Unveiling the Ion Conduction Mechanism in Imidazolium-Based Poly

Article pubs.acs.org/Macromolecules

Unveiling the Ion Conduction Mechanism in Imidazolium-Based Poly(ionic liquids): A Comprehensive Investigation of the Structureto-Transport Interplay Virginie Delhorbe,† Dominic Bresser,*,§ Hakima Mendil-Jakani,§ Patrice Rannou,§ Laurent Bernard,§ Thibaut Gutel,† Sandrine Lyonnard,§ and Lionel Picard*,† †

University Grenoble Alpes, CEA, LITEN, DEHT, STB, F-38000 Grenoble, France University Grenoble Alpes, CEA, CEA, SyMMES, F-38000 Grenoble, France

§

S Supporting Information *

ABSTRACT: Polymerized ionic liquids (poly(ILs)) are considered highly promising for the realization of high-performance and intrinsically safer electrolytes for rechargeable batteries due to their high charge density. However, to date little is known about the ion conduction mechanism for this class of solid polymer electrolytes (SPEs). Herein, we performed an in-depth characterization of a homologous series of 1-alkyl-3-vinylimidazolium bis(trifluoromethane)sulfonimide-derived homopolymers, i.e., p(CnVIm-TSI) with n = 2, 4, 6, 8, and 10, serving as a model compound family. A particular focus was set on the interplay of the physicochemical properties, nanostructure, and ionic conductivity as well as on the impact of the additional incorporation of a lithium salt, LiTFSI. The results reveal that the nanostructure of these selfassembling poly(ILs) plays a decisive role for the ion conduction mechanism, allowing for a (partial) decoupling of charge transport and segmental relaxation of the polymer backbone.

1. INTRODUCTION Lithium-based batteries are nowadays the power source of choice for portable electronic devices, such as smartphones or laptops, and are moreover steadily gaining momentum in the field of electric vehicles.1−4 One of the major issues with respect to the practical implementation in large-scale applications, however, concerns their safety, which is largely determined by the employed electrolyte.5 Two potential candidates for realizing intrinsically safer rechargeable batteries are solid polymer- and ionic liquid-based electrolyte systems.5,6 Solid polymer electrolytes (SPEs) are considered advantageous due to their mechanical strength, the avoidance of potential leakage in the case of mechanical cell failure, and the possibility of realizing thin films serving simultaneously as electrolyte and separator, thereby allowing for enhanced energy densities.7 Ionic liquids (ILs), on the contrary, provide higher ambient temperature ionic conductivities, commonly excellent thermal stability, and greatly extended electrochemical stabilities particularly toward oxidationas well as tailorable physicochemical properties by carefully selecting the ionic species.8−11 In an attempt to combine these advantageous properties within SPEs, polymerized ionic liquids (abbreviated hereinafter as poly(ILs)) were designed,12 ideally resulting in highly conductive and electrochemically stable free-standing membranes that are easily processed and able to dissolve large amounts of conducting lithium salts.13−18 However, the ionic conductivity commonly drops by several orders of magnitude © XXXX American Chemical Society

after the polymerization of the ionic liquid monomers owing to a significant increase of the glass transition temperature (Tg) and the immobilization of one of the two ionic species.12,15,19 In order to address this issue, two main strategies have been followed: (i) the design of new poly(ILs) with macromolecular architectures of increasing complexity (e.g., comb-shaped block copolymer poly(ILs)) including a smart selection of the ionic species and (ii) the introduction of a plasticizer like, e.g., an ionic liquid (commonly referred to as “ion gels”).20−25 Indeed, the second approach and in particular the “ion gel” route appear very elegant considering the high chemical affinity of the two compounds. Accordingly, very wide composition ranges can be prepared without the occurrence of phase separation,26,27 resulting in an enhanced segmental motion of the polymer chain and, hence, higher ionic conductivity.28 Confirming these beneficial characteristics and reporting ionic conductivities up to 10−3 S cm−1, several studies revealed the suitability of these ion gels for lithium batteries containing a LiFePO4 cathode.27,29,30 Nevertheless, one must underline that the addition of large amounts of such a liquid plasticizers eventually compromises the mechanical properties of the membrane, leading to a necessary trade-off for optimizing the electrolyte performance in the final device.24,25 Received: January 27, 2017 Revised: April 30, 2017

