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Solid State Ionics 176

Ionic association in liquid (polyether–Al2O3–LiClO4)

composite electrolytes

Marek Marcineka,*, Marcin Cioseka, Grayyna Zukowskab, Wyadysyaw Wieczoreka,

Kenneth R. Jeffreyb, Jim R. Stevensb

aWarsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warszawa, PolandbThe Guelph-Waterloo Physics Institute, University of Guelph, Physics Department, Guelph, Ontario, Canada N1G 2W1

Received 14 May 2004; accepted 27 August 2004

Abstract

The influence of surface-modified Al2O3 particles on the formation of different ionic aggregates in poly (ethylene glycol) methyl ether

(PEGME)–LiClO4 and poly (ethylene oxide) dimethyl ether (PEODME)–LiClO4 electrolytes is discussed. Three independent methods have

been used to estimate the fractions of free ions and ionic associates. The first two methods are based on the deconvolution of the FTIR 624

cm�1 and Raman 930 cm�1 perchlorate anion modes. The third method uses a Fuoss–Kraus semiempirical method involving the salt

concentration dependence of ionic conductivity. Results are compared for two polyether systems to explain interactions in polymer

electrolytes based on low molecular weight polyglycols. The temperature dependence of the fractions of ionic species is also analysed.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Composites; Polymer electrolytes; Ionic associations; Ion pairs; Raman spectroscopy

1. Introduction

Charge carrier concentration and ionic mobility are two

important parameters which influence the conductivity of

the electrolyte. Due to the relatively low dielectric constant

of most polymer matrices, typically long-range Coulomb

forces give rise to extensive ion–ion interactions, and in

general, several different types of ion species can be present

in the polymer salt complexes:

– bfreeQ anions,– solvated cations,

– solvent-separated ion pairs, contact ion pairs,

– triplet ion clusters and higher order aggregates.

The fraction of ion species belonging to each member of

the group mentioned above has been found through various

spectroscopic techniques [1]. Nevertheless, researchers

0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.ssi.2004.08.013

* Corresponding author. Tel.: +48 6605637.

E-mail address: marekmar@soliton.ch.pw.edu.pl (M. Marcinek).

agree that the picture created from this kind of experimental

evidence is oversimplified [2].

Ion associations have been so far thoroughly studied for

nonaqueous liquid electrolytes and low or medium molec-

ular weight amorphous polymeric electrolytes based on

polyether matrices [3–12].

Examination of the ion–ion, ion–polymer interactions

are of great interest. Infrared (IR) and Raman spectros-

copies have been widely used to investigate associations

of oxyanions with cations. Numerous reports have also

shown that spectroscopy is a useful tool to study ion–

solvent interactions [13,14]. The formation of ion

aggregates is seen in the Raman spectra through

frequency shifts and spectral components related to the

conjugated anion dissociated from the lithium salt. In

previous studies, the symmetric Cl–O stretch of the bfreeQperchlorate anion was observed at 933 cm�1, and spectral

components at 934 and 938 cm�1 were assigned

respectively to the solvent-shared ion pairs and contact

ion pairs. It had been also reported by Ducasse et al. [15]

that the Raman spectra of the perchlorate anion are

(2005) 367–376

M. Marcinek et al. / Solid State Ionics 176 (2005) 367–376368

interesting probes of the local lithium coordination and

the degree of crystallinity of the sample.

Apart from these studies, the structures of ion pairs and

complexes are unclear. Ab initio calculations have been

compared with experimental results obtained from Raman

spectroscopy for LiClO4+DC+EC, establishing the bidenate

structure of ion pairs [13].

Johansson and Jacobsson noticed that the anion–cation

polymer models are in better agreement with observed

spectral features than the contact ion pair model for ab initio

Hartree–Fock studies for Li with anions ClO4�, PF6

�, AsF6�,

CF3SO3�, [(CF3SO2)2N]

�[16]. Apparently, contact ion pairs

as ab initio models for calculating the vibrational shifts upon

cation coordination in solid polymer electrolytes (SPEs)

may be improved by adding a molecule that mimics the

local environment and provides a more realistic coordina-

tion number for the Li cation.

1.1. Ionic associates–salt concentration dependence

When the concentration of dissolved salt in the polymer

matrix increases, the average separation between dissolved

ions decreases, and increasing ion–ion interaction is

expected. Fuoss and Kraus [3] originally postulated the

formation of triplets with increasing salt concentration in

dioxane water solutions of tetraisoamylammonium nitrate.

Upon the addition of salt to solutions of low permittivity, the

relative dielectric constant increases. This is contrary to the

decrease observed in aqueous solutions. If the increase in

the permittivity value is fast enough, it is possible that the

fraction of dissociated ions increases rather than decreases

with the increasing salt concentration. This is due to a

reduced Coulomb interaction, and the effect is referred to as

bredissociationQ. It has been noticed [17] that ion species are

not well defined in terms of the spatial separation of

potential energies. For example, a concept that aggregates

such as triplets should be defined in terms of short- and

long-range molecular interactions rather than as bmolecularQspecies has been proposed [4].

