,~viHhetic MetaA'. 2S (1989) C647 C654 C647

SECONDARY ORGANIC BATTERIES MADE WITH THICK FREE STANDING

FILMS OF ELECTROCHEMICALLY PREPARED POLYANILINE

E. GENIES*, P. HANY* and Ch. SANTIER**

* Electrochimie Mol4culaire, Laboratoires de Chimie,

** Dynamique de Spin et Propri~t~s Electroniques, Service de Physique,

D~partement de Recherche Fondamentale,

Centre d'Etudes Nucl@aires, 85X, 38041 Grenoble (France)

ABSTRACT

Free standing films of polyaniline (FSF-PANi) are synthesized in an eutectic

acid NH4F,2.35 HF solution of aniline (0.6M) on stainless steel and Monel alloy

electrode supports. These films which display fibrillar and crystalline

structures can be made to any dimensions and are promising materials for use as

positive electrodes in lithium-aluminium secondary batteries, because they

deliver 130 Ah/kg and 200 Ah/l (after lamination of the film) at a current of

0.25 mA/cm 2 at 25°C. This corresponds to an average voltage of 3 V with energy

densities close to 400 Wh/kg and 600 Wh/l (after lamination of the film).

With propylene carbonate IM LiCI04, the maximum useful current density is 5

mA/cm 2. Several hundred charge-discharge cycles can be carried out, if the

current density is maintained under i mA/cm 2 at 80Z of the full capacity. Self

discharge is estimated to be in the range of 15~ after one year. The behaviour

of the battery vs temperature indicates that at -0.5°C the capacity is still 26Z

of the capacity at 25°C. A button-cell battery (24/50) of 11.2 mAh and a

cylindrical battery (Sub-C) of 68 mAh made by rolling the FSF-PANi and a

lithium-aluminium sheet, have been made.

I INTRODUCTION

Various kinds of conducting organic polymers (i) and in particular

polyaniline (2,3) have been examined as material for secondary lithium

batteries.

Polyaniline (PANi) can be made by several processes: chemically, by

polymerizing aniline in aqueous acid solutions (4) or in an eutectic NH4F,2.3HF

mixture (4) in the presence of an oxidant (6,7); electrochemically in an aqueous

0379-6779/89/$3.50 © Elsevier Sequoia/Printed in The Nctherhnds

C648

or organic acid solution under constant voltage, potential cycling or constant

current. The working electrode generally used is platinum (5,8,9), but several

investigations have been carried out with other electrodes such as iron (i0),

copper (ii), chromium-gold (12), palladium (13) and graphite (14).

The morphology of electrochemically prepared PANi strongly depends on the

kind of experimental conditions: supporting electrolyte, solvent, current

density, constant or scanning potential, nature of electrodes, agitation or not,

temperature, etc... For example PANi synthesized in aqueous HCIO 4 (15) and HBF 4

(16) solutions shows a fibrillar morphology, while PANi prepared in HNO 3 (17),

HCI (17) and H2SO 4 solutions (18) has a granular surface.

In this paper we report a method for producing "free-standing" polyaniline

films to any dimensions. The material, characterized by FTIR, elemental

analysis, SEM observations and X-ray diffraction appears to be a good candidate

for use as a positive electrode in a secondary lithium-aluminium battery.

II EXPERIMENTAL PART

FSF-PANi was synthesized by electrochemical oxidative polymerization of

freshly distilled aniline (0.6M) in eutectic NH4F, 2.35 HF mixture. The cell

employs two stainless steel or Monel alloy sheets 7 cm wide, 25 cm long and 1.5

mm thick as working and counter electrodes. The polymerization was carried out

at room temperature under controlled current and potential conditions. The

current was limited to 5 mA/cm 2 and the potential limited to 2V. Of course it

can also be carried out using a three-electrode assembly, employing a Cu/CuF 2

reference electrode. In this case the working electrode (anode) was then

controlled at 0.7 V vs the reference electrode.

FSF-PANi easily separates itself from the Monel anode, because during the

electrolysis the electrode is covered by a conductive layer of NiF 2 which

dissolves when the electrode is first washed with water. It is possible that the

fibrillar morphology of the free standing polyaniline films and their

processability can be correlated to the composition of the anodic alloy. We have

observed that with different alloys, the morphology of the film was different.

