Ionic association in liquid (polyether–Al2O3–LiClO4) composite electrolytes

Solid State Ionics 176 (2005) 367 – 376 www.elsevier.com/locate/ssi

Ionic association in liquid (polyether–Al2O3–LiClO4) composite electrolytes Marek Marcineka,*, Marcin Cioseka, Grayyna Z˙ukowskab, Wyadysyaw Wieczoreka, Kenneth R. Jeffreyb, Jim R. Stevensb b

a Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warszawa, Poland The 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 * Corresponding author. Tel.: +48 6605637. E-mail address: [email protected] (M. Marcinek). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.08.013

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 molecular 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 spectroscopies 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 bfreeQ perchlorate 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

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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 coordination 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 bmolecularQ species 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.

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 conductivity 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 superpositions 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

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

Poly (ethylene glycol) methyl ether CH3(OCH2CH2)nOH (PEGME, M w=350, Aldrich) and poly (ethylene oxide) dimethyl ether CH3(OCH2CH2)nOCH3 (PEODME, M w= 500, Aldrich) were filtered, then stringently freeze-dried

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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. Temperaturedependent 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).

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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.

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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.

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;

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the reverse is true on the high-concentration side. The fraction of triplets increases with increasing concentration but decreases with increasing temperature. The concentration dependence indicates that there is a trend, first to ion pairs and then to higher aggregates. The temperature dependence indicates a redissociation at lower concentrations 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, and the 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 increasing 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 bfreeQ anions 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=410 4 S/cm) were found for concentrations around 1 mol/kg. At the higher concentrations, 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 PEGMEbased 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.

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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 analyzing Raman and FTIR spectra, the bands attributed to the m 1 symmetrical stretching and m 4 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 m 1 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 m 1 peak at 933 cm 1 is ascribed to bfreeQ anions and solventshared ion pairs. This feature has a shoulder on the highfrequency 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

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-

Fig. 2. Deconvolution of fully symmetric stretching mode (m 1) of perchlorate anion. Spectrum recorded at 25 8C for PEODME–LiClO4 (5 mol/kg)–acidic Al2O3 electrolyte.

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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 concentrations, 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 deconvolution of the m 1 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 electrolyte 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 m 1 characteristic Raman mode deconvolution Electrolyte

Salt concentration (mol/kg)

Ion pairs (%)

Free ions (%)

PEGME–LiClO4

4 3 4 3 3 4 3 4 3 2 4 3 2 4* 3 2 4 3

49 26 38 20 25 38 23 56 39 29 47 28 13 49 41 5 22 14

51 75 62 80 75 62 77 44 61 71 53 72 87 51 59 95 78 86

PEGME–LiClO4–acidic Al2O3 PEGME–LiClO4–basic Al2O3 PEGME–LiClO4–neutral Al2O3 PEODME–LiClO4

PEODME–LiClO4–acidic Al2O3 PEODME–LiClO4–basic Al2O3 PEODME–LiClO4–neutral Al2O3

* 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 PEODMEbased 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 concentrations. 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 percentages 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 m 4 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 wavenumbers for m 4 of the 35ClO4 and 37ClO4 isotopomers [34]. The low Dm between the peak attributed to bfreeQ anions and the 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 m 4 band for the electrolyte PEODME–LiClO4–Al2O3 basic. The isotopo-

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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 deconvolution of the m 4 band, are presented in Table 3. Table 2 Ion fractions Polymer matrix

Fig. 3. Peak fitting for the m 4 ClO4 FTIR region for the PEODME–LiClO4 (4 mol/kg PEODME)–basic Al2O3. Sample at 25 8C.

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 various temperatures 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 m 4 mode in the FTIR spectra are evident in Table 2, especially for electrolytes based on monomethyl-capped PEGME, Table 2a. This m 4 band is much more complex than the m 1 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

Temperature (8C)

Type of filler

Ion pairs (%)

Free ions (%)

(a) Data for PEGME-based electrolytes PEGME 20 None 0 None 20 None 50 None 70 None PEGME 20 Basic 0 Basic 20 Basic 50 Basic 70 Basic PEGME 20 Acidic 0 Acidic 20 Acidic 50 Acidic 70 Acidic PEGME 50 Neutral 0 Neutral 20 Neutral 50 Neutral 70 Neutral

33 29 51 28 18 54 53 53 24 30

67 71 49 72 82 46 47 47 76 70

22 31 32 34 26 30 38 50 55

78 69 68 66 74 70 62 50 45

(b) Data for PEODME PEODME 20 0 20 50 70 PEODME 20 0 20 50 70 PEODME 20 0 20 50 70 PEODME 20 0 20 50 70

35 35 30 18 15 39 42 40 41 44 57 61 64 68 69 17 16 17 11 11

65 65 70 82 85 61 58 60 59 56 43 39 36 32 31 83 84 83 89 89

None None None None None Basic Basic Basic Basic Basic Acidic Acidic Acidic Acidic Acidic Neutral Neutral Neutral Neutral Neutral

Results of the deconvolution of the band of m 4 ClO4 vibration mode (FTIR) at various temperatures. Salt concentration is equal to 5 mol/kg.

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Fig. 4. Shape of the m 4 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.

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 and bfreeQ 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 bfreeQ anions increases after the addition of the filler, and, for the system with acidic surface groups, the fraction of bfreeQ anions 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 m 4 vibration mode (FTIR) compared to the m 1 (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 deconvolution of the FTIR band less certain.

The Fuoss–Kraus estimation for the PEODME–LiClO4 electrolytes studied shows that the fraction of bfreeQ anions decreases 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 m 4 band. In the composite systems, there is a problem with the low intensity in the FTIR anion m 4 asymmetric mode for samples containing more than 1 mol/kg of salt, and moisture was a problem in the measurements. The Raman m 1 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 with an 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 includes the 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 Table 3 Percentages of free anions and ion pairs found on the basis of deconvolution of the m 4 ClO4 band (FTIR) Polymer matrix

Salt concentration mole per kilogram of polymer

Type of filler

Ion pairs (%)

Free ions (%)

PEGME

5 4 5 4 5 4 5 4 5 4 3 5 4 3 5 4 3 2 5 4 3 2

None None Basic Basic Acidic Acidic Neutral Neutral None None None Basic Basic Basic Acidic Acidic Acidic Acidic Neutral Neutral Neutral Neutral

33 21 31 10 33 15 35 38 35 35 30 39 42 40 57 61 64 68 17 16 17 11

67 79 69 90 67 85 65 62 65 65 70 61 58 60 43 39 36 32 83 84 83 89

PEGME PEGME PEGME PEODME

PEODME

PEODME

4. Discussion 4.1. Concentration dependence Results originating from three methods, FTIR and Raman deconvolution and Fuoss–Kraus are compared.

PEODME

Deconvolution of 624 cm

1

peak in a room temperature.

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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 anions decreases 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 aggregates 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 bion pairsQ 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 bion pairsQ 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.

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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 bion pairsQ 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 temperature. 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 concentrations, there is a higher concentration of bion pairsQ in the 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 aggregates; 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 interactions 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].

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5. Conclusions There is good agreement between the Fuoss–Kraus predictions and the results from both spectroscopic techniques 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 concentration dependence of the ion fractions in the filled electrolytes. 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 electrolytes 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. References [1] P.G. Bruce, F.M. Gray, in: P.G. Bruce (Ed.), Solid State Electrochemistry, Cambridge University Press, Cambridge, UK, 1995, p. 119.

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