Solid State Ionics 124 (1999) 29–35

Effect of crystallinity on lithium insertion behaviour of Na V O11x 3 8

*Jin Kawakita , Takashi Miura, Tomiya KishiDepartment of Applied Chemistry, Faculty of Science and Technology, Kejo University, Hiyoshi 3-14-1, Kouhoku-ku,

Yokohama 223-8522, Japan

Received 26 March 1999; accepted 25 May 1999

Abstract

The amorphous sodium vanadate, Na V O was prepared by quenching of the melt and its lithium insertion behaviour11x 3 8

was compared with those of poorly crystalline and highly crystalline ones. The experimental results showed that thecrystallinity of Na V O determined the role of sodium on moving of lithium upon lithiation and the flexibility of adjacent11x 3 8

units in the crystalline host lattice. 1999 Elsevier Science B.V. All rights reserved.

Keywords: Amorphous; Vanadate; Quenching; Lithium; Insertion

1. Introduction higher diffusion rate of lithium ions and the reducedinteraction between the interlayer cations.

The lithium trivanadate, Li V O is a promising According to the reports concerning the crystal11x 3 8

alternative to vanadium oxides such as V O and structure of Na V O , sodium ions as pillar cations2 5 11x 3 8

V O for positive electrodes of the lithium sec- are situated at octahedral sites between the layers6 13

ondary batteries since Besenhard et al. reported its composed of vanadium and oxygen atoms, andsuitability as lithium insertion host [1]. This vanadate vacant tetrahedral sites are capable for occupation ofhas vanadium atoms in the pentavalent state and lithium and sodium ions [2,3].lithium ions acting as pillars between the vanadium Lithium insertion behaviour of Na V O has11x 3 8

oxide layers [2]. The pillar ions contribute to stabi- been studied by using electrochemical methods,lize the host structure during lithium insertion and XRD, IR and XAS measurements [4,5]. The authorsextraction processes and to expand the interlayer, reported the presence of the primary single phase forleading to increase in the amount of acceptable guest 0 , x , 2.8 and the consecutive multi-phase forspecies and the ion diffusion rate in the material [3]. 2.8 , x , 3.0 in Li Na V O [6]. In these studies,x 1.2 3 8

The isostructural sodium trivanadate, Na V O has Na V O has been prepared by the conventional11x 3 8 11x 3 8˚ high temperature synthesis and by applying the samea larger interlayer distance (7.08 A) than lithium

˚ solution technique used for the synthesis of amor-trivanadate (6.36 A) [2]. Therefore, it can providephous Li V O . Unlike the case of the lithiumadditional advantages over Li V O , which are the 11x 3 811x 3 8

trivanadate, Na V O by the solution technique11x 3 8

was a low crystalline material and had a larger*Corresponding author. capacity related to a limited long-range order com-

0167-2738/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PI I : S0167-2738( 99 )00135-6

30 J. Kawakita et al. / Solid State Ionics 124 (1999) 29 –35

pared with Na V O by the high temperature Thermogravimetry and differential thermal analy-11x 3 8

synthesis [4]. Furthermore, the results of electro- sis (TG–DTA, Mac Science TGDTA 2000) were7chemical and Li NMR measurements showed that carried out on the AM sample to detect the transition

diffusion of lithium in Na V O is faster than that from the amorphous to the crystalline state. The11x 3 8

in Li V O [7]. Other researchers found out the samples were examined by X-ray diffraction (XRD,11x 3 8

ability of Na V O as a host material for sodium Rigaku RINT 1300) measurement by CuKa radia-11x 3 8

[8,9] and magnesium [10,11]. Recently, it was tion with a nickel filter, and Fourier transformreported that chemically lithiated Na V O infrared (FT–IR, Bio-Rad FTS- 165) spectroscopic11x 3 8

functioned as a lithium source in lithium-ion bat- measurement using KBr disk method. The chemicalteries [3]. composition of all the samples was determined to be

In this paper, the authors reported the synthesis of Na V O by elemental analysis using atomic ab-1.2 3 8

amorphous NaV O by quenching of the melt and its sorption for vanadium and flame emission spec-3 8

characteristics as lithium insertion host. In addition, troscopy for sodium (Hitachi, 180-55 AAS).this amorphous vanadate was compared with the Electrochemical measurements were carried out bycrystalline ones obtained by its heat treatment and using the three-electrode cell, as presented elsewherethe high temperature synthesis. The effect of the in details [14]. The working electrode was preparedcrystallinity on lithium insertion behaviour of by pressing a powder mixture of the oxide sample,Na V O was elucidated. acetylene black (Denka Black, Denkikagaku Kogyo)11x 3 8

