metal clustering was also studied in the layers of transition metal ditellurides. A clear dependence of the structural modulations on the d-electron count was found in these compounds. The specific structural distortions of the group 5 metal ditellurides MTe 2 originate from the excess valence electron density available for M–M bonding. De- populated antibonding states of Te p character give rise to interlayer Te–Te interactions, thus enhancing the d-electron count of the metal ions from d 1 to about d 4 / 3 . According to the model of hidden 1D bands [15], the
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00812-3
94
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
resulting d-electron count leads to metal–metal interactions in two different directions within each layer, which result in metal ribbon-chains running parallel to the layers and defining an in-plane monoclinic C2 / m supercell. As well as in other structurally simpler two-dimensional dichalcogenides, alkali metal intercalation has been reported in layered ditellurides [16,17]. The aim of this work is to study comparatively the intercalation aptitude towards alkali metals (Li and Na) and / or hydration at the interlayer space of the quasi-twodimensional tantalum chalcogenides Ta 2 Te 3 and Ta 2 Se. By using their metal-to-chalcogen ratio and their different distribution of metal atoms in the layers as compared with previously studied solids MQ 2 , a new insight on the factors relating intercalation to metal–metal interactions is presented.
2. Experimental details
2.1. Materials Polycrystalline Ta 2 Se and Ta 2 Te 3 samples were prepared as described elsewhere [8,13]. Sample manipulation was always carried out inside a glove box (M. Braun, water content monitored to be below 3 ppm) to avoid hydrolysis / oxidation.
2.2. Sample characterization X-ray powder diffractometry was carried out using a Siemens D5000 diffractometer furnished with Cu Ka radiation and a graphite monochromator. For intercalated phases, a plastic fiber was used to cover the sample in order to avoid undesirable reactions with air during the recording. Electron diffraction patterns were obtained by using a JEOL 200 CX apparatus. Thermogravimetric analysis was carried out with a Cahn 2000 electrobalance.
initial relaxation of the cells was allowed until the condition DV /Dt#1 mV h 21 was attained and the average alkali metal composition of the cathodic material was calculated by using Faraday’s law on the assumption that no current flow was due to side reactions. The SPES data were recorded with 210 mV h 21 voltage steps. The spectra were recorded in duplicate to ensure reproducibility. In the galvanostatic experiments, constant current densities were applied to the cells to achieve a charge transfer of 1 F / mol of active material in 30 h (C / 30).
3. Results and discussion
3.1. Characterization of starting materials: Ta2 Se and Ta2 Te3 Powder X-ray diffraction data of as-prepared Ta 2 Se reveals a set of 00l intense lines and few hkl reflections. The relative intensities basically agree with a LAZYPULVERIX simulation by using the structure model derived from single-crystal data [8], and assuming an enhanced effect of preferred orientation of the layered particles with the basal plane parallel to the substrate. Selected area electron diffraction data of plate-like particles recorded with zero tilt angle reveal preferred k001l zone patterns (Fig. 1), consisting of a square pattern of basic spots that can be indexed in the P4 / nmm space group. The powder X-ray diffraction data for Ta 2 Te 3 could be indexed in the monoclinic space group C2 / m. The unit cell dimensions refined from diffractometer data by using the AFFMA computer program were in excellent agreement with those reported from Guinier camera data [13] (Table 1). Fig. 2 shows k001l zone electron diffraction patterns for Ta 2 Te 3 and a Li-intercalated sample, exhibiting the extinctions of a base-centred monoclinic cell.
3.2. Electrochemical reactions with alkali metals 2.3. Electrochemical intercalation The electrochemical intercalation of lithium and sodium was studied in Li / 1 M LiClO 4 in propylene carbonate (PC):ethylene carbonate (EC)::1:1 /(Ta 2 Se or Ta 2 Te 3 ) and Na / 1 M NaClO 4 in PC /(Ta 2 Se or Ta 2 Te 3 ) test cells. The electrochemical cells were prepared inside the drybox by placing a clean alkali metal disk, two glass fibre separators soaked with the electrolyte solution and a pellet of the chalcogenide concerned into a Teflon container with two stainless steel terminals. The cathode pellets (7 mm diameter, 0.3–0.5 mm thick) were prepared by pressing 12–20 mg of metal chalcogenide on an inert steel grid. Galvanostatic discharge–charge cycles and step potential electrochemical spectroscopy (SPES) [18] of the cells were carried out at 258C by using a multi-channel microprocessor-controlled system (MacPile). In all experiments, an
3.2.1. Li /Ta2 Se Two-electrode test cells of the type Li / LiClO 4 (PC:EC) / Ta 2 Se were studied by recording the first galvanostatic discharge–charge cycle at C / 30 rate (Fig. 3). The discharge profile consists of a flat pseudo-plateau located at an average potential of 0.5 V, which extends up to nearly 1 F / mol. Further discharge results in a sharp decrease in cell voltage. The charge branch was recorded after discharge to ca. 0.9 F / mol. A poor reversibility of the reaction with lithium was evidenced by an enhanced polarization of the cell. This effect did not allow the extraction of significant amounts of lithium. Attempts to improve this behaviour by changing the experimental conditions (current density in galvanostatic mode or potentiostatic measurements) were unsuccessful. Powder X-ray diffraction and electron diffraction data of
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
Fig. 1. k001l zone electron diffraction patterns of (a) Ta 2 Se crystals and (b) Na 0.8 Ta 2 Se particles showing superstructure spots along [100].
the discharged cathodes reveal a loss of long-range order. New diffraction lines ascribable to poorly crystallized Ta metal occur almost exclusively for the deepest discharge. No selenium-containing phase was detected by powder X-ray diffraction, which can be considered indicative of the formation of non-crystalline lithium selenides. The amorphization process can be correlated with the shape of
95
Fig. 2. k001l zone electron diffraction patterns of (a) Ta 2 Te 3 and (b) LiTa 2 Te 3 .
