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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 gel
electrolyte membranes based on polymer
PVdF-HFP, BMIMTFSI ionic liquid and the
Li-salt with improved electrical, thermal and
structural 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 TFSIÀ
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 Â 10À3
S cmÀ1
at 30 1C
and B3 Â 10À2
S cmÀ1
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 for
potential application in solid state devices like rechargeable
batteries, 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 a
wide electrochemical window. In technological applications,
polymer electrolytes are preferred over liquid electrolytes, as they
overcome the problem associated with liquid electrolytes like
leakage, corrosion and portability.1–8
Generally, polymer electro-
lytes are obtained by polar polymers (like PEO, PEG, PVA, PMMA
and 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, and
good electrode–electrolyte contact but their low room tempera-
ture ionic conductivity limits application in devices. In order to
increase the ionic conductivity of the polymer electrolytes, a
number of approaches such as (i) use of conventional plasticizers
like EC, PC, DEC etc. (ii) dispersion of inorganic filler like SiO2,
Al2O3, CNT, TiO2 etc. (iii) copolymerization (iv) blending etc. are
adopted. Incorporation of the conventional organic plasticizers
can increase the ionic conductivity of the polymer electrolyte by
increasing the flexibility and amorphous phase of the polymer
which in turn increases the volume within the electrolyte system
and decreases the viscosity of the electrolyte making the mobility
of ions easier. It is found that the dispersion of inorganic ceramic
fillers 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 ionic
conductivity of polymer electrolytes at moderate temperatures but
these polymer electrolytes still have some technical drawbacks
such as (i) the ionic conductivity values are still low for practical
applications at room temperature compared with liquid electro-
lytes (ii) their low temperature range of operation due to the
volatile nature of organic solvents that limits the thermal stability
and reduces the electrochemical potential window, (iii) flammability,
(iv) toxicity, (v) environmentally hazardous nature, which are the
main problems of these electrolytes and limit their application in
solid state devices.9–17
Department of Physics, Banaras Hindu University, Varanasi-221005, India.
E-mail: rksingh_17@rediffmail.com; 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
www.rsc.org/MaterialsC
Journal of
Materials Chemistry C
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J. Mater. Chem. C This journal is ©The Royal Society of Chemistry 2015
Recently, room temperature ionic liquids (RTILs) have emerged
as suitable candidates that replace the organic solvents due to their
unique properties like high ionic conductivity, non flammability,
non-toxicity, non-volatility, good chemical and thermal stability
and a wide electrochemical potential window etc. Ionic liquid (ILs)
entirely consist of dissociated anions and cations and are generally
present in the molten state below 100 1C. Despite their high ionic
conductivity, their liquidus nature prevents their direct application
in devices due to leakage and portability problems. So, it is very
important to immobilize the ILs into some organic/inorganic
matrices that provide good mechanical stability along with preser-
ving the main properties of ILs that can significantly provide a
large range of applications to these materials. Therefore in the
present study, IL BMIMTFSI has been incorporated in the polymer
matrix (resulting matrices termed as polymer gel electrolyte
membranes (PGEs)) in which IL acts as a good plasticizer as
well as a supplier of free charge carriers.18–21
Recently, some reports are available in literature on room
temperature ionic liquids (e.g. methyl N-methylpyrrolidinium-
N-acetate trifluoromethanesulfonimide [MMEPyr][TFSI], 1-ethyl
3-methyl imidazolium trifluoro-methane sulfonate, etc.) based
polymer gel electrolytes having high ion conductivity, good
thermal and electrochemical stability. Cheng (2007) et al. reported
the value of ionic conductivity B6.9 Â 10À4
S cmÀ1
at 40 1C in a
pyridiniumimide ionic liquid (BMPyTFSI) based polymer electrolyte
and they also reported an improvement in the electrochemical and
interfacial stability with the incorporation of ionic liquid in the PEO–
LiTFSI electrolyte. Yang et al. reported that the ionic conductivity for
gel polymer electrolytes based on (1-butyl-4-methylpyridinium bis(tri
fluoromethanesulfonyl) imide/lithium bis(trifluoromethanesulfonyl)
imide) B4MePyTFSI/LiTFSI–PVdF-HFP at room temperature is
(2 Â 10À4
S cmÀ1
). Navarra et al. showed that the ionic conductivity
value was of the order of 3 Â 10À4
S cmÀ1
at 50 1C for Li-TFSA/
N-butyl-N-ethyl pyrrolidinium (trifluoromethylsulfonyl) amide
TFSA/PVdF-HFP. Similarly, Jung et al. reported that the conductivity
value for the polymer gel membrane based on (1-butyl-1-methyl-
pyrrolidinium bis(trifluoromethanesulfonyl) imide) PYR14TFSI/
LiTFSI/PVdF-HFP was B3.6 Â 10À4
S cmÀ1
at 30 1C.22–27
In the present work, we have synthesized high ion conducting
polymer gel electrolyte membranes based on polymer poly(vinyl-
idene fluoride-co-hexafluoropropylene) PVdF-HFP, ionic liquid,
1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide
BMIMTFSI with and without Li-salt (having same anion i.e. TFSIÀ
anion). The copolymer PVdF-HFP has been used in the present
study because of its high dielectric constant, which helps in the
dissociation of salts and also due to its HFP units which reduce
the degree of crystallinity.28,29
These synthesized polymer gel
electrolyte membranes were characterized by various techniques
like scanning electron microscopy, X-ray diffraction, differential
scanning calorimetry, thermogravimetric analysis, Fourier trans-
form infrared (FTIR) spectroscopy, complex impedance spectro-
scopy etc. The ionic conductivity for the polymer gel electrolyte
membrane i.e. PVdF-HFP + 80% BMIMTFSI was of the order of
B6.6 Â 10À4
S cmÀ1
at 30 1C and for (PVdF-HFP + 20% LiTFSI) +
70% BMIMTFSI about 2.1 Â 10À3
S cmÀ1
. These polymer gel
electrolyte 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.
