conductivity of polyacrylonitrile based lithium polymer...

AD-A283 965

Conductivity of Polyacrylonitrile BasedLithium Polymer Electrolytes

Steve Slane and Eric Shapow

ARL-TR-36 1 August 1994

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I August 1994 I Technical Report: Mar to Aug 934 TITLE AND SUBITME S. FUNDING NUMBRS

CONDUCTIVITY OF POLYACRYLONITRILE BASED LITHIUM POLYMER PE: 612705ELECTROLYTES PR: 52MP02P

4. AUTHOA(S)i

Steve Slane and Eric Shapow

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US Army Research Laboratory (ARL) UPORTNU• MR

Electronics and Power Sources Directorate (EPSD) ARL-TR-361ATTN: AMSRL-EP-PBFort Monmouth, NJ 07703-5601

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13. ABSTRACT (Maimum200won*)

Preparation of solid state polymer lithium electrolytes based on polyacrylonitrile(PAN) have achieved room temperature conductivities equal to that of liquid organicelectrolytes. Polymer films of ethylene carbonate, propylene carbonate, PAN, andlithium salts have yielded conductivities as high as 0.004 S/cm at 25 deg C. Thesehigh conductivities make the use of polymer electrolytes a viable possibility inadvanced batteries. Reported here are film preparation technique, conductivitiesfrom -70 to 70 deg C, and Vogel-Tammann-Fulcher (VTF) relationships.

14. SUBJECT TERMS 15. NUMBER OF PAGES

Polymer electrolyte; polyacrylonitrile; lithium electrolyte; 18rechargeable batteries; ionic conductivities ILPRKICODE

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CONTENTS

PAGE

INTRODUCTION .................. ..................... 1

EXPERIMENTAL .I.................. ..................... 1

RESULTS AND DISCUSSION .............. ................ 2

CONCLUSIONS ................ ....................... 10

REFERENCES .................... ...................... 10

FIGURE 1. Arrhenius plot of electrolyteconductivities ........... .............. 3

FIGURE 2. Arrhenius plot of electrolyteconductivities ........... ............... 5



FIGURE 5. VTF Plot ............... .................. 9

TABLES

TABLE 1. Electrolyte Conductivities at 25*C ... ..... 4

TABLE 2. VTF Equation Values ......... ............. 7

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INTRODUCTION

The use of solid polymer electrolytes (SPE) inrechargeable lithium batteries is currently being widelyinvestigated. Polymer electrolytes can be prepared intovery thin films possessing large surface area yielding highpower densities. An energy advantage of a solid flexibleelectrolyte is its ability to enable the design of morevolume efficient battery configurations. In anelectrochemical cell, especially in a reversible cell, aflexible electrolyte can accommodate the volume changes thatoccur with charge/discharge cycles. Polymer electrolytescan increase cell safety by preventing ignition by acting asa shut down separator, if thermal runaway should occurwithin a cell. A thin Li+ ion conducting polymer film actsas both the electrolyte and a separator between the lithiumanode and a lithium insertion compound as the cathode. Theuse of high energy cathode films of reversible compoundssuch as LiCoO2 , LiNiO2 , LiMn 2 0 4 , V2 0 5 , or V60 1 3 with thinlithium foil anodes and the structural flexibility ofpolymer electrolytes make the lithium polymer battery apromising candidate for advanced battery systems forelectric vehicles (EV) or consumer/military electronicsapplications.

One of the first polymer electrolyte chemistriesconsisted of poly(ethylene oxide) PEO-LiX complexes (1,2)which need to operate at around 100°C. New multiphasesystems involve adding plasticizing solvents to PEO-LiX ortrapping liquid electrolyte solutions in a polymer matrix toform a "gel" electrolyte. The latter more "liquid like"chemistries can operate at room temperature and thereforeare of interest for further research. One cf the basicfundamental problems in the development of solid state ionicmaterials based on polymers is the conductivities of thesematerials. Recently, room temperature conductivities ashigh as 10- 3 S cm- 1 have been reported for polyacrylonitrile(PAN) based lithium salt complexes (3). It is ionicconductivity measurements on this chemistry that is thefocus of this report.

EXPERIMENTAL

The preparation of the solid gel electrolytes involvedthe immobilization of LiCIO4 , LiAsF 6 , or LiN(CF 3 SO2 ) 2 inethylene carbonate (EC) and propylene carbonate (PC)mixtures with PAN. The LiAsF 6 (Lithco "Lectro-salt") andLiN(CF 3SO2 ) 2 (3M) were dried under vacuum at 60 0 C for 24 h.ýLiClO4 (Alfa reagent grade) was recrystallized in distilledwater, then dried under vacuum at 150 0 C for 24 h. PC(Burdicke and Jackson) was dried with type 4A molecular

1

sieves for 48 h then distilled under vacuum. EC (Fluka AG)was fractionated under vacuum. Dimethyl carbonate (DMC)(Burdick and Jackson) was fractionated in an argonatmosphere. Karl Fisher titration for EC, PC, and DMCindicated water contents of <24 ppm. Poly(acrylonitrile)(Polyscience Inc.) with an average molecular weight of150,000 was dried under vacuum at 60 0 C for 48 h.

