Preparation of sodium molybdenum oxides by a solution technique and their electrochemical performance in lithium intercalation

Solid State Ionics 106 (1998) 11–18

Preparation of sodium molybdenum oxides by a solution technique and their electrochemical performance in lithium intercalation a, b a c Aishui Yu *, Naoaki Kumagai , Zhaolin Liu , Jim Y. Lee a

Institute of Materials Research and Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 1192600, Singapore b Department of Applied Chemistry and Molecular Science, Faculty of Engineering, Iwate University, Morioka 020, Japan c Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 3 September 1997; accepted 15 September 1997

Abstract Several sodium molybdenum oxides with different structures were prepared by acidification of Na 2 MoO 4 solutions with 1–3.5 N of strong acids (HCl, HNO 3 ) at 1008C followed by heat treatment. Structural and compositional analyses of the oxides show that their structures were determined by preparation conditions such as acid type and concentration, and heat-treatment temperature. The electrochemical intercalation of lithium into these oxides was attempted using them as the positive electrode materials for secondary lithium batteries. The results show reversible lithium intercalation and electrochemical performance that greatly depends on the oxide structure and composition. Keywords: Sodium molybdenum oxide; Solution technology; Electrochemical lithium intercalation

1. Introduction Molybdenum oxides exist in many crystal forms notably the orthorhombic and the monoclinic [1–4]. Orthorhombic molybdenum trioxide was the first to be used as the intercalation host for many monovalent and multivalent cations that are inserted chemically or electrochemically. Since the electrochemical insertion of lithium in MoO 3 is reversible, the application of this material in secondary lithium batteries and electrochromic devices has been suggested. Several other molybdenum oxides such as Mo 18 O 52 , Mo 8 O 23 and Mo 17 O 47 have also been *Corresponding [email protected]

author.

Fax:

165-872-0785;

email:

studied as potential cathode materials because of the high energy density associated with the Limolybdenum oxide cells. The reversible electrochemical behaviour of the MoO 3 electrodes can be explained by the following redox reaction which is known to be topotactic: xLi 1 1 e 2 1 MoO 3 5 Li x MoO 3 .

(1)

Relatively low specific energy of the Li / MoO 3 couple and poor stability of the lithium intercalated oxide are recognized drawbacks of MoO 3 as an electrode material in secondary lithium batteries [5]. Improvements in material performance have been made by using solution techniques to modify the basic MoO 3 structure [6–9]. It is known that oxy

0167-2738 / 98 / $19.00  1998 Elsevier Science B.V. All rights reserved. PII S0167-2738( 97 )00491-8

12

A. Yu et al. / Solid State Ionics 106 (1998) 11 – 18

acids such as MoO 3 ?2H 2 O lose the water molecules topotactically in dehydration reactions. The dehydrated oxides should contain more vacancies in the structure into which Li 1 ions can be reversibly incorporated. These oxy acids and their dehydrated products are therefore potentially useful cathode materials for secondary lithium batteries and electrochronomic devices. In this paper, several sodium molybdenum oxides were prepared by a solution technique and characterized by X-ray diffraction and chemical composition analysis. Furthermore, these materials were also evaluated for their suitability as the cathodes of secondary lithium batteries.

2. Experimental

weight ratio of 1:1, and compression moulded onto Ni nets under ca. 50 Mpa to achieve a typical loading of ca. 20 mg cm 22 (active material). The pellets thus obtained were used as the positive electrodes after drying in a vacuum at 808C for one day. Lithium pellets were used for both the negative and the reference electrodes. The electrolyte was 1 M LiClO 4 in propylene carbonate (PC) with less than 100 mg dm 23 of water. The electrodes were charged and discharged galvanostatically on a Hokuto Denko model HJ-201B charge-discharge unit. Quasi-equilibrium open-circuit potentials were measured after 24 h on open circuit, when the variations in cell potential were less than 0.2 m Vh 21 . The cells were also characterized by a.c. impedance measurements in the frequency range of 63 kHz to 1 mHz. The details of measurements can be found in the literature [10].

