Highly efficient gel polymer electrolytes for all solid ...

Electrochimica Acta 278 (2018) 137e148

Contents lists available at ScienceDirect

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

Highly efficient gel polymer electrolytes for all solid-state electrochemical charge storage devices P. Pal, A. Ghosh* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 February 2018 Received in revised form 12 April 2018 Accepted 2 May 2018 Available online 4 May 2018

Gel polymer electrolytes are prepared using poly(vinylidene fluoride-co-hexafluoropropylene) polymer matrix and 1- propyl-3-methyleimidazolium bis(trifluromethylesulfonyl)-imide ionic liquid and also by adding lithium bis(trifluoromethanesulfonyl)imide salt and plasticizer mixture (ethylene carbonate: propylene carbonate in the ratio 1:1). Thermal and electrical properties of these electrolytes are first investigated. The electrical conductivity of these electrolytes is analyzed over a wide frequency range using a universal power law coupled with modified Poisson-Nernst-Planck model for electrode polarization. All electrolytes show excellent thermal stability up to 340 C, high ionic conductivity (~1 103 S cm1) and wide potential window (~4.0 V). Supercapacitors are fabricated with these electrolytes using activated carbon as electrodes and their electrochemical properties are studied. The cyclic voltammetry curves show almost box-like shape corresponding to an ideal and reversible capacitive characteristic. The specific capacitance of supercapacitors increases with the addition of lithium salt and plasticizer mixture. The cycle stability of these supercapacitors up to 4000 cycles confirms their electrochemical stability. A lithium ion coin cell is also fabricated using electrolytes containing plasticizer mixture. At ambient temperature, the fabricated cell delivers high specific discharge capacity (~165.8 mAh g1) for the first discharge cycle at a constant current rate C/12. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Gel polymer electrolytes Ionic conductivity Supercapacitor Lithium ion battery

1. Introduction Nowadays, a rapid growing demand has been received from our modern society to develop high performing solid-state electrochemical charge storage devices operating at ambient condition due to the limited source of fossil fuels and global warming, etc. Over last few years, researchers have delivered their efforts in developing alternative green energy storage devices, such as polymer based supercapacitors, lithium ion batteries, etc. [1e4]. There are some basic differences in their charge storage mechanisms of polymer supercapacitors and lithium ion batteries. Supercapacitors store charge through electrostatic interaction between electrode surfaces and charge carriers of polymer electrolytes, whereas lithium ion batteries store energy via electrochemical reactions [5]. The cycle life of supercapacitors is higher than that of batteries [6], while batteries can store more energy than supercapacitors. Currently, gel polymer electrolytes (GPEs) have received extensive attention for their applications in

* Corresponding author. E-mail address: [email protected] (A. Ghosh). https://doi.org/10.1016/j.electacta.2018.05.025 0013-4686/© 2018 Elsevier Ltd. All rights reserved.

energy storage devices as an electrolyte/separator due to their higher ionic conductivity (>1 103 S cm1) at ambient temperature, wider electrochemical voltage window and better stability of electrode-polymer interface than that of liquid electrolytes [7,8]. Besides device applications, the studies of Liþ ion conduction mechanism in GPEs are an important aspect for investigation [9e11]. In general, among various polymer matrices [12e17], poly(vinylidene fluoride-co-hexafluoropropylene) [P(VdF-HFP)] copolymer is the most widely studied host polymer matrix used for the preparation of high performance GPEs, because the crystalline part P(VdF) of this polymer supports the mechanical property and amorphous part P(HFP) helps to trap the liquid electrolytes [16,18]. Furthermore, to improve the ionic conductivity, interfacial stability, thermal/mechanical stability, flexibility and electrochemical voltage stability different approaches, such as addition of ceramic fillers (Al2O3, SiO2, TiO2, graphene oxide) [16,17,19], blending with other polymers [20], aqueous and organic liquid electrolyte consisting of different salts and polar solvents [21e24], etc. have been adopted. However, development of high performance gel polymer electrolytes has been an active research area. Recently, ionic liquids

138

P. Pal, A. Ghosh / Electrochimica Acta 278 (2018) 137e148

(ILs) have received considerable attention as an electrolyte component or single liquid electrolyte due to their high ionic conductivity, good electrochemical stability, wide electrochemical voltage range, non-flammability, non-volatility, negligible vapor pressure and environmental compatibility [24,25]. It is important to note that ILs plays dual roles in polymer electrolytes such as supplier of additional charge carriers as well as critical role of plasticizers [26,27]. In this paper, we have prepared GPEs using copolymer P(VdFHFP) and 1- propyl-3-methyleimidazolium bis(trifluromethy lesulfonyl)-imide (PMIMTFSI) ionic liquid. We have investigated the effect of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt and a mixture of plasticizers ethylene carbonate (EC): propylene carbonate (PC) on physicochemical properties and electrochemical performance of the GPEs. We have fabricated three supercapacitor cells using gel polymer electrolytes and activated carbon (AC) as an electrode material and studied electrochemical performance of these supercapacitors. Finally, we have also fabricated a lithium ion battery using best performing gel electrolyte in the configuration: graphite//gel electrolyte//LiFePO4 and studied its electrochemical performance. 2. Experimental 2.1. Materials Poly(vinylidene fluoride-hexafluoropropylene) (P(VdF-HFP), having average molecular weight ~ 455,000 g/mol was used as a host polymer. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 99.0%) was used as salt. The used ionic liquid was 1- propyl-3methyleimidazoliuum bis(trifluromethylesulfonyl)-imide (PMIM TFSI, 98.0%) which offers low viscosity (40 cP at 30 C) [28]. Plasticizers were ethylene carbonate (EC, anhydrous 99.0%) and propylene carbonate (PC, anhydrous 99.7%). All chemicals were procured from Sigma Aldrich and stored inside the argon gas filled glove box (mBRAUN LAB star ECO) with uc(GPE-2) > uc(GPE-3). The lowest value of uc for GPE3 is due to fast polymer chain dynamics. It is also observed in Table 2 that the value of power law exponent n is independent of compositions. The temperature dependence of the ionic conductivity for all gel polymer electrolytes obtained from above fits is shown in Fig. 2(c). It is observed that the ionic conductivity increases with increase of temperature and exhibits Vogel-Tammann-Fulcher (VTF) relationship [39e41]:

sdc ðTÞ ¼ s0 exp

Es ; kB ðT T0 Þ

(4)

where s0 is the pre-exponential constant, kB is the Boltzmann constant, Es is pseudo-activation energy related to the critical free volume for ion transport, T is the absolute temperature and T0 is the equilibrium Vogel scaling temperature. The ionic conductivity for all gel polymer electrolytes has been fitted to Eq. (4) in Fig. 2(c). The values of Es and T0 obtained from the best fits are shown in Table 2. It is observed in Table 2 that the value of Es is minimum for GPE-3. The electrochemical voltage stability (i.e. working voltage window) of the gel polymer electrolytes is a vital parameter from application viewpoint in supercapacitors, lithium ion batteries, etc. Fig. 2(d) shows the cyclic voltametry (CV) of all the gel polymer electrolytes films. It is observed that the working voltage of GPE-1 is in the range from - 2.1 V to þ2.2 V (i.e. 4.3 V). The voltage range of GPE-2 is slightly reduced from e 2.0 V to þ2.1 V (i.e. 4.1 V) due to introduction of LiTFSI salt. The voltage range of GPE-3 increases again from e 2.1 V to þ2.2 V (i.e. 4.3 V) due to plasticization. The Li-ion transference number (tþ Li) in these gel polymer electrolytes is a crucial issue for the Li-ion battery application. Fig. 3(a) and (b) show the electrochemical impedance spectra at 25 C of the symmetric cells using GPE-2 and GPE-3 gel electrolytes before and after DC polarization respectively. The time dependence of response of current (I-t curves) of DC polarization at 10 mV for the two symmetry cells is shown in Fig. 3(c). This figure shows that ions are accumulated at the electrode-electrolyte interface when DC polarization voltage is applied across the cells and form a passive layer. It is noted that the time dependence of the polarization current decreases due to this passive layer and current reached at a steady-state condition due to Liþ ion flow through gel polymer electrolytes the between electrodes. The different measured parameters and apparent lithium ion transference number are summarized in Table 3. The Li ion transference number of the gel electrolytes is determined using to Bruce-Vincent-Evans method [42]:

Fig. 3. Nyquist plots for symmetric cells (a) LijjGPE-2jjLi and (b) LijjGPE-3jjLi before and after polarization. (c) Time dependence of current response (I-t curves) of DC polarization at 10 mV at room temperature at 25 C.

142


Table 3 Summary of different measured parameters and apparent Liþ ion transference number obtained from impedance spectroscopy analysis and time dependence current response for Li jj GPE-2, 3 jj Li symmetric cells at 25 C. Gels

І0 (mA)

Іss (mA)

R0 (Ohm)

Rss (Ohm)

DV (V)

tLiþ

GPE-2 GPE-3

2.85 4.18

1.04 2.15

2304 1516

4003 2332

0.01 0.01

~0.22 ~0.38

tLiþ ¼

½Iss ðDV R0 I0 Þ ; ½I0 ðDV Rss Iss Þ

(5)

where I0 and Iss are the initial and steady-state current respectively, DV is the polarization voltage, R0 and Rss are the electrodeelectrolyte interfacial resistances measured before and after the DC polarization respectively. The apparent values of Li-ion transference number obtained from Eq. (5) for GPE-2 and GPE-3 electrolytes are ~ 0.22 and ~0.38 at 25 C respectively. Therefore, the Liþ ions carry out about 22.0% and 38.0% conductivity in GPE-2 and GPE-3 respectively. The value of Liþ ion number increases for GPE-3 gel polymer electrolyte due to plasticizer mixture (EC/PC). For GPE2 (without plasticizer) some Liþ ions are surrounded by anions and form effectively large ionic species which trap the Liþ ions, resulting in a low value of transference number. On the other hand, the plasticizer mixture having high dielectric constant completely dissociates the large ionic atmosphere into Liþ cations and anions.

3.2. Electrochemical performance of supercapacitor cells (up to 340 C), high

ionic conductivity The high thermal stability at 30 C (>103 S cm1) and good electrochemical voltage stability

(~4.0 V) are important properties of the present gel polymer electrolytes for fabrication of efficient electrochemical storage devices such as all solid state supercapacitor, lithium ion battery, etc. [43]. These properties directly impact the specific capacity, power density and energy density of the devices. Thus, we have proceeded to fabricate the solid state supercapacitors and evaluated their electrochemical performance.

