- Email: [email protected]
Characteristics of gel electrolytes formed by self-aggregating comb-shaped polyethers with end-functionalised side chains
Solid State Ionics 166 (2004) 417 – 424 www.elsevier.com/locate/ssi
Characteristics of gel electrolytes formed by self-aggregating comb-shaped polyethers with end-functionalised side chains Patric Jannasch *, Wendy Loyens Department of Polymer Science and Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden Received 2 September 2003; received in revised form 6 November 2003; accepted 6 November 2003
Abstract Gel electrolytes based on comb-shaped polyethers have been prepared and characterised. The comb-shaped polyethers consisted of poly(ethylene oxide) (PEO) or poly(ethylene oxide-co-propylene oxide) (PEOPO) side chains tethered to styrenic backbone polymers. All the polyether side chains were terminated by hydrocarbon (C16) chain ends. Polymer gel electrolytes were prepared by adding 10 – 70 wt.% of an electrolyte solution with 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt in g-butyrolactone (g-BL) or in a mixture of ethylene carbonate and diethyl carbonate (EC/DEC). Thermal analysis by differential scanning calorimetry (DSC) showed endotherms arising from the melting of hydrocarbon phase domains, indicating polymer self-aggregation in the gels by microphase separation of the chain ends. As the concentration of electrolyte solution was increased, the melt temperature of the hydrocarbon phase domains was found to decrease, while the heat of fusion in joules per gram polyether remained essentially constant. The latter finding implied a strong propensity for the hydrocarbon chains to phase separate in the electrolytes. This was further supported by small-angle X-ray scattering measurements which ˚ in the gels. The ion conductivity of the electrolytes with 70 wt.% of showed a characteristic diffraction distance of approximately 60 A electrolyte solution added typically reached 10 2.5 S/cm at 20 jC. D 2004 Elsevier B.V. All rights reserved. Keywords: Self-associating graft copolymer; Hydrophobically modified poly(ethylene oxide); Polymer gel electrolyte; Crystallisation; Ionic conductivity
1. Introduction In the design of polymer electrolytes, several properties, such as ionic conductivity, mechanical strength, and electrode compatibility, have to be efficiently combined in a single material. For example, electrolytes with a combination of high ionic conductivity and mechanical strength have been obtained by using chemically cross-linked polymer networks [1 – 3]. An alternative, in some regards more useful, way is to employ aggregating or self-assembling polymers to form physically cross-linked polymer electrolyte systems. This has for example been successfully achieved using self-assembling block copolymers with ionophilic blocks, usually of poly(ethylene oxide) (PEO), to complex the salt and facilitate the ion conduction, and ionophobic blocks which induce a phase-separated system with mechanical strength [4– 10]. Basically, these systems
* Corresponding author. Tel.: +46-46-222-9860; fax: +46-46-222-4115. E-mail address: [email protected] (P. Jannasch). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.11.008
resist flow because this would mean mixing the ionophobic blocks with the ionophilic blocks, and vice versa, which is thermodynamically unfavourable. If the block copolymers are well designed, ionic conductivities close to the corresponding neat complex of salt and ionophilic polymer can be reached [10]. Recently, a series of comb-shaped polymers consisting of backbone copolymers with densely grafted polyether side chains have been prepared and studied as ion-conducting solid polymer electrolytes in combination with lithium salts [11,12]. The side chains of the comb-shaped polyether were functionalised with hexadecanoyl chains. These ionophobic chain ends were immiscible in the polyether –salt complexes and were found to phase separate from the polyether – salt complexes. This phase separation induced the formation of physically cross-linked polyether networks through interpolymeric aggregation. Recent simulations of comb-shaped copolymers with attracting, ‘‘sticky,’’ side chain ends have shown that strong attraction between the chain ends stabilises micellar aggregates that serve as junction points of physical networks forming so-called ‘‘micellar gels’’ [13].
418
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
2. Experimental
(DSC) system under N2 purge. After calibration using standard reference materials (indium, zinc, and lead), samples were placed in aluminium containers that were sealed under Ar atmosphere. The samples were first annealed at 60 jC for 5 min, then cooled to 150 jC, and finally heated to 60 jC. The scan rate was at 10 jC/min during the two scans. The glass transition temperature (Tg) was taken as the temperature at the inflexion point of the heat capacity change detected during the heating scan.
2.1. Materials
2.4. X-ray scattering
The preparation, purification, and characterisation of the comb-shaped polyethers have been reported previously [11], and the molecular data of the two polymers used in the present study are shown in Table 1. The solvents g-butyrolactone (g-BL), ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) were all of SelectipurR battery grade purchased from Merck. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt was kindly supplied by 3M. All the solvents and the LiTFSI salt were used as-received and were handled under dry Ar atmosphere in a glove box.
Structural information was gathered from small-angle Xray scattering (SAXS) experiments. They were performed using a Kratky compact camera equipped with a linear position sensitive detector (OED 50M from MBraun, Graz). A Seifert ID 3000 X-ray generator provided the CuKa ˚ . The samples were radiation with a wavelength of 1.542 A placed in a sealed solid sample cell between mica sheets, and data were collected during 2 h at 25 jC.
In the present work, the aggregating comb-shaped polyethers were studied as hosts for electrolyte solutions, i.e., a salt dissolved in organic aprotic solvents. The main motive for the preparation of gel electrolytes was to obtain electrolytes with conductivities sufficiently high for a general use in electrochemical devices.