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DOI: 10.1021/acs.macromol.7b00197 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules The first approach (vide supra) may be considered as a more fundamental one, as it targets the improvement of the charge transport through advanced macromolecular strategies. When considering the different strategies successfully applied to date for enhancing the ion conduction, the vast majority relies on lowering the Tg by increasing the segmental motion of the polymer chain and/or facilitating ion dissociation, e.g., by introducing (oligo-)ether-type side chains or spacers.23,25 More recently, however, it has been reported that an accurate control of the (nano)segregation of ionic and aliphatic domains, i.e., the nanoscale structural arrangement, plays an equally crucial role for enhancing the overall ion mobility. Relying on the fact that this nanoscale segregation is largely impacted by the nature of the pendant groups,16 Green et al.,31 for instance, studied 1-alkyl-3-vinylimidazolium bis(trifluoromethane)sulfonimide-derived homopolymers (p(CnVIm-TFSI), with n = 2, 4, and 8). They found that the trend for the development of the ionic conductivitybeing the highest for n = 4did not follow the trend observed for the Tg. For the same three samples, Salas-de la Cruz et al.32 investigated the relationship between the polymer nanostructure and ionic conductivity. Their study revealed a Tgindependent contribution to the overall charge transport linked to the backbone-to-backbone correlation distance. On the basis of their results, they proposed that longer alkyl chains lead to a reduced contribution of the anion hopping, especially from one polymer chain to another, i.e., a higher hopping energy barrier. While the segmental relaxation still substantially determines the ion conduction process within polymer electrolytes, these results indicate that the common focus on the Tg only may be reconsidered in the light of the resulting intrinsic conductivity limitations at ambient temperature for solid polymer electrolytes (SPEs).33 As a matter of fact, the successful decoupling of charge transport and segmental relaxation is considered to be a very promising, maybe the only research direction toward plasticizer-free SPEs with suitable room temperature conductivities.33−35 Following this strategy, we report herein an in-depth systematic investigation of a homologous series of p(CnVImTFSI) homopolymers (n = 2, 4, 6, 8, and 10), focusing on the interplay of their physicochemical properties, nanostructure, and ionic conductivity. Subsequently, the impact of introducing a lithium salt (LiTFSI) in p(CnVIm-TFSI) poly(ILs) is studied, revealing that rather general conclusions like, for example, “the longer the alkyl chain, the lower is...” are to be reconsidered.

corresponding monomer and 1.5 wt % of AIBN were dissolved in cyclohexanone (monomer/cyclohexanone = 1:1 w/w) and placed into a 100 mL round-bottom flask. The obtained solution was then, under stirring, subjected to three vacuum/argon cycles in order to remove any moisture and oxygen in the experimental setup. Subsequently, the system was kept under argon, and the temperature was increased to 70 °C to initiate the free radical polymerization. After 24 h, a small amount of cyclohexanone was added to the polymerization medium. In case of p(C2VIm-TFSI), the obtained product was precipitated three times in an excess of chloroform for purification. For the other samples, the resulting polymerization medium was washed three times with a large excess of a cyclohexane/dichloromethane mixture (5:1 v/ v). After having separated the poly(IL)-containing phase and evaporating the solvent medium, the polymers were thoroughly dried under high vacuum at 80 °C until no weight loss was observed anymore. The overall polymerization yield was between 70 and 85% for the herein synthesized p(CnVIm-TFSI) poly(IL) homopolymers. 2.3. Synthesis of Cross-Linked Poly(ionic liquids). Generally, the synthesis of cross-linked poly(ionic liquids) (p(CnVIm-TFSI)cl) was performed in an MBraun glovebox, displaying a H2O and O2 content of 500 °C. The glass transition temperature (Tg) of the synthesized p(CnVIm-TFSI) poly(ILs) was determined by DSC (Figure 2

Figure 3. Development of the density of the p(CnVIm-TFSI) poy(ILs) as a function of temperature, with n = 2 (blue), 4 (green), 6 (purple), 8 (red), and 10 (orange), normalized by the density at T = 30 °C (see Table S2).

The two polymers p(C2VIm-TFSI) and p(C4VIm-TFSI) show the expected trend of decreasing densities with increasing temperatures,41 which was previously assigned to reduced intermolecular interactions between the ionic macromolecules due to the rising local molecular mobility.42−46 However, for the other three poly(ILs), with n = 6, 8, or 10, the opposite trend is observed; i.e., the density increases slightly with an increasing temperature. We may assume at this point that for sufficiently long alkyl chains the impact of the aliphatic domains and the resulting van der Waals interactions on the overall density becomes increasingly important, which is basically in agreement with a previous study of Filippov et al.,43 investigating the impact of the alkyl chain length on the density of pyrrolidinium-based ILs. According to these considerations, the peculiar behavior of poly(ILs) with longer alkyl chains, revealing an increasing packing density when increasing their mobility by rising the temperature may be associated with a further increasing impact of the van der Waals forces between the aliphatic chains. To further investigate this divergent density development for different alkyl chain lengths, we studied the nanostructure of these poly(ILs) as a function of temperature by means of smallangle X-ray scattering (SAXS). A comparison of the obtained 1D patterns and the corresponding structural correlation distances is presented in Figure 4. For clarity reasons, only the three SAXS patterns for p(CnVIm-TSI) with n = 2, 6, and 10 are given in Figure 4a, while the comparison of the corresponding correlation distances as a function of the alkyl chain length n, depicted in Figure 4b, includes all the samples studied herein. Generally, all scattering patterns show three reflections within the investigated q range, i.e., at positions q1 ≈ 0.2−0.4 Å−1, q2 ≈ 0.8−0.9 Å−1, and q3 ≈ 1.35 Å−1 (Figure 4a). The dependence of each peak position, intensity, and shape upon the alkyl chain