Experimental
Materials
The starting materials poly(vinylidene fluoride-co-hexafluoro-
propylene) PVdF-HFP (molecular weight = 400 000 g molÀ1
),
LiTFSI (purity 4 99.9%) salt, and the IL BMIMTFSI (purity 4 99%)
were procured from Sigma-Aldrich (Germany). The IL was dried
in vacuum at about 10À6
Torr for 2 days before use.
Synthesis of polymeric gel membranes
In the present paper, two sets of polymer electrolyte membranes
have been prepared by the conventional solution cast technique.
(a) The polymer gel electrolyte membranes of PVdF-HFP + x
wt% 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 polymer
gel electrolyte membranes, the polymer PVdF-HFP, salt LiTFSI and
IL BMIMTFSI were vacuum dried (B10À3
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 clear
homogeneous solution was obtained. Different amounts of IL were
then added in the above solution and again stirred for 2–4 h at
50 1C until a viscous solution of PVdF-HFP + IL was obtained. For
the preparation of polymer electrolyte gel membranes (PVdF-HFP
+ 20 wt% LiTFSI) + x wt% BMIMTFSI, PVdF-HFP was first dissolved
in dried acetone and stirred for 3–4 hours at 50 1C, after that, an
appropriate amount of LiTFSI salt was added to it and again
stirred for 3–4 hours. After complete dissolution of the salt with the
polymer PVdF-HFP, a desired amount of IL was added and again
stirred for 4–5 hours for obtaining a complete homogeneous
mixture. These viscous solutions were poured in polypropylene
Petri dishes and solvent was allowed to evaporate slowly at room
temperature for a week. After complete evaporation of the solvent,
flexible, thin, free standing and semi-transparent polymer gel
electrolyte membranes (see Fig. 1) PVdF-HFP + x wt% BMIMTFSI
and (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 B10À6
Torr
for 2–3 days to remove any traces of moisture present in the
membranes. A typical photograph of the polymer gel electrolyte
membranes is shown in Fig. 1.
An X’Pert PRO X-ray diffractometer (PANalytical) with CuKa
radiation (l = 1.54 Å) in the range 2y = 101 to 801 was used to
record the X-ray diffraction profiles of polymer gel electrolyte
membranes. A scanning electron microscope (model Quanta
C-200) was used to examine the surface morphology of the
polymeric gel electrolyte membranes.
Paper Journal of Materials Chemistry C
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Bulk elastic modulus (E) of the prepared polymeric gel
electrolyte membranes was measured using the pulse-echo
technique at room temperature in order to determine the eﬀect
of the IL/salt on the mechanical stability of the resulting
membranes. 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 as
receiving ultrasonic waves was coupled to the disc-shaped
membrane (thickness, d B 200–400 mm). The return echo was
received by the pulser/receiver and both the echo pulse and the
input pulse were displayed on a 500 MHz Agilent digital storage
oscilloscope DSO5052A. The transit time of the echo pulse was
recorded and velocity of propagation of ultrasonic waves in
polymeric membranes was calculated using the relation v = 2d/t.
The E values of the samples were calculated using the relation
E = v2
r, (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 scanning
calorimeter using the Mettler DSC 1 system in the temperature
range À110 to 160 1C at a heating rate of 10 1C minÀ1
and
thermogravimetric analysis (TGA) (Mettler DSC/TGA 1 system)
under continuous purging of nitrogen. The FTIR spectra of the
polymeric gel membranes were recorded with the help of Perkin-
Elmer FTIR spectrometer (Model RX 1) from 3500 to 400 cmÀ1
.
