Ionic-electronic mixed conduction in LixV2O5

Solid State Ionic, 20 (1986) 135-139 North-Holland, Amsterdam

IONIC-ELECTRONIC MIXED CONDUCTION IN LixV205 Katsumi KUWABARA, Motold ITOH and Kohzo SUGIYAMA Department of Applied Chemistry, Faculty of Engineering, Nagoya University, Furo¢ho, Chikusa4cu, Nagoya 464, Japan Received 30 September 1985 Accepted for publication 30 October 1985

The dc conductivity of lithium vanadium bronze, LixV20 s was measured on polycrystal prepared by solid-state reaction in the x region 0.25-0.70. Both electronic and ionic conduction was observed. The former increased with increase of lithium content and was nearly equal to the total conductivity 10-1-10 ° S/crn. The ionic conductivity (~10 -4 S/ern) measured by dc four-probe technique decreased as the lithium content increased in the range 400-500"C. The apparent activation energy for ionic conduction varied from 57 kJ/mol for x of 0.25 to 82 kJ/mol for x of 0.50.

1. Introduction In a recent study [1 ], the present authors examined the behavior of a lithium concentration cell by using lithium superionic conductor, Lisicon, [2], LixV205/Lisicon/Lio.25V205 . The open circuit voltage of the cell increased monotonously with lithium content in the anode, and the cathode polarization was larger than the anode polarization. In the cell, Lisicon was satisfactorily stable in contact with LixV205 . These experimental facts suggested that the lithium vanadium bronze worked effectively as an electrode material. Much attention has been paid to various cathode materials, such as intercalation compounds, in the field of rechargeable lithium cells. The materials should satisfy various requirements [3,4], e.g. (1) high conductivity of lithium ion to gain a high cathode utility of a cell, (2) high electronic conductivity to increase the charge-discharge efficiency, (3) high energy density to raise the discharge capacity, (4) good reversibility of reaction with lithium ion to elongate the lifetime in charge-discharge cycle. In addition, a wide range of homogeneous phase or little structural change on reaction must be needed. This item was confirmed to be satisfied in a physico0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

chemical study of sintered fl-LixV205 [5]. On the other hand, the first and the second items mentioned above seem to be promising characteristics of LixV205 from the study of the cell performance

[i]. Dudley et al. investigated the electronic and ionic conductivity in the series of studies of potassium ferrite K1+xFellOl7 [6,7] and moreover, studied the mixed conductivity in LiFe508 [8]. Except these papers, studies on mixed conduction in compounds with alkali ion are few [9]. One of the authors of the present paper studied the electronic and ionic conduc. tion in potassium ferrites with beta.alumina structure and found the ionic conduction of about 1% in the mixed conductor [10], and in addition, examined the mixed conduction in sodium vanadium bronze, NaxV205 [11]. According to the latter paper, the electronic conductivity of NaxV205 was nearly equal to the total conductivity and depended on the V 4+ ion concentration. However, the ionic transport number was not obtained in the study. On the basis of these studies, the total conductivity and lithium ion conduc. tivity of LixV205 will be described in this paper relating to the lithium content x.

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K. Kuwabara et al./Ionic-electronic mixed conduction in Lix V20s

2. Experimental procedure 2.1. Materials

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LixV205 was prepared by the following solid-state reaction: I[11 ¢i.. J

xLiVO 3 + (1 - ~x)V205 + ~-xV203 -+ LixV205 . The starting material LiVO 3 was obtained by mixing V205 and Li2CO 3 powders, pressing at 200 MPa into tablets and heating at 560°C for 11 h in dried oxygen flow. V203 was prepared by reducing V205 in hydro. gen gas at 900°C. The prescribed amounts of powders of LiVO3, V205 and V203 were weighed, mixed, dried and pressed to form tablets 13 mm diameter and 3 mm thick. The tablets were heated at 560°C for 18 h in a purified nitrogen gas flow. The samples thus obtained were ground sufficiently and pressed isostaticaUy into tablets of 10 mm in diameter and 3 - 1 0 mm long, and then the tablets were heated at 560°C for 11 h in a purified nitrogen gas flow. The electrolyte, Lisicon, was prepared from GeO2, ZnO and Li2CO 3 according to following reaction: 4GeO 2 + ZnO + 7Li2CO 3 ~ Lil4Zn(GeO4) 4 + 7CO 2. Reagents were pressed into tablets of 13 mm in diameter and 3 mm thick and heated at 1050°C for 11 h in air. The prefired tablets were ground again and pressed isostatically into tablets which were buried in powder of the same composition and sintered at 1050°C for 2 h in air. All materials were identified by X-ray diffraction. 2.2. Measurement o f conductivity

