Electrochemical study of the reaction of lithium with Aurivillius and related phases

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1 Pergamon Materials Research Bulletin 36 (2001) Electrochemical study of the reaction of lithium with Aurivillius and related phases F.E. Longoria Rodríguez, A. Martínez-de la Cruz* Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, Apartado Postal 1625, Monterrey, N.L., Mexico (Refereed) Received 20 September 2000; accepted 19 December 2000 Abstract Some Aurivillius and Sillen phases have been tested as positive electrode materials for secondary lithium batteries through an electrochemical study. We have carried out an electrochemical and structural study of the lithium reaction with some of the most representative phases of the systems WO 3 -Bi 2 O 3,MoO 3 -Bi 2 O 3, and Nb 2 O 5 -Bi 2 O 3. Although the amount of lithium atoms incorporated in all cases was large, due to irreversible structural transformations in the matrix of the host, specific capacity of the cells were dramatically lost after the first cycle. On the basis of in situ X-ray diffraction data of lithiated phases, we propose a mechanism of reaction Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; B. Intercalation reactions; C. Electrochemical measurements; C. X-ray diffraction; D. Electrochemical properties 1. Introduction The Aurivillius and Sillen phases have been studied extensively since its discovery in order to know more about the chemistry of bismuth and its lone pair electrons [1 2]. The family of mixed bismuth layered oxides that belongs to the homologous series Bi 2 A n O 3n 3 (A W or Mo) can be visualised as an special case of the general Aurivillius series of phases [3]. The structural arrangement of these compounds is formed by (Bi 2 O 2 ) 2 layers inter- * Corresponding author. address: azmartin@ccr.dsi.uanl.mx (A. Martínez-de la Cruz) /01/$ see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S (01)

2 1196 Longoria Rodríguez, Martínez-de la Cruz / Materials Research Bulletin 36 (2001) Fig. 1. Structural representation of Bi 2 WO 6, an Aurivillius phase (a); and Bi 2 Nb 2 O 8, a related Aurivillius phase. leaved by perovskite-like layers (A n O 3n 1 ) 2-, where n represents the number of perovskite layers presents (typically n 1 or 2 for W, and n 1 for Mo), see Figure 1a. On the other hand, in the Sillen phases the layers (A n O 3n 1 ) 2- consist of tetrahedra, instead of octahedra, having no anions in common. Due to its interesting ferroelectric properties, these phases have been the subject of many studies including structural studies by X-ray diffraction, neutron diffraction, and high resolution transmission microscopy [3 6]. In the present work we have carried out an electrochemical lithium insertion study in some phases in the Bi 2 O 3 -WO 3 and Bi 2 O 3 -MoO 3 systems with Aurivillius and Sillen structures. We have chosen these compounds in order to take advantage a priori of different features exhibited by these compounds: (a) its crystalline structure based in perovskite layers interleaved by (Bi 2 O 2 ) 2- sheets produces an open structure suitable to experiment lithium insertion reactions, (b) the high oxidation state of the cations that form these compounds, i.e. Bi 3,W 6, and Mo 6, suggests a large amount of lithium that could be inserted, and (c) its very interesting electrical properties studied and related in early works. In the same way we have evaluated the ability of Bi 2 Nb 2 O 8, an related Aurivillius phase where the central cation has a valence of 5, as positive electrode. In contrast to Aurivillius and Sillen phases, in Bi 2 Nb 2 O 8 the (Bi 2 O 2 ) 2 layers are missing and a simple layer of bismuth atoms exist between the perovskite layers, see Figure 1b. 2. Experimental Powdered samples were prepared by solid state reactions from ground stoichiometric mixtures of Bi 2 O 3,WO 3,MoO 3, and Nb 2 O 5. The mixtures were heated in alumina crucibles within an electric furnace as have been described in previous works [4,7 8]. After each

