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Polymer solid electrolytes - an overview
745
Solid State tonics 9 & 10 (1983) 745-754 North-Holland Publishing Company
POLYMER SOLID ELECTROLYTES- AN OVERVIEW
Michel Armand Laboratoired'Energ6tiqueElectrochimique- CNRS LA 265 ENSEE - Grenoble - B.P. 75 - Domaine Universitaire 38402 - Saint-Martin-d'H&es- France Polymers containingsolvatingheteroatomsform conductivecomplexeswith low lattice energy alkali salts. Despite the apparent complexityof the phase diagram and the influence of preparation history, only the amorphous domains show an appreciable mobility. With low interfacial resistance and a broad stability window, these materials open a new field in solid-stateelectrochemistry. A still debated question is the extend of anion mobility and how it limits performancesfor DC utilizations.
INTRODUCTION The advantagesof associatinga plastic electrolyte and a solid electrode were first mentioned in the earlier studies on intercalationelectroA material with viscoelastic chemistry [11. propertiescan accomodatefor the volume change during the ion-electron exchange process and such mechanical property needs a disordered structure.a feature favorable to ionic conductivity. However. the simplest.polyelectrolytes used in ion-exchange resins show an extremely low conductivity z 10_14(Rcml~ when anhydrous, due to extensive ion pairing. These compounds function rather like thermally depolarizable electrets than solid electrolytes. Also, the ternary systems in which an aprotic solution is gelled by a polymer network [21 [31 behave similarly to the parent liquid with high solvent activity and the same voltage stability window. The formationof complexesbetween poly(ethylene oxide), PEO, poly(propylene oxide), PPO and potassium salts was recognized as early as 1966 [41 151. but they remained mainly curiosities even after their ionic nature was demonstrated in 1973 [61. Later. the PEO and PPO complexes were proposed as solid electrolytes[71 [81 as the generality of alkali metal salts-polyethersinteractions A practical was established and defined. emphasis was of course placed on lithium conductors. the largest.majority of solid electrode materialsworking via Li+ insertion. The polymer electrolyte concept has since generated a widespread interest among both polymer specialists and electrochemists,with the perspectiveof high energy density batteries (> 150 Wh/kg) in a thin film configuration. However a complete understandingof such materials is only emerging after the importantparameters have been defined and studied. In this 0 167-2738/83/0000-OOOO/$ 03.00~ 1983North-Holland
paper, the four main electrochemicalproperties i.e. conductivity.stabilityuindow, interfacial kinetics and transference number will be addressed separately. I - PARAMETERS GOVERNING THE FORMATION AND CONDUCTIVITYOF THE COMPLEXES PEO (CH2-CH2-O),as the simplest of the class of solvating polymers has received maximum attention. However, due to its stereoregularitythe polymer is highly crystalline,thus the study of its properties as a solvent is quite complex, and not a simple extrapolationof those on the qesomolecular glymes or crown ethers with similar repeat.units [91. FH3 On the other hand, PPO (CH2-CH-01, obtained by non stereospecific polymerization (Vandenberg catalyst) is completely amorphous and forms apparently continuous solid solutions with low lattice energy salts. The complexes have allowed a precise demonstration [81 of the validityof free volume concept.expressedby the VTF (Vogel Tamman Fulcher) equation: B w = A exp Go where w are the reduced transport parameters:
DT-+
,
oT+,
Such lau introduces TO* the ideal glass transition temperature. a relatively new concept in solid-state ionics; the Arrhenius plots of u vs l/T feature a typical curvature. The PPO-NaCF3S03,LiCF3S03 systems have shown a continuousvariation of the parametersA. B, To with salt concentration,To decreasing monotonously with dilution towards To of the pure polymer. Though, no simple dilute-electrolyte model has been proposed to quantify these
146
M. Armand
/ Polymer
results. A more widely used law for transport mechanisms in polymers has been derived, the WLF (Williams Landef Ferry) equation:
Cl (T-T’ ) w. = w I lT ’ ?2+(T-T’ )
Cl’
c* = cts
allowing to deduce one transport property at temperature T from the same parameter at temperature T’. Since any arbitrary T’ can be chosen, this equation is extremely useful to correlate the transport properties with reference to the glass transition temperature Tg obtained from measurements of viscoelastic modulus. the Killis et al. have used this model to study extensively the conductivity/mechanical properties relation in polymer networks [lOI [121. Both VTF and WLF equations are in fact formally equivalent (ClC2 = 8, C2 = T!-To) and do not need to identify the transpor ed particles on the implicit assumption that the salt is fully ionized, concentration of giving a constant mobile species throughout temperature the range. More sophisticated models including an Arrhenius activation energy for dissociation have been proposed by Miyamoto et al. [131 for regular polymers and could reflects even better the conductivity behavior in the amorphous (T> Tg) phases [141. PEO is far more difficult to study, two phases can be present in the slow interconversion processes.
as more than system, with
The equilibrium constants for complexation are only known for solutions, with reference to water or a polar solvent as competing donor (1ogK = 3 for K+, z 2 for Na+) 1151 1161, the limit O/M ratio being found close to 4 : 1 1181 For lithium, the enthalpy for or 8 : 1 [191. salvation is large compared to the entropy term, so multidentate ligands like PEO only show a 1ogK -- 1 in solvents of basicity comparable to the ether oxygen; thus, the compounds are hygroscopic and do not exist in water or methanol solutions. As for PPO, PEO forms complexes with salts of large anions, the most useful of which were tentatively listed [81; a detailed comparison of the salvation vs the salt lattice energy [201 [211 shows a strikingly sharp delineation for isomorphous salts LiCl( +1; NaCl(‘) (LiF(-1 The maximum oxygen/metal ratio was NaBr(+),l. believed to be invariably 4 : 1 for complexes of Li, Na or K, when the 19.3 A helical thread of the pure polymer is shrunk to 7.2A(Na+l, 8.1A (K+) to solvate the cation [221 [231. It is noteworthy that a 4 : 1 O/M ratio can mean a 6 with two cations sharing the fold coordination, Heavier alkalis are too large same oxygen atom. to be accomodated in regular helix and tend to This give amorp’rous complexes [81 [211 1241.
solid electrolyres
4 : 1 stoichiometric value appears invalid in some cases when salts of polarizable (soft base) anions like LiSCN [241 or NaBH4 [251 give more concentrated complexes, amorphous or crystalline. In both cases,che more covalent M-X bond competes with ion pair separation by the polymer strand and should correspond to a different helical structure, yet unstudied. Mostly three types of conductivity behavior were observed for the PEO complexes 181 [241. Few compounds like (PE015 LiSCN show a free volume curvature of lno vs l/T, and behave like PPO in the whole temperature range (Type Il. Such material do not show any crystallization exotherm when studied by DSC [261. Type II compounds on the other hand show Arrhenius conductivity below 315-330 K, and turn to VTF law this above temperature; the LiClOq (PE018 provides a good example, but most Li and K salts are included. Finally, and within the precision conductivity measurements, the transition allows only a change of activation energy for an Arrhenius behavior (Type III). In all cases, the 315-330 K transition below the melting of pure PEO (338 Kl.
