r AD-A094 039 NORTHWESTERN UNIV EVANSTON IL F/6 10/9T7RANSFERENCE NUMBER MEASUREMENTS FOR THE POLYMER ELECTROLYTE PO--ETC(U)JAN 81 R DUPONM 0 H WHITMORE, D F SHRIVER NOSOIN-SO-C-0532UNCL ASSIFIDIE N

NONNIL~llD

%ECU01ITY C -ASSi Fir ATICN P 1.,'% PAf.E I4

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REPOR0T DOCUMENTIO I P EFORE rOPL?:1N'G C,.I. REPORT NUMBEr2. GOVT ACCESSION NO. kEIPIEtTS CATA-0. NUMBER

001 IT1 E 'd Sbell -PEOF RZ-IZRT A PERIOD COVERED

tfransrerence Number Measurements for the T.cnnica. RptPolymer Electrolyte Poly(ethylene oxide).Na-SCu.interim 1-:nnical Rep rto,

6. PERFORMING ORG. REPORT NUMBER

7. AUTHOR(@, A. CONTRACT OR 3RA,- NUMB,ERrs)

0 :R./Dupon, D.. hitmore -w6D.F./Shriver-NOO14-rQ-C.0'532!-

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Departments of Chemistry and Materials Science,and Materials Research Center, Nortnwestern Univ NR 359-746Evanston, IL 60201

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Accpntpd fnr nuhlication in J. Electrochemical Society E

19. KEY WORDS (Continue on reverse atde I! neceesary and Identify by block number)

Solid Electrolyte Electrical ConductivityPolymer ElectrolytePoly(ethylene oxide)Polymer-salt ComDlexes

"I11,00a ABSTRACT (Conti..je on reverse aide If necescary and IdentIft, by block number)'

Fmf measurpmnnt ,," been determined in a Na/Hg amalgam concentration Cell1..-I containing the so*d electrolyt poly(ethylene oxide).NaSCN. These resuitt

in conjunction with measurements made with blocking electrodes of the frec-uepcy dependent conductivity, demonstrate that the main charge carrier is

DD ' 1473 EDITION OF I NOV 65 IS OBSOLETE

SN 0102-L.F-014-6601 SECURITY CLASSIFICATION OF THIS PAGE ("on Data Jnltrso)

4-

Transference Number Measurements for the Polymer

- Electrolyte Poly(ethylene oxide)'NaSCN

R. Dupon, D. H. Whitmore, and D. F. ShriverDepartments of Chemistry and Materials Sciences, andMaterials Research Center, Northwestern University

Evanston, Illinois 60201

Current interest in the development of high energy density batteries has

focused considerable attention on solid electrolytes. The leading solid

electrolytes in developmental batteries are hard ceramic materials, which

usually are employed at elevated temperatures with liquid electrodes. A

more compliant solid electrolyte, such as a polymer electrolyte, should

enable contact to be maintained at the electrode-electrolyte interfaces in

an all solid state battery and lower operating temper tures might be facili-

tated. Potential application of ion-containing polymers as battery electro-

lytes, and electrical measurements on salt complexes with poly(ethylene

oxide) and with poly(propylene oxide) have been presented by Armand (1,2),

and structural studies of poly(ethylene oxide) salt complexes have been

reported from our laboratory (3). The original synthesis and charac-

terization of these materials was reported by Wright (4,5) and extended by

James (6).

A fundamental parameter in the characterization of any solid electro-

lytes is the fraction of the total current which is carried by the mobile

ion, i.e. the transference number. Prior to the present work, a rigorous

determination of the transference number for poly(ethylene oxide) based

electrolytes was not available. The present study is based on the measure-

ment of emf between ion reversible amalgam electrodes which are separated by

the solid electrolyte in a concentration cell (7,8).

fty words: Solid electrolyte, polymer electrolyte, transference number,

poly(ethylene oxide).

81 1 22 028

-2-

EXPERIMENTAL

Poly(ethylene oxide) with an average molecular weight 600,000 was

obtained from Aldrich. The polymer was purified by ion-exchange of an

aqueous solution and the solvent was subsequently removed under high vacuum.

Reagent grade NaSCN (B & A) was recrystallized from methanol and dried under

high vacuum. Reagent grade methanol (MCB) was dried by distillation from

iodine-activated magnesium under a nitrogen atmosphere.

A PEO'NaSCN complex, 4.5:1 mole ratio of polymer repeat unit to salt,

was prepared from stoichiometric quantities of the polymer and salt in

anhydrous methanol. Following complete dissolution of the two solids, the

methanol was removed under vacuum. All manipulations were carried out using

standard inert atmosphere techniques (9). The identity of the PEO-NaSCN

complex was confirmed by infrared spectroscopy and differential scanning

calorimetry, and in previous research the material prepared by this same

procedure has been characterized by optical microscopy and x-ray diffrac-

tion, which demonstrate the absence of free NaSCN.

Amalgams were prepared with triple distilled mercury and reagent grade

sodium (MCB). Molten sodium was filtered through fritted glass to remove

sodium oxide. The concentration of the stock amalgam, solution No. 1,

Table I, was determined gravimetrically and checked by the gasometric method

based on the amount of hydrogen evolved from the reaction of the sodium

amalgam with hydrochloric acid. Concentrations of the remaining solutions

were determined gravimetrically by successive dilution of the stock amalgam.[ 7

The sample and amalgams were placed in a cell which permitted exclusion [7

of the atmosphere, Fig. 1. A pressed pellet of the PEO*NaSCN complex,

4.5:1, was fitted into an o-ring which was then clamped between the side

arm . All loading of the polymer complex was carried out in a nitrogen

-- s~ IS p---

-3-

filled dry box. One side arm was filled with the stock amalgam solution

while the other was successively filled with the dilutions of the stock.

