Complex formation and ionic conductivity of polyphosphazene solid electrolytes

Solid State lonics 18 & 19 (1986) 258-264 North-Holland, Amsterdam

258

COMPLEX FORbGTION AND IONIC CONDUCTIVITY OF POLYPHOSPHAZENE SOLID ELECTROLYTES

Peter M. BLONSKY + and Duward F. SHRIVER Department of Chemistry and Materials Research Center, Northwestern University, Evanston, Illinois 60201, U.S.A. Paul AUSTIN # and Harry R. ALLCOCK Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A. The l i n e a r poly[(alkoxy)phosphazene], [NP(OC2H4OC2H4OCH3)2] n (MEEP), has b e e n synthesized and i n v e s t i g a t e d as a polymeric e l e c t r o l y t e host m a t e r i a l . Amorphouss o l v e n t f r e e polymers a l t complexes formed with a v a r i e t y of mono-, d i - , and t r i v a l e n t s a l t s and e x h i b i t high i o n i c c o n d u c t i v i t y . The c o n d u c t i v i t y v a r i e s with changes in the i d e n t i t y of the cation, the anion and the s a l t concentration. I t a l s o e x h i b i t s a non-Arrhenius temperature dependence. For a l k a l i - m e t a l s a l t complexes the c a t i o n s and anions both c o n t r i b u t e to the i o n i c c o n d u c t i v i t y . The complex (LiSO3CF3)o.25.MEEP e x h i b i t s a c o n d u c t i v i t y a t room temperature which i s 2.5 orders of magnitude g r e a t e r t h a t the corresponding p o l y ( e t h y l e n e oxide) complex.

I.

INTRODUCTION

t r o l y t e s based on s i l o x a n e and phosphazene

Most r e p o r t s o f high i o n i c c o n d u c t i v i t y in

systems.12 Of the two, the phosphazene poly-

polymers have been concerned with p o l y ( e t h y l -

mers are more a t t r a c t i v e for the i n t r o d u c t i o n

ene oxide),

of p o l a r s i d e - c h a i n s and for

es. I - I I

PEO, a l k a l i - m e t a l s a l t complex

We r e p o r t here a new s o l v e n t - f r e e

stability.

t h e i r chemical

L i n e a r polyphosphazenes c o n s i s t s

polymer e l e c t r o l y t e host which e x h i b i t s i o n i c

of a backbone of a l t e r n a t i n g phosphorus and

conductivity

nitrogen atoms with two side groups a t t a c h e d

2-3 o r d e r s o f

magnitude h i g h e r

than the PEO s a l t complexes a t moderate temp-

to each phosphorus atom.13

e r a t u r e s (<60 °C).

Our

Factors which i n f l u e n c e complex formation

initial

work on amino s u b s t i t u t e d

(NHCH3;N(CH3)2),

short-chain alkoxy-substi-

and c o n d u c t i v i t y of s o l v e n t - f r e e polymer e l

tuted

e c t r o l y t e s have been discussed p r e v i o u s l y ,3-8

NHC5H4 N) p o l y p h o s p h a z e n e s i n d i c a t e d t h a t

among these, a low g l a s s temperature i s espe-

these

c i a l l y important.

as

Owing to the importance of

(OCH2CF3) and

systems

electrolytes.

do n o t

mixed l i g a n d (OC6H5,

perform

The p o l y m e r

satisfactorily systems,

after

low Tg in both p o l y m e r - s a l t complex formation

a d d i t i o n of a l k a l i - m e t a l s a l t s , had r e l a t i v e -

and ion t r a n s p o r t , we have i n v e s t i g a t e d e l e c -

l y poor i o n i c c o n d u c t i v i t i e s (a < 10 6 ohm I_

+ #

Current Address: Union Carbide Corporation, Westlake, Ohio. Current Address: Union Carbide Corporation, Tarrytown, New York.

0 167-2738/86/$ 03.50 © ElsevierSciencePublishem B.V. (North-HoRand PhysicsPubUshmg DNision)

P.M. Blonsky et al. / Polyphosphazene solid electrolytes

cm- I

@ I00

°C).