Viscosity of the ionic liquid was measured using a Brook-
field DV-III Ultra Rheometer in the temperature range À10 to
80 1C. The instrument was calibrated with standard viscosity
fluid supplied by the manufacturer before each measurement.
Ionic conductivity of the polymeric gel membranes was
measured by the complex impedance spectroscopy technique
using a NOVO Control Impedance Analyzer in the frequency
range 1 Hz–40 MHz. The bulk resistance was determined from
the complex impedance plots. The electrical conductivity (s)
can be calculated by using the following relation:
s ¼
1
Rb
Á
l
A
(1)
where l is the thickness of the sample, A is the cross sectional
area of the disc shaped sample and Rb is the bulk resistance
obtained from complex impedance plots. For temperature
dependent conductivity studies, disc shaped polymeric gel
membranes were placed between two stainless steel electrodes
and 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 voltage
of 10 mV was applied across the disc shaped polymeric gel
membranes placed between two stainless steel electrodes and the
corresponding current was monitored as a function of time. The
cationic transport number (i.e. tLi+) of the polymer gel electrolyte
membrane containing a higher amount of IL (i.e. PVdF-HFP + 20%
LiTFSI + 70% BMIMTFSI) was calculated by using the combined
a.c./d.c. technique. The Li/PVdF-HFP + 20% LiTFSI + 70%
BMIMTFSI/Li cell was subjected to polarization by applying a
voltage DV = 10 mV for 2 hours and resultant currents were
calculated (i.e. initial and final current). The cell resistances
were also measured before and after polarization using ac
impedance spectroscopy.
The cyclic voltammetric studies were carried out using
an electrochemical analyzer with an AUTOLAB PGSTAT 302N
controlled by NOVA 1.8 software version (Methohm Lab) to
estimate the ‘electrochemical stability window (ECW)’ of the
polymer 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% BMIMTFSI
and (PVdF-HFP + 20 wt% LiTFSI) + 70 wt% BMIMTFSI is shown
in Fig. 2(a)–(d). It can be seen from Fig. 2(a) that the pure PVdF-
HFP consists of large crystalline grains with lamellar structure
which are equally distributed and pure PVdF-HFP as well as
polymer gel electrolyte membrane has solvent swollen structure.
When we add IL in to the polymer matrix the size of the grains
starts decreasing and the membrane became flexible. When
20 wt% of LiTFSI salt was added in the polymer matrix, the size
of the grains decreased and some white crystallites were observed
(shown by green circles in Fig. 2(b)), which were absent in the
pure polymer PVdF-HFP micrograph (see Fig. 2(a)) that may
be 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 semi
transparency of the prepared membranes.
Journal of Materials Chemistry C Paper
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electrolyte. Furthermore, when we add both IL, BMIMTFSI and
LiTFSI salt (say 70% IL in PVdF-HFP + 20% LiTFSI), the membrane
became more amorphous with no crystalline grains and no undis-
solved LiTFSI (Fig. 2(d)). The evidence of the enhanced amorphicity
with increasing IL content was also obtained from XRD results as
discussed in the next section. The reduction in crystallinity of the
polymer matrix also affects the thermal and ion transport behavior
which 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 pure
PVdF-HFP showed a semi-crystalline nature (i.e. crystalline and
amorphous both phases are present simultaneously) having
characteristic 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 39.931
remain and some crystalline phase related peaks disappear.
From Fig. 3(c and d), it can be seen that the IL based polymer
gel electrolyte membranes consist of less intense and broader
halos. As reported in literature,30–32
the sharp intense peaks of
LiTFSI salt at 2y = 14.11, 15.91, 18.61, 18.91 and 21.41 reveal the
crystalline character of LiTFSI salt.33
It can be seen that when
20 wt% LiTFSI salt was added in the polymer matrix, all
crystalline peaks of LiTFSI salt are disappear and only one at
2y = 14.11 (see Fig. 3(b)) remains. This remaining peak also
disappeared when IL was incorporated in the polymer electro-
lyte (i.e. PVdF-HFP + 20 wt% LiTFSI) which indicates a complete
dissolution of the salt in the polymer gel electrolyte membranes
in the presence of IL (see Fig. 3(e and f)). Further, upon increasing
the amount of IL in polymer gel electrolyte membranes, the
intensity of the crystalline phase related peaks of PVdF-HFP start
decreasing due to the enhanced amorphicity of the prepared
membranes (see Fig. 3(e and f)). The FWHM of the halo was also
found to increase with increasing amounts of ionic liquid in
the polymer gel electrolyte membranes. The broadening of the
peaks and reduction in intensity/or absence of some crystalline
phase related peaks in prepared gel membranes indicates that
the crystallinity of the polymer PVdF-HFP is decreased upon
increasing the amount of IL in the membranes. The amorphous
polymer–salt mixtures are ideally suited for battery electrolyte
applications. The evidence of increased amorphicity with the
increasing concentration of IL was also confirmed by SEM
analysis discussed above.
