Accepted Manuscript

Title: Cross-linking of polymer and ionic liquid ashigh-performance gel electrolyte for flexible solid-statesupercapacitors

Authors: Xiongwei Zhong, Jun Tang, Lujie Cao, WeiguangKong, Zheng Sun, Hua Cheng, Zhouguang Lu, Hui Pan,Baomin Xu

PII: S0013-4686(17)31106-4DOI: http://dx.doi.org/doi:10.1016/j.electacta.2017.05.110Reference: EA 29545

To appear in: Electrochimica Acta

Received date: 10-3-2017Revised date: 16-5-2017Accepted date: 17-5-2017

Please cite this article as:XiongweiZhong, JunTang, LujieCao,WeiguangKong,ZhengSun, Hua Cheng, Zhouguang Lu, Hui Pan, Baomin Xu, Cross-linking of polymer andionic liquid as high-performance gel electrolyte for flexible solid-state supercapacitors,Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.05.110

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1

Cross-linking of polymer and ionic liquid as high-performance

gel electrolyte for flexible solid-state supercapacitors

Xiongwei Zhonga,b, Jun Tanga, Lujie Caoa,b, Weiguang Konga, Zheng Suna, Hua

Chenga, Zhouguang Lua, Hui Pan*b, Baomin Xu*a

a Department of Materials Science and Engineering, Southern University of Science and

Technology of China, Shenzhen, Guangdong Province 518055, China

b Institute of Applied Physics and Materials Engineering, University of Macau, Macao

* Corresponding authors. Tel.: +86 755 88018980;

E-mail address: xubm@sustc.edu.cn (Baomin Xu); huipan@umac.mo (Hui Pan)

Highlights

A facile method to prepare gel polymer electrolyte with high conductivity by

ultraviolet triggering and cross-linking between ionic liquid and poly (ethylene

oxide) is proposed.

A flexible symmetric capacitor based on the prepared GPE shows ultra-

flexibility.

The capacitor with high voltage can power up a 3.0V LED even bended to a

angle of 180o.

2

Abstract:

It is highly desirable to develop flexible solid-state electrochemical double-layer

capacitors (EDLCs) with non-liquid electrolyte. However, it is still a great challenge to

prepare gel polymer electrolyte (GPE) possessing high ionic conductivity and good

mechanical property. In this work, a simple and novel method to improve the

conductivity and mechanical properties of GPE film for their applications as electrolyte

and separator in EDLC is presented. The GPE film is prepared by cross-linking ionic

liquid (IL) with poly (ethylene oxide) (PEO) and benzophenone (Bp) followed by

ultraviolet (UV) irradiation. Then, a non-woven cellulose separator (FPC) is used to

absorb the GPE. By tuning the mass ratio (n) between IL and PEO, the flexible EDLC

cooperated with low-cost active carbon and the electrolyte film with n=10 has a high

capacitance of 70.84F·g-1, a wide and stable electrochemical window of 3.5V, an

energy density of 30.13Wh∙kg-1 and a power density of 874.8W∙kg-1 at a current density

of 1A∙g-1, which can drive a 3.0V light-emitting diode (LED). Importantly, the excellent

performance of the flexible and low-cost EDLC can be maintained at a bending angle

up to 180o, indicating the ultra-flexibility. It is expected that the IL-PEO-FPC

electrolyte film is a promising candidate of GPE for flexible devices and energy storage

systems.

3

Keywords: Electric double-layer capacitor; gel polymer electrolyte; flexible; ionic

liquid

1 Introduction

The flexible and wearable electrochemical storage device is one of the hottest

research topics in wearable electronics and related multidisciplinary fields, such as

flexible displays, implantable medical devices and flexible solar cells [1-9]. Among

various wearable electrochemical devices, electric double-layer capacitors (EDLCs)

has been considered as one of the most potential candidates due to its long cycle

lifetime, high power density, excellent reliability and environmentally friendly [10-21].

Recently, it has been attracted a majority of scientists to develop non-liquid electrolytes

which with high ionic conductivity, excellent flexibility, wide electrochemical window,

physicochemical stability and good mechanical integrity for flexible and wearable

supercapacitors [9, 22, 23].

