PolyMOB /lithium salt complexes: from salt-in-polymer to polymer-in-salt electrolytes

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1 Electrochimica Acta 48 (2003) 2037/ PolyMOB /lithium salt complexes: from salt-in-polymer to polymer-in-salt electrolytes Wu Xu, Li-Min Wang, C. Austen Angell * Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ , USA Received 19 May 2002; accepted 8 October 2002 Abstract Lithium polymob has been investigated as the polymer in a polymer-in-salt type electrolyte incorporating the salts lithium perchlorate (LiClO 4 ), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium tetrafluoroborate (LiBF 4 ). While all salts give rubbery solids at high salt contents, only LiClO 4 provides high conductivity because only in the case of LiClO 4 is the lithium cation motion highly decoupled from the structural relaxation. The crystallization of the salt at high salt contents prevents a favorable combination of mechanical and electrical properties, but the system provides an excellent example of the principle of the polymer-in-salt ionic rubber electrolyte and the factors determining its performance. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: PolyMOB; Salt-in-polymer electrolytes; Polymer-in-salt electrolytes 1. Introduction Polymer /salt complexes or salt-in-polymer electrolytes have been studied extensively since the suggestion by Armand et al. [1] in 1978 that they could be used as solid electrolytes in electrochemical devices. In these electrolytes a low polymer glass transition temperature is required because the ionic mobility is determined by the polymer segmental motion. The glass transition temperature (T g ) of this type electrolyte increases rapidly with salt concentration in the domain. The ambient conductivity of solvent-free salt-in-polymer electrolyte is in the range of 10 8 /10 4 S cm 1 and the cation transport number (t ) is far below 0.5 [2,3]. The observation that many polymer/salt mixtures must exhibit a maximum value of T g at intermediate salt contents lead to the suggestion [4] that a second domain of high conductivity may exist at salt-rich compositions particularly if, in the high salt region, the lithium cation motions are highly decoupled from the anion matrix. Thus the conductivity can in principle become much higher than that of any salt-in-polymer electrolytes at * Corresponding author. address: caa@asu.edu (C.A. Angell). the same temperature, while adding the advantage that the lithium ion transport number approaches unity [4,5]. Since only a small amount of high-molecular-weight polymer is needed to create a solid in polymer-in-salt electrolytes, these materials should combine the high conductivity of the fast-ion-conducting glasses and the good mechanical properties of the polymer. They are actually rubbery versions of glassy electrolytes. Many examples of polymer-in-salt type cation-conducting polymer electrolytes have now been reported [5 /16], though cases suitable for battery applications remain to be developed. Recently we prepared a novel polyanionic electrolyte, poly(lithium oligoetherato mono-oxalato orthoborate) called polymobs i.e. P(LiOEG n B) where n represents the repeating number of oxyethylene units [17]. The polymob with 14 oxyethylene unit spacer showed high single ionic conductivity of 10 5 S cm 1 at 25 8C, and wide electrochemical stability above 4.5 V versus Li / Li. In a companion paper [18], we have studied the effect of plasticization by organic solvents on the ionic conductivities and electrochemical properties of poly- MOBs, and also investigated gel forms prepared using high molecular weight poly(methyl methacrylate) (PMMA). The conductivity increases greatly on plasti /03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi: /s (03)00183-x

2 2038 W. Xu et al. / Electrochimica Acta 48 (2003) 2037/2045 cization with molecular liquid solvents while the single ion character is maintained since the polyanions cannot move significantly in the resulting gel. In the present paper, we report the results of plasticizing the polymob with appropriate lithium salts like LiClO 4. The thermal behavior and ionic conductivities of these polymob / lithium salt solutions, from low to high salt content, is reported, and the importance of the particular salt type used in the plasticization is made clear. 