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Effect of nano-porous Al2O3 on thermal, dielectric and transport properties of the (PEO)9LiTFSI polymer electrolyte system
Electrochimica Acta 47 (2002) 3257 /3268 www.elsevier.com/locate/electacta
Effect of nano-porous Al2O3 on thermal, dielectric and transport properties of the (PEO)9LiTFSI polymer electrolyte system P.A.R.D. Jayathilaka a,b, M.A.K.L. Dissanayake a,b, I. Albinsson a, B.-E. Mellander a,* a
Physics and Engineering Physics, Chalmers University of Technology, S-412 96 Go¨teborg, Sweden b Department of Physics, University of Peradeniya, Peradeniya, Sri Lanka Received 20 December 2001; received in revised form 26 April 2002
Abstract Thermal, electrical conductivity and dielectric relaxation measurements have been performed on (PEO)9LiTFSI/10 wt.% Al2O3 nano-porous polymer electrolyte system. It is observed that the conductivity enhances substantially due to the presence of the filler particles with different surface groups. The highest enhancement is found for the filler particles with acidic groups followed by basic, neutral, and weakly acidic. The results reveal that the filler particles do not interact directly with poly(ethelene) oxide (PEO) chains indicating that the main chain dynamics governing the ionic transport has not significantly affected due to the filler. The results are consistent with the idea that the conductivity enhancement is due to the creation of additional sites and favourable conduction pathways for ionic transport through Lewis acid /base type interactions between the filler surface groups and the ionic species. This is reflected as an increase in the mobility rather than an increase in the number of charge carriers. A qualitative model has been proposed to explain the results. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polymer electrolyte; (PEO)9LiTFSI; Nano-porous Al2O3; Conductivity; Dielectric relaxation
1. Introduction Poly(ethelene) oxide (PEO) based solid polymer electrolyte membranes have received much attention in the recent past due to the possibility of using them in compact, light weight, high energy density lithium rechargeable batteries [1 /9]. One of the major drawbacks of PEO /LiX electrolytes, however, is their low ionic conductivity at ambient temperatures. This is due to the presence of crystalline phases at temperatures below 60 8C which impedes lithium ion transport while the high ionic conductivity is generally associated with the high temperature amorphous phase. However, it should be mentioned here that, in a recent paper, the possibility of relatively higher conductivity in the (PEO)6LiSbS6 crystalline phase, compared to the amorphous phase with the same composition has been demonstrated [10]. The most widely used technique to lower the operational temperature of PEO based
* Corresponding author. Tel.: /46-31-772-3340; fax: /46-31-7723354. E-mail address: [email protected] (B.-E. Mellander).
electrolytes has been to add liquid plasticizers, but this gives rise to electrolyte films with poor mechanical properties and higher reactivity towards lithium anode. Much of the more recent research efforts to improve the ambient temperature conductivity while retaining the mechanical properties and the stability towards metallic lithium anode have been directed towards the use of nano-scale ceramic fillers such as Al2O3, SiO2, TiO2 and g-LiAlO2 in PEO based polymer electrolytes [11 /31]. These systems are generally known as nano-composite polymer electrolytes. It has been shown that nano-sized ceramic powders incorporated into the PEO based electrolytes can act as solid plasticizers inhibiting crystallization kinetics and promoting the retention of the amorphous phase down to sub-ambient temperatures [7]. These nano-composite PEO based polymer electrolytes have shown enhanced ionic conductivities and improved mechanical and thermal stability. They also have better electrochemical stability towards lithium metal and an enhanced cation transport number [14,15]. All these physical characteristics, once optimised, would make them suitable candidates for rechargeable solid state lithium polymer batteries.
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 2 4 3 - 8
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Although much attention has recently been focused on nano-composite polymer electrolyte systems by various groups, the mechanism of ionic conductivity enhancement and the role played by the nano-sized ceramic fillers such as Al2O3, SiO2 and TiO2 is still not well understood. In the case of the PEG /LiClO4 /Al2O3 composite polymer electrolyte system, it has been shown that the conductivity enhancement depends on the nature of the filler surface group and results from the Lewis acid /base type interactions between the filler surface centres, ions, and ether oxygen base groups [22,23]. In the PEO /LiClO4 /Al2O3 system, the authors have suggested that the acid sites on the surface of the ceramic particles may compete with the lithium cations to form complexes with the basic oxygens on the PEO chains thereby promoting salt dissociation [7]. These authors have also suggested that the ceramic particles may act as cross-linking centres for PEO segments, thereby reducing the possibility for the PEO chains to reorganize themselves and also favouring the lithium ion transport through the grain boundaries. This could explain the enhanced conductivities and cation transport numbers obtained for this system. The same group, in a more recent paper have reported the conductivity enhancement in the (PEO)20 /LiCF3SO3 system due to the addition of nano-sized Al2O3 particles [8]. They have attributed the mechanism of enhancement to different types of interactions existing between filler grains with different surface groups and the salt species, Li ions, and PEO. Conductivity enhancement due to Al2O3 and TiO2 nano-particulate fillers in a fully amorphous trifunctional polyether and also in a PEO complex with LiClO4 and LiCF3SO3 have been interpreted in terms of electrostatic interactions between the ionic species and the fillers [9]. Another promising method to enhance the ionic conductivity of PEO based polymer electrolytes is to incorporate lithium salts with low lattice energy and bulky anions such as lithium(bis)trifluoromethanesulfonate imide (LiTFSI or Li(CF3SO2)2N), which are expected to slow down the re-crystallization kinetics of PEO /LiTFSI complexes and thereby enhance the conductivity [32 /37,48 /51]. Ionic conductivities of the order of 10 5 S cm 1 at 25 8C have been obtained for these polymer/salt complexes [33]. It would be interesting to try to make use of both these approaches, that is to incorporate nano-sized fillers into a PEO /LiTFSI polymer salt complex, with a view to optimise the favourable physical properties mentioned above and at the same time to understand the fundamental processes responsible for the improved transport properties due to the incorporation of nanosized fillers. In order to do this we have chosen the composite polymer electrolyte system, (PEO)9LiTFSI / Al2O3 incorporating 10 wt.% of nano-porous Al2O3. The idea of using nano-porous filler grains is to provide
the maximum possible surface area for a fixed amount of filler material, as the conductivity enhancement is believed to be associated with the nature and the number of Lewis acid /base groups on the surface of the grains. The alumina fillers used were of four different types: basic, neutral, weakly acidic and acidic. We have measured the ionic conductivity, dielectric relaxation and thermal properties of this composite electrolyte material. One unique advantage of this system, compared to the well-studied nano-composite systems based on PEO and other lithium salts such as LiClO4 or LiCF3SO3 is that one can ignore the effect of crystalization. This is because the parent system, (PEO)9LiTFSI, is supposed to be amorphous even below the PEO crystalline melting temperature of / 60 8C, due to extremely slow re-crystalization kinetics [38]. Broadband dielectric spectroscopy has been used as an alternative tool to probe the ion/polymer interactions in several polymer electrolyte systems [39 /47]. The dielectric relaxation studies used in the present work is intended to study the behaviour of ion pairs in relation to the local environment of mobile ionic species, modified by the presence of alumina fillers in the PEO /LiTFSI polymer electrolyte system. To our knowledge, such dielectric studies on composite polymer electrolyte systems incorporating nano-sized ceramic fillers have also not been reported so far.
2. Experimental PEO (molecular weight 5 /106), LiTFSI and Al2O3, all from Aldrich were used as starting materials. Four different types of Al2O3 powders, namely basic, neutral, weakly acidic and acidic, having the same particle size (104 mm), pore size (5.8 nm) and surface area (155 m2 g1), were used as fillers. Prior to use, PEO and LiTFSI was vacuum dried for 24 h at 50 and 120 8C, respectively. Appropriately weighed quantities of PEO and LiTFSI required for ether oxygen to Li ratio of 9:1 were dissolved in anhydrous MeCN. All four types of Al2O3 powders were vacuum dried at 200 8C for 24 h and added to the above solution which was magnetically stirred at room temperature at least for 24 h, until a homogenous solution was obtained. The amount of the Al2O3 filler added was fixed at 10 wt.% of the total PEO/LiTFSI weight. All the weighings were done inside the glove box. The resulting slurry after stirring was cast on to a Teflon plate and then left in order to let the solvent slowly evaporate over a molecular sieve for about 24 h. This procedure yielded visually homogenous composite polymer electrolyte films of average thickness 100 /200 mm which were stored in the glove box. The glass transition temperature of different samples were determined using a Mettler Toledo DSC 30
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differential scanning calorimeter. The measurements were carried out at a heating rate of 10 8C min 1 from /120 to 100 8C in the heating cycle and from 100 to /120 8C in the cooling cycle. A flow of nitrogen gas was maintained over the perforated pan to avoid any contact with atmospheric moisture. Complex impedance measurements were made on disc shaped samples sandwiched between two stainless steel electrodes of 5 mm diameter, using a HP 4291 A RF impedance analyser in the 1 MHz /1.8 GHz high frequency range. The temperature of the sample was varied from 0 to 100 8C and the measurements were taken at approximately 10 8C intervals on heating as well as on cooling. The ionic conductivity and the complex permittivity (o ? and o ƒ) were derived from the complex impedance data. The oƒcorr values were obtained after subtracting the dc conductivity contribution from the measured oƒtotal spectrum according to the relation, oƒcorr oƒtot so =o o v; where so and o o refer to the dc conductivity and the static dielectric constant, respectively.