3.2.1. Electrochemical impedance spectroscopy of supercapacitors We have first explored the bulk and interfacial resistance of the fabricated supercapacitor cells by using electrochemical impedance spectroscopy (EIS). The Nyquist plots i. e. Z0 (real part) versus - Z00 (imaginary part) plots of the supercapacitor cells obtained at room temperature using an ac signal of 1 V are shown in Fig. 4(a) in the frequency window 10 mHz - 1 MHz. It is observed from this figure that the plots for all cells rise steeply at low frequencies indicating capacitive nature of the interface between the activated carbon electrode and the gel polymer electrolyte. In the high frequency region, a small semicircular arc is observed in the impedance spectra shown in inset of Fig. 4(a). The value of the bulk resistance (Rb) (i.e. high frequency intercept on Z0 axis), charge transfer resistance (Rct) (i.e. diameter of the small semicircle) and total resistance (Rt) at frequency 10 mHz are listed in Table 4. The overall EIS capacitance ðCoverall Þ of the cells is calculated using the following relation

1 EIS Coverall ¼ 00 ; 2pf Z

(6)

where f is the frequency in Hz and Z00 is the imaginary part of the complex impedance in Ohm. The single electrode specific capaciEIS (F g1) were calculated by multiplying the overall tance Csp

Fig. 4. (a) Complex impedance plots of different supercapacitor cells at room temperature. The inset shows the enlarge view of the complex impedance plots. (b) Frequency dependence of specific capacitance for different cells obtained from impedance spectra analysis. (c) and (d) Frequency dependence of Z0 and -Z00 of supercapacitor Cell-1 and Cell-3 respectively.


143

Table 4 EIS ) at 10 mHz, response time (t ), available energy density at The bulk resistance (Rb), charge transfer resistance (Rct), total resistance (Rt) at 10 mHz, specific capacitance (Csp 0 response time and pulse power of all three solid state supercapacitor cells. Cells

Rb (U cm2)

Rct (U cm2)

Rt (U cm2)

EIS (F g1) Csp

t0 (s)

Available energy density (Wh kg1)

Pulse power (kW kg1)

Cell-1 Cell-2 Cell-3

19 11 6

14 8 4

203 93 80

92 122 134

21 12 8

9.33 15.49 19.73

1.60 4.65 8.88

capacitance by a factor of 2 and divided by mass of the active electrode material in g. Fig. 4(b) show the frequency dependence of the calculated specific capacitance for all supercapacitor cells. The values of specific capacitance at 10 mHz are listed in Table 4. It is noted that the value of specific capacitance increases with addition of salt and plasticizers mixture. We have studied performance characteristics of super-capacitor cells using complex impedance analysis. Fig. 4(c) and (d) show the frequency dependence of Z0 and Z00 plots for Cell-1 and Cell-3. It is noted that the real part Z0 (u) and imaginary part Z00 (u) of the complex impedance Z*(u) spectra cross to each other at a particular frequency which is the reciprocal of the response time (t0) of the cells [44,45]. At the crossing point the capacitance and resistance of the cells are equal and their phase difference is 45 [45]. In these figures response time t0 is denoted by blue symbols. The available energy (E0) has been calculated using the relation E0 ¼ 1/2CV2,

where C is overall capacitance at response time t0 and V is rate voltage of the cells. The pulse power which is the available energy density (E0/mass of the active material) divided by the response time t0 has been also calculated [45]. The values of the response time, available energy density at response time and pulse power for different cells are shown in Table 4, which indicates that Cell-3 exhibits the fastest response and also delivers best pulse power of all other cells. 3.2.2. Cyclic voltammetry of supercapacitors The electrochemical behavior of the supercapacitor cells has been evaluated by cyclic voltammetry (CV). Fig. 5(a), (b) and (c) show the CV profiles of Cell-1, Cell-2 and Cell-3 respectively at different scan rates of 10, 30, 50, 80, 100, 150 and 200 mV s1 in the voltage range from 1.0 V to þ1.0 V at room temperature. It is observed in these figures that the CV curves for the all cells show

Fig. 5. Cyclic Voltammetry (CV) of different supercapacitor cells: (a) Cell-1, (b) Cell-2 and (c) Cell-3 at different scan rates. (d) Comparison of Cyclic Voltammetry for different cells at a scan rate of 50 mV s1.

144


rates. Thus, specific capacitance decreases with the increase of cv for Cell-1, speed of the scan rates [47]. For example, the values of Csp Cell-2 and Cell-3 are 55.3, 81.4 and 123.4 Fg-1 respectively at a common scan rate of 10 mVs-1, indicating that the specific capacitance increases after addition of lithium salt LiTFSI and plasticizer mixture. The addition of salt into the GPE-1 electrolyte increases the specific capacitance due to enhancement of electrostatic interaction between the activated carbon electrode and charge carriers of gel electrolytes which increases the double layer capacitance. After addition of plasticizers mixture, the gel polymer electrolyte shows more flexible and high ionic conductivity. The flexibility of the gel polymer electrolyte GPE-3 provided a very good contact between the electrode-gel electrolyte for which rapid charge carrier adsorption takes place at electrode-polymer electrolytes interface [8,48]. Fig. 6. Specific capacitance obtained from CV at different scan rates such as 10, 30, 50, 80, 100, 150 and 200 mV s1.

almost box - like in shape and correspond to an exceptionally ideal and reversible capacitive characteristic of a double layer at the carbon electrode-polymer electrolyte interface [45]. Fig. 5(d) shows the comparison of the CV of the supercapacitor cells at 50 mV s1 cv of the supercapacitors has scan rate. The specific capacitance Csp been calculated using the following equation [46].