2.1. Preparation of sodium molybdenum oxides Analytical reagent grade chemicals were used as the starting materials. Sodium molybdenum oxides were prepared by the acidification of Na 2 MoO 4 solutions. 1.0 to 3.5 N of HNO 3 or HCl was added to 0.25 M of Na 2 MoO 4 with constant stirring and heating at 1008C for 1 h. The solutions were then aged for 1 day before the precipitates were removed by filtration, copiously washed with distilled water, and heated at different temperatures for 6 h.

2.2. Analysis of the products The chemical composition of the oxides was analyzed by the atomic absorption method using a Hitachi model 180-60 / 80 sprectophotometer. The water content in the oxides was calculated from the weight loss in the sample after heating at 3508C for 3 h. The determination of crystal structure was carried out on a Rigaku Geigerflux model 20 X-ray diffractometer using the CuKa line and a Ni filter.

2.3. Electrochemical characterizations The test battery was a three-electrode glass cell in Ar atmosphere. The preparation of the electrodes and electrolyte, the cell design, and the detailed test procedures have been described elsewhere [10]. The molybdenum oxides were mixed with graphite in the

3. Results and discussion

3.1. Preparation and structural analysis of the compounds The conditions for obtaining precipitates from the Na 2 MoO 4 solution with HCl and the compositions of the precipitates (sodium molybdenum oxide precursors) are given in Table 1. Fig. 1 gives the X-ray diffraction patterns of these oxide precursors. It is evident from Table 1 and Fig. 1 that the sodium and water contents of the oxide precursors decrease with the increase in HCl concentration. The crystal structures of the oxide precursors are also dependent on the acid concentration. The X-ray diffraction pattern of the oxide precursor 0.10Na 2 O?MoO 3 ?0.68H 2 O corresponding to Na / H51:2 in acidification indicates a totally cubic structure. The cubic structure persists up to a Na / H ratio of 1:3. A further increase in the ratio to 1:4, however, introduces the orthorhombic phase which becomes all predominant at Na / H$1:5. For instance at Na / H ratio51:7, the resulting precursor has a stoichiometry of 0.029Na 2 O?MoO 3 ?0.39H 2 O and a completely orthorhombic structure. These results indicate the importance of acid concentration control in attaining a certain crystallographic structure in the final products.

A. Yu et al. / Solid State Ionics 106 (1998) 11 – 18

13

Table 1 The compositions and the structures of the products prepared at different conditions and then dried at 508C Reaction condition

Composition xNa 2 O?MoO 3 ? yH 2 O

Na 1 / H 1 ratio

Na 2 MoO 4 (M) (aq., 100 ml)

HCl (M) (aq., 100 ml)

x

y

1:2 1:3 1:4 1:5 1:6 1:7

0.25 0.25 0.25 0.25 0.25 0.25

1.0 1.5 2 2.5 3 3.5

0.100 0.087 0.064 0.056 0.049 0.029

0.681 0.654 0.512 0.439 0.401 0.394

Fig. 1. XRD patterns of sodium molybdenum oxide precursors prepared from HCl with different Na / H ratios.

Product phase

cubic cubic cubic1orthorhombic orthorhombic orthorhombic orthorhombic

The effect of acid type on the composition and structure of the oxide precursors can be readily examined from Table 2 and Fig. 2. In general HCl is more effective in the decomposition of the Na 2 MoO 4 , as precursors with lower Na 2 O content can be produced with a given acid concentration. On the other hand, HNO 3 is more capable of transforming the cubic structure in the starting material. Only one cubic peak remained in the X-ray diffraction patterns of the oxide precursors whereas five cubic peaks were clearly visible when HCl was the acid used. The effect of acid type is compound with the effect of acid concentration to determine the composition and structure of the oxide precursor under a given set of preparation conditions. The X-ray patterns of the oxide precursor 0.087Na 2 O?MoO 3 ?0.654H 2 O with a cubic structure before and after heat treatment at different temperatures are shown in Fig. 3. The structure is predominantly cubic unless the precursor is heated above 3258C where conversion into the orthorhombic form occurs. The conversion appears to be complete at 5508C and the final oxide is 100% orthorhombic. As the inception temperature for the cubic–ortho-

