Solid State Ionics 124 (1999) 21–28

Comparison of Na V O with Li V O as lithium insertion host11x 3 8 11x 3 8

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

Yokohama 223-8522, Japan

Received 5 January 1999; accepted 25 May 1999

Abstract

Lithium insertion behaviour of Na V O was studied and compared with that of Li V O . The electrode potential and11x 3 8 11x 3 8

structural changes upon lithiation showed the presence of the primary single phase for 0 , x , 2.8 and the consecutive multiphases for 2.8 , x , 3.0 in Li Na V O . Larger interlayer distance in Na V O contributed to a relaxation of interactionx 1.2 3 8 11x 3 8

1 1between inserted Li ions and did not improve a diffusivity of Li ion, compared with Li V O . Lithium insertion and11x 3 8

extraction was a reversible reaction. 1999 Elsevier Science B.V. All rights reserved.

Keywords: Sodium; Vanadate; Lithium; Insertion; Diffusion

1. Introduction further improvement of its electrochemical perform-ance. With a view to fastening diffusion of lithium

Vanadium oxides such as V O and V O have during discharge and charge cycle, the interlayer gap2 5 6 131been investigated as cathode materials for lithium was expanded by substituting part of Li ions at

secondary batteries [1–3]. Lithium trivanadate, octahedral sites with other cations having larger ionicLi V O , is a promising alternative to these van- radii in the interlayer [8–10]. These trials, however,11x 3 8

adates mainly because of its higher rate capability had not brought out more favorable results comparedand cycleability [4–6]. Wadsley [7] reported that it with the case of the original Li V O .11x 3 8

has the layered structure where there are two kinds In an isostructural sodium trivanadate, Na V O ,11x 3 81 1of site for occupation of Li ions between the layers. Na ions are situated at octahedral and tetrahedral

These are octahedral and tetrahedral sites. One mole sites between the layers. Though sodium trivanadateof lithium ion per formula unit is essential for a obviously takes advantage of a larger interlayerbalance of the charge, as described by the formula distance than lithium trivanadate, the former does not

1 51 22 1Li V O . Such Li ions preexist at octahedral exceed the latter extensively in terms of the chemical3 8

sites and link strongly the adjacent layers composed diffusion coefficient of lithium and the discharge1of both V and O atoms. Excess Li ions corre- capacity [11]. When the concept of introduction of a

1sponding to the amount x are accommodated at larger cation was extended to K ion, Na K V O12x x 3 8

tetrahedral sites. Many researchers have attempted and KV O were investigated as lithium insertion3 8

hosts [12,13]. Pistoia et al. [14] reported synthesis*Corresponding author. and lithium intercalation of Na V O by applying11x 3 8

0167-2738/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PI I : S0167-2738( 99 )00134-4

22 J. Kawakita et al. / Solid State Ionics 124 (1999) 21 –28

the same solution technique used for preparation of quantity of the electricity passed through the workingamorphous Li V O . This sodium vanadate was a electrode.11x 3 8

low crystalline material unlike the case of lithium The crystal structure of all the samples wasvanadate and had a larger capacity related to a characterized by X-ray powder diffraction (XRD)limited long-range order, compared with Na V O measurement using the Rigaku apparatus (RlNT-1311x 3 8

prepared by conventional high temperature synthesis 00) with CuKa radiation source filtered by a Ni thin[15]. Other researchers found the ability of plate. To avoid contamination by and reaction withNa V O as a host material for sodium [16,17] and humid air, the glass sample holder for the XRD11x 3 8

magnesium [18,19]. Recently, it was reported that measurement was covered with an X-ray Mylar filmchemically lithiated Na V O functioned as a (Chemplex industry, inc., 25 mm in thickness). The11x 3 8

lithium source in lithium-ion batteries [20]. powder of the chemically and electrochemicallyCompared with Li V O , there are left some lithiated products was set to the sample holder in a11x 3 8

unknown aspects on lithium insertion behaviour of glove box filled with argon gas and in a glove bagNa V O , for example, the range of the primary filled with nitrogen gas.11x 3 8

single-phase region, the presence or absence of the Electrochemical properties were investigated bysecond phase, and the determinant factor of lithiation the galvanostatic discharge and the discharge /chargelimit. To clarify them, the potential and structural cycle experiments at the current density of 2 0.05 to

2changes of sodium trivanadate upon lithiation were 2 1.0 mA cm . The open circuit potential (OCP) offocused on and compared between trivanadates in chemically lithiated products was measured using thethis paper. In addition, a mobility of lithium in electrochemical cell described above. The chemical