The liquid electrolyte EC:PC:LiX was prepared in a vialwith a stirring bar. PAN powder was then added and themixture stirred to ensure wetting of the PAN. The mixturewas heated slowly in an oil bath to 100WC, avoiding over-heating and decomposing the PAN. The mixture turned to aclear highly viscous gel and was cast between glass plates,with 0.25 mm spacers, and allowed to cool. The resultingpolymer electrolyte was an elastomeric mechanically stablefilm. Two general film compositions were prepared with molepercentages of 40EC:34.75PC:21PAN:4.25LiX (17.6:1 EC+PC:LiX)and 38EC:33PC:21PAN:8LiX (8.8:1 EC+PC:LiX). Variations tothese compositions include the use of DMC with EC/PCmixtures, the addition of ground molecular sieves to form acomposite film, and a film with a lower concentration of PAN(12%).

Electrolyte conductivities were determined from acimpedance measurements using an EG&G PAR model 388 impedancesystem with a frequency range of 5 Hz to 1000 kHz. The testcell was made of ceramic with an electrode configuration ofSS/SPE/SS. A thermocouple was in close approximity to theSPE in the cell. The cell assembly was inserted into awide-mouthed glass reaction vessel packed with molecularsieves, and nitrogen bubbled through. The temperaturetesting (700C to -70 0 C) was performed in a Tenneyenvironmental chamber. All chemical storage, film casting,and cell assembly was performed in a Vacuum Atmospheresargon-filled dry box.

RESULTS AND DISCUSSION

For polymer electrolytes to be of practical use, Li-ionmobility must be high enough to enable useful ratecapabilities in lithium batteries. As a general comparisonof two types of polymer electrolyte chemistries, Figure 1shows an Arrhenius plot of the ionic conductivities of PEO-LiClO4 film, a PAN:EC:PC:LiClO4 film, and a EC:PC:LiClO4liquid electrolyte. The PAN-based electrolyte filmdemonstrates conductivities approaching that of the liquidand a significant increase in ion mobility over the PEOelectrolyte. It is this result, first demonstrated byAbraham and Alamigir for this chemistry (3), that makes amechanically stable free-standing film a possible battery

2

Temperature, 0C

O 60 12 -23 -5010-

10-2

E -3

~10

b_ 10-1

i-5-oC 100

0 Liquid, LiCIO 4 /EC:PC

10- 1 Polymer, LiCIO 4/EC:PC:PAN

v Polymer, LiCIO4/P(EO) 81 0 - 8 1 , I ,,.- , - - -

2.5 3.0 3.5 4.0 4.5 5.0

10 3(T/K)-1

Figure 1. Arrhenius plot of electrolyte conductivities.

3

electrolyte. The difference in conductivity of the PAN-based electrolyte and the liquid at decreasing temperaturesis due to the lower viscosity of the polymer.

Having established that PAN-based films can be preparedand can produce ionic conductivities close to liquid organicelectrolytes, further studies of Li-ion mobility in thesetypes of electrolytes were performed. Table 1 lists thepolymer electrolyte molar compositions, solvent:LiX ratio,and conductivities at 250C of the films studied.

TABLE 1. Electrolyte Conductivities at 250C

ELECTROLYTE Solvent:LiX CONDUCTIVITY(S cm- 1 )

45EC:45PC:LiClO 4 (liquid) 9:1 5.2 x 10-338EC:33PC:21PAN:8LiClO 4 8.8:1 2.9 x 10-340EC:34.75PC:21PAN:4.25LiClO 4 17.6:1 4.5 x 10-456.5:EC:23PC:16PAN:4.25LiN(CF 3 SO2 ) 2 17.6:1 2.0 x 10-340EC:34.75PC:21PAN:4.25LiAsF 6 17.6:1 3.7 x 10-340EC:34.75PC:21PAN:4.25LiAsF 6 :5wt%3A 17.6:1 1.6 X l0-333EC:28PC:13.5DMC:20PAN:5.5LiAsF 6 13.5:1 9.9 x 10-444.3EC:39PC:12PAN:p4.7LiAsF 6 17.6:1 2.8 x 10-338EC:33PC:21PAN:8LiAsF 6 8.8:1 1.0 x 10-3

In Figure 2, the conductivities of three LiX saltcomplexes, LiAsF 6 , LiCIO4 , and LiN(CF 3 SO2 ) 2 with EC:PC:PAN,are shown. As in liquid organic electrolytes, the LiAsF 6electrolyte gives the highest conductivity. The imide,LiN(CF 3 SO2 ) 2 , known as a high temperature stabilizing salt,demonstrated poor conductivity at low temperatures. This isbelieved to be due to precipitation of the imide salt fromthe EC:PC mixture resulting in loss of lithium ions forcharge transfer.

