The Effect of Type of Cation and Salt Concentration on Ion− Ion and

J. Phys. Chem. B 2001, 105, 5847-5851

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ARTICLES The Effect of Type of Cation and Salt Concentration on Ion-Ion and Ion-Polymer Interactions in PEG-MSCN (M ) Li, Na, K) Polymer Electrolytes A. Zalewska, J. Stygar, E. Ciszewska, M. Wiktorko, and W. Wieczorek* Faculty of Chemistry, Warsaw UniVersity of Technology, ul. Noakowskiego 3, 00-664 Warszawa, Poland ReceiVed: September 13, 2000; In Final Form: February 23, 2001

The effect of the type of alkali metal cation and salt concentration on ion-ion and ion-polyether interactions has been examined for poly(ethylene glycol) (PEG)-MSCN (M ) Li, Na, K) electrolytes by means of impedance spectroscopy, DSC, rheological, and FT-IR techniques. Studies were performed in wide salt (from 10-5 to 5 mol/kg PEG) and temperature (from 20 to 90 °C) ranges. It is shown that for low salt concentrations of approximately up to 10-3 mol/kg PEG and for high salt concentrations (>1 mol/kg PEG), conductivities measured for LiSCN-doped electrolytes are higher than those for other systems. In the intermediate salt concentration range, the conductivities of all electrolytes studied are comparable.

Introduction It is commonly accepted that the interest in polymeric electrolytes arises from the possibility of their applications in various electrochemical devices working from subambient (e.g., down to -50 °C) to moderate (e.g., up to 200 °C) temperatures.1 To this end, ionic conductivity is a crucial feature of polymer ionic conductors. These systems are often considered as solutions of a dopant salt in a polymeric solvent.2 Due to the low dielectric constant of the latter, polymeric electrolytes belong to the group of weak electrolytes in which ionic associations are commonly present.3 Therefore, ion-ion interactions play an important role in the understanding of the ion conduction mechanism in polymeric electrolytes. On the other hand the dissociation of the salt leading to the creation of charge carriers results from the ion-polymer interactions of the iondipole type.1,3 These interactions can be alternatively classified as the acid-base reactions between alkali metal cations (acid type molecules) and base centers on the polyether oxygen atoms. Unfortunately, up to our knowledge, there is a limited number of studies covering a wide salt concentration range with the use of complementary experimental techniques, which are related to the explanation of the role of ion-ion and ionpolymer interactions in the conduction process.4-6 Most of these studies are related to polymer electrolytes doped with alkali metal thiocyanates.5-8 However, even in these works, the salt concentration does not cover a wide enough range. Therefore, in the present work, the salt concentration range is extended compared to the previously used limits.5-6 By using a variety of experimental techniques, we examined the relation between the microstructure of polymer conductors, the flexibility of polymeric chains, and the ion-ion and ion-polymer interactions. It is shown that the importance of various types of interactions for ionic conduction at various salt concentration * Corresponding author. Tel.: + 48 22 6607572. Fax: + 48 22 628 2741. E-mail: [email protected].

ranges is different. The experimental results are compared with previously published data.5 Experimental Section Sample Preparation. PEG (Mw ) 350, Aldrich, monomethyl-capped) was filtered, dried on a vacuum line at ∼60 °C for 72 h, and then, under a vacuum of 10-5 Torr, stringently freeze-dried using freeze-pump-thaw cycles. While still under vacuum, the polymer was transferred to an argon-filled drybox (moisture content lower than 2 ppm), where the salt was dissolved into the polymer using a magnetic stirrer. The salt concentration varied from 10-6 to 5 mol/kg of polymer. Samples of the salt concentration from 5 mol/kg down to 0.5 mol/kg were prepared by the direct dissolution of the salt in a polymer. Samples of the highest salt concentration were heated to 50 °C to facilitate the dissolution process. Samples of lower salt concentration were prepared by the successive dilution of a batch containing the electrolyte with 0.5 mol/kg of alkali metal salt. LiSCN, NaSCN, and KSCN (Aldrich, reagent grades) were dried under a vacuum of 10-5 Torr at 120 °C prior to the dissolution. All samples were equilibrated at ambient temperature for at least a month before undertaking any experiments. Experimental Techniques DSC Studies. DSC data were obtained between -110 and 150 °C using a UNIPAN 605 M scanning calorimeter with a low-temperature measuring head and liquid nitrogen cooled heating element. Samples in aluminum pans were stabilized by slow cooling down to -110 °C and then heated at 10 °C/min up to 150 °C. An empty aluminum pan was used as a reference. The estimated experimental error of the determination of the glass transition temperature (Tg) is (2 °C. Conductivity Measurements. Ionic conductivity was determined using the complex impedance method in the temperature range from 20 to 90 °C. The samples were sandwiched between stainless steel blocking electrodes and placed in a temperature-

