A new strategy for preparing oligomeric ionic liquid gel polymer electrolytes for high-performance and nonflammable lithium ion batteries

Journal of Membrane Science 499 (2016) 462–469

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

A new strategy for preparing oligomeric ionic liquid gel polymer electrolytes for high-performance and nonflammable lithium ion batteries Ping-Lin Kuo n, Chih-Hao Tsao, Chun-Han Hsu, Szu-Ting Chen, Huang-Ming Hsu Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC

art ic l e i nf o

a b s t r a c t

Article history: Received 15 September 2015 Received in revised form 6 November 2015 Accepted 7 November 2015 Available online 11 November 2015

In the present work, a new strategy is used to economically synthesize an oligomeric ionic liquid from conventional phenolic epoxy resin. This oligomeric ionic liquid is further blended with PVdF-co-HFP and organic liquid electrolyte to prepare a high performance, nonflammable gel polymer membrane. Although the liquid electrolyte uptake is low ( o 50%) for this novel gel polymer electrolyte, it possesses high ionic conductivities of 2.0 mS cm
1 at 30 °C and 6.6 mS cm
1 at 80 °C, respectively. The AC impedance results show that the interfacial compatibility between this gel polymer electrolyte and the electrodes is good. These two factors result in high cell capacity under different charge/discharge rates. Further, excellent cell-cycle stability after being charged and discharged 100 cycles is also demonstrated with the columbic efficiency to be up to 99. Due to the existence of the oligomeric ionic liquid, this novel gel polymer electrolyte exhibits superior dimensional stability; that is, at high temperature (150 °C) the dimensional change is less than 1%. Notably, the electrolyte’s limiting oxygen index can be as high as 29, meaning that it achieves the flame-retardant requirement under a normal atmosphere, which is essential to the safety of lithium ion batteries. These features allow this novel gel polymer electrolyte to function as a high performance and high safety lithium ionic conductor as well as a separator for lithium-ion batteries. & 2015 Elsevier B.V. All rights reserved.

Keywords: Lithium-ion battery Flame-retardant Polymer electrolyte Poly(ionic liquid)

1. Introduction Over the past decades, lithium ion batteries have been considered as an attractive power source for a variety of wide applications such as consumer electronics, electric vehicles and energy storage systems [1,2]. Although organic liquid electrolytes are the most common employed electrolytes in such batteries due to their high conductivities, they have a high risk of leakage during packaging and use. Particularly for large capacity batteries, the existence of highly flammable organic liquid electrolytes raises serious safety concerns [3]. For this reason, gel polymer electrolytes are being explored as replacements for liquid electrolytes since they can maintain high ionic conductivity, good mechanical strength without the risk of leakage [4–6]. Pyrrolidinium- and imidazolium-based ionic liquid hosts are an important class of electrolytes due to their sufficient ionic transport abilities, good electrochemical stability, and non-flammable/ non-volatile property [7–12]. Typical ionic liquid similar to n

Corresponding author. E-mail address: [email protected] (P.-L. Kuo).

http://dx.doi.org/10.1016/j.memsci.2015.11.007 0376-7388/& 2015 Elsevier B.V. All rights reserved.

molecular solvents have poor mechanical strength when applied as polymer-membrane host. And currently, ionic liquid-based gel polymer electrolytes have low conductivity (  10
4 S cm
1) due to the high viscosity of the ionic liquid [10,13–16]. Further, polyionic liquid electrolytes show even worse conductivity as a result of the low movability of ionic moiety [15,17,18]. Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-coHFP) based electrospinning and non-woven separators feature flexibility, non-flammability and electrochemical stability, and so are widely used as host polymer membranes for lithium batteries [19–22]. Although the high porosity retains large amount of liquid electrolytes and results in high conductivity, the liquid electrolyte-leakage problem remains [23,24]. To overcome the leakage safety issue, PVDF-co-HFP with ionic liquid has been studied as a replacement of traditional gel polymer electrolytes. However, the ionic conductivity and capacity are still insufficient [13,25]. Accordingly, to ensure PVDF-co-HFP is a viable gel polyelectrolyte, solving these disadvantages becomes critical. In the present work, a series of oligomeric ionic liquids are blended with PVDF-co-HFP and an electrolyte solution to form an oligomeric ionic liquid-type gel-polymer electrolytes to solve the