No complex f o r m a t i o n was

i s one of t h e m o s t f l e x i b l e

observed between [NP(OCH2CF3)2]n and a v a r -

backbones known.

i e t y of s a l t s .

3. R e s u l t s

The p r e s e n t r e s e a r c h concerns the p r e p a r a t i o n 12 and c h a r a c t e r i z a t i o n o f t h e p o l y m e r

259

3.1

macromolecular

and D i s c u s s i o n

High P o l y m e r

The s o d i u m

salt

Reaction. of 2-(2-methoxyethoxy)-

and p o l y m e r - s a l t c o m p l e x e s formed between

ethanol

underwent reaction

poly(bis-(methoxyethoxyethoxide)phosphazene),

r o p h o s p b a z e n e ) (Vl)

MEEP (V) and a v a r i e t y of mono-, d i - and t r i -

n-butylammonium bromide,

v a l e n t s a l t s . 14

substituted,

2. Theory.

stable

with

in the

thermally

poly(dichlo-

p r e s e n c e of t e t r a to

yield

the

fully

and h y d r o l y t i c a l l y

h i g h p o l y m e r MEEP (V).

The mechanism of ion t r a n s p o r t in polymer

[N=PCI2]n + 2nNaOC2H4OC2H4OCH3 e l e c t r o l y t e s i s thought to i n v o l v e conforma-

n-Bu4NBr ........ %--> THF, 60 C

(vl) tion

f l u c t u a t i o n s of

the polymer. 1 ' 3 ' 9

This

[N=P(OC2H4OC2H4OCH3)2] n + 2nNaCl

(v)

t y p e o f t r a n s p o r t i s r e p r o d u c e d by e i t h e r a free

volume, 1 ' 9 ' 1 5 or excess e n t r o p y model, 6

r a t h e r t h a n A r r h e n i u s law.

The i o n i c c o n -

Evidence

of c o m p l e t e

obtained

from 31p NMR,

halogen

analysis.

h i g h e r in amorphous p o l y m e r - s a l t complexes as

involve

compared to p a r t i a l l y c r y s t a l l i n e m a t e r i a l s

bonds c o u l d be c o n v e r t e d

of the same composition.7'8

moieties.

The c o n d u c t i v i t y

was

13C NMR and elemental

d u c t i v i t y has been shown to be s i g n i f i c a n t l y

Since

exchange

the purification

treatment with water,

procedures

residual

P-C1

to POH or P(O)-NH

Hence the 31p spectrum was of par-

o f an amorphous p o l y m e r , as g i v e n by a c o n -

ticular

interest

f i g u r a t i o n e n t r o p y model,

singlet

u p f i e l d 6.3 ppm from 85% H 3 P O 4 / D 2 0 ,

f o l l o w s e q u a t i o n 1,

and

consisted

of a sharp

where the A term i s p r o p o r t i o n a l to the num-

no evidence of a P-C1 or P(O)-NH moieties was

b e r of c h a r g e c a r r i e r s

seen.

and t h e TO term,

in

The

13C NMR spectrum contains

5 sing-

the e x p o n e n t i a l , is c l o s e l y r e l a t e d to the

lets expected for the alkoxide sidegroup,

glass

the elemental

transition

t e m p e r a t u r e of

the

sam-

p l e . 1,6

analysis

fit the values

and

for a

f u l l y s u b s t i t u t e d polymer.12

a = AT-I/2exp [ B/(T-To) ]

(1)

The i n f r a r e d s p e c t r u m of MEEP c o n t a i n s

The g l a s s t r a n s i t i o n temperature of the p o l y -

t h e P-N-P a s y m m e t r i c s t r e t c h a t

mer and polymer s a l t complexes a r e a f u n c t i o n

which i s t y p i c a l f o r an a l k o x y s u b s t i t u t e d

of r e o r i e n t a t i o n a l m o b i l i t y of t h e b a c k b o n e

l i n e a r p h o s p h a z e n e .14

and sidegroups.

