Polymerized Ionic Liquids: Correlation of Ionic ... - ACS Publications

Aug 16, 2017 - Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United ... Dep...
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Polymerized Ionic Liquids: Correlation of Ionic Conductivity with Nanoscale Morphology and Counterion Volume Ciprian Iacob,† Atsushi Matsumoto,‡ Marissa Brennan,† Hongjun Liu,§ Stephen J. Paddison,§ Osamu Urakawa,‡ Tadashi Inoue,‡ Joshua Sangoro,*,§ and James Runt*,† †

Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ Department of Macromolecular Science, Osaka University, Toyonaka, Osaka 560-0043, Japan § Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *

ABSTRACT: The impact of the chemical structure on ion transport, nanoscale morphology, and dynamics in polymerized imidazolium-based ionic liquids is investigated by broadband dielectric spectroscopy and Xray scattering, complemented with atomistic molecular dynamics simulations. Anion volume is found to correlate strongly with Tgindependent ionic conductivities spanning more than 3 orders of magnitude. In addition, a systematic increase in alkyl side chain length results in about one decade decrease in Tg-independent ionic conductivity correlating with an increase in the characteristic backbone-to-backbone distances found from scattering and simulations. The quantitative comparison between ion sizes, morphology, and ionic conductivity underscores the need for polymerized ionic liquids with small counterions and short alkyl side chain length in order to obtain polymer electrolytes with higher ionic conductivity.

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scale structure. They observed that the Tg-independent ionic conductivity decreases by about 1 order of magnitude when the alkyl chain length is varied from ethyl to octyl and proposed a model relating the Tg-independent ionic conductivity to backbone-to-backbone spacing.30 In a related study, Evans et al.31 investigated the influence of positioning the charged species either along the polyIL backbone or in pendant groups, and observed that “backbone” polyILs result in a nearly 10-fold increase in the Tg-independent ionic conductivity. This presumably arises from differences in morphology accompanying the placement of the charged groups in the polymer backbone.31 Although these studies highlight the impact of polymer morphology on ionic conduction, the resultant order of magnitude increase in the Tg-independent ionic conductivity cannot adequately explain the experimentally observed disparity between molecular and polyILs. What other molecular parameters control the Tg-independent ionic conductivity? In the current investigation, broadband dielectric spectroscopy and wide-angle X-ray scattering, along with atomistic molecular dynamics (MD) simulations, are employed to probe the impact of molecular structure and nanoscale morphology on Tg-independent ionic conductivity in a series of imidazolium

olymerized ionic liquids (polyILs) combine the attractive mechanical characteristics of polymers with the unique physicochemical properties of low molecular weight ionic liquids (ILs).1−15 These properties make polyILs appealing candidates for various applications, such as electrolytes in electrochemical devices including lithium ion batteries, supercapacitors, fuel cells, and dye-sensitized solar cells.16−18 High ionic conductivity coupled with robust mechanical properties is one of the main requirements of electrolytes for these applications. As a consequence, concerted efforts have focused on establishing the link between the molecular structure, morphology, and ionic transport in polyILs, with the aim of developing new polymer electrolytes with high ionic conductivities.19−29 One approach is to lower the glass transition temperatures (Tg) through lower molecular weights or inclusion of flexible functional groups such as silanes.21 Such polyILs usually have relatively high ionic conductivity but poor mechanical properties at ambient conditions. As high mechanical modulus is typically achieved below Tg it is therefore of interest to develop strategies to increase ionic conductivity independent of Tg. In a recent study, Salas de la Cruz et al.30 probed the X-ray scattering profiles and ionic conductivity of poly(1-alkyl-3-vinylimidazolium)-based ionic liquids as a function of the alkyl chain length (and for two different counteranions). They concluded that three characteristic length scales (associated with backbone-to-backbone, ionto-ion and pendant-to-pendant spacings) describe the nano© XXXX American Chemical Society

