Polyethylene oxide electrolyte membranes with pyrrolidinium-based ionic liquids

Electrochimica Acta 55 (2010) 5478–5484

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Polyethylene oxide electrolyte membranes with pyrrolidinium-based ionic liquids E. Abitelli a , S. Ferrari a , E. Quartarone a , P. Mustarelli a,∗ , A. Magistris a , M. Fagnoni b , A. Albini b , C. Gerbaldi c a b c

Dept. of Physical Chemistry, University of Pavia, Via Taramelli 16, 27100 Pavia, Italy Dept. of Organic Chemistry, University of Pavia, Via Taramelli 12, 27100 Pavia, Italy Dept. of Material Science and Chemical Engineering, Politecnico di Torino, C.so Duca degli Abruzzi, 24, 10129 Torino, Italy

a r t i c l e

i n f o

Article history: Received 27 February 2010 Received in revised form 19 April 2010 Accepted 24 April 2010 Available online 17 May 2010 Keywords: Polyethylene oxide Pyrrolidinium Ionic liquids Ether moieties Polymer electrolytes Lithium batteries

a b s t r a c t Two pyrrolidinium-based ionic liquids (ILs) with ether groups, and namely N-methoxyethylN-methylpyrrolidinium bis-trifluoromethanesulfonimide (PYRA1,2O1 ) and 1-(2-(2-(2-methoxyethoxy) ethoxy)ethyl)-1-methylpyrrolidinium bis-trifluoromethanesulfonide (PYRA1,2(O2)2O1 ), were used as plasticizers for the PEO20 –LiTFSI solid polymer electrolyte. The ionic liquids differ for the number of oxygens and lateral chain length. The properties of the plasticized polymer electrolytes were investigated by means of thermal analysis, impedance spectroscopy, XRD, FTIR spectroscopy and voltammetry. Both the ILs enhanced the conductivity of PEO20 –LiTFSI of about one order of magnitude at 40 ◦ C. The polymer electrolyte plasticized with PYRA1,2O1 showed a higher transport number, and a wider electrochemical window. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Rechargeable lithium batteries based on polymer electrolytes (Li-ion polymer batteries) are needed as power sources for portable electronic devices, telecommunication equipments and hybrid electric vehicles (HEVs). Among the polymers available on the market, polyethylene oxide (PEO) is still widely studied as a promising base for electrolyte membranes [1], mainly because of the good thermal properties and interfacial stability. The typical PEObased solid polymer electrolytes (SPE) are prepared by dissolving a lithium salt with a large anion in a high-molecular weight PEO matrix [2]. The resulting materials are more stable toward lithium metal than the liquid electrolytes. However, these polymers are often highly crystalline (or undergo rapid crystallization) at room temperature; as a consequence, conductivity values higher than 10−4 S/cm are obtained only above the polymer melting (T ∼ 70 ◦ C). Several approaches were developed in order to increase the conductivity at lower temperatures, such as the addition of inorganic phases to obtain (nano)composite polymer electrolytes, or the polymer plasticization by means of high-boiling liquids (e.g. lowmolecular-weight polyethylene glycol, or organic carbonates) [3].

∗ Corresponding author. Tel.: +39 0382 987205; fax: +39 0382 507575. E-mail address: [email protected] (P. Mustarelli). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.04.099

Organic carbonates like propylene carbonate (PC) may also improve the ion dissociation due to the presence of oxygen atoms available for the Li+ coordination [4,5]. Unfortunately, these organic carbonates are not safe and stable, since they can give highly exothermic reactions when interfaced with Li metal at the anode. At present, room temperature ionic liquids RTILs are considered as the most promising solvents/plasticizers for lithium batteries due to their nonvolatility and nonflammability, as well as to their high room temperature ionic conductivity, combined to wide electrochemical windows. Several RTILs, mainly based on imidazolinium and pyrrolidinium cations, were used as plasticizing agents in PEOn –LiTFSI [6–11]. Ionic conductivities higher than 10−5 S/cm were easily obtained at room temperature for IL molar contents, x = molIL /molLi+ , >1. Significant improvements in terms of interfacial stability and cycling behaviors were also obtained, with specific capacity exceeding 120 mAh g−1 at moderate temperature in case of methylpropylpyrrolidinium based salts (PYRA13 X) [12]. In this paper, we test two new pyrrolidinium-based ionic liquids as plasticizers for the PEO20 –LiTFSI SPE, and namely Nmethoxyethyl-N-methylpyrrolidinium bis-trifluoromethanesulfonimide (PYRA1,2O1 ) and 1-(2-(2-(2-methoxyethoxy)ethoxy) ethyl)-1-methylpyrrolidinium bis-trifluorometha-nesulfonimide (PYRA1,2(O2)2O1 ). Both of them contain ether functionalities, chemically similar to the CH2 CH2 O– unit of the PEO backbone, which differ for the number of oxygens and the overall chain length. In a previous work, we showed that the –O– atoms in

