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EISEYIER Solid State Ionics 96 (I 997) 195-200

Superionic conductivity of glass-ceramics Al,O,-TiO,-P,O,

in the system L&O-

Jie Fu”

&search Institute, OHARA Inc., I- 15-30 Oycrma, Sqatnihara-shi, Kanqawu 229, Japan

Received 14 May 1996; accepted 14 October 1996

Abstract

A Li,O-AI,O,-TiO,-PzO, glass was prepared and converted into glass-ceramics through heat-treatment at 700- 1000°C for 12 h. X-ray analysis indicated that the resultant glass-ceramics were mainly composed of Li, + \Al,Ti> \(PO,), phases.

The lithium ion conductivity of the glass-ceramics increased with the growth of the Li, +.AI,Ti,..,(PO,), phase and a

extremely high value, 1.3 X 10m7 S cm-‘, was obtained at room temperature. This value is as large as that of Li,N which is

well known to exhibit the highest lithium ion conductivity for a solid electrolyte.

Kewwd.~: Ionic conductivity - lithium; LiZO-Al,O,-TiO?-P20, system: Glass-ceramics; Li, ,\AI%Ti,_ >(PO,), phase

1. Introduction

In recent years numerous studies have been de-

voted to the development of high Li+ ion solid

electrolytes for their potential applications in high

energy density batteries and other electrochemical

devices. Some materials such as Li,N [l] single crystal and SiS,-P*S,-Li,S-LiI [2] glasses have

been found to show very high conductivity of over lo-’ S cm-’ at room temperature, but they are not easily fabricated and have poor chemical durability,

largely restricting their practical use. The sintered

LiTi,tPO,),, which possesses a NASICON-type

structure, was reported to exhibit a relatively high

conductivity of 10m6 S cm-’ at room temperature and an enhancement of more than two orders of magnitude in conductivity was observed in the M1’

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substituted Li , +,M,TiZ_rtPOJ)3 (M = Al, In, etc.) systems [3,4]. These sintered materials have become

attractive for use as solid electrolytes because of

their good chemical stability and easy handling. In

comparison with the sintered materials, glass-

ceramics have a much large advantage because they

can be easily manufactured into a desired size or shape and have a dense microstructure. Therefore,

the development of high conducting glass-ceramics is of more practical significance. Glass-ceramics

generally exhibit lower ionic conductivity than the

corresponding mother glasses [ 5,6], but remarkable

enhancement can be realized when conductive crys- tals are precipitated from the glasses [7,8]. Recently,

the author explored the glass formation in the system Li,O-Al,O,-TiO,-P,O, and examined the crys- talline phases after heat-treatment of the obtained glasses. It was found that a composition consisting of 14Li,0-9A1,0,-38Ti0,-39P,O, formed a glass and after heat-treatment glass-ceramics containing

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196 J. Fu / Solid State lonics 96 (1997) 19.5-200

Li,+,Al,Ti,_,(PO,), as a major crystalline phase

were obtained. It is worthwhile to note and surpris-

ing that the resultant glass-ceramics exhibit extreme-

ly high conductivity over lo-’ S cm-’ at room

temperature. In the present paper, the effect of heat-

treatment conditions on the conductivity will be

reported.

2. Experimental procedure

Glasses were prepared using a conventional melt-

quenching method. Reagent grade chemicals of

L&CO,, AI(O TiO, and NH,H,PO, were used as starting materials. A 30 g batch consisting of

14Li,0-9A1,0,-38Ti0,-39P,O, (in mol%) was placed in a platinum crucible and melted in an

electrical furnace. The batch was initially kept at

700°C for 1 h in order to release volatile products

coming from the starting materials. Then, it was

heated up to 1450°C and melted at this temperature for 1.5 h. The melt was poured onto an preheated

stainless steel plate and pressed with another one.

The resultant glasses were annealed at 550°C for 2 h.

The glass transition temperature (T,) and the onset

crystallization temperature (T,) were determined

using differential thermal analyses (DTA) at a heat-

ing rate of 10 K min-‘. Heat-treatments were

performed at the temperatures above T, for 12 h.

