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A mechanism of ionic conduction in cross-linked polyethers
Solid State lonics 28-30 (1988) 1032-1037 North-Holland, Amsterdam
A MECHANISM OF IONIC CONDUCTION IN CROSS-LINKED POLYIETHERS J.F. LE N E S T , H. C H E R A D A M E and A. GANDINI
Laboratoire de Chimie Macromoldculaire et Papetibre, Ecole Franeaise de Papeterie (INPG), BP 65, 38402 Saint Martin d'Heres Cedex, France
Received 4 August 1987
A wide variety of polyether-based networks were prepared and tested in terms of ionic conductivity as a function of the type and concentration of the salt added, the structure of the reticulate and the temperature. This study includes ionomeric networks. The results indicate the occurrence of polyether chain partitioning by the dissociated salt and formation of quadrupoles binding two such chains. A model is proposed to rationalize the conduction mechanism on the basis of the overall experimental evidence.
1. Introduction
Growing interest in polymer electrolytes is undisputable [ 1,2]. The optimization of their performance in view of possible applications in solid-state batteries requires a ~horough knowledge of the ionic conduction mechanism. In order to avoid the problems intrinsic te linear polyethers (crystallization,creep, etc.), we begun to work several years ago on cross-li nked materials prepared flom oligo- or polyether polyols and multifunctional isocyae, ates [3]. These networks were either mixed with metal salts or converted into ionomeric structures with the anions covalently fLxed to the matrix [ 31. The advantages of this approach are primarily the amorphous character of the materials at all temperatures and their dimensional stabil"y [ 4 ]. The results obtained therefrom have been quite stimulating as already reported in part [ 3 ]. This communication deals specifically with the conductivity behaviour of a wide variety of networks and salts and with an atteml)t to unify the results gathmodel 1,~r the ionic a?~regation state and of'.~ eir inte~action with the polyether chains as well as a mechanism of ionic conduction.
2. Experimental
The experimental procedures followed to prepare both network-plus-salts [ 5 ] and i o n o m e r i c networks
[3] have been described. All starting materials for these syntheses are given in table 1. The dried salts added to the non-ionomeric networks were: LiCIO4, LiSO3CF3, LiBF4, LiOCOCF3, LiSCN, NaCIO4, KCIO4, NH4C104, NaBPh4, Mg(CIO4)2. Five anionic moieties were fixed to the chains of ionomeric networks [ 3 ] and used as lithium salts: phosphate, thiophosphate, - O - C O ( CF2 ) 3COO -, - O (SbCIs) and -O(SbFs) -. Ionic conductivities were obtained by the classical complex-impedance method using the setup described [ 6 ]. These measurements also allow the determination of the dielectric constant of the material.
3. Results and discussion We ~:ave previously shown that ionic conduction in poiyether-based networks containing an aikaii me~al salt follows the relationship [ 7]" G=Cq 2 Ba exp
+
0 167-2738/88/$ 03.50 © Elsevier Science Publishers B.V.
(North-Holland Physics Publishing Division)
exp
L +
fs + o ~ ( T - T g ) - R
r-
"
1033
J.F. Le Nest et al.llonic conduction in cross-linked polyethers
Table 1 Network precursors. Compound, formula (abbreviation) (a) polyether-ols poly(ethylene oxide)a, to-diol HO-(CH2-CH2-O-),,H (PEO)
tris(poly(PPO-b-PEO) to-ol) glycidyl ether CH 2-O ( C~H~O) ~(C2H40) ioH
Source
f,'l a )
J~n
Merck
1050 1550 2100 3800
2.03-2.04 2.02 2.05-2.06 2.07
Dow Chemical
2590
3.0
BASF, Wyandotte, Ugine Kuhlmann
4980
2.03
8390
2.04
:lT,,,/J-L 1.05 1.05 ! .06
I CH-O (C3H60) 7( C 2 H 4 0 ) loll
I CH2-O ( C3H60) 7( C2H40 ) loll triblock copolymers a, to-diol PEO-b-POP-b-PEO ( l ) HO (C2H40) 4s(CsH60) i v(C2H40) 4sH (Pluronic F 38) (2) HO(C2H40)78(CsH60)26C2H40)TsH (Pluronic F 68) graft-block copolymers polydimethylsiloxane-g-polyether (CHs)sSi(OSi(CHs) 2)2s(('SiCHsR)14OSi(CHs)s R = -CHz-CH 2-CH2- ( OC3 H6) Io( OC2H4) I2OH (PDMS--g-(POP-b-POE-OH)) (b) multifunctional isocyanates hexamethylene diisocyanate OCN(CH2)6NCO (HMDI) 4,4' ,4"-methylidyne tris( phenylisocyanate ) HC(C6H4NCO) 3 ( Desmodur R) OCN (CH2)6N(CONH(CH2)6NCO)2 (Desmodur N)
Rh6ne-Poulenc
19400
Merck
168
Bayer
367
Bayer
47~,
14
" ~ Average functionality (OH) as determined by acety!ation or i H NMR.
