Electrochimica Acta 47 (2002) 1275–1281

www.elsevier.com/locate/electacta

Dipoles and their possible effects on conductivity inpolymer-ceramic composite electrolytes

Binod Kumar a,*, Stanley J. Rodrigues a, Robert J. Spry b

a Uni�ersity of Dayton Research Institute, 300 College Park, KL 501, Dayton, OH 45469-0170, USAb Air Force Research Laboratory, Materials Directorate, Wright-Patterson AFB, OH 45433-7750, USA

Received 24 July 2001; received in revised form 27 September 2001

Abstract

This paper conceptualizes and discusses the physical existence of electrically active dipoles in polymer-ceramic compositeelectrolytes. The concept of physical existence of dipoles is supported by experimental evidences that include time dependence, dcfield assisted variation, and mechanically induced effects on conductivity. © 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Composite electrolytes; Conductivity; Mechanical stretching; Dipole orientation; Charge carriers

1. Introduction

Dipoles are electrically active structural componentsin gases, liquids, and solids. The dipole and externalelectric field interaction is often quantified by a determi-nation of the dielectric constant. The water molecule isa classic dipole which enables the liquid water to exhibita high dielectric constant. The dipolar structure ofwater molecule and its effects are retained even in ice[1].

The polymer electrolytes for applications in lithiumrechargeable batteries, fuel cells, and electrochemicalsensors have been investigated for two decades [2,3] yetlittle is known and understood about their dipolarstructure and its influence on conductivity. Takeuchi, etal. [4] have reported that 5 mol% of ferroelectric bar-ium titanate (BaTiO3) when dispersed in Na4Zr2Si3O12

increases its sodium ion conductivity. They attributedthe conductivity enhancement to electrostatic forcesoriginating from dipole moments of ferroelectricBaTiO3 and subsequent increase in the hopping rate ofthe sodium ion. Sun, et al. [5,6] also report enhance-ments in conductivity and lithium transport number ofPEO:LiClO4 when BaTiO3 is added as a filler whichwere explained on the basis of spontaneous polarizationof ferroelectric BaTiO3.

The impetus of this paper is driven by a long-term,reproducible, and consistent experimental observationthat isothermal stabilization (annealing) of polymer(PEO:LiBF4 complex)–ceramic (MgO, TiO2, ZrO2)composite electrolytes at a temperature near the meltingpoint of PEO enhances conductivity. The enhancementin conductivity may approach four orders of magnitudeat subambient temperatures. The annealing time andcooling rate dependence of conductivity have beendemonstrated in the PEO:LiBF4–TiO2 [7],PEO:LiBF4–ZrO2 [8], and PEO:LiBF4–MgO [9] sys-tems. It has been suggested [10] that thermally frozen,nonequilibrium, and randomly oriented dipoles tend toapproach equilibrium, oriented configuration—a dipo-lar structure conducive for enhanced conductivity. Thetransition rate from a non-equilibrium to an equi-librium state is believed to be ceramic particle volumeand size, and temperature dependent [10]. At low tem-peratures and for larger volume fraction and particlesize of ceramic phase, the time dependence of conduc-tivity may be very small and may even be unobservablebecause the dipoles become immobile in a rigid polymermatrix.

The dipoles respond to electric field and mechanicalforce. On a macroscopic level, dipoles associated withpolymer chains and ceramic particles possess randomorientation. Under an applied dc field, the dipolesshould orient in the direction of the field and subse-quent measurement of conductivity using ac impedance

* Corresponding author. Fax: +1-937-229-3433.E-mail address: kumar@udri.udayton.edu (B. Kumar).

0013-4686/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.PII: S 0 0 13 -4686 (01 )00840 -4

B. Kumar et al. / Electrochimica Acta 47 (2002) 1275–12811276

should theoretically provide an evidence of physicalpresence of dipoles. Mechanical stretching leads to achange in molecular structure of the polymer and thusan oriented dipolar configuration ensues. These align-ments do not lead to crystallization of the polymerphase, but rather order ion hopping sites in the amor-phous phase along the chain dimension, thus increasingionic conductivity along polymer chains [10]. In view ofthe preceding background, this paper presents results ofan exploratory investigation with an objective to studythe effects of dc field and mechanical stretching onconductivity. The underlying assumption is that thepolymer-ceramic composite electrolytes are an assem-blage of dipoles and these dipoles can be oriented undersuitable conditions which in turn should influence theconductivity.