Figure 2. Glass transition temperature Tg as a function of the alkyl chain length for the different p(CnVIm-TFSI) poly(IL) homopolymers.

and Table 1), and the Tg values for the different p(CnVIm-TSI) samples show a pronounced variation with the alkyl chain length. While the Tg is around 49 °C for p(C2VIm-TFSI), it decreases substantially for p(C4VIm-TFSI) (3 °C) and further to −18 °C and −22 °C for p(C6VIm-TFSI) and p(C8VImTFSI), respectively, before reaching 0 °C again for p(C10VImTFSI). Such a general decrease in Tg with an increasing alkyl chain length is indeed, expected, considering the increasing molecular volume of the repeating unit.40 More remarkable, however, is the increase in Tg from n = 8 to n = 10. To explain this increase, we may reconsider the amount of remaining monomer in the poly(ILs) as reported in Table S1, revealing a significantly lower value for n = 10 compared to n = 8. Since the monomer acts as plasticizer, the reduced amount in n = 10 may, thus, explainat least partiallythis increase of the glass transition temperature. D


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Figure 4. Comparison of the SAXS results for p(CnVIm-TFSI) poly(ILs) at ambient temperature (for clarity reasons, only three patterns are presented in panel (a)): (a) SAXS patterns (plotted on logarithmic scale) for p(CnVIm-TFSI) poly(ILs) with n = 2 (red), n = 4 (green), and n = 10 (blue); (b) the corresponding correlation distances plotted vs the alkyl chain length n.

Figure 5. Schematic illustration of the correlation distances d1, d2, and d3 related to the observed reflections in Figure 4, exemplified for the p(CnVIm-TFSI) poly(ILs) with n = 2 (left panel, in red) and n = 10 (right panel, in green). In the middle panel, the correlation distance d1 is plotted as a function of the alkyl chain length in comparison with the theoretical length of the pendant alkyl chain multiplied by a factor of 2, thus providing evidence whether the alkyl chains are or are not interdigitated.

and poly(ILs),32,37,53 this reflection may be attributed to the anion−anion correlation distance. In fact, considering that d1 increases, the minor increase of d2 may be related to slightly reduced Coulombic repulsions, i.e., a less dense arrangement of the anions within the ionic domain, while the decreasing intensity may be assigned to a dilution effect due to the substantial extension of the aliphatic domain, i.e., its overall volumetric ratio. The third reflection (q3) appears to be rather independent of the alkyl chain length and does not vary concerning its maximum intensity or its position, corresponding to a correlation distance d3 of ∼4.6 Å. Such value for the mean correlation distance was also found in previous studies on ILs54,55 and poly(ILs)32,37,53 and was ascribed to the pendantto-pendant correlation distance,32,37,53 i.e., the superimposition of two contributions: (i) the π−π stacking interaction between neighboring imidazolium rings and (ii) the van der Waals interaction between the alkyl chains. On the basis of the peak variations and in consistency with earlier reported findings for similar compounds, we attribute the observed three reflections, as schematically depicted in Figure 5, exemplified for n = 2 (left panel) and 10 (right panel). When comparing the experimentally determined correlation distances d1 with the theoretical length of the alkyl chains (multiplied by a factor of 2 in order to take into account the two neighboring polymer chains; see the middle panel), it appears, moreover, that the pendant alkyl chains are increasingly interdigitating for increasing n, starting from n =

length can be usefully analyzed in greater detail to gain further insight into the structural organization of these polymers. First, we notice that the position of the small-angle peak (q1) is highly dependent on the length of the alkyl chain, in accordance with earlier studies on nanoscale-segregated ammonium-46 and imidazolium-based ILs.47−49 For increasing n, the peak position is continuously shifting toward lower q values. Transforming the q1 values in the corresponding correlations distances d1 using Bragg’s law reveals a linear increase in Bragg spacing from 13.1 to 23.3 Å for n = 2 and 10, respectively (Figure 4b). Moreover, the small-angle peak simultaneously sharpens and gains in intensity (Figure 4a). It appears noteworthy at this point that such a sharpening and intensity increase were reported also for ILs with sufficiently long alkyl chains and assigned to the formation of oily domains, which are embedded in a charged three-dimensional network, thus forming a spongelike bicontinuous nanostructure50−52 with an increasing electron density contrast between these two domains.37,53 However, in contrast to ILs, poly(ILs) reveal such nanoscale structural heterogeneities also for n = 2 and generally a relatively more pronounced small-angle peak (q1) for all n,32,41,52 indicating the absence of a shell-to-shell local structure even for small n.49,51 Compared to the small-angle peak q1, the medium-angle peak (q2) is shifting only slightly to lower q values for increasing n; i.e., d2 increases from ca. 6.9 to 7.6 Å for n = 2 and 10, respectively, while experiencing a simultaneous decrease of its intensity. Following previous studies on ILs46 E