Here the subscripts a and c refer to the anion and the cation respectively. In the case of ionomeric polyanions all a-related variables vanish and eq. (1) reduces to a single term. Moreover, we showed that with most systems involving a network-salt mixture, the cationic transpo~ numbers are ve~, !ow~ i~e. 0.1 to 0.25 [ 8]. It becomes then reasonable to ignore the cationic exponential term in eq. (1). Thus, in both situations pertaining to the present study, oniy one term of eq. (1) needs to be considered for further development. All evidence points clearly to a free-volume behaviour of such dynamic properties of these systems
as viscoelasticity, nuclear magnetic relaxation and ionic conductivity [3]. Therefore one can write an expression of the reduced conductivity as developed in a previous paper [ 7 ]: log(o'T/arg) = - log a7+, where log a t = -C1( T - Tg)/( C2 + "f'- T~), i.e. the classical WLF relationship [ 7 tTable 2 gives the values of Ct and C2 obtained from conductivity measurements on a wide variety of systems. It is possible to conclude that, within the experimental errors implicit in the way Ct and C2 are determined, these characteristic constants are unique
1034
J.F. Le Nest et aL/lonic cgnduction in cross-linked polyethers
Table 2 Average values of WLF constants for polyether-based networks. System (see table 1)
C,
Ca (K)
PEO 1050/Desmodur R/LiCIO4 PEO 1050/Desmodur N/LiCIO4 PEO 2100/Desmodur R/LiCIO4 PEO 3800/Desmodur R/LiCIO4 PEO 1050/Desmodur N/LiCIO4 LiSO3CF3 LiBF4 LiSCN LiOCOCF3 NaCIO4 KCIO4 PEO 1050/Desmodur R/NH4CIO4 PEO 3800/Desmodur R/NH4CIO4 PEO-b-PPO-b-PEO/Desmodur R/LiCIO4 (AI',,= 4980) PEO-b-PPO-b-PEO/Desmodur R/LiCIO4 (~,=8390) PEO-b-.PPO-b-PEO/Desmodur N/LiCIO, (~,=8390) PDMS-g(PPO-b-PEO)/HMDI/LiCIO4 PEO 10501Desmodur R/Mg(CIOa)., polyphosphate and thiophosphate ionomers with Li ÷ as counterion
10.6 10.3 10.6 10.0 10.3 11.5 10.3 I 1.5 10.0 13.1 13.8 10.3 9.7 I 1. l
52 56.5 52 52.5 56.5 53 6l 54 67 40 30 50.5 5! 69
11.5
51
9.1
60
11.1 11.0 ! 0.0
49 55 61
for these polyether-based networks. In fact, no significant trend could be detected when changing the DP of the polyether chains, the structure of these chains, their topology, the type of salt added, its concentration and finally the situation of the anions, i.e. their attachment or not to the network. The average values representative of all situations are C~ = 11 and 7"~= 5 5 K. These unified features emphasize the paramount importance of the glass transition temperatures as the single basic parameter influencing the value of ionic conductivity around room temperature. The lower the Tg of the system, the higher the conductivity. Such a trend had already been observed [6], but the present work widens considerably the scope of the investigation. The changes of cr as a function of temperature for s5 stems as those defined in table 2 have already been p,ablished [6,7]. It is interesting here to compare them with the corresponding behaviour of ionomeric networks. Fig. 1 shows such a comparison: some i ar~omers gave quite low conductivities which could however be improved by swelling with a good liquid
-4
-7
1
I
3.0
3.5
I - -
z
~
Fig. 1. A comparison of the conductivity bchaviour of various iol~umeric networks with that of a network-plus-salt system. Sr Polyether network based on PEO-b-PPO-b-PEO (M~ = 8390' containing 12% LiCIOa (w/w). White star in black dot: Polyether-phosphate network with Li+..~- Former system swollen wit'a 40% propylene carbonate (w/w). [] Polyether-thiophosphate network with Li +. A. Polyether network based on the triol of table 1, bearing -(CF,,)3COO- Li +. , Polyether network based on the triol of table 1, bearing lithium alcoholate functions ¢lissociated by SbCI,.