occurs
the study of these Recently, complexes became complicated by the apparent dual behavior of nominally identical compounds prepared out of different solvents [271 t281 [291, suggesting the existence of allotropes; for instance, a low melting (340 Kl and high melting (430 Kl PEOLiBF4 could be observed, though with identical X-ray diffraction lines. Optical or audio frequencies spectroscopy offers an insight of the different dissipative modes [231 1291 [301 supporting the idea of a crownether type interaction, but precise NMRmeasurementsappear most useful for understanding the phase diagram of these complexes. Berthier et al. [311 [321 have shown that at all temperature an elastomeric phase is responsible Above 330 K, the molten for the ionic motion. excess PEO as in (PE018 NaI or LiCF3S03 forms a dilute phase (O/M > 201 becoming progressively as the temperature is inmore concentrated creased to the melting point of the pure complex. Again the solid phase was found metric (3.5 : 11, generalizing partially separated ion pairs.
over-stoichiothe idea
of
The occurence of the 315-330 K transition must thus be associated with the existence of a Though rare examples of such behavior eutectic. are known for polymers, PEO forms a low melting LiClO4 is a compound with glutaric acid [331. special case as the eutectic is far more concenIt is probable trated than for other salts. that the solubility in the elastomeric phase reflects the lattice energy of the salts: (C104-
< I-
< CF3S03- < CF3CO2-1
147
M. Armand / Polymer solid electrolytes
the amorphous phase left below PEO Conversely, concentrated relatively recrystallization is and thus three phases are pres(O/M = 8 : 1). ent. 1341 showed that the Recently, Sorensen et al. conductivity of LiCF3S03 in PEO could be very simply modelized above 340 K by the dissolution of the crystalline complex into the molten PEO The calculation implies a constant phase. and such model can only mobility of the ion, apply to dilute phases O/M>15; obviously, as seen for PPO and PEO-LiC104 (O/H = 4.5 or 8) linear concentration-conductivity [243 a relationship in a single phase domain is not valid for more concentrated solutions. Application of either a VTF or Arrhenius law is subject to extreme caution in any region were > two phases are present, as the number of charges considerably, changing the conductivity vary Also the amorphous domains can be parameters. considered as confined in a given volume by the a situation where a Fulcher law crystallites, Though the influshould revert to Arrhenius. ence of pressure have been studied [351 1361, more data is needed for a conclusion. The NHR results are quite surprising but unambiguous in denying any participation to the ionic conductivity from the crystalline phase. It is thus probable that the size of the domains governs the kinetics of the observed phases transition in the usual time range (lo4 sl. If diffusion coefficient of we consider a as found for oxygen in crystalline 10_16cm2/s, polyethylene or for doping ions (AsF6’. Li+) in polyacetylene, a penetration length of only 100 A can be expected for 2 hours equilibration. On the other hand, the size of the domains depends strongly on the chain entanglement allowed during solvent evaporation. We suggest that if low dielectric constant solvents (CHC13acetone mixtures1 or low temperatures are emspherulites of the complex get ployed I large organized; in this case, these domains dissolve only very slowly in the melt. On the contrary, better solvents or higher temperatures for casting result in no phase separation until the last traces of volatiles are removed; in this respect, the role of water contamination is especially important, as its lower vapor preasure allows it to concentrate in the final stage of film formation. In such case. microscopic adjacent domains of PEO and complex can melt almost congruently, giving access to the true equilibrium diagram; however, no water remains in the compounds under vacuum 1371. Furthermore. it must be noted that the chain entanglement is “locked in”, and thus the polymer keeps memory of the possible domain sizes, allowing reproductlbility of the results over repeated temperature scans. This specific behavior of high MWmacromolecules provides thus a likely explanation for the observed discrepancies between different groups.
is whether or not An interesting question where “stages” (PEOl4, - MK can be formed. either vacancies or uncomplexed chains could coIf no evidence of such exist with the complex. larger is found with the Li salts tested, cations and/or anions with weaker interactions Recently, (CH), has might show this behavior. been found to give also stages during doping with alkali metals. Another possibility is the formation of double helical stands 1381 as in DNA, though this hypothesis needs further verification. The total conductivity results are summarized Table I for the most studied anions.
TRBLE
m+ Li+
X-
I-
11 II 11 u 1,
in
I
r(r=10-5
o= lo-’
("C)
(“C)
8
35
55
4.5
70
110
e I
31
50
4.5
75
120
8
70
loo
14
65
es
10
45
50
am
N?lC
I-
K+
CF3503-
4.5
40
70
Li+
I-
8
65
85
Li+
CF3503
9
55
95
Nat
tF3503-
12
45
95
iv
in
(skd)
Other polymers than PEO and PPO are candidates for complex formation. with the prospect of improved conductivity. especially at lower temperatures. Poly(oxymethylene1 (CH2-01, and poly(oxetane1 (CH2-CH2-CH2-01, were tested and showed no solvate formation [71 because of a non-optimal spacing of the hetero atom. Silicon based polymers are well known for their stabillty and low Tg; furthermore. the 0-Si-0 length is 75% of that of the 0-CH2-CH2-0 unit due to longer bonds. The siloxane cyclomers that have been studied for their complexing abilities were found far less effective than crown ethers [391. due to dx - pa character of the Sl-0 bond, suppressing the donor character of the oxygen. In our experiments, high MW poly( dimetbyl siloxane) showed no compatibility with any alkali metal salt even in a co-solvent, like
748
THF. For similar electronic group) poly(epichlorhydrin) solvating properties.
M. .4rmand
/ Polymer
.solid eleclrolyres
effects (-IS of Cl (CH2-CH-O), has poor CH2Cl
The sulfur homolog of PEO, poly(ethylene sulfide) was shown [201 to solvate Ag+, a soft Lewis acid able to interact with the highly nucleophilic sulfide. The reported conductivity is quite low as expected from the rigidity of the C-S-C linkage as compared to its oxygen homolog . Nitrogen hetero-atoms polymers look quite promising when considering the high donor number (dn) of the amine group (dn I 60 for :: N- ; dn : 22 for -0-1, as shown by the complexing ability of the nitrogen homolog of glymes [401. High MW equivalent of PEO, poly(N-methylaziridine) (CH2-CH2-N-1, is extremely difficult to preCH3 due to chain competition between pare, propagation and quaternarization at the nitrogen Furthermore, atom. space filling models show that the N-methyl group, as in PPO, tends to hinder the formation of a complete solvation helix, resulting in extensive ion pairing and low conductivities. new complexes with polyesters like Recently, poly(ethylene succinate) (C2H4-OCO-C,H4-COO-), were formed but somewhat less conductive than PEO t411, as the solvating chain again cannot fold easily into an helical structure. It appears evident that a multitude of maoromolecular structures can be built with conformaFor intions favorable to ion solvation. a variety of poly(cyclooxalkane)-diyl stance, where shown to have cation (THF, dioxolane) binding properties [421 [431 in solution, but these polymers have probably high Tg due to their polycyclic structure. The most appealing method for controlling both the crystallinity and mechanical properties of electrolytes is to form the macromolecular PEO, PPO and PEO-PPO arrays. cross-linked copolymers have been reacted with di or tri isocyanates (difunctional groups requiring trio1 units) to form urethane linkages [lOI [ill [121. excellent mechanical polymers exhibit Such short PEO segments (MW properties and with <1500) completely amorphous materials can be The urethane group contains the obtained. unit CO-NH, leading to hydrogen bond protic the bulky Also, formation as in polyamides. toluene diisocyanatel may starting unit (i.e. fraction of the represent a non-negligible probably influencing the anion/cation volume, Though, threemobility as discussed below. dimensional network with either small or solvating cross-links may represent the ultimate choice for high performance materials. A simpler possibility to strengthen the polymer electrolyte is to add inert powdered fillers, as in this case, [441; shown by Weston et al. surface charge of the solid component must be
