High Conductivity, High Strength Solid Electrolytes Formed by in Situ

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High Conductivity, High Strength Solid Electrolytes Formed by in situ Encapsulation of Ionic Liquids in Nanofibrillar Methyl Cellulose Networks Ramya Mantravadi, Parameswara Rao Chinnam, Dmitriy A Dikin, and Stephanie L. Wunder ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02903 • Publication Date (Web): 06 May 2016 Downloaded from http://pubs.acs.org on May 13, 2016

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High Conductivity, High Strength Solid Electrolytes Formed by in situ Encapsulation of Ionic Liquids in Nanofibrillar Methyl Cellulose Networks Ramya Mantravadi1, Parameswara Rao Chinnam1*, Dmitriy A. Dikin2 and Stephanie L. Wunder1* 1 Department of Chemistry and 2Department of Mechanical Engineering Temple University, Philadelphia, PA 19122 *[email protected] *[email protected] KEYWORDS: Energy Storage Materials; Ion Gels; Nanostructures; Ionic liquids; Renewable Polymers; Supercapacitors ABSTRACT: Strong, solid polymer electrolyte ion gels, with moduli in the MPa range, a capacitance of 2 µF/cm2 and high ambient ionic conductivities (> 1 x 10-3 S/cm), all at room temperature, have been prepared from butyl-N-methyl pyrrolidinium bis(trifluoromethylsulfonyl) imide (PYR14TFSI) and methyl cellulose (MC). These properties are particularly attractive for supercapacitor applications. The ion gels are prepared by co-dissolution of PYR14TFSI and MC in N,N-dimethyl formamide (DMF), which after heating and subsequent cooling form a gel. Evaporation of DMF leave thin, flexible, self-standing ion gels with up to 97 wt% PYR14TFSI, which have the highest combined moduli and ionic conductivity of ion gels to date, with an excellent electrochemical stability window (5.6 V). These favorable properties are attributed to the immiscibility of PYR14TFSI in MC, which permits the ionic conductivity to be independent of the MC at low MC content, and the formation in situ of a volume spanning network of semicrystalline MC nano-fibers, which have a high glass transition temperature (Tg = 190 oC) and remain crystalline until they degrade at 300 oC. INTRODUCTION Room temperature ionic liquids (RTILs) are salts or mixtures of salts composed of ionic species that are large and asymmetric with delocalized charge distributions, and thus have low melting temperatures1. They have been investigated as electrolytes in electrochemical devices2-5 such as solar and dye sensitized solar cells, lithium ion batteries, electrical double layer capacitors (EDLCs) and fuel cells2-5. This is because they are nonvolatile (and as a result often nonflammable) and therefore safer than conventional aprotic liquid electrolytes, and also have useful electrochemical stability windows (> 4V)6. However, they are typically more viscous (30-200 cP vs ~ 1 cP) than conventional aprotic or aqueous solvents, and therefore less conductive (0.1 to 18 1 ACS Paragon Plus Environment


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mS/cm). When used in battery applications, a lithium salt (LiX) with the same anion as in the RTIL is dissolved in the RTIL, which can decrease the ionic conductivity, and further increase the viscosity2. The many benefits of RTILs and their usefulness in device applications are greatly enhanced by their formation into solid films that are referred to as ion gels. Ion gels are ion conducting liquids (such as RTILs) immobilized in a polymer matrix, forming a solid polymer electrolyte (SPE). In this solid form RTILS have become increasingly important as components in polymer electrolytes7 for potential use as flexible solid-state electrochemical devices such as separators in lithium ion batteries8, dye sensitized solar cells9-10, electrochemical mechanical actuators and electrochromic windows (using pi-conjugated polymers)11, gate insulators in organic thin-film transistors (OTFTs)12-13 and membranes for fuel cells14. Immobilization of RTILs can be achieved using gelators that participate in chemical crosslinking reactions or by the formation of physical crosslinks. Chemical cross-linking is achieved using in situ polymerization of: (i) vinyl monomers with difunctional crosslinkers7, 15-17; (ii) triethoxysilylethyl-terminated, polydimethylsiloxane oligomers18; (iii) amine end-capped polyimide-poly(ethylene oxide) copolymers crosslinked with triisocyanates19; or by (iv) UV irradiation/crosslinking of polyethylene oxide (PEO)20. Physical gelation can be achieved using: (i) block copolymers, where the polar block (typically PEO) forms the conductive phase and solvates the ionic liquid and the hydrophobic block (often polystyrene) forms the structural phase21-22; (ii) semicrystalline homopolymers such as polyethylene oxide (PEO)8, 23, polyacrylonitrile (PAN)24, polyvinyl alcohol (PVA)24, microcellulose25, or poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HPF)26-28, where the crystalline component forms the crosslink sites and the amorphous component solvates the ionic liquid and can also form entanglements; (iii) silica nanoparticles, which form interconnected silica particulate networks2931 ; (iv) glycol-lipids, which form bilayer structures (fibrils or vesicles) in carbohydrate dissolving ionic liquids32-33. A hybrid method incorporates simultaneous phase separation and crosslinking, by polymerizing a mixture of styrene and divinylbenzene in a RTIL34-35. Since the gelator further decreases the ionic conductivity of the RTIL and only serves to ensure formation of the solid, a minimum amount of gelator is desirable. In general, chemical crosslinking requires more “nonconductive” polymer to achieve the same mechanical integrity that can be obtained from physical crosslinking using block copolymers. If the gelator is compatible with the RTIL, partial dissolution of the polymer in the RTIL will increase the viscosity and decrease the conductivity of the RTIL. Here MC is used as the gel former for the RTIL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (referred to here PYR14TFSI) (Figure 1), with which it is insoluble, to form tough, temperature stable ion gels. MC, a type of modified cellulose, belongs to a class of renewable, environmentally friendly, abundant and inexpensive natural polymers 2 ACS Paragon Plus Environment