the electrochemical discharge profile and the poor cyclability of the cells. Thus, it can be concluded that a true intercalation reaction of lithium does not take place under the different experimental conditions used in this study. Instead de-insertion of selenium leading to a struc-
Table 1 Lattice parameters of the host tantalum tellurides and intercalation compounds Compound
Lattice parameters
Reference
Ta 2 Se
a5337.5(1) pm c5983.2(1) pm a5337(2) pm c51155(5) pm a52047.1(5) pm b5349.5(2) pm c51222.4(3) pm b 5143.8(1)8 a52077(4) pm b5351.0(9) pm c51239(2) pm b 5144.3(9)8 a52049(2) pm b5350(1) pm c51361(9) pm b 5144(1)8
[8]
Na 0.8 Ta 2 Se Ta 2 Te 3
LiTa 2 Te 3
Na 0.5 Ta 2 Te 3
This work [13]
This work
This work Fig. 3. First discharge–charge cycle of lithium and sodium cells using Ta 2 Se as active electrode material.
96
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
tural collapse and the formation of poorly crystalline b.c.c. tantalum takes place.
3.2.2. Na /Ta2 Se Na / NaClO 4 (PC) / Ta 2 Se test cells were used in this study. Galvanostatic recordings reveal a more complex profile than in lithium cells (Fig. 3). The discharge profile consists of two low-slope regions at 0.4 V (between 0 and 0.4 F / mol) and 0.2 V (0.4–0.8 F / mol) separated by a more pronounced decrease in cell voltage. Reversibility is somehow better here than in the lithium case. Thus, ca 0.4 F / mol can be extracted after discharge with a profile located at higher voltages but which resembles the discharge profile. The voltage difference between both profiles can be a consequence of poor electronic transport properties of the solid. X-ray diffraction data support the intercalation reaction of sodium into Ta 2 Se. The discharged cathodes show a complex diffraction pattern. First, a set of 00l lines leading to a c parameter of ca 980 pm, which is ascribable to unreacted selenide. Besides, a new set of multiple-order reflections also appears in the diffraction pattern of the cathode material, which leads to a unit cell parameter of ca 1150 pm (Table 1). If this set of lines is ascribed to the sodium intercalated phase, an interlayer expansion of ca 172 pm can be derived. This value is slightly larger than that reported for layered dichalcogenides after sodium intercalation, e.g. from 636.2 pm in 2H–TaSe 2 to 770 pm in Na 0.6 TaSe 2 [19]. However, the particular geometry of the insertion sites of Ta 2 Se (square pyramidal if P4 / nmm 2c sites with z51 / 2 are assumed), as compared with ‘classical’ hosts (trigonal prismatic or trigonal antiprismatic), may account for this difference. The existence of two steps in the discharge curve may also have a structural origin. However, powder X-ray diffraction does not allow to recognise the phases involved in each step and it was not possible to prepare single crystals of the intercalated product with appropriate size for a deeper structural study. A more detailed information in the axb plane was obtained by electron diffraction. The product Na x Ta 2 Se of sodium intercalation into Ta 2 Se for x max (¯0.8) shows a complex pattern in the electron microscope, which is probably due to a nonhomogeneous reaction of the complete mass of the electrode material. The layered particles exhibit electron diffraction patterns which depart from that of the pristine solid in different extension. However, the following features were clearly discernible in the k001l zone patterns: the projection of the basic tetragonal structure onto (001) is complicated by different related superstructures which develop along [100]. These were progressively identified as 2axa and 4axa (Fig. 1b). Further departure from the pristine structure was also derived from the occurrence of streaks along [100], which are indicative for disorder phenomena. Still further reaction results in a complete loss of long-range order, as is concluded from the lack of
diffraction intensity. This multiphase mechanism affecting successive ordered stages agrees well with the electrochemical data. A description of these data may be carried out by using the concept proposed by Armand [20] of ‘insertion pressure’, a factor which is opposite to the existence of a large domain of non-stoichiometry and thus implies the ordering of the cations intercalated in layered materials. The pseudo-multiphase mechanism is complicated here by the possible changes in metal–metal interactions resulting from electron donation from the incoming sodium atoms to the transition metal energy levels. This effect is known to induce modulations in layered dichalcogenides.
3.2.3. Li /Ta2 Te3 The galvanostatic discharge curves of two-electrode cells of the type Li / LiClO 4 (PC:EC) / Ta 2 Te 3 consist of a two-step profile (Fig. 4). A first pseudo-plateau is located at ca 1.2 V and extends to nearly 1 F / mol. At the end of this step, the diffractometer traces of the cathodes reveal a slight layer expansion, leading to the C2 / m parameters collected in Table 1. Electron diffraction data show that the basic k001l zone pattern remains basically unchanged (Fig. 2b). The cell dimension values may be consistent with the topotactic intercalation of small size Li 1 ions in the interlayer space. The small expansion along [001] is also consistent with the relatively long interlayer distances in the pristine solid, resulting from the absence of close Te–Te contacts. These contact have to be broken in other tellurides such as TaTe 2 [18], which lead to a slightly more pronounced expansion. At the end of the first discharge plateau, there is a switch to cell voltages close to 0.4 V leading to an extended plateau up to 3 F / mol. The nature of this second step is completely different to the first
Fig. 4. First discharge–charge cycle of lithium cells using Ta 2 Te 3 as active electrode material and discharged to two different depths in F / mol.
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
Fig. 5. Successive galvanostatic cycles of Li / Ta 2 Te 3 and Na / Ta 2 Te 3 cells.
