J. MATER. CHEM., 1991, 1(2), 157-162 157 FEATURE ARTICLE Solid Electrolytes and Mixed Ionic-Electronic Conductors: An Applications Overview

Anthony R. West Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen AB9 2UE, UK

A review of the various applications of solid electrolytes and mixed conductors is presented. Discussion focuses on their uses in: sensors; high-density batteries; solid-oxide fuel cells; ion-exchange materials; optical waveguides; solid-state batteries; thin-film electrochromic devices; and high-T, ceramic superconductors. The potential for future development is also discussed.

Keywords: Feature article; Solid electrolyte; Ionic-electronic conductor

Solid electrolytes are an unusual group of materials which indicate some likely future developments. Specifically excluded have high ionic conductivity with negligible electronic conduc- from consideration here are polymeric electrolytes and poly- tivity. Examples are now known involving high conductivity meric mixed conductors; these have been reviewed recently in of most monovalent and some divalent ions, e.g. Ag+ in ref. 3 and 4. RbAg415, Na' in /?-alumina, Li' in H-doped Li,N, 02-in 9Zr0, * 1 Y 203(yttria-stabilised zirconia), F- in PbSnF4 and Batteries H+ in HU02P04*4H20(hydrogen uranyl phosphate, HUP) (Fig. 1). There is another group of materials, mixed ionic- The great upsurge of research into solid electrolytes (also electronic conductors, that have high conductivities of both called superionic conductors or fast-ion conductors) in the ions and electrons. Solid electrolytes and mixed conductors 1960s, which commenced with the discovery of the ion- have been surveyed recently in ref. 1 and more comprehen- conducting properties of fl-alumina' and various Ag salts,6 sively in ref. 2. was motivated to a great extent by the possibility of building The purpose of this review is to summarise the current new high-density secondary battery systems. The concept of status of the applications of these materials (Table 1) and to the Na/S cell, using molten electrodes separated by a solid p- alumina electrolyte, originated from the Ford Motor Com- pany and has since been developed vigorously in several Table I Applications of solid electrolytes and mixed ionic/electronic conductors major laboratories worldwide. The main Na/S cell designs that have been tested contain a p-alumina tube, closed at one batteries, primary or secondary end, with one molten electrode inside and one outside (Fig. 2). sensors, gas pumps The cell operating temperature is 300-350 "C, in order to fuel cells, especially solid-oxide fuel cells maintain both electrodes and the sodium polysulphide dis- electrochemical reactors charge products in the liquid state (Fig. 3). The cell voltage supercapaci tors synthesis of new materials by ion exchange waveguide fabrication by ion exchange optimisation of superconductivity by oxygen (de)intercalation lithium (de)intercalation, new materials, solid solution electrodes electrochromics, smart windows and displays Ak03 insulator

Na

Al can

j3-alumina electrolyte

sulphur + C felt -5 t \ 1 103 KIT Fig. 2 Schematic drawing of a sodium-core, Na/S cell: Fig. 1 Some conductivity Arrhenius plots 2Na + XS e Na,S, 158 J. MATER. CHEM., 1991, VOL. 1 reaction is 2Na + (Ni, Fe)Cl, +2NaC1+ (Ni, Fe) The purpose of the NaAlCl, is to act as an ionically con- I I I I I I ducting liquid contact between the solid electrolyte and the I solid electrode, i.e. the NaC1-FeC1,-Fe, mixture. The cell may be readily assembled in the discharge state, from a mixture of NaCl and Fe/Ni, impregnated with NaAlCl,. The I i cell has a higher voltage, 2.35 V for Fe and 2.57 V for Ni, - cell 400 operating and lower operating temperature, 250 "C,than the Na/S cell; region prototype batteries giving several hundred cycles of operation, have been tested. It remains to be seen whether the perform- 300 - ance (densities of 88 W h kg-' and 65 W kg-' have been 0 quoted for a 66 cell Na/NiCl, battery) can be optimised to i= exceed that of Na/S batteries. A variety of alternative battery systems using solid electro- lytes have been proposed over the years but none has received the same amount of attention as the 'a batteries' described above. The only example that has achieved commercialisation I Ill I 1 1 is the Li/12 miniature heart pacemaker primary battery. It +Na Na2S2 Na2S4Na2S5 S contains as the solid electrolyte a thin film of LiI electrolyte composition which forms in situ on assembly when the two electrodes are Fig. 3 Phase diagram for the Na-S system and cell open-circuit placed in contact. This and other potential batteries, many voltage at 350 "C based on polymeric materials, have been reviewed.' '