2.2. Electrolyte preparation Gel electrolytes were prepared by swelling salt-containing polyethers with electrolyte solutions. Samples of the neat polyethers were first transferred to the glove box, and then dried at 60 jC under vacuum for 3 days. Precise amounts of a solution of LiTFSI in DMC were added to glass ampoules containing the polyethers to obtain electrolytes with a salt concentration corresponding to 20 coordinating ether oxygens per lithium ion. The DMC was allowed to evaporate under Ar for 2 days at ambient temperature before the samples were transferred to a glass oven where the residual solvent was removed under vacuum at 60 jC for 3 days. Gel electrolytes were subsequently prepared by adding precise amounts of 1 M LiTFSI in either g-BL, or an EC/DEC (2:1 wt./wt.) mixture, to the salt-containing polyethers. In this way, gel electrolytes containing 10, 30, 50, and 70 wt.% of the respective electrolyte solutions were prepared. 2.3. Calorimetry The thermal properties of the electrolytes were analysed using a Mettler TA 3000 differential scanning calorimetry Table 1 Molecular data of the self-aggregating polyethers Designation
Mn (kg/mol)
Side chain Mna,b (kg/mol)
Side chain contenta,b (wt.%)
EO/PO (wt./wt.)
hCPEO hCPEOPO
730 670
2.1 1.9
96 95
100:0 80:20
a b
Calculated from 1H NMR results. Includes the hydrocarbon chain end (Mn = 0.239 kg/mol).
2.5. Impedance spectroscopy The ion conductivity (r) of the electrolytes was evaluated by measuring the temperature dependence of impedance spectra during first heating from 40 to 100 jC, and then cooling to 40 jC. Samples with a diameter of 18 mm and a thickness of 90 Am were sandwiched between two goldplated stainless steel blocking electrodes spaced by a PTFE ring. The measurements were carried out using a computercontrolled Novocontrol BDC40 high-resolution dielectric analyser equipped with a Novocool cryostat unit. Samples were analysed in the frequency range 10 1 – 107 Hz at 100 mV AC amplitude, and the conductivities were subsequently evaluated using the Novocontrol software WinDeta.
3. Results and discussion The chemical structures of the comb-shaped polyethers used in the present work are shown in Scheme 1, and their molecular data are collected in Table 1. As seen, hCPEO had side chains of poly(ethylene oxide) (PEO), while hCPEOPO had side chains of poly(ethylene oxide-co-propylene oxide) (PEOPO). Furthermore, the average molecular weight of the side chains of the two comb-shaped polyethers were approximately the same, 2 kg/mol, and they both contained about 95 wt.% of side chains, the remaining part being the polystyrenic backbone. The phase behaviour of the different materials was first studied by DSC. The traces of the two electrolyte solutions, 1 M LiTFSI in EC/DEC and 1 M LiTFSI in g-BL, are shown in Fig. 1. As seen, the former solution displayed a glass transition at 105 jC and a melting endotherm peak at 8 jC, while the latter solution had a glass transition at 135 jC, a ‘‘cold’’ crystallisation exotherm at 100 jC,
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
419
Scheme 1. Structure and aggregation of the comb-shaped polyethers.
and finally a melting endotherm peak at 55 jC. Thus, the electrolyte solution based on g-BL had much lower transition temperatures than the EC/DEC solution. Gel electrolytes were prepared by adding different amounts of the respective electrolyte solution to the comb-shaped polymers. All the gel electrolytes were optically transparent, but appeared to scatter light. This is usually an indication of microphase separation. Fig. 2 shows the DSC heating traces of the polymer electrolytes. First, the electrolytes based on hCPEOPO will be discussed. The trace of the solid electrolyte, marked ‘‘0%’’ in Fig. 2A, revealed a glass transition at 50 jC, originating from the amorphous polyether side chains, and melting peak at 13 jC. This peak has previously been identified as arising from the melting of the hydrocarbon chain ends [11,12]. When 1 M LiTFSI in g-BL was added and the amount subsequently increased, the Tg decreased as the polymer was plasticised by the solvent. Furthermore, the integral of the melting peak of the hydrocarbon chain ends decreased, and melting occurred at decreasing temperatures. No signs of crystallisation or melting of the
electrolyte solution were noted in any of the gel electrolytes. It can also be observed in Fig. 2A that the traces of hCPEOPO containing 10 wt.% of either of the two electrolyte solutions were very similar. However, after addition of 30 wt.% EC/DEC the resulting gel showed signs of crystallisation and melting of the solution. The propensity for EC/DEC crystallisation then increased at 50 and 70 wt.% added solution. The traces of the electrolytes based on hCPEO are shown in Fig. 2B. The solid electrolyte displayed a glass transition at 50 jC, a ‘‘cold’’ crystallisation and melting in the temperature interval between 25 and 30 jC. The melting of the hydrocarbon chain ends probably occurred in this temperature interval, but the peak was overlapped by the PEO transitions. When increasing amounts of 1 M LiTFSI in g-BL were added to hCPEO, the crystallisation and melting of PEO, as well as the Tg, were depressed. In the electrolyte containing 50 wt.% of the solution, the remaining peak at 30 jC most probably arose from the melting of the hydrocarbon chain ends. This peak was further depressed and broadened at 70 wt.% added solution. In the case of the
420
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
Fig. 1. DSC heating traces of 1 M LiTFSI salt in g-BL (a) and 1 M LiTFSI salt in the EC/DEC mixture (b). The solutions were cooled from 25 down to 150 jC prior to the heating scan. Scan rate: 10 jC/min.