Among the various non-liquid electrolytes, including solid [24-26] and gel

electrolyte [27, 28], the gel polymer electrolyte (GPE) is considered as one of the ideal

candidates for flexible devices because it exhibits high conductivity and prevents the

leakage from the solution [29, 30]. The fast growth of flexible energy devices with high

performance triggers increasing demands of GPE [31]. To fabricate GPE, host polymer

matrix materials, such as poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-

4

co-hexafluoropropylene) (P(VDF-HFP)), poly(ethylene oxide) (PEO), and supporting

electrolytes are key components [32]. Normally, the conventional electrolytes in GPE

system, including aqueous and organic electrolytes, have many serious drawbacks. For

example, the aqueous electrolytes (e.g., H2SO4, KOH, and Na2SO4) have a narrow

electrochemical window (~1.2V) [33, 34], leading to narrow working voltage and very

limited energy density. The organic electrolytes (e.g., acetonitrile and propylene

carbonate) suffer from serious health and safety problems. Recently, room temperature

ionic liquid (IL) has been widely pursued as an electrolyte in applications for energy

storage devices [35-37], such as supercapacitors [38], lithium batteries [39] and sodium

batteries [40], etc. Particularly, IL as electrolytes in GPEs has been a hot research topic

because of the excellent thermal stability, high ionic conductivity, non-flammability,

negligible vapour pressure, low melting point and wide electrochemical window [41].

IL composed of dissociated ions with no intervening solvent can be obtained from

molten salts and is a liquid at room temperature [42]. Liu [43] et.al reported that the 1-

ethyl-3-methylimidazolium (EMImCl) gel formed by UV irradiating the homogeneous

solution of (EMImCl), hydroxyethyl methacrylate(HEMA), Chitosan and water. This

gel demonstrates good tensile property but a low electrochemical window (1.0V).

Pandey and his colleagues [44] synthesized polymer electrolyte by incorporating ionic

liquid, PEO, magnesium and lithium salt without UV irradiation. The EDLCs cell with

the polymer electrolytes had a specific capacitance of 1.7-3.0 F/g of multi-walled

carbon nanotube. Lewandowski [45] simply mixed EMImBF4, PEO and sulpholane to

prepare polymer electrolyte. The operating voltage of EDCL was 1.5V. The EDLC with

5

high energy density and operating voltage by incorporated ionic liquid gel polymer

electrolyte (ILGPE) has not been reported yet.

In this work, we propose a novel ILGPE forming by cross-linking of IL and PEO.

Then, a non-woven separator (FPC3018) adsorbs the ILGPEs to form nIL-PEO-FPC (n

is the mass ratio of IL/PEO) electrolyte film for flexible EDLCs. The high ionic

conductivity up to 6.7mS/cm is realised in the as-fabricated 10EMImTFSI-PEO-FPC

film at room temperature. The kind of IL and the ratio of IL/PEO in a GPE are optimised

by the comprehensive evaluation of the electrochemical performances and capacitance

[46]. The EDLC with the optimized GPE shows high performance in electrical energy

storage and high flexibility.

2 Experimental

2.1 Chemicals

Aqueous solutions were prepared by using deionized water with a purification of

18.3MΩ∙cm. PTFE powder and carbon black were purchased from Sigma-Aldrich.

Zeolite (ZSM-5) was purchased from Alfa. Active carbon (YP-80F type, the content of

carbon > 95%) and non-woven cellulose separator (FPC3018) was obtained from

Kuraray Chemical Co. Ltd and SAM industrial Chemical Co. Ltd (Shenzhen, P.R

China), respectively. Poly(ethylene oxide) (PEO, MW = 4,000,000), Benzophenone

(Bp), LiTFSI (> 99%) and NaBF4 (> 99%) were purchased from Energy Chemical. All

reagents were used without any handle.

6

2.2 Preparation of materials

The procedure to prepare 1.3-dimethylimidazolium bis(trifluoromethyl

sulfonyl)imide (DMImTFSI) following these steps. Firstly, the equal molar amount of

iodomethane and 1-methylimidazole were added to a round-bottomed flask fitted with

a reflux condenser for 24h-36h at 10oC with stirring until no more crystal produced [47,

48]. Secondly, the bottom phase, 1.3-dimethylimidazolium iodide (DMImI), was

washed with ethyl acetate and dried at 70oC under vacuum. Thirdly, the DMImI was

transferred to a plastic bottle; then a 1:1 molar ratio of LiTFSI was added, followed by

adding appropriate deionized water. Finally, the colourless bottom phase was washed

with fresh deionized water and dried under a vacuum at 100-120oC for 24-36h [49].

