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This journal is©The Royal Society of Chemistry 2015 J. Mater. Chem. C

Cite this:DOI: 10.1039/c5tc00940e

Development of ion conducting polymer gelelectrolyte membranes based on polymerPVdF-HFP, BMIMTFSI ionic liquid and theLi-salt with improved electrical, thermal andstructural properties†

Shalu, Varun Kumar Singh and Rajendra Kumar Singh*

Ion conducting polymer gel electrolyte membranes based on polymer poly(vinylidene fluoride-co-hexa-

fluoropropylene) PVdF-HFP, ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide

BMIMTFSI with and without the Li-salt (having the same anion i.e. the TFSI� anion) have been synthesized.

Prepared membranes have been characterized by scanning electron microscopy, X-ray diffraction, Fourier

transform infrared (FTIR), differential scanning calorimetry, thermogravimetric analysis (TGA) and complex

impedance spectroscopic techniques. Incorporation of IL in the polymer PVdF-HFP/polymer electrolyte

(i.e. PVdF-HFP + 20 wt% LiTFSI) changes different physicochemical properties such as melting temperature

(Tm), glass transition temperature (Tg), thermal stability, degree of crystallinity (Xc), and ionic transport

behaviour of these materials. The ionic conductivity of polymeric gel electrolyte membranes has been found

to increase with increasing concentration of IL and attains a maximum value of 2 � 10�3 S cm�1 at 30 1C

and B3 � 10�2 S cm�1 at 130 1C. A high total ionic transference number 40.99 and the cationic

transference number (tLi+) B 0.22 with a wider electrochemical window (ECW) B 4.0–5.0 V for the

polymer gel electrolyte membrane containing higher loading of IL (B70 wt% of IL) have been obtained.

Temperature dependent ionic conductivity obeys Arrhenius type thermally activated behaviour.

Introduction

The development of ion-conducting polymer electrolytes forpotential application in solid state devices like rechargeablebatteries, fuel cells, solar cells etc. has received worldwide atten-tion because of their intrinsic properties such as thin-film form-ing ability, flexibility, transparency, high ionic conductivity and awide electrochemical window. In technological applications,polymer electrolytes are preferred over liquid electrolytes, as theyovercome the problem associated with liquid electrolytes likeleakage, corrosion and portability.1–8 Generally, polymer electro-lytes are obtained by polar polymers (like PEO, PEG, PVA, PMMAand PVdF-HFP etc.) with ionic salts (like LiBF4, LiClO4, NH4ClO4

etc.). Polymer electrolytes so obtained offer a number of advan-tages in terms of good mechanical stability, lightweight, andgood electrode–electrolyte contact but their low room tempera-ture ionic conductivity limits application in devices. In order toincrease the ionic conductivity of the polymer electrolytes, a

number of approaches such as (i) use of conventional plasticizerslike EC, PC, DEC etc. (ii) dispersion of inorganic filler like SiO2,Al2O3, CNT, TiO2 etc. (iii) copolymerization (iv) blending etc. areadopted. Incorporation of the conventional organic plasticizerscan increase the ionic conductivity of the polymer electrolyte byincreasing the flexibility and amorphous phase of the polymerwhich in turn increases the volume within the electrolyte systemand decreases the viscosity of the electrolyte making the mobilityof ions easier. It is found that the dispersion of inorganic ceramicfillers in polymer electrolytes not only improves electrical conduc-tivity but also improves the mechanical strength of the systems.The abovementioned approaches result in a relatively good ionicconductivity of polymer electrolytes at moderate temperatures butthese polymer electrolytes still have some technical drawbackssuch as (i) the ionic conductivity values are still low for practicalapplications at room temperature compared with liquid electro-lytes (ii) their low temperature range of operation due to thevolatile nature of organic solvents that limits the thermal stabilityand reduces the electrochemical potential window, (iii) flammability,(iv) toxicity, (v) environmentally hazardous nature, which are themain problems of these electrolytes and limit their application insolid state devices.9–17

Department of Physics, Banaras Hindu University, Varanasi-221005, India.

E-mail: [email protected]; Fax: +91 542 2368390; Tel: +91 542 2307308

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5tc00940e

Received 3rd April 2015,Accepted 10th June 2015

DOI: 10.1039/c5tc00940e

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J. Mater. Chem. C This journal is©The Royal Society of Chemistry 2015

Recently, room temperature ionic liquids (RTILs) have emergedas suitable candidates that replace the organic solvents due to theirunique properties like high ionic conductivity, non flammability,non-toxicity, non-volatility, good chemical and thermal stabilityand a wide electrochemical potential window etc. Ionic liquid (ILs)entirely consist of dissociated anions and cations and are generallypresent in the molten state below 100 1C. Despite their high ionicconductivity, their liquidus nature prevents their direct applicationin devices due to leakage and portability problems. So, it is veryimportant to immobilize the ILs into some organic/inorganicmatrices that provide good mechanical stability along with preser-ving the main properties of ILs that can significantly provide alarge range of applications to these materials. Therefore in thepresent study, IL BMIMTFSI has been incorporated in the polymermatrix (resulting matrices termed as polymer gel electrolytemembranes (PGEs)) in which IL acts as a good plasticizer aswell as a supplier of free charge carriers.18–21

Recently, some reports are available in literature on roomtemperature ionic liquids (e.g. methyl N-methylpyrrolidinium-N-acetate trifluoromethanesulfonimide [MMEPyr][TFSI], 1-ethyl3-methyl imidazolium trifluoro-methane sulfonate, etc.) basedpolymer gel electrolytes having high ion conductivity, goodthermal and electrochemical stability. Cheng (2007) et al. reportedthe value of ionic conductivity B6.9 � 10�4 S cm�1 at 40 1C in apyridiniumimide ionic liquid (BMPyTFSI) based polymer electrolyteand they also reported an improvement in the electrochemical andinterfacial stability with the incorporation of ionic liquid in the PEO–LiTFSI electrolyte. Yang et al. reported that the ionic conductivity forgel polymer electrolytes based on (1-butyl-4-methylpyridinium bis(trifluoromethanesulfonyl) imide/lithium bis(trifluoromethanesulfonyl)imide) B4MePyTFSI/LiTFSI–PVdF-HFP at room temperature is(2� 10�4 S cm�1). Navarra et al. showed that the ionic conductivityvalue was of the order of 3 � 10�4 S cm�1 at 50 1C for Li-TFSA/N-butyl-N-ethyl pyrrolidinium (trifluoromethylsulfonyl) amideTFSA/PVdF-HFP. Similarly, Jung et al. reported that the conductivityvalue for the polymer gel membrane based on (1-butyl-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide) PYR14TFSI/LiTFSI/PVdF-HFP was B3.6 � 10�4 S cm�1 at 30 1C.22–27