97
intercalation process, as both X-ray and electron diffraction data evidence a dramatic amorphization of the solid. An indirect proof of this interpretation comes from the fact that the reversibility of the first step is high, while the second process is clearly irreversible. In order to check the cathode performance, several cycles were recorded near the 1-V region (Fig. 5). A good capacity retention was obtained. Such behaviour may be interesting from the battery technology point of view. Of course, these materials have the limitations which are inherent to their large weights and not being environment friendly, but for certain applications, such as microbatteries, this may not be more important than cycling performance. In order to complement the characterization of the lithium intercalation reaction into Ta 2 Te 3 , the chemical diffusion coefficients of lithium were evaluated in the 1.5–1.85-F / mol interval, where the galvanostatic discharge curves did not show evidence of a multiphase mechanism. For D evaluation purposes, step potential electrochemical spectroscopy (SPES) studies of the lithium cells were carried out at 10-mV/ h steps (Fig. 6). It is worth noting that the intensity versus potential plots for lithium show two well-defined reduction peaks which agree well with the two steps found in the galvanostatic experiments. Moreover, current relaxation was recorded for each potential step and were characteristic of a slow lithium ion diffusion into the host structure previous to the first peak of the voltammogram (Fig. 7). The values of D obtained in this range of composition were 10 26.2 to 10 26.9 cm 2 / s,
Fig. 6. Step potential electrochemical spectroscopy (SPES) of lithium and sodium cells using Ta 2 Te 3 as active cathode material. Left, voltage versus composition; right, cell current versus voltage.
98
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
Fig. 7. Current relaxation during discharge (left) and charge (right) of Li / Ta 2 Te 3 and Na / Ta 2 Te 3 cells.
which are larger in magnitude to those previously reported for the composition-related compound TaTe 2 . The difference may be related with the fact that tantalum ditelluride shows a more pronounced interlayer interaction resulting in shorter interlayer Te–Te distances. The diffusion of lithium may be inhibited by these interactions.
3.2.4. Na /Ta2 Te3 Finally, the possible sodium intercalation into Ta 2 Te 3 was evaluated in Na / NaClO 4 (PC) / telluride test cells by both galvanostatic and SPES recordings. These techniques reveal a complex profile with a first quasi-plateau at 1.5 V (Fig. 6). The X-ray diffraction data evidence a significant expansion of the interlayer space (Table 1), while electron microscopy shows that lattice dimensions in the axb plane remain basically unaffected, thus giving additional evidence of the topotactic intercalation process. Current relaxation in the potential steps of the SPES experiments also show a smooth relaxation by sodium ion diffusion (Fig. 7). The lower diffusion coefficients computed for sodium (10 28.1 to 10 28.6 cm 2 / s) as compared with lithium (Fig. 8) are indicative of the larger size of the latter ions and the more pronounced interlayer expansion required for the intercalation of sodium. Nevertheless, capacity retention during galvanostatic cycling is better as compared with lithium cells (Fig. 5). Irreversible reactions with lithium may take place for prolonged cycling due to the larger polarization effect caused by small lithium ions.
3.3. Sodium hydration in the interlayer space On exposing the sodium intercalated solid Na 0.5 Ta 2 Te 3 to water vapour, an increase in weight was monitored in
Fig. 8. Plot of the diffusion coefficients versus composition obtained from the first steps of potentiostatic discharge of Li / Ta 2 Te 3 and Na / Ta 2 Te 3 cells.
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
99
layer of water. Similar processes have also been established in other complex layered chalcogenides, like misfit layer compounds [21].
4. Conclusions
Fig. 9. Successive powder X-ray diffraction patterns of electrochemically intercalated Na 0.5 Ta 2 Te 3 exposed to water vapour: 001 Miller indices correspond to unreacted solid; 0019 indices to a 1020-pm phase with Na 0.5 (H 2 O) 0.3 Ta 2 Te 3 stoichiometry and 0010 and 0020 to Na 0.5 (H 2 O) 0.6 Ta 2 Te 3 with a bilayer of water.
the electrobalance. In addition, the XRD patterns (Fig. 9) reveal the progressive occurrence of two sets of extra lines. The occurrence of a 1275-pm phase, which can be interpreted as an intercalated phase, in which the expansion of 476 pm relative to c?sin b of the anhydrous intercalate in Table 1 (799 pm), suggests possible cointercalation of solvating water molecules. After the equilibrium was reached, thermogravimetric recordings were performed (Fig. 10). From this plot, a two-step process is found that can be associated to the occurrence of the above two phases: with ca 1275-pm basal spacing Na 0.5 (H 2 O) 0.6 Ta 2 Te 3 with a bilayer of water, and with a 1023-pm phase with Na 0.5 (H 2 O) 0.3 Ta 2 Te 3 with a mono-
From the present knowledge on the intercalation chemistry of tantalum chalcogenides and the results shown above, different relationships can be derived: (1) Irrespective of its layered structure, the metal-rich selenide Ta 2 Se, possessing four-atom thick layers structurally related to b.c.c. tantalum, is less prone to intercalation than other solids with higher Q / Ta ratio. The electrochemical reaction with lithium extends up to 2 F / mol and leads to the irreversible elimination of the selenium atoms separating consecutive layered blocks, which results in a collapse of the quasi-two-dimensional structure with the formation of microcrystalline tantalum and—presumably— amorphous lithium selenide, etc. The process may be regarded as a de-insertion of the chalcogen by reaction with lithium ions simultaneous to a reduction of the host tantalum. Nevertheless, sodium can be intercalated to a limited extent, fixed by the low oxidation state of tantalum in the chalcogenide. (2) Layered Ta 2 Te 3 shows a more extended intercalation behaviour, as lithium and sodium can be incorporated in the interlayer space. It should be noted that the presence of metal–metal bonds within the layers of the tantalum sublattice does not inhibit the intercalation of alkali metals. A similar behaviour was also found in tantalum ditelluride [17]. However, as compared with TaTe 2 , the absence of strong interlayer interactions is more evident for Ta 2 Te 3 . Nevertheless, lithium intercalation takes place at cell potentials unusually low with broad signals in the SPES spectra. This phenomenon may be indicative of an incomplete oxidation of lithium. Recently a similar effect was reported in carbon hosts, although in that case, lithium intercalation takes place closer to 0 V [22]. A poor lithium diffusivity is also in agreement with this assumption. On the contrary, the more electropositive sodium allows a complete electron donation to the host, irrespective of the high packing density of metal atoms in the one-atom thick tantalum layers of Ta 2 Te 3 .