3. Results and discussion 3.1. Thermal properties Fig. 1, curve (a) shows the DSC traces of pure PEO taken during the heating (lower curve) and cooling (upper curve) cycles. The melting of PEO crystallites occurs at 60.4 8C in the heating run. Re-crysallization occurs at 38.5 8C in the cooling run. Fig. 1(b) shows the DSC traces of the (PEO)9LiTFSI polymer salt complex taken during the heating and cooling runs. The glass transition temperature, Tg, of the material, as obtained from both of these traces, is /41.7 8C. This value
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agrees well with the Tg values reported in literature for this material [38 /40]. There is no melting/re-crystallization peak appearing around 60 8C in agreement with the presence of a crystallinity gap for a narrow composition range from n/6 to 12 in (PEO)n LiTFSI [39,40]. The material used in this study (n /9) therefore, remains completely amorphous. DSC traces of the composite polymer electrolyte, (PEO)9LiTFSI/10 wt.% Al2O3 incorporating the four different types of nano-porous alumina fillers, are shown in Fig. 1(c /f). The glass transition temperatures for all samples taken in both heating and cooling runs remains around / 41 8C (see Table 1). From these results it is clear that no appreciable change in the Tg values have occurred due to the addition of alumina fillers into the (PEO)9LiTFSI polymer salt complex. This suggests that the presence of nano-porous alumina fillers have not appreciably changed the main chain dynamics governing the glass transition of the material. The above observations can be compared with the thermal behaviour of some PEO based nano-composite electrolytes reported in the literature. No appreciable change in Tg has been seen in the (PEO)8LiBF4 system due to the addition of 10 wt.% of Al2O3 of particle size 13 nm [18]. For the composite electrolyte system (PEO)16LiClO4 incorporating various types and sizes of filler grains, it has been found that Tg has decreased by a few degrees due to the addition of 10 wt.% of Al2O3 of particle size 37 nm [27]. Effect of nano-scale SiO2 on (PEO)8LiTFSI has shown that the Tg has increased by a few degrees due to the incorporation of the filler grains [21]. Some recent studies using several PEO /LiX complexes with nano-scale SiO2 have shown that the addition of fillers does not change significantly the thermal properties of the material but it gives to the electrolyte better resistance to crystallization [52,53]. A review of a large number of PEO based composite electrolyte systems has revealed that there is no clear direct interaction between the filler grains and the host polymer [20]. According to a more recent paper, the thermal studies on nano-composite polymer electrolytes Table 1 Glass transition temperature and ionic conductivity (at 25 8C) of the composite polymer electrolyte, (PEO)9LiTFSI10 wt.% Al2O3 for four different types of Al2O3 fillers Type of Al2O3 filler (10 Tg (8C) wt.%)
Conductivity at 25 8C (S cm 1)
Heating Cooling Heating Fig. 1. DSC traces taken on heating and cooling cycles at 10 8C min 1 rate for: (a) pure PEO; and (b) (PEO)9LiTFSI without alumina. Traces for the nano-composite electrolyte, (PEO)9LiTFSI/10 wt.% Al2O3 with different surface groups are shown as: (c) acidic; (d) basic; (e) neutral; and (f) weakly acidic. The peak onset temperatures are 60.4 and 38.5 8C.
Acidic Basic Neutral Weakly acidic Without filler
41.3 41.4 41.8 41.4 41.7
40.8 41.6 40.1 41.8 41.7
5.61 10 5 4.95 10 5 3.47 10 5 1.98 10 5 7.03 10 6
Cooling 2.17 10 4 1.68 10 4 1.52 10 4 1.07 10 4 4.17 10 5
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based on an amorphous polymer, LiClO4 or LiCF3SO3 salts and Al2O3 and TiO2 (/20 nm) fillers, have shown that the Tg is essentially unaffected due to the addition of the filler particles [9]. These authors have suggested that there are no extraordinary interactions between the PEO /salt complex and the ceramic particles and that polymer/ceramic grain boundaries do not contribute appreciably to ionic conduction. NMR studies on PEO/LiClO4/nano-sized Al2O3, TiO2 and SiO2 fillers have demonstrated that the increased ionic conductivity is not attributable to a corresponding increase in polymer segmental motion, but more likely to a weakening of the polyether-cation association induced by the nano-particles [15]. According to the DSC results, our nano-composite electrolyte system also appears to behave similarly, suggesting that the contribution to the conductivity enhancement from the segmental flexibility of the PEO chains is essentially unaffected by the addition of the filler grains. The interactions responsible for the observed conductivity enhancement, therefore, appear to involve largely the Lewis acid /base type interactions between the surface groups of alumina grains and the salt ionic species. Extensive studies on composite PEO /LiClO4 and / LiCF3SO3 polymer electrolyte systems incorporating nano-sized Al2O3, TiO2 and g-LiAlO2 have shown that once these materials are annealed at temperatures above the melting temperature of PEO the ceramic additive causes the retention of the amorphous phase at ambient temperatures enhancing the conductivity [11 /14]. However, in the case of the nano-sized alumina added (PEO)9LiTFSI composite electrolyte, we do not see the melting transition around 60 8C, because, even the material without alumina is already in the amorphous state at ambient temperatures as already stated [39]. Therefore, generally in all PEO based nano-composite electrolytes, a different type of conductivity enhancement mechanism appears to play a major role. This probably involves surface interactions on alumina grains as mentioned earlier, which could enhance the conductivity in the amorphous phase as well. 3.2. Ionic conductivity The ionic conductivity of the solid polymer electrolyte, (PEO)9LiTFSI has been studied by several groups since it was first reported by Armand et al. in 1989 [32 / 37,48,49,53]. In this system, the large, flexible anion of LiTFSI is believed to act as a plasticizer inhibiting recystallisation kinetics and thereby retaining the amorphous phase down to ambient temperature. Fig. 2(a) shows the variation of conductivity with inverse temperature for the composite polymer electrolyte system, (PEO)9LiTFSI/10 wt.% Al2O3 for the four different types of nano-porous Al2O3 fillers taken on the heating run. Data taken on the cooling run are shown in Fig.
Fig. 2. Variation of the ionic conductivity with inverse temperature for the system (PEO)9LiTFSI/10 wt.% Al2O3 with different filler surface groups: (a) heating; (b) cooling.
2(b). Corresponding curves for the system without alumina are also included for comparison. According to these observations, the addition of nano-porous alumina fillers has obviously increased the ionic conductivity of the polymer /salt complex considerably and the maximum enhancement is found for acidic alumina. The room temperature (25 8C) conductivity of this electrolyte, measured on the cooling run, is 2.2 /104 S cm 1, which is among the highest conductivity values reported so far for a PEO based, plasticizer free, polymer electrolyte. All the conductivity curves exhibit similar curvature expected for an amorphous electrolyte obeying the VTF relation. The basic alumina gives the next highest conductivity enhancement followed by the neutral and the weakly acidic over the entire temperature range studied. The conductivity has increased by about fivefold at 20 8C and by about an order of magnitude at 0 8C due to the incorporation of the nano-porous, acidic alumina filler. The VTF type behaviour indicated in the log sT versus 1/T plots suggests that, even in the nano-composite polymer electrolyte, the ionic transport takes place predominantly by the same mechanism as that in the filler-free electrolyte, namely by the segmental motion of the polymer chains. Table 1 shows the comparison of the
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ionic conductivity values (at 25 8C) of the composite polymer electrolyte, (PEO)9LiTFSI/10 wt.% Al2O3 for the four different types of Al2O3 fillers. The ionic conductivity values obtained in this work for the (PEO)9LiTFSI polymer electrolyte without alumina are comparable with the values reported by others for the same system [21,32,33,49,53]. The relatively higher conductivity values of the PEO /LiTFSI system, compared to the widely studied systems such as PEO /LiClO4 and PEO /LiCF3SO3 must evidently be due to the strong polarizing effect of the large, flexible TFSI anion, which inhibits recrystalization kinetics and causes the retention of the amorphous phase down to ambient temperatures [49]. This salt is supposed to be among the most dissociated lithium salts presently available. In the case of PEO based, filler free polymer electrolytes such as PEO /LiClO4, PEO /LiCF3SO3 and PEO / LiBF4 the conductivity drops to low values at temperatures below the crystallite melting temperature of about 60 8C due to recrystallization. This conductivity drop is rather abrupt as seen from the discontinuity in the log s versus 1/T graphs where the amorphous phase with high conductivity at higher temperatures changes to a crystalline phase with low conductivity. However, when nanosized ceramic fillers are added to these systems, two important changes appear in the log s versus 1/T graphs. The discontinuity in the log s versus 1/T graphs disappears because the amorphous phase extends down to ambient temperatures exhibiting higher conductivity values at these temperatures. There is also a significant conductivity enhancement even in the high temperature amorphous phase. It is believed that the nano-sized ceramic filler, due to its large surface area, prevents local PEO chain reorganization with the result of locking in at ambient temperature, a high degree of disorder characteristic of the amorphous phase, which in turn favours high ionic transport [11 /15]. However, this interpretation cannot be applied for the conductivity enhancement observed above the crystallite melting temperature where the electrolyte is believed to be amorphous by its own nature. Therefore, as proposed by Wieczorek et al. and further developed by Croce et al., other effects involving the ceramic filler, such as Lewis acid /base interactions, must also be taken into account in order to explain the overall conductivity enhancement observed in these PEO based nano-composite polymer electrolytes at temperatures above as well as below the melting temperature [7,8,22,23]. In a recent paper, Croce et al. have proposed an impressive qualitative model to explain the conductivity enhancement caused by nano-sized Al2O3 ceramic fillers in the PEO20 /LiCF3SO3 system [8]. In this system, the conductivity enhancement in the amorphous phase, above 60 8C, decreases in the order: neutral/acidic /basic /filler free while below 60 8C it decreases as:
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acidic /neutral/filler free /basic. The authors have suggested that nano-sized alumina grains help to increase the conductivity in two ways: (a) by creating cross linking centres for the PEO segments and for X anions, leading to lowering of the PEO reorganizing tendency, retaining the amorphous phase down to ambient temperatures and promoting Li ion conducting pathways at the ceramic surface; and (b) by creating Lewis acid /base centres with ionic species, leading to ionic dissociation and generating more cations. The authors have supported their results with lithium transference number measurements which follows the decreasing order: acidic /neutral/basic /filler free. Best et al. have studied microscopic interactions in some nano-composite polymer electrolytes using Raman measurements [9]. In this study, DSC, Raman and impedance measurements on three different nano-composite polymer electrolyte systems based on a fully amorphous trimethyl polyether (2PEG), poly(methylene ethylene oxide) and PEO complexed with LiClO4 and LiCF3SO3 and incorporating 20 nm sized Al2O3 and TiO2 fillers have been analysed. In these systems, the authors have suggested that the leading contribution for the conductivity increase does not come from the effect of Tg or from the change in the number of ions through dissociation but rather from an increase in the mobility brought about by the filler grains. They have also suggested that the filler grains modify the potential field seen by the cations providing them with a favourable conduction path in the vicinity of the grains and the polymer segments. Their Raman results have shown that the fillers did not produce any significant decrease in ion pairing at any temperature. The work of Wieczorek and co-workers on several PEO based nano-composite polymer electrolyte systems have suggested that above the melting point of the crystalline polymer, the conductivity enhancement is caused by the increase in ionic mobility in samples with low salt concentration (O/M /300) and by the creation of additional charge carriers in samples with high salt concentrations (O/M /3) [22,26]. The work by the same group on PEG /LiClO4 /Al2O3 system have emphasised the role of filler surface groups on the ionic conductivity enhancement. The difference in conductivity enhancement has been attributed to different interactions by different surface groups of filler grains with Lewis base centres on polyether oxygens and ions. At the maximum conductivity composition, the conductivity enhancement caused by the alumina fillers decreased in the order: neutral /basic /acidic /filler free. It has been suggested that the conductivity increase results from an increase of the number of charge carriers arising from the decrease of the fraction of ion pairs due to the presence of the filler grains. Based on the results reported by different groups, as discussed above, it is not clear whether the conductivity
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Table 2 The conductivity enhancements by nano-sized Al2O3 fillers with different surface groups incorporated into polymer electrolytes, as reported by different laboratories This work; (PEO)9LiTFSI10 wt.% Al2O3 (5.8 nm)
Acidic Basic Neutral Weakly acidic Filler free
Croce et al. (Ref. [8]); (PEO)20LiCF3SO310 Marcinek et al. (Ref. [22]); PEGLiClO410 wt.% Al2O3 wt.% Al2O3 (5.8 nm) ( B 5 mm) (2 mol salt kg1 of PEG, molecular weight 350) Above 60 8C
Below 60 8C
Neutral Acidic Basic Filler free /
Acidic Neutral Basic Filler free /
Neutral Basic Acidic Filler free /
In each case, the surface groups are listed in decreasing order of ionic conductivity enhancement.
enhancement caused by the filler is due to an increase in the number of charge carriers or due to the increased mobility. In Table 2, we have compared the conductivity enhancements in some nano-composite polymer electrolyte systems reported by other groups along with the data from the present work. A simple comparison of data, see Table 2, shows that there is no clear trend when comparing the results obtained by different laboratories with regard to the degree of conductivity enhancement by different surface groups of alumina. Some difference in the conductivity enhancement can, of course, result from the nature, source and the thermal history of the alumina grains, conditions of preparation and the thermal history of the polymer electrolyte samples, and also from the type of the polymer and the salt used. Although the composite polymer electrolyte systems studied are different, one would expect a common trend if the same conductivity enhancement mechanism is generally operating in all these systems. In Section 3.3, we shall try to make use of the results of our dielectric relaxation measurements in order to better understand the conductivity enhancement in these nano-composite polymer electrolytes. 3.3. Dielectric relaxation As described in Sections 1 and 2, dielectric relaxation spectra can provide important information regarding the behaviour of polymer electrolytes. In this work, we have obtained the oƒcorr spectra of the (PEO)9LiTFSI system as well as the systems incorporating 10 wt.% Al2O3 fillers with four different types of surface groups for both heating and cooling runs. The oƒcorr spectra taken on the heating run for the filler free electrolyte and for the electrolyte incorporating 10 wt.% of acidic Al2O3 fillers are shown in Fig. 3 as representative examples. The appearance of a dominant dielectric relaxation peak, whose maximum shifts gradually to higher frequencies with increasing temperature, can be clearly seen for both these systems. This peak with a similar temperature dependence can be seen for the samples
Fig. 3. Variation of the intensity of the oƒcorr spectrum with frequency at different temperatures measured on heating: (a) (PEO)9LiTFSI; (b) (PEO)9LiTFSI/10 wt.% Al2O3(acidic).
with other three types of alumina fillers as well (not shown). As this peak was not present in the o ƒ spectrum of pure PEO, and started to appear only after introducing the salt to the polymer, we believe that it originates from dielectric relaxations of ion pairs. This identification is consistent with several polymer electrolyte systems, including the (PEO)9LiTFSI, studied by us and by others previously [40 /47]. The presence of a considerable number of ion pairs in this system has also been confirmed by Raman measurements [35]. Analysis of Fig. 3(a and b) first reveals the following results: (i) as expected, the ion pair relaxation frequency,
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fm, increases with increasing temperature for both systems; (ii) the intensity of the ion pair peak also shows the expected behaviour, that is, it decreases gradually with increasing temperature for the filler free electrolyte, although this change is rather small for the composite electrolyte; (iii) the intensity of the peak (at a given temperature) is somewhat higher for the sample that contains the filler. A similar temperature dependence has been observed for the o ƒ spectra of samples with the other three types of alumina particles (not shown). The values of ion pair relaxation frequency, fm, have also been verified by directly obtaining the derivative of the o ? spectrum in order to make sure that the procedure used for dc conductivity correction on o ƒ peaks has not affected the fm values. An interesting result emerges from the analysis of the temperature dependence of the relaxation frequency of the samples with different alumina surface groups (see Fig. 4). This is due to the fact that the ion pair relaxation frequency can be taken as a measure of the average ionic mobility in the electrolyte medium, provided that the polymer chain flexibility is rate determining for both ion pair rotation and ionic translation [45]. Thus, the results shown in Fig. 4(a) suggest that the mobility has increased significantly due to the addition of the filler. Also, the order of increase of fm for fillers with different surface groups appears to follow the same order as the conductivity increase shown in Fig. 2(a). The data taken
Fig. 4. Variation of the ion pair peak frequency with inverse temperature for (PEO)9LiTFSI/10 wt.% Al2O3 with different filler surface groups corresponds to: (a) heating; and (b) cooling.
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on cooling (Fig. 2(b)) also follow the same trend although the curves for basic and neutral appears to overlap at some temperatures. The highest conductivity enhancement as well as the highest increase in fm is observed for acidic alumina followed by basic, neutral, weakly acidic and filler free systems. From these results, it can be inferred that the major contribution to the observed conductivity enhancement due to the addition of nano-porous alumina fillers comes from the mobility increase in the electrolyte. These results seem to be in agreement with the recent findings reported by Best et al. based on Raman measurements on similar systems [9]. The intensity of the ion pair peak in the oƒcorr spectrum can be taken as a measure of the concentration of ion pairs in the electrolyte [45]. However, comparison of peak intensity at different temperatures is not trivial due to several factors. The intensity is not only a function of the concentration of dipoles but also of temperature. It is possible to obtain a rough estimate of the variation of the number of dipoles with temperature by multiplying the peak intensity by the absolute temperature. This has been done in the present case for samples with all four types of filler particles and the results show that the number of dipoles increases only marginally with temperature. However, at a given temperature, the intensity of the oƒcorr peak may be used for comparison. Although a slight increase in the peak intensity can be observed in samples with all four types of filler particles, a definitive conclusion cannot be made due to the fact that the oƒcorr peak intensity is very sensitive to the correction procedure used to compensate for the dc conductivity. Anyhow, for the present system, the general conclusion can be made that, the number of ion pairs has not changed significantly due to the addition of the filler particles. However, in the case of the system (PEO)20LiClO4 with 10 wt.% nano-sized Al2O3 fillers, Croce et al. have attributed the observed conductivity enhancement to salt dissociation creating more Li ions [8]. Fig. 5 shows the variation of log s and log fm of the ion pair peak with inverse temperature for the nanocomposite electrolyte with acidic Al2O3 fillers for the heating run. It should be noted that the vertical scales in both figures (a and b) have been set to three decades in order to allow comparison. A comparison of the two curves clearly shows that the temperature dependence of the ion pair peak frequency follows very closely the temperature dependence of the conductivity, both obeying the VTF type behaviour with essentially the same rate of increase with temperature. This, we believe, is an important observation concerning the mechanism of ion transport in this nano-composite polymer electrolyte, since it suggests that the local environment in which the ion pair relaxation occurs and the ionic transport takes place is the same.
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Fig. 5. Comparison of the variation of ionic conductivity and ion pair peak frequency with inverse temperature for (PEO)9LiTFSI/10 wt.% Al2O3 (acidic) measured on heating.