2. Experimental

The PEO:LiBF4/LiCF3SO3–MgO composite elec-trolyte films were made by the solvent casting techniqueusing reagent grade poly(ethylene) oxide (PEO), lithiumtetrafluoroborate (LiBF4) or lithium methyl fluorosul-fonate (LiCF3SO3) and nanosize magnesium oxide(MgO). The [O]:[Li] ratio of the polymer complex was8:1 and the average particle size of MgO was 19 nm. Asolution of PEO (Union Carbide, mol. wt. 2,000,000)and LiBF4 or LiCF3SO3 in AR grade acetonitrile(Aldrich) was prepared in which MgO was dispersedand sonicated. After sonication, a homogenized col-loidal solution was obtained which was cast and driedinto a film of about 75 �m thickness.

The ac impedance measurement was carried out us-ing an EG&G impedance spectrometer model 398 inthe frequency range of 0.1 Hz to 100 kHz on stainlesssteel (ss)/composite electrolyte/ss cells. The cells werecontained in a glass vessel which was heated in anenvironmental chamber that allowed a temperature de-pendence measurement in the −40 to 100 °C range.The set temperature was maintained within �1 °C.

The effect of the dc field assisted cooling and subse-quent effect on conductivity was determined. In thisexperiment, the electrolyte specimen in blocking elec-trode configuration was heated to 100 °C and a dc fieldof 10 V cm−1 was applied across the specimen. Subse-quently, the specimen was cooled down to 20 °C andafter removal of the dc field, conductivity was measuredby the ac impedance technique.

The dried films were mechanically stretched at 50 °Cin the dry box using a hand-made mechanical stretcher.The films were uniaxially stretched from 5 to 15%. Theimpedance was measured perpendicular to the stretch-ing direction.

Infrared (IR) vibrational spectra were obtained onthe films using a Nicolet 740 FTIR spectrometer in the4000–500 cm−1 range.

3. Results and discussion

3.1. Conducti�ity data of the composite electrolyte(without the dc field)

Conductivity data of composite electrolyte films ob-tained from PEO:LiBF4 (8:1)–MgO (10 wt.%) materialare shown in Fig. 1. The specimen was heated to andstabilized at 100 °C for 30 min, cooled down to 20 °C,and held at the temperature overnight. Subsequently, itwas again heated to and stabilized at 100 °C for 30 minbefore the conductivity measurement began at this tem-perature. The conductivity at each temperature wasobtained after stabilizing the specimen for 30 min. Afterthe temperature reached 20 °C, the specimen was al-

Fig. 1. Temperature dependence of conductivity of PEO:LiBF4 (8:1)–MgO (10 wt.%) during heating and cooling cycles. Also shown is thetemperature dependence of conductivity of the PEO:LiBF4 complex.

B. Kumar et al. / Electrochimica Acta 47 (2002) 1275–1281 1277

Fig. 2. Conductivity of PEO:LiBF4 (8:1)–MgO (10 wt.%) (a) duringcooling scan and isothermal stabilization at 20 °C, and (b) dc field-assisted, rapidly quenched specimen isothermally stabilized at 20 °C.

tallization thereby imparting a large, positive effect onconductivity. The concentration of MgO can be in-creased up to 20 wt.% without any adverse effect onconductivity [9].

3.2. Effects of the dc field

Fig. 2 shows the temperature dependence of conduc-tivity of the PEO:LiBF4 (8:1)–MgO (10 wt.%) com-posite electrolyte specimen with and without applied dcfield. During a cooling scan, the specimen without anapplied dc field was held at 20 °C for 16 h and theconductivity increased by almost an order of magni-tude—a situation similar to the results of Fig. 1.