In the present work, we have synthesized high ion conductingpolymer gel electrolyte membranes based on polymer poly(vinyl-idene fluoride-co-hexafluoropropylene) PVdF-HFP, ionic liquid,1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imideBMIMTFSI with and without Li-salt (having same anion i.e. TFSI�

anion). The copolymer PVdF-HFP has been used in the presentstudy because of its high dielectric constant, which helps in thedissociation of salts and also due to its HFP units which reducethe degree of crystallinity.28,29 These synthesized polymer gelelectrolyte membranes were characterized by various techniqueslike scanning electron microscopy, X-ray diffraction, differentialscanning calorimetry, thermogravimetric analysis, Fourier trans-form infrared (FTIR) spectroscopy, complex impedance spectro-scopy etc. The ionic conductivity for the polymer gel electrolytemembrane i.e. PVdF-HFP + 80% BMIMTFSI was of the order ofB6.6 � 10�4 S cm�1 at 30 1C and for (PVdF-HFP + 20% LiTFSI) +70% BMIMTFSI about 2.1 � 10�3 S cm�1. These polymer gelelectrolyte membranes are thermally stable (B300–400 1C),

flexible, transparent and free standing in nature. In this study,we report that the incorporation of the IL in polymer electrolyte(PVdF-HFP + 20% LiTFSI) changes the crystallinity, thermal stability,melting temperature (Tm), glass transition temperature (Tg), com-plexation behaviour and also increases the ionic conductivity.

ExperimentalMaterials

The starting materials poly(vinylidene fluoride-co-hexafluoro-propylene) PVdF-HFP (molecular weight = 400 000 g mol�1),LiTFSI (purity 4 99.9%) salt, and the IL BMIMTFSI (purity 4 99%)were procured from Sigma-Aldrich (Germany). The IL was driedin vacuum at about 10�6 Torr for 2 days before use.

Synthesis of polymeric gel membranes

In the present paper, two sets of polymer electrolyte membraneshave been prepared by the conventional solution cast technique.

(a) The polymer gel electrolyte membranes of PVdF-HFP + xwt% BMIMTFSI where x = 20, 40, 60 and 80.

(b) The polymer electrolyte membranes (PVdF-HFP + 20%LiTFSI) + x wt% BMIMTFSI where x = 0, 20, 40, 60 and 70.

All the materials which are used for the preparation of polymergel electrolyte membranes, the polymer PVdF-HFP, salt LiTFSI andIL BMIMTFSI were vacuum dried (B10�3 Torr) at 50 1C overnight.For the preparation of polymer gel electrolyte membranes, i.e.PVdF-HFP + x wt% BMIMTFSI, a desired amount of polymer PVdF-HFP was dissolved in acetone under stirring at 50 1C until a clearhomogeneous solution was obtained. Different amounts of IL werethen added in the above solution and again stirred for 2–4 h at50 1C until a viscous solution of PVdF-HFP + IL was obtained. Forthe preparation of polymer electrolyte gel membranes (PVdF-HFP+ 20 wt% LiTFSI) + x wt% BMIMTFSI, PVdF-HFP was first dissolvedin dried acetone and stirred for 3–4 hours at 50 1C, after that, anappropriate amount of LiTFSI salt was added to it and againstirred for 3–4 hours. After complete dissolution of the salt with thepolymer PVdF-HFP, a desired amount of IL was added and againstirred for 4–5 hours for obtaining a complete homogeneousmixture. These viscous solutions were poured in polypropylenePetri dishes and solvent was allowed to evaporate slowly at roomtemperature for a week. After complete evaporation of the solvent,flexible, thin, free standing and semi-transparent polymer gelelectrolyte membranes (see Fig. 1) PVdF-HFP + x wt% BMIMTFSIand (PVdF-HFP + 20 wt% LiTFSI) + x wt% BMIMTFSI of thick-ness B200–400 mm were obtained and before characterization,prepared membranes were finally vacuum dried at B10�6 Torrfor 2–3 days to remove any traces of moisture present in themembranes. A typical photograph of the polymer gel electrolytemembranes is shown in Fig. 1.

An X’Pert PRO X-ray diffractometer (PANalytical) with CuKaradiation (l = 1.54 Å) in the range 2y = 101 to 801 was used torecord the X-ray diffraction profiles of polymer gel electrolytemembranes. A scanning electron microscope (model QuantaC-200) was used to examine the surface morphology of thepolymeric gel electrolyte membranes.

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Bulk elastic modulus (E) of the prepared polymeric gelelectrolyte membranes was measured using the pulse-echotechnique at room temperature in order to determine the effectof the IL/salt on the mechanical stability of the resultingmembranes. Radio frequency pulses were sent by the pulser/receiver (Olympus model 5900PR) to excite a 6 MHz piezo-electric transducer (D6HB-10) to generate longitudinal ultra-sonic waves. The transducer used for transmitting as well asreceiving ultrasonic waves was coupled to the disc-shapedmembrane (thickness, d B 200–400 mm). The return echo wasreceived by the pulser/receiver and both the echo pulse and theinput pulse were displayed on a 500 MHz Agilent digital storageoscilloscope DSO5052A. The transit time of the echo pulse wasrecorded and velocity of propagation of ultrasonic waves inpolymeric membranes was calculated using the relation v = 2d/t.The E values of the samples were calculated using the relationE = v2r, (where v = velocity of the longitudinal wave and r =density of the samples (i.e. r = mass/volume)).

Thermal analyses were carried out by differential scanningcalorimeter using the Mettler DSC 1 system in the temperaturerange �110 to 160 1C at a heating rate of 10 1C min�1 andthermogravimetric analysis (TGA) (Mettler DSC/TGA 1 system)under continuous purging of nitrogen. The FTIR spectra of thepolymeric gel membranes were recorded with the help of Perkin-Elmer FTIR spectrometer (Model RX 1) from 3500 to 400 cm�1.

Viscosity of the ionic liquid was measured using a Brook-field DV-III Ultra Rheometer in the temperature range �10 to80 1C. The instrument was calibrated with standard viscosityfluid supplied by the manufacturer before each measurement.