P.-L. Kuo et al. / Journal of Membrane Science 499 (2016) 462–469

above-mentioned problems. Herein, we use a novel strategy to economically synthesize the oligomeric ionic liquid from epoxy resin of suitable molecular weight. The oligomerization enhances the mechanical properties of the ionic liquid and also offer high conductivity. Most importantly, these gel polymer electrolytes feature non-flammability and stable heat resistance. Moreover, they also shows high conductivity, low interfacial resistance and high long-term durability. Therefore, they hold promise to be used as high safety and high performance gel polymer electrolytes.

2. Experimental 2.1. Ring-opening bromination reaction of phenolic epoxy resin Phenolic epoxy resin (PNE 177, Chang Chun Plastics, average molecular weight 1600 g mol
1, EEW 172-182 g eq
1) was washed by chloroform and precipitated in hexane before using. Subsequently, the purified phenolic epoxy resin was dissolved in chloroform (Tedia, 99.9%), the solution of which was cooled in an ice bath. Then, hydrobromic acid (Acros Organics, 48.0%) was slowly added into the solution and stirred for 6 h to obtain a reaction mixture. Thereafter, the reaction mixture was washed with water to remove unreacted hydrobromic acid, and the solvents removed using a rotary evaporator. A brominated intermediate polymer was thus obtained.

NMR spectra were recorded on a Bruker AMX600 MHz Digital NMR, for which an appropriate deuterated solvent was chosen. Further, the morphologies of the PVdF-HFP/x%OIL membranes were investigated by field-emission scanning electron microscopy (FESEM) (JEOL, JSM-6380LV). The amount of liquid electrolyte uptake (η) is calculated using Eq. (1):

η = ( Wt − W0 )/Wt x 100%

The brominated intermediate polymer was dissolved in dimethyl sulfoxide (DMSO), mixed with 1-methylimidazole (Alfa Aesar, 99.0%) at 80 °C, and then stirred for 36 h. Subsequently, the oligomeric ionic liquid with bromide counter-ion (OIL-Br anion) was precipitated in ethyl acetate. The product was then dried under vacuum at 80 °C for 24 h. 2.3. Ion exchange of OIL-Br anion The OIL-Br anion obtained in the previous step was dissolved in deionized (DI) water to obtain an OIL-Br solution. Bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) (Solvay) was dissolved in the DI water to obtain a LiTFSI solution, which was added drop-wise to the OIL-Br solution and stirred for 6 h. The final product, oligomeric ionic liquid with bis(trifluoromethane)sulfonamide (OIL) was precipitated in DI water, and then t dried under vacuum at 80 °C for 24 h before use. 2.4. Preparation of PVdF-HFP blend OIL membranes Both PVdF-HFP and OIL were dissolved in N-methyl-2-pyrrolidon (NMP), the solutions of which were mixed at different weight ratios (70/30, 50/50, 30/70). The polymer mixtures were then treated at 80 °C for 24 h to evaporate the solvent and form the polymer membranes, which were prepared by solution casting. Subsequently, the membranes were dried under vacuum at 100 °C for 24 h before use. The three prepared samples are identified as PVdF-HFP/x%OIL, where x represents the OIL weight ratio of the membranes. 2.5. Characterization Fourier transform infrared spectroscopy (FTIR) was recorded with a Nicolet Magna II 550 spectrometer. Thermal gravimetric curves (TGA) were obtained by using a Perkin Elmer TGA7, with a heat increment of 20 °C min
1 under a nitrogen atmosphere. 1H

(1)

where W0 and Wt are membrane weights before and after absorbing the electrolyte solution, respectively. Weights were measured in a glove box. 2.5.1. Methods of electrochemical performance To characterize of the electrochemical performance of the PVdF-HFP/x%OIL gel polymer electrolytes (GPEs), a 1.0 M LiPF6 in EC/DEC (1/1 v/v) liquid electrolyte was employed. The GPEs were then analyzed by ac-impedance spectroscopy on an electrochemical instrument (CHI604A, CH Instrument, Inc.) assembled between two stainless steel electrodes to characterize the ionic conductivities from 30 to 80 °C. Measurement were carried out at 0 V with an ac-impedance spectroscopy of 10 mV and a frequency variation ranging from 0.1 Hz to 1 MHz. The ionic conductivity (s) was calculated according to the bulk electrolyte resistance (R) using Eq. (2):