2. Experimental section 2.1. Materials PolyMOBs with different length of oxyethylene spacer, i.e. P(LiOEG n B) where n/3, 5, 9 and 14, were prepared and dried following the procedures described in ref. [17]. The numbers 5, 9 and 14 are used to represent products obtained from syntheses using PEG 200, 400 and 600, respectively, and are not be thought as integral numbers. The lithium salts, lithium perchlorate (LiClO 4 ) and lithium tetrafluoroborate (LiBF 4 ) from Aldrich and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) from 3 M as a gratis, were used as received Preparation of Li salt/p(lioeg n B) solutions For polymobs with spacer of n :/5, 9 and 14, the solutions were prepared by directly dissolving the Li salt in the polymob at high temperatures. Measured amounts of polymob and Li salt were weighed into Pyrex vials in a VAC dry box filled with purified nitrogen. Compositions are reported as mole fraction of the lithium salt, using the definition X salt /moles salt/ (moles salt/moles boron). Thus the physical concentration, in mol l 1, of Li will be lower in the solutions with polymobs of higher n value at the same value of X salt. The mole fraction of the Li salt in the solution (X Li salt) was varied from 0.1 to 0.9 in 0.1 increments. The vials were sealed with caps and heated in an oven at around 100 8C for 4 days, during which time the vials were stirred occasionally (by turning them upside down), till homogeneous solutions were obtained. For polymob with n /3, the direct dissolution method was unsatisfactory as the temperatures needed to dissolve the LiClO 4 completely were too high. In these cases the samples were prepared by an organic solution intermediate route. The mixture of LiClO 4 and P(LiOEG 3 B) with X LiClO4 /0.1/0.9 with 0.1 increments, was first dissolved in anhydrous acetonitrile to obtain a homogeneous solution. The solvent was then removed in two stages. The major part was removed by evaporation on a rotary evaporator under reduced pressure, and then the residual almost solid material was further dried in a vacuum oven at 100 8C for 1 week. We also prepared LiClO 4 /P(LiOEG 5 B) samples using the same solution method, in order to compare the effect of the two preparation methods. The polymer/salt materials obtained in this work ranged from light brown viscous liquids to hard glassy materials (details below) Calorimetric and conductimetric studies The thermal behavior of the complexes was studied by differential scanning calorimetry (DSC), in the temperature range of /100 to 80 8C, using a Perkin/Elmer DSC-7 in sub-ambient mode with liquid nitrogen reservoir. The instrument temperature scale was calibrated using the crystal /crystal transition of cyclopentane ( / C) and melting of indium (/ C). Samples were sealed in aluminum pans, purged with helium gas, and scanned from /100 to 80 8C. The heating rate was 10 K min 1. Glass transition temperatures (T g ) were recorded as the onset of the heat capacity jump on the thermograms during up-scan. The conductivities of the complexes were measured by a.c. impedance measurements during cooling from 120 8C to ambient or sub-ambient temperatures, using (i) dip-type conductance cells containing two parallel platinum discs when the samples were viscous liquids, or (ii) block-type cells with two stainless steel rod electrodes compressing a circular disc-shaped film sample, when the complexes were glassy or stiff rubbery materials. The measurements were carried out on a HP 4192A LF Impedance Analyzer in a frequency range from 5 Hz to 13 MHz. The cell constants of the dip-type cells were from 0.5 to 0.8 cm 1, calibrated by a 0.1 m KCl aqueous solution. The cell constants of the block type cells were obtained by measuring the thickness and radii of the complex films, which typically were about 0.3 cm 1. In order to get good contact of the solid films with the electrodes, the cells were pre-heated in an oven to a temperature of about 100 8C. Test measurements during heating and cooling at constant temperature yielded the same results within the data noise. 3. Results and discussion 3.1. Effect of preparation method Despite the fact that there are noticeable differences in the color of the samples obtained by the two procedures described above, there are generally no systematic differences in the physical properties provided that the drying procedure for the acetonitrile is strict enough. In Fig. 1 the effects of the two different sample preparation methods on the glass transition temperature and ionic conductivities of the LiClO 4 /P(LiOEG 5 B) solutions are compared. The values of conductivity and T g for pure