E. Abitelli et al. / Electrochimica Acta 55 (2010) 5478–5484

5479

solution of LiTFSI in degassed water and the resulting biphasic system was stirred overnight. The resulting IL6 was first dissolved in methanol and then mixed to active charcoal. The solution was refluxed overnight in order to remove the eventual reaction impurities. After charcoal filtration and solvent evaporation, the ionic liquid was deeply dried under vacuum at 80 ◦ C for 24 h. 1 H NMR (ı, CDCl ): 2.10–2.20 (bs, 4H), 3.00 (s, 3H), 3.25 (s, 3H), 3 3.30–3.60 (m, 14H), 3.75–3.85 (br s, 2H). 13 C NMR (ı, CDCl3 ): 21.0 (CH2 ), 48.4 (CH3 ), 58.5 (CH3 ), 63.2 (CH2 ), 64.7 (CH2 ), 65.3 (CH2 ), 69.8 (CH2 ), 69.9 (CH2 ), 70.0 (CH2 ), 71.4 (CH2 ), 119.0 (q, CF3 , J = 240 Hz). Anal. Calcd. for C14 H26 F6 N2 O7 S2 : C 32.81, H 5.11. Found: C 32.8, H 5.2. 2.2. Preparation of the polymer electrolytes

Scheme 1. Chemical structure of the solvents and liquid electrolyte. IL2: N-methoxyethyl-N-methylpyrrolidiniumbis-trifluoromethanesulfonimide (PYRA1,2O1 ); IL6: 1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-1-methylpyrrolidinium bis-trifluoromethanesulfonimide (PYRA1,2(O2)2O1 ); M = solution of LiTFSI in IL2, m = 0.41 mol kg−1 ; PC = propylene carbonate.

the lateral chain of the pyrrolidinium cation give better thermal stability, higher conductivity and lower viscosity than the common alkylpyrrolidinium-based ILs. These positive effects are not altered by the presence of a lithium salt up to molal concentration, m, ∼0.4 mol kg−1 [13]. Here, we report on the preparation and characterization of PEO20 –LiTFSI polymer electrolytes plasticized with different molar contents of PYRA1,2O1 and PYRA1,2(O2)2O1 . To our knowledge, this is the first time these ILs are used to plasticize PEO. The role of both the concentration and the molecular structure of the RTILs on the thermal, structural, and electrochemical properties of the polymer electrolytes are investigated by means of thermal analysis, infrared spectroscopy, impedance spectroscopy and voltammetry. The properties of these new systems were also compared with a PEO20 –LiTFSI plasticized with a more conventional solvent (PC), and with a gel polymer electrolyte PEO–(PYRA1,2O1 TFSI–LiTFSI) 37/63 (w/w) with respect to PEO. This last sample will be called GPE in the following. 2. Experimental 2.1. Synthesis of the ionic liquids Scheme 1 shows all the plasticizers used in the preparation of the PEO-based polymer electrolytes. PYRA1,2O1 , PYRA1,2(O2)2O1 and Li-based PYRA1,2O1 solution were synthesized in our lab. PEO (4 M), lithium bis-trifluoromethanesulfonimide (LiTFSI) and propylene carbonate (PC) were commercial products (Fluka, Aldrich). 2.1.1. Synthesis of N-methoxyethyl-N-methylpyrrolidinium bis-trifluoromethanesulfonimide (PYRA1,2O1 , IL2) The ionic liquid PYRA12O1 TFSI (IL2) was synthesized as reported in details elsewhere [13]. The lithium salt–ionic liquid solution of molality m = 0.41 mol kg−1 (PYRA12O1 -LiTFSI) was prepared by adding a proper amount of LiTFSI, previously dissolved in acetone, to the purified ionic liquid. The solution was dried at 70 ◦ C in order to remove completely the solvent. Each operation was carried out in a dry-box (MBraun, O2 < 1 ppm, H2 O < 1 ppm) under argon atmosphere. 2.1.2. Synthesis of 1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)1-methylpyrrolidinium bis-trifluorometha-nesulfonimide [PYRA1,2(O2)2O1 , IL6] 1-Iodo-3,6,9-trioxadecane was dissolved in a solution of N-methylpyrrolidine in ethyl acetate to form N-methoxyethyl-Nmethylpyrrolidinium iodide. After purification, it was mixed to a

