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Electrochimica Acta 54 (2009) 5660–5665

Contents lists available at ScienceDirect

Electrochimica Acta

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ighly conductive ionic liquid based ternary polymer electrolytesbtained by in situ photopolymerisation

zabela Stepniak ∗,1, Ewa Andrzejewskaoznan University of Technology, Faculty of Chemical Technology, Piotrowo3, PL 60-965 Poznan, Poland

r t i c l e i n f o

rticle history:eceived 6 February 2009eceived in revised form 27 April 2009ccepted 4 May 2009

a b s t r a c t

New ternary polymer electrolytes based on a commercial cross-linking resin dianol diacrylate (DDA),tetramethylene sulphone (TMS) as a compatibilizing solvent and ionic liquids (ILs) were prepared byin situ photopolymerisation. The electrolytes containing two polymer-to-TMS-to-IL ratios, 20:30:50 and20:20:60 by weight, respectively and six various ILs were investigated. The obtained materials were flex-

vailable online 13 May 2009

eywords:onic liquidsolymer electrolyteshotopolymerisation

ible, self-standing with good mechanical properties and a long-term stability. They showed high ionicconductivities, in the range of ca. 7 × 10−3 to 3 × 10−2 S cm−1 at 25 ◦C. A very important result is that theconductivities of all the prepared polymer electrolytes exceeded the conductivities of the correspond-ing neat ILs by 2–3.5 times. The temperature dependence of the ionic conductivity correlates with VTFequation. All the materials prepared were characterized by a broad electrochemical stability window

onductivityTF

(3.3–3.7 V).

. Introduction

Ionic liquids (ILs) are compounds consisting of an organic cationnd an organic or inorganic anion, with melting points below theoiling point of water. When they are liquid at room temperature,hey are called room temperature ionic liquids (RTILs). ILs havenusual properties including non-volatility, non-flammability, high

onic density, conductivity, chemical and electrochemical stabil-ty, etc. [1]. These attractive properties have led many scientists toxploit ILs as electrolytes in batteries [2–5], in double-layer capac-tors [6–8], fuel cells [9,10], dye-sensitive solar cells [11–15] orumerical displays with conducting polymers [16].

In contrast to conventional polymer electrolytes, which are solidolutions of electrolyte salts in polymers (e.g. polyethylene oxides)17], the IL-based polymer electrolytes belong to gel polymer elec-rolytes, which consist of polymer matrix and liquid electrolyte. Theatter serves both as the charge carrier and a plasticizer. Sometimesnon-volatile solvent (like carbonates) is added to plasticize and toissolve IL in the system [18].

IL-based solid polymer electrolytes can be prepared by:

(i) dissolution of a polymer and an IL in a volatile solvent and thenvaporization of the latter [19,20],

∗ Corresponding author. Tel.: +48 616652317; fax: +48 616652571.E-mail address: izabela.stepniak@put.poznan.pl (I. Stepniak).

1 ISE member.

013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.05.004

© 2009 Elsevier Ltd. All rights reserved.

(ii) in situ polymerisation of a monomer-IL mixture (alternativelyionic liquids with vinyl groups):• induced by thermal radical initiators,• induced by UV (or visible) light in the presence of suitable

photoinitiators.

Preparation of polymer electrolytes in the polymerisationprocess induced by thermal initiators usually needs relativelyhigh temperatures (∼70–80 ◦C) and long reaction times (2–24 h)[21–26]; such conditions are the main drawbacks of the method.On the other hand, the light-induced polymerisation seems tobe especially useful due to the short polymerisation time (min-utes or seconds) and low reaction temperature (room or ambient).Moreover, it enables the full control of the process because the poly-merisation starts when the light is switched on and stops when thelight is cut off. This technique is still one of the most rapid growingpolymerisation methods as it offers a broad application spectrum inthe industrial area, including coatings, printing inks, stereolithogra-phy, dental materials, optical devices, medicine, microelectronics,etc. [27]. Only in several works (in this number those from ourgroup) photochemical reactions were used for preparation of solidpolymer electrolytes [28–37]. IL-containing solid polymer elec-trolytes were based mainly on poly(meth)acrylates.

The usage of popular methacrylate and acrylate monomers often

results in poor mechanical properties of the resulting conductingfilm [38]. Another difficulty in application of the in situ method islimited miscibility of monomers with ILs.