3 W. Xu et al. / Electrochimica Acta 48 (2003) 2037/ Effects of different length of oxyethylene spacer in the polymob component Fig. 1. Effect of the two preparation methods on the glass transition temperature and ionic conductivities of LiClO 4 /P(LiOEG 5 B) complexes. Conductivities lower than Scm 1 were obtained by extrapolations of higher temperature data, using fitted VFT equation. Also, values of T g and log s for pure LiCIO 4 are obtained by extrapolations of binary solution data in ref. [19] and refs. [4,5], respectively. LiClO 4 seen in Fig. 1 were obtained by extrapolation, as described in refs. [4,5,19]. Since the conductivities less than S cm 1 cannot be measured directly with the present instrumentation, data points below this value were obtained by extrapolation of the conductivitytemperature plots using best fitting to the Vogel / Fulcher/Tamman (VFT) equation. It is seen from Fig. 1 that the T g values are almost the same for the samples with the same LiClO 4 content (X LiClO4 ), except for the case of X LiClO4 /0.7 where the sample from direct dissolution has a T g about 20 8C lower than that from the acetonitrile solution preparation. This is difficult to understand since any residual solvent (acetonitrile) in the solution would be expected to lower T g rather than raise it. The difference in the conductivities for the different preparations is in the direction expected from the T g difference. The stepwise increases in T g are unexpected but they are reproduced by samples prepared by each method. The steps in T g are generally not reflected in the conductivity behavior, and are not seen for other n values. Overall, the variation of isothermal conductivity with salt concentration for the samples obtained by direct dissolution shows the form expected from the behavior of T g, i.e. the initial decrease of T g with salt addition is accompanied by increase in conductivity. The conductivities then decrease as T g increases and finally increases again after the T g passes through its maximum. This occurs near X LiClO4 /0.6 after which T g decreases to the value given elsewhere for pure LiClO 4. Details will be given in Section 3.3. Table 1 lists the physical appearances and glass transition temperatures (T g ) of the LiClO 4 / P(LiOEG n B) solutions from direct dissolution (n :/5, 9, and 14) and from the acetonitrile solution method (n /3). With increasing Li salt content, the solutions from polymob with very short EO spacer i.e. n/3 change from a rubbery material to a stiff rubbery solid, then a glassy material and finally a stiff rubbery solid again. For solutions from polymobs with long EO spacer such as n :/9 and 14, they show changes with salt content from a very viscous liquid to a sticky rubber, a soft rubber, then a rubbery material and finally a stiff rubbery solid. For solutions from polymobs with moderate EO length e.g. n :/5, the physical appearance, with increasing X LiClO4, changes from a sticky rubber to a rubbery solid to a stiff rubber. The high salt content electrolytes evidently would be favored by good mechanical properties. Fig. 2 shows the variations of T g with LiClO 4 content for the systems of LiClO 4 /P(LiOEG n B) of variable n. It is seen that for each P(LiOEG n B) there is a maximum value of T g. The maximum shifts to the left with decreasing n, which is consistent with the idea that it is the transient crosslinking of oxyethylene chains by interaction with Li that raises T g. The smaller the linker the lower in concentration the maximum effect is reached and the sooner the subsequent decrease towards the low T g (LiClO 4 ) can commence. For n/3, the maximum T g appears at X LiClO4 /0.5. For n :/5, the maximum T g is located at X LiClO4 /0.6. The maximum T g is found at X LiClO4 :/0.8 for both n :/9 and 14. The stepwise variation in T g for the case n :/5 is not seen with other polymers, but it is found for glasses prepared by independent methods. On the other hand, the experimental error of T g values from our DSC measurement is within 9/0.2 8C. Therefore, the description of stepwise is not just within the experimental error, but apparently true and observable. However, its explanation must await further studies. The temperature dependence of ionic conductivities of the complexes with different length of spacer n is shown in Figs. 3/6. The curved plots are consistent with the usual VFT behavior, which applies to almost all liquids approaching their glass temperatures. The curvature is greatest in the case containing the least polyether (n /3, X LiClO4 /0.8), which emphasizes that this is not merely a property of polymeric systems. Figs. 7 /10 show the variations of isothermal conductivities with LiClO 4 mole fraction at different temperatures for the solutions, and also the variation in T g. Data points below S cm 1 were obtained by extrapolation of the conductivity plots shown in Figs. 3/6 using best fitting to the VFT equation. Although