The preparation of 1-ethyl-3-methylimidazolium bis (trifluoromethyl sulfonyl)-

imide (EMImTFSI) and 1.3-diethylimidazolium bis(trifluoromethyl sulfonyl)imide

(DEImTFSI) follow the same process as for DMImTFSI described above, where

iodomethane was replaced by bromoethane and 1-ethylimidazole was instead of 1-

methylimidazole, respectively.

The 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) was synthesised

by following steps. Firstly, the preparation of 1-ethyl-3-methylimidazolium bromide

(EMImBr) followed the same process as DMImI, where 1-ethylimidazole and

bromoethane were instead of 1-methylimidazole and iodomethane, respectively.

Secondly, the EMImBr and NaBF4 (molar ratio=1:1.3) was stirred for 24h at 25-70oC

in acetone solvent. Thirdly, the precipitated bromide salt (NaBr) and excess NaBF4 was

removed by filtering under vacuum. The filtrate was evaporated at 40-50oC under a

7

vacuum to remove any residual acetone. Then, the filtrate was washed with

dichloromethane and dried under uninterrupted vacuum (-0.1MPa) at 100-120oC for

24-36h. The synthesis of 1-propyl-3-methylimidazolium tetrafluoroborate (PrMImBF4)

follows the same procedure as EMImBF4, with 1-bromopropane used instead of 1-

bromoethane. All prepared ionic liquids were confirmed by 1HNMR and 13CNMR and

stored in an argon glovebox before use.

To prepare electrolyte film, firstly, Benzophenone (Bp) was dissolved into IL by

heating under vacuum. Bp was used the initiator for the cross-linking process.

Secondly, PEO was dissolved to the mixture. the Bp/PEO weight ratio was kept equal

to 0.05. The PEO-IL-Bp mixture was annealed under vacuum at 100°C for 2h to obtain

a homogeneous semi-solid gel material. Finally, the polymer gel electrolytes were put

at flat glass dish and cross-linked by UV irradiation (BZS250GF-TC UV photo-

irradiator equipped with a 250 W Hg lamp) for 4 min. The GPE is denoted as nIL-PEO,

where n represents the mass ratio of IL/PEO. The non-woven cellulose separator (FPC)

hold the nIL-PEO inside to prepare nIL-PEO-FPC electrolyte film, which is used as

both of GPE and separator. The GPEs were dried under vacuum at 80oC for 4h and

stored in a glove box before use.

Briefly, 80wt% active carbon as an active material, 10wt% carbon black as

conductive addition and 10wt% PTFE as a binder were uniformly mixed in deionized

water and rolled into ~100μm thickness sheets on aluminium foil (20μm) which works

as a current collector, and then the water was evaporated at 120oC under vacuum for 6-

8h. Finally, the foil punched into Ф12mm discs as electrodes for coin cell and cut into

8

30mm*40mm rectangle as electrodes for the flexible capacitor. All electrodes were

stored in the glovebox.

2.3 Materials Characterization

The scanning electron microscopy (SEM, TESCAN MIR3) was used to analyse

micro-morphology. The ionic liquid structure was confirmed by nuclear magnetic

resonance (NMR, Bruker Avance 400 MHz), where the (CH3)4Si and CDCl3 were used

as an external standard and solvent, respectively. Nitrogen adsorption/desorption was

carried out at 77K on a Micrometric ASAP 2020 apparatus. The surface area was

calculated using the BET method within the relative pressure (P/Po) range of 0.05-0.45.

The total pore volume was determined from the amount of nitrogen adsorbed at P/Po

as close to 1, and the pore size distribution was analysed by using the density functional

theory model with slit pore geometry.

2.4 Electrochemical measurements

The FPC3018 dips in GPE and then hold GPE on its surface and inside as an

electrolyte film. The measurement of ionic conductivity was done by using a cell that

sandwiched the electrolyte film between two stainless steel electrodes, where the

electrolyte film has a size of Ф12mm and a thickness of 0.03mm. The conductivity of

GPEs was measured by electrochemical impedance spectroscopy (EIS) measurements

(CHI660E) at 0V under alternating current with a potential amplitude of 5mV and a

frequency range of 100 kHz to 0.01Hz. The external resistance of the cell is negligible.

The intercept of the measured curve on the real axis (x-axis) is the intrinsic resistance

9

of IL as the ohmic resistance [50]. The conductivity of IL is calculated according to the

equation (1).