In the following parts, the crystallinity is classified and poly(tetrafluoroethylene) (PTFE, Mitsui-Duinto three types. The first is the amorphous vanadate Pont) in a weight ratio of 70:25:5. Metallic lithiumprepared by quenching of the melt, termed as AM. rods (Aldrich, . 99.9% purity, 3.2 mm in diameter)The second is the low crystalline one obtained by the were used as the counter and reference electrodes.

23heat treatment of the AM sample, termed as LC. The The electrolyte was 1 mol dm lithium perchlorate /last is the highly crystalline material prepared by propylene carbonate solution (LiClO /PC, Mit-4

slow cooling of the melt, termed as HC. subishi Chemical, battery grade) containing waterwithin 20 ppm. All the cells were fabricated in aglove box filled with an argon gas. Galvanostatic

2. Experimental discharge and charge cycle test was performed withan aid of galvanostat (Toho Technical research PS-

The amorphous sodium trivanadate, Na V O 08) controlled by a personal computer (NEC, PC-11x 3 8

(AM) was prepared by quenching of the melt, as 980 1). The rest potential was regarded as the quasi-described in preparation of Li V O (AM) [12]. A open circuit potential (OCP) when its change was11x 3 8

21mixture of sodium metavanadate (NaVO , Soekawa within 0.001 V h after discharge experiment. XRD3

Chemicals, . 99.9% purity) and vanadium pentox- measurement was also performed after electrochemi-ide (V O ) in a molar ratio of 1.2:0.9 was heated in a cal experiment, as described elsewhere [14].2 5

platinum crucible at 9008C for 30 h in air. V O used2 5

in this experiment was synthesized by thermaldecomposition of ammonium metavanadate 3. Results and discussion(NH VO , Soekawa Chemicals, . 99% purity) at4 3

5008C for 24 h. The crucible was swung for some 3.1. Preparationtimes to attain a homogeneous composition of themelt. Then, the melt was quenched by putting the The TG–DTA curves of NaV O (AM) are pre-3 8

crucible on ice water rapidly. The resulting sample sented in Fig. 1. Two exothermic peaks appear nearwas ground and heated at 220 and 3508C for 12 h in 200 and 2508C on the DTA curve. A similar DTAair to produce the low crystalline samples, LC220 curve was observed in the case of amorphous LiV O3 8

and LC350, respectively. The reference material, prepared by the quenching method [12]. The TGhigh crystalline Na V O (HC) was prepared by curve does not have any weight change above 2008C,11x 3 8

slow cooling of the melt [13]. All the products were although a weight loss can be seen below aboutsieved under 38 mm. 1508C and caused by vaporization of adsorbed water.

J. Kawakita et al. / Solid State Ionics 124 (1999) 29 –35 31

peaks at 200 and 2508C in this experiment would bedue to crystallization of the NaV O phase and to a3 8

kind of ordering phenomenon, respectively.Fig. 2 shows XRD patterns of NaV O (AM,3 8

LC220, LC350 and HC). NaV O (AM) has an3 8

amorphous nature because no obvious diffractionlines appear in the XRD pattern except for a hallowcharacteristic of the amorphous substance, as shownin Fig. 2a. Accordingly, the quenching methodpermits the gain of amorphous NaV O , which could3 8

not be obtained even by the solution technique. Asfor the LC220, LC350 and HC samples, all thediffraction lines in the XRD patterns (Figs. 2b–d)are attributed to Na V O , which belongs to a11x 3 8

monoclinic system. The two LC samples, however,have broader diffraction lines than the HC sample,indicating the smaller grain size and imperfection ofthe crystal. As the temperature of heat treatment

Fig. 1. TG–DTA curves of amorphous NaV O .3 8 increases, some peaks of LC become a bit clear and]

sharp, for example, (103) and (305). Furthermore,the relative intensities of the LC samples seem to be

As discussed later, XRD patterns of the samples considerably different from those of the HC sample.heat-treated at 220 and 3508C (see Fig. 2) are Then, the relative intensities of the diffraction lines

