12364
J. Phys. Chem. C 2010, 114, 12364–12369
Melting Behavior of Pyrrolidinium-Based Ionic Liquids and Their Binary Mixtures Miriam Kunze, Sangsik Jeong, Elie Paillard, Martin Winter, and Stefano Passerini* Institute of Physical Chemistry, Westfa¨lische Wilhelms-UniVersita¨t Mu¨nster, Corrensstrasse 28/30, 48149 Muenster, Germany ReceiVed: April 26, 2010; ReVised Manuscript ReceiVed: May 28, 2010
To understand the low-temperature behavior of pyrrolidinium-based ionic liquids (ILs), ILs, including N-butylN-methyl pyrrolidinium (Pyr14+) and N-methyl-N-propyl pyrrolidinium (Pyr13+) cations and bis(fluorosulfonyl)imide (FSI-), bis(trifluoromethanesulfonyl)imide (TFSI-), bis(pentafluoroethanesulfonyl)imide (BETI-), and (trifluoromethanesulfonyl)(nonafluorobutyl-sulfonyl) imide (IM14-) anions, were investigated. Pyr14FSI, Pyr13FSI, Pyr14TFSI, Pyr13TFSI, Pyr14BETI, Pyr13BETI, Pyr14IM14, and Pyr13IM14 were prepared and intensively studied by means of differential scanning calorimetry (DSC). Four of these ILs (Pyr14FSI, Pyr13FSI, Pyr14TFSI, Pyr13TFSI) were used to prepare binary mixtures, which were examined by DSC measurements. For these mixtures, reduced melting transitions and an enhanced liquidus range were detected, which represents a great advantage for low-temperature applications. In addition, it was observed that the crystallization process of the mixtures is mainly influenced by the anion. Introduction In 1914, the first ionic liquid [EtNH3]+[NO3]- with a melting temperature of 12 °C was introduced by Walden.1 However, it is in the early 2000s that a wide development in this field has taken place and the number of reports and applications concerning ionic liquids has increased tremendously. Nowadays, ionic liquids are defined as salts with a melting temperature below 100 °C. Their extensive properties, such as a wide liquidus range associated with a low melting point, negligible vapor pressure and lack of a boiling point, chemical and electrochemical stabilities up to high temperatures, and high ionic conductivity (even at low temperatures), make them attractive not only for catalytic reactions2 and synthesis but for energy applications as well. Because of the low volatility and high thermal stability, ionic liquids are often labeled as “green electrolytes” for energy applications3-5 even if their chemical nature is not always benign. These energy applications are energy conversion in solar cells6-8 and fuel cells9,10 and energy storage with lithium batteries11-17 and super or ultracapacitors.18-20 To be mentioned as well in the field of electrochemical devices are the electrochemical actuators21-23 and light-emitting electrochemical cells.24-26 A widely used class of ionic liquids in electrochemistry is that based on pyrrolidinium cations. The advantage of these ionic liquids is a very low melting point, TM, which can even be well below 0 °C. This enables the ILs to be used in low-temperature devices. For the aforementioned electrochemical applications and especially for lithium batteries, it is, however, important to widen further the liquidus range by shifting the TM to lower temperatures. This can be achieved, for example, by dissolving a lithium salt in the ionic liquid. Mixtures of these ionic liquids with lithium TFSI have been proved successfully for lithium insertion in graphite.27-29 However, for some IL/Li salt mixtures, just a small decrease of the TM is observed and only at a low lithium salt content. A substantial increase of the system’s melting transition is, in fact, observed by adding a large fraction * Towhomcorrespondenceshouldbeaddressed.E-mail:stefano.passerini@ uni-muenster.de.