σ = l/RA 2.2. Synthesis of phenolic epoxy resin-based oligomeric ionic liquid with bromide ion

463

(2)

where l is the GPEs thickness and A is the contact area between the GPEs and steel electrodes. Battery performance tests of the membranes were conducted by fabricating 2032 coin-type cells with LiFePO4 cathode and lithium metal anodes. The cathodes were made by pouring a NMPbased slurry onto aluminum foil, for which contained 80 wt% LiFePO4 powder (Aleees, Taiwan), 10 wt% polyvinylidene fluoride (PVDF), and 10 wt% Super P. The electrodes were dried at 120 °C for 48 h under vacuum and were roll-pressed to ensure superior particulate contact and foil adhesion. The cell assembly took place in a dry, Ar-filled glove box (the moisture content is o5 ppm), and charge–discharge battery testing was conducted in the range of 2.5–4.2 V at room temperature, according to the Battery Automatic Test System (Acu Tech Systems, BAT-750B). Interfacial resistance was measured by ac-impedance spectroscopy at 10 mV before and after the battery charge/discharge, with a frequency variation ranging from 0.01 Hz to 1 MHz at the open circuit potential (OCP) of the battery. 2.5.2. Limiting oxygen index test The limiting oxygen index (LOI) values were measured by Atlas Limiting Oxygen Index Chamber to evaluate the flammability of the samples. The percentage of O2 in the O2–N2 mixture was just sufficient to enable combustion; that is the value of LOI.

3. Results and discussion In this paper, a novel imidazolium based OIL with phenyl polymer backbone was designed using phenolic epoxy resin as reactant, and synthesis process of which is shown in Fig. 1. Fig. S1 (a) and (b) shows the 1H NMR spectra of the phenolic epoxy resin and brominated phenolic epoxy resin with all assignment, respectively. The phenolic epoxy resin shows the protons of epoxy group at 2.5–2.8 and 3.1–3.3 ppm; after bromination, the protons of the brominated resin are observed at the peaks of 3.4–3.7 and 4.1 ppm. Furthermore, the obtained OIL is a light yellow, honeylike polymer, and was characterized by 1H NMR, as shown in Fig. 2.

464

P.-L. Kuo et al. / Journal of Membrane Science 499 (2016) 462–469

Fig. 1. (a)Synthesis and preparation of PVdF-HFP/X%OIL membrane. (b)Scheme of PVdF-HFP/X%OIL membrane.

70

(a) 60

Transmittance (%)

As can be seen, there is a chemical shift of the OIL's 1H NMR spectra, which can be assigned to aliphatic proton (3.1–4.1 ppm), phenyl proton (6.6–7.3 ppm), and imidazolium ring (8.6–9.1 ppm, and 7.3–7.8 ppm); moreover, all signals are assigned to the novel imidazolium-based OIL and depicted in the figure. In order to further confirm the synthesis of OIL from phenolic epoxy resin, the FTIR spectra of the phenolic epoxy resin and brominated phenolic epoxy resin are shown in the supporting information (Fig. S2). The phenolic epoxy resin shows a strong band of epoxy ring stretching vibration at 917 cm
1. After bromination, the peak of epoxy ring is disappear, and a new peak of C–Br stretches appears at 652 cm
1, which shows the brominated phenolic epoxy resin was prepared. Moreover, the FTIR spectra of the OIL are presented in Fig. 3. The characteristic peaks of the OIL-Br anion and OIL-TFSI anion, including the C–O and aromatic ring stretching vibrations at 1128 cm
1 and 1609 cm
1, respectively, correspond to phenolic resin. The bands at 3159 and 3117 cm
1 are assigned to the C–H, while those at 1575 and 1459 cm
1 are due to the C ¼N and C ¼ C skeleton stretching vibrations of the imidazole ring, respectively. The new bands appearing at 1349, 1136 cm
1 and 1139 cm
1 are attributed to S¼O and C–F of TFSI anion. The appearance of all typical peaks of novolac resin and imidazole indicates that the imidazolium based OIL was successfully synthesized. The TGA

50 40

(b)

30 20 10

(a) OIL-Br anion (b) OIL-TFSI anion

0 3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm ) Fig. 3. The IR spectra of OIL-Br anion and OIL-TFSI anion.