1.2. Ionic associates–temperature dependence

In several electrolyte solutions in the higher temperature

ranges, precipitation of the salt has been observed [18].

Furthermore, the dynamic equilibria governing the ionic

speciation generally shift towards an increasing abundance

of associated species at higher temperatures [19]. The work

of Jacobsson and Lundin [20] indicates that the observation

of increasing ion association with increasing temperature is

attributed to volume effects.

1.3. Ionic associates–effect of the fillers

Studies devoted to composite polymeric electrolytes

based on semicrystalline polyether matrices have been

limited to a narrow salt concentration range (usually for

polyether oxygen-to-metal cation ratios equal to 8 or 10;

[21–23]). Ionic associations and filler concentrations have a

considerable effect on the ionic conductivity of electrolytes

containing low permittivity solvents. Therefore, it is

important to know whether the effect of the filler on

conductivity enhancement is limited only to this narrow salt

concentration range or can be extended over larger salt

concentration ranges. Thus, the main goal of the research

reported here is to study the salt concentration dependence

of the molal conductivity of various composite polymeric

electrolytes which contain fillers. We compare these results

with the salt concentration dependence of molal conductiv-

ity obtained for the base PEO–LiClO4 electrolyte without a

filler. An increase in the conductivity observed for

composite systems can be discussed in terms of the

formation and redissociation of contact ion pairs and higher

ionic aggregates.

It has been shown by Wieczorek et al. [24] that the

conductivity of the PEO–LiClO4 electrolytes changes upon

the addition of various organic or inorganic fillers. The

effect of a filler is to change the fraction of available oxygen

sites, which in turn results in changes to the formation of

ionic aggregates. The region in which the enhancement of

ionic conductivity is observed corresponds to a lowering of

the fraction of contact ion pairs and higher aggregates; this

is due to the placement of filler molecules in the vicinity of

the coordination sphere of the Li+ cations.

Results based on Raman light-scattering studies showed

that the addition of nanoparticles of TiO2 and Al2O3 to a

trifunctional polyether had no influence on ionic association

as a function of temperature [25]. These authors were unable

to discriminate between interactions among anions, cations,

filler and polymer and mentioned that only small volumes of

the material are likely to be affected at the polymer

electrolyte/ceramic interface. These results [25] indicate

that it may not be possible to detect small or localized

changes via Raman light scattering where the information

arising from different environments is observed as super-

positions with relative intensities being proportional to the

actual scattering volume of the environment [26]. However,

it should be noted that we use micron-sized filler particles,

and this may make a difference.

The aim of this paper is to compare three independent

methods of estimation of ionic fractions in electrolytes

based on low molecular weight polyglycols doped with

LiClO4 with or without Al2O3 fillers.

2. Experimental

2.1. Sample preparation

Poly (ethylene glycol) methyl ether CH3(OCH2CH2)nOH

(PEGME, Mw=350, Aldrich) and poly (ethylene oxide)

dimethyl ether CH3(OCH2CH2)nOCH3 (PEODME, Mw=

500, Aldrich) were filtered, then stringently freeze-dried

M. Marcinek et al. / Solid State Ionics 176 (2005) 367–376 369

using several freeze–pump–thaw cycles on a vacuum line.

Afterward, they were dried under high vacuum (10�5 Torr)

at ~60 8C for 72 h.

Lithium perchlorate LiClO4 (reagent grade, Aldrich)

was dried under vacuum at 80 8C for 48 h prior to the

dissolution. After the drying procedure, still under vacuum,

the polymer was transferred to an argon-filled dry box

(moisture content lower than 2 ppm) where the LiClO4

was dissolved into the polymer, using a magnetic stirrer.

Aluminum oxides Al2O3 (Aldrich, reagent grade; grain

size b5 Am), with the various types of surface groups,

were dried under vacuum (about 10�5 Torr) at 150 8C for

over 72 h prior to being added to the polymer–salt

mixture. Technical information from the producer (Sigma

Aldrich) about the different surface modifications of the

fillers indicates that these modifications are not caused by

surface crystalline structures but by the processes used to

make them.

The salt concentration varied from 10�6 to 5 mol of

LiClO4 per kilogram of polymer. Samples with salt

concentrations from 5 mol/kg down to 0.5 mol/kg were

prepared through direct dissolution of the salt in the

polyether. Samples with the highest salt concentration were

heated up to 50 8C to facilitate the dissolution process.

Samples at low salt concentrations were prepared by the

successive dilution of an electrolyte with a concentration of

0.5 mol/kg LiClO4. Composite electrolytes were obtained

by the dispersion of Al2O3 in each polyether–LiClO4

solution, and the concentration of Al2O3 in the composite

electrolytes was equal to 10 mass %.

2.2. Experimental techniques

2.2.1. Impedance spectroscopy in blocking electrodes

Ionic conductivity was determined using the complex

impedance method in the temperature range from 20 to 90

8C. The samples were sandwiched between stainless steel

blocking electrodes and placed in a temperature-controlled

oven. The experiments were performed in a constant volume

cylindrical cell with an electrode diameter equal to 7.8 mm

and electrodes separation of 1.6 mm. The impedance

measurements were carried out on a computer-interfaced

Solartron-Schlumberger 1255 impedance analyzer over the

frequency range of 1 Hz to 1 MHz.