]attributed to the monoclinic Na V O . Therefore, of (020), (003), (111) to (100) for LC220 and11x 3 8

these observations suggest that the two exothermic LC350 are compared with those for HC in Fig. 3.The HC sample has much smaller intensities than theLC samples because of extremely strong diffractionline on (100) plane. This phenomenon might be

Fig. 2. XRD patterns of NaV O , (a) prepared by quenching3 8

(AM), heat treated at (b) 2208C (LC220) and (c) 3508C (LC350),and (d) prepared by slow cooling (HT). Fig. 3. Relative intensity in some planes of NaV O .3 8

32 J. Kawakita et al. / Solid State Ionics 124 (1999) 29 –35

There seemed to be no obvious differences betweenLC 220 and LC350. In the case of LiV O , it is3 8

reported that the exothermic peak at higher tempera-ture on the DTA curve is related to an arrangementof the layer to gain some long-range orders [12].Accordingly, a similar structural rearrangementmight occur also in NaV O by the heat treatment3 8

above 2508C where the second exothermic peakappears (see Fig. 1). Fast crystal growth, however,does not proceed even at 3508C mainly because of aslower moving rate of bulky Na atoms in thecompound.

3.2. Lithium insertion and extractioncharacteristics

Fig. 5 shows the relation of open circuit potential(OCP) of the oxides versus composition x inLi Na V O obtained by electrochemical lithiumx 1.2 3 8

insertion. As for the AM sample, the continuousFig. 4. IR spectra of NaV O , (a: HC, b: LC3SO, c: LC220 and d:3 8

AM). decrease in potential with x is characteristic of theamorphous material, and indicates a progress oflithium insertion up to x 5 4.0. The shape of the

caused by a preferred orientation of the crystal due to curve of HC expresses the phase state of the lithiateda plate-like shape of the powdered HC sample. product, as reported by the presence of the primary

Fig. 4 shows FT–IR spectra of NaV O (AM, single phase for 0 , x , 2.8 and the consecutive3 8

LC220, LC350 and HC). According to the previousstudies on IR spectra of this oxide [5,15], the two

21main peaks around 1000 and 950 cm were as-cribed to the stretching vibration between V and Oatoms, n(V5 O), corresponding to the existence ofthe short-range order of VO unit. Although these6

two peaks were observed obviously for the HCsample, they are broadened and overlapped byadditional peaks for the LC samples. Vanadiumatoms in LC are not surrounded by oxygen ones soregularly and specifically as in HC. As for AM in

21Fig. 4d, an ambiguous peak near 950 cm confirmsa disordered configuration of the atoms, as seen fromthe result of the XRD measurement. On the other

21hand, the peak near 730 cm is attributed to thestretching vibration between V and O atoms, n(V–O–V), indicating the existence of the long-range order,and it is observed clearly for HC, as presented in Fig.4a. This peak becomes broad for the LC samples,and disappears for the AM sample. Contrary to AM,LC has a long-range order similar to HC. Its degree, Fig. 5. OCP plots of Li Na V O , (s: AM, ,: LC220, n:x 1.2 3 8

however, is smaller than that in the HC sample. LC3SO, and h: HC).

J. Kawakita et al. / Solid State Ionics 124 (1999) 29 –35 33

multiphase for 2.8 , x , 3.0 in Li Na V O upon the long-range order and might be related to forma-x 1.2 3 8

lithiation [6]. The LC samples have a potential curve tion of the multi-phase. No diffraction lines in thesimilar to the HC sample for 0 , x , 3.0 and plateau pattern of AM show the successive absence of thepotentials near 2.4 V and 2.25 V for LC350 and crystalline phase upon lithiation.LC220, respectively. This result shows that insertion Fig. 7 shows the first discharge and charge cyclereaction of the LC samples proceeds up to x 5 2.8 by curves of the samples. As for HC, an identical shapethe same mechanism as HC, followed by further of the curves during discharge and charge cyclelithiation in the multi-phase. The potential gap indicates the reversibility of lithium insertion andbetween the LC samples for x . 3.0 might be extraction reactions. In the case of AM, an absencedescribed by the reason that LC220 has also a of any plateaus during the cycle indicates non-occur-characteristic similar to AM. rence of crystallization. The charging capacity of

The XRD patterns of electrochemically lithiated AM smaller than the discharge one corresponds toproducts, Li Na V O are compared in Fig. 6. the remaining of part of lithium in the compound3.0 1.2 3 8