carefully controlled to avoid aggregation of the particles, and the effect of inhomogeneities in current distribution for such filled electrolyte are difficult to predict. II
- ELECTROCHEMICAL STABILITY
With the apparent (metal stability of various organic liquids in contact with lithium, the problem of solvent stability in batteries has long been overlooked but it is now well recognized. Polymers appear at once to have the advantage over liquids that no diffusion of the solvent brings the same molecule in contact with two electrodes of widely different chemical activiunless extensive chain degradation occurs. ties, Cyclic voltammetry has emerged as the simplest screening technique for stability studies. It must be kept in mind, however, that the time span for an experiment, (2 102s) is not representative of long term (lOas) tests needed for battery cycling life . Since polymer electrolytes have a non-zero transference number for the anion, as discussed below, the first consequence is a possible reaction of the negative charge at the electrode. This is in contrast to the stability of Li3N beyond its thermodynamic domain, and the resistance to oxydation of conducting glasses when no anion in the network former is mobile [451. The possible reactions summarized in table II
at
the
interface
are
TABLE II High Electron
Low Electron
Activity
Activity
I
t
t
Only processes A and D are required for battery It is unlikely on thermodynamical operation. considerations that the reduction of a polyatomic anion or of the macromolecular network leads to reversible reactions. Also, there is no evidence that beyond the stability domain, any electronic conductivity appears (n or p) as in solid electrolytes, crystalline but more likely a limited chemical reaction takes place at the interface. Rigaud 141 described a cyclic adapted to solid electrolytes,
voltammetry set-up with the use of a
M. Arrnand / Pol.vmer solid electrolytes
solid-state reference electrode (Ag3SI or Li) under high vacuum. Using non reducible anions, like I-, the observed reversible reduction peaks of lithium or sodium proved the stability of the Also, the polymer backbone 1461 [471 [481. reversible oxidation of I- to Is- and 12 occurs before observable degradation of the any polymer; this is in contrast with cyclic ethers, like THF or dioxolane whose polymerization via a cationic mechanism can be initiated in the 2.8 LiBr in PEO showed an 3.5 V/Li:Li+ potential. unexpected peak in the cathodic region, and this observation must be brought together with the darkening of Li metal in presence of the same the highly hydrocomplex [481; most probably, philic Br- ions bind to water molecules under LiCl, the only chloride the conditions used. soluble in PEO has a conductivity too low for cyclic voltammetry. anodic domain can be obtained with A larger anions polyatomic containing electronegative like are species, SCN-. All C104or thermodynamically unstable towards lithium in the cathodic region, but a kinetic hindrance may especially with be expected in some cases, highly symmetrical anions. Table III summarizes the, results obtained from Additional information on LiCF3S03 ref. [461. was obtained from North et al. [491.
749
the lithium redissolution peak has been attributed to the formation of a Li-Pt alloy 1461. Such diagram has been used to determine the The interesting anion transference number. trifluoromethane sulfonate ion, a safe analog to displays apparently a different behavior c104the observed in presence of Na or Li ions; reduction of the lithium salt at + 0.5 V/Li:Li+ has been explained in terms of the high stabiliThe alkali metal polymer ty of LiF 1461. interface is in this case probably protected by an ionically conducting film of the reduction similarly to most liquid electroproducts, lytes.
4-
POE-LI(I~>CIO,~,)
+8
2- hc 5 P
O-
2TABLE
III 1OOt 4.10.‘v
-
4Cathodic
Limii
Anodic
I
Limit
l&al --
T (“c)
Li+
a5
0
(ho)
+ 2.8 ox. I-
Nat
60
0
(Na')
+ 3
Lit
100
0
(ii')
+ 4.3
Na'
80
0
(Na')
+ 4.0
;; ."
80
+ 0.5
+ 4.8
;.
A
I
Li+
CF SO 3 3 CF3S03-
s 130
0
80
0
N03-
110
+ 1
SCN-
80
N,g+
CF SO 3 3
Ll+ K--
(red.X-:
+ 1.5
(Li')
+ 3.5
(Na')
+ 4.9
(red.X-)
t4.0
(red.X-
V I
-2
,
-1
I
0
I
1
I
1
2
3
I
4
ox. I-
Fig
Li+
1
-3
-4
d’
I
2 z
+ 3.5
perchlorate ion displays As shown in Table III, an extremely good kinetic stability upon reducas usually found, but handling of more tion, than lab. quantities of any oxygen-chlorine derivative is probably hazardous. A cyclic voltammogram for a mixture of perchlorate and iodide ions shows a superposition of the characteristic features (Fig. [ll). The splitting of
(1)
Cyclic doped
voltmmetry truce PIEO18 LiCZOq
for
iodide
As expected from molten salt data, nitrate and thiocyanate ions reduce at quite positive potenand this restricts their use in contact tial, with any alkali metal. All anodic oxidation peaks beyond +4 Volts appear irreversible, and it is not possible to distinguish between the pirect oxidation of the polymer (formation of -0-l or the formation of active radicals like CF3.303.. PPO showed similar results [461 on anodic and cathodic sweeps. Intrinsically, the polymer electrolytes show a stability window of at least 3.5 V, starting from metal activity, a domain including most insertion cathode materials. Though. only a few stable or metastable anions with a delocalized charge are known, and the synthetic approach to new salts has already been attempted with
limited success in the case of liquid electroDerivatives of B(C6H5)4-, acceptable lytes. substitutes of C104- in dioxolane [491, yield relatively poor conductors in solution in PEO or PPO due to the high crystallinity and/or Tg imposed by the bulky rigid anion. III
- INTERFACIAL KINETICS
In the intercalation compound concept, there is a continuous ionic path from the electrolyte into the electrode material, though the environment of the charged species changes abruptly at the interface. In crystalline solid electrolytes the transfer kinetics are known to be fast in the absence of solvation and charge screening Liquid electrolytes, on (unipolar conduction). by a thick the other hand, are characterized diffuse layer containing ions of opposite inducing a large double layer capacicharges, and the desolvation step might be limittance, ing for small ions strongly attached to polar Co-intercalation can be observed in 1 solvents. or 2D electrodes. the conduction In the polymer electrolyte, mechanism is a solvation-desolvation process, so it can be expected that the ultimate shedding of at the interface should be of donor groups relatively low penalty on the total process. no systematic study of With this presumption, rather but kinetics was made, interfacial verifications were given for specific experiments.
0
spectrum reversible impedance with Complex electrodes displays two semicircles corresponding to the bulk resistance and the interfacial transfer resistance before any concentration polarization takes place in the electrodes or the electrolyte [501. cells using two Li metal With PEO electrolytes, electrodes [331 [441 [501 [51l Fig. [21, or LixTiS2 [511 and unsymmetrical assemblies Liinterfacial all show a limited LiAl [521, smaller than the bulk resistance, that of Following the conclusions of the electrolyte. it must be remembered that preceeding chapter, the transfer impedance may include the crossing of the passivation layer formed where the anion is and this applies to SCN- or reducible, CF3S03-. No study has been made with LiI, though the relatively stable LiC104 electrolyte shows a = Li and 50 n resistance in contact with both LiAl electrodes at 65-C 1521; at 1lO’C this value is = 7 Q for Li-TiS2 151 3, this value is less than 10% of the total electrolyte resistance at that temperature. a larger PPO, on the other hand might exhibit For the same transfer impedance (Fig. [21). both bulk and interfacial resisttemperature, ance appear markedly higher than for PEO (at P(EO)g LiC104 = 3.10-4, P(P0)6 LiC104 = 60°C
g.lo-6~ncmr’).