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that are increasingly being considered as alternatives to synthetic polymers in electrochemical device applications. These natural polymers have been investigated as gel separators for electrochromic devices36, flexible paper electronics25, lithium ion batteries (LIBs)37-41 and Mg-air batteries42. Other biopolymers have been incorporated as binder materials (carboxymethyl cellulose) for cathodes43 and anodes44-46 in LIBs. PYR14TFSI was chosen since it has the highest ionic conductivity of the pyrrolidinium salts and has a wide stability window (about -3.0 C to + 2.5 V 47-48).

Methyl Cellulose (MC)

PYR14TFSI

DMF

Figure 1. Structures of methyl cellulose (MC), showing hydrogen bonding interactions, 1-butyl-1methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR14TFSI) and N,N-dimethyl formamide (DMF).

Although PYR14TFSI does not directly dissolve in MC, it can be incorporated by an entrapment mechanism, after co-dissolution in and subsequent removal of N,N -dimethylformamide (DMF). MC forms gels in DMF49-50 upon cooling and this gelation phenomena at low temperature (in contrast to the well-known thermos-reversible gelation of methylcellulose in water that occurs upon heating) also occurs in DMF in the presence of the PYR14TFSI, as shown schematically in 3 ACS Paragon Plus Environment


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Figure 2. Schematic of method to prepare ion gels. Separate solutions of MC and PYR14TFSI in DMF were mixed, stirred at 90oC and cooled to RT, where they formed a gel. After DMF removal, the PYR14TFSI was trapped in the MC nanofibril network. Pictures of PYR14TFSI/MC/DMF: (a) before gelation (90oC); (b) after gelation (RT); and (c) flexible ion gel after removal of DMF for PYR14TFSI/MC = 90/10 sample. SEM images of MC from PYR14TFSI/MC = 90/10 sample after removal of PYR14TFSI with acetonitrile (ACN) by (d) evaporation of ACN; and (e),(f) freeze drying of ACN, enabling observation of the collapsed nano-fibril network.

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Figure 2. The PYR14TFSI/MC remaining in the pores formed by the network of MC nano-fibrils after removal of DMF, form strong, self-supporting films that are no longer thermo-reversible and stay solid until ~ 300 o C, the onset of the decomposition of MC51. As expected, the elastic moduli increase and ionic conductivities decrease with MC content, but solid films can be formed with as little as 3 wt% of MC. Room temperature moduli > 1GPa are achieved for all compositions with < 60% PYR14TFSI and the PYR14TFSI/MC = 90/10 and 80/20 have reasonable RT moduli of 0.15 GPa and 0.75GPa, respectively. EXPERIMENTAL Chemicals