Mechanical test. The elastic modulus (E) of the polymer gel
electrolyte membranes was calculated as described in the Experi-
mental section. The value of the elastic modulus (E) of pure
PVdF-HFP (as calculated in our earlier study46
), PVdF-HFP + 20%
LiTFSI, and (PVdF-HFP + 20% LiTFSI) + 70 wt% of BMIMTFSI are
14.6 Â 1010
, 7.5 Â 1010
and 3.2 Â 1010
dynes per cm2
respectively.
The elastic modulus of the membranes decreased with the
addition of the salt and IL in PVdF-HFP matrix. This was due
to the plasticization eﬀect of the IL.
Ionic transport behavior. The ionic conductivity (s) of pure
IL is 5.8 Â 10À3
S cmÀ1
at 30 1C which increases with increasing
temperature as shown in Fig. 4. This increase in conductivity is
closely related to the decrease in viscosity with temperature (as
given 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 membranes
containing PVdF-HFP + 40% BMIMTFSI and (PVdF-HFP + 20%
LiTFSI) + 40% BMIMTFSI at room temperature are shown in
Fig. 5 (a similar behaviour was also observed for all the prepared
membranes). The depressed semicircle was observed in the
high-frequency region due to the parallel combination of bulk
resistance (Rb) and the double layer capacitance Cdl followed by
an inclined spike in the low-frequency region that indicates the
diﬀusion of ions. The angle of inclination of the straight line and
angle of the depressed semicircle are due to the presence of
distributed macroscopic material properties termed as constant
phase element (CPE) (see inset of Fig. 5).
CPE = K/(jo)a
(2)
where K is the Warburg coeﬃcient related to the properties of
the electrode surface and the ionic species of the electrolytes, o
is the angular frequency and a is an exponent (i.e. the slope of
the log Z vs. log f plot) whose value lies between 0 and 1 and is
related to the roughness of the electrode.
The composition and temperature dependent ionic conductivity
of 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 the
polymer gel electrolyte membranes i.e. PVdF-HFP + 20 wt%
BMIMTFSI is found to be B5.76 Â 10À7
S cmÀ1
at 30 1C. From
Fig. 6(A), it can be seen that the ionic conductivity was found
to increase with increasing amount of IL in PVdF-HFP and
reaches at B6.58 Â 10À4
S cmÀ1
at room temperature for the
membrane containing higher amount of IL (i.e. 80%). The ionic
conductivity of polymer electrolyte i.e. PVdF-HFP + 20 wt%
LiTFSI was found to be B1.01 Â 10À7
S cmÀ1
at 30 1C. From
Fig. 6(B), it can be seen that the highest conductivity value of
the polymer gel electrolyte membrane (i.e. PVdF-HFP + 20 wt%
LiTFSI + 70% BMIMTFSI) B2.07 Â 10À3
S cmÀ1
at 30 1C and
B2.91 Â 10À2
S cmÀ1
at 130 1C, was obtained. From the above
discussion it can be concluded that the ionic conductivity value
of the membranes was quite low when we use IL, BMIMTFSI
and LiTFSI salt alone but a drastic increment (about four order
of magnitude) was observed in the conductivity value when
both were used to prepare polymer gel electrolyte membrane
because in the present system, IL and Li-salt are containing same
anion. 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 its
equivalent 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 that
the mixed-anion systems (i.e., in which a dopant salt and added IL
having a diﬀerent anion) have a chance to form contact/cross-
contact ion pairs (which do not take part in the conduction
mechanism) and hence significantly decrease the ionic conductivity
of the system. These IL-based polymer gel electrolyte membranes
(in which the IL and salt have the same anion) are suitable choices
for applications in rechargeable batteries.9
The above discussed
increment in conductivity was not only because of the choice
of the same anion system but it was also due to the choice of the
Li-salt (i.e. LiTFSI salt). The LiTFSI, has a high salt dissociation
due to the strong electron-withdrawing SO2CF3 groups present at
both sides of the imide anion. This salt has low lattice energy and
also has a low tendency to form ion-pairs. It also acts as a
plasticizer for the polymer matrix by creating free-volume leading
to the enhanced ionic mobility and hence the ionic conductivity.