Fig. 2. 1H NMR of phenolic resin-based OIL.

500

P.-L. Kuo et al. / Journal of Membrane Science 499 (2016) 462–469

465

Fig. 4. SEM micrographs of surface of (a) PVdF-HFP, (b) PVdF-HFP/30%OIL, (c) PVdF-HFP/50%OIL, (d) PVdF-HFP/70%OIL; cross-section of (e) PVdF-HFP, (f) PVdF-HFP/30%OIL, (g) PVdF-HFP/50%OIL, (h) PVdF-HFP/70%OIL.

measurements (Fig. S3) show that the temperature during the initial 5% weight loss of OIL reached 422 °C, indicating that OIL has excellent thermal stability. The novel imidazolium-based OIL was blended with PVdF-HFP to prepare flame-retardant ionized polymer membranes (PVdFHFP/x%OIL) that are flexible and semi-transparent. SEM photos of the PVdF-HFP and PVdF-HFP/x%OIL membranes are shown in Fig. 4 (a) and (b)–(d). As can be seen, SEM surface images of the PVdFHFP/x%OIL (Fig. 4(b)–(d)) show some nano-size pores, which are reported as being be beneficial for lithium ion transportation and high performance separators. Compared with PVdF-HFP and PVdFHFP/x%OIL, the surface of the PVdF-HFP membrane is nonporous as presented in Fig. 4(a), thus restricting ion transport. Accordingly, it appears that the grain size of the membrane decreases with the OIL content increases. The cross-section SEM image of the PVdF-HFP/x%OIL membranes (Fig. 4(e)–(h)) show that they comprised small porous structures at 30 and 50%OIL content. However, the pores disappear when the OIL content surpassed 70% (Fig. 4 (h)), which might be attributable to the phase separation of the OIL and PVdF-HFP. Fig. 5 shows the temperature dependence of ionic conductivity of PVdF-HFP/x%OIL. As seen, the room-temperature ionic

(S/cm)

0.01

1E-3

PVdF-HFP/70% OIL PVdF-HFP/50% OIL PVdF-HFP/30% OIL PVdF-co-HFP

1E-4 2.8

2.9

3.0

3.1

3.2

3.3

1000/T (1/K) Fig. 5. Temperature dependence of ionic conductivity on different OIL content.

