Gel Polymer Electrolyte Based on Poly(vinylidene fluoride

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Gel Polymer Electrolyte Based on Poly(vinylidene fluoride) /Thermoplastic Polyurethane/Polyacrylonitrile by Electrospinning Technique Yuewen Liu, Xiuxiang Peng, Qi Cao, Bo Jing, Xianyou Wang, and Yuanyuan Deng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03411 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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The Journal of Physical Chemistry

Gel Polymer Electrolyte Based on Poly(vinylidene fluoride) /Thermoplastic Polyurethane/Polyacrylonitrile by Electrospinning Technique Yuewen Liu, Xiuxiang Peng, Qi Cao ∗, Bo Jing, Xianyou Wang, Yuanyuan Deng (Key Laboratory of Environmentally Friendly Chemistry and Applications of Minister of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China)

∗

Corresponding author. Tel.: +86 731 58298090; Fax: +86 731 58298090; E-mail: [email protected]. 1

ACS Paragon Plus Environment


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ABSTRACT: Poly(vinylidene fluoride) (PVdF), thermoplastic polyurethane (TPU) and polyacrylonitrile (PAN) are blended in different ratios by electrospinning technique. The corresponding composite gel polymer electrolytes (GPEs) are obtained after activating the electrospun membranes in 1.0 mol dm-3 LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC). Their morphology and performance are systematically investigated by scanning electron microscopy (SEM), thermal analysis (TGA), charge-discharge tests, and so on. The gel polymer electrolyte (GPE) based on PVdF/TPU/PAN (2/2/1, wt.%) exhibits a high ionic conductivity of 6.91×10-3 S cm-1, and its electrochemical stability window reaches up to 5.7 V. Furthermore, the Li/GPE/LiFePO4 cell shows a first discharge capacity of 167.52 mAh g-1 at 0.1C rate at 25 °C. The film also exhibits a high tensile strength (10.3± 0.2 MPa) and elongation (102.5 ± 0.2%). It is observed that the PVdF/TPU/PAN (2/2/1, wt.%) based GPE is perfectly suitable for high-performance lithium ion rechargeable batteries.

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INTRODUCTION Lithium ion batteries have been applied widely with the rapid growth of energy sources, due to their high specific energy, high efficiency, nonmemory effect and long cycle life.

1-3

However, there are some safety issues to be solved for lithium ion

batteries. The safety issues of lithium ion battery are mainly caused by liquid organic solvent usage in the electrolyte system. Recently, researchers have focused on replacing common liquid electrolytes with GPEs 4,5 to achieve the full plastic batteries. However, the main obstacles are the poor mechanical strength and ionic conductivity. Many ways can obtain GPEs such as phase inversion method, solvent casting technique, electrospinning technique, γ-ray irradiation method etc. Among these methods, electrospinning is a simple, controllable and efficient approach to get GPEs with excellent properties. 6-11 PVdF is a semicrystalline polymer with the repeated unit of –(CH2CF2)n–. It exhibits some prominent advantages, such as good flexibility, good chemical resistance and superior thermal porosities. excellent overall performance by far.

17

12-16

It is the best polymer monomer with

Nevertheless, it is expensive.

18

Researchers

often use it to graft or blend with one or two polymers to obtain superior GPEs with good mechanical properties and electrochemical performance, such as the blending like

PVdF/Poly

(vinyl

chloride)

(PVC),

19

PVdF/polysulfone

(PSF),

20

PVdF/polyurethane (PU). 21 TPU is one of the most versatile engineering thermoplastics with elastomeric properties. TPU is widely used in various branches of industry owing to its superior physical performances, chemical resistance, abrasion resistance, good adhesion and ease of processing.

22-26

The GPE based on TPU/poly(vinylidene fluoride-co-

3



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hexafluoropropylene) (PVdF-HFP)

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for rechargeable lithium batteries has been

reported. PAN has been adopted as polymer electrolytes in lithium ion batteries with high ionic conductivity, thermal stability, good morphology for electrolyte uptake and low price, Besides the above-mentioned advantages, PAN can reduce the dendrite growth during the charging-discharging process of lithium ion batteries.

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However, the

mechanical properties of PAN based polymer electrolytes are greatly reduced after adding the plasticizer, and it is difficult to obtain free-standing membranes for analysis.

