Imidazolium-based Mono and Dicationic Ionic Liquid Sodium Polymer Gel Electrolytes

Electrochimica Acta 241 (2017) 517–525

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Imidazolium-based Mono and Dicationic Ionic Liquid Sodium Polymer Gel Electrolytes J.F. Véleza,* , L.V. Álvareza , C. del Ríoa , B. Herradónb , E. Mannb , E. Moralesa a b

Instituto de Ciencia y Tecnología de Polímeros (CSIC), C/Juan de la Cierva 3 28006 Madrid, Spain Instituto de Química Orgánica General (CSIC), c/Juan de la Cierva 3 28006 Madrid, Spain

A R T I C L E I N F O

Article history: Received 5 January 2017 Received in revised form 17 April 2017 Accepted 19 April 2017 Available online 26 April 2017 Keywords: Ionic liquids Imidazolium Electrolytes Oligo oxyethylene Gel polymer electrolyte Sodium-ion batteries

A B S T R A C T

The synthesis and characterization of novel ionic liquid (ILs) based gel polymer electrolytes for application in sodium-ion (Na+) batteries is reported. It comprises a 0.2 M solution of NaN(CF3SO2)2 (sodium bis(trifluoromethylsulfonyl) imide) in mono and dicationic imidazolium-based ionic liquids immobilized in a poly(vinylidene fluoride)-hexafluoropropylene copolymer matrix. The membranes offer acceptable ionic conductivity, up to 2.2
104 S cm1 at room temperature, with excellent thermal and electrochemical stabilities. The useful thermal stability reached up 150  C and decomposition only starts at about 400  C, while electrochemical stability window comprises from 1.5 V to 5.0 V vs. Na/Na+. Na+ transference values (tNaþ ) are in the range of 0.1 to 0.5, indicating a contribution of anionic transport (TFSI anion) and component ions of ionic liquid to the total ionic conductivity. The nature of Na+ coordination and the formation of Na+-complexes were elucidated through Raman spectroscopy. The average number of TFSI coordinated to Na+ (n) allowed to know the contribution of the formed complexes in ionic mobility mechanism in the gel electrolytes. The effect of side chain length in oxyethylenefunctionalized imidazolium mono and dicationic RTILs on both structural and electrochemical properties were discussed. Results obtained demonstrate promising characteristics of the membranes, being suitable candidates to be used in rechargeable sodium batteries. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction The indiscriminate use of natural resources, evidenced mainly by the dependence of fossil fuels on the current energy system, has led to serious environmental problems and is one of the causes of the global warming. Therefore, it is necessary to take urgent actions to mitigate the damage; among proposed actions the proper integration of renewable energies with energy storage systems in order to improve the efficiency of the current energy system has growing a big interest [1,2]. To assure commercial acceptance of these advanced energy storage systems it is necessary to improve also the safety and abuse tolerance. Within these systems, lithium-ion batteries (LIBs) have had a prominent role in research during the last decades and have been extensively used, in particular for portable electronic devices [3,4]. However, for larger scale applications lithium-ion batteries might not be the

* Corresponding author at: Instituto de Ciencia y Tecnología de Polímeros (CSIC), C/Juan de la Cierva 3. 28006 Madrid, Spain. Tel.: +34 912587568; fax: +34 91 5644853. E-mail address: [email protected] (J.F. Vélez). http://dx.doi.org/10.1016/j.electacta.2017.04.096 0013-4686/© 2017 Elsevier Ltd. All rights reserved.

best solution as lithium sources are geographical limited, its price is high and future availability is not assured [4–6]. Sodium and sodium-ion batteries (SIBs) have emerged as low-cost candidates for medium and large scale stationary energy storage applications of renewable energy and smart grid [7,8], because of their safety, long-cycle life, abundant natural resources (the fifth most abundant element in earth's crust) with homogenous geographic distribution [9] and versatile geometries [10–14] as well as the similarity in the chemical properties to Li, has making Na-ion batteries (SIBs) a promising storage devices for intermittent renewable power sources. In addition, sodium has low atomic mass (23.0) and very suitable redox potential (E0Na+/Na  2.71 V vs. SHE) [7,8,15]. The electrolyte is a crucial component of electrochemical energy storage systems, responsible for the hazards associated with lithium ion batteries. In order to enhancing the safety of these systems it is necessary to replace the common flammable carbonate solvents with more stable component [16]. An ideal replacement choice are ionic liquids (ILs) [17] due to their chemical and electrochemical stability, the strong ionic bonding nature of ILs allows them to be nonflammable as well as nonvolatile. In fact, ILs have been added to organic carbonates to reduce the flammability