conductivity of OIL/PVdF-HFP GPEs is significantly higher than that of pure PVdF-HFP GPE (0.12 mS cm
1); further, the conductivity progressively rises as the OIL content increases. Plots of PVdF-HFP/ 30%OIL and PVdF-HFP/50%OIL are linear, indicating their conductivities follow the Arrhenius behavior due to the small porosity shown by the cross-section of these membranes. This means the ion transport decoupled from the polymer chain segmental motion but the porous of membranes. Plots of ionic conductivities for pure PVdF-HFP and PVdF-HFP/70%OIL electrolytes obey the Vogel– Tamman–Fulcher (VTF) equation. As can be seen in Fig. 4(h), the cross-sectional SEM images of the PVdF-HFP and PVdF-HFP/70% OIL membranes display no pores inside, which indicates that local segmental motion and chain relaxation play important roles in these GPEs. Pure PVdF-HFP GPEs were prepared by solution casting; however, since its nonporous structure cannot absorb large amount of liquid electrolytes, the ion transportation is limited. The VTF behavior of PVdF-HFP/70%OIL is demonstrated since the OIL chain mobility increases significantly with rising temperature, which means that ionized oligomer easily softens as the temperature increases. Consequently, a fast-charge transport may occur in PVdF-HFP/70%OIL. Interfacial compatibility is an important factor of battery performance. Fig. 6(a)–(c) shows the interfacial resistance of the gel electrolytes with the electrodes, for which impedance measurements were obtained by AC-impedance in a Li|electrolyte|LiFePO4 cell before and after cycling. The intercept of the real axis and the diameter of the semicircle are assigned to bulk (Rb) and interfacial (Ri) resistance, respectively, as given in Table 1. The Rb of the PVdFHFP/x%OIL decreased as the OIL content rose, which is attributed to the ascending electrolyte conductivity caused by the higher OIL content of the polymer membranes. The Ri of the PVdF-HFP/30% OIL GPE significantly increased after 5 cycles charge–discharge (Δ ZRe  71 Ω). By comparison, the PVdF-HFP/70%OIL GPE has smaller Ri and strongly mitigates the increase in cell impedance (ΔZRe  16 Ω), which also suggests a stable interface between the PVdFHFP/70%OIL GPE and the electrodes. This indicates the PVdF-HFP/ 30%OIL GPE reacted with the electrodes and formed the passivation layer; however, a higher OIL content could suppress the growth of the cell impedance, mainly due to the ease of ion transportation. In this manner, the interface compatibility between the GPE and electrodes is enhanced. Accordingly, the above suggests that the PVdF-HFP/70%OIL GPE is the blend for application. The linear sweep voltammetry curve of the PVdF-HFP/X%OIL

466

P.-L. Kuo et al. / Journal of Membrane Science 499 (2016) 462–469

Table 1 Impedance parameter of PVdF-HFP/x%OIL before and after charge–discharge.

90

PVdF-HFP/30% OIL 80

Rb (Ω)

Sample

70 60

PVdF-HFP/30%OIL PVdF-HFP/50%OIL PVdF-HFP/70%OIL

Z''( )

50 40

19 8 6

Ri (Ω)

Ri (Ω)

Ri (Ω)

Initial

1 cycle

5 cycles

52 48 45

82 78 69

123 82 66

ΔZRea(Ω)

71 34 16

a ΔZRe is equal to Ri 5 cycle minus Ri Initial, which is corresponded to the increase interfacial resistance after charge–discharge

30 20

Charge 0.1C

10

initial after 1 cycle after 5 cycle 0

50

100

150

200

Z'( )

90

PVdF-HFP/50% OIL

80

+

-10

4000

Voltage (Li/Li )(mV)

0

3500

3000

Discharge 0.1C 0.3C 0.5C 1C 3C

2500

70 60

2000

Z''( )

50

0 40

20

40

60

80

100

120

140

160

Capacity (mAh/g) Fig. 7. Charge–discharge profiles of Li/ PVdF-HFP/70%OIL /LiFePO4 batteriy tests at various C-rates. LiFePO4 was assumed a maximal theoretical capacity of 170 mA h g
1 for this study.

30 20 10

initial after 1 cycle after 5 cycle

0 -10 0

50

100

150

200

Z'( )

membranes (Fig. S4) have an onset oxidation voltage at 4.5 V corresponding to the electrochemical oxidative degradation of the organic liquid electrolytes. Fig. 7 plots the charge/discharge test profiles of PVdF-HFP/70%OIL with a lithium-metal anode and LiFePO4 cathode cell at various C-rates. The PVdF-HFP segment acts as a binder to combine with the OIL and provides a high-

90

PVdF-HFP/70% OIL

80 70 60

Z''( )

50 40 30 20 10

initial after 1 cycle after 5 cycle

0 -10 0

50

100

150

200

Z'( ) Fig. 6. Nyquist plots before and after charge–discharge profiles of PVdF-HFP/x%OIL GPEs, (a) 30%, (b) 50%, (c) 70%. The cell was assembled by Li/ GPE/LiFePO4.

Fig. 8. Cyclic performance of Li/ PVdF-HFP/70%OIL /LiFePO4 at constant current density of 0.5 C.

P.-L. Kuo et al. / Journal of Membrane Science 499 (2016) 462–469

467

Fig. 9. Dimensional stability of the PVdF-HFP/70%OIL tested at various temperatures.