electrolytes based on EC/DEC, crystallisation and melting of the PEO side chains were noted at 10 wt.% added. The trace for the electrolyte with 30 wt.% added solution only showed a Tg at 50 jC, and a melting peak from the hydrocarbon chain ends at 23 jC. Thus, no crystallisation or melting of the PEO or the EC/DEC was detected. However, at 50 and 70 wt.% solution added, the crystallisation and melting of the EC/DEC solutions were evident. These traces were very similar to the corresponding traces of the electrolytes formed by hCPEOPO. From the thermal analysis of the electrolytes it was concluded that the thermal properties of the various gel electrolytes were clearly influenced by their respective components. Thus, hCPEO and the LiTFSI/EC/DEC solution crystallised, given a sufficiently high concentration in the electrolytes. In contrast, hCPEOPO and the LiTFSI/gBL solution was in the amorphous and liquid state, respectively, with the exception of the hydrocarbon chain ends of
hCPEOPO. Of cause, the reason for the amorphous state of hCPEOPO side chains was their irregular copolymeric configuration. Crystallinity of the hydrocarbon chain ends was observed in all electrolyte systems, although in many cases, the melting peak was overlapped by the melting peak of the polyether side chains. This revealed that the chain ends phase separated both in the EC/DEC and g-BL based electrolyte solutions. It is feasible that, in connection with this process, the chain ends formed ‘‘micellar’’-like microdomains which physically cross-linked the polyether network that hosted the electrolyte solution. A schematic picture of the self-aggregated comb-shaped polyethers is shown in the lower part of Scheme 1, where the ‘‘micellar’’-like microdomains (medium thick lines) are linked to the polyether side chains (thin lines) which are connected to the backbone polymers (thick lines). From the molecular weight values of the comb-shaped polyethers in Table 1, it was calculated that each molecule carried an average of 350 hydrocarbon chain ends. Thus, there is a considerable likelihood of intermolecular aggregation of the polyethers. As mentioned in Section 1, the process of self-aggregation in comb-shaped polymers with strongly attracting groups located at the chain ends have previously been simulated by Shirvanyanz et al. [13]. They found that the formation of a ‘‘micellar’’-like phase was dependent on both the chemical composition and on the polymer concentration. They also found that the ‘‘micelle’’ formation observed in sufficiently dense systems resulted mainly in intermolecular aggregation. In the case of LiTFSI/g - BL in hCPEOPO, the effect of the addition of various amounts of electrolyte solution to the comb-shaped polyethers could be studied because the melting endotherms of the hydrocarbon phase were isolated from other transitions. In Fig. 3, the melting peak temperature (Tm) and the heat of fusion (DHm) are plotted versus the amount of electrolyte solution added. As already noted above, Tm decreased as electrolyte solution was added. This may indicate a decrease in size, or possibly a more irregular crystalline structure of the hydrocarbon phase domains. On the other hand, the value of DHm was essentially constant at a value between 9 and 9.5 J/g hCPEOPO in the gel. This indicated that the degree of phase separation of the hydrocarbon chain end, and thus the degree of aggregation, did not change as the amount of electrolyte solution was increased in the gel. Structural information concerning the possible ‘‘micelle’’ formation was obtained from SAXS experiments. As expected, a solid polymer electrolyte based on the nonmodified form of hCPEOPO, i.e., without hydrocarbon C16 chain ends, did not display any diffraction pattern, most probably due to structural homogeneity. Fig. 4 presents the diffraction patterns of the solid polymer electrolyte based on hCPEOPO pure and of gel electrolytes containing different concentrations of 1 M LiTFSI in g-BL. The presence of diffraction peaks in the SAXS spectra indicated quite a high
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
421
Fig. 2. DSC heating traces of electrolytes based on hCPEOPO (A), and hCPEO (B) containing 0, 10, 30, 50, and 70 wt.% of 1 M LiTFSI in the EC/DEC mixture and 1 M LiTFSI in g-BL, respectively. The electrolytes were cooled from 25 down to 150 jC prior to the heating scan. Scan rate: 10 jC/min.
degree of order in the system. All these electrolytes exhibited distinct diffraction peaks, indicative of ordered systems. As expected, the intensity of the peak became lower upon increasing the content of the electrolyte solution.
Fig. 3. Influence of the content of 1 M LiTFSI in g-BL on the heat of fusion and melt temperature of the hexadecanoyl phase of hCPEOPO.
Fig. 4. SAXS diffraction patterns of a solid polymer electrolyte based on hCPEOPO (0%) and of hCPEOPO gel electrolytes containing various concentrations of 1M LiTFSI in g-BL (10 – 70%).
422
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
The thermodynamic driving force for phase separation in many heterogeneous systems is the formation of a crystalline phase. It was thus interesting to note that although the measurements were performed at 25 jC, well above the melt temperature of the hydrocarbon phase, the microdomains still remained phase separated to physically cross-link the polyether—also when the concentration of the electrolyte solution was increased to 70 wt.%. This was most probably
a consequence of the low solubility of the hydrocarbon chain ends in the salt-containing phase. The simulations performed by Shirvanyanz et al. [13] showed that the ‘‘micellar’’-like microdomains had a spherical shape. These simulations further suggested that if a critical interaction energy between the chain ends is reached, the ‘‘micellar’’ aggregates will form clusters of a larger size [13]. These clusters were shown to have a close to spherical
Fig. 5. Arrhenius conductivity plots for gel electrolytes containing various amounts (10 – 70 wt.%) of the electrolyte solutions: hCPEOPO/LiTFSI/g-BL (a), hCPEOPO/LiTFSI/EC/DEC (b), hCPEO/LiTFSI/g-BL (c) and hCPEO/LiTFSI/EC/DEC (d). Also included are the conductivities of the solid polymer electrolytes based on hCPEOPO and hCPEO, respectively, having [Li]/[O] = 0.050 (0 wt.%). The conductivities were evaluated by impedance spectroscopy during heating, from 40 to 100 jC (solid lines and filled symbols), and then cooling to 40 jC (dashed lines and open symbols). Please note the high degree of overlapping of the heating and cooling curves in some cases.