3 Longoria Rodríguez, Martínez-de la Cruz / Materials Research Bulletin 36 (2001) thermal treatment the products were cooled to room temperature in air, reground and analysed by X-ray powder diffraction. X-ray diffraction data were collected in a SIEMENS D-5000 diffractometer using CuK radiation with a nickel monochromator. The experiments were run in the 2 range 5 to 90 with a scan rate of 0.05 /2 s. Electrodes in the form of 7mm diameter pellets were made containing active material/ carbon black/ethylene-propylene-diene terpolymer (EPDT) in a proportion of 90/9/1. These pellets were used in a Swagelok type cell as positive electrode. A lithium pellet was used as negative and reference electrode and a 1 mol dm -3 solution of LiPF 6 in ethylene carbonate/ dimethyl carbonate (50/50) as electrolyte. All cells assembling were carried out into a glove box filled with argon to avoid the decomposition of lithium. In a typical galvanostatic experiment a current density of 150 A/cm 2 was applied to cycle the cell between different values of voltage vs Li /Li 0. The evolution of structural changes in the framework of the host as reaction with lithium proceeded was followed by in situ X-ray diffraction technique. For this propose we have used a laboratory made electrochemical cell that has a dual function: as a sample holder and as electrochemical cell [9]. Due to the poor crystallinity of the lithiated compounds, X-ray diffraction data collection was performed slowly. Typically we used a scan rate of 0.05 /12 s in the 2 range from 20 to Results and discussions 3.1. The system WO 3 -Bi 2 O 3 The most representative phase of this system, Bi 2 WO 6, was selected to study its ability to incorporate lithium through a lithium insertion reaction. When some cells with configuration Li/electrolyte/Bi 2 WO 6 were charged-discharged, electrochemical reaction with lithium proceeded in Bi 2 WO 6 through several steps as shown in the voltage-composition plot, see Figure 2. When a cell was discharged just above 0.0 V vs Li /Li 0, approximately 17.5 lithium atoms reacted per formula. Such amount of lithium leads to a high specific capacity of 670 Ah Kg -1. Nevertheless, as is shown in Figure 2, such capacity was dramatically lost after the first cycle. During the charge of the cell, i.e. oxidation process, only 11.5 lithium ions can be removed. We have attributed the origin of this irreversibility to the existence of the first plateau, labelled A, presents during the discharge of the cell around 1.5 V vs Li /Li 0. A similar situation was observed when another cell with the same configuration was cycled between V. In this case, Bi 2 WO 6 reacts with 13.5 lithium atoms producing a specific capacity of the cell of 515 Ah kg -1. Note that in both cells the first 6 lithium atoms incorporated can not be removed during the first charge. After several cycles the capacity to remove lithium incorporated from the positive electrode was abruptly lost. When the second member of this family was tested as cathode, Bi 2 W 2 O 9, it showed a voltage-composition plot that resembled lithium reaction with Bi 2 WO 6. During the discharge of the cell between V, Bi 2 W 2 O 9 accepted 16 lithium atoms per formula (455 Ah kg -1 ). This process took place through several steps that involved three semiconstant

4 1198 Longoria Rodríguez, Martínez-de la Cruz / Materials Research Bulletin 36 (2001) Fig. 2. Voltage-composition plot obtained from galvanostatic cycling of two cells Li//Bi 2 WO 6 until 0.5 and 0.01 VvsLi /Li 0 respectively. potential regions separated by regions where the potential drops abruptly. As it was observed in Bi 2 WO 6, about 6 lithium atoms kept into the positive electrode after the first charge, see Figure 3. So, seems clear the role that plays these first six lithium in the subsequent reversibility of the cell. To obtain some additional information about the irreversibility of the lithium reaction with these Aurivillius tungsten oxides, we have carried out a structural study by X-ray diffraction during the discharge of the cells Li/Bi 2 WO 6 and Li/Bi 2 W 2 O 9. Figure 4 shows several X-ray diffraction patterns that have been taken for different compositions Li x Bi 2 WO 6 (where 0 x 12). As reaction with lithium proceeds material become amorphous and for a composition Li 6 Bi 2 WO 6 all reflections of the pristine oxide are missing. On the basis of these results we can confirmed that plateau labelled A, in Figure 2, is due to an irreversible structural transformation of the host matrix. This situation was confirmed through an analysis of the I(t) plot in each potential level which showed that current follows a far behaviour from t -1/2 law [10]. Similar results were obtained when we studied the lithium insertion in Bi 2 W 2 O 9. So we can conclude that both oxides can not be considered like as solid solution electrodes. By means of a careful analysis of the diffraction plot we have detected some weak reflections that can be assigned to metallic bismuth when x 6, see insert in Figure 4. Formation of Bi o during the discharge of the cell is not surprising taking into account previous works about lithium insertion in Bi 4 V 2 O 9, a related Aurivillius phase. Although Arroyo et al. [11] do not detected Bi o during the discharge of the cell, they reported its formation when samples with lithium were exposed to air, thus indicating that the reduction of Bi 3 occurs before that the metal transition, V 3. Additionally for higher

5 Longoria Rodríguez, Martínez-de la Cruz / Materials Research Bulletin 36 (2001) Fig. 3. Voltage-composition plot obtained from galvanostatic cycling of a cell Li//Bi 2 W 2 O 9 until just above 0.0 VvsLi /Li 0. Fig. 4. X-ray diffraction patterns taken during the discharge of a cell Li//Bi 2 WO 6 to form Li x Bi 2 WO 6.