Received: May 2, 2017 Accepted: August 14, 2017

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ACS Macro Letters Scheme 1. Synthesis of Imidazolium Polymerized Ionic Liquids with Various Alkyl Substituent Lengths and Anions

polycations having different alkyl side chain lengths and counteranions. It is found that while the variation of the alkyl chain length results in systematic changes in the backbone-tobackbone spacing and up to 1 order of magnitude difference in the Tg-independent ionic conductivity, the sizes of the anions have a more dramatic effect and result in more than three orders of magnitude difference in the Tg-independent ionic conductivity for the same set of imidazolium-based polymerized ILs. These findings highlight the need for polymerized ionic liquids with small counterions and shorter alkyl side chain lengths in order to obtain materials with higher ionic conductivity. The polyILs [poly(1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide) (PC4VITFSI), poly(1-butyl3-vinylimidazolium trifluoromethansulfonate) (PC4VITfO), poly(1-butyl-3-vinylimidazolium tetrafluoroborate)

Figure 1. Temperature dependence of segmental relaxation times τ0 obtained from rheological measurements, dielectric relaxation time τσ for conduction and dc conductivity from dielectric spectroscopy for polyILs with PC4VI cations with different anions, TfO− and NfO−. Glass transition temperatures from dielectric measurements (TBDS g ) are determined at the crossover temperature from VFT to Arrhenius thermal activation.

Figure 3. X-ray scattering profiles for (a) PCnTFSI, where n = 2−6, and (b) PC4VI X−, where X− = BF4−, TfO−, NfO−, TFSI−, PFSI−, and HFSI−. db, dp, and di correspond to the mean distances from backboneto-backbone, pendant-to-pendant, and anion-to-anion. Data are shifted vertically for clarity.

Figure 2. dc conductivity as a function of Tg/T for (a) poly(1-n-alkyl3-vinylimidazolium TFSI), where n = 2−6, and (b) poly(1-butyl-3vinylimidazolium-X−), where X− = BF4−, TfO−, NfO−, PFSI−, and HFSI−. 942

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Figure 4. (a) Simulated X-ray scattering profiles of PCnVI TFSI (n = 2, 4 and 6) as a function of alkyl side chain length, compared with experimental results. 3D tortuous contour surface (white colored) marks the ion transport channels (including all polar parts): see SI, section 3, for additional details. Space-filling spheres in orthographic viewpoint are color-coded: anions (red), cationic imidazolium rings (yellow), and alkyl tails are represented by cyan segmented lines. All H atoms are omitted for clarity. Increasing apolar domain size poses more restrictions to the interconnectivity of the polar network, thereby lowering the ionic conductivity. (b) Left: Correlation lengths db, dp, and di for the polyILs with various side chains and counterions as indicated. Right: dc conductivities at Tg as a function of backbone-to-backbone correlation distances. A similar trend is also observed for the concentration-normalized dc ionic conductivity (see Figure S4). An exponential correlation is found between dc conductivity and d2b for varying alkyl side chain lengths and d2p for the series of anions (see Figure S4).

(PC 4 VIBF 4 ), poly(1-butyl-3-vinylimidazolium bis(pentafluoroethanesulfonyl)imide) (PC4 VIPFSI), poly(1butyl-3-vinylimidazolium bis(heptafluoropropanesulfonyl)imide) (PC4VIHFSI), poly(1-butyl-3-vinylimidazolium nonafluorobuthanesulfonate) (PC4VINfO), and (PCnVITFSI) (n = 2, 3, 4, and 5)] investigated in this work were synthesized according to the procedure illustrated in Scheme 130 and the method described by Marcilla et al.32,33 For PC6VITFSI we used ethyl acetate (in the first step of the reaction) and dimethyformamide (in the second step) as solvents instead of methanol and water, respectively. Finally, the counterion conversion was conducted using methanol. Salient synthetic details are provided in section 1 of the accompanying SI.

