Polymeric insertion electrodes

Solid State lonics 28-30 (1988) 1192-1196 North-Holland, Amsterdam

POLYMERIC INSERTION EI.~ECTRODES M.G. MINETT and J.R. OWEN Department of Chemistry and Applied Chemistry, University of Salford, The Crescent, Manchester M5 4 WT, UK Received ,1 August 1987; in revised version 15 November 1987

Modified forms of an electronically conducting polymer, polypyrrole, have been prepared in an attempt to form a mixed (ionicelectronic) conducting matrix. The novel polypyrrole polymers have been shown to exhibit superior discharge characteristics when incorporated as positive electrodes in polymer electrolyte cells. The results have been explained in terms of an improved ionic conductivity for polypyrrole.

1. Introduction

Ionically conducting polymers are the basis of a promising new technology in battery manufacture, which is expected to lead to improved performance at lower cost [ l ]. Thin films of suitable polymers such as polyethylene oxide (PEO) can act as the electrolyte in all solid-state cells, which have a number of advantages over constructions containing liquid electrolytes, e.g. long shelf and cycle life, high durability, fewer sealing problems, possibility of miniaturisation for circuit elements, etc. Elecu'onicaiiy conducting polymers (e.g. polypyrrole, polyacetylene, etc.) have been proposed as insertion electrode materials to be incorporated into advanced batteries using liquid electrolytes [2]. However, polymeric electrode materials, having very low intrinsic ionic mobility, rely on the interpenetration of a liquid electrolyte, and such peeetration by a polymeric electrolyte would not occur under normal conditions. A single polymeric material combining both elec~roactive and ion diffusion properties would be expected to show many advantages over polymeric electrode materials currently availab!e, since :~a~c.r diffusion of ions within the electroactive polymer would enhance the rate of electrode utilization. The successful development of a material of this type may lead to a new class of"mixed" conducting polymers which could be useful for many applica-

tions, apart from insertion electrodes, such as electrochromics, sensors, etc.

2. Synthesis and characterization of mixed conducting polymers

2.1. Mixed conducting polymer composites There are many examples in the literature of the synthesis of composite materials of polypyrrole with a wide variety of polymeric materials. This may be achieved by ei;her electrochemical [ 3] or chemical [ 4] polymerization of pyrrole directly into the polymer matrix. Conductive composite films have been synthesized here by exposing a film of PEO soaked in pyrrole to aqueous FeCI3 solution; a black conductive composite ( cr= 10- 4 S era- ~) was formed. However, it was difficult to remove excess FeCI3 and unpoly.m,e~zed pyrroie without removing PEO. A more suiiable n,cthod was to expose a film of FeC13 impregnated PEO to pyrro!e vapour. A composite film with a conductivity of 10-~ S cm-I was formed in this way.

2.2. Mixed cor~duceing polymers A novel polymer having an electron conducting backbone with pendant solvating moieties should exhibit good ionic and electronic conduction. The

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M.G. Minett, J.R. Owen/Polymeric insertion electrodes

backbone is here based on polypyrrole and the pendant groups ate similar to PEO. Two N-substituted pyrrole monomers of this type have been prepared by reactien of N-pyrryl potassium with the requisite di- and mono-chlorinated ethers. ~ N - (CH2) 20-(CH2) 2 - O - ( C H 2 ) 2 - N ~ 1,8-( 3,6-oxaoctan-)..N,N-dipyrrole ( O N D P ) ,

( 1)

~ N - ( C H 2 ) 20- (CH2) 2-O- (CH2) 2-OCH3 N-( 3,6,9-trioxadecyl)-pyrrole ( N T P ) .

(2 )

The polymers were synthesized by anodic oxidation of the monomers in a one-compartment electrochemical cell onto a pbtinum elec,~ode. Zinc triflate in acetonitrile was used as the supr arting electrolyte. Platinum was again used as the counter electrode and a zinc rod was used as the reference. The advantages of this cell were believed to be: (i) The system eliminates the unknown liquid junction potential which occurs when an aqueous saturated calomel electrode is used as the reference. (it) Since reduction of Zn 2+ ~ Z n is the counter electrode reaction (i.e. clean counter electrode chemistry) it is possible to simplify cell design and use a single compartment cell. The polymer~ have been characterized by IR, elemental analysis and four point conducfivily measurements. A possible structure of polyNTP is illustrated in fig. 1.

1193

The electronic conductivity values obtained were as follows: oolypyrrole, 50 S cm- t; polyONDP, 10- 4 S cm- t; polyNTP, 10- 3 S cm3. Electrochemical characterization of polymers Cyclic voltammograms were produced from 0.1 pm fi!-ls of the polymers grown potentiostaticaUy in the ceil as previously described. The films were rinsed in acetonitrile to remove any residual monomers or oligomers and transferred to fresh electrolyte solution before measuring the voltammograms. The zinc reference electrode potential was checked for any drift by checking the potential of the ferrocene/ferrocinium redex couple at regular intervals throughout the experiments. Figs. 2 and 3 show a comparison of the voltammograms obtained for polypyrrole and polyNTP (the voltammograms for the polyNTP and the polyONDP were v e ~ similar). The following points of interest were noted from the voltammograms (the results are summarized in table 1 ):

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Fig. 1. Proposed polyNTP slructt, re showing ionic and electronic conducting pathways.