1Note that the peaks overlapped with the dashed lines because of existence of the strong bond between Li22in the pattern are due to the existence of metallic and O ions. The LC samples exceed the AM and

silver (Ag) which was used for connecting a copper HC ones in terms of the discharge capacity. Thelead with a pellet of the working electrode and could higher capacity of the LC samples is caused bynot be washed out completely. As for HC sample in having a lower degree of the long-range order than

1Fig. 6a, the presence of the multi-phase is confirmed the HC one. In the structure of LC, Na ion works asby observation of additional diffraction lines near 158 a pillar at the octahedral site in the interlayer and notand 238 in 2u, and yet were not ascribed to already as a hindrance of diffusion of lithium while it is aknown materials. For the LC samples, obvious network modifier in the AM sample and disturbs a

1diffraction lines are not observed in the patterns. movement of Li ion. The behaviour of LC220This phenomenon is caused by a considerable loss of seems situated between LC350 and AM samples.

Consequently, a regular atomic arrangement, i.e.1Na ions are situated at octahedral sites, is essential

for improvement of insertion kinetics. A long-range

Fig. 6. XRD patterns of chemically lithiated samples, Fig. 7. Discharge and charge cycle curves of NaV O at 61003 822Li Na V O , (a: HC, b: LC350, c: LC215, and d: AM). mA?cm , (a: AM, b: LC220, c: LC350, and d: HC).3.0 1.2 3 8

34 J. Kawakita et al. / Solid State Ionics 124 (1999) 29 –35

order, which is sufficient enough to give a preferred units or by the long-linked rigid structure. Theorientation, leads to less flexibility of adjacent units dependence of the coulombic efficiency of the firstand retardation of insertion reaction together with cycle on the c.d. is given in Fig. 8b. While the c.d.

22formation of the multi-phase. increases from 50 to 100 mA ? cm , the efficiency ofThe dependence of the discharge capacity on the all the samples becomes large. In this current range,

current density (c.d.) is shown in Fig. 8a, where the the discharge capacity has a larger decreasing ratefirst discharge and charge is cycled between 3.5 and than the charge capacity because of a larger decre-1.5 V. The discharge capacity of AM decreases ment of the region corresponding to existence of therapidly as the c.d. increases, compared with the multi-phase. There is a tendency that the AM sampleothers. This phenomenon is explained by the reason has a smaller efficiency than the crystalline ones.

1 1that Na ions prevent Li ions from jumping This is because a considerably attractive interaction1 22between the sites. The LC350 sample has a profile of between inserted Li and O ions is responsible for

the capacity decay similar to the LC215 one. This an irreversible capacity loss of AM during charging.indicates that both samples have an identical reaction Although the LC samples are comparable to the HCmechanism of insertion, including no hindrance by sample in terms of the efficiency, they have more

1 1Na ions at octahedral sites and possible formation Li ions are remained in the LC samples. Theirof the multi-phase. Presumably, LC350 has a more amount corresponds to a difference between theregular arrangement of the layered structure than for discharge and charge capacity, as expressed by theLC220, resulting in a larger capacity of the former in following equation,an almost whole c.d. range. Smaller capacity and less

122 Residual Li ions 5 discharge capacitysteep slope of the HC sample over 100 mA ? cmare due to a difficulty in formation of the multi-phase efficiency

]]]S D3 1 2 .and comparatively high insertion rate in the original 100host lattice, caused by less flexibility of adjacent

1Accordingly, more remaining of Li ions in the LCsamples is explained by formation of the strong

1 22bonds between Li and O ions resulted fromdisordering of the atomic arrangement and irrevers-ible structural change to the multi-phase duringlithium insertion. On the other hand, the residual Liion in HC is within 0.2 mole per formula unit except

22at 50 mA?cm . In the case of Li V O , it was11x 3 8

reported that the structure of a slightly lithiatedcompound, Li V O (x,0.5) is a less distorted11x 3 8

configuration, compared with the ‘non’-lithiated one,LiV O [16]. This consideration can be applied to3 8

NaV O with a similar structure to LiV O , and3 8 3 8

probably a stable vanadate is formed with the2composition Li Na V 0 . At 50 mA?cm , the0.2 1.2 3 8

1remaining Li ion in HC is related to irreversiblestructural transformation from the multi-phase to theoriginal phase.

