Polymers for Advanced Technologies Volume 4, pp. 194-197

Paper Form Copper Ion Conductive Solid Electrolyte T. Hara,*’ N. Yasuda,’ M. Nagata,’ S. Kondo2 and T. Sotomura3 ‘Electronic Research Laboratory, Japan Synthetic Rubber Co. Ltd, 3-5-1 Higashi-Yurigaoka, Asao-Ku, Kawasaki, Kanagawa 215, Japan ‘Technology Laboratory, Matsushita Battery Ind. Co. Ltd, 1 Matsushita-Cho, Moriguchi, Osaka 570 Japan 3Living Systems Research Center, Matsushita Electric lnd. Co. Ltd, 3-15 Yagumo-Nakamachi, Moriguchi, Osaka 570, Japan

ABSTRACT

A paper form solid electrolyte (PFSE) with good conduc- tivity and flexibility was developed. PFSE is made of an inorganic-organic composite consisting of a copper ion conductive solid electrolyte of Rb,Cu,,17Cl13 ( S E ) and a polymer binder embedded in a synthetic fibre network. A PFSE with styrene-butadiene based synthetic rubber ( S B ) was found to give the highest conductivity among the binding materials that we examined. The S B content was changed from 10 vol% to 90 vol% in the PFSE, and the electrical properties and the damp resistance of PFSE were evaluated. Electrical conductivity decreased slightly with an S B content below 70 ~ 0 1 % and abruptly above 70 ~ 0 1 % . The damp resistance of PFSE improved with the increase of the S B content and was superior to that of the raw powder. The PFSE with 65 vol% S B showed relatively high conductivity, 3 x S/cm, which was sufficient for battery use, and good damp resistance.

KEYWORDS: Solid electrolyte, Copper ion conductor, Composite

INTRODUCTION Solid electrolytes with a high ionic conductivity at ambient temperature could potentially be used for solid-state batteries, memory devices, display panels, electrochemical capacitors, etc. In particular, using solid-state batteries in microelectronics is an attractive application because of their advantages of

* To whom correspondence should be addressed.

reliability and choice of device geometries which result from the absence of leaking liquids.

It is well known that some inorganic materials have a high ionic conductivity equivalent to that of a liquid electrolyte. Inorganic solid electrolytes were usually used in the form of a press-molded pellet. The pellet is brittle and rigid, so the fabrication of a solid-state cell was very troublesome. Intimate con- tact between electrolyte and electrode is difficult because of the brittleness of the pellet. Moreover, the reproducibility of cell performance is usually poor because the electrode shows a volume change during discharge-charge cycles and the rigid solid electro- lyte hardly compensates for this volume change. A solid electrolyte with elasticity was needed to over- come these disadvantages.

A solid polymer electrolyte was proposed by Wright ([I]) and extensively studied by Armand and Duclot [2]. A solid polymer electrolyte, which is a polymer containing an ethyrene oxide unit com- plexed with the metal salt, can be made in flexible film form. Much research work has been aimed at developing a solid polymer electrolyte with high conductivity. However, the conductivity of solid polymer electrolytes is still too low for practical battery application at room temperature.

We have developed the paper form solid electro- lyte (PFSE), which is a composite of an inorganic solid electrolyte and a polymer and has high conduc- tivity and good elasticity at ambient temperature. PFSE is a mixture of copper ion conductor of Rb4Du1617Cl13 and synthetic rubber which is embed- ded in a synthetic fiber network. Rb,Cul,I,C1,, is known to exhibit an ionic conductivity of 3 x lo-’ S/cm at room temperature [3] and to be mois- ture sensitive [4]. It is important for the synthetic

Received 1 June 1992 Revised 8 August 1992

1042-7147/93/030194-04 $07.00 0 1993 by John Wiley & Sons, Ltd

Paper Form Copper Ion Conductive Solid Electrolyte / 195

Electrolyte & Binder Fiber Network

FIGURE 1. Photograph and schematic cross-section of paper form solid electrolyte.

rubber to hold solid electrolyte particles firmly and to be inactive towards them.

This paper describes the effect of a rubber binder on conductivity and damp resistance of PFSE.