˜vanadates was evaluated using the chemical diffusion diffusion coefficient of lithium (D ) was determinedLI

coefficient of lithium. Furthermore, reversibility of by the galvanostatic intermittent titration techniquelithium insertion and extraction reactions was con- (GITT) [23]. The OCP of electrochemically lithiatedfirmed during discharge and charge cycle. samples was obtained additionally by using this

technique. The rest potential was also regarded as the21OCP when potential change was within 0.001 V? h

2. Experimental after galvanostatic discharge up to each x value. Inthis paper, discharge capacity was denoted by the

1Sodium and lithium trivanadates, Na V O and amount of inserted Li ions, i.e. x in Li M V O .1.2 3 8 x 1.2 3 8

Li V O were prepared as described in detail M is Na or Li.1.2 3 8

elsewhere [9,21]. The mixtures of Na CO and V O2 3 2 5

and of Li CO and V O in a molar ratio of 1.2: 32 3 2 5

were melted at 700 and 6808C, respectively. Con- 3. Results and discussionsecutively, the melts were cooled slowly down toroom temperature. All powder samples were sieved The relation between the potential and structuralunder 38 mm in particle size by grinding using an changes of LiV O upon lithiation had been reported3 8

agate mortar and pestle. elsewhere [22]. Lithium insertion proceeds as theLithium insertion into trivanadates was carried out single phase reaction for 0 , x , 1.8 in the nominal

using both electrochemical and chemical methods by composition of Li V O to Li V O , followed by1.2 3 8 3.0 3 8

means of a cylindrical glass cell with three electrodes the two phase reaction for 1.8 , x , 3.0 in theand an n-butyl lithium/n-hexane solution, respec- composition of Li V O to Li V O . The original3.0 3 8 4.2 3 8

tively. All the procedures for both methods were LiV O and the second Li V O phases co-existed in3 8 4 3 8

presented in detail elsewhere [22]. the two-phase region. The shape of OCP profile vs.The lithium content of chemically lithiated prod- composition expresses well these changes of phase

ucts was determined by elemental analysis for Li and during lithium insertion. On the other hand, the XRDV using atomic absorption analysis (Hitachi, 180-55 measurement on NaV O seems to show the remain-3 8

AAS). In the case of the electrochemical method, the ing of the original phase during chemical andamount of inserted lithium was calculated from the electrochemical lithium insertion to NaV O , as3 8

J. Kawakita et al. / Solid State Ionicns 124 (1999) 21 –28 23

Fig. 2. XRD patterns of lithiated products, Li Na V O , byFig. 1. XRD patterns of lithiated products, Li Na V O , by x 1.2 3 8x 1.2 3 8

electrochemical insertion, (a, x50.0 not immersed; b, x50.0chemical insertion (a, x50.0; b, x50.60; c, x51.72; and d,immersed; c, x51.0; and d, x52.0).x52.97).

shown in Figs. 1 and 2. An additional peak of To evaluate the structural modification of NaV O3 8

metallic silver in Fig. 2 is due to a silver paste used during lithium insertion, dependence of the interlayerfor preparation of the working electrode. Once the spacing on x value (Fig. 3) is studied and comparedsample electrode was immersed in the electrolyte with that of LiV O . Note that the interlayer spacing3 8

(i.e. no lithium is inserted), some diffraction lines are was calculated using diffraction lines on (001) andshifting in the XRD pattern compared with those of (003) planes in the XRD pattern, i.e. d and d ,100 003

no immersed sample, as compared in Figs. 2a and b. respectively. The spacing of NaV O has a slightly3 8

Probably, this phenomenon is explained by the ion different value according to lithiation methods. This1exchange that occurs between Na ions in the oxide phenomenon resulted from the ion exchange, as

and Li1 ions in the electrolyte solution to some described above. Almost constant values in bothextent during immersion for about 1 day. As lithium OCP and interlayer spacing for 2.0,x,3.0 indicateinsertion proceeds in NaV O , the diffraction line that electricity flowed during discharge at lower3 8

near 148 in 2u is broadened and all the lines except potential than 1.5 V was consumed for a sidefor this one decrease in relative intensity rapidly and reaction in preference to electrochemical lithiation.some lines are not distinguishable. This result indi- While the interlayer spacing of chemically lithiatedcates a gradual loss of the long-range order that NaV O shrinks for 0,x,2.8, that of LiV O de-3 8 3 8

means not amorphization but subtle distortion of the creases linearly in the single phase region and retainsregular arrangement of atoms. When an additional two constant values in the two phase region. Theline (marked by an arrow) is observed near 158 in the shrinking for NaV O is explained by a strong3 8

1 22XRD pattern of lithiated samples over x52.8 ob- attraction between inserted Li and O ions, whichtained by the chemical method, as shown in Fig. 1d are constituent atoms of VO polyhedron formingn

