Solid State Ionics 99 (1997) 165-171

SOLID STATE IONICS

Preparation and lithium insertion behaviour of Li,Na,.,_,V308

Jin Kawakita”, Koji Makino, Yasushi Katayama, Takashi Miura, Tomiya Kishi

Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku-ku, Yokohama 223,

Japan

Received 28 August 1996; accepted 11 March 1997

Abstract

The layered compounds, Li,,Na,,,_,V,O,, have been obtained by the ion exchange reaction of Na,.,V,O, in the molten nitrates containing Li+ and Naf ions with various ratios. Electrochemical measurements showed only two steps of the lithium insertion process of these vanadates and increase in the discharge capacity for y = 0.21. The third step, with the phase transition, has been found to be a characteristic of Li,,,V,O,.

Keywords: Lithium; Sodium; Vanadate; Ion exchange; Molten salt

1. Introduction

Lithium vanadate, Li,+,V,O,, with a layer struc- ture has been regarded as one of the attractive cathode materials for lithium rechargeable batteries [l-3]. In this material, Lit ions occupy the octahed- ral sites between the layers consisting of V,O, units and excess Lit ions denoted by x in Li,+,V,O, occupy the tetrahedral sites in the interlayer [4]. Li+ ions at the octahedral sites link the layers strongly by ionic bonds, and this ‘pinning effect’ gives a stability of the crystal structure with discharge and charge cycling [3]. It was reported that up to more than three moles of lithium per formula unit are able to be inserted and extracted reversibly with the high average potential (2.63 V) [2].

Sodium vanadate, Na, +rV30x, has the layered structure similar to Li l+rVs08. Although Na,+,V30s

*Corresponding author. Fax: + 81-45-563-0446; e-mail: rm310@applc.keio.ac.jp

0167-2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO167-2738(97)00255-5

takes advantage of the larger interlayer distance than Li,+,V30s [4], it is inferior to the lithium vanadate in terms of electrochemical performance such as discharge capacity, i.e. less than 2.0 moles of Lit ions accommodated per formula unit [5,6]. West et al. reported preparation of Na,+,V,O, by ion ex- change reaction of Li, +xV308 in molten sodium nitrate and its sodium insertion behaviour [7]. Pistoia et al. reported synthesis of Na,+,V,O, with low crystallinity by a solution technique and lithium insertion behaviour of it [5,8,9]. Pistoia et al. also reported preparation of the vanadates of Na+ or K’ ions between the layers of V,O, and improvement of discharge capacity [lo]. The reason, however, has not been explained why sodium vanadate with the merit of the larger interlayer distance was inferior to lithium vanadate in terms of discharge capacity. Moreover, the details have not been confirmed clearly on the lithium insertion behaviour of lithium sodium vanadate, Li,Na,.,_,V,O,.

In the present paper the authors report preparation

166 J. Kawakita et al. I Solid State Ionics 99 (1997) 165-171

of Li,Na,.,_,V,O, with various Li/Na ratios by the ion exchange reaction of Na,+,V,O, in molten LiNO,-NaNO, and characterization of the structural and insertion behaviour of these compounds.

2. Experimental

Na,._,V,O, was prepared as described by West et al. 173. A mixture of Na,CO, (Junsei Chemical Co., > 99.5% purity) and V,O, (Junsei Chemical Co., > 99.0% purity) at the molar ratio of 1.2:3 was powdered in an agate mortar and heated at 700°C for 24 h in air. Na,,,V,O, was obtained after cooling the mixture slowly down to room temperature. Na’ ions in Na, ._,V,O, were exchanged by Li’ ions by dispersing 0.03 mol% of the powdered sample in molten LiNO,-NaNO, (both reagents, Kanto Chemical Co., >99.0% purity) at 350°C for 6 h. The extent of exchange was controlled by varying the mole fraction of Li ’ ions in the molten salts from 0 to 1. The samples were washed with water, filtered and dried at both 60°C for over 12 h followed by 90°C for the same time. Lithium content (y) of the sample, Li,Na,.,_,.VJO,. was determined by atomic absorption analysis for lithium and flame atomic emission spectroscopy for sodium using an instrument for elemental analysis (Hitachi, 180-50 AAS). The X-ray powder diffraction patterns of the samples were obtained by Rigaku apparatus (RINT- 1300) using Cu Ka radiation with Ni filter.