EXPERIMENTAL A copper ion conductive solid electrolyte, Rb, cu1617c113, was prepared by the method reported by Takahashi et al. [3]. R ~ , C U , ~ I , C ~ , ~ was mixed with a solution dissolving SB in dehydrated organic sol- vent and then pulverized by a ball mill. The slurry obtained was coated on a synthetic polymer mesh and then dried to remove the solvent. A photograph and schematic cross-section of PFSE are shown in Fig. 1.

The conductivity measurements were made for PFSE sandwiched between electrode sheets, which were a composite of the copper chevrel phase com- pound (CU,MO,S~,~), the solid electrolyte and SB. They were prepared in the same manner as the PFSE. Intimate contact between the PFSE and electrode sheets was obtained by hot-pressing them at 120°C for 10 min at a pressure of 3 MPa. The conductivity was estimated from the impedance at 1 kHz mea- sured by impedance analyzer (YHP 4194A).

The influence of moisture on solid electrolyte in the PFSE was examined by the change of the X-ray diffraction pattern. The intensity of the (311) peak of Rb4Cu,,17C1,, was monitored for the PFSE placed in a humid atmosphere.

RESULTS AND DISCUSSION Binding Materials for Rb4Cu,,I,Cl,,

We selected styrene-butadiene based synthetic rubber (SB) as the binding material for Rb4Cul6I7ClI3. The screening test of polymer was

made as follows. The choice of solvent for the binding polymer was limited because of the reacti- vity of the solid electrolyte, e.g. a polar solvent, such as ketone and alcohol, reacts with this solid electro- lyte. Some nonpolar aromatic or aliphatic hydrocar- bon solvents are usable. The trial preparation of PFSE was made for many kinds of polymers dis- solved in these solvents. Most of them did not hold the solid electrolyte particles in a flexible sheet form sufficiently firmly. The synthetic rubbers of SB, butadiene rubber and acrylonitrile-butadiene based rubber (NB) gave a good result in terms of flexibility. However, in terms of the electrical conductivity, only the PFSE with SB provided a sufficient value required for practical battery use. The conductivity of PFSE with the other binding materials was at least one order of magnitude lower than that with SB at a polymer content of 10 ~ 0 1 % .

0 50 1 SB content ( vol% )

10

FIGURE 2. Conductivities of PFSE at 20°C with various volume fractions of SB.

196 / Hara et al.

h - I E 3 -2 a v 01 0 -

-3

-1 c -

-

SB content(vol%)

20

40

65 - 1 #K/T

3 4 5

FIGURE 3. Temperature dependence of conductivities of PFSE with various volume fractions of SB.

Conductivity of PFSE The conductivity was measured for PFSE sand- wiched between copper chevrel electrode sheets. The conductivity of PFSE sandwiched between copper plates was also measured for the reference. The value of the apparent conductivity of PFSE measured with the copper plate electrode was about two orders of magnitude lower than that for copper chevrel elec- trode sheet. The difference was too large and was hardly attributable to the difference in the effective electrode area between the electrode and electrolyte sheet. The contact resistance, which is probably ascribed to the deposition and dissolution resistance of copper ions at the interface of electrolyte and electrode, might be the reason for the difference. The copper chevrel electrode sheet, which was found to have a good compatibility with PFSE and to provide a lower contact resistance, was used to obtain elec- trical conductivity of PFSE with various polymer contents.

The conductivity of PFSE with various SB con- tent measured at room temperature is shown in Fig.

2. The conductivity decreased slightly with SB con- tent below 70 vol% and abruptly above 70 ~01%.

The temperature dependence of conductivity of PFSE with various SB content is shown in Fig. 3. The conductivity vs. temperature plot for the PFSE with 20 vol% of SB shows a transition at about -25°C. The activation energy was calculated to be 13 kcmal/mol in the lower temperature region and 7.2 kcal/mol in the higher temperature region. The conductivity knee was reported for the press-molded pellet of the raw solid electrolyte powder and the cause of the knee for the raw solid material was found to be associated with the transition from the low ordered to high ordered crystal structure or amorphous to crystalline [3]. The conductivity knee for PFSE with 20 volo/~ of SB must be due to the same reason. It seemed that the conduction of copper ion in the PFSE with 20 vol% of SB took place between particles in contact with each other, similar to that of the press- molded pellet. The conductivity knee was also observed for the PFSE with 40 vol% of SB. However, the knee was observed at about O"C, which is fairly high compared with that of raw solid electrolyte. The conductivity knee was not clear for PFSE with 65% of SB compared with that of the PFSE with lower content.

