Physical and Electrochemical Properties of N-Alkyl-N

Oct 2, 2008 - ... reproducible in some scans and completely absent (flat baseline) in others ..... A series of EDLCs with only 0.4% deviation in the c...
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J. Phys. Chem. B 2008, 112, 13577–13580

13577

Physical and Electrochemical Properties of N-Alkyl-N-methylpyrrolidinium Bis(fluorosulfonyl)imide Ionic Liquids: PY13FSI and PY14FSI Qian Zhou,† Wesley A. Henderson,*,† Giovanni B. Appetecchi,‡ Maria Montanino,‡ and Stefano Passerini*,‡,§ ILEET (Ionic Liquids and Electrolytes for Energy Technologies Laboratory), Department of Chemical & Biomolecular Engineering, North Carolina State UniVersity, Raleigh, North Carolina 27695, USA, ENEA, Casaccia Research Center, Rome 00060, Italy and Institute of Physical Chemistry, UniVersity of Muenster, Muenster, D 48149, Germany ReceiVed: June 19, 2008; ReVised Manuscript ReceiVed: August 7, 2008

Two ionic liquids based upon N-alkyl-N-methylpyrrolidinium cations (PY1R+) (R ) 3 for propyl or 4 for butyl) and the bis(fluorosulfonyl)imide (FSI-), N(SO2F)2-, anion have been extensively characterized. The ionic conductivity and viscosity of these materials are found to be among the highest and lowest, respectively, reported for aprotic ionic liquids. Both ionic liquids crystallize readily on cooling and undergo several solid-solid phase transitions on heating prior to melting. PY13FSI and PY14FSI are found to melt at -9 and -18 °C, respectively. The thermal stability of PY13FSI and PY14FSI is notably lower than for the analogous salts with the bis(trifluoromethanesulfonyl)imide (TFSI-), N(SO2CF3)2-, anion. Both ionic liquids have a relatively wide electrochemical stability window of ∼5 V. Introduction Ionic liquids (ILs) are being incorporated as electrolyte materials into a wide variety of electrochemical devices including lithium batteries,1-12 electrochemical (super or ultra) capacitors,13-15 electrochemical actuators,16-18 light-emitting electrochemical cells,19-21 and so forth. Such electrochemical applications have stringent requirements including high chemical, thermal, and electrochemical stability; wide liquid range for operation at low and high temperatures; and high ionic conductivity. The latter is necessary, even for lithium batteries, because the Li+ conductivity (when a lithium salt is added to the IL) is generally found to scale with the conductivity of the IL. Thus, batteries can be discharged at faster rates with more conductive ILs.12 Of particular interest are a new set of ILs based upon the bis(fluorosulfonyl)imide (FSI-), N(SO2F)2-, anion.1-4 A variety of synthesis techniques have been reported for the FSIanion22-32 and crystal structures are known for LiFSI, KFSI, and CsFSI.23,33,34 Alkali metal salts (K, Rb, Cs) with FSI- have relatively low melting points for inorganic salts and CsFSI is reported to be thermally stable to 300 °C.35 The IL with 1-ethyl3-methylimidazolium cations (C2mimFSI) has a low melting point of -13 °C and (at 25 °C) a low viscosity of 18-25 cP and high conductivity of 15.4-16.5 mS/cm.1,3 Unfortunately, the C2mim+ cation is known to have a relatively low electrochemical stability (both cathodic and anodic).7,10,11 In addition, C2mimFSI has a poorer thermal stability in contact with active electrode materials than N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (PY13FSI).36 In contrast with the C2mim+ cation, the PY13+ and PY14+ cations are known to have a high electrochemical stability suitable for lithium-battery electro* To whom correspondence should be addressed. E-mail: whender@ ncsu.edu. Tel: (919) 513-2917. Fax: (919) 515-3465 (W.A.H.), E-mail: [email protected] (S.P.). † North Carolina State University. ‡ ENEA. § University of Muenster.

