(CH)x battery with unsymmetrical tetraalkyl ammonium salt

Synthetic Metals, 18 (1987) 6 1 9 - 6 2 4

61 9

(CH)x/(CH)x BATTERY WITH UNSYMMETRICAL TETRAALKYL AMMONIUM SALT

YUKIO KOBAYASHI,

TOSHIKAZU SHISHIKURA,

Central Research Laboratory,

H I D E N O R I N A K ~ M U R A and H I R O S H I K O N U M A

Showa Denko K.K., 2-24-25, Tamagawa, 0hta-ku,

Tokyo 146 (Japan) KAZUNORI

FUZITA

Hitachi Research Laboratory,

Hitachi Ltd., 4026 Kuji-cho,

Hitachi,

Ibaraki

319-12 (Japan)

ABSTRACT The performance of nonaqueous

(CH)x/(CH)x battery operated at a high current

density of 5 mA/cm 2 has been investigated. a solvent because of its good solubility, wide electrochemical

Nitrile was found to be suitable as highly electrical conductivity and

stability window. Acetonitrile showed the best dielectric constant

but benzonitrile exhibited better battery performance due to its lower reactivity with (CH)x. As a supporting electrolyte,

unsymmetrical

tetraalkyl ammonium salts

showed better solubility and higher conductivity Than symmetrical ones. Three types of (CH)x (film, gel and powder) with fibrillar morphology were used as electrode materials.

The battery performance was as follows, gel > powder > film.

At a low temperature of -28 °C, cycle life and self discharge performance were greatly improved.

The self discharge ratio was increased by the addition of RsN

in electrolyte.

INTRODUCTION Application of conductive polymers,

especially polyacetylene,

(CH)x,

to a

secondary battery has attracted a great deal of attention because of the potentials of high energy and high power secondary battery

[i] . Many investigations on

Li/(CH)x battery have been done [2] and promising results have been obtained. However,

a few studies on (CH)x/(CH)x battery have been undertaken

cycle life and self discharge performance, of the battery have not yet been clarified. about the effect of species of solvents,

[3]

and the

which are of great practical importance, This paper summarizes our studies

supporting

electrolytes,

operation

conditions and types of (CH)x on the performance of (CH)x/(CH)x battery operated at a high current density of 5 mA/cm 2.

0379-6779/87/$3.50

© Elsevier Sequoia/Printed in The Netherlands

620

EXPERIMENTAL Materials

and reagents

Polyacetylene catalyst

film was prepared

consisted

of AIEt3(0.79

using Shirakawa

mol/l)

technique

and Ti(OBu)4(0.30

[4]

mol/l).

film was washed with toluene and dried at 25 °C under vacuum. was prepared

by a modified

(0.Ii mol/l)

and

into a reactor without was continued swelling

Shirakawa

stirring

as a gel-(CH)x

electrode.

The catalyst

Polyacetylene

consisted

above.

tetraalkyl

ammonium

mol/l)

was washed with toluene, The sheet was used

salts were synthesized

was pressed

mol/l).

stirred

was obtained

into a pellet as

electrode.

as described

Shirakawa

and Ti(OBu)4(O.O035

with fibrillar morphology

The powdery polyacetylene

as usual methods

polyacetylene

solution which was mechanically

The pellet was used as a powder-(CH)x

Solvents were purified

gel-like

into a sheet.

of AIEt3(O.OI4

Powdery polyacetylene

and washed with toluene.

of AIEt 3

powder also prepared by a modified

Acetylene gas was blown into the catalyst at room temperature.

consisted

gel

solution and the polymerization

The swollen polyacetylene

ferro plate and pressed

The obtained

Polyacetylene

acetylene gas was introduced

the polymerization

in toluene was obtained.

described

The catalyst

Purified

for 6 hrs. On the surface of the solution,

held between chromated

technique.

technique.

Ti(OBu)4(O.044mol/l).

at -78 °C. The

Unsymmetrical

in the literature

[5] .

and the content of water was less than

30 ppm. Electrochemical

studies

Cycle life tests were carried out with the cylindrical The standard

conditions

of (CH)x electrode = 4 mol%,

of the tests were as follows

cell shown in Fig.

: thickness

and bulk density

are 200 - 300 ~ m and 0.3 - 0.4 g/cc,respectively,

charge and discharge

doping level

current density = 5.0 mA/cm 2, discharge

voltage = 1.0 V, rest time after charge and discharge

= 2.0 min.,cycle

defined as the cycle number when coulombic

becomes

self discharge standing

ratio is defined

efficiency

current collector

(Pt net)

CH)x separator

(glass filter)

(CH)x ~nt

Fig. I. Cylindrical

test cell

cut off life is

less than 50 % and

as the loss of charged coulomb after 15 hrs

time.

collector

(Pt net)

(Inner diameter = 9 mm)

i.

621 RESULTS The physical properties of electrolytes required for (CH)x/(CH)x battery worked at high current density

(5 mA/cm 2) are wide electrochemical stability window,

highly electrical conductivity,

good solubility,

low freezing point and high boiling

point. On the basis of these points, we chose nitrile as a solvent and found that among nitrile

benzonitrile was more suitable than acetonitrile because of its low

reactivity with (CH)x electrode.

As a supporting electrolyte,

unsymmetrical

tetra-

alkyl ammonium salts showed better solubility and higher electrical conductivity than symmetrical ones (Table i and Fig. 2).

TABLE i Physical properties of nitrile

Nitrile

Boiling point (°C)

Acetonitrile Benzonitrile m-Tolunitrile

Melting point (°C)

81.6 190.7 >200

8

Dielectric constant at 25 °C

-43.8 -12.9 -23.0

37.5 25.2

Abbr.