29

We often modify it by copolymerization or blending to improve its

mechanical stability. we are the first people to prepare GPEs from the blending system of PVdF/TPU/PAN by electrospinning technique. Electrospun membranes based on PVdF/TPU/PAN are soaked in 1.0 mol L-1 LiPF6-EC/DMC liquid electrolyte solution and activated to form polymer electrolytes. We examine the electrochemical performances of these GPEs for lithium ion batteries. The results indicate that PVdF/TPU/PAN (2/2/1, wt.%) based electrospun membranes exhibit better properties than the other two samples. EXPERIMENTAL SECTION Materials PVdF (Alfa Aesar), TPU (yantaiwanhua) and PAN (Sigma-Aldrich) were dehydrated under vacuum at 80 °C for 24 h before using. A certain weight of LiPF6 was dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1/1, v/v) to yield 1.0 mol L-1 electrolyte solution. N, N-Dimethylforamide (DMF) and Acetone were analytical purity and used as received. Preparation of the PVdF/TPU/PAN Films 4


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10 g TPU, PVdF and PAN powder (1/1/1, 2/2/1 and 3/3/1, wt.%) were dissolved in N, N-Dimethylforamide/acetone (3/1, w/w) forming 9 wt.% solution, respectively. The mixture was transfered to the syringe and conducted with a flow rate of 1.0 mL h−1. A high voltage of 24 kV was kept between the nozzle tip and the collector (aluminum drum) at room temperature. Dry fibers that accumulated on the collector were obtained as fibrous membranes. The electrospun films were dried under vacuum at 60 °C for 12 h to remove the residual solvents prior to futher use. Preparation of Gel Polymer Electrolytes The dried porous fibrous films were transformed into polymer electrolytes by soaking the fibrous membranes in 1.0 mol L-1 LiPF6–EC/DMC liquid electrolyte solution at 25 °C in an Ar-filled glove box. Wipe the surface of swelled films by filter paper and then get GPEs. Membrane Characterization The morphology of films was investigated by SEM (Hitachi S-3500 N, Japan). The membranes should be gold sprayed before the SEM measurement. The structure can be explored using the FTIR spectra (Spectrum One, PerkinElmer Instruments). The thermal property of the fibrous films was tested by thermogravimetric analysis (model TGAQ 50, TA Company, USA). The TGA measurements were conducted under dry nitrogen atmosphere with a speed of 20 °C/min from 30 °C to 800 °C. The porosity can be calculated using the formula (A) after immersing the fims into n-butanol for 1 h. ܲ=

ௐೢ ିௐ೏ ఘ್ ௏೛

× 100%

(A)

Where Ww and Wd represent the weights of the wet and dry separators, respectively, ρb is the density of n-butanol, Vp is the volume of the dry film. The electrolyte uptake was determined by the weight increase after immersing the 5



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membranes into the LiPF6-EC/DMC electrolyte and calculated by the following equation: Uptake(%) =

ௐିௐబ ௐబ

× 100%

(B)

In this equation, W and W0 are the weights of the wet and dry membranes, respectively. 30

The ionic conductivity of the porous films was examined with SS/GPE/SS blocking cells by AC impedance measurement applying the Zahner Zennium electrochemical analyzer with a frequency range from 0.1 Hz to 1.0 MHz. The ionic conductivity could be worked out according to the following Eq. (C): ߜ=

௛

(C)

ோౘ ௌ

In this equation, δ expresses the ionic conductivity, Rb represents the bulk resistance of the membrane, h is the thickness and S is the area. The mechanical strength of the membranes was investigated employing the universal testing machines (UTM, Instron Instruments) with a extension rate of 5 mm/min. The sheets' dimensions were about 2 cm × 5 cm × 100 µm (width×length×thickness). Performance of the Assembled Cells Electrochemical stability was investigated by linear sweep voltammetry (LSV). It means studying the Li/GPE/SS cells by a Zahner Zennium electrochemical analyzer at a scanning rate of 5 mV s−1, with a voltage range of 0-8 V. The charge–discharge cycle test of Li/GPE/LiFePO4 cells was carried out using the Neware battery testing system (model BTS-51, ShenZhen, China) with voltage from 2.0 V to 4.2 V at 0.1 C. The assembly of the testing cells was conducted in an argon-filled glove box, where water and oxygen levels were kept less than 5 ppm. 6