In order to illustrate the dependence of conductivity and ion pair frequency on the pH value of the filler surface groups, we have plotted these quantities (at 25 8C) in Fig. 6. It is clear from this figure also that a correlation exists between the conductivity and the ion pair frequency for the four types of filler surface groups. In order to gain a better understanding of the ionic conductivity enhancement observed in this system, we shall try to make use of our results on thermal, electrical conductivity and dielectric relaxation measurements along with some of the ideas proposed and developed by the other groups, especially by Wieczorek et al., Scrosati et al., and Best et al., as described earlier. As discussed earlier, our dielectric relaxation results suggests that the major contribution to the observed conductivity enhancement due to nano-sized fillers comes from the increased mobility rather than from the increased charge carrier concentration resulting from the dissociation of ion pairs. 3.4. The mechanism of conductivity enhancement In a PEO /LiX polymer electrolyte, at any instant of time, a Li ion is co-ordinated to about four ether
Fig. 6. Dependence of ionic conductivity and ion pair peak frequency at 25 8C on the pH value of the filler surface groups of (PEO)9LiTFSI/10 wt.% Al2O3.
oxygen atoms of the same or different PEO chains. As the ion moves along the polymer chain, old co-ordination links break and the new links form. This cation motion is facilitated by the flexing of the polymer chain segments producing a strong coupling between the segmental motion of the polymer and the ionic transport. As the polymer segments are flexible in the amorphous phase, the polymer electrolyte generally has a higher conductivity in the amorphous phase. The anion is only weakly bound to the polymer chain but the flexibility of the polymer chain is rate determining also for the anion transport. This situation is pictorially illustrated in Fig. 7(a). The different types of alumina grains we have used in this work have the same particle size (104 mm) and the same pore size (5.8 nm). Due to their ultra fine nanostructure, these nano-porous grains are expected to provide effectively a large surface area with a large number of surface groups for the surface interactions to take place. However, for simplicity, we assume that this large nano-porous surface area may be compared to a large number of nano-sized alumina grains. In the case of nano-composite polymer electrolytes incorporating Al2O3 grains, as suggested by Wieczorek et al. and Croce et al., the dependence of the conductivity enhancement on the nature of the filler surface group may be satisfactorily explained in terms of the Lewis acid /base type interactions involving different types of surface groups [7,8,23,25]. Using these ideas, we shall now try to explain the effect of different types of surface groups in conductivity enhancement in the present PEO /LiTFSI/10 wt.% Al2O3 system. Fig. 7(b) illustrates the case with acidic Al2O3 filler. We denote a grain of alumina with acidic surface groups as consisting of OH polar groups. As suggested by Wieczorek et al. for the system PEG /LiClO4 /Al2O3, based on FTIR studies, the anions have a larger affinity towards Al2O3 surface acid groups than cations [19]. Due to the strongly polarisable nature of the TFSI anion, a stronger affinity can be expected between the TFSI anion and the Al2O3 surface acid groups. During their migration under an external potential, the polar TFSI anions dissociated from the salt can interact with these polar OH groups via transient hydrogen bonding. The situation is somewhat similar to the migration of cations through intermittent co-ordination to ether oxygens. The anions will remain H-bonded to the OH groups only temporarily. In the filler free electrolyte the anion motion is only assisted to some extent by the segmental flexibility of the polymer chains. However, now the anions will have additional sites to occupy during their migration effectively creating new favourable conduction pathways in the vicinity of the filler grains. The anion migration is therefore facilitated and streamlined compared to the filler free situation. This we believe is reflected as an increase in mobility as inferred
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Fig. 7. A pictorial model to illustrate the proposed mechanism of ionic conductivity enhancement in the nano-composite polymer electrolyte (PEO)9LiTFSI/10 wt.% Al2O3: (a) filler free; (b) acidic; (c) basic; (d) neutral; and (e) weakly acidic. Note that for simplicity the OH groups are marked with H in the figure.
from our dielectric relaxation data as described earlier. According to our model, the involvement of the polymer chain in ionic transport is rather unaffected due to the addition of the filler in agreement with the essentially unchanged Tg value observed in our DSC results. Cation migration may remain essentially unaffected by the presence of these acidic fillers. Furthermore, according to this model, the number of ion pairs also does not change appreciably due to the presence of the filler. The observed highest conductivity enhancement due to the filler with acidic surface groups, therefore, appears to come from the above mechanism, which is experimentally seen as an increase in the effective average mobility of carriers. For the filler free
(PEO)8LiTFSI system at 67 8C, Gorecki et al. have reported an anionic mobility of 1.1 /109 m2 s 1 V 1 and a cationic and anionic transport numbers of 0.57 and 0.43, respectively [48]. It is interesting to note the substantial anionic conductivity in this system, despite the large sized, flexible TFSI anion. Although we have not attempted to measure the transport numbers for the present system with nano-sized fillers, we expect it to be higher than the filler free system due to the increased mobility. The system with Al2O3 grains with basic surface groups is illustrated in Fig. 7(c). In an alumina grain with basic surface groups the strong basic centres are due to oxygens at the surface of the alumina. FTIR
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Fig. 7 (Continued)
measurements on the PEG /LiClO4 /Al2O3 system have suggested that the basic surface groups in alumina interact with Li cations [22]. In our system, we propose that during their migration, the Li cations form temporary, weaker transient bonds with the oxygen atoms of the surface basic groups of alumina grains, very much similar to the co-ordinated transient links they form with ether oxygens. This provides extra sites for the cations, which would, otherwise, be migrating only via forming and breaking co-ordination links with ether oxygens of the PEO chains. During the cationic transport process the bonds between Li ions and oxygens in alumina surface groups are also subjected to breaking and making. Effectively an additional favourable conduction pathway for the cations is created in the vicinity of the basic alumina grains, which may involve both oxygen sites on alumina surface as well as ether oxygens. This improved ionic transport, we believe, is reflected as an increase in the average mobility of the carriers as seen from our dielectric measurements. The degree of conductivity enhancement brought about by the acidic alumina grains is expected to be higher compared to the enhancement by basic alumina grains because the acidic alumina increases the anionic contribution to the conductivity. In the filler free system, the anionic migration is only weakly influenced by the polymer whereas the cationic migration is already assisted by making/breaking of links with ether oxygens.
Therefore, acidic alumina grains are expected to give rise to the highest conductivity enhancement through anionic contribution. In all cases, we assume that the concentration of carriers, both cations and anions, remains essentially the same. The case of neutral alumina is shown in Fig. 7(d). Here, each alumina grain surface can be expected to have about equal number of acidic and basic sites. The acidic sites are expected to promote the anion migration and the basic sites the cation migration, as already explained earlier. However, now the anions are transiently ‘bonded’ to acidic sites and cations to basic sites. Because the sites are randomly distributed on the surface it is likely that a cation and anion, both transiently ‘bonded’ are in close proximity so that cation /anion interactions occur and decrease the mobility of both species, compared to the pure acidic and basic cases. Fig. 7(e) shows the case of alumina fillers with weakly acidic surface groups. Here, each alumina grain is expected to have more acidic than basic groups exposed from its surface. Therefore, there are relatively fewer additional sites available for the Li ions. Also, in this case, the random distribution of these surface groups is likely to decrease the conductivity even more. Both these factors may contribute to reduce the conductivity enhancement more effectively for the samples with weakly acidic alumina.
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4. Conclusion The results of the thermal, electrical conductivity and dielectric relaxation measurements on the (PEO)9LiTFSI/10 wt.% Al2O3 nano-porous polymer electrolyte system reported in this paper demonstrates the following. The filler particles do not interact directly with the PEO chains as revealed by the essentially unchanged Tg values suggesting that the main chain dynamics governing the ionic transport is unaffected. The presence of filler particles enhances the ionic conductivity substantially, and the degree of enhancement depends on the nature of the filler surface group and decreases as acidic /basic /neutral/weakly acidic. The peak observed in the oƒcorr spectrum has been attributed to ion pair dielectric relaxations. As revealed from the intensity of the ion pair peak, the number of ion pairs remains essentially unchanged due to the presence of the filler particles and also with increasing temperature ruling out the possibility of ion pair dissociation contributing to the conductivity enhancement. The ion pair peak frequency follows temperature dependence very much similar to the conductivity, suggesting that the increased mobility is largely responsible for the observed conductivity enhancement. A qualitative model based on Lewis acid / base type interactions between filler surface groups and cations/anions has been proposed in order to explain the difference in conductivity enhancement caused by different surface groups. It is proposed that the Lewis acid /base surface groups interact with cations and anions and provides additional sites creating favourable conduction pathways for the migration of ions, effectively enhancing the ionic mobility.
Acknowledgements PARDJ and MAKLD wish to thank the International Programmes in the Physical Sciences (IPPS), Uppsala University Sweden for the award of fellowships to work in Sweden. We would also like to thank the Swedish Research Council for financial support.
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[41] L.R.A.K. Bandara, M.A.K.L. Dissanayake, M. Furlani, B.-E. Mellander, Ionics 6 (2000) 239. [42] T. Furukawa, K. Yoneya, Y. Takahashi, K. Ito, H. Ohno, Electrochim. Acta 45 (2000) 1443. [43] P.A.R.D. Jayathilaka, M.A.K.L. Dissanayake, I. Albinsson, B.E. Mellander, to be published. [44] B.-E. Mellander, I. Albinsson, in: B.V.R. Chowdari, M.A.K.L. Dissanayake, M.A. Careem (Eds.), Solid State Ionics: New Developments, World Scientific, Singapore, 1996, p. 83. [45] I. Albinsson, B.-E. Mellander, J.R. Stevens, Solid State Ionics 72 (1994) 177. [46] B.-E. Mellander, I. Albinsson, in: B.V.R. Chowdari, M.A.K.L. Dissanayake, M.A. Careem (Eds.), Solid State Ionics: New Developments, World Scientific, Singapore, 1996, p. 97.