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[18]. On the other hand, the use of solid electrolytes offer many advantages regarding liquid electrolytes such as ease of manufacturing, immunity from leakage, suppression of lithium dendrite formation, elimination of volatile organic liquids, high mechanical flexibility, less corrosive with the electrodes and the ability to function as the separator between the electrodes [19]. Among these, gel polymer electrolytes (GPEs) based on ionic liquid solvents occupy a front row position due to their unique properties such as wide chemical, electrochemical and thermal stability and adequate ionic conductivity. In order to make sodium-ion batteries commercially viable, high performance electrolytes with acceptable room temperature ionic conductivity and wide electrochemical stability windows are required. Additionally the development of full solid state devices require additionally reduce interfacial resistance between the solid electrolytes and the electrode materials [20]. Gel polymer electrolytes (GPE), obtained by confining liquid electrolytes into different polymer hosts have become good potential candidates for all-solid batteries, since they can accommodate volume changes of the electrodes during charging/discharging processes, facilitate the flexible designs of batteries in any desired configurations and avoid electrolyte leakage [21,22]. In this case, the metal salt provides free-mobile ions which take part in the conduction process and the polymer provides mechanical stability [21–24]. Ionic liquid electrolytes based on imidazolium cation and bis(trifluoromethylsulfonyl) imide anion have become available for electrochemical research and device development [4,25–27]. Therefore, new gel polymer electrolytes produced by incorporating Na-doped ionic liquid electrolytes into an appropriate polymer matrix have been explored. Among the host materials used in GPEs, poly(vinylidene fluoride)-hexafluoropropylene copolymer P(VdF-co-(HFP) has been widely employed because its crystalline structure assure good mechanical stability while amorphous regions retain high concentrations of the liquid electrolytes [24]. Gel polymer electrolytes based on P(VdF-co-HFP) matrix swollen by alkylsubstituted imidazolium-based ionic liquids (1-ethyl 3-methyl imidazolium trifluoro-methane sulfonate or EMI-triflate) were studied by D. Kumar and S. A. Hashmi. [28]. Similar materials has been recently studied by M. Forsyth's group to produce gel type Na+ ionic conductors using 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide (C4mpyrTFSI) [29]. Those materials have demonstrated good thermal and electrical properties and have the same focus of the present work. In this work, we report on the synthesis, thermal and electrochemical characterization of sodium gel polymer electrolytes (Na-GPEs) based on imidazolium-based mono and dicationic ionic liquids doped with sodium bis(trifluoromethylsulfonyl) imide (NaTFSI), entrapped in a P(VdF-co-HFP). Mono and dicationic imidazolium-based room-temperature ionic liquids (RTILs), having different oligo(ethylene glycol) side chain length substituents were synthesized, and the 0.2 M NaTFSI corresponding ionic liquid electrolytes were incorporated to the polymer matrix. The effect of ionic liquid structure on parameters such as the thermal stability, ionic conductivity, the electrochemical stability windows and the Na+-ion transport of the gel electrolytes was studied.

40–63 mm, Merck) or deactivated alumina (Brockmann I, SigmaAldrich). Reactions were followed using thin-layer chromatography (TLC) on silica gel-coated plates (Merck 60 F254). Detection was performed with UV light and/or by charring at ca. 150  C after dipping into an aqueous solution of potassium permanganate (KMnO4), or an ethanolic solution of phosphomolybdic acid (PMA). Yields refer to chromatographically and spectroscopically (1H-NMR) homogeneous material, unless otherwise stated. NMR spectra were recorded on Mercury-400 and Bruker-300 instruments and are calibrated using residual undeuterated solvent as an internal reference. The molecular structures and nomenclature of the imidazolium-based mono and dicationic ILs used in this paper are shown in Fig. 1. Compounds MIL-1, MIL-2 and MIL-3 were synthesized and characterized following the method described by Bara et al. [30]. NMR spectral data are in agreement with those previously reported [30] (see supplementary information Figures S1 to S6). Compounds DIL-1, DIL-2 and DIL-3 were synthesized and characterized following the method described by Zhang et al. [31]. NMR spectral data are in agreement with those previously reported [31] (see supplementary information Figures S8 to S12). IL-based gel electrolytes,having 30 wt% of the P(VdF-co-HFP) copolymer and 70 wt% of the 0.2 M NaTFSI IL electrolyte, were obtaining by dissolving in acetone, stirring for 12 h, casting on PTFE molds and drying under controlled conditions. According with the previous nomenclature of mono and dicationic ionic liquids, gels were called GMIL and GDIL respectively. All operation and handling was made inside an argon-filled dry box with a humidity level below 1 ppm. The phase transition and thermal stability of the ionic liquidbased gel polymer electrolytes were studied using differential scanning calorimetry and thermogravimetric analysis. Thermogravimetric analyses (TGA) were performed, under nitrogen atmosphere, using TA Q500 equipment in the temperature range between 25 and 800  C, at a heating rate of 10  C min1. DSC measurements were carried out using a Mettler TA4000 calorimeter. The GPE samples, housed in sealed aluminum pans, were cooled (10  C min1) from room temperature down to -150  C and, successively, heated (10  C min1) up to 100  C, under a nitrogen flow of 100 cm3 min1. Raman spectra were recorded by using a Renishaw InVia Reflex Raman system, employing a grating spectrometer with a 1200 l/ mm Peltier-cooled charge-coupled device (CCD) detector, coupled to a confocal microscope. The Raman scattering was excited using a diode laser wavelength of 785 nm. The laser beam was focused on the sample with a 0.85
100 microscope objective, with a laser power at the sample of approximately 150 mW. The exposure and

2. Experimental Poly(vinylidene fluoride-co-hexafluoropropylene) copolymer matrix P(VdF-co-HFP), trade name KF2801, was supplied by ElfAtochem and dried at 80 C under vacuum along 48 hours. Sodium bis(trifluoromethylsulfonyl)imide (99.5% purity, Solvionic S.A, France) was used without further purification. All other reagents were purchased at the highest commercial quality and used without further purification, unless otherwise stated. Flash column chromatography was performed using silica gel (60-Å pore size,

Fig. 1. Molecular structures of the imidazolium-based room temperature ionic liquids. (a) Monocationic Ionic Liquids (MILs); (b) Dicationic Ionic Liquids (DILs).