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RESULTS AND DISCUSSION Morphology and Structure Fig. 1 presents the morphology of the films prepared from mixed solution (PVdF/TPU/PAN) by electrospinning technique . It can be seen that the interlaid network and nearly straightened tubular structure fibers are observed for all membranes. The average fiber diameter (AFD) of Electrospun PVdF/TPU/PAN (1/1/1) fibrous membrane [Fig. 1(a)] is about 550 nm. As it is shown in Fig. 1(c), the PVdF/TPU/PAN (3/3/1) nonwoven film has interconnected multifibrous layers with ultrafine porous structure. The AFD of PVdF/TPU/PAN (3/3/1) nonwoven film is much more uniform about 530 nm. While the PVdF/TPU/PAN (2/2/1) membrane in Fig. 1(b) has the narrowest fiber diameter range, with the diameter distribution value about 470 nm.

(a)

(b)

(c)

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Fig. 1 SEM images of electrospun PVdF/TPU/PAN membranes (a) 1/1/1, (b) 2/2/1 and (c) 3/3/1. There are many parameters influence the morphology and structure of the electrospun sheets. For instance, the gap between the nozzle tip and the collector, the applied voltage, the concentration and dielectric constant of the blending. In this study, the first three factors are set to the same. So they can 't explain the differences of the morphology. We should consider the following reasons. The increase of PVdF causes the increase of the dielectric constant of the mixture for electrospinning, because of the high dielectric constant of PVdF. 31 The easier electrospun jet formation is, the lower average fiber diameter is. With the further increase of PVdF and TPU, the viscosity and surface tension of the solution decrease. The solvent is not evaporated completely, and then the droplet is deposited on the collector. Hence it increases the AFD of the PVdF/TPU/PAN (3/3/1) nonwoven film. The FT-IR spectrum of eletrospun membranes is shown in Fig. 2. The characteristic absorption peaks of TPU are clearly observed to be 2939 cm−1 (N-H stretching in hard phase) and 1700 cm−1 (C=O stretching). The characteristic peaks of PVdF are 1400 cm−1 (–CH2– bending vibration), 1073 cm−1 (stretching band of C–C) and 873 cm−1 (band of amorphous phase). The typical peaks of PAN are 2243 cm−1 (stretching band of C≡N) and 1453 cm−1 (deformation vibration band of –CH2–). However, in the case of the composite membrane of PVdF/TPU/PAN (2/2/1), the characteristic absorption peaks of TPU and PVdF are found to be shifted, corresponding to 2922 cm−1, 1699 cm−1, 1399 cm−1, 1071 cm−1. Hence, we consider the following reasons. There are molecular level interactions between the amino-group (–NH) in the hard segments of TPU and cyanogroup (–CN) in the PAN. It may form the hydrogen bond between the –NH of TPU and the electronegative fluorine atom in PVdF. 8


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Fig. 2 FTIR spectra of electrospun membranes (a) TPU, (b) PVdF, (c) PAN and (d) PVdF/TPU/PAN (2/2/1). Thermal Behavior Thermograms of different electrospun PVdF/TPU/PAN membranes are exhibited in Fig. 3. The decomposition of the three membranes follows a similar pattern in thermograms. All of the three membranes start losing weight at 292 °C. The PVdF/TPU/PAN (1/1/1) membrane has 37.8% residual mass at 503 °C, while the PVdF/TPU/PAN (2/2/1) and PVdF/TPU/PAN (3/3/1) membranes have 39.6% and 29.1%

weight

left,

respectively.

The

decomposition

temperature

of

our

PVdF/TPU/PAN composite films is higher than the value about 240 °C reported by Santhosh et al. 21 for PVdF/PU composite polymer electrolyte film system.