References
Fig. 10. Thermogravimetric analysis curves of the fully hydrated derivative Na 0.5 (H 2 O) 0.6 Ta 2 Te 3 .
[1] W.S. Sheldrick, M. Wachhold, S. Jobic, R. Brec, E. Canadell, Adv. Mater. 9 (1997) 669. [2] H.F. Franzen, T.A. Beineke, B.R. Conrad, Acta Crystallogr., Sect. B 4 (1968) 412. ¨ [3] T.E. Weirich, A. Simon, R. Pottgen, Z. Anorg. Allg. Chem. 622 (1996) 630. [4] B. Harbrecht, R. Leersch, J. Alloys Compounds 238 (1996) 13. ¨ [5] T.E. Weirich, R. Ramlau, A. Simon, S. Hovmoller, X. Zou, Nature 382 (1996) 144.
100
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
[6] P.A. Maggard, J.D. Corbett, Inorg. Chem. 37 (1998) 814. [7] T. Degen, B. Harbrecht, Angew. Chem. Int. Ed. Engl. 34 (1995) 2703. [8] B. Harbrecht, Angew. Chem. Int. Ed. Engl. 28 (1989) 1660. [9] B. Harbrecht, T. Degen, M. Conrad, J. Alloys Compounds 246 (1997) 37. [10] T. Degen, B. Harbrecht, Angew. Chem. 107 (1995) 1226. [11] M. Conrad, B. Harbrecht, Z. Anorg. Allg. Chem. 623 (1997) 742. [12] T. Degen, B. Harbrecht, Acta Crystallogr., Sect. C 51 (1995) 2218. [13] M. Conrad, B. Harbrecht, J. Alloys Compounds 187 (1992) 181. [14] B.E. Brown, Acta Crystallogr. 20 (1966) 264. [15] M.H. Whangbo, E. Canadell, J. Am. Chem. Soc. 114 (1992) 9587.
´ J. Morales, J.L. Tirado, Inorg. Chem. 33 (1994) 3164. [16] R. Guzman, ´ J. Morales, J.L. Tirado, Chem. Mater. 7 (1995) 1171. [17] R. Guzman, [18] Y. Chabre, D. Djurado, M. Armand, W.R. Romanow, N. Coustel, J.P. McCauley, J.E. Fischer, A.B. Smith, J. Am. Chem. Soc. 114 (1992) 764. [19] T. Hibma, Intercalation Chemistry in: M.S. Whittingham, A.J. Jacobson (Eds.), Academic Press, New York, 1982. [20] M. Armand, Ph.D. Thesis, Univ. Grenoble, 1978. [21] P. Lavela, J. Morales, J.L. Tirado, Chem. Mater. 4 (1992) 2. [22] Y. Nakagawa, S. Wang, Y. Matsumura, C. Yamaguchi, Synthetic Metals 85 (1997) 1363.
L
Electrochemical lithium and sodium intercalation into the tantalum-rich layered chalcogenides Ta 2 Se and Ta 2 Te 3 a b b b a, P. Lavela , M. Conrad , A. Mrotzek , B. Harbrecht , J.L. Tirado * a
´ ´ ´ ´ , Facultad de Ciencias, Universidad de Cordoba , Avda. San Alberto Magno s /n, 14004 Cordoba , Spain Laboratorio de Quımica Inorganica b ¨ Marburg, Fachbereich Chemie, Hans-Meerwein-Strasse 1, D-35043 Marburg, Germany Philipps-Universitat Received 19 August 1998
Abstract Two-layered tantalum chalcogenides are evaluated as alkali metal intercalation hosts in lithium and sodium electrochemical cells. The metal-rich pseudo-two-dimensional solid Ta 2 Se shows a poor intercalation behaviour. Lithium reacts with the selenide by deintercalating selenium from the blocks of Ta-related b.c.c. structure leading to a collapse of the structure and the formation of tantalum metal. Sodium is reversibly intercalated to a limited extent leading to complex structural changes in the selenide, as revealed by electron diffraction. The two-dimensional telluride Ta 2 Te 3 allows a topotactic intercalation of lithium below 1 F / mol, while a more extended reaction leads to sample amorphization. The better intercalation behaviour of this solid can be related with the one-atom thick metal layer and the van der Waals gap separating tellurium atoms of successive layers. Sodium can be reversibly intercalated into Ta 2 Te 3 in sodium cells which show a good cycling behaviour. Exposure of the intercalated solid to water vapour allows the preparation of hydrated products with a monolayer or a bilayer of water molecules solvating sodium in the interlayer space. 1999 Elsevier Science S.A. All rights reserved. Keywords: Metal chalcogenides; Tantalum selenide; Tantalum telluride; Alkali metal intercalation
1. Introduction A remarkable progress in the understanding of the structure and chemistry of metal chalcogenides has been achieved during the last decade. The stoichiometry of these compounds span from chalcogen-rich solids, particularly of highly electropositive alkali metals such as Cs 3 Te 22 [1], to metal-rich chalcogenides of the transition elements (e.g. Ta 6 S [2], Ti 9 Se 2 [3], Zr 3 Te [4], Ti 11 Se 4 [5], Sc 8 Te 3 [6], Ta 5 Q 2 [7], M 2 Q [8,9], M 9 Q 5 [10], Ta 5 Q 3 [11] and Ta 3 Q 2 [12]), through different intermediate compositions (e.g. Ta 2 Te 3 [13] and TaTe 2 [14] in the Ta–Te system). Among transition metal chalcogenides, low-dimensional structures and metal clustering are frequently found for different stoichiometries. A relevant example of a layered metal-rich material is provided by Ta 2 Se [8] and the structurally similar ternary compounds Ta 2 (Se,Te) and Ta 2 (S,Se) [9]. These are quasi-two-dimensional solids and their structures depart largely from the sandwich structures of layered MQ 2 solids. Remarkably, the layers of metal and chalcogen atoms are arranged in the same way as in *Corresponding author. Tel.: 134-957-218637; fax: 134-957-218606; e-mail: [email protected]