The 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (C11H20F6N2O4S2, MW 422.41, CAS Number 223437-11-4, was used as received (Sigma-Aldrich). This compound will be referred to here as PYR14TFSI. In the literature, 1-butyl-1-methylpyrrolidinium is also referred to as PYR14, P14 (where subscripts describe number of carbons in each alkyl substituent47), [BuMePy]+, [BMP], and [bmpyr]; and bis(trifluoromethylsulfonyl)imide is referred to as TFSI, TFSA, and [NTf2]. Methyl cellulose (MC) (Sigma-Aldrich, primary supplier DOW, METHOCEL A), average molecular weight of 86,000, 27.5-31.5 wt% methoxy and a degree of substitution of 1.6-1.9 mol methoxy per mol cellulose (as specified by manufacturer) was used as received after heating to 70 C in vacuum overnight. Their structures are shown in Figure 1. Characterization Differential scanning calorimetry (DSC) was obtained on a TA Instruments Hi-Res DSC 2920 at 100C /min under N2. Except as noted, samples were scanned from 25 oC to 100 oC, 100 oC to 100 oC, and -100 oC to 100 oC, with the second heating scans reported. The glass transition temperature, Tg, was taken as the midpoint of the heat capacity (Cp) versus temperature plots. Thermogravimetric analysis (TGA) data was obtained on a TA Instruments 2950 (TA Instruments, New Castle, DE), scanned from 25 to 800 oC at a rate of 10 oC/min under N2. Gel points were determined by heating samples to 90 oC in the Zetasizer Nano-ZS, cooling samples in predetermined decrements, and observing if they flowed. Wide angle x-ray scattering (WAXS) data was collected using a Bruker AXS D8 Discover X-ray diffractometer under N2 purge. Scanning electron microscopy (SEM) images were obtained with the Quanta 450F (FEI Co.) using secondary (SE) and backscatter (BS) detectors. “Bulk” material cross sections were prepared by the method of freeze-fracture in liquid nitrogen. Elemental composition was analyzed over the membrane cross section using energy dispersive X-ray spectrometer (EDS) (Oxford Aztec Energy Advanced SDD detector). Dynamic Mechanical Analysis (DMA) of the blends was obtained with a TA Instruments DMA Q800 under N2 purge. A pre-load force of 2.0 mN, a constant frequency of 1 Hz and a 15 µm amplitude were applied. Samples, ~ 12 mm 5 ACS Paragon Plus Environment


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length x 6.3 mm width x (0.01- 0.1mm) thickness, were equilibrated at -100 °C for 10 minutes. Measurements were taken from -100 °C to 250 °C with a heating rate of 2° C/min. Electrochemical Measurements: Ionic conductivities were measured by AC impedance spectroscopy using a Gamry potentiostat/galvanostat/ZRA (model interface 1000) in the frequency range from 0.01 to 1 MHz. Control of the equipment was through Gamry framework software and data was analyzed with Gamry Echem analysis software purchased from Gamry. Temperature dependent conductivities were obtained in a homemade electrochemical cell52 with 1 cm2 stainless steel blocking electrodes that was thermostatted in the oven of a gas chromatograph (GC), or using dry ice at below ambient temperatures. The electrochemical cell was placed in the oven of a GC and annealed overnight at 90oC. Conductivity measurements were made on the cooling cycle and heating cycles (the resistances on the heating and cooling cycles were very close) and the heating cycles are reported. At each temperature above RT, the sample was equilibrated for about 30 minutes. Conductivities, σ (mS/cm), were obtained using σ = (l /A)•(1/R), where l is the separator thickness in cm, A is the separator cross-sectional area in cm2 and R is the bulk resistance in mΩ. The electrochemical stability window of the 90/10 PYR14TFSI/MC composition was measured at 25 oC by linear sweep voltammetry using stainless steel as the working electrode and stainless steel as the counter/reference electrode, with the voltage swept between -4 to 5V at 5 mV/sec. Sample Preparation Sample preparation and storage were in a N2 or argon purged MBraun glove box. PYR14TFSI is not soluble in MC in the temperature range tested, room temperature (RT) to 90 oC. However, PYR14TFSI and MC are each soluble in DMF, and were dissolved separately in DMF at RT. The PYR14TFSI dissolves immediately, but the MC takes ~1 h to dissolve. After complete dissolution of both the MC and PYR14TFSI, the two solutions were mixed in appropriate ratios. In all cases, the MC was a 1wt/vol% solution in DMF, and only the PYR14TFSI was varied. For example, to prepare the 50/50 wt/wt PYR14TFSI/MC composition, the PYR14TFSI solution (50 mg PYR14TFSI in 0.5 mL of DMF) was added to the MC solution (50 mg in 4.5 mL of DMF). The DMF/PYR14TFSI/MC solution was left stirring overnight at RT, heated to 90 oC for 45 min and cooled to RT. The phase transition temperatures (where gelation occurred) cannot be monitored by UV-Vis53, so gelation was assessed by the inability of the material to flow. All of the samples were gels at RT. In order to cast films, the gels were reheated briefly to 80-90 oC and cast onto Teflon™ sheets. Upon cooling they reformed a gel. The thin gel films were left overnight in the MBraun glove box at RT, during which time the DMF evaporated, leaving a clear film. Any residual DMF and/or adsorbed water were removed by heating the samples in vacuum at ~ 140 oC -150 oC for 18-24 h; no residual DMF confirmed by TGA and NMR.


