Structure and ionic conductivity in evaporated thin films of poly(ethylene oxide) complexed with LiCF3SO3

Solid State Ionics I8 & 19 (1986) 277-281 North-Holland, Amsterdam

277

STRUCTURE AND IONIC CONDUCTIVITY IN EVAPORATEDTHIN FILMS OF POLY(ETHYLENE OXIDE) COMPLEXED WITH LiCF3SO3 Yukio ITO, Kiyoaki SYAKUSHIRO, Masahiko HIRATANI, Katsuki MIYAUCHI and Tetsuichi KUDO Central Research Laboratory, H i t a c h i , L t d . , P.O. Box 2, Kokubunji, Tokyo 185, Japan Thin polymer e l e c t r o l y t e f i l m s are formed by evaporating a s u i t a b l e mixture of poly(ethylene oxide) (PEO) an~ LiCF3SO3. The f i l m s are found to e x h i b i t a considerably high i o n i c c o n d u c t i v i t y of about 3xlO- S/m at room temperature. The evaporated f i l m s are p a r t i a l l y composed of a structure s i m i l a r to that of the molten state of PEO, the melting point being d i f f u s e in the range -20 - 50°C. The c o n d u c t i v i t y enhancement at room temperature in the evaporated f i l m s can probably be a t t r i b u t e d to the molten state s t r u c t u r e regions. 1. INTRODUCTION The salts

complexes and

formed

polyethers,

oxide)(PEO), of

This

have

between such

as

a l k a l i metal poly(ethylene

been a t t r a c t i n g a great deal

a t t e n t i o n , p a r t l y because of the i n t e r e s t i n g

scientific because

problems of

materials Armand3

potential

in

electrochemical

application

batteries

devices I .

were to

found

and

characterization

electrolyte LiCF3SO3, structure

These

by

Wright

as

films

of

~

preparation

evaporated

obtained

from

along with information and conduction properties.

polymer PEO and on

their

polymeric et

( IO0°C ).

2. EXPERIMENTAL

other a l . 2 and

e x h i b i t fast ion conduction at ~gh

temperatures

reports on the

which they pose and p a r t l y

their

electrolytes

and

paper

The

complex

Starting

materials

were PEO with an average

molecular weight of 6xlO 5 (Seitetsu Kagaku L t d . ) and

LiCF3SO3

amounts mixed

of

(ICN Biomedicals I n c . ) .

PEO and

under

a

Required

LiCF3SO3 were weighed and

dry argon atmosphere in a glove

(PEO)xLiCF3SO3 ( 8 ~ X ~ 2 0 ) is known to have good electrochemical stability and an ionic

box to provide an oxygen in PEO-to-lithium r a t i o

conductivity

to

of

10- 2 - 10- I S/m at temperatures

around lO0°C4'5 far

The polymer salt complexes SO

reported have been a l l obtained by s o l u t i o n

casting

onto

a

s u i t a b l e substrate and heating

under vacuum. w e r e produced using

evapora~on

method.

evaporating

a suitable

LiCF3SO3 were found h/gh 25°C,

ionic

3 : I and 200 : i .[(PEO)x-LiCF3S03, X=3

200].

Thin

polgmer electrolyte the

conventional

films

mixture of

formed by PEO and

to exhibit a considerably

conductivity of about 3xi0-2 S/m at

which

was

much greater

than

the

Thin f i l m s were formed by evaporating

small amounts of the mixtures ( - I a

room of

g) charged in

tungsten boat onto s i l i c a glass substrates at temperature.

0.5 nm/s

In the present work, thin f//ms

between

in a

resulting

composition

films of

The

deposition

- 1 0 -4 Pa vacuum. was t y p i c a l l y

rate

-l~m.

The

evaporated f i l m s was determined

by i n d u c t i v e l y coupled argon plasma spectroscopy (ICPS). In

addition

to

the

evaporated f i l m s , some

(PEO)xLiCF3SO3 complex f i l m s were prepared using a s o l u t i o n casting technique s i m i l a r to that

conductivity of (PEO)xLiCF3SO 3 complexes prepared by casting. The reason the evaporated films show such a c h a r a c t e ~ c conducting

described by Weston et a l . 4, f o r comparison.

behavior is of particular interest to us.

scanning

0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)

was

The thickness

Thermal analysis of polymer f i l m s was carried out using a Rigaku DSC 8240 d i f f e r e n t i a l calorimeter

with

a

heating

rate of

Y. Ito et al. / Structure and ionic conductivity in evaporated thin films

278

5°C/min. 1 um

Film specimens with thickness of about were

substrate

deposited

onto

(100 um t h i c k ) .