After the specimen was rapidly quenched under a dcfield of 10 V cm−1 from 100 to 20 °C, the measuredconductivity (log �= −5.64) was lower than the value(log �= −5.24) obtained during the standard coolingscan by 33%. There is an effect of dc field assistedcooling on conductivity, however it is negative. Al-though the dipoles have been aligned by the dc field,the lower conductivity of the dc field-assisted, rapidlyquenched specimen is attributed to polarization anddepletion of ionic charge carriers. But, the interestingresult to be noted here is that the isothermal stabiliza-tion for 13 h of the specimen led to a further decreasein conductivity. The effect may be attributed to arelative disorientation of dipoles driven by the thermalenergy.

It is imperative from the observations of this experi-ment that the active concentration of ionic chargecarriers and orientation of dipoles both are affected bythe dc field. The coupling of these two electrically activespecies makes interpretation of the data a cumbersometask, and therefore other experimental techniques mayhave to be employed to study each of the electricallyactive species independently.

3.3. Effect of mechanical stretching

When a solid assemblage of dipoles is subjected to amechanical stress, each dipole is subjected to the stressin a localized manner and if a rotation of a dipole isallowed, an orientation of dipoles will take place andthe conductivity will be affected.

The ac impedance spectra of a PEO:LiCF3SO3–MgOspecimen prior to and after uniaxial mechanical stretch-ing at 26 °C are shown in Fig. 3(a) and (b), respec-tively. The measurement was conducted without anyprior heat treatment of the films and represents atypical observation on the composite electrolytes. Ingeneral, specimens with no heat treatment exhibit highresistance because of the presence of a crystalline PEOphase. A comparison of the two spectra reveals that theresistivity of the composite electrolyte after stretchingdecreased from 1.29×109 to 2.90×108 � cm. A reduc-

lowed to remain at the temperature overnight. It isnoted that at 20 °C log � increased from −5.09 to−4.65 (by a factor of 2.75) in 17 hours. This conduc-tivity enhancement is typical of composite electrolytesand has been proposed to be related to the alignment ofdipolar structure [10]. As the temperature was loweredto −40 °C, the conductivity decreased monotonically.Again, the specimen was held overnight at −40 °Cbefore the conductivity was measured during the heat-ing cycle. The conductivities during heating cycle arehigher at temperatures above −20 °C than that of theconductivities during the cooling cycle.

Also shown in Fig. 1 is the conductivity data ofPEO:LiBF4 (8:1) polymer complex for a comparisonpurpose. The conductivity of the complex drops precip-itously below 70 °C and at 20 °C this specimen dis-plays about four orders of magnitude lowerconductivity than that of the composite electrolyte. Thecomparison clearly points out the role of ceramic com-ponent, MgO, which retards and eliminates PEO crys-

B. Kumar et al. / Electrochimica Acta 47 (2002) 1275–12811278

tion of the electrolyte resistance by a factor of almostfive (5) was achieved while the thickness of the filmdecreased by only 15%.

The temperature dependence of conductivity (inverseof resistance normalized by geometrical parameters ofthe film) data of the two specimens whose impedancedata were presented in Fig. 3 are shown in Fig. 4. Thecells containing these two specimens were contained ina controlled atmosphere glass vessel. The two speci-mens were thermally stabilized and characterized simul-

taneously. The total thermal treatment time was 132 hat 60 °C which is believed to be sufficient to stabilizethe structure of composite electrolytes. The conductiv-ity measurement after thermal stabilization was ini-tiated at 100 °C and terminated at 0 °C. In Fig. 4, it isnoted that in spite of the long annealing time, theconductivity of the stretched specimen remained about40% greater than the unstretched specimen over theentire temperature range. Thus, the effect of mechanicalstretching on conductivity is permanent and compli-

Fig. 3. Ac impedance spectra of PEO:LiCF3SO3 (8:1)–MgO (10 wt.%) films (a) as-prepared and (b) after stretching. The specimen was notsubjected to any prior heat treatment.

B. Kumar et al. / Electrochimica Acta 47 (2002) 1275–1281 1279

Fig. 4. Temperature dependence of conductivity of PEO:LiCF3SO3

(8:1)–MgO (10 wt.%) films after annealing at 60 °C for 132 h: �,unstretched, �, stretched.

mentary to the effects rendered by isothermal stabiliza-tion leading to the formation of a predominantly amor-phous polymer structure.