Electrochemical measurements were performed using a cylindrical glass cell at 25 and 100°C in a thermostatic water and oil bath, respectively. Lithium wires (Aldrich, > Y9.9% purity, 3.2 mm diameter) were used as the counter and reference electrodes. The electrolyte was a I mol. dmm3 LiClO,/pro- pylene carbonate (PC) solution (Mitsubishi Chemi- cal, water content [H,O] < 20 ppm). A sample pellet was prepared by pressing the mixture of Li,Na,.,_,V,O,, acetylene black (Denka black, De- nkikagaku Kogyo) and poly(tetratluoroethylene) (PTFE) at a weight ratio of 80:15:5 on a porous nickel net at 2 X IO4 N. cmm2. The pellet was used as the working electrode (surface area of 0.196 cm’) in which the surfaces except the working arca were insulated with silicone resin. The test cells were fabricated in the glove box filled with argon. All the

electrochemical measurements were performed with the aid of a potentiostat (Toho Technical Research, 2092 or PS-08). Galvanostatic discharge was per- formed at a current density of 0.05 and 0.1 mA . cm -*. Cyclic voltammetry was performed at a scanning rate of 0.01 mV. s-’ with the potentiostat connected with a function generator (Hokuto Denko, HB-104).

3. Results and discussion

Fig. 1 shows the XRD patterns of Li,.Na,,2 )V308 with various r lialues. It has been reported that the crystal structure of the starting material, Na,,,V,O, (y = 1.2) is closely related to that of Li,,,V30s (y = 0) [4], and lattice parameters have been re- ported by Wadsley [4] and West et al. 171. The spacing hkl of Na,+,V,O, was refined in this work. The unit cell parameters were listed in Table I and

compared to those of Na, zV,O, ]4,7], and Li,.,V,O,

Fig. 1. XRD patterns of Li,Na,,z_, v 0, (a: L’=o, h: J”O.34, c: ) y=O.41, d: y-0.87, e: ~=1.2).

J. Kawakita et al. I Solid State Ionics 99 (1997) 165-171 167

Table 1

Unit cell parameters of M,+;V,O, (M=Li, Na)

a (A)

b (A)

c (8)

P (deg.)

%2W8

7.36(l)

3.61(4)

12.19(l)

107.7(9)

Na, +,V,O, [41

7.38

3.62

12.12

106.36

Na,.,V,O, 171

7.31(l)

3.61(l)

12.22(3)

107.0(2)

Li,+xV& 141

6.68

3.60

12.03

107.50

obtained for a single crystal [4]. Observed and calculated crystal data of Na,.,V,O, were also listed in Table 2. The determination of the detailed crystal structure of Na,+,V,O, is under way.

The vanadium oxide layers are stacked along the

a-axis of the monoclinic unit cell of Na, +,V30,. The strongest peak around 12.6” in 28 was assigned to the diffraction of 100 plane (d,,,) corresponding to the distance between the adjacent vanadium oxide

Table 2

Observed and calculated spacing hkl for Na, zV,O,

hkl Calc. (A) Obs. (A, III, a

100 7.009 6.981 vvs

003 3.869 3.860 M

20i 3.681 3.675 M

202 3.513 3.502 s

011 3.451 3.452 W

iii 3.214 3.209 S

203 3.114 3.109 M

201 3.103 3.106 M

012 3.068 3.065 M

103 3.019 3.013 S

111 2.990 2.990 M

2ii 2.579 2.574 W

302 2.441 2.435 M

300 2,336 2.331 s

203 2.275 2.271 s

304 2.173 2.178 w

114 2.017 2.016 W

305 1.976 1.972 M

015 1.953 1.957 W

402 1.840 1.837 M

403 1.818 1.814 W

020 1.807 1.814 M

303 1.774 1.771 W

120 1.750 1.750 M

312 1.729 1.726 M

401 1.659 1.657 s

023 1.637 1.636 W

208 1.514 1.507 W

403 1.439 1.437 W

‘Relative intensities were affected by preferential orientation of

powder particles.

layers. The ion exchange reaction proceeded in the single phase keeping the crystal structure of Na,.,V,O, for y CO.34, since the position of the

peak of d,,, was shifted slightly to the higher angle with increase in the lithium content (y) and no obvious peak appeared other than those of the original phase, as shown in Fig. lb. Beyond y = 0.41, two phases existed since the peaks ascribed to Li,.,V,O, were also seen in addition to those corre- sponding to Na,,,V,O,, as seen in Fig. lc. After the lithium content exceeded 0.64, an unknown peak appeared near 11 S” in 28 in addition to the peaks assigned to Li,.,V,08, as shown in Fig. Id. Fig. 2 shows the relationship between the lithium content (y) and layer distance of Li,Na,,,_,,V,O, for 0 <y < 0.34, where the primary single phase region was observed in the XRD patterns. It is found that the interlayer distance decreases almost linearly with increase of the content (y) of substituted Li + ions. This was simply due to substitution of smaller Li+ in

I I I I

? 6.95 -

s E 3 z 6.90 - 5 H E

6.85 -

6.601 ; ’ I I 0.1 0.2 0.3

y in Li,Nal.z&08

Fig. 2. Dependence of the interlayer distance on the lithium

content of Li,Na,_,_,V,O,.