Ionic conductivity of the polymeric gel membranes wasmeasured by the complex impedance spectroscopy techniqueusing a NOVO Control Impedance Analyzer in the frequencyrange 1 Hz–40 MHz. The bulk resistance was determined fromthe complex impedance plots. The electrical conductivity (s)can be calculated by using the following relation:

s ¼ 1

Rb� lA

(1)

where l is the thickness of the sample, A is the cross sectionalarea of the disc shaped sample and Rb is the bulk resistanceobtained from complex impedance plots. For temperature

dependent conductivity studies, disc shaped polymeric gelmembranes were placed between two stainless steel electrodesand the whole assembly was kept in a temperature controlled oven.

The d.c. polarization technique was used for the determina-tion of the total ionic transport number (tion) in which a voltageof 10 mV was applied across the disc shaped polymeric gelmembranes placed between two stainless steel electrodes and thecorresponding current was monitored as a function of time. Thecationic transport number (i.e. tLi+) of the polymer gel electrolytemembrane containing a higher amount of IL (i.e. PVdF-HFP + 20%LiTFSI + 70% BMIMTFSI) was calculated by using the combineda.c./d.c. technique. The Li/PVdF-HFP + 20% LiTFSI + 70%BMIMTFSI/Li cell was subjected to polarization by applying avoltage DV = 10 mV for 2 hours and resultant currents werecalculated (i.e. initial and final current). The cell resistanceswere also measured before and after polarization using acimpedance spectroscopy.

The cyclic voltammetric studies were carried out usingan electrochemical analyzer with an AUTOLAB PGSTAT 302Ncontrolled by NOVA 1.8 software version (Methohm Lab) toestimate the ‘electrochemical stability window (ECW)’ of thepolymer electrolyte membrane.

Results and discussion(a) Structural characterization

SEM study. Surface morphology of the pure PVdF-HFP,PVdF-HFP + 20 wt% LiTFSI, PVdF-HFP + 80 wt% BMIMTFSIand (PVdF-HFP + 20 wt% LiTFSI) + 70 wt% BMIMTFSI is shownin Fig. 2(a)–(d). It can be seen from Fig. 2(a) that the pure PVdF-HFP consists of large crystalline grains with lamellar structurewhich are equally distributed and pure PVdF-HFP as well aspolymer gel electrolyte membrane has solvent swollen structure.When we add IL in to the polymer matrix the size of the grainsstarts decreasing and the membrane became flexible. When20 wt% of LiTFSI salt was added in the polymer matrix, the sizeof the grains decreased and some white crystallites were observed(shown by green circles in Fig. 2(b)), which were absent in thepure polymer PVdF-HFP micrograph (see Fig. 2(a)) that maybe due to the undissolved LiTFSI salt present in the polymer

Fig. 1 A typical photograph of polymer gel electrolyte membrane PVdF-HFP + 20 wt% LiTFSI + 70 wt% BMIMTFSI that shows flexibility and semitransparency of the prepared membranes.

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electrolyte. Furthermore, when we add both IL, BMIMTFSI andLiTFSI salt (say 70% IL in PVdF-HFP + 20% LiTFSI), the membranebecame more amorphous with no crystalline grains and no undis-solved LiTFSI (Fig. 2(d)). The evidence of the enhanced amorphicitywith increasing IL content was also obtained from XRD results asdiscussed in the next section. The reduction in crystallinity of thepolymer matrix also affects the thermal and ion transport behaviorwhich will be discussed later in another section.

XRD study. Fig. 3 shows the XRD profile of PVdF-HFP +x wt% BMIMTFSI (where x = 20 and 60) and (PVdF-HFP + 20%LiTFSI) + x wt% BMIMTFSI (where x = 0, 20 and 60). The purePVdF-HFP showed a semi-crystalline nature (i.e. crystalline andamorphous both phases are present simultaneously) havingcharacteristic crystalline peaks at 16.751, 18.211, 20.041, 26.711,and 38.831, corresponding to the crystalline phase of a-PVDF.30–32

When IL was added in different amounts in the polymer PVdF-HFP, only two broad peaks/halos at 2y = 20.471 and 39.931remain and some crystalline phase related peaks disappear.From Fig. 3(c and d), it can be seen that the IL based polymergel electrolyte membranes consist of less intense and broaderhalos. As reported in literature,30–32 the sharp intense peaks ofLiTFSI salt at 2y = 14.11, 15.91, 18.61, 18.91 and 21.41 reveal thecrystalline character of LiTFSI salt.33 It can be seen that when20 wt% LiTFSI salt was added in the polymer matrix, allcrystalline peaks of LiTFSI salt are disappear and only one at2y = 14.11 (see Fig. 3(b)) remains. This remaining peak alsodisappeared when IL was incorporated in the polymer electro-lyte (i.e. PVdF-HFP + 20 wt% LiTFSI) which indicates a completedissolution of the salt in the polymer gel electrolyte membranes

in the presence of IL (see Fig. 3(e and f)). Further, upon increasingthe amount of IL in polymer gel electrolyte membranes, theintensity of the crystalline phase related peaks of PVdF-HFP startdecreasing due to the enhanced amorphicity of the preparedmembranes (see Fig. 3(e and f)). The FWHM of the halo was alsofound to increase with increasing amounts of ionic liquid inthe polymer gel electrolyte membranes. The broadening of thepeaks and reduction in intensity/or absence of some crystallinephase related peaks in prepared gel membranes indicates thatthe crystallinity of the polymer PVdF-HFP is decreased uponincreasing the amount of IL in the membranes. The amorphouspolymer–salt mixtures are ideally suited for battery electrolyteapplications. The evidence of increased amorphicity with theincreasing concentration of IL was also confirmed by SEManalysis discussed above.

Mechanical test. The elastic modulus (E) of the polymer gelelectrolyte membranes was calculated as described in the Experi-mental section. The value of the elastic modulus (E) of purePVdF-HFP (as calculated in our earlier study46), PVdF-HFP + 20%LiTFSI, and (PVdF-HFP + 20% LiTFSI) + 70 wt% of BMIMTFSI are14.6� 1010, 7.5� 1010 and 3.2� 1010 dynes per cm2 respectively.The elastic modulus of the membranes decreased with theaddition of the salt and IL in PVdF-HFP matrix. This was dueto the plasticization effect of the IL.

Ionic transport behavior. The ionic conductivity (s) of pureIL is 5.8 � 10�3 S cm�1 at 30 1C which increases with increasingtemperature as shown in Fig. 4. This increase in conductivity isclosely related to the decrease in viscosity with temperature (asgiven in Fig. 4), which leads to increase in ionic mobility.