Broadband dielectric spectroscopy was employed to characterize ionic conduction in these materials and the spectra were analyzed within the framework of different formalisms, including the complex dielectric, conductivity and modulus functions (see Figure S3 for more details). The dielectric spectra provide information about the dc ionic conductivity σ0 as well as the conductivity relaxation time τσ. The temperature dependence of these key ion transport quantities is shown in Figure 1 for PC4VI cations with two different anions, TfO− and NfO− (PC4VITfO and PC4VINfO). Both τσ and σ0 exhibit Vogel−Fulcher−Tammann (VFT) temperature dependence at higher temperatures (above the calorimetric Tg), described by τ = τ0 exp[BT0/(T − T0)] where τ0, B, and T0 are fitting 943

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group correlation length (pendant-to-pendant, qp), which varies with anion volume. The intermediate peak (qi = 7.4−8.7 nm−1) is associated with the polar group correlation length (anion-toanion). The assignment of the three characteristic peaks is in accord with previous experimental and computational studies.30,38 The scattering profiles in Figure 3 (also see the corresponding spacings in Figures 4 and S4) demonstrate that variation of the alkyl pendant length has an important influence on backbone-to-backbone spacing while variation in counterion size appears to be most strongly reflected in pendant-to-pendant correlation lengths. As n increases from ethyl to hexyl (for PCnVI TFSI)), qb shifts from 4.75 to 3.43 nm−1. For the same series of polyILs, the side group correlation length remains constant at 0.46 nm, and the polar group correlation length (di ≈ 0.76 nm) increases slightly with n. Our atomistic simulations indicate a progressive change in the nanoscale organization with increasing alkyl chain length. Structural analysis from the MD simulations yields correlation lengths in quantitative agreement with the experimental X-ray data (Figure 4a). Representative configuration snapshots reveal a tortuous polar network and progressive growth of nonpolar domains with increasing alkyl chain length. With increasing alkyl pendant length, discrete apolar islands first form within the continuous polar network and then grow beyond the percolation threshold, finally intertwining with the polar domains into a bicontinuous nanostructure (see Figures S6 and S7).38 These complex 3D networks are especially important in the context of ionic conductivity, as the actual pathways for the mobile ions are governed by geometric constraint. The apolar islands, which grow with increasing backbone-to-backbone correlation lengths, can be viewed as obstacles in the anion diffusion pathways resulting in reduction of the ionic conductivities. Similar transition to percolated structures with increasing volume fraction of polar groups was also observed in computational studies (coarse-grained and fully atomistic molecular dynamics) of ionomers by Frischknecht and co-workers.39−41 Molar concentration-normalized ionic conductivities (at Tg/ T = 1) were calculated using the density values described in refs 19 and 42. Figure 5 displays the strong relationship between Tgindependent normalized molar conductivity and repeat unit molecular volumes. Variation of the anion molecular volume (directly related to Vm) leads to differences of somewhat more than 3 orders of magnitude in concentration-normalized conductivity, in contrast to a one decade change arising from variation of alkyl side chain lengths. The fact that the same imidazolium-based polyILs require two different relations linking ionic conductivity to the morphology and anion molecular volume highlights the need for a more effective approach to parametrize the morphology. The complex 3D network morphology and the related counterion volume are the principal factors controlling ionic conductivity in these polyILs. This study underscores the need for smaller counterions to obtain polymer electrolytes with higher ionic conductivity. In summary, the interplay between molecular structure, morphology and ionic conduction in a systematic series of poly(1-alkyl-3-vinylimidazolium)-based ionic liquids is investigated by employing broadband dielectric spectroscopy, X-ray scattering and atomistic molecular dynamics simulations. Tgindependent ionic conductivities are found to increase with decreasing counterion volume and alkyl pendant length. Variation of anion molecular volume is found to result in