Thus, the polymer gel electrolyte membranes that contain LiTFSI
salt show high ionic conductivity.34–36
From both the figures (i.e. Fig. 6(A) and (B)), it can also be seen
that the conductivity increases with increasing temperature and
follows an Arrhenius type thermally activated process. After
definite temperature, a sudden jump in conductivity value was
observed for all the prepared membranes. The temperature at
which the sudden conductivity jumps occur corresponds to the
melting temperature, Tm (i.e., semi-crystalline to amorphous
transition) of the polymer gel electrolyte membranes as
obtained from DSC thermograms. The ionic conductivity
at T o Tm obeys Arrhenius type thermally activated process
and can be expressed as:
s = s0 exp(ÀEa/kT) (3)
where s0 is the pre-exponential factor, Ea is the activation
energy, k is the Boltzmann constant and T is the temperature
in Kelvin. Fig. 6 (which is the plot between log s and 1/T) was
used to calculate activation energy (Ea). It was found that the
activation energy (Ea) decreases as we increase the concentration
of 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 the
enhancement 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 À35 34
PVdF-HFP + 20% BMIMTFSI 141 À47 23.2 0.42
(PVdF-HFP + 20% LiTFSI) +
20 wt% BMIMTFSI
147 À58 31.8 0.41
40 wt% BMIMTFSI
137 À67 19.5 0.26
60 wt% BMIMTFSI
126 À75 10.4 0.15
70 wt% BMIMTFSI
113 À84 4.1 0.13
PVdF-HFP + 20% LiTFSI 150 À39 44.7 0.52
Pure BMIMTFSI — À87
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energy (Ea) on increasing the IL content indicates easier ionic
transport in the system due to the plasticization/amorphization
eﬀect of IL which is responsible for reducing the crystallinity of
the polymer matrix, as described earlier in the present work from
the structural and thermal studies. A phenomenological model
as described below, explains the observed behaviour of ionic
conductivity in these polymer electrolyte gel membranes. Fig. 7
systematically shows (a) the polymeric chains, (b) polymer
electrolyte membrane (i.e. PVdF-HFP + LiTFSI salt) and (c) IL
based polymer gel electrolyte membrane (i.e. PVdF-HFP +
LiTFSI + BMIMTFSI). Fig. 7(a) shows that the semi-crystalline
nature of the polymer PVdF-HFP and Fig. 7(b) shows that the
polymer 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 high
ionic conduction (because of more availability of ions) in the
system resulting in enhancement of the ionic conductivity (see
Fig. 7(c)).
The total ionic transport number (tion) of the polymer gel
electrolyte membrane ((PVdF-HFP + 20% LiTFSI) + 70% BMIMTFSI)
has also been determined (see Fig. 8(A)) using eqn (4) as
given below:
tion = (iT À 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 indicates
that the total ionic conductivity is mainly due to the flow of
Fig. 7 Schematic representation of (a) polymeric chains, (b) the polymer
electrolyte membrane (i.e. PVdF-HFP + LiTFSI salt) and (c) the IL based
polymer 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 mV
recorded 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 after
polarization of the cell (i.e. Li|polymer gel electrolyte membrane containing 70% of IL|Li) at room temperature. (C) Cyclic voltammograms of the cells
containing polymer gel electrolyte membrane sandwiched between two symmetrical stainless steel (blocking) electrodes and inset of Figure [C] shows
the cyclic voltammograms of the membrane sandwiched between two non-blocking electrodes i.e., Li||polymer gel electrolyte membrane||LiMnO2 for
various cycles at room temperature at a scan rate of 10 mV sÀ1
.
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ions. In the present case, it is expected that three different
charge carriers (i.e. BMIM+
, Li+
and TFSIÀ
) can conduct in the
system to give ionic conductivity. Therefore, it is very important
to calculate the cationic transport number (tLi+). The transport
number 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, the
Li|polymer gel electrolyte membrane|Li cell was polarized at
constant potential by applying a voltage, DV = 10 mV and the
resulting currents were monitored that fall from an initial value
(I0) to a final value i.e. steady state value (Iss) w.r.t time (see
Fig. 8(B)). The cell resistances (i.e. before (R0) and after (Rss)
the polarization at room temperature as given in the inset
of Fig. 8(B) were evaluated by a.c. impedance spectroscopy).
The following expression has been used for the estimation of
the value of tLi+.
tLi+ = Iss(DV À I0R0)/(I0(DV À IssRss))
The value of tLi+ for the PVdF-HFP + 20% LiTFSI + 70%
BMIMTFSI gel polymer electrolyte has been found to be B0.22
at room temperature. This value shows that the ionic conduc-
tivity of the system is the contribution of other ions present
in the system also (like triflate anions which is common for
both the lithium salt and ionic liquid and imidazolium cations
(BMIM+
ions)). Li et al. also reported that the lithium ion
transport decreases with increasing concentration of IL in the
system and they have reported that tLi+ = 0.3 for the system
containing 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 of
the prepared Li+
-ion based polymer gel electrolyte membrane
consisting of (PVdF-HFP + 20% LiTFSI salt) + 70% BMIMTFSI
(this is the optimized composition out of all prepared polymer
gel electrolyte membranes that has high ionic conductivity along
with good mechanical stability), has been analyzed at room
temperature by cyclic voltammetric studies. Fig. 8(C) shows the
cyclic voltammograms (CV) (i.e. the current–voltage plot), traced on
the cells containing polymer gel electrolyte membrane sandwiched
between two symmetrical stainless steel (blocking) electrodes and
inset of Fig. 8(C) shows the cyclic voltammograms (CV) traced on
the cells containing polymer gel electrolyte membrane sandwiched
between two non-blocking electrodes i.e., Li8polymer gel electrolyte
membrane8LiMnO2 for various cycles at room temperature at a
scan rate of 10 mV sÀ1
. The electrochemical stability window
has been found B4.0–5.0 V (i.e. only as the estimation for the
electrochemical stability in Li+
-ion-cells) which is considerably
good from the application point of view predominantly in Li-ion
rechargeable battery applications. The value of ECW provides the
information about the polymer gel electrolyte membrane up to
which 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. The
DSC thermogram of the polymer electrolyte membrane (i.e.