the s J d e c h a i n s a r e a l m o s t i d e n t i c a l to t h o s e

The polyphosphazene s k e l e t o n

1240 cm- I ,

Bands a t t r i b u t e d t o

P.M. Blonsky et al. / Polyphosphazene solid electrolytes

260

PEO.4

for

salt

X-ray d i f f r a c t i o n , DSC, and o p t i c a l micro-

complexes

formed the

polymer-salt

phous a t room temperature and above.

ray

diffraction

r e f r i n g e n c e i s observed when p o l a r i z e d l i g h t

material

i s employed. The DSC r e s u l t s

values

c o n s i s t of a

s i n g l e 2nd o r d e r h e a t c a p a c i t y t r a n s i t i o n ,

Nd2(S04) 3

a crystalline

scopy i n d i c a t e t h a t MEEP i s c o m p l e t e l y amorNo bi

of

phase

were were

to

a ratio

C. The

ter

than

s e a l e d s a m p l e s were c y c l e d a minimum of 3

ray

diffraction

t i m e s a t each h e a t i n g r a t e w i t h no e v i d e n c e

MSO3CF 3 c o m p l e x e s ,

where

of polymer d e g r a d a t i o n , s h i f t i n g Tg or add-

As e x p e c t e d ,

glass

assigned as a g l a s s t r a n s i t i o n a t

83

of

or

the

but

the

repeat

salt

the

pure

salt.

unit

grea-

was observed

optical

b y X-

microscopy, M = Li,

Na,

transition

for

Ag. tempera-

s t r a t e t h a t t h e polymer e x h i b i t s no s i g n i f

a higher

i c a n t weight l o s s

due to a reduction in the polymer f l u i d i t y .9

at

temperature

complexes

upon

shift

addition

of

to

salt,

The change in Tg i s r e f l e c t e d in the mechan

which point complete decomposition occurs. 3.2

polymer-salt

2e

tures

303 °C,

the

no excess

The X-

semicrystalline broad

salt/polymer

after

i t i o n a ] e x o / e n d o t h e r m s . TGS s t u d i e s demon-

(<2%) up to

of

2,

this

those

At

o

was formed.

generally

close of

of

Gd2(S04) 3

ca_:. 7 2 h o u r s

complex peaks

or

i c a l r i g i d i t y of the complexes.

ComplexFormation and Glass T r a n s i t i o n Temperatures.

In g e n e r a l ,

the complexes change from e]astomers to t h e r MEEP forms p o l y m e r - s a l t complexes with a m o p l a s t i c s with i n c r e a s i n g s a l t concentra v a r i e t y of mono , d i

and t r i v a l e n t

salts, t i o n . T a b l e 2 c o n t a i n s some r e p r e s e n t a t i v e

T a b l e 1.

As w i t h t h e p a r e n t polymer, t h e s e p o l y m e r - s a l t complexes and t h e i r r e s p e c t i v e

complexes are c o m p l e t e l y amorphous, s i n g l e g l a s s t r a n s i t i o n temperatures. phase s y s t e m s e x h i b i t i n g a s i n g l e Tg and no 3.3

b i r e f r i n g e n c e under p o l a r i z e d l i g h t

Conductivity Measurements.

when Complex

first

prepared.

Complexes

with

i m p e d a n c e a n a l y s i s was performed

Na3Co(NO2) 6 for the pure p o l y m e r and p o l y m e r

s a l t com

s l o w l y formed a c r y s t a l l i n e phase after applexes, proximately two weeks. not

match

that

1:1 Li: Na:

Rb: Ag:

of

on data which was c o l l e c t e d between

The X-ray pattern did

the

pure

Table

1.

salt.

