Ionic conductivity of a poly(vinylpyridinium)/silver iodide solid ...

Solid State Ionics 171 (2004) 41–44www.elsevier.com/locate/ssiIonic conductivity of a poly(vinylpyridinium)/silver iodide solidpolymer electrolyte systemTed M. Pappenfus, Wesley A. Henderson, Boone B. Owens,Kent R. Mann*, William H. Smyrl*Departments of Chemistry, Chemical Engineering, and Materials Science, University of Minnesota, 139 Smith Hall,Minneapolis, MN 55455, USAReceived 25 February 2003; received in revised form 15 April 2003; accepted 26 May 2003AbstractA series of quaternized poly(2-vinylpyridine) (PVP) polymers has been synthesized with the general formula [(PVP)R]I, where R=H,Me, Bu. Addition of AgI to the polymers results in new silver ion conducting solid polymer materials. Conductivities as high as 610 3S/cm have been observed for the 3:1 AgI/[(PVP)Me]I composition. Analogous materials with a 3:1 composition for the protonated andbutyl-substituted polymers display lower conductivities near 210 4 S/cm. Conductivity studies, as a function of temperature, show thesematerials to be thermodynamically stable with a low activation energy for ion conduction. Differential scanning calorimetry experimentssuggest that a new phase with the general formula (PVP)RAg 3 I 4 forms with silver ions in a new amorphous state.D 2003 Elsevier B.V. All rights reserved.Keywords: Solid polymer electrolyte; Ionic conductivity; Silver ion conductors; Electrochemical impedance analysis; DSC1. IntroductionSolid polymer electrolytes continue to be extensivelystudied for their many potential applications in sensors,solid state batteries, and fuel cells [1,2]. Recent studieshave been dominated by lithium ion conducting systems fortheir potential use in solid state batteries with the high cellvoltages and energy densities that are characteristic oflithium batteries. As a result, far fewer studies have beenperformed with other metal ion conducting polymers suchas silver, and only a limited number of silver ion conductingpolymer electrolyte systems have been studied [3–10].More recently, numerous studies of materials that containsilver ions in a polymer matrix have shown these to be* Corresponding authors. K.R. Mann is to be contacted at theDepartment of Chemistry, University of Minnesota, 139 Smith Hall,Minneapolis, MN 55455, USA. Tel.: +1-612-625-3563; fax: +1-612-626-7541. W.H. Smyrl is to be contacted at the Corrosion Research Center,Department of Chemical Engineering and Materials Science, University ofMinnesota, 221 Church Street SE, Minneapolis, MN 55455, USA. Tel.: +1-612-625-0717; fax: +1-612-626-7246.E-mail addresses: mann@chem.umn.edu (K.R. Mann),smyrl001@tc.umn.edu (W.H. Smyrl).useful as transport membranes for olefin/paraffin separations(e.g., see Ref. [11], and references therein).Silver-substituted ammonium halide conductors of thegeneral formula QAg n I n+1 (where Q is an ammoniumcation) have been studied previously. The crystalline solidelectrolytes exhibit high conductivities that are greater than10 2 S/cm at temperatures well below 25 jC [12]. Includedin the previous studies were tetraalkylammonium as well asboth saturated and unsaturated azacyclic cations. An alternativeto the use of monomeric ammonium cations is to usepolycations to create a new Ag + ion conducting solidpolymer electrolyte system. This modification may lead toenhanced material properties such as flexibility, adhesion,and thermal stability in an ionically conductive polymersheet.Selection of the QAg n I n +1 systems mentioned abovewas motivated by the early work on the solid electrolytesystem MAg 4 I 5 (where M is K + , Rb + , or NH + 4 ), whichdisplays high Ag + conductivity of 0.2 S/cm at 20 jC [13].Recently, binary lithium halide systems such as Li 2 Rb 3 I 5were reported to exhibit conductivities near 0.01 S/cm at65 jC [14]. These results suggest a strong possibility thatAg + may be replaced by Li + to produce QLi n I n +1 systemswith high lithium conduction at room temperature. Con-0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0167-2738(03)00210-8

