Polymerizable Ionic Liquid with State of the Art ... - ACS Publications

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Polymerizable Ionic Liquid with State of the Art Transport Properties Sebastian Jeremias,* Miriam Kunze, Stefano Passerini, and Monika Schönhoff Institute of Physical Chemistry, University of Muenster, Corrensstrasse 28/30, 48149 Münster, Germany ABSTRACT: The physicochemical properties of diallyldimethylammoniumbis(trifluoromethanesulfonyl)imide (DADMATFSI) and its binary mixture with LiTFSI are presented herein, also showing this novel compound as a polymerizable room temperature ionic liquid with excellent transport properties for Li+ ions. In particular, results of pulsed field gradient (PFG)-NMR diffusion experiments and impedance measurements show that DADMATFSI exhibits state of the art properties of ionic liquids. Similar ionic diffusion coefficients and a similarly high conductivity as seen in the benchmark compound N-butyl-N-methylpyrrolidiniumbis(trifluoromethanesulfonyl)imide (PYR14TFSI) are observed. In accordance, the Li transference number in the binary mixture matches the trend seen for PYR14TFSI−LiTFSI mixtures. In addition to these impressive properties as ionic liquid, DADMATFSI was polymerized by UV treatment. The polymerization is demonstrated and the ion conducting properties of the resulting gel polymer electrolyte are investigated, showing that DADMATFSI can be transformed into an ionogel and may have applications where polymerization is desirable.

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INTRODUCTION Ionic liquids (IL) and their polymeric analogues, the polymeric ionic liquids (PIL), are compounds entirely composed of ions. The IL cations are generally bulky organic molecules varying in shape. ILs reported in the past decade are mainly based on imidazolium, pyrrolidinium, piperidinium, or ammonium derivatives as cations. The anions are mostly voluminous, soft molecules, which may possess organic and inorganic groups.1 Ionic liquids exhibit a broad variety of promising properties, for example, high flame resistance, low vapor pressure, high thermal and electrochemical stability and high conductivity.2 For PIL a broad variety of polyanions and polycations are known and intensively studied.3−5 They possess structures very similar to known ionic liquids where at least one type ion is included in the polymer backbone or a part of the side chains in graft copolymers.4 The unique properties of both, ionic liquids and polymeric ionic liquids provide possible applications in secondary batteries, dye sensitized solar cells, actuators and other electrochromic devices.4−6 For their use in electrochemical cells, mixtures of IL and PIL might represent the suitable compromise for a long-term use with reasonable conductivity, good mechanical properties, and safety.7 The addition of a polymeric compound to an ionic liquid commonly results in an increase of the viscosity, which can avoid leakage of the electrolyte in a damaged battery.8 A drawback, however, is the decrease of the conductivity occurring as well. Such mixtures have been named “ionic gels” by Susan et al.9 The concept of gels is already well-known for common electrolytes.8 However, using an ionic liquid as plasticizer and simultaneously as the conductive species is a rather new approach. For example, Joost et al. have prepared PEO/IL/Li-salt mixtures as electrolyte.10 It could be shown that low amounts of ionic liquid decrease the Tg and increase the conductivity in © 2013 American Chemical Society

comparison to binary PEO/Li-salt mixtures. The ionic liquid acts as plasticizer and increases the free volume. Furthermore, Raman spectroscopy could verify that the Coulomb interaction between TFSI− and the lithium cation enhances the mobility of lithium. Another known gel is composed of PMMA and EMITFSI.9 Here the gel is prepared in situ by photopolymerization of MMA monomers dissolved in EMITFSI. The resulting gel shows good thermal, mechanical and electrochemical properties. A gel system based on a charged polymer has been developed by Pont et al., they use the polycation (PDADMATFSI) as polymer in mixtures with PYR14TFSI and LiTFSI.7 The synthesis route is different to the PMMA gels. Pont et al. start with an anion exchange of polydiallyldimethylammoniumchloride (PDADMAC), replacing Cl− by TFSI−, followed by an extensive purification, mixing and annealing process. The resulting gels show promising properties and have already been tested for battery application in half cells in combination with different electrode materials.11 In the present work, the physicochemical properties of the monomeric compound DADMATFSI are studied with respect to a later use as electrolyte component. We show that on the one hand DAMDATFSI is a state-of-the-art IL with excellent transport properties in comparison to other commonly used ILs. Furthermore, we show that a successful in situ polymerization of DADMATFSI, dissolved in PYR14TFSI, can be used as an easy route to synthesize ionogels with variable polymer content and appealing ion transport properties. Received: July 17, 2013 Revised: August 7, 2013 Published: August 19, 2013 10596