Polymer

Representative

2:1

Salts

BF4, B r , SCN, NO 3, C I , SCN, CF3SO 3 I , SCN NO3 , CF3SO 3

Salts

CF3CO0. CF3SO 3

Li: Ca: Sr: Zn:

Which Form Complexes and

1:2

Salts

O2C(CF2)3CO 2 S03CF 3 SO3CF 3 SO3CF 3

with

MEEP 3:1

and

2:3

Salts

Na: C o ( N 0 2 ) 6 Gd: SO 4 Nd: SO 4

P.M. Blonsky et al. / Polyphosphazene solid electrolytes

2.

Table

Glass

Transition

Temperatures

M:P a

Tg b

(x:l)

(°C)

AgSO3CF 3 AgSO3CF 3 AgSO3CF 3 AgSO3CF 3 AgSO3CF 3 AgS03CF 3 AgS03CF 3

0 0.083 0.125 0.167 0.250 0.500 1.000 2.000

-83.5 -78.3 -74.3 -68.7 -60.0 -35.2 -11.6 +11.4

8.1x10 -8 1.6x10 -4 2.6x10 -4 .... .... 2.8x10 -5 2.5x10 -6 < 10 - 9 c

LiSO3CF 3 LiSO3CF 3 LiSO3CF 3 LiSO3CF 3

0.125 0.167 0.250 0.500

-69.4 -65.7 -62.4 -58.9

2.2x10 2.2x10 2.7x10 1.2x10

Salt

a30 °

and

Conductivities

for

a55 °

261

selected

MEEP C o m p l e x e s .

a70 °

a90°

1.6x10 -7 3.2x10 -4 5.3x10 -4 6.8x10 -4 4.8x10 -4 1.5xlO -4 3.3x10 -5 < 10 - 9 c

1.9xlO -7 4.8x10 -4 8.4x10 -4 9.8x10-4 9.4x10 -4 3.1xlO -4 1.OxlO -4 < 10 - 9 c

2.1xlO -7 7.5x10 -4 1.4x10 -3 .... d 1.4x10 -3 6.7x10 -4 3.7x10 -4 < 10 -9c

5.7x10 6.2x10 7.5x10 5.7x10

8.5x10 8.7x10 1.2x10 1.OxlO

1.3xlO 1.5x10 2.2x10 1.9x10

(ohm-cm) -I

-5 -5 -5 -5

-5 -5 -5 -5

-5 -5 -4 -4

-4 -4 -4 -4

a c t u a l s t o i c h i o m e t r i e s +0.02. b E x t r a p o l a t e d t o O°/min. stoichiometry; c Conductivity t o o l o w to measure, d Sample f~owed and s h o r t e d c e l l .

a Ideal

o

25 and 100

C.

a

of

variety

P l o t s o f i n (aT) v s . 1/T f o r

data

are

(trifluoromethyl)sulfonate,

1/T.

as

Curvature

S03CF3 , s a l t c o m p l e x e s a r e shown i n F i g u r e

ity

data

I.

due

to

S i m i l a r g e n t l e curves are obtained i f the

plotted

is

In

of

the

expected

(aT 1 / 2 )

or

In

electrical for

conductiv-

amorphous

the

T-T o temperature

low

but

(g) vs.

polymers,

dependence

(eq.

conductivity

of

1). The x -1

x

O

zl

significant

×

~.

+

the ×

D ~.

r7

~'4

MEEP h o s t

polymer

appears

to

be

due

to

x

D

X

x

Ag

residual

NaC1

trapped

in

the

polymer.

Thus,

-3 +

~

~

LI

+

passing

MEEP t h r o u g h

ion

exchange

columns

in

ANa +

the

+ +

to

+ S¢

-7

H+ a n d 8.1

x 10 -8

factor ple.