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Figure 6. SAXS patterns recorded for p(C2VIm-TFSI) (a) and p(C10VIm-TFSI) (c) for varying temperatures (30−232 °C; both plotted on logarithmic scale) and the corresponding development for the resulting correlation distances ((b) and (d) for n = 2 and 10, respectively).

increasing temperatures (from 22.33 to 20.89 Å), while d2 remained constant and d3 increases (from 4.36 to 4.68 Å; see Figure 6d), revealing an anisotropic dilatation for n = 10 and volume decrease perpendicularly to the polymer backbone. In an intermediate important conclusion, these results provide a well-grounded explanation for the divergent density development shown in Figure 3, confirming the increasing importance of the van der Waals interactions for sufficiently long pendant alkyl chains, i.e., n > 4. 3.3. Relationship between Nanostructure, Viscosity, and Ionic Conductivity. One of the most important physicochemical properties to be investigated when studying electrolyte systems is certainly the viscosity, as it has a direct impact on the charge transport. Therefore, in a next step, we carried out rheological measurements on the p(CnVIm-TFSI) poly(ILs) as a function of shear rate and temperature. The variation of the shear rate while keeping the temperature constant (exemplarily plotted for p(C6VIm-TFSI) at 90 °C in Figure S4) revealed an initial non-Newtonian, shear-thinning behavior at low shear rates, illustrating the cohesive nature of the polymer; i.e., it is prone to undergo a shear-induced alignment and/or slipping behavior. The increasing ionic conductivity when applying a shear stress (not presented herein) is in favor of such a shear-flow alignment. For sufficiently high shear rates, the viscosity then appears to remain constant, suggesting an apparent Newtonian behavior. Interestingly, when plotting the shear rate vs the shear stress, it appears that these poly(ILs) are characterized by a certain threshold stress, indicating a Bingham-like viscositya result which we observed particularly for poly(ILs) bearing longer

4 onward. For n = 2, however, the correlation distance d1 is larger than twice the theoretical alkyl chain length, indicating that in this case the Coulombic repulsion between the ionic domains is too large to allow for the interdigitation of such short pendant alkyl chains. This latter result is in good agreement with the relatively lower intensity of the small-angle reflection (q1) for n = 2 (Figure 4a) due to the less sharp electron density contrast in this case. Furthermore, these results shed some light on the divergent density behavior for varying temperatures (Figure 3), since the impact of the van der Waals interactions is becoming increasingly important for increasingly interdigitating alkyl chains. Nevertheless, for establishing a direct link between the density measurements and the samples’ nanostructure, the development of the latter as a function of temperature has to be investigated. These results are presented in Figure 6; once again for clarity reasons, exemplified for n = 2 (Figure 6a,b) and n = 10 (Figure 6c,d). We may briefly note at this point that we did not observe any path-dependent hysteresis for the recorded data; i.e., the structural changes appeared fully reversible. In fact, the overall shape of the SAXS patterns did not change for different temperaturesneither for p(C2VIm-TFSI) nor for p(C10VIm-TFSI). In the case of n = 2 we observe that all intensity maxima shifted linearly to lower q values when increasing the temperature, i.e., larger correlation distances. This observation is well expected and assigned to the dilatation of the polymer, caused by the increasing motion of the macromolecule in its entirety and the polymer backbone in particular (see Figure 6b). Differently and rather unexpectedly, the small-angle peak (q1) of p(C10VIm-TFSI) shows the opposite trend; i.e., the correlation distance d1 is decreasing for F


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Figure 7. Viscosity of the p(CnVIm-TFSI) poly(ILs) as a function of temperature ((a); fitting was performed using either the Arrhenius equation (for n = 2) or the VFT equation (n = 4, 6, 8, and 10)) and alkyl chain length n (b), studied for a temperature range from 30 to 90 °C.