(for example at x52.97). Presumably, this is due to the layers. The existence of two linear relationshipsformation of the new phase, which was not iden- of interlayer distance vs. x value might be due totified. inequivalency of available sites for occupation be-

24 J. Kawakita et al. / Solid State Ionics 124 (1999) 21 –28

Fig. 3. Relation between interlayer spacing and x in Li M V O , 2x 1.2 3 8 Fig. 4. OCP plots and discharge curves at 20.1 mA cm of(d, M5Na, chemical insertion; m, M5Na, electrochemical M V O , (d, M5Na, chemical insertion; —, m, M5Na,1.2 3 8insertion; s, M5Li, chemical insertion; and n, M5Li, electro- electrochemical insertion, s; M5Li, chemical insertion; and . . .chemical insertion).

n, M5Li, electrochemical insertion).

tween the layers. Beyond x52.8, the interlayerspacing of NaV O increases probably owing to a3 8

presence of the new phase. These results revealedlithium insertion range of the single phase reactionfor 0,x,2.8, followed by a coexistence of the multiphases for 2.8,x,3.0.

The potential vs. composition profiles upon lithia-tion of Na V O and Li V O are shown in Fig. 4,1.2 3 8 1.2 3 8

which include their OCP plots and galvanostatic2discharge curves at 20.1 mA cm . The OCP data of

both chemically and electrochemically lithiated prod-ucts are plotted. In the case of the lithium trivana-date, a decrease of the OCP with x value wasindependent of the lithiation method. On the otherhand, NaV O has the slight potential gap between3 8

OCPs of chemically and electrochemically lithiatedsamples. This phenomenon cannot be simply ex-plained by the deduction that the electrochemically

Fig. 5. Dependence of chemical diffusion coefficient of lithium,lithiated sodium vanadates have not reached theD̃ , on x in Li M V O , (s, M5Na; and b, M5Li).Li x 1.2 3 8equilibrium state yet. This is confirmed by the fact

that the diffusion of lithium in the sodium vanadatewas as approximately fast as that in the lithium vanadates have a common decreasing tendency andvanadate, as seen in Fig. 5. Although LiV O has a an almost same order of magnitude for 0,x ,1.5.3 8

larger apparent chemical diffusion coefficient of Such an OCP gap is even observed at x50 because a˜lithium, D , than NaV O in some x ranges, both rest potential of the electrode immersed for about 1Li 3 8

J. Kawakita et al. / Solid State Ionicns 124 (1999) 21 –28 25

day is lower than that for a few hours. Consequently,the gap will be a result of the ion exchange, as citedabove and thus real x value becomes larger than thatcalculated from the amount of flowed electricity.

1In LiV O , Li ions at the octahedral sites attach3 8

the adjacent layers and exert no hindrance over1incoming Li ions to the tetrahedral sites. These

were explained by the reason that the presence of1Li ions in the original lattice allows further metal–

oxygen bonds in addition to V–O bonds [5]. Further-1more, Na ions at the octahedral sites in NaV O3 8

1were not substituted with Li ions upon lithiation1[14], and thus these Na ions also have the similar

characteristics, i.e. attachment and no hindrance, asdescribed above. To examine the interaction between

1inserted Li ions in both vanadates, OCPs wereplotted after the equation proposed by Armand [24](Eq. (1)),

x /xRT 10 Fig. 6. Dependence of E 1 (RT /F ) ln(z /1 2 z) on z( 5 x /x ) value1] ]]]E 5 E 2 ln 1 K(x /x ) (1)1nF 1 2 x /x of M V O , (a, M5Na; and b, M5Li).1 1.2 3 8

0where E, E , x, x and K were the electrode potential11(OCP), its standard state value, extent of lithium implying a presence of inserted Li ions in the ideal

insertion, its limiting value and inter-guest inter- state. Although the interlayer distance decreasesaction coefficient, respectively and other letters had during lithium insertion, formation of LiO tetra-4

respective usual meanings. In this case, OCPs ob- hedra makes a little contribution to the stabilization1tained by GITT was used as the value most close to energy of inserted Li ions because of a less stability

one in the equilibrium state. The experimental values of LiO tetrahedra on the basis of a larger interlayer4

fitted Eq. (1) with n51 and x 51.5 for 0 , x , 1.5 distance of sodium vanadate compared with lithium1

are given in Fig. 6. Steep slopes are observed for vanadate. The potential of NaV O is lower than that3 8

z , 0.25 (x , 0.3) in both vanadates. These negative of LiV O at any x value upon lithium insertion,3 8

slopes are not derived from the repulsive interaction including the starting material at x50 (see Fig. 4).1 0between inserted Li ions. An initial steep decline of Furthermore, the standard electrode potential, E , is