This must be due to the formation in solution of complexes between aniline and

metallic cations coming from electrochemical corrosion of the alloy.

Effectively, the electropolymerization on platiunum of aniline in NH3/HF mixture

gave very strongly adherent coating which can not be separated as a film,

however if the electrolysis is made in solution containing copper salts as

additive, it is possible to obtain free standing films which separate easily

even on platinum electrode.

For battery applications, we choose to work with propylene carbonate (IM

LiCiO4) and with a negative electrode made by electrodeposition of lithium on

aluminium. The amount of lithium and electrolyte concentration were in large

C649

excess so these materials did not limit the capacity of the cell. Positive

electrodes of icm 2 area were prepared from pellets cut from the free standing

film. The pellets or the full FSF-PANi were washed with an aqueous solution of

ammonia and rinced with water. They were then doped with HClO 4 (pH=0) in water,

washed in a soxhlet apparatus with acetonitrile and finally dried under vacuum.

It is very important to remark that no acetylene black was necessary, whereas

20Z of it was used with chemically prepared PANi powder when forming similar

pellets. The crude FSF-PANi are very porous and thick because of their fibrillar

structure, thus inducing a poor electric capacity per volume of the material;

however it is possible to laminate the film and to decrease the thickness, but

preferably by no more than a factor of 5, in order to maintain enough favourable

ionic transport properties necessary for battery electrodes.

III RESULTS AND DISCUSSION

After washing, FSF-PANi is in the basic form as the polyemeraldine (7). No

significant differences are found between FSF-PANi and chemically prepared PANi

(C-PANi). The surface of the film in contact with the electrolytic solution is

very porous while the surface in contact with the electrode is very compact. On

the SEM micrograph of a cross-section shown in Fig. I. we observe fibrils with

diameters of about 0.3 ~m. The density of fibrils gradually increases going from

the surface in contact with the solution to the surface in contact with the

electrode. FSF-PANi has an average specific surface area of i0 mZ/g, whereas it

is 30 m2/g for C-PANi.

Diffuse reflectance FTIR spectra from 200 to 1800 cm -I of FSF-PANi showed

features similar to C-PANi. On such IR spectra, we observe all valence vibration

frequencies of C=C and C-N bonds at the same frequencies (19). In the out-of-

plane deformation vibration range for C-H bonds (700 to 800 cm-l), the spectrum

of FSF-PANi displays a lower amount of vibrations (700 and 740 cm -I) in

arb. unit

\

8 d~j.

Fig. i. SEM cross-section of Fig. 2. X-ray diffraction of

a undoped FSF-PANi. undoped FSF-PANi.

C650

comparison to C-PANi. These vibrations are attributed to the presence of

trisubstituted benzenic rings (which are probably due to the formation of

phenazine rings issue from cross-linking reactions between polymer chains (20)).

X-ray diffraction profiles performed on C-PANi pellets and on FSF-PANi films

are different (Fig. 2). The electrochemically synthesized polymer (a) displays a

higher degree of crystallinity than the chemically prepared polymer (b). C-PANi

has two peaks at 0 = 10.25 and 12.42 ° and FSF-PANi has three peaks at 0 = 7.43,

10.05 and 12.44 ° (Fig. 2). The peak at 0 = 12.42 or 12.44 ° (corresponding to a

D-value of 3,58 A) can be attributed to the scattering from PANi chains,

resulting from interplanar spacings close to the Van der Waals distance for

aromatic groups (21).

After extraction of FSF-PANi with dimethylformamide, subsequent concentration

of the solution and precipitation of the polymer with acetonitrile, the

benzidine (4,4'-diaminobiphenyl) content in FSF-PANi was determined by liquid

chromatography and found to be about 1 ppm which is comparable to the value for

chemically prepared PANi. The Ames test has been performed on FSF-PANi with

Salmonella Typhimurium TA i00 and Salmonella Typhimurium TA 98 bacteria. The

results indicate that FSF-PANi is not toxic.