J.F. Vélez et al. / Electrochimica Acta 241 (2017) 517–525

number of accumulations for the Raman measurements were 10 seconds and 5 times respectively. The spectral resolution was better than 1 cm1. All spectra were processed and fitted using Renishaw WiRE 3.3 software. The temperature dependence of the ionic conductivity of neat ionic liquids, and the liquid and gel polymer electrolytes was measured within the -40 to 100  C temperature range using a Novocontrol Alpha analyzer in combination with a Novocontrol Quatro temperature controller in steps of 5  C after a stabilization time of 10 minutes, the frequency range used was between 107 and 0.1 Hz (10 point/decade) with an ac perturbation of 10 mV. A homemade stainless steel sealed cell (10.27 mm diameter, 1.44 mm in thickness) was used for both neat ionic liquid and the corresponding 0.2 M NaTFSI liquid electrolytes, while a goldcoated cell was used for testing gel polymer electrolyte membranes (20 mm diameter, 40 mm in thickness). The real and imaginary parts of the complex impedance were plotted and the ionic conductivity was calculated by using the ZPlot fitting software. The electrochemical stability window (ESW) of the GPEs was evaluated by cyclic voltammetry (CV) at low potentials (from -4.0 to 3.0 V vs. Na/Na+) and by linear sweep voltammetry (LSV) at high potentials (from 2.5 to 6.0 V vs. Na/Na+) employing sealed Swagelok1 type two electrode cells, assembled in argon-filled dry box, using sodium metal as counter and reference electrodes and Cu (low voltage) or Al (high voltage) as current collectors. The measurements were performed at room temperature and 2 mV s1 scan rate using Autolab PGSTAT30 potentiostat/galvanostat equipment. The room temperature sodium-ion transference number (tNaþ ) of the gel polymer electrolytes was estimated by a combination of ac impedance and dc polarization, as described by Watanabe and Bruce-Evans [32–34], using a Swagelok1 type cell with a symmetric Na0 | GPE | Na0 configuration. The surface of sodium metal was polished with a scalpel prior to use and placed on the stainless steel current collectors, then the GPE samples were sandwiched between the electrodes. The cell was assembled in a glove box (H2O and O2 < 1 ppm). A dc voltage (DV = 10 mV in this study) was applied until a steady state current was obtained (the current was stabilized after 2 h in this study), the initial (Is) and steady (I0) currents, which flow through the cell, were measured. Simultaneously, the impedance spectra of the cell were recorded in the frequency range from 106 to 0.1 Hz with an amplitude voltage of 10 mV, before and after the dc polarization, to obtain the initial (Ri) and final (Rf) resistances of the electrolyte, and the initial (R0e ) and final (Rse ) resistances of interfacial layers of the Na metal electrode/electrolyte. On the base of these values measured for the parameters above, transport number of Na+ in ILs-based GPE was then calculated by using Eq. (1): h i DV  I0 R0e   ð1Þ tNaþ ¼ Is Rf I0 Ri DV  Is Rse This equation is a slightly modified version of the original Evans et al. equation that takes in account the change in the resistance of the gel polymer electrolyte [34].

3. Results and discussion All synthesized imidazolium-based ionic liquids (mono- and dicationic) are low viscosity pale-yellow liquids, their molecular structures agree with previous reported data (see supplementary information for details) [30,31]. 0.2 M NaTFSI ionic liquid electrolytes, prepared from different imidazolium-based ILs were used to obtain the corresponding gel polymer electrolytes (30 wt% P(VdFco-HFP) copolymer, 70 wt% ionic liquid electrolyte). The procedure

519

Fig. 2. Macroscopic morphology of the GMIL-1 gel polymer electrolyte (thickness  40 mm).

provided electrolytes as self-supported, mechanically stable and flexible semi-transparent low thickness (around 40 mm) GPE membranes, which can be applied in solid state sodium-ion batteries. Fig. 2 shows the photograph of GMIL-1 gel polymer electrolyte film with solid-like appearance. The homogeneous surface of the sample indicates the uniform distribution of P(VdFco-HFP) and satisfactory confinement of the IL in the polymeric matrix (no apparent liquid disaggregated). The thermal stability of self-supported gel electrolytes was investigated by thermogravimetric analysis (TGA) in inert atmosphere (N2). Fig. 3(a). Mono- and dicationic oligo(oxyethylene)functionalized gels show at first sight one-step decomposition, similarly to the corresponding pristine (Fig. S13 and Table S1) and imidazolium-based ionic liquids electrolytes (Fig. S14 and Table S1), thus indicating that the addition of NaTFSI salt does not affect this property to a large extent. Similar thermal behavior is observed for all samples, no significant differences were observed between mono- and dicationic ILs gels. Nevertheless, dicationic ionic liquids are slightly more stable than monocationic ones and minimal effect of the chain length was observed. The maximum temperature decomposition was located at 442  C for GDIL-1, that is, the gel based on the dicationic IL with the shortest chain length, the temperature decreasing to 425  C and 382  C on increasing the oxyethylene side chain length. Similar trends have been detected for pyrrolidinium, and imidazolium and thiazolium gel polymer systems, where lower decomposition onsets were detected in presence of larger substituents [25,35,36]. Thereby, such differences in the thermal stability are associated mainly with the chain length instead the oxygen units. On the other hand, the spatial confinement of the dicationic ionic liquids in gel matrix increases the thermal stability of these electrolytes while the same decomposition mechanism is observed. The onset temperature (Tonset) values and maximum decomposition temperature (Tmax) of the GPEs are compiled in Table 1. The decomposition temperature is, in all cases, well above operating temperature of rechargeable sodium batteries (40–80  C) [37,38]. This stability range is similar to the corresponding alkoxy-substituted pyrrolidinium [39] and imidazolium [25] ionic liquids, and remarkably higher than that of the alkyl-substituted pyrrolidinium (e. g. PY13FSI and PY14FSI) [36,40], imidazolium (e.g. 1-Ethyl-3-methylimidazolium dicyanamide, EMIMDCA) [41], pyrazolium [42], and much more stable than carbonate-based gel polymers composed of PVdF-EC/PC [41]. Fig. 3(b) shows DSC heating traces of the membranes based on mono- and dicationic ILs-based gel polymer electrolytes. No signal of crystallization or melting was detected over the studied temperature range, indicating that these materials are not able to crystallize despite lowering the temperature down to -150 C, in