10.1021/jp003248j CCC: $20.00 © 2001 American Chemical Society Published on Web 05/31/2001

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controlled oven. The experiments were performed in a constant volume cylindrical cell of the electrode diameter equal to 7.8 mm and fixed electrolytes thickness equal to 1.6 mm. The impedance measurements were carried out on a computerinterfaced Solartron-Schlumberger 1255 impedance analyzer over the frequency range of 1 Hz to 1 MHz. The reproducibility of impedance spectroscopy results were checked by multiple experiments performed at room temperature. The results obtained for samples of the same composition do not differ by more than 10%. FT-IR. Infrared absorption spectra were recorded on a computer-interfaced Perkin-Elmer 2000 FT-IR system with a wavenumber resolution of 2 cm-1. FT-IR studies were performed at 25 °C. Electrolytes were sandwiched between two NaCl plates and placed in the FT-IR temperature-controlled cell; the accuracy of the temperature was estimated to be (1 °C. Rheological Experiments. Rheological experiments were conducted at 25 °C using a Bohlin Visco 88BV viscometer in two coaxial cylinders geometry. The measurements were performed within a shear rate range of 24-1200 cm-1. The estimated error of rheological experiments is 10%. Results Figure 1a-c presents a comparison of the conductivity isotherms obtained for PEG-MSCN (Li, Na, K) electrolytes at 25 °C (a), 50 °C (b), and 90 °C (c). On each of the presented graphs, three salt concentration regions are distinguished. A clear difference in the conductivities measured for electrolytes doped with various alkali metal thiocyanates is seen at salt concentrations above ∼1 mol/kg PEG and below ∼10-3 mol/kg PEG. In these regions, conductivities measured for samples doped with LiSCN are the highest. At high salt concentrations, conductivities for PEG-KSCN electrolytes are higher than those for the PEG-NaSCN system, whereas at low salt concentration, the conductivities obtained for both sets of electrolytes are close to each other. In the intermediate salt concentration range, e.g., from ∼10-3 to 1 mol/kg PEG, the conductivity for all three sets of electrolytes are similar to those for PEG-KSCN electrolytes, being usually slightly higher (especially at 50 and 90 °C) than those for the other two types of electrolytes. The latter observation is consistent with the previous studies of Cameron et al.,5 who observed the conductivity trend σKSCN > σNaSCN > σLiSCN for amorphous ethylene oxide-propylene oxide copolymer-based electrolytes at ∼ 55 °C. The concentration range in these studies varied from 10-2 to 1.7 mol/kg. Outside these concentration limits, the trends in conductivity seems to change (see Figure 5b in ref 5). Figure 2 presents the changes in molar conductivity measured as a function of the square root of molar concentration for all three sets of electrolytes at 25 °C. Three characteristic regions described originally by Fuoss and Kraus10 and discussed in details by their followers can easily be distinguished.11 It should be emphasized that the initial low salt concentration region (in which a decrease in conductivity is observed, due to the formation of ion pairs) corresponded to the low salt concentration region already pointed out with regard to the results shown in Figure 1. Moreover, the final (e.g., high salt concentration region) decrease in conductivity (assigned by most of the authors to the viscosity increase) roughly corresponded to the high salt concentration conductivity region described in Figure 1. Using the formalism developed by Fuoss and Kraus10 and adjusted to polymer electrolytes by Vincent and co-workers,4 we calculated the fraction of ion pairs, free ions, and charged

Figure 1. Changes in ionic conductivity as a function of salt concentration measured at 25 °C (a), 50 °C (b), and 90 °C (c) for (B) PEG-KSCN, (0) PEG-LiSCN, and (4) PEG-NaSCN electrolytes.

triplets for all three sets of electrolytes. The details of the calculation procedure are described elsewhere.4,12 The data used in our calculations are included in Table 1.