dx.doi.org/10.1021/jp407083z | J. Phys. Chem. B 2013, 117, 10596−10602

The Journal of Physical Chemistry B

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4.33 × 10−3 m−1. The measurements were run by performing a 2 °C/h heating scan in steps of 10 °C from 0 to 70 °C in a frequency range of 10−2 Hz up to 106 Hz. Calorimetry. Differential scanning calorimetric (DSC) measurements were performed using a calorimeter model (TA Instruments). The samples were prepared in a glovebox and sealed in Al pans. The IL samples were cooled with 10 °C/min from room temperature to −150 °C, then heated with 2 °C/min to −40 °C, again cooled to −150 °C and finally heated to 70 °C with a heating rate of 2 °C/min, while measurements were taken. Density. The density of the samples was measured from 10 to 40 °C in steps of 10 °C using a density meter (Mettler Toledo, DE40) in a dry room.

MATERIALS AND METHODS Materials. The chemicals diallyldimethylammoniumchloride (DADMAC) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were purchased from Aldrich and used without further purification. LiTFSI was dried 24 h at 90 °C under vacuum to remove remaining water and then stored in a dry room. Alumina and dichloromethane (DCM) were purchased from Aldrich and used with no further purification. The ionic liquid PYR14TFSI was purchased from Solvionics with a purity of 99.9%. It was used after drying under high vacuum conditions. The Photoinitiator DAROCURE 1173 was kindly provided by Roberta Bongiovanni Ionic Liquid Synthesis. To prepare DADMATFSI, 60 mL of a solution of DADMAC (100.0 mmol) in ultrapure water was added to a solution of lithium bis(trifluoromethanesulfonyl)imide (100.0 mmol). The mixture was stirred for 24 h at 50 °C. The resulting IL (organic phase) was separated from the aqueous phase and subsequently washed several times with ultrapure water. To the IL/water mixture, DCM (80 mL) was added and the organic phase, including the IL, was extracted. The yellowish solution was filtered through neutral alumina to remove remaining impurities of lithium salts. The DCM was removed using a rotary evaporator at 40 °C under vacuum. Finally, the IL was dried at 50 °C under high vacuum conditions for 24 h. The water content of the IL was less than 10 ppm, confirmed by Karl Fischer titratino. The IL (DADMATFSI) was stored in a glovebox under dry argon atmosphere. A binary sample ((1 − x) DADMATFSI) (x LiTFSI) with a molar fraction of x = 0.091 was prepared by adding LiTFSI to DADMATFSI. The resulting mixture was stirred for 12 h at 50 °C and then stored in a glovebox under dry argon atmosphere. Samples for NMR measurements were filled into 5 mm NMR tubes, evacuated, and then flame-sealed. Samples for impedance measurements were filled into measurement cells under dry argon atmosphere. Ionogel Synthesis. The iongels were prepared in a dryroom atmosphere by photopolymerization. DADMATFSI was mixed in different molar ratios with PYR14TFSI. The studied compositions were (x DADMATFSI) ((1−x) PYR14TFSI) with x = 0.3 and 0.5 molar fraction of DADMATFSI. As photoinitiator DAROCURE 1173 was used. The added amount of DAROCURE was 5 mol % with respect to DADMATFSI. For UV-curing, a mercury lamp with an effective power of 14.3 mW/cm2 was used. The curing time was always fixed to 10 min. After they were cured, the samples were stored overnight at 80 °C and then transferred into NMRtubes and impedance cells, respectively. All sample handling was performed in a dry room (R.H. < 0.2%). NMR Diffusion Experiments. NMR measurements were performed using a Bruker spectrometer Avance 400 with permanent field strength of 9.4 T. For the diffusion measurements a gradient probe head (Bruker, “Diff 30”) with selective radiofrequency inserts for 1H, 7Li, and 19F detection and a maximum gradient strength of 1180 G/cm was used. The PGSTE-sequences was used for ionogels to determine the diffusion coefficients of the different species. For the liquids, the dstegp3s-sequence was used to suppress possible convection artifacts.12 Conductivity. Conductivity measurements were carried out using an impedance spectrometer “Alpha analyzer” (Novocontrol). The impedance cell had a cell constant of