(cf. fig. 1 ); others were more satisfactory but did not quite reach the values of the best salt + network systems. Nevertheless, the fact of having prepared purely cation-conducting solid electrolytes is a major qualitative step forward, and quantitative improvements should be forthcoming. The measurements of ionic conductivity have provided another important insight into the microscopic interactions occurring within these systems. The ionic conductivity inside a polymer matrix is a function of both the concentration and the mobility
J.F. Le Nest et al./Ionic conduction in cross-linked polyethers
of charged species. Because an increase in salt concentration induces an increase in the Tg of the network, the ionic mobility follows a trend opposit~ to that displayed by the abundance of charge carders. In order to compensate for this dichotomy, it is necessary to correlate the measured conductivity values to the salt concentration at equal mobility, viz. at ( T - T , ) = c o n s t a n t . When such plots are construtted, as shown in fig. 2, three features become apparent: the linearity of the plots up to a critical salt concentration, the slope of these straight lines, and the occurrence of singularities above the critical concentration characterized by conductivity minima at specific values of the salt concentration. With reference to the plot in fig. 2, its linear portion and its slope of uni. ' will first be discussed. The determination of the concentration of conducting species implies a detailed knowledge of the dissociation equilibria involving ionisable species. For the present situation, characterized by a neutral network containing an alkali metal salt, the following equilibria must be considered: KI
nMA.
"(MA),,
|og~"
(2)
1035
K2
(MA),.
'M + + ( M A ) , _ t A - ,
(3)
:h- + (MA),_IM +
(4)
K3
(MA),.
If as a first approximation activities are assimilated to concentrations, it follows that [M + ]2 =KIK:,[MA]" and [A-]2=KIK3[MA] ". Experimentally we find for PEO-based networks that tr=K[MA]o (cf. slope l of log-log plot in fig. l ). Since tr was measured at constant mobility, it must be directly proportional to the concentration of charge carriers, i.e. to [ A - ] , because the contribution of [ M + ] in these systems is very much smaller (t + < 0.2). Thus, [A- ] = K'[ MA ]o. Comparing this experimental correlation with the general expression for [ A - ] derived above, it follows that if [MA] ~ [MA]o, i.e. if the salt is essentially in lhe form of ion pairs, n = 2. This interpretation describes a situation in which most of the salt added, in the concentration range of linearity, remains in the form of undissociated ion pairs at temperatures above 35°C. A small fraction of the salt is associated in the form of"dimers", i.e. quadruplets which in turn dissociate into the respective i,~ns and triplets. Measurements of the dielectric constant in these media
'
(,~-lcm-1)
-4
_5
~
•
K
lo~C(moLdm-~ ~
Fig. 2. Log-log plots of ionic conductivity versus LiCIO4 concentration for three values of T - Tg in a network based on PEO 1050 and Desmodur R triisocyanate.
