New polymer electrolyte systems

Ekctrochimica

Arm,

Printedin &ear

Vol.

Britain.

37. No. 9. pp. 153% 1544, 1992 6

0013-4686/92 15.00 + 0.00 1992. Pergamon Press Ltd.

NEW POLYMER ELECTROLYTE SYSTEMS J. M. G.

COWIE,*

A. T. ANDERSON, M. ANDREI, A. C. S. MARTIN and C. ROBERTS

Department of Chemistry, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, U.K. (Received 20 December 1991) Abstract-Conductivities in a number of polymer electrolyte systems have been investigated using divalent cation salts such as Ma(ClO,),. Ca(ClO,),. Ni(Cl0.L and ZnCI,. The polymers used were comb-branched structures with oligoTkthyl% oxide) -&d iligo(piopylene oxide) aidd chains and polyphosphazene networks linked by tetra-aza-macrocycles. The systems using Mg and Ca salts achieved levels as high as 10-3-10-4 S cm-’ but the aza-macrocycles were about two orders of magnitude poorer. Key words: ion conduction, electrolytes.

comb-branch

polymers, aza-macrocycles,

INTRODUCTION A considerable body of work has now been accumulated on ion conductivity in solid solutions of polymers and salts. The criteria for the development of systems that can achieve good working conductivities in the range 10-3-10-6Scm-‘, over a temperature range of ambient to 390 K, are now well established. The polymer must be capable of dissolving the salt and ionizing it to produce sufficient numbers of the charge carrying species to reach these conductivity levels; it must be amorphous to allow easy movement of the ions; it must have a flexible chain (ie low glass transition temperature r,) to assist the ion transport, and a diverse range of polymer structures have been developed that are capable of achieving these ends. In the majority of the polymer electrolyte systems reported so far there appears to be a maximum of just over 10m3Scm-’ for the conductivity which is difficult to exceed unless small molecule plasticizers are added to the system. Even this modest value can only be reached at higher temperatures rather than at ambient, and so various areas must be explored in an attempt to overcome this apparent barrier. One method is to seek new polymer systems, another is to investigate the use of multivalent cation salts (particularly divalent) where the isolated cations could each carry more charge. Work in this area[ l-71 has highlighted some interesting characteristics of these divalent cation salt systems. Farrington and co-workers[&71 have found that hard, acid-base, combinations such as halides of M$+ and Ca2+ tend to be pure anion conductors when dissolved in combipoly(ethylene oxide), whereas soft-hard nations, eg Pb2+, Cd’+, Zn2+, Co*+, exhibit both anion and cation transport. Some preliminary work on divalent cation salt systems mixed with wholly amorphous comb-branch oligo(alkylene oxide) polymers is described here, where the use of short side chains of ethylene oxide *Author to whom all correspondence should be addressed.

divalent cation salts, polymer

and propylene oxide avoids the complication of crystallinity encountered in poly(ethylene oxide). Another possible way to increase conductivity in polymer electrolytes is to try and maximize ion pair separation in the polymer matrix and improved cation complexing units such as crown ether polymers have been studied in this laboratory[8,9]. This has now been extended to include poly(azamacrocyclic) systems and the initial studies are reported in this paper.

EXPERIMENTAL Comb-branch polymers

The experimental procedures for monomer and polymer synthesis of the comb-branch polymers with oligo(ethylene oxide) side chains, based on the itaconate backbone[lO] and vinyl ether macromonomers[l l] are as detailed in the original references, and these are coded PDMEO(3)I and PVE0(3), respectively, where the side chains contain three ethylene oxide units. Similarly a poly(itaconic acid ester) with oligo(propylene oxide) side chains was prepared as outlined previously[l2] and coded PDPPGI. All of these structures are shown in Fig. 1. Macrocycle networks

The macrocycle 1,4,8,1 l-tetra-aza-cyclotetradecane was reacted with excess chlorohexanol to form the tetra substituted alcohol derivative. This was then used to crosslink poly[bis(trifluoroethoxy)phosphazene] (to give a structure with the idealized schematic form shown in Fig. 11). Full experimental details are given elsewhere[l3]. The polymer samples used here contained 10 wt% of the macrocycle and the Tg was 193 K. ac conductivity The UC conductivity of the polymer-salt was made by complex impedance analysis Solartron 1286 electrochemical interface Solartron 1255 frequency response analyser.

1539

mixtures using a with the The real

1540

J. M. G.

&WIE

er

al.

compressed teflon ring was inserted as a spacer. This served two purposes; it prevented arcing of the current across the electrodes and it prevented sample flow at high temperatures.

