Introduction

Polymer electrolytes are "complexes" of electrodonor polymers with various inorganic or

organic salts or acids.1 The main requirements for a polymer to be used as a matrix in polymer

electrolyte systems are :

the presence of an heteroatom (usually O, N, S) with lone electron pairs of a donor power

sufficient to complex cations,

appropriate distances between coordinating centres to insure the hopping of charge carriers

and

sufficiently flexible polymer chain segments to facilitate the movement of ion carriers.

In polymer electrolytes ion transport occurs in a highly viscoelastic (solid) state. The most

intensively studied polymer electrolytes are based on poly (oxa alkanes), poly (aza alkanes) or

poly (thia alkanes). The present work contains the studies of polymer electrolytes based on low

molecular weight poly (oxa alkanes) - polyethers and particularly on alkali metal salt complexes

with poly (ethylene glycols) (PEG).

The increased interest in polymer electrolytes results from a variety of possible applications

of these materials.2 Various application possibilities have stimulated further investigation of solid

polymer ionic conductors. Among them, the following seem to be of major importance:

lithium polymer-ion batteries,

fuel cells

electrochemical sensors,

electrochromic devices (windows or displays)

The properties of novel polymeric electrolytes should fulfil the requirements necessary for

application in at least one of the above mentioned devices. The most important and universal of

these requirements are listed below:

1[9]. Armand, M.B.; Chabagno, J. M.; Duclot, M.: Fast Ion Transport in Solids (P.Vashita, J.N. Mundy and G.K.

Shenoy, Eds.) Elsevier, North - Holland, New York 1979, 131.

2 [154]. Polymer Electrolyte Reviews - 1 and Polymer Electrolyte Reviews 2, J.R. MacCallum and C.A. Vincent

Eds., Elsevier, London 1987 and 1989.

[192]. Vincent C. A., Prog. Solid. St. Chem. 17, 1987, 145.

Figure 1.1.: Phase diagram of PEO-NaI electrolyte.

After: [154]. Polymer Electrolyte Reviews 1 and 2 (J. R.

Mac Callum and C. A. Vincent Eds.), Elsevier, London 1987

and 1989.

chemical and mechanical stability over a wide temperature range,

electrochemical stability of at least 4V versus a Li electrode; especially important for battery

applications3

low activation energies for conduction

high cation transport numbers

good electrode - electrolyte characteristics

ease of sample preparation

The range of conductivities

required depends on the kind of

application and is, for example

10-3

-10-4

S/cm for batteries and

10-1

-10-2

S/cm for fuel cells;

conductivities can be lower for

sensors and electrochromic window

applications.

As can be expected it is not easy

to find an electrolyte fulfilling all the

desirable properties. Despite a very

intensive search there is still a

considerable number of unsolved

problems connected with the

fundamental understanding, synthesis

and application of polymeric

electrolytes. This results mostly from the complicated phase structure of the materials. Even one

of the simplest polymeric electrolytes, PEO-NaI, is structurally complicated.4 (See Figure 1.1.).

The structure consists of an amorphous phase, a crystalline polymer phase and at least one of a

range of crystalline complex phases formed between the polymer and the salt. The contribution

of each particular phase changes with temperature. Such a complicated phase structure causes

3 [96]. Imanishi Q. Li , Hirano A., Takeda Y., Yamamoto O.: Journal of Power Sources 110, 2002, 38.

4 see for instance [154]. Polymer Electrolyte Reviews - 1 and Polymer Electrolyte Reviews 2, J.R. MacCallum and

C.A. Vincent Eds., Elsevier, London 1987 and 1989.

difficulties in the interpretation of ion transport phenomena in polymeric electrolytes. Therefore

the mechanism of conductivity in polymeric electrolytes is difficult to establish.