Z idV cv Csp ¼

2 m S DV

;

(7)

where !idV is the integrated area of the CV curve, m is the single electrode mass of active material (activated carbon) in g, S is the scan rate and DV is cell voltage range. Fig. 6 shows the variation of cv with scan rate for different cells. It is observed that the value of Csp cv decreases with increasing scan rate. Actually, the ions have Csp enough time to pass through the pores of activated carbon materials at low speed scan rates, while the ions accumulate on the external surface of the carbon materials at the high speed scan

3.2.3. Galvanostatic charge-discharge (GCD) characteristics of supercapacitors Fig. 7(a), (b) and (c) represent the galvanostatic chargedischarge (GCD) characteristics recorded from 0 to 2.0 V at room temperature for all supercapacitors Cell-1, Cell-2 and Cell-3 respectively at different constant currents of 1, 1.5, 2, 3 and 4 mA cm2. For comparison the charge-discharge characteristic for different cells is shown in Fig. 7(d) at a current rate of 1.5 mA cm2. It is found that all supercapacitor cells show nearly linear discharge characteristics with small Ohmic drop for all discharge currents, indicating non-Faradic capacitive behavior of the cells. It is also observed in each discharge curve that a sudden voltage drop occurs due to an Ohmic internal resistance, referred to as equivalent series resistance (ESR) of the cell. The value of ESR has been calculated from the following relation

ESR ¼

DVIR ; 2i

(8)

where DVIR is internal Ohmic voltage drop and i is the applied discharge current. The values of ESR at 1.5 mAcm2 discharge current are listed in Table 5 for different cells. It is found that the cells

Fig. 7. Galvanostatic charge-discharge (GCD) of supercapacitors: (a) Cell-1, (b) Cell-2 and (c) Cell-3 at different constant currents. (d) Comparison of galvanostatic charge-discharge (GCD) of different cells at a constant current of 1.5 mA cm-2 at room temperature.


145

Table 5 dis , energy density and power density at a constant current 1.5 mA cm2 obtained from charge-discharge characteristics. Value of ESR, specific capacitance Csp Cells

ESR (U cm2)

dis (F g1) Csp

Energy density (Wh kg1)

Power density (kW kg1)

Cell-1 Cell-2 Cell-3

12 10 5

101 134 169

13.47 18.08 23.07

0.491 0.528 0.533

show small internal resistance drop, which gradually decreases in the order Cell -1, Cell -2 and Cell -3. Cell-1 and Cell-3 show the highest and lowest ESR values respectively. The overall capacitance dis Þ have been calculated from the discharge curves of the cells ðCcell using the relation

dis Fg 1 ¼ Ccell

i

; mtam DDVt

(9)

where i is the discharge current in Ampere, mtam is the total active mass (mass of activated carbon in both electrodes) in supercapacitor in g and DV/Dt is slope of the discharge characteristic after internal resistance drop. In symmetrical cell system, the dis is related to specific capacitance referred to a single electrode Csp dis overall capacitance of the cells (Ccell ) by the following relation [27].

dis dis Fg 1 ¼ 4 Ccell Csp

(10)

dis with discharge current for Fig. 8(a) shows the variation of Csp dis at 1.5 mA cm2 discharge the different cells. The values of Csp

current are listed in Table 5 for different cells. It is observed that the dis increase due to addition of lithium salt and plastivalues of Csp cizers mixture. These results are consistence with those obtained from electrochemical impedance measurements discussed earlier in section 3.2.2. It is also found that specific capacitance obtained from charge-discharge characteristics is slight higher than that obtained from CV patterns due to different time taken for a CV response and a charge-discharge cycle [8]. dis of all supercapacitor cells as a function of Fig. 8(b) shows Csp charge-discharge cycles (upto 4000 cycles) at a current of 1.5 mAcm2 to test the rechargeability of the supercapacitor cells [49]. It is observed in Fig. 8(b) that the values of specific capacitances of all cells decrease with increasing number of chargedischarge cycles due to accumulation of ions at the electrodepolymer electrolyte interface coupled with irreversible oxidation/ reduction of insecurely bound OH (most likely) surface groups on the porous activated carbon electrodes [23]. After 1200 cycles, specific capacitance values become saturated at 49, 57 and 56% of the initial value for Cell-1, Cell-2 and Cell-3 respectively. It is also observed from chargeedischarge profiles that both charging and

dis obtained from GCD with discharge current for all supercapacitor cells. (b) The cycling performance of all cells at a constant chargeFig. 8. (a) Variation of specific capacitances Csp discharge current of 1.5 mA cm2. (c) Energy density versus power density plots for all supercapacitor cells. (d) Digital photograph demonstrating that series connection of the five supercapacitor cells was able to light up five green LEDs connected in parallel. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

146


Table 6 Comparison of performance of different gel polymer electrolytes with activated carbon (AC) electrode. Gel polymer electrolytes (GPEs)

sdc (mS/cm)

ESWc (V)

dis (F/g) Csp

Voltage (V)

E (Wh/kg)

Pf (kW/kg)

Ref.