[47] W.A. Samantha, H. Eliasson, M. Furlani, I. Albinsson, M.A.K.L. Dissanayake, B.-E. Mellander, to be published. [48] W. Gorecki, M. Jeannin, E. Belorizky, C. Roux, M. Armand, J. Phys.: Condens. Matter 7 (1995) 6823. [49] S. Lascaud, M. Perrier, A. Vallee, S. Besner, J. Prud’homme, M. Armand, Macromolecules 27 (1994) 7469. [50] M. Armand, Solid State Ionics 89 (1996) 5. [51] W. Gorecki, C. Roux, E. Belorizky, Solid State Commun. 106 (1998) 681. [52] E. Quartano, C. Tomasi, P. Mustarelli, A. Magistris, Electrochim. Acta 43 (1998) 677. [53] P. Mustarelli, E. Quartarone, C. Tomasi, A. Magistris, Solid State Ionics 135 (2000) 81.
Effect of nano-porous Al2O3 on thermal, dielectric and transport properties of the (PEO)9LiTFSI polymer electrolyte system P.A.R.D. Jayathilaka a,b, M.A.K.L. Dissanayake a,b, I. Albinsson a, B.-E. Mellander a,* a
Physics and Engineering Physics, Chalmers University of Technology, S-412 96 Go¨teborg, Sweden b Department of Physics, University of Peradeniya, Peradeniya, Sri Lanka Received 20 December 2001; received in revised form 26 April 2002
Abstract Thermal, electrical conductivity and dielectric relaxation measurements have been performed on (PEO)9LiTFSI/10 wt.% Al2O3 nano-porous polymer electrolyte system. It is observed that the conductivity enhances substantially due to the presence of the filler particles with different surface groups. The highest enhancement is found for the filler particles with acidic groups followed by basic, neutral, and weakly acidic. The results reveal that the filler particles do not interact directly with poly(ethelene) oxide (PEO) chains indicating that the main chain dynamics governing the ionic transport has not significantly affected due to the filler. The results are consistent with the idea that the conductivity enhancement is due to the creation of additional sites and favourable conduction pathways for ionic transport through Lewis acid /base type interactions between the filler surface groups and the ionic species. This is reflected as an increase in the mobility rather than an increase in the number of charge carriers. A qualitative model has been proposed to explain the results. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polymer electrolyte; (PEO)9LiTFSI; Nano-porous Al2O3; Conductivity; Dielectric relaxation
1. Introduction Poly(ethelene) oxide (PEO) based solid polymer electrolyte membranes have received much attention in the recent past due to the possibility of using them in compact, light weight, high energy density lithium rechargeable batteries [1 /9]. One of the major drawbacks of PEO /LiX electrolytes, however, is their low ionic conductivity at ambient temperatures. This is due to the presence of crystalline phases at temperatures below 60 8C which impedes lithium ion transport while the high ionic conductivity is generally associated with the high temperature amorphous phase. However, it should be mentioned here that, in a recent paper, the possibility of relatively higher conductivity in the (PEO)6LiSbS6 crystalline phase, compared to the amorphous phase with the same composition has been demonstrated [10]. The most widely used technique to lower the operational temperature of PEO based
* Corresponding author. Tel.: /46-31-772-3340; fax: /46-31-7723354. E-mail address: [email protected] (B.-E. Mellander).
electrolytes has been to add liquid plasticizers, but this gives rise to electrolyte films with poor mechanical properties and higher reactivity towards lithium anode. Much of the more recent research efforts to improve the ambient temperature conductivity while retaining the mechanical properties and the stability towards metallic lithium anode have been directed towards the use of nano-scale ceramic fillers such as Al2O3, SiO2, TiO2 and g-LiAlO2 in PEO based polymer electrolytes [11 /31]. These systems are generally known as nano-composite polymer electrolytes. It has been shown that nano-sized ceramic powders incorporated into the PEO based electrolytes can act as solid plasticizers inhibiting crystallization kinetics and promoting the retention of the amorphous phase down to sub-ambient temperatures [7]. These nano-composite PEO based polymer electrolytes have shown enhanced ionic conductivities and improved mechanical and thermal stability. They also have better electrochemical stability towards lithium metal and an enhanced cation transport number [14,15]. All these physical characteristics, once optimised, would make them suitable candidates for rechargeable solid state lithium polymer batteries.
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 2 4 3 - 8
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Although much attention has recently been focused on nano-composite polymer electrolyte systems by various groups, the mechanism of ionic conductivity enhancement and the role played by the nano-sized ceramic fillers such as Al2O3, SiO2 and TiO2 is still not well understood. In the case of the PEG /LiClO4 /Al2O3 composite polymer electrolyte system, it has been shown that the conductivity enhancement depends on the nature of the filler surface group and results from the Lewis acid /base type interactions between the filler surface centres, ions, and ether oxygen base groups [22,23]. In the PEO /LiClO4 /Al2O3 system, the authors have suggested that the acid sites on the surface of the ceramic particles may compete with the lithium cations to form complexes with the basic oxygens on the PEO chains thereby promoting salt dissociation [7]. These authors have also suggested that the ceramic particles may act as cross-linking centres for PEO segments, thereby reducing the possibility for the PEO chains to reorganize themselves and also favouring the lithium ion transport through the grain boundaries. This could explain the enhanced conductivities and cation transport numbers obtained for this system. The same group, in a more recent paper have reported the conductivity enhancement in the (PEO)20 /LiCF3SO3 system due to the addition of nano-sized Al2O3 particles [8]. They have attributed the mechanism of enhancement to different types of interactions existing between filler grains with different surface groups and the salt species, Li ions, and PEO. Conductivity enhancement due to Al2O3 and TiO2 nano-particulate fillers in a fully amorphous trifunctional polyether and also in a PEO complex with LiClO4 and LiCF3SO3 have been interpreted in terms of electrostatic interactions between the ionic species and the fillers [9]. Another promising method to enhance the ionic conductivity of PEO based polymer electrolytes is to incorporate lithium salts with low lattice energy and bulky anions such as lithium(bis)trifluoromethanesulfonate imide (LiTFSI or Li(CF3SO2)2N), which are expected to slow down the re-crystallization kinetics of PEO /LiTFSI complexes and thereby enhance the conductivity [32 /37,48 /51]. Ionic conductivities of the order of 10 5 S cm 1 at 25 8C have been obtained for these polymer/salt complexes [33]. It would be interesting to try to make use of both these approaches, that is to incorporate nano-sized fillers into a PEO /LiTFSI polymer salt complex, with a view to optimise the favourable physical properties mentioned above and at the same time to understand the fundamental processes responsible for the improved transport properties due to the incorporation of nanosized fillers. In order to do this we have chosen the composite polymer electrolyte system, (PEO)9LiTFSI / Al2O3 incorporating 10 wt.% of nano-porous Al2O3. The idea of using nano-porous filler grains is to provide
the maximum possible surface area for a fixed amount of filler material, as the conductivity enhancement is believed to be associated with the nature and the number of Lewis acid /base groups on the surface of the grains. The alumina fillers used were of four different types: basic, neutral, weakly acidic and acidic. We have measured the ionic conductivity, dielectric relaxation and thermal properties of this composite electrolyte material. One unique advantage of this system, compared to the well-studied nano-composite systems based on PEO and other lithium salts such as LiClO4 or LiCF3SO3 is that one can ignore the effect of crystalization. This is because the parent system, (PEO)9LiTFSI, is supposed to be amorphous even below the PEO crystalline melting temperature of / 60 8C, due to extremely slow re-crystalization kinetics [38]. Broadband dielectric spectroscopy has been used as an alternative tool to probe the ion/polymer interactions in several polymer electrolyte systems [39 /47]. The dielectric relaxation studies used in the present work is intended to study the behaviour of ion pairs in relation to the local environment of mobile ionic species, modified by the presence of alumina fillers in the PEO /LiTFSI polymer electrolyte system. To our knowledge, such dielectric studies on composite polymer electrolyte systems incorporating nano-sized ceramic fillers have also not been reported so far.
2. Experimental PEO (molecular weight 5 /106), LiTFSI and Al2O3, all from Aldrich were used as starting materials. Four different types of Al2O3 powders, namely basic, neutral, weakly acidic and acidic, having the same particle size (104 mm), pore size (5.8 nm) and surface area (155 m2 g1), were used as fillers. Prior to use, PEO and LiTFSI was vacuum dried for 24 h at 50 and 120 8C, respectively. Appropriately weighed quantities of PEO and LiTFSI required for ether oxygen to Li ratio of 9:1 were dissolved in anhydrous MeCN. All four types of Al2O3 powders were vacuum dried at 200 8C for 24 h and added to the above solution which was magnetically stirred at room temperature at least for 24 h, until a homogenous solution was obtained. The amount of the Al2O3 filler added was fixed at 10 wt.% of the total PEO/LiTFSI weight. All the weighings were done inside the glove box. The resulting slurry after stirring was cast on to a Teflon plate and then left in order to let the solvent slowly evaporate over a molecular sieve for about 24 h. This procedure yielded visually homogenous composite polymer electrolyte films of average thickness 100 /200 mm which were stored in the glove box. The glass transition temperature of different samples were determined using a Mettler Toledo DSC 30
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differential scanning calorimeter. The measurements were carried out at a heating rate of 10 8C min 1 from /120 to 100 8C in the heating cycle and from 100 to /120 8C in the cooling cycle. A flow of nitrogen gas was maintained over the perforated pan to avoid any contact with atmospheric moisture. Complex impedance measurements were made on disc shaped samples sandwiched between two stainless steel electrodes of 5 mm diameter, using a HP 4291 A RF impedance analyser in the 1 MHz /1.8 GHz high frequency range. The temperature of the sample was varied from 0 to 100 8C and the measurements were taken at approximately 10 8C intervals on heating as well as on cooling. The ionic conductivity and the complex permittivity (o ? and o ƒ) were derived from the complex impedance data. The oƒcorr values were obtained after subtracting the dc conductivity contribution from the measured oƒtotal spectrum according to the relation, oƒcorr oƒtot so =o o v; where so and o o refer to the dc conductivity and the static dielectric constant, respectively.