Fig. 2 Surface morphology of the (a) pure PVdFHFP, (b) PVdF-HFP + 20 wt% LiTFSI, (c) PVdF-HFP + 80 wt% BMIMTFSI and (d) PVdF-HFP + 20 wt%LiTFSI + 70 wt% BMIMTFSI.


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The typical Nyquist plots of the two prepared membranescontaining PVdF-HFP + 40% BMIMTFSI and (PVdF-HFP + 20%LiTFSI) + 40% BMIMTFSI at room temperature are shown inFig. 5 (a similar behaviour was also observed for all the preparedmembranes). The depressed semicircle was observed in thehigh-frequency region due to the parallel combination of bulkresistance (Rb) and the double layer capacitance Cdl followed byan inclined spike in the low-frequency region that indicates thediffusion of ions. The angle of inclination of the straight line andangle of the depressed semicircle are due to the presence ofdistributed macroscopic material properties termed as constantphase element (CPE) (see inset of Fig. 5).

CPE = K/(jo)a (2)

where K is the Warburg coefficient related to the properties ofthe electrode surface and the ionic species of the electrolytes, ois the angular frequency and a is an exponent (i.e. the slope ofthe log Z vs. log f plot) whose value lies between 0 and 1 and isrelated to the roughness of the electrode.

The composition and temperature dependent ionic conductivityof the polymer gel electrolyte membranes PVdF-HFP + x wt%BMIMTFSI (where x = 20, 40, 60 and 80) and (PVdF-HFP + 20%LiTFSI) + x wt% BMIMTFSI (where x = 0, 20, 40, 60 and 70) are

shown in Fig. 6(A) and (B). The ionic conductivity of thepolymer gel electrolyte membranes i.e. PVdF-HFP + 20 wt%BMIMTFSI is found to be B5.76 � 10�7 S cm�1 at 30 1C. FromFig. 6(A), it can be seen that the ionic conductivity was foundto increase with increasing amount of IL in PVdF-HFP andreaches at B6.58 � 10�4 S cm�1 at room temperature for themembrane containing higher amount of IL (i.e. 80%). The ionicconductivity of polymer electrolyte i.e. PVdF-HFP + 20 wt%LiTFSI was found to be B1.01 � 10�7 S cm�1 at 30 1C. FromFig. 6(B), it can be seen that the highest conductivity value ofthe polymer gel electrolyte membrane (i.e. PVdF-HFP + 20 wt%LiTFSI + 70% BMIMTFSI) B2.07 � 10�3 S cm�1 at 30 1C andB2.91 � 10�2 S cm�1 at 130 1C, was obtained. From the abovediscussion it can be concluded that the ionic conductivity valueof the membranes was quite low when we use IL, BMIMTFSIand LiTFSI salt alone but a drastic increment (about four orderof magnitude) was observed in the conductivity value whenboth were used to prepare polymer gel electrolyte membranebecause in the present system, IL and Li-salt are containing sameanion. Therefore, in such a system, chances of cross-contact ion pair

Fig. 3 XRD profile of (a) pure PVdF-HFP, (b) PVdF-HFP + 20 wt% LiTFSI,PVdF-HFP + x wt% BMIMTFSI (c) x = 20 (d) x = 60, (PVdF-HFP + 20%LiTFSI) + x wt% BMIMTFSI (e) x = 20 and (f) x = 60.

Fig. 4 Temperature dependent conductivity and viscosity of pure IL.

Fig. 5 Typical Nyquist plots of (a) PVdF-HFP + 40% BMIMTFSI and (b)(PVdF-HFP + 20% LiTFSI) + 40% BMIMTFSI at room temperature and itsequivalent circuit (inset of the figure).


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formation are minimal and hence the enhancement in the con-ductivity value was observed. It is well documented in literature thatthe mixed-anion systems (i.e., in which a dopant salt and added ILhaving a different anion) have a chance to form contact/cross-contact ion pairs (which do not take part in the conductionmechanism) and hence significantly decrease the ionic conductivityof the system. These IL-based polymer gel electrolyte membranes(in which the IL and salt have the same anion) are suitable choicesfor applications in rechargeable batteries.9 The above discussedincrement in conductivity was not only because of the choiceof the same anion system but it was also due to the choice of theLi-salt (i.e. LiTFSI salt). The LiTFSI, has a high salt dissociationdue to the strong electron-withdrawing SO2CF3 groups present atboth sides of the imide anion. This salt has low lattice energy andalso has a low tendency to form ion-pairs. It also acts as aplasticizer for the polymer matrix by creating free-volume leadingto the enhanced ionic mobility and hence the ionic conductivity.Thus, the polymer gel electrolyte membranes that contain LiTFSIsalt show high ionic conductivity.34–36

From both the figures (i.e. Fig. 6(A) and (B)), it can also be seenthat the conductivity increases with increasing temperature and

follows an Arrhenius type thermally activated process. Afterdefinite temperature, a sudden jump in conductivity value wasobserved for all the prepared membranes. The temperature atwhich the sudden conductivity jumps occur corresponds to themelting temperature, Tm (i.e., semi-crystalline to amorphoustransition) of the polymer gel electrolyte membranes asobtained from DSC thermograms. The ionic conductivityat T o Tm obeys Arrhenius type thermally activated processand can be expressed as:

s = s0 exp(�Ea/kT) (3)

where s0 is the pre-exponential factor, Ea is the activationenergy, k is the Boltzmann constant and T is the temperaturein Kelvin. Fig. 6 (which is the plot between log s and 1/T) wasused to calculate activation energy (Ea). It was found that theactivation energy (Ea) decreases as we increase the concentrationof the IL in polymer gel electrolyte membranes (i.e. PVdF-HFP +x wt% BMIMTFSI and PVdF-HFP + 20 wt% LiTFSI + x wt%BMIMTFSI) (see Table 1). So, in brief, we can conclude that theenhancement in conductivity and decrease in the activation

Fig. 6 (A) Show the composition and temperature dependent ionic conductivity of the polymer gel electrolyte membranes PVdF-HFP + x wt% BMIMTFSI(a) x = 20, (b) x = 40, (c) x = 60 (d) x = 80 and (B) (PVdF-HFP + 20% LiTFSI) + x wt% BMIMTFSI (a) x = 20, (b) x = 40, (c) x = 60 (d) x = 70 respectively.