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
shape. By assuming a spherical geometry of the entities formed by the chain ends of the presently studied materials, it is possible to calculate an average diffraction distance from the scattering data by using q¼
4p h sin k 2
ð1Þ
combined with Braggs law 1 2 sinh ¼ d k
ð2Þ
where q and d represent the scattering vector and the diameter, respectively. h is obtained from the measured diffraction angle 2h, and k is the wavelength of the X-rays [14]. The calculation gave a characteristic diffraction ˚ (see Fig. 4) for the present electrodistance of f 60 A lytes, almost regardless of the concentration of the electrolyte solution. It is, however, unclear whether this dimension is characteristic of singular micelles formed by the C16 chain ends or clusters of aggregated micelles. As a comparison, the length of two fully stretched C16 ˚ . Because chains was calculated to be approximately 40 A the micelles formed by hCPEOPO can be expected to be quite mobile due to the flexible polyether side chains, the existence of the cluster structure remains likely. The broadness of the diffraction peak provided further information because it reflected the distribution of the scattering distances in the electrolytes. In addition, a second less intense peak was observed at higher q-values, which may indicate the presence of two different populations [13]. Arrhenius plots of the ion conductivity of the gel electrolytes and the solid electrolytes are shown in Fig. 5. As expected, r for the gels increased with increasing concentration of the electrolyte solution. At the addition of electrolyte solution, the transport of ions became less coupled to the segmental motions of the polymers and instead became increasingly coupled to the dynamics of the solvent. The conductivites were evaluated first during heating and then during cooling to study hysteresis effects due to melting and crystallisation, respectively. As seen, the hysteresis in the values of r corresponded well with the behaviour seen in the DSC study. Thus, the electrolytes containing hCPEO showed hysteresis at low concentrations of electrolyte solution, and gel electrolytes containing more than 30 wt.% of LiTFSI/EC/DEC showed hysteresis between 20 and 20 jC. Consequently, the electrolytes based on g-BL and hCPEOPO displayed no difference between the data measured during heating and cooling, respectively. Notably, all the electrolytes showed quite similar values of r above 30 jC. In a previous study, physically cross-linked gel electrolytes based on a self-assembling ABA triblock copolymer having a middle (B) block of PEOPO and outer (A) blocks of polyethylene were investigated [15]. A block copolymer gel electrolyte containing 70% of 1 M LiTFSI
423
in g-BL was found to have a conductivity of 10 2.8 S/cm at 40 jC. Moreover, Aihara et al. [16] have studied gel electrolytes based on chemically cross-linked PEO-networks containing 80% 1 M LiTFSI in g-BL. They measured r of the gel to be 10 2.6 S/cm at 40 jC. In comparison, the value of r measured in the present study for the gels containing 70% electrolyte solution was 10 2.4 S/cm at 40 jC.
4. Conclusions Comb-shaped polyethers having hydrocarbon-terminated side chains were found to self-aggregate in the electrolyte solutions to form gel electrolytes. In this way, the polyethers behaved like associative thickeners and sharply increased the viscosity of the electrolytes. The aggregation of the polyethers occurred through phase separation of the hydrocarbon chains, whereby discrete crystalline junctions in a physically cross-linked network were formed. Furthermore, the heat of fusion of the hydrocarbon domains remained constant when the amount of electrolyte solution was increased, indicating that the hydrocarbon chain ends had a large propensity to phase separate in the gel electrolytes. This was further supported by the X-ray scattering data. Gel electrolytes based on the present comb-shaped polyethers were found to have conductivities slightly higher than similar gel electrolytes based on chemically cross-linked polyethers, possibly because the former electrolytes possess a polymer network with a higher segmental mobility.
Acknowledgements We thank the Dept. of Physical Chemistry 1 at Lund University for the use of their X-ray scattering equipment. P.J. is grateful for the financial support from the Swedish Foundation for Strategic Environmental Research, MISTRA. The work was done within the framework of the Jungner Centre for batteries and fuel cells.
References [1] M. Kono, S. Furuta, S. Mori, M. Watanabe, N. Ogata, Polym. Adv. Technol. 4 (1993) 85. [2] A. Nishimoto, K. Agehara, N. Furuya, T. Watanabe, M. Watanabe, Macromolecules 32 (1999) 1541. [3] M. Kono, M. Nishiura, E. Ishiko, T. Sada, Electrochim. Acta 45 (2000) 1307. [4] J. Li, I. Khan, Macromol. Chem. 192 (1991) 3043. [5] J.R.M. Giles, F.M. Gray, J.R. MacCallum, C.A. Vincent, Polymer 28 (1987) 1977. [6] F.M. Gray, J.R. MacCallum, C.A. Vincent, J.R.M. Giles, Macromolecules 21 (1988) 392. [7] I.M. Khan, D. Fisch, Y. Delaviz, J. Smid Makromol, Chem.- Macromol. Chem. Phys. 190 (1989) 1069.
424
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
[8] K. Hirahara, A. Takano, M. Yamamoto, T. Kazama, Y. Isono, T. Fujimoto, O. Watanabe, React. Funct. Polym. 37 (1998) 169. [9] P. Jannasch, Chem. Mater. 14 (2002) 2718. [10] P.P. Soo, B. Huang, Y.-I. Jang, Y.-M. Chiang, D.R. Sadoway, A.M. Mayes, J. Electrochem. Soc. 146 (1999) 32. [11] P. Jannasch, Macromolecules 33 (2000) 8604. [12] P. Jannasch, Electrochim. Acta 46 (2001) 1641.
[13] D.G. Shirvanyanz, A.S. Pavlov, P.G. Khalatur, A.R. Khokhlov, J. Chem. Phys. 112 (2000) 11069. [14] K. Kakudo, N. Kasai (Eds.), X-ray Diffraction by Polymers, Elsevier, Amsterdam, 1972(chap. 4). [15] P. Jannasch, Polymer 43 (2002) 6449. [16] Y. Aihara, S. Arai, K. Hayamizu, Electrochim. Acta 45 (2000) 1321.