1036
J.F. Le Nest et al./lonic conduction in cross-linked polyethers
at these temperatures gave very high values. Thus, a PEO-based network centaining 0.8 mol dm -3 of LiCIO4 displayed an e of about 500 at 65°C. On cooling to 20°C, e decreased sharply to about 10. Whereas the high polarity at 65 °C confirms the predominance of ion pairs (dipoles), its very low value at room temperature indicates a sudden change in the state of the salt, namely towards a more neutral form of aggregation. The most obvious conformation seems to be quadrupoles, i.e. a predominance of quadruplets (salt "dimers"). Other systems, e.g. PPO and PEO.-b-PPO-b-PEO based networks with alkali metal salts, showed slopes of the log a versus log C plots higher than unity and even as high as 3 for PPO-based networks. Obviously, the degree of association of the salt can vary considerably according to the solvating power of the polyether chains: PPO is known to be a poorer complexing agent for alkali metal ions than PEO [ 9 ] and this would explain the change in behaviour described above. As for the discontinuities and in particular the minima of conductivity, they were found to correspond to discrete values of the ratio between Li + and ether units in the chains. In the specific context of fig. 2, the first minimum corresponds to one Li + per PEO chain, i.e. to its partition into two segments of about twelve ethylene oxide units each. The second minimum corresponds to two Li + per PEO chain. i.e. to a partition into three segments of about eight ethylene oxide units each. Similar observations were made with ne,tworks prepared with PEO of M, = 2100. The appearance of"waves" such as those shown in fig. 2 above the critical concentration, has been tentatively rationalized in terms of the attainment of ordered structures at each discrete ratio. Thus, a modification of the order pattern within the salt-polyether matrix probably takes place every time each PEO chain has been filled with the same number of Li + (minima in condut~tivity). Further ad-
cies, the aggregation of the latter, the mode of ionic fluctuation and their migration under an electric field. We envisage the segmental motions of polyether chains either bearing an ionised salt molecule (dipole) at the oxygen site or bound in pairs by ionic
quadruplets (see picture below). Statistical encounters between these two types of associations can give rise to exchange of conformation as indicated: \/ x /
0¢
(13
/\
+
\
Thus, segmental motions (free volume) induce continuous dipole transfers. As for charge transfers, they obviously require these motions but also the presence of vacant sites (defaults, 'triplets, etc.) in order to occur and will be random unless an electric field is applied across the material. The fact that the cationic transport number is much lower than its anionic counterpart is due to slower charge-transfer events because of the strong binding of the cations to the electron doublet of the ether oxygen. However, it is not the charge transfer which determines the overall conductivity (ionic transport). indeed, the perfect compliance of these systems to free-volume laws is a proof that dipole transfers, i.e. segmental motions, determine "Lheoverall rate of ionic movements.
4. Conclusion Tl,e results obtained in this study have established beyond doubt the free-volume character of the ionic transi~ort in polye~her-based networks. From a more pra,:tical point of view, two essential requirements IlUII ~ . , ~ u l t y l H k , |
situations induces a perturbation in the order attained and a corresponding increase in conductiviW is observed. In view of all the evidence obtained in this work and in previous investigations [31 it seems appropriate to suggest a model to visualize the binding of polyether chain through complexation by ionic spe-
-...->
~lk,~.~gltk$1t~
t,'~,-.~.OLlk,
Lltl~li
lltl~.< ;~%~'Jtv q ~ . g l l t | ~ Q V J i t - -
ity towards alkali metal ions and their very low 7g. Moreover, if cationic conduction is to be maximized, the ideal materials are obviously the same type of networks, but with anions as integral part of their structure. As for the salt concentration, best performances wi!l be obtained by working a little below the critical c~ncentration, defined fox" PEO-based
J.F. Le Nest et al./Ionic conduction in cross-linked polyethers
networks as that which provokes complete chain partition into subchains of 12 EO units.
References [ 1 ] M.B. Armand, Ann. Rev. Mater. Sci. 16 (1986) 245. [ 2 ] J.R. MacCailum and C.A. Vincent, eds., Polymer electrolyte reviews, Vol. 1 (Elsevier, Applied Science Publ., London, 1987). [ 3 ] J.F. Le Nest, A. Gandini and H. Cheradame, Brit. Polymer J., to be published.
1037
[4] A. Killis, J.F. Le Nest, A. Ga, dini and H. Cheradame, Makromol. Chem. 183 (1982) i037. [ 5 ] A. Killis, .LF. Le Nest, A. Gandini and H. Cheradame, J. Polym. Sci. Polym. Phys. Ed. 19 (1981) 1073. [6] A. Killis, J.F. Le Nest, H. Cheradame and A. Gandini, Makromol. Chem. 183 (1982) 2835. [7] A. Kiilis, J.F. Le Nest, A. Gandini, H. Cheradame and J.P. Cohen-Addad, Solid State Ionics 14 (1984) 231. [8] M. Lev~que, J.F. Le Nest, A. Gandini and H. Cheradame, J. Power Sources 14 (1985) 27. [9] J.F. Le Nest, A. Gandini, H. Gheradame and J.P. CohenAddad,. Macromolecules, to be published.