Figure 8. Ionic conductivity of the p(CnVIm-TFSI) poly(ILs) as a function of temperature ((a); fitting was performed using the VFT equation) and alkyl chain length n (b), studied for a temperature range from 30 to 90 °C.

alkyl chains (n ≥ 4). Though at first sight rather fundamental, this finding is rather important for the targeted application as solid electrolyte and, simultaneously, separating membrane in electrochemical energy storage devices, since a flowing polymer provides the severe risk of short circuits of the two separated electrodes, thus leading to a rapid heat evolution and, finally, a thermal runaway of the cell.5 However, a more detailed investigation will be required to further analyze this Bingham plastic behavior, including a careful determination and evaluation of the temperature-dependent viscous and elastic modulus as a function of frequency,56 which is beyond the scope of this study. Nonetheless, these preliminary findings highlight the cohesive behavior of these p(CnVIm-TFSI) polymers. When further analyzing the temperature dependency of the viscosity η, generally, all samples reveal a decreasing viscosity with an increasing temperature (Figure 7a; the absolute viscosity values for T = 30 °C are provided in Table S2). Nonetheless, only the results obtained for n = 2 can be well fitted using the Arrhenius equation (1) for correlating the temperature dependency of η, including the change in slope at the glass transition temperature of about 49 °C as determined by DSC. ⎡ −E ⎤ ⎡ B⎤ η = η0 exp⎢ a ⎥ = η0 exp⎢ ⎥ ⎣T ⎦ ⎣ RT ⎦

obtained for n = 4−10 using the VFT equation revealed a very good agreement between the calculated and the experimentally determined results.57 ⎡ B ⎤ η = η0 exp⎢ ⎥ ⎣ T − T0 ⎦

(2)

for which T is the absolute temperature, η0 is the viscosity at infinite temperature, B is a constant, and T0 is the Vogel temperature, i.e., the temperature at which the viscosity is infinite. All fitting parameters are summarized in Table S3. Plotting the viscosity results as a function of the alkyl chain length n (Figure 7b) highlights that the order of the p(CnVImTSI) homopolymers isindependent of the temperaturethe following: 2 > 4 > 10 > 8 > 6, i.e., not linearly correlated to the length of the alkyl chain. Given a certain temperature, the viscosity, in fact, decreases from n = 2 to n = 6, before slightly increasing again from n = 6 to n = 10, following a roughly parabolic shape. Such an increase in viscosity with an increasing pendant alkyl chain length has been observed also for ammonium-45 and imidazolium-based42 ILs and was assigned to the increasing inductive contribution of the van der Waals interactionsjust as confirmed also by our previous density and SAXS results. It appears that an increase from n = 2 to n = 4 and 6 results in decreased electrostatic interactions, while for sufficiently long alkyl chains (n > 6), the van der Waals forces between the interdigitated aliphatic chains become substantially larger and dominate the physicochemical properties. The determination of the ionic conductivity by means of complex electrochemical impedance spectroscopy (EIS) revealsas expectedthe opposite trend; i.e., the conductivity increases along with the temperature, showing a VFT-type

(1)

We may note that the temperature-dependent viscosities above the Tg are commonly fitted using the Vogel−Fulcher− Tammann (VFT) equation (2),31 though in the present case the Arrhenius fit showed the better agreement with the experimental data. We may, at this point, assume that this might be linked to the relatively limited number of data points above the Tg. As a matter of fact, the fitting of the viscosity data G


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⎡ − B′ ⎤ σ0 exp⎢ ⎥ T ⎣ T − T0′ ⎦

(3)

for which σ0 is the fitting parameter, B′ is a factor related to the activation energy, and T0′ is the ideal glass transition temperature. All fitting parameters are summarized in Table S3. Overall, the conductivity increases for all poly(ILs) by about 2 orders of magnitude, when increasing the temperature from 30 to 90 °C. Moreover, the conductivity data obtained for T = 30 °C nicely resemble the trend observed for the viscosity; i.e., it increases from n = 4 to n = 6 and subsequently decreases slightly again for n = 8 and 10 (see also Figure 8b). However, for elevated temperatures, the conductivity increase is significantly more pronounced for n = 10 and substantially more pronounced for n = 4, compared to n = 8 and 6, even though it remains to be the highest for all temperatures for the latter one. These results clearly show that the ion transport in these poly(ILs) is not solely determined by the viscosity, i.e., the mobility of the ionic species, which is directly related to the segmental motion of the polymer backbone. In fact, while the more pronounced increase in conductivity for n = 4 may be explained by the relatively higher charge carrier density compared to n = 6, 8, and 10, the more pronounced increase for n = 10 compared to n = 6 and 8 appears remarkable. Sala-de la Cruz et al.32 have recently demonstrated that, in addition to the segmental motion (i.e., the viscosity) and the charge carrier concentration, the backbone-to-backbone correlation distance affects the ionic conductivity. According to their findings, the ionic conductivity decreases exponentially with increasing backbone-to-backbone distances, since this hampers the intramolecular anion hopping/dissociation. This is as a matter of fact in excellent agreement with the development of the structural organization of these poly(ILs) with temperature (Figure 6), showing a decrease of the backbone-to-backbone correlation distance with temperature for n = 10, additionally contributing to a further enhanced conductivity for p(C10VImTSI) at elevated temperatures. Even though the temperaturedependent SAXS results for n = 4, 6, and 8 are not discussed and analyzed herein, we may underline at this point that this trend is certainly more pronounced for longer alkyl chains with stronger van der Waals interactions than for shorter ones. Indeed, the conductivity data and the different slopes for n = 10 compared to n = 6 and 8 indicate that this additional effect is more pronounced for n = 10, while n = 6 and 8 reveal a similar increase with temperature. In order to further investigate the underlying contributions to the recorded conductivity, the Walden rule (Λη = constant) was applied, based on the assumption that ionic conductivity is strongly coupled to viscosity,59 i.e., the local segmental relaxation in the case of polymer electrolytes.33,34,60,61 For this purpose, the molar conductivity (Λ) was calculated using eq 4 and plotted vs the fluidity, i.e., the reciprocal value of the viscosity (Figure 9). Λ = σVe