The conducting film can be useful in practice when its ionicconductivity is above 10−3 S cm−1. For the IL-based polymeric

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I. Stepniak, E. Andrzejewska / Elec

onducting systems prepared by in situ polymerisation, conduc-ivities in the range of 10−6 to 10−3 S cm−1 were reported [39–41].he same order of conductivity revealed solid polymer electrolytesbtained by means of photopolymerisation method. Our inves-igations focused on the preparation of IL-based photocurableystems forming flexible films with high ionic conductivity andood mechanical properties. The preliminary results were pub-ished earlier [32]; the present article is the first full paper in theeries presenting the results of our studies.

To obtain good flexibility and sufficient mechanical strengthf the polymer electrolyte we decided to test commercial mul-ifunctional resins as the monomers. The first resin selected bys, dianol diacrylate (DDA, trade mark Actilane 420 from Akzoobel) is widely used in production of offset lithographic inks oretal decorating and paper and board applications [42]. Six ILsith conductivities in the range of 3.3 × 10−3 to 14.5 × 10−3 S cm−1

onsisting of substituted imidazolium or pyrrolidonium cationsnd two types of anions were selected as the conducting media.owever, the miscibility of DDA with the selected ILs was poornd to obtain homogeneous solutions before the polymerisationn organic solvent had to be added. In this work it was tetram-thylenesulphone (TMS), which is expected to form stable systemsn combining with the polymer and ILs. TMS is a polar solvent

ith high boiling (287 ◦C) and flash (177 ◦C) points, which makest acceptable for applications in electrochemical devices. As men-ioned earlier, highly boiling polar solvents, like cyclic carbonates orutyrolactone, are used as solvents for ILs in polymer electrolytes18], but they can also be applied as additives to ionic liquids inithium or graphite–lithium electrodes to improve their properties43].

The aim of the work was to investigate electrochemicalroperties (ionic conductivity with its temperature dependence,tability windows) of three-component cross-linked solid polymerlectrolytes (poly-DDA)-TMS-IL prepared by the in situ photopoly-erisation method, containing a fixed polymer and (TMS + IL)

ontents, at two different TMS:IL ratios. It should be stressed thatapers devoted to electrochemical properties of IL-based solidolymer electrolytes focused mostly on linear or slightly cross-

inked polymers, which may results from the assumption on highhain mobility in such systems. Systems of this type, based on

copolymer of a monomethacrylate with small amounts of aimethacrylate were obtained also by means of photopolymerisa-ion [30,35]. Another cross-linked polymer electrolyte, based onpolyurethane oligomer, was obtained by thermal curing [38]. In

he present paper we will show that the use of a properly selectedntirely cross-linking resin will enable to obtain in a UV-initiatedrocess a highly conductive and mechanically resistant solid poly-er electrolyte with an exceptional electrochemical behaviour.

. Experimental

.1. Materials

Dianol diacrylate (DDA) (Actilane 420), a highly viscousonomer with viscosity of 1200–1500 mPa s at 25 ◦C [42], was a

ift from Akzo Nobel. The structure of this monomer was not given;t is probably ethoxylated bisphenol A diacrylate (the number ofthoxy groups is unknown). Before use it was dried under vacuum atlevated temperature (80oC). The photoinitiator, 2′,2′-dimethoxy-′-phenylacetophenone, was the product of Ciba ChemicalsIrgacure 651). TMS (viscosity 10 mPa s at 30 ◦C) was purchased

rom Merck and was purified before use according to the procedureescribed in Ref. [44]. Ionic liquids: 1-ethyl-3-methylimidazoliumetrafluoroborate ([EMIm][BF4]), 1-butyl-3-methy-limidazoliumetrafluoroborate ([BMIm][BF4]), 1-ethyl-3-methylimidazoliumis(trifluoromethane-sulfonyl)imide ([EMIm][NTf2]), 1-butyl-3-

mica Acta 54 (2009) 5660–5665 5661

methylimidazolium bis(trifluoromethanesulfonyl) imide ([BMIm][NTf2]), 1-methyl-1-propylpyrrolidinium bis(trifluoromethane-sulfonyl)imide ([MPPy][NTf2]), 1-butyl-1-methylpyrrolidiniumbis(trifluoromethanesulfonyl)imide ([BMPy][NTf2]), all of highpurity (≥99.0%, water content and halide content ≤100 ppm) weredelivered by Merck.