of lithium salt.30 The decrease of the melting transition for application in low-temperature devices can be also obtained by introducing a second ionic liquid in the system. Furthermore, the mixing of ionic liquids with each other can be followed by additional property changes like, for example, an increase of ionic conductivity or a widening of the electrochemical stability window. Here, we report the physical and thermal properties of four different ILs based on the two cations, N-butyl-N-methyl pyrrolidinium cation (Pyr14+) and N-methyl-N-propyl pyrrolidinium cation (Pyr13+), and on the four anions, bis(fluorosulfonyl)imide (FSI-), bis(trifluoromethanesulfonyl)imide (TFSI-), bis(pentafluoroethanesulfonyl)imide (BETI-), and (trifluoromethanesulfonyl)(nonafluorobutylsulfonyl) imide (IM14-) (Scheme 1), and their resulting binary mixtures. Hence, the melting temperatures TM and further phase transitions were extensively studied by using differential scanning calorimetry (DSC) to understand the melting behavior of the pure ILs and their related mixtures. Experimental Section Sample Preparation. The eight investigated ionic liquids (Pyr13FSI, Pyr14FSI, Pyr13TFSI, Pyr14TFSI, Pyr13BETI, Pyr14BETI, Pyr13IM14, and Pyr14IM14) (cf. Scheme 1) are synthesized via a method developed at ENEA (Agency for the New Technologies, Energy and the Environment) and described in detail in a previous work.31 The chemicals N-methylpyrrolidine (97 wt %), 1-propylbutane (99%), 1-bromobutane (99 wt %), and ethylacetate (ACS grade, >99.5 wt %) were purchased from Aldrich and previously purified (with the exception of ethyl acetate) using activated carbon (Aldrich, Darco-G60) and alumina (acidic, Aldrich Brockmann I). Lithium bis(trifluoromethanesulfonyl)imide, LiTFSI (99.9 wt %, battery grade), lithium bis(pentafluoroethanesulfonyl)imide, LiBETI (99.9 wt %, battery grade), and acidic (trifluoromethanesulfonyl)(nonafluorobutylsulfonyl)imide, HIM14 (59 wt % solution in water), were obtained from 3M and used as received. Potassium bis(fluorosulfonyl) imide, KFSI (99.9%, battery grade) was purchased from Dai-Ichi Kogyo Seiyaku Co. Ltd. and used as
10.1021/jp103746k 2010 American Chemical Society Published on Web 06/28/2010
Melting Behavior of Pyrrolidinium-Based ILs SCHEME 1: Structures of the Investigated Cations (a) Pyr13 and (b) Pyr14 (Both Given in Twist Conformation) and the Anions (c) FSI, (d) TFSI, (e) BETI, and (f) IM14 (Displayed in cis Conformation)a
a Further description on the conformation is given elsewhere.39-42 The structures are scaled to offer a comparison of the ion sizes.
received. Deionized H2O was obtained using a Millipore ionexchange resin deionizer. The N-alkyl-N-methylpyrrolidinium bromide precursors (Pyr13Br and Pyr14Br) were synthesized by reacting N-methylpyrrolidine with the appropriate amount of bromoalkyl in the presence of ethyl acetate. The precursors were repeatedly rinsed with ethyl acetate to remove the excess reagents and the soluble impurities. The four ionic liquids were obtained by reacting aqueous solutions of the precursors (Pyr13Br and Pyr14Br) with the appropriate amounts of the imide salts (KFSI, LiTFSI, LiBETI, and LiIM14). The lithium or potassium content in the ionic liquids was tested to be below 2 ppm by atomic absorption spectroscopy (AAS). The reactions led to the formation of the hydrophobic ionic liquids and hydrophilic LiBr or KBr. After removal of the aqueous phase, the ionic liquids were washed several times with deionized water to remove the water-soluble bromide salts (LiBr, KBr, or HBr) and excess imide salts. The ionic liquids were then added to ethylacetate to reduce their
J. Phys. Chem. C, Vol. 114, No. 28, 2010 12365 viscosity and purified with activated carbon and acidic alumina. The liquid fractions were separated from the solid phases by vacuum filtering and then placed in a rotary evaporator at 80 °C under vacuum to remove the solvent (ethyl acetate). Finally, the ionic liquids were dried using an oil-free vacuum pump at 60 °C for at least 2 h and then at 120 °C for at least 18 h. The overall yields ranged from 85 to 90 mol %. The materials were stored in sealed glass tubes in a controlled environment (dry room, RH < 0.1% at 20 °C, or drybox, H2O < 1 ppm). The water content in the ionic liquids was measured using the standard Karl Fischer method. The titrations were performed by an automatic Karl Fischer coulometer titrator (Mettler Toledo DL32) in a dry room (RH < 0.1%) at 20 °C. The Karl Fisher titrant was a one-component reagent purchased from Aldrich (Hydranal 34836 Coulomat AG). Ionic