BATTERY APPLICATION

A typical charge-discharge curve for a cell containing FSF-PANi at a constant

current of 0.25 mA/cm2 is given in fig. 3. The negative electrode is a

lithium-aluminium electrode and the electrolyte is propylene carbonate /IM

LiCIO 4. The discharge capacity is about 130 Ah/kg at 25°C. It must be noted that

there is about 1.5 cm 3 of solution, the distance between the positive and

negative electrode is 0.8 cm and there is no separator. The variation of the

discharge capacity with the temperature is represented in Fig. 4. It can be

~A POTENTIAL

3513 ~ charge 2.5 ~ g e

2 CAPACITY (Ah.Kg 1)

' 2o ' 4o ' 6o ' ao' foo' 1)o' f4o = Fig. 3. FSF-PANi/PC+LiCIO 4 IM/Li-AI battery

behaviour (0.25 mA.cm -2, 10th cycle, 25°C).

C651

correlated to the increase in the ionic conductivity of the propylene carbonate

/ IM LiCI04 solution and to the increase in the diffusion coefficient value of

the ions in the polymer. The usable capacity is obviously a function of the

current density (Fig. 5). The capacity decreases by a factor of 2.6 when the

discharge current density goes from 0.i to 5 mA/cmZ, the charging current

density being maintained at 0.25 mA.cm-2. We notice that with discharges of 3 to

5 mA/cm 2, it is not possible to carry out a large number of cycles, since a

rapid mechanical splitting of the FSF-PANi occurs. This phenomena is a

consequence of the volume variations of conducting polymers between the reduced

and oxidized states.

If the current density is low enough, the material has time to change its

geometry; if not, the electrode breaks up into particles. With a charge capacity

limited to about 80Z of the maximum capacity and at 0.25 mA.cm -2, the coulombic

efficiency is maintained at nearly 100Z over several hundred cycles without any

degradation of the polymer. The self discharge of the battery is quite low. It

can be 15 % after i year's storage. Such self dischage is probably due to the

degradation of the negative electrode because the cell is not sufficiently

moisture-proof.

We have also made button-cell batteries (24/50) with FSF-PANi. Disks of PANi

of diameter 20 mm and 0.7 to 1.5~mn thickness are cut from FSF-PANi. The negative

electrode was lithium-aluminium alloy made by electrochemical coating of lithium

on aluminium with 100 C. The separator was a disk of polyamide (0.07 or 0.15

n=n). The difficulty in making such button-cells results from the difficulty in

making the electric connections between the electrodes and the body of the cell.

Typical results concerning this type of battery are represented in fig. 6. The

capacity after i0 cycles of the cell was 11.2 mAh. Taking into account that the

total weight of the battery was 5.38 g, the full battery capacity was 2.08

° ~ t / ~

/

/

Ah .kg .

DISCHARGE CAPACITY 150 (Ah.Kg-1) 140 130

120

110

IO0

90

80

70

6O 5O

Dis¢harge capacity

T E M P E R A T U R E : °c

40 6 5 10 15 20 25 3~ 35 40 4'5 50 55 -~'- Fig. 4. Capacity of FSF-PANi/PC+LiCI04 IM/Li-AI battery vs

the temperature (Between 1.7 and 3.9 V, 0.25 mA.cm-2).

C652

Discharge capacity (Ah.kg-l)

\

120

I00

80

60

4O

20 Currenf density

0 05 1 15 2 25 3 35 4 45 5 mAcro

Fig. 5. Capacity as function of the discharge current.

(Charging current 0.25 mA/cm 2, between 1.7 and 3,9 V).

This was our first experience in making a commercial size battery. An

improvement in the technology would of course increases the capacity possibly by

a factor 2 or 3.

However, the interest in free standing films lies in the possibility of

developing a battery by rolling the components and for soft flat batteries. A

cylindrical battery (Sub-C) was made with the help of SORAPEC (22). The positive

electrode was made from two free standing PANi films of 2.9/ilcm (ig) which were

laminated on a Nickel sponge (500g/cm 2) to 0.8 mm thickness. The negative

electrode was also a lithium-aluminium sheet and we used polyamide separator

(0.15 mm). Fig. 7 represent the 50th charge-discharge cycle of the battery in

which the capacity was still increasing. For this initial attempt at fabricating

a cylindrical battery made by rolling the components, the resulting capacity (of

PANi) was 68 Ah/kg.