Figure 1. Chemical structures of (a) PY13FSI and (b) PY14FSI.

lytes.1,3,5,6,9,37 At present, however, only limited data is available regarding the physical characteristics of PY1RFSI ILs. Here, we report the physical and electrochemical properties of PY13FSI and PY14FSI (Figure 1) over a wide temperature range. Experimental Section Sample Preparation. 1-Methylpyrrolidine (97%), 1-bromopropane (99%), and 1-bromobutane (99%) were purchased from Aldrich and used as received. LiFSI was purchased from Dai-ichi Kogyo, Seiyaku Co., Ltd., Japan (DKS). PY13FSI and PY14FSI were prepared according to our previously reported IL preparation process.37 The ILs were dried under vacuum at 60 °C for 2 h and then at 120 °C for 20 h. The ILs were clear, colorless, and odorless with a moisture content below 1 ppm. The materials were stored in sealed glass tubes in a controlled environment (dry room, RH < 0.1% at 20 °C). Thermal Measurements. DSC measurements were performed using a TA Instruments Q1000 differential scanning calorimeter with liquid N2 cooling. The instrument was calibrated with cyclohexane (solid-solid phase transition at -87.06 °C, melt transition at 6.54 °C) and indium (melt transition at 156.60 °C). Hermetically sealed aluminum pans were prepared in an argon glovebox. TGA measurements were performed using a TA Instruments Q500 thermogravimetric analyzer. The thermal stability of the ILs was initially analyzed by heating from ambient temperature to 600 °C under either an air or N2 atmosphere. Separate samples of each IL were then analyzed isothermally at different

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13578 J. Phys. Chem. B, Vol. 112, No. 43, 2008 temperatures for 24 h under an N2 (>99.99%; water content between 2–5 ppmv) atmosphere. Viscosity. Viscosity measurements were conducted using a HAAKE RheoStress 600 rheometer located in the dry room. The tests were performed from 20 to 80 °C with a 1 °C min-1 heating rate in the 100 to 2000 s-1 rotation speed range. Measurements were taken after 10 °C steps. Ionic Conductivity. The ionic conductivity was measured with an AMEL 160 conductivity meter located in the dry room. The ILs were placed in sealed glass conductivity cells (AMEL 192/K1) equipped with two porous platinum electrodes (cell constant of 1.00 ( 0.01 cm). The cells were assembled in the dry room, and conductivity tests were performed between -40 and 100 °C using a climatic test chamber (Binder GmbH MK53). The entire setup was controlled by software developed at ENEA. To fully crystallize the samples, the cells were immersed in liquid N2 for a few seconds and then transferred to the climatic test chamber at -40 °C. After a few minutes of storage at this temperature, the amorphous solids turned back into liquids. This procedure was repeated until the ILs remained (crystalline) solids at -40 °C. In previous work, it was demonstrated that incomplete crystallization of ILs result in nonequilibrium behavior, which dramatically influences the conductivity measurements. The crystallized samples were stored at -40 °C for at least 1 h before measurements began. The samples were then heated at 2 °C hr-1. Electrochemical Stability. Linear sweep voltammetry (LSV) (5 mV s–1) of the salts was performed to determine the electrochemical stability of the ILs. A sealed 3-electrode, glass microcell described previously38 was used. The cells were loaded with about 0.5 mL of the ILs. A glass-sealed platinum working microelectrode (active area ) 0.78 mm2) and a platinum foil counter electrode (about 0.5 cm2) were used. The reference electrode was a silver wire immersed in a 0.01 M solution of AgCF3SO3 in PY14TFSI, separated from the cell compartment with a fine glass frit (reference electrode potential is 3.40 V ((5 mV) vs Li/Li+).39,40 High purity argon (3 ppmv water and 2 ppmv oxygen) was flowed over the ILs for 30 min before the start of the measurements and continued during the experiments. This was necessary because we have previously shown that other gases such as N2 and O2 result in an increase in the current.38 Separate LSV tests were carried out on each IL. The cell potential was scanned either anodically or cathodically from the open circuit potential (OCP). Clean electrodes and fresh samples were used for each test. To confirm the results obtained, the LSV tests were performed at least twice on different fresh samples of each IL. The measurements were performed at 20 °C using a Schlumberger (Solartron) Electrochemical Interface (model 1287) controlled by software developed at ENEA. Results and Discussion DSC heating and cooling traces for PY13FSI and PY14FSI are shown in Figure 2. Although both salts significantly supercool (hysteresis between the melting point and crystallization temperature), crystallization does occur readily on cooling. In fact, it was not possible to quench the salts into an amorphous phase using cooling rates up to 40 °C min-1. In all cases the salts crystallized. PY13FSI undergoes several exothermic transitions on cooling. On heating, several endothermic transitions were observed including a low energy solid-solid phase transition (phase III-II) with a peak at -83 °C, a solid-solid phase transition at -19 °C (phase II-I), and a melt transition at -9 °C (phase I-L). The identity of the broad endothermic transition with a

Zhou et al.

