Lithium intercalation in polyacetylene

Solid State Ionics 8 (1983) 165-168 North-Holland Publishing C o m p a n y

LITHIUM INTERCALATION IN POLYACETYLENE M. FOULETIER, P. DEGOTT and M.B. ARMAND Laboratoire d'Energ~tique Electrochimique, Laboratoire Associd au CNRS No. 265, ENSEEG, BP 44, 38401 Saint Martin d'Hdres, France

Received 16 August 1982 Revised manuscript received 2 December 1982

The coulometric titration curve for lithium intercalation in polyacetylene has been determined using poly(ethylene oxide) based solid electrolyte. The plateaus observed suggest the existence of stages, comparable to graphite. This interpretation seems also supported by the cyclic voltammetry scans on polyacetylene. Lithium intercalation appears as a very slow process with ~ estimated at = lO-13cmZ.s-1.

1. INTRODUCTION Polyacetylene (CH)x , the simplest conjugated polymer is raising considerable interest in fundamental physics (1,2), as well as in electrochemistry. (CH) x is like graphite an amphoteric ~ system, giving intercalation compounds either with cations (Li +, Na +, K +) or anions (CIO~, PF~). The so-called "doped" polymer exhibits a quasi-metallic conductivity. With an energy band narrower than that of graphite, it has been anticipated that reduction (cation insertion) or oxidation (anion insertion) occurs at less extreme chemical activities than for the former compound, allowing the electrochemical preparation of the doped material in organic solvents. Doped (CH)x can be used as a source or a sink of electrons, as for example : (CH) x + xy Li + + xy e- = (LiyCH) x

(I)

(CH) x + xy CIO~ = xy e- + (CH[CIO4]y) x

(2)

Such reactions have been proposed for high energy density batteries (3). However, anion intercalation takes place in the 3.2 - 3.8 V/Li:Li + range for 0 < y < 0.06, a potential where solvents oxidation threshold is observed. For cations, solvent reduction (cyclic esters) or co-intercalation (ethers) is almost certain for Na + and Li + (4-5). The polymer solid electrolytes (7,8) are emerging as a new tool in e]eetrochemistry for precise studies of intercalation, without the drawbacks of liquid solvents. Yazami et al (9) already reported the preparation of non solvated graphite-lithium compounds using such

0 167-2738]83/0000-0000/$ 03.00 © 1983 North-Holland

electrolyte. The feasibility of the first all polymeric all solid-state cell (CH)x/PEO-NaI/ (CH)x was proven by Chiang (10) but no precise determination of the electrode potential was given. We have followed the thermodynamics and kinetics of lithium intercalation in polyacetylene, using the cell : (PEO)8 / (LiyCH)x Li / LiCI04 as a function of the lithium content. Also, cyclic voltammetry adapted to solid-state cells was used to ascertain the results.

2. EXPERIMENTAL -Polyacetylene was obtained from the described Shirakawa's technique (|l) in a |15~ (coulometry) and ]8B (cyclic voltammetry) film. Cis-trans isomerization proceeded from annealing at 80°C prior to experiments. PEO-LiCIO~ (O/Li = 8) complex was poured . from an acetonltrile solution of PEO (MW : 5.106 ) (Polyscience) and LiCIO 4 onto a PTFE plate and the solvent evaporated to yield a ca 200~ thick film (7). - Lithium : 2 mm thick foil (Ventron Corp.) -

The electrodes and the electrolyte were punched in an argon dry box (02, H20 < ] vpm)

M. Fouletier et al. / Lithium intercalation in polyacetylene

166

to 7 mm disks and stacked in a grommeted button cell container. The system was thermostated at 83± 3°C with a transparent B~chi TO 50 oven, above the softening point of the polymer electrolyte (70°C) to ensure good electrical contacts. The cell circuit was closed on a coulometer and a resistance load adjusted to give a ca lO~A.cm -2 discharge current, for a charge injection of 1OO mC. The oc voltage was recorded after equilibrium was reached i.e., a potential drift smaller than ]mV.hour -I (typically 2 to 3 days).

Details on the three electrode rig for solid electrolytes have been published elsewhere (8, 12). In our case, the reference and counterelectrode were of lithium metal.'Operating temperature was llO°C.