Qualitatively, in low-yield stress material such aspolymers and composites used in this investigation, ahigher level of stress will lead to a greater elongation ofthe film in the stretching direction. In these compositeelectrolytes there is an optimum elongation (�10%)beyond which the conductivity remains constant.

3.4. Infrared absorption spectroscopy

The dielectric constants of PEO and MgO at ambienttemperature are 5 [2] and 9.65 [11], respectively. Thedielectric constant difference between the two phasesraises a possibility of chemical interaction. Thus, thechemical bonds in the polymer complex (PEO:LiBF4)are expected to be affected by the addition of MgO.These bonds may even be further influenced by anelongating stress. To explore and assess this possibility,the IR absorption spectra on as-prepared PEO:LiBF4

(8:1) complex, PEO:LiBF4 (8:1)–MgO (20 wt.%) com-posite, and stretched PEO:LiBF4 (8:1)–MgO (20 wt.%)electrolyte films were obtained.

A comparison of IR spectra of as-prepared films ofPEO:LiBF4 (8:1) and PEO:LiBF4 (8:1)–MgO (20 wt.%)material is shown in Fig. 5. A broad absorption in

Fig. 5. IR absorption spectra of PEO:LiBF4 (8:1) complex and PEO:LiBF4 (8:1)–MgO (20 wt.%) composite films.

B. Kumar et al. / Electrochimica Acta 47 (2002) 1275–12811280

Fig. 6. IR absorption spectra of PEO:LiBF4 (8:1)–MgO (20 wt.%) composite electrolyte; as-prepared and stretched.

Table 1Infrared bands and assignments in PEO:LiBF4:MgO complexes (2000–400 cm−1)

PEO:LiBF4 (8:1)PEO PEO:LiBF4–MgO (20 wt.%) Assignments

Unstretched Stretched

1961.4, 1647.7 1965.81965.119601469.01473 1466.8 1469.5 � (CH2)a

1349.9 1350.91342 � (CH2)a1349.91281.6 1281.91282.6 t (CH2)a1283

1241.91244 1245.7 1241.9 t (CH2)a

1147 1154.6 1128.7 1147.11083.8, 1033.0 1112.4, 1097, 1047, 992.41113.0, 1088.1, 1017.9, 992.4 � (COC)a (1088.1, 1083.8, 1097)1103

950.4958 958.8 964.5 r (CH2)a, � (COC)a

843.9, 766.7844 843.8, 777.4842.4, 778.3 r (CH2)a

514.8, 478.3, 462.8 536.1, 500.4, 482.4528.4 OCC–MgO, � (OCC)a; OCC–MgO; OCC–MgO530

r (rocking), t (twisting), � (stretching), � (wagging), � (bending), a (asymmetric).

650–750 cm−1 and bands in the 450–520 cm−1 rangesresult due to the addition of MgO. The absorption bandat 528.4 cm−1 in the PEO:LiBF4 (8:1) complex isattributed to the bending mode of the OCC group [12].With the addition of MgO in the complex, a chemicalinteraction takes place and additional bands appear at514.8, 478.3 and 462.8 cm−1. The 514.8 cm−1 band mayhave resulted from a shift of the 528 cm−1 band. Thus,it is evident that the addition of MgO leads to aformation of a ternary PEO:LiBF4 (8:1)–MgO complex.

After a film of PEO:LiBF4 (8:1)–MgO (20 wt.%) wasmechanically stretched, there are additional changes in

the IR absorption spectra, as shown in Fig. 6. The slopeof the broad absorption in the 650–750 cm−1 rangeincreases and the individual bands shift by approxi-mately 21 cm−1 to higher frequencies. A slight shift insome other absorption bands also occur which aresummarized along band assignments in Table 1. Forreference, Table 1 also presents absorption bands associ-ated with PEO as reported by Papke et al. [12].

Chung et al. [13] have conducted a nuclear magneticresonance study on a film of PEO:LiI (20:1) and re-ported that short-range structural changes such as Li–Hinternuclear distances and local Li–O bond lengths and

B. Kumar et al. / Electrochimica Acta 47 (2002) 1275–1281 1281

angles are affected when a uniaxial stress is applied. Thestructural changes are accompanied with reduced seg-mental chain motion of the host polymer.