Several concepts have been proposed and summarized in review papers5 but none of

them is generally valid for a wide range of materials. Berthier et al.6 have shown that fast ionic

transport takes place in the amorphous phase of the electrolyte. Here ion diffusion coefficients

are about three orders of magnitude higher than in the crystalline phase. This assumption by

Berthier concerning the crucial role played by the amorphous phase of the polymer in ion

conductivity forms the basis for some of the proposed conductivity mechanisms, such as free

volume7, configurational entropy

8 and dynamic bond percolation

9. These models are mainly

successful in quantitatively describing conductivity mechanisms for simple monophase

amorphous electrolytes and are not valid for multiphase systems.

Berthier's assumption10

requires high amorphous phase content for fast ionic transport in

polymeric electrolytes. Also the flexibility of the amorphous polymer phase is of crucial

importance for conduction since ion and polymer segmental motions are coupled for good

conductivity. Therefore a low glass transition temperature (Tg) for the amorphous polymer phase

is a desirable property. Unfortunately, the polyether-salt complexes which are the most widely

studied systems are those which have high crystallinity at ambient temperatures. PEO poly-

(ethylene oxide) is still one of the most extensively studied polyether matrices due to its

relatively low melting point and low Tg, its ability to be a matrix for a variety of lithium salts

over a range of concentrations and its capacity to act as a binder for other phases. However

PEO is semicrystalline, which still inhibits conduction of the lithium ion.11

The highest

conductivity of PEO based electrolytes is found above its melting point. In this temperature

range (above 65C) segmental motion and local relaxation of the polymer can assist

conductivity.

5 [154]. Polymer Electrolyte Reviews - 1 and Polymer Electrolyte Reviews 2, J.R. MacCallum and C.A. Vincent

Eds., Elsevier, London 1987 and 1989.

[192]. Vincent, C.A.: Progress in Solid State Chemistry 17, 1987, 145.

6 [19]. Berthier, C.; Gorecki, W.; Minier, M.; Armand, M. B.; Chabagno, J M; Rigaud, P.: Solid State Ionics 11,

1983, 91.

7 [41]. Cohen, M.H.; Turnbull D.: Journal of Chemical Physics, 31, 1959, 1164.

8 [1]. Adam, G.; Gibbs, J. H.: Journal of Chemical Physics 43, 1965, 139.

9 [160]. Ratner, M. A.: Polymer Electrolyte Reviews,Vol. 1, ed. J.R. MacCallum and C.A. Vincent, Elsevier,

London, 1987, 173, Chapter 7.

10 Ref [19].

11 [83]. Gray, F. M. Polymer Electrolytes; Royal Society of Chemistry: Cambridge, UK, 1997.

The theory that ion transport mainly occurs in amorphous liquid like regions of

polymer electrolytes12

is the logic for using low molecular weight liquid matrices as model

systems in fundamental studies of microscopic properties and transport processes.

However in order to extrapolate correlations observed in model low molecular weight

polymer based electrolytes to higher molecular weight analogue systems, it is crucial that

specific features associated with polymer terminal groups are eliminated.

The conductivities of the PEO based electrolytes are in the range 10-7

-10-8

S/cm which is

too low for most applications. The amount of flexible amorphous phase increases after

approaching the melting point of the crystalline polymer phase i.e. in the range 65-68oC.

However, at temperatures exceeding the melting point the mechanical stability of electrolytes is

much lower and membranes often creep under any pressure applied in electrochemical devices

resulting in shortcircuing effects.

A number of methods for the modification of the structure leading to the enhancement

of ionic conductivity of polymer electrolytes has been realized to this end.13

For example, the

addition of inorganic fillers is one of the most commonly used and effective methods of

modification.14

Such composite electrolytes are important for their application in

microbatteries owing to an enhancement in conductivity and an improved electrochemical

stability over the pure PEO-based systems. For these semicrystalline systems, the increase in

conductivity has been attributed to a lowering of their degree of crystallinity; this results in

the formation of a highly conducting amorphous phase. A similar effect for various fillers has

been observed in amorphous oxymethylene linked PEO systems and was attributed to

changes in polymer-ion interactions leading to the increase in the flexibility of the polymer

host.15

Scrosati and co-workers demonstrated that the addition of various fillers (such as

zeolites or aluminates) to polymeric electrolytes leads to an improvement in the electrode-

electrolyte interfacial behavior, electrochemical stability and the suppression or limitation of

12

[19]. Berthier, C.; Gorecki, W.; Minier, M.; Armand, M. B.; Chabagno, J M; Rigaud, P.: Solid State Ionics 11,

1983, 91.