Figure 2. DSC cooling and heating traces (5 °C min-1) of PY13FSI (upper plot) and PY14FSI (lower plot).

peak temperature of -50 °C and subsequent exothermic transition near -45 °C is not clear. These latter transitions were reproducible in some scans and completely absent (flat baseline) in others. One possibility is that the peak at -50 °C corresponds to the crystallization of another (kinetic) phase, which is dependent upon the thermal history of the salt. After melting this phase (at -50 °C), the resulting liquid then crystallizes (the exothermic transition) into the thermodynamically stable phase II structure. Thermal decomposition occurs on heating the PY13FSI above 150 °C. The melting point of PY13FSI has been previously reported to be -18 °C.1 The method used to determine this melting point was not provided. It is possible that the onset of the solid-solid phase transition was mistakenly identified as the onset of melting. PY14FSI also undergoes several endothermic transitions on heating. Sharp endothermic transitions at -45, -28, and -18 °C correspond to phase III-II, phase II-I, and phase I-L transitions, respectively. The identity of the small peak near -34 °C is not clear. This peak is not found to be reproducible in repeated heating/cooling traces. It may be that this peak is associated with a kinetically (but not thermodynamically) stable phase as suggested above for PY13FSI. Figure 3 compares the variable-temperature ionic conductivity of the ILs. At 25 °C, the values are 6.4 and 4.8 mS cm-1 for PY13FSI and PY14FSI, respectively. The conductivity of PY13FSI is somewhat lower than the previously reported conductivities (mS/cm) values of 8.2 (25 °C),3 8.3 (25 °C),1 and 9.14 (20 °C)2 (possibly due to lower level of impurities and/or water; samples of PY13FSI purchased from Dai-ichi Kogyo, Seiyaku Co., Ltd. had a higher anodic current prior to Ea (below) than samples

PY13FSI and PY14FSI

J. Phys. Chem. B, Vol. 112, No. 43, 2008 13579

Figure 5. Variable-temperature TGA traces (5 °C min-1) of (a) PY13FSI and (b) PY14FSI in an air or N2 atmosphere. Figure 3. Arrhenius plot of ionic conductivity of (a) PY13FSI (open circles) and (b) PY14FSI (filled circles).

Figure 4. Viscosity of (a) PY13FSI (open circles) and (b) PY14FSI (filled circles).

prepared at ENEA). Note that, at -38 °C, both salts are crystalline solids and have a conductivity near 10-8 S cm-1, this despite the fact that both salts undergo solid-solid phase transitions at lower temperature, presumably forming crystalline phases with disordered ions. Figure 4 compares the variable-temperature viscosity of the ILs. The following equations fit the data between 20-80 °C:

PY13FSI:η ) (89.561)-(3.1808)T + (5.9269e-2)T2(5.7737e-4)T 3 + (2.2585e-6)T 4 PY14FSI:η ) (129.39)-(4.3875)T + (7.1815e-2)T 2(6.0301e-4)T 3 + (2.0577e-6)T 4 where the units are η (mPa s) and T (°C). The higher viscosity of PY14FSI, relative to PY13FSI, may account for the lower conductivity of the former. The viscosity of PY13FSI is intermediate between the previously reported viscosity (mPa s) values of 40 (25 °C),3 53 (25 °C),1 and 39 (20 °C).2 The thermal stability of the ILs was analyzed using both variable-temperature and isothermal TGA measurements. Figure 5 indicates that the initial mass loss for both ILs is independent of the atmosphere (air or N2). The onset point for mass loss (1 wt%) is about 219-231 °C for PY13FSI and 189-198 °C for PY14FSI. Approximately 15-20 wt% of the original IL remains after decomposition when heated under an N2 atmosphere. If, instead, air is present, a second thermal process beginning near 400 °C and finishing near 600 °C leads to the complete volatilization of the thermal degradation products (Figure 5).