PILTFSI/PYR14TFSI PILTFSI/PYR14FSI PTFE/[EMIM][Tf2N] PVdF-HFP/[EMIM][Tf2N] PVdF-HFP/EMITf-LiTf PVdF-HFP/IL/SN(1:2:2) PVA-CH3COONH4-BmImI PVdF-HFP/SN/BMPTFSI PVdF-HFP/SN/EMITf PVdF-HFP/IL-LiTFSI-EC/PC

0.41a 2.1a 1.1b 1.5b 4.5b 6.4a 9.63a 3.5b 8.7b 14.8b

3.7 3.5 4.2 4.1 4.1 5.4 3.8 6.4 5.2 4.3

110d 150d e e 108d 176e 50.25d 234d 253d 210d

3.5 3.5 3.0 3.0 2.0 2.5 1.0 2.0 2.0 2.0

35 36 17 15 15 ~39 6.92 33 35 28.75

0.25 0.23 0.15 0.15 0.238 ~18.5 50.25 7.0 12 0.331

[27] [27] [50] [50] [23] [51] [48] [52] [52] Current work

a b c d e f

Ionic conductivity (sdc) at 25 C. Ionic conductivity (sdc) at room temperature. ESW- electrochemical stability window. Specific capacitance at 1 mA/cm2 discharge current. Specific capacitance at 0.18 A/g discharge current. Projected according to P ¼ E/t at lowest discharge current.

discharging times are almost equal for all supercapacitor cells. The Coulombic Efficiency is calculated using following relation

h% ¼

td 100%; tc

(11)

where td and tc are discharging and charging times respectively obtained from the charge-discharge curve. It has been noted that all supercapacitor cells exhibit 100% Coulombic efficiency throughout 4000 cycles. The energy density (E) and power density (P) of the supercapacitor cells have been calculated from charge-discharge characteristics at different current rates using following relations

E¼

dis ðDVÞ2 Ccell 1000 3600 2

(12)

E 3600 ; Dt

(13)

and

P¼

dis is the overall capacitance of the cell, DV is the cell where Ccell voltage and Dt is the discharge time. Fig. 8(c) shows the power density versus energy density plots (Ragone plot) for the supercapacitor cells. It is noted in Fig. 8(c) that the addition of salt and plasticizers enhances performance of the supercapacitor cells and saves energy at high power density. The values of energy and power densities at a current 1.5 mA cm2 for different cells are listed in Table 5. The power density increases due to high ionic conductivity of the gel electrolytes and good contact between electrode and electrolytes which improves the electrostatic interaction between ions of polymer electrolyte and carbon electrode. To demonstrate the application of the solid state supercapacitor cells, five supercapacitor cells (Cell-3) were connected in series. It is observed in Fig. 8(d) that series connection of the five supercapacitor cells was able to light up five green LEDs connected in parallel. This device (series connected five cells) provided 5 V and after charging for 15 s, the five green LEDs were lighted up very well for 1 min. This device also lighted up the five yellow LEDs for about 1.5 min and five red LEDs for 2.2 min. This demonstration indicates high power and energy density of the cell-3. Finally, Table 6 shows a comparison of the best performance of the solid state supercapacitor (Cell-3) in this present work with those of other reported works in which activated carbon (AC) was used as a active electrode material. It is noted in Table 6 that the supercapacitor fabricated with GPE-3 gel electrolyte shows the best

electrochemical performance in terms of specific capacitance and energy density. The solid state supercapacitor containing GPE-3 electrolyte exhibits good capacitance and energy density which are comparable with those reported in the literature due to its high ionic conductivity and flexibility which provided good contact between electrode and electrolyte. 3.3. Performance of lithium ion coin cell Encouraged by the above very good physicochemical properties and electrochemical performance of the supercapacitor cells fabricated using the present gel polymer electrolytes, we have also fabricated a lithium ion full coin cell (type CR2032) using gel polymer electrolyte GPE-3 with the configuration graphite//GPE3//LiFePO4 and investigated the performance of the coin cell at room temperature (25 C). Fig. 9(a) shows the charge-discharge performance of the coin cell at different current rates in the voltage range of 1.8 Ve3.6 V. The theoretical capacity of the cathode for the cell was determined as C (mAh) ¼ specific theoretical capacity of LiFePO4 (170 mAh g1) weight of the active material (g) [53]. It is observed in Fig. 9(a) that the cell exhibits plateau voltages of 2.75 V and 2.52 V respectively for the 1st charging and discharging cycles at constant current rate C/12. The plateau voltage is nearly 2.80 V for charging and 2.4 V for discharging of 1st cycle at current rate 1C. It is also observed that the DV (plateau voltage for charging - plateau voltage for discharge) increases with the increase in charge-discharge current rate. For example, the values of DV for C/12 and 1C are 0.22 V and 0.40 V respectively. The specific charge capacity of the coin cell is 167.07, 149.12, 110.07, 81.2, 62.9 and 47.2 mAh g1 and specific discharge capacity is 165.8, 143.44, 108.9, 74.35, 57.53 and 45.37 mAh g1 at current rates C/12, C/10, C/ 8, C/5, C/3 and 1C respectively. This charge-discharge capacity of the coin cell decreases with increasing current rate due to slow diffusion of Liþ ions within the electrodes and formation of potential barrier at electrode-electrolytes interface [54]. Finally, we have recorded at room temperature the cycling performance of the coin cell up to 20th cycle at low current rate C/12 in the voltage range 1.8 Ve3.6 V. The 1st and 20th charge and discharge cycles at constant current rate C/12 are shown in Fig. 9(b), while the specific charge/discharge capacity of the coin cell up to 20 cycles is also shown in Fig. 9(c). It is noted that the coin cell delivers specific discharge capacity of 165.8 mAh g1 and 124 mAh g1 for 1st and 20th cycles respectively with respect to the active LiFePO4 cathode material [55]. The discharge capacity remains 72.9% of the theoretical capacity of the cathode after 20th cycle. This high capacity retention and cycling stability of the coin cell arises from the high