Gas Sensors and Pumps is 2.08 V during initial discharge, while the discharge products Oxygen sensors have been important in various applications are in the region of liquid immiscibility (Fig. 3), and then falls for determining oxygen contents of gases and liquids. Most gradually to 1.78 V, at which point Na2S2starts to precipitate. are fabricated from a tube of an oxide ion conductor such as For practical reasons, the onset of such precipitation is taken yttria-stabilised zirconia (YSZ), bismuth oxide (in oxidising as the fully discharged state. environments) or thoria (in reducing environments), Fig. 4. Na/S cells have a theoretical power density of 760 W h The tube is coated with inner and outer electrodes of porous kg-'; in practice, power densities of 150-200 W h kg-' are Pt and the potential difference that develops between the routinely achieved in individual cells. The principal research electrodes may be related to the difference in oxygen partial and development groups that have been assembling and pressure in the two compartments. One of the compartments testing multicell batteries, including Brown Boveri (West contains a reference gas, e.g. air, 0, or a metal-metal oxide Germany), Chloride Silent Power and Beta R & D (UK), mixture such as Ni-NiO. Ford (USA) and Yuasa/NGK (Japan), report performance Oxygen sensors are used commercially for monitoring gas levels that approach the 1000 cycles of reliable operation that compositions in combustion-plant and metallurgical processes are required for electric vehicle applications. For instance, the and for determining the amount of oxygen dissolved in molten most recent data available on the 360 cell battery from Brown metals.12 They are also used in car exhaust systems (2 probe) Boveri reveal densities of 85 W h kg-' and 120 W kg-'.7 This to help optimise the fuel : air ratio. compares favourably with the power density of lead acid The same cell principle that is illustrated in Fig. 4 is also batteries, typically 20-40 W h kg-'. used in oxygen pumps.13 The two electrodes are short- There are many problems that can lead to a decrease in circuited and oxygen gas may then be pumped from one battery performance, e.g. short circuiting through the electro- electrode compartment to the other. Commercial devices are lyte walls, resistance rise associated with the precipitation of available and are used to purify oxygen or to provide con- impurities (especially Ca leached from the ceramic electrolyte), trolled oxygen atmospheres in studies of, for example, the corrosion of the container and consequent loss of sulphur. corrosion of metals and the cultivation of micro-organisms. Individually these problems appear not to be insurmountable; however, there is still the necessity to improve cell reliability. The early fears that the p-alumina electrolyte may be thermo- dynamically unstable when in contact with molten Na (owing to the leaching of oxygen from the conduction planes, followed Yl by partial collapse of the crystal structure and loss of ionic conductivity) appear to be groundless. Given the twin factors of diminishing fossil-fuel supplies and increasing environmental awareness surrounding atmos- pheric pollution, it seems reasonable to expect some degree of commercialisation of Na/S batteries within the next decade, either for electric-vehicle or power-station load-levelling appli- cations. An interesting alternative to the Na/S cell that has recently environment been announced*-" is the Zebra cell, Na/MCl,: M = Fe, Ni. /c to be measured It uses much the same design and technology as the Na/S -1 cell but the cathode compartment has, instead, a mixture of Fig. 4 Design of an oxygen sensor based on yttria-stabilised zirconia liquid NaAlC1, and solid Fe (or Ni) Cl,. The cell discharge solid electrolyte. I/= (RT/nF)In [p"(O,)/p'(O,)] J. MATER. CHEM., 1991, VOL. 1 159 Solid Oxide Fuel Cells (SOFC) with electrolyte management, corrosion and maintenance. (4) Air pollution should be small. (5) High-grade waste heat is Dramatic advances have been made in the development of produced, leading to possible combined heat and power solid oxide fuel cells in the past few years, as signalled by the (CHP) applications for SOFCs. first international SOFC conference in 1989.14 Much of this Recent targets for both Japan and the USA include 25 kW impetus has come from Westinghouse, USA but recently, cells for 1990; it therefore seems likely