Poly(ethylene oxide) - LiCF3SO3 - polystyrene electrolyte systems

Solid State lonics 18 & 19 (1986) 282-286 North-Holland, Amsterdam

282

POLY(ETHYLENE OXIDE) - LiCF3SO 3 - POLYSTYRENE ELECTROLYTE SYSTEMS

Fiona M. GRAY, James R. MacCALLUM and Colin A. VINCENT Department of Chemistry, University of St. Andrews, St. Andrews, Fife, KY16 9ST, Scotland

Polymer electrolytes based on poly(ethylene oxide), polystyrene and LiCF3SO 3 have been prepared. Thin films formed by a hot-pressing technique had superior conductivities to solvent cast systems. The inclusion of the polystyrene support polymer at concentrations up to 60% by volume, improved the physical strength of the electrolytes without seriously impairing the conductivity at temperatures above 60 °C.

i. INTRODUCTION

2. EXPERIMENTAL

Polymer-salt electrolytes based on alkali metal salt complexes of linear polyethers

2.1. Materials PEO-based electrolytes were formed using two

exhibit conducting properties which have

molecular weights of polymer:

recently received considerable attention for

molecular weight 1×106 and molecular weight

potential application in solid-state batteries.I-3

l×lO 5 (Aldrich).

In particular, poly(ethylene oxide)

further purification.

(PEO) has

BDH Polyox 301,

These were used without Lithium trifluoromethane

shown very promising electrical behaviour, but

sulphonate

a major obstacle to its use lies in its poor

grade) was dried for 8 hours at ii0 °C. Styrene 7 was purified in the usual manner and was used

mechanical properties.

At elevated temperatures,

(L~CF3S03)

(3M U.K. Ltd., battery

(>60 ~C), thin films become very flexible and

within 24 hours of distillation.

readily undergo creep.

were stored and subsequent preparations were

Several authors have

suggested methods of improving mechanical sta4-6 The

bility of polymer electrolyte systems.

approach described in this study is to form a

All materials

carried out in a drybox. 2.2. Preparation of Polymer Systems Polymer electrolyte systems, PEOIoLiCF3SO3,

polymer of high Tg in the presence of PEO-

Y% styrene, were prepared by thermally poly-

e'lectrolyte complex, with the object of

merising styrene in the presence of the PEO com-

establishing a structural framework at a mole-

plex.

cular level.

prepared.

Such a method might also bring

PEO, Y% styrene systems were also The polymer was first milled under

about some graft copolymerisation of the two

liquid nitrogen in a specially constructed ball-

polymers which would help to prevent phase

mill. 8

separation. Polystyrene support system.

Appropriate quantities of electrolyte

and PEO (to give an O:Li ratio of lO:l) were (Tg=lO0 °C) was chosen as the Since there was the possibility

milled together for a further period to ensure thorough mixing.

A measured amount of purified

of phase separation if films were prepared by

styrene was added and the mixture transferred to

solvent casting, a hot-pressing preparation

a polymerisation tube which was degassed and

technique was developed which had the added

sealed.

benefit of eliminating any influence which

at 120 or 150 °C for 48 hours for purposes of

residual solvent might have on the conductivity.

comparison.

Polymerisation was carried out in vacuo

Samples containing no styrene were

annealed for 48 hours at 120 and 1SO °C. The following abbreviations have been adopted:

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

F.M. Gray et al. / Poly(ethylene oxide) - LiCF3SO 3 - polystyrene electrolyte systems

PEO(M)I20 or 150 and PEO(K)I20 or 150 where

(M)

positioned

such that the tip just made contact

and (K) refer to systems containing molecular

with the sample surface.

weight IxlO 6 and ixlO 5 respectively

was then monitored

refers to the temperature 2.3. Preparation a)

and 120, 150

of polymerisation

(°C).

of Films

The probe penetration

as a function of sample

temperature. 2.5. Composition

Hot Pressed Films

a)

Analysis

Degree of Polymerisation

Films were prepared by a two-stage hot pressing 8 process. 5 mm diameter pellets were formed by

The peak integral ratio,

means of a hydraulic press,

compared with the calculated

15.OO (Analytical

Specac model P/N

Accessories

Ltd.).