Characteristics of gel electrolytes formed by self-aggregating comb-shaped polyethers with end-functionalised side chains Patric Jannasch *, Wendy Loyens Department of Polymer Science and Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden Received 2 September 2003; received in revised form 6 November 2003; accepted 6 November 2003
Abstract Gel electrolytes based on comb-shaped polyethers have been prepared and characterised. The comb-shaped polyethers consisted of poly(ethylene oxide) (PEO) or poly(ethylene oxide-co-propylene oxide) (PEOPO) side chains tethered to styrenic backbone polymers. All the polyether side chains were terminated by hydrocarbon (C16) chain ends. Polymer gel electrolytes were prepared by adding 10 – 70 wt.% of an electrolyte solution with 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt in g-butyrolactone (g-BL) or in a mixture of ethylene carbonate and diethyl carbonate (EC/DEC). Thermal analysis by differential scanning calorimetry (DSC) showed endotherms arising from the melting of hydrocarbon phase domains, indicating polymer self-aggregation in the gels by microphase separation of the chain ends. As the concentration of electrolyte solution was increased, the melt temperature of the hydrocarbon phase domains was found to decrease, while the heat of fusion in joules per gram polyether remained essentially constant. The latter finding implied a strong propensity for the hydrocarbon chains to phase separate in the electrolytes. This was further supported by small-angle X-ray scattering measurements which ˚ in the gels. The ion conductivity of the electrolytes with 70 wt.% of showed a characteristic diffraction distance of approximately 60 A electrolyte solution added typically reached 10 2.5 S/cm at 20 jC. D 2004 Elsevier B.V. All rights reserved. Keywords: Self-associating graft copolymer; Hydrophobically modified poly(ethylene oxide); Polymer gel electrolyte; Crystallisation; Ionic conductivity
1. Introduction In the design of polymer electrolytes, several properties, such as ionic conductivity, mechanical strength, and electrode compatibility, have to be efficiently combined in a single material. For example, electrolytes with a combination of high ionic conductivity and mechanical strength have been obtained by using chemically cross-linked polymer networks [1 – 3]. An alternative, in some regards more useful, way is to employ aggregating or self-assembling polymers to form physically cross-linked polymer electrolyte systems. This has for example been successfully achieved using self-assembling block copolymers with ionophilic blocks, usually of poly(ethylene oxide) (PEO), to complex the salt and facilitate the ion conduction, and ionophobic blocks which induce a phase-separated system with mechanical strength [4– 10]. Basically, these systems
* Corresponding author. Tel.: +46-46-222-9860; fax: +46-46-222-4115. E-mail address: [email protected] (P. Jannasch). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.11.008
resist flow because this would mean mixing the ionophobic blocks with the ionophilic blocks, and vice versa, which is thermodynamically unfavourable. If the block copolymers are well designed, ionic conductivities close to the corresponding neat complex of salt and ionophilic polymer can be reached [10]. Recently, a series of comb-shaped polymers consisting of backbone copolymers with densely grafted polyether side chains have been prepared and studied as ion-conducting solid polymer electrolytes in combination with lithium salts [11,12]. The side chains of the comb-shaped polyether were functionalised with hexadecanoyl chains. These ionophobic chain ends were immiscible in the polyether –salt complexes and were found to phase separate from the polyether – salt complexes. This phase separation induced the formation of physically cross-linked polyether networks through interpolymeric aggregation. Recent simulations of comb-shaped copolymers with attracting, ‘‘sticky,’’ side chain ends have shown that strong attraction between the chain ends stabilises micellar aggregates that serve as junction points of physical networks forming so-called ‘‘micellar gels’’ [13].
418
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
2. Experimental
(DSC) system under N2 purge. After calibration using standard reference materials (indium, zinc, and lead), samples were placed in aluminium containers that were sealed under Ar atmosphere. The samples were first annealed at 60 jC for 5 min, then cooled to 150 jC, and finally heated to 60 jC. The scan rate was at 10 jC/min during the two scans. The glass transition temperature (Tg) was taken as the temperature at the inflexion point of the heat capacity change detected during the heating scan.
2.1. Materials
2.4. X-ray scattering
The preparation, purification, and characterisation of the comb-shaped polyethers have been reported previously [11], and the molecular data of the two polymers used in the present study are shown in Table 1. The solvents g-butyrolactone (g-BL), ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) were all of SelectipurR battery grade purchased from Merck. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt was kindly supplied by 3M. All the solvents and the LiTFSI salt were used as-received and were handled under dry Ar atmosphere in a glove box.
Structural information was gathered from small-angle Xray scattering (SAXS) experiments. They were performed using a Kratky compact camera equipped with a linear position sensitive detector (OED 50M from MBraun, Graz). A Seifert ID 3000 X-ray generator provided the CuKa ˚ . The samples were radiation with a wavelength of 1.542 A placed in a sealed solid sample cell between mica sheets, and data were collected during 2 h at 25 jC.
In the present work, the aggregating comb-shaped polyethers were studied as hosts for electrolyte solutions, i.e., a salt dissolved in organic aprotic solvents. The main motive for the preparation of gel electrolytes was to obtain electrolytes with conductivities sufficiently high for a general use in electrochemical devices.