Introduction of amalgam solutions was done with a gas-tight syringe under a

flush of dry nitrogen.

The potentials of the cell were measured using a Keithley 177 multimeter

together with an electrometer containing an RCA CA3140 E MOS/FET operational

amplifier designed to increase the input impedance of the measuring device

to 1012 ohms. All measurements were made at room temperature, 23.5 ± 0.2 *C,

which was monitored using an iron-constantan thermocouple and millivolt

meter. The cell and connecting leads were placed in an aluminum box

designed to shield the system from extraneous radiation. Reversing the cell

polarity produced insignificant changes in the magnitude of the observed

potentials. The emf was read after the potential had maintained a stable

( -+ 1.3 mV) value over a period of one hour. Typical stabilizaton times

were of the order of one to two hours. The measured results are given in

Table I. As a check on these measurements, emf values were determined on a

different batch of the polymer electrolyte using different amalgam prepara-

tions. Agreement between the data sets was good.

A value of the cell impedance was obtained by loading standard resistors

in parallel with the cell and observing the change in emf. The observed

room temperature cell impedance of 3.9 x 106 0 was well within the capabi-

lity of the measuring device and yielded a D.C. conductivity of 6.3 x

10"8 (i cm)- , which is in general agreement with the A.C. value shown by

Armand (2).

The amf of the concentration cell Hg-Na(x l) l PEO'NaSCNI Hg-Na(x 2) is

expressed by (8):

-w - .

-4-

'Eb -(- T a1 RT

Kobe -te)Eid - (1 - t) -T-In -- (I te) -IT In YxKos ( -teid0nF &2 nF y2 x2

The activity coefficients for dilute amalgams (present case) may be

calculated from the relationship (10):

Log - Qx

where

Q - 18.720 - 8.00 x l0-3 T

From this, Eid may be calculated:

-RT X l RTEid - 2 . 3 03 log x2-.*-- 2 . 3 03 Q(xl - x2 )

A plot of Eobs vs. Eid for each x2 (xl being fixed as the stock amalgam,

solution No. 1) yields a straight line (Fig. 2) of slope (l-re).

DISCUSSION

For a purely ionic conductor the value of (1 - te) should be unity.

Least squares treatment of Eob s vs. Eid produced a line of slope 1.005

1 0.005, Fig. 2. This result establishes that the electronic contribution

to the total electrical conductivity of PEO'NaSCN is negligible and that the

ionic component is responsible for the conduction. Since sodium-reversible

electrodes were employed and since the conductivity is close to the

C3Xv-r; epublished ac data, the sodium ion is implicated as the major charge in the

PEO'NaSCN complex.

4. r_ _ _ _ _ _ _ _

kinition of variables:

lobs - the measured emf

to electronic transference number

lid - ideal emf, value of Eob s when te = 0

R - universal gas constant

T - absolute temperature

n - number of electrons transferred

F - Faraday constant

al, a2 - activities of sodium in the amnalgams

TI, Y2 - activity coefficients of sodium in the amalgams

zi, x2 - mole fractions of sodium in the amalgams

ACKNOWLEDGMENT

We wish to thank Mr. J. R. Anderson for discussions and help with the

instrumentation. This research was supported in part by the Office of Naval

Research.

-6-

REFERENCES

1. M. Armand, J. M. Chabagno, and M. J. Duclot, Abstract 6.5, Extended

Abstracts, Second International Conference on Solid Electrolytes, St.

Andrews, Scotland, Sept. 20-22, 1978.

2. M. Armand, J. M. Chabagno, and M. J. Duclot, in "Fast Ion Transport in

Solids," P. Vashishta, J. N. Mundy, and G. K. Shenoy, Editors, p. 131,

Worth-Holland, Amsterdam (1979).

3. B. L. Papke, M. A. Ratner, and D. F. Shriver, J. Phys. Chem. Solids, in

press.

4. D. E. Fenton, J. M. Parker, and P. V. Wright, Polymer, 14, 589 (1973).

5. P. V. Wright, Br. Polym. J., 7, 319 (1975).

6. D. B. James, R. S. Stein, and W. J. Macknight, Bull. Am. Phys. Soc., 24,

479 (1979).

7. L. Rsueh and D. N. Bennion, This Journal, 118, 1128 (1971).

8. R. Galli, F. A. Tropeano, P. Bazzarin, and U. Mirarchi, in "Fast Ion

Transport in Solids," W. van Gool, Editor, p. 573, North-Holland,

Amsterdam (1973).

9. D. F. Shriver, "The Manipulation of Air-Sensitive Compounds,"

McGraw-Hill, New York (1969).

10. T. Mussini, A. Mains, and A. Pagella, J. Chem. Thermodyn. 3, 281

(1971).

-7-

Table I. Ideal and observed emf's of concentration cell at room

temperature.

Solution No. xNa Eid (mV) Eobs (mV)

1 0.04103 - -

2 0.03426 11.13 10.98

3. 0.02944 19.64 19.47

4 0.02406 29.99 29.11

5 0.01928 40.25 40.89

6 0.01440 52.36 52.10

7 0.009477 67.84 68.34

8 0.004753 90.00 89.83

FIGURE LEGENDS

Figure 1. Cell for transference number measurements. A, nickel electrode;

B, polymer electrolyte; C, stopcock; D, teflon in glass valves

for N2 inlet.

Figure 2. Observed vs. ideal emf values for a Na/Hg concentration cell

with NaSCN*PEO polymer electrolyte.

90-

75

.o 60

E4530-

0

w15

010 15 30 45 60 75 90

E (ideal) millivolts

A A

D D

B

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