2.2.2. Viscosity measurements

Rheological experiments were conducted at 25 8C using

a Bohlin Visco 88BV viscometer in the two coaxial

cylinders geometry. The measurements were performed

within a shear rate range of 24–1200 cm�1. Temperature-

dependent rheological measurements were performed on an

AR 2000 Advanced Rheometer system equipped with a

Peltier plate temperature controller. Experiments were

performed from 5 to 90 8C in the cone and plate geometry

at a constant shear stress equal to either 10 Pa (pure

polyglycols) or 150 Pa (polymer electrolytes).

2.2.3. Formalism of Fuoss–Kraus

Combining the data obtained from conductivity and

viscosity measurements, we are able to calculate the

fractions of charge carriers using a semiempirical formalism

proposed by Fuoss and Kraus. Details of the calculation

have been described previously [8,10,11,27,28].

2.2.4. Raman spectroscopy

Raman spectra were recorded using a Renishaw System

2000 Raman spectrometer equipped with a confocal

Raman microscope, an 1200 lines/mm holographic grating

and a CCD camera. The diode laser operating at 785 nm

was used as the excitation source, and the spectral

resolution was about 2 cm�1. To avoid contact with air

during the measurements, samples were kept in sealed

glass bottles. Because of the fact that the experiment was

performed in a closed argon-filled flask, there were no

changes in the geometry of the sample during the

experiment.

2.2.5. FTIR

Infrared (IR) absorption spectra were recorded on a

computer-interfaced Perkin-Elmer 2000 FTIR system with a

wavenumber resolution of 2 cm�1. FTIR studies were

performed in the �50 to 80 8C temperature range using a

vacuum isolated temperature-controlled cell. Samples were

sandwiched between two KBr plates. The system was

pumped for 0.5 h prior to use, and the accuracy of the

temperature measurements was estimated to be F1 8C.FTIR and Raman spectra were analysed using a Galactic

Grams Research software package using a Gaussian–

Lorentzian function. To avoid the influence of the variance

of the geometry of the samples, spectra were normalized to

the peak of CH2 scissoring vibration (1470 cm�1) which

was invariant with the change in the salt concentration. The

main limitation for this procedure is the relatively low

intensity of this band for samples containing less than 1 mol

of LiClO4 per kilogram of polyglycol.

3. Results

3.1. Studies of ionic associations using Fuoss–Kraus

formalism

3.1.1. Distribution of ion species

Fig. 1a–f presents the results of calculations of the

fraction of QfreeQ anions, ion pairs and triplets using the

Fuoss–Kraus formalism. In the Fuoss–Kraus procedure, we

can distinguish among QfreeQ anions, ion pairs and higher

aggregates, including charged triplets. These calculations

were done for data in the low concentration range and

extrapolated for higher concentrations for PEODME–

LiClO4 and PEODME–LiClO4–Al2O3acidic electrolytes.

The percentages in Fig. 1a–c add to 100 and the same for

Fig. 1d–f.

Fig. 1. Fraction of ion pairs calculated on the basis of the Fuoss–Kraus formalism for PEODME–LiClO4- and PEODME–acidic Al2O3–LiClO4-based systems

in 3D plots as a function of concentration and temperature.

M. Marcinek et al. / Solid State Ionics 176 (2005) 367–376370

For the PEODME–LiClO4 electrolyte, the fraction of

bfreeQ anions decreases with increasing concentration and

increases with increase in temperature. The fraction of ion

pairs reaches a maximum in the range ~10�3 to 10�2 mol/

kg. On the low concentration side of this maximum, the

fraction of ion pairs decreases with increase in temperature;

Fig. 2. Deconvolution of fully symmetric stretching mode (m1) of

perchlorate anion. Spectrum recorded at 25 8C for PEODME–LiClO4 (5

mol/kg)–acidic Al2O3 electrolyte.

M. Marcinek et al. / Solid State Ionics 176 (2005) 367–376 371

the reverse is true on the high-concentration side. The

fraction of triplets increases with increasing concentration

but decreases with increasing temperature. The concentra-

tion dependence indicates that there is a trend, first to ion

pairs and then to higher aggregates. The temperature

dependence indicates a redissociation at lower concentra-

tions with increasing temperature; at concentrations above

the maximum in Fig. 1b, the main redissociation with the

increase in temperature affects the triplets where the fraction

of triplets decreases, and the fraction of ion pairs increases.

The concentration dependence for the PEODME–

LiClO4–Al2O3acidic electrolyte is the same as for the

PEODME–LiClO4 electrolyte. However, at the lower

concentrations, as the temperature increases, the fraction

of bfreeQ anions reaches a maximum at around 25 8C, andthe fraction of ion pairs and triplets is a minimum. For the

higher concentrations, the fraction of ion pairs increases

with increase in temperature, and the fraction of triplets

decreases. Again, redissociation of the triplets with increas-

ing temperature is the defining behaviour.