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RESULTS Polymerizable Ionic liquid. To prove that DADMATFSI is a state of the art ionic liquid, we first performed a standard characterization of the physicochemical properties and compare these to those of PYR14TFSI, which is taken as a benchmark.

Figure 1. Temperature dependent density of neat DADMATFSI (squares) and ((0.909) DADMATFSI−(0.091) LiTFSI) (triangles), lines represent fit.

Basic Physicochemical Properties. Density. Figure 1 shows the density of DADMATFSI (neat) and ((0.909 DADMATFSI) (0.091 LiTFSI)). The temperature dependence is linear for both samples and could be well fitted with linear regression (1). Fit parameters are shown in Table 1. ρ = a − bT

(1)

Table 1. Density Fit Parameters of Neat DADMATFSI and Binary Mixture DADMATFSI (neat) ((0.909) DADMATFSI)(0.091 LiTFSI)

a [g/cm3]

b [g/cm3 K]

1.68 1.71

9.2 × 10−4 9.4 × 10−4

With ρ(30 °C)= 1.40 g/cm3 the neat IL has a similar density as PYR14TFSI and [(n-C4H9)(CH3)3N]TFSI, both ρ(30 °C) = 1.39 g/cm3.13,14 The binary mixture, including the lithium salt, has a slightly higher density compared to the neat IL. Phase Transitions. The DSC heating traces for the neat IL and the binary mixture from −70 to 30 °C are shown in Figure 2. These data were extracted from the final heating step (−150 to 70 °C), no further transitions were observed. 10597



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conductivity of the binary mixture is lower than that of neat DADMATFSI, however, the latter offers conductivity comparable to that of PYR14TFSI. The conductivity of PYR14TFSI matches over the whole temperature range very well with literature data.13,16 The lower conductivity of binary mixtures of IL with corresponding Li salts was observed for other ILs as well.17−19 It is directly connected to the mobility changes of each ionic species in the presence of lithium salts and will be discussed in more detail in the next section. In conclusion, the polymerizable compound DADMATFSI shows conductivity properties comparable with those of state-of-the-art ionic liquids. Ion Transport. Diffusion Coefficients. In PFG-NMR diffusion experiments, the echo decays for all species show exponential signal attenuation (data not shown). The diffusion coefficients of DADMA, TFSI, and Li in the neat IL and the binary mixture show no significant variation when performing PFG-NMR measurements with varying observation time, that is, 50, 150, and 300 ms (data not shown). The observation time is the evolution period for diffusion, given by the spacing of the two gradient pulses in the STE and dste3gs sequences. Therefore, the experiments do not distinguish between different states in which the ions might exist, e.g.: ion independently moving, ion in pairs or clusters. The observed diffusion coefficient is an average over all these states of Li, TFSI, or DADMA, respectively, and can be expressed by (2)

Figure 2. Heating traces of neat DADMATFSI (solid line) and the binary mixture (scattered line).