RESULTS

AND DISCUSSION

PDPPGI

The dissolution of hard-soft salts such as Ca(ClO,), and Mg(ClO,), in the comb-branch oligo(a1kylene oxide) polymers is readily achieved and the conductivity levels measured are often comparable to the monovalent salt/polymer mixtures studied[lO-121. PDMEO(3)1 mixed with Ca(ClO,),

Fig. 1. Some of the polymer structures used as polymer

electrolyte systems. and imaginary parts of the complex impedance were plotted as shown in Fig. 2, from which the conducdvity can be derived. Polymer samples were contained between two stainless steel electrodes, and in order to maintain constant sample thickness throughout the course of the measurements which were carried out as a function of temperature, a

Z” x

Homogeneous mixtures of PDMEO(3)I and Ca(ClO,), can be prepared which exhibit a large increase in Tg when compared with the undoped polymer, and this usually indicates that the polymer is an effective solvent for the salt. Thus for [Ca2+]/[EO] = 0.05 the Tg = 280 K which represents an increase (AT) of 61 K. This is much higher than for equivalent systems containing lithium or sodium perchlorates, where the respective values of T, are 253 and 256K at [M+]/[EO] = 0.05. In some systems the higher AT,, the higher the conductivity[8], but this is not reflected in the conductivity levels recorded for the present system. Typical data, presented in Fig. 3, show that the conductivity rarely exceeds 10e4 Scm-‘. Comparison with the data for a sodium cation-PDMEO(3)I mixture, shown in Fig. 4, illustrates the inferior performance of the calcium salts, at a fixed salt concentration of 0.05 and comparable chain flexibilities when plotted under the reduced temperature conditions (T - T,). The reason for the lower conductivities when the calcium salts are used could be either that the number of effective charge carriers is lower or that the ionic

Id/n

0.0

I.2

3.6

2.4 zx

4.8

-I 6.0

ldrf2

2.4

z’x

104/R

Fig. 2. Examples of the complex impedance plots obtained ‘for the aza-macrocycle systems with NaClO, (bottom) and 4. Ni(C10+)2 (top) salts.

I

I

I

2.6

2.6

3.0

T

1 3.2

/l@K

Fig. 3. Log (conductivity)-reciprocal temperature plots for the PDEMEO(3)1/Ca(ClO,), system with [Ca2+]/[EO] ratios of 0.0125 (0); 0.05 (O), and 0.125 (A).

1541

New polymer electrolyte systems

-4

-

-^

‘6 Y

Log 0 Iscrn-t

P z z s 0 0 J0 .E

-6-5 -

k” --2.6

-71

28

i

i2

3:4

3.6

lO%- /K-r 20

40

60

80

100

120

Fig. 5. Log conductivity-reciprocal temperature plots for PVEG(3)-Ca(ClO,), mixtures: (a) 0.0625, (0) 0.125, and (0) 0.25.

140

T-Tg,/K

Fig. 4. Reduced temperature-conductivity plots for PDMEO(3)I mixed with calcium (0) and sodium (0) perchlorates at a fixed salt-polymer ratio of 0.05.

-2I

species are larger and more difficult to transport. As the size of the Ca*+ ion (99 pm) is similar to that of the Na+ (95 pm) ion, it may indicate instead that the ionization of the calcium salt, as represented by /Scm- t

Ca(ClO,), * CaCIO,+ + ClO; z$ Ca*+ + 2ClOi, only goes as far as the first step and that the larger monovalent CaCIO: species contributes to the conductivity but is slower to respond to site interchange in the low dielectric medium. PVEO(3) mixed with Ca(ClO,), and Mg(ClO,)* Polymers prepared from vinyl ether macromonomers of oligo(ethylene oxide) tend to be the most successful of the comb-branch polymer structures we have studied and give the highest conductivity levels with alkali metal perchlorates[l 11. When PVEO(3) was mixed with Ca(CIO,), at a ratio of [Ca*+ ]/[EO] = 0.0625 the glass transition temperature increased by 44 K, which is less than that for the itaconate polymer system, but greater than for PVE0(3bLiC104 where AT, u 31 K. Conductivity data for the Ca(C104)* system plotted against (l/T) are shown in Fig. 5 and are generally higher than for the PDME0(3)I/Ca(CIO,), system. The trends of conductivity with salt concentrations

I

-1OJ 2.6

2.8

3

3.2

Calcium IPV EG(3) 2PV EO(3) 3PV EG(3) Magnesium 4PV EO(3) 5PV EO(3) 6PV EO(3)

