Solid State Ionics 104 (1997) 191–194

1Fast Li ion conducting glass-ceramics in the systemLi O–Al O –GeO –P O2 2 3 2 2 5

*Jie FuR&D Centre, Ohara Inc., 1-15-30 Oyama, Sagamihara-shi, Kanagawa 229-11, Japan

Received 30 June 1997; accepted 31 July 1997

Abstract

Fast lithium ion conducting glass-ceramics were successfully prepared by heat-treatment of glasses in the system41Li O–Al O –GeO –P O . The phase present in the glass-ceramics was LiGe (PO ) in which Ge ions were partially2 2 3 2 2 5 2 4 3

31 31 41replaced by Al ions. Composition dependence of conductivity showed that substitution of Al ions for Ge ions resulted24in a considerable enhancement in conductivity and in a wide composition range extremely high conductivity over 10

21S cm was obtained at room temperature.

Keywords: Ionic conductivity–lithium; Glass-ceramics; Li O–Al O –GeO –P O system; LiGe (PO ) phase2 2 3 2 2 5 2 4 3

1. Introduction materials, glass-ceramics have much advantage be-cause they can be easily manufactured into desired

Fast lithium ion conductors are potential elec- size or shape and have dense microstructure. There-trolyte materials for utilization in high energy density fore, the development of fast ion conducting glass-batteries and other electrochemical devices. For ceramics is of greater practical significance. Recent-practical use, however, the conductors will also be ly, the present author successfully prepared glass-required to possess good chemical stability and to be ceramics containing LiGe (PO ) phase from the2 4 3

easily manufactured. LiGe (PO ) -based solid solu- system Li O–Al O –GeO –P O . Conductivity2 4 3 2 2 3 2 2 5

tions such as Li M Ge (PO ) (M 5 Al, Cr) not measurements show the glass-ceramics are excellent11x x 22x 4 3

only are chemically stable but also exhibit high lithium ion conductors. In the present paper, com-24 21lithium ion conductivity up to 2.4 3 10 S cm at position dependence of conductivity will be reported.

room temperature [1,2]. LiGe (PO ) , which posses-2 4 3

ses NASICO-type structure, consists of both GeO6

octahedra and PO tetrahedra. Both units are linked 2. Experimental4

by their corners to form a three-dimensional networkstructure [3], and this structure results in cavities Glasses were prepared using a conventional melt-where lithium ions reside and in bottlenecks in which quenching method. Reagent grade chemicals ofthey pass through. In comparison with the sintered Li CO , Al(OH) , GeO and NH H PO were used2 3 3 2 4 2 4

as starting materials. The batches were placed in*Tel.: 1 81 427 72 2101; fax: 1 81 427 72 7622 platinum crucibles and melted in an electrical fur-

0167-2738/97/$17.00 1997 Elsevier Science B.V. All rights reserved.PII S0167-2738( 97 )00434-7

192 J. Fu / Solid State Ionics 104 (1997) 191 –194

Table 1nace. The batches were initially kept at 7008C for 1 hGlass transition (T ) and crystallization temperature (T ) ofg xin order to release volatile products coming from theglasses in the system Li Al Ge (PO )11x x 22x 4 3starting materials. Then, they were heated up tox T (8C) T (8C) T 2 Tg x x g1350–14508C and melted at the temperature for 1.5

h. The melts were poured onto preheated stainless 0 597 651 540.1 592 643 51steel plates and pressed into plates of 2 mm thick.0.2 575 646 71The resultant glasses were immediately placed in0.3 561 635 74

furnace at 500–5608C for annealing; after holding 0.4 550 630 80for 2 h the furnace was turned off and the glasses 0.5 539 622 83were cooled at the cooling rate of the furnace. 0.6 531 611 80

0.7 521 600 79Glass transition temperature (T ) and crystalliza-g

tion temperature (T ) were determined using dif-x

ferential thermal analyses at a heating rate of 1021K min . Heat treatment was carried out for 12 h at Fig. 1 shows the X-ray diffraction patterns of

the temperatures above T . Since the conductivity Li Al Ge (PO ) glass-ceramics. Only conduc-x 11x x 22x 4 3

changes with the heat-treatment temperature, for tive phase LiGe (PO ) precipitates in all samples.2 4 331every composition the heat-treatment temperature Since no new peak appears as Al ions replace

41 31was progressively elevated at an interval of 258C Ge ions, it is inferred that the added Al ionsuntil the maximum conductivity is reached. Crys- were incorporated into the structure of LiGe (PO )2 4 3

41talline phases were identified by X-ray diffraction by replacing Ge ions. A slight shift of diffraction(XRD) analyses at room temperature using CuKa

radiation. The microstructures of the crystallinephases were observed by means of scanning electronmicroscopy (SEM) on the fracture surfaces.