Table 2 Comparison of the composition of the products prepared using different acid agents Reaction

Condition

Composition

xNa 2 O?MoO 3 ? yH 2 O

0.25 M Na 2 MoO 4 0.25 M Na 2 MoO 4 0.25 M Na 2 MoO 4 0.25 M Na 2 MoO 4

2.5 N HCl 2.0 N HCl 2.5 N HNO 3 2.0 N HNO 3

x50.056 x50.064 x50.059 x50.072

y50.439 y50.512 y50.486 y50.553

14

A. Yu et al. / Solid State Ionics 106 (1998) 11 – 18

rhombic transformation is very close to the temperature for the complete removal of water in the precursor (|3508C), the residual water in the precursor may be linked to the stabilization of the cubic structure. The original cubic structure is no longer viable for sodium molybdenum oxide once water is completely removed and conversion into the orthorhombic form becomes an inevitable event. The X-ray patterns of the orthorhombic precursor 0.056Na 2 O?MoO 3 ?0.439H 2 O before and after heat treatment are likewise shown in Fig. 4. The heat treatment here only improves the oxide crystallinity without any change in the lattice structure. The removal of water from the original layered structure is inconsequential so far as structure is concerned. The oxide that is formed at 5508C has almost the same structure as that of orthorhombic MoO 3 [4]. In summary, sodium molybdenum oxides with two kinds of structures, i.e., orthorhombic and cubic, can be prepared by acidification of Na 2 MoO 4 solution. The orthorhombic sodium molybdenum oxide shows better thermodynamic stable properties than that of the cubic one. Fig. 2. A comparison of the XRD patterns of sodium molybdenum oxide precursors prepared from different acids.

Fig. 3. XRD patterns of the cubic oxide precursor and its heattreated products.

Fig. 4. XRD patterns of the orthorhombic oxide precursor and its heat-treated products.

A. Yu et al. / Solid State Ionics 106 (1998) 11 – 18

3.2. Electrochemical characterization of sodium molybdenum oxides as lithium intercalation hosts The basic structures of cubic and orthorhombic MoO 3 are schematized in Fig. 5 [11,12]. The cubic structure is one of ReO 3 -type consisting of a threedimensional array of corner-sharing MoO 6 octahedrons (Fig. 5a). For the orthorhombic MoO 3 that is shown in Fig. 5b, the oxide adopts an unique layer structure which is constructed from linked distorted MoO 6 octahedrons sharing both edges and corners. In each MoO 6 octahedron, one oxygen atom is unshared, two oxygen atoms are common to two octahedrons and three are edge-sharing and common to three octahedrons resulting in the overall stoichiometry MoO 1 (O 1 / 2 ) 2 (O 1 / 3 ) 3 . Fig. 5 also shows that both MoO 3 structures have 3D and 1D tunnels that lithium ions can intercalate the crystal vacant sites. Fig. 6 shows the initial discharge–charge curves of various sodium molybdenum oxides that were heat treated at 508C. The discharge capacity is found to be the lowest for cubic 0.087Na 2 O?MoO 3 ? 0.654H 2 O (210 mAh g 21 ) and the highest for orthorhombic 0.056Na 2 O?MoO 3 ?0.439H 2 O (330

