Phase Behavior and Ionic Conductivity in Lithium Bis

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J. Phys. Chem. B 2009, 113, 11247–11251

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Phase Behavior and Ionic Conductivity in Lithium Bis(trifluoromethanesulfonyl)imide-Doped Ionic Liquids of the Pyrrolidinium Cation and Bis(trifluoromethanesulfonyl)imide Anion Anna Martinelli,* Aleksandar Matic, Per Jacobsson, and Lars Bo¨rjesson Department of Applied Physics, Chalmers UniVersity of Technology, 41296 Go¨teborg, Sweden

Alessandra Fernicola and Bruno Scrosati UniVersity of Rome La Sapienza, P.le Aldo Moro 5, 00185 Rome, Italy ReceiVed: June 19, 2009; ReVised Manuscript ReceiVed: June 28, 2009

The phase behavior and the ionic conductivity of ionic liquids (ILs) of the N-alkyl-N-alkylpyrrolidinium (PYRxy) cation and the bis(trifluoromethanesulfonyl)imide (TFSI) anion are investigated upon addition of LiTFSI salt. We compare the case of two new ILs of the PYR2y cation (where 2 is ethyl and y is butyl or propyl) with that of the PYR14 (where 1 is methyl and 4 is butyl). We find that the addition of LiTFSI increases the glass transition temperature, decreases the melting temperature and the heat of fusion and, in the ILs of the PYR2y family, suppresses crystallization. In the solid state, significant ionic conductivities are found, being as high as 10-5 S cm-1, strongly increasing with Li+ concentration. The opposite trend is found in the liquid state, where the conductivity is on the order of 10-3-10-2 S cm-1 at room temperature. A Tg-scaled Arrhenius plot shows that the liquid-state ionic conductivity in these systems is mainly governed by viscosity and that the fragility of the liquids is slightly influenced by the structural modifications on the cation. I. Introduction There is a considerable interest in ionic liquids (ILs) as electrolytes in battery applications.1,2 If intended to be used in Li batteries, ILs are required to meet two important requirements. The first is the ability to dissolve a Li salt to have a high Li+ conductivity; the second is to show a good electrochemical stability toward the metal electrodes. The electrochemical stability toward metal electrodes is still a severe limitation, mainly due to the reduction potential of the cation.3 It has recently been demonstrated that improved stability can be achieved by chemical modification of the cation. For instance, LiTFSI-doped N-butyl-N-ethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR24TFSI) shows a better stability in time toward lithium metal electrodes than the previously investigated imidazolium-based systems and also has a wider electrochemical stability window.4 Thus, there is an interest to investigate the influence of Li salt addition also on other properties of ionic liquids of the PYR2yTFSI family. The addition of a Li salt to an IL usually reduces the ionic conductivity due to an increased viscosity and, possibly, to enhanced ion-ion interactions. As a consequence, the understanding of the compositional influence on the phase behavior and the ionic conductivity in Li salt/IL mixtures is an issue of some interest. These have been partly addressed for the ILs of the PYR1yTFSI family.5-10 These studies reveal complex phase diagrams with the existence of several crystalline phases, recognized as plastic crystal phases. Here, we extend these studies by the investigation of the ionic conductivity and the phase behavior as a function of composition in three pyrrolidinium based ILs doped with LiTFSI: two are of the new PYR2y family (i.e., N-butyl-N-ethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR24TFSI) and N-propyl* Corresponding author. E-mail: [email protected].

Figure 1. Molecular structure of the constituting ions in the three investigated ILs. The TFSI anion is shown in the two possible conformations: C1 (felt) and C2 (right).