Polished samples were served for ionic conduc-tivity measurements. Gold electrodes were sputteredon both sides of the samples. The measurementswere carried out with the complex impedance meth-od using a Solartron 1260 impedance analyzerconnected to a computer. The frequency range used

22 7was 10 –10 Hz. Ionic conductivity values werecalculated with s 5 d /AR, where d is the samplethickness, A is the area of the electrodes and R is thesample resistivity corresponding to the total of thebulk and the grain-boundary components.

3. Results and discussion

Compositions studied in the present paper wereexpressed by Li Al Ge (PO ) . Glass formation11x x 22x 4 3

was first investigated. Glasses were obtained in therange of x 5 0–0.8. Table 1 shows the glass transi-tion temperature (T ) and crystallization temperatureg

(T ). Increasing the x results in a decrease in both Tx g

and T , but an increase in T 2 T , indicating thex x g

thermal stability of the glass increases with the Fig. 1. X-ray diffraction patterns of Li Al Ge (PO ) glass-11x x 22x 4 331 41substitution of Al ions for Ge ions. ceramics.

J. Fu / Solid State Ionics 104 (1997) 191 –194 193

peak position towards low angle indicated the re-placement indeed took place because of the larger

31 ˚ionic radius of Al (0.535 A) compared with that of41 ˚Ge (0.530 A).Fig. 2 shows the complex impedance plot (Cole–

Cole plot) for the sample with x 5 0.2. Two semicir-cles were observed. The semicircle in the lowfrequency range was attributed to the grain boundary(R ) and the semicircle in the high frequency rangeg

was attributed to the bulk of the grain (R ) [4].b

However, only one distorted semicircle which wascomposed of the above mentioned two semicircleswas observed for some samples. R could not beb

obtained in these samples. From the complex impe-dance plots the total resistivity (R 1 R ) was de-g b

termined and used to calculate the conductivity ofthe present glass-ceramics.

Fig. 3 shows the temperature dependence ofthe conductivity of Li Al Ge (PO ) glass-11x x 22x 4 3

ceramics. The conductivity data fit the equation Fig. 3. Temperature dependence of the conductivity forsT 5 A exp(2E /KT ) where A is the preexponential Li Al Ge (PO ) glass-ceramics.a 11x x 22x 4 3

factor, E the activation energy for conduction and Ka

the gas constant. Both the conductivity at roomtemperature and the activation energy are shown inFig. 4. The conductivity increases by more than fourorder of magnitude with the initial increase in x,

24 21reaches a maximum value, 4.0 3 10 S cm ataround x 5 0.5 and then decreases. In a wide com-position range the glass-ceramics exhibit conduc-

24 21tivity over 10 S cm . The composition depen-dence of conductivity of the present glass-ceramics ismuch similar to that of the sinteredLi Al Ge (PO ) conductors, but the highest11x x 22x 4 3

conductivity is more than two times as large as that

Fig. 4. Electrical conductivity at room temperature and activationenergy of Li Al Ge (PO ) glass-ceramics as a function of x.11x x 22x 4 3

of the sintered ones [1,2]. The E , on other hand,a21passes through a minimum value, 36 kJ mol , at

around x 5 0.5 and the composition dependence ofactivation energy corresponds to that of conductivity,indicating that changes in the conductivity are con-Fig. 2. Complex impedance plot for the sample with x 5 0.2 at

room temperature. trolled by the changes in the activation energy.

194 J. Fu / Solid State Ionics 104 (1997) 191 –194

contact between grains can result in a higher mobili-1ty of Li ions at grain boundary and hence a higher

total conductivity. This is the reason why the presentglass-ceramics exhibits higher conductivity than thesintered conductors.

4. Conclusions

Fast lithium ion conducting glass-ceramics con-taining LiGe (PO ) crystalline phase have been2 4 3

successfully obtained in the systems Li O–Al O –2 2 3

GeO –P O . The glass-ceramics exhibit very high2 2 524 21conductivity over 10 S cm in a wide com-

position range at room temperature. The high con-ductivity, good chemically stability and easy fabrica-tion of various shapes suggest they are promisingsolid electrolytes.

Fig. 5. SEM micrographs of the fracture surfaces ofLi Al Ge (PO ) glass-ceramics.1.4 0.4 1.6 4 3

ReferencesFig. 5 shows the fracture surfaces observed by

SEM for Li Al Ge (PO ) glass-ceramics. Evi- [1] S. Li, J. Cai, Z. Lin, Solid State Ionics 28–30 (1998) 1265.1.4 0.4 1.6 4 3

[2] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G. Adachi,dently, the present glass-ceramic possesses a veryBull. Chem. Soc. Jpn. 65 (1992) 2200.dense microstructure without any pores and cracks.

[3] L. Hagman, P. Kierkegaard, Acta Chem. Scand. 22 (1968)Furthermore, it is observed the contact between1822.

grains is much close. These characteristics are con- [4] P.G. Bruce, A.R. West, J. Electrochem. Soc. 130 (1983) 662.trast to that of the sintered conductors which have [5] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G. Adachi,

Solid State Ionics 47 (1998) 257.pores and poor contact between grains [5]. A closer