liquid binary mixtures of Pyr13FSI, Pyr14FSI, Pyr13TFSI, and Pyr14TFSI were prepared by mixing these ionic liquids in a 1:1 molar ratio. This yields six mixtures, in which only five have different compositions. By mixing either Pyr14TFSI and Pyr13FSI or Pyr13TFSI and Pyr14FSI, the same composition was obtained. In both cases, all four ions with the same molar ratio of 25 mol % are present in the prepared mixture. Besides this mixture, two samples with only one anionic species (either TFSI- or FSI-) and both cations and two samples with only one cationic species (either Pyr14+ or Pyr13+) and both anions are obtained. Thermal Measurements. DSC measurements were performed using a TA Instruments differential scanning calorimeter model Q2000 with liquid nitrogen cooling. The temperature range for this instrument ranges from -150 °C up to 500 °C. The temperature calibration was done using adamantane (Merck Chemicals, >99% with solid-solid phase transition at -64.56 °C) and indium (TA Instruments calibration set with the melt transition at 156.61 °C). The pans used for all measurements were Al pans, which could be hermetically sealed. The pans were filled and sealed in the glovebox under an argon atmosphere with the water content below 2 ppm. The pure ionic liquids were cooled to -150 °C with a 2 °C/ min cooling rate, then heated to 200 °C using a 10 °C/min rate. In addition to this procedure, the pure ionic liquids were quenched by cooling the sample from 40 to -150 °C using a cooling rate of roughly 40 °C/min. This was done to determine possible glass transition temperatures and additional phase transitions. The pans containing the binary mixtures of the ionic liquids were equilibrated at 40 °C in the DSC system for 30 min prior cooling to -150 °C. The cooling was performed as well as for the pure ionic liquids very slowly with a 2 °C/min rate. The system was kept at -150 °C for 2 min and afterward heated to 200 °C with a heating rate of 10 °C/min. Finally, another cooling and heating cycle was performed with 10 °C/min, as well. The glass transition temperatures were taken at the reversal point of the transition step. All the other transition temperatures (solid-solid phase transition and melting) were taken at the peak’s onset. Results Pure Ionic Liquids. The heating traces of some pure ionic liquids have been already published previously30,32 but are displayed in Figure 1 and listed in Table 1 for a quicker comparison. The melting transition of the ILs containing the smaller anions, Pyr13FSI and Pyr14FSI, are observed at -9 and -18 °C, respectively. For these ionic liquids, two additional solid-solid phase transitions besides the melting transition are
12366
J. Phys. Chem. C, Vol. 114, No. 28, 2010
Kunze et al.
Figure 1. DSC heating traces of the pure ionic liquids Pyr13FSI, Pyr14FSI, Pyr13TFSI, Pyr14TFSI, Pyr13BETI, Pyr14BETI, Pyr13IM14, and Pyr14IM14 (exotherm down).
TABLE 1: Glass Transition, Tg, Recrystallization, TC, and Melting Transition, TM, Temperatures of the Pure Ionic Liquids FSI-
TFSI-
BETI-
IM14-
-75 -30 7
-73
-75 -25 5
-72
+
Tg/°C TC/°C TM/°C
-104 -72 -18
Pyr14 -85 -53 -7 (-18) Pyr13+
Tg/°C TC/°C TM/°C
-9
10
detected. The ionic liquids based on the TFSI- anion show only melting transitions of the solid phase. These are located at 10 °C for Pyr13TFSI and at -7 °C for Pyr14TFSI. A further increase of the anion size does not simply result in an increase of the melting point of the corresponding ILs. In fact, although an increase is detected for Pyr14BETI, the TM of this IL is detected at 7 °C; with respect to Pyr14TFSI, a decrease in the melting transition is detected on going from Pyr13TFSI to Pyr13BETI (TM ) 5 °C). Even more anomalous is the behavior of the IM14-based ILs. In fact, these materials only showed a glass transition temperature below -70 °C. Although apparently anomalous, the observed behavior is clearly associated with the different sizes of the ions constituting the ionic liquids. In particular, the melting temperature is seen to increase when the anion and cation sizes are matched. As a matter of fact, Pyr13TFSI and Pyr14BETI, whose ions match fairly well in size (see Scheme 1), show the highest melting temperatures, whereas Pyr13IM14 and Pyr14IM14 do not show any feature because of the large dimensional mismatch combined with the asymmetry and flexibility of the IM14- anion. Overall, the melting point of the Pyr14+-based ILs is lower than that of the Pyr13+-based ones except for the BETI--based ionic liquids. Here, the melting transition of Pyr13BETI is 2 °C lower than that of Pyr14BETI. This can be explained by the higher flexibility and steric hindrance of the butyl side chain compared with the propyl side