Golodnitsky and Peled [14] reported a fivefold in-crease in conductivity of a PEO:LiI (20:1) specimenalong the stretching direction when the specimen wasmechanically stretched. They also observed developmentof fibrous morphology after stretching.

It is expected that the dipoles associated with polymerchain segment and MgO particles are aligned in thedirection of stretching. However, the polymer chains areapproximately 1000 times larger than the MgO particlesthat may introduce additional bends and curvature inthe long polymer chains. The polymer chains cross eachother in the amorphous, bulk structure. An anisotropyof conductivity in the stretched specimen is a possibilitywhich is yet to be characterized. This paper presentedand discussed conductivity data which have been mea-sured perpendicular to the stretching direction.

4. Summary and conclusions

This investigation presented and discussed three ex-perimental evidences which support the existence ofdipoles in composite electrolytes. These evidences in-clude time dependence, dc field-assisted variation, andmechanically-induced effects on conductivity. Specificconclusions of this investigation are:

1. The composite electrolyte may be considered as anassemblage of permanent dipoles whose average orienta-tion depends upon the temperature and prior thermalhistory of the specimen.

2. The conductivity enhancement is proposed to berelated to an interaction of the dipoles associated withthe polymer and ceramic phases.

3. The conductivity of a specimen at 20 °C quenchedunder a dc field was lower and it decreased further withisothermal stabilization. The lower conductivity wasattributed to depletion of ionic charge carriers. Theconductivity decay was linked to thermally-induced dis-orientation of dipoles.

4. The medium of ionic transport remains the poly-mer phase. The stretching affects only the dipolar struc-ture of the composite material.

5. The enhancement in conductivity is related to thedegree of elongation. The peak in conductivity occursaround 10% elongation.

6. Absorption bands in the range of 460–530 cm−1

develop as a result of MgO addition in the PEO:LiBF4

complex. These bands shift to higher frequencies byapproximately 21 cm−1 due to stretching.

Acknowledgements

BK and SJR acknowledges the financial support byEagle-Picher Technologies, LLC and the NASA-GlennResearch Center under purchase order no. C-778999-J.The authors express their appreciation to A.E. Turnerfor conducting the experimental work.

References

[1] P. Debye, Polar Molecules, Dover Publications, New York,1945, pp. 102–104.

[2] J.R. MacCallum, C.A. Vincent (Eds.), Polymer Electrolyte Re-views 1 and 2, Elsevier Applied Science, Barking, UK, 1987(for dielectric constant see Review 1, p. 3).

[3] F.M. Gray, Polymer Electrolytes, Royal Society of Chemistry,UK, 1997.

[4] T. Takeuchi, K. Ado, Y. Saito, M. Tabuchi, H. Kageyama, O.Nakamura, Solid State Ionics 89 (1996) 345.

[5] H.Y. Sun, H.-J. Sohn, O. Yamamoto, Y. Takeda, N. Iman-ishi, J. Electrochem. Soc. 146 (5) (1999) 1672.

[6] H.Y. Sun, Y. Takeda, N. Imanishi, O. Yamamoto, H.-J.Sohn, J. Electrochem. Soc. 147 (7) (2000) 2662.

[7] B. Kumar, L.G. Scanlon, SAE Aerospace Power Systems Con-ference, Williamsburg, VA, 9–11 April, 1997, pp. 71–82.

[8] B. Kumar, L.G. Scanlon, Solid State Ionics 124 (3) (1999)239.

[9] B. Kumar, L.G. Scanlon, R.A. Marsh, R. Mason, R. Higgins,R.S. Baldwin, Electrochim. Acta 46 (10–11) (2001) 1515.

[10] B. Kumar, L.G. Scanlon, R.J. Spry, J. Power Sources 96/2(2001) 337.

[11] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction toCeramics, Wiley, New York, 1976, p. 933.

[12] B.L. Papke, M.A. Ratner, D.F. Shriver, J. Electrochem. Soc.129 (7) (1982) 1434.

[13] S.H. Chung, Y. Wang, S.G. Greenbaum, D. Golodnitsky, E.Peled, Electrochem. Solid State Lett. 2 (11) (1999) 553.

[14] D. Golodnitsky, E. Peled, Electrochem. Acta 45 (2000)1431.