Homogeneous films of (PEO)20 –LiTFSI–xL (L = IL2, IL6, PC) were obtained with the hot-pressing technique. In the case of IL2 and IL6, 4 molar contents, x = nIL /nLi+ were chosen, and namely 0, 0.5, 1.0, 1.5. At high IL content (x > 0.5) free-standing films were obtained only when IL2 was used as the plasticizer. All the products were mixed in a dry-box using a agate mortar. The mixture was then pressed by using a thin film-maker accessory at 100 ◦ C and 1 ton for about 10 min to form a film of about 100 ␮m. A similar procedure was adopted in the preparation of the gel polymer electrolyte PEO–(IL2–LiTFSI), where a solution of IL2–LiTFSI of m = 0.41 mol kg−1 was used in a weight ratio of 37/63 (w/w) with respect to PEO. This weight ratio corresponds to an IL molar content x ∼ = 0.85. 2.3. Characterization Differential scanning calorimetry (DSC) measurements were performed on a 2910 MDSC (TA Instrument) in the temperature range between −100 ◦ C and 300 ◦ C under nitrogen purge at a heating rate of 5 ◦ C/min. Thermogravimetry (TGA) measurements were carried out by a 2950 TGA (TA instrument) with a scan rate of 5 ◦ C/min up to 600 ◦ C in N2 atmosphere. X-ray powder diffraction (XRPD) patterns were collected by using a Bruker-D8 Advance Diffractometer with Cu anticathode radiation. Measurements were performed in the 2 range from 10◦ to 30◦ with a scan step of 0.02◦ and a fixed counting time of 2 s for each step. FTIR spectra were collected by means of a Fourier Transform Infrared spectrometer Jasco FTIR-410 in reflectance mode. A 30◦ specular reflectance accessory (Pike Technology) was used for the analysis of polymer films with thickness of ∼30 ␮m. Impedance spectroscopy sweeps were carried out to measure the ionic conductivity of the polymer electrolytes, by using a frequency response analyser (FRA Solartron 1255), connected to an electrochemical interface (Solartron 1287), over the frequency range 1 Hz–1 MHz at a voltage of 100 mV. The impedance scans were performed onto a two SS electrodes cell (cell constant 2 cm−1 ) in the temperature range between −25 ◦ C and 100 ◦ C. Voltammetry linear sweeps on the polymer electrolytes were performed with stainless steel electrodes, using Ag as the reference electrode. In the case of liquid electrolyte the three-electrodes cell (Cypress Systems) was made of glassy carbon as the working electrode, a Pt wire as the counter one and Ag/Ag+ (CH3 CN, 1 mM AgNO3 ) as the reference. The effective electrode area of the glassy carbon electrode, 0.033 cm2 , was calculated from cyclic voltammograms on a 1 mM ferrocene solution in CH3 CN (0.1 M tetrabutylammonium perclorate), by applying the Randles–Sevcik equation to the resulting peak current: ip = 0.4463nF

 nF 1/2 RT

AD1/2 v1/2 c

(1)

5480

E. Abitelli et al. / Electrochimica Acta 55 (2010) 5478–5484

where ip is the peak current, n is the number of electrons in the charge transfer step, A is the electrode area, D is the diffusion coefficient of Fc (2.5 × 10−5 cm2 s−1 ), c is the concentration and the v the scan rate (10 mV s−1 ). The lithium transference number, TLi+ , was determined by dc polarization combined with impedance spectroscopy, as proposed by Evans et al. [14]. The method consists in applying a small dc pulse, V, to a symmetrical Li/electrolyte/Li cell and measuring the initial, I0 , and the steady state, Iss , currents which flow through the cell. The same cell was also monitored by impedance spectroscopy to detect the initial, R0 , and the final, Rss , resistances of the two Li interfaces in order to account for the resistance of passivation layers and the eventual increase of this value upon the duration of the dc pulse. Under these circumstances, the lithium transference number, TLi+ , is given by: TLi+ =

Iss V − I0 R0 I0 V − Iss Rss

Fig. 1. Arrhenius plots of the ionic conductivity for IL2 (open circles) and IL6 (filled circles).