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RESULTS AND DISCUSSION The MC powder is fibrillar as received, and swell/dissolves in the presence of DMF readily at RT. Under these conditions it does not form a gel phase, even after prolonged storage (weeks) for solutions up to 5 wt% (higher concentrations were not tested). Evaporation of DMF in DMF/MC solutions at RT that were never heated did not result in a gel phase during the drying process, despite the increasing MC concentration. Heating is necessary to affect the gel formation that occurs on cooling, as previously observed for MC/DMAc53 and MC/DMF50 gels. Once heated (> 90 oC.; the fibrils completely disappear at 110 oC) and then cooled, gel formation occurs for all compositions ≥ 1% MC, with the reappearance of the fibrils. As suggested for cellulose/aprotic tertiary amine solvents and MC/DMAc53, the requirement to heat the solutions before gel formation can occur during the cooling step may be due to a solvation/dissolution mechanism in which it is necessary to raise the temperature to decrease the dipole-dipole interactions between the solvent molecules, freeing them to form H-bonds with the hydroxyl groups of cellulose or MC54. In the case of DMF, interaction with MC can result from both hydrogen bonding with the OH groups (Figure 1) and a dipole-dipole interaction with the methoxy groups49. Upon cooling, fibrils reform and the solution gels. The hydrogen bonding and hydrophobic interactions in MC fibers have been previously discussed51, 55. We suggest that the reformation of the fibrils upon cooling occurs with MC chains participating in more than one fibril, creating crosslinking bridges between the fibrils, and thus form an interconnected network that gels. Addition of PYR14TFSI, which was only 1% of the total volume, did not affect gel formation. PYR14TFSI/MC remaining after removal of DMF form strong, self-supporting films that are no longer thermo-reversible and stay solid until ~ 300 o C, the onset of the thermal decomposition of MC51. TGA data (Figure S1) indicate that the thermal stability of the blends is determined by the MC component, since the major weight loss of the PYR14TFSI occurs at Tdmax (PYR14 TFSI) = 450 C, while Tdmax (MC) = 375 oC. Self-supporting films of PYR14TFSI/MC could be formed with as little as 3wt% MC, with no leakage. SEM Data The morphology of the ion gels can be obtained by solvent substitution of PYR14TFSI with acetonitrile, followed by freeze drying to remove the acetonitrile. In order to observe the fibrillar network, the PYR14TFSI was first removed by solvent extraction with acetonitrile (ACN), which is a solvent for PYR14TFSI and a non-solvent for MC. We confirmed the solvent substitution by TGA, since ACN is removed at T = 82 oC and PYR14TFSI at T = 450 C. Since the decomposition temperature of PYR14TFSI is greater than that of MC (Figure S1), it is not possible to directly remove the PYR14TFSI. In Figure 2d, the ACN is simply evaporated, and it is difficult to see the nanofiber network. As the ACN evaporates, the fibers come together by capillary action, and the 7 ACS Paragon Plus Environment


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fibrous network (which should be only 10% of the total weight) collapses. However, if the MC/ACN is frozen and then freeze dried, it is possible to see the collapsed fibrils. The result of this procedure, shown for the 90/10 composition (Figure 2), is a volume spanning, random 3-D network of nanometer diameter MC fibrils. Linear fibers have been shown to be capable of gel formation through their topological interactions alone, i.e., without cross-links, provided the fibers are sufficiently long56. However, it is possible that MC chains link the fibrils, providing additional structural strength. Although the fiber network looks dense (Figure 2), it is the result of its collapse upon removal of the 90% liquid component. The nanofibers themselves were observed as grouped into larger bundles and some of the material looked like diaphanous sheets (Figure S2). When there was incomplete removal of the PYR14TFSI (as the result of insufficient soaking in acetonitrile), the PYR14TFSI could be observed as an amorphous structure in the SEM images (Figure S3), with EDAX confirming the presence of N, S and F in these regions. The PYR14TFSI (before removal) is expected to reside in the interstices between the fibrils that form the network. However, it should be noted that some swelling of the fibrils themselves with PYR14TFSI, as is the case for MC fibrils in aqueous solution57, cannot be ruled out. Differential Scanning Calorimetry and X-ray Diffraction The phase separation behavior of the MC fibrils and PYR14TFSI components was confirmed by differential scanning calorimetry (DSC) and x-ray scattering experiments. The DSC and x-ray characteristics of the neat materials are shown in Figures 3 and 4, respectively. Pure MC is semicrystalline and degrades before it melts. It exhibits only a very weak Tg (~ 170 oC, not shown) in DSC traces. The x-ray diffraction data show peaks at 2θ = 8.5o, 12.8o and 20.8o. The 7.5-10o reflection is from the 101 plane of MC, which was shown to decrease (i.e. d spacing increase) with degree of substitution53, and was suggested to be from crystallization of trisubstituted units in the gel phase58. The 2θ reflection between 21.8-22.0 lies close to the 002 reflection for cellulose53. The assignment of the 2θ = 12.5 o peak is uncertain. PYR14TFSI is amorphous at low temperatures when cooled from the melt at 10 oC/min (see cooling trace, Figure 3), and has characteristic broad x-ray scattering peaks at 2θ = 10.5o and 20o (Figure 4). These peaks remain the same above the melt temperature (Tm = -18 oC) of PYR14TFSI (not shown), since both the glass and melt are disordered. Supercooling of PYR14TFSI, i.e. the formation of cooling-rate dependent glassy materials (Tg = - 87 oC), has previously been reported59. In order to kinetically enable crystallization of PYR14TFSI, it was cooled to -173 oC, heated to -40 oC, so that it crystallizes but does not remelt, and recooled to 173 oC. In this case, the x-ray diffraction pattern at -173 oC remains crystalline (Figure 4), and a much smaller crystallization exotherm and a much larger single melt endotherm (at -18 oC) are observed upon heating (Figure S4). 8 ACS Paragon Plus Environment