Figure 9. Walden plot (molar conductivity vs fluidity) for the different p(CnVIm-TFSI) poly(ILs); the “ideal” Walden line (in black) was obtained for a 10 mM aqueous solution of LiCl.

a diluted aqueous solution of LiCl, separating the Walden plot into a superionic and a subionic regime (above and below the “ideal” line, respectively). Independent of the alkyl chain length n, it is observed that all poly(ILs), having a comparable Mw, reveal a slope different from 1, i.e., obey the fractional Walden rule (Ληα = constant; with 0 < α < 1, being constant)62 rather than the classic one. More strikingly, however, all p(CnVImTFSI) poly(ILs) show a superionic behavior, even for n = 2, confirming the previously discussed findings that the ion conduction is partially decoupled from the structural dynamics of the polymer, i.e., the macroscopic viscosity and glass transition temperatureboth varying substantially with the alkyl chain length, which is not reflected in the superionic behaviormeaning that the conductivity has weaker temperature dependence than the segmental relaxation time. As this superionic behavior is apparently independent of n and, thus, best illustrated for n = 2, for which the glass transition temperature is the highest, we may assign it to the frustration in polymer packing related to the bulky imidazolium cation, closely bonded to the polymer backbone, by this allowing for ion diffusion through the polymer matrix even when the segmental dynamics slow down.63 Such packing frustration favored superionic behavior is in excellent agreement with recent studies of Sokolov and co-workers34,35,64,65 on poly(ILs) and rigid, sterically demanding polymers, using inter alia broadband dielectric spectroscopy, dynamic mechanical spectroscopy, and depolarized dynamic light scattering, i.e., complementary experimental techniques, and in clear contrast to the ion conduction mechanism of standard polymer electrolytes like poly(ethylene oxide)60,61 or monomeric ionic liquids.42,66 We may finally note that indeed, for polymer electrolytes, the molar conductivity is usually plotted versus the local relaxation rate rather than the reciprocal value of the macroscopic viscosity.67 Nevertheless, Fan et al.65 have very recently shown that in the case of poly(ILs) the traditional Walden plot reveals highly similar results, indicating that both plots may be used in this case. 3.4. Impact of Incorporating a Lithium Salt, LiTFSI. While the understanding of the structure-to-transport interplay for the pure p(CnVIm-TFSI) poly(ILs) is of fundamental significance and importance for any further application of these SPEs, the utilization as lithium battery electrolyte requires the addition of a lithium salt, i.e., the introduction of a mobile lithium cation in the system. Thus, we prepared blends of p(CnVIm-TFSI) incorporating lithium bis(trifluoromethane)-

(4)

with σ being the ionic conductivity and Ve corresponding to the polymer molar volume (Ve = M/ρ). The calculated molar conductivities for all p(CnVIm-TFSI) poly(ILs) at 30 °C are provided in Table S2. The “ideal” Walden line was obtained for H


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Figure 10. Comparison of the SAXS patterns for (a) p(C2VIm-TFSI) and (b) p(C10VIm-TFSI) with (red patterns) and without (black patterns) the addition of LiTFSI.