chain. The longer side chain in the cation can more easily change its conformation, and different conformations hinder the crystallization. Furthermore, this side chain constricts the arrangement of the cation in the crystal structure. Substantial differences in the DSC traces were observed on fast quenching the Pyr14-based ionic liquids (with the exception of Pyr14IM14, which does always show only the glass transition feature) and Pyr13BETI. The changes induced by quenching are displayed in Figure 2 (solid lines) and compared with the slow cooling results (cf. dotted lines in Figure 2). For Pyr14FSI, additionally, to the melting and the two solid-solid phase transitions, a glass transition at -104 °C and a recrystallization peak at -72 °C are detected. The same additional features, a glass transition at -75 °C and a cold recrystallization peak at -30 °C, are observed for Pyr14BETI. However, the other features remain unaffected by the fast quenching procedure of Pyr14FSI and Pyr14BETI. On the other hand, several effects are observed for Pyr14TFSI and Pyr13BETI. A glass transition, a recrystallization peak, and a solid-solid phase transition are detected at, respectively, -85, -53, and -30 °C for Pyr14TFSI and -75, -26, and -20 °C for Pyr13BETI. In addition to the supplementary transitions, a very peculiar shift in the melting temperature from -7 °C down to -18 °C is observed only for Pyr14TFSI. This shift and other changes due to fast quenching of Pyr14TFSI were mentioned in the literature before but not discussed further.30 Henderson et al.33 have proposed that the TM shift in Pyr14TFSI results from the melting of a metastable phase that is formed upon heating the quenched IL (solid-solid phase transition at -30 °C). This metastable phase originates from the quenched crystallization of the TFSI- anions with a conformation (C1) typical of the liquid state (C1 conformathe CF3 groups are located on opposite sides of the S-N-S plane). Upon heating, the formation of the most thermodynamically stable phase (C2 conformation where the CF3 groups are located on the same side of the S-N-S plane) is sterically hindered (prior melting), thus resulting in a low-TM material. The direct comparison of quenched Pyr14FSI and Pyr14TFSI, including glass transition temperatures, shows some similarities. Both ILs have a solid-solid phase transition at -30 °C and a melting transition at -18 °C after the quenching process. In addition, the areas of the solid-solid phase transition at -30 °C and the melting transition at -18 °C cover the same area for both, Pyr14FSI and Pyr14TFSI. There is 4 J/g detected for the transition at -30 °C and 32 J/g at -18 °C, respectively. Pyr13FSI and Pyr13TFSI did not show substantial changes upon fast quenching. These ILs seemed to crystallize directly and did not show any glass transition in the temperature range of the DSC instrument. Furthermore, no additional changes in the heating traces compared to the slow cooling experiment of these two Pyr13 systems were registered. Finally, the ILs based on the larger anions (Pyr13IM14 and Pyr14IM14) show no changes between the slow and the fast cooling experiments because, in both cases, a glass transition is already detected after a slow cooling. Mixtures with the Same Anion. Mixing Pyr14TFSI with Pyr13TFSI and Pyr14FSI with Pyr13FSI results in samples with only one anionic species and two different cationic species. In Figure 3, the DSC heating traces of these two ionic liquid mixtures are displayed. Comparing the heating curve of the FSIbased mixture after slow cooling (Figure 3a) with the heating traces of the pure ionic liquids in Figure 1, one reveals a shift to lower temperatures of the melting transition and solid-solid phase transitions. However, the presence of a Tg (-105 °C) indicates that the sample is not fully crystallized, which also
Melting Behavior of Pyrrolidinium-Based ILs
J. Phys. Chem. C, Vol. 114, No. 28, 2010 12367
Figure 2. DSC heating traces of Pyr14FSI, Pyr14TFSI, Pyr14BETI, and Pyr13BETI after quenching (solid lines) and after slow cooling (dashed lines) (exotherm down).
Figure 3. Heating and cooling cycle of the mixtures based only on one anion and two different cations: (Pyr14FSI)0.5(Pyr13FSI)0.5 (a-c) and (Pyr14TFSI)0.5(Pyr13TFSI)0.5 (d-f) (exotherm down).
Figure 4. Heating and cooling cycle of the mixtures based only on one cation and two anions: (Pyr13FSI)0.5(Pyr13TFSI)0.5 (a-c) and (Pyr14FSI)0.5(Pyr14TFSI)0.5 (d-f) (exotherm down).