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Figure 3. DSC data ( 2nd heating cycle) of PYR14TFSI and PYR14TFSI /MC blends, which have previously been cooled to the glassy state at 10 C/min (only shown for neat PYR14TFSI).

Figure 4. X-ray diffraction data for PYR14TFSI slow cooled to -173 oC (same at RT); heated to -40 oC; recooled to -173 oC.

In all PYR14TFSI/MC mixtures, MC peaks in the x-ray scattering data remain at the same positions as in the neat MC (Figures 5 and 6), although they become weaker as the MC content decreases. For mixtures where the PYR14TFSI is an amorphous glass at low temperature (Figure 5), i.e. slow cooled from the melt, x-ray diffraction data for all compositions show the same amorphous scattering as observed for neat PYR14TFSI in the glassy (Figure 4) state. The 2θ = 20o of amorphous PYR14TFSI and the 2θ = 20.8o of MC are distinguishable in the x-ray data (as shown by the guides to the eye in Figure 5). DSC data (Figure 3) show similar but weaker crystallization/melting phenomena, compared with neat PYR14TFSI, for the PYR14TFSI/MC = 90/10 and 80/20 compositions, while the 70/30 sample shows no crystallization and only a very weak melt endotherm, and a Tg at -87 oC. These results indicate that when excess PYR14TFSI is present, i.e. for PYR14TFSI/MC ≥ 80/20, it exists as a separate phase in these blends, although slightly perturbed by the MC, since the phase transitions are shifted up in temperature. As the MC content increases (PYR14TFSI = 60/40, 50/50, 30/70 and 10/90 compositions), there are no phase transitions indicative of crystallization/melting of the PYR14TFSI. However, a weak Tg. is always observed in the PYR14TFSI blends, which does not shift in temperature from that of neat PYR14TFSI, indicating that the PYR14TFSI and MC are phase separated. The same trends are observed for PYR14TFSI/MC blends containing crystalline PYR14TFSI (first cooled to -173 oC, 9 ACS Paragon Plus Environment


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heated to -40 oC. and recooled to -173 oC). X-ray diffraction peaks characteristic of crystalline PYR14TFSI are observed (Figure 6) for the PYR14TFSI/MC = 90/10 and 80/20 samples, indicating phase separation in the blends with high PYR14TFSI content. The PYR14TFSI/MC = 70/30 shows a predominantly amorphous x-ray pattern, with a small amount of PYR14TFSI crystallinity. For the IL/MC = 60/40 sample and samples containing greater amounts of MC, the PYR14TFSI remains amorphous. The existence of distinct crystalline MC peaks (black dashed vertical lines in Figure 5 and 6) and distinct amorphous (red dashed vertical lines in Figure 5) or crystalline (red solid vertical lines in Figure 6) PYR14TFSI peaks indicates the persistence of separate semi-crystalline crystalline MC and amorphous or crystalline PYR14TFSI phases at all compositions. As the MC content increases, it is possible that the accessible volume between the MC nano-fibrils is too small to permit nucleation of stable PYR14TFSI crystallites, so that no PYR14TFSI crystals can grow. Alternatively, interfacial interactions of the PYR14TFSI with the MC suppress these processes, as has previously been reported in more strongly interacting RTIL/polymer ion gels7.

Figure 5. X-ray diffraction data for PYR14TFSI and PYR14TFSI/MC blends slow cooled to -173 oC. Guide to eye, vertical lines indicate: ---- major MC peaks; ---- major amorphous PYR14TFSI peaks.