sulfonimide (LiTFSI) with a constant cation ratio of [Im+]: [Li+] = 3:1 and studied the impact on the beforehand investigated physicochemical, electrochemical, and structural properties. For the latter, we performed a comparative SAXS analysis of the p(CnVIm-TFSI) poly(ILs) with LiTFSI and without LiTFSI. Generally, no substantial modification of the structural organization was observed for any of the samples. The results obtained for n = 2 and n = 10 as the end members of our experimental series are presented in Figure 10. In the case of p(C2VIm-TFSI), a slight shift of the intensity maximum to lower q values is observed for the small-angle reflection (q1), i.e., from 0.46 to 0.43 Å−1, which corresponds to an increase of the backbone-to-backbone correlation distance d1 from 13.6 to 14.6 Å (Figure 10a). Such an increase of the ionic domain size is well expected when introducing an additional ionic species as a result of the increasing Coulombic repulsion. Accordingly, the intensity of the following two reflections (q2 and q3) increases due to the relatively larger number of scattering entities and the increasing contrast for the ionic and aliphatic interactions, while no significant shift of the intensity maxima was observed. For p(C10VIm-TFSI), on the contrary, no change was detected for the small-angle peak (q1; Figure 10b), confirming that the backbone-to-backbone correlation distance d1 is dominated by van der Waals rather than Coulombic interactions for sufficiently long pendant alkyl chains. Besides, also for n = 10, a slight increase in intensity is recorded for the second and third reflection (q2 and q3), assigned to the relatively higher concentration of ionic species. More interestingly, however, the intensity maximum of the medium-angle peak (q 2 ), related to the anion−anion correlation distance d2, is slightly shifted to higher q values, suggesting that the constant size of the ionic domain (i.e., d1 = constant) leads to a relatively higher ion concentration within the ionic domains and, hence, to shorter anion−anion correlation distances. These results are in good agreement with the modified glass transition temperatures when adding LiTFSI (Figure 11). As a matter of fact, it is decreasing for n = 2 from about 49 to 29.8 °C upon the incorporation of the lithium salt as a result of the increase in free volume, indicated by the increasing backboneto-backbone correlation distance. For all the other samples, however, the glass transition temperature is, as commonly expected,68 slightly increasing, which is assigned to the formation of an increased ionic interaction between the polymer chains, originating from the increase in charge

Figure 11. Glass transition temperature Tg as a function of the alkyl chain length for the different p(CnVIm-TFSI) poly(IL) homopolymers incorporating LiTFSI.

concentration in the ionic domain, for which the correlation distance remains constant. Besides, the general overall trend as depicted earlier in Figure 2 is observed, i.e., a slight increase from n = 8 to n = 10, which we may, as well, assign to the presence of less unreacted monomer in the case of n = 10 (Table S1). Prior to the eventual investigation of the influence of the introduced LiTFSI on the ionic conductivity, in a next step, its effect on the viscosity was studied (Figure 12). For ILs, usually, an increase in viscosity is observed when adding a lithium salt.9,69 For p(CnVIm-TFSI), however, this trend is reversed. While generally maintaining a non-Newtonian VFT-type behavior (Figure 12a; see Table S4 for the fitting parameters), Bingham fluid character (not shown here), and the overall trend for an increasing alkyl chain length (decreasing from n = 2 over n = 4 to n = 6 and subsequently slightly increasing again for n = 8 and 10; Figure 12b; as also in accordance with the trend for the DSC data; see Figure 11), overall, the viscosity decreases after the incorporation of LiTFSIthough not in the same manner for all n. At 60 °C, for instance, the viscosity of pure p(C2VIm-TFSI) is 7.5 × 107 Pa s, while it is 6.3 × 103 Pa s after blending with the lithium salt; i.e., a decrease by about 4 orders of magnitude. For p(C10VImTFSI) it decreases from 1.4 × 103 to 4.6 × 102 Pa s, i.e., by only slightly more than 1 order of magnitude. In light of the earlier discussed SAXS results, this increased plasticizing effect for short alkyl chain lengths may be rationalized by the increasing backbone-to-backbone correlation distance and, thus, weakened interactions between the polymer backbone bundles. I


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Figure 12. (a) Viscosity of the p(CnVIm-TFSI)/LiTFSI blended poly(ILs) ([Im+]:[Li+] = 3:1) as a function of temperature (30−90 °C; the fitting was performed using the VFT equation) and (b) in a logarithmic plot as a function of the alkyl chain length n.

Figure 13. (a) Ionic conductivity of the p(CnVIm-TFSI)/LiTFSI blended poly(ILs) ([Im+]:[Li+] = 3:1) as a function of temperature (30−90 °C; fitting was performed using the VFT equation) and (b) in logarithmic scale as a function of the alkyl chain length n.

networks p(CnVIm-TSI)cl were synthesized, providing higher mechanical stability and resistivity compared to the previously discussed homopolymers, while at the same time allowing for the incorporation of e.g. ILs to further enhance the ionic conductivity without losing their mechanical properties.70 A complete characterization analogously to the homopolymers would exceed the scope of this paper. Nonetheless, we may highlight briefly that the overall structural organization and the trend for varying n (i.e., the decrease in intensity of q1, accompanied by a shift to larger q values, and an increase in intensity of q2 and q3 for decreasing n) is well preserved (see the corresponding SAXS patterns in Figure S5). Considering the great impact of the structural organization on the charge transport mechanism, we may thus anticipate that the previously discussed findings of a partially decoupled ionic conductivity are presumably applicable also for the cross-linked samples, which is of great significance for the potential application of poly(ILs) in practical electrochemical energy storage devices.