15

mAh g 21 ). The charge capacity also varies likewise. The 50% more capacity in the orthorhombic form can be attributed to the higher Li 1 diffusion coefficient in the more open layered structure, and lower polarization during the discharge process. As also can be seen form Fig. 6, the orthorhombic sodium molybdenum oxide gives highly recharge efficiency than that of cubic one. The recharge efficiency of the orthorhombic molybdenum is 74% and the cubic one is 72%. The effect of acid type on the electrochemical behaviour of the oxides is shown in Fig. 7a and Fig. 7b. The discharge capacity of the HCl derived product is somewhat higher than the HNO 3 derived product. This may reflect the different amounts of Na 2 O and H 2 O remaining in the layers of sodium molybdenum oxide (Fig. 2, Table 2). In general, the lesser this amount is, the higher is the discharge capacity as residual Na 2 O and H 2 O are likely to occupy the same sites as that for lithium intercalation. Furthermore, the HCl derived product gives more orthorhombic sodium molybdenum oxide phase than that of the cubic one shown in Fig. 2 and Table 2. The effect of heat-treatment temperature on the

Fig. 5. Structural models of molybdenum trioxide.

16

A. Yu et al. / Solid State Ionics 106 (1998) 11 – 18

Fig. 6. The initial charge–discharge curves of several sodium molybdenum oxides with different structures. (a) No. 1, cubic 0.087Na 2 O? MoO 3 ?0.654H 2 O; (b) No. 2, mixed 0.064Na 2 O?MoO 3 ?0.512H 2 O; (c) No. 3, orthorhombic 0.056Na 2 O?MoO 3 ?0.439H 2 O.

electrochemical behaviour of the oxides (cubic 0.087Na 2 O?MoO 3 ?0.654H 2 O and orthorhombic 0.056Na 2 O?MoO 3 ?0.439H 2 O is shown in Fig. 8a Fig. 8b where the discharge capacity is plotted as a function of cycle number. For both cubic and orthorhombic forms of the oxides, those without the heat treatment perform better in electrochemical cycling. In general, the discharge capacity decreases with the increase in cycle number. About 50% of the initial discharge capacity will be gained in the 10th cycle. The discharge capacity of both of the precusors heated at 3508C (orthorhombic 0.056Na 2 O?MoO 3 and cubic 0.087Na 2 O?MoO 3 ) in the 5th cycle is shown as a function of current density in Fig. 9. For current densities greater than 0.5 mA cm 22 , the orthorhombic oxide has better rate capacity than the cubic oxide. The openness of the layer structure of the orthorhombic oxide, which provides faster lithium ion transport within the host lattice, may be the main contributing factor. Interestingly at a current density of 0.2 mA cm 22 , the trend is reversed and the cubic oxide is better than the orthorhombic oxide in terms of discharge capacity. The variation of the OCVs of the orthorhombic

sodium molybdenum oxide Li x [0.087Na 2 O?MoO 3 ] electrode with the depth of lithium intercalation, x, at 258C is given in Fig. 10. The figure shows that the OCVs decrease linearly with an increase in the xvalue in Li x [0.087Na 2 O?MoO 3 ]. The OCV–x curve mainly consists of two straight lines with different slopes at about x,0.7 and x.0.7. The relationship between the OCV(E) and the x-value are replaced by: E 5 2.99 2 0.99x

(x , 0.7),

(2)

E 5 2.44 2 0.87x

(x . 0.7).

(3)

This observation is similar to the behaviour of hexagonal sodium tungstate prepared by solution techniques [15]. It is likely that lithium ions may occupy more than one kind of crystallographic sites and the single phase of Li x [0.087Na 2 O?MoO 3 ], where the x-value varies continuously, is formed. The typical a.c. impedance response of the sodium molybdenum oxide electrode consists of an irregular semi-circular arc at the high frequency end and a near vertical line at the low frequency end. Using the

A. Yu et al. / Solid State Ionics 106 (1998) 11 – 18

Fig. 7. A comparison of the charge–discharge curves of sodium molybdenum oxides prepared from different acids. The structure and composition of these compounds are shown in Table 2 Fig. 2.