The XRD patterns of NaV O (AM, LC and HC)3 8

after the first discharge and charge cycle between 3.6V and 1.5 V are compared in Fig. 9. As for HC, some

Fig. 8. Dependence of (a) discharge capacity, x in Li Na V Ox 1.2 3 8 diffraction lines observed clearly and ascribed toup to 1.5 V and (b) coulombic efficiency during first cycleNaV O show a retaining of the host lattice duringbetween 3.5 and 1.5 V on current density, (d: AM, .: LC220, m: 3 8

LC3SO, and j: HC). lithium insertion and extraction. They, however, are

J. Kawakita et al. / Solid State Ionics 124 (1999) 29 –35 35

long-range order and a less preferred orientationcompared with Na V O prepared by the conven-11x 3 8

tional high temperature synthesis.1In amorphous Na V O , Na ion is a network11x 3 8

modifier in the amorphous structure and prevents1inserted Li ion from moving in the compound.

1Furthermore, parts of inserted Li ions are bonded22strongly with O ions and become unextractable.

1In crystalline Na V O , Na ion acts as a pillar11x 3 8

in the interlayer and is not a hindrance of moving of1inserted Li ions. When a long-range order is limited

to some extent, adjacent units of the host lattice areflexible and the atomic arrangement is liable to bedisordered during lithiation. Once the crystal ofNaV O is grown largely enough to have a preferred3 8

orientation, it becomes a rigid structure, which losesflexibility.

ReferencesFig. 9. XRD patterns of NaV O after first cycle between 3.6 and3 8

1.5 V, (a: HT, b: LC350, c: LC220, and d: AM). ¨[1] J.O. Besenhard, R. Schollhorn, J. Power Sources 1 (1976)267.

[2] A.D. Wadsley, Acta Cryst. 10 (1985) 281.shifting slightly, which is caused by not only the´[3] M.E. Spahr, P. Novak, W. Scheifele, O. Haas, R. Nesper, J.remaining lithium in the host structure after cycling

1 Electrochem. Soc. 145 (1998) 421.but also substitution of part of pre-existing Na ions[4] M. Pasquali, G. Pistoia, Electrochim. Acta 36 (1991) 1549.1with inserted Li ions. Although the LC samples [5] R. Tossici, R. Marassi, M. Berrettoni, S. Stizza, G. Pistoia,

have diffraction lines assigned to NaV O , their Solid State Ionics 67 (1993) 77.3 8

[6] J. Kawakita, T. Miura, T. Kishi, Solid State Ionics, in press.relative intensities decrease considerably. In addition,[7] G. Wang, J. Roos, D. Brinkrnann, M. Pasquali, G. Pistoia, J.these lines are shifting and broadening, compared

Phys. Chem. Solids 54 (1993) 851.with the lines of the original material before cycling.[8] K. West, B. Zachau-Christiansen, T. Jacobsen, Solid State

This phenomenon indicates that a remarkable dis- Ionics 28/30 (1988) 1128.ordering of the atomic arrangement upon lithiation [9] K. West, B. Zachau-Christiansen, T. Jacobsen, S. Skaarup, J.

Power Sources 26 (1989) 341.leads to having a difficulty in the recovery of the´[10] P. Novak, W. Scheifele, O. Haas, J. Power Sources 54 (1995)original structural configuration. On the other hand,

479.no diffraction lines in the pattern of the AM sample´[11] P. Novak, W. Scheifele, F. Joho, O. Haas, J. Electrochem.

result in keeping the amorphous state during charg- Soc. 142 (1995) 2544.ing as well as during discharge. [12] J. Kawakita, T. Kato, Y. Katayama, T. Miura, T. Kishi, J.

Power Sources, in press.[13] J. Kawakita, K. Makino, Y. Katayama, T. Miura, T. Kishi,

Solid State Jonics 99 (1997) 165.4. Conclusion[14] J. Kawakita, Y. Katayama, T. Miura, T. Kishi, Solid State

Ionics 107 (1998) 145.The sodium trivanadate, Na V O was obtained [15] Y. Kera, J. Solid State Chem. 51 (1984) 205.11x 3 8

[16] M. Pasquali, G. Pistoia, V. Manev, R.V. Moshtev, J. Electro-in an amorphous state by quenching of the melt ofchem. Soc. 133 (1986) 2454.NaVO and V O . The heat treatment changed this3 2 5

vanadate to a crystalline one, which has a less