3. RESULTS AND DISCUSSION Spontaneous

discharge of the cell

:

(PEO)8 / (CH) x Li / LiCIO4 corresponds to reaction (1) where lithium is intercalated in the (CH) x matrix. Fig. I shows

0

0t 2

1

2

3

i

i

,

4 Q {mC) i

li

\\

,,e.%

\ \ °°eeeo

the potential vs composition curve for this system. The rapid decline in potential at very low lithium concentration could be attributed to high electron affinity of the delocalized system or possibly to a slight oxygen contamination (<3 at %), unavoidable in a rapidly flushed (40 m3.h -I) dry box. The striking feature of the curve" in Fig. l is the appearance of plateaus instead of a monotonously decreasing potential. Thermodynamical models for intercalation predicts in this case a pseudo-two phase behavior (13), expressing the fact that not all solute (Li) compositions are allowed in the host (CH)x matrix. In the case of graphite, the well known formation of stages gives similar titration curves (9, 14). The similarities between 2D graphite and ID (CH)x , both well crystallized compounds gives strong clues for the existence of "stages" in the Li(CH) x system. In this case, a model where an integer number of polyacetylene chains in bundles or planes are separated by alkali metal atoms can be proposed. The end points of the plateaus correspond in our hypothesis to roughly y = 0.06 (4 th stage) and y = O.13 (2 d stage). Up to now, a stage model for (CH) x has not been proposed, with no precise data available on the alkali metal intercalation. Also, X rays diffraction patterns on Na(CH) x and Li(CH) x system show only an amorphous phase (15) due to solvent co-intercalation, non solvated potassium giving a tetragonal unit cell. The maximum lithium incorporation corresponds to y = 0.25 (CH/Li = 4) at 500 mV/Li:Li +. This value of the potential agrees well with the principle of chemical intercalation of Li + via naphtalene (500 mV) anion radical. Only the benzophenone di-anion ( 1 V / L i : L i +) yields partially lithiated compounds (5) but not the mono-anion (1.5 V). Beyond y = 0.25, the oc voltage rises very slowly to a higher value ca l V/Li:Li +. This surprising feature arises probably from an irreversible degradation of the (CH)x conjugated chain. Morrison et al (16) have shown that lithium vapor can react on conjugated systems to yield poly-lithiated compounds. Polyacetylene would give at the interface non-conjugated segments :

• • • ~ o e~, -

1

Li

Li

i I J J - C - C - C - C - C I I E I

Li

i

H

0 Fig.

01.

0.2

y

I. Potential vs composition curve for (LiyCH) x ref. : Li °, temperature : 83°C, Electrolyte : (PEO) 8 LiCIO 4.

Li

H

Li

H

H

J

H

With no electronic (e-) or ionic (Li +) conductivity due to covalent C - Li bonds, similar to those in n-butyl lithium. The kinetics for Li diffusion in the host polyacetylene can be evaluated from the oc relaxation after charge i n j e c t i o n ; this potential reflecting the composition of the electrode-electrolyte interface. According to the thermodynamical model for intercalation compounds (13), the potential is given by the Nernst equation. In the case of a small

M. Fouletier et al.

/

Lithium intercalation in polyacetylene

167

departure ~ from the equilibrium composition Yi, we have : RT Yi - E e(t) = e i +--~- In Yi

RT e . (3) F Yl

ei

As the intercalated lithium diffuses away from the interface, the diffusion coefficient can be expressed, according to the treatmen~ of W i n n e t al (17) : e(t) = e i

-

RT ~Q.e F Qi ~ V ~ t

-0.1

(4)

with 6Q : elementary amount of charge injected Qi : total amount of charge injected e : thickness of the sample

-0.2

D~ V.mn-, de/=d0.r5

-0.3

= 10 -13 cm 2 s -| at 83°C.

-04

.,

"~.. Fig. 3. Cyclic voltammogram

for (CH) x.

ref. : Li °, temperature : IIO°C, S : O.13 cm 2, scan rate 500 mV.mn -I, electrolyte : (PEO)8 LiCIO 4.

1650

°~, "~. 1600

1550 5.5

'

'

6.0

6.5

'

t "v2

,

(lO'3xS"v2)

Fig. 2. Potential recovery after charge injection, e = f(t-I/2). ref.

: Li °, y = 0.04, temperature

= 83°C.

This ~ value appears considerably lower for (CH)x than for 2D intercalation compounds (10 -8 cm2.s -I for TiS2, 5 x 10 -8 cm2.s -I for graphite), a phenomenon tentatively attributed to the considerable covalent character of the polarized C--Li + bond. Our determination is however in good agreement with the "slow" mechanism for2intra-fibrillar diffusion (3 x 10 -16 cm .s -I at room temperature) observed by Rachdi (6) from EPR measurements.

The trace for the Li(CH) x system is shown in Fig. 3. Graphite was tested in similar conditions, Fig. 4, to compare the two conjugated polymers.