2.2. Electrolyte preparation Gel electrolytes were prepared by swelling salt-containing polyethers with electrolyte solutions. Samples of the neat polyethers were first transferred to the glove box, and then dried at 60 jC under vacuum for 3 days. Precise amounts of a solution of LiTFSI in DMC were added to glass ampoules containing the polyethers to obtain electrolytes with a salt concentration corresponding to 20 coordinating ether oxygens per lithium ion. The DMC was allowed to evaporate under Ar for 2 days at ambient temperature before the samples were transferred to a glass oven where the residual solvent was removed under vacuum at 60 jC for 3 days. Gel electrolytes were subsequently prepared by adding precise amounts of 1 M LiTFSI in either g-BL, or an EC/DEC (2:1 wt./wt.) mixture, to the salt-containing polyethers. In this way, gel electrolytes containing 10, 30, 50, and 70 wt.% of the respective electrolyte solutions were prepared. 2.3. Calorimetry The thermal properties of the electrolytes were analysed using a Mettler TA 3000 differential scanning calorimetry Table 1 Molecular data of the self-aggregating polyethers Designation
Mn (kg/mol)
Side chain Mna,b (kg/mol)
Side chain contenta,b (wt.%)
EO/PO (wt./wt.)
hCPEO hCPEOPO
730 670
2.1 1.9
96 95
100:0 80:20
a b
Calculated from 1H NMR results. Includes the hydrocarbon chain end (Mn = 0.239 kg/mol).
2.5. Impedance spectroscopy The ion conductivity (r) of the electrolytes was evaluated by measuring the temperature dependence of impedance spectra during first heating from 40 to 100 jC, and then cooling to 40 jC. Samples with a diameter of 18 mm and a thickness of 90 Am were sandwiched between two goldplated stainless steel blocking electrodes spaced by a PTFE ring. The measurements were carried out using a computercontrolled Novocontrol BDC40 high-resolution dielectric analyser equipped with a Novocool cryostat unit. Samples were analysed in the frequency range 10 1 – 107 Hz at 100 mV AC amplitude, and the conductivities were subsequently evaluated using the Novocontrol software WinDeta.
3. Results and discussion The chemical structures of the comb-shaped polyethers used in the present work are shown in Scheme 1, and their molecular data are collected in Table 1. As seen, hCPEO had side chains of poly(ethylene oxide) (PEO), while hCPEOPO had side chains of poly(ethylene oxide-co-propylene oxide) (PEOPO). Furthermore, the average molecular weight of the side chains of the two comb-shaped polyethers were approximately the same, 2 kg/mol, and they both contained about 95 wt.% of side chains, the remaining part being the polystyrenic backbone. The phase behaviour of the different materials was first studied by DSC. The traces of the two electrolyte solutions, 1 M LiTFSI in EC/DEC and 1 M LiTFSI in g-BL, are shown in Fig. 1. As seen, the former solution displayed a glass transition at 105 jC and a melting endotherm peak at 8 jC, while the latter solution had a glass transition at 135 jC, a ‘‘cold’’ crystallisation exotherm at 100 jC,
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
419
Scheme 1. Structure and aggregation of the comb-shaped polyethers.
and finally a melting endotherm peak at 55 jC. Thus, the electrolyte solution based on g-BL had much lower transition temperatures than the EC/DEC solution. Gel electrolytes were prepared by adding different amounts of the respective electrolyte solution to the comb-shaped polymers. All the gel electrolytes were optically transparent, but appeared to scatter light. This is usually an indication of microphase separation. Fig. 2 shows the DSC heating traces of the polymer electrolytes. First, the electrolytes based on hCPEOPO will be discussed. The trace of the solid electrolyte, marked ‘‘0%’’ in Fig. 2A, revealed a glass transition at 50 jC, originating from the amorphous polyether side chains, and melting peak at 13 jC. This peak has previously been identified as arising from the melting of the hydrocarbon chain ends [11,12]. When 1 M LiTFSI in g-BL was added and the amount subsequently increased, the Tg decreased as the polymer was plasticised by the solvent. Furthermore, the integral of the melting peak of the hydrocarbon chain ends decreased, and melting occurred at decreasing temperatures. No signs of crystallisation or melting of the
electrolyte solution were noted in any of the gel electrolytes. It can also be observed in Fig. 2A that the traces of hCPEOPO containing 10 wt.% of either of the two electrolyte solutions were very similar. However, after addition of 30 wt.% EC/DEC the resulting gel showed signs of crystallisation and melting of the solution. The propensity for EC/DEC crystallisation then increased at 50 and 70 wt.% added solution. The traces of the electrolytes based on hCPEO are shown in Fig. 2B. The solid electrolyte displayed a glass transition at 50 jC, a ‘‘cold’’ crystallisation and melting in the temperature interval between 25 and 30 jC. The melting of the hydrocarbon chain ends probably occurred in this temperature interval, but the peak was overlapped by the PEO transitions. When increasing amounts of 1 M LiTFSI in g-BL were added to hCPEO, the crystallisation and melting of PEO, as well as the Tg, were depressed. In the electrolyte containing 50 wt.% of the solution, the remaining peak at 30 jC most probably arose from the melting of the hydrocarbon chain ends. This peak was further depressed and broadened at 70 wt.% added solution. In the case of the
420
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
Fig. 1. DSC heating traces of 1 M LiTFSI salt in g-BL (a) and 1 M LiTFSI salt in the EC/DEC mixture (b). The solutions were cooled from 25 down to 150 jC prior to the heating scan. Scan rate: 10 jC/min.