The die

was heated to a temperature between 70 and iiO °C, depending on the polystyrene the PEO molecular weight. pellet was re-pressed

concentration

After cooling,

and the

in a 13 mm diameter die

iH nmr spectra of the materials were obtained. 8

percentage b)

polystyrene

possible

This

in thickness. b)

in water.

However,

for sufficiently

Solvent Cast Films

and solvent cast material,

films of PEOIoLiCF3SO 3

were prepared by casting a 4 wt% acetonitrile solution into glass formers on teflon sheets. solvent was removed and films annealed

(80 °C) for 6 hours.

is

it was considered

short chains of poly-

styrene grafted onto a PEO backbone,

Polymer systems were agitated of hot-pressed

to be drawn

of the PEO.

in hot water

After filtering,

a u.v.

spectrum in the range 200-400 nm was obtained on a Pye Unicam SP8-150 spectrophotometer. 2.6 Differential

Scanning Calorimetry

Thermal behaviour was studied with a Perkin

by heating for 24 hours at 120 °C in a dry

Elmer DSC-2 differential

nitrogen

The heating rate was either iO or 20 ° min -I

c)

flow.

Unannealed

films

scanning calorimeter.

The enthalpy change involved in melting PEO was

Hot pressed films were prepared material was not annealed.

as in a) but the

Annealing above

iO0 °C brings about the conversion of a crystalline to an amorphous

free

from the polymer-

Unlike PEO, polystyrene

into aqueous media by dissolution

In order to compare conductivities

Residual

evaluated.

was first separated

ised material.

as before under a reduced load of 7.6 MPa.

150-3OO ~m

was

value and the

Graft Copolymer

insoluble

films,

conversion

iHarom.:iHaliph,

To detect the presence of graft copolymer,

by first cold pressing at 19 MPa, then heating

technique produced homogeneous

283

structure which is, to some

extent retained after cooling. film acted as a 'blank'

An unannealed

for examining this effect.

2.4. Penetrometry

evaluated using indium metal as a standard. 2.7 A.C. Conductivities Films were mounted within a cell holder which has been described previously. 4 measurements Solartron

were carried out using a i174

frequency response analyser,

by a Tektronix 4052 microcomputer

The relative physical

strengths of the poly-

Conductivity

controlled

and the com-

plex impedance measured as a function of fre-

mer systems were measured in the temperature

quency from 1 H z

range 70-75 °C using a Perkin Elmer TMS-I pene-

conductivity

trometer.

the low frequency spike associated with electrode

5 mm diameter pellets were prepared

as described

earlier.

dry nitrogen atmosphere

Samples were held in a and a furnace,

raised

to 1 M H z .

The zero frequency

was found from the intersection

impedance and the high frequency semicircle associated

with the bulk relaxation. 4

around the sample, was controlled by a modified

After

preheating

samples at 95 °C for 12 hours,

Perkin Elmer DSC~I differential scanning calorimeter. A weighted 1 mm quartz probe was

measurements

were made over the temperature

range 20-150 eC.

of

F.M. Gray et al. / Poly(ethylene oxide) - LiCF3SO 3 - polystyrene electrolyte systems

284

3. RESULTS AND DISCUSSION

1 shows the variation in conductivity over the

3.1. Composition

temperature range 20-1OO °C.

Assuming equivalence of weight and volume%,

ivities were exhibited by cast films, with only

[density of polystyrene = 1.05 gcm-3), values

Poorest conduct-

slight improvement for the unannealed material.

of polystyrene content were calculated from IH

Films prepared by the hot-pressing method

nmr spectra and were found in every case to be

detailed here, exhibited consistently higher

within ±5% of the initial volume% of styrene

conductivities than reported elsewhere 2 and, by

monomer.

comparing with figures 2 and 3, it can be seen

Within the limits of uncertainty,

conversion was complete.

U.v. spectra of

that as much as 40% styrene may be added to the

aqueous solutions showed some absorption at

system before conductivities fall to levels of

270 rim, indicating that the mixed polymer

previously reported data.

materials contained a percentage of graft copolymer. 3.2. Physical Properties

-3

Penetrometry measurements of the physical

-o~o

strength of polymer systems were normalised to

~,

allow direct comparison with PEO(M) IoLiCF3SO 3.