specimens

were

atmosphere

in

loaded an

a

cover

The

under

aluminum

a pan

glass

as-deposited

evaporated

from

that of the cast complex.

for

consistent with the molecular weight measurement

the

DSC

p a r a l l e l to the f i l m surfaces

measured on specimens with i n t e r d i g i t a l Au

state

at

obtained

LiCF3SO3 d e c r e a s e s

previous

reports

prepared

by

described

in

the authors 6'7.

using

a

of

the of

it

AC impedance measurements in the frequency range

analysis

that

of

partially

composed of

5 Hz - 13 MHz

were

made

using

a

However, films

3.1 Structure Evaporated mixtures

PEO.

thin

with

films

slightly

All

brownish

and

had a w a x - l i k e

the

ICPS analysis,

from

in color, transparent,

it

in

the

the

Li

mixtures,

two

lower C-H

changed

although

the

s l i g h t l y lower than the i n i t i a l

in

values

the

the not

evaporated

structure

to

with

binding a

films

were

of

PEO.

the

evaporated

to that of the molten

the a

One

PEO carbons and the other

s h i f t of about 1.9 eV towards

energy.

This s h i f t is expected

C-O bond in PEO is replaced by a C-C or

bond.

F r o m this r e s u l t , i t is considered

in

that

a

PEO

coexists.

ones.

In

a disordered structure,

identical

the were

by and

In theEsCA spectrum f o r evaporated f i l m s ,

appeared

found that the Li

films

is

formed

PEO (6xlO 5)

carbon Is electron lines were observed.

when

concentration

films

was confirmed by /nfrarea spectra

corresponded

From the r e s u l t of

was

proportion

to

thin f i l m s obtained

texture.

concentrations starting

prepared

various molar r a t i o s X of PEO to

LiCF3SO3 ( 3 ~ X ! 2 0 0 ) . were

were

is

permeation

such as that observed in the m o l t e n

3. Results and discussion

This

to a value of 200 - 2000.

addition,

Hewlett-Packard 4192A impedance analyzer.

gel

mixture

weight

molecular

using

as

room temperature.

chromatography, which indicates that the average evaporating

same procedure

par*i~77g

molten

blocking electrodes fabricated on t h e i r surfaces the

In other words,

the evaporated films are probably in a

result

Conductivities

films is considerably d i f f e r e n t

flow nitrogen

measurements. were

the

structure region d i f f e r e n t

determined

on

Details the

basis

of

from that of the

structure

of infrared and ESCA

studies w i l l be reported elsewhere8.

Typical Dsc traces of t h i n films prepared by evaporating LiCF3S03)

a

Fig.l.

For

for

or

pure

comparison,

(PEO)I5LiCF3SO3 also

(PEO)I5-LiCF3SO3

mixture

given.

complex

(6.3 mol%

PEO are a

DSC trace

formed

Three e n d o t h e r m i c

by

of

a

casting is

peaks are found

the evaporated f i l m prepared using the pure

PEO.

These

(PEO)ls-LiCF3SO~evaporated film

shown in

peaks s h i f t to lower temperatures

~J Ip -

0 a z

W

when LiCF3SO3 are

three

vicinity

is added to the specimen.

diffuse

endothermic

There

peaks in

the

of -16 - 41°C f o r the evaporated films

with a s t a r t i n g composition of (PEO)15-LiCF3S03, whereas sharp

-50

- () 5'0 TEMPERATURE (°C)

100

the cast (PEO)I5LiCF3SO3 complex has a endothermic peak at 58°C, which

corresponds

to

pure PEO. This

the

melting

suggests

that

point the

of

nearly

state

of

FIGURE 1 DSC traces f o r evaporated and cast films with various compositions in PEO-LiCF3SO3 system.