Crystalline phases were identified by X-ray diffrac-

tion (XRD) analyses at room temperature using CuKcv radiation. In order to determine the lattice

constant, the diffraction peak positions (219) were

precisely measured by scanning between 20 = 10 and 40” in 0.01” steps and counting for 5 s at each step. High purity silicon was used as a standard. The

microstructures of the crystalline phases were ob- served by means of scanning electron microscopy

(SEM) on the fracture surfaces.

Polished samples were used for ionic conductivity measurements. Gold electrodes were sputtered on both sides of the samples. The measurements were carried out with the complex impedance method using a solartron 1260 impedance analyzer connected to a computer. The frequency range used was lo-‘-- 10’ Hz. Ionic conductivity values were calculated from complex impedance plots.

I I I

400 600 800 1000

Temperature (“C)

Fig. 1. DTA curve of the cast glass.

3. Results and discussion

Fig. 1 shows the DTA cure of the cast glass. From this curve, the glass transition (T,) and crys-

tallization temperatures (T,) were estimated to be

650°C and 690°C. respectively. X-ray diffraction analysis revealed that the sharp crystallization peak

was solely due to LiTi,(P0,)3 (conducting phase).

No melting temperature was observed up to lOOO”C, indicating the present material has high resistance to

heating.

Fig. 2 shows the XRD patterns of the specimens

heat-treated at various temperatures higher than the

crystallization temperature for 12 h. Diffraction

peaks arising from LiTi,(PO,), were observed in all

the specimens. Only one diffraction peak due to

AlPO, was also observed, but it could be detected

clearly only at heat- treatment temperatures higher than 800°C. These results indicate that the LiTi,(PO,), precipitates easier than AlP04 and that

LiTi,(PO,), is the major crystalline phase in all the specimens. The intensity of the peaks due to

LiTi,(PO,), increased with increasing the heat-treat-

ment temperature, indicating gradual growth of the

LiTi,(PO,), phase. From the diffraction peak posi- tions, the lattice constants were estimated for

LiTi,(PO,), present in the specimens heat-treated at 900 and 950°C. The a- and c-parameter for the former specimen were about 8.491 A and 20.839 A,

respectively, and for the latter 8.495 A and 20.818 A, respectively. These two values are smaller than those of the sintered LiTi,(PO,), (a = 8.5 12 A, c = 20.858 A) [4], suggesting that the A13+ ions replace the Ti4’ ions in the present LiTi,(PO,),. Thus the

.I. Fu I Solid State Ionics 96 (1997) 195-200 197

0 LlT12(P04)3

& Alp04

Fig. 2. X-ray diffraction patterns of the specimens heat-treated at

various temperatures, for 12 h. The values indicated in the figure

denote the heat-treatment temperature. The open circles and the

triangle denote the diffraction peaks due to LiTi?(PO,), and

AIPO,, respectively.

precipitated conducting phase can be expressed as Li , +ATi,-,W,L.

Fig. 3 shows complex impedance plots for the

specimen heat-treated at 950°C as an example.

Similar behaviors were observed for the other speci- mens. Only one semicircle was observed at measure-

ment temperature above - lO”C, but another small

semicircle in the higher frequency range appeared

below - 10°C. The semicircle in the low frequency

range was attributed to the grain boundary (R,) while the small semicircle in the high frequency range was attributed to the bulk of the grain (R,,) [9,10]. Disappearance of the small semicircle at relatively high temperature is due to the very low resistivity of the bulk part, which requires very high frequencies

0 150 3M) 450 600 ZKI

-23% 27kHI K 37wr

e3

2 GkHZ

0 3 6 9 12 Z’lkn

J 0 ‘0 30&,+R9 40

Fig. 3. Complex impedance plots at various temperatures for the

specimen heat-treated at 950°C.

for measuring it. The total resistivity (Rb + Rp) used

in the present paper was obtained from the intercept of the semicircle with the real axis in the low

frequency range.

Fig. 4 shows the temperature dependence of conductivity for the mother glass and the glass-

ceramic specimens. The conductivity, g, obeys the Arrhenius equation:

UT = A exp(-E,lkT),

where T is the absolute temperature, A is the pre- exponential factor, E, the activation energy for conduction, and k the gas constant. It is noted that the conductivity of the glass-ceramics is many orders of magnitude greater than that of the mother glass.