3. Results and discussion 3.1. The ionic liquids, IL2 and IL6 Table 1 compares some physical properties of IL2 and IL6 with those ones of PC and the LiTFSI–IL2 (m = 0.4 mol kg−1 ) solution. Both the ionic liquids show a glass transition temperature, Tg , below −70 ◦ C. However, a lower value is observed for the IL with the shorter ether chain (IL2). Accordingly, lower values of viscosity and density and are obtained by decreasing the ether-based chain length. The addition of LiTFSI to IL2 (m = 0.41 mol kg−1 ) leads to a relevant decrease of the Tg and  values, as better discussed elsewhere [13]. In contrast to what observed for alkylpyrrolidinium systems, IL2 and IL6 do not show any crystallization and/or melting phenomena, as proved by DSC measurements reported in our recent paper [13]. This is likely due to the presence of the etheric oxygen which gives origin to polar electrostatic interactions inside the liquid. Fig. 1 shows the ionic conductivity of the pure IL2 and IL6 vs. the temperature. Conductivity values in the range 1–7 mS cm−1 are obtained between 20 ◦ C and 60 ◦ C. However, the conductivity slightly decreases by increasing the ether chain length, as expected from the higher viscosity measured at room temperature (see Table 1). Contrary to the alkylpyrrolidinium and other similar electrolytes for which a VTF relationship generally fits well the ionic transport [15,16], the conductivity behavior of IL2 and IL6 may be fitted in terms of the Arrhenius equation, at least in the relatively small temperature range we explored. Activation energies, Ea ∼ 0.3 eV are obtained for both the samples. In contrast, the activation energy dramatically decreases (Ea < 0.15 eV) for LiFFSI-IL2 solution. Fig. 2 reports the linear voltammetry curves of IL2 and IL6. As already stated, the electrochemical data were collected by using an Ag+ /Ag reference electrode calibrated to the ferrocene–ferrocinium redox couple in order to obtain an absolute reference. This was

made in order to rule out possible ILs reactivity with lithium metal. For a more detailed discussion see Ref. [13]. Both the liquids show electrochemical windows wider than 5 V vs. Fc/Fc+ . However, IL6 is more stable from the cathode point of view (about 1 V) than IL2. The presence of a longer ether chain therefore improves the reduction limit and protects the pyrrolidinium cation against decomposition. Similar findings were reported for the alkylpyrrolidinium systems, which showed a dependence of the electrochemical stability from the lateral chain length of the cation [16]. 3.2. The plasticized polymer electrolytes 3.2.1. Thermal behavior Several physico-chemical properties of some polymer electrolytes are reported in Table 2. Fig. 3 shows the DSC heating traces of the plasticized polymer electrolytes PEO20 –LiTFSI–xIL2. The curve of PEO20 –LiTFSI SPE is also reported for the sake of comparison. The thermograms of the plasticized polymer electrolytes are similar to that of the SPE. In all the cases, only one glass transition and the melting of the polymer crystalline fraction are seen. The glass transition temperature of PEO–LiTFSI decreases by increasing the molar content of IL2, approaching the value measured for the pure ionic liquid (see Table 1). The melting temperature decreases from ∼60 ◦ C up to ∼40 ◦ C in presence of the ionic liquid. Following the literature [11,17], we can conclude that the ionic liquid has a plasticizing role and does enhance the amorphous

Table 1 Physical parameters of the chosen solvents and liquid electrolytes. Tg , glass transition temperature; d and , density and viscosity measured at 25 ◦ C, respectively; , ionic conductivity at 25 ◦ C; Ea , activation energy from the conductivity data. Liquid (L)

Tg (◦ C)

d (g cm−3 )

 (Pa s)

 (mS cm−1 )

Ea (eV)

IL2 IL6 PCa LiTFSI–IL2 (m = 0.41 mol kg−1 )

−87 −79 −107 −92

1.40 1.59 1.19 1.41

0.047 0.073 0.002 0.028

1.30 0.89 – 1.27

0.30 0.28 – 0.12

a

Data taken from: [22,23].