PVdF-HFP + 20% LiTFSI) is given in the inset of Fig. 10(A). An
endothermic peak corresponding to the melting of the crystal-
line phase of the polymer PVdF-HFP is observed at 145 1C and
the glass transition temperature (Tg) (i.e. the transition from
brittle or hard state at lower temperature to rubbery behaviour or
flexible at high temperatures) of polymer PVdF-HFP is observed
at À35 1C (see Fig. 9(A) (curve a) and (B) (curve a)). The melting
temperature (Tm) and the glass transition temperature (Tg) of the
prepared membranes shifted towards lower temperature side on
the 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 interaction
between the cations of the IL, Li-salt and polymer which
significantly enhanced the segmental motion of the polymer
network by making polymer matrix more flexible. This decrease
in 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 electrolyte
membrane (PVdF-HFP + 20% LiTFSI) was expected to decrease
the 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 the
phase transition) to the melting heat DH
m
À Á
of 100% crystalline
PVdF-HFP. The value of DH
m is 104.7 J gÀ1
.38
The ratio of DHm
to DH
m gives Xc as,
Xc ¼ DHm

DH
m
À Á
Â 100% (5)
The value of degree of crystallinity (Xc) was found to decrease
from 34 to 4% (see Table 1). Initially when 20 wt% of LiTFSI salt
was added in the polymer matrix, the degree of crystallinity (Xc)
and melting temperature (Tm) both were increased but this
increased crystallinity and melting temperature eﬀectively got
suppressed by the addition of IL and reached to the lowest
value (approximately about B4%) for the membrane contains
higher 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 (where
x = 0, 20, 40, 60 and 70) are shown in Fig. 11 and the TGA plots
of polymer gel electrolyte membranes, PVdF-HFP + x wt%
BMIMTFSI (where x = 20, 40, 60 and 80) are shown in the inset
of Fig. 11 (high thermal stability (B300–350 1C) of all the
prepared polymer gel electrolyte membranes is confirmed from
Fig. 11). PVdF-HFP and IL BMIMTFSI decompose in a single
step 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 of
added IL in the polymer electrolyte membranes, thermal stability
decreases by a small amount but is still suitable for practical
application. Interestingly, when we increase the amount of IL in
the membranes, the thermal stability of the prepared mem-
branes start increasing and reached upto B460 1C for higher
concentration (B70 wt%) of the added IL in the polymer gel
electrolyte membranes. From Fig. 11, it can be seen that when
IL was present in small amount in polymer electrolyte, some
amount of LiTFSI salt gets complexed with the polymer back-
bone but when we increase the amount of the IL in polymer
electrolyte it reduces the complexing ability of the salt with
polymer. It may be due to the presence of IL which reduces the
complexing 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 = 60
and (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 gel
membrane for x = 20, 40, 60 and 80 respectively.
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salt, which has low lattice energy and also a low tendency to form
ion-pairs and complex as discussed earlier. Therefore, on the
basis of above discussions we can conclude that in the present
study, IL BMIMTFSI and LiTFSI salt both provide more free ions
to 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 electrolyte
membranes reduces the complexing ability of Li-salt with
polymer backbone we have carried out a detailed discussion on
TGA supported by the first derivative of TGA (i.e. DTGA) in ESI†
(see Fig. S1 and S2). To account for this, complex formation
between cations of the Li-salt and the IL with polymer cannot
be 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 pure
PVdF-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 the
range of 400–1600 cmÀ1
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 polymer
therefore, FTIR spectra of pure PVdF-HFP contain some crystalline
(a-phase) and amorphous (b-phase) phase related peaks. The bands
of pure polymer PVdF-HFP due to the crystalline phase (a-phase) are
observed at 489, 534, 614, 762, 796 and 976 cmÀ1
, while the bands
related to the amorphous phase (b-phase) are observed at 840 cmÀ1
and 880 cmÀ1
. From Fig. 12(A) and 13(A), it can be seen that
when IL is incorporated in the polymer PVdF-HFP and in polymer
gel electrolyte membranes ((PVdF-HFP + 20 wt% LiTFSI) + x wt%
BMIMTFSI), almost all the crystalline phase related bands of
PVdF-HFP (i.e. at 976, 796, 762, 614, and 534 cmÀ1
) are disappear
and/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 the
region 400–1800 cmÀ1
and in the region 2800–3200 cmÀ1
respectively. Inset of (B) shows the FTIR spectra of pure IL in the region 2800–3200 cmÀ1
.