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Fig. 3 Termograms of the electrospun PVdF/TPU/PAN membranes (a) 1/1/1, (b) 2/2/1 and (c) 3/3/1. This demonstrates that the PVdF/TPU/PAN (2/2/1) membrane has better thermal stability than the other two samples. Additionally, the initial decomposition temperature is higher than 290 °C and the final decomposition temperature is larger than 500 °C for all samples, which proves that the PVdF/TPU/PAN film is suitable for lithium ion batteries as a separator. Porosity, Electrolyte Uptake and Ionic Conductivity The porosity of PVdF/TPU/PAN (1/1/1) membrane is about 45%. And the porosity of PVdF/TPU/PAN (3/3/1) membrane is 73%. Among all these electrospun membranes, the PVdF/TPU/PAN (2/2/1) membrane shows the largest porosity (88%). Fig. 4 shows the uptake behaviors of these fibrous films. We can work out the electrolyte uptake amount basing on Eq (B). Within 2 min, the PVdF/TPU/PAN 10


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(1/1/1) electrospun membrane owns an electrolyte uptake of about 173%, meanwhile the PVdF/TPU/PAN (3/3/1) membrane is 207% and the PVdF/TPU/PAN (2/2/1) film is 253%. After 15 min, the electrolyte uptake of all films becomes stable, the PVdF/TPU/PAN (1/1/1) membrane reaches up to 227%, while the PVdF/TPU/PAN (2/2/1) and PVdF/TPU/PAN (3/3/1) reach up to 311% and 263%, respectively. Plainly, the PVdF/TPU/PAN (2/2/1) membrane always possesses the highest electrolyte uptake percentage. In a word, higher electrolyte uptake of the PVdF/TPU/PAN (2/2/1) membrane is attributed to the fully connected pore structure.

Fig. 4 The uptake behaviors of the PVdF/TPU/PAN electrospun membranes (a) 1/1/1, (b) 2/2/1 and (c) 3/3/1. Fig. 5 shows the impedance spectra of the cells of PVdF/TPU/PAN based fibrous polymer electrolytes. It is typical AC impedance for GPEs. We can identify clearly from Fig. 5(a) that the bulk resistance (Rb) of PVdF/TPU/PAN (1/1/1) fibrous polymer electrolyte is 4.8 Ω. And in Fig. 5(c), the bulk resistance of the 11



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PVdF/TPU/PAN (3/3/1) is 2.1 Ω. However, the PVdF/TPU/PAN (2/2/1) fibrous polymer electrolyte is observed to have the minimum value of 1.3 Ω (Fig. 5(b)).

Fig. 5 Impedance spectra of the gel polymer electrolytes based on PVdF/TPU/PAN at 25 °C (a) 1/1/1, (b) 2/2/1 and (c) 3/3/1. The ionic conductivity can be calculated according to Eq. (C). The ionic conductivity of PVdF/TPU/PAN (1/1/1) and PVdF/TPU/PAN (3/3/1) fibrous polymer electrolytes are 2.13×10-3 S cm-1 and 4.88×10-3 S cm-1, respectively. Obviously, the PVdF/TPU/PAN (2/2/1) GPE has the largest ionic conductivity of 6.91×10-3 S cm-1. It is much larger than the value ∼3.6×10-3 S cm-1 reported by Gophlan et al. 32 for Poly(vinylidene fluoride)–polydiphenylamine (PVdF-PDPA) electrospun membrane wetted in 1M LiClO4-PC electrolyte and the value ∼4.7×10-3 S cm-1 reported by Li et al. 33 for poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) incorporating LiPF6-EC/PC gel polymer electrolyte system.

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Table 1. Ionic Conductivity and Porosity of Different Films

samples

surface area (cm-2)

thickness (cm)

ionic conductivity (mS·cm-1) at 25 °C

porosity (%)