agrees with the broad cold recrystallization after the first solid-solid phase transition at -70 °C. Trying to fully crystallize the system Pyr14FSI with Pyr13FSI, by cycling with slow cooling and heating around the -50 °C, was not successful. In Figure 3c, a small melting transition at -42 °C is detected besides the more pronounced glass transition at -105 °C. Hence, this mixture has, in addition to the glassy state, crystalline regions. For the TFSI--based mixture, only a single melting transition is detected (see Figure 3d) as it was found for the pure ionic liquids Pyr14TFSI and Pyr13TFSI; however, the melting temperature of the mixture is lower than those of the pure ILs. The comparison of the heating traces (Figure 3c,f) of the FSIand TFSI-based mixtures after fast cooling (illustrated in Figure 3b,e) with those of Pyr14TFSI and Pyr14FSI after quenching (Figure 2, left and center panels) shows a good agreement for the glass transition temperatures. The (Pyr14TFSI)0.5(Pyr13TFSI)0.5 mixture shows a Tg at about -86 °C, that is, practically coincident of that of pure Pyr14TFSI (-85 °C), whereas the (Pyr14FSI)0.5(Pyr13FSI)0.5 mixture shows a Tg at about -105 °C that is, once again, practically coincident of that of pure Pyr14FSI (-104 °C). Both binary mixtures show the same melting temperature independent of the cooling rate (compare Figure 3, panels a and c, and d and f). The TFSIbased mixture also shows a recrystallization peak, like for Pyr14TFSI. However, the melting transition of this mixture is not shifted as a result of the cooling rate, unlike for Pyr14TFSI. This evidence indicates that the most thermodynamically stable conformation of TFSI- is not hindered as it was in pure Pyr14TFSI. In summary, the TFSI-based mixture shows a thermal behavior that is a combination of the pure components (slow crystallization but
without formation of metastable phases) with the great advantage of a resulting melting point 18 °C lower than the lowest among the two components. Mixtures with the Same Cation. In Figure 4, the DSC curves for the ionic liquid mixtures with one cation and two different anions, (Pyr13FSI)0.5(Pyr13TFSI)0.5 (Figure 4a-c) and (Pyr14FSI)0.5(Pyr14TFSI)0.5 (Figure 4d-f), are displayed. In the mixture with just Pyr13+, a small glass transition at -97 °C is detected and a melting transition within two steps starting at -26 °C and second at -13 °C are detected in the heating scan following the initial slow cooling. Once more, the presence of a Tg indicates that the material is not fully crystallized. Similar Tg values are, in fact, observed by cooling the sample again from 200 to -150 °C with 10 °C/min and in the second heating cycle as well. For the (Pyr14FSI)0.5(Pyr14TFSI)0.5 system, no differences after slow and fast cooling rates are visible. In all cases, just a glass transition is present at about -96 °C. 4-Ion Mixture. Mixing Pyr14FSI with Pyr13TFSI and Pyr13FSI with Pyr14TFSI is, as mentioned before, supposed to yield the same mixture. This is proved true by the same registered DSC curves, which are displayed for these samples in Figure 5. This evidence indicates that there is no preferential organization of cations and anions surviving. Although this observation might appear trivial, it indicates that there is no preferential organization of cations and anions surviving a simple mixing procedure (magnetic stirring) even at room temperature. In fact, mixtures prepared at ambient temperature and directly subjected to DSC measurements with a slow cooling as the first step show the
12368
J. Phys. Chem. C, Vol. 114, No. 28, 2010
Figure 5. Heating and cooling cycle of the mixtures based only on all cations and anions: (Pyr14FSI)0.5(Pyr13TFSI)0.5 (a-c) and (Pyr13FSI)0.5(Pyr14TFSI)0.5 (d-f) (exotherm down).