PEG-MSCN (M ) Li, Na, K) Polymer Electrolytes

Figure 2. Changes in molar conductivity as a function of the square root of molar concentration at 25 °C for (B) PEG-KSCN, (0) PEGLiSCN, and (4) PEG-NaSCN electrolytes.

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Figure 4. Changes in the Tg as a function of salt concentration. Data for (B) PEG-KSCN, (0) PEG-LiSCN, and (4) PEG-NaSCN electrolytes.

TABLE 1: Physicochemical Data Used for the Calculation of Ionic Association on the Basis of the Fuoss-Kraus Formalism PEG-NaSCNa PEG-KSCNa PEG-LiSCNa cm-1 kg-1

Λo/S mol λo/S cm-1 kg-1 mol γ/g cm-3 η/Pas KI/mol-1 kg KT/mol-1 kg

0.00382b 0.0025c 1.094d 0.020d 54255e 60e

0.00384b 0.0026c 1.094d 0.020d 36864e 90e

0.00345b 0.0023c 1.094d 0.020d 1190e 124e

All calculations for 25 °C. b Calculated on the basis of the average Walden product value Λoη ) 0.6999 for NaSCN, 0.7049 for KSCN, and 0.652 for LiSCN solutions in low permittivity nonaqueous solvents. c λ ) 2/37Λ , following refs 9 and 10. d Data for PEG obtained o o experimentally from rheological and picnometric studies. e Calculated on the basis of experimental data from the Fuoss-Kraus formalism.4,10 a

Figure 5. Viscosity as a function of salt concentration. Data obtained at 25 °C for (B) PEG-KSCN, (0) PEG-LiSCN, and (4) PEGNaSCN electrolytes.

Figure 3. Changes in the fraction of ion pairs as a function of salt concentration at 25 °C for (B) PEG-KSCN, (0) PEG-LiSCN, and (4) PEG-NaSCN electrolytes.

For the clarity of presentation, Figure 3 shows the changes in the fraction of ion-pairs for each type of electrolytes studied as a function of salt concentration. This is under the assumption that ion-pairs do not contribute themselves to ionic conductiv-

ity. For salt concentrations up to 0.1 mol/kg PEG, the fraction of ion pairs found for PEG-LiSCN electrolytes is much smaller than that for the other two systems studied. For higher salt concentrations, the calculated fractions of ion pairs overlapped and are not higher than the estimated experimental errors of the impedance spectroscopy studies. Figure 4 presents changes in the glass transition temperature measured for each type of electrolytes as a function of salt concentration. At salt concentration up to ∼10-2 mol/kg PEG, Tg increases in the order KSCN < NaSCN e LiSCN, with the values for PEG-KSCN electrolytes being about 10 °C lower than those for the other two systems. For higher salt concentrations, the differences in Tg between particular electrolytes measured in the same salt concentration range are within an experimental error. The only exception are higher Tg values measured for the highest salt concentrations for PEG-KSCN electrolytes. Figure 5 presents changes in the electrolytes viscosity as a function of the salt concentration measured for all three types of electrolytes. Up to the salt concentration of 1 mol/kg PEG,

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Figure 6. Exemplary FT-IR spectrum at 25 °C for PEG-MSCN electrolytes.

Zalewska et al.

Figure 8. Changes in the fraction contact ion pairs and dimers calculated from the deconvolution of νSCN- as a function of salt concentration. Data for (B) PEG-KSCN, (0) PEG-LiSCN, and (4) PEG-NaSCN electrolytes.

mated experimental error. At higher salt concentrations, the downshift in the maximum is observed for PEG-LiSCN electrolytes. Figure 8 presents the fraction of contact ion pairs and dimers calculated from the deconvolution of νSCN- envelope for each type of the electrolytes using the procedure previously described for LiClO4-doped electrolytes.12 The positions of peaks characteristic for spectroscopic free anions, contact ion pairs, and dimers are assigned according to the assignments suggested by Chabanel et al.13 Although, the fractions of these ionic associates differ from the ones calculated on the basis of the Fuoss-Krauss formalism and have more qualitative than quantitative meaning, it can be noticed that for salt concentrations higher than 0.1 mol/kg PEG, the fractions of ion pairs calculated for each type of electrolytes are close to each other and are within the 0.40.6 range for most of the systems studied. Figure 7. Changes in the maximum of the C-O-C stretching mode as a function of salt concentration. Data for (B) PEG-KSCN, (0) PEG-LiSCN, and (4) PEG-NaSCN electrolytes.