Table 1 Tm, Tg, Xc and activation energy (Ea) of the polymeric gel membranes, PVdF-HFP + x wt% BMIMTFSI and (PVdF-HFP + 20 wt% LiTFSI) + x wt%BMIMTFSI for different values of x

Samples Tm (1C) Tg (1C) Degree of Xc (%) Ea (eV)

Pure PVdF-HFP 145 �35 34PVdF-HFP + 20% BMIMTFSI 141 �47 23.2 0.42PVdF-HFP + 40% BMIMTFSI 132 �73 21.1 0.29PVdF-HFP + 60% BMIMTFSI 125 �81 12.1 0.27PVdF-HFP + 80% BMIMTFSI 114 �86 4.5 0.28(PVdF-HFP + 20% LiTFSI) +20 wt% BMIMTFSI

147 �58 31.8 0.41

(PVdF-HFP + 20% LiTFSI) +40 wt% BMIMTFSI

137 �67 19.5 0.26


126 �75 10.4 0.15


113 �84 4.1 0.13

PVdF-HFP + 20% LiTFSI 150 �39 44.7 0.52Pure BMIMTFSI — �87


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energy (Ea) on increasing the IL content indicates easier ionictransport in the system due to the plasticization/amorphizationeffect of IL which is responsible for reducing the crystallinity ofthe polymer matrix, as described earlier in the present work fromthe structural and thermal studies. A phenomenological modelas described below, explains the observed behaviour of ionicconductivity in these polymer electrolyte gel membranes. Fig. 7systematically shows (a) the polymeric chains, (b) polymer

electrolyte membrane (i.e. PVdF-HFP + LiTFSI salt) and (c) ILbased polymer gel electrolyte membrane (i.e. PVdF-HFP +LiTFSI + BMIMTFSI). Fig. 7(a) shows that the semi-crystallinenature of the polymer PVdF-HFP and Fig. 7(b) shows that thepolymer chain became flexible on the addition of LiTFSI salt.Furthermore, on the addition of IL in polymer electrolyte mem-branes, the membranes became more flexible and provide highionic conduction (because of more availability of ions) in thesystem resulting in enhancement of the ionic conductivity (seeFig. 7(c)).

The total ionic transport number (tion) of the polymer gelelectrolyte membrane ((PVdF-HFP + 20% LiTFSI) + 70% BMIMTFSI)has also been determined (see Fig. 8(A)) using eqn (4) asgiven below:

tion = (iT � ie)/iT (4)

where iT and ie are the total and residual currents respectively.The value of tion has been found to be 40.99, which indicatesthat the total ionic conductivity is mainly due to the flow of

Fig. 7 Schematic representation of (a) polymeric chains, (b) the polymerelectrolyte membrane (i.e. PVdF-HFP + LiTFSI salt) and (c) the IL basedpolymer gel electrolyte membrane (i.e. PVdF-HFP + LiTFSI + BMIMTFSI).

Fig. 8 (A) DC polarization curves of symmetric cells: (a) SS|polymer gel electrolyte membrane containing 70% of IL|SS with an applied voltage of 10 mVrecorded at room temperature. (B) dc polarization curve at the applied voltage of 10 mV, and inset of (B) is the ac impedance plot before and afterpolarization of the cell (i.e. Li|polymer gel electrolyte membrane containing 70% of IL|Li) at room temperature. (C) Cyclic voltammograms of the cellscontaining polymer gel electrolyte membrane sandwiched between two symmetrical stainless steel (blocking) electrodes and inset of Figure [C] showsthe cyclic voltammograms of the membrane sandwiched between two non-blocking electrodes i.e., Li||polymer gel electrolyte membrane||LiMnO2 forvarious cycles at room temperature at a scan rate of 10 mV s�1.


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ions. In the present case, it is expected that three differentcharge carriers (i.e. BMIM+, Li+ and TFSI�) can conduct in thesystem to give ionic conductivity. Therefore, it is very importantto calculate the cationic transport number (tLi+). The transportnumber of lithium ions in the polymer gel electrolyte mem-branes is determined by the combination of a.c./d.c. techniques(as described in experimental section). In this technique, theLi|polymer gel electrolyte membrane|Li cell was polarized atconstant potential by applying a voltage, DV = 10 mV and theresulting currents were monitored that fall from an initial value(I0) to a final value i.e. steady state value (Iss) w.r.t time (seeFig. 8(B)). The cell resistances (i.e. before (R0) and after (Rss)the polarization at room temperature as given in the insetof Fig. 8(B) were evaluated by a.c. impedance spectroscopy).The following expression has been used for the estimation ofthe value of tLi+.

tLi+ = Iss(DV � I0R0)/(I0(DV � IssRss))

The value of tLi+ for the PVdF-HFP + 20% LiTFSI + 70%BMIMTFSI gel polymer electrolyte has been found to be B0.22at room temperature. This value shows that the ionic conduc-tivity of the system is the contribution of other ions presentin the system also (like triflate anions which is common forboth the lithium salt and ionic liquid and imidazolium cations(BMIM+ ions)). Li et al. also reported that the lithium iontransport decreases with increasing concentration of IL in thesystem and they have reported that tLi+ = 0.3 for the systemcontaining 50% IL (i.e., PVdF-HFP/LiTFSI/50% PYR14TFSI).37

From the application point of view, it is necessary to inves-tigate the electrochemical stability of the prepared membranes.The electrochemical window (ECW) or working voltage limit ofthe prepared Li+-ion based polymer gel electrolyte membraneconsisting of (PVdF-HFP + 20% LiTFSI salt) + 70% BMIMTFSI(this is the optimized composition out of all prepared polymergel electrolyte membranes that has high ionic conductivity alongwith good mechanical stability), has been analyzed at room

temperature by cyclic voltammetric studies. Fig. 8(C) shows thecyclic voltammograms (CV) (i.e. the current–voltage plot), traced onthe cells containing polymer gel electrolyte membrane sandwichedbetween two symmetrical stainless steel (blocking) electrodes andinset of Fig. 8(C) shows the cyclic voltammograms (CV) traced onthe cells containing polymer gel electrolyte membrane sandwichedbetween two non-blocking electrodes i.e., Li8polymer gel electrolytemembrane8LiMnO2 for various cycles at room temperature at ascan rate of 10 mV s�1. The electrochemical stability windowhas been found B4.0–5.0 V (i.e. only as the estimation for theelectrochemical stability in Li+-ion-cells) which is considerablygood from the application point of view predominantly in Li-ionrechargeable battery applications. The value of ECW provides theinformation about the polymer gel electrolyte membrane up towhich membranes are electrochemically stable.