same thermal traces as those equilibrated at 40 °C or higher temperatures. For this mixture, where all ions are present with 25 mol %, only a glass transition temperature at about -97 °C is registered independent of the DSC procedure. This glass transition is in agreement with the glass transitions of the mixtures that possess only one cationic species (cf. Figure 4). Discussion The crystalline structure of pyrrolidinium-based ionic liquids is known only for a few members of the TFSI family. The ion crystal packing of Pyr11TFSI, Pyr12TFSI,34 Pyr13TFSI,33 and Pyr14TFSI,35 however, has been reported. In all materials, the ions arrange themselves in layers of anions and cations. Each ion fits within a cage of eight counterions (four in the layer above and four below).All of the anions have the C2 conformation, whereas the rings of the cations have the envelope (CS) conformation. In Pyr13TFSI and Pyr14TFSI, there is no close packing between the neighboring cation alkyl chains, which might suggest favorable van der Waals interactions. In Pyr14TFSI, the cations adopt a herringbone arrangement relative to one another with the cation rings positioned near one another. Regarding the organization in the liquid phase, no reports are available on pyrrolidinium-based ionic liquids. Triolo et al.36 and Mizuhata et al.,37 however, have reported on the structural order of N-alkyl-N-methylpiperidinium-bis(trifluoromethanesulfonyl) imide (PIP1RTFSI with R ranging from propyl to heptyl) ionic liquids in the molten state using small- and wideangle X-ray scattering (SWAXS). These ionic liquids differ from the pyrrolidinium-based ones only for one additional CH2 unit in the cation ring. As a matter of fact, Pyr11TFSI and Pip11TFSI are isostructural in the solid phase and have a very analogous thermal behavior.34 The SWAXS study indicates that the cation aliphatic alkyl chains tend to aggregate, forming alkyl domains embedded into polar regions where the polar heads of the cations are facing the negatively charged anions.38 The transition from the liquid to the solid phase of the ionic liquids can be seen as the rearrangement of the anions and the cations from the alkyl chain segregation structure in the liquid phase to the layered structure with no close packing of the neighboring cation alkyl chains. The driving (but also the
Kunze et al. inhibiting) force of this process could be either the formation of stable anion layers around which the cations adjust to minimize the energy or vice versa. However, the comparison of the melting behaviors of Pyr13FSI and Pyr14FSI with that of Pyr13TFSI and Pyr14TFSI (Figure 1) appears to indicate that the anion interactions influence the crystal frame strength of the solid ionic liquid. Whereas there are supplementary solid-solid phase transitions in the pure FSI-based materials, there are none for the pure TFSI materials. This is consistent with the FSIanions generating a less strong crystalline structure than that of the TFSI-anions, thus leaving the cations with a higher degree of freedom to change their conformation during heating. The fact that the anion, either TFSI- or FSI-, is building the matrix for the crystallization process is also supported by the fast cooling experiments of the pure ILs, Pyr14FSI and Pyr14TFSI (cf. Figure 2). The different anions are frozen in their conformations of the liquid state, which are different for TFSI- and FSI-, and that results in different glass transition and recrystallization temperatures. The main points of interest are the transition peaks at -30 °C and at -18 °C for both, Pyr14FSI and Pyr14TFSI. These, in fact, can be associated only with the presence of Pyr14+. The cation is rearranging its conformation at these temperatures. This would not be possible if the anion is not forming the layered frame in the solid ionic liquid. The shift of the melting transition of Pyr14TFSI depends on the stability of the crystalline configuration. By quenching, the most stable configuration, which is build up during the slow cooling process, is not achieved and the melting transition is lowered by 11 °C. Pyr13BETI and Pyr14BETI show only one solid-liquid melting transition as the TFSI-based ILs. However, being that BETI is larger than TFSI, some differences are observed. First of all, the melting transition of Pyr13BETI is lower than that of Pyr13TFSI, which can be attributed to the mismatch of the Pyr13+ and BETI- sizes. Pyr13+ and TFSI- have nearly the same size, and consequently, the