σ = L/(R ∙ S) (1)

Where 𝞼 is the ionic conductivity (mS/cm), L is the distance between the two

electrodes (the thickness of electrolyte film) (cm), R is the resistance of ionic liquids

(Ω), and S is the area of the electrolyte film (cm2).

All coin cells (CR2025 coin-type cell) were assembled by a conventional process

with two electrodes and one electrolyte film in a glovebox. The flexible EDLCs were

fabricated by sandwiching one electrolytes film between two electrodes under ~1MPa

pressure for 30min, then sealed with polydimethylsiloxane (PDMS). All

supercapacitors were prepared in the argon glove box (O2 < 0.1 ppm and H2O < 0.1

ppm).

All the electrochemical tests were carried out at room temperature (23±2oC). The

electrochemical measurements were performed by using an electrochemical

workstation (CHI660E, shanghai, P.R. China) in a two-electrode cell system. Cyclic

voltammetry (CV) tests were conducted in the potential window, ranging from 0 to

3.5V. Galvanostatic charge and discharge (GCD) were conducted within the potential

range from 0 to 3.5V at a current density of 1A/g, where the current density is the ratio

of the real current value to the mass of one electrode. EIS spectra were measured within

the frequency range of 100 kHz to 0.01Hz with an alternating potential amplitude of 5

mV.

10

The electrode specific capacitance (Cm, F∙g-1), energy density (E, Wh∙kg-1),

equivalent series resistance (ESR, Ω), power density (P, W∙kg-1) were calculated

according to the following formula (2-3):

C𝑚 = 4I∆t/(M ∙ ∆V)

(2)

E = (Cm∆V2)/8

(3)

Where I (A) is the discharge current, △t (s) is the discharge time, M (g) is the total

weight of two electrodes, ∆V (V) is the actual work voltage. The IR drop (iRdrop, V) is

defined as the electrical potential difference between the two ends of a conducting phase

during the charge-discharge process.

3 Results and Discussion

The GPE can be prepared by irradiating the mixed homogeneous solution under

ultraviolet (UV) light (Figure 1(a)) and the high-resolution image of electrolyte film is

shown in Figure S1(b). The kind of IL and the ratio of IL/PEO in a GPE optimised by

the comprehensive evaluation of the electrochemical performances and capacitance.

To find the suitable IL candidate for GPE film exhibiting the best electrochemical

and practical performance, we have synthesised five kinds of high conductivity ionic

liquid (DMImTFSI, EMImTFSI, DEImTFSI, EMImBF4, PrMImBF4, the molecular

formulas of ionic liquid are shown in Figure S2).

11

Initially, the CV and GCD curves for coin cells fabricated with 5IL-PEO-FPC and

10IL-PEO-FPC electrolyte films and activated carbon are shown in Figure 2. In 5IL-

PEO-FPC and 10IL-PEO-FPC electrolyte film, EMImBF4 and EMImTFSI display

nearly rectangular CV responses (Figure 2(a) and 2(b)), demonstrating that the two

ionic liquids have high electrochemical stability [51, 52]. In a voltage range from 0 V

up to 3.5 V, the curves of EDLC with 10EMImBF4-PEO-FPC and 10EMImTFSI-PEO-

FPC electrolyte film are almost symmetrical, indicating an almost completely

reversible ion adsorption/desorption process and no side-reaction at the surface of the

porous activated carbon. In GCD measurements, the IR drop appears at the beginning

of the discharge due to the internal resistance of devices. The EDLCs with 5IL-PEO-

FPC electrolyte film indicate high IR drop, because of low conductivity and high

internal resistance induced by the high viscosity. The EDCLs with 10IL-PEO-FPC

electrolyte film show low IR drop and high specific capacitances. The IR drop increase

as a sequence of EMImBF4< EMImTFSI< DMImTFSI< DEImTFSI< PrMImBF4, and

the specific capacitance increases as a sequence of PrMImBF4< DEImTFSI <

DMImTFSI < EMImTFSI < EMImBF4 (Table S2).