1/1/1

1.96

0.02

2.13

45

2/2/1

2.0

0.018

6.91

88

3/3/1

1.95

0.02

4.88

73

From the SEM images of the fibrous PVdF/TPU/PAN membranes in Fig. 1, we can see that many interconnected pores exist in the electrospun membranes. The existence of porous structure is beneficial to absorption of electrolyte solution to form gel polymer electrolytes. The decrease of the AFD also contributes to the high electrolyte uptake and the increase of ionic conductivity. The electrolyte is well encapsulated in the swollen phase, besides the pore walls in the fibrous membrane are also wetted excellently by LiPF6-EC/DMC electrolyte. Mechanical Performance Fig. 6 reflect the mechanical properties of the electrospun membranes. The PVdF/TPU/PAN (1/1/1) fibrous film shows a breaking tensile strength of 9.37 MPa, while the PVdF/TPU/PAN (3/3/1) membrane is 9.74 MPa and the PVdF/TPU/PAN (2/2/1) film is 10.31 MPa. It implies higher toughness of the PVdF/TPU/PAN (2/2/1) blend membrane. The elongation reaches up to 85.15% (1/1/1), 97.98% (3/3/1) and 102.51% (2/2/1), respectively. The increase of the break strength and elongation at break for PVdF/TPU/PAN (2/2/1) membrane may be attributed to the addition of TPU content. TPU belongs to an elastomer class possessing high tensile strength and elasticity. The microstructure of TPU comprises the hard section and the soft section. The hard phase and the soft phase are interconnected in the entire polymer molecular chain. In addition, the hard segments are responsible for maintaining the GPEs’ 13



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dimensional stability. The decrease of the break strength and elongation at break for the PVdF/TPU/PAN (3/3/1) membrane may be due to the decrease of the percentage of PAN content. However, further study is necessary to explore the cause.

Fig. 6 Stress–strain curves of electrospun PVdF/TPU/PAN membranes (a) 1/1/1, (b) 2/2/1 and (c) 3/3/1. Table 2. Mechanical Properties of All Electrospun Fibrous Membranes

samples

thickness (µm)

tensile strength (MPa)

elongation at break (%)

1/1/1

100

9.37

85.15

2/2/1

100

10.31

102.51

3/3/1

100

9.74

97.98

Evaluation in Li/LiFePO4 Cells Fig. 7 gives the initial charge-discharge state of the Li/LiFeO4 cells containing GPEs of PVdF/TPU/PAN with different weight ratios. It is the typical characteristic 14


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of LiFePO4.

34

The theoretical specific capacity of LiFePO4 is 170 mAh g-1. The

PVdF/TPU/PAN (1/1/1) GPE and the PVdF/TPU/PAN (3/3/1) GPE deliver charge capacities of 163.69 mAh g-1 and 165.91 mAh g-1, respectively. Accordingly, the discharge capacities are 162.41 mAh g-1 and 165.27 mAh g-1. While the GPE with PVdF/TPU/PAN (2/2/1) shows the highest charge capacity of 168.56 mAh g-1 and the highest discharge capacity of 167.52 mAh g-1, which may be due to the high ionic conductivity. We all know the high ionic conductivity facilitates the repeated intercalation/deintercalation of carrier ions in/from the electrode materials. Furthermore, we study the rate capability of the PVdF/TPU/PAN (2/2/1) GPE, and the results appear in Fig. 8. We have known the charge and discharge capacities of LiFePO4 are 168.56 and 167.52 mAh g-1 at 0.1 C. With the increase of current density, the polarization is greatly enhanced, and the discharge capacities decrease accordingly. However the anode material still deliver high discharge capacities of 164.51 mAh g-1, 161.90 mAh g-1 and 153.23 mAh g-1 at relatively larger current rates of 0.2 C, 0.5 C and 1 C, respectively. We also evaluate the cycle stability of the three different GPEs under 0.1 C rate at 25 ºC. It can be seen from Fig. 9 that the discharge capacity gradually decreases as the cycle number increases. Compared to the cells with GPEs containing PVdF/TPU/PAN (1/1/1) and PVdF/TPU/PAN (3/3/1) respectively, the cell owning PVdF/TPU/PAN (2/2/1) polymer electrolyte shows higher discharge capacity and better capacity retention. After 50 cycles, the cell with the GPE based on PVdF/TPU/PAN (2/2/1) retains 96.2% of initial discharge capacity, while the batteries with GPEs based on PVdF/TPU/PAN (1/1/1) and PVdF/TPU/PAN (3/3/1) nanofibrous membranes deliver 87.6% and 89.3% of the first discharge capacity, respectively. The capacity attenuation of the cell with GPE based on PVdF/TPU/PAN (2/2/1) is very small after 15