N-ethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR23TFSI)), and the third is N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI). The latter has previously been extensively investigated, but we here add data for the influence of LiTFSI concentration on the glass transition temperatures as well as on the universality of the ionic conductivity. II. Experimental Section A. Sample Preparation. The synthesis procedure to obtain the pure ILs is described in detail elsewhere.4 The molecular structures of the constituting ions are shown in Figure 1, the TFSI anion being depicted in the two possible conformations,

10.1021/jp905783t CCC: $40.75  2009 American Chemical Society Published on Web 07/22/2009

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TABLE 1: Thermal Properties of the Pure ILs and the Corresponding LiTFSI Added Solutionsa

Tm (°C)

Tg (°C)

Hf (J g-1)

LiTFSI (x) 0 0.02 0.04 0.08 0.16 0 0.02 0.04 0.08 0.16 0 0.02 0.04 0.08 0.16

PYR24

PYR23

PYR14

-8 -8 -11

25 22 18 11 -2

-13 -14 -14 -15 -18 -81 -81 -78 -78 -71 18 13 10 5 4

-84 -82 -82 -79 -75 15 14 2

-74 9 6 3 3 3

a x, molar fraction; Tm, melting temperature; Tg, glass transition temperature; and Hf, heat of fusion.

i.e. the cisoid and the transoid also known as forms C1 and C2, respectively.11 To prepare the xLiTFSI(1 - x)IL solutions, the LiTFSI salt was added to the pure ILs at 60 °C under magnetic stirring. Solutions were prepared in the range x ) 0-0.16, where x designates the concentration in molar fraction (see Table 1). B. Differential Scanning Calorimetry. Differential scanning calorimetric (DSC) experiments was performed in the temperature range -120 to 80 °C in hermetically sealed Al pans prepared in an Ar-filled glovebox. The samples were first cooled from 25 to -120 °C at a rate of 10 °C min-1, and the DSC traces were recorded during the subsequent heating scan up to 80 °C at a rate of 5 °C min-1. The rapid cooling is performed to avoid crystallization and thus possibly detect a glass transition during the heating scan. C. Dielectric Spectroscopy. Conductivity measurements were performed on a Novocontrol broadband dielectric spectrometer covering the frequency range 10-1-106 Hz and the temperature range -70 to 90 °C. Data were collected every 10 °C. Samples were sandwiched between two gold-plated electrodes with a L of 20 mm. A Teflon spacer of 1 mm in thickness was used to keep the cell dimensions constant. III. Results and Discussion A. Phase Behavior. Figure 2 shows the DSC traces of the pure ILs recorded during the heating scan. The PYR14TFSI system shows a glass transition at ∼-81 °C and a sharp crystallization peak before two endothermic peaks, the last one being the melting that occurs at a subambient temperature. The phase behavior of this material has previously been investigated in detail and is shown to depend on the cooling rate applied to the sample.10 The DSC profile recorded by us agrees well with the previous results for rapidly cooled samples. When a fast cooling is applied, the liquid is brought into the glassy state; hence, a glass transition is observed upon heating, followed by crystallization and then melting. A slow cooling, on the other hand, promotes crystallization; thus, no Tg is detected upon heating, and a melting peak only is observed. The presence of two endothermic peaks in the DSC trace indicates that there are at least two different solid phases for this material. The substitution of the methyl with an ethyl group (PYR14 f PYR24) does not change the overall behavior. The glass transition is detected at a somewhat lower temperature, ∼-84 °C, and is followed by crystallization of the liquid and then

Figure 2. DSC traces of the pure ILs during the heating scan.