1OOK
20K zcodp
in1
M. Armand / Polymer solid electrolytes
A possible explanation is that the deaolvation step at the interface involves a close contact between the aolvating whorl and the electrode PEO with a minimum ateric hindrance surface. can lie flat at the interface, while the methyl group in PPO prevents a close approach (Fig. A similar effect has been observed with 131). compound showing THF and 2-Me-THF, the latter If this increased interfacial resistance 1531. mechanistic aspect of interfacial polarization a careful study must be appears to be valid, undertaken for cross-linked networks or aemicrystalline electrolytes for which a restricted motion of the polymer strands may be expected at the interface.
c = concentration a_= anionic conductivity
It is thus mandatory to choose an electrolyte conductivity. This is with a low anionic especially true when the electrolyte is mixed with the intercalation compounds, draining then an effective current density inversely proportional to its volume fraction. The problem of the transference number in electrolytes has been addressed since, relatively conflicting results.
Fig (3) b!odeZ "forinterfaciaZdesolvationat the electrodeinterf'ace
IV - TRANSFERENCE NUMBER If the electronic conductivity of polymer baaed electrolytes was found negligible [241 1461 [541 [551 from emf measurements, the importance of the respective qabilities of the anion and the Since intercalacation emerged only recently. tion compounds, Li metal or alloys, are reveraible electrodes for the positive charges only, a whenever a arises concentration polarization current is passed through the electrolyte. In accumulation of negative charges at this case, the anode and depletion at the cathode is partially compensated by back diffusion from Atlung and co-workers [561 chemical gradient. [57l have shown that in the ideal solute aituathis phenomenon is characterized by a tion, limiting current: 4RT o+ ‘lim = F d U+ cationic conductivity’ d electrolyte thickness beyond cathodic
which salt interface.
depletion
For current i across the cell, depletion occurs at the time t:
occurs the
at
the
onset
of
751
these with
Early T2 NMR or PEO-LiCF3S03 showed a t- close to 0.1 [71, but the precision of this experiment Using cycling voltammetry was relatively low. Rigaud [461 determined the oxidation peak current of doping iodide ion in P(EO)gLiC104. The corresponding diagram is shown in Fig. 111. Assuming a similar diffusion for I- and C104-, of comparable size, Delahay’a equation gave a tThese results are to be brought of 0.13. together with the studies of Fauteux et al. [521, using ion selective electrodes to determine the iodide diffuaivity in C104’, AaF6- or If homogenoua P(EO) LiC104 electroCF3S03-. lytes gave corresponding t--_ 0.3 the calculated value for semi-crystalline LiCF3S03 is surpriaingly high and does not correlate to the conducbut solving the equations for tivity data, diffusion in a salt-lean molten PEO occupying ok 50% of the volume, a reasonable model for P(EOJ6LiCF3.303 at 75’ C, yields the expected value of t- = 0.35. These results should give an upper limit of the t- since the diffusion of neutral MX is also accounted for in such experiments. However, all reports of complex impedance spectroscopy using McDonald analysis 1501 1261 1441 yield higher values ranging from 0.5 to 0.9. Besides the fact that SCN- or CF3S03baaed electrolytes probably form thin films on the lithium electhe model requires an ideal solution trade, behavior in the whole concentration range; in view of the total immobility of both ions, as checked by NMR [321 in the salt rich phase, a more likely explanation for polarization in the formation of a thin blocking layer of the complex at crystalline the anodic interface since no back diffusion takes place. Again, the semi-crystalline adducta where a larger effective current density is carried by the amorphous phase are especially susceptible to clogging of the conductive paths. Thus t in the absence of further study, the results from either diffusion or cyclic voltammetry can be considered valid for an amorphous non cross-linked elastomer, with t- ranging from 0.15 to 0.4; such values can be expected from the ratios of anion/cation
the important parameter for YTF diffuand place the polymer electrolytes between sicn, like I3 alumina or the unipolar conductors glasses and liquids with t+ = t- = 0.5.
volumes,
A different situation is expected cation pathway is blocked, as for: semi-crystalline chain folding. - cross-linked units.
complexes
networks
with
when
the
extensive
contribution
from BF4” could
The problem of the transference number remains thus the most controversial issue in the polymer electrolytes and will probably need elaborate techniques like NMR or neutron diffraction for settlement. CONCLUSIONS
with bulky non sclvating
- non dissociated anions helical)
interpreted as the not be evaluated.
ion pairs, as with pclarizable or with polymers forming an open (non sclvaticn structure.
In these three cases, long range caticnie motion can only cccur after chain transfer, while the anion mobility is unrestricted; or‘. more likely, cation hopping takes place concomitantly with as shown in Fig. 141, a situation anion release, leading to t+ :: t- = 0.5. An extreme example is given with halide salt - crown-ether complexes which were shown to carry only the negative charges [591, as there is no solvaticn continuum
As summarized in this paper, a considerable amount of information has been generated in six years on these new electrolytes. An interesting point is that both solid-state electrcchemists, NMR physicists and polymer chemists are invclved, SO no aspect of the properties of these compounds have been overlooked. It appears quite likely that slow kinetics in the ccncentrated phase and chain entanglement are mainly responsible for discrepancies the observed concerning the co-existing phases and their conductivity. high Most molecular weight polymers show a marked relationship between preparation history and properties, due to the possibility of a “locked in” disorder. If this appears for elaboration for reproducible
clX-
traces of prctic sclfor the cations. Also, vents iH20, MeOH), if left, have certainly a immediate beneficial effect on anion mobility, On0 et since they sclvate the negative charges. 1601 determined the stability constant of al. PEO complexes in methanol from conductivity measurements assuming a t- I 1. Such assumption is reasonable if we consider that the viscosity inside the chains is only slightly affected while the anions move in a liquid environment. The interchain hopping time is still unknown; Dupcn et al. 1611 have tried to estimate it from conductivity measurement of mixed LiBFb-LiBH4 the bcrohydride forming tight ion complexes, thus blocking the pathway of dissociated pairs, Unfortunately, such experiment cannot be Li*.
to be true, specific procedure should be selected and followed results.
The wide stability window of the polyethers in general is certainly an excellent incentive for continued interest in these materials. In fact, in most oases the stability domain is limited by anion, thus linked to the the and is A global solution contransference numbers. sists in attaching the anion to a macromolecular chain, so that the negative charges would remain and inactive. immobile electrochemically However, since only salts of extremely strong acides are suitably dissociated in such media, the choice of such molecules is limited and their synthesis quite delicate. The state-of-the-art polymers, like PEO-LiCF3S03 and LiClO4 have the advantage of their extreme simplicity in preparation and handling; they are emerging as readily accessible tools in electrcfor precise equilibrium and kinetics chemistry, measurements I551 [621 1631 specially to determine the titration voltage curve of intercalaThin film of redox tion compounds and alloys. containing solid electrolytes have found use in photoelectrochemical cells [641 rendering these likely Most to solid-state. systems back improvement in the-already respectable efficienAnother advancies (1 - 2%) can be expected. tage of the PEG electrolytes is their compatibility with high vacuum systems, allowing in situ observation (electron microscopy or ESCAf of electrode surfaces f651. solid-state battery The main goal, an all as the limiting current appears now feasible, polarization, 0.1 concentration due to in the range needed for ImA/cm2, are already medium drain, thin film cells [661 [671.
153
M. Armmd /Polymer solid electrolytes
Chem.
Only little data have yet been reported on Cycle life and performance of polymer batteries and this should probably be interpretedas a symptom of a continuous international effort in this domain.
t181 H. Awano, K. Ono, K. Murakami, Bull. Sot. Jap. 55 2530 (1982).