3. Results and discussion 3.1. Thermal properties Fig. 1, curve (a) shows the DSC traces of pure PEO taken during the heating (lower curve) and cooling (upper curve) cycles. The melting of PEO crystallites occurs at 60.4 8C in the heating run. Re-crysallization occurs at 38.5 8C in the cooling run. Fig. 1(b) shows the DSC traces of the (PEO)9LiTFSI polymer salt complex taken during the heating and cooling runs. The glass transition temperature, Tg, of the material, as obtained from both of these traces, is /41.7 8C. This value
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agrees well with the Tg values reported in literature for this material [38 /40]. There is no melting/re-crystallization peak appearing around 60 8C in agreement with the presence of a crystallinity gap for a narrow composition range from n/6 to 12 in (PEO)n LiTFSI [39,40]. The material used in this study (n /9) therefore, remains completely amorphous. DSC traces of the composite polymer electrolyte, (PEO)9LiTFSI/10 wt.% Al2O3 incorporating the four different types of nano-porous alumina fillers, are shown in Fig. 1(c /f). The glass transition temperatures for all samples taken in both heating and cooling runs remains around / 41 8C (see Table 1). From these results it is clear that no appreciable change in the Tg values have occurred due to the addition of alumina fillers into the (PEO)9LiTFSI polymer salt complex. This suggests that the presence of nano-porous alumina fillers have not appreciably changed the main chain dynamics governing the glass transition of the material. The above observations can be compared with the thermal behaviour of some PEO based nano-composite electrolytes reported in the literature. No appreciable change in Tg has been seen in the (PEO)8LiBF4 system due to the addition of 10 wt.% of Al2O3 of particle size 13 nm [18]. For the composite electrolyte system (PEO)16LiClO4 incorporating various types and sizes of filler grains, it has been found that Tg has decreased by a few degrees due to the addition of 10 wt.% of Al2O3 of particle size 37 nm [27]. Effect of nano-scale SiO2 on (PEO)8LiTFSI has shown that the Tg has increased by a few degrees due to the incorporation of the filler grains [21]. Some recent studies using several PEO /LiX complexes with nano-scale SiO2 have shown that the addition of fillers does not change significantly the thermal properties of the material but it gives to the electrolyte better resistance to crystallization [52,53]. A review of a large number of PEO based composite electrolyte systems has revealed that there is no clear direct interaction between the filler grains and the host polymer [20]. According to a more recent paper, the thermal studies on nano-composite polymer electrolytes Table 1 Glass transition temperature and ionic conductivity (at 25 8C) of the composite polymer electrolyte, (PEO)9LiTFSI10 wt.% Al2O3 for four different types of Al2O3 fillers Type of Al2O3 filler (10 Tg (8C) wt.%)
Conductivity at 25 8C (S cm 1)
Heating Cooling Heating Fig. 1. DSC traces taken on heating and cooling cycles at 10 8C min 1 rate for: (a) pure PEO; and (b) (PEO)9LiTFSI without alumina. Traces for the nano-composite electrolyte, (PEO)9LiTFSI/10 wt.% Al2O3 with different surface groups are shown as: (c) acidic; (d) basic; (e) neutral; and (f) weakly acidic. The peak onset temperatures are 60.4 and 38.5 8C.
Acidic Basic Neutral Weakly acidic Without filler
41.3 41.4 41.8 41.4 41.7
40.8 41.6 40.1 41.8 41.7
5.61 10 5 4.95 10 5 3.47 10 5 1.98 10 5 7.03 10 6
Cooling 2.17 10 4 1.68 10 4 1.52 10 4 1.07 10 4 4.17 10 5
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based on an amorphous polymer, LiClO4 or LiCF3SO3 salts and Al2O3 and TiO2 (/20 nm) fillers, have shown that the Tg is essentially unaffected due to the addition of the filler particles [9]. These authors have suggested that there are no extraordinary interactions between the PEO /salt complex and the ceramic particles and that polymer/ceramic grain boundaries do not contribute appreciably to ionic conduction. NMR studies on PEO/LiClO4/nano-sized Al2O3, TiO2 and SiO2 fillers have demonstrated that the increased ionic conductivity is not attributable to a corresponding increase in polymer segmental motion, but more likely to a weakening of the polyether-cation association induced by the nano-particles [15]. According to the DSC results, our nano-composite electrolyte system also appears to behave similarly, suggesting that the contribution to the conductivity enhancement from the segmental flexibility of the PEO chains is essentially unaffected by the addition of the filler grains. The interactions responsible for the observed conductivity enhancement, therefore, appear to involve largely the Lewis acid /base type interactions between the surface groups of alumina grains and the salt ionic species. Extensive studies on composite PEO /LiClO4 and / LiCF3SO3 polymer electrolyte systems incorporating nano-sized Al2O3, TiO2 and g-LiAlO2 have shown that once these materials are annealed at temperatures above the melting temperature of PEO the ceramic additive causes the retention of the amorphous phase at ambient temperatures enhancing the conductivity [11 /14]. However, in the case of the nano-sized alumina added (PEO)9LiTFSI composite electrolyte, we do not see the melting transition around 60 8C, because, even the material without alumina is already in the amorphous state at ambient temperatures as already stated [39]. Therefore, generally in all PEO based nano-composite electrolytes, a different type of conductivity enhancement mechanism appears to play a major role. This probably involves surface interactions on alumina grains as mentioned earlier, which could enhance the conductivity in the amorphous phase as well. 3.2. Ionic conductivity The ionic conductivity of the solid polymer electrolyte, (PEO)9LiTFSI has been studied by several groups since it was first reported by Armand et al. in 1989 [32 / 37,48,49,53]. In this system, the large, flexible anion of LiTFSI is believed to act as a plasticizer inhibiting recystallisation kinetics and thereby retaining the amorphous phase down to ambient temperature. Fig. 2(a) shows the variation of conductivity with inverse temperature for the composite polymer electrolyte system, (PEO)9LiTFSI/10 wt.% Al2O3 for the four different types of nano-porous Al2O3 fillers taken on the heating run. Data taken on the cooling run are shown in Fig.
Fig. 2. Variation of the ionic conductivity with inverse temperature for the system (PEO)9LiTFSI/10 wt.% Al2O3 with different filler surface groups: (a) heating; (b) cooling.