6 1200 Longoria Rodríguez, Martínez-de la Cruz / Materials Research Bulletin 36 (2001) values in lithium content, when the cell across the region B and x 12, a slow scan in 2 showed the formation of the alloy Li-Bi. This reaction is well documented in literature and is known that proceed at 0.7 V vs Li /Li 0 through the formation of LiBi, and Li 3 Bi [12 13]. On the basis of these results is obvious that lithium insertion in these tungsten phases is not feasible. Nevertheless between plateaus labelled as A and B in Bi 2 WO 6 and between plateaus B and C in Bi 2 W 2 O 9, a region where the potential varies continuously with composition was observed. These region, according with its I(t) plot, correspond to a solid solution phase with an interval of composition near to 2 and 4 lithium atoms for Bi 2 WO 6 and Bi 2 W 2 O 9 respectively. We suggest that the reduction of W 6 to W 4 (2 electrons associated with each W) could occurs in this interval, and likely this could be the only region where lithium insertion proceeds truly. Note that a reaction with a very large lithium amount implied the reduction to lower oxidation states of the elements that build the framework of the oxide. So the remainder lithium to be justify in Bi 2 WO 6 at 0.0 V could be associated with the electrons provided from the reduction of W 4 to form W The system Nb 2 O 5 -Bi 2 O 3 Electrochemical lithium reaction with the more simple member of this system, Nb 2 Bi 2 O 8, proceeds through similar steps that reaction with tungsten phases. A maximum of 15 lithium atoms per formula (570 Ah kg -1 ) reacted with Nb 2 W 2 O 8 when the cell was discharged until just above 0.0 V vs Li /Li 0 and near to 13.5 lithium atoms when voltage of the cell was limited to 0.5 V. The large plateau detected in E-x plot, labelled as A in Figure 5, was again easily associated with bismuth reduction through X-ray diffraction data. In the same way, for higher lithium contents, i.e. x max. 12, diffraction studies shown the presence of Li 3 Bi. Taking into account the different structural arrangements between Bi 2 Nb 2 O 8 and the tungsten Aurivillius oxides seem clear that reaction with lithium is governed by the chemistry of bismuth more than structural aspects. In the same way, we want to remark that on some data of literature which relate that Bi 2 O 3 reacts with lithium through similar steps [14 15] The system MoO 3 -Bi 2 O 3 The phase diagram and compounds of the system MoO 3 -Bi 2 O 3 have been the subject of several studies. Its well known that MoO 3 reacts with Bi 2 O 3 in a 1:1 molar stoichiometric ratio to form Bi 2 MoO 6, a phase with Aurivillius type-structure. When Bi 2 MoO 6 is heated at 640 C, it undergoes an irreversible phase transition to the Sillen type structure where Mo changes its coordination from octahedral to tetrahedral [16]. Figure 6 shows E-x plots obtained when two cells were discharged until 0.01 V vs Li /Li 0 using as active material each oxide (hereafter A-Bi 2 MoO 6 and S-Bi 2 MoO 6 ). These experiments have showed that structural differences between A-Bi 2 WO 6 and S-Bi 2 WO 6 almost do not exhibited different electrochemical behaviours. Although both plots resemble the E-x

7 Longoria Rodríguez, Martínez-de la Cruz / Materials Research Bulletin 36 (2001) Fig. 5. Voltage-composition plot obtained from galvanostatic cycling of two cells Li//Bi 2 Nb 2 O 8 until 0.5 and 0.01 VvsLi /Li 0 respectively. Fig. 6. Voltage-composition plot obtained from galvanostatic cycling of the cells Li//A-Bi 2 MoO 6 and Li//S- Bi 2 MoO 6 until 0.01 V vs Li /Li 0 respectively.