When heating neat DADMATFSI, DSC reveals two peaks. The first can be assigned to a solid−solid phase transition (−8.2 °C) while the second peak indicates the melting point (Tm = −0.3 °C). This melting temperature does not agree to that determined by Yim et al., they reported Tm = −4 °C.15 We assume this discrepancy is related to the much higher water content (50 ppm) present in their studies. Additionally, in our measurements we observe a solid−solid phase transition at −8.2 °C which cannot be seen in the DSC data by Yim et al.15 The binary mixture shows no solid−solid phase transition when heated, the melting point is observed at Tm = −5.2 °C.15 Thus, the melting point is shifted to lower temperatures for the Lidoped system compared to the neat IL. This kind of behavior is found for binary mixtures of (x LiTFSI) ((1 − x) PYR14TFSI) as well, and results from strong Coulombic interactions between Li and TFSI. In the studies of Henderson et al., the addition of small amounts of LiTFSI decreases the melting point leading to an eutectic with a minimum Tm at (0.12· LiTFSI)(0.88·PYR14TFSI).16 Further addition of LiTFSI in these mixtures is leading to a continuously increasing melting point, which lies above the Tm of the neat PYR14TFSI. There might be a similar behavior in the case of (x LiTFSI) ((1 − x) DADMATFSI) mixtures. Conductivity. Figure 3 depicts the temperature dependent ionic conductivities of neat DADMATFSI and its binary mixture with LiTFSI, and neat PYR14TFSI. The values of σ(T) were obtained from the σdc plateaus of the σ(ν) measurements. For all samples σ(T) clearly shows a VTF-behavior. The ionic

Dobs(X) =

∑ ai × Di(X)

(2)

Here Dobs is the diffusion coefficient observed by NMR for the different species (X = DADMA, TFSI, Li) and ai represents the fraction of different states (i = free, pair, complex). Figure 4a shows the cation diffusion coefficients of neat DADMATFSI, neat PYR14TFSI and the binary mixture in

Figure 4. Arrhenius plots displaying the temperature dependent diffusivities of DADMATFSI (squares), binary mixture (triangles), PYR14TFSI (open, circles) and Li+ (open triangles) of (panel a) cations and (panel b) anions and lithium.

dependence of temperature. In Figure 4b the anion and lithium diffusion coefficients are displayed. In the neat ILs, for both ions the D(T) behavior is non-Arrhenius like. A slight but significant difference in the diffusion of cationic and anionic species occurs in the neat IL, while it is always Dcation > Danion. A reasonable explanation of this difference might relate to the distinct sizes of both ions, when assuming Stokes−Einstein behavior. However, the agreement of the diffusivity with the

Figure 3. Arrhenius plot of the temperature dependent conductivities of neat DADMATFSI (squares), binary mixture (triangle), and PYR14TFSI (open circle). 10598



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molecular size in IL is not generally valid.19 For example, in EMIMBF4 the cation diffuses faster than the anion, although it has a larger radius. Ionic diffusion in IL is probably much more controlled by cluster or pair formation than by single ion hydrodynamic radii. The ion diffusion coefficients in DADMATFSI match very well with those of the corresponding ions in PYR14TFSI, as seen by the overlap of the data in Figure 4a and b, respectively. This correlates with almost identical conductivities of both ILs. It can thus be concluded that the aggregation behavior of the DADMA cation is very similar to that of PYR14+. In the binary mixture with the Li salt, the diffusion coefficients follow the order Dcation > Danion > DLi. This is again in good agreement with other Li-doped ILs studied in literature.17−19 Compared to the neat IL, DADMA+ and TFSI− both diffuse significantly slower. For several ILs this phenomena was related to the presence of [Li(TFSI)n]1−n complexes, as verified by Raman spectroscopy.20,21 The Li+ ion with a localized charge is strongly coordinated by TFSI anions in a tetrahedral manner.20 Through this coordination a significant amount of Li+ has to diffuse with a TFSI solvation shell that possesses a multiple radius compared to the single Li ion. A more detailed insight into lithium transport in Li/IL mixtures is given by MD simulations by Borodin et al.22 Their results reveal two different transport mechanisms. One is the diffusion of the before mentioned [Li(TFSI)n]1−n complex (vehicle mechanism), but it accounts only 30%. The main, faster mechanism is governed by a fast exchange of TFSI anions in the first solvation shell of lithium (structure mechanism). The drop in mobility of DADMA+ and TFSI− in the binary mixture explains the decreasing conductivity behavior. Apparent Transference Numbers. For use in electrochemical devices the transference number of lithium tLi, that is, the fraction of charge carried by Li+ ion, is of major interest. We calculate the transference numbers at different temperatures via the individual ion mobilities obtained from PFGNMR using ts =