melting. Here, one solid phase only is observed. Compared to PYR14, the melting point shifts to higher temperatures. The substitution of the butyl by a propyl group (PYR24 f PYR23) introduces clear differences to the DSC trace. For PYR23TFSI, the cooling rate of 10 °C/min is not enough to avoid crystallization; thus, no glass transition is observed upon heating. The DSC trace shows four endothermic peaks, the last one of which, at ∼25 °C, corresponds to the solid-liquid transition. Thus, four solid phases can be identified for PYR23TFSI. Figure 3 shows the effect on the phase behavior upon addition of LiTFSI. For the PYR14TFSI series, the melting marginally shifts to lower temperatures upon increasing LiTFSI concentration. However, at the highest concentration, x ) 0.16, multiple endothermic peaks are observed, pointing toward the existence of more than one crystalline phase, in agreement with the phase diagram of xLiTFSI(1 - x)PYR14TFSI mixtures proposed in ref 10. That phase diagram is here complemented by the dependence of the glass transition temperatures on composition. For all concentrations, a glass transition can be observed, shifting to higher temperatures with the LiTFSI content (see also Table 1). For the PYR24TFSI series, the addition of LiTFSI results in a reduction of the heat of fusion, becoming negligible for x g 0.04. Thus, for high Li salt concentrations, no crystallization occurs in the solutions, and the temperature window of the liquid phase is considerably increased. As for the PYR14-based solutions, the glass transition temperature slightly increases with the LiTFSI concentration. Also for the PYR23TFSI series, there is a continuous decrease in the heat of fusion, but here, it is only for the highest concentration, x ) 0.16, that we can avoid crystallization and observe the glass transition. The phase behavior as a function of LiTFSI concentration is summarized in the phase diagram sketched for the three ILs in Figure 4. The common behavior is of an increasing glass transition temperature, Tg, and a decreasing melting temperature, Tm, upon increased Li+ content. The explanation of such thermal behavior is not straightforward. The increase in the glass transition temperature is still not well understood, but it has already been observed in other Li salt-doped ILs.12 The decrease in the melting temperature and the corresponding heat of fusion, on the other hand, might be a consequence of an increased ionic disorder in the solid phases preceding the melting. B. Conductivity. In Figure 5, the conductivity of the pure ILs is shown in an Arrhenius plot. The solid-liquid transitions are recognized as sharp steps in the conductivity occurring at temperatures that, for the PYR24TFSI and PYR23TFSI, agree well with those of melting determined by DSC. For PYR14TFSI,

Behavior and Conductivity of LiTFSI-Doped ILs

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Figure 5. Arrhenius plot of the conductivity in the pure ILs: PYR14TFSI (]), PYR24TFSI (°), and PYR23TFSI (4).

the sensitivity of this IL to the heating rate, which is different in the dielectric instrument as compared to the DSC. The fact that the step in the conductivity is small at the solid-liquid transition for PYR14TFSI is a result of two factors: (i) we have supercooled the liquid considerably before crystallization, and (ii) the conductivity in the solid state is relatively high for this IL. Our results are in agreement with the conductivities previously reported for PYR14TFSI in ref 5. In the crystalline solid phases, the conductivities approximately show an Arrhenius dependence. In the liquid phase, the conductivity is non-Arrhenius and can be well-described by a VFT (Vogel-Fulcher-Tamman) function, as expected for systems in which the conductivity is essentially governed by viscosity. In the VFT equation written as

σ ) σ0 · e-D · T0/T-T0 Figure 3. DSC traces during the heating scan of the xLiTFSI(1 - x)IL solutions, where x is the molar fraction and equals 0, 0.02, 0.04, 0.08, and 0.16.

Figure 4. Phase diagrams of the investigated xLiTFSI(1 - x)IL mixtures showing the endothermic phase transition temperatures (•) and the glass transition temperatures (() as a function of x. Solid lines are guides to the eye only.

this transition is observed only as a marginal, though discrete, step at a temperature (marked in the figure) considerably lower than Tm (see Table 1). This discrepancy might be ascribed to

(1)

D is a parameter related to the fragility of the liquid (fragility ∝ D-1), and T0 is a temperature closely related to the glass transition temperature. The liquid phase conductivities are relatively high, being in the range 10-3-10-2 S cm-1 for temperatures between 20 and 80 °C. There is only a slight dependence of conductivity on the cationic structure, and it increases in the order σPYR24 < σPYR14 e σPYR23. The dependence of the conductivity on LiTFSI concentration is shown in Figures 6 and 7. In general, in the liquid state, the conductivity decreases with increasing LiTFSI concentration (see Figure 7, where the conductivities are plotted as a function of LiTFSI concentration at 37 °C and where all samples are in the liquid phase). The decrease of the overall ionic conductivity upon addition of a Li salt is in agreement with previous studies on pyrrolidinium-based ILs8,9 and is most likely a direct consequence of the increased viscosity of the liquid. It should be mentioned, however, that the formation of diffusing [Li(TFSI)n]-(n-1) ionic species in the solutions13 might be an additional cause for the decrease in conductivity. The extent to which the Li ion contributes to the overall ionic conductivity is an issue of interest that might be further investigated by, for example, 7Li NMR experiments.9 These issues, however, are outside the scope of this paper and will be treated separately. In the PYR23TFSI and PYR24TFSI series, the conductivity shows a liquidlike behavior in the whole temperature range