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 cmÀ1
and in the region 2800–3200 cmÀ1
respectively.
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amorphous phase (b-phase) (i.e. at 841 and 879 cmÀ1
) become
prominent.39–46
From the above discussions, it can be said that
crystallinity of the polymer PVdF-HFP decreases upon adding
IL in the polymer PVdF-HFP matrix and in the polymer gel
electrolyte membranes (PVdF-HFP + x wt% BMIMTFSI) as
confirmed 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) in
the range of 2800–3200 cmÀ1
respectively, and their respective
assignments are listed in Table 2.47–52
From Fig. 12(A) and 13(A), it can be seen that the peaks
related to the IL (i.e. 511, 572, 600, 619, 654, 740, 1140, 1195,
1232, 1332, 1354 and 1573 cmÀ1
) become prominent and also
shift to lower wavenumber side with the increasing content of
IL in polymer and polymer gel electrolyte membranes. These
changes in shift and changes in intensity have revealed the
interaction between cation of the IL/or cation of the Li-salt
with polymer backbone. The peak of the polymer PVdF-HFP at
1068 cmÀ1
lies very near to the peak of IL at 1058 cmÀ1
. Hence,
they tend to merge in a single peak at B1056 cmÀ1
and no
definite conclusion can be drawn from this peak. Two new
peaks also appear in the prepared membranes at 1468 and
1630 cmÀ1
. The intensity of the peak at 1468 cmÀ1
increased
upon increasing the concentration of IL in the polymer matrix.
However, the peak at 1630 cmÀ1
(which may be due to the
complexation formed between polymer backbone and LiTFSI
salt) was first visible in the polymer electrolyte (i.e. PVdF-HFP +
20% LiTFSI) and then intensity of this peak gets reduced upon
incorporation of IL in the polymer electrolyte because as
discussed earlier in our TGA/DTGA study that when IL and
LiTFSI salt both were present together in the membranes, they
have 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–H
stretching vibrations of butyl chain of IL (also those of polymer
backbone stretching) and the imidazolium cation ring of IL.
The region II (i.e. the C–H stretching vibration of imidazolium
cation ring) which is more expected to be affected due to
complexation. Hence, a detailed deconvolution has been done
for this region and is given in Fig. 14(A). The C–H stretching
vibrations of polymer backbone are at 2985 and 3025 cmÀ1
,
while for pure IL, three peaks at 2969, 2942 and 2880 cmÀ1
in
the region I are due to the alkyl C–H stretching of butyl chain of
IL. It can be seen that C–H stretching vibrations related to the
butyl chain of IL at 2880 and 2942 cmÀ1
also shift towards lower
wavenumber side. The peak of IL at 2969 cmÀ1
cannot be
clearly seen since it is near the peak of polymer chain vibration
at 2985 cmÀ1
. Hence, they tend to merge and no definite
conclusion can be drawn from this peak.
In region II, we have also found some significant changes in
the peak position of the vibrational bands related to the C–H
stretching vibration of imidazolium cation ring. Apart from
these changes we have seen asymmetry in the peak which is
present at 3104 cmÀ1
when IL is incorporated in polymer matrix.