4 ! Voltage

25 Capacity " mAh

Fig. 6. Behaviour of a button-cell battery (24/50)

(constant current, 0.25 mA, 25°C, lOth cycle)

V o l t a g e

3

2

c a p a c i t y m A h

o ~ 21 ~1 ~1 91 60 70

Fig. 7. Behaviour of a cylindrical (Sub C) battery made by rolling

free standing films of PANi and Lithiual-Aluminium sheet.

(50th cycle, 16 mA, 25°C, electrode area 64 cm2).

C 6 5 3

IV CONCLUSIONS

A free standing film of PANi is a suitable material for use as the active

constituent of an electrode in a secondary battery because of its fibrillar

structure, crystallinity and processability. Its discharge capacity was about

130 Ah/kg (not including the electrolyte) at 250C. This capacity has been

obtained with iOO mg of polymer without any addition of carbon black. The usable

voltage of a such battery with lithium-aluminium negative electrode is about 3 V

and the energy density for the polymer itself is in the order of 400 Wh/kg and

600 Wh/l (after lamination of the film). The feasability of button-cell, and

rolled cylindrical batteries was demonstrated. The characteristics for a fully

commercial battery are a function of the the behaviour of the negative electrode

with respect to the electrolyte and of the level of technology to make the

batteries.

REFERENCES

i L.W. Shacklette, T.R. Jow, M. Maxfield and R. Hatami, Synth. Met., 28 (1989)

C655 (these Proceedings).

2 T. Nakajima and T. Kawagoe, ibid.

3 M. Mizumoto, M. Namba, S. Nishimura, H. Miyadera and Y. Kobayashi, ibid.

4 A.G. MacDiarmid, N.L.D. Somasiri, W.R. Salaneck, I. LundstrSm, B. Liedberg,

M.A. Hasan, R. Erlandsson and P. Konrasson, SprinKer Series in Solid State

Sciences, 63 (1985) 218.

5 E.M. Genies, C. Tsintavis and A.A. Syed, Mol. Cryst. Liq. Cryst., 121 (1985)

181.

A.G. MacDiarmid. J.C. Chiang, M. Halpern, W.S. Huang, S.L. Mu, N.L.D.

Somasiri, W. Wu and S.I. Yaniger, Mol. Cryst. Liq. Cryst., 121 (1985) 173.

7 P. Hany, C. Santier and E. M. Genies, J. Appl. Electrochem., in press.

6

C654

8 J. Bacon and R. N. Adams, J. Am. Chem. Soc., 90 (1968) 6596.

9 B. Wang, J. Tang and F. Wang, Synt. Met., 13 (1986) 329.

i0 M. Musiani, G. Mengoli and F. Furlaneto, J. Appl. Polym. Sci., 29 (1984)

4433.

ii G. Mengoli, M.T. Munari and C. Folonari, J. Electroanal. Chem., 124 (1981)

237.

12 E.W. Paul, A.J. Ricco and M. S. Wrighton, J. Phys. Chem., 89 (1985) 1441.

13 B. Pfeiffer, A. Thyssen, M. Wolf and J.W. Schultze, Electrochemistry of

Polymer Layers, DuisburK, 15-17 Sept. 1986.

14 C.M. Carlin, L.J. Kerley and A.J. Bard, J. Electrochem. Soc., 132 (1985)

353.

15 F. Goto, K. Abe, K. Okabayashi, T. Yoshida and H. Morimoto, J. Power

Sources., 20 (1987) 243.

16 S.-A. Chen and T.-S. Lee, J. Folym. Sc., Part C, 25 (1987) 455.

17 A. Kitani, J. Izumi, J. Yano, Y. Hiromoto and K. Sasaki, Bull. Chem. Soc.

Jpn., 57 (1984) 2254.

18 S. Taguchi and T. Tanaka, J. Power Sources., 20 (1987) 249.

19 H. Kuzmani, N.S. Sariciftci, H. Neugebauer and A. Neckel, Phys. Rev. Let.

60 (1988) 212.

20 J.F. Penneau, E.M. Geni~s and M. Lapkowski, J. Electroanal Chem., in

press.

21 K.J. Wynne and G.B. Street, Macromol., 18 (1985) 2361.

22 SORAPEC: "Soci@t~ de Recherche et d'Applications Electrochimiques"

192 rue Carnot, 94124 Fontenay-Sous-Bois, France.