highest melting transition in this homologous series of the Pyr13+ can be achieved with Pyr13TFSI. Increasing the difference between the cation and anion sizes results in lowering the melting transition temperature that ends in the impossible (or very difficult) crystallization like in Pyr13IM14 or Pyr14IM14, for which only a glass transition is detected even upon very slow cooling and repeated lowtemperature thermal cycles. In practice, the thermal motion of this large anion at low temperatures is so reduced that it cannot rearrange in any stable configuration. Pyr13BETI and Pyr14BETI can also be quenched on fast cooling (cf. Figure 2), thus indicating that the rearrangement of the anions to form the layered structure might be slow. The melting point of these two ionic liquids, however, is unaffected by the quenching. At temperatures between the Tg and TM, both materials showed thermal features associated with structural rearrangements, an exothermal cold recrystallization and, in the case of Pyr13BETI, an endothermic solid-solid phase transition, leading to the formation of the thermodynamically stable crystalline phase. The results of the mixtures with one anionic and two cationic species confirm the formation of the anion layered frame that is followed by the cation ordering. The measured glass transition temperatures in these two mixtures are the same as those observed in the quenched Pyr14FSI and Pyr14TFSI. Consequently, both anions are again frozen in their liquid conformation. From the heating traces a and d in Figure 3, one can also see that the TFSI anion forms again the more stable host, although, not only Pyr14+ but also Pyr13+ is involved in the crystallization process. However, the frame built up by FSI- is
Melting Behavior of Pyrrolidinium-Based ILs again less strong than that of TFSI-. Consequently, there are additional phase transitions besides the glass and melting transitions in the (Pyr14FSI)0.5(Pyr13FSI)0.5 sample like in the pure ILs. The mixtures that consist of only one cationic but two anionic species point as well into the direction of a layered frame driven by the anions. The anionic matrix made of two types of anions is different from the matrices, which are formed by either FSIor TFSI- species. Into this mixed anion frame only the Pyr13+ fits and the system can crystallize (cf. Figure 4a). This is in contrast to the mixture with Pyr14+. Whereas the smaller cation can rearrange itself in the anion frame, Pyr14+ does not fit into it and the mixture (Pyr14FSI)0.5(Pyr14TFSI)0.5 cannot crystallize. The whole system becomes a glass by cooling the sample either very slowly with 2 °C/min or fast with 10 °C/min. The glass transition temperature of (Pyr14FSI)0.5(Pyr14TFSI)0.5 is -97 °C, which is comparable with the Tg of (Pyr13FSI)0.5(Pyr13TFSI)0.5 at -98 °C, and both are in good agreement with the glass transition temperatures of the mixture containing all four ions, where the Tg is again -97 °C, independent of the origin of the pure ionic liquid. The agreement of the glass transition temperatures for the samples containing 25 mol % of FSI- and TFSI- (independent of the cation nature and content) confirms again that the frame of the solid IL is formed by the anions in the system. Conclusion Eight pyrrolidinium-based ionic liquids and six 50 mol % mixtures based on the ILs Pyr13FSI, Pyr14FSI, Pyr13TFSI, and Pyr14TFSI were investigated using the DSC method. The DSC heating and cooling traces and their comparisons show that the matrix for crystallization of the ionic liquid or ionic liquid mixture is built up by the anionic species in the system. It does not matter if there is only the TFSI anion, only the FSI anion or a mixture of both present. Consequently, the cation has to fit into the anion frame. If the cation fits into this matrix, the system can crystallize, and if the cation does not fit, due to size and configuration effects, only a glassy state of the ionic liquid is achieved. Finally, several IL mixtures with melting temperatures below -40 °C (which is considered to be the lower limit for portable electrochemical power sources) were identified, which are presently under further investigation for application in electrochemical devices. Acknowledgment. We are thankful for the financial support of the European Commission within the FP6 STREP Project ILLIBAT (Contract No. NMP3_CT_2006_033181) and the FP7 project ORION (Contract No. NMP3-LA-2009_229036). References and Notes (1) Walden, P. Bulletin de l’Acade´mie Impe´riale des Sciences de St. Pe´tersbourg; 1914; Vol. 8, p 405. (2) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667. (3) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem. Soc. 2002, 124, 14247. (4) Earle, M. J.; Seddon, K. R.; McCormac, P. B. Green Chem. 2000, 2, 261. (5) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359.
