Electrochimica Acta 45 (2000) 12651270

Highly conductive polymer electrolytes prepared by in situpolymerization of vinyl monomers in room temperature

molten salts

Akihiro Noda, Masayoshi Watanabe *Department of Chemistry and Biotechnology, Yokohama National Uni6ersity, 79-5 Tokiwadai, Hodogaya-ku,

Yokohama 240-8501, Japan

Received 27 November 1998; received in revised form 19 April 1999

Abstract

In order to achieve highly conductive polymer electrolytes, room temperature molten salts with high ionicconductivity have been explored, and in situ polymerization of vinyl monomers in the molten salts have beenconducted. It is found in this study that 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and 1-butylpyri-dinium tetrafluoroborate (BPBF4) form room temperature molten salts, and these molten salts exhibit high ionicconductivities of 2102 and 3103 S cm1 at 30C, respectively. Certain vinyl monomers can be polymerizedin the molten salts by radical polymerization. In situ polymerization of suitable vinyl monomers gives transparent,mechanically strong and highly conductive polymer electrolyte films. For example, 2-hydroxyethyl methacrylatenetwork polymers in which BPBF4 is dissolved exhibit an ionic conductivity of 10

3 S cm1 at 30C. 2000 ElsevierScience Ltd. All rights reserved.

Keywords: Polymer electrolyte; Room temperature molten salt; Ionic conductivity; 1-Ethyl-3-methylimidazolium tetrafluoroborate;1-Butylpyridinium tetrafluoroborate

www.elsevier.nl:locate:electacta

1. Introduction

Conventional ion-conducting polymers, likepolyether-based polymer electrolytes, are solid solutionof electrolyte salts in polymers [15]. Ionic motion inthese polymer electrolytes is coupled with the localsegmental motion, and the increases in carrier-ion den-sity and the mobility are inconsistent, because of theincrease in glass transition temperature (Tg) with in-creasing ionic concentration. These facts are reflectedby the appearance of maximum ionic conductivity inpolyethers with increasing salt concentration.

On the other hand, in certain saltpolymer systems,in the range of high salt concentrations, the ionic

conductivity increases again and Tg decreases again.These electrolyte salts are characterized by low Tg andTm, and form supercooled liquids or molten salts withhigh conductivity at room temperature. In the saltpolymer systems, so-called polymer-in-salt electrolytes[6,7], the number of carrier ions and their mobilityincrease with increasing the electrolyte concentration.As a result, a high ionic conductivity that is not cou-pled with the segmental motion of the polymers can beexpected to be achieved.

Certain pyridinium chlorides [810] or certain imida-zolium chlorides [11,12] react with aluminum chlorideto form molten salts at room temperature, so-calledroom temperature molten salts. The room temperaturemolten salts exhibit high ionic conductivity, wide elec-trochemical window, non-volatility, thermal stability* Corresponding author.

0013-4686:00:$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.

PII: S0013 -4686 (99 )00330 -8

A. Noda, M. Watanabe : Electrochimica Acta 45 (2000) 126512701266

and nonflammability. The polymer-in-salt electrolytes,which are made by dissolving compatible polymers inthe chloroaluminate molten salts, have already beenreported [6,13] to afford polymer electrolytes with highionic condutivity as well as rubbery electrolyte prop-erty. However, chloroaluminate molten salts have quitehigh moisture sensitivity, and the decomposed productby hydrolysis, HCl, is highly corrosive. Recently, non-chloroaluminate room temperature molten salts havebeen explored. It is known that certain combinations ofimidazolium cations and bulky and soft anions formionic liquids at or near room temperature [1421].Polymer gel electrolytes which contain non-chloroalu-minate molten salts are also reported [22,23].

The objectives of this study are to find new roomtemperature molten salts and compatible polymers withthe molten salts for achieving highly ion-conductivepolymer-in-salt electrolytes. This paper deals with thepreparation and characterization of 1-ethyl-3-methylim-idazolium tetrafluoroborate (EMIBF4) and 1-butylpyri-dinium tetrafluoroborate (BPBF4) and with thepolymerization of vinyl monomers in the room temper-ature molten salts in order to present a new methodol-ogy affording highly conductive polymer electrolytes.