Immediately before measurement of the electrical conductivity, the surface of the sintered tablet of LixV205 was polished to prepare a specimen of 7 - 8 mm diameter and 2 - 8 mm in length by use of emery paper. The current collector was gold foil and the tablet was held by compression in a stainless-steel frame. The total conductivities were measured by dc two terminal method. A constant current was passed through the specimen and the resultant potential difference between the two electrodes was corrected by a blank voltage at zero current. The temperature dependence of the total conductivity was examined over the temperature region from 150 to 500°C in purified

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I

Fig. 1. Schematic arrangement of tablets for measuring ionic conductivity by dc four-probe technique: (M) LixV2Os, (E) Lisicon, (Me) platinum foil, (S) spacing material, (I) mica plate.

nitrogen flow. The polarity of the electrodes was alternately changed every three seconds to eliminate some effect of thermal electromotive force. The ionic conductivity was measured by dc fourprobe technique. An arrangement of the specimens is shown in fig. 1. The tablet of Lisicon was used in order to block electronic current. The ionic voltage probe was tightly set to hold the point contact by using a stainless-steel frame. The measurement was carried out in the range 400-500°C in purified nitrogen flow.

3. Results and discussion 3.1. Total conductivity o f Li x V205

Fig. 2 shows the total conductivities obtained on cooling. The conductivity increased with increase of lithium content in the one-phase region (0.22 ~ x < 0.55) [5 ], but decreased with subsequent increase o f x in the mixed phases (0.55 < x < 1.0). The gradient of the linear curve decreased with increasing lithium content in the one-phase. Kapustkin et al. [12] measured electrical conductivities along the b axis in single crystals of vanadium oxide bronzes MxV205 (M = Li, Na, K) by de potentiometry [ 12]. The compounds showed reversible semiconductor-metai transitions near 500°C, but the hysteresis of the conductivity perpendicular to the b axis of the sodium bronze was on a small scale. The authors explained the hysteresis behavior of the conductivity-temperature curves in relation to the

137

K. Kuwabara et al./Ionic-electronic mixed conduction in Lix V20 s

500

300

t (°c) 100

10

E

0

lO~ >

E

-10

I-.-

b f

102

IO~/T

(K-~ )

Fig. 2. Total conductivities in sintered LixV205 . x in LixVzOs axe: (a) 0.25, (b) 0.30, (c) 0.40, (d) 0.50, (e) 0.60 and (f) 0.70. crystallographic orientation of a specimen. However, the polycrystalline samples in this study did not show the conductivity hysteresis, although X-ray diffraction patterns of the samples revealed the strong peaks which mostly correspond to (hOl) planes parallel to the b axis such as (00 2) and (104). The difference of the conduction behavior between the polycrystal and a single crystal may depend highly on whether the grain boundary exists or not. In other words, the grain boundary can be considered to play a role of relaxation for the semiconductor-metal transition in each crystaUite. The curves shown in fig. 2 are regarded approximately as the electronic conductivities, since the ionic conductivity is smaller by about three orders of magnitude than the electronic conductivity (see later). The deviation from electrical neutrality produced by the difference in the lithium content is compensated by the variation of the oxidation number of vanadium atom. The electrons participating in the conduction process come from the transition V 4+ ~ V s+ + e - [ 11]. Thus, the sample with the higher concerttration of V 4+ ion has the higher concentration of electrons. This means that the samples with larger value o f x will have the higher concentration of conductive electrons. 3.2. Ionic conductivity

Fig. 3 shows a typical behavior of the potential

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Time (s) Fig. 3. ,xVi versus time curves for the sample of Lio.2sV2Os obtained by a current of 30 #A at 450°C. Polarity of the main electrodes was changed alternately. AVi means the potential difference between the ion probes.

difference between ionic probes in the four-probe cell. Charging and decaying curves were obtained similarly when the current was supplied inversely. The potential difference varied in proportion to the magnitude of the current passed through the main assemble of the specimens. Such behaviors were also observed in other samples with different lithium content. Ionic conductivity can be calculated from the steady-state potential difference and the current. Arrhenius plots of the ionic conductivity are indicated in fig. 4 for four specimens. As can be seen, t(=~) 500

450

400

"E v

6ld 1

113

114 103/T (K -1 )

115

Fig. 4. Lithium ion ¢onductivities in LixV=Os . x are (a) 0.25, (b) 0.30, (c) 0.40 and (d) 0.50.