42T.M. Pappenfus et al. / Solid State Ionics 171 (2004) 41–44sequently, we initiated a two-phase strategy for the synthesisof conductive Li + solid polymer electrolytes. Thefirst phase includes the preparation of highly conductiveAg + polymer electrolytes. Once these materials are characterized,Ag + will be replaced with Li + to produce highlyconductive lithium solid polymer electrolytes at ambienttemperature.During the first phase, we have prepared a series ofpoly(vinylpyridinium) electrolytes of the general formula[(PVP)R]I, where R=H, Me, Bu (see Fig. 1). Upon additionof silver iodide to the polyelectrolyte, increased ionicconductivity is obtained relative to pure silver iodide. Thispaper reports the results of the first phase studies: aninvestigation of the conductivity and thermal properties ofthe silver ion containing materials to gain insight intoproperty/composition relationships.2. Experimental2.1. Chemicals, reagents, and analysisReagent grade silver iodide (99.9%), 1-iodobutane,acetone, tetrahydrofuran (THF), and N,N V-dimethylformamide(DMF) were obtained from Aldrich and were usedas received. Hydriodic acid (HI) (57 wt.% aqueous solution)and iodomethane were obtained from Acros andwere used as received. A commercial grade of poly(2-vinylpyridine) (PVP), with a MW=387,000–520,000, wasutilized in the syntheses. Elemental analyses were performedby Quantitative Technologies (Whitehouse, NJ).Infrared spectra were obtained by the attenuated totalreflectance (ATR) method with a Nicolet Magna-FTIRSystem 550 spectrometer.2.2. Polymer synthesisAll reactions were performed under an argon atmosphereand were covered with foil to prevent exposure tolight. The [(PVP)Me]I and [(PVP)Bu]I compounds wereprepared by a method similar to a previously reportedprocedure [15]. The degree of quaternization was determinedby elemental analysis and confirmed by IR spectroscopyby comparison of the absorption peaks of‘‘unquaternized’’ PVP (1588 cm 1 ) and quaternized PVP(ca. 1610–1625 cm 1 ) [15].2.2.1. Synthesis of [(PVP)Me]IIn a typical reaction, 1 g of PVP was dissolved in 35 mlof DMF. Iodomethane (3 ml) was added, and the reactionwas stirred under argon for 48 h at 60 jC at which time alight-yellow precipitate formed. Acetone (60 ml) was addedto ensure complete precipitation. The precipitate was filteredand washed with acetone (510 ml) and diethyl ether(310 ml) to afford 2.2 g of [(PVP)Me]I as a light-yellowsolid. The degree of quaternization was approximately 98%of the pyridine monomeric units. Larger-scale reactions (f4g of PVP) resulted in approximately 88% quaternization ofthe pyridine rings under identical conditions. IR (ATR) 1625(vs), 1577 (m), 1510 (s), 1457 (s), 1435 (m) cm 1 .2.2.2. Synthesis of [(PVP)Bu]IPVP (4 g) and 21.7 ml of 1-iodobutane in 225 ml ofDMF were stirred at 110 jC for 48 h. The reaction mixturewas poured into 500 ml of acetone, which resulted in ayellow precipitate. The solid was filtered and washed withacetone (420 ml) to afford 8.8 g of [(PVP)Bu]I as a lightyellowsolid. The degree of quaternization was determinedto be approximately 80%. IR (ATR) 1624 (vs), 1610 (s),1576 (w), 1510 (w), 1465 (m) cm 1 .2.2.3. Synthesis of [(PVP)H]IPVP (1 g) was dissolved in 30 ml of THF. Hydroiodicacid (3 ml) was added dropwise with stirring at which timean immediate precipitate formed. The reaction mixture wasstirred for an additional 5 min, and the solid was filtered andwashed with acetone (525 ml) to afford 2.2 g of[(PVP)H]I. The degree of quaternization of the polymer,analyzed as a monohydrate, is 100%. IR (ATR) 1610 (vs),1535 (m), 1464 (m) cm 1 .2.3. Addition of silver iodide to polyelectrolytesAll sample preparation steps and storage of the materialswere conducted in a dry room (

2.5. Differential scanning calorimetryDSC measurements were performed using a liquid-nitrogen-cooledPerkin-Elmer Pyris 1 Differential ScanningCalorimeter. Small amounts of the samples (8–25 mg) wereadded to aluminum pans that were then crimped sealed.Typically, the sample pans were slowly heated from ambienttemperature at 10 jC/min. The background has been subtractedfrom the reported DSC traces.T.M. Pappenfus et al. / Solid State Ionics 171 (2004) 41–44 433. Results and discussion3.1. Ionic conductivityFig. 3. Arrhenius plots of the ionic conductivity (26–100 jC) of the[(PVP)R]I–AgI systems: (5) R=H, (n) R =Me, and () R=Bu.The pure polymer electrolytes synthesized here([(PVP)R]I, where R=H, Me, Bu) do not display measurableconductivities (