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dozens of cycles. It proves a better reversibility. This may be due to the higher electrolyte uptake and more facile lithium ions transfer. Finally, we conduct the morphology test of the GPE based on the PVdF/TPU/PAN (2/2/1, wt.%) electrospun film after 50 charge/discharge cycles. The SEM image is exhibited in Fig. 10. We find that the appearance of the PVdF/TPU/PAN (2/2/1) GPE is well retained, still maintaining the original three-dimensional network and hierarchically mesoporous structure, which guarantees the fast transport of lithium ions. The AFD of the PVdF/TPU/PAN-based GPE (2/2/1) after the 50th cycle is about 600 nm, which is slightly greater than that before the charge and discharge test. It can be attributed to the presence of the swollen polymer chains. 31 Electrochemical Stability Fig. 11 shows the electrochemical stability test of the GPEs by LSV. The PVdF/TPU/PAN (1/1/1) and PVdF/TPU/PAN (3/3/1) fibrous polymer electrolytes exhibit the electrochemical stability of up to 5.4 V and 5.5 V, respectively. While the stability of the PVdF/TPU/PAN (2/2/1) GPE is further enhancing. Their electrochemical stability is ranked as follows: the membrane of PVdF/TPU/PAN (1/1/1) (5.4 V) < PVdF/TPU/PAN (3/3/1) (5.5 V) < PVdF/TPU/PAN (2/2/1) (5.7 V). The values of electrochemical stability for PVdF/TPU/PAN-based GPEs are much higher than the value ∼4.8 V reported by Li et al. 33 for PVdF-HFP incorporating 1 M LiFP6-EC/DMC gel polymer electrolyte system and the value ∼4.7 V reported by Jung et al.

35

for poly(methyl methacrylate)/polyvinyl chloride (PMMA/PVC) wetted

by EC/DMC of 1 M LiFP6 polymer electrolyte system. The better anodic stability of PVdF/TPU/PAN-based GPEs is in part due to the excellent affinity to the carbonates (EC and DMC) of the liquid electrolyte. The interactions between the ester groups of the carbonate molecules and VdF groups of 16


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PVdF or -CN groups of PAN contribute significantly to the enhancement of the electrochemical stability. What’s more, the increase in stability of the electrolytes may result form large surface areas of PVdF/TPU/PAN-based swollen fibers, although the ionic conduction is carried out through the entrapped liquid electrolytes in a fully interconnected pore structure. In other words, electrochemical stability of the GPEs increases when the AFD decreases. The values of decomposition potential show that the blend PVdF/TPU/PAN (2/2/1) polymer electrolyte prepared in this study is more suitable for application in lithium ion batteries than the other two samples. CONCLUSIONS The three kinds of GPEs based on PVdF/TPU/PAN with different mass ratios were prepared by electrospinning technique in this study. The optimum blend composition has been observed as PVdF/TPU/PAN (2/2/1, wt.%) for the electrolyte by performing a series of tests. Firstly, the PVdF/TPU/PAN (2/2/1) based GPE has the highest ionic conductivity of 6.91 × 10−3 S cm−1 among the three samples. Besides, the excellent mechanical performances to make it a outstanding separator applied in rechargeable polymer lithium ion batteries. Finally, the cell with the GPE of PVdF/TPU/PAN (2/2/1), its first charge and discharge capacities can be up to 168.56 mAh g-1 and 167.52 mAh g-1, and after 50 cycles, its capacity drops very little. The cell shows a high discharge capacity retention percentage under current constant voltage condition. Considering these results, the PVdF/TPU/PAN-based GPE (2/2/1, wt.%) appears as a promising candidate for polymer lithium ion batteries.

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Fig. 7 First charge-discharge capacities of the GPEs based on the three different fibrous membrane (a) 1/1/1, (b) 2/2/1 and (c) 3/3/1.

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Fig. 8 First charge–discharge capacities of the PVdF/TPU/PAN (2/2/1, wt.%) GPE at different capacity rates (1C = 170 mA h g -1).

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Fig. 9 The cycle stability of GPEs based on the three different electrospun membranes (a) 1/1/1, (b) 2/2/1 and (c) 3/3/1.

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Fig. 10 Typical SEM image for the PVdF/TPU/PAN-based GPE (2/2/1) after 50 cycles.

Fig. 11 Linear sweep voltammograms of the gel polymer electrolytes based on electrospun PVdF/TPU/PAN membranes (a) 1/1/1, (b) 2/2/1 and (c) 3/3/1.

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Gel Polymer Electrolyte Based on Poly(vinylidene fluoride

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