4 2040 W. Xu et al. / Electrochimica Acta 48 (2003) 2037/2045 Table 1 Physical appearances and glass transition temperatures of LiClO 4 /P(LiOEG n B) complexes from direct dissolution except for n/3 X LiClO4 (mol.%) Appearance and T g (in parentheses) n/3 n :/5 n :/9 n :/14 0 Yellow brown, rubber (/0.3 8C) Yellow brown, sticky rubber (/14.6 8C) liquid (/44.4 8C) Yellow, viscous liquid (/49.4 8C) 0.1 Yellow brown, rubber (1.6 8C) (/20.7 8C) liquid (/41.9 8C) Yellow, viscous liquid (/46.4 8C) 0.2 Yellow brown, rubber (9.41 8C) (/7.9 8C) liquid (/38.1 8C) liquid (/42.4 8C) 0.3 Brown, stiff rubber (15.8 8C) (/5.7 8C) liquid (/33.3 8C) Yellow, very viscous liquid (/39.0 8C) 0.4 Yellow brown, glass (22.9 8C) (7.0 8C) Yellow brown, soft rubber (/28.3 8C) Yellow brown, sticky solid (/33.8 8C) 0.5 Yellow brown, glass (31.7 8C) (10.0 8C) Brown, rubber (/17.7 8C) Yellow brown, sticky solid (/25.5 8C) 0.6 Yellow brown, glass (26.3 8C) Dark brown, rubber (24.2 8C) Brown, rubber (/6.2 8C) (/14.8 8C) 0.7 Yellow brown, stiff rubber (15.9 8C) Dark stiff rubber (13.3 8C) Dark brown, rubber (11.3 8C) (/2.5 8C) 0.8 Dark brown, stiff rubber (/5.3 8C) Dark stiff solid (/11.4 8C) Brown, stiff rubber (25.7 8C) (19.1 8C) 0.9 Dark brown, stiff rubber (not measured) Dark stiff solid (not measured) Dark brown, stiff rubber (/10.3 8C) Dark brown, rubber (6.0 8C) 1.0 (/16.1 a ) (/16.1 a ) (/16.1 a ) (/16.1 a ) Fig. 2. Variations of T g with LiClO 4 mole percentage for the systems of LiClO 4 /P(LiOEG n B). these extrapolated values are subject to uncertainty, the variation of the conductivity with salt content at low temperatures is consistent with that at high temperatures and is very reasonable. The solutions of LiClO 4 in the shortest spacer polymob, n/3, illustrates most clearly the important characteristics of this type of system. The variation of conductivity with LiClO 4 content shows the trend: Fig. 3. Temperature dependence of ionic conductivity of LiClO 4 / P(LiOEG 3 B) complexes with different salt concentration. The samples were obtained from solution method. increase, decrease and then increase again. This behavior can be explained in terms of a balance of the opposing effects, free ion number and microviscosity, on conductivity. However, the final increase (seven orders of magnitude at room temperature for 0.5 mole fraction of LiClO 4 ) is so great that a third influence must be involved. The initial increase, which occurs despite a

5 W. Xu et al. / Electrochimica Acta 48 (2003) 2037/ Fig. 4. Temperature dependence of ionic conductivity of LiClO 4 / P(LiOEG 5 B) complexes with different salt concentration. The samples were obtained from direct dissolution. weak increase in T g, could be a free carrier number effect or a decoupling effect. It is not obvious that a free carrier number effect should be present since the polymob is already quite rich in Li. However, when estimates of the conductivity at T g are made to assess the decoupling index (see below) it does not seem to provide an explanation for an increase in conductivity in this low salt range. Thus the free carrier number must be responsible for the initial increases. When the spacer of the polymob is short, the high salt content solution has higher conductivity than the low salt content solutions. This must be because the short spacer polymob is less able to chelate the Li ions Fig. 6. Temperature dependence of ionic conductivity of LiClO 4 / P(LiOEG 14 B) complexes with different salt concentration. The samples were obtained from direct dissolution. added in the plasticizing LiClO 4. Then, beyond X LiClO4 /0.2 the increase in T g causes the mobility, hence conductivity, to decrease until T g passes its maximum. A large composition range is found beyond the T g maximum in which the addition of LiClO 4 systematically increases the conductivity towards the extrapolated pure salt limit (see in particular, Fig. 7) Onset of decoupling in the polymer-in-salt domain By combining conductivity and T g data we can learn the effect of the structural parameters on the freedom of Fig. 5. Temperature dependence of ionic conductivity of LiClO 4 / P(LiOEG 9 B) complexes with different salt concentration. The samples were obtained from direct dissolution. Fig. 7. Variations of isothermal conductivities and T g with LiClO 4 mole content for LiClO 4 /P(LiOEG 3 B) complexes. The samples were prepared by long vacuum drying of acetonitrile solutions.