A MECHANISM OF IONIC CONDUCTION IN CROSS-LINKED POLYIETHERS J.F. LE N E S T , H. C H E R A D A M E and A. GANDINI
Laboratoire de Chimie Macromoldculaire et Papetibre, Ecole Franeaise de Papeterie (INPG), BP 65, 38402 Saint Martin d'Heres Cedex, France
Received 4 August 1987
A wide variety of polyether-based networks were prepared and tested in terms of ionic conductivity as a function of the type and concentration of the salt added, the structure of the reticulate and the temperature. This study includes ionomeric networks. The results indicate the occurrence of polyether chain partitioning by the dissociated salt and formation of quadrupoles binding two such chains. A model is proposed to rationalize the conduction mechanism on the basis of the overall experimental evidence.
1. Introduction
Growing interest in polymer electrolytes is undisputable [ 1,2]. The optimization of their performance in view of possible applications in solid-state batteries requires a ~horough knowledge of the ionic conduction mechanism. In order to avoid the problems intrinsic te linear polyethers (crystallization,creep, etc.), we begun to work several years ago on cross-li nked materials prepared flom oligo- or polyether polyols and multifunctional isocyae, ates [3]. These networks were either mixed with metal salts or converted into ionomeric structures with the anions covalently fLxed to the matrix [ 31. The advantages of this approach are primarily the amorphous character of the materials at all temperatures and their dimensional stabil"y [ 4 ]. The results obtained therefrom have been quite stimulating as already reported in part [ 3 ]. This communication deals specifically with the conductivity behaviour of a wide variety of networks and salts and with an atteml)t to unify the results gathmodel 1,~r the ionic a?~regation state and of'.~ eir inte~action with the polyether chains as well as a mechanism of ionic conduction.
2. Experimental
The experimental procedures followed to prepare both network-plus-salts [ 5 ] and i o n o m e r i c networks
[3] have been described. All starting materials for these syntheses are given in table 1. The dried salts added to the non-ionomeric networks were: LiCIO4, LiSO3CF3, LiBF4, LiOCOCF3, LiSCN, NaCIO4, KCIO4, NH4C104, NaBPh4, Mg(CIO4)2. Five anionic moieties were fixed to the chains of ionomeric networks [ 3 ] and used as lithium salts: phosphate, thiophosphate, - O - C O ( CF2 ) 3COO -, - O (SbCIs) and -O(SbFs) -. Ionic conductivities were obtained by the classical complex-impedance method using the setup described [ 6 ]. These measurements also allow the determination of the dielectric constant of the material.
3. Results and discussion We ~:ave previously shown that ionic conduction in poiyether-based networks containing an aikaii me~al salt follows the relationship [ 7]" G=Cq 2 Ba exp
+
0 167-2738/88/$ 03.50 © Elsevier Science Publishers B.V.
(North-Holland Physics Publishing Division)
exp
L +
fs + o ~ ( T - T g ) - R
r-
"
1033
J.F. Le Nest et al.llonic conduction in cross-linked polyethers
Table 1 Network precursors. Compound, formula (abbreviation) (a) polyether-ols poly(ethylene oxide)a, to-diol HO-(CH2-CH2-O-),,H (PEO)
tris(poly(PPO-b-PEO) to-ol) glycidyl ether CH 2-O ( C~H~O) ~(C2H40) ioH
Source
f,'l a )
J~n
Merck
1050 1550 2100 3800
2.03-2.04 2.02 2.05-2.06 2.07
Dow Chemical
2590
3.0
BASF, Wyandotte, Ugine Kuhlmann
4980
2.03
8390
2.04
:lT,,,/J-L 1.05 1.05 ! .06
I CH-O (C3H60) 7( C 2 H 4 0 ) loll
I CH2-O ( C3H60) 7( C2H40 ) loll triblock copolymers a, to-diol PEO-b-POP-b-PEO ( l ) HO (C2H40) 4s(CsH60) i v(C2H40) 4sH (Pluronic F 38) (2) HO(C2H40)78(CsH60)26C2H40)TsH (Pluronic F 68) graft-block copolymers polydimethylsiloxane-g-polyether (CHs)sSi(OSi(CHs) 2)2s(('SiCHsR)14OSi(CHs)s R = -CHz-CH 2-CH2- ( OC3 H6) Io( OC2H4) I2OH (PDMS--g-(POP-b-POE-OH)) (b) multifunctional isocyanates hexamethylene diisocyanate OCN(CH2)6NCO (HMDI) 4,4' ,4"-methylidyne tris( phenylisocyanate ) HC(C6H4NCO) 3 ( Desmodur R) OCN (CH2)6N(CONH(CH2)6NCO)2 (Desmodur N)
Rh6ne-Poulenc
19400
Merck
168
Bayer
367
Bayer
47~,
14
" ~ Average functionality (OH) as determined by acety!ation or i H NMR.