The change of the activation energy with the SB content indicates that there are different methods for copper ion conduction in the PFSE. The most prob- able one is method (a) in which copper ions are conducted through solid electrolyte particles directly contacting with one another. Another probable method is (b) in which copper ions are conducted through an insulating binder layer which is formed over each solid electrolyte particle and prevents direct contact them. Method (a) might become domi- nant when the SB content is lower and (b) might become dominant as the SB content increases.

The abrupt increase of the conductivity around 70 vol% SB indicated that the thickness of the insulating binder layer became critical at this value for the copper ion conduction across the layer, i.e., at

I 1 (b)rawSEpowder

10 20 30 40 50 60 28 /degree

FIGURE 4. X-ray diffraction pattern of (a) PFSE and (b) the raw SE powder.

Paper Form Copper Ion Conductive Solid Electrolyte / 197

Damp Resistance of PFSE

Rb4Cu,,I,C1,, is one of the most conductive solid electrolytes. For practical applications, the stability of the solid electrolyte is also very important. However, the solid electrolyte is reported to decom- pose in a humid atmosphere [4]. If the damp resis- tance of PFSE is superior to that of the raw solid

0.1 - T O 40T, 60!!H material, devices using PFSE will be more profitable than devices using the raw solid material, from the

The PFSE immediately after preparation gave the X-ray diffraction pattern with raw solid electrolyte Powder shown in Fig. 4. The Pattern did not change over more than six months, after storage in a dry N2 atmosphere at room temperature.

The PFSE and the raw solid electrolvte uowder

65 A 4 0

20

0) > 5 P) K

I I I industrial viewpoint. 1 5 10 50 100 5001000

0.05-

Time(rnin)

FIGURE 5. The damp resistance of PFSE at 40°C in air with 60% RH.

around 70 vol%, the conductivity becomes highly sensitive to the uniformity of PFSE. We have experi- enced a poor reproducibility in conductivity of PFSE around 70 vol% SB; the PFSE with poorly dispersed solid electrolyte particles provided much lower con- ductivity as compared with the one in which they were finely and uniformly dispersed.

We have already reported that the thickness of the insulating binder layer and the volume fraction of SB can be correlated in a simple manner [5]. The thickness of the layer becomes over 0.4L around 70 vol0/o SB ( L is the length of the side of a solid electrolyte cube). The distance separating the cubic solid electrolyte particles having 1 pm sides was calculated to be 0.4 X 1 X 2 = 0.8 pm. Up to this value copper ion conduction through the insulating binder layer might be permitted.

, I

were placed in a humid atmosphere, and the inten- sity of the (311) peak of Rb, Cu,,I, Cl,, was monitored. As shown in Fig. 5, the X-ray diffraction peak of the raw solid electrolyte powder began to decrease after only 25 min in the open air atmosphere at 60% relative humidity (RH) and 40°C. In the case of the PFSE with 65 vol% SB, the intensity of the X-ray diffraction peak hardly decreased even after 20 hr storage. The rate of decrease of the intensity or the decomposition rate of the solid electrolyte greatly depended on the SB content; the decomposition rate became smaller as the SB content increased. The results given under the different conditions of 90% RH and 40°C are shown in Fig. 6. The PFSE with 65 vol% SB hardly decomposes within 3 min even when it was exposed to the highly humid atmosphere, while the raw solid electrolyte powder instanta- neouslv decomposed. The rubber binder coated over the soiid electiolyte particles worked effectively to prevent direct contact between the raw solid electro- lyte powser and the humid atmosphere.

Thus the good damp resistance of PFSE might reduce the chances of failure in the production of the solid-state devices, providing a higher reliability in the final products.

REFERENCES 1. P. V. Wright, Polymer, 7, 319 (1975). 2. M. B. Armand and M. Duclot, USP 4, 303, 748 (1978).

1 5 10 50 3. T. Takahashi, 0. Yamamoto, S. Yamada and S. T i rne(rni n) Hayashi, J. Electrochem. Soc., 126, 1654 (1979).

4. T. Turkovic, Solid State lonics, 28-30, 900 (1988). 5. T. Sotomura, S. Itoh, S. Kondo and T. Iwaki, Denki FIGURE 6. The damp resistance of PFSE at 40°C in air

with 90% RH. Kugaku, 59, 129 (1991).