3.1.2. Ionic conductivity

Comparing the two PEODME electrolytes, the fraction

of bfreeQ anions and triplets available for ionic conduction is

about the same at 25 8C. However, at 35 8C, more bfreeQanions are available in the unfilled electrolyte, but there are

fewer triplets. The filler seems to result in more triplets in

the Fuoss–Kraus model. This result implies that one should

not expect a significant change in the ionic conductivity

with the addition of filler or with increasing temperature.

As was previously seen, the fraction of neutral ion pairs

is much lower for composite electrolytes based on the

PEGME polymer matrix [29]. For the PEGME-based

samples with the highest salt concentration, the fraction of

charge carriers is approximately twice as high for systems

containing Al2O3.

The variation of ionic conductivity with salt concentration

for PEODME- and PEGME-based systems containing

LiClO4 and Al2O3 filler was very similar. The highest room

temperature conductivities (r=4�10�4 S/cm) were found for

concentrations around 1 mol/kg. At the higher concentra-

tions, PEODME–LiClO4–Al2O3acidic conductivities were

slightly higher [30]. The only difference was that the

conductivities for PEGME–LiClO4 were much lower than

for the PEODME–LiClO4 electrolyte due to the polar OH

terminal groups on the PEGME. Thus, there was an increase

in conductivity with the addition of filler for the PEGME-

based system. No significant change was observed for the

PEODME-based system on the addition of the acidic filler

[30], agreeing with the Fuoss–Kraus prediction noted above.

3.2. Analysis of ionic association estimated on the basis of

Raman and FTIR spectra analysis

Ionic associations in LiClO4 solutions can be studied by

the analysis of those spectral features which are character-

istic of the anions and those of the polymer matrix. As

known from spectrochemical data [31], bfreeQ ClO4� anions

can be excited in four modes of which three are both Raman

and infrared active and the fourth, a nondegenerate

symmetric stretching vibration, which is only Raman active.

The strongest of the anion characteristic infrared vibrations,

corresponding to the asymmetrical stretch with maximum at

~1102 cm�1, is highly overlapped by the bands of the

solvent C–O–C stretching modes. Therefore, when analyz-

ing Raman and FTIR spectra, the bands attributed to the m1symmetrical stretching and m4 asymmetrical bending modes

were used; these peak, respectively, at ~932 and ~624 cm�1.

3.2.1. Ionic association studied by Raman spectroscopy

The maximum in the intensity of the m1 band at 932 cm�1

(Fig. 2) is slightly shifted to higher wavenumbers for

samples with increasing salt concentration. In the spectra of

the samples with high (2–4 mol/kg) salt content, this band is

accompanied by a shoulder at higher wavenumbers; the m1peak at 933 cm�1 is ascribed to bfreeQ anions and solvent-

shared ion pairs. This feature has a shoulder on the high-

frequency side. According to the studies of Chabanel et al.

[14] and Schantz et al. [19], this shoulder, with maximum at

~938 cm�1, should be attributed to contact ion pairs. Other

authors [13] and references cited therein suggest that the

~938 cm�1 mode should be ascribed to the solvent-shared

ion pairs, and that the mode at 944 cm�1 should be assigned

to contact ion pairs with higher aggregates occurring at 955

cm�1. However, these authors [13] used simple carbonate

solvents or water to dissolve the LiClO4 salt. Such solvents

have considerably higher dielectric constants than the

M. Marcinek et al. / Solid State Ionics 176 (2005) 367–376372

polyethers usually used in solid polymer electrolytes or the

aprotic solvents used by Chabanel et al. [14], so one might

expect the results obtained. Nevertheless, triplets and higher

aggregates should be present, especially at higher concen-

trations, so intensities at higher wavenumbers would

contribute to the tail of the 938 cm�1 feature but not be

resolvable in any deconvolution.

Carefully comparing sets of data for composites based on

PEGME and PEODME, the fractions of higher aggregates,

ion pairs and bfreeQ anions were determined. The deconvo-

lution of the m1 band was done using the Gaussian–

Lorentzian mixed function. In the salt concentration range

studied (up to 4 mol/kg), detection of the peak attributed to

the higher agglomerates (at 948 cm�1) [32] was impossible.

Therefore, it was found necessary to fix the location of the

938 cm�1 feature in the deconvolution and attribute it to

bion pairsQ with the caveat that the higher aggregates could

also be present. An example of the deconvolution of the

Raman spectra for the PEODME–LiClO4–Al2O3acidic elec-

trolyte is shown in Fig. 2.

Also seen in Fig. 2 is a peak at ~910 cm�1, which is

ascribed to first overtone of the bending vibration of ClO4�

anion with a maximum at 459 cm�1.

