Melting Behavior and Ionic Conductivity in Hydrophobic Ionic Liquids

Melting Behavior and Ionic Conductivity in Hydrophobic Ionic Liquids

(Parte 1 de 2)

Melting Behavior and Ionic Conductivity in Hydrophobic Ionic Liquids

Miriam Kunze,† Maria Montanino,‡ Giovanni B. Appetecchi,‡ Sangsik Jeong,† Monika Schonhoff,† Martin Winter,† and Stefano Passerini*,†

Department of Physical Chemistry, Westfälische Wilhelms-UniVersitat Munster, Corrensstra e 28/30, 48149 Munster, Germany, and Agency for the New Technologies, Energy and the EnVironment (ENEA), Via Anguillarese 301, 00123 Rome, Italy

ReceiVed: October 16, 2009; ReVised Manuscript ReceiVed: December 9, 2009

Four room-temperature ionic liquids (RTILs) based on the N-butyl-N-methyl pyrrolidinium (Pyr14+) and

N-methyl-N-propyl pyrrolidinium cations (Pyr13+) and bis(trifluoromethanesulfonyl)imide (TFSI-) and bis(fluorosulfonyl)imide (FSI-) anions were intensively investigated during their melting. The diffusion coefficients of 1H and 19F were determined using pulsed field gradient (PFG) NMR to study the dynamics of the cations, anions, and ion pairs. The AC conductivities were measured to detect only the motion of the charged particles. The melting points of these ionic liquids were measured by DSC and verified by the temperature-dependent full width at half-maximum (FWHM) of the 1H and 19F NMR peaks. The diffusion and conductivity data at low temperatures gave information about the dynamics at the melting point and allowed specifying the way of melting. In addition, the diffusion coefficients of 1H( DH) and 19F( DF) and conductivity were correlated using the Nernst-Einstein equation with respect to the existence of ion pairs.

Our results show that in dependence on the cation different melting behaviors were identified. In the Pyr14- based ILs, ion pairs exist, which collapse above the melting point of the sample. This is in contrast to the

Pyr13-based ILs where the present ion pairs in the crystal dissociate during the melting. Furthermore, the anions do not influence the melting behavior of the investigated Pyr14 systems but affect the Pyr13 ILs. This becomes apparent in species with a higher mobility during the breakup of the crystalline IL.

Introduction

Ionic liquids (ILs) are molten salts with a melting point below 100 °C. Not only this fact, but also that these liquids have a low vapor pressure and a high chemical and electrochemical stability make them labeled as green solvents.1-5 Therefore, ILs are being investigated for a wide range of applications. One field is the utilization as electrolyte component in electrochemical devices. This includes lithium batteries,6-14 fuel cells,15 electrochemical (super or ultra) capacitors,16 electrochemical actuators,17 light-emitting electrochemical cells,18 etc. For these electrochemicalapplications,one has to have exact requirements includinga high chemical,thermal,and electrochemicalstability, a wide liquid range for operation at low and high temperatures, and a high ionic conductivity. Especially for lithium batteries the ionic conductivity plays an important role. In case a lithium salt is added to the IL, it is generally found that the Li+ conductivity scales with the conductivity of the IL. Thus, batteries can be discharged at faster rates with high conductivity ILs. Furthermore, the use (charge and discharge) of the lithium batteries shall be possible at very low temperatures. Because of that fact, it is important to gain knowledge of the molecular motionat low temperaturesand throughoutthe meltingtransition of the material. Until now, in the literature it is often reported

that above the melting temperature TM ion pair formation is taking place in the IL.19-2

Here, we report the physical and electrochemical properties of four different ILs based on the N-butyl-N-methyl pyrroli- dinium cation (Pyr14+), N-methyl-N-propyl pyrrolidiniumcation

(Pyr13+), bis(trifluoromethanesulfonyl)imide (TFSI-), and bis- (fluorosulfonyl)imide (FSI-) anion (Scheme 1) over a wide temperature range including the dynamics below TM. Mixtures of these ionic liquids with lithium TFSI have been proved successfully for lithium insertion in graphite.23-25 Hence, the melting temperatures TM were determined by DSC and NMR, and for the dynamic processes the measured diffusion coef-

* Corresponding author. E-mail: stefano.passerini@uni-muenster.de † Westfaelische Wilhelms-Universitat Munster. ‡ ENEA.

SCHEME 1: Cations and Anions Used for the Ionic Liquids

J. Phys. Chem. A X, x, 0 A

10.1021/jp9099418 X American Chemical Society ficients of 1H and 19F in the solid and liquid state with the correspondingAC conductivitieswere correlated and discussed.