2(b). Corresponding curves for the system without alumina are also included for comparison. According to these observations, the addition of nano-porous alumina fillers has obviously increased the ionic conductivity of the polymer /salt complex considerably and the maximum enhancement is found for acidic alumina. The room temperature (25 8C) conductivity of this electrolyte, measured on the cooling run, is 2.2 /104 S cm 1, which is among the highest conductivity values reported so far for a PEO based, plasticizer free, polymer electrolyte. All the conductivity curves exhibit similar curvature expected for an amorphous electrolyte obeying the VTF relation. The basic alumina gives the next highest conductivity enhancement followed by the neutral and the weakly acidic over the entire temperature range studied. The conductivity has increased by about fivefold at 20 8C and by about an order of magnitude at 0 8C due to the incorporation of the nano-porous, acidic alumina filler. The VTF type behaviour indicated in the log sT versus 1/T plots suggests that, even in the nano-composite polymer electrolyte, the ionic transport takes place predominantly by the same mechanism as that in the filler-free electrolyte, namely by the segmental motion of the polymer chains. Table 1 shows the comparison of the
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ionic conductivity values (at 25 8C) of the composite polymer electrolyte, (PEO)9LiTFSI/10 wt.% Al2O3 for the four different types of Al2O3 fillers. The ionic conductivity values obtained in this work for the (PEO)9LiTFSI polymer electrolyte without alumina are comparable with the values reported by others for the same system [21,32,33,49,53]. The relatively higher conductivity values of the PEO /LiTFSI system, compared to the widely studied systems such as PEO /LiClO4 and PEO /LiCF3SO3 must evidently be due to the strong polarizing effect of the large, flexible TFSI anion, which inhibits recrystalization kinetics and causes the retention of the amorphous phase down to ambient temperatures [49]. This salt is supposed to be among the most dissociated lithium salts presently available. In the case of PEO based, filler free polymer electrolytes such as PEO /LiClO4, PEO /LiCF3SO3 and PEO / LiBF4 the conductivity drops to low values at temperatures below the crystallite melting temperature of about 60 8C due to recrystallization. This conductivity drop is rather abrupt as seen from the discontinuity in the log s versus 1/T graphs where the amorphous phase with high conductivity at higher temperatures changes to a crystalline phase with low conductivity. However, when nanosized ceramic fillers are added to these systems, two important changes appear in the log s versus 1/T graphs. The discontinuity in the log s versus 1/T graphs disappears because the amorphous phase extends down to ambient temperatures exhibiting higher conductivity values at these temperatures. There is also a significant conductivity enhancement even in the high temperature amorphous phase. It is believed that the nano-sized ceramic filler, due to its large surface area, prevents local PEO chain reorganization with the result of locking in at ambient temperature, a high degree of disorder characteristic of the amorphous phase, which in turn favours high ionic transport [11 /15]. However, this interpretation cannot be applied for the conductivity enhancement observed above the crystallite melting temperature where the electrolyte is believed to be amorphous by its own nature. Therefore, as proposed by Wieczorek et al. and further developed by Croce et al., other effects involving the ceramic filler, such as Lewis acid /base interactions, must also be taken into account in order to explain the overall conductivity enhancement observed in these PEO based nano-composite polymer electrolytes at temperatures above as well as below the melting temperature [7,8,22,23]. In a recent paper, Croce et al. have proposed an impressive qualitative model to explain the conductivity enhancement caused by nano-sized Al2O3 ceramic fillers in the PEO20 /LiCF3SO3 system [8]. In this system, the conductivity enhancement in the amorphous phase, above 60 8C, decreases in the order: neutral/acidic /basic /filler free while below 60 8C it decreases as:
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acidic /neutral/filler free /basic. The authors have suggested that nano-sized alumina grains help to increase the conductivity in two ways: (a) by creating cross linking centres for the PEO segments and for X anions, leading to lowering of the PEO reorganizing tendency, retaining the amorphous phase down to ambient temperatures and promoting Li ion conducting pathways at the ceramic surface; and (b) by creating Lewis acid /base centres with ionic species, leading to ionic dissociation and generating more cations. The authors have supported their results with lithium transference number measurements which follows the decreasing order: acidic /neutral/basic /filler free. Best et al. have studied microscopic interactions in some nano-composite polymer electrolytes using Raman measurements [9]. In this study, DSC, Raman and impedance measurements on three different nano-composite polymer electrolyte systems based on a fully amorphous trimethyl polyether (2PEG), poly(methylene ethylene oxide) and PEO complexed with LiClO4 and LiCF3SO3 and incorporating 20 nm sized Al2O3 and TiO2 fillers have been analysed. In these systems, the authors have suggested that the leading contribution for the conductivity increase does not come from the effect of Tg or from the change in the number of ions through dissociation but rather from an increase in the mobility brought about by the filler grains. They have also suggested that the filler grains modify the potential field seen by the cations providing them with a favourable conduction path in the vicinity of the grains and the polymer segments. Their Raman results have shown that the fillers did not produce any significant decrease in ion pairing at any temperature. The work of Wieczorek and co-workers on several PEO based nano-composite polymer electrolyte systems have suggested that above the melting point of the crystalline polymer, the conductivity enhancement is caused by the increase in ionic mobility in samples with low salt concentration (O/M /300) and by the creation of additional charge carriers in samples with high salt concentrations (O/M /3) [22,26]. The work by the same group on PEG /LiClO4 /Al2O3 system have emphasised the role of filler surface groups on the ionic conductivity enhancement. The difference in conductivity enhancement has been attributed to different interactions by different surface groups of filler grains with Lewis base centres on polyether oxygens and ions. At the maximum conductivity composition, the conductivity enhancement caused by the alumina fillers decreased in the order: neutral /basic /acidic /filler free. It has been suggested that the conductivity increase results from an increase of the number of charge carriers arising from the decrease of the fraction of ion pairs due to the presence of the filler grains. Based on the results reported by different groups, as discussed above, it is not clear whether the conductivity
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Table 2 The conductivity enhancements by nano-sized Al2O3 fillers with different surface groups incorporated into polymer electrolytes, as reported by different laboratories This work; (PEO)9LiTFSI10 wt.% Al2O3 (5.8 nm)
Acidic Basic Neutral Weakly acidic Filler free
Croce et al. (Ref. [8]); (PEO)20LiCF3SO310 Marcinek et al. (Ref. [22]); PEGLiClO410 wt.% Al2O3 wt.% Al2O3 (5.8 nm) ( B 5 mm) (2 mol salt kg1 of PEG, molecular weight 350) Above 60 8C
Below 60 8C
Neutral Acidic Basic Filler free /
Acidic Neutral Basic Filler free /
Neutral Basic Acidic Filler free /
In each case, the surface groups are listed in decreasing order of ionic conductivity enhancement.
enhancement caused by the filler is due to an increase in the number of charge carriers or due to the increased mobility. In Table 2, we have compared the conductivity enhancements in some nano-composite polymer electrolyte systems reported by other groups along with the data from the present work. A simple comparison of data, see Table 2, shows that there is no clear trend when comparing the results obtained by different laboratories with regard to the degree of conductivity enhancement by different surface groups of alumina. Some difference in the conductivity enhancement can, of course, result from the nature, source and the thermal history of the alumina grains, conditions of preparation and the thermal history of the polymer electrolyte samples, and also from the type of the polymer and the salt used. Although the composite polymer electrolyte systems studied are different, one would expect a common trend if the same conductivity enhancement mechanism is generally operating in all these systems. In Section 3.3, we shall try to make use of the results of our dielectric relaxation measurements in order to better understand the conductivity enhancement in these nano-composite polymer electrolytes. 3.3. Dielectric relaxation As described in Sections 1 and 2, dielectric relaxation spectra can provide important information regarding the behaviour of polymer electrolytes. In this work, we have obtained the oƒcorr spectra of the (PEO)9LiTFSI system as well as the systems incorporating 10 wt.% Al2O3 fillers with four different types of surface groups for both heating and cooling runs. The oƒcorr spectra taken on the heating run for the filler free electrolyte and for the electrolyte incorporating 10 wt.% of acidic Al2O3 fillers are shown in Fig. 3 as representative examples. The appearance of a dominant dielectric relaxation peak, whose maximum shifts gradually to higher frequencies with increasing temperature, can be clearly seen for both these systems. This peak with a similar temperature dependence can be seen for the samples
Fig. 3. Variation of the intensity of the oƒcorr spectrum with frequency at different temperatures measured on heating: (a) (PEO)9LiTFSI; (b) (PEO)9LiTFSI/10 wt.% Al2O3(acidic).
with other three types of alumina fillers as well (not shown). As this peak was not present in the o ƒ spectrum of pure PEO, and started to appear only after introducing the salt to the polymer, we believe that it originates from dielectric relaxations of ion pairs. This identification is consistent with several polymer electrolyte systems, including the (PEO)9LiTFSI, studied by us and by others previously [40 /47]. The presence of a considerable number of ion pairs in this system has also been confirmed by Raman measurements [35]. Analysis of Fig. 3(a and b) first reveals the following results: (i) as expected, the ion pair relaxation frequency,
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fm, increases with increasing temperature for both systems; (ii) the intensity of the ion pair peak also shows the expected behaviour, that is, it decreases gradually with increasing temperature for the filler free electrolyte, although this change is rather small for the composite electrolyte; (iii) the intensity of the peak (at a given temperature) is somewhat higher for the sample that contains the filler. A similar temperature dependence has been observed for the o ƒ spectra of samples with the other three types of alumina particles (not shown). The values of ion pair relaxation frequency, fm, have also been verified by directly obtaining the derivative of the o ? spectrum in order to make sure that the procedure used for dc conductivity correction on o ƒ peaks has not affected the fm values. An interesting result emerges from the analysis of the temperature dependence of the relaxation frequency of the samples with different alumina surface groups (see Fig. 4). This is due to the fact that the ion pair relaxation frequency can be taken as a measure of the average ionic mobility in the electrolyte medium, provided that the polymer chain flexibility is rate determining for both ion pair rotation and ionic translation [45]. Thus, the results shown in Fig. 4(a) suggest that the mobility has increased significantly due to the addition of the filler. Also, the order of increase of fm for fillers with different surface groups appears to follow the same order as the conductivity increase shown in Fig. 2(a). The data taken
Fig. 4. Variation of the ion pair peak frequency with inverse temperature for (PEO)9LiTFSI/10 wt.% Al2O3 with different filler surface groups corresponds to: (a) heating; and (b) cooling.
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on cooling (Fig. 2(b)) also follow the same trend although the curves for basic and neutral appears to overlap at some temperatures. The highest conductivity enhancement as well as the highest increase in fm is observed for acidic alumina followed by basic, neutral, weakly acidic and filler free systems. From these results, it can be inferred that the major contribution to the observed conductivity enhancement due to the addition of nano-porous alumina fillers comes from the mobility increase in the electrolyte. These results seem to be in agreement with the recent findings reported by Best et al. based on Raman measurements on similar systems [9]. The intensity of the ion pair peak in the oƒcorr spectrum can be taken as a measure of the concentration of ion pairs in the electrolyte [45]. However, comparison of peak intensity at different temperatures is not trivial due to several factors. The intensity is not only a function of the concentration of dipoles but also of temperature. It is possible to obtain a rough estimate of the variation of the number of dipoles with temperature by multiplying the peak intensity by the absolute temperature. This has been done in the present case for samples with all four types of filler particles and the results show that the number of dipoles increases only marginally with temperature. However, at a given temperature, the intensity of the oƒcorr peak may be used for comparison. Although a slight increase in the peak intensity can be observed in samples with all four types of filler particles, a definitive conclusion cannot be made due to the fact that the oƒcorr peak intensity is very sensitive to the correction procedure used to compensate for the dc conductivity. Anyhow, for the present system, the general conclusion can be made that, the number of ion pairs has not changed significantly due to the addition of the filler particles. However, in the case of the system (PEO)20LiClO4 with 10 wt.% nano-sized Al2O3 fillers, Croce et al. have attributed the observed conductivity enhancement to salt dissociation creating more Li ions [8]. Fig. 5 shows the variation of log s and log fm of the ion pair peak with inverse temperature for the nanocomposite electrolyte with acidic Al2O3 fillers for the heating run. It should be noted that the vertical scales in both figures (a and b) have been set to three decades in order to allow comparison. A comparison of the two curves clearly shows that the temperature dependence of the ion pair peak frequency follows very closely the temperature dependence of the conductivity, both obeying the VTF type behaviour with essentially the same rate of increase with temperature. This, we believe, is an important observation concerning the mechanism of ion transport in this nano-composite polymer electrolyte, since it suggests that the local environment in which the ion pair relaxation occurs and the ionic transport takes place is the same.