that, within a few years, major EEC and Japanese programmes have also commenced. fuel cells based on the SOFC principle, may finally make a The basic principle of the SOFC is shown in Fig. 5." It major contribution to energy management programmes. uses the oxide ion conducting ceramic, yttria-stabilised zir- conia, to act as separator and solid electrolyte between the fuel (CO, H,, CH, etc.) and air/oxygen. The air electrode is Electrochemical Reactors based on the mixed conductor perovskite, LaMnO,, and the fuel electrode is an electronically conducting Ni/ZrO, cermet. Yet another application of YSZ and similar oxide ion conduc- Cell operating temperature is 1000 "C, at which the electrolyte, tors, in a configuration similar to that of Fig.4, is for the the most resistive component in the cell, has a conductivity electrochemical partial oxidation of hydrocarbons, e.g. natural gas, to give industrially useful products such as CH30H and of 0.1 $2-' cm - Typical cell voltage is 0.7 V. The choice of cell components is such that they should be C2H4.Development work is still at an early stage and yields compatible with each other over long periods of time at high are low, but this is, nevertheless, seen as an important growth temperatures and it may be that the compositions of the a~ea.'~,'~Choice of electrode is critical so as to catalyse the components are not yet optimised. The original Westinghouse oxidation. Vayenas has shown, in the NEMCA (non-Faradaic design, which has operated at the 3 kW level for 2500 h, is a electrochemically modified catalytic activity) technique, that tubular design, but there is now increasing interest in a flat- application of a voltage to mixed conducting electrodes such plate configuration (Fig. 6). This is made feasible by advances as Bi2O3-Pr60,, leads to greatly enhanced rates for the in ceramic fabrication, using tape-casting methods to fabricate oxidation of methane.I7 The mechanism of oxidation is the planar components, although there are still doubts about unclear but may involve the active 0- species as an inter- the long-term mechanical stability of such structures. The mediate. individual three-layer cell 'sandwiches' are separated by a bipolar plate of electronically conducting LaCrO, which has Supercapacitors a corrugated structure on either side to permit the easy flow of air and fuel over the respective electrode surfaces. The amount of charge that can be stored in a capacitor is There are several intrinsic features of SOFCs that make limited by the area of the electrodes; double-layer capacitances them attractive as power sources. (1) At the temperature of formed between an ionically conducting electrolyte and a cell operation, 1000°C, methane (natural gas) may be used metal electrode are typically optimised at a value between directly as the fuel. In the competing molten carbonate fuel 1 and 10 pF cmV2,since capacitance, C, is proportional to cell (MCFC) fuel reforming is necessary thereby leading to a A/d, where A is the electrode area and d is the double-layer reduction in its efficiency. (2) Fuel conversion efficiencies of thickness. Greatly enhanced apparent capacitances have been 50-60% should be possible, which is much higher than is achieved by using as the electrode a fine mixture of electrode obtainable with, e.g. MCFCs. (3) There should be few problems and solid electrolyte. Thus, in cells of the type: graphite, RbAg41, /R bAg,I ,/grap hi te, R bAg,I , a finely ground mixture of electronically conducting graphite and ionically conducting RbAg,I, is used as the electrode 10 Ni.Zr02 cermet anode 1 material. With this, interfacial contact areas of many square metres per gram are obtained and capacitances as high as 1-10 F are achieved in small, gram-size devices.18 I I -40 LaMn03 cathode EXCESS The time constant, z, of a capacitor is given by the magni- e- ,, ,, 02' 4e - 20~- * tude of the RC product, which for example may have a value / AIR AIR of 100 for a supercapacitor containing an electrolyte of Fig. 5 Schematic of a solid oxide fuel cell. Adapted from ref. 15 resistance 1OR. Such a capacitance can be effective only at low frequencies, since from the relation or= 1, co corresponds to the frequency at which the charge stored on a capacitor CURRENT FLOW reaches lje of its limiting value. In the above case, the full magnitude of the capacitance would be observed only at angular frequencies, o,considerably less than 10 - ' Hz.