The arrhenius plots in Figure 3 show the conductivitiesof three chemical variations with the LiAsF 6 electrolyte.Changing the solvent to LiAsF 6 ratio from 17.6:1 to 8.8:1(i.e., doubling the amount of salt while holding the PANconcentration at 21 mole percent) lowers the conductivityover the whole temperature range. This again is a viscosityeffect on the ionic mobility. The addition of DMC, which inliquids makes the electrolyte more stable with the lithiummetal anode, lowers the conductivity. This is expectedsince LiAsF 6 /DMC electrolyte has a lower conductivity than

4

Temperature, °C

60 12 -23 -5010-

10-2

E• 1 0-3 A

o 10 40EC:34.75PC:21PAN:4.25LiX0N

10 0 LiAs Fs 6 .10-7 - 3 LiCIO 4

&• LIN(CF 3so02)2

1 0-8 - - I I A

2.5 3.0 3.5 4.0 4.5 5.0

103(T/K)


5

Temperature, 0C

60 12 -23 -5010-1

10-2

0 -o-3

10.

U

CI)

:E10-7 V

,- 0-0

S10

2.5 3.0 3.5 4..0 4..5 5.0103 (T/K)-

0 EC:PC:PAN:LiAsF. (17.6:1)

o EC:PC:PAN:LiAsF6 (17.6:1) w/5wt% 3A sieves

V EC:PC:PAN:LiAsF6 (8.8:1)

0 EC:PC:DMC:PAN:LiAsF 6 (17.6:1)


6

LiAsF 6 /EC:PC in liquids due to its much lower dielectricconstant (E: EC-89.6, PC=64.9, DMC=3.1). A film with a 12%concentraticr of PAN was prepared. Its conductivity (seenin Figure ' at lower temperatures was greater than filmsprepared with 21% PAN. This is expected since the film withless PAN is more liquid like. The electrolyte was "wetter,"but still had the physical integrity of a solid film.

All the arrhenius plots for the gel electrolytesexhibit significant curvature, which is characteristic ofVogel-Tammann-Fulcher (VTF) behavior (1). The VTFrelationship is an application of free-volume distributionto conductivity in polymer dynamics and is expressed by theequation

a = AT-1exp[-Ea/(T-To)] (1)

where A is a prefactor related to the transport coefficient,To is the idealized temperature corresponding to zeroconfigurational entropy, and Ea is the activation energy forcharge transport within the gel electrolytes. Table 2 givesthese calculated values for some of the electrolytesstudied.

TABLE 2. VTF Equation Values

ELECTROLYTE A To (0 C) Ea (eV)

EC:PC:LiClO4 (Liquid) 2.68 176 0.036EC:PC:PAN:LiClO4 (17.6:1) 5.38 130 0.093EC:PC:PAN:LiAsF 6 (17.6:1) 4.12 176 0.043EC:PC:PAN:LiAsF 6 (8.8:1) 5.10 162 0.066

Figure 5 shows the straight line VTF plots from which theactivation energies were determined. These calculationswere performed over a temperature range of -20 0 C to 70 0 C.As shown, the higher the ionic conductivity the lower theactivation energy. This is expected since Ea is calculatedfrom the conductivity. What Figure 5 does show is that theactivation energy of the electrolytes does not change overthe temperature range. This may indicate the solid gel ishomogenous.

7

Temperature, "C

60 12 -23 -50

E 10

b 10 - 2

E 10-.

U 0

0- 4403EC:39PC:12PAN:4.7LiAsF6

0 4OEC:34.75PC:21 PAN'4.25LiAs%.

10-I I I

2.5 3.0 3.5 4.0 4.5 5.010 -8

1O00/T/ (K 1 )


8

-2

-3

C- -4 -

x -5

-6

-7V

0.000 0.005 0.010 0.015 0.020

1 /(T-To)

o EC:PC:LiCIO 4 (liquid)o EC:PC:PAN:LiAsF 6 (17.6:1)A EC:PC:PAN:LiAeF. (8.8:1)

V7 EC:PC:PAN:LiCIO 4 (17.6:1)

Figure 5. VTF Plot.

9

CONCLUSIONS

It can be concluded from this study that solid gelpolymers based on PAN and LiX salt complexes possessadequate ionic conductivities to be used in lithiumrechargeable batteries. The next logical step in thedevelopment of these electrolytes is to study their lithiumstability and interfacial properties and their use in actualanode/electrolyte/cathode configurations.

REFERENCES

1. F.M. Gray, "Solid Polymer Electrolytes," VCH PublishersInc., New York, 1991.

2. J.R. MacCallum and C.A. Vincent, eds. "PolymerElectrolyte Reviews," Vol. 2, Elsevier Applied Science,London, 1988.

3. K.M. Abraham and M. Alamgir, J. Electrochem. Soc., 106,1657 (1990).

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