In order to find the above discussed asymmetry in the peak and
exact peak positions of C–H stretching vibrations of imidazolium
cation ring and to know the role of IL complexation. We have
carried out a detailed deconvolution of the spectra of region II in
the spectral range 3070 to 3200 cmÀ1
. The deconvolution was
done with the help of Peakfit software.53
In all the cases, the
deconvolution was carried out using multiple Gaussian peaks to
extract 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 (cmÀ1
) Assignment
Pure PVdF-HFP 489, 532 Bending and wagging vibrations of the CF2 group
614 Mixed mode of CF2 bending and CCC skeletal vibration
762 CH2 rocking vibration
796 CF3 stretching vibration
839 Mixed mode of CH2 rocking
879 Combined CF2 and CC symmetric stretching vibrations
976 C–F stretching
2983, 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 LiTFSI
654 C–H vibrational mode of cyclic BMIM+
of BMImTFSI
740 Overlapping of symmetric bending mode of CF3 and combination
of C–S of BMIMTFSI and S–N stretching of LiTFSI
789 Combination of C–S and S–N stretching mode of BMImTFSI and LiTFSI
1058 Asymmetric S–N–S stretching of LiTFSI and BMImTFSI
1140 C–SO2–N bonding mode of LiTFSI and BMIMTFSI
1195 CF3 symmetric stretching mode of LiTFSI and C–H vibrational mode
for cyclic BmIm+
of BMImTFSI
1232 N–H stretching mode of BMImTFSI
1332 C–SO2–N bonding mode of LiTFSI and BMIMTFSI
1354 Asymmetric SO2 stretching mode of LiTFSI and BMIMTFSI
1573 C–C and C–N bending mode of BMImTFSI
2880 S–CH3 bonding mode of BMImTFSI
2942, 2969 CH2 stretching mode of BMImTFSI
3123, 3160 C–H vibrational mode for cyclic BMIm+
of BMImTFSI
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deconvoluted spectra of pure IL and prepared membranes
containing different amounts of IL (with the value of square of
regression 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 gel
electrolyte membranes (PVdF-HFP + 20 wt% LiTFSI) + x wt%
BMIMTFSI (where x = 20 and 60) respectively. The deconvoluted
FTIR 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 cmÀ1
. (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 membranes
for different values of x.
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at 3171, 3154, 3128 and 3104 cmÀ1
. However, the deconvoluted
spectra of PVdF-HFP + IL gel membranes (see Fig. 14(A)(b–d))
consist of an additional peak at 3093 cmÀ1
(marked as x
in Fig. 14(A)(b–d)). In PVdF-HFP + IL gel membranes, peak at
3104 cmÀ1
split into two peaks 3093 and 3102 cmÀ1
. Therefore,
it can be concluded that IL might be present in two different
forms in PVdF-HFP + IL gel membranes (i) IL cation complexed
with the polymer chain (marked as x) and (ii) uncomplexed IL
(marked as y). Further, we also expect that with the increasing
content of IL in PVdF-HFP + IL gel membranes, the ‘‘amount’’
of uncomplexed IL (marked as y) will be more. Therefore, we
can estimate the relative intensity of the peak corresponding
to 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 is
increasing with the increasing concentration of IL. So, in view
of above discussions we can conclude that when IL is present in
small amount in PVdF-HFP + IL gel membrane, most of the IL
complexes with the polymer and less uncomplexed IL is present
as such in the matrix. The amount of uncomplexed IL increases
as the concentration of IL in the polymer gel membrane
increases. Fig. 14(A)(e and f) shows the deconvoluted spectra of
polymer gel electrolyte membranes (PVdF-HFP + 20 wt% LiTFSI)
+ x wt% BMIMTFSI (where x = 20 and 60). From Fig. 14(A)(e and
f), it can be seen that there no additional peak appears which
belongs to the complexation in the deconvoluted spectra of
polymer 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 the
LiTFSI salt both were present in the prepared membranes there
is very less or no chance to form complexes. Thus, on the basis of
the above discussion we can say that in the present study both
IL, BMIMTFSI and the LiTFSI salt provide more free ions to
polymer gel electrolyte membranes which play a significant role
in enhancing the overall ionic conductivity of the system as we
have discussed in the TGA/DTGA study (see ESI†).
Conclusions
High ion conducting polymer gel electrolyte membranes based
on 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. TFSIÀ
anion) have been synthesized
and characterized. The ionic conductivity of the polymer gel
electrolyte membrane i.e. PVdF-HFP + 80% BMIMTFSI was of
the order of B6.6 Â 10À4
S cmÀ1
and that of (PVdF-HFP + 20%
LiTFSI) + 70% BMIMTFSI was B2.1 Â 10À3
S cmÀ1
at 30 1C. The
total ionic transference number is 40.99 and the cationic
transference number (tLi+) B 0.22 for the above mentioned
electrolyte membrane. A wide electrochemical window (ECW)
B 4.0–5.0 V for polymer gel electrolyte membrane containing
higher loading of IL (B70 wt% of IL) has been obtained.
The ionic conductivity of the polymeric gel membranes was
found to increase as the amount of IL increased in the
membranes. Temperature dependent ionic conductivity seems
to obey Arrhenius type thermally activated behaviour. These
polymer gel electrolyte membranes are found to be thermally
stable (B300–400 1C), flexible, transparent and free standing
in nature. Incorporation of the IL in the polymer electrolyte
(PVdF-HFP + 20% LiTFSI) changes the crystallinity, thermal
stability, melting temperature (Tm), complexation behaviour
besides increasing the ionic conductivity of the prepared
membranes. FTIR and DTGA studies showed the complex
formation between the polymer and the cation of the IL or
the salt in the membrane.
Acknowledgements
One of us RKS gratefully acknowledges financial support from
BRNS-DAE, Mumbai and DST, New Delhi, India, to carry out
this work.
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