Moreover, the ionic conductivity of different ILs adsorbed in the 10IL-PEO-FPC

electrolyte films was measured and shown in Table S1. To data, the conductivity of

10IL-PEO-FPC gel electrolyte is higher than PEO/IL/LiTFSI gel electrolyte [53]. The

moderate chain of cation shows high electrochemical performance, such as EMImBF4

and EMImTFSI, the EMImBF4 exhibited high conductivity and capacitance, however,

it is hydrophilic and therefore moisture was easily absorbed leading to narrow

12

electrochemical window. Whereas the EMImTFSI is hydrophobic and relatively stable

in the air in short time, also the 10EMImTFSI-PEO-FPC electrolyte film revealed

nearly 80% retention of ionic conductivity of neat EMImTFSI. Therefore, the

EMImTFSI was found to be the best candidate among the ILs we have obtained for

preparing GPE electrolyte film.

Then, we fabricated coin cell type supercapacitors by using the nEMImTFSI-PEO-

FPC as electrolyte and active carbon (Figure S1(a)) as electrodes to optimise the ratio

of IL/PEO. As a reference, an EDLC with pristine EMImTFSI and a separator is also

fabricated using the same electrode. Figure 3(a) and Figure 3(b) shows that the

capacitance increases with the increment of IL/PEO ratio. The specific capacitance of

EMImTFSI is slightly higher than that of 15EMImTFSI-PEO-FPC electrolyte film.

However, the IR drop of 15EMImTFSI-PEO-FPC electrolyte film increase

dramatically because large internal resistance induced by the low viscosity of

15EMImTFSI-PEO-FPC and little amount of GPE adsorbed inside the separator (Table

S3). The 2EMImTFSI-PEO-FPC electrolyte film shows poor capacitance because of

high internal resistance and low ion diffusion. For the rectangular CV curves and

straight GCD curves of these capacitors consisting of 10EMImTFSI-PEO-FPC exhibit

standard capacitive behaviour of EDLC [54]. These devices reveal low IR drop, which

is slightly higher than that of the device contains neat EMImTFSI. As a result, we have

found that the optimal IL/PEO ratio for GPE was 10:1, which showed the best

comprehensive performance among the GPEs we have obtained.

13

To further demonstrate the flexibility and stability of the current gel polymer

electrolyte, full solid symmetric supercapacitors were assembled by employing the

10EMImTFSI-PEO-FPC as an electrolyte, activated carbon as electrode and the

electrochemical performance was systematically measured under different bending

angles as shown in Figure 4. The CV curves and GCD curves for flexible EDLC

containing the 10EMImTFSI-PEO-FPC electrolyte film with the bending angles of 0o,

50o, 100o, 150o and 180o are shown in Figure 4a and Figure 4b, respectively. The CV

curves are rectangular and the GCD curves are perfectly straight and symmetry at

different angles. The flexible EDCLs with 10EMImTFSI-PEO-FPC electrolyte film at

different angles show similar specific capacitance with the capacitance slightly

decreased in the order of 0o > 50o > 100o > 150o > 180o. The flexible EDCL with

10EMImTFSI-PEO-FPC electrolyte film at different angles also show similar IR drop,

which manifests the similar internal resistance at different angles. The EIS of the EDLC

with 10EMImTFSI-PEO-FPC electrolyte film at different bending angles is shown in

Figure 4c. The intercept on the real axis on the Nyquist plots at high frequency (close

to 100 kHz) is the intrinsic internal resistance of the electrode material, connection

resistance and electrolyte of the device. The similar resistance at high frequency at

different bending angles demonstrates that the bending has no impact on intrinsic

internal resistance. An approximate semi-circular curve from the high to intermediate

frequency region, which is relative to the interface between electrolyte and electrode

material, the flexible EDLC at different bending angles show slightly different, because

of a trifle interface changes between the electrolyte and electrode material, but this is

14

no effect of bending angle on electrochemical performance. The tail is an almost

vertical line at low-frequency region (Warburg impedance), which is related to the

diffusion of the ions into the bulk of electrodes, indicating a typical response of a perfect

performance of supercapacitor with the porous electrode. The CV, GCD and EIS curves

reveal that the EDLC with the 10EMImTFSI-PEO-FPC electrolyte film is completely

flexible. The flexible EDLC shows a specific capacitance of 70.84F∙g-1, an energy

density of 30.13Wh∙kg-1 and a power density of 874.8W∙kg-1 at a current density of

1A∙g-1 at a bending angle of 180o, respectively, which is better than reported flexible

EDLC (0.12Wh/kg [28] and 3Wh/kg [55]). Furthermore, the flexible EDLC was

fabricated with 10EMImTFSI-PEO-FPC electrolyte film, then the EDLCs was bent in

between 180o and 0o for 1500 times. After 100, 500,1000 and 1500 times bending, the

capacitance of this EDLC was measured at 0o. The capacitance after 1500 times

bending can be retained 83% of initial specific capacitance (Figure S7), indicating that

the capacitances are well maintained at a current density of 1A/g and the packing

technique is effective.