Figure 6. Isothermal TGA traces of (a) PY13FSI and (b) PY14FSI in an N2 atmosphere. The temperature for each scan is indicated in the figure.

The isothermal TA measurements indicate that neither IL loses mass on storage at 100 °C for 24 h. Both ILs, however, lose approximately 9-10 wt% of their mass on heating at 150 or 175 °C. Note that PY14FSI did not lose any significant mass on heating at 125 °C, whereas PY13FSI did but only after a long delay. This suggests that a nucleation process occurs for the decomposition. The data further suggest that the same degradation mechanism occurs for both ILs up to 175 °C, resulting in a thermally stable degradation product. Heating to higher temperature, however, leads to greater mass loss. Heating both ILs at 250 °C results in an initial rapid mass loss down to about 18 wt%, after which the mass loss is minor, in agreement with the variable temperature data (for an N2 atmosphere; Figures 5 and 6). The measurement of the electrochemical stability of electrolyte solvents and ions is a complicated task, which is influenced by the electrode material, scan rate, and other factors.41 One simplification in the present measurements is the absence of a solvent. If a cutoff current density of 0.1 mA cm-2 is considered,41 the anodic/cathodic (Ea/Ec) limits are 0.03/5.37 V (vs Li+/Li) for PY13FSI and -0.01/5.35 V (vs Li+/Li) for PY14FSI, respectively. Note that for lithium-battery electrolyte applications, the addition of LiBF4 or LiTFSI to ILs led to an improved cathodic electrochemical stability.9,38,42 This may also occur by the addition of LiTFSI to the PY1RFSI, but this has

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Figure 7. Electrochemical stability of (a) PY13FSI (black) and (b) PY14FSI (gray).

not yet been confirmed. Also note that carbon electrodes with a C2mimFSI-LiTFSI electrolyte were able to intercalate and release Li+ cations, whereas a C2mimTFSI-LiTFSI electrolyte performed poorly, suggesting that the FSI- anions formed a stable SEI layer on carbon, which was highly conductive to Li+ cations but prevented further electrolyte degradation.1,3 Conclusions The PY13FSI and PY14FSI ionic liquids have a low melting point, low viscosity, high conductivity, and high electrochemical stability, making them desirable materials for many electrochemical technologies. These salts have a lower thermal stability than the corresponding TFSI--based ionic liquids, but the stability is acceptable for most applications. Acknowledgment. This material is based upon work supported by, or in part by, the European Commission under Contracts TST4-CT-2005-518307 and NMP3-CT-2006-033181, and the U. S. Army Research Laboratory and the U. S. Army Research Office (ARO) under Contract Grant Number W911NF07-1-0556. References and Notes (1) Ishikawa, M.; Sugimoto, T.; Kikuta, M.; Ishiko, E.; Kono, M. J. Power Sources 2006, 162, 658. (2) Guerfi, A.; Duchesne, S.; Kobayashi, Y.; Vijh, A.; Zaghib, K. J. Power Sources 2008, 175, 866. (3) Matsumoto, H.; Sakaebe, H.; Tatsumi, K.; Kikuta, M.; Ishiko, E.; Kono, M. J. Power Sources 2006, 160, 1308. (4) Saint, J.; Best, A. S.; Hollenkamp, A. F.; Kerr, J.; Shin, J.-H.; Doeff, M. M. J. Electrochem. Soc. 2008, 155, A172. (5) Shin, J.-H.; Henderson, W. A.; Scaccia, S.; Prosini, P. P.; Passerini, S. J. Power Sources 2006, 156, 560. (6) Shin, J.-H.; Henderson, W. A.; Tizzani, C.; Passerini, S.; Jeong, S.-S.; Kim, K.-W. J. Electrochem. Soc. 2006, 153, A1649. (7) Sakaebe, H.; Matsumoto, H.; Tatsumi, K. Electrochim. Acta 2007, 53, 1048.

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