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[201 D.F. Shriver, B.L. Papke, M.A. Ratner, R.
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[191
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(1983).
San
Solid State tonics 9 & 10 (1983) 745-754 North-Holland Publishing Company
POLYMER SOLID ELECTROLYTES- AN OVERVIEW
Michel Armand Laboratoired'Energ6tiqueElectrochimique- CNRS LA 265 ENSEE - Grenoble - B.P. 75 - Domaine Universitaire 38402 - Saint-Martin-d'H&es- France Polymers containingsolvatingheteroatomsform conductivecomplexeswith low lattice energy alkali salts. Despite the apparent complexityof the phase diagram and the influence of preparation history, only the amorphous domains show an appreciable mobility. With low interfacial resistance and a broad stability window, these materials open a new field in solid-stateelectrochemistry. A still debated question is the extend of anion mobility and how it limits performancesfor DC utilizations.
INTRODUCTION The advantagesof associatinga plastic electrolyte and a solid electrode were first mentioned in the earlier studies on intercalationelectroA material with viscoelastic chemistry [11. propertiescan accomodatefor the volume change during the ion-electron exchange process and such mechanical property needs a disordered structure.a feature favorable to ionic conductivity. However. the simplest.polyelectrolytes used in ion-exchange resins show an extremely low conductivity z 10_14(Rcml~ when anhydrous, due to extensive ion pairing. These compounds function rather like thermally depolarizable electrets than solid electrolytes. Also, the ternary systems in which an aprotic solution is gelled by a polymer network [21 [31 behave similarly to the parent liquid with high solvent activity and the same voltage stability window. The formationof complexesbetween poly(ethylene oxide), PEO, poly(propylene oxide), PPO and potassium salts was recognized as early as 1966 [41 151. but they remained mainly curiosities even after their ionic nature was demonstrated in 1973 [61. Later. the PEO and PPO complexes were proposed as solid electrolytes[71 [81 as the generality of alkali metal salts-polyethersinteractions A practical was established and defined. emphasis was of course placed on lithium conductors. the largest.majority of solid electrode materialsworking via Li+ insertion. The polymer electrolyte concept has since generated a widespread interest among both polymer specialists and electrochemists,with the perspectiveof high energy density batteries (> 150 Wh/kg) in a thin film configuration. However a complete understandingof such materials is only emerging after the importantparameters have been defined and studied. In this 0 167-2738/83/0000-OOOO/$ 03.00~ 1983North-Holland
paper, the four main electrochemicalproperties i.e. conductivity.stabilityuindow, interfacial kinetics and transference number will be addressed separately. I - PARAMETERS GOVERNING THE FORMATION AND CONDUCTIVITYOF THE COMPLEXES PEO (CH2-CH2-O),as the simplest of the class of solvating polymers has received maximum attention. However, due to its stereoregularitythe polymer is highly crystalline,thus the study of its properties as a solvent is quite complex, and not a simple extrapolationof those on the qesomolecular glymes or crown ethers with similar repeat.units [91. FH3 On the other hand, PPO (CH2-CH-01, obtained by non stereospecific polymerization (Vandenberg catalyst) is completely amorphous and forms apparently continuous solid solutions with low lattice energy salts. The complexes have allowed a precise demonstration [81 of the validityof free volume concept.expressedby the VTF (Vogel Tamman Fulcher) equation: B w = A exp Go where w are the reduced transport parameters:
DT-+
,
oT+,
Such lau introduces TO* the ideal glass transition temperature. a relatively new concept in solid-state ionics; the Arrhenius plots of u vs l/T feature a typical curvature. The PPO-NaCF3S03,LiCF3S03 systems have shown a continuousvariation of the parametersA. B, To with salt concentration,To decreasing monotonously with dilution towards To of the pure polymer. Though, no simple dilute-electrolyte model has been proposed to quantify these
146
M. Armand
/ Polymer
results. A more widely used law for transport mechanisms in polymers has been derived, the WLF (Williams Landef Ferry) equation:
Cl (T-T’ ) w. = w I lT ’ ?2+(T-T’ )
Cl’
c* = cts
allowing to deduce one transport property at temperature T from the same parameter at temperature T’. Since any arbitrary T’ can be chosen, this equation is extremely useful to correlate the transport properties with reference to the glass transition temperature Tg obtained from measurements of viscoelastic modulus. the Killis et al. have used this model to study extensively the conductivity/mechanical properties relation in polymer networks [lOI [121. Both VTF and WLF equations are in fact formally equivalent (ClC2 = 8, C2 = T!-To) and do not need to identify the transpor ed particles on the implicit assumption that the salt is fully ionized, concentration of giving a constant mobile species throughout temperature the range. More sophisticated models including an Arrhenius activation energy for dissociation have been proposed by Miyamoto et al. [131 for regular polymers and could reflects even better the conductivity behavior in the amorphous (T> Tg) phases [141. PEO is far more difficult to study, two phases can be present in the slow interconversion processes.
as more than system, with
The equilibrium constants for complexation are only known for solutions, with reference to water or a polar solvent as competing donor (1ogK = 3 for K+, z 2 for Na+) 1151 1161, the limit O/M ratio being found close to 4 : 1 1181 For lithium, the enthalpy for or 8 : 1 [191. salvation is large compared to the entropy term, so multidentate ligands like PEO only show a 1ogK -- 1 in solvents of basicity comparable to the ether oxygen; thus, the compounds are hygroscopic and do not exist in water or methanol solutions. As for PPO, PEO forms complexes with salts of large anions, the most useful of which were tentatively listed [81; a detailed comparison of the salvation vs the salt lattice energy [201 [211 shows a strikingly sharp delineation for isomorphous salts LiCl( +1; NaCl(‘) (LiF(-1 The maximum oxygen/metal ratio was NaBr(+),l. believed to be invariably 4 : 1 for complexes of Li, Na or K, when the 19.3 A helical thread of the pure polymer is shrunk to 7.2A(Na+l, 8.1A (K+) to solvate the cation [221 [231. It is noteworthy that a 4 : 1 O/M ratio can mean a 6 with two cations sharing the fold coordination, Heavier alkalis are too large same oxygen atom. to be accomodated in regular helix and tend to This give amorp’rous complexes [81 [211 1241.
solid electrolyres
4 : 1 stoichiometric value appears invalid in some cases when salts of polarizable (soft base) anions like LiSCN [241 or NaBH4 [251 give more concentrated complexes, amorphous or crystalline. In both cases,che more covalent M-X bond competes with ion pair separation by the polymer strand and should correspond to a different helical structure, yet unstudied. Mostly three types of conductivity behavior were observed for the PEO complexes 181 [241. Few compounds like (PE015 LiSCN show a free volume curvature of lno vs l/T, and behave like PPO in the whole temperature range (Type Il. Such material do not show any crystallization exotherm when studied by DSC [261. Type II compounds on the other hand show Arrhenius conductivity below 315-330 K, and turn to VTF law this above temperature; the LiClOq (PE018 provides a good example, but most Li and K salts are included. Finally, and within the precision conductivity measurements, the transition allows only a change of activation energy for an Arrhenius behavior (Type III). In all cases, the 315-330 K transition below the melting of pure PEO (338 Kl.