Figure 5. Lithium transference numbers for different ionic liquids (see legend) and mole fractions of lithium salt (lines are guide to the eye).2,17−19,23,24

In a first step, an apparent molar conductivity resulting from the diffusion coefficients can be calculated via the Nernst− Einstein equation ΛNMR =

(4)

Here, NA is the Avogadro number, e the elementary charge, kB the Boltzmann constant, and T the temperature. This equation was used assuming that the ions have a unitary activity. On the other hand, the molar conductivity resulting from impedance measurements is calculated via σ Λ m = dc c ion (5) where cion is the ion concentration in solution, which is calculated for each temperature using the results from the density measurements (Table 1). The Nernst−Einstein equation allows the characterization of the “ionicity” of ionic liquids.

Nv, sDs ∑ Nv, sDs

(NAe 2) (Dcation + Danion) (kBT )

r=

(3)

Here, Nv,s is the number density of the different ions, Ds is the diffusion coefficient of the species s. Li transference numbers in LiTFSI/DADMATFSI are independent of temperature, resulting in a value of tLi = 0.023. For a better classification, Figure 5 shows this result in comparison to Li transference numbers of other IL−Li salt mixtures.2,17−19,23 Here the transference numbers (at ∼30 °C) are plotted against the mole fraction of the Li salt. For all mixtures a concentration dependency can be observed. The transference number of Li+ increases with mole fraction (see trend lines). For both ILs, the dependency seems to be best described as linear. But, these results were extracted from other publications, the error of measurement/extraction is not clear. The lithium transference number for EMIBF4 is clearly lower compared to PYR14TFSI and increases with a different slope. In the case of DADMATFSI the transference number fits in the trend seen for PYR14TFSI. This is reasonable, because conductivity and diffusion of the neat IL agree quite well with those of PYR14TFSI pointing to a similar ionic radius or structure in the IL. Molar Conductivities. It is now possible to compare the diffusivity of each species in the neat IL with the conductivity.

Λm ΛNMR

(6)

The ratio r represents the fraction of ions, which are present in a dissociated state and are thus contributing to conductivity, as opposed to ion pairs or, more in general, neutral clusters, which contribute to diffusion but not to conductivity. In Figure 6, the ratio r and the molar conductivities are plotted against temperature. While the molar conductivities Λdc and ΛNMR increase with temperature, their ratio remains constant over the whole temperature range investigated, with an average value of r = 0.68 (solid line). It is close to the r value of PYR14TFSI, r ≈ 0.7, and expresses a similar high degree of ion dissociation.24 The observed temperature independent behavior of the r-value in the case of DADMATFSI cannot be generalized for other ionic liquids. In agreement with our results PYR14TFSI shows no temperature dependency.13 But, various authors report for other ionic liquids a decreasing r-value toward higher temperatures.25,26 So far, it is open what is causing this phenomenon. In conclusion, DADMATFSI is an ionic liquid with very similar transport properties as compared to PYR14TFSI, which is considered a benchmark. The reason is possibly due to very similar properties of the respective cations, concerning the 10599