the structure of b.c.c. tantalum. Alternatively, these solids can be formally seen as the result of an oxidative insertion of Q into b.c.c. tantalum. A different structural arrangement is provided by Ta 2 Te 3 [14]. In this compound, Ta atoms are arranged in corrugated single layers, sandwiched by puckered, closepacked layers of Te atoms. The presence of short Ta–Ta intralayer distances (297.1–309.7 pm) allows a denser packing of metal atoms in the layers as compared to the CdI 2 - type-related solid TaTe 2 . However, the distances between tellurium atoms belonging to consecutive layers are sufficiently long (.372.9 pm) to consider this solid as a quasi-two-dimensional solid with van der Waals interlayer interactions. Metal clustering was also studied in the layers of transition metal ditellurides. A clear dependence of the structural modulations on the d-electron count was found in these compounds. The specific structural distortions of the group 5 metal ditellurides MTe 2 originate from the excess valence electron density available for M–M bonding. De- populated antibonding states of Te p character give rise to interlayer Te–Te interactions, thus enhancing the d-electron count of the metal ions from d 1 to about d 4 / 3 . According to the model of hidden 1D bands [15], the
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00812-3
94
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
resulting d-electron count leads to metal–metal interactions in two different directions within each layer, which result in metal ribbon-chains running parallel to the layers and defining an in-plane monoclinic C2 / m supercell. As well as in other structurally simpler two-dimensional dichalcogenides, alkali metal intercalation has been reported in layered ditellurides [16,17]. The aim of this work is to study comparatively the intercalation aptitude towards alkali metals (Li and Na) and / or hydration at the interlayer space of the quasi-twodimensional tantalum chalcogenides Ta 2 Te 3 and Ta 2 Se. By using their metal-to-chalcogen ratio and their different distribution of metal atoms in the layers as compared with previously studied solids MQ 2 , a new insight on the factors relating intercalation to metal–metal interactions is presented.
2. Experimental details
2.1. Materials Polycrystalline Ta 2 Se and Ta 2 Te 3 samples were prepared as described elsewhere [8,13]. Sample manipulation was always carried out inside a glove box (M. Braun, water content monitored to be below 3 ppm) to avoid hydrolysis / oxidation.
2.2. Sample characterization X-ray powder diffractometry was carried out using a Siemens D5000 diffractometer furnished with Cu Ka radiation and a graphite monochromator. For intercalated phases, a plastic fiber was used to cover the sample in order to avoid undesirable reactions with air during the recording. Electron diffraction patterns were obtained by using a JEOL 200 CX apparatus. Thermogravimetric analysis was carried out with a Cahn 2000 electrobalance.
initial relaxation of the cells was allowed until the condition DV /Dt#1 mV h 21 was attained and the average alkali metal composition of the cathodic material was calculated by using Faraday’s law on the assumption that no current flow was due to side reactions. The SPES data were recorded with 210 mV h 21 voltage steps. The spectra were recorded in duplicate to ensure reproducibility. In the galvanostatic experiments, constant current densities were applied to the cells to achieve a charge transfer of 1 F / mol of active material in 30 h (C / 30).
3. Results and discussion
3.1. Characterization of starting materials: Ta2 Se and Ta2 Te3 Powder X-ray diffraction data of as-prepared Ta 2 Se reveals a set of 00l intense lines and few hkl reflections. The relative intensities basically agree with a LAZYPULVERIX simulation by using the structure model derived from single-crystal data [8], and assuming an enhanced effect of preferred orientation of the layered particles with the basal plane parallel to the substrate. Selected area electron diffraction data of plate-like particles recorded with zero tilt angle reveal preferred k001l zone patterns (Fig. 1), consisting of a square pattern of basic spots that can be indexed in the P4 / nmm space group. The powder X-ray diffraction data for Ta 2 Te 3 could be indexed in the monoclinic space group C2 / m. The unit cell dimensions refined from diffractometer data by using the AFFMA computer program were in excellent agreement with those reported from Guinier camera data [13] (Table 1). Fig. 2 shows k001l zone electron diffraction patterns for Ta 2 Te 3 and a Li-intercalated sample, exhibiting the extinctions of a base-centred monoclinic cell.