y YSZ Synthesis of New Materials by Ion Exchange Solid electrolytes are ideal materials for carrying out ion- CELL f PLATE exchange reactions since they have mobile ions of one type REPEAT within a rigid host framework. Using ion-exchange methods, new materials can be synthesised that, thermodynamically, 1 are metastable and could not be synthesised by other means, such as direct reaction of the components. Sometimes, the new materials have propertiesjstructures that are of techno- logical importance. FUEL Following on from the discovery of the high Na+ ion Fig. 6 Schematic of a parallel plate SOFC design. Adapted from conductivity in /?-alumina, there was considerable interest in ref. 15 studying the structures and properties of ion-exchanged 160 J. MATER. CHEM., 1991, VOL. 1

/?-aluminas.l9 This remained a topic of essentially academic crystals by partial Na Ag exchange,28 but the interiors only interest until the discovery, by Farrington and Dunn, that of the crystals are allowed to ion exchange. This is done by Na+ ions in Nap"-alumina could be ion exchanged for a first coating the crystals with a diffusion mask of Ni-Cr, range of divalent and trivalent cation^.^'-^^ These materials with a polyamide overcoat, and then etching the crystal end provided the first examples of mobile divalent cations in solid faces uia laser lithography to expose a selected set of conduc- electrolytes; the most remarkable is Pb2+p"-alumina whose tion planes. The masked crystal is immersed in a bath of conductivity is comparable to that of Nab"-alumina over a molten AgN03 and these inner conduction planes undergo very wide temperature range (Fig. 7). ion exchange. The resulting material has a high refractive The synthesis of trivalent p"-aluminas has led to a new index inner region, associated with the Ag p-alumina and this family of solid-state laser materials. In particular, Nd3+pff- buried waveguide structure is found to be very efficient for alumina has luminescence properties that compare very coupling and guiding light. favourably with those of Nd-YAG. For instance, its absorption spectrum contains an anomalously large absorption coefficient Optimisation of Superconductivity by Oxygen at 573 nm, with an oscillator strength nearly 10 times that of (De)intercalation Nd-YAG at the same wavelength. Other transitions in the two materials have comparable oscillator strengths.24 The The high T, ceramic superconductors such as YBa2Cu30, high oscillator strength, coupled with long fluorescence life- and Bi2Sr2CaCu20dare mixed oxide ionic/electronic conduc- times at high Nd concentrations, leads to potential appli- tors. Their composition 6 is variable and the different values cations as lasers. One of the current problems that prevents are achieved by processing the materials at different tem- full commercial exploitation is associated with the difficulty peratures and oxygen partial pressures. The critical tempera- in growing large single crystals of p"-alumina: crystals tend tures, T,, generally vary greatly with oxygen content. In to be small plates of cross-sectional area several mm2 but of Bi2Sr2CaCu,0B,T, is optimised at 87 K for 6 = 8.1 85 (Fig. 8). thickness < 1 mm. The wide range of optical properties exhib- Such a value of 6 can be obtained, for example, in air at ited by the lanthanide, transition metal and Cu' /I"-aluminas 820 "C or in N2 at 400 0C.29 have been reviewed in ref. 23. At temperatures above 400-500 "C, especially in fine- grained powders, oxide-ion diffusion rates are sufficiently Fabrication of Waveguide Materials by Ion Exchange rapid that samples can respond fairly rapidly to changing T and p(02),but in dense ceramics full equilibration and hom- The ion-exchange process discussed above may be used, under ogenisation may be difficult. In the well studied YBa2CuJOd closely controlled conditions, to give inhomogeneous mater- materials, there is still confusion as to how T, varies with 6 ials. The associated compositional variations may lead to (Fig. 9). In samples that have been quenched after high- variations in refractive index and therefore, potential appli- temperature equilibration, T, varies approximately linearly cations in waveguides. with 8.30 But in samples that have been prepared by oxygen LiNb03 single crystals may be converted into high-index deintercalation at relatively low temperatures (400 "C) there optical waveguides by proton exchange of the surface lay- is evidence for plateaux at 90 and 60 K in the plot of T, us. er~.~~.~~The surface layers form a solid solution 8.31 Very recent results suggest that the plateaux are caused (Li, -xHx)Nb03whose structure depends on both the crystal- by ordering of oxygen ions at low temperatures, together with lographic orientation of the surface and the composition x.~' associated changes in the electronic structure. Thus, it is LiNb03 is not usually regarded as a solid electrolyte, but the possible to take a quenched material, anneal it for several Li+ ions have sufficient mobility under the conditions used, days at 100-200 "C and generate increased T, values, with 200 "C in benzoic acid for several days, for partial ion plateaux in plots of T, us. x similar to those in Fig. 9. exchange to occur. It is clear that oxide ion conduction is fundamental to the Waveguides have been fabricated from p-alumina single processing and optimisation of the properties of the ceramic superconductors, and is important not only in controlling the overall oxygen content, 8, and therefore the hole concen-