For all the systems studied, the intensity of the shoulder

and, hence, the concentration of ion pairs increase with

increasing salt concentration (see Table 1). Our general

observation is that the percentage of contact ion pairs is

higher for all salt concentrations for electrolytes prepared on

the basis of PEODME matrix except for the system with

neutral Al2O3 which seems to have an anomalously low

percentage of ion pairs. The higher concentration of ion

Table 1

Percentage of bfreeQ ions and ion pairs calculated on the basis of the ClO4

m1 characteristic Raman mode deconvolution

Electrolyte Salt

concentration

(mol/kg)

Ion pairs

(%)

Free ions

(%)

PEGME–LiClO4 4 49 51

3 26 75

PEGME–LiClO4–acidic

Al2O3

4 38 62

3 20 80

PEGME–LiClO4–basic Al2O3 3 25 75

PEGME–LiClO4–neutral

Al2O3

4 38 62

3 23 77

PEODME–LiClO4 4 56 44

3 39 61

2 29 71

PEODME–LiClO4–acidic

Al2O3

4 47 53

3 28 72

2 13 87

PEODME–LiClO4–basic

Al2O3

4* 49 51

3 41 59

2 5 95

PEODME–LiClO4–neutral

Al2O3

4 22 78

3 14 86

* PEODME–LiClO4-basic Al2O3 with the deconvolution in three

fractions—7% of triplets, 22% ion pairs, 71% of bfreeQ ions.

pairs in the PEODME-based system may be explained by

the absence of a hydroxyl group; more ions are available for

ion pair and higher aggregate formation. Therefore, there is

a weaker polymer–salt interaction in this system, as

compared to that in PEGME. Another reason for the

increase of the ion pair concentration in the PEODME-

based system is the higher flexibility of PEGME–LiClO4

systems [30]. After the addition of fillers with various types

of surface groups, the fraction of ion pairs decreases by up

to 10% over the entire salt concentration range except for

the anomaly mentioned above. As would be expected, these

changes are most pronounced at the highest salt concen-

trations. In PEGME-based systems, the type of surface

group is less important in its effect on the fraction of ion

pairs. The percentages of bfreeQ anions and bcontact ion

pairsQ obtained from the deconvolution are shown in Table

1. The percentages in Table 1 add up to 100%, but there

should be triplets and higher aggregates present at these

high concentrations. Therefore, it is likely that the percen-

tages listed for bion pairsQ include triplets and higher

aggregates as mentioned above.

3.2.2. FTIR spectroscopy

The fractions of ion associates were estimated by the

deconvolution of the peak attributed to the m4 ClO4 vibration

mode. In spectra of samples with higher salt concentration

(above 1 mol/ kg), the peak of the bfreeQ anion, with

maximum at 624 cm�1, is associated with a shoulder at 635

cm�1, ascribed to contact ion pairs and higher aggregates

[14]. The shape and intensity of this shoulder depends on

salt concentration and temperature. For samples with the

highest (3–5 mol/ kg) salt concentration, the deconvoluted

peaks are usually well separated, whereas for other samples,

only an asymmetry on the higher wavenumber side can be

observed. It should be stressed that the band ascribed to the

bspectroscopically freeQ anions originates both from free

ions and solvent-separated ion pairs [33].

In spectra of PEODME-based electrolytes with high (3–5

mol/kg) salt concentration, especially those recorded at low

temperatures (see below), additional splitting was observed,

with peaks at ~630 and 620 cm�1. The latter band is

probably due to the isotope effect, i.e., different wave-

numbers for m4 of the35ClO4

� and 37ClO4� isotopomers [34].

The low Dm between the peak attributed to bfreeQ anions andthe described peak allows us to suppose that this band

corresponds to bfreeQ 37ClO4�. As the temperature increases,

the bandwidth becomes broader, and only an asymmetry of

the band on the low-wavenumber side can be observed. The

peak observable at ~630 cm�1 indicates a loss of

degeneracy of the F2 mode of the bfreeQ perchlorate anion

rather than a splitting caused by an isotope effect. The latter

only gives rise to a splitting of 3 cm�1. Such a conclusion is

supported by the fact that the area ratio of the component

peaking at 630 cm�1 is larger than that at 638 cm�1. See

Fig. 3 for the deconvolution of the m4 band for the

electrolyte PEODME–LiClO4–Al2O3 basic. The isotopo-

Fig. 3. Peak fitting for the m4 ClO4 FTIR region for the PEODME–LiClO4

(4 mol/kg PEODME)–basic Al2O3. Sample at 25 8C.