occurs
the study of these Recently, complexes became complicated by the apparent dual behavior of nominally identical compounds prepared out of different solvents [271 t281 [291, suggesting the existence of allotropes; for instance, a low melting (340 Kl and high melting (430 Kl PEOLiBF4 could be observed, though with identical X-ray diffraction lines. Optical or audio frequencies spectroscopy offers an insight of the different dissipative modes [231 1291 [301 supporting the idea of a crownether type interaction, but precise NMRmeasurementsappear most useful for understanding the phase diagram of these complexes. Berthier et al. [311 [321 have shown that at all temperature an elastomeric phase is responsible Above 330 K, the molten for the ionic motion. excess PEO as in (PE018 NaI or LiCF3S03 forms a dilute phase (O/M > 201 becoming progressively as the temperature is inmore concentrated creased to the melting point of the pure complex. Again the solid phase was found metric (3.5 : 11, generalizing partially separated ion pairs.
over-stoichiothe idea
of
The occurence of the 315-330 K transition must thus be associated with the existence of a Though rare examples of such behavior eutectic. are known for polymers, PEO forms a low melting LiClO4 is a compound with glutaric acid [331. special case as the eutectic is far more concenIt is probable trated than for other salts. that the solubility in the elastomeric phase reflects the lattice energy of the salts: (C104-
< I-
< CF3S03- < CF3CO2-1
147
M. Armand / Polymer solid electrolytes
the amorphous phase left below PEO Conversely, concentrated relatively recrystallization is and thus three phases are pres(O/M = 8 : 1). ent. 1341 showed that the Recently, Sorensen et al. conductivity of LiCF3S03 in PEO could be very simply modelized above 340 K by the dissolution of the crystalline complex into the molten PEO The calculation implies a constant phase. and such model can only mobility of the ion, apply to dilute phases O/M>15; obviously, as seen for PPO and PEO-LiC104 (O/H = 4.5 or 8) linear concentration-conductivity [243 a relationship in a single phase domain is not valid for more concentrated solutions. Application of either a VTF or Arrhenius law is subject to extreme caution in any region were > two phases are present, as the number of charges considerably, changing the conductivity vary Also the amorphous domains can be parameters. considered as confined in a given volume by the a situation where a Fulcher law crystallites, Though the influshould revert to Arrhenius. ence of pressure have been studied [351 1361, more data is needed for a conclusion. The NHR results are quite surprising but unambiguous in denying any participation to the ionic conductivity from the crystalline phase. It is thus probable that the size of the domains governs the kinetics of the observed phases transition in the usual time range (lo4 sl. If diffusion coefficient of we consider a as found for oxygen in crystalline 10_16cm2/s, polyethylene or for doping ions (AsF6’. Li+) in polyacetylene, a penetration length of only 100 A can be expected for 2 hours equilibration. On the other hand, the size of the domains depends strongly on the chain entanglement allowed during solvent evaporation. We suggest that if low dielectric constant solvents (CHC13acetone mixtures1 or low temperatures are emspherulites of the complex get ployed I large organized; in this case, these domains dissolve only very slowly in the melt. On the contrary, better solvents or higher temperatures for casting result in no phase separation until the last traces of volatiles are removed; in this respect, the role of water contamination is especially important, as its lower vapor preasure allows it to concentrate in the final stage of film formation. In such case. microscopic adjacent domains of PEO and complex can melt almost congruently, giving access to the true equilibrium diagram; however, no water remains in the compounds under vacuum 1371. Furthermore. it must be noted that the chain entanglement is “locked in”, and thus the polymer keeps memory of the possible domain sizes, allowing reproductlbility of the results over repeated temperature scans. This specific behavior of high MWmacromolecules provides thus a likely explanation for the observed discrepancies between different groups.
is whether or not An interesting question where “stages” (PEOl4, - MK can be formed. either vacancies or uncomplexed chains could coIf no evidence of such exist with the complex. larger is found with the Li salts tested, cations and/or anions with weaker interactions Recently, (CH), has might show this behavior. been found to give also stages during doping with alkali metals. Another possibility is the formation of double helical stands 1381 as in DNA, though this hypothesis needs further verification. The total conductivity results are summarized Table I for the most studied anions.
TRBLE
m+ Li+
X-
I-
11 II 11 u 1,
in
I
r(r=10-5
o= lo-’
("C)
(“C)
8
35
55
4.5
70
110
e I
31
50
4.5
75
120
8
70
loo
14
65
es
10
45
50
am
N?lC
I-
K+
CF3503-
4.5
40
70
Li+
I-
8
65
85
Li+
CF3503
9
55
95
Nat
tF3503-
12
45
95
iv
in
(skd)
Other polymers than PEO and PPO are candidates for complex formation. with the prospect of improved conductivity. especially at lower temperatures. Poly(oxymethylene1 (CH2-01, and poly(oxetane1 (CH2-CH2-CH2-01, were tested and showed no solvate formation [71 because of a non-optimal spacing of the hetero atom. Silicon based polymers are well known for their stabillty and low Tg; furthermore. the 0-Si-0 length is 75% of that of the 0-CH2-CH2-0 unit due to longer bonds. The siloxane cyclomers that have been studied for their complexing abilities were found far less effective than crown ethers [391. due to dx - pa character of the Sl-0 bond, suppressing the donor character of the oxygen. In our experiments, high MW poly( dimetbyl siloxane) showed no compatibility with any alkali metal salt even in a co-solvent, like
748
THF. For similar electronic group) poly(epichlorhydrin) solvating properties.
M. .4rmand
/ Polymer
.solid eleclrolyres
effects (-IS of Cl (CH2-CH-O), has poor CH2Cl
The sulfur homolog of PEO, poly(ethylene sulfide) was shown [201 to solvate Ag+, a soft Lewis acid able to interact with the highly nucleophilic sulfide. The reported conductivity is quite low as expected from the rigidity of the C-S-C linkage as compared to its oxygen homolog . Nitrogen hetero-atoms polymers look quite promising when considering the high donor number (dn) of the amine group (dn I 60 for :: N- ; dn : 22 for -0-1, as shown by the complexing ability of the nitrogen homolog of glymes [401. High MW equivalent of PEO, poly(N-methylaziridine) (CH2-CH2-N-1, is extremely difficult to preCH3 due to chain competition between pare, propagation and quaternarization at the nitrogen Furthermore, atom. space filling models show that the N-methyl group, as in PPO, tends to hinder the formation of a complete solvation helix, resulting in extensive ion pairing and low conductivities. new complexes with polyesters like Recently, poly(ethylene succinate) (C2H4-OCO-C,H4-COO-), were formed but somewhat less conductive than PEO t411, as the solvating chain again cannot fold easily into an helical structure. It appears evident that a multitude of maoromolecular structures can be built with conformaFor intions favorable to ion solvation. a variety of poly(cyclooxalkane)-diyl stance, where shown to have cation (THF, dioxolane) binding properties [421 [431 in solution, but these polymers have probably high Tg due to their polycyclic structure. The most appealing method for controlling both the crystallinity and mechanical properties of electrolytes is to form the macromolecular PEO, PPO and PEO-PPO arrays. cross-linked copolymers have been reacted with di or tri isocyanates (difunctional groups requiring trio1 units) to form urethane linkages [lOI [ill [121. excellent mechanical polymers exhibit Such short PEO segments (MW properties and with <1500) completely amorphous materials can be The urethane group contains the obtained. unit CO-NH, leading to hydrogen bond protic the bulky Also, formation as in polyamides. toluene diisocyanatel may starting unit (i.e. fraction of the represent a non-negligible probably influencing the anion/cation volume, Though, threemobility as discussed below. dimensional network with either small or solvating cross-links may represent the ultimate choice for high performance materials. A simpler possibility to strengthen the polymer electrolyte is to add inert powdered fillers, as in this case, [441; shown by Weston et al. surface charge of the solid component must be