17

Fig. 8. The dependence of discharge capacity on cycle number for the following oxide precursors before and after heat-treatment. (d) cubic 0.087Na 2 O?MoO 3 ?0.654H 2 O; (e) orthorhombic 0.056Na 2 O? MoO 3 ?0.439H 2 O.

methods of Ho and Weppner [13,14], we obtain the following equations: Zw 5 A w v 21 / 2 , A w 5Vm (dE / dx) /zFAD˜

(4) 21 / 2

,

(5)

where Zw is the Warburg impedance of the intercalated electrode, v is the frequency of the ac response, Vm is the molar volume of the oxides, dE / dx is the slope of the OCV–x curve, z is the charge transfer number for lithium intercalation reaction, which is equal to 1, F is the Faraday constant, and A is the electroactive surface area of the electrode

Fig. 9. Discharge capacity in the 5th cycle as a function of current density.

18

A. Yu et al. / Solid State Ionics 106 (1998) 11 – 18

borhood of 10 28 cm 2 s 21 . The results are in agreement on the rate capacity shown in Fig. 9. These values are comparable to the values determined from tungstates of different structures [16] and are reflections of the structure differences of the orthorhombic and cubic oxides shown in Fig. 5. The electrochemical behaviours of the sodium molybdenum oxides are mainly decided by the structure and composition of the product. The solution technology may provide a novel method to prepare molybdenum oxides with new phases that are interested in electrochemical devices.

Fig. 10. The open circuit potential of sodium molybdenum oxide electrodes Li x [0.087Na 2 O?MoO 3 ] as a function of the depth of lithium intercalation.

which is substituted by the geometric area in the present study. The chemical diffusion coefficients of lithium ˜ calculated from the above equations intercalation, D, for oxides of different structures, are shown in Fig. 11. The D˜ values for cubic sodium molybdenum oxide 0.087Na 2 O?MoO 3 are generally of the order of 10 210 cm 2 s 21 . For orthorhombic sodium molybdenum oxide 0.056Na 2 O?MoO 3 , the D˜ values are two orders of magnitude higher, in the neigh-

Fig. 11. The chemical diffusion coefficient of lithium ion in sodium molybdenum oxide as a function of the depth of lithium intercalation.

Acknowledgements The authors would like to express their thanks to Mrs. Nobuko Kumagai and Mr. Joji Aritsumi for their experimental assistance.

References [1] F.W. Dampier, J. Electrochem. Soc. 121 (1974) 656. [2] M.S. Whitingham, J. Electrochem. Soc. 123 (1976) 315. [3] J.O. Besenhard, J. Heydecke, H.P. Fritz, Solid State Ionics 6 (1982) 215. [4] N. Malgalit, J. Electrochem. Soc. 121 (1974) 1460. [5] J.O. Besenhard, R. Schollhorn, J. Power Sources 1 (1976) 267. [6] V.B. Krebs, Acta Cryst. B28 (1972) 2222. [7] J.R. Gunter, J. Solid State Chem. 5 (1972) 354. [8] H.R. Oswald, J.R. Gunter, E. Dubler, J. Solid State Chem. 13 (1975) 330. [9] N. Kumagai, N. Kumagai, K. Tanno, Electrochim. Acta 32 (1987) 1521. [10] N. Kumagai, N. Kumagai, Y. Umetzu, K. Tanno, J.P. Pererira-Ramos, Solid State Ionics 86–88 (1996) 1443. [11] L. Kihlborg, Kemi 21 (1963) 357. [12] M. Figlarz, Prog. Solid State Chem. 19 (1989) 1. [13] C. Ho, J. Electrochem. Soc. 127 (1980) 343. [14] W. Weppner, R.A. Huggins, J. Electrochem. Soc. 124 (1977) 1569. [15] N. Kumagai, A. Yu, N. Kumagai, H. Yashiro, Thermochim. Acta 229 (1997) 19. [16] N. Kumagai, A. Yu, H. Yashiro, Solid State Ionics 98 (1997) 159.