the differences in the electrolytes viscosities are not very much higher than experimental error. For higher salt concentrations, the viscosity increases in the order η LiSCN < η KSCN e ηNaSCN. For the highest salt concentration, the viscosity of PEG-LiSCN electrolytes is approximately 3 times lower than that for PEG-KSCN systems and 20 times lower than that for PEG-NaSCN electrolytes. Figure 6 presents an exemplary FT-IR spectrum measured for PEG-MSCN electrolytes. Two regions are of interest for studies of ion-ion and ion-polymer interactions. These are ∼2050 cm-1 and 1100 cm-1, respectively, which characterizes the C-N stretching vibrations of the SCN- anion and stretching vibrations of C-O-C bonds of the polyether chains, respectively. The positions of the latter change with the formation of transient cross-links due to the interactions of alkali metal cations or positively charged triplets with base centers on polyether oxygen atoms. Figure 7 present changes in the position of the maximum of C-O-C stretch as a function of salt concentration for all sets of electrolytes. Up to the salt concentration of ∼0.1 mol/kg PEG, the position of the maximum is roughly constant for each type of the electrolyte. The differences between electrolytes containing various salts are within 6 cm-1, with the highest values measured for lithium and the lowest for sodium thiocyanate doped samples. These differences are almost within the esti-

Discussion It has been demonstrated that either at low (e.g., below 10-3 mol/kg PEG) or high (e.g., above 1 mol/kg PEG) salt concentration, the conductivities of PEG-LiSCN electrolytes are considerably higher than those measured for PEG-NaSCN and PEG-KSCN systems, the latter being superior to PEG-NaSCN conductivities at salt concentrations above 1 mol/kg PEG. In the intermediate salt concentration range (between 10-3 and 1 mol/kg PEG), the trends in conductivity behavior are similar to those previously described by Cameron et al.5 for electrolytes based on ethylene oxide-propylene oxide copolymers. The observed trends can be discussed in the view of supporting DSC, viscosity, and FTIR experiments as well as on the basis of the results calculated from impedance spectroscopy data using the Fuoss-Krauss formalism. At low salt concentrations, the fraction of ion pairs calculated on the basis of the Fuoss-Krauss formalism is much lower for the PEG-LiSCN system than for other electrolytes. According to the Fuoss-Krauss formalism, the decrease in molar conductivity observed for our electrolytes in the discussed salt concentration range is predominantly due to the ion-pair formation. Because of the comparable viscosities (closely related to the mobility of charge carriers) of all electrolytes studied (for this concentration range), the highest conductivity should be (and it is) observed for the electrolyte with the highest fraction of charge carriers, e.g., PEG-LiSCN. The conductivities measured for PEG-KSCN and PEG-NaSCN electrolytes are similar in this salt concentration range, most probably due

PEG-MSCN (M ) Li, Na, K) Polymer Electrolytes to the similar fractions of mobile charge carriers calculated for both sets of electrolytes. For salt concentrations of ∼10-2 mol/kg PEG, there is an increase in the Tg values observed on DSC traces (see Figure 4). This corresponds to the region in which the formation of charged triplets or redissociation of ion pairs should occur according to the Fuoss-Krauss concept (see data showing an increase in the molar conductivity corresponding to this salt concentration range in Figure 2). In this salt concentration range, conductivities for all sets of electrolytes are similar, which is consistent with the similar values of the formation of ion pairs (see Figure 2) and viscosities (see Figure 5). For salt concentrations higher than 1 mol/kg PEG, the viscosity changes in the order ηNaSCN g KSCN > ηLiSCN. In our opinion, this behavior can be assigned to the increase in the dielectric constant (polarity) of the electrolyte, leading to the redissociation of ionic aggregates. This trend is the strongest for LiSCN-doped samples containing a cation of the highest polarizability. Therefore, for electrolytes of the highest salt concentrations, the conductivity of the LiSCN doped systems is the highest. The differences in conductivities of PEG-NaSCN and PEG-KSCN electrolytes in this salt concentration range cannot easily be explained on the basis of IR, DSC, and rheological experiments since both systems exhibit similar viscosities as well as Tg values. The only feature which accounts for the higher conductivity of the PEG-KSCN system is the higher fraction of mobile charge carriers, as demonstrated in Figure 3. As was mentioned before, the results presented differ from those obtained previously by Cameron and co-workers.5,6 The difference is especially evident at low and high salt concentration ranges. There are several reasons which might account for this difference. The electrolytes studied by Cameron et al. were based on the EO-PO copolymer (EO-PO ratio ) 1:1) of molecular weight 3300 bearing n-butane and COCH3 end groups. Our system is based on a much shorter PEG chain with an OH group on one end. The end OH groups are capable of complexation of both anions and cations, as shown by Bernson and Linndgren on the base of IR studies.14 Therefore, these OH groups compete with polyether oxygen atoms in the complexation of alkali metal cations. This competition is partially supported by changes in the position of the C-O-C stretch mode, as shown in Figure 7. The maximum of the C-O-C stretch decreases only slightly with an increase in salt concentration for KSCN- and NaSCNdoped electrolytes. A larger downshift is observed for the LiSCN doped system, thus evidencing stronger interactions with polyether oxygen atoms. These various types of interactions most probably account for more complex changes in the viscosity and, therefore, conductivity than has been observed by Cameron