2. Experimental

2.1. Preparation of room temperature molten salts andpolymer electrolytes

EMIBF4 and BPBF4 were prepared according toScheme 1[17]. For EMIBF4, 1-ethyl-3-methylimida-zolium chloride (EMICl) was first prepared by thequaternization reaction of 1-methylimidazole withethylchloride at 110C for 12 h in cyclohexane in anautoclave. Crude product was purified by repetitiousrecrystallization. An anion exchange reaction fromchloride to tetrafluoroborate (BF4

) was completed bymixing equimolar quantities of EMICl and AgBF4, inethanol:water mixtures at room temperature for 12 h.Precipitated AgCl was filtered off and obtained

EMIBF4 was dried under reduced pressure. In the caseof BPBF4, 1-butylpyridinium bromide was first pre-pared by the quaternization reaction of pyridine withbutyl bromide at 100C for 2 days in cyclohexane, andthe post-treatments were similar to those for EMIBF4.The structures of EMIBF4 and BPBF4 were identifiedby 1H-NMR and fast atom bombardment mass spectra.Purified EMIBF4 and BPBF4 were stored in an argonatmosphere glove box (VAC, [O2]B1 ppm, [H2O]B1ppm).

In order to obtain polymer-in-salt electrolytes, wetried to polymerize common vinyl monomers inEMIBF4 and BPBF4 as the polymerization solvents.Methyl methacrylate, acrylonitrile, vinyl acetate,styrene and 2-hydroxyethyl methacrylate (HEMA) wereadopted as the vinyl monomers and were purified bydistillation. In the cases where either EMIBF4 orBPBF4 and the equimolar vinyl monomers were com-patible, radical polymerization was performed by heat-ing the solutions at 80C for 12 h in the presence of 0.5wt.% benzoyl peroxide as an initiator. Because of com-paratively good compatibility of poly(HEMA) with themolten salts in the resulting vinyl polymers, the net-work polymer electrolytes based on HEMA and eitherEMIBF4 or BPBF4 were prepared as follows. HEMA,ethylene glycol dimethacrylate (EGDMA, 2 mol%based on HEMA), and benzoyl peroxide (2 mol% basedon the monomers) as an initiator were dissolved in themolten salts. The mixtures were spread between twoglass plates, which were separated by a 0.5 mm thickspacer, and heated at 80C for 12 h. By this procedure,solid polymer electrolytes with a variety of composi-tions could be obtained.

2.2. Measurements

Differential scanning calorimetry (DSC) was mea-sured by using a Seiko Instruments DSC 220C. Thesamples for the DSC measurements were tightly sealedin Al pans in the argon atmosphere glove box. Ther-mograms were recorded during cooling (100 to 150C) and then heating (150100C) scans at acooling or heating rate of 10 K min1.

Scheme 1. Preparation procedures of EMIBF4 and BPBF4.

A. Noda, M. Watanabe : Electrochimica Acta 45 (2000) 12651270 1267

Fig. 1. DSC thermograms of EMIBF4 and BPBF4. Coolingand heating rates are 10 K min1.

container in the glove box. For polymer electrolytes, afilm (13 mm diameter, ca. 0.5 mm thickness) was sand-wiched between mirror-finished stainless-steel elec-trodes, sealed in a Teflon container, and subjected tothe impedance measurements.

Electrochemical windows of EMIBF4 and BPBF4were checked by cyclic voltammetry using a three-mi-croelectrode cell. The three-microelectrode cell consistsof a tip of a Pt wire (50 mm diameter, sealed in a glasscapillary) as a working electrode, a Pt wire (400 mmdiameter) for a counter electrode and an Ag:AgCl wire(400 mm diameter) for a pseudo-reference electrode.The cyclic voltammetry was performed at room temper-ature by using a BAS 100 B:W electrochemicalworkstation.

Dynamic mechanical analysis was performed underN2 atmosphere at 10 Hz by using a Seiko InstrumentsDMS 210.

3. Results and discussion

3.1. Characterization of room temperature molten salts

Fig. 1 shows DSC thermograms of EMIBF4 andBPBF4 during cooling and successive heating scans.During cooling from 100 to 150C, neither exother-mic nor endothermic peak could be observed except forthe heat capacity change corresponding to glass transi-tion temperature (Tg) for both EMIBF4 and BPBF4. Inthe heating scans, the thermograms showed Tg, crystal-lization point (Tc) and melting point (Tm). Their ther-mal properties are summarized in Table 1. Since themelting points of both of EMIBF4 and BPBF4 arearound 15C, we can confirm that these ionic liquidsare room temperature molten salts. Below the meltingpoints during the cooling scans, EMIBF4 and BPBF4form supercooled liquids. We have already exploredthat the crystallization rates of these molten salts arevery slow and that these supercooled liquids are fairlystable.