Fig. 10. Limiting oxygen index test of PVdF-HFP/70%OIL gel electrolyte at LOI 28. (a) before combustion, (b) during combustion (c) after combustion.

modulus skeleton of the composite electrolytes. The PVdF-HFP/ 70%OIL-membrane cell contains only a small amount of liquid electrolyte (uptake  13%, as shown in Table S1), yet delivers satisfactory discharge capacities of 152 mA h g
1 and 141 mA h g
1 at 0.1 and 1 C, respectively. Further, despite a high C-rate (3 C), the cell featured the discharge capacity of 117 mA h g
1. Under the charge–discharge procedure, the profile displays a flat plateau at 3.4 V and 3.2–3.4 V, which corresponds to the Fe2 þ /Fe3 þ oxidation–reduction reaction of LiFePO4. However, at 3 C, the discharge curve decreases at an earlier stage because of the poor lithium-ion transference of the GPE caused by the polarization. Conventional

ionic liquid-based GPE has larger differences between charging voltages and discharging voltages and discharge curve decreases at high C-rate is obvious. Moreover, the rate performance results demonstrate that the higher ionic conductivity and superior compatibility improve the PVdF-HFP/70%OIL gel membrane. The cycling performance test was carried out of the cell containing the PVdF-HFP/70%OIL gel membrane at a constant charge/ discharge current density of 0.5/0.5 C, the results of which are shown in Fig. 8. The PVdF-HFP/70%OIL features a steady cycle life with the columbic efficiency of nearly 99% after 100 cycles. The high capacity retention after 100 cycles is due to the high ionic

468

P.-L. Kuo et al. / Journal of Membrane Science 499 (2016) 462–469

conductivity and stable interfacial layer in the cell. To further confirm the cycling performance, the AC impedance spectrum of the PVdF-HFP/70%OIL gel electrolyte was analyzed after 100 cycles. The cell impedance was suppressed by the PVdF-HFP/70%OIL host, which can be observed that the Ri did not change largely after 100 cycles. This result indicates the electrolyte/electrodes interface was optimization after the cycling test. The thermal stability of the separators and GPE membranes is an important factor for the battery safety. To this end, shrinkage of the separator was evaluated on heating at different temperatures. Samples were stored in an oven for 30 min at a set temperature, after which their dimensional change are noted. Fig. 9 shows the dimensional change of the PE/PP separator and PVdF-HFP/70%OIL membrane. As can be seen, the dimensional change of PVdF-HFP/ 70%OIL membrane was negligible during the test; in contrast, the PE/PP separator curled and shrank at the temperatures of 120 °C and 150 °C, respectively, because the PE and PP melt at around 120 °C and 150 °C. Hence, the PE/PP encounters significant dimensional reduction when temperatures exceed 120 °C. The improvement improved thermal stability of PVdF-HFP/70%OIL is attributed to the high melting temperature of PVdF-co-HFP. In addition, the dimensional stability of PVdF-HFP/70%OIL membrane before and after the absorption of liquid electrolyte for 24 h is shown in Fig. S5. As can be seen, the dimensional change of PVdFHFP/70%OIL membrane after absorption of liquid electrolyte is negligible. This result indicates the PVdF-HFP/70%OIL is ideal for lithium lithium-ion battery safety. Moreover, the flame-retardant ability of the polymer gel electrolyte is a key factor for safety issue of lithium-ion batteries. To clarify the flame retardant degree of the PVdF-HFP/70%OIL gel electrolyte, the LOI test was administered, which is an useful tool to quantitatively evaluate the minimum concentration of oxygen to support the combustion of the material. Generally, if the LOI value is between 22 and 27, it is considered an non-flammable material; value higher than 27 indicate the material is flame-retardant; and, LOI value below 22 indicate the material is flammable material [26]. Fig. 10 shows the LOI test procedure; as seen, when the lighter was removed, the PVdF-HFP/70%OIL gel electrolyte exhibits excellent flame resistance, with the gel membrane still intact. The LOI value of a dry PVdF-HFP/70%OIL membrane can reach 31, while the LOI value for the PVdF-HFP/70%OIL mixed liquid electrolyte solutions (  13%) was constant at 29, suggesting that OIL can improve non-flammablility of both the PVdF-HFP/70% OIL membrane and gel electrolyte system. In addition, the small amount liquid electrolyte provides satisfactory cycling and rate performance. Moreover, the addition of OIL yields an excellent flame retarding electrolyte and reasonable conductivity, as well as highly improving safety of the battery application.