AN BN m-TN

(C)

.,-t 4~

~

2

u 0

20

4.0

Concentration Fig.

2.

Concentration

vs.

60

(mol/1)

(A) Bu4NBF 4

in

BN

(B) Bu4NBF 4

in

m-TN

(C) Et3BuNBF 4 in

BN

(D) Et3BuNBF 4 i n

m-TN

conductivity.

Effect of operation conditions on performance of (CH)x/(CH)x cell with film The effect of the kinds of electrolytes,

charging cut off voltage, concentration

of salts in electrolytes and working temperature on the cycle life and self discharge performance of (CH)x/(CH)x battery with Shirakawa film was examined. As stated earlier,

the cell with AN solvent showed poor performance because of

its high reactivity with (CH)x electrodes.

On the other hand, the cell with BN

solvent exhibited good cycle life and the cycle life was extremely improved by ]owering the discharge cut off vo]tage from 1.0 V to 0.0 V (Fig. 3).

622 The self discharge ratio decreased with the increase of the concentration of salts in electrolytes,

in other words, with the increase in the electrical conductivity

of electrolytes, while the cycle life was not dependent on the concentration of salts (Table 2).

"~¢w ~ 100[

C,so!

(A) Me3BuNBF4/AN (B) Et3BuNBF4/BN

o

1;0

500 600

2~'

(C) Me3(Hep)NBF4/BN (D) Et3BuNBF4/BN (Cut off voltage = 0.0 V)

Cycle number

Fig. 3. Effect of kinds of electrolytes on battery performance

TABLE

2

Effect of concentration of salt in electrolyte on battery performance

Electrolyte

Conductivity (mS/cm)

Cycle life (number)

Self discharge (%, 15 hrs)

IM Et3BuNBF4/BN

5.1

107

36.5

3M Et3BuNBF4/BN

7.5

123

26.1

5M Et3BuNBF4/BN

7.2

119

23.8

i

The cycle life and self discharge performance were greatly improved by lowering the operation temperature from 20 °C to -28 °C (Table 3). This result suggests that the reaction of electrolyte

with (CH)x electrode is prevented at low

temperature. TABLE 3 Effect of operation temperature on battery performance Electrolyte

Cycle life (number)

Self discharge (%,15 hrs)

20 °C

-28 °C

20 °C

-28 °C

IM Et4NBF4/AN

66

106

52.1

18.0

IM Bu4NBF4/BN

130

186

38.4

19.9

IM Et3BuNBF4/BN

107

620

36.5

9.5

623 Effect of types of (CH)x on battery performance The battery performance gel and powder

was

for three types of (CH)x electrodes

compared.

GeI-(CH)x

the lowest self discharge ratio of physical

and chemical

electrode showed the best cycle life and

properties

(molecular weight,

crystallinity,

length of conjugated

were measured.

we could not find big difference.

However,

from film,

(Table 4). For these three types (CH)x, a number

surface area, porosity,

electrodes

prepared

double bonds,

showed such good battery performance

specific

content of oxygen etc.) The reason why gel-(CH)x

has not been clarified.

TABLE 4 Effect of types of (CH)x on battery performance (electrolyte: IM Bu4NBF4/BN) (CH)x

Bulk density (g/cc)

Thickness (~m)

Cycle life (number)

Self discharge (%, 15 hrs)

Film Gel Powder

0.35 0.36 0.93

180 220 250

118 158 133

38.2 21.3 22.8

Influence

of the concentration

of salts in electrolytes

on the performance

of (CH)x/(CH)x battey with ge-(CH)x electrodes was examined. self discharge conductivity

performance

The cycle life and

were improved with the increase of the electrical

(Table 5).

TABLE 5 Effect of conductivity Electrolyte

of electrolyte

on performance

of gel-(CH)x/(CH)x

cell

Conductivity (mS/cm)

Cycle life (number)

Self discharge (%, 15 hrs)

IM Bu4NBF 4

2.9

118

21.9

2M Bu4NBF 4

4.0

132

16.9

IM EtsBuNBF 4

5.1

143

16.0

5M EtsBuNBF 4

7.2

165

12.1

The influence of the bulk density of (CH)x electrode on the performance (CH)x/Bu4NBF 4 in BN/(CH)x cell with gel- or powder-(CH)x self discharge

performance

of the electrodes

was investigated.

of The

of gel-(CH)x battery did not depend on the bulk density

in the range between 0.2 and 0.6 g/cc even at a high current

density of 5 mA/cm 2, while the self discharge ratio of powder-(CH)x

battery

increased with the increase of bulk density at 5 mA/cm 2 current density

(Fig. 4).

624 lOO

0

811

6o

,1~ o tll

40

"~

20

ul

0

Sample ! 0.1

I 0.2

I o~

I 0.4

I o,6

Bulk density (g/cc)

o.e

Current density

(i) GeI-(CH)x

5.0 mA/cm 2

(2) Powder-(CH)x

5.0 mA/cm 2

(3) Powder-(CH)x

l.OmA/cm 2

Fig. 4 . Effect of bulk density of electrode on self discharge ratio for (CH)x/1M Bu4NBF 4 in BN/(CH)x cell.

Influence of addition of tributylamine in electrolyte on self discharge performance The formation of R3N in electrolyte was found after cycle life test. R3N was assumed to increase the self discharge ratio by compensation reactions with doped BF4-.

As predicted, the self discharge ratio was increased with the concentration

of 8u3N added into electrolyte.

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