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GMIL - 1 GMIL - 2 GMIL - 3 GDIL - 1 GDIL - 2 GDIL - 3

70 60 50 40 30 20 10 0 0

100 200 300 400 500 600 700 800 900

Temperature / °C

(b)

GMIL - 1 GMIL - 2 GMIL - 3 GDIL - 1 GDIL - 2 GDIL - 3

Tg= -78.5°C

Exo

Tg= -60.5°C Tg= -69.9°C

Heat Flow

Tg= -52.3°C Tg= -48.8°C Tg= -48.5°C

-100

-50

0

50

100

Temperature / °C

ATFSI Coordinated

Table 1 Parameters obtained from the thermal characterization of the mono and dicationic imidazolium-based RTIL's gel polymer electrolytes. Sample

TGA Tonset/ C

Monocationic IL's based GPE's GMIL - 1 413 GMIL - 2 385 GMIL - 3 381 Dicationic IL's based GPE's GDIL - 1 442 GDIL - 2 425 GDIL - 3 382

DSC GPE/(Neat RTIL) Tmax/ C

Tg/ C

485(442) 478(426) 478(420)

-78.5 (-84.4) -60.5 (-71.0) -69.9 (-73.4)

464 478(447) 478(424)

-52.3 (-55.4) -48.8 (-59.4) -48.4 (-48.1)

good agreement with reported pyrrolidinium and pyrazolinium based membranes behaviour [39,42,43]. This fact, together with the low glass transition temperature (Tg) values suggests that at room temperature GPEs have a rubbery state that facilitates the movement of ions across the membrane. The Tg of the dicationic ionic liquid gel membranes are higher than those of the corresponding monocationic ones, in agreement with reported data [44], higher than those of the corresponding pristine ionic liquids (Fig. S13 and Table S1) and quite similar to those of the 0.2 M NaTFSI ionic liquid electrolytes (Fig. S14 and Table S1), the value being displaced to higher temperatures when increasing the

f TFSI Coordinated Atotal ¼ x molsalt x molsalt

ð2Þ

where ATFSI Coordinated is the area of the high frequency component, corresponding to the TFSI anions coordinated to Na+ cations, Atotal is the total area under the curve (fTFSI Coordinated is the fraction of the coordinated TFSI) and x mol is the mole fraction of the salt (Na concentration in GPEs, in this experiment x molsalt = 0.2). Table 2 summarizes the average coordination numbers of GPEs resulting from fitting areas of the 740 cm1 Raman band (see supplementary information Fig. S16). As can be seen, the results do not have a clear relation to the oxyethylene

741

GMIL - 1 EMIL - 1 MIL - 1 NaTFSI

Intensity / a.u.

Fig. 3. (a) TGA traces; (b) DSC traces of the synthesized gel polymer electrolytes based on P(VDF-co-HFP)-imidazolium- mono- and dicationic RT ionic liquids having different oxyethylene side chain systems with an ionic liquid/NaTFSI mol ratio 1:0.2.

n¼

740

Residual weight / %

80

742

90

oxyethylene chain length in both mono an dicationic ILs-based membranes, this fact explained in terms of the intermolecular interactions between Na+ cations and oligo(oxyethylene) chains in the IL. The extent of the intermolecular interactions are related to presence of oxygen units in ILs and exhibit a progressive increase in Tg [45]. Values of Tg, measured at the inflexion point of the Cp jump, are summarized in Table 1. Fig. 4 shows Raman spectra of GMIL-1 gel polymer electrolyte and the corresponding sodium salt (NaTFSI), neat IL and 0.2 M NaTFSI IL-based electrolyte precursors measured at room temperature. As can be seen Raman profiles of 0.2 M NaTFSI IL-based electrolyte is quite similar to the gel polymer, indicating that the confinement of the ionic liquid in the polymeric matrix does not greatly modify the structural properties associated to ionic liquids electrolytes. However, the most intense band located at around 740 cm1 shift at lower wavelength indicating important differences in the coordination shell in gel polymer electrolytes respect to neat IL, this frequency shifts upon coordination with Na+ have been well characterized in terms of TFSI anion expansioncontraction normal mode [46,47]. Further study of this band enables to relate the ILs structure with their electrical properties. The band located at 740 cm1 is commonly attributed to mixed vibrations with some share of S–N–S symmetrical stretching modes of common TFSI anion and it is very sensitive to the addition of Na+ [48,49]. The width and shift of this band are related to the interactions of Na+ ions in the GPE structure. When NaTFSI salt is added to neat IL, this band is split into three bands centered at 740, 742 and 745 cm1, corresponding to free TFSI anions, ion pairs and aggregates, respectively [50]. The ratio between A745/(A740+ A742 + A745), that is ATFSI coordinated/(Atotal) of fitted areas is associated to the degree of interactions of the sodium ion in the membrane, so called fTFSI coordinated. A quantitative determination of the fraction of coordinated anions (n) can be obtained by applying Eq. (2):