J. Phys. Chem. C, Vol. 114, No. 28, 2010 12369 (6) Bonhote, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram, K.; Gratzel, M. Inorg. Chem. 1996, 35, 1168. (7) Papageorgiou, N.; Athanassov, Y.; Armand, M.; Bonhote, P.; Pettersson, H.; Azam, A.; Gratzel, M. J. Electrochem. Soc. 1996, 143, 3099. (8) Wang, P.; Zakeeruddin, S. M.; Exnar, I.; Gratzel, M. Chem. Commun. 2002, 2972. (9) Xu, W.; Angell, C. A. Science 2003, 302, 422. (10) Yoshizawa, M.; Xu, W.; Angell, C. A. J. Am. Chem. Soc. 2003, 125, 15411. (11) Ishikawa, M.; Sugimoto, T.; Kikuta, M.; Ishiko, E.; Kono, M. J. Power Sources 2006, 162, 658. (12) Shin, J. H.; Henderson, W. A.; Passerini, S. Electrochem. SolidState Lett. 2005, 8, A125. (13) Shin, J. H.; Henderson, W. A.; Scaccia, S.; Prosini, P. P.; Passerini, S. J. Power Sources 2006, 156, 560. (14) Matsumoto, H.; Sakaebe, H.; Tatsumi, K.; Kikuta, M.; Ishiko, E.; Kono, M. J. Power Sources 2006, 160, 1308. (15) Nakagawa, H.; Fujino, Y.; Kozono, S.; Katayama, Y.; Nukuda, T.; Sakaebe, H.; Matsumoto, H.; Tatsumi, K. J. Power Sources 2007, 174, 1021. (16) Sakaebe, H.; Matsumoto, H.; Tatsumi, K. J. Power Sources 2005, 146, 693. (17) Fernicola, A.; Croce, F.; Scrosati, B.; Watanabe, T.; Ohno, H. J. Power Sources 2007, 174, 342. (18) Balducci, A.; Henderson, W. A.; Mastragostino, M.; Passerini, S.; Simon, P.; Soavi, F. Electrochim. Acta 2005, 50, 2233. (19) Lazzari, M.; Mastragostino, M.; Soavi, F. Electrochem. Commun. 2007, 9, 1567. (20) Yuyama, K.; Masuda, G.; Yoshida, H.; Sato, T. J. Power Sources 2006, 162, 1401. (21) Ding, J.; Zhou, D. Z.; Spinks, G.; Wallace, G.; Forsyth, S.; Forsyth, M.; MacFarlane, D. Chem. Mater. 2003, 15, 2392. (22) Bennett, M. D.; Leo, D. J. Sens. Actuators, A 2004, 115, 79. (23) Wang, J.; Xu, C. Y.; Taya, M.; Kuga, Y. Smart Mater. Struct. 2007, 16, S214. (24) Yang, C. H.; Sun, Q. J.; Qiao, J.; Li, Y. F. J. Phys. Chem. B 2003, 107, 12981. (25) Shin, J. H.; Edman, L. J. Am. Chem. Soc. 2006, 128, 15568. (26) Shao, Y.; Bazan, G. C.; Heeger, A. J. AdV. Mater. 2007, 19, 365. (27) Lux, S. F.; Schmuck, M.; Appetecchi, G. B.; Passerini, S.; Winter, M.; Balducci, A. J. Power Sources 2009, 192, 606. (28) Appetecchi, G. B.; Montanino, M.; Balducci, A.; Lux, S. F.; Winter, M.; Passerini, S. J. Power Sources 2009, 192, 599. (29) Schmuck, M.; Balducci, A.; Rupp, B.; Kern, W.; Passerini, S.; Winter, M. J. Solid State Electrochem. 2009. (30) Henderson, W. A.; Passerini, S. Chem. Mater. 2004, 16, 2881. (31) Appetecchi, G. B.; Scaccia, S.; Tizzani, C.; Alessandrini, F.; Passerini, S. J. Electrochem. Soc. 2006, 153, A1685. (32) Zhou, Q.; Henderson, W. A.; Appetecchi, G. B.; Montanino, M.; Passerini, S. J. Phys. Chem. B 2008, 112, 13577. (33) Henderson, W. A.; Choudhury, A. R.; Steiner, A.; Winterton, N.; Young, V. G., Jr.; Reichert, W. M.; De Long, H. C.; Trulove, P. C. Linking together ion structure, crystal packing, and salt thermal properties. 214th ECS Meeting, Honolulu, HI, 2008. (34) Henderson, W. A.; Young, V. G., Jr.; Pearson, W.; Passerini, S.; De Long, H. C.; Trulove, P. C. J. Phys.: Condens. Matter 2006, 18, 10377. (35) Choudhury, A. R.; Winterton, N.; Steiner, A.; Cooper, A. I.; Johnson, K. A. J. Am. Chem. Soc. 2005, 127, 16792. (36) Triolo, A.; Russina, O.; Fazio, B.; Appetecchi, G. B.; Carewska, M.; Passerini, S. J. Chem. Phys. 2009, 130, 164521. (37) Mizuhata, M.; Maekawa, M.; Deki, S. ECS Trans. 2007, 3, 89. (38) NMR spin-lattice and spin-spin relaxation measurements in progress in our laboratories are supporting the stacking of the cation alkyl chains in the liquid state. (39) Fujii, K.; Seki, S.; Fukuda, S.; Takamuku, T.; Kohara, S.; Kameda, Y.; Umebayashi, Y.; Ishiguro, S. J. Mol. Liq. 2008, 143, 64. (40) Fujimori, T.; Fujii, K.; Kanzaki, R.; Chiba, K.; Yamamoto, H.; Umebayashi, Y.; Ishiguro, S. I. J. Mol. Liq. 2007, 131, 216. (41) Fukuda, S.; Takeuchi, M.; Fujii, K.; Kanzaki, R.; Takamuku, T.; Chiba, K.; Yamamoto, H.; Umebayashi, Y.; Ishiguro, S. J. Mol. Liq. 2008, 143, 2. (42) Umebayashi, Y.; Mitsugi, T.; Fujii, K.; Seki, S.; Chiba, K.; Yamamoto, H.; Lopes, J. N. C.; Padua, A. A. H.; Takeuchi, M.; Kanzaki, R.; Ishiguro, S. J. Phys. Chem. B 2009, 113, 4338.
JP103746K