electrolytes based on EC/DEC, crystallisation and melting of the PEO side chains were noted at 10 wt.% added. The trace for the electrolyte with 30 wt.% added solution only showed a Tg at 50 jC, and a melting peak from the hydrocarbon chain ends at 23 jC. Thus, no crystallisation or melting of the PEO or the EC/DEC was detected. However, at 50 and 70 wt.% solution added, the crystallisation and melting of the EC/DEC solutions were evident. These traces were very similar to the corresponding traces of the electrolytes formed by hCPEOPO. From the thermal analysis of the electrolytes it was concluded that the thermal properties of the various gel electrolytes were clearly influenced by their respective components. Thus, hCPEO and the LiTFSI/EC/DEC solution crystallised, given a sufficiently high concentration in the electrolytes. In contrast, hCPEOPO and the LiTFSI/gBL solution was in the amorphous and liquid state, respectively, with the exception of the hydrocarbon chain ends of
hCPEOPO. Of cause, the reason for the amorphous state of hCPEOPO side chains was their irregular copolymeric configuration. Crystallinity of the hydrocarbon chain ends was observed in all electrolyte systems, although in many cases, the melting peak was overlapped by the melting peak of the polyether side chains. This revealed that the chain ends phase separated both in the EC/DEC and g-BL based electrolyte solutions. It is feasible that, in connection with this process, the chain ends formed ‘‘micellar’’-like microdomains which physically cross-linked the polyether network that hosted the electrolyte solution. A schematic picture of the self-aggregated comb-shaped polyethers is shown in the lower part of Scheme 1, where the ‘‘micellar’’-like microdomains (medium thick lines) are linked to the polyether side chains (thin lines) which are connected to the backbone polymers (thick lines). From the molecular weight values of the comb-shaped polyethers in Table 1, it was calculated that each molecule carried an average of 350 hydrocarbon chain ends. Thus, there is a considerable likelihood of intermolecular aggregation of the polyethers. As mentioned in Section 1, the process of self-aggregation in comb-shaped polymers with strongly attracting groups located at the chain ends have previously been simulated by Shirvanyanz et al. [13]. They found that the formation of a ‘‘micellar’’-like phase was dependent on both the chemical composition and on the polymer concentration. They also found that the ‘‘micelle’’ formation observed in sufficiently dense systems resulted mainly in intermolecular aggregation. In the case of LiTFSI/g - BL in hCPEOPO, the effect of the addition of various amounts of electrolyte solution to the comb-shaped polyethers could be studied because the melting endotherms of the hydrocarbon phase were isolated from other transitions. In Fig. 3, the melting peak temperature (Tm) and the heat of fusion (DHm) are plotted versus the amount of electrolyte solution added. As already noted above, Tm decreased as electrolyte solution was added. This may indicate a decrease in size, or possibly a more irregular crystalline structure of the hydrocarbon phase domains. On the other hand, the value of DHm was essentially constant at a value between 9 and 9.5 J/g hCPEOPO in the gel. This indicated that the degree of phase separation of the hydrocarbon chain end, and thus the degree of aggregation, did not change as the amount of electrolyte solution was increased in the gel. Structural information concerning the possible ‘‘micelle’’ formation was obtained from SAXS experiments. As expected, a solid polymer electrolyte based on the nonmodified form of hCPEOPO, i.e., without hydrocarbon C16 chain ends, did not display any diffraction pattern, most probably due to structural homogeneity. Fig. 4 presents the diffraction patterns of the solid polymer electrolyte based on hCPEOPO pure and of gel electrolytes containing different concentrations of 1 M LiTFSI in g-BL. The presence of diffraction peaks in the SAXS spectra indicated quite a high
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
421
Fig. 2. DSC heating traces of electrolytes based on hCPEOPO (A), and hCPEO (B) containing 0, 10, 30, 50, and 70 wt.% of 1 M LiTFSI in the EC/DEC mixture and 1 M LiTFSI in g-BL, respectively. The electrolytes were cooled from 25 down to 150 jC prior to the heating scan. Scan rate: 10 jC/min.
degree of order in the system. All these electrolytes exhibited distinct diffraction peaks, indicative of ordered systems. As expected, the intensity of the peak became lower upon increasing the content of the electrolyte solution.
Fig. 3. Influence of the content of 1 M LiTFSI in g-BL on the heat of fusion and melt temperature of the hexadecanoyl phase of hCPEOPO.
Fig. 4. SAXS diffraction patterns of a solid polymer electrolyte based on hCPEOPO (0%) and of hCPEOPO gel electrolytes containing various concentrations of 1M LiTFSI in g-BL (10 – 70%).
422
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
The thermodynamic driving force for phase separation in many heterogeneous systems is the formation of a crystalline phase. It was thus interesting to note that although the measurements were performed at 25 jC, well above the melt temperature of the hydrocarbon phase, the microdomains still remained phase separated to physically cross-link the polyether—also when the concentration of the electrolyte solution was increased to 70 wt.%. This was most probably
a consequence of the low solubility of the hydrocarbon chain ends in the salt-containing phase. The simulations performed by Shirvanyanz et al. [13] showed that the ‘‘micellar’’-like microdomains had a spherical shape. These simulations further suggested that if a critical interaction energy between the chain ends is reached, the ‘‘micellar’’ aggregates will form clusters of a larger size [13]. These clusters were shown to have a close to spherical
Fig. 5. Arrhenius conductivity plots for gel electrolytes containing various amounts (10 – 70 wt.%) of the electrolyte solutions: hCPEOPO/LiTFSI/g-BL (a), hCPEOPO/LiTFSI/EC/DEC (b), hCPEO/LiTFSI/g-BL (c) and hCPEO/LiTFSI/EC/DEC (d). Also included are the conductivities of the solid polymer electrolytes based on hCPEOPO and hCPEO, respectively, having [Li]/[O] = 0.050 (0 wt.%). The conductivities were evaluated by impedance spectroscopy during heating, from 40 to 100 jC (solid lines and filled symbols), and then cooling to 40 jC (dashed lines and open symbols). Please note the high degree of overlapping of the heating and cooling curves in some cases.