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Figure 7. Dependence of the conductivity on the LiTFSI concentration in the xLiTFSI(1- x)IL mixtures of the PYR14 (]), the PYR24 (O), and the PYR23 (∆) cation, at a temperature of 37 °C.

Figure 6. Arrhenius plots of the conductivity in the three IL series: from top to bottom the PYR14, PYR24, and the PYR24 series. Within each series, the pure ILs are shown as open circles (O), whereas the corresponding xLiTFSI(1- x)IL mixtures are shown as upward triangles (4, x ) 0.02), open squares (0, x ) 0.04), downward triangles (∇, x ) 0.08), and diamonds (], x ) 0.16).

investigated for high LiTFSI concentrations (x ) 0.16 and x ) 0.04-0.16, respectively) in accordance with the suppression of crystallization observed in the calorimetric experiments. In the solid state, the general trend is the opposite, with an increase in ionic conductivity for higher Li salt concentrations. This is particularly clear for the PYR23TFSI and PYR24TFSI series. However, for the highest Li salt concentration in the PYR14TFSI series, x ) 0.16, there is a large decrease in the ionic conductivity in the solid state. One can note that for this composition the DSC trace is different, and we can expect the material to have a different crystal structure, which might influence the ionic conductivity. The fact that the solid state conductivities are considerable, being as high as 10-5 S cm-1; that they increase with the Li+ content; and that they show an Arrhenius dependence points toward a conductivity mainly related to the diffusion of the Li+ ions, possibly assisted by rotational disorder of the crystal. To closer investigate the role of viscosity on conductivity, a Tg-scaled Arrhenius representation is plotted in Figure 8, where the liquid-state conductivities of the investigated systems are brought together on a common temperature scale for comparison. We find that, independently of the LiTFSI concentration, the conductivities of the solutions of the same cation tend to

Figure 8. Tg-scaled Arrhenius plot of the logarithm of conductivity, here showing the liquid-phase data points only. The different pyrrolidinium series are labeled with the following open symbols: triangles (4), PYR14; circles (O), PYR24; and diamonds (]), PYR23. Each symbol includes conductivity data of the xLiTFSI(1- x)IL mixtures for which a Tg was detected. These solutions correspond to x ) 0, 0.02, 0.04, 0.08, and 0.16 for PYR24; x ) 0.16 for PYR23; and x ) 0, 0.02, 0.04, and 0.08 for PYR14. Dashed lines are the best VFT fits of the scaled conductivity data.

collapse onto a single curve. Thus, the Tg scaling cancels out the effects of LiTFSI concentration (as observed in Figures 6 and 7), underlining that the observed decrease in conductivity with higher LiTFSI concentration is mainly a consequence of higher viscosity. The Tg-scaled representation is commonly employed to discuss the viscosity of glass-forming materials, including polymers and ILs.14-17 Depending on the strength of the Tg/T dependence, liquids are classified as strong or fragile. In a Tgscaled representation, a fragile liquid is characterized by a strong curvature near Tg, which analytically corresponds to a low value of D in the VFT equation best fitting the data points.14,15 By fitting the Tg-scaled data with the appropriately modified VFT equation,

log(σ) ) A -

D · T0 Tg /T - T0

(2)