85

80 s L" 75

70

I I I 1 I I I I I 1 8.15 8.16 8.17 8.18 8.19 8.20 2 3 4 5 6 6 in Bi,Sr,CaCu,O, lo3 KIT Fig. 8 Critical temperature us. composition for Bi,Sr,CaCu,O, Fig. 7 Conductivity Arrhenius plots of p"-aluminas, from ref. 23 (ref. 29) J. MATER. CHEM., 1991, VOL. 1 161

groups dimerise to forin octane. This is essentially an internal redox/intercalation reaction. The reverse process of deintercalation or delithiation may be carried out using a variety of methods, e.g. electrochemical deintercalation or treatment of a sample with a solution of I2 in acetone (LiI is insoluble in acetone and gradually precipitates as Li is removed). As well as acting to reverse intercalation reactions, this method may be used to synthesise entirely new materials. For example, a new form of COO, has been synthesised by deintercalation of Li from LiCoO,.

Electrochromics, Smart Windows and Displays Commercial electrochromic devices are now available based on the reversible intercalation of protons into thin films of W03, yielding a coloured tungsten bronze.33 The device structure is shown schematically in Fig. 11 and is made by evaporation onto a glass substrate of successive layers of indium tin oxide electrodes and W03. The proton source is 7.0 6.8 6.6 6.4 hydrated Ta205. In the OFF state, the device is colourless and transparent. In the ON state, H+ ions intercalate the 6 in YBa,Cu,O, W03 and the accompanying electrons enter the 5d band of Fig. 9 Critical temperature us. composition for Y Ba,Cu,O, for W, giving a bronze of nominal stoichiometry samples prepared by (0)quenching from high temperatures3' and HXW~!-,W~O3.Absorption of light by the 5d electrons is by (0)low-temperature deintercalation of oxygen3' responsible for darkening, which occurs in a matter of seconds. Such structures have been proposed as large-scale coatings on 'smart windows' for better heating/lighting management tration, or average oxidation state of copper, but also in of buildings, but none are yet at the commercial stage owing achieving structures with ordered defect arrangements and to problems in fabricating large-area thin-film structures of modified T, values. sufficient quality and uniformity. Two small-area devices are commercially available, antidazzle car mirrors from Schotts Lithium (De)intercalation, New Materials, Solid and electrochromic spectacles from Nikon. Solution Electrodes The ability to introduce lithium ions into or remove lithium Future Prospects ions from certain transition metal compounds gives rise to a All of the possible applications listed in Table 1 have been variety of new phases and solid solutions, some of which find demonstrated convincingly in laboratory-scale experiments. application as reversible electrodes in prototype high-density Several are undergoing development and testing on a larger battery systems.32 The principle is illustrated in Fig. 10 in scale, and others, such as sensors and electrochromic thin- which TiS, behaves as an intercalation host, accepting lithium film devices, are actually on the market place. Since these ions from the electrolyte/anode and electrons from the external various feasibility studies have already been successfully car- circuit to form solid solutions, LixTiS2.The requirements for ried out, future developments are likely to depend on economic the host structure (TiS,) are to (a) accept electrons and be and environmental factors and on whether sufficient resources electronically conducting and (b)accept Li' ions and for them are made available to turn laboratory-scale devices into to exhibit high mobility. Many host (oxide, sulphide, etc.) commercial products. It is therefore difficult to make predic- structures have been intercalated successfully with lithium. tions as to which applications are likely to achieve full Currently there is much interest in V,OI3 as a possible battery commercialisation, other than for those which are already cathode since its Li+ diffusion rate is higher than that of undergoing large-scale development. TiS2. Of more interest is to enquire whether any further develop- A convenient means of carrying out 'lithiation' is to immerse ments in new materials or improved properties are possible samples in n-butyl lithium dissolved in hexane. The n-butyl or desirable. Many research groups are looking for new solid lithium acts as a source of lithium and the residual n-butyl electrolytes with the particular objectives of finding (a) high oxide ion conductivity at intermediate temperatures (200-500 "C), (b) high protonic conductivity at similar tem- peratures, since most materials with high proton conductivity useful power at room temperature have a high water content and are not