4. Conclusion A novel strategy to synthesize the OIL from phenolic epoxy resin was prepared and presented in this work. The polymer membrane features high safety and superior performance, and is also cost-effective; consequently, it is a promising candidate for safe and reliable energy storage. The PVdF-HFP/70%OIL ionic gel electrolyte has the high conductivity of 2  10
3 S cm
1 at room temperature and exhibits satisfactory charge/discharge performance with a LiFePO4 cell. Moreover, the ionic gel polymer electrolyte is thermal stable and nonflammable (LOI  29), which holds great promise for to lithium-ion battery requiring high safety and low cost.

Acknowledgment The authors would like to thank the Ministry of Science and Technology, Taipei, ROC within the project “Development of HighPerformance, High Voltage and High Safety Lithium Ion Electrolytes” (MOST 104-2221-E-006-245 and MOST 104-3113-E-006-01) for their generous financial support of this research.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2015.11.007.

References [1] B. Scrosati, J. Garche, Lithium batteries: status, prospects and future, J. Power Sources 195 (2010) 2419–2430. [2] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy Environ. Sci. 4 (2011) 3243. [3] H.W. Rollins, M.K. Harrup, E.J. Dufek, D.K. Jamison, S.V. Sazhin, K.L. Gering, D. L. Daubaras, Fluorinated phosphazene co-solvents for improved thermal and safety performance in lithium-ion battery electrolytes, J. Power Sources 263 (2014) 66–74. [4] P.L. Kuo, C.A. Wu, C.Y. Lu, C.H. Tsao, C.H. Hsu, S.S. Hou, High performance of transferring lithium ion for polyacrylonitrile interpenetrating crosslinked polyoxyethylene network as gel polymer electrolyte, ACS Appl. Mater. Interfaces 6 (2014) 3156–3162. [5] S.H. Wang, P.L. Kuo, C.T. Hsieh, H.S. Teng, Design of poly(acrylonitrile)-based gel electrolytes for high performance lithium ion batteries, ACS Appl. Mater. Interfaces 6 (2014) 19360–19370. [6] H. Lee, M. Yanilmaz, O. Toprakci, K. Fu, X. Zhang, A review of recent developments in membrane separators for rechargeable lithium-ion batteries, Energy Environ. Sci. 7 (2014) 3857–3886. [7] Y.S. Ye, J. Rick, B.J. Hwang, Ionic liquid polymer electrolytes, J. Mater. Chem. A 1 (2013) 2719–2743. [8] D.R. MacFarlane, N. Tachikawa, M. Forsyth, J.M. Pringle, P.C. Howlett, G. D. Elliott, J.H. Davis Jr., M. Watanabe, P. Simon, C.A. Angell, Energy applications of ionic liquids, Energy Environ. Sci. 7 (2014) 232. [9] Y. Lu, S.S. Moganty, J.L. Schaefer, L.A. Archer, Ionic liquid-nanoparticle hybrid electrolytes, J. Mater. Chem. 22 (2012) 4066–4072. [10] K. Yin, Z. Zhang, L. Yang, S.I. Hirano, An imidazolium-based polymerized ionic liquid via novel synthetic strategy as polymer electrolytes for lithium ion batteries, J. Power Sources 258 (2014) 150–154. [11] Y. Lu, K. Korf, Y. Kambe, Z. Tu, L.A. Archer, Ionic-liquid–nanoparticle hybrid electrolytes: applications in lithium metal batteries, Angew. Chem. 126 (2014) 498–502. [12] A.L. Pont, R. Marcilla, I.D. Meatza, H. Grande, D. Mecerreyes, Pyrrolidiniumbased polymeric ionic liquids as mechanically and electrochemically stable polymer electrolytes, J. Power Sources 188 (2009) 558–563. [13] J. Pitawala, M.A. Navarra, B. Scrosati, P. Jacobsson, A. Matic, Structure and properties of Li-ion conducting polymer gel electrolytes based on ionic liquids of the pyrrolidinium cation and the bis(trifluoromethanesulfonyl)imide anion, J. Power Sources 245 (2014) 830–835. [14] P.X. Yang, L. Liu, L.B. Li, J. Hou, Y.P. Xu, X. Ren, M.Z. An, N. Li, Gel polymer electrolyte based on polyvinylidenefluoride-co-hexafluoropropylene and ionic liquid for lithium ion battery, Electrochim. Acta 115 (2014) 454–460. [15] M. Li, L. Yang, S. Fang, S. Dong, S. Hirano, K. Tachibana, Polymer electrolytes containing guanidinium-based polymeric ionic liquids for rechargeable lithium batteries, J. Power Sources 196 (2011) 8662–8668. [16] I. Stepniak, E. Andrzejewska, A. Dembna, M. Galinski, Characterization and application of N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl) imide ionic liquid–based gel polymer electrolyte prepared in situ by photopolymerization method in lithium ion batteries, Electrochim. Acta 121 (2014) 27–33. [17] A.S. Shaplov, P.S. Vlasov, E.I. Lozinskaya, D.O. Ponkratov, I.A. Malyshkina, F. Vidal, O.V. Okatova, G.M. Pavlov, C. Wandrey, A. Bhide, M. Sch€onhoff, Y. S. Vygodskii, Polymeric ionic liquids: comparison of polycations and polyanions, Macromolecules 44 (2011) 9792–9803. [18] Y.S. Vygodskii, A.S. Shaplov, E.I. Lozinskaya, K.A. Lyssenko, D.G. Golovanov, I. A. Malyshkina, N.D. Gavrilova, M.R. Buchmeiser, Conductive polymer electrolytes derived from poly(norbornene)s with pendant ionic imidazolium moieties, Macromol. Chem. Phys. 209 (2008) 40–51. [19] W. Li, Y. Xing, X. Xing, Y. Li, G. Yang, L. Xu, PVDF-based composite microporous gel polymer electrolytes containing a novel single ionic conductor SiO2(Liþ), Electrochim. Acta 112 (2013) 183–190. [20] F. Zhang, X. Ma, C. Cao, J. Li, Y. Zhu, Poly(vinylidene fluoride)/SiO2 composite membranes prepared by electrospinning and their excellent properties for