Fig. 2. Linear voltammetry sweeps at room temperature of IL2 and IL6. Scan rate 10 mV s−1 .

E. Abitelli et al. / Electrochimica Acta 55 (2010) 5478–5484

5481

Table 2 Physical properties of some polymer electrolytes: glass transition temperature, Tg ; melting enthalpy, Hm , crystallinity degree of the polymer, Xc , and lithium transport number, TLi+ . Ea is the activation energy calculated from the conductivity data in the temperature range from −20 to 40 ◦ C. Polymer electrolyte

Tg (◦ C)

Hm (kJ g−1 )

Xc (%)

TLi+

 40 ◦ C (mS cm−1 )

Ea (eV)

PEO20 –LiTFSI PEO20 –LiTFSI-0.5IL2 PEO20 –LiTFSI-0.5IL6 PEO20 –LiTFSI–0.5PC PEO–(PYRA1,2O1 TFSI–LiTFSI) (GPE sample)

−45 −56 −62 −56 −81

124 80 69 67 21

69 44 38 37 17

0.29 0.23 0.17 0.06 0.37

0.03 3.4 6.2 0.13 0.025

0.95 0.84 0.75 0.69 0.60

Hm values are normalized with respect to the value of the pure PEO.

Fig. 3. DSC thermograms of the plasticized polymer electrolytes PEO20 –LiTFSI–xIL2.

fraction of the polymer, as also demonstrated by the progressive decrease of the melting enthalpy. This last effect is better evidenced in Fig. 4, where the behaviors of the melting enthalpy of the crystalline fraction, Hm , and of the glass transition temperature, Tg , are plotted vs. the ionic liquid molar content. A Hm decrease of about 30% is observed from x = 0.0 to x = 0.5, while further increases of ionic liquid do produce only minor effects. In contrast, a linear decrease of Tg from −45 to −73 ◦ C is observed by increasing the ionic liquid amount. Fig. 5 reports the DSC traces of PEO20 –LiTFSI–0.5L films (L = IL2, IL6 and PC) and on the GPE sample. Each sample exhibits a neat glass transition below −50 ◦ C followed by endothermic effects, which can be ascribed to melting processes, starting just above room temperature. The melting enthalpies and the Tg values obtained by the DSC scans are reported in Table 2. By comparing the different

Fig. 4. Plot of the melting enthalpy, Hm , and of the glass transition temperature, Tg , of the PEO20 –LiTFSI–xIL2 polymer electrolytes vs. the IL2 molar content, x. Hm values are normalized with respect the value of the pure PEO.

curves, we see that in the case of GPE all thermal phenomena are shifted at temperatures appreciably lower than those of the corresponding effects in the plasticized systems containing PC, IL2, and IL6. In particular, the Tg value of −81 ◦ C is not far from to the one recorded on the ionic liquid/salt solution (−92 ◦ C, see Table 1). As reported in Section 2, the IL molar content of GPE is x = 0.85, which falls inside the explored range of the PEO20 –LiTFSI–xIL electrolytes (0 ≤ x ≤ 1.5). Now, if we compare the data of Table 2 and Fig. 4, we see that the GPE electrolyte displays a Tg about 15 ◦ C lower than that linearly extrapolated for a polymer electrolyte with composition PEO20 –LiTFSI–0.85IL2 (−64 ◦ C). This calls for a multi-phase system with regions where the Tg is intermediate between that of the IL–LiTFSI and that of PEO20 –LiTFSI–0.85IL2. We can infer that the addition of the solution to the PEO matrix leads to the formation of a cage structure in which the IL-salt solution is contained. Another indirect evidence of the cage structure of the GPE sample is given by the narrow endotherm at ∼180 ◦ C in curve (d), which can be ascribed to PEO decomposition. Both the samples GPE and PEO20 –LiTFSI–0.5IL6 present a double endothermic melting peak likely due to inhomogeneous crystalline fractions. Such an inhomogeneity seem to be less evident in the other samples. Finally, the broad endotherm at ∼100 ◦ C, which is particularly evident in sample IL6, can be attributed to the mixing enthalpy of the amorphous phase above its glass transition and the melted crystalline one. 3.2.2. Structural features The influence of the ionic liquids on the polymer structure was also investigated on some selected samples by X-rays diffraction and FTIR spectroscopy. Fig. 6 reports the X-rays diffractograms of pure PEO, PEO20 –LiTFSI, PEO20 –LiTFSI–0.5IL2 and of the GPE sample. The neat diffraction peaks at  ∼ 19◦ and  ∼ 23◦ are characteristic of the crystalline structure of the pristine polymer. The

Fig. 5. DSC thermograms of the system PEO20 –LiTFSI–0.5L [(a) IL6; (b) IL2; (c) PC] and of the GPE sample PEO–(PYRA1,2O1 TFSI–LiTFSI) (d).