carefully controlled to avoid aggregation of the particles, and the effect of inhomogeneities in current distribution for such filled electrolyte are difficult to predict. II
- ELECTROCHEMICAL STABILITY
With the apparent (metal stability of various organic liquids in contact with lithium, the problem of solvent stability in batteries has long been overlooked but it is now well recognized. Polymers appear at once to have the advantage over liquids that no diffusion of the solvent brings the same molecule in contact with two electrodes of widely different chemical activiunless extensive chain degradation occurs. ties, Cyclic voltammetry has emerged as the simplest screening technique for stability studies. It must be kept in mind, however, that the time span for an experiment, (2 102s) is not representative of long term (lOas) tests needed for battery cycling life . Since polymer electrolytes have a non-zero transference number for the anion, as discussed below, the first consequence is a possible reaction of the negative charge at the electrode. This is in contrast to the stability of Li3N beyond its thermodynamic domain, and the resistance to oxydation of conducting glasses when no anion in the network former is mobile [451. The possible reactions summarized in table II
at
the
interface
are
TABLE II High Electron
Low Electron
Activity
Activity
I
t
t
Only processes A and D are required for battery It is unlikely on thermodynamical operation. considerations that the reduction of a polyatomic anion or of the macromolecular network leads to reversible reactions. Also, there is no evidence that beyond the stability domain, any electronic conductivity appears (n or p) as in solid electrolytes, crystalline but more likely a limited chemical reaction takes place at the interface. Rigaud 141 described a cyclic adapted to solid electrolytes,
voltammetry set-up with the use of a
M. Arrnand / Pol.vmer solid electrolytes
solid-state reference electrode (Ag3SI or Li) under high vacuum. Using non reducible anions, like I-, the observed reversible reduction peaks of lithium or sodium proved the stability of the Also, the polymer backbone 1461 [471 [481. reversible oxidation of I- to Is- and 12 occurs before observable degradation of the any polymer; this is in contrast with cyclic ethers, like THF or dioxolane whose polymerization via a cationic mechanism can be initiated in the 2.8 LiBr in PEO showed an 3.5 V/Li:Li+ potential. unexpected peak in the cathodic region, and this observation must be brought together with the darkening of Li metal in presence of the same the highly hydrocomplex [481; most probably, philic Br- ions bind to water molecules under LiCl, the only chloride the conditions used. soluble in PEO has a conductivity too low for cyclic voltammetry. anodic domain can be obtained with A larger anions polyatomic containing electronegative like are species, SCN-. All C104or thermodynamically unstable towards lithium in the cathodic region, but a kinetic hindrance may especially with be expected in some cases, highly symmetrical anions. Table III summarizes the, results obtained from Additional information on LiCF3S03 ref. [461. was obtained from North et al. [491.
749
the lithium redissolution peak has been attributed to the formation of a Li-Pt alloy 1461. Such diagram has been used to determine the The interesting anion transference number. trifluoromethane sulfonate ion, a safe analog to displays apparently a different behavior c104the observed in presence of Na or Li ions; reduction of the lithium salt at + 0.5 V/Li:Li+ has been explained in terms of the high stabiliThe alkali metal polymer ty of LiF 1461. interface is in this case probably protected by an ionically conducting film of the reduction similarly to most liquid electroproducts, lytes.
4-
POE-LI(I~>CIO,~,)
+8
2- hc 5 P
O-
2TABLE
III 1OOt 4.10.‘v
-
4Cathodic
Limii
Anodic
I
Limit
l&al --
T (“c)
Li+
a5
0
(ho)
+ 2.8 ox. I-
Nat
60
0
(Na')
+ 3
Lit
100
0
(ii')
+ 4.3
Na'
80
0
(Na')
+ 4.0
;; ."
80
+ 0.5
+ 4.8
;.
A
I
Li+
CF SO 3 3 CF3S03-
s 130
0
80
0
N03-
110
+ 1
SCN-
80
N,g+
CF SO 3 3
Ll+ K--
(red.X-:
+ 1.5
(Li')
+ 3.5
(Na')
+ 4.9
(red.X-)
t4.0
(red.X-
V I
-2
,
-1
I
0
I
1
I
1
2
3
I
4
ox. I-
Fig
Li+
1
-3
-4
d’
I
2 z
+ 3.5
perchlorate ion displays As shown in Table III, an extremely good kinetic stability upon reducas usually found, but handling of more tion, than lab. quantities of any oxygen-chlorine derivative is probably hazardous. A cyclic voltammogram for a mixture of perchlorate and iodide ions shows a superposition of the characteristic features (Fig. [ll). The splitting of
(1)
Cyclic doped
voltmmetry truce PIEO18 LiCZOq
for
iodide
As expected from molten salt data, nitrate and thiocyanate ions reduce at quite positive potenand this restricts their use in contact tial, with any alkali metal. All anodic oxidation peaks beyond +4 Volts appear irreversible, and it is not possible to distinguish between the pirect oxidation of the polymer (formation of -0-l or the formation of active radicals like CF3.303.. PPO showed similar results [461 on anodic and cathodic sweeps. Intrinsically, the polymer electrolytes show a stability window of at least 3.5 V, starting from metal activity, a domain including most insertion cathode materials. Though. only a few stable or metastable anions with a delocalized charge are known, and the synthetic approach to new salts has already been attempted with
limited success in the case of liquid electroDerivatives of B(C6H5)4-, acceptable lytes. substitutes of C104- in dioxolane [491, yield relatively poor conductors in solution in PEO or PPO due to the high crystallinity and/or Tg imposed by the bulky rigid anion. III
- INTERFACIAL KINETICS
In the intercalation compound concept, there is a continuous ionic path from the electrolyte into the electrode material, though the environment of the charged species changes abruptly at the interface. In crystalline solid electrolytes the transfer kinetics are known to be fast in the absence of solvation and charge screening Liquid electrolytes, on (unipolar conduction). by a thick the other hand, are characterized diffuse layer containing ions of opposite inducing a large double layer capacicharges, and the desolvation step might be limittance, ing for small ions strongly attached to polar Co-intercalation can be observed in 1 solvents. or 2D electrodes. the conduction In the polymer electrolyte, mechanism is a solvation-desolvation process, so it can be expected that the ultimate shedding of at the interface should be of donor groups relatively low penalty on the total process. no systematic study of With this presumption, rather but kinetics was made, interfacial verifications were given for specific experiments.
0
spectrum reversible impedance with Complex electrodes displays two semicircles corresponding to the bulk resistance and the interfacial transfer resistance before any concentration polarization takes place in the electrodes or the electrolyte [501. cells using two Li metal With PEO electrolytes, electrodes [331 [441 [501 [51l Fig. [21, or LixTiS2 [511 and unsymmetrical assemblies Liinterfacial all show a limited LiAl [521, smaller than the bulk resistance, that of Following the conclusions of the electrolyte. it must be remembered that preceeding chapter, the transfer impedance may include the crossing of the passivation layer formed where the anion is and this applies to SCN- or reducible, CF3S03-. No study has been made with LiI, though the relatively stable LiC104 electrolyte shows a = Li and 50 n resistance in contact with both LiAl electrodes at 65-C 1521; at 1lO’C this value is = 7 Q for Li-TiS2 151 3, this value is less than 10% of the total electrolyte resistance at that temperature. a larger PPO, on the other hand might exhibit For the same transfer impedance (Fig. [21). both bulk and interfacial resisttemperature, ance appear markedly higher than for PEO (at P(EO)g LiC104 = 3.10-4, P(P0)6 LiC104 = 60°C
g.lo-6~ncmr’).