T E

Table 1 shows that increasing polystyrene content markedly improved the physical strength, with slightly poorer properties for PEO(K) systems. Addition of electrolyte alone to PEO is also

-4 - .S.~j~m_

,.

" ~ o \ o\

c~

-S

b

-6

i x

•

o

AN~£C,, HOT

•

"%R~u~" \ ,

• LINA.NIIEAI.ED, MOT

\o\\

-7

seen to improve the strength of the polymer:

-- ~-r.~'~

2

-8

this is consistent with higher Tg values 9

2.4

2.6

2.8

3

exhibited by complexes.

3.2

3.4

1000/T (K-I)

3.3. Conductivity Measurements A.c. conductivity measurements were made on polystyrene-free PEO-electrolyte films.

Fig. 1

Conductivities of PEOIoLiCF3SO 3

Figure Conductivities for the mixed polymer materials PEO(M)I20 styrene and PEO(K)120 styrene are shown in figures 2 and 3.

Table Vol% Styrene

1

Those polymer-

ised at 150 °C gave similar results.

Normalised Penetration PEO(M) PEO(K)

Figure 4

shows the temperature variation of the conductivity in the range 60-130 °C for PEO(M) I20

0

l

i. 29

20

O.ll

O.15

40

0.18

0.26

60

0.067

80

0.0089

O

3.34 a

a. Value for PEO containing no electrolyte

(40, 60%) styrene. With increasing polystyrene concentration, there is an accompanying decrease in the volume of conducting material.

Figure S shows figure

1 corrected for this dilution. a)

Temperature region <60 °C

Polymer electrolytes based on PEO have been shown to exist generally as a mixture of phases, 2'9 principally the highly conducting amorphous complex, poorly conducting crystalline

/

F.M. Gray et al.

-3.5

-3

• "~•---

•

•

0%

STYRENE

20~(

"

-4 ¥

285

Poly(ethylene oxide) - LiCF3SO 3 - polystyrene electrolyte systems

~

• 40% STYRENE • BOX

-5

• vt..'--.~

E 0

co v

-6

b

-7

O)

o

~

-4

CO

o_

-8

v. \i~v. "%'-...

"\ m')'"~

oo) - 4 . S

mm'~-m-.._ 'o

-g

\'\ °~'~o ~

-f0

.S 1000/T

Fig. 2

2.6

2.7

~\\"v

2.g

28

1000/T

(K-';

PEO(M)-based electrolyte conductivity

complex and uncomplexed crystalline PE0.

"\ •

The

Fig. 4

Conductivities in the range 60-130 °C

temperature range until the styrene volume is between 60 and 80%.

introduction of polystyrene into the system

3

(K-l)

This may be explained in

contributes to the volume of non-conducting

terms of two distinct percolation limits. I0

material.

Around 60 °C, the non-conducting crystalline

The volume-corrected conductivity spectra

PEO phase melts and consequently becomes a

show that, between 0 and 40% styrene volume,

conducting volume.

the decrease in conductivity can largely be

support polymer can thus be incorporated into

attributed to a fall in relative proportion of

the materials before conductivities are adversely

conducting phase, although some loss is due to

affected. b)

other effects of introducing support polymer

A higher percentage of

Temperature region 60-100 °C

into the system, such as tortuosity and limit-

Two explanations may be considered for the

ations of the free segmental motion of the PEO

change in conductivity at the melting point:-

chains.

i)

Between 40 and 60% styrene volume, the

conductivity falls markedly.

the number of sites for ions to move between.

An equivalent

decrease is not apparent in the higher (>60 °C) -3

• • •

-o-o

r~ -4

•

- •

e~.•

This relative importance of availability of suitable sites in comparison to higher ion conll centration is a finding of a seFarate study.

0% STYRENE 20X ' 40~( "

ii) by removing non-conducting domains, there is a decrease in tortuosity.