Graphite shows two reduction waves at 500 mV (A) and < IOO mV (B) associated with the staged intercalation of the alkali metal, the two steps appearing reversible upon reoxidation (peaks B' and A' respectively). Peak B' is suppressed if the cathodic limit is set at 300 mV, before the appearance of the corresponding second reduction step B. A similar treatment for (CH) x shows also two reversible reduction waves at IV(C) and 500 mV (D) corroborating the idea of "stages" formation. However, the current densities are two orders of magnitude lower than for graphite due to either the low electronic conductivity of pristine (CH)x or to the slow lithium diffusion. When the anodic limit is extended to + 4.4 V, the onset of (CH) x oxidation with anion insertion appears at + 3.5 V in relative agreement with the published value (18). However, this reaction is electrochemically irreversible as no reduction peak is seen in this potential range on subsequent cathodic sweep ; these results are in conflict with the accepted idea of a reversible "p" type doping in aprotic media. It is thus most likely that t h e a n i o n i c mobility is strongly solvent assisted. An exception may be found with iodide ion, as "p" doping can proceed with gaseous molecular iodine diffusing in the open (CH)x structure (Io). In our case, the charge of the immobile anion can only be compensated upon reduction

168

M. Fouletier et al.

/ Lithium

intercalation in polyacetylene investigation to evaluate the feasibility of all solid state cell.

A" B"

2

5. ACKNOWLEDGEMENTS We thank Dr. P. Bernier and F. Rachdi from the Groupe de Dynamique des Phases Condens~es (Montpellier, France) for kindly providing the (CH)x samples.

-20f

-40

6. REFERENCES II

-60

A

0

I

I

1

2

I

e (V)

Fig. 4. Cyclic voltammogram for graphite. ref. : Li °, temperature : IIO°C, S = 0.13 cm 2 , scan rate : 500 mV.mn -I electrolyte : (PEO)8LiCIO 4.

through Li + insertion, with nucleation. Indeed, the height of the first reduction wave (A) becomes dependent upon the anodic current passed.

4. CONCLUSION The advantages of PEO-based electrolytes appear evident to avoid the problems of solvent reactivity and co-intercalation for insertion electrode materials. The Li-(CH)x and Li-graphite systems appear quite similar and the postulated existence of "stages" or any types of sublattice ordering for one-dimensional systems is an important point of theory, especially when dealing with the semi-conductor metal transition. When evaluated as a candidate for negative electrode material, polyacetylene is penalized if we consider the existence of a high voltage plateau at 1 V for 0.06 < y < O.13. Also, the apparent irreversibility of lithium intercalation beyond x " 0.3 is a problem for overcharge cannot be avoided locally in practical battery electrodes. Our findings of irreversible anion intercalation in the absence of solvent has not yet been observed and deserves some futher

(I) Molecular Metals, ed. W.E. Hatfield (Plenum Press, New York, 1979). (2) Polymgres Electroactifs, ed. P. Bernier, B. Payet (CNRS, Montpellier, 1982). (3) D. Mac Innes, M.A. Druy, P. Nigrey, D.P. Nairns, A. Mac Diarmid and A.J. Heeger, J.C.S. Chem. Comm. (1981) 317. (4) G.C. Farrington, D. Frydrych and R. Huq, in : International Meeting on Lithium Batteries, Roma, 1982, abstract n ° 22. (5) J.J. Andre, B. Franqois and M. Bernard, in : (2), p. 104. (6) F. Rachdi, Thesis, Montpellier (1981) (7) M.B. Armand, J.M. Chabagno and M.J. Duclot in : Fast Ion Transport in Solids, ed. P. Vashista, J.N. Mundy, G.K. Shenoy. (8) M.B. Armand, M.J. Duclot and Ph. Rigaud, Solid State lonics 3/4 (1981) 429. (9) R. Yazami and Ph. Touzain, in : International Meeting on Lithium Batteries, Roma, 1982, abstract n o 23. (10) C.K. Chiang, Polymer 22 (1981) 1454 (11) H. Shirakawa and S. Ikeda, Polym. J. 2 (1971) 231. (12) A. Kone, M. Armand and J.L. Souquet, Electrochim. Acta 27 (1982) 653. (13) M.B. Armand, in : Materials for Advanced Batteries, ed. D.W. Murphy, J. Broadhead, B.C.H. Steele (Plenum Press, New York, 1980) p. 145. (14) S. Aronson, F.J. Salzano and D. Bellafiore, J. Chem. Phys. 49 (1969) 434. (15) R.H. Baughman, R.R. Chang, H. Eckardt, R.L. Elsenbaumer, J.E. Frommer, D.M. Ivory, G.G. Miller, A.F. Preziosi and L.W. Shacklette, in : Am. Chem. Soc. Meeting, Las Vegas, 1982, abstract n ° 170. (16) J.A. Morisson, C. Chung and C.J. Lagow, J. Am. Chem. Soc. 97 (1975) 5015. (17) D.A. Winn, J.M. Shemilt and B.C.H. Steele Mat. Res. Bull. 11 (1976) 559.