All the ILs were dried at 60 ◦C under vacuum before use.

2.2. Preparation of polymer electrolytes

The monomer and the photoinitiator (1% based on the entiremixture) were dissolved in the appropriate TMS-IL mixture. Thesolution obtained was spread between two PET foils and exposedto UV-light (400 W medium pressure Hg lamp, light intensity2 mW cm−2) in an argon atmosphere for 10 min. The thickness ofthe solid polymer electrolyte film was ca. 0.05 cm.

2.3. Methods

2.3.1. Ionic conductivity measurementsElectrochemical measurements were performed with the aid

of electrochemical system �AutoLab FRA III, EcoChemie – TheNetherlands. The ionic conductivity (�) of ILs and IL-containingpolymer electrolytes was measured by impedance method using animpedance analyser over the frequency range from 1 Hz to 99 kHzand the potential amplitude 10 mV. The IL-polymer electrolyteswere sandwiched between two platinum electrodes (a surface areaof the working electrode was 1.77 cm2). The measurements werecarried out in the temperature range of −5 ◦C to +85 ◦C. The ionicconductivities of the prepared polymer electrolytes were calculatedusing the following equation:

� = d

SRb(1)

where d is the electrolyte thickness (cm), S is the activity area (cm2),and Rb is the resistance of the electrolyte (�).

Ionic conductivities of the ILs were measured at 25 ◦C in a glassconductivity cell between two platinum electrodes.

2.3.2. Electrochemical stability measurementsThe measurements of the electrochemical stability of the

IL-polymer electrolytes were carried out at 25 ◦C by cyclic voltam-metry, with a 10 mV s−1 potential sweep rate and using athree-electrode cell. Platinum electrodes were used as counter andworking electrodes (geometric surface area of the working elec-trode was 0.0177 cm2). As a reference electrode the silver/silver(I)couple in dimethylsulphoxide (Ag/Ag+ 0.01 mol dm−1 in DMSO;E(Ag/Ag+) = 3.7 V versus Li/Li+) was applied.

2.3.3. Viscosity measurementsViscosities of the investigated ILs were measured at 25 ◦C using

a Brookfield Digital Viscometer model DV-II at the share rate10–160 rpm (no dependence on the share rate was observed).

3. Result and discussion

The ionic conductivities (�) and viscosities (�) of the ILs used inthis work are given in Table 1.

As mentioned in the Introduction part, the acrylate resin usedby us was poorly miscible with the selected ILs and the additionof the compatibilizing solvent, TMS, was necessary. Preliminary

examinations of various polymer-to-(TMS + IL) ratios showed thatthe lower polymer content is, the higher is the conductivity ofthe solid electrolyte. Thus, for further investigations, we selectedconducting films containing only 20 wt.% of the polymer. In thisway, in the initial monomer-TMS-IL ternary mixtures the monomer

5662 I. Stepniak, E. Andrzejewska / Electrochimica Acta 54 (2009) 5660–5665

Table 1Conductivities and viscosities of the ILs used at 25 ◦C.

IL � (mS cm−1) � (mPa s)

[EMIm][BF4] 14.5 45.1[BMIm][BF4] 3.3 77.1[[[[

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EMIm][NTf2] 11.0 34.2BMIm][NTf2] 3.5 45.6MPPy][NTf2] 2.5 63.9BMPy][NTf2] 3.0 73.9

ontent was 20 wt.%; the remaining 80 wt.% of the formulation com-rised of TMS and IL in two proportions (30:60 and 20:60). Thiseans that the weight ratios of the components in the investi-

ated ternary solid polymer electrolytes (polyDDA)-TMS-IL were0:30:50 and 20:20:60, respectively In the case of [EMIm][BF4] the

nitial mixture with the component ratio 20:20:60 was not compat-ble and the corresponding solid electrolyte could not be prepared.espite the low polymer content, the resulting conducting filmsere self-standing and flexible with mechanical strength sufficient

or practical applications (Fig. 1). All the films were opaque indi-ating a phase separation. The inhomogeneous, porous structure ofhe polymer electrolyte is clearly seen on an exemplary SEM imageFig. 2).