T E -5 O o*3 -6 b

Both factors may

contribute to some extent although the controlling factor is unlikely to be tortuosity:

the

knee at 60 °C is not due to a transition through \L~.

o~-7

xn

a percolation limit.

xm~ -8

It should be noted that

there is a variable volume of non-conducting

~lL.. n

crystalline PEO complex in addition to the 60%

-9

2.7

2.9

3.1

3.3 1000/T

Fig. 3

conductivity may be enhanced by increasing

3.5 (K-I)

PEO(K)-based electrolyte conductivity

polystyrene, so that the actual volume of conducting material can be lower than 40% before the conductivity becomes significantly impaired.

F.M. Gray et al. / Poly(ethylene oxide) - LiCF3SO 3 - polystyrene electrolyte systems

286

4. CONCLUSIONS (i) Films prepared by hot pressing have -3 "

•

g~

STYRENE

superior conductivities to solvent-cast material

-4

(ii) The physical strengths of PEO-based polymer

T

E

electrolytes may be improved by including a

-S

o~ ,j

support polymer in the matrix.

(9

(iii) As much as 40 volume% polystyrene may be '\

-7

-~

v~

~o

included into the systems before conductivities

o --

fall to those of PEOIoLiCF3SO 3 solvent-cast -8

films.

c-_ --o

-g

2.7

2.9

31

(iv) The fall in conductivity over the concen-

3.3 1000/T

(K -~ )

tration range 0-60 volume% styrene is less than a decade in the higher temperature range and

Fig. 5

Volume-corrected condu~tivities

thus may be a viable modification to PEO electrolytes in the temperature range where physical

The percentage of crystalline PEO was calculated from DSC traces by evaluating the

stability is poorest. ACKNOWLEDGEMENTS

enthalpy of melting from the area under the

The authors gratefully acknowledge the

endothermic peak at 60 °C and taking a value of

financial support of the Ministry of Defence

8.36 kJ mol -I as the enthalpy of melting of 1OO%

and the Science and Engineering Research Council

crystalline PEO. 12

REFERENCES

By adding salt to PEO, the

crystallinity is reduced from 53 to 30%.

With

increasing polystyrene concentration, there is

i. Armand M., Chabagno M., Duclot M.J.: Fast Ion Transport in Solids, eds Vashishta, Mundy, Shenoy (North Holland, Amsterdam, 1979).

only a marginal decrease in crystallinity (~5%) although a certain amount of inhomogeneity was found for materials of higher polystyrene content.

Overall, the presence of the support

polymer has minimal effect on PEO crystallinity. c)

Temperature region >IO0 °C

Above iOO °C, which corresponds to the Tg of polystyrene, a small discontinuity is observed in the conductivity spectra which is accompanied by a change in the temperature dependence of the conductivity (figure 3).

The conductivity is

enhanced above iO0 °C by the greater freedom of movement of polystyrene molecules above their Tg, thus allowing greater mobility of the conducting chains.

Contribution from the free

polystyrene bulk as well as grafted side chains was verified by measuring the conductivity over this temperature range for a co-mixed system of PEOIQLiCF3S03 and polystyrene, which is thus free of graft copolymer.

2. Steele B.C.H., Weston J.E.: Solid State Ionics, (1981), 2, 347. 3. Armand M.B.: Solid State Ionics, iO, 745.

(1983), 9 &

4. MacCallum J.R., Smith M.J., Vincent C.A.: Solid State Ionics, (1984), ii, 307. 5. Steele B.C.H., Weston J.E.: Solid State lonics, (1982), 7, 75. 6. Tsuchida E., Ohno H., Tsunemi K., Kobayashi N.: Solid State Ionics (1983), iI, 227. 7. Perrin D.D., Armarego W.L.F., Perrin D.R.: Purification of Laboratory Chemicals (Pergamon Press, Oxford, 1966). 8. Gray F.M., MacCallum J.R., Vincent C.A.: submitted for publication. 9. Berthier C., Minier M., Gorecki W.: J. de Phys., (1984), 45, 739. iO. Kirkpatrick S.: Reviews of Modern Physics, (1973), 45, 574. ll. Gray F.M., MacCallum J.R., Vincent C.A.: unpublished work. 12. Griffin Lewis 0.; Physical Constant of Linear Homopolymers (Springer-Verlag, Berlin, 1968).