Here the subscripts a and c refer to the anion and the cation respectively. In the case of ionomeric polyanions all a-related variables vanish and eq. (1) reduces to a single term. Moreover, we showed that with most systems involving a network-salt mixture, the cationic transpo~ numbers are ve~, !ow~ i~e. 0.1 to 0.25 [ 8]. It becomes then reasonable to ignore the cationic exponential term in eq. (1). Thus, in both situations pertaining to the present study, oniy one term of eq. (1) needs to be considered for further development. All evidence points clearly to a free-volume behaviour of such dynamic properties of these systems
as viscoelasticity, nuclear magnetic relaxation and ionic conductivity [3]. Therefore one can write an expression of the reduced conductivity as developed in a previous paper [ 7 ]: log(o'T/arg) = - log a7+, where log a t = -C1( T - Tg)/( C2 + "f'- T~), i.e. the classical WLF relationship [ 7 tTable 2 gives the values of Ct and C2 obtained from conductivity measurements on a wide variety of systems. It is possible to conclude that, within the experimental errors implicit in the way Ct and C2 are determined, these characteristic constants are unique
1034
J.F. Le Nest et aL/lonic cgnduction in cross-linked polyethers
Table 2 Average values of WLF constants for polyether-based networks. System (see table 1)
C,
Ca (K)
PEO 1050/Desmodur R/LiCIO4 PEO 1050/Desmodur N/LiCIO4 PEO 2100/Desmodur R/LiCIO4 PEO 3800/Desmodur R/LiCIO4 PEO 1050/Desmodur N/LiCIO4 LiSO3CF3 LiBF4 LiSCN LiOCOCF3 NaCIO4 KCIO4 PEO 1050/Desmodur R/NH4CIO4 PEO 3800/Desmodur R/NH4CIO4 PEO-b-PPO-b-PEO/Desmodur R/LiCIO4 (AI',,= 4980) PEO-b-PPO-b-PEO/Desmodur R/LiCIO4 (~,=8390) PEO-b-.PPO-b-PEO/Desmodur N/LiCIO, (~,=8390) PDMS-g(PPO-b-PEO)/HMDI/LiCIO4 PEO 10501Desmodur R/Mg(CIOa)., polyphosphate and thiophosphate ionomers with Li ÷ as counterion
10.6 10.3 10.6 10.0 10.3 11.5 10.3 I 1.5 10.0 13.1 13.8 10.3 9.7 I 1. l
52 56.5 52 52.5 56.5 53 6l 54 67 40 30 50.5 5! 69
11.5
51
9.1
60
11.1 11.0 ! 0.0
49 55 61
for these polyether-based networks. In fact, no significant trend could be detected when changing the DP of the polyether chains, the structure of these chains, their topology, the type of salt added, its concentration and finally the situation of the anions, i.e. their attachment or not to the network. The average values representative of all situations are C~ = 11 and 7"~= 5 5 K. These unified features emphasize the paramount importance of the glass transition temperatures as the single basic parameter influencing the value of ionic conductivity around room temperature. The lower the Tg of the system, the higher the conductivity. Such a trend had already been observed [6], but the present work widens considerably the scope of the investigation. The changes of cr as a function of temperature for s5 stems as those defined in table 2 have already been p,ablished [6,7]. It is interesting here to compare them with the corresponding behaviour of ionomeric networks. Fig. 1 shows such a comparison: some i ar~omers gave quite low conductivities which could however be improved by swelling with a good liquid
-4
-7
1
I
3.0
3.5
I - -
z
~
Fig. 1. A comparison of the conductivity bchaviour of various iol~umeric networks with that of a network-plus-salt system. Sr Polyether network based on PEO-b-PPO-b-PEO (M~ = 8390' containing 12% LiCIOa (w/w). White star in black dot: Polyether-phosphate network with Li+..~- Former system swollen wit'a 40% propylene carbonate (w/w). [] Polyether-thiophosphate network with Li +. A. Polyether network based on the triol of table 1, bearing -(CF,,)3COO- Li +. , Polyether network based on the triol of table 1, bearing lithium alcoholate functions ¢lissociated by SbCI,.