Figure 6. X- ray data for PYR14TFSI/MC films; All data taken at -173 oC: ▬, ▬, ▬, ▬, ▬ after cooling to -173C, heating to -40C and re-cooling to -173C. ••−••, ▬, ▬ after first cool to -173C. Guide to eye, vertical lines indicate: ---- major MC peaks; ---major amorphous PYR14TFSI peaks; ___ major crystalline PYR14TFSI peak. 10

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DMA DMA data confirm the phase separated morphology of the PYR14TFSI/MC blends. Tan δ (Figure 7) shows more clearly Tg of MC, which is independent of PYR14TFSI content, indicating that there is little mixing of the PYR14TFSI with amorphous MC. The Tg of pure MC is 196.9 oC and the average value in the blends is 196.7 oC. The transition at -61o in pure MC is not due to DMF, since it also occurs when MC is cast from water and DMSO, and when the film cast from DMF is extracted with acetone (Figure S5). Further, no DMF is observed by TGA weight loss, and DMF is not observed in 1H NMR/DMSO-d5 spectra. Last, x-ray data (Figure S6) show no changes in this temperature range. Therefore, this is either a secondary, sub-Tg transition of MC, or an artifact due to slippage of the MC in the clamps. Since films for DMA measurements were prepared by cooling the samples from room temperature to -100 oC, where the PYR14TFSI is amorphous, and then heating to obtain the data, the PYR14TFSI is expected to follow the heating profiles shown in Figure 3. The Tg of PYR14TFSI has a peak at ~ -71 oC in the tanδ plots, independent of composition, and the crystallization/melt transition can be observed in the PYR14TFSI/MC = 90/10, 80/20 and 70/30 films. The Tg is present but the melt/crystallization absent in the films with PYR14TFSI/MC < 60/40 film. For the PYR14TFSI/MC = 30/70, 20/80 and 0/100 films, the DMF transition associated with the MC also appears at ~ -60 oC. As the MC content increases, the loss peak associated with its Tg (and that from DMF) increases relative to the loss peak associated with the Tg of PYR14TFSI. Figure 7. Tanδ vs temperature for PYR14TFSI/MC as a function of composition. Guide to eye, vertical lines indicate: ---- MC (Tg) transition at 190 and either a secondary MC transition or an artifact; ---Tg of PYR14TFSI.



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DMA data also confirm that solids were obtained at all compositions for PYR14TFSI/MC ≤ 90/10. The storage and loss moduli for the PYR14TFSI/MC = 90/10 composition shows solid behavior over the whole frequency range (Figure 8 top). Although solid, self-standing films are formed for compositions with as little as 3 wt% MC, the films were too fragile to be tested using the tension clamp. All the films are solids up until the degradation temperature (375 oC) of the MC (in N2), and are strong and flexible when PYR14TFSI/MC ≤ 90/10.

Figure 8. (top) Storage and loss moduli as a function of frequency for PYR14TFSI/MC = 90/10 composition at 25 oC; (bottom) storage moduli vs temperature for PYR14TFSI/MC as a function of composition.

The storage moduli (Figure 8 bottom) decrease as the PYR14TFSI content increases, as expected. The values of the storage moduli at 30 oC, 90 oC and 150 oC are summarized in Table 1. For the 12 ACS Paragon Plus Environment

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PYR14TFSI/MC = 90/10, 80/20 and 70/30 compositions, the peak at -25oC corresponds to the crystallization (moduli increase) and melting (moduli decrease) of the PYR14TFSI, as observed in both the DSC (Figure 3) and X-ray data (Figure 6). For the PYR14TFSI/MC = 60/40, 50/50 and 40/60, this peak does not appear since the PYR14TFSI remains amorphous, as also observed in the DSC (Figure 3) and x-ray (Figure 6) data. The drop in modulus at ~ 175 oC, seen most prominently in the pure MC sample, occurs at the glass transition temperature of the MC. Nevertheless, the moduli (Table 1) remain in the MPa range at this temperature. Table 1. Moduli and Conductivity (σ σ) of PYR14TFSI/MC films Composition Modulus Modulus σ σ (MPa) PYR14TFSI/MC (MPa) (S/cm) (S/cm) 90 oC 30 oC 30 oC 90 oC. 100/0 4.3 x 10-3 1.55 x 10-2 97/03 * 3.3 x 10-3 * 9.8 x 10-3 90/10 150 1.4 x 10-3 109 6.0 x 10-3 85/15 6.9 x10-4 3.0 x 10-3 80/20 740 1.3 x 10-4 482 1.0 x 10-3 70/30 745 7.0 x 10-5 485 4.6 x 10-4 60/40 1772 5.8 x 10-6 1088 1.8 x10-4 50/50 1985 5.6 x 10-6 1302 5.7 x 10-5 40/60 3052 2004 30/70 3648 2346 20/80 5270 3881 10/90 5033 4204 0/100 6741 5270 *Moduli were not measured since the films were too soft for the tensile clamp used.