[M’+ I/PO1

T,/K*

0.250 0.125 0.0625

250 259 252

0.250 0.125 0.062

-

*Undoped polymer T, = 208 K

3.6

1dr’lK” Fig. 6. Log conductivity-reciprocal temperature plots for PVE0(3)-Mg(ClO& mixtures: (A) 0.0625, (0) 0.125, and (0) 0.25.

are rather unusual. When the [Ca2+]/[EO] ratios are 0.25 and 0.0625 there is little difference in conduction levels, but there is a noticeable decrease at 0.125. This is mirrored in the behaviour of the Tg for each mixture, as given in Table 1, where T, is seen to pass through a maximum at 0.125 concentration. This trend is not so obvious when the salt used is Mg(CIO,), where now the samples with

Table 1. Glass transition temperatures, activation energies, and VT’F parameters for comb-branch oligo (ethylene oxide) polymers PVEG(3) mixed with calcium and magnesium perchlorate Sample

3.4

EJkJmol-

S/K

To/K

(Ts - T,)/K

75.2 107.4 79.3

1256 1712 1938

209 205 177

41 55 75

47.8 97.6 74.8

857 4388 2236

197 125 161

-

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J. M. G. C~WIE ef al.

PVEO(3) with 4: 1 M(ClO&.

(a)

-10 .I 2.6

28

3

3.2

3.4

PVEO(3) with 8:l M(ClO,)s

(b)

2.6

3.6

2.6

ldr’/K-’

3 lo%-’

32

3.4

3.6

/K-l

PVEG(3) with 16:l M(ClO,),

Cc)

d

-10

4 2.6

2.8

3

33

3.4

3.6

Fig. 7. Log conductivity-reciprocal temperature plots for PVE0(3)-salt mixtures with salt concentrations (a) 0.25, (b) 0.125, (c) 0.0625; Mg (0) and Ca (m).

[Mg2+ ]/[EO] = 0.25 clearly give the highest conductivity (see Fig. 6). A comparison of the conductivities measured for both salts is made in Fig. 7a-c where it can be seen that the PVE0(3)/Mg(ClO.J, systems are generally superior (N.B. the Md+ ion is smaller than Ca2+). It is also interesting to note that when compared with the conductivities achieved using LiClO, , the latter are better only when the salt/polymer concentrations are 60.125, whereas the divalent cation systems have the edge, as can be seen in Fig. 8, at concentrations of 0.25. The conductivity-temperature plots tend to be non-Arrhenius and although the curvature in the plots is slight the data are best analysed using the Vogel-Tammann-Fulcher (VTF) equation: IJ = a, exp( - B/T - To).

(1)

Values of the constants B and To are listed in Table 1 and for the calcium salt system the magnitude of 7’, - To) conforms to the Adam-Gibbs[ 141configura-

tional entropy model in which To is the temperature at which the configurational entropy becomes negligible and is predicted to be 50 K lower than Ts. This model also interprets the constant B in the form B = T,S,*A~/kTAC,,,

(2)

where k is the Boltzmann constant, AC, is the difference in heat capacity between the glass and the rubber state at the transition, S: is the configurational entropy of the segmental motion of a minimum segment capable of undergoing a spatial rearrangment whose activation energy is A/A. However, the data can be treated using a linear approximation and the activation energies derived in this way are higher than those calculated for comparable systems[l5]. PDPPGI mixed with ZnCl, Dissolution of ZnCl, in PDPPGI leads to a monotonic increase in T . The value of ATx is 25 K at [Zn’+]/[PO] = 0.05 wbch is lower than for the

New polymer electrolyte

systems

1543

10%-‘/K-

Idr’ IF’

Fig. 8. Comparison of conductivity levels for Ca, Mg and Li perchlorates at concentraton levels of 0.25

and 0.125. previous systems and this is also reflected in the conductivities recorded in Fig. 9. The general levels of conductivity fall below the monovalent perchlorates but are larger than the polymer mixed with LiCl where very poor values are obtained. These comparisons are illustrated in Fig. 10. A VTF analysis of the data is summarized in Table 2 for a fixed salt composition. The systems appear to conform to the Adam-Gibbs model with (T - T.) values close to the theoretically predicted 50 k. The activation energies are of comparable magnitudes for

both mono and divalent ion systems although the lower conductivities for the ZnCl, system may suggest that the ZnCl+ ion is one of the species present. Poly(aza-macrocycle)

mixed with NaClO, and Ni(ClO,)* Polymers have been prepared with macrocyclic ethers (ldcrown-5) pendant to the main polymer chain[8,9] which gave reasonably good levels of ionic conductivity when mixed with alkali metal salts. By

4

-5

Log a /sem-’ d

-7

0.2

0.1

CtVTl/CPOl

Fig. 9. Conductivity-temperature plots for PDPPGI-ZnCl, mixtures at concentrations of (0) 0.125, and (m) 0.25.