Thermal analysis(a) Differential scanning calorimeter (DSC)

The DSC thermograms of polymer gel electrolyte membranes,PVdF-HFP + x wt% BMIMTFSI (where x = 0, 20, 40, 60 and 80)and (PVdF-HFP + 20% LiTFSI) + x wt% BMIMTFSI (where x = 20,40, 60 and 70) are shown in Fig. 9 and 10 respectively. TheDSC thermogram of the polymer electrolyte membrane (i.e.PVdF-HFP + 20% LiTFSI) is given in the inset of Fig. 10(A). Anendothermic peak corresponding to the melting of the crystal-line phase of the polymer PVdF-HFP is observed at 145 1C andthe glass transition temperature (Tg) (i.e. the transition frombrittle or hard state at lower temperature to rubbery behaviour orflexible at high temperatures) of polymer PVdF-HFP is observedat �35 1C (see Fig. 9(A) (curve a) and (B) (curve a)). The meltingtemperature (Tm) and the glass transition temperature (Tg) of theprepared membranes shifted towards lower temperature side onthe incorporation of different amounts of IL in the polymer,PVdF-HFP and in polymer gel electrolyte membrane (PVdF-HFP+ 20% LiTFSI) (see Fig. 9 and 10).

Fig. 9 (A and B) DSC thermograms of polymer gel electrolyte membranes, PVdF-HFP + x wt% BMIMTFSI (a) x = 0, (b) x = 20, (c) x = 40, (d) x = 60 and (e)x = 80. Inset of (B) shows the glass transition temperature of pure PVdF-HFP.


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The decrease in Tg of the polymer with increasing concen-tration of IL indicates the weaker intermolecular interactionbetween the cations of the IL, Li-salt and polymer whichsignificantly enhanced the segmental motion of the polymernetwork by making polymer matrix more flexible. This decreasein Tm and Tg of polymer gel electrolyte membranes upon incor-poration of IL is due to the plasticization effect of IL.13,18

The addition of IL in PVdF-HFP and in polymer gel electrolytemembrane (PVdF-HFP + 20% LiTFSI) was expected to decreasethe degree of crystallinity (Xc). The degree of crystallinity (Xc)was calculated from the ratio of the area under melting peak(which is a measure of melting heat (DHm) involved in thephase transition) to the melting heat DH�m

� �of 100% crystalline

PVdF-HFP. The value of DH�m is 104.7 J g�1.38 The ratio of DHm

to DH�m gives Xc as,

Xc ¼ DHm

�DH�m

� �� 100% (5)

The value of degree of crystallinity (Xc) was found to decreasefrom 34 to 4% (see Table 1). Initially when 20 wt% of LiTFSI saltwas added in the polymer matrix, the degree of crystallinity (Xc)and melting temperature (Tm) both were increased but thisincreased crystallinity and melting temperature effectively gotsuppressed by the addition of IL and reached to the lowestvalue (approximately about B4%) for the membrane containshigher amount of added IL.

(b) Thermogravimetric analysis (TGA)

The TGA plots of the pure PVdF-HFP, pure IL, PVdF-HFP + 20%LiTFSI and (PVdF-HFP + 20% LiTFSI) + x wt% BMIMTFSI (wherex = 0, 20, 40, 60 and 70) are shown in Fig. 11 and the TGA plotsof polymer gel electrolyte membranes, PVdF-HFP + x wt%BMIMTFSI (where x = 20, 40, 60 and 80) are shown in the insetof Fig. 11 (high thermal stability (B300–350 1C) of all theprepared polymer gel electrolyte membranes is confirmed fromFig. 11). PVdF-HFP and IL BMIMTFSI decompose in a singlestep respectively at B475 1C and B465 1C (see Fig. 11). However,

the PVdF-HFP + IL gel membranes exhibit two step decomposi-tion mechanisms as shown in Fig. 11.

From Fig. 11, it can be found that for the lower amount ofadded IL in the polymer electrolyte membranes, thermal stabilitydecreases by a small amount but is still suitable for practicalapplication. Interestingly, when we increase the amount of IL inthe membranes, the thermal stability of the prepared mem-branes start increasing and reached upto B460 1C for higherconcentration (B70 wt%) of the added IL in the polymer gelelectrolyte membranes. From Fig. 11, it can be seen that whenIL was present in small amount in polymer electrolyte, someamount of LiTFSI salt gets complexed with the polymer back-bone but when we increase the amount of the IL in polymerelectrolyte it reduces the complexing ability of the salt withpolymer. It may be due to the presence of IL which reduces thecomplexing ability of Li-salt with polymer and because of LiTFSI

Fig. 10 (A and B) DSC thermograms of polymer gel electrolyte membranes, (PVdF-HFP + 20% LiTFSI) + x wt% BMIMTFSI (a) x = 20, (b) x = 40, (c) x = 60and (d) x = 70. Inset of (A) shows the DSC thermograms of polymer electrolyte, (PVdF-HFP + 20% LiTFSI).

Fig. 11 TGA curves for pure PVdF-HFP, pure IL, PVdF-HFP + 20 wt%LiTFSI + x wt% IL gel membrane for x = 0, 20, 40, 60 and 70 respectively.Inset of Fig. 11 shows the TGA curves for PVdF-HFP + x wt% IL gelmembrane for x = 20, 40, 60 and 80 respectively.


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salt, which has low lattice energy and also a low tendency to formion-pairs and complex as discussed earlier. Therefore, on thebasis of above discussions we can conclude that in the presentstudy, IL BMIMTFSI and LiTFSI salt both provide more free ionsto the polymer gel electrolyte membranes which plays a signifi-cant role to enhance the overall ionic conductivity of the system.In order to explain how the addition of IL in the polymer electrolytemembranes reduces the complexing ability of Li-salt withpolymer backbone we have carried out a detailed discussion onTGA supported by the first derivative of TGA (i.e. DTGA) in ESI†(see Fig. S1 and S2). To account for this, complex formationbetween cations of the Li-salt and the IL with polymer cannotbe excluded and spectroscopic studies are given in the follow-ing section to confirm this hypothesis.