Modulus (MPa) 150 oC

σ (S/cm) 140 oC 3.18 x 10-2

* 35

1.13 x 10-2

271 276 518 672 1103 2280 2147 2395 3142

Electrochemical Data: Conductivity and stability window Electrochemical data were analyzed using both Bode and Nyquist plots. The magnitude (│Z│) and phase angle of the impedance data for PYR14TFSI/MC as a function of composition are shown in Bode plots Figure 9. The magnitude of Z, Z''contains both capacitive and resistive components: │Z│ = [(Z’)2 + (Z”)2]1/2. The phase angle, tan(θ) = Z”/Z’, is -90o or 0o for pure capacitive or resistive behavior, respectively. However, a phase angle of 90o is almost never obtained (here the average value is 80o) and so it is typically modelled using a constant phase element. At high frequencies │Z│ is independent of frequency and can be characterized by the resistive component (Z'). It occurs at about 3 kHz in our system (Figure 9), with the resistance increasing with increasing MC content. This is shown more clearly in Figure 10, where the conductivity (where the resistance at a frequency of 100,000 Hz was used to calculate conductivity) values are plotted as a function of PYR14TFSI/MC composition. The resistance for neat PYR14TFSI, and the PYR14TFSI/MC = 13 ACS Paragon Plus Environment


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97/3 and 90/10 compositions are close, indicating little perturbation by the MC. At low frequencies │Z│ is frequency dependent, which indicates capacitive behavior (Z''). The average value of the slopes of the │Z│ vs frequency plots is -0.82, indicative of capacitive behavior, as previously observed17(where the ideal value is -1). The capacitive behavior originates from the formation of an electrostatic double layer near the electrodes resulting from electric field-driven ion arrangement. The onset of capacitive behavior increases with PYR14TFSI content (Figure 9), indicating that the ion arrangement or mobility near the electrodes becomes easier as the pure liquid content increases. The specific capacitance, obtained using C’ = (2πf│ Z''│A)-1, at f = 10 Hz is presented in Figure 10.

Figure 9. Frequency dependence of the (top) magnitude, (│Z│); and (bottom) phase angle of PYR14TFSI/MC films as a function of composition at 40 o C. Capacitive behavior increases with increasing PYR14TFSI content.


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Figure 10. Conductivity and specific capacitance as a function of PYR14TFSI/MC composition at 40 oC. Conductivity data, obtained from Nyquist plots (where the bulk resistance was obtained as the point where the high frequency semicircle or straight line in the plot of Z” as a function of Z’ intersects the Z’ axis), are presented in Figure 11. The conductivity of the neat PYR14TFSI is the same as previously reported47, 60. The conductivity of the PYR14TFSI/MC blends increases with PYR14TFSI, as expected. The highest ionic conductivity is achieved for the PYR14TFSI/MC = 97/03 composition, nearly 1 x 10-2 S at 90 oC and 3.3 x 10 -3 S/cm at 30 oC., only 1.6-1.3 less than the conductivity of neat PYR14TFSI. The moduli could not be measured for the 97/03 composition (since the films were too soft for the tensile clamp used), although a self-standing film was formed. At PYR14TFSI/MC = 90/10, respectable conductivities of 1.4 x 10-3 S/cm at 30 o C, 6 x 10-3 S/cm at 90 oC and 1.13 x 10-2 S/cm at 140 oC., a factor of ~3 less than the neat PYR14TFSI, were achieved with films that had moduli of 150 MPa, 109 MPa, and 57 MPa, respectively. Very strong films, with moduli > 1 GPa at 30 oC were formed for compositions with PYR14TFSI/MC < 60/40, but these films had relatively low ionic conductivities (~ 6 x 10-6 S/cm at 30 oC). Thus the PYR14TFSI/MC = 90/10 composition has a reasonable mechanical property/conductivity trade-off, i.e. σ > 10-3 S/cm and a modulus > 100 MPa at RT. The logarithm of conductivity plots with and without MC are linear over the limited temperature interval (10 to 90 oC), suggesting that the conduction mechanism follows Arrhenius behavior, as observed for some ionic liquids2. Addition of MC up to ~ 10 wt% does not affect the conduction mechanism, with the same values of the activation energy (Ea = 22 kJ/mol), and indicates that 15 ACS Paragon Plus Environment


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ion migration is through the liquid component. Values of activation energy are in good agreement with those reported in the literature for PYR14TFSI48, 61 and other PYR+TFSI- ILs62. For PYR14TFSI/MC < 80/20, the activation energy increases with MC content (to 35 kJ/mol), indicating some coupling of the two components, perhaps at the interface of the phases, and is consistent with the DSC (Figure 3) and X-ray data (Figure 5 and 6).

Figure 11. Conductivity versus temperature plots of neat PYR14TFSI and PYR14TFSI/MC as a function of composition.