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successful polymerization. Thus, the polymerized mixtures can be defined as ionogels. However, the DADMATFSI peaks clearly show that a small fraction of not polymerized DADMA+ remains in the ionogels (Figure 7b). We assume that increasing viscosity is kinetically inhibiting the propagation process of the polymerization. This is in agreement with the conversion of neat DADMATFSI, with no additional solvent, recently published by Jovanovski et al.27 The conversion in their studies is clearly below 70% after 6 h of polymerization at 70 °C.27 We used the 1H-spectra data to determine the conversion of DADMATFSI in the binary mixtures. The integrals of the peaks 1 and 10 were evaluated before and after UV treatment. The fraction of polymerized DADMATFSI is given by 7. cr =

Figure 6. Arrhenius plot displaying the temperature dependent molar conductivities Λdc (open circles), ΛNMR (squares), and r (filled hexagon).

∫ peak 10(before) ∫ peak 1(after) × ∫ peak 1(before) ∫ peak 10(after)

(7)

Here, the value cr reflects the conversion rate of DADMATFSI and is summarized in Table 2.

interaction with TFSI, such that pair formation and clustering, which control the transport properties, are similarly effective. However, because of its polymerizability DADMATFSI might have advantages in several applications. In the following, we, therefore, demonstrate the polymerization and the properties of a resulting ionogel. Ionogels. Polymerization. Ionogels are obtained via photopolymerization of DADMATFSI/PYR14TFSI mixtures. We chose two compositions (x DADMATFSI) ((1 − x) PYR 14 TFSI) with x = 0.3 and 0.5 mol fraction of DADMATFSI. In Figure 7, the 1H NMR-spectra of the 0.5:0.5 mixture before and of both mixtures after polymerization are shown.

Table 2. Conversion of DADMATFSI sample

conversion rate (%)

PDADMATFSI content (mol %)

Ionogel 0.3:0.7 Ionogel 0.5:0.5

81 ± 10 78 ± 10

24.3 39.0

It can be seen that the conversion is similar for both compositions and amounts to around 80%. The error is due to the overlapping of the different peaks after polymerization. Possible conversion products of the DADMA cation are wellknown to be isomers of pyrrolidinium ring and a conversion product that is not result of ring closure.28 In this case, one of the allylic groups allows a branching with other polymer chains. In conversion studies of aqueous solutions of DADMAC, the amount of branched reaction products is less than 2%.28 A direct detection of poly(DADMA)n+ and distinguishing different polymerization products was not possible due to the similar structure of the PYR14+-cation and the line broadening. Conductivity. Figure 8 depicts the temperature dependent ionic conductivities of neat PYR14TFSI and the polymerized ionogels. The values of σ(T) were obtained from the σdc plateaus of the σ(ν) measurements. For all samples σ(T) clearly shows a VTF-behavior. The ionic conductivity of the ionogels is lower than that of neat PYR14TFSI. Furthermore,

Figure 7. 1H NMR spectra of DADMATFSI/PYR14TFSI 0.5: 0.5 mixture before (solid line) and ionogels after polymerization (dashed lines) a: complete spectra b: specific range showing remaining allylic peaks.

The 1H-spectra of polymerized samples (see dashed lines) show a significant line broadening compared to the mixture of the neat ionic liquids. This is an effect of a reduced molecular mobility even of low molecular weight compounds in a viscous polymeric environment. Furthermore, the signal intensity of the DADMA+ peaks 1 and 2 vanish almost completely, allowing these peaks to be assigned to the allylic protons (see the inset of Figure 7). Therefore, the general line broadening and the decrease of the peaks 1 and 2 are clear indications of the

Figure 8. Arrhenius plot of the temperature dependent conductivities of neat PYR14TFSI (open circles), ionogel 0.3:0.7 (left triangle), and ionogel 0.5:0.5 (diamond). 10600