(cf. fig. 1 ); others were more satisfactory but did not quite reach the values of the best salt + network systems. Nevertheless, the fact of having prepared purely cation-conducting solid electrolytes is a major qualitative step forward, and quantitative improvements should be forthcoming. The measurements of ionic conductivity have provided another important insight into the microscopic interactions occurring within these systems. The ionic conductivity inside a polymer matrix is a function of both the concentration and the mobility
J.F. Le Nest et al./Ionic conduction in cross-linked polyethers
of charged species. Because an increase in salt concentration induces an increase in the Tg of the network, the ionic mobility follows a trend opposit~ to that displayed by the abundance of charge carders. In order to compensate for this dichotomy, it is necessary to correlate the measured conductivity values to the salt concentration at equal mobility, viz. at ( T - T , ) = c o n s t a n t . When such plots are construtted, as shown in fig. 2, three features become apparent: the linearity of the plots up to a critical salt concentration, the slope of these straight lines, and the occurrence of singularities above the critical concentration characterized by conductivity minima at specific values of the salt concentration. With reference to the plot in fig. 2, its linear portion and its slope of uni. ' will first be discussed. The determination of the concentration of conducting species implies a detailed knowledge of the dissociation equilibria involving ionisable species. For the present situation, characterized by a neutral network containing an alkali metal salt, the following equilibria must be considered: KI
nMA.
"(MA),,
|og~"
(2)
1035
K2
(MA),.
'M + + ( M A ) , _ t A - ,
(3)
:h- + (MA),_IM +
(4)
K3
(MA),.
If as a first approximation activities are assimilated to concentrations, it follows that [M + ]2 =KIK:,[MA]" and [A-]2=KIK3[MA] ". Experimentally we find for PEO-based networks that tr=K[MA]o (cf. slope l of log-log plot in fig. l ). Since tr was measured at constant mobility, it must be directly proportional to the concentration of charge carriers, i.e. to [ A - ] , because the contribution of [ M + ] in these systems is very much smaller (t + < 0.2). Thus, [A- ] = K'[ MA ]o. Comparing this experimental correlation with the general expression for [ A - ] derived above, it follows that if [MA] ~ [MA]o, i.e. if the salt is essentially in lhe form of ion pairs, n = 2. This interpretation describes a situation in which most of the salt added, in the concentration range of linearity, remains in the form of undissociated ion pairs at temperatures above 35°C. A small fraction of the salt is associated in the form of"dimers", i.e. quadruplets which in turn dissociate into the respective i,~ns and triplets. Measurements of the dielectric constant in these media
'
(,~-lcm-1)
-4
_5
~
•
K
lo~C(moLdm-~ ~
Fig. 2. Log-log plots of ionic conductivity versus LiCIO4 concentration for three values of T - Tg in a network based on PEO 1050 and Desmodur R triisocyanate.