Ion–polymer interaction by FTIR. The FTIR spectra of the purePVdF-HFP, polymer gel electrolyte membranes PVdF-HFP + x wt%

BMIMTFSI (where x = 0, 20, 40 and 60) and (PVdF-HFP + 20%LiTFSI) + x wt% BMIMTFSI (where x = 0, 20, 40 and 60) in therange of 400–1600 cm�1 are shown in Fig. 12(A) and 13(A)respectively and their respective assignments are listed in Table 2.Polymer PVdF-HFP is known to be a semi-crystalline polymertherefore, FTIR spectra of pure PVdF-HFP contain some crystalline(a-phase) and amorphous (b-phase) phase related peaks. The bandsof pure polymer PVdF-HFP due to the crystalline phase (a-phase) areobserved at 489, 534, 614, 762, 796 and 976 cm�1, while the bandsrelated to the amorphous phase (b-phase) are observed at 840 cm�1

and 880 cm�1. From Fig. 12(A) and 13(A), it can be seen thatwhen IL is incorporated in the polymer PVdF-HFP and in polymergel electrolyte membranes ((PVdF-HFP + 20 wt% LiTFSI) + x wt%BMIMTFSI), almost all the crystalline phase related bands ofPVdF-HFP (i.e. at 976, 796, 762, 614, and 534 cm�1) are disappearand/or become weak and the intensity of the peaks belonging to

Fig. 12 (A and B) FTIR spectra of (a) pure PVdF-HFP, (b) pure IL and PVdF-HFP + x wt% IL gel membrane for (c) x = 20, (d) x = 40 and (e) x = 60 in theregion 400–1800 cm�1 and in the region 2800–3200 cm�1 respectively. Inset of (B) shows the FTIR spectra of pure IL in the region 2800–3200 cm�1.

Fig. 13 (A and B) FTIR spectra of (a) pure PVdF-HFP, (b) pure IL, (c) LiTFSI salt and PVdF-HFP + 20 wt% LiTFSI + x wt% IL gel membrane for (d) x = 0, (e) x =20, (f) x = 40 and (g) x = 60 in the region 400–1800 cm�1 and in the region 2800–3200 cm�1 respectively.


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amorphous phase (b-phase) (i.e. at 841 and 879 cm�1) becomeprominent.39–46 From the above discussions, it can be said thatcrystallinity of the polymer PVdF-HFP decreases upon addingIL in the polymer PVdF-HFP matrix and in the polymer gelelectrolyte membranes (PVdF-HFP + x wt% BMIMTFSI) asconfirmed by XRD and DSC results discussed earlier.

Fig. 12(B) and 13(B) show the FTIR spectra of the pure PVdF-HFP, pure IL, polymer gel electrolyte membranes PVdF-HFP +x wt% BMIMTFSI (where x = 0, 20, 40 and 60) and (PVdF-HFP +20% LiTFSI) + x wt% BMIMTFSI (where x = 0, 20, 40 and 60) inthe range of 2800–3200 cm�1 respectively, and their respectiveassignments are listed in Table 2.47–52

From Fig. 12(A) and 13(A), it can be seen that the peaksrelated to the IL (i.e. 511, 572, 600, 619, 654, 740, 1140, 1195,1232, 1332, 1354 and 1573 cm�1) become prominent and alsoshift to lower wavenumber side with the increasing content ofIL in polymer and polymer gel electrolyte membranes. Thesechanges in shift and changes in intensity have revealed theinteraction between cation of the IL/or cation of the Li-saltwith polymer backbone. The peak of the polymer PVdF-HFP at1068 cm�1 lies very near to the peak of IL at 1058 cm�1. Hence,they tend to merge in a single peak at B1056 cm�1 and nodefinite conclusion can be drawn from this peak. Two newpeaks also appear in the prepared membranes at 1468 and1630 cm�1. The intensity of the peak at 1468 cm�1 increasedupon increasing the concentration of IL in the polymer matrix.However, the peak at 1630 cm�1 (which may be due to thecomplexation formed between polymer backbone and LiTFSIsalt) was first visible in the polymer electrolyte (i.e. PVdF-HFP +20% LiTFSI) and then intensity of this peak gets reduced uponincorporation of IL in the polymer electrolyte because asdiscussed earlier in our TGA/DTGA study that when IL and

LiTFSI salt both were present together in the membranes, theyhave less tendency of complex formation.

Fig. 12(B) and 13(B) are divided into two regions (i.e. region I(red box) and region II (blue box)) respectively due to the C–Hstretching vibrations of butyl chain of IL (also those of polymerbackbone stretching) and the imidazolium cation ring of IL.The region II (i.e. the C–H stretching vibration of imidazoliumcation ring) which is more expected to be affected due tocomplexation. Hence, a detailed deconvolution has been donefor this region and is given in Fig. 14(A). The C–H stretchingvibrations of polymer backbone are at 2985 and 3025 cm�1,while for pure IL, three peaks at 2969, 2942 and 2880 cm�1 inthe region I are due to the alkyl C–H stretching of butyl chain ofIL. It can be seen that C–H stretching vibrations related to thebutyl chain of IL at 2880 and 2942 cm�1 also shift towards lowerwavenumber side. The peak of IL at 2969 cm�1 cannot beclearly seen since it is near the peak of polymer chain vibrationat 2985 cm�1. Hence, they tend to merge and no definiteconclusion can be drawn from this peak.

In region II, we have also found some significant changes inthe peak position of the vibrational bands related to the C–Hstretching vibration of imidazolium cation ring. Apart fromthese changes we have seen asymmetry in the peak which ispresent at 3104 cm�1 when IL is incorporated in polymer matrix.In order to find the above discussed asymmetry in the peak andexact peak positions of C–H stretching vibrations of imidazoliumcation ring and to know the role of IL complexation. We havecarried out a detailed deconvolution of the spectra of region II inthe spectral range 3070 to 3200 cm�1. The deconvolution wasdone with the help of Peakfit software.53 In all the cases, thedeconvolution was carried out using multiple Gaussian peaks toextract the exact peak positions of prepared gel membranes. The

Table 2 Possible assignment of some significant peaks in the FTIR spectra of the pure PVdF-HFP, pure IL BMImTFSI and LiTFSI salt

Sample Wavenumber (cm�1) Assignment

Pure PVdF-HFP 489, 532 Bending and wagging vibrations of the CF2 group614 Mixed mode of CF2 bending and CCC skeletal vibration762 CH2 rocking vibration796 CF3 stretching vibration839 Mixed mode of CH2 rocking879 Combined CF2 and CC symmetric stretching vibrations976 C–F stretching2983, 3024 Symmetric and antisymmetric stretching vibrations of CH2

