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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: [email protected] (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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