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Fig. 5. Comparison of the variation of ionic conductivity and ion pair peak frequency with inverse temperature for (PEO)9LiTFSI/10 wt.% Al2O3 (acidic) measured on heating.
In order to illustrate the dependence of conductivity and ion pair frequency on the pH value of the filler surface groups, we have plotted these quantities (at 25 8C) in Fig. 6. It is clear from this figure also that a correlation exists between the conductivity and the ion pair frequency for the four types of filler surface groups. In order to gain a better understanding of the ionic conductivity enhancement observed in this system, we shall try to make use of our results on thermal, electrical conductivity and dielectric relaxation measurements along with some of the ideas proposed and developed by the other groups, especially by Wieczorek et al., Scrosati et al., and Best et al., as described earlier. As discussed earlier, our dielectric relaxation results suggests that the major contribution to the observed conductivity enhancement due to nano-sized fillers comes from the increased mobility rather than from the increased charge carrier concentration resulting from the dissociation of ion pairs. 3.4. The mechanism of conductivity enhancement In a PEO /LiX polymer electrolyte, at any instant of time, a Li ion is co-ordinated to about four ether
Fig. 6. Dependence of ionic conductivity and ion pair peak frequency at 25 8C on the pH value of the filler surface groups of (PEO)9LiTFSI/10 wt.% Al2O3.
oxygen atoms of the same or different PEO chains. As the ion moves along the polymer chain, old co-ordination links break and the new links form. This cation motion is facilitated by the flexing of the polymer chain segments producing a strong coupling between the segmental motion of the polymer and the ionic transport. As the polymer segments are flexible in the amorphous phase, the polymer electrolyte generally has a higher conductivity in the amorphous phase. The anion is only weakly bound to the polymer chain but the flexibility of the polymer chain is rate determining also for the anion transport. This situation is pictorially illustrated in Fig. 7(a). The different types of alumina grains we have used in this work have the same particle size (104 mm) and the same pore size (5.8 nm). Due to their ultra fine nanostructure, these nano-porous grains are expected to provide effectively a large surface area with a large number of surface groups for the surface interactions to take place. However, for simplicity, we assume that this large nano-porous surface area may be compared to a large number of nano-sized alumina grains. In the case of nano-composite polymer electrolytes incorporating Al2O3 grains, as suggested by Wieczorek et al. and Croce et al., the dependence of the conductivity enhancement on the nature of the filler surface group may be satisfactorily explained in terms of the Lewis acid /base type interactions involving different types of surface groups [7,8,23,25]. Using these ideas, we shall now try to explain the effect of different types of surface groups in conductivity enhancement in the present PEO /LiTFSI/10 wt.% Al2O3 system. Fig. 7(b) illustrates the case with acidic Al2O3 filler. We denote a grain of alumina with acidic surface groups as consisting of OH polar groups. As suggested by Wieczorek et al. for the system PEG /LiClO4 /Al2O3, based on FTIR studies, the anions have a larger affinity towards Al2O3 surface acid groups than cations [19]. Due to the strongly polarisable nature of the TFSI anion, a stronger affinity can be expected between the TFSI anion and the Al2O3 surface acid groups. During their migration under an external potential, the polar TFSI anions dissociated from the salt can interact with these polar OH groups via transient hydrogen bonding. The situation is somewhat similar to the migration of cations through intermittent co-ordination to ether oxygens. The anions will remain H-bonded to the OH groups only temporarily. In the filler free electrolyte the anion motion is only assisted to some extent by the segmental flexibility of the polymer chains. However, now the anions will have additional sites to occupy during their migration effectively creating new favourable conduction pathways in the vicinity of the filler grains. The anion migration is therefore facilitated and streamlined compared to the filler free situation. This we believe is reflected as an increase in mobility as inferred
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Fig. 7. A pictorial model to illustrate the proposed mechanism of ionic conductivity enhancement in the nano-composite polymer electrolyte (PEO)9LiTFSI/10 wt.% Al2O3: (a) filler free; (b) acidic; (c) basic; (d) neutral; and (e) weakly acidic. Note that for simplicity the OH groups are marked with H in the figure.
from our dielectric relaxation data as described earlier. According to our model, the involvement of the polymer chain in ionic transport is rather unaffected due to the addition of the filler in agreement with the essentially unchanged Tg value observed in our DSC results. Cation migration may remain essentially unaffected by the presence of these acidic fillers. Furthermore, according to this model, the number of ion pairs also does not change appreciably due to the presence of the filler. The observed highest conductivity enhancement due to the filler with acidic surface groups, therefore, appears to come from the above mechanism, which is experimentally seen as an increase in the effective average mobility of carriers. For the filler free
(PEO)8LiTFSI system at 67 8C, Gorecki et al. have reported an anionic mobility of 1.1 /109 m2 s 1 V 1 and a cationic and anionic transport numbers of 0.57 and 0.43, respectively [48]. It is interesting to note the substantial anionic conductivity in this system, despite the large sized, flexible TFSI anion. Although we have not attempted to measure the transport numbers for the present system with nano-sized fillers, we expect it to be higher than the filler free system due to the increased mobility. The system with Al2O3 grains with basic surface groups is illustrated in Fig. 7(c). In an alumina grain with basic surface groups the strong basic centres are due to oxygens at the surface of the alumina. FTIR
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Fig. 7 (Continued)
measurements on the PEG /LiClO4 /Al2O3 system have suggested that the basic surface groups in alumina interact with Li cations [22]. In our system, we propose that during their migration, the Li cations form temporary, weaker transient bonds with the oxygen atoms of the surface basic groups of alumina grains, very much similar to the co-ordinated transient links they form with ether oxygens. This provides extra sites for the cations, which would, otherwise, be migrating only via forming and breaking co-ordination links with ether oxygens of the PEO chains. During the cationic transport process the bonds between Li ions and oxygens in alumina surface groups are also subjected to breaking and making. Effectively an additional favourable conduction pathway for the cations is created in the vicinity of the basic alumina grains, which may involve both oxygen sites on alumina surface as well as ether oxygens. This improved ionic transport, we believe, is reflected as an increase in the average mobility of the carriers as seen from our dielectric measurements. The degree of conductivity enhancement brought about by the acidic alumina grains is expected to be higher compared to the enhancement by basic alumina grains because the acidic alumina increases the anionic contribution to the conductivity. In the filler free system, the anionic migration is only weakly influenced by the polymer whereas the cationic migration is already assisted by making/breaking of links with ether oxygens.
Therefore, acidic alumina grains are expected to give rise to the highest conductivity enhancement through anionic contribution. In all cases, we assume that the concentration of carriers, both cations and anions, remains essentially the same. The case of neutral alumina is shown in Fig. 7(d). Here, each alumina grain surface can be expected to have about equal number of acidic and basic sites. The acidic sites are expected to promote the anion migration and the basic sites the cation migration, as already explained earlier. However, now the anions are transiently ‘bonded’ to acidic sites and cations to basic sites. Because the sites are randomly distributed on the surface it is likely that a cation and anion, both transiently ‘bonded’ are in close proximity so that cation /anion interactions occur and decrease the mobility of both species, compared to the pure acidic and basic cases. Fig. 7(e) shows the case of alumina fillers with weakly acidic surface groups. Here, each alumina grain is expected to have more acidic than basic groups exposed from its surface. Therefore, there are relatively fewer additional sites available for the Li ions. Also, in this case, the random distribution of these surface groups is likely to decrease the conductivity even more. Both these factors may contribute to reduce the conductivity enhancement more effectively for the samples with weakly acidic alumina.
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4. Conclusion The results of the thermal, electrical conductivity and dielectric relaxation measurements on the (PEO)9LiTFSI/10 wt.% Al2O3 nano-porous polymer electrolyte system reported in this paper demonstrates the following. The filler particles do not interact directly with the PEO chains as revealed by the essentially unchanged Tg values suggesting that the main chain dynamics governing the ionic transport is unaffected. The presence of filler particles enhances the ionic conductivity substantially, and the degree of enhancement depends on the nature of the filler surface group and decreases as acidic /basic /neutral/weakly acidic. The peak observed in the oƒcorr spectrum has been attributed to ion pair dielectric relaxations. As revealed from the intensity of the ion pair peak, the number of ion pairs remains essentially unchanged due to the presence of the filler particles and also with increasing temperature ruling out the possibility of ion pair dissociation contributing to the conductivity enhancement. The ion pair peak frequency follows temperature dependence very much similar to the conductivity, suggesting that the increased mobility is largely responsible for the observed conductivity enhancement. A qualitative model based on Lewis acid / base type interactions between filler surface groups and cations/anions has been proposed in order to explain the difference in conductivity enhancement caused by different surface groups. It is proposed that the Lewis acid /base surface groups interact with cations and anions and provides additional sites creating favourable conduction pathways for the migration of ions, effectively enhancing the ionic mobility.
Acknowledgements PARDJ and MAKLD wish to thank the International Programmes in the Physical Sciences (IPPS), Uppsala University Sweden for the award of fellowships to work in Sweden. We would also like to thank the Swedish Research Council for financial support.
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