Y. Ito et al. / Structure and ionic conductivity in evaporated thin films

3.2 Conductivity The were At

complex

conductivities

of evaporated thin films

measured using ion-blocking Au electrodes. any

plots

given of

temperature,

the

specimens

conductivity

consisted

semicircular

complex

impedance

response

of

an

in

all

the

representative

approximately

considerable

sCng/e arc with the high frequency

conductivity being heating

led

to

were

value.

open f i g u r e .

separated

This

resistivity

an

value

of

The

two

at the real resistance

was taken

as

the

bulk

the f i l m s and used to calculate

the ionic conductivity. conauc~vity

Ionic

evaporated

films

data obtained by cooling with various starting

observed every

10

T

This

data obtained by also given as a Below

about

is that

hysteresis

45°C

found,

when cooling than

For

composition

the

the

film

obtained

when

phenomenum was

a s i m i l a r temperature region f o r

specimen

measured,

except

with

low

LiCF3SO3 concentrations (below 3 mole %). These cooling conductivity curves are not linear

but bowed.

Thus,

the evaporated f i l m

specimens do not e x h i b i t Arrhenius behavior, but

dependence

TEMPERATURE 80 40

the is

example.

over

comparison are

120

,

obtained

it.

seem

a (PEO)15LiCF3SO3

a starting

hgstez'es:Ls

much higher

compositions of PEO-LiCF3SO3 are plotted as Iog~ versus 1/T in Fig.2. Also included for d a t a for

with

6.3 mole % LiCF3SO3 heating the film

intercept at the o r i g i n , whereas data in the low regions

prepared by casting (dotted l i n e ) .

specimen

frequency

279

to

follow

equation

which of

a Vogel-Tam mann-Fulcher

describes

the

(VTF)

temperature

ionic conductivities observed in

T (°C) I

0

LiCF3S03 (mol%)

©

0.5 6.3

~

7.7

'O

14.3 25.0 6.3

CO01klg

-0.1

10.0

•

heatin

~

"T

~10-1

-0.2

R (,-) 10-2

-0,3

1 0 -3

2.4

216 2:8 310 3.2 3.4 103/T ( K -11

3.6

3.8

FIGURE 2 Temperature dependence of conductivity in evaporated f i l m s with various s t a r t i n g compositions in PEO-LiCFRSOR system. Data for (PE0)I5LiCF3SO3 complex prepared by casting are shown for comparison (dotted l i n e ) .

-0.41 0

2o 40 so so 16o liO14O TEMPERATURE

(°C)

FIGURE 3 Plot of ~n(~T1/2/A~-I versus temperature for evaporated films with various starting compositions in PEO-LiCF3SO3 system.

Y. Ito et al. / Structure and ionic conductivity in evaporated thin films

280

fused

salts 9

and

amorphous

polymer

e l e c t r o l y t e s 3, a =AT-I/2exp(-Ea/T-To ) where

A,E a

and

plots

shown in

when

[In( ~ T I / 2 / A ~ -1

temperature A,E a

Fig.2

for

and TO,

TO are

constant.

The curved

can give s t r a i g h t lines is

p l o tt e d

against

with the mole percent in Fig.4. increases

almost

increases

more or

concentration the

shown in

Fig.3.

The three

Ea also as LiCF3SO3

linearly

less

is increased.

On the other hand,

of

Another

striking

behavior

of

feature

evaporated

parameters obtained, together with the values of

conductivities

conductivity

at 25°C, are given as functions of

compositions

the s t a r t i n g

LiCF3SO3 concentrations

extremely

expressed

and

T are about 200 K and almost 0 independent of LiCF3SO3 concentration.

the most appropriate choice of as

values

The parameter A

exponentially

for

of the conduction

films

is

that

the

specimens with the s t a r t i n g

3.2 - 10.0 mole %

LiCF3SO3

are

(3xlO -2 S/m) even at 25°C and

large

decrease steeply when these specimens are cooled

below

10-1

a

certain

temperature

in

the

region

5 - 15°C. On the

10-2 /~c~,~.~

other

(PEO)15LiCF3SO3 found

hand,

in

complex

the case of a cast

Arrhenius

behavior is

above 60°C, and in the 50 - 60°C region a

pronounced decrease in conductivity is observed,

E

°~°~k~

L~

Q 10-3 z 0

which

greatly

value. with

diminishes

and Weston et a l . 4 . that

the room temperature

This conducting behavior is in agreement 10 the results obtained by Berthier et a l. an

Recently, i t has been shown

elastomeric phase or a molten phase is

responsible f o r the ion transport process 1 0 ' I I . In

the

sudden

case

of

the

evaporated f i l m s , the

change in conductivity corresponds to the

endothermic

peaks caused

by the melting which

were seen in the DSC measurements. above,

infrared

As mentioned

absorption bands corresponding

to the molten state of PEO are observed at 25°C. N

From these

o

.i

250~

o

evaporated

I

0

4

facts,

A

J

8 12 16 2 0 Li CF3SO3 (mol °4,)

28

films

molten

state

results

that

films

l

24

the

is

is

concluded

mainly

structure the

that the

behavior

for

a t t r i b u t e d to the

regions.