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
shape. By assuming a spherical geometry of the entities formed by the chain ends of the presently studied materials, it is possible to calculate an average diffraction distance from the scattering data by using q¼
4p h sin k 2
ð1Þ
combined with Braggs law 1 2 sinh ¼ d k
ð2Þ
where q and d represent the scattering vector and the diameter, respectively. h is obtained from the measured diffraction angle 2h, and k is the wavelength of the X-rays [14]. The calculation gave a characteristic diffraction ˚ (see Fig. 4) for the present electrodistance of f 60 A lytes, almost regardless of the concentration of the electrolyte solution. It is, however, unclear whether this dimension is characteristic of singular micelles formed by the C16 chain ends or clusters of aggregated micelles. As a comparison, the length of two fully stretched C16 ˚ . Because chains was calculated to be approximately 40 A the micelles formed by hCPEOPO can be expected to be quite mobile due to the flexible polyether side chains, the existence of the cluster structure remains likely. The broadness of the diffraction peak provided further information because it reflected the distribution of the scattering distances in the electrolytes. In addition, a second less intense peak was observed at higher q-values, which may indicate the presence of two different populations [13]. Arrhenius plots of the ion conductivity of the gel electrolytes and the solid electrolytes are shown in Fig. 5. As expected, r for the gels increased with increasing concentration of the electrolyte solution. At the addition of electrolyte solution, the transport of ions became less coupled to the segmental motions of the polymers and instead became increasingly coupled to the dynamics of the solvent. The conductivites were evaluated first during heating and then during cooling to study hysteresis effects due to melting and crystallisation, respectively. As seen, the hysteresis in the values of r corresponded well with the behaviour seen in the DSC study. Thus, the electrolytes containing hCPEO showed hysteresis at low concentrations of electrolyte solution, and gel electrolytes containing more than 30 wt.% of LiTFSI/EC/DEC showed hysteresis between 20 and 20 jC. Consequently, the electrolytes based on g-BL and hCPEOPO displayed no difference between the data measured during heating and cooling, respectively. Notably, all the electrolytes showed quite similar values of r above 30 jC. In a previous study, physically cross-linked gel electrolytes based on a self-assembling ABA triblock copolymer having a middle (B) block of PEOPO and outer (A) blocks of polyethylene were investigated [15]. A block copolymer gel electrolyte containing 70% of 1 M LiTFSI
423
in g-BL was found to have a conductivity of 10 2.8 S/cm at 40 jC. Moreover, Aihara et al. [16] have studied gel electrolytes based on chemically cross-linked PEO-networks containing 80% 1 M LiTFSI in g-BL. They measured r of the gel to be 10 2.6 S/cm at 40 jC. In comparison, the value of r measured in the present study for the gels containing 70% electrolyte solution was 10 2.4 S/cm at 40 jC.
4. Conclusions Comb-shaped polyethers having hydrocarbon-terminated side chains were found to self-aggregate in the electrolyte solutions to form gel electrolytes. In this way, the polyethers behaved like associative thickeners and sharply increased the viscosity of the electrolytes. The aggregation of the polyethers occurred through phase separation of the hydrocarbon chains, whereby discrete crystalline junctions in a physically cross-linked network were formed. Furthermore, the heat of fusion of the hydrocarbon domains remained constant when the amount of electrolyte solution was increased, indicating that the hydrocarbon chain ends had a large propensity to phase separate in the gel electrolytes. This was further supported by the X-ray scattering data. Gel electrolytes based on the present comb-shaped polyethers were found to have conductivities slightly higher than similar gel electrolytes based on chemically cross-linked polyethers, possibly because the former electrolytes possess a polymer network with a higher segmental mobility.
Acknowledgements We thank the Dept. of Physical Chemistry 1 at Lund University for the use of their X-ray scattering equipment. P.J. is grateful for the financial support from the Swedish Foundation for Strategic Environmental Research, MISTRA. The work was done within the framework of the Jungner Centre for batteries and fuel cells.
References [1] M. Kono, S. Furuta, S. Mori, M. Watanabe, N. Ogata, Polym. Adv. Technol. 4 (1993) 85. [2] A. Nishimoto, K. Agehara, N. Furuya, T. Watanabe, M. Watanabe, Macromolecules 32 (1999) 1541. [3] M. Kono, M. Nishiura, E. Ishiko, T. Sada, Electrochim. Acta 45 (2000) 1307. [4] J. Li, I. Khan, Macromol. Chem. 192 (1991) 3043. [5] J.R.M. Giles, F.M. Gray, J.R. MacCallum, C.A. Vincent, Polymer 28 (1987) 1977. [6] F.M. Gray, J.R. MacCallum, C.A. Vincent, J.R.M. Giles, Macromolecules 21 (1988) 392. [7] I.M. Khan, D. Fisch, Y. Delaviz, J. Smid Makromol, Chem.- Macromol. Chem. Phys. 190 (1989) 1069.
424
P. Jannasch, W. Loyens / Solid State Ionics 166 (2004) 417–424
[8] K. Hirahara, A. Takano, M. Yamamoto, T. Kazama, Y. Isono, T. Fujimoto, O. Watanabe, React. Funct. Polym. 37 (1998) 169. [9] P. Jannasch, Chem. Mater. 14 (2002) 2718. [10] P.P. Soo, B. Huang, Y.-I. Jang, Y.-M. Chiang, D.R. Sadoway, A.M. Mayes, J. Electrochem. Soc. 146 (1999) 32. [11] P. Jannasch, Macromolecules 33 (2000) 8604. [12] P. Jannasch, Electrochim. Acta 46 (2001) 1641.
[13] D.G. Shirvanyanz, A.S. Pavlov, P.G. Khalatur, A.R. Khokhlov, J. Chem. Phys. 112 (2000) 11069. [14] K. Kakudo, N. Kasai (Eds.), X-ray Diffraction by Polymers, Elsevier, Amsterdam, 1972(chap. 4). [15] P. Jannasch, Polymer 43 (2002) 6449. [16] Y. Aihara, S. Arai, K. Hayamizu, Electrochim. Acta 45 (2000) 1321.