13 [28]. Solid State Electrochemistry; Ed. P.G. Bruce; Cambridge University Press: Cambridge, 1995.

[85]. Gray, F. M.: Solid Polymer Electrolytes - Fundamentals and Technological Applications VCH:

Weinheim, Germany, 1991.

14 [158]. Quartarone, E.; Mustarelli, P.; Magistris, A.: Solid State Ionics 110, 1998, 1.

[203]. Wieczorek, W.; Florjanczyk, Z.; Stevens, J. R.: Electrochimica Acta 40, 1995, 2251.

15 [208]. Wieczorek, W.; Such, K.; Florjaczyk, Z.; Stevens, J.R.: Journal Physical Chemistry 98, 1994, 6840.

[207]. Wieczorek, W.; Such, K.; Chung, S. H.; Stevens, J. R.: Journal Physical Chemistry 98, 1994, 9047.

the formation of passive layers at the alkali metal electrode-polymer electrolyte interface.16

It

has also been shown that the use of fillers improves mechanical stability and extends the

thermal stability range of polymer electrolytes.17

The same authors18

as well as Kumar and

co-workers19

showed that the addition of a filler leads to an enhancement in the cation

transport number for polymer electrolytes thus improving their performance in alkali metal

batteries.

The phase structure, being a system of ordered and amorphous phases makes interpreting

the direct effects of filler on conductivity complicated due to:

the degree of crystallinity

the sophisticated mechanism by which the filler might influence ion concentration or

mobility in the fully amorphous phase.

It has been recently described that similar benefits can be reached by the addition of

inorganic filler to amorphous high or low molecular weight polyether based electrolytes.20

The fully amorphous model system was chosen in order to separate out different mechanisms

for conductivity enhancement; the primary one being changes related to crystallinity.

So far there is a lack of in-depth studies of ion transport in low molecular weight polyglycols

containing dispersed filler particles. The attempts made by Scrosati's and Fedkiw's groups

were limited to narrow filler and salt concentration ranges and did not show major differences

between pure polyether electrolytes and those with dispersed filler particles.21

Despite

intensive research, mechanism of ion transport in polymeric electrolytes as well as electrode-

electrolyte interfacial behavior are still under discussion. In most cases the results suggest a

nonspecific mechanism for the conductivity

16

[170]. Scrosati, B.; Neat, R.: Applications of Electroactive Polymers; Scrosati, B., Ed.; Chapman and Hall:

London, 1993, Chapter 6.

[208]. Wieczorek, W.; Such, K.; Florjaczyk, Z.; Stevens, J.R.: Journal Physical Chemistry 98, 1994, 6840.

[207]. Wieczorek, W.; Such, K.; Chung, S. H.; Stevens, J. R.: Journal Physical Chemistry 98, 1994, 9047.

[33]. Capuano, F.; Croce, F.; Scrosati, B.: Journal of Electrochemical Society 138, 1991, 1918.

17 [198]. Weston, J. E.; Steele, B. C.: Solid State Ionics 7, 1982, 75.

18 [43]. Croce, F.; Appetecchi, G. B.; Persi, L.; Scrosati, B.: Nature 394, 1998, 456.

19 [113]. Kumar, B.; Scanlon, L. G.: Journal of Power Sources 52, 1994, 261.

20 Ref [208].

[202]. Wieczorek, W.; Lipka, P.; ukowska, G.; Wycilik, H.: Journal Physical Chemistry B 102, 1998, 6968.

21 [147]. Panero, S.; Scrosati, B.; Greenbaum, S. G.: Electrochimica Acta 37, 1992, 1533.

[60]. Fan J.; Fedkiw P.S.: Journal of Electrochemical Society 144 (2), 1997, 399.