745

(a) 100

720

730

740

750

760

Raman Shift / cm

200

400

600

800

1000

1200

1400

1600

-1

Raman Shift / cm

Fig. 4. Raman spectra of GMIL–1 gel polymer electrolyte and its material precursors (sodium salt: NaTFSI; 0.2 M NaTFSI IL's based electrolyte: EMIL-1 and neat IL: MIL-1).

J.F. Vélez et al. / Electrochimica Acta 241 (2017) 517–525 Table 2 Areas obtained from the deconvolution of the 740 cm1 Raman band of the mono and dicationic imidazolium-based RTIL's gel polymer electrolytes.

Sample

Fitted Areas 740  1 cm1 742  1 cm1 745  1 cm1 fTFSI

Monocationic IL's based GPE's GMIL - 1 1.7684 7.7313 GMIL - 2 1.7286 6.7165 GMIL - 3 1.5550 7.4100 Dicationic IL's based GPE's 3.5802 GDIL - 1 4.7465 GDIL - 2 5.1197 3.2950 GDIL - 3 5.0092 2.8069

coordinated

n

2.4640 3.2967 2.0726

0.2060 0.2808 0.1878

1.0 1.4 0.9

3.6928 3.3780 3.8978

0.3072 0.2864 0.3327

1.5 1.4 1.7

chain length in both the mono and dicationic-based gel electrolytes. A reduced n values (e.g. for GMIL-3) suggest that there is a lower proportion of TFSI anions coordinated to Na+ cations and hence a preferential coordination of Na+ by the ether oxygen atoms in the IL side chain. Higher n values were obtained for dicationic ILs-based gel polymer electrolytes, which implies a mixed Na+ coordination in the first solvation shell, indicating that IL cation cannot displace TFSI anions found in the first Na+ coordination shell and hence these gel electrolytes present greater preference for the formation of more stable ionic pairs (TFSI anions coordinated to Na+ cations and/or higher aggregates) [51]. Fig. 5 (a) and (b) shows the variation of ionic conductivity (s ) with the temperature obtained for neat ILs and gel polymer electrolytes respectively. As can be seen the P(VdF-co-HFP) polymeric matrix causes a considerably reduction in ionic

(a) 10-1

T = 25 ºC

-2

10

-3

10

-4

10

σ / S cm

-1

-5

10

-6

10

-7

10

-8

10

-9

10

-10

10

-11

MIL - 1 MIL - 2 MIL - 3 DIL - 1 DIL - 2 DIL - 3

10

2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 -1

(1000/T) / K

(b) 10-1

T = 25 ºC

-2

10

-3

10

-4

10

σ / S cm

-1

-5

10

-6

10

-7

10

-8

10

-9

10

-10

10

-11

GMIL - 1 GMIL - 2 GMIL - 3 GDIL - 1 GDIL - 2 GDIL - 3

10

2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 -1

(1000/T) / K

Fig. 5. (a) Ionic conductivities obtained for neat RT mono and dicationic ionic liquids; (b) Ionic conductivities obtained for the gel polymer electrolytes based on P (VDF-co-HFP)-imidazolium, ionic liquid/0.2 M NaTFSI solutions.

521

conductivity compared to neat ionic liquids. Room temperature ionic conductivity for neat ILs reach values in the 103 S cm1 range for monocationic ILs electrolyes and 104 S cm1 for dicationic ones (Table S3), while gel polymer electrolytes reach only values of 104 S cm1 and 105 S cm1 for mono- and dicationic based ILsbased membranes, respectively (Table 3). Room temperature values obtained for the membranes being up to two orders of magnitude lower than those of the corresponding neat ILs (this study, Table S3). These differences are smaller than those reported for pyrrolidinium and imidazolium based gel polymer electrolytes [52,53], and are comparable to the conductivities reported for other gel polymer electrolyte systems [54]. If the ionic conductivity of all gels resembles that of 0.2NaTFSI/ILs electrolytes, suggesting similar transport mechanism to that of the liquid component [55], the decrease in conductivity can be due to both lower fraction of the conducting component (0.2 M NaTFSI doped IL present in the membrane) and interactions between the liquid electrolyte and the P(VdF-co-HFP) polymer matrix. On the other hand, no significant differences between neat ILs and 0.2 M NaTFSI doped ILs electrolytes were detected (Fig. S17), both the pristine ILs and the electrolytes reach values about 103 S cm1 at room temperature and their variation with the temperature show the same profile, making clear that the addition of the alkali metal salt (at least for 0.2 M NaTFSI concentration) to the IL does not significantly decrease the ionic conductivity as expected when sodium salt is added at low concentration. At low salt concentrations ionic conductivity are similar to that of the pristine ionic liquid, while high salt concentrations lead to high viscosity solutions with restricted ion mobility resulting in lower conductivities. The variation of ionic conductivity with salt concentration usually has a bell shape with a maximum at intermediate salt concentration [4,29,56,57]. Higher conductivity values correspond to the electrolytes based on the mono substituted imidazolium ILs, the value decreasing on increasing the oxyethylene side chain length (Table 3). The highest room temperature conductivity value 2.2
104 S cm1 was obtained for the GMIL-1 membrane. For dicationic-based GPEs, the ionic conductivity values of the gels (GDILs) are quite similar throughout the temperature range (Fig. 5(b)), no significant effect on the increase the oxyethylene chain length being detected (Table 3). This may be due to a greater restriction in ion mobility as chain length increases. Higher steric hindrance restricts the mobility of Na+ ions and the polymer segmental motion; hence the total ionic conductivity decreases. Considering that sodium conductivity is directly proportional to s TOT
tNaþ (Table 4), the larger sodium conductivity corresponds to the monocationic ionic liquid gel electrolyte with the largest side chain, which can be explained in terms of the interaction between the sodium ion and ether oxygen that improves sodium mobility [58,59]. Thus, the variation in sodium ion conductivity is attributable to differences in self-organization of the ether-substituted side chain. The conductiviy values of monocationic liquid electrolytes are of the same order of those reported in the literature for pyrrolidinium ionic liquids doped with LiTFSI [51]. Parallel studies on ethersubstituted pyrrolidinium ILs reveals the effect of the side chain length on the Li+ coordination responsible of ionic mobility, and exhibit a significant effect on the ionic conductivity [51]. In the actual state, the sodium conductivities of gel electrolytes are far away of the requirements needed for practical sodium ion batteries. These results are in agreement with thermal properties previously discussed, Tg increases with oxyethylene chain length in monocationic-based GPEs resulting in more rigid materials in which Na+ mobility is reduced, while long chain substitution results in higher steric hindrance. On the other hand, dicationicbased GPEs are much more rigid structures (higher Tg values) and