168 J. Kawakita et al. I Solid State tonics 99 (1997) 16.5-171

size for Na’ ions between the layers. Consequently, these results revealed that Li,Na,,,_,Na,.,_,V,O, was obtained as the single phase in the range of o<y<o.34.

Extent of substitution (y) changed with Li/(Li + Na) ratio in the molten salts, as shown in Fig. 3. When the mole fraction of Li ’ ions in the nitrate salts was less than 0.2, the lithium contents (y) in the

oxides were on the upside of the line representing y = 1.2 X Li/(Li + Na). On the other hand, when the fraction exceeded 0.2, the lithium content (y) was on the downside of the line. These phenomena seemed to be explained in terms of the activities of Li- and Na- ions in the nitrate salts. For detailed discussion, however, the data for the activity coefficient of both Li ’ and Na ’ ions in the molten nitrate is needed.

Fig. 4 shows the discharge curves of Li,Na, .>_ ,.V,O, obtained at a current density of 0.1

mA* cm.-2. The amounts of inserted lithium in- creased with the content (y) of substituted Lif ions in Li,Na ,.2_,.V,0,, for 0 < y < 0.21, but decreased for y > 0.21. It was supposed to be due to electro- static interaction between cations in the interlayer. Na- ions at the octahedral sites were linked to 02- ions by ionic bonds in contrast to Li- ions linked by

a stronger bond like a covalent one. This was supported by the fact that the difference [0.64(7) A] in the interlayer distance d of Na,.,V,O, and

0.55 0.4 t

0.1 .

0 .

0 _

8

:/ i

a

0

I . I * I . I * I

0 0.1 0.2 0.3 0.4

Li I (Li+Na)

Fig. 3. Dependence of the lithium content of Li,Na, ,_,V,O, on

the cation composition of molten nitrate salts.

3

i

l i

? >

w

2

I

0 I I I

1.0 2.0 3.0 x in LipxNal,t.,,VJO~

Fig. 4. Discharge curves of Li,Na, ~ _,V,O, at -0.1 mA.cm -’

(--: y-0, : y=0.21. “‘: );=0.34, -. -: y=l.2).

Li, ,lV,O, was larger than the value (2r,, - 2r,.i - =

0.52 A) expected using the ionic radii of both Na+ (2rNa+ = 1.16 A [Ill) and Lif ions (2r,,+ =0.90 A 1111) at the octahedral coordination, where d was calculated from the equation of d =(I .sin( 180 - 0). Accordingly, strong repulsion between ionic Na+ ions and inserted Li- ions seemed to lead to decrease in the amounts of accommodated Li ions. Although substitution of Li’ for part of Na’ ions caused decrease in the interlayer distance up to y=O.21 as shown in Fig. 2, relaxation of repulsion between cations presumably enabled additional Li+ ions to be inserted. For y>O.21, larger shrinkage of the interlaycr distance exceeded the effect of relaxa- tion of repulsion by substitution of Ii+ for part of Na- ions.

In the case of Li , ~ xV30a, insertion of Li ’ ions proceeded in the three steps, as described by Picciot- to et al. [12]. In the first and second steps, inserted Li ’ ions occupied two kinds of tetrahedral sites with different stabilization energies. In the third step, lithium insertion occurs with a phase transition from its original structure to defect rock-salt one, whcrc all metal components are coordinated octahcdrally.

Pistoia et al. reported that cyclic voltammctry at a low scan late gave separate energies for site occupa- tion of Li+ ions for M, ,V,O, (M =Li and Na) [3,5,1 I, 13.141. Cyclic voltammograms were shown

J. Kawakita et ul. I Solid &are Ionics 99 (1997) 165-171 169

I I I 1.0 - 0

v El 0.5-

2 5 2 -z o- E g (3

-0.5 -

I

2.0 3.0 4.0

E I V vs. Li+ /Li

Fig. 5. cyclic vohammograms of Li! ,V,O, (a) and Na, ,V,O, (b) at 0.01 mV.s-I.