Behavior and Conductivity of LiTFSI-Doped ILs we find that the D values of the PYR24TFSI and the PYR14TFSI series are 6.37 and 5.78, respectively. These values can be compared with that of toluene (D ) 5.6),14 which is known to be a very fragile glass-forming liquid. This means that within the framework of strong/fragile classification, these ILs display a fragile behavior. Moreover, these D values show that fragility slightly changes with the structural modifications on the cation. Even though this structural effect is small, the trend might prove interesting for the development of IL-based electrolytes of higher fragility, as required for performance as high-fluidity liquids at room temperature. IV. Conclusions In this paper, we investigate the phase behavior and the ionic conductivity in LiTFSI-doped ionic liquids of the TFSI anion and of the N-alkyl substituted pyrrolidinium cation. We find that the addition of LiTFSI to the pure ILs increases the glass transition temperature; decreases the melting temperature and the heat of fusion; and in PYR23TFSI and PYR24TFSI, suppresses crystallization. The latter results in larger temperature windows of liquid phase and, consequently, in wider temperature ranges for practical applications. In the solid states, significant ionic conductivities are observed that increase with the LiTFSI content, suggesting the presence of plastic crystal phases and a conductivity mainly due to diffusing Li+ ions. The opposite trend is found in the liquid states, where the addition of the Li salt decreases the conductivity, most likely due to an increase in viscosity. In a Tg-scaled Arrhenius representation of the ionic conductivities, the effect of the Li salt concentration is canceled out, and all data points fall onto a single curve for each cation family. This suggests that in the xLiTFSI(1- x)ILs systems here investigated, the conductivity is mainly governed by viscosity over the whole liquid range, all the way down to the glass transition temper-

J. Phys. Chem. B, Vol. 113, No. 32, 2009 11251 ature. In the Tg-scaled plot, the fragility of the ILs appears to be slightly dependent on the cationic structure, but not influenced by the LiTFSI concentration. Acknowledgment. The financial support from the Swedish Research Council (VR) and the Italian Ministry for University and Research (MIUR, Project PRIN 2007) are both gratefully acknowledged. References and Notes (1) Alessandrini, F.; Appetecchi, G. B.; Conte, M.; Passerini, S. ECS Trans. 2006, 1, 67. (2) Hagiwara, R.; Lee, J. S. Electrochemistry 2007, 75, 23. (3) Holzapfel, M.; Jost, C.; Prodi-Schwab, A.; Krumeich, F.; Wursig, A.; Buqa, H.; Novak, P. Carbon 2005, 43, 1488. (4) Fernicola, A.; Croce, F.; Scrosati, B.; Watanabe, T.; Ohno, H. J. Power Sources 2007, 174, 342. (5) MacFarlane, D. R.; Meakin, P.; Sun, J.; Amini, N.; Forsyth, M. J. Phys. Chem. B 1999, 103, 4164. (6) Henderson, W. A.; Young, V. G.; Passerini, S.; Trulove, P. C.; Long, H. C. D. Chem. Mater. 2006, 18, 934. (7) Henderson, W. A.; Young, V. G., Jr.; Pearson, W.; Passerini, S.; Long, H. C. D.; Trulove, P. C. Phys. Rep. 1979, 53, 1. (8) Huang, J.; Forsyth, M.; MacFarlane, D. R. Solid State Ionics 2000, 136, 447. (9) Forsyth, M.; Huang, J.; MacFarlane, D. R. J. Mater. Chem. 2000, 10, 2259. (10) Henderson, W. A.; Passerini, S. Chem. Mater. 2004, 16, 2881. (11) Herstedt, M.; Smirnov, M.; Johansson, P.; Chami, M.; Grondin, J.; Servant, L.; Lassegues, J. J. Raman Spectrosc. 2005, 36, 762. (12) Hayamizu, K.; Aihara, Y.; Nakagawa, H.; Nukuda, T.; Price, W. S. J. Phys. Chem. B 2004, 108, 19527. (13) Lassegues, J. C.; Grondin, C. A. J.; Johansson, P. J. Phys. Chem. A 2009, 113, 305. (14) Angell, C. A. J. Res. Natl. Inst. Stand. Technol. 1997, 102, 171. (15) Angell, C. A. Polymer 1997, 38, 6261. (16) Xu, W.; Cooper, E. I.; Angell, C. A. J. Phys. Chem. B 2003, 107, 6170. (17) Martinez, L. M.; Angell, C. A. Nature 2001, 410, 663.

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