5482

E. Abitelli et al. / Electrochimica Acta 55 (2010) 5478–5484

Fig. 6. X-rays diffractograms of pure PEO (a); PEO20 –LiTFSI (b); PEO20 –LiTFSI–0.5IL2 (c); GPE sample PEO–(PYRA1,2O1 TFSI–LiTFSI) (d).

presence of these peaks suggests that the lithium salt and the ionic liquid do not change the PEO crystalline structure. Li+ complexation takes place in the amorphous phase, as expected from the literature [18]. However, the intensity of the diffraction peaks remarkably decreases, because of the increase of the amorphous fraction, both in the plasticized polymers and in the SPE sample. In the last case, this is due to the plasticizing nature of the TFSI− anion. The phase changes induced by the presence of the TFSI− anion and by the ionic liquids are better evidenced by FTIR spectroscopy. Fig. 7 shows the FTIR absorption spectra of the same samples of Fig. 6 in the spectral regions between 3500 and 2500 cm−1 (part a) and 1600 and 400 cm−1 (part b). Basically, the main vibration modes of polyethylene oxide are assigned to the methylene (CH2 ) and ether (C–O–C) groups. In particular, the broad peaks around 2900 cm−1 , at 1465 cm−1 and 850 cm−1 are assigned to the symmetric and asymmetric C–H stretching, asymmetric C–H bending and the CH2 rocking, respectively [19]. In presence of the ionic liquid and LiTFSI–IL2 solution, the peak at 1465 cm−1 becomes wider and more intense. This feature is generally ascribed to the increase of the amorphous phase, as already observed for similar IL-based PEO systems [17]. The same trend is also observed for the CH2 rocking modes. The double peak at 1355 cm−1 in the PEO spectrum (a), which is assigned to CH2 wagging, is a marker of the crystalline phase in the pure polymer [20]. In presence of LiTFSI, and mostly of the ionic liquids, the wagging vibrations turn in a single and neat peak, which further confirms crystalline-to-amorphous phase changes. The small band at 1215 cm−1 , observed in the spectra of PEO20 –LiTFSI (b), PEO20 –LiTFSI–0.5IL2 (c) and GPE (d) is generally attributed to the Li+ –TFSI− ion pair association [17]. The intensity of this peak is slightly decreased in presence of the ionic liquids. This is probably due to the capability of the solvent to weaken the interactions between Li+ and TFSI− . As already stated, the region between 1100 and 1150 cm−1 is assigned to the C–O–C stretching, which is greatly affected by the Li+ complexation. In general, speculations on the interaction between Li+ and polymer are complicated by the presence of the TFSI− absorptions, which are particularly strong in this spectral area. Generally speaking, we see that the intensity of the stretching bands decreases in the PEO–salt system, as expected by taking into account the Li complexation. The presence of the ionic liquid further weakens the C–O–C stretching, in particular in the case of PEO20 –LiTFSI–0.5IL2, and this can recall eventual coordination of PEO oxygen with the pyrrolidinium cation, which is less important in the GPE sample, in agreement with the DSC results.

Fig. 7. FTIR reflectance spectra of pure PEO (a); PEO20 –LiTFSI (b); PEO20 –LiTFSI–0.5IL2 (c); GPE sample PEO–(PYRA1,2O1 TFSI–LiTFSI) (d) in two spectral regions.