in Fig. 5a,b, which were obtained for both Li,,,V,O, (a) and Na,,,V,O, (b) at 0.01 mV*s-’ in our experi- ment. In the case of Li,,,V,O,, two cathodic peaks near 2.65 and 2.45 V (marked by filled circles) were ascribed to lithium insertion in the first and second step of lithium insertion process, respectively, as mentioned above. Since these cathodic peaks also appeared near 2.60 and 2.40 V (marked by filled triangles) for Na,.,V,O,, both vanadates seemed to have almost the same energies for site occupation in the first and second steps. The shoulder-like cathodic peak and the corresponding anodic peak were ob- served characteristically near 2.25 and 2.45 V (mark- ed by tilled and open blocks) for only Li,,,V,O,, respectively. These redox peaks arc related with the phase transition, accompanying small displacement of Li and 0 atoms. No appearance of such peaks in the case of Na,,,V,O, will presumably be caused by a lack in the ability of displacement for Na atoms, and will be the possible explanation for decrease in the amounts of Li’ ions inserted to Na,.,V,O,. The anodic peak near 3.00 V for Li,,,V,O, (marked by open circle), corresponding to the two cathodic peaks near 2.65 and 2.45 V, showed extraction of Ii’ ions occupying the sites which resulted in having the resembling stabilization energy for site occupation during lithium insertion. The main anodic peaks appeared at the different potentials near 3.00 and

2.80 V for Li, ,V,O, and Na,.,V,O, (marked by open circle and triangle), respectively. This indicated that extraction of Li- ions was more difticult from the former than the latter, owing to enhanced bond strength of Li + and 02- ions by displacement of Li’. and O2 ions with the phase transition occurring

during lithium ins&on in the case of Li,,2V308. Consequently, Na,,2V,0, might have no region of lithium insertion with the phase transition. This assumption was confirmed by absence of the region in the discharge curve in Fig. 6b when galvanostatic discharge at a smaller current density was performed, and by no appearance of the phase other than the original structure (marked by filled blocks) in the XRD pattern of the sample after discharge, as shown in Fig. 7b. Moreover, discharge at the higher tem- perature (100°C) proceeded further over the two steps observed usually, as shown in Fig. 6c, and in the XRD pattern of the discharged sample shown in Fig. 7c, unknown new phase (marked by filled circles) appeared in addition to the same original structure as the observed one in Fig. 7b. This unknown phase was not ascribed to defect rock salt structure such as Li,,,Na,.,V,O, simulated by as- suming phase transition of Na, ,,V,O,. Accordingly, the phase transition did not occur in this third step of the discharge curve obtained at 100°C. The study on

b

I I I

0 2.0 4.0 6.0

x in L&+,Na, 2_yV308

Fig. 6. Discharge curves of Na I ,V,O, operating at 25°C at -0.1

mA.cm-* (a) and at -0.05 mA.cm-’ (b), and operating at IW”C

at -0.1 mA.cm-* (c).

170 J. Kawakita et al. I Solid State Ionics 99 (1997) 165-171

m : original phase

uu a

I s 1 I I I I I I I I 10 20 30 40 50 60

28

Fig. 7. XRD patterns of Na,.,V,O, before discharge (a), after

discharge up to 1.5 V at 25°C at -0.05 mA.cm-’ (b), and after

discharge up to 1.5 V at 100°C at -0.1 mA.cm-* (c).

discharge behaviour of Na,.,V,O, at the high tem- perature is continued now in our laboratory.

Fig. 8 shows the cyclic voltammograms of Li,Na,,,_,V,O, [y=O (a) and y=O.22 (b)]. Both voltammograms essentially have two cathodic peaks near 2.40 and 2.60 V and one anodic peak near 2.80 V. All of these peaks for y = 0.22, however, became broad compared to y=O. It is suggested that this indicated diversification on stabilization energy for

-0.4 - ,v , I 1.5 2.0 2.5 3.0 3.5

E I V vs. LieI Li

Fig. 8. Cyclic voltammograms of Li,Na,,,_“V,O, at 0.01 mVs_’

(a: y=O.31 and b: y=O.68).

site occupation of inserted Lit ions in the first and second steps. The phase transition did not occur when Li+ was substituted for a part of Na+ ions, since Li,Na, .2_,,V30S ( y = 0.22) has no additional redox peaks which were observed at lower potential in the cyclic voltammogram of Li, ,,V,O,. Conse- quently, substitution of Lit for part of the Nat ions in Na,.,V,O, caused an increase in the amount of

inserted Li+ ions in the discharge curve of Li0,21Na0.99V308 because of relaxation of repulsion between cations in the interlayer, and not because of the phase transition as seen in the lithium insertion process of Li,.,V,O,. It was found that such phase

transition is characteristic of Li 1 .,V,O,.

4. Conclusion

Li,Na ,.2_-yV308 was prepared by the ion exchange reaction in molten LiNO,-NaNO, salt at 350°C. The ratio of Li+ /Na+ in the layer can be controlled by the equilibrium composition in the molten salt. Li,Na,,,_,V,O, had the primary single phase for O<y<O.34. Cyclic voltammograms showed that lithium insertion of Li,Na1_2_,,V30X proceeded in the first and second steps. When y = 0.21 the amounts of inserted Li+ ions increase in the first and second steps owing to relaxation of repulsion between cations in the interlayer. The characteristic phase transition with lithium insertion in the additional

third step was observed only for Li,.,V,O,.

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