6 2042 W. Xu et al. / Electrochimica Acta 48 (2003) 2037/2045 Fig. 8. Variations of isothermal conductivities and T g with LiClO 4 mole content for LiClO 4 /P(LiOEG 5 B) complexes. The samples were obtained by direct dissolution of LiClO 4 in the hot polymer. Fig. 10. Variations of isothermal conductivities and T g with LiClO 4 mole content for LiClO 4 /P(LiOEG 14 B) complexes. The samples were obtained by direct dissolution of LiClO 4 in the hot polymer. very high, even at T g where the segmental relaxation time is ca. 100 s. When R t is near unity, however, the conductivity at T g will be of the order of S cm 1 To estimate the conductivity at T g we extrapolate the data of temperature-dependence of conductivities using the well-known VFT equation: ss o exp[d s T o =(TT o )] (1) Fig. 9. Variations of isothermal conductivities and T g with LiClO 4 mole content for LiClO 4 /P(LiOEG 9 B) complexes. The samples were obtained by direct dissolution of LiClO 4 in the hot polymer. Li cations to migrate independently of the relaxation of the remainder of the structure. We obtain this information from the decoupling index R t which is defined as the ratio of the structural relaxation time (t s ) to the conductivity relaxation time (t s ) [20,21]. At a T g where the structural relaxation time is 100 s [22], the decoupling index is given by the approximate relation log R t /14.3/log s Tg [23]. A high value of R t means the ionic motion is only weakly controlled by the immobile elements of the structure, meaning that the conductivity is highly decoupled from the segmental motion of the polymer chains and also from the motion of anionic groups. In this case the conductivity can be where s o is the pre-exponent conductivity; T o, the vanishing mobility temperature; D s is inversely proportional to the fragility of the liquid [19], provided conductivity is coupled to viscosity, i.e. R t /1 [24] (see below). The fragility is a measure of how rapidly the structure changes with increasing temperature above T g. High fragility permits a liquid to be very fluid even when T g is relatively high. D is normally in the range from 2 to 20, for polymeric and ionic systems. Values of the VFT parameters such as s o, T o and D s are summarized in Table 2. Also included in Table 2 are the differences between T g and T o, the estimated conductivity of the polymer electrolytes at T g, s Tg, and the corresponding decoupling indexes R t. We note that for X LiClO4 /0.5 the values of some s o are unphysically large ( /100, the value found for lithium superionic glasses). This would normally imply some additional phenomenon is involved in the conductivity-temperature dependence. However, the range of data is so small for these cases that the application of Eq. (1) is probably inappropriate. Data fit parameters for these compositions are entered in italics in Table 2 and are disregarded in the analysis. Excluding the above data sets, the values of s Tg show some significant trends. They indicate that as the Li ion concentration increases, the logarithmic decoupling index (log R t ) first decreases and then increases to

7 W. Xu et al. / Electrochimica Acta 48 (2003) 2037/ Table 2 VFT parameters from best fitting for LiClO 4 /P(LiOEG n B) complexes n Value X LiClO4 s o (S cm 1 ) T o (K) D s T g /T o (K) log s Tg log R t / / / /14.82 / /15.88 / / / / / / / / /15.96 / /15.95 / / / / / / / / / / /14.35 / / / / / / /20.96 / / / / / / / / / / Pure LiClO / values of order 3/6 at high LiClO 4 contents, with fluctuations that reflect the uncertainties of Eq. (1) extrapolations. A rapid increase in log R t was seen in other salt-in-polymer electrolytes [17,25] beyond the transition from salt-in-polymer to polymer-in-salt domains, and is a feature essential for successful polymerin-salt electrolytes. It is only by such decoupling that a high ambient conductivity can be obtained when glass temperatures lie above /50 8C. In the present case the data show that, to obtain superior conductivites, (s at 25 8C/10 4 S cm 1 ) the salt would need to remain uncrystallized up to LiClO 4 contents above 90 mol.