It should be stressed that the polymer gels were stable even afterne year of storage and did not show any syneresis.

.1. Ionic conductivity

The � values of the prepared (polyDDA-TMS-IL) solid elec-rolytes along with the conductivities of the ILs used (0:0:100omposition), all measured at 25 ◦C, are shown in Fig. 3.

As can be seen, the conductivities of the prepared polymerlectrolytes are high and lie in the range of 7 × 10−3 S cm−1 to.9 × 10−2 S cm−1; these values are much higher than the valueseported for conventional polymer electrolytes at ambient tem-eratures. It was suggested [26] that the high ionic conductivity

f polymer gels is due to the high self-dissociating and ion-ransporting abilities of the IL component and the decoupling ofhe ion transport from polymer segmental motion. Obviously, theigher was the conductivity of the IL applied, the higher was theonductivity of the solid electrolyte. However, the conductivities

ig. 1. Self-standing film of the (polyDDA)-TMS-IL (20:30:50 weight ratio) solidolymer electrolyte prepared by photopolymerisation.

Fig. 2. SEM image of the polyDDA-TMS-[BMIm][BF4] system (in 20:30:50 weightratio).

of the polymer electrolytes obtained by us are higher than thosereported in the literature for other polymer gels containing ILswith similar conductivities. Our preliminary investigations showedthat the lower is the TMS:IL ratio (within the frame of entire 80%content of the liquid component), the higher is the sample conduc-tivity. The rapid increase in conductivity with the rising amountof IL was observed till 50% of IL concentration; further rise from50 wt.% to 60 wt.% increased the conductivity only slightly. This isin agreement with observation that a high content of IL may not beconnected with the improvement in conductivity of an IL-solventmixture [41].

A striking and very important result of our investigations is thatfor each solid polymer electrolyte its ionic conductivity was sig-nificantly higher that the conductivity of the corresponding neatIL. With one exception ([EMIm][NTf2]) the � values of the ternarypolymer electrolytes exceeded the � values of ILs even 2–3.5 times.A substantial increase in polymer electrolyte conductivity com-pared with IL conductivity was observed also at lower IL contents. Aqualitatively similar phenomenon (but much less pronounced) wasreported in Ref. [45] for a polymer electrolyte consisting of 12.9 wt.%

of polyacrylonitrile, 48.3 wt.% of [EMIm][BF4] and 38.8 wt.% ofTMS; only slight increase in the conductivity was noticed, from13.8 × 10−3 S cm−1 (taken from lit.) to 15 × 10−3 S cm−1 (measuredby Authors). Our result is in contrast with the statement that thepolymer phase does not bring any remarkable contribution to the

Fig. 3. Ionic conductivities of the ternary polymer electrolytes polyDDA-TMS-IL(component weight ratios 20:30:50 and 20:20:60) and corresponding neat ionicliquids (0:0:100) at 25 ◦C.

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ig. 4. Ionic conductivities of neat ionic liquids, TMS-IL mixtures (30:50 weightatio), ternary solutions DDA-TMS-IL before the polymerisation and ternary polymerlectrolytes polyDDA-TMS-IL at 25 ◦C (component weight ratios 20:30:50).

verall conductivity of the material due to location of the motion inhe immobilized microscopic phase [46]. To the best of our knowl-dge, our work is the first report on preparation of IL-based solidolymer electrolytes, which conductivity exceeds significantly theonductivity if neat ILs.

It was often observed that a dilution of ILs with an aprotic solventan lead to an increase in ionic conductivity of the resulting solutionue to the strong decrease of viscosity, which reduces the ion–ionssociation in IL [41] (theory of the influence of system viscosityn its conductivity is described in Ref. [47]). We observed a simi-ar phenomenon for four of the TMS-IL mixtures, those containingBMIm][NTf2], [MPPy][NTf2], [BMPy][NTf2] and [BMIm][BF4]. Con-uctivities of the TMS-ILs mixtures in the same ratio as in the solidlectrolyte are shown in Fig. 4. It is interesting that the conduc-ivities of the mixtures containing [EMIm][BF4] and [EMIm][NTf2]ere lower than the conductivities of the neat ILs. This may suggest

hat the dilution of these two ILs does not result in dissociation ofossible ionic aggregates, despite the reduction of the viscosity ofhe system (which for these two ILs is the lowest).