Table 2

Ion fractions

Polymer

matrix

Temperature

(8C)Type of

filler

Ion pairs

(%)

Free ions

(%)

(a) Data for PEGME-based electrolytes

PEGME �20 None 33 67

0 None 29 71

20 None 51 49

50 None 28 72

70 None 18 82

PEGME �20 Basic 54 46

0 Basic 53 47

20 Basic 53 47

50 Basic 24 76

70 Basic 30 70

PEGME �20 Acidic

0 Acidic 22 78

20 Acidic 31 69

50 Acidic 32 68

70 Acidic 34 66

PEGME �50 Neutral 26 74

0 Neutral 30 70

20 Neutral 38 62

50 Neutral 50 50

70 Neutral 55 45

(b) Data for PEODME

PEODME �20 None 35 65

0 None 35 65

20 None 30 70

50 None 18 82

70 None 15 85

PEODME �20 Basic 39 61

0 Basic 42 58

20 Basic 40 60

50 Basic 41 59

70 Basic 44 56

PEODME �20 Acidic 57 43

0 Acidic 61 39

20 Acidic 64 36

50 Acidic 68 32

70 Acidic 69 31

PEODME �20 Neutral 17 83

0 Neutral 16 84

20 Neutral 17 83

50 Neutral 11 89

70 Neutral 11 89

Results of the deconvolution of the band of m4 ClO4 vibration mode (FTIR)

at various temperatures. Salt concentration is equal to 5 mol/kg.

M. Marcinek et al. / Solid State Ionics 176 (2005) 367–376 373

meric 37ClO4 anion should give the band a much weaker

satellite than that due to the 35ClO4 anion.

3.2.3. Influence of the temperature in FTIR studies

The influence of temperature is the most pronounced in

samples with the highest salt concentration. Table 2a–b

presents percentage of ion pairs and bfreeQ anions at varioustemperatures for electrolytes with a salt content equal to 5

mol/ kg. Again, inasmuch as these percentages add up to

100%, one should consider that the values given for d% ion

pairsT include triplets and higher aggregates.

The temperature dependency is different for systems

based on PEGME (Table 2a) and PEODME (Table 2b). The

difficulties in deconvoluting the m4 mode in the FTIR

spectra are evident in Table 2, especially for electrolytes

based on monomethyl-capped PEGME, Table 2a. This m4band is much more complex than the m1 band used in the

deconvolution of the Raman spectra, being a superposition

of the spectral features of the bfreeQ 35ClO4 and 37ClO4

isotopomers, ion pairs, dimers as well as species forming

complexes with filler particles. Roughly speaking, for the

PEGME-based electrolytes, one can say that for no filler or

for fillers with basic groups, an increase in temperature is

followed by an increase in the percentage of bfreeQ anions.However, for the PEGME-based electrolytes containing

fillers with acidic or neutral surfaces, the reverse is true.

For those containing double-methyl-capped PEODME,

the trend also depends on the presence and type of inorganic

filler (see Table 2b). A comparison of spectra for PEODME–

Al2O3–basic LiClO4 recorded at different temperatures is

shown in Fig. 4. For samples without filler, the percentage of

bfreeQ anions is increasing with temperature. When fillers

with neutral surface groups were used, a similar, less

pronounced trend was observed, but the percentage of ion

pairs was distinctly lower than for the other PEODME-based

electrolytes. For samples doped with Al2O3 with acidic

groups, the percentage of bfreeQ anions decreases with the

increase in temperature, and, for those with Al2O3 with basic

groups, the percentage of ion species is approximately

independent of temperature. The use of Al2O3 with acidic

groups significantly increases the fraction of ion pairs.

3.2.4. Influence of the salt concentration

The fractions of bfreeQ anions and ion aggregates, as a

function of concentration and determined by the deconvo-

lution of the m4 band, are presented in Table 3.

Fig. 4. Shape of the m4 anion characteristic mode for the PEODME–

LiClO4–basic Al2O3 (4 mol/kg) at different temperatures. The broadening

of the band and the decrease in the intensity at higher temperatures is

clearly observed.

Table 3

Percentages of free anions and ion pairs found on the basis of

deconvolution of the m4 ClO4 band (FTIR)

Polymer

matrix

Salt concentration

mole per kilogram

of polymer

Type of

filler

Ion pairs

(%)

Free ions

(%)

PEGME 5 None 33 67

4 None 21 79

PEGME 5 Basic 31 69

4 Basic 10 90

PEGME 5 Acidic 33 67

4 Acidic 15 85

PEGME 5 Neutral 35 65

4 Neutral 38 62

PEODME 5 None 35 65

4 None 35 65

3 None 30 70

PEODME 5 Basic 39 61

4 Basic 42 58

3 Basic 40 60

M. Marcinek et al. / Solid State Ionics 176 (2005) 367–376374

The effect of the filler is influenced by salt concentration.

The most pronounced differences observed are between

electrolytes with and without filler in the 3–5 mol/kg salt

concentration range. The fractions of contact bion pairsQ andbfreeQ anions calculated for the highest (5 mol/kg) salt

content are similar for the PEODME-and PEGME-based

systems without filler. For the PEGME-based system, the

addition of Al2O3 filler does not affect the percentages of

bfreeQ anions and contact ion pairs for salt concentrations of

5 mol/kg. The trend is similar for the PEODME system with

no filler and with a filler with basic groups. However, for

this system with neutral surface groups, the fraction of bfreeQanions increases after the addition of the filler, and, for the

system with acidic surface groups, the fraction of bfreeQanions decreases to values less than any of the other

systems.