3.2. Electrochemical reactions with alkali metals 2.3. Electrochemical intercalation The electrochemical intercalation of lithium and sodium was studied in Li / 1 M LiClO 4 in propylene carbonate (PC):ethylene carbonate (EC)::1:1 /(Ta 2 Se or Ta 2 Te 3 ) and Na / 1 M NaClO 4 in PC /(Ta 2 Se or Ta 2 Te 3 ) test cells. The electrochemical cells were prepared inside the drybox by placing a clean alkali metal disk, two glass fibre separators soaked with the electrolyte solution and a pellet of the chalcogenide concerned into a Teflon container with two stainless steel terminals. The cathode pellets (7 mm diameter, 0.3–0.5 mm thick) were prepared by pressing 12–20 mg of metal chalcogenide on an inert steel grid. Galvanostatic discharge–charge cycles and step potential electrochemical spectroscopy (SPES) [18] of the cells were carried out at 258C by using a multi-channel microprocessor-controlled system (MacPile). In all experiments, an
3.2.1. Li /Ta2 Se Two-electrode test cells of the type Li / LiClO 4 (PC:EC) / Ta 2 Se were studied by recording the first galvanostatic discharge–charge cycle at C / 30 rate (Fig. 3). The discharge profile consists of a flat pseudo-plateau located at an average potential of 0.5 V, which extends up to nearly 1 F / mol. Further discharge results in a sharp decrease in cell voltage. The charge branch was recorded after discharge to ca. 0.9 F / mol. A poor reversibility of the reaction with lithium was evidenced by an enhanced polarization of the cell. This effect did not allow the extraction of significant amounts of lithium. Attempts to improve this behaviour by changing the experimental conditions (current density in galvanostatic mode or potentiostatic measurements) were unsuccessful. Powder X-ray diffraction and electron diffraction data of
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
Fig. 1. k001l zone electron diffraction patterns of (a) Ta 2 Se crystals and (b) Na 0.8 Ta 2 Se particles showing superstructure spots along [100].
the discharged cathodes reveal a loss of long-range order. New diffraction lines ascribable to poorly crystallized Ta metal occur almost exclusively for the deepest discharge. No selenium-containing phase was detected by powder X-ray diffraction, which can be considered indicative of the formation of non-crystalline lithium selenides. The amorphization process can be correlated with the shape of
95
Fig. 2. k001l zone electron diffraction patterns of (a) Ta 2 Te 3 and (b) LiTa 2 Te 3 .
the electrochemical discharge profile and the poor cyclability of the cells. Thus, it can be concluded that a true intercalation reaction of lithium does not take place under the different experimental conditions used in this study. Instead de-insertion of selenium leading to a struc-
Table 1 Lattice parameters of the host tantalum tellurides and intercalation compounds Compound
Lattice parameters
Reference
Ta 2 Se
a5337.5(1) pm c5983.2(1) pm a5337(2) pm c51155(5) pm a52047.1(5) pm b5349.5(2) pm c51222.4(3) pm b 5143.8(1)8 a52077(4) pm b5351.0(9) pm c51239(2) pm b 5144.3(9)8 a52049(2) pm b5350(1) pm c51361(9) pm b 5144(1)8
[8]
Na 0.8 Ta 2 Se Ta 2 Te 3
LiTa 2 Te 3
Na 0.5 Ta 2 Te 3
This work [13]
This work
This work Fig. 3. First discharge–charge cycle of lithium and sodium cells using Ta 2 Se as active electrode material.
96
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
tural collapse and the formation of poorly crystalline b.c.c. tantalum takes place.
3.2.2. Na /Ta2 Se Na / NaClO 4 (PC) / Ta 2 Se test cells were used in this study. Galvanostatic recordings reveal a more complex profile than in lithium cells (Fig. 3). The discharge profile consists of two low-slope regions at 0.4 V (between 0 and 0.4 F / mol) and 0.2 V (0.4–0.8 F / mol) separated by a more pronounced decrease in cell voltage. Reversibility is somehow better here than in the lithium case. Thus, ca 0.4 F / mol can be extracted after discharge with a profile located at higher voltages but which resembles the discharge profile. The voltage difference between both profiles can be a consequence of poor electronic transport properties of the solid. X-ray diffraction data support the intercalation reaction of sodium into Ta 2 Se. The discharged cathodes show a complex diffraction pattern. First, a set of 00l lines leading to a c parameter of ca 980 pm, which is ascribable to unreacted selenide. Besides, a new set of multiple-order reflections also appears in the diffraction pattern of the cathode material, which leads to a unit cell parameter of ca 1150 pm (Table 1). If this set of lines is ascribed to the sodium intercalated phase, an interlayer expansion of ca 172 pm can be derived. This value is slightly larger than that reported for layered dichalcogenides after sodium intercalation, e.g. from 636.2 pm in 2H–TaSe 2 to 770 pm in Na 0.6 TaSe 2 [19]. However, the particular geometry of the insertion sites of Ta 2 Se (square pyramidal if P4 / nmm 2c sites with z51 / 2 are assumed), as compared with ‘classical’ hosts (trigonal prismatic or trigonal antiprismatic), may account for this difference. The existence of two steps in the discharge curve may also have a structural origin. However, powder X-ray diffraction does not allow to recognise the phases involved in each step and it was not possible to prepare single crystals of the intercalated product with appropriate size for a deeper structural study. A more detailed information in the axb plane was obtained by electron diffraction. The product Na x Ta 2 Se of sodium intercalation into Ta 2 Se for x max (¯0.8) shows a complex pattern in the electron microscope, which is probably due to a nonhomogeneous reaction of the complete mass of the electrode material. The layered particles exhibit electron diffraction patterns which depart from that of the pristine solid in different extension. However, the following features were clearly discernible in the k001l zone patterns: the projection of the basic tetragonal structure onto (001) is complicated by different related superstructures which develop along [100]. These were progressively identified as 2axa and 4axa (Fig. 1b). Further departure from the pristine structure was also derived from the occurrence of streaks along [100], which are indicative for disorder phenomena. Still further reaction results in a complete loss of long-range order, as is concluded from the lack of
diffraction intensity. This multiphase mechanism affecting successive ordered stages agrees well with the electrochemical data. A description of these data may be carried out by using the concept proposed by Armand [20] of ‘insertion pressure’, a factor which is opposite to the existence of a large domain of non-stoichiometry and thus implies the ordering of the cations intercalated in layered materials. The pseudo-multiphase mechanism is complicated here by the possible changes in metal–metal interactions resulting from electron donation from the incoming sodium atoms to the transition metal energy levels. This effect is known to induce modulations in layered dichalcogenides.