Following the viscosity results, the temperature-dependent ionic conductivities also show a VFT-type behavior (the fitting parameters are summarized in Table S4), with p(C6VIm-TFSI) having the highest ionic conductivity across the whole studied temperature range (Figure 13a). Surprisingly, however, p(C2VIm-TFSI) reveals an ionic conductivity higher than expected considering the viscosity trend (Figure 13b), but reflecting the substantial reduction of the Tg, while Li-doped p(C10VIm-TFSI) shows a slightly lower conductivity than p(C4VIm-TFSI), though its viscosity and Tg are lower (see Figure 12a). This result is, moreover, in contrast to the relatively higher conductivity for the lithium-free samples (see Figure 8b). In fact, even though the conductivity is generally increasing for all poly(ILs) after adding LiTFSI, we observe a slight decrease for n = 10. We may assume at this point that, once more, the different modification of the structural organization for varying alkyl chain lengths plays a decisive role for the ion transport. In this case, the increase in ion “packing” for Li-doped p(C10VIm-TFSI) as a result of the constant ionic domain size, presumably, leads to a relatively increased ion pair association and aggregation, i.e., a reduced ion mobility, being in accordance with the comparatively lowest increase of the glass transition temperature upon adding LiTFSI (2.3 °C compared to 8.7, 12.7, and 19.3 °C for n = 4, 6, and 8, respectively). On the contrary, the expansion of the ionic domain in the case of Li-doped p(C2VIm-TFSI), i.e., a gain in free volume, facilitates the ion mobility, resulting in an increase in ionic conductivity, higher than expected from the viscosity data only. 3.5. Effect of Cross-Linking the P(CnVIm-TSI) Homopolymers. Finally, once again with respect to the desired application as lithium battery electrolytes, cross-linked polymer

4. CONCLUSIONS A continuous series of p(CnVIm-TSI) homopolymers with n = 2, 4, 6, 8, and 10 were synthesized and thoroughly characterized. Their physicochemical properties were scrutinized with a particular focus on the impact of these characteristics like inter alia the density and viscosity on the ionic conductivity as well as the interplay of structure and ion mobility. The results reveal that the charge transport in p(CnVIm-TFSI) poly(ILs) is not only determined by the segmental motion of the polymer backbone, i.e., the glass transition temperature, and the charge carrier density, but moreover by the self-assembling nanostructure of such J


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Macromolecules

LPPI and Ms. Marlou Keller, respectively, for performing DSC measurements.

poly(ILs), specifically, the backbone-to-backbone correlation distance. This nanostructure, in turn, is determined by the sum of Coulombic repulsion and van der Waals interactions and, hence, depending on the pendant alkyl chain. Based on these insights into the structure-to-transport interplay, the effect of incorporating a lithium salt like LiTFSI was studied, and once more, the obtained results reveal a great impact of the nanostructure on the ion conduction mechanism. While, on the one hand, these results show that ion transport in poly(ILs) is determined by a complex interplay of physicochemical properties, charge carrier concentration, and nanostructure, they may, on the other hand, pave the way for successfully decoupling charge transport and segmental motion in polymer electrolyte systems. We may thus anticipate that the herein reported results will help to understand also the charge transport in related self-assembling ionic (cross-linked) polymers and, eventually, allow for the realization of safer electrolytes with enhanced ionic conductivities at ambient temperature.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00197. ATR-FTIR and 1H NMR spectra as well as TGA thermograms for the characterization of the purified poly(ILs), rheological behavior at a constant shear rate at 90 °C for p(C6VIm-TFSI), SAXS patterns for crosslinked poly(ILs) at ambient temperature, summary of the physicochemical characteristics and remaining monomer, fitting parameters for the viscosity and conductivity data for the pure poly(ILs) and the lithium-doped ones (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (D.B.). *E-mail [email protected] (L.P.). ORCID

Dominic Bresser: 0000-0001-6429-6048 Sandrine Lyonnard: 0000-0003-2580-8439 Present Address

D.B.: Helmholtz Institute Ulm (HIU), 89081 Ulm & Karlsruhe Institute of Technology (KIT), 76021 Karlsruhe, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the NanoSciences Programme (CEA) within the framework of the ELLIPSE project. D.B. acknowledges the EU/CEA Enhanced Eurotalents Fellowship for financial support. In addition, the authors thank Dr. Arnaud de Geyer and Dr. Alain Farchi as well as Dr. Cyrille Rochas for setting up the SAXS experiments at CEA-INAC and at the ESRF (BM02-D2AM), respectively. Generally, the authors acknowledge the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, for granting beamtime to perform part of the herein reported SAXS experiments (proposal no. 0201-854). Finally, V.D. and D.B. gratefully acknowledge the K


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Unveiling the Ion Conduction Mechanism in Imidazolium-Based Poly

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