%. Unfortunately, this cannot happen with pure LiClO 4 as salt, so the development of successful polymer-in-salt electrolytes will require the use of eutectic salt mixtures, or the discovery of new lithium salts that combine noncrystallizing characteristics with high melt conductivity. This point is emphasized by the results we have obtained for two other polymob systems using different lithium salts described in the next section. The variation of D s value with LiCIO 4 content shows a maximum, and this maximum appears at the salt content where the maximum T g is located. A similar and more precisely defined maximum was observed in an earlier study of salt-in -polymer systems [26]. There it was interpreted in terms of increased intermediate range order due to Li crosslinking of chains, and the same interpretation can be applied to the present case Dependence of polymer-in-salt phenomenology on lithium salt The dependence of T g on salt type, in systems incorporating LipolyMOB as polymer, is shown in

8 2044 W. Xu et al. / Electrochimica Acta 48 (2003) 2037/2045 Fig. 11. Variations of the glass transition temperatures with salt content for different Li salt/p(lioeg 5 B) complexes. Fig. 11. Data are for solutions in the n :/5 polymob. Data for LiClO 4 from earlier figures are compared with those for systems in which the LiClO 4 is replaced by LiTFSI or LiBF 4. In each case the T g values of the pure Li salts were obtained by an extrapolation method described in ref. [19]. The T g plot for the LiTFSI solutions shows monotonous increase with salt content. It is seen that no maximum exists and T g is always above ambient. Accordingly this system can exhibit none of the rubbery electrolyte properties desirable in a polymer-in-salt electrolyte. In the LiBF 4 system, the value of T g for the pure salt lies well below the T g maxima of Fig. 2 (and also 25 8C) so a viable ionic rubber range should exist. Only one composition has been prepared in this system and it has a T g comparable to that of LiClO 4 /P(LiOEG 5 B) solutions prepared by the acetonitrile solution route (Fig. 1). The variations of the isothermal conductivities for these solutions of different lithium salts are plotted in Fig. 12. Variations of the isothermal conductivities at different temperatures for different Li salt/p(lioeg 5 B) complexes with salt content. Fig. 12. It is seen that it is only at high salt content that the LiClO 4 /P(LiOEG 5 B) system yields the highest conductivity of these three salt systems. The LiTFSI / P(LiOEG 5 B) system shows a monotonous decrease in conductivity at ambient temperature after a weak maximum conductivity at X LiTFSI /0.1. This variation in conductivity is as expected from the variation of T g with salt content and confirms that LiTFSI cannot provide the basis for an ionic rubber solid electrolyte. By employing the VFT equation, we also obtained the VFT parameters for LiTFSI/P(LiOEG 5 B) and LiBF 4 / P(LiOEG 5 B) systems and the results are listed in Table 3. Compared with LiClO 4 /P(LiOEG 5 B) system, these two systems have lower s o and D s values at each salt concentration. The LiTFSI/P(LiOEG 5 B) also shows lower decoupling index. For LiBF 4 /P(LiOEG 5 B) system, the range of conductivity data available ( B/1 order of magnitude) is too small to extract meaningful parameters. It cannot be decided from the data in Fig. 12 whether or not LiBF 4 -rich solutions have the decoupling char- Table 3 VFT parameters from best fitting for other Li salt/p(lioeg n B) complexes Salt X Li salt s o (S cm 1 ) T o (K) D s T g /T o (K) log s Tg log R t LiTFSI / /16.15 / /15.36 / / / / / / / LiBF /