Comparing Tables 1 and 3 at a salt concentration of 4

mol/kg, the percentage of ion pairs is generally lower in

the Raman results for all the systems studied. The one

exception is the PEODME system with Al2O3 filler with

acidic surface groups. One of the reasons of such

behaviour might be the higher sensitivity of the m4vibration mode (FTIR) compared to the m1 (Raman)

towards changes in the anion environment. The band

which is observed in Raman is nondegenerate, whereas the

band in FTIR is triply degenerate. In the FTIR spectra, an

isotopic effect becomes important, making the deconvolu-

tion of the FTIR band less certain.

PEODME 5 Acidic 57 43

4 Acidic 61 39

3 Acidic 64 36

2 Acidic 68 32

PEODME 5 Neutral 17 83

4 Neutral 16 84

3 Neutral 17 83

2 Neutral 11 89

Deconvolution of 624 cm�1 peak in a room temperature.

4. Discussion

4.1. Concentration dependence

Results originating from three methods, FTIR and

Raman deconvolution and Fuoss–Kraus are compared.

The Fuoss–Kraus estimation for the PEODME–LiClO4

electrolytes studied shows that the fraction of bfreeQ anionsdecreases with an increase in concentration. The same

results are obtained from both spectroscopic techniques. For

the PEODME–LiClO4–Al2O3acidic electrolyte, a similar

trend is observed from Fuoss–Kraus and Raman but an

opposite trend (percentage of bfreeQ anions increasing with

an increase in concentration) is observed from the FTIR

deconvolution of the m4 band. In the composite systems,

there is a problem with the low intensity in the FTIR anion

m4 asymmetric mode for samples containing more than 1

mol/kg of salt, and moisture was a problem in the

measurements. The Raman m1 symmetric stretch mode is

relatively strong.

The fraction of ion pairs calculated from the Fuoss–

Kraus estimation reaches a maximum in the range ~10�3 to

10�2 mol/kg so that dpercentage of ion pairsT decreases withan increase in concentration for the concentrations used in

the spectroscopic techniques; the fraction of triplets

increases with increasing concentration. FTIR and Raman

see both triplets and ion pairs as dion pairsT. If one includesthe possibility of a high percentage of triplets (as predicted

by Fuoss–Kraus) and higher aggregates at the high

concentrations used in the Raman and FTIR deconvolutions,

it is felt that, generally, the trend for dion pairsT and higher

aggregates should be to increase with the increase in

concentration. We suggest this in spite of the relatively

small increases of the fraction of triplets with concentration

seen in Fig. 1c and f. An increase in the fraction of dion

M. Marcinek et al. / Solid State Ionics 176 (2005) 367–376 375

pairsT with increase in concentration is what is observed in

the PEODME–LiClO4 electrolytes in both Raman (Table 1)

and FTIR (Table 3). For the PEODME electrolyte with

acidic filler, results from the Raman deconvolution show a

similar trend.

Fuoss–Kraus estimations for the PEGME electrolytes

were given previously by Marcinek et al. [29] where the

Al2O3 filler used was rich with Lewis acid-type groups.

Generally, the trends are the same as found in Fig. 1a–f.

Using the results from the spectroscopic techniques (Tables

1 and 3), the concentration dependence for the ion fractions

in the PEGME–LiClO4 and PEGME–LiClO4–Al2O3acidic

electrolytes have similar trends; the fraction of bfreeQ anionsdecreases with the increase in concentration, and the

fraction of dion pairsT increases with the increase in

concentration. An explanation for these results is helped if

one recognizes the inclusion of triplets and higher aggre-

gates in the fraction of bion pairsQ, which fraction is higher

for the PEODME electrolytes due to the absence of OH

terminal groups.

4.2. Temperature dependence

The Fuoss–Kraus estimation predicts an increase in the

fraction of bfreeQ anions with the increase in temperature for

the PEODME–LiClO4 electrolytes in agreement with the

FTIR results (Table 2b). FTIR results for the PEGME–

LiClO4 electrolyte follow a similar trend.

We observe mixed trends in the variation of the fraction

of bion pairsQ with temperature from the FTIR results. At

the highest salt concentrations, both the PEGME and the

PEODME electrolytes with fillers with acidic surface

groups and the PEGME system with neutral fillers follow

this trend and show an increase in the fraction of bionpairsQ with an increase in temperature (see Table 2). On the

other hand, within broad error limits, others show very

little effect, or the percentage of bion pairsQ even decreases

with an increase in temperature. The fractions of bionpairsQ obtained from the FTIR and Fuoss–Kraus formalism

are not the same; FTIR sees both triplets and ion pairs as

bion pairsQ. Using FTIR and Raman techniques, we studied

anion characteristic modes. One of the reasons for the

apparent disagreement in the Raman and FTIR results is

the nature of the techniques used. In the composite

systems, there is a problem with the low intensity in the

FTIR anion asymmetric mode for samples containing more

than 1 mol/kg of salt. For the Raman symmetric mode,

which is relatively strong, it is possible to estimate the

ionic fractions for samples in the lower salt in the lower

salt concentration ranges. This is the advantage of the

Raman technique. In both techniques, it is difficult to

distinguish between bfreeQ anions and the solvent-separated

ion pairs due to the low dielectric constant of these

electrolytes. In particular, it is evident when the additional

interactions originated from the presence of the filler

particles in the composite electrolytes.