Figure 5. Relationship between anion concentration-normalized dc conductivity at Tg/T = 1 and repeat unit molecular volume Vm (including the counterion) for imidazolium-based polyILs with various side chains and counterions as indicated. The dashed lines are to guide the eye.

parameters. In addition, the mechanical segmental relaxation times of the same polyILs also follow VFT temperature dependence as shown in Figure 1. Below Tg, τσ, and σ0 are observed to follow Arrhenius-type thermal activation.34 The transition temperature from VFT to Arrhenius behavior in the dielectric data is found to be in quantitative agreement with Tgs determined from differential scanning calorimetry, DSC (see Table S2 for DSC Tgs). Both polymer dynamics and counterion mobility contribute to ion transport in polyILs above Tg. The transition from VFT to Arrhenius thermal activation of the transport parameters results from the “freezing out” of the segmental dynamics at the relevant experimental time scales below Tg.34 Below Tg, ionic conductivity is dominated by contributions from hopping of the counterions within the glassy polymeric matrix. By plotting the ionic conductivity as a function of Tg/T, differences in polyIL Tg, and consequently polymer segmental dynamics, are suppressed. Figure 2 shows the T g -independent ionic conductivity for all polyILs investigated in the current work. For the same bis(trifluorosulfonyl)imide anion, the Tgindependent ionic conductivity decreases systematically with increasing the alkyl chain length from ethyl to hexyl, spanning a range of 1 order of magnitude at Tg/T = 1. This finding is consistent with a previous report by Salas de la Cruz et al.30 More importantly, systematic variation of the molecular volume Vm of the anions leads to differences in the Tg-independent conductivity of about 3 orders of magnitude after normalizing for anion molar concentration. The van der Waals volumes of the anions in this work increase in the order: BF4− < TfO− < TFSI ≅ NfO− < PFSI− < HFSI−, an inverse trend to the measured Tg-independent ionic conductivity (see Table S2). This behavior is in contrast to low molecular weight aprotic ionic liquids for which the Tg-independent ionic conductivity remains comparable across a wide variety of anions.35 We will return to the key influence of counterion volume later in the Letter. X-ray scattering profiles in Figure 3 display three peaks arising from the correlation lengths characterizing the morphology.30,36,37 These lengths are related to the scattering wave vector qx according to Bragg’s law (dx = 2π/qx). The lowest q scattering maxima (3.4−4.8 nm−1) reflect the main chain correlation length (backbone-to-backbone) and is observed to vary with alkyl pendant length yielding: db = (0.12 nm/CH2)n + d0, where d0 ≈ 1.08 nm and n = 2−6. The scattering peaks at highest q (12−15.3 nm−1) reflect the side 944

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differences in the Tg-independent molar conductivity spanning ∼3 orders of magnitude, in contrast to the one decade change from variation in alkyl chain side lengths. Atomistic MD simulations reveal a systematic change in the morphology from discrete apolar islands within a continuous polar network for shorter alkyl chains, to bicontinuous sponge-like nanostructures for longer alkyl groups.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00335. Materials, Methods (Thermal Analysis, Broadband Dielectric Spectroscopy, Molar Concentration-Normalized dc Ionic Conductivity, X-ray Scattering, Rheology), and Molecular Dynamics (MD) Simulations (Cluster Analysis of Polar and Nonpolar Domains; PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ciprian Iacob: 0000-0001-7982-0586 Hongjun Liu: 0000-0003-3326-2640 Stephen J. Paddison: 0000-0003-1064-8233 Joshua Sangoro: 0000-0002-5483-9528 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation, Division of Materials Research, Polymers Program through DMR-1505953. J.S. gratefully acknowledges the National Science Foundation for financial support through the Polymers Program Award DMR-1508394. S.J.P. gratefully acknowledges financial support of the U.S. Army Research Office under Contract No. W911NF-15-1-0501. Computing resource is provided through XSEDE allocation DMR130078. This work was partially supported by JSPS KAKENHI Grant Number 16H04204 and ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).

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