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M.G. Minett, J.R. OwenlPolymeric insertion electrodes

i 194

for that of polypyrrole (yellow~black). This is believed to be the only example of a double colour change in substituted polypyrroles. (iii)Polypyrrole peaks are much sharper than those of the substituted polymers. However, the broadness of the substituted polymer peaks may be due to the overlap of two oxidation peaks associated with the double colour change. (iv) The lower values of AEp for the substituted polymers when compared to polypyrrole suggests faster redox kinetics.

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100

4. Solid state polymer cells

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Fig. 3. Cyclic voltammogram for polyNTP. Table 1 Cyclic voltammetry data. All potentials are quoted versus Zn reference (Eo for ferrocene/ferrocinium = 1.06 V versus Zn/Zn 2÷ ). Polypyrrole PolyONDP Po|yNTP Monomer oxidation potential (V) 1.65

1.75

1.70

Polymer Epa (V) Epc (V) AEp (mY)

0.62 0.33 290

1.30 1.20 100

1.25 1.15 100

Colour changes reduced--, oxidized

yellow--, black

yellow--, red-green

yellow--, red--, green

(i) lne ~ormauon ano oopmg potenuals of the substituted polymers are more positive than those of polypyrrole alone (these results are consistent with electron withdrawal from the pyrrole ring by oxygen atoms). ('~i) On cycling the films undergo an interesting electrochromic effect: two colour changes are observed ( yellow ~ red --,green ) compared with only one

Films of the mixed conducting polymers were assembled into all solid state cells using lithium as the anode and PEOsLiCIO4 as the electrolyte in order to assess their performance as lithium insertion electrodes. The cells were placed inside a specially designed sample holder inside a Buchii furnace under an Ar atmosphere. Cell discharge experiments were performed at 80°C. (Results performed at higher temperatures resulted in rapid degradation of the polymer.)

4. I. Composite material cell performance The discharge characteristics of a polypyrrolePEO composite were compared with those of polypyrrole alone by discharging across a 104 ~ load. The voltage of the polypyrrole cell fell almost immediately to less than 1 V and then quickly fell to almost zero, whereas the composite cell retained a voltage greater than 2 V for several minutes before gradually tailing off. The Coulombic capacity of the composite electrode was greatly imprnved when compared to polypyrrole alone and this was attributed to the interpenetration of the electrolyte into the electroac.: ...... tlVl~

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surface contact (i.e. an analogous situation to liquid electrolyte cells). The performance of composite electrode cells of this type, from blends of solubilized polypyrrole and PEO, has recently been studied by lnganas et al. [ 5 ], who have also demonstrated an improved utilization of polypyrrole.

M.G. Minett, J.R. Owen/Polymeric inser:ion electrodes

1195

3.5 3.4

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Charge/Coulombs x I0e--3 Fig. 4. Polypyrrole versus polyNTP discharge curves.

4. 2. Mixed conducting polymer cell performance 10 I~m films of the polymers were grown potentiostatically onto a 1 c m 2 platinum disc, and assembled into a lithium polymeric electrolyte cell as previously described (NB no results were obtained for polyONDP since the as-picpatcd fih'ns cl'acked into small pieces and peeled away from the Pt electrode during the drying procedure). A typical result for a constant current discharge comparing polyNTP to polypyrrole is shown (fig. 4). The poiyNTP even when discharged at 10 times the rate of a polypyrrole electrode still gave a much higher capacity.

5. Discussion The experimental results indicate improved discharge characteristics of the substituted polypyrroles

and polypyrrole-PEO composites when compared to ",hose of polypyrrole alone. A possible explanation can be given in terms of an improved ionic conductivity. The discharge rate of a polymeric electrode is limited by the smaller of the two constituent (ionic and electronic) charge mobilities. In the initial state of discharge, electrode overpotentials are generated mainly by the electronic resistance path which will be sufficiently low in all cases. However, after a partial discharge of surface material, subsequent discharge requires conduction through the ionic r~sistance. It is proposed that ionic resistance in polypyrro|e leads to a substantial overootential oreventing further discllarge. This limitation is not as prevalent in the substituted polypyrroles and composite materials since ion conducting pathways are incorporated into the structure.

I 196

M.G. Minett. J.R. Owen/Polymeric insertion electrodes

Acknowledgement We wo'Jld like to thank the S E R C and Chloride Advanced Research for financial support.

References [ 1] M. Armand, Solid State lonics 9/10 (1983) 745.

[ 2 ] F. Bonino and B. Scrosati, in: Materials for solid state ba:teries, eds. B.V.R. Chowdari and S. Radhakrishna (World Scientific, Singapore, 1986) pp. 53-68. [ 3 ] O. Niwa and T. Tamamura, J. Chem. Soc., Chem. Commun. (1984) 817. [4] V. Bocchi and G.P. Gardini, J. Chem. Soc. Chem. Commun. (1986) 148. [ 5 ] O. Inganiis, P. Novak and R. Bjorklund, J. Electrochem. Soc. 134 (1987) 1341.