3.2.3. Li /Ta2 Te3 The galvanostatic discharge curves of two-electrode cells of the type Li / LiClO 4 (PC:EC) / Ta 2 Te 3 consist of a two-step profile (Fig. 4). A first pseudo-plateau is located at ca 1.2 V and extends to nearly 1 F / mol. At the end of this step, the diffractometer traces of the cathodes reveal a slight layer expansion, leading to the C2 / m parameters collected in Table 1. Electron diffraction data show that the basic k001l zone pattern remains basically unchanged (Fig. 2b). The cell dimension values may be consistent with the topotactic intercalation of small size Li 1 ions in the interlayer space. The small expansion along [001] is also consistent with the relatively long interlayer distances in the pristine solid, resulting from the absence of close Te–Te contacts. These contact have to be broken in other tellurides such as TaTe 2 [18], which lead to a slightly more pronounced expansion. At the end of the first discharge plateau, there is a switch to cell voltages close to 0.4 V leading to an extended plateau up to 3 F / mol. The nature of this second step is completely different to the first
Fig. 4. First discharge–charge cycle of lithium cells using Ta 2 Te 3 as active electrode material and discharged to two different depths in F / mol.
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
Fig. 5. Successive galvanostatic cycles of Li / Ta 2 Te 3 and Na / Ta 2 Te 3 cells.
97
intercalation process, as both X-ray and electron diffraction data evidence a dramatic amorphization of the solid. An indirect proof of this interpretation comes from the fact that the reversibility of the first step is high, while the second process is clearly irreversible. In order to check the cathode performance, several cycles were recorded near the 1-V region (Fig. 5). A good capacity retention was obtained. Such behaviour may be interesting from the battery technology point of view. Of course, these materials have the limitations which are inherent to their large weights and not being environment friendly, but for certain applications, such as microbatteries, this may not be more important than cycling performance. In order to complement the characterization of the lithium intercalation reaction into Ta 2 Te 3 , the chemical diffusion coefficients of lithium were evaluated in the 1.5–1.85-F / mol interval, where the galvanostatic discharge curves did not show evidence of a multiphase mechanism. For D evaluation purposes, step potential electrochemical spectroscopy (SPES) studies of the lithium cells were carried out at 10-mV/ h steps (Fig. 6). It is worth noting that the intensity versus potential plots for lithium show two well-defined reduction peaks which agree well with the two steps found in the galvanostatic experiments. Moreover, current relaxation was recorded for each potential step and were characteristic of a slow lithium ion diffusion into the host structure previous to the first peak of the voltammogram (Fig. 7). The values of D obtained in this range of composition were 10 26.2 to 10 26.9 cm 2 / s,
Fig. 6. Step potential electrochemical spectroscopy (SPES) of lithium and sodium cells using Ta 2 Te 3 as active cathode material. Left, voltage versus composition; right, cell current versus voltage.
98
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
Fig. 7. Current relaxation during discharge (left) and charge (right) of Li / Ta 2 Te 3 and Na / Ta 2 Te 3 cells.
which are larger in magnitude to those previously reported for the composition-related compound TaTe 2 . The difference may be related with the fact that tantalum ditelluride shows a more pronounced interlayer interaction resulting in shorter interlayer Te–Te distances. The diffusion of lithium may be inhibited by these interactions.
3.2.4. Na /Ta2 Te3 Finally, the possible sodium intercalation into Ta 2 Te 3 was evaluated in Na / NaClO 4 (PC) / telluride test cells by both galvanostatic and SPES recordings. These techniques reveal a complex profile with a first quasi-plateau at 1.5 V (Fig. 6). The X-ray diffraction data evidence a significant expansion of the interlayer space (Table 1), while electron microscopy shows that lattice dimensions in the axb plane remain basically unaffected, thus giving additional evidence of the topotactic intercalation process. Current relaxation in the potential steps of the SPES experiments also show a smooth relaxation by sodium ion diffusion (Fig. 7). The lower diffusion coefficients computed for sodium (10 28.1 to 10 28.6 cm 2 / s) as compared with lithium (Fig. 8) are indicative of the larger size of the latter ions and the more pronounced interlayer expansion required for the intercalation of sodium. Nevertheless, capacity retention during galvanostatic cycling is better as compared with lithium cells (Fig. 5). Irreversible reactions with lithium may take place for prolonged cycling due to the larger polarization effect caused by small lithium ions.
3.3. Sodium hydration in the interlayer space On exposing the sodium intercalated solid Na 0.5 Ta 2 Te 3 to water vapour, an increase in weight was monitored in
Fig. 8. Plot of the diffusion coefficients versus composition obtained from the first steps of potentiostatic discharge of Li / Ta 2 Te 3 and Na / Ta 2 Te 3 cells.
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
99
layer of water. Similar processes have also been established in other complex layered chalcogenides, like misfit layer compounds [21].