The most important factor discussed is the effect of the

filler surface groups on the ion fractions present in the

electrolytes. No significant difference in the spectral features

related to the ClO4� modes are visible upon the addition of

acidic or basic aluminas. For the filled PEGME systems, in

the room-temperature FTIR spectra, the fraction of bionpairsQ is approximately unchanged (see Table 2). For the

PEODME systems, at room temperature, there is an increase

in the fraction of bion pairsQ with the addition of basic and

acidic filler particles; for fillers with neutral surfaces, there

is a significant decrease. More promising results were

obtained from the Raman spectra analysis at room temper-

ature. A lower content of ion pairs is noticed, as compared

with the unfilled systems independent of the polymer matrix

used. Again, the addition of neutral filler particles to the

PEODME system results in a significant decrease in the

fraction of ion pairs; we have no explanation for this.

Fractions of bion pairsQ (ion pairs, triplets and higher

aggregates) obtained from deconvoluting the FTIR spectra

at 25 8C for concentrations of 5 mol/kg show no

significant differences between PEGME–LiClO4 and

PEODME–LiClO4 electrolytes. However, for lower con-

centrations, there is a higher concentration of bion pairsQ inthe PEODME–LiClO4 electrolyte. This is in agreement

with results from Raman spectroscopy, which show that

the fraction of bion pairsQ present in PEODME–LiClO4 is

relatively higher than that obtained for PEGME–LiClO4.

This is in disagreement with predictions based on

calculations using the Fuoss–Kraus formalism, which

show that the fraction of ion pairs for PEGME–LiClO4

(Fig. 6 in Ref. [29]) is much higher than for PEODME–

LiClO4 (Fig. 1b). The ion conductivity for the PEODME–

LiClO4 electrolyte at the higher concentrations is higher

than for the PEGME–LiClO4 electrolyte [30]. The

confusion here is in the fact that the FTIR and Raman

results for bion pairsQ include triplets and higher aggre-

gates; one cannot know whether a change in the fraction

of bion pairsQ means a change in any of ion pairs, triplets

or higher aggregates, all three or combinations. Based

upon the Fuoss–Kraus and ion conductivity results, one

should conclude that there is a higher fraction of triplets

and bfreeQ anions in the PEODME–LiClO4 electrolyte at

the higher concentrations due to the absence of OH

terminal groups.

We connect the variation in the conductivity with the

presence or absence of the OH end groups in the polymer

matrix of the composite electrolytes. We have noticed that

an increase in the conductivity of the composites containing

micron-sized alumina particles is only found in the PEGME

(i.e., OH terminated) electrolytes [29,30]. The nature of

these changes originates from the Lewis acid–base inter-

actions between different species present in the electrolyte

and surface groups of the filler. Considering the results of

the Fuoss–Kraus calculation, this increase in conductivity

was assisted by a decrease in the fraction of ion pairs in the

filled PEGME-based electrolytes [28,29].

M. Marcinek et al. / Solid State Ionics 176 (2005) 367–376376

5. Conclusions

There is good agreement between the Fuoss–Kraus

predictions and the results from both spectroscopic techni-

ques for the PEODME–LiClO4 and PEGME–LiClO4 [29]

electrolytes for changes in ion fractions as a function of

concentration. Results from Raman spectroscopy generally

agree with the Fuoss–Kraus predictions for the concen-

tration dependence of the ion fractions in the filled electro-

lytes. This is not the case for the results from FTIR which

are at variance with Fuoss–Kraus for some of the filled

electrolytes. Difficulties with the FTIR technique were

noted in the text.

Results for ion fractions as a function of temperature

were only obtained from Fuoss–Kraus predictions and

FTIR. For the PEODME–LiClO4 electrolytes without filler,

both techniques show that the fraction of bfreeQ anions

increases with an increase in temperature; the results for ion

pairs, triplets and higher aggregates are also in agreement if

one considers the FTIR results for bion pairsQ to include

triplets and higher aggregates. For PEGME–LiClO4–

Al2O3acidic electrolytes at the high concentrations studied

with FTIR, the fraction of bfreeQ anions decreases with

increasing temperature; this fraction reaches a maximum at

about 25 8C in the Fuoss–Kraus predictions but, for higher

temperatures, decreases with an increase in temperature in

agreement with the FTIR results. Results for ion pairs,

triplets and higher fractions are in agreement between the

two methods of the electrolyte with acidic surface on the

Al2O3 filler.

From this study and those published previously [29,30],

we conclude that, generally, the PEODME-based electro-

lytes are superior to the PEGME-based electrolytes. The

strong interaction of the polar OH terminal group on the

PEGME with ions and filler surfaces is a detriment.

Acknowledgements

This work was financially supported by the President of

the Warsaw University of Technology according to the 504/

164/853/8, 503/G/0020, 503/G/0021 research grant and by

the Natural Sciences and Engineering Research Council of

Canada.

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