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the ionic conductivity decreases with increasing fraction of polymerized DADMATFSI. This is connected to the obvious viscosity increase with increasing polymer fraction. The feasibility of PDADMATFSI/PYR14TFSI ionogels has already been reported by Pont et al.7 However, in that work PDADMATFSI was prepared via anion exchange of the polymer poly(diallydimethylammoniumchloride) (PDADMAC) and not via in situ polymerization of DADMATFSI. Pont et al. report a similar conductivity tendency when increasing the polymer content.7 Diffusion Coefficients. Figure 9a shows the PYR14+ diffusion coefficients in the neat IL and in the ionogels as a function of Figure 10. Cation (filled symbols) and anion (open symbols) transference numbers in the neat IL (PYR14TFSI) and the ionogels.

however, the transference numbers of cation and anion approximate against each other. This tendency is the result of increasing amounts of mobile anions, while the cation fraction is continuously decreasing because of DADMA+ conversion.

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CONCLUSIONS The compound DADMATFSI is an ionic liquid with several remarkable physicochemical properties. First of all, compared to other ILs, it shows high conductivities in the neat form and in binary mixtures with LiTFSI. Particularly noteworthy is the similarity to PYR14TFSI, which is considered a benchmark compound for battery applications. Both ILs have comparable conductivity, ion mobility and high ionicity. Even for binary IL/Li-salt mixtures DADMATFSI fits the trend seen for PYR14TFSI/Li-salt mixtures. Thus, the ionic liquid DADMATFSI has state of the art properties as an IL, while in addition it provides the ability to polymerize. Here, we proved the successful polymerization of DADMATFSI with high rates of conversion in mixtures with PYR14TFSI, forming ionogels. The transport properties are strongly influenced by the content of polymer and can be tuned by the composition. The polymerizability in combination with excellent IL properties can be interesting for application in electrochemical devices (batteries and supercapacitors), wherever an in situ polymerization process is desired.

Figure 9. Arrhenius plots of the temperature dependent diffusion coefficients of neat PYR14TFSI (circle), ionogel 0.3:0.7 (triangle) and ionogel 0.5:0.5 (pentagon). (a) PYR14+ and (b) TFSI−.

temperature. In Figure 9b, the TFSI− diffusion coefficients are displayed. The order Dcation > Danion is observed in all sample compositions. Furthermore, a decrease of the diffusion coefficients is visible with increasing polymer content, similar to the observation in the impedance measurements (Figure 8). Apparent transference numbers. The transference numbers of PYR14+ and TFSI− have been calculated in the same way as for Li+, see above. However, some important assumptions are made for the ionogels. In analogy to aqueous PDADMAC solutions, it is assumed complete dissociation of PDADMATFSI in presence of PYR14TFSI. Thus, the TFSI− anions from PDADMATFSI are mobile and can contribute to conductivity while the polymer chains (PDADMAn+) are considered as immobile. Furthermore, it is assumed, in accordance with the PFG-NMR results (Figure 4a), that nonpolymerized DADMA+ can contribute to the conductivity with the same mobility of PYR14+. Therefore, we summarize the contributions of remaining DADMA+ and PYR14+ and do not distinguish between them when discussing about cation transference numbers (Figure 10). For the neat IL, it can be seen that the transference number of the cation is slightly higher compared to the anion. This reflects the trend seen for the diffusion coefficients. Here, the error bars represent the fitting error of the PFG-NMR experiments. For the ionogels the error bars furthermore include the uncertainty in the conversion determination of DADMATFSI. Thus, it can be seen that for low polymer content the transference number remains similar to the neat IL and the contribution of the cation is higher. At higher polymer content,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 251 83-36757. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Björn Brüske for support with the density measurements and Roberta Bongiovanni for providing the photo initiator.

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REFERENCES

(1) Tokuda, H.; Hayamizu, K.; Ishii, K.; Abu Bin Hasan Susan, M.; Watanabe, M. Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 1. Variation of Anionic Species. J. Phys. Chem. B 2004, 108, 16593−16600.

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