Experimental Section

Sample Preparation. The four ILs (Pyr13FSI, Pyr13TFSI,

Pyr14FSI, and Pyr14TFSI) (cf., Scheme 1) were synthesized through a procedure developed at ENEA and described in detail elsewhere.26 The chemicals N-methylpyrrolidine (97 wt %), 1-propylbutane (9%), 1-bromobutane (9 wt %), and ethyl acetate (ACS grade, >9.5 wt %) were purchased from Aldrich and previously purified (with the exception of ethyl acetate) using activated carbon (Aldrich, Darco-G60) and alumina (acidic, Aldrich Brockmann I). Lithium bis(trifluoromethanesulfonyl)imide, LiTFSI (9.9 wt %, battery grade), and lithium (or potassium) bis(fluorosulfonyl)imide, Li(K)FSI (9.9 wt %, battery grade), were purchased from 3 M and Dai-Ichi Kogyo Seiyaku Co. Ltd., respectively, and used as received. Deionized

H2O was obtained using a Millipore ion-exchange resin deionizer.

The N-alkyl-N-methylpyrrolidinium bromide precursors

(Pyr13Br and Pyr14Br) were synthesized by reacting N-methylpyrrolidine with the appropriate amount of bromoalkyl in the presence of ethyl acetate. The precursors were repeatedly rinsed with ethyl acetate to remove the reagents excess and the soluble impurities. The four ionic liquids were obtained by reacting aqueous solutions of the precursors (Pyr13Br and Pyr14Br) with the appropriate amounts of LiTFSI or Li(K)FSI. The lithium to potassium content in the ionic liquids was tested to be below 2 ppm by atomic absorption spectroscopy (AAS). The reactions led to the formation of the hydrophobic ionic liquids and hydrophilic LiBr or KBr. After removal of the aqueous phase, the ionic liquids were rinsed several times with deionized water to remove water-soluble LiBr or KBr and excess of LiTFSI or Li(K)FSI. Next, the ionic liquids were purified with activated carbon and acidic alumina. The liquid fractions were separated from the solid phases by vacuum filtering and then placed in a rotary evaporator at 80 °C under vacuum to remove the solvent (ethyl acetate). Finally, the ionic liquids were dried using an oil-free vacuum pump at 60 °C for at least 2 h and then at 120 °C for at least 18 h with yields ranging from 85 to 90 mol %. The materials were stored in sealed glass tubes in a controlled

The water content in the ionic liquids was measured using the standard Karl Fischer method. The titrationswere performed by an automaticKarl Fischercoulometertitrator(MettlerToledo DL32) in dry-room (R.H. <0.1%) at 20 °C. The Karl Fisher titrant was a one-component reagent purchased from Aldrich (Hydranal 34836 Coulomat AG).

Thermal Measurements. DSC measurements were performed using a TA Instruments (model Q100) differential scanning calorimeter (DSC). Hermetically sealed Al pans were prepared in the dry room. Typically, the electrolyte samples were cooled at 20 K/min from room temperature to -140 °C and then heated at 10 K/min to 100 °C.

Ionic Conductivity. The ionic conductivity measurements were performed as follows: the samples were placed in sealed glass conductivity cells (AMEL 192/K1) equipped with two porous platinum electrodes (cell constant of (1.0 ( 0.1) cm). The cells were assembled in the dry room. The samples were then held at -40 °C overnight (18 h). The measurements were run by performing a 1 K/h heating scan from -40 to 100 °C using an AMEL 160 conductivity meter and a climatic test chamber (Binder GmbH MK53) located in the dry room. The entire setup was controlled by software developed at ENEA.

NMR Measurements. A Bruker Avance 400 MHz spectrometer with an Ultra Shield 89 m cryomagnet with a magnetic flux density of 9.4 T was employed for all NMR measurements.A gradientprobehead (Bruker,Diff 30) provided a maximum gradient strength of 12 T/m for diffusion measurements. The gradient coils were cooled by a water circulation unit. For both 1H and 19F experiments, a tunable RF insert (376-400 MHz) was used, which, by appropriateisolation, was especiallyconfiguredfor low and hightemperaturemeasurements.

Self-diffusioncoefficients of 1H and 19F were measured using pulsedfieldgradientNMR (PFG-NMR).27,28The stimulatedecho pulse sequence was used, which contains three 90° pulses: {π/2-τ-π/2-T-π/2-τ-ECHO}. This sequence was combined with two gradient pulses of strength g and duration δ after the first and the third 90° pulse within the delay time τ, respectively. τ was kept constant for all measurements, τ ) 6.2 ms. The waiting time between the two gradient pulses is the diffusion time ∆. For the 1H and 19F diffusion measurements, ∆ ranged from 100 ms at low temperatures to 70 ms at high temperatures, respectively. The gradient strength g was varied at 1180 G cm-1 for all experiments, and δ was set to 4 ms below the melting point and 2.5 ms above the melting point of the ionic liquid.

The analysis of the echo decays was based on the area of the resonances of 1H and 19F for the cation and anion, respectively. The decay of the signal as a function of g resulted in the diffusion coefficient D by fitting the exponentialdecay function:

In the case of 1H, the values obtained for different resonances were averaged to yield a final diffusion coefficient.