147

Fig. 9. (a) Rate capability of lithium ion coin cell (CR2032) at room temperature. (b) 1st and 20th charge-discharge curves of the coin cell at C/12 current rate at room temperature. (c) Cycling performance of specific capacitance and Columbic efficiency for the coin cell at C/12.

ionic conductivity and good contacts between gel electrolyte and electrodes. However, there are several issues for the decay of specific capacity and cyclic durability. The interfacial stability and compatibility of the electrolytes with electrodes are the most serious barriers for using these electrolytes in devices, leading to poor cycle-ability of the lithium ion cell despite high ionic conductivity of the gel electrolytes at ambient temperature [56e58]. It is also observed in Fig. 9(c) that capacity for charging and discharging is almost equal, providing nearly 99% Coulomb efficiency of the present lithium ion cell.

cells have good electrochemical performance at ambient temperature in the aspect of large potential window, small internal resistance, high specific capacitance and long cycle stability. The specific capacity of the prepared lithium ion coin cell for 1st cycle is 165.8 mAh g1 at a constant current rate C/12 at ambient temperature. The discharge capacity remains 72.9% of the theoretical capacity of the cathode material after 20th cycle. The results suggest that these gel polymer electrolytes have excellent potential to be used in different types of electrochemical devices. Acknowledgements

4. Conclusions The gel polymer electrolytes are prepared using copolymer P(VdF-HFP), ionic liquid PMIMTFSI, LiTFSI salt and plasticizer mixture EC:PC (1:1 v/v). The gel polymer electrolytes show high thermal stability up to 340 C, high ionic conductivity and electrochemical potential window of ~4 V, confirming their suitability in electrochemical devices. The ionic conductivity obtained for GPE-1, GPE-2 and GPE-3 electrolytes are about 6.09 103, 9.5 103 and 1.48 102 S cm-1 at 30 C respectively. The shape of CV curves of all fabricated supercapacitor cells at room temperature is almost box-like corresponding to exceptionally ideal and reversible capacitive characteristics of the cells. The supercapacitor

P. Pal acknowledges DST INSPIRE (IF 131179) program for providing him research fellowship. A. Ghosh acknowledges financial support from J. C. Bose Fellowship (Grant No. SB/S2/JCB-33/ 2014) of DST, Government of India. References [1] C. Fasciani, S. Panero, J. Hassoun, B. Scrosati, Novel configuration of poly(vinylidenedifluoride)-based gel polymer electrolyte for application in lithium-ion batteries, J. Power Sources 294 (2015) 180e186. [2] G.P. Pandey, S.A. Hashmi, Ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate-based gel polymer electrolyte for electrochemical capacitors, J. Mater. Chem. A 1 (2013) 3372e3378. guin, Carbon materials for the electrochemical storage of [3] E. Frackowiak, F. Be