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Table 3 VTF fitting parameters for ionic conductivity of mono and dicationic imidazolium-based RTIL's gel polymer electrolytes. Values of the sodium-ion transference number (tNaþ ) and ionic conductivity (s ) for GPE's at room temperature. Sample

A/ S cm1 K1/2

Monocationic IL's based GPE's GMIL - 1 118.20 GMIL - 2 20.51 GMIL - 3 28.77 Dicationic IL's based GPE's GDIL - 1 75.03 GDIL - 2 54.40 GDIL - 3 53.18

B/K

To/K

R2

Ea/ KJ mol1

tNa+

s25 C/ S cm1

1809.34 1324.81 1403.61

124.2  3 164.2  2 156.2  2

0.9983 0.9997 0.9995

15.04 11.02 11.67

0.10 0.16 0.49

2.2
104 5.8
105 8.3
105

1828.11 1629.54 1622.13

157.7  3 166.7  2 168.7  2

0.9987 0.9998 0.9998

15.20 13.55 13.49

0.12 0.11 0.12

9.2
106 1.2
105 1.1
105

Table 4 Parameters used for determining tNaþ for IL's based gel polymer electrolytes and the tNaþ , sTOTAL and sNa+ values at room temperature. DV = 10 mV GPE sample

I0/A

Monocationic IL's based GPE's GMIL-1 1.1
10-5 GMIL-2 2.3
10-6 GMIL-3 4.7
10-6 Dicationic IL's based GPE's GDIL-1 3.4
10-6 GDIL-2 1.8
10-6 GDIL-3 1.3
10-6

a

.

Is/A

Ri/V

Rf/V

Re0/V

Res/V

tNa+

sTOTAL/S cm1

s(Na + )/S cm1

1.7
10-6 6.2
10-8 3.9
10-7

30.89 164.1 15.79

35.11 1340 119.8

454.4 1280 739.1

821.2 6831 4566

0.10 0.16 0.49

2.2
10-4 5.8
10-5 8.3
10-5

2.2
10-5 9.3
10-6 4.1
10-5

3.2
10-7 2.1
10-7 8.4
10-8

310.4 354.8 274.3

523.2 472.4 684.4

924.3 1817 2404

3169 4726 5032

0.12 0.11 0.12

9.2
10-6 1.2
10-5 1.1
10-5

1.1
10-6 1.4
10-6 1.3
10-6

a I0: initial current; Is: steady state current; Ri: electrolyte resistance in the initial state; Rf: electrolyte resistance in the steady state; Re0, Res: initial (0) and final (s) resistances of interfacial layers of the Na metal electrode/electrolyte; tNaþ : transport number of Na+; sTOTAL: total conductivity; sNa+: Na+ conductivity.

the mobility of polymer chains may be reduced by the nature of the IL (linked cations by oxyethylene chain) and its packing inside the polymer matrix. The temperature dependence of the ionic conductivity of both neat ILs and gel electrolytes is expressed through an Arrhenius conductivity plot as depicted in Fig. 5(a) and (b), showing a nonlinear behavior. These plots are well described and fitted by the Vogel–Tamman–Fulcher (VTF) relationship [60–62], Eq. (3):