Fig. 2 shows the temperature dependence of ionicconductivity for EMIBF4 and BPBF4. Both of theArrhenius plots of the ionic conductivity exhibit posi-tively curved-profiles, as can be expressed by WLF orVTF equations. The ionic conductivities for EMIBF4and BPBF4 are quite high and are 210

2 and 3

The ionic conductivities of EMIBF4 and BPBF4 weredetermined by means of the complex impedance mea-surements with stainless-steel blocking electrodes, usinga computer controlled Hewlett-Packard 4192A LFimpedance analyzer over the frequency range from 5Hz to 13 MHz. A sample was filled between mirror-finished stainless-steel electrodes with using a Teflon

ring spacer (13 mm outer diameter, 7 mm inner diame-ter, 2 mm thickness) and was sealed in a Teflon

Table 1Thermal properties of EMIBF4 and BPBF4

Tm (C) Tg (C)DHm (J g1) DSm (J deg1 g1) Tc (C) DHc (J g1) DSc (J deg1 g1)

2.210163.214.6 89.4EMIBF4 2.410153.450.5

15.3 45.8 1.6101 11.9 37.5 1.4101BPBF4 66.7

A. Noda, M. Watanabe : Electrochimica Acta 45 (2000) 126512701268

Fig. 2. Arrhenius plots of ionic conductivity for EMIBF4 andBPBF4.

their polymers which are polymerized in EMIBF4 andBPBF4 by radical polymerization. First, the compatibil-ity of these monomers with EMIBF4 and BPBF4 werechecked, where equimolar amounts of EMIBF4 orBPBF4 and a monomer were mixed. A circle in Table 2means that the mixture is transparent and compatible,and a cross means that the mixture is not compatibleand phase-separated. For example, in the case ofBPBF4, methylmethacrylate, acrylonitrile, vinyl acetateand 2-hydroxyethyl methacrylate (HEMA) are compat-ible, whereas styrene is not compatible. When amonomer and the molten salt is compatible, radicalpolymerization was performed with heating in the pres-ence of a radical initiator. Interestingly, it was foundthat these monomers were polymerized in EMIBF4 orBPBF4, except for vinyl acetate. However, most of theresulting polymers were phase-separated from EMIBF4or BPBF4, as indicated by crosses. In the vinylmonomers used in this study, poly(HEMA) showed

Fig. 3. Cyclic voltammograms of EMIBF4 and BPBF4 at roomtemperature. Scan rate is 50 mV s1.

103 S cm1 at 30C, respectively. For EMIBF4, theobtained values are in good agreement with the data byCarlin et al. [21]. The measurements were carried outwith cooling from 100 to 10C, and the samples werethermally equilibrated at each temperature for at least 1h before the measurements. However, we did not ob-serve any remarkable change in the conductivities, cor-responding to the melting transitions at ca. 15 C. Theconductivity results also support the stability of thesupercooled liquids.

The cyclic voltammetry for the room temperaturemolten salts was carried out to investigate the electro-chemically stable potential windows. Fig. 3 shows thecyclic voltammograms of EMIBF4 and BPBF4. ForEMIBF4, the irreversible reduction starts from ca. 2V versus Ag:AgCl, and the irreversible oxidation ap-pears at ca. 2 V versus Ag:AgCl. As the result, EMIBF4exhibits an electrochemical potential window widerthan 4 V, which agrees with the results reported byCarlin et al. [16,17,21]. The electrochemical potentialwindow of BPBF4 is narrower than that of EMIBF4and is ca. 3.4 V. The anodic limit does not largely differfrom that of EMIBF4, whereas the cathodic limit ismuch higher than that of EMIBF4. The pyridiniumstructure is more easily reduced than the imidazoliumstructure. It is also interesting to note the reports byCarlin et al. [21] statingthat EMIBF4 is not reducedwith the addition of a trace amount of H2O down tothe lithium redox potential.