198 J. Fu I Solid State Ionics 96 (1997) 195-200

E T E 10-2 3 %

10-3

10-4

10-5 ; 1.6 2.0 2.4 2.8 3.2 3.6 4.0

1oooK/T

Fig. 4. Temperature dependence of the conductivity for the as-cast

glass and the heat-treated specimens. The values indicated in the

figure denote the heat-treatment temperature.

Fig. 5 shows the conductivity of the specimens at room temperature as a function of the heat-treatment

temperature. The conductivity increases rapidly with

increasing the heat-treatment temperature to 950°C

and then decreases with further increases of the

heat-treatment temperature. A maximum conductiv- ity, 1.3 X 10e3 S cm-‘, was obtained. This value is

twice as large as the highest one of the sintered

Li, +.,Al,Ti,_,(PO,), [4] and is almost the same as that of Li,N [I] which is well known to possess the

highest conductivity for a solid electrolyte. However, it should be pointed out that the present glass- ceramics have a much larger advantage than Li,N because they are easily fabricated and have very good chemical durability at ambient atmosphere.

Fig. 6 shows the variation of (a) the pre-exponen- tial factor, A, and (b) the activation energy, E,, of the glass-ceramics with the heat-treatment temperature. Comparing Figs. 5 and 6, it is seen that the A shows an identical variation while the E, experiences an

P 10-4

5

o 10-5

10-6

10‘7 I I I I , 1

650 750 850 950 1050

Heat-treatment temperature (‘C)

Fig. 5. Variation of conductivity at room temperature with the

heat-treatment temperature.

opposite variation to the conductivity, indicating that

the conductivity of the present glass ceramics is

determined by both the A and E,. Further, it is noted that the change in A is much smaller than that in u

while the change in E, is remarkable; the E, of the

mother glass was 69.4 kJ mall’ and it decreases to 31.5 kJ mall’ after heat-treatment. It is therefore concluded that the E, rather than A is the main factor

governing the conductivity. With the increase of the

heat-treatment temperature, as shown in Fig. 3, the conducting phase gradually grows. This would result

in an increase of mobile Li+ concentration and a

longer path for the Lif migration. As a result, the A

increases and the E, decreases, and hence, the

conductivity increases. Fig. 7 shows the fracture surfaces observed by

SEM for the specimens heated at (a) 950°C and (b)

1000°C. The specimen heated at 950°C was char- acterized by very dense structure without pores and cracks. Especially, it should be noted that the contact between grains is so close that the grain boundary can not be distinguished clearly. These are the main reasons for the very low activation energy and very high conductivity. In the specimens heated at 1000°C the grains grew further, but some cracks occurred along the grain boundary. The occurrence of the

J. Fu I Solid State hits 96 (1997) 193-200 199

,04\ 650 750 850 950 1050

Heat-treatment temperature (‘C)

55

50

” g 45

3

w” 40

35

30

650 750 850 950 1050

Heat-treatment temperature (‘C)

Fig. 6,Variation of (a) pre-exponential factor, A, and (b) activation

energy, Ez, of the glass-ceramics with the heat-treatment tempera-

ture.

cracks, which may block the conduction path for lithium ions are responsible for the decreases in the

conductivity (Fig. 3).

4. Conclusions

Through the heat-treatment of L&O-AI,O,-

TiO,-P,O, glass, high lithium conducting glass- ceramics containing Li, +XAl,Ti,_ ,(POd), as a major crystalline phase have been successfully prepared.

Fig. 7. SEM micrographs of the fracture surfaces of the specimens

heat-treated at (a) 950°C and (b) 1000°C.

The maximum conductivity obtained at room tem-

perature is 1.3 X IO ~’ S cm- ’ . This value is one of the highest conductivities for a solid electrolyte.

Acknowledgments

The author thanks Dr. K. Yamashita at Faculty of Technology, Tokyo Metropolitan University, Dr. M.

200 J. Fu I Solid Siate Ionics 96 (1997) 195-200

Itoh and Dr. Y. Inaguma at Research Laboratory of Engineering Materials, Tokyo Institute of Technolo- gy, for the helpful suggestions.

[4] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G. Adachi,

J. Electrochem. Sot. 137 (1990) 1023.

[5] M. Tatsumisago, A. Hamada, T. Minami, M. Tanaka, J. Am.

Ceram. Sot. 65 (1982) 575.

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