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A High-Performance Electrospun Poly (propylene carbonate)/ Poly (vinylidene #uoride) Gel Polymer Electrolyte for Lithium Ion Batteries Xueyan Huang, Songshan Zeng, Jingjing Liu, Ting He, Luyi Sun, Donghui Xu, Xiaoyuan Yu, Ying Luo, Wuyi Zhou, and Jianfeng Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09130 • Publication Date (Web): 12 Nov 2015 Downloaded from http://pubs.acs.org on November 15, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

A High-Performance Electrospun Poly (propylene carbonate)/ Poly (vinylidene fluoride) Gel Polymer Electrolyte for Lithium Ion Batteries Xueyan Huang a, Songshan Zeng b, Jingjing Liu b, Ting He a, Luyi Sun b, Donghui Xu a, Xiaoyuan Yua*, Ying Luoa, Wuyi Zhoua**, Jianfeng Wuc a

Institute of Biomaterial, College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China b

Department of Chemical & Biomolecular Engineering and Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, United States c

State Key Laboratory of Motor Vehicle Biofuel Technolog，Nanyang, 473000, China

ACS Paragon Plus Environment

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Abstract: A novel high-performance gel polymer electrolyte (GPE) based on an electrospun polymer membrane of poly (propylene carbonate)/ poly (vinylidene fluoride) (PPC/PVdF) was prepared and investigated for high-performance lithium ion battery applications. This study presents a methodology for introducing PPC into PVdF based GPEs designed for highperformance lithium ion batteries. SEM images and porosity measurements showed that the electrospun membrane had a uniform and highly interconnected porous structure with an average fiber diameter of 300~850 nm. Such a morphology resulted in excellent electrolyte uptake amount (500 wt%) and retention in PVdF/PPC membrane. The DSC result indicated that the PVdF crystallinity was deteriorated by the incorporation of PPC. The PVdF/PPC electrospun membrane showed significantly higher ionic conductivity (4.05 mS cm−1) than that of the PVdF electrospun membrane (2.11 mS cm−1) at 30°C. The PVdF/PPC GPE is stable at the potential higher than 5.2 V (versus Li+/Li). The capacity of Li/CGE-20/LiFePO4 was 160, 151, 133, 119, and 102 mAh g−1 at the charge/discharge rate of 0.1, 0.2, 0.5, 1, and 2 C, respectively.


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INTRODUCTION Lithium ion batteries (LIBs) have been considered as a main energy storage device for hybrid electric vehicles (HEVs) and electric vehicles (EVs) owing to their high energy density, long cycle life, and minimal memory effects.

1-4

One of the critical components that affect the safety

of LIBs is the liquid electrolyte. Commercial LIBs use oligomers like aliphatic carbonic acid esters as the main component of electrolyte which is highly hazardous, flammable and explosive.5-6 Hence, the improvement in electrolyte composition is critical for the increasing demand for the safety of LIBs. 7 Different levels of successes in this field have been achieved by introducing the polymer electrolytes with no-leakage, high flexibility and high physical and chemical stability.8 Among these, gel polymer electrolytes (GPEs) have been considered as a promising candidate because of their lower risk of electrolyte leakage and higher ionic conductivity (ca.10-3 S cm-1, over tenfold than that of solid polymer electrolytes (SPEs)), which can potentially meet the demanding requirements in HEVs and EVs. So far, several different methods of preparing GPEs have been investigated, including phase inversion method, the Bellcore process, γ-ray irradiation method, solvent casting technique, thermally induced phase (TIPS) technique, and electrospinning.9-11 Among them, electrospinning technique is probably the most popular method to prepare the GPEs with good mechanical properties and high ionic conductivity. A facile, controllable, and low-cost technique, the electrospinning allows for the production of continuous GPEs by applying a strong electric field between the nozzle and collector when injecting a polymer solution. The resulting electrospun membranes contain numerous nanofibers with interconnected pores, providing sufficient channels for ion conduction.


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Recently, poly (vinylidene fluoride) (PVdF) oxide) (PEO)

16,17

12,13

, polyacrylonitrile (PAN)

, poly (methyl methacrylate) (PMMA)

(TPU)19,20 and poly (propylene carbonate) (PPC)

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21-23

18

14,15

, poly (ethylene

, thermoplastic polyurethane

have been used for the preparation of

polymer electrolytes. Notably, PPC has received considerable attentions due to its similar structure to carbonate-based solvents applied in conventional gel electrolytes 24, which suggests that it may have good compatibility with the lithium salts as well as offer good interfacial contact with commonly used electrodes. By comparison, PPC is also a type of completely biodegradable polymer which can contribute to prepare environmentally friendly polymer electrolytes.

25,26

Moreover, PPC is synthesized from carbon dioxide (CO2) and propylene oxide (PO) 27-29, which can efficiently reduce the amount of CO2 released into atmosphere, serving to mitigate the greenhouse effect. PPC has a polar group in the backbone structure: the ester group is beneficial for ionic conduction by trapping and storing liquid electrolytes. As a new amorphous aliphatic polycarbonate with a low glass transition temperature (Tg), the local relaxation and segmental motion of PPC chain is favorable for the transport of Li+ ions. The ester groups allow PPC chains to a solvent gelator owing to their strong interactions with the liquid electrolyte. However, PPC often shows high electrolyte uptake but low mechanical strength in the GPEs system, which restricts its success in practical battery application. To overcome this problem ， the most straightforward approach is to modify the PPC to achieve a better mechanical strength and thermal stability, while retaining a high volume fraction of the conductive amorphous phase and the outstanding electrolyte uptake. PVdF has also been widely studied due to its anodic stability, superior thermal stability, decent mechanical strength, outstanding electrochemical stability, and high affinity to liquid electrolytes. 30-32 It contains -C-F groups with the fluorine atoms thus high


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dielectric constant (ε =8.4).

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Hence, blending with PVdF is beneficial in dissociating lithium

salt and conducting lithium-ions in the polymer electrolytes. In this study, PVdF was blended with PPC by electrospinning technique. The presence of PVdF in the polymer blend enhanced the mechanical performance and thermal stability of the membranes. Given the high crystallinity of PVdF, blending with the amorphous PPC is an effective way to reduce the crystallinity and facilitate ion motion in the polymeric framework. The effect of the ratio of PPC to PVdF on the properties of the microporous membrane including morphology, porosity, liquid uptake capability were systematically studied. We also investigated the ionic conductivity, electrochemical performance, cycle performances, and rate capabilities of different PVdF/PPC electrospun fibrous polymer electrolytes for lithium ion batteries. The results show that our developed route has several distinct advantages: 1) the electrospun membranes present a threedimensional network porous structure, which can greatly enlarge the specific surface area, promote the adsorption electrolyte and gelatinization efficiently; 2) the segregation of PVdF chains by polymeric chains of PPC can reduce the crystallization of PVdF-based polymers and increase the segmental mobility of the polymer, which benefit for the transport of lithium ions; 3) the formation of Li+••• (δ−) F-C (δ+) and PF6−••• (δ+) C=O (δ−) complexes can separate the Li+ and PF6− anions, which prevent the reconnection between Li+ and PF6− anions simultaneously and create more free Li+, thus the ability of lithium ion transference is improved. Therefore, the large surface area, well-developed microporous structure, sufficient electrolyte uptake, low crystallinity and appropriate porosity allow the electrospun PVdF/PPC polymer electrolyte exhibit a significantly higher ionic conductivity and excellent electrochemical performances.


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EXPERIMENTAL SECTION Materials. PPC (average Mw = 120k, from Henan Tianguan, China) and PVdF (Kynar 760, from Arkema) were dried under vacuum at 80°C for 24 h prior to using. 1.0 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, v/v) was purchased from Tinci Materials Technology Co., Ltd. Acetone (A.R.) and N, N-dimethylforamide (DMF) (A.R.) were purchased from Baishi Chemical Industry Co., Ltd. All the solvents were used as received without further treatment. Preparation of Nanofiber Membrane. The electrospun PVdF and PVdF /PPC fibrous composite membranes were prepared by electrospinning technique as follows. Mixtures of PVdF and PPC at different weight ratios (100:0; 90:10; 80:20; 70:30 and 60:40) were dissolved in a mixed solvent of DMF/acetone (1:1, w/w) and magnetic stirred at room temperature for 24h at concentration of 12 wt%. After that, the resulting homogeneous solution was transferred to a plastic syringe and electrospun on a spinning machine (KATO Tech. Co., Japan) at ambient atmosphere. A high voltage of 17 kV was applied to the needle tip with a flow rate of 0.5 mL h−1. The collector placed at a distance of 15 cm to collect a nanofiber film on an aluminium foil wrapped around a drum collector rotating at a speed of 10r/s. The average thickness of electrospun membranes was ca.150 µm. Subsequently, the membranes were dried under vacuum at 60°C for 24h to remove the residual solvents and prevent the collapse of the fiber structure. The obtained nanofibrous membranes are denoted as PVdF-100 and PPC-X, where X is 10, 20, 30 and 40, representing the weight concentration of PPC in polymer blends. After being activated by liquid electrolyte, the GPEs are donated as VGE-100, CGE-10, CGE-20, CGE-30 and CGE-40, respectively.


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Characterization. Surface microstructures and morphologies of the membranes were characterized using a scanning electron microscope (SEM, JEOL JSM-6380LA). IR spectra (4000-500 cm-1, resolution 2 cm-1) of the samples were acquired on a Nicolet Magna II 550 spectrophotometer. Differential scanning calorimetry (NETZSCH DSC-200PC Instrument) analysis was carried out under N2 atmosphere to study the phase transitions and thermal properties of the dry membranes. The samples were subjected to repeated heating/cooling cycle in the range of -10°C to 200°C at a rate of 10°C min-1. The results were recorded on the second heating DSC curve. Based on the DSC data, the crystallinity of the samples was calculated according to Eq. (1): X c (%) =

∆H Sample m × 100% ∆H *

Eq. (1)

where Xc is crystallinity, ∆HmSample is the enthalpy of fusion of the polymer samples, ∆H* is the enthalpy of fusion for the totally crystalline PVdF, 104.7 J g−1. The porosity of the samples was determined by immersing the membranes into n-butanol for 1 h and then calculated by using Eq. (2): Porosity (%) =

W wet - W dry ρ b V dry

× 100%

Eq. (2)

where Wwet and Wdry represent the weights of the separator before and after immersion in nbutanol, ρb is the density of n-butanol, and Vdry is the apparent volume of the dry membrane. The electrolyte uptake (%) was analyzed by measuring the weight increase of the membranes and then calculated by using Eq. (3): Uptake (%) =

W - W0 × 100 % W0


Eq. (3)

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where W0 and W represent the weights of the membranes before and after soaking the membranes in a liquid electrolyte (1.0 M LiPF6 in EC/ DMC (1:1, v/v)). In order to study the dimensional stability, dried membranes of known volume, V, was immersed in liquid electrolyte for 1 h at 25 °C. The swollen membranes were removed from the liquid electrolyte and the excess electrolyte removed using a filter paper. The volume of the swollen membranes, V0, was recorded and the swelling ratio was calculated according to Eq. (4): Swelling

ratio (%) =

V - V0 × 100 % V0

Eq. (4)

where V and V0 are the volume of the swollen and dry membrane, respectively. Electrochemical measurements. The ionic conductivity (σ) of gel polymer electrolytes (GPEs) was measured by AC impedance measurement using a Zahner Im6ex electrochemical analyzer at the amplitude of 5 mV over a frequency range of 0.1 Hz–1.0 MHz. The polymer electrolytes were cut into 2 cm2 size and sandwiched between two blocking stainless steel electrodes for impedance measurement. The ionic conductivity was calculated using Eq. (5): σ=

d(cm) R b ( Ω )S( cm 2 )

Eq. (5)

where σ is the ionic conductivity, Rb is the bulk resistance, d and S are the thickness and area of the GPEs, respectively. Furthermore, ionic conductivity of the GPEs was also measured between 30°C and 90°C. To study the interface phenomena, time dependant electrode/electrolyte interfacial resistance (Rf) between the electrolyte and the electrodes was evaluated by investigating the resistance change of the cell Li/GPE/Li based on electrospun PVdF/PPC, PVdF and Celgard PE membranes at room temperature over a period of 7 days.


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Linear sweep voltammetry (LSV) was determined by using a stainless steel working electrode and lithium foil as the counter electrode at a scanning rate of 0.5mV s−1 over the potential range of 2.0–6.0 V vs. Li/Li+. The cathode mixture was composed of 80% LiFePO4 powder, 10% conductive carbon and 10% PVdF binder with N-methyl-2-pyrrolidone (NMP). The slurry was coated on aluminum foil using the doctor blade technique and dried at 80 °C for 24 h in vacuum for further use. The fabrication of test cells was carried out in an argon-filled glove box with oxygen and moisture level lower than 0.1 ppm. The charge–discharge tests of the Li/GPEs/LiFePO4 2025 coin cells were carried out using Battery Testing System (BTS XWJ, Neware Tech. Co., Ltd. China). All electrochemical measurements on the Li/LiFePO4 batteries were conducted between 2.5 and 4.0 V at room temperature with various current densities of 0.1, 0.2, 0.5, 1.0 and 2.0 C at room temperature.

RESULTS AND DISCUSSION Characterization of PVdF/PPC Electrospun Membranes. Figure 1 shows the IR spectra of the pure PPC, pure PVdF membrane, and PVdF/PPC composite membrane. The characteristic absorption peaks of PPC were observed at 3000-2800 cm-1 (C-H stretching in CH2 and CH3), 1747 cm-1 (stretching vibration band of C=O), 1453 cm-1 (C-H bending in CH2), and 1233 and 1069 cm-1 (stretching vibration band of C−O−C). The characteristic peaks of PVdF were at 3020-2800 cm-1 (stretching vibration band of C-H), 1400 cm-1 (bending vibration band of -CH2), 1071 cm-1 (stretching band of C-C), 883 cm-1 (band for amorphous phase) and 841 cm1

(bending vibration band band of -CH2-). The spectra of the PVdF/PPC composite membrane


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showed all the characteristic peaks of PPC and PVdF, indicating the composite membrane was successfully prepared.

Figure 1. FTIR spectra of (a) PPC; (b)PVdF/PPC; (c)PVdF. As it is well known that a number of parameters influence the size and morphology of electrospun nanofibers, which include the applied voltage, flow rate, gap between the tip of the needle and collection target, molecular weight and molecular weight distribution of the polymers, the formation, concentration of polymer solution. In the present study, all of the above parameters were kept constant except for the molecular weight and molecular weight distribution of the polymers and the formation of polymer solution. Figure 2a-e shows the morphology of the porous electrospun PVdF and PVdF/PPC membranes with different weight ratios. The electrospun membranes present a three-dimensional network composed of nanofibers with random orientation, which results in a porous structure. Presence of a porous structure in the membrane can greatly increase the specific surface area, accordingly leading to an enhanced the adsorption capacity of liquid electrolyte and gelatinization efficiently as it soaked in electrolyte solution. Moreover, the porous structure enables the formation of numerous ionic transport channels and provides a path way for fast Li+ ions transport. The nanofibrous membrane can hold


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solvent molecules to avoid solvent leakage or evaporation. Meanwhile, the fiber structure assures the GPE of its mechanical integrity. The as-spun membranes can be peeled off readily from the aluminum foil as a free-standing nanofiber mat for the application of the separator in lithium ion batteries. The average fiber diameter (AFD) of the electrospun membranes was measured to be between 300 and 850 nm. It can be seen that the average diameter of nanofibers decreased as the ratio of PPC to PVdF increased.

Figure 2. SEM images of the electrospun membranes (a) PVdF-100; (b) PPC-10; (c) PPC-20; (d) PPC-30, (e) PP-40, and (f) the variation of electrolyte uptake of Celgard PE and the electrospun membranes Porosity and wettability of the nanofibrous membrane are two important factors for the performance of polymer lithium-ion batteries. It is well known that porosity is strongly


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dependent on the fiber diameter and density. In this study, the smaller fiber size of PVdF/PPC membrane resulted in a higher porosity of 87% (PPC-40), as compared with PVdF-100 (77%), PPC-10 (80%), PPC-20 (85%) and PPC-30 (85.4%). Apparently, the electrospun membranes possess a higher porosity than that of Celgard PE (only about 42%) because its pore size is apparently anisotropic with one dimension. Figure 2f displays the electrolyte uptake of typical electrospun membranes and commercial separator. All the electrospun membranes quickly reached their maximum uptake within 20 min, while it took Celgard PE more than 40 min. The uptake amount of PVdF-100, PPC-10, PPC-20, PPC-30, and PPC-40 electrospun membranes can respectively reach 422%, 492%, 501%, 471%, and 450%, which are significantly higher than that of Celgard PE (147 %). This indicates that the three-dimensional network morphology and the high porosity of electrospun membranes allow fast liquid infusion into the membrane. Nevertheless, PPC-40 electrospun membrane which has the highest porosity, did not show the biggest electrolyte uptake, so did PPC-30 electrospun membrane. Thus, proper level of porosity is necessary for separator due to the leakage and uneven current distribution of electrolyte in excessively high porous film. A good dimensional stability of the GPEs during soaking in electrolyte solution is crucial to the safety of lithium-ion battery. Unfortunately, the PVdF-100 electrospun membrane obviously swelled after trapping the liquid electrolyte as shown in Figure 3. It is believed that two-phases exist in PVdF based GPE system, i.e., the swollen polymer chains and liquid electrolyte retained in the cavities of the porous polymer membrane. 30 Due to the high crystallinity of the pristine PVdF, it generally tends to the swollen polymer chains for reducing its crystallinity. It can be proved that lowering the crystallinity of the PVdF based GPE can reduce its volume expansion ratio.


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Figure 3. Photographs of (a) PVdF-100 and PPC-40 before trapping the liquid electrolyte; and (b) VGE-100 and CGE-40 after trapping the liquid electrolyte. Figure 4 displays the DSC thermograms of the prepared electrospun membranes, and the calculated thermodynamic data with the corresponding volume expansion rate are listed in Table 1. The effect of incorporation of PPC on the crystallinity of PVdF was investigated through the thermal properties of the polymer membranes. Along with an increasing PPC content, the melting enthalpies (∆Hm) decreased as shown in Table 1. The crystallinity of PVdF-100, PPC10, PPC-20, PPC-30 and PPC-40 membrane were calculated to be 20.4%, 19.1%, 15.6%, 14.7% and 11.2%, respectively. This reduction in crystallinity may result from partial inhibition of PPC chains on the crystal formation of PVdF segment, which will be discussed in detail in Figure 6 below. The segregation of PVdF chains by polymeric chains of PPC reduced the likelihood of crystallization in PVdF-based polymers and created free volume for the transport of lithium ions. Besides, the volume expansion rate of the electrospun membranes decreases as the crystallinity


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was deteriorated, shown that the volume expansion rate is further decreased by blending with PPC. The incorporation of PPC changed the formation of the polymer chains, which led to the generation of cavities in the porous membrane. And the cavities can effectively trap the liquid electrolyte rather than being overly swollen. Furthermore, the liquid electrolyte retained in the cavities is beneficial for high ionic conductivity, whereas the swollen polymer chains locking the liquid electrolyte inside will hinder the transportation of Li+ ion and eventually lower the ionic conductivity.

Figure 4. DSC curves of the electrospun membrane (a) PVdF-100; (b) PPC-10; (c) PPC-20; (d) PPC-30; (e) PPC-40. Table 1. Tm, ∆H, Crystallinity, and volume expansion rate of the electrospun membranes

Sample name

Melting potin Melting enthalpy Crystallinity Volume expansion rate/% Tm/°C ∆Hm/(J·g-1) Xc/%

PVdF-100

162.3

21.4

20.4

16.3

PPC-10

168.3

20

19.1

10.8

PPC-20

167.4

16.3

15.6

6.0

PPC-30

167.4

15.4

14.7

5.1

PPC-40

167.3

11.7

11.2

2.4


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Electrochemical Performance of PVdF/PPC Electrospun Membranes. The ionic conductivity was determined by electrochemical impedance spectroscopy analysis in this work. Figure 5(a, b and c) shows the AC impedance spectra of the commercial Celgard PE VGE-100, and CGE-20 at 30 °C to 90 °C. The impedance spectra of all samples are almost linear along with the real x-axis. The disappearance of semicircular portion in the high frequency range suggests that the current carriers are ions. 34 The typical plot of ionic conductivity versus inverse temperature (1000/T) for gel polymer electrolyte was depicted in Figure 5d. In general, the ionic conductivities of all samples increase with the increase of temperature, obeying the Arrhenius plot of conductivity, which suggests that the conductivity is thermally activated and indicate that blending PPC with PVdF enhanceds solvent storage performance of the polymer framework resulting GPE is more thermally stable. The data of electrolyte uptake amount and ionic conductivity are summarized in Table 2. The ionic conductivity values were observed to increase with an increