The well known ReO3-type structure is the basis to describe many compounds within the extended family called “tungsten bronzes”. The common structural arrangement of these materials is the WO6 unit which is repeated along three directions through shared corners. This provides empty tunnels, running parallel to b-axis, where ions can be accommodated to form a great variety of compounds called then perovskite type bronzes (PTB). Due to their interesting chemical, electrical and optical properties many bronzes AxWO3 (A= Li, Na, K, Mg, Zn, etc.) with the ReO3-type structure have been extensively studied [2-4].
Different structural arrangements can be produced if WO6 units are replaced by monophosphate (PO4) or diphosphate (P2O7) groups, and then the compounds called phosphate tungsten bronzes are formed [5-8].
In the present work we have carried out an electrochemical lithium insertion study on K2P4W12O44, a ternary oxide of the family of compounds so-called diphosphate tungsten bronzes with hexagonal tunnels (DPTBh). The structural arrangement of K2P4W12O44, see Figure 1, can be described by a layered structure built up of regular WO3-type slabs with the same width, linked together through isolated PO4 units. Each slab is formed from a set of linear strings of three WO6 octahedra sharing corners, all oriented along the same direction within the layer and also in the different layers . Different features of K2P4W12O44, i.e. a) its framework based in the perovskite type structure and b) the high oxidation state of the tungsten, suggests a large amount of lithium may be inserted in this oxide. These features make a priori that this material can be considered as material active in electrodes for lithium batteries.
Figure 1. Projection along b of the K2P4W12O44 monoclinic structure. The filled circles represent K atoms.
The diphosphate tungsten bronze K2P4W12O44 was obtained in two steps. In first instance, a mixture of (NH4)2HPO4, WO3 and K2CO3 in the correspondence molar relation was heated in air at 650oC for 24 h in order to decompose the ammonium phosphate [9-10]. Then, a stoichiometric amount of W was added to the starting mixture to reach the composition K2P4W12O44. After grinding, the mixture was placed in a quartz tube that was evacuated, sealed and heated at 1000oC for 5 days and then slowly cooled to room temperature .
Structural characterization was carried out by X-ray diffraction using a Siemens D-5000 diffractometer with Cu Kα (λ= 1.5418 Å) radiation. X-ray diffraction data of the pristine oxide were collected between 2θ range of 5 to 90º with a scan rate of 0.05º/2s.
The electrochemical lithium insertion was studied by means of an electrochemical cell of the Swagelok type bearing metallic lithium as the negative electrode. The positive electrode was a 7 mm diameter pellet obtained by pressing 15-25 mg of a mixture containing ternary oxide, carbon black and ethylene-propylene-diene-terpolymer (EPDT) in a 89:10:1 ratio. For the electrolyte, a 1 mol dm-3 solution of LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) 50:50 was used. Due to the high reactivity of metallic lithium, the assemblage of the cells was carried out in an argon filled glove box (MBraun) with a content in oxygen and water less than 1 ppm. The assembled cell was then removed and connected to a multichannel galvanostatic-potentiostatic system (MacPile II).
Results and Discussion
The product of the solid state reaction between the reactant oxides was a fine powder whose X-ray diffraction pattern corresponded with that previously reported for K2P4W12O44 , see Figure 2. On the basis of this result we have taken the ternary oxide to performance electrochemical lithium insertion experiments.
Figure 2. X-ray diffraction pattern of K2P4W12O44 obtained by solid state reaction.
When lithium is electrochemically inserted in K2P4W12O44 the voltage of the Li/LiPF6 (1mol dm-3 in EC+DMC(50:50)/ K2P4W12O44) cell varies with composition as shown in Figure 3. The reaction proceeds through different processes as can be deduced from the evolution of the cell voltage with x in LixK2P4W12O44; different regions labelled as I, II, A, and B are observed. The regions where a continuous variation of the potential with composition is observed corresponds to a single phase region (I and II) whereas the voltage plateaus, A and B, are assigned to two-phase regions which are bounded by points of inflection at definite changes in slope of the potential-composition curve.
Figure 3. Voltage-Composition plot for a charge-discharge cycle of a cell with configuration Li/electrolyte/ K2P4W12O44 when it was cycled until 1.0 V vs Li+/Lio.
The maximum amount of lithium inserted (~15 ions Li/K2P4W12O44) leads to a specific capacity of the cell of 120 Ah kg-1, when cycled in the range 3.4-1.0 V vs Li+/Li0. Nevertheless, after the first cycle, the cell was unable to maintain this specific capacity for subsecutive cycles, fading until near 30% of the initial value after six cycles.
Figure 4 shows the complete discharge of the cell K2P4W12O44 through their I-E curves. For high potential values, only small currents are detected and the cell exhibits a low capacity. For low potential values, below 2.0 V vs Li+/Lio, the current increases reaching a maximum value. In this region, peak A, the behaviour of current with time was far from a t-1/2 law. Hence we deduced that the diffusion of the lithium ions is not the process that rules the insertion reaction. This behaviour confirms that the system is crossing a two-phase domain. In this case, the shape of the peak A in I-E diagram is asymmetrical with a linear slope whose intercept with the zero current is at the open-circuit two-phase equilibrium value. A similar situation was observed for B process.