OCP was described by the opinion that initial estimated by extrapolation of a straight line in Fig. 61inserted Li ions in the LiV O structure contributed for 0.25,z,1.0 to be z50 using Armand equation.3 8

0to reorganize it towards less distorted or stable In fact, E is equal to 2.80 and 2.71 V for lithiumconfiguration, and a composition Li V O was and sodium vanadates, respectively, and thus LiV O1.5 3 8 3 8

reached [8,25]. Probably, this interpretation can is more stable than NaV O by approximately 0.1 eV3 8

apply to the case of NaV O . Beyond z50.25, These results are explained by a difference of the3 8

LiV O has a positive slope (K 5 1 0.075 V ) corre- stabilization energy for site occupation of the in-3 81sponding to possession of the attractive interaction serted Li ion because both vanadates belong to an

estimated as 0.075 eV This result suggests an ordered isostructural monoclinic system.occupation of specific tetrahedral sites by inserted Fig. 7 shows a dependence of the discharge

1Li ions and simultaneous formation of stable LiO capacity on the current density in comparison with4

tetrahedra, leading to a decrease of the interlayer that of LiV O when discharged up to 1.5 V.3 8

distance upon lithiation. On the other hand, NaV O Although NaV O has smaller capacity than LiV O3 8 3 8 3 8

keeps the almost constant value (K¯0 V) against z, at every current density, the former is different from

26 J. Kawakita et al. / Solid State Ionics 124 (1999) 21 –28

Fig. 8. Discharge and charge cycle curves of M V O at 60.11.2 3 8Fig. 7. Dependence of x in Li M V O on current density, (a,x 1.2 3 8 22mA cm , (a, M5Na; and b, M5Li).M5Na; and b, M5Li).

sites in the starting material, In a high lithiumthe latter with respect to the rate of decrease with the content, the charging process is considered to becurrent density. Taking into account the agreement lithium extraction from unstable tetrahedral sites.

1 1of the diffusion coefficient of lithium between both When both Na and Li ions occupy equivalentvanadates, this phenomenon is mainly caused by an sites in small x value (i.e. x,0.5), the former areabsence of the phase transition and a decay of the extracted more easily than the latter cation. This is

1long-range order during lithium insertion to NaV O . expected by the fact that Li ions are bonded to3 822 1Rapid decrease of NaV O in capacity from 0.05 to surrounding O ions more strongly than Na ions3 8

22 10.2 mA cm may be caused by a considerably slow because of a larger charge density of Li ions. Whendiffusion of lithium in the new phase formed beyond the upper limit of charging potential is 3.6 V, a smallx52.8, as in the case with LiV O [26]. amount of lithium is still remained in the host3 8

Fig. 8 shows first discharge and charge cycle structure of NaV O , as is the same in LiV O . It3 8 3 822curves of NaV O and LiV O at 60.1 mA cm , may be extracted if charged up to a higher potential,3 8 3 8

and current direction is reversed at 1.5 V. Most steps similarly to the case of Li V O .3 8

on the discharge curve of LiV O disappear on the XRD patterns of electrode samples of NaV O3 8 3 8

charging curve. This phenomenon is explained by subjected to the electrochemical measurement arethe reason that electrochemical lithium extraction of presented in Fig. 9. After first discharge and chargeLiV O has a reaction path different from insertion, cycling, the patterns seem to be analogous to that of3 8

yet leading to the return to the original structure [27]. the starting material, i.e. sample before cycle.In the case of NaV O , the charging curve returns Though all the diffraction lines of samples after3 8

through a resembling shape of the discharge curve. cycling are ascribed to the original NaV O , some3 8

This result indicates a reversible nature of lithium diffraction lines are shifting slightly. This is causedinsertion and extraction reactions of NaV O . How- by not only the remaining lithium in the host3 8

ever, the charging curve of NaV O has steps for structure after cycling but also substitution of preex-3 81 1x,0.5 and rises up rapidly. This might be related to isting Na ions with inserted Li ions. Consequent-

1an extraction of Na ions preexisting at tetrahedral ly, lithium insertion and extraction reactions of

J. Kawakita et al. / Solid State Ionicns 124 (1999) 21 –28 27

and was almost similar to that in LiV O for 0,x,3 8

1.5 with respect to the order of magnitude.Lithium insertion and extraction processes are

1reversible and Na ions preexisting at the tetrahedralsites might be extracted or substituted with inserted

1Li ions.

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1occupation of inserted Li ion than LiV O . A larger Electrochem. Soc. 145 (1998) 421.3 8[21] J. Kawakita, H. Katagiri, T. Miura, T. Kishi, J. Powerlayer distance lead to a relaxation of the interaction

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1Diffusivity of Li ion in NaV O decreased with x 1569.3 8

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