44T.M. Pappenfus et al. / Solid State Ionics 171 (2004) 41–44ever, of both the (PVP)HAg 3 I 4 and (PVP)BuAg 3 I 4 materialswere more than an order of magnitude lower inconductivity with nearly identical values of 210 4 S/cm.Some of the samples at a given composition for thematerials of all three polymers did not yield consistentlyhigh conductivities. This may be attributed to incompletereaction of the solid state reactants.The temperature dependence of the conductivity of thenew materials is shown in Fig. 3 for the temperature rangeof 26–100 jC. The conductivity increases slowly withincreasing temperature, as expected for a solid electrolytematerial with a low activation energy for conduction. Theapparent activation energies for the methyl and protonatedsystems are 0.16 and 0.19 eV, respectively, and are inexcellent agreement with previously reported silver ionpolymer electrolytes [9]. The activation energy for thebutyl-substituted material was not calculated, but it isanticipated to be similar to the other two materials, basedon the slope of the data presented. No thermal transitionsover this temperature range are evident, suggesting that theconducting phase is thermodynamically stable in thisrange.3.2. Differential scanning calorimetryDSC results for the (PVP)MeAg n I n+1 system are shownin Fig. 4. Silver iodide has a well known h–a transformationtemperature near 148 jC that is clearly displayed inthe DSC plot for pure AgI (Fig. 4, top). This is theendothermic transition from the low conductivity phaseto the highly conducting phase that exists above 148 jC.The binary mixtures corresponding to 4 or 5 mol AgI/molpolymer unit also exhibit this solid–solid AgI transition,but with smaller specific heats. At or below the compositionof 3 AgI units/polymer unit (75 mol% AgI), little orno thermal evidence of crystalline AgI is detected in thesamples. [(PVP)Me]I with no added silver iodide shows aglass transition temperature (T g ) near 175 jC, well abovethe T g for the non-quaternized PVP, which has a T g near100 jC [17]. Furthermore, the T g of the polymer decreaseswith increasing silver iodide content. These data areconsistent with the formation of a new discrete materialof the composition (PVP)MeAg 3 I 4 .Monomeric systems, such as the pyridinium iodide–silver iodide system, form highly crystalline solid electrolytes.For example, the PyI–AgI system (where Py ispyridinium) form crystalline materials with compositionsof PyAg 5 I 6 and Py 5 Ag 18 I 23 [18]. In these structures, theiodide ions form face-sharing tetrahedra, which are thesuggested pathways for silver ion conduction. Althoughour materials appear to be amorphous, they may have asimilar short range order that accounts for the highconductivity.4. ConclusionSilver iodide forms new solid state materials withquaternized poly(2-vinylpyridine) polymers of the generalformula [(PVP)R]I. The maximum room temperature conductivityhas been found to be 610 3 S/cm for theaddition of three equivalents of silver iodide to themethylated polymer. The protonated and butyl-substitutedpolymers exhibit enhanced conductivity, but they are bothmore than an order of magnitude lower in conductivitythan the methylated analog. DSC data, coupled with theconductivity data, suggest the formation of new discretematerials of the general formula (PVP)RAg 3 I 4 . Thesematerials represent a new class of ion conducting solidpolymer electrolytes.AcknowledgementsThis research was supported by NASA ResearchProgram NRA-00-GRC-1.References[1] B. Scrosati (Ed.), Application of Electroactive Polymers, Chapman &Hall, London, 1993.[2] F.M. Gray, Solid Polymer Electrolytes: Fundamentals and TechnologicalApplications, VCH, New York, 1991.[3] H. Eliasson, I. 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