Over a wider temperature range (10 oC to 140 oC) Vogel-Fulcher-Tamann (VTF) behavior is observed (Figure S7). Conductivities were measured to 140 oC for the PYR14TFSI/MC = 90/10 composition, and followed VTF behavior over this temperature interval, and reached 1.13 x 10-2 S/cm compared with 3.18 S/cm x 10-2 S/cm for the neat Pyr14TFSI. The electrochemical stability Figure 12. Stability window of Pyr14TFSI/MC = 90/10 using stainless steel working, counter and reference electrodes at 25 oC., with voltage sweep between -4 to 5 V at 5 mV/sec


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window for the Pyr14TFSI/MC 90/10 composition (Figure 12) is from -2.8V to 2.8V (potential window ~ 5.6V) at 25 co., where the threshold current is 50 µA/cm2. The moduli and conductivities of PYR14TFSI/MC gels are greater, at comparable weight percents, than those obtained with gels composed of RTILs and homopolymers24, 26, 42, 63-64 or phase separated diblock copolymers21, 65, gels that are chemically crosslinked16-17, 66, and crosslinked using silica67. Gels with high ionic conductivities typically exhibited rubbery mechanical properties, and stronger gels with MPa moduli had lower ionic conductivities. In general, if the conductive phase contains a polymer component, e.g. as in the case of polyethylene oxide (PEO)22, 68, poly(methyl methacrylate) (PMMA)22 or poly(ethyl acrylate) (PEA) 22, the viscosity will increase and hence the conductivity will decrease. Miscibility of the polymer and RTIL is manifest as a single lowered glass transition temperature (compared with the neat polymer), observed by DSC or DMA (tan δ)7, with lower Tg polymers desirable for higher ionic conductivity. The lack of solubility of MC in PYR14TFSI, which enables the neat RTIL to be trapped in the MC nano-fibrillar network, is related to the inability of the hydrophobic PYR14TFSI (which is also not soluble in water) to break the intra- and intermolecular hydrogen bonds in MC. For cellulose, breaking these hydrogen bonds typically requires ionic liquids containing halide, carboxylate and phosphate anions for dissolution69. This suggests that other hydrophobic RTILs may similarly be entrapped by MC. The great strength of the MC gels is suggested here to be related to the high Tg and crystallinity of the MC fibrils, which may be connected during gelation by MC tie chains. CONCLUSIONS The reported combination of high ionic conductivity (> 1 x 10-3 S/cm), high capacitance (2 µF/cm2), wide voltage stability window (5.6V) and excellent mechanical properties (flexible but with moduli > 100 MPa) at RT for the 90/10 PYR14TFSI/MC composition are the best reported to date using an ionic liquid (PYR14TFSI), properties that are desirable for electrochemical capacitor applications70. The procedure used to make the separators, namely the use of low viscosity solvents (compared with viscous ionic liquids), will allow permeation of the electrolyte into the electrodes (e.g. carbon in supercapacitors) and thus decrease interfacial resistance at the electrode/electrolyte interface. This combination of properties will facilitate easy device fabrication into thin film supercapacitors. The robustness and high conductivity of the PYR14TFSI/MC ion gels, at high incorporation of PYR14TFSI, can be attributed to the network of crystalline, high Tg, and high modulus MC nanofibers spanning the whole volume of the films (Figure 2). There is little interaction of the hydrophobic PYR14TFSI with the MC matrix, particularly at high PYR14TFSI loading, as evidenced by the appearance of distinct glass transition temperatures for the components near those of the neat materials, so that the conductive and structural phases are independent. The small diameters of the MC fibers enable low weight percent of MC to span the whole volume of 17 ACS Paragon Plus Environment


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the gels. Although there is phase separation between the PYR14TFSI and MC, the presence of MC does suppress the low temperature (< 0 oC) crystalline transitions in the PYR14TFSI at increasing MC content. While this should not affect the ionic conductivity (since the PYR14TFSI is a liquid at T > 0 oC), it does provide insight on the phase morphology. The suppression of PYR14TFSI crystallinity may result from the inability of the PYR14TFSI to form stable nuclei in the confined space of the nano-fibrillar network, or from increased interfacial interactions between the MC and PYR14TFSI. SUPPORTING INFORMATION: TGA data of PYR14TFSI/MC; SEM images of freeze dried PYR14TFSI/MC 90/10; DSC data of neat PYR14TFSI; DMA of MC cast from different solvents; x-ray diffraction of MC as a function of temperature; conductivity plots in VTF format. ACKNOWLEDGEMENTS Steven Patrick DiLuzio obtained the gel temperatures for DMF/MC and DMF/PYR14TFSI/MC. We thank Dr. Schafmeister for use of his freeze drying apparatus. REFERENCES 1.

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