3.2.3. The ionic conductivity Fig. 8 shows the conductivity values of the PEO20 –LiTFSI–IL2 electrolytes vs. the molar content, x, of ionic liquid. The logarithm of the conductivity increases quasi-linearly with the amount of IL2, in particular at temperature below 0 ◦ C, where PEO is still crystalline. Values up to ∼7 × 10−5 S cm−1 are obtained at 20 ◦ C in presence of x = 1.5 of IL2. Similar results were found by Passerini et al. in case of the system PEO20 –LiTFSI–xPYRA14 (methylbutylpyrrolidinum bistrifluorosulfonimide) [9]. The conductivity enhancement is reasonably related to the increase of the polymer amorphous phase, and to the decrease of the Tg and consequently, of the viscosity at a given temperature. Table 2 reports the conductivity values measured at 40 ◦ C of PEO20 –LiTFSI–0.5L (L = IL2, IL6, PC) and of GPE. By comparing the electrolytes plasticized with the ILs, we observe that IL6 scores better than IL2, likely because of the lower Tg and the higher amorphous fraction. However, the best electrolyte from the point of view of the conductivity is that plasticized with propylene carbonate,

E. Abitelli et al. / Electrochimica Acta 55 (2010) 5478–5484

5483

Fig. 8. Conductivity behavior of PEO20 –LiTFSI–xIL2 against IL2 molar content, x, at three different temperatures. The error bar is given by the symbols dimension.

which shows  40 ◦ C > 10−4 S cm−1 . Such a result may be related to the fact that PC is a free-anion solvent, contrary to IL2 and IL6 where the ion transport may suffer of additional Li+ –TFSI− ion pair interactions due to the presence of the anions provided by the ionic liquid. The lower viscosity of PC with respect to the ionic liquids is another reason to explain the higher conductivity of the membrane plasticized with propylene carbonate. Finally, the GPE shows a conductivity value much lower than the other electrolytes. This seems to suggest that the ion transport chiefly occurs in the liquid electrolyte, whereas the polymer plays the role of a inert cage which hinders the conductivity due to dilution effects. Similar conductivity values were observed for PVDF-based gel systems with comparable polymer/solution weight ratio [20]. The conductivity behavior may be considered as nearly Arrhenian both before and after the polymer melting. Fig. 9a shows, as an example, the conductivity values of the IL2-based electrolytes in the temperature range between −25 and 100 ◦ C. The resulting activation energies, Ea , obtained by the linear fit in the region from −20 to 40 ◦ C, are plotted in Fig. 9b as a function of IL2 molar content, x. As expected, Ea decreases by increasing the IL2 content. The activation energies of the other electrolytes for x = 0.5 are reported in Table 2.

Fig. 9. Conductivity Arrhenius plots of the PEO20 –LiTFSI–xIL2 electrolytes (part a), and activation energies, Ea , vs. the IL2 molar content, x (part b). x = 0 (squares); x = 0.5 (circles); x = 1.0 (triangles); x = 1.5 (stars).

made in the literature [8]. The SPE electrolyte displays a value of 4.2 V in agreement with the literature [21]. The addition of the ionic liquid causes a significant decrease of the cathodic stability, which is reduced to ∼0.6 for x = 1.5. At the IL content x = 0.5 the stability limit is 2.7 V, which is much better than that of the corresponding

3.2.4. The transport number, TLi+ As already stated in the experimental section, the lithium transport number was calculated by chronoamperometric technique and impedance spectroscopy, performed on a Li/PE/Li symmetrical cell at 40 ◦ C. In the case of IL2-based systems, the lithium transport number, TLi+ , decreases by increasing the IL molar content, contrary to the conductivity behavior. TLi+ s of 0.3, 0.23 and 0.1 were obtained for PEO20 –LiTFSI–xIL2 at x = 0, 0.5 and 1.0, respectively. Similar trends were found by Cheng et al. for the same polymer electrolyte, plasticized by PYRA14 TFSI (butylmethylpyrrolidinium), in the molar range, x = 0–1.0. In particular, the authors reported TLi+ values of 0.396 and 0.15, for the samples with x = 0 and x = 1, respectively, in good agreement with our results [11]. We obtained a lower transport number (∼0.17) for the system containing the more viscous IL6. In contrast, TLi+ near to that of PEO20 –LiTFSI was determined in the case of the GPE electrolyte. 3.2.5. The cathodic electrochemical stability limit (CESL) of the plasticized electrolytes Fig. 10 shows the cathodic electrochemical stability limits of the PEO20 –LITFSI–xIL2 electrolytes. The values obtained vs. Ag/Ag+ (see Section 2) have been rescaled in terms the couple Li/Li+ , as usually

Fig. 10. Cathodic stability limits (vs. Li/Li+ ) of the PEO20 –LiTFSI–xIL2 electrolytes vs. the IL2 molar content, x.