The maximum voltage of flexible EDCL with 10EMImTFSI-PEO-FPC electrolyte

film is ~3.5V, which is comparable with about three regular AAA Ni-MH rechargeable

batteries (with a height of 43.6mm, diameter of 10.1mm and an output voltage of 1.2V).

As is shown in Figure 4(d) and Figure 4(e), the single flexible EDLC can drive a white

or a blue light emitting diode (LDE) bulb over 2min and 5min, respectively. Therefore,

it clearly shows that the as-fabricated gel polymer electrolytes derived from the cross-

linking of IL and PEO, which exhibits high ionic conductivity, wide electrochemical

15

window, excellent flexibility and stability as a promising solid electrolyte for wearable

energy storage devices.

4 Conclusions

In summary, a novel nIL-PEO-FPC solid electrolyte film is presented by fast

cross-linking IL and PEO under UV irradiation. The EMImTFSI remains the optimal

electrochemical performance after irradiation due to the moderate chain and

hydrophobicity. The 10EMImTFSI-PEO-FPC electrolyte film displayed high ionic

conductivity (around 6.7mS∙cm-1) and outstanding mechanical property for the flexible

device. The flexible EDLC assembles 10EMImTFSI-PEO-FPC electrolyte film and

two activated carbon electrodes that show a very broad electrochemical window (3.5V),

a high energy density of 30.13Wh∙kg-1 and a high power density of 874.8W∙kg-1 at a

current density of 1A∙g-1. This flexible EDLC shows same performance at different

bending angles, indicating perfect flexibility. We firmly believe that the excellent

performances of nIL-PEO-FPC shall bring new design opportunities of EDLC device

configuration for clean energy storage systems for wearable and flexible electronics

Note

The authors declare no competing financial interest.

16

Acknowledgements

This work is supported by the startup funding of Southern University of Science

and Technology (Grants Nos. 25/Y01256112 and 25/Y01256212), the Peacock Team

Project funding from Shenzhen Science and Technology Innovation Committee (Grant

No. KQTD2015033110182370), the National Key Research and Development Project

funding from the Ministry of Science and Technology of China (Grants Nos.

2016YFA0202400 and 2016YFA0202404), and the National Natural Science

Foundation of China (No. 21671096, and No. 21603094). Prof. Hui Pan thanks the

support from the Science and Technology Development Fund from Macau SAR

(Grants Nos. FDCT-068/2014/A2, FDCT-132/2014/A3, and FDCT-110/2014/SB) and

Multi-Year Research Grants (Grants Nos. MYRG2014-00159-FST and MYRG2015-

00017-FST) from Research & Development Office at University of Macau.

17

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Figure 1 Schematic representation of the preparation procedure and structure of the gel polymer

electrolyte formed via the cross-linking of ionic liquid and PEO chains. a) The digital photo of a fresh

GPE. b) The optimal ratio of IL/PEO in GPE formed by UV irradiation. c) The low ratio of IL/PEO in

GPE formed by UV irradiation.

22

Figure 2 a), b) CV curves of the EDLC with a different kind of IL in 5IL-PEO-FPC and 10IL-PEO-FPC

electrolyte films, respectively. c), d) GCD curves of the EDLC with a different kind of IL in 5IL-PEO-

FPC and 10IL-PEO-FPC electrolyte films, respectively.

23

Figure 3 The CV (a) and GCD (b) measurements of the nEMImTFSI-PEO-FPC electrolyte film and neat

EMImTFSI.

24

Figure 4 Electrochemical properties of the flexible EDLC making of the 10EMImTFSI-PEO-FPC

electrolyte film under different bending angles. a) CV curves at a scan rate of 10mV∙s-1. b) GCD curves

at a current density of 1A∙g-1. c) Nyquist plots in a frequency range from 10mHz to 100kHz with a

potential amplitude of 5mV, the inset shows the equivalent circuit used to simulate the Nyquist plots. d)

The LED (operation voltage is 3V) driven by single flexible EDLC. e) The blue LED (working voltage

is 2.5V) driven by single flexible EDLC

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