1OOK
20K zcodp
in1
M. Armand / Polymer solid electrolytes
A possible explanation is that the deaolvation step at the interface involves a close contact between the aolvating whorl and the electrode PEO with a minimum ateric hindrance surface. can lie flat at the interface, while the methyl group in PPO prevents a close approach (Fig. A similar effect has been observed with 131). compound showing THF and 2-Me-THF, the latter If this increased interfacial resistance 1531. mechanistic aspect of interfacial polarization a careful study must be appears to be valid, undertaken for cross-linked networks or aemicrystalline electrolytes for which a restricted motion of the polymer strands may be expected at the interface.
c = concentration a_= anionic conductivity
It is thus mandatory to choose an electrolyte conductivity. This is with a low anionic especially true when the electrolyte is mixed with the intercalation compounds, draining then an effective current density inversely proportional to its volume fraction. The problem of the transference number in electrolytes has been addressed since, relatively conflicting results.
Fig (3) b!odeZ "forinterfaciaZdesolvationat the electrodeinterf'ace
IV - TRANSFERENCE NUMBER If the electronic conductivity of polymer baaed electrolytes was found negligible [241 1461 [541 [551 from emf measurements, the importance of the respective qabilities of the anion and the Since intercalacation emerged only recently. tion compounds, Li metal or alloys, are reveraible electrodes for the positive charges only, a whenever a arises concentration polarization current is passed through the electrolyte. In accumulation of negative charges at this case, the anode and depletion at the cathode is partially compensated by back diffusion from Atlung and co-workers [561 chemical gradient. [57l have shown that in the ideal solute aituathis phenomenon is characterized by a tion, limiting current: 4RT o+ ‘lim = F d U+ cationic conductivity’ d electrolyte thickness beyond cathodic
which salt interface.
depletion
For current i across the cell, depletion occurs at the time t:
occurs the
at
the
onset
of
751
these with
Early T2 NMR or PEO-LiCF3S03 showed a t- close to 0.1 [71, but the precision of this experiment Using cycling voltammetry was relatively low. Rigaud [461 determined the oxidation peak current of doping iodide ion in P(EO)gLiC104. The corresponding diagram is shown in Fig. 111. Assuming a similar diffusion for I- and C104-, of comparable size, Delahay’a equation gave a tThese results are to be brought of 0.13. together with the studies of Fauteux et al. [521, using ion selective electrodes to determine the iodide diffuaivity in C104’, AaF6- or If homogenoua P(EO) LiC104 electroCF3S03-. lytes gave corresponding t--_ 0.3 the calculated value for semi-crystalline LiCF3S03 is surpriaingly high and does not correlate to the conducbut solving the equations for tivity data, diffusion in a salt-lean molten PEO occupying ok 50% of the volume, a reasonable model for P(EOJ6LiCF3.303 at 75’ C, yields the expected value of t- = 0.35. These results should give an upper limit of the t- since the diffusion of neutral MX is also accounted for in such experiments. However, all reports of complex impedance spectroscopy using McDonald analysis 1501 1261 1441 yield higher values ranging from 0.5 to 0.9. Besides the fact that SCN- or CF3S03baaed electrolytes probably form thin films on the lithium electhe model requires an ideal solution trade, behavior in the whole concentration range; in view of the total immobility of both ions, as checked by NMR [321 in the salt rich phase, a more likely explanation for polarization in the formation of a thin blocking layer of the complex at crystalline the anodic interface since no back diffusion takes place. Again, the semi-crystalline adducta where a larger effective current density is carried by the amorphous phase are especially susceptible to clogging of the conductive paths. Thus t in the absence of further study, the results from either diffusion or cyclic voltammetry can be considered valid for an amorphous non cross-linked elastomer, with t- ranging from 0.15 to 0.4; such values can be expected from the ratios of anion/cation
the important parameter for YTF diffuand place the polymer electrolytes between sicn, like I3 alumina or the unipolar conductors glasses and liquids with t+ = t- = 0.5.
volumes,
A different situation is expected cation pathway is blocked, as for: semi-crystalline chain folding. - cross-linked units.
complexes
networks
with
when
the
extensive
contribution
from BF4” could
The problem of the transference number remains thus the most controversial issue in the polymer electrolytes and will probably need elaborate techniques like NMR or neutron diffraction for settlement. CONCLUSIONS
with bulky non sclvating
- non dissociated anions helical)
interpreted as the not be evaluated.
ion pairs, as with pclarizable or with polymers forming an open (non sclvaticn structure.
In these three cases, long range caticnie motion can only cccur after chain transfer, while the anion mobility is unrestricted; or‘. more likely, cation hopping takes place concomitantly with as shown in Fig. 141, a situation anion release, leading to t+ :: t- = 0.5. An extreme example is given with halide salt - crown-ether complexes which were shown to carry only the negative charges [591, as there is no solvaticn continuum
As summarized in this paper, a considerable amount of information has been generated in six years on these new electrolytes. An interesting point is that both solid-state electrcchemists, NMR physicists and polymer chemists are invclved, SO no aspect of the properties of these compounds have been overlooked. It appears quite likely that slow kinetics in the ccncentrated phase and chain entanglement are mainly responsible for discrepancies the observed concerning the co-existing phases and their conductivity. high Most molecular weight polymers show a marked relationship between preparation history and properties, due to the possibility of a “locked in” disorder. If this appears for elaboration for reproducible
clX-
traces of prctic sclfor the cations. Also, vents iH20, MeOH), if left, have certainly a immediate beneficial effect on anion mobility, On0 et since they sclvate the negative charges. 1601 determined the stability constant of al. PEO complexes in methanol from conductivity measurements assuming a t- I 1. Such assumption is reasonable if we consider that the viscosity inside the chains is only slightly affected while the anions move in a liquid environment. The interchain hopping time is still unknown; Dupcn et al. 1611 have tried to estimate it from conductivity measurement of mixed LiBFb-LiBH4 the bcrohydride forming tight ion complexes, thus blocking the pathway of dissociated pairs, Unfortunately, such experiment cannot be Li*.
to be true, specific procedure should be selected and followed results.
The wide stability window of the polyethers in general is certainly an excellent incentive for continued interest in these materials. In fact, in most oases the stability domain is limited by anion, thus linked to the the and is A global solution contransference numbers. sists in attaching the anion to a macromolecular chain, so that the negative charges would remain and inactive. immobile electrochemically However, since only salts of extremely strong acides are suitably dissociated in such media, the choice of such molecules is limited and their synthesis quite delicate. The state-of-the-art polymers, like PEO-LiCF3S03 and LiClO4 have the advantage of their extreme simplicity in preparation and handling; they are emerging as readily accessible tools in electrcfor precise equilibrium and kinetics chemistry, measurements I551 [621 1631 specially to determine the titration voltage curve of intercalaThin film of redox tion compounds and alloys. containing solid electrolytes have found use in photoelectrochemical cells [641 rendering these likely Most to solid-state. systems back improvement in the-already respectable efficienAnother advancies (1 - 2%) can be expected. tage of the PEG electrolytes is their compatibility with high vacuum systems, allowing in situ observation (electron microscopy or ESCAf of electrode surfaces f651. solid-state battery The main goal, an all as the limiting current appears now feasible, polarization, 0.1 concentration due to in the range needed for ImA/cm2, are already medium drain, thin film cells [661 [671.
153
M. Armmd /Polymer solid electrolytes
Chem.
Only little data have yet been reported on Cycle life and performance of polymer batteries and this should probably be interpretedas a symptom of a continuous international effort in this domain.
t181 H. Awano, K. Ono, K. Murakami, Bull. Sot. Jap. 55 2530 (1982).
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