Fig. IO.Isothermal conductivity-salt concentration plots for PDPPGI mixed with LiClO, (0). NaClO, (O), Z&l, (m), and LiCl (+) at a temperature of 403 K.

Table 2. Comparison of the systems PDPPGI mixed with ZnCl, , NaClO, and LiClO, using a VTF analysis salt

ZnCl, NaClO, LiClO.

M/[pol

T,IK

B

To/K

(T, - T,)/K

ACp/J K-i mol-i

0.125 0.125 0.125

262 290 u13

1546 1728 1302

215 223 234

41 67 49

34.3 39.8 37.3

A@J mol-i 94 129 85

J. M. G. Cowt~ et al.

1544 I -r-N-_l:=N-

I

I

! -P-O

+ZH,),-0

-I:

N

i

(T;H,16

d I

-P=N

I

Fig. 11. Schematic diagram of the likely structure of the polyphosphazene-azamacrocyclic polymer systems.

tivities were measured at a fixed salt concentration of one cation per ring using NaClO, and Ni(ClO,),. The results are displayed in Fig. 12. The conductivities are relatively poor, being approximately two orders of magnitude lower than the calcium and magnesium salts in the linear polyethers. However, this may not be a valid comparison to make. Under these conditions the monovalent ion salts showed higher conductivity levels but this may mean that the effective binding of the nickel ion by the macrocycle impedes its transport and lowers the conductivity. Activation energies were estimated from Arrhenius approximations and found to be 9 1 k 4 kJ mol - ’ for both systems. These are similar in magnitude to the other polymer-salt mixtures studied here. While the initial results indicate disappointing levels of conductivity in the axa-macrocyclic systems this is far from being a comprehensive study and work in this area is ongoing.

REFERENCES

IScm-t

1.6

2.8

3

lo?’

3.2

3.4

3.6

/ K-t

Fig. 12. Conductivity-temperature plots for the polyphosphazene-azamacrocycle $xe_tG;ith (0) NaClO, and (0) 4 2’

1. A. Patrick, M. Glass, R. Latham and R. Linford, Solid St. Ionics 18/19, 1063 (1986). 2. T. M. A. Abrantes, L. T. Alcacer and C. A. C. Sequeira, Solid Sr. Ionics 18/19, 315 (1986). 3. P. M. Blonsky, D. F. Shriver, P. Austin and H. R. Allcock, I. Am. &em. Sot. 106, 6854 (1984). 4. L. L. Yane. R. Hua and G. C. Farrinaton. _ , I. electrochem.-hoc. 133, i380 (1986). 5. L. L. Yang, R. Huq and G. C. Farrington, Solid St. Ionics 18/19, 291 (1986). 6. R. Huq, G. Chiodelli, P. Ferloni, A. Magistris and G. C. Farrington, I. electrochem. Sot. 134, 364 (1987). 7. R. Huq and G. C. Farrington, Solid St. Ionics 28-38, 990 (1988). 8. J. M. G. Cowie and K. Sadaghianizadeh, Mukromol. Chem. 9, 387 (1988). 9. J. M. G. Cowie and K. Sadaghianizadeh, Polymer Commun. 29, 126 (1988).

10. J. M. G. Cowie and A. C. S. Martin, Polymer Commun. 26, 298 (1985).

way of a contrast it was decided to examine systems in which nitrogen replaced oxygen as the co-ordinating atom in the macrocycle which should also improve the complexing with the transition element cations. Tetra-aza-crowns were used to crosslink polyphosphaxene chains and produce a network structure similar to that shown in Fig. Il. Conduc-

11. J. M. G. Cowie, A. C. S. Martin and A.-M. Firth, Br. Polym. J. M, 247 (1988). 12. J. M. G. Cowie and A. C. S. Martin, Polymer 28,627 (1987).

13. J. M. G. Cowie and C. Roberts, to be published. 14. G. Adam and J. H. Gibbs. I. them. Phvs. 43.139 (1965). 15. J. M. G. Cowie and K. Sadaghianizadeh, kolyker i, 509 (1989).