OH- f o r m s

of

reduces

ohm-l-cm

25 less

Complexes

than of

-1

the

at

the

conductivity

25 ° , which unexchanged

MEEP w i t h

is

a

sam-

triflate

salts

-9 O 0

-1i

l 2.7

,

~.~

0

~I~

O

O i

~.o

of

0

~J~

0

MEEP

~I~

~.~

q

IO00/T (degrees K)

simple

exhibited the

conductivity, vs 1/T for

ions,

ionic

with Sr;

cations,

Zn;

Li;

Na; Ag,

conductivities

the

salts

Ca,

and

thar~

having

dipos-

a further

reduc-

1. tion

Electrical as in (aT) complexes.

higher

complexes

itive Figure

monopositive

a(fl-l-cm-1), plotted [M(SO3CF3)x]O.25,MEEP

for

of

the

complexes

ionic

conductivity

of tripositive

is metal

observed salts.

P.M. Blonsky et al. / Polyphosphazene solid electrolytes

262

Figure

2 displays

the influence

rent anions on the ionic conductivity. data

are

compared at

met

ratio

and

only

a constant

transport,

in the

lithium

anions

accounts

decreasing

The i o n i c

were

nature

relative

complexes,

are

carrier

species

tigated

using

A.C.

by

complex

When i o n

X

~~.

× ÷+

+

]I

~"

x

0

+S03CF

t-

C

conductivity

the

blocking

and

of the different

vs anion)

and

Numbers.

D.C.

were inves-

potentials

and

impedance/admittance range

of

5-500,000

electrodes,

Pt,

were

Hz. em-

the complex

impedance plots

were con-

I

×

-5.0

ployed

!

+

~-'°

(cation

and

I

+

×

a nd T r a n s p o r t

contributions

over

Tg v a l u e s

6'9

of the

the

techniques

+

Species

ion

analyzed

+ X

the rising

conductivities.

3.4 C a r r i e r

almost i d e n t i c a l .

-3.0 ' I

for

for

energies

salt

The

cation/poly-

mononegative

The a c t i v a t i o n

employed.

ation

of diffe-

| SCN×

-6.Q

with a model

sisting

of

a

equivalent

parallel

circuit

geometric

con-

capacitance

3

x

O

sistent

and bulk

resistance

layer

capacitance.

sults

i n an a r c

in series

with

a double

Such a combination

followed

by a s p u r ,

re

with

the

-7.0 spur -8.0

2~7

'

2 8

!0 3.

2.-9

!O00/T

I

3J"l

(degcees

3.2

I

at

layer

3.3

K]

low

frequencies

ascribed

c a p a c i t a n c e . 16 When c a t i o n

electrodes

are

employed

the

to

double-

reversible

spur,

for

ion

F i g u r e 2. blocking Electrical conductivity, a(-1-cm-1), plotted a s I n (aT) v s 1/T f o r [LiX]0.167.MEEP complexes.

arc,

electrodes,

which

can

charge-transfer The

conductivity

plexes

increase

beyond

t h a t . 12

for with

a series

of

(AgSO3CF3)x. MEEP com-

up t o x = 0 , 1 7 , A similar

trend

to

lower

(Table

higher 2).

glass

around

ionic

complexes with high salt ited

is observed

of (LiSO3CF3)x°MEEP complexes,

maximum c o n d u c t i v i t y

addition

and decrease

conductivity, concentration

transition

The p r o g r e s s i v e

polymer chains,

x=0.25.

the exhib-

of

complex form-

be

ascribed

resistance

interface.

This

response

is

for

trolyte

observed

used to estimate and

to a second a parallel

double

layer

electrode

elec-

type

of

other

dual

arc

polymer elec-

reversible

elec-

11

A potentiostatic

obtained

and

systems with cation

trodes.2,10,

of this

to

at the reversible

trolyte

anions

temperatures

immobilization

by p o l y m e r - s a l t

In

capacitance

is converted

polarization

m e t h o d was

the transference

numbers for

cations.

technique with

In a r e c e n t reasonable

transference

application

agreement

was

n u m b e r s from Tu

P.M. Blonsky et al. / Polyphosphazene solid electrolytes

bandt,

pulsed field gradient NMR,

tracer techniques. 17

applied to a cell with the electrolyte wiched

between

Complete

cell

cation-reversible polarization

anion diffusion, exactly

is

Pt e l e c t r o d e s .

sand-

measured

electrodes.