Figure 4. Voltamperogram obtained by discharging a cell Li/electrolyte/AxP4W8O32 at a scan rate of -10 mV/ 0.5h.
On the basis of the experiment described above, we can conclude that lithium insertion in K2P4W12O44 proceeds through successive steps that involve the existence of single and two-phase regions. The single phase domains correspond to the abrupt drops of the potential in the E-x plot (regions labelled as I and II) and with regions of minimum current in the E-I plot. On the other hand, plateaus in the voltage-composition plot and maxima of current in E-I plot, where the behaviour of current with time does not follow a t-1/2 law, correspond to two-phase domains. Note that through the different charge-discharge cycles the reduction and oxidation peaks tend to dissappear which is indicative of the irreversible nature of process.
We have carried out the cycling of two cells Li/electrolyte/K2P4W12O44 under galvanostatic conditions to a minimum voltage limit of 1.2 and 0.3 V vs Li+/Lio respectively, i.e. before and after that A process takes place. As can be observed in Figure 5, the origin of system irreversibility is associated with the process previously labeled as B. On the other hand this plot shows the reversible nature of the process to inserted until 10 lithium, i.e. to form Li10K2P4W12O44.
Figure 5. Voltage-Composition plot for a charge-discharge cycle of two cells with configuration Li// K2P4W12O44 when its were cycled until a) 1.2 V and b) 0.3 V vs Li+/Lio.
Electrochemical lithium insertion in K2P4W12O44 leads to a specific capacity of the cell of 75, 120 and 240 Ah kg-1 when cells of the type Li/electrolyte/ K2P4W12O44 were discharged until 1.2, 1.0 and 0.3 V vs Li+/Lio, respectively. Nevertheless, the specific capacity of the cells underwent a decrease in its value when it was discharged until 1.0 and 0.3 V after the first cycle due to an irreversible process. Taking this behaviour into account we suggest that this bronze only can be considered as a cathode in primary lithium batteries.
We wish to thank to CONACYT for supporting the project 43800 and the Universidad Autónoma de Nuevo León (UANL) for its invaluable support through the project PAICYT CA839-04.
1. P. Hagenmuller, “Tungsten bronzes, vanadium bronzes, and related compounds”, in Comprehensive inorganic chemistry, ed. Pergamon, Oxford, 1973, p. 541.
2. M.S. Whittingham, “Intercalation chemistry: an introduction” in Intercalation Chemistry, ed. Academic Press, New York, 1982, p. 8.
3. A. M. Chippindale, P. G. Dickens and A. V. Powell, “Insertion compounds of transition-metal and uranium oxides”, Prog. Solid St. Chem., 21 (1991) 133-198.
4. A. Martínez-de la Cruz, Leticia M. Torres-Martínez, F. García Alvarado, E. Morán and M. A. Alario-Franco, “Formation of new tungsten bronzes: electrochemical zinc insertion in WO3”, J. Mater. Chem., 8  (1998) 1805-1807.
5. E. Canadell and M. H. Whangbo, “On the possible electronic instability of the monophosphate tungsten bronze (WO3)4(PO2)4”, J. Solid St. Chem., 86 (1990) 131-134.
6. E. Canadell, M. H. Whangbo and I. E. Rachidi, “Similarity of the electronic properties of the monophosphate tungsten bronzes”, Inorg. Chem., 29 (1990) 3871-3875.
7. M. Greenblatt, “Phosphate tungsten bronzes: a new family of quasi-low-dimensional metallic oxides”, International Journal of Modern Physics, B7 (1993) 3937-3971.
8. P. Roussel, P. Labbé and D. Groult, “Symmetry and twins in the monophosphate tungsten bronze series (PO2)4(WO3)2m (2 < m < 14)”, Acta Cryst., B56 (2000) 377-391.
9. P. Roussel, D. Groult, A. Maignan and Ph. Labbé, “Phase relations, crystal structure, and electron transport properties of phosphate tungsten bronzes (KxNay)(PO2)4(WO3)2m (m = 4,6)”, Chem. Mater., 11 (1999) 2049-2056.
10. E. Wang, M. Greenblatt, I. El-Edrissii Rachidi, E. Canadell and M. Whangbo, “Anisotropic electronic properties of the diphosphate tungsten bronzes K2P8W24O88, K2P8W28O100 and their substituted compounds”, J. Solid St. Chem., 80 (1989) 266-275.
11. B. Domengès, M. Hervieu and B. Raveau, “Monophosphate tungsten bronzes with hexagonal tunnels, Nax(PO2)4(WO3)2m : X-ray diffraction and HREM study”, Acta Cryst., B46 (1990) 610-619.
12. P. Roussel, S. Drouard, D. Groult, P. Labbé, J. Dumas and C. Schlenker, “Crystal structure and electrical properties of K2P4W12O44, m = 6 member of the series of low-dimensional conductors Kx(PO2)4(WO3)2m”, J. Mater. Chem., 9 (199) 973-978.