3.2. Preparation and ionic conducti6ity of polymerelectrolytes

Table 2 shows the compatibility of EMIBF4 andBPBF4 with five common vinyl monomers and with

A. Noda, M. Watanabe : Electrochimica Acta 45 (2000) 12651270 1269

Table 2Compatibility a of EMIBF4 and BPBF4 with monomers

b and their polymers c

BPBF4EMIBF4

PolymerMonomer Monomer Polymer

Methyl methacrylate Acrylonitrile

Vinyl acetate No polymerization No polymerizationStyrene

2-Hydroxyethyl methacrylate

a, transparent; , translucent; , phase-separated.b Molten salts and monomers were mixed at a 1:1 molar ratio.c Polymerization was carried out in the presence of 0.5 wt.% BPO at 80C for 12 h.

comparatively good compatibility and the mixtures af-ter polymerization gave translucent gels. Thus, we at-tempted to obtain a new solid polymer electrolyte byusing HEMA. Transparent and mechanically strongpolymer-in-salt electrolytes could be obtained by usingHEMA, ethylene glycol dimethacrylate (EGDMA) androom temperature molten salts. By this procedure, solidpolymer electrolytes with a variety of compositionscould be obtained. The amount of unreacted monomersafter the polymerization was checked but was found tobe negligible for both of the EMIBF4 and BPBF4polymer electrolytes.

Figs. 4 and 5 show the temperature dependence ofionic conductivity for the polymer electrolytes based onthe HEMA network polymers and the room tempera-ture molten salts. The remarkable decrease of the ionicconductivities at the melting points of the room temper-ature molten salts is not observed in these results. Theionic conductivity of the EMIBF4 electrolyte films areconsiderably lower than that of the molten salt itself(Fig. 4). The electrolyte films were mechanically hard,and the phase-separation occurred at the higher compo-sitions than [EMIBF4]:[Monomer]4:6, giving translu-cent films. On the other hand, the compatibility ofBPBF4 with the HEMA network polymers is rathergood, though the ionic conductivity of BPBF4 itself islower than that of EMIBF4. The ionic conductivity ofthe BPBF4 polymer electrolytes increases with increas-ing the concentration of BPBF4 (Fig. 5). The ionicconductivity of the 6:4 ([BPBF4]:[Monomer]) electrolytereaches 103 S cm1 at 30C, and obtained polymerelectrolyte film was optically compatible, transparentand mechanically strong. Fig. 6 shows the temperaturedependence of dynamic mechanical properties for aBPBF4 polymer electrolyte film ([BPBF4]:[Monomer]4:6), as a typical example. A large relaxation in tensilemodulus (E %) and a peak in tan d are observed at ca.20C. Although this temperature is close to Tm ofBPBF4, the DSC thermogram did not exhibit any melt-ing transition at this temperature. Thus, this relaxation

temperature is assigned to Tg of the polymer electrolyte.This temperature is much higher than Tg of BPBF4(66.7C) and is much lower than Tg of PHEMA(90C), indicating the compatibility between BPBF4and the HEMA network polymer.

4. Conclusion

EMIBF4 and BPBF4 form room temperature moltensalts with melting points of ca. 15C and exhibit a highionic conductivity of 2102 and 3103 S cm1 at30C, respectively. Electrochemically stable potentialwindow of EMIBF4 and BPBF4 is ca. 4 and 3.4 V,respectively. Certain vinyl monomers can be poly-merized in the molten salts by radical polymerization. Bythe radical polymerization, highly conductive solidelectrolytes based on the PHEMA network polymers, inwhich molten salts are dissolved and compatible, can beobtained.

Fig. 4. Arrhenius plots of ionic conductivity for EMIBF4 andtheir polymer-in-salt electrolytes ([EMIBF4]:[HEMA]).

A. Noda, M. Watanabe : Electrochimica Acta 45 (2000) 126512701270

Fig. 5. Arrhenius plots of ionic conductivity for BPBF4 andtheir polymer-in-salt electrolytes ([BPBF4]:[HEMA]).

(No. 282:10131228) from the Japanese Ministry ofEducation, Science, Sports and Culture, and by NEDOInternational Joint Research Grant.

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Fig. 6. Temperature dependence of E % and tan d for HEMAnetwork polymers with dissolved BPBF4 ([BPBF4]:[Monomer]4:6).

Acknowledgements

This research was supported in part by Grant-in-Aidfor Scientific Research (No. 10650878) and that onPriority Area Electrochemistry of Ordered Interfaces

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