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K. Kuwabara et al./lonic-electronic mixed conduction in L i x V 2 0 s

t (*C) 500

450

400

I

80

_--" 7c 0 E ,;,60

G

1:4

+:5

1 0 3 / T ( f 1)

030

o.;+o

o.so

x in LixV205

Fig. 5. Transport numbers in LixV20 s . x are (a) 0.25, (b) 0.30, (c) 0.40 and (d) 0~50.

Fig. 6. Apparent activation enezgy fez lithium ion conduction in LixV20 s .

the curves are linear and the ionic conductivity decreases as the lithium content increases. Such a relation between the ionic conductivity and the alkali content was observed in the study of the mixed conduction in NaxV205 [11]. The lithium bronze has the same ionic conductivity of the order of 10 -4 S/cm at 450°C as the sodium ion conductivity of the sodium bronze though the ionic species and the crystal structure of the lithium bronze are different from those of the sodium bronze. The transport number (ti) of the lithium ion in LixV205 can be calculated from both the total conductivity (fig. 2) and the lithium ion conductivity (fig. 4). t i thus obtained is shown in fig. 5. The temperature dependence of t i seems to reflect mainly the ionic conductivity and t i has the order of magnitude of 10-4. Moreover, t i decreases with increasing lithium content as did the ionic conductivity. The apparent activation energy for ionic conduction can be obtained from the gradient of the Arrhenius plot. Fig. 6 shows the apparent activation energy (Ei) as a function o f x in LixV205 . The energy value increases with increasing lithium content, but the increment, AEi, decreases gradually as the lithium content approaches the upper limit of the one-phase region. The energy, E i, consists of the energy for produc-

tion (Ep) and the energy for migration (Era) of the conductive species. These energies may relate to the crystal structure of the sample. The lithium bronze with/i-phase structure have three possible sites for lithium ion location, i.e. a seven-coordinated site (L1), an eight~oordinated site (L2) and a tetrahedral site (L3) [13]. According to the study on the structure of the LixV205 by Galy et al. [14], a small excess of lithium ions in the/1-phase (0.22 =
K. Kuwabara et aL/1onic-electronic mixed conduction in Li x V20s

oxygen atoms more than those in the L3 sites. This m a y lead to a condition that the energy Ep decreases and in turn, the concentration o f the conductive species increases with increase o f the lithium content. The ionic conductivity is proportional to the product o f the concentration o f the mobile ions and the m o b i l i t y o f the concerned ion. If the mobility does not vary with increase o f lithium content, the conductivity should increase with increase o f the conductive ions. However, fig. 4 displayed the inverse tendency in the ionic conduction. Therefore, the mobility o f the lithium ion must decrease as the lithium content increases. This reflects that E m increases with lithium ion packing. The increasing curve shown in fig. 6 suggests that the contribution o f E m to E i is larger than the contribution o f E_ and that the increment, AEm, decreases as the ~thium packing progresses. However, precise discussion, such as ionic conduction mechanisrn or conduction path was impossible for the present polycrystalline sample.

139

References [ 1] K. Kuwabara, M. Itoh and K. Sugiyama, J. Appl. Electrochem. 14 (1984) 759. [2] H.Y.-P. Hong, Mater. Res. Bull. 13 (1978) 117. {3] M.S. Whittingham, J. Solid State Chem. 29 (1979) 303. [4] D.W. Murphy and P.A. Christian, Science 205 (1979) 651. [5] K. Kuwabaxa, K. Sugiyama and M. Itoh, J. Ceram. Soc. Japan 93 (1985) 1. [6] G.J. Dudley, B.C.H. Steele and A.T. Howe, J. Sofid State Chem. 18 (1976) 141. [7] G.J. Dudley and B.C.H. Steele, J. Solid State Chem. 21 (1977) 1. [8] G.J. Dudley and B.C.H. Steele, J. Electrochem. Soc. 125 (1978) 1994. [9] W.L. Roth and R.J. Romanczuk, J. Electrochem. Soc. 116 (1969) 975. [10] T. Takahashi and K, Kuwabara, J. Solid State Chem. 29 (1979) 27. [11] T. Takahashi, K. Kuwabara and Y. Abe, Solid State lonics 2 (1981) 139. [12] V.K. Kapustkin, V.L. Volkov and A.A. Fotiev, J. Solid State Chem. 19 (1976) 359. [13] A.D. Wadsley, Aeta Cxyst. 8 (1955) 695. [ 14] J. Galy, J. Darriet, A. Casalot and J.B. Goodenough, J. Solid State Chem. 1 (1970) 339.