P.-L. Kuo et al. / Journal of Membrane Science 499 (2016) 462–469

nonwoven separators for lithium-ion batteries, J. Power Sources 251 (2014) 423–431. [21] Y. Zhu, F. Wang, L. Liu, S. Xiao, Z. Chang, Y. Wu, Composite of a nonwoven fabric with poly(vinylidene fluoride) as a gel membrane of high safety for lithium ion battery, Energy Environ. Sci. 6 (2013) 618. [22] H.S. Jeong, E.S. Choi, S.Y. Lee, J.H. Kim, Evaporation-induced, close-packed silica nanoparticle-embedded nonwoven composite separator membranes for high-voltage/high-rate lithium-ion batteries: advantageous effect of highly percolated, electrolyte-philic microporous architecture, J. Membr. Sci. 415–416 (2012) 513–519. [23] J. Shi, H. Hu, Y. Xia, Y. Liu, Z. Liu, Polyimide matrix-enhanced cross-linked gel separator with three-dimensional heat-resistance skeleton for high-safety and high-power lithium ion batteries, J. Mater. Chem. A 2 (2014) 9134–9141.

469

[24] M. Xia, Q. Liu, Z. Zhou, Y. Tao, M. Li, K. Liu, Z. Wu, D. Wang, A novel hierarchically structured and highly hydrophilic poly(vinyl alcohol-co-ethylene)/ poly(ethylene terephthalate) nanoporous membrane for lithium-ion battery separator, J. Power Sources 266 (2014) 29–35. [25] 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) 559–566. [26] B. Chen, W. Gao, J. Shen, S. Guo, The multilayered distribution of intumescent flame retardants and its influence on the fire and mechanical properties of polypropylene, Compos. Sci. Technol. 93 (2014) 54–60.