148


energy in capacitors, Carbon 39 (2001) 937e950. [4] M. Que, Y. Tong, G. Wei, K. Yuan, J. Wei, Y. Jiang, H. Zhu, Y. Chen, Safe and flexible ion gel based composite electrolyte for lithium batteries, J. Mater. Chem. A 4 (2016) 14132e14140. guin, V. Presser, A. Balducci, E. Frackowiak, Carbons and electrolytes for [5] F. Be advanced supercapacitors, Adv. Mater. 26 (2014) 2219e2251. [6] V.L. Martins, A.J.R. Rennie, R.M. Torresi, P.J. Hall, Ionic liquids containing tricyanomethanide anions: physicochemical characterisation and performance as electrochemical double-layer capacitor electrolytes, Phys. Chem. Chem. Phys. 19 (2017) 16867e16874. [7] H. y. Sung, Y.y. Wang, C.c. Wan, Preparation and characterization of poly(vinyl chloride-co-vinyl acetate)-based gel electrolytes for Li-Ion batteries, J. Electrochem. Soc. 145 (1998) 1207e1211. [8] G.P. Pandey, S.A. Hashmi, Y. Kumar, Performance studies of activated charcoal based electrical double layer capacitors with ionic liquid gel polymer electrolytes, Energy Fuels 24 (2010) 6644e6652. [9] M. Egashira, H. Todo, N. Yoshimoto, M. Morita, Lithium ion conduction in ionic liquid-based gel polymer electrolyte, J. Power Sources 178 (2008) 729e735. [10] P.R. Chinnam, V. Chatare, S. Chereddy, R. Mantravadi, M. Gau, J. Schwab, S.L. Wunder, Multi-ionic lithium salts increase lithium ion transference numbers in ionic liquid gel separators, J. Mater. Chem. A 4 (2016) 14380e14391. [11] P. Pal, A. Ghosh, Influence of TiO2 nano-particles on charge Carrier transport and cell performance of PMMA-LiClO4 based nano-composite electrolytes, Electrochim. Acta 260 (2018) 157e167. [12] J. Sharma, S.A. Hashmi, Magnesium ion transport in poly(ethylene oxide)based polymer electrolyte containing plastic-crystalline succinonitrile, J. Solid State Electrochem. 17 (2013) 2283e2291. [13] D. Kumar, S.A. Hashmi, Ion transport and ionefiller-polymer interaction in poly(methyl methacrylate)-based, sodium ion conducting, gel polymer electrolytes dispersed with silica nanoparticles, J. Power Sources 195 (2010) 5101e5108. [14] S. Mitra, A.K. Shukla, S. Sampath, Electrochemical capacitors with plasticized gel-polymer electrolytes, J. Power Sources 101 (2001) 213e218. [15] C.-C. Yang, S.-T. Hsu, W.-C. Chien, All solid-state electric double-layer capacitors based on alkaline polyvinyl alcohol polymer electrolytes, J. Power Sources 152 (2005) 303e310. [16] X. Yang, F. Zhang, L. Zhang, T. Zhang, Y. Huang, Y. Chen, A high-performance graphene oxide-doped ion gel as gel polymer electrolyte for all-solid-state supercapacitor applications, Adv. Funct. Mater. 23 (2013) 3353e3360. [17] P.F.R. Ortega, J.P.C. Trigueiro, G.G. Silva, R.L. Lavall, Improving supercapacitor capacitance by using a novel gel nanocomposite polymer electrolyte based on nanostructured SiO2, PVDF and imidazolium ionic liquid, Electrochim. Acta 188 (2016) 809e817. [18] V.K. Thakur, G. Ding, J. Ma, P.S. Lee, X. Lu, Hybrid materials and polymer electrolytes for electrochromic device applications, Adv. Mater. 24 (2012) 4071e4096. [19] G.B. Appetecchi, P. Romagnoli, B. Scrosati, Composite gel membranes: a new class of improved polymer electrolytes for lithium batteries, Electrochem. Commun. 3 (2001) 281e284. [20] Sellam, S.A. Hashmi, High rate performance of flexible pseudocapacitors fabricated using ionic-liquid-based proton conducting polymer electrolyte with Poly(3, 4-ethylenedioxythiophene):Poly(styrene sulfonate) and its hydrous ruthenium oxide composite electrodes, ACS Appl. Mater. Interfaces 5 (2013) 3875e3883. [21] A. Manuel Stephan, Review on gel polymer electrolytes for lithium batteries, Eur. Polym. J. 42 (2006) 21e42. [22] B.G. Choi, J. Hong, W.H. Hong, P.T. Hammond, H. Park, Facilitated ion transport in all-solid-state flexible supercapacitors, ACS Nano 5 (2011) 7205e7213. [23] Y. Kumar, G.P. Pandey, S.A. Hashmi, Gel polymer electrolyte based electrical double layer capacitors: comparative study with multiwalled carbon nanotubes and activated carbon electrodes, J. Phys. Chem. C 116 (2012) 26118e26127. [24] C. Sirisopanaporn, A. Fernicola, B. Scrosati, New, ionic liquid-based membranes for lithium battery application, J. Power Sources 186 (2009) 490e495. ski, A. Lewandowski, I. Ste˛ pniak, Ionic liquids as electrolytes, Elec[25] M. Galin trochim. Acta 51 (2006) 5567e5580. [26] H. Ohno, M. Yoshizawa, W. Ogihara, A new type of polymer gel electrolyte: zwitterionic liquid/polar polymer mixture, Electrochim. Acta 48 (2003) 2079e2083. ~ oz-Torrero, J. Palma, M. Anderson, R. Marcilla, Performance [27] G.A. Tiruye, D. Mun of solid state supercapacitors based on polymer electrolytes containing different ionic liquids, J. Power Sources 326 (2016) 560e568. [28] J.-K. Kim, Hybrid gel polymer electrolyte for high-safety lithium-sulfur batteries, Mater. Lett. 187 (2017) 40e43. [29] K. Ekramul, M. Khatun, L. Nasrin, J.R. Mustafa, M. Rahman, Pure b -phase formation in polyvinylidene fluoride (PVDF)-carbon nanotube composites, J. Phys. D Appl. Phys. 50 (2017), 163002. [30] K. Karuppasamy, P.A. Reddy, G. Srinivas, A. Tewari, R. Sharma, X.S. Shajan, D. Gupta, Electrochemical and cycling performances of novel nonafluorobutanesulfonate (nonaflate) ionic liquid based ternary gel polymer electrolyte membranes for rechargeable lithium ion batteries, J. Membr. Sci. 514 (2016) 350e357.