hours.

salt gradient,

the m i g r a t i o n

of this

a D.C. p o t e n t i a l

plied to a symmetric

o c c u r s when the

created by a

opposes

the c o n d u c t i v i t y ,

and radio-

A constant potential

263

cell

current

decayed

and

ion

the electronic

conductivity

general

only,

phazenes are electrical

due to the cat-

current vs.

time polarization

result

fits

with

the

that ion-free polyphosinsulators. 14'18

system, MEEP, has been

s y n t h e s i z e d by a p p e n d i n g s h o r t - c h a i n

A typical

curve

observation

The polyphosphazene

number. Table 3 lists transport numbers for a complexes.

This

is f o u n d to

4. Conclusions.

ion and anion, yields the cation transference

of M E E P - s a l t

16

tally polarized with respect to ionic motion

The ratio of final current, due to the cation

variety

to z e r o w i t h i n

the

At this point the c e l l has b e e n to-

be n e g l i g i b l e .

current,

ion blocking

U n d e r these c o n d i t i o n s

under the influence of the applied potential.

to the inital

with

was ap-

ether g r o u p s to the p h o s p h a z e n e

exhibits is

bone.

polarized,

after which a constant current

is

complexes

observed.

The cation

is

display very good ionic conductivity at room

gradual

decay

until

transference

number

forms

(PN)x back-

the c e l l

a smooth,

MEEP

poly-

with

single-phase

alkall-metal

amorphous

salts,

which

affected by the cation and anion employed but

temperature.

appears

on polyphosphazene

may provide

a wide

of

for p o l y m e r

electro!yte

to be concentration

a slightly

independent.

modified experiment,

In

to test for

Salt

3.

Cation

hosts

formation.

the possibility of an electronic component to

Table

polymer

The variation of the sidechatns

Transport

Numbers a

M:P c

Temp.

Ia

(x:l)

(°C)

(~)

Initial

Idc(/iA )

Final

T+ b

Run T i m e

(hours)

.... AgS03CF 3 AgSO3CF 3 AgSO3CF 3

0 0.125 0.250 1.000

52.4 56.3 54.8 64.7

0.03 1.35 2.20 4.33

0.04 2.40 4.31 10.32

0.00 0.05 0.11 0.36

<.00 .02 .03 .03

16 42 50 44

LiSO3CF 3 LiSCN LJBF 4 LiBr Zn(SO3CF3) 2

0.167 0.167 0.167 0.167 0.167

54.0 58.5 55.0 55.0 61.6

3.48 1.52 2.71 1.38 1.55

3.59 1.60 2.94 1.37 1.73

1.15 0.68 0.49 0.79 0.01

.32 .42 .17 .58 <.01

92 105 94 100 15

a In all cases 0.01V was the applied potential, c Ideal stoichiometry, actual stoichiometries

b Errors +.02.

in

transport

numbers

+.02.

range

P.M. Blonsky et al. / Polyphosphazene solid electrolytes

264

ACKNOWLEDGEMENTS

8.

C. B e r t h i e r , W. Gorecki, M. M i n i e r , M.B. Armand, J.M. Chabagno and P. Rigaud, S o l i d S t a t e Ionics 11 (1983) 91.

9.

H. Cheradame, IUPAC M a c r o m o l e c u l e s , eds. H. B e n o i t and P. Rempp (Pergamon P r e s s , New York, 1982) p. 251.

This research was supported by the Office of Naval R e s e a r c h and t h e NSF through t h e N o r t h w e s t e r n U n i v e r s i t y M a t e r i a l s Center. Work a t P e n n s y l v a n i a S t a t e U n i v e r s i t y was

lO. L.C. H a r d y a n d D.F. S h r i v e r , S o c . 107 ( 1 9 8 5 ) 3 8 2 3 .

J.

Am. Chem.

supported by the P u b l i c Health Service. 11. J.E. Weston and B.C.H. S t e e l e , S o l i d S t a t e I o n i c s 7 (1982) 81.

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