The addition of the monomer of very high viscosity, as DDA,as to increase the viscosity of the DDA-TMS-IL ternary system andhould reduce in this way its conductivity. This was really observedn our case: the conductivities of the ternary solutions before theolymerisation were markedly lower than the conductivities of theorresponding neat ILs (Fig. 4).

The strongest reduction of the conductivity of the monomer-MS-IL mixtures compared with neat ILs conductivities wasbserved in the case of ILs containing [EMIm]+ cation. It can bexplained by the fact that these ILs have lowest viscosities and high-st conductivities (see Table 1), thus, they will be most sensitive tony changes in viscosity. On the other hand, the strongest increasen solid electrolyte conductivity compared with IL conductivity wasoted for ILs containing [BF4]− anion (ILs with the highest polarity).

However, as mentioned above, the conductivities of the resultingolymer electrolytes exceeded those of ILs significantly. Fur-hermore, they exceeded even the conductivities of the TMS-IL

ixtures. This unexpected result is difficult to explain. One couldpeculate that the increase in ionic conductivity of the polymerlectrolyte over the value of this parameter for the neat IL resultsrom the polymer separation in the polymer-TMS-IL systems, whicheads to the formation of the TMS + IL liquid electrolyte withnhanced conductivity (due to a decrease of its viscosity), embed-

ed in the porous polymer matrix. However, this cannot explainhe fact that the conductivities of polyDDA-TMS-IL ternary systemsre higher than those of TMS-IL binary mixtures. Thus, we need toonsider the influence of the polymer matrix.

mica Acta 54 (2009) 5660–5665 5663

It was suggested for instance, that the existence of a maxi-mum on the dependence of carrier numbers on mole fraction of[EMIm][NTf2] in poly(methyl methacrylate) matrix can be associ-ated with the possibility to some specific interactions between thepolymer matrix and either cation or anion, which should decreasethe tendency to form ion clusters or aggregates in the IL and increasethe number of ion carriers in the polymer gel [48]. Reduction ofaggregation in ILs by interactions with the polymer matrix cannotrather explain our result, because similar interactions as betweenthe polymer and the IL occur also between the monomer and theIL. One of the possible explanations is a change in the conductiv-ity mechanism. This, however, needs more investigations; they arenow in progress.

The temperature dependence of the ionic conductivity of theprepared polymer electrolytes was measured in the range of −5 ◦Cto +85 ◦C. When an ion transport involves an intermolecular hop-ping, the conductivity shows Arrhenius temperature behaviour.However, when the ion transport is dominated by the mobil-ity of the solvent molecule, the conductivity will correlate withVogel–Tamman–Fulcher (VTF) equation [48,49]:

�T1/2 = A exp( −B

T − T0

)(2)

where A is related to the number of charge carriers in theelectrolyte, B is related to the activation energy of ion transportassociated with the configurational entropy of the polymer chains,and T0 is a ideal glass transition temperature at which the conduc-tivity goes to zero.

The Arrhenius plots of conductivity–temperature data in thetemperature range of −5 ◦C to +85 ◦C for the prepared solid polymerelectrolytes are shown in Figs. 5a and 6a. The plots are not lin-ear and exhibit convex upward curved profiles, what is commonlyobserved for amorphous materials [50]. Thus, the conductivity can-not be correlated with temperature with the Arrhenius equation.However, the data can be fitted with VTF equation very well (Figs.5b and 6b), which indicates that the VTF interpretation is more validthan the Arrhenius type in describing the conductivity-temperaturedependence of our ternary polymer electrolytes.

The conductivity increases with temperature rise. This well-known phenomenon results from faster movement of the ionswhen temperature increases and this causes an increase in theionic conductivity. The solid polymer electrolytes of lower con-ductivity reached the conductivity level of 10−2 S cm−1 at 45 ◦C,whereas the polymer electrolytes containing ILs with [EMIm]+

cation exceeded this threshold � value just at 0 ◦C ([EMIm][BF4])or 25 ◦C ([EMIm][NTf2]).

The excellent agreement of the temperature dependence of ionicconductivity with the VTF equation suggests a solvent-assistedionic conductivity mechanism [24]. Thus, both the interactions ofILs with the polymer as well as with the polar solvent (TMS) seemto influence conductivity values. Interactions of the cations of ILswith carbonyl groups of methacrylates have been described in thelitearture [50,51].