Pure BMImTFSI and pure LiTFSI 515, 574 (CF3 asymmetric bending mode of LiTFSI)600–620 Deformation mode of SO2 of LiTFSI654 C–H vibrational mode of cyclic BMIM+ of BMImTFSI740 Overlapping of symmetric bending mode of CF3 and combination

of C–S of BMIMTFSI and S–N stretching of LiTFSI789 Combination of C–S and S–N stretching mode of BMImTFSI and LiTFSI1058 Asymmetric S–N–S stretching of LiTFSI and BMImTFSI1140 C–SO2–N bonding mode of LiTFSI and BMIMTFSI1195 CF3 symmetric stretching mode of LiTFSI and C–H vibrational mode

for cyclic BmIm+ of BMImTFSI1232 N–H stretching mode of BMImTFSI1332 C–SO2–N bonding mode of LiTFSI and BMIMTFSI1354 Asymmetric SO2 stretching mode of LiTFSI and BMIMTFSI1573 C–C and C–N bending mode of BMImTFSI2880 S–CH3 bonding mode of BMImTFSI2942, 2969 CH2 stretching mode of BMImTFSI3123, 3160 C–H vibrational mode for cyclic BMIm+ of BMImTFSI


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deconvoluted spectra of pure IL and prepared membranescontaining different amounts of IL (with the value of square ofregression coefficient i.e. r E 0.999) are given in Fig. 14(A).Fig. 14(A) shows the deconvoluted spectra of pure IL, PVdF-HFP

+ x wt% of IL (where x = 20, 40 and 60) and polymer gelelectrolyte membranes (PVdF-HFP + 20 wt% LiTFSI) + x wt%BMIMTFSI (where x = 20 and 60) respectively. The deconvolutedFTIR spectra of pure IL (see Fig. 14(A)(a)) have four strong peaks

Fig. 14 (A) Deconvoluted FTIR spectra of (a) pure IL, PVdF-HFP + x wt% IL gel membranes for (b) x = 20, (c) x = 40, (d) x = 60 and PVdF-HFP + 20 wt%LiTFSI + x wt% IL gel membrane for (e) x = 20 and (f) x = 60 for CH stretching vibrational mode of imidazolium cation ring of IL in the region 3070–3200 cm�1. (B) Ratio of the relative intensities of uncomplexed to the complexed IL (y/x) vs. concentration of IL in PVdF-HFP + x wt% IL gel membranesfor different values of x.


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at 3171, 3154, 3128 and 3104 cm�1. However, the deconvolutedspectra of PVdF-HFP + IL gel membranes (see Fig. 14(A)(b–d))consist of an additional peak at 3093 cm�1 (marked as xin Fig. 14(A)(b–d)). In PVdF-HFP + IL gel membranes, peak at3104 cm�1 split into two peaks 3093 and 3102 cm�1. Therefore,it can be concluded that IL might be present in two differentforms in PVdF-HFP + IL gel membranes (i) IL cation complexedwith the polymer chain (marked as x) and (ii) uncomplexed IL(marked as y). Further, we also expect that with the increasingcontent of IL in PVdF-HFP + IL gel membranes, the ‘‘amount’’of uncomplexed IL (marked as y) will be more. Therefore, wecan estimate the relative intensity of the peak correspondingto the uncomplexed IL to the IL complexed with the polymer(i.e. y/x). From Fig. 14(B), it can be seen that y/x intensity ratio isincreasing with the increasing concentration of IL. So, in viewof above discussions we can conclude that when IL is present insmall amount in PVdF-HFP + IL gel membrane, most of the ILcomplexes with the polymer and less uncomplexed IL is presentas such in the matrix. The amount of uncomplexed IL increasesas the concentration of IL in the polymer gel membraneincreases. Fig. 14(A)(e and f) shows the deconvoluted spectra ofpolymer gel electrolyte membranes (PVdF-HFP + 20 wt% LiTFSI)+ x wt% BMIMTFSI (where x = 20 and 60). From Fig. 14(A)(e andf), it can be seen that there no additional peak appears whichbelongs to the complexation in the deconvoluted spectra ofpolymer gel electrolyte membranes (i.e. (PVdF-HFP + 20 wt%LiTFSI) + x wt% BMIMTFSI (where x = 20 and 60)). Therefore,from Fig. 14(A)(e and f), it can be concluded that when IL and theLiTFSI salt both were present in the prepared membranes thereis very less or no chance to form complexes. Thus, on the basis ofthe above discussion we can say that in the present study bothIL, BMIMTFSI and the LiTFSI salt provide more free ions topolymer gel electrolyte membranes which play a significant rolein enhancing the overall ionic conductivity of the system as wehave discussed in the TGA/DTGA study (see ESI†).

Conclusions

High ion conducting polymer gel electrolyte membranes basedon polymer poly(vinylidene fluoride-co-hexafluoropropylene)PVdF-HFP, ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoro-methanesulfonyl)imide BMIMTFSI with and without the Li-salt(having same anion i.e. TFSI� anion) have been synthesizedand characterized. The ionic conductivity of the polymer gelelectrolyte membrane i.e. PVdF-HFP + 80% BMIMTFSI was ofthe order of B6.6 � 10�4 S cm�1 and that of (PVdF-HFP + 20%LiTFSI) + 70% BMIMTFSI was B2.1 � 10�3 S cm�1 at 30 1C. Thetotal ionic transference number is 40.99 and the cationictransference number (tLi+) B 0.22 for the above mentionedelectrolyte membrane. A wide electrochemical window (ECW)B 4.0–5.0 V for polymer gel electrolyte membrane containinghigher loading of IL (B70 wt% of IL) has been obtained.The ionic conductivity of the polymeric gel membranes wasfound to increase as the amount of IL increased in themembranes. Temperature dependent ionic conductivity seems

to obey Arrhenius type thermally activated behaviour. Thesepolymer gel electrolyte membranes are found to be thermallystable (B300–400 1C), flexible, transparent and free standingin nature. Incorporation of the IL in the polymer electrolyte(PVdF-HFP + 20% LiTFSI) changes the crystallinity, thermalstability, melting temperature (Tm), complexation behaviourbesides increasing the ionic conductivity of the preparedmembranes. FTIR and DTGA studies showed the complexformation between the polymer and the cation of the IL orthe salt in the membrane.

Acknowledgements

One of us RKS gratefully acknowledges financial support fromBRNS-DAE, Mumbai and DST, New Delhi, India, to carry outthis work.

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53 Peakfit is a commercially available product from SeasolveSoftware Inc.


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