[31] Z. He, Q. Cao, B. Jing, X. Wang, Y. Deng, Gel electrolytes based on poly(vinylidenefluoride-co-hexafluoropropylene)/thermoplastic polyurethane/poly(methyl methacrylate) with in situ SiO2 for polymer lithium batteries, RSC Adv. 7 (2017) 3240e3248. [32] A. Serghei, M. Tress, J.R. Sangoro, F. Kremer, Electrode polarization and charge transport at solid interfaces, Phys. Rev. B 80 (2009), 184301. [33] B.K. Money, K. Hariharan, J. Swenson, Glass transition and relaxation processes of nanocomposite polymer electrolytes, J. Phys. Chem. B 116 (2012) 7762e7770. [34] D.P. Almond, A.R. West, Anomalous conductivity prefactors in fast ion conductors, Nature 306 (1983) 456e457. , Debye length dependence of the [35] R.J. Kortschot, A.P. Philipse, B.H. Erne anomalous dynamics of ionic double layers in a parallel plate capacitor, J. Phys. Chem. C 118 (2014) 11584e11592. [36] J.R. Macdonald, Electrical response of materials containing space charge with discharge at the electrodes, J. Chem. Phys. 54 (1971) 2026e2050. [37] J.-K. Kim, A. Matic, J.-H. Ahn, P. Jacobsson, An imidazolium based ionic liquid electrolyte for lithium batteries, J. Power Sources 195 (2010) 7639e7643. [38] A. Fernicola, F.C. Weise, S.G. Greenbaum, J. Kagimoto, B. Scrosati, A. Soleto, Lithium-ion-Conducting electrolytes: from an ionic liquid to the polymer membrane, J. Electrochem. Soc. 156 (2009) A514eA520. [39] H. Vogel, Temperature dependence of viscosity of melts, Z. Phys. 22 (1921) 645e646. €ngigkeit der Viscosita €t von der Temperatur [40] G. Tammann, W. Hesse, Die Abha bie unterkühlten Flüssigkeiten, Z. Anorg. Allg. Chem. 156 (1926) 245e257. [41] G.S. Fulcher, Analysis of recent measurements of the viscosity of glasses, J. Am. Ceram. Soc. 8 (1925) 339e355. [42] J. Evans, C.A. Vincent, P.G. Bruce, Electrochemical measurement of transference numbers in polymer electrolytes, Polymer 28 (1987) 2324e2328. [43] G.P. Pandey, S.A. Hashmi, Solid-state supercapacitors with ionic liquid based gel polymer electrolyte: effect of lithium salt addition, J. Power Sources 243 (2013) 211e218. [44] J.R. Miller (Ed.), Proceedings of the Eighth International Seminar on Double Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, Florida Educational Seminars, Boca Raton, FL, 1998. Dec 7 - 9, 1998. [45] F. Lufrano, P. Staiti, M. Minutoli, Evaluation of nafion based double layer capacitors by electrochemical impedance spectroscopy, J. Power Sources 124 (2003) 314e320. [46] A. Yu, I. Roes, A. Davies, Z. Chen, Ultrathin, transparent, and flexible graphene films for supercapacitor application, Appl. Phys. Lett. 96 (2010), 253105. [47] M. Nasibi, M.A. Golozar, G. Rashed, Nano zirconium oxide/carbon black as a new electrode material for electrochemical double layer capacitors, J. Power Sources 206 (2012) 108e110. [48] C.-W. Liew, S. Ramesh, A.K. Arof, Enhanced capacitance of EDLCs (electrical double layer capacitors) based on ionic liquid-added polymer electrolytes, Energy 109 (2016) 546e556. €tz, M. Carlen, Principles and applications of electrochemical capacitors, [49] R. Ko Electrochim. Acta 45 (2000) 2483e2498. [50] W. Lu, K. Henry, C. Turchi, J. Pellegrino, Incorporating ionic liquid electrolytes into polymer gels for solid-state ultracapacitors, J. Electrochem. Soc. 155 (2008) A361eA367. [51] G.P. Pandey, T. Liu, C. Hancock, Y. Li, X.S. Sun, J. Li, Thermostable gel polymer electrolyte based on succinonitrile and ionic liquid for high-performance solid-state supercapacitors, J. Power Sources 328 (2016) 510e519. [52] M. Suleman, Y. Kumar, S.A. Hashmi, Flexible electric double-layer capacitors fabricated with micro-/mesoporous carbon electrodes and plastic crystal incorporated gel polymer electrolytes containing room temperature ionic liquids, J. Solid State Chem. 19 (2015) 1347e1357. [53] S. Ferrari, E. Quartarone, P. Mustarelli, A. Magistris, M. Fagnoni, S. Protti, C. Gerbaldi, A. Spinella, Lithium ion conducting PVdF-HFP composite gel electrolytes based on N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide ionic liquid, J. Power Sources 195 (2010) 559e566. [54] W. Zhai, Y.-w. Zhang, L. Wang, F. Cai, X.-m. Liu, Y.-j. Shi, H. Yang, Study of nano-TiO2 composite polymer electrolyte incorporating ionic liquid PP12O1TFSI for lithium battery, Solid State Ionics 286 (2016) 111e116. [55] Y.-S. Lee, W.-K. Shin, J.S. Kim, D.-W. Kim, High performance composite polymer electrolytes for lithium-ion polymer cells composed of a graphite negative electrode and LiFePO4 positive electrode, RSC Adv. 5 (2015) 18359e18366. [56] H.-J. Ha, E.-H. Kil, Y.H. Kwon, J.Y. Kim, C.K. Lee, S.-Y. Lee, UV-curable semiinterpenetrating polymer network-integrated, highly bendable plastic crystal composite electrolytes for shape-conformable all-solid-state lithium ion batteries, Energy Environ. Sci. 5 (2012) 6491e6499. [57] K. Shono, T. Kobayashi, M. Tabuchi, Y. Ohno, H. Miyashiro, Y. Kobayashi, Proposal of simple and novel method of capacity fading analysis using pseudo-reference electrode in lithium ion cells: application to solvent-free lithium ion polymer batteries, J. Power Sources 247 (2014) 1026e1032. [58] R.E. Sousa, M. Kundu, A. Goren, M.M. Silva, L. Liu, C.M. Costa, S. LancerosMendez, Poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) lithium-ion battery separator membranes prepared by phase inversion, RSC Adv. 5 (2015) 90428e90436.