3.2. Electrochemical stability window (EW)

The electrochemical stability window was evaluated for eachpolyDDA-TMS-IL ternary electrolyte. This parameter is very impor-tant for applications of polymer electrolytes as the potentialconductive media in lithium batteries, capacitors, fuel cells, andsolar cells. The main advantage of using ILs in modern chemical

son with aqueous and several non-aqueous systems [39]Electrochemical stability windows were measured by cyclic

voltammetry toward Ag/Ag+ as the reference electrode and plat-inum as the working electrode. The current threshold for both the

5664 I. Stepniak, E. Andrzejewska / Electrochimica Acta 54 (2009) 5660–5665

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ig. 5. Conductivity–temperature plots for polyDDA-TMS-IL (20:30:50 weight ratio)ystems: (a) Arrhenius plots, (b) VTF plots.

athodic and anodic potential limits was set when the reduction or−2 −1

xidation current reached 1 mA cm at the scan rate of 10 mV s .

he obtained values of the anodic and catodic limits are summa-ized in Table 2. They depend both on the reduction and oxidationotentials of the conducting ions as well as on the electrochem-

cal properties of all the other components, like polymer matrix

able 2otential limits of the polyDDA-TMS-IL ternary electrolytes on platinum versus Ag/Ag+, at

olid polymer electrolyte Polymer-TMS-IL (wt.%)

olyDDA-TMS-[EMIm][BF4] 20:30:5020:20:60

olyDDA-TMS-[BMIm][BF4] 20:30:5020:20:60

olyDDA-TMS-[EMIm][NTf2] 20:30:5020:20:60

olyDDA-TMS-[BMIm][NTf2] 20:30:5020:20:60

olyDDA-TMS-[MPPy][NTf2] 20:30:5020:20:60

olyDDA-TMS-[BMPy][NTf2] 20:30:5020:20:60

Fig. 6. Conductivity–temperature plots for polyDDA-TMS-IL (20:20:60 weight ratio)systems: (a) Arrhenius plots, (b) VTF plots.

and the solvent. As can be seen, the EWs of all the ternary poly-

mer electrolytes are at the level of 3.3–3.7 V, which is sufficient forapplication in electrochemical devices (e.g. in capacitors). Similarresults were reported by Li et al. [52] for polyHEMA-[BMIm][BF4]and McEwen et al. [6] and Song and co-workers [53] for liquidsystems based on IL.

25 ◦C.

Cathodic limit (V) Anodic limit (V) EW range (V)

−1.8 1.7 3.5– – –

−1.8 1.7 3.5−1.7 1.6 3.3

−1.8 1.8 3.6−1.7 1.8 3.5

−1.9 1.7 3.6−1.8 1.9 3.7

−2.1 1.7 3.7−1.7 1.9 3.6

−2.1 1.6 3.7−2.0 1.7 3.7

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. Conclusions

New solid polymer electrolytes based on a commercial resinDDA), TMS as the compatibilizing solvent and ILs (20:30:50nd 20:20:60 weight ratio, respectively) were prepared by thehotopolymerisation method. All the materials showed a phaseeparation. Despite the low polymer content, they were flexible,elf-standing, with proper mechanical strength and the long-termtability. Their ionic conductivities were high, on the order of0−2 S cm−1 at 25 ◦C. A very important result was that the con-uctivities of all the prepared polymer electrolytes exceeded theonductivities of the corresponding neat ILs by 2–3.5 times andere higher than the conductivities of the corresponding TMS + ILixtures. The highest conductivity (2.92 × 10−2 S cm−1 at 25 ◦C)as obtained for polyDDA-TMS-[EMIm][BF4] (20:30:50 wt.%)

ystem. The electrolytes of lowest conductivity reached the0−2 S cm−1 level at about 45 ◦C. The temperature dependence ofhe ionic conductivity showed the VTF behaviour. All the materialsrepared were characterized by a broad electrochemical stabilityindow (3.3–3.7 V). To sum up, the high ionic conductivity, broad

W, sufficient mechanical stability and good adhesive characteris-ics of the polymer electrolytes obtained make them promising forpplications in advanced electrochemical devices.

cknowledgement

This work was supported by research grant 3T08E 028 29 forears 2005–2008

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