The

above

c o n d u c t i v i t i e s of evaporated

can be f a i r l y s a t i s f a c t o r i l y described by VTF equation

conclusion. PEO-LiCF3SO3

FIGURE 4 Va ria t i o n in c o n d u c t i v i t y and in best f i t values of the parameters, A, Ea and TO, of the VTF equat i o n f o r evaporated films as a function of the s t a r t i n g LiCF3SO3 concentration.

it

conductivity

characteristic

coopera~ve

The

are ion

films motion

also in support of t h i s transport probably

of

polymer

in evaporated occurs chains

through in the

molten state regions, according to the mechanism similar

to

that

proposed

f or

the

PEO-salt complexes by Papke et al. 12.

amorphous

Y. Ito et al. / Structure and ionic conductivity in evaporated thin films

in isothermal conductivity of the

A maximun

evaporated

films

occurs

in

the

region

prepared enhancement

by

281

casting.

at

The

3 - 10 mole % LiCF3S03, as can be seen in Fig.4.

evaporated

At

the molten state structure regions.

low LiCF3SO3 concentrations (below 3 mole %)

the

low conductivity

charge carriers The decrease in LiCF3SO3

in

probably

ACKNOWLEDGMENT The authors

above with

10 mole % is the inorease in

apparent activation energy Ea term as shown Fig.4.

It

polymer chain

caused

by

a progressive

motions which are thought to be

coupled with ion motion.

polymer electrolyte films were prepared

by evaporating LiCF3SO3. is

a suitable

Their

properties

can probably be attributed to

would like

to acknowledge the

support and encouragement of Drs. Mitsuhiro Kud% Tsuneo Suganuma and Toshikatsu Manabe (Technical Research Center, Hitachi Maxell Ltd.) REFERENCES I. M.B. Armand, Solid 745

State Ionics 9/10 (1983)

2. D.E. Fenton, J . M . Parker and P.V. Wright, Polymer 14 (1973) 589

4. CONCLUSION Thin

the

is l i k e l y that this decrease in is

in the volume available for long-range

reduction

in

due to few

conductivity with increasing

associated

conductivity

films

responsible for ion transport.

concentration

predominantly the

is

conductivity

r o o m temperature

mixture of PEO and

structures

and conduction

were investigated and discussed.

It

3. M.B. Armand, J.M. Chabagno and M.J. Duclot, Fast ion transport in solids, eds. P. Vashishta, J.N. Mundy and G.K. Shenoy (North-Holland, Amsterdam, 1979) p.131 4. J.E. Weston and B.C.H. Steele, lonics 2 (1981) 347

Solid State

concluded f r o m the following three results

that

the

molten

evaporated films state

temperature.

structure 1) Diffuse

contain regions

partially at

room

endothermic peaks

appeared in the range -20 - 50°C on a DSC trace. 2) The average molecular obtained

weight

of the films

by evaporating a mixture of PEO(6xlO5)

and LiCF3SO3 decreased to a value of 200 - 2000. 3) Infrared

absorption

the molten state temperature.

band characteristics of

of PEO were observed at room

The evaporated films obtained from starting mixtures PEO-LiCF3SO3 with 3 -10 mole % LiCF3SO3 were found to exhibit a considerably h/gh ionic conductivity of temperature conductivity

about 3x10-2 S/m even at room

which was much greater of

(PEO)xLiCF3SO 3

5. P.R. Sorensen and T. Jacobsen, Solid State Ionics 9/10 (1983) 1147

than the complexes

6. K. Miyauchi, Phys. Lett.

T. Kudo and T. Suganuma, Appl. 37 (1980) 799

7. Y. I t o , K. Miyauchi and T. Oi, Non-Crystalline Solids 57 (1983) 389

J.

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9. C.A. Angell, J.

Phys. Chem. 68 (1964) 1917

10.C. Berthier, W. Gorecki, M. Minier, M.B. Armand, J . M . Chabagno and P. Rigaud, Solid State lonics I i (1983) 91 11.P.R. Sorensen and Bulletin 9 (1983) 47

T. Jacobsen,

Polymer

12.B.L. Papke, M.A. Ratner and D.F. Shriver, J. Electrochem. Soc. 129 (1982) 1694