1036
J.F. Le Nest et al./lonic conduction in cross-linked polyethers
at these temperatures gave very high values. Thus, a PEO-based network centaining 0.8 mol dm -3 of LiCIO4 displayed an e of about 500 at 65°C. On cooling to 20°C, e decreased sharply to about 10. Whereas the high polarity at 65 °C confirms the predominance of ion pairs (dipoles), its very low value at room temperature indicates a sudden change in the state of the salt, namely towards a more neutral form of aggregation. The most obvious conformation seems to be quadrupoles, i.e. a predominance of quadruplets (salt "dimers"). Other systems, e.g. PPO and PEO.-b-PPO-b-PEO based networks with alkali metal salts, showed slopes of the log a versus log C plots higher than unity and even as high as 3 for PPO-based networks. Obviously, the degree of association of the salt can vary considerably according to the solvating power of the polyether chains: PPO is known to be a poorer complexing agent for alkali metal ions than PEO [ 9 ] and this would explain the change in behaviour described above. As for the discontinuities and in particular the minima of conductivity, they were found to correspond to discrete values of the ratio between Li + and ether units in the chains. In the specific context of fig. 2, the first minimum corresponds to one Li + per PEO chain, i.e. to its partition into two segments of about twelve ethylene oxide units each. The second minimum corresponds to two Li + per PEO chain. i.e. to a partition into three segments of about eight ethylene oxide units each. Similar observations were made with ne,tworks prepared with PEO of M, = 2100. The appearance of"waves" such as those shown in fig. 2 above the critical concentration, has been tentatively rationalized in terms of the attainment of ordered structures at each discrete ratio. Thus, a modification of the order pattern within the salt-polyether matrix probably takes place every time each PEO chain has been filled with the same number of Li + (minima in condut~tivity). Further ad-
cies, the aggregation of the latter, the mode of ionic fluctuation and their migration under an electric field. We envisage the segmental motions of polyether chains either bearing an ionised salt molecule (dipole) at the oxygen site or bound in pairs by ionic
quadruplets (see picture below). Statistical encounters between these two types of associations can give rise to exchange of conformation as indicated: \/ x /
0¢
(13
/\
+
\
Thus, segmental motions (free volume) induce continuous dipole transfers. As for charge transfers, they obviously require these motions but also the presence of vacant sites (defaults, 'triplets, etc.) in order to occur and will be random unless an electric field is applied across the material. The fact that the cationic transport number is much lower than its anionic counterpart is due to slower charge-transfer events because of the strong binding of the cations to the electron doublet of the ether oxygen. However, it is not the charge transfer which determines the overall conductivity (ionic transport). indeed, the perfect compliance of these systems to free-volume laws is a proof that dipole transfers, i.e. segmental motions, determine "Lheoverall rate of ionic movements.
4. Conclusion Tl,e results obtained in this study have established beyond doubt the free-volume character of the ionic transi~ort in polye~her-based networks. From a more pra,:tical point of view, two essential requirements IlUII ~ . , ~ u l t y l H k , |
situations induces a perturbation in the order attained and a corresponding increase in conductiviW is observed. In view of all the evidence obtained in this work and in previous investigations [31 it seems appropriate to suggest a model to visualize the binding of polyether chain through complexation by ionic spe-
-...->
~lk,~.~gltk$1t~
t,'~,-.~.OLlk,
Lltl~li
lltl~.< ;~%~'Jtv q ~ . g l l t | ~ Q V J i t - -
ity towards alkali metal ions and their very low 7g. Moreover, if cationic conduction is to be maximized, the ideal materials are obviously the same type of networks, but with anions as integral part of their structure. As for the salt concentration, best performances wi!l be obtained by working a little below the critical c~ncentration, defined fox" PEO-based
J.F. Le Nest et al./Ionic conduction in cross-linked polyethers
networks as that which provokes complete chain partition into subchains of 12 EO units.
References [ 1 ] M.B. Armand, Ann. Rev. Mater. Sci. 16 (1986) 245. [ 2 ] J.R. MacCailum and C.A. Vincent, eds., Polymer electrolyte reviews, Vol. 1 (Elsevier, Applied Science Publ., London, 1987). [ 3 ] J.F. Le Nest, A. Gandini and H. Cheradame, Brit. Polymer J., to be published.
1037
[4] A. Killis, J.F. Le Nest, A. Ga, dini and H. Cheradame, Makromol. Chem. 183 (1982) i037. [ 5 ] A. Killis, .LF. Le Nest, A. Gandini and H. Cheradame, J. Polym. Sci. Polym. Phys. Ed. 19 (1981) 1073. [6] A. Killis, J.F. Le Nest, H. Cheradame and A. Gandini, Makromol. Chem. 183 (1982) 2835. [7] A. Kiilis, J.F. Le Nest, A. Gandini, H. Cheradame and J.P. Cohen-Addad, Solid State Ionics 14 (1984) 231. [8] M. Lev~que, J.F. Le Nest, A. Gandini and H. Cheradame, J. Power Sources 14 (1985) 27. [9] J.F. Le Nest, A. Gandini, H. Gheradame and J.P. CohenAddad,. Macromolecules, to be published.