5484

E. Abitelli et al. / Electrochimica Acta 55 (2010) 5478–5484

electrolytes plasticized with IL6 (CESL = 1 V), and PC (CESL = 0.5 V). The GPE electrolyte displays a stability limit of 3 V. 4. Conclusions Two ether-functionalized pyrrolidinium ionic liquids (PYRA1,2O1 and PYRA1,2(O2)2O1 ), which differ for the ether chain length, were characterized and used to plasticize the PEO20 –LiTFSI electrolyte. Homogeneous and plastic films were obtained for different IL molar contents (up to 1.5) by means of the hot pressing technique. Thermal, electrochemical and structural properties of the plasticized electrolytes were investigated. Room temperature ionic conductivity values approaching 10−4 S cm−1 can be obtained by proper amounts of the ionic liquid. The best performances in terms of cation transport number and cathodic electrochemical stability were obtained by using the PYRA1,2O1 ionic liquid. Our results show that a proper tailoring of the ionic liquid can lead to improve the electrochemical properties of plasticized polymer electrolytes. Acknowledgement We gratefully acknowledge funding from CARIPLO Foundation (Project 2006.0688/10.8485 “Nuove membrane elettrolitiche nanocomposite a base di liquidi ionici”). References [1] C. Zhang, S. Gamble, D. Ainsworth, A.M.Z. Slawin, Y.G. Andreev, P.G. Bruce, Nature Materials 8 (2009) 580.

[2] A.M. Stephan, K.S. Nahm, Polymer 47 (2006) 5952. [3] For a review see: E. Quartarone, P. Mustarelli, A. Magistris, Solid State Ionics 110 (1998) 1, and references therein. [4] L.R.A.K. Bandara, M.A.K.L. Dissanayake, B.-E. Mellander, Electrochimica Acta 43 (1998) 1447. [5] S. Chintapalli, R. Frech, Solid State Ionics 86–88 (1996) 341. [6] J.-H. Shin, W.A. Henderson, S. Passerini, Electrochemical Communication 5 (2003) 1016. [7] Y.H. Kim, G. Cheruvally, J.W. Choi, J.H. Ahn, et al., Macromolecular Symposium 249–250 (2007) 1883. [8] J.-H. Shin, W.A. Henderson, S. Passerini, Journal of Electrochemical Society 152 (2005) A978. [9] J.-H. Shin, W.A. Henderson, G.B. Appetecchi, F. Alessandrini, S. Passerini, Electrochimica Acta 50 (2005) 3859. [10] J.W. Choi, G. Cheruvally, Y.H. Kim, et al., Solid State Ionics 178 (2007) 1235. [11] H. Cheng, C. Zhu, B. Huang, M. Lu, Y. Yang, Electrochimica Acta 52 (2007) 5789. [12] G.T. Kim, G.B. Appetecchi, F. Alessandrini, S. Passerini, Journal of Power Sources 171 (2007) 861. [13] S. Ferrari, E. Quartarone, P. Mustarelli, A. Magistris, S. Protti, S. Lazzaroni, M. Fagnoni, A. Albini, Journal of Power Sources 194 (2009) 45. [14] J. Evans, C.A. Vincent, P.G. Bruce, Polymer 28 (1987) 2324. [15] H. Ohno, M. Yoshizawa, Solid State Ionics 154–155 (2002) 303. [16] K.M. Johansson, J. Adebahr, P.C. Howlett, M. Forsyth, D.R. MacFarlane, Australian Journal of Chemistry 60 (2007) 57. [17] C. Zhu, H. Cheng, Y. Yang, Journal of Electrochemical Society 155 (2008) A569. [18] S.J. Wen, T.J. Richardson, D.I. Ghantous, K.A. Striebel, P.N. Ross, E.J. Cairns, Journal of Electroanalytical Chemistry 408 (1996) 113. [19] W. Wieczorek, D. Raducha, A. Zalewska, J.R. Stevens, Journal of Physical Chemistry B 102 (1998) 8725. [20] E. Quartarone, P. Mustarelli, A. Magistris, Journal of Physical Chemistry B 106 (2002) 10828. [21] See, for example, Y. Zhao, R. Tao, T. Fujinami, Electrochimica Acta 51 (2006) 6451. [22] J.J. Fontanella, M.C. Wintersgill, J.J. Immel, Journal of Chemical Physics 110 (1999) 5392. [23] J. Wang, Y. Zhao, K. Zhuo, R. Lin, Zeitschrift für Physikalische Chemie 217 (2003) 637.