s ¼ AT 2 expðB=ðT  T 0 Þ 1

ð3Þ

where A is the conductivity at infinite temperature, B is a constant, related to the charge carrier number and the activation energy, and T0 represents the “ideal glass transition temperature”, lower than the glass-transition temperature observed during the DSC measurements [63]. Parameters A and B can be easily obtained from the typical log(sT1/2) vs. 1/(T  T0) plot, Table 3 summarizes the calculated VTF parameter values for the gel polymer electrolytes. The fitting was excellent, as shown by R2 values, indicating that the ion conduction mechanism can be explained in terms of free volume and configurational entropy theories [64,65], and controlled by the viscous properties of the liquid [66]. The same trend is observed for neat ILs (Fig. 5(a)). The parameter B obtained from VTF fitting, (Eq. 2) defines the temperature dependence of transport properties and is dependent on the interactions of the system [67,68]. B value increases when NaTFSI electrolytes are added to polymer matrix (Table 3, c.f. Table S4), this effect being more pronounced for monocationic GPEs than for the dicationic ones. It has been reported in the literature that increasing on the B parameter is associated with an increase in the interactions in the liquid phase [66]. In our study, it was detected that B decrease with oxyethylene chain length, indicating lower liquid electrolytepolymer matrix interactions. Thus, the confinement of 0.2 M NaTFSI doped IL electrolyte in the matrix affect the basic physical properties following the Tg increment discussed above. Table 3 shows the room temperature total ionic conductivity and calculated activation energy for studied GPEs compositions. Quite similar activation energy values (from 11.02 to15.20 KJ mol1) were obtained for all membranes, indicating a similar behavior in ionic

conduction mechanism. The minimum activation energy is obtained for the gel electrolyte with the intermediate oxyethylene side chain for the monocationic ILs-based gel (11.02 KJ mol1), but differences are small. The conductivity values are lower than alkylsubstituted pyrrolidinium-based (C4mpyrTFSI + NaTFSI) and imidazolium-based (EMI-Tf + NaTf) sodium gel polymer electrolytes (about 103 S cm1 at r.t) previously reported [28,29]. The electrochemical stability of the polymer electrolyte in contact with electrode materials is an essential parameter for providing satisfactory performance in metal-ion battery applications. In terms of safety, secondary and decomposition reactions should be avoided; therefore the electrochemical stability window plays a key feature in battery design. Room temperature cyclic voltammogram (low potentials) and linear sweep voltammogram (high potentials) for GMIL-1 gel polymer electrolyte are depicted in Fig. 6 (a) and (b), respectively. Low voltage test shows a two oxidative processes centered at about -3.06 and -0.14 V vs. Na/Na+ for GMIL-1 sample together with a clear reduction peak at -3.44 V and three broad peaks. No other significant peaks are observed at higher potentials (from 1.0 to 3.0 V vs. Na/Na+) in the CV curves, suggesting that no secondary chemical or decomposition reactions against sodium take place. The stability of the gel polymer electrolyte at high potentials was evaluated by using linear sweep voltammetry (LSV) from 2.5 to 6.0 V vs. Na/Na+, Fig. 6(b). No peaks are observed, indicating that no secondary or decomposition reactions occur in this potential range for all samples [54]. If we assume that the -0.14 V corresponds to the stripping of sodium, and the -3.44 V peak to the plating, it is clear that plating/stripping process (reduction and oxidation reactions for the Na/Na+ couple) is not fully reversible (low Na cycling efficiency of the plating/ stripping process) [69,70], hence the electrolyte is not adequate to use in sodium-metal batteries, but the profile obtained indicate that the electrolyte is stable in a voltage range between 1.5 to 5.0 V then making suitable to be use in Na-ion rechargeable batteries. The other samples tested showed a similar behavior. Slight differences in oxidation peak potential values being detected when changing the chemical structure of the ILs. The position of the oxidation peak depends on the length of the oxyethylene chain

J.F. Vélez et al. / Electrochimica Acta 241 (2017) 517–525

(a) 0.25

523

-1

Current collector: Cu, dE/dt = 2 mV s

GMIL-1 0.20

j / mA cm

-2

0.15 0.10 0.05 0.00 -0.05 -0.10 -4

-3

-2

-1

0

1

2

3

+

E vs. (Na/Na ) / V

(b) 0.8

-1

Current collector: Al, dE/dt = 2 mV s

GMIL - 1

Fig. 7. Time-dependence response of dc polarization for GMIL-1 gel polymer electrolyte Na0 | GMIL-1 | Na0 cell, polarization voltage: 10 mV; (inlet) impedance response of the same cell before and after the dc polarization measured at room temperature and proposed equivalent circuit for fitting.

j / mA cm

-2

0.6

combined with a constant phase element in parallel (CPEe). Diffusion process can be described as Warburg W0 element in second semicircle, that leads to an equivalent circuit in the form

0.4

0=s

0.2

0.0 3

4

5

6

+

E vs. (Na/Na ) / V Fig. 6. Electrochemical stability window at room temperature: (a) cyclic voltammetry (CV); (b) linear sweep voltammetry (LSV) of the GMIL-1 gel polymer electrolyte.

attached to the imidazole ring and may be associated to the solid electrolyte interphase (SEI) film formation and surface reactions [71,72]. In the case of monocationic-based GPEs, this peak appears at the same position but intensity decrease with oxyethylene chain length. Nevertheless, in dicationic-based GPEs the peak position changes (shifted to higher values with oxyeylene chain length) but keeping similar intensities. This demonstrates the ability of these electrolytes to support reversible processes in 1.5 to 5.0 V voltage range enabling higher density technologies. The sodium-ion transference number (tNaþ ) of gel polymer selfstanding membranes was calculated by Eq. (1). The time dependence plot of the current during polarization, the Nyquist plots (before and after polarization) and their equivalent circuits for GMIL-1 obtained at room temperature are shown in Fig. 7, (for more detail see chronoamperograms and Nyquist plots for same sample in Fig. S18). The calculated data are summarized in Table 4. Nyquist plots show several electrochemical process associated to charge transfer phenomena and the development of SEI layer. These processes can be described as circuit elements in Nyquist plots by equivalent circuit fit. As can be seen, both initial and steady state impedance response before and after polarization test follow the same behavior, namely two semicircles followed by a diffusion process. The equivalent circuit to fit impedance data represents 0=s

bulk resistance Rb (before and after polarization test) in series with Ri,f (initial or final) electrolyte resistance in parallel with a capacitor element Ci,f in series with the passivation layer resistances Re0=s in the initial and final states, respectively,