4. Conclusions
Fig. 9. Successive powder X-ray diffraction patterns of electrochemically intercalated Na 0.5 Ta 2 Te 3 exposed to water vapour: 001 Miller indices correspond to unreacted solid; 0019 indices to a 1020-pm phase with Na 0.5 (H 2 O) 0.3 Ta 2 Te 3 stoichiometry and 0010 and 0020 to Na 0.5 (H 2 O) 0.6 Ta 2 Te 3 with a bilayer of water.
the electrobalance. In addition, the XRD patterns (Fig. 9) reveal the progressive occurrence of two sets of extra lines. The occurrence of a 1275-pm phase, which can be interpreted as an intercalated phase, in which the expansion of 476 pm relative to c?sin b of the anhydrous intercalate in Table 1 (799 pm), suggests possible cointercalation of solvating water molecules. After the equilibrium was reached, thermogravimetric recordings were performed (Fig. 10). From this plot, a two-step process is found that can be associated to the occurrence of the above two phases: with ca 1275-pm basal spacing Na 0.5 (H 2 O) 0.6 Ta 2 Te 3 with a bilayer of water, and with a 1023-pm phase with Na 0.5 (H 2 O) 0.3 Ta 2 Te 3 with a mono-
From the present knowledge on the intercalation chemistry of tantalum chalcogenides and the results shown above, different relationships can be derived: (1) Irrespective of its layered structure, the metal-rich selenide Ta 2 Se, possessing four-atom thick layers structurally related to b.c.c. tantalum, is less prone to intercalation than other solids with higher Q / Ta ratio. The electrochemical reaction with lithium extends up to 2 F / mol and leads to the irreversible elimination of the selenium atoms separating consecutive layered blocks, which results in a collapse of the quasi-two-dimensional structure with the formation of microcrystalline tantalum and—presumably— amorphous lithium selenide, etc. The process may be regarded as a de-insertion of the chalcogen by reaction with lithium ions simultaneous to a reduction of the host tantalum. Nevertheless, sodium can be intercalated to a limited extent, fixed by the low oxidation state of tantalum in the chalcogenide. (2) Layered Ta 2 Te 3 shows a more extended intercalation behaviour, as lithium and sodium can be incorporated in the interlayer space. It should be noted that the presence of metal–metal bonds within the layers of the tantalum sublattice does not inhibit the intercalation of alkali metals. A similar behaviour was also found in tantalum ditelluride [17]. However, as compared with TaTe 2 , the absence of strong interlayer interactions is more evident for Ta 2 Te 3 . Nevertheless, lithium intercalation takes place at cell potentials unusually low with broad signals in the SPES spectra. This phenomenon may be indicative of an incomplete oxidation of lithium. Recently a similar effect was reported in carbon hosts, although in that case, lithium intercalation takes place closer to 0 V [22]. A poor lithium diffusivity is also in agreement with this assumption. On the contrary, the more electropositive sodium allows a complete electron donation to the host, irrespective of the high packing density of metal atoms in the one-atom thick tantalum layers of Ta 2 Te 3 .
References
Fig. 10. Thermogravimetric analysis curves of the fully hydrated derivative Na 0.5 (H 2 O) 0.6 Ta 2 Te 3 .
[1] W.S. Sheldrick, M. Wachhold, S. Jobic, R. Brec, E. Canadell, Adv. Mater. 9 (1997) 669. [2] H.F. Franzen, T.A. Beineke, B.R. Conrad, Acta Crystallogr., Sect. B 4 (1968) 412. ¨ [3] T.E. Weirich, A. Simon, R. Pottgen, Z. Anorg. Allg. Chem. 622 (1996) 630. [4] B. Harbrecht, R. Leersch, J. Alloys Compounds 238 (1996) 13. ¨ [5] T.E. Weirich, R. Ramlau, A. Simon, S. Hovmoller, X. Zou, Nature 382 (1996) 144.
100
P. Lavela et al. / Journal of Alloys and Compounds 282 (1999) 93 – 100
[6] P.A. Maggard, J.D. Corbett, Inorg. Chem. 37 (1998) 814. [7] T. Degen, B. Harbrecht, Angew. Chem. Int. Ed. Engl. 34 (1995) 2703. [8] B. Harbrecht, Angew. Chem. Int. Ed. Engl. 28 (1989) 1660. [9] B. Harbrecht, T. Degen, M. Conrad, J. Alloys Compounds 246 (1997) 37. [10] T. Degen, B. Harbrecht, Angew. Chem. 107 (1995) 1226. [11] M. Conrad, B. Harbrecht, Z. Anorg. Allg. Chem. 623 (1997) 742. [12] T. Degen, B. Harbrecht, Acta Crystallogr., Sect. C 51 (1995) 2218. [13] M. Conrad, B. Harbrecht, J. Alloys Compounds 187 (1992) 181. [14] B.E. Brown, Acta Crystallogr. 20 (1966) 264. [15] M.H. Whangbo, E. Canadell, J. Am. Chem. Soc. 114 (1992) 9587.
´ J. Morales, J.L. Tirado, Inorg. Chem. 33 (1994) 3164. [16] R. Guzman, ´ J. Morales, J.L. Tirado, Chem. Mater. 7 (1995) 1171. [17] R. Guzman, [18] Y. Chabre, D. Djurado, M. Armand, W.R. Romanow, N. Coustel, J.P. McCauley, J.E. Fischer, A.B. Smith, J. Am. Chem. Soc. 114 (1992) 764. [19] T. Hibma, Intercalation Chemistry in: M.S. Whittingham, A.J. Jacobson (Eds.), Academic Press, New York, 1982. [20] M. Armand, Ph.D. Thesis, Univ. Grenoble, 1978. [21] P. Lavela, J. Morales, J.L. Tirado, Chem. Mater. 4 (1992) 2. [22] Y. Nakagawa, S. Wang, Y. Matsumura, C. Yamaguchi, Synthetic Metals 85 (1997) 1363.