9 W. Xu et al. / Electrochimica Acta 48 (2003) 2037/ acteristics needed for successful polymer-in-salt electrolytes, though the high melting point of LiBF 4 itself must eliminate systems based on the pure salt from consideration. It is possible that a system in which the melting point of LiBF 4 is lowered by complexing with suitable Lewis acids, or mixing with suitable low melting second components, could provide a suitable liquid salt for these purposes, and the investigation of this possibility is being investigated in current work. 4. Conclusions Using a polyanionic polymer and LiClO 4 as ionic plasticizer, a wide range of polymer-in-salt (ionic rubber) electrolyte behavior has been made available for study, particularly by using polymobs with short spacer groups. High conductivity and high decoupling index ca. 10 6, have been measured at high salt content. The increase in conductivity is a direct consequence of decreased glass transition temperature and increased decoupling index. These electrolytes are physically robust materials, with rubbery solid characteristics in most cases because the glass transition temperature is below ambient temperature. The mechanical properties of these materials appear to be excellent. If lithium salt that had similar decoupling characteristics to LiClO 4 but lower T g were available, a superior solid electrolyte could be obtained. Acknowledgements This work was supported by a grant from Mitsubishi Chemical Corporation of Japan. References [1] M.B. Armand, J.M. Chabagno, M. Dulcot, in: Extended Abstracts, The Second International Conference on Solid Electrolytes, St. Andrews, Scotland, September [2] P.G. Bruce, M.T. Hardgrave, C.A. Vincent, Solid State Ionics 53/56 (1992) [3] W. Gorecki, M. Jeannin, E. Belorizky, C. Roux, M. Armand, J. Phys.: Condens. Matter 7 (1995) [4] C.A. Angell, C. Liu, E. Sanchez, Nature 362 (1993) 137. [5] C.A. Angell, J. Fan, C. Liu, Q. Lu, E. Sanchez, K. Xu, Solid State Ionics 69 (1994) 343. [6] J. Fan, R.F. Marzke, C.A. Angell, J. Non-Crystallogr. Solid 172/ 174 (1994) 178. [7] K. Xu, C.A. Angell, Mater. Res. Soc. Symp. Proc. 369 (1995) 505. [8] J. Fan, C.A. Angell, Electrochim. Acta 40 (1995) [9] (a) M. Watanabe, K. Yamada, K. Sanui, N. Nogata, J. Chem. Soc. Chem. Commun. (1993) 929.; (b) M. Watanabe, T. Mizumura, Solid State Ionics 86/88 (1996) 353. [10] L. Feng, H. Cui, J. Power Sources 63 (1996) 145. [11] M. Forsyth, J. Sun, D.R. MacFarlane, Solid State Ionics 112 (1998) 161. [12] D.R. MacFarlane, F. Zhou, M. Forsyth, Solid State Ionics 113/ 115 (1998) 193. [13] A. Ferry, L. Edman, M. Forsyth, D.R. MacFarlane, J. Sun, J. Appl. Phys. 86 (1999) [14] A. Ferry, L. Edman, M. Forsyth, D.R. MacFarlane, J. Sun, Electrochim. Acta 45 (2000) [15] M. Forsyth, D.R. MacFarlane, A.J. Hiller, Electrochim. Acta 45 (2000) [16] Z. Wang, W. Gao, X. Huang, Y. Mo, L. Chen, Electrochem. Solid-State Lett. 4 (2001) A148. [17] W. Xu, M.D. Williams, C.A. Angell, Chem. Mater. 14 (2002) 401. [18] W. Xu, C.A. Angell, Electrochim. Acta, in press. [19] M. Videa, C.A. Angell, J. Phys. Chem. B 103 (1999) [20] C.T. Moynihan, N. Balitactac, L. Boone, T.A. Litovitz, J. Chem. Phys. 55 (1971) [21] C.A. Angell, Solid State Ionics, 9 /10 (1983) 3 and 18/19 (1986) 72. [22] C.T. Moynihan, P.B. Macedo, C.J. Montrose, P.K. Gupta, M.A. DeBolt, et al., Annu. New York Acad. Sci. 279 (1976) 15. [23] C.A. Angell, Annu. Rev. Phys. Chem. 43 (1992) 693. [24] C.A. Angell, C.T. Imrie, M.D. Ingram, Polym. Int. 47 (1998) 9. [25] M.G. McLin, C.A. Angell, Solid State Ionics 53/56 (1992) [26] M.G. McLin, C.A. Angell, J. Phys. Chem. 95 (1991) 9464.