8 1202 Longoria Rodríguez, Martínez-de la Cruz / Materials Research Bulletin 36 (2001) Fig. 7. Voltage-composition plot obtained from galvanostatic cycling of a cell Li//S-Bi 2 MoO 6 discharged until 1.6 VvsLi /Li 0. behaviour observed in Bi 2 WO 6, in these case the reaction with lithium proceeds through a different mechanism. X-ray diffraction data were collected while some cells with configuration Li/A- Bi 2 MoO 6 and Li/S-Bi 2 MoO 6 were discharged. All reflections of the parent oxide were missing in both cases for x 2 and until a maximum of composition about x 12 none reflection of metallic bismuth or Li 3 Bi were observed. These data suggested that reaction with lithium of molybdenum phases proceeds through a different mechanism. This thought is in agreement with the results observed when we have charged-discharged several cells until 1.6 V vs Li /Li 0. Unlike the process A in Bi 2 WO 6 and Bi 2 Nb 2 O 8 ;and B in Bi 2 W 2 O 9 ; more than 50% of the total amount of lithium that have reacted during this process can be removed after the first charge for A-Bi 2 MoO 6 and S-Bi 2 MoO 6 see Figure 7. So this process has to involve the formation of a new phase which offers a good capacity to inserted-deinserted lithium although such capacity was lost after several charge-discharge cycles. Unfortunately the nature of this phase can not be elucidate through X-ray diffraction experiments and for this reason now some electron diffraction experiments are in progress. Nevertheless, the formation of this new phase involves the reduction from Bi 3 to Bi 0 as was observed in the tungsten and niobium phases. This situation was confirmed by means of crystallisation at 550 C in an evacuated ampoule of the amorphous phase with 2 x 6. In all cases the X-ray diffraction shown the formation of Bi 0.

9 Longoria Rodríguez, Martínez-de la Cruz / Materials Research Bulletin 36 (2001) Conclusions Lithium insertion is only feasible in molybdenum oxides. On the contrary, the reaction of tungsten and niobium phases with lithium seems to be governed by the chemistry of bismuth more than structural arrangement aspects of the host. Due to the high oxidation state of the elements involved in the oxide framework the amount of lithium atoms incorporated in tungsten and niobium oxides was large, but due to irreversible structural transformations in the matrix of the host, specific capacity of the cells were dramatically lost after the first cycle. Taking into account this behaviour we suggest that this phases only can be considered as cathode in primary lithium batteries. The origin of the irreversibility of reaction with lithium in tungsten Aurivillius and niobium related Aurivillius phases can be associated with the irreversible reduction of the Bi 3 to Bi 0 as was confirmed by X-ray diffraction experiments. When similar experiments were run in molybdenum Aurivillius phases, we detected a different lithium insertion mechanism. In this case, a new phase was formed at high values of voltage and it had the capacity to inserted-deinserted lithium with certain reversibility. Due to this phase is amorphous we cannot conclude about the origin of this reversibility. Nevertheless, as was showed by X-ray diffraction experiments, important structural arrangement take place for small lithium amounts inserted and these changes involve reduction of bismuth. Having in mind this result, is obvious the importance of this new phase in the reversibility of the first process. Although Bi is essential to form the framework of the Aurivillus phases, it plays a negative role in the lithium insertion process. For this reason we conclude that better results could be obtained if as host an opened Aurivillius structure is used but without bismuth in its framework. In addition to bismuth compounds only antimony oxides has exhibited this type of framework, but it has showed that follow a similar behaviour that bismuth compounds. Acknowledgments We wish to thank CONACYT for supporting the project J28162-E and the Universidad Autónoma de Nuevo León for its invaluable support through the projects PAICYT CA and CA References [1] B. Aurivillus, Arkiv. Kemi. 1 (1949) 463. [2] L. Sillen, Z. Anorg. Allgmen. Chem. 246 (1941) 331. [3] D.A. Jefferson, J. Gopalakrishnan, A. Ramanan, Mater. Res. Bull. 17 (1982) 269. [4] R.W. Wolfe, R.E. Newnahm, M.I Kay, Solid State Comm. 7 (1969) [5] Y. Bando, A. Watanabe, Y. Sekikawa, M. Goto, S. Horiuchi, Acta Cryst. 35 (1979) 142. [6] S.N. Hoda and L.L.Y. Chang, J. Amer. Ceram. Soc. 57 (1974) 323. [7] T. Chen, G.S. Smith, J. Solid State Chem. 13 (1975) 288. [8] Y. Bando, A. Watanabe, Y. Sekikawa, M. Goto, S. Horiuchi, Acta Cryst. A35 (1979) 142.

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