Rb (Ri,f Ci,f)(R0=s e CPEeW0), see Fig S18(b). The growth of the passivation layer is evidenced by the increase in the second resistance after polarization test [55]. The fitting parameters for Nyquist plots and observed current densities (I0 and Is) related to the transport properties of Na+ ions are listed in Table 4. The calculated Na+-ion transference numbers measured at room temperature of the gel polymer electrolytes are also summarized in the same table. The Na+-transference number for GMIL-1 composition results in tNaþ ¼ 0:10 = and increase with oxyethylene chain length in monocationic imidazolium based GPEs, while the values for dicationic-based gels are quite similar (about 0.12). Higher tNaþ value was obtained for GMIL-3 self-supported membrane. Na+ transference values are in the range of 0.1 to 0.5 indicating a contribution of TFSI anion and organic cation of ILs to the total ionic conductivity. These results are in good agreement to Raman analyses previously discussed. The transference number estimates the fraction of current carried by the different ionic species formed in gel polymer electrolytes. Raman studies enables to make a more comprehensive study of the Na-ion mobility mechanism in gel polymer electrolytes. The n values reported above from Raman analyses allow to estimate the proportion of each ionic species (charge carriers) and to know the nature of Na+ coordination and the formation of Na+-complexes responsible of the transference numbers values. As discussed above, low n values indicating that there is a lower proportion of TFSI anions coordinated to Na+ cations and hence a greater coordination of Na+ to the ether oxyethylene side chain. The Na+—O interaction combined with segmental motion in oxyethylene side chain ILs produces greater ionic mobility and therefore higher Na-transport values. On the other hand, Passerini and co-workers suggests that higher n values implies a mixed Na+ coordination in the first solvation shell (the ether chains are too short to fully coordinate the Na+ ion, but long enough to displace some of the TFSI anions) [51]. In the case of dicationic ILs-based gel polymer electrolytes, n values are greater than the corresponding monocationic ones (Table 2), suggesting mixed Na+ coordination in the solvation shell, so the ionic mobility mechanism is inhibited by the formation of ion-pairs and aggregates (In addition to a greater steric hindrance due to the

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molecular size of the IL cations) resulting in lower Na-transference values (see Table 4). The GMIL-3 composition results in lower n values and the highest tNaþ value (see Table 4) This composition also exhibited high ionic conductivity and a wide electrochemical window, becoming a potential candidate for sodium storage systems [73,74]. In this case higher values in transference number (up to 0.49) was obtained at room temperature and low NaTFSI concentration in gels, optimizing the composition in order to ensure charge-discharge properties in solid state sodium batteries. 4. Conclusions Free standing gel electrolytes membranes based on oxyethylene substituted mono and dicationic ionic liquid electrolytes swollen in a P(VdF-co-HFP) polymer matrix were synthesized. Thermal analysis confirms that the membranes are stable up to 380  C, well above the operation temperature of rechargeable sodium batteries; regarding the glass transition temperature, results indicate that the value of Tg depends both on the length and type of the side chains, being larger for the dicationic gel polymer electrolytes compared with the monocationic ones. Room temperature ionic conductivity of 2.2
104 S cm1 was obtained for the membrane based on the monocationic imidazolium with the shorter oxyethylene side chain, two orders of magnitude lower than those of the corresponding neat IL, similar behavior being detected for the other GPEs studied. In all cases ionic conductivity of GPEs based on monocationic ILs are higher than that of the corresponding dicationic ones. Cyclic voltammograms show two oxidative processes centered at about -3.1 and -0.2 V vs. Na/Na+ and several reduction processes, but plating/striping is not fully reversible. The Na+-transference number values are in the 0.1-0.5 range. Low values of tNaþ are associated to high degree of interaction between Na+ cations and both the oxygen from the IL side chains and TFSI anion, while high tNaþ reflects that the Na+ cations are mainly coordinated to the TFSI anion. Results obtained are in agreement with coordination parameter (n) obtained from Raman spectra. As a final conclusion we can stated that the synthesized GPEs are potential candidates to be use as electrolytes in sodiumion rechargeable batteries, but ionic conductivity need to be improve (i.e. by incorporation of large dielectric constant plasticizers, such as organic carbonates). Acknowledgements This work has been supported by the Spanish Ministry of Economy and Competiveness under project MAT2014-54994-R. L. V. Álvarez also thanks the international mobility scholarship given by Secretaría de Educación Pública of the Mexican Federal Government (SEP). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2017.04.096. References [1] F. Bonaccorso, L. Colombo, G. Yu, M. Stoller, V. Tozzini, A.C. Ferrari, R.S. Ruoff, V. Pellegrini, Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage, Science 347 (2015) 1246501. [2] J.-M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [3] M. Armand, J.-M. Tarascon, Building better batteries, Nature 451 (2008) 652– 657.

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