J. Phys. Chem. B, Vol. 105, No. 25, 2001 5851 et al. in their studies.5 Moreover, the PEG chain containing seven ethylene oxide monomeric units might be too short to effectively complex Na+ and especially K+ cations. It has been demonstrated that in the previously studied systems cross-linking of polyether chains was most probably caused by positively charged triplets (see Figure 6 in ref 5). The cross-linking was considered to be preferentially intramolecular (within one polymeric chain). Such a cross-linking mechanism explains the similar viscosities obtained by Cameron et al.5 at the same salt concentration for all the studied electrolytes, independently of the type of cation. In our case, intermolecular cross-links involving oxygen atoms from various polyether chains cannot be excluded, especially for KSCN- and NaSCN-doped electrolytes. This also explains the differences in viscosity and conductivity values of our systems compared with those of the previously studied electrolytes.5 It has too be emphasized that even in the previous studies, changes in the conductivity trends seem to occur outside the concentration limits discussed (see Figure 5b in ref 5). The extension of concentration limits leads to more complex electrolyte behavior, which has been demonstrated in the presented work. Acknowledgment. This work is financially supported by the Polish State Committee for Scientific Research (PBZ-KBN 15/ 09/T09/99/01b) and Warsaw University of Technology (504/ G/1020/0126). References and Notes (1) In Applications of ElectroactiVe Polymers; Scrosati, B., Ed.; Chapman and Hall: London, 1993. (2) Armand, M.; Gauthier, M. In High ConductiVity Solid Ionic Conductors - Recent Trends and Applications; Takahashi, T., Ed.; World Scientific Publication: Singapore, 1989; pp 114-165. (3) Bruce, P. G. Solid State Electrochemistry; Cambridge University Press: Cambridge, 1995; Chapters 5 and 6. (4) MacCallum, J. R.; Tomlin, A. S.; Vincent, C. A. Eur. Polym. J. 1986, 22, 787. (5) Cameron, G. C.; Ingram, M. D.; Sorrie, G. A. J. Chem. Soc., Faraday Trans. 1987, 83, a3345. (6) Cameron, G. C.; Ingram, M. D. In Polymer Electrolytes ReViews -2; Vincent, C. A., MacCallum, J. R., Eds.; Elsevier: London, 1989; Chapter 5. (7) Maynard, K. J.; Irish, D. E.; Eyring, E. M.; Petrucci, S.; J. Phys. Chem. 1984, 88, 729. (8) Saar, D.; Brauner, J.; Farber, H.; Petrucci, S. J. Phys. Chem. 1980, 84, 341. (9) Xu, M.; Eyring, E. M.; Petrucci, S. J. Phys. Chem. 1995, 99, 14589. (10) Fuoss, R. M.; Accasina, F. Electrolytic Conductance; Interscience: New York, 1959. (11) Petrucci, S.; Eyring, E. M. J. Phys. Chem. 1991, 95, 1731. (12) Wieczorek, W.; Lipka, P.; Zukowska, G.; Wycis´lik, H. J. Phys. Chem. B 1998, 102, 6968. (13) Chabanel, M.; Wang, Z. J. Phys. Chem. 1984, 88, 1441. (14) Bernson, A.; Lindgren, J. Polymer 1994, 35, 4842.