I TO I 1 H+SOURCE

electrolyte TIS~ + LI LI + Li+ +

Fig. 10 Intercalation of Li into TiS2, adapted from ref. 32 Fig. 11 Schematic construction of a thin-film electrochromic device 162 J. MATER. CHEM., 1991, VOL. 1 stable at high temperatures, (c) high lithium ion conductivity 10 R. J. Bones, J. Coetzer, R. C. Galloway and D. A. Teagle, J. in atmosphere stable materials at ambient temperature. Such Electrochem. SOC., 1987, 134, 2379. 11 R. G. Linford, p. 564 in ref. 2. materials could find applications in improved fuel cells (a),(b) 12 P. Jagannathan et al., in Solid Electrolytes and Their Applications, or batteries (c). There is also much interest in finding new ed. E. C. Subbarao, Plenum Press, New York, 1980, p. 201. mixed conductors especially those with (a) high oxide ion 13 H. Iwahara, in Solid State Ionic Devices, ed. B. V. R. Chaudari conductivity for catalyst and reversible electrode applications and S. Radhakrishna, World Scientific, Singapore, 1989, p. 309. and (b) high Li', Na' ion conductivity for reversible elec- 14 Proc. Int. Symp. Solid Oxide Fuel Cells, Nagoya, Japan, 1989, trodes in solid-state batteries. New sensor materials and Science House, Tokyo. 15 J. T. Brown, p. 630 in ref. 2. devices are required that are selective to, for instance, oxides 16 B. C. H. Steele, I. Kelly, H. Middleton and R. Rudkin, Solid State of sulphur, oxides of nitrogen or C02. Zonics, 1988, 28-30, 1547. There is also much activity in developing multilayer, thin- 17 C. G. Vayenas, Solid State Zonics, 1988, 28-30, 1521. film solid-state devices particularly for miniature power 18 R. A. Huggins, p. 664 in ref. 2. sources. An advantage of miniaturisation is that the electrolyte 19 J. T. Kummer, Prog. Solid State Chem., 1972, 7, 141. resistance decreases linearly as thickness decreases and there- 20 B. Dunn and G. C. Farrington, Mater. Res. Bull., 1980, 15, fore, the normally stringent requirements of having low specific 1773. 21 G. C. Farrington and B. Dunn, Solid State Zonics, 1982, 7, 267. resistivity for ionic conduction can be relaxed somewhat. 22 G. C. Farrington, B. Dunn and J. 0. Thomas, Appl. Phys. A, In summary, there are probably a considerable number of 1983, 32, 159. potential ionic conductors/mixed conductors waiting to be 23 G. C. Farrington, B. Dunn and J. 0. Thomas, p. 327 in ref. 2. discovered and a sufficient number of perceived applications 24 M. Jansen, A. J. Alfrey, 0. M. Stafsudd, D. L. Yang, B. Dunn to ensure continued vigorous activity in this area. and G. C. Farrington, Opt. Lett., 1984, 9, 119. 25 J. L. Jackel, A. M. Glass, G. E. Peterson, C. E. Rice, D. H. Olson and J. J. Veeselka, J. Appl. Phys., 1984, 55, 269. References 26 J. L. Jackel, C. E. Rice and J. J. Veeselka, Electron. Lett., 1983, 19, 387. A. R. West, Ber. Bunsenges. Phys. Chem., 1989,93, 1235. 27 M. Ito and H. Takei, Jpn. J. Appl. Phys., 1989, 28, 144. High Conductivity Solid Ionic Conductors, Recent Trends and 28 B. Dunn, G. C. Farrington and J. 0. Thomas, ZSSZ Lett., 1990, Applications, ed. T. Takahashi, World Scientific, Singapore, 1989. 1, 1. J. R. MacCallum and C. A. Vincent, Polymer Electrolyte Reviews, 29 C. Namgung, J.T. S. Irvine, J.H. Binks, E. E. Lachowski and Elsevier, Barking, 1987, 1989, vol. 1 and 2. A. R. West, Supercond. Sci. Tech., 1989, 2, 181. S. Etemad, A. J. Heeger and A.G. MacDiarmid, Rev. Phys. 30 C. Namgung, J.T. S. Irvine and A. R. West, Physica C, 1990, Chem., 1982, 33, 443. 168, 346. Y. F. T. Yao and J. T. Kummer, J. Znorg. Nucl. Chem., 1967, 29 31 R. J. Cava, B. Batlogg, C. H. Chen, E. A. Rietman, S. M. Zahurak 2453. and D. Werder, Phys. Rev. B, 1987, 36, 5719. T. Takahashi, p. 1 in ref. 2. 32 K. West, p. 447 in ref. 2. W. Fischer, p. 595 in ref. 2. 33 F. G. K. Baucke and J. A. Duffy, Chem. Br., 1985, 643. J. Coetzer, J. Power Sources, 1986, 18, 377. R. C. Galloway, J. Electrochem. SOC.,1987, 134, 256. Paper 0/02205E; Received 17th May, 1990