Besides the self-diffusion coefficients, the full width at half-maximum (FWHM) of the 1H and 19F signals was determined. For the measurements of the corresponding onedimensional spectra, a simple 90° pulse-acquisition was employed. In addition, these spectra were used to determine the liquid fraction in the ionic liquid system below the melting point. At room temperature, the signal of the ionic liquid equals 100% of the excited spins. Below the melting point, the NMR signal only represents the liquid-like part of the ionic liquid; less spins are excited than at room temperature or in the molten state. This result can be correlated to the signal where all spins are registered and the liquid fraction can be assigned.

For NMR measurements, the ILs were filled into NMR tubes in an Ar glovebox and subsequently sealed under vacuum (10-2 bar) and liquid N2 cooling. Temperaturedependent NMR experiments were always started from frozen samples. To induce crystallization, the samples were quenched and kept in liquid N2 for about 5 min, and then placed into the NMR spectrometer at a temperature about 10 K below their TM. After measurement at one temperature, the samples were heated by 1 K and equilibrated at the new temperature for about 15 min. The sample temperature was calibrated by a GMH 3710 with a Pt100 thermocouple (Greisinger electronics, Germany) and controlled with a precision of 0.25 °C.

Results and Discussion

The DSC traces of all four ionic liquids are displayed in Figure 1. In the two FSI-based ionic liquids, besides the

melting, there are two additional solid-solid phase transitions, respectively. In the two TFSI-based ILs, only the melting is registered. A detailed discussion concerning different phase transition and melting in the DSC traces is given in the literature. 29,30 The melting temperatures TM of the four ILs range from 254 to 285 K in dependence of the ion sizes. The melting points of the TFSI-anion-based ionic liquids are higher than those of the FSI-anion-based ones.

At the same time, the Pyr14-cation-based ionic liquid shows lower TM values than the Pyr13-cation-based ionic liquids. This means the larger is the difference between the size of

the cation and the anion, the lower is TM. The Pyr13+ has nearly the same size as the TFSI-. Because of this fact, the lattice can more easily be formed in Pyr13TFSI than in the IL Pyr14FSI, where Pyr14+ is much larger than FSI-. This results in the following order in TM: Pyr14FSI < Pyr13FSI < Pyr14TFSI < Pyr13TFSI.

This sequence of TM for these pyrrolidinium-based ionic liquids is confirmed by NMR measurements. In Figure 2a-d, the comparison of the DSC heating traces and the changes in the FWHM of the 19F signal during melting are displayed for the different ionic liquids. The filled symbols are the FWHM of the NMR signal. At temperatures below the melting point, the registered FWHM values range from 160 to 260 Hz, which is clearly larger than expectedfor the liquid state. Representative for all nuclei, Figure 2 shows the FWHM of the 19F signal. Qualitatively, the same behavior is found for 1H (data not shown). In the liquid state, above TM, the FWHM is about 80

Hz. The melting temperature determined by NMR (TM(NMR))i s defined as the temperature where the decay of FWHM is one- half of the difference in the solid and in the liquid. The melting temperatures are indicated by dash-dotted lines for TM and by dashed lines for TM(NMR) in Figure 2b-d. In Figure 2a, TM and TM(NMR) are the same, and due to that just the dashed line for TM(NMR) is displayed. Both TM and TM(NMR) have the same values within their errors. In addition to TM, a solid-solid phase transition at the phase transition temperature TP for Pyr13FSI was registered by DSC measurement and by NMR. Here, the

FWHM of the NMR signaldecreasesas well.The decreaseitself is less steep than the change in the peak’s FWHM at TM (cf., Figure 2b). The temperatures measured by DSC and NMR are given again by a dash-dotted line (TP) and a dashed line

(TP,NMR). An example for the changes in the peak’s FWHM during the melting for the 1H and the 19F signal in Pyr14TFSI is given

Figure 1. DSC traces of the different ionic liquids: (a) Pyr14FSI; (b) Pyr13FSI; (c) Pyr14TFSI; and (d) Pyr13TFSI.

Figure 2. Comparison of the melting points determined with DSC (O) and NMR (blue 9) via the full width at half-maximum (FWHM).

The dotted lines represent the melting temperatures. (a) Pyr14FSI; (b) Pyr13FSI; (c) Pyr14TFSI; and (d) Pyr13TFSI.

Melting Behavior and Ionic Conductivity in Hydrophobic ILs J. Phys. Chem. A, Vol. x, No. x, X C

in Figure 3. One can see that the proton and fluorine spectra of

Pyr14TFSI show a continuouschange of the spectralshape while the sample is melting. Narrowing of the proton resonances upon heating becomes mainly evident by the resolution enhancement allowing the separation of single peaks. Generally, the peak width decreases with increasing temperature by a factor of 2-3, depending on the investigated ionic liquid. In addition, there is no chemical shift present for the 1H and 19F spectra as a function of temperature.Consequently,no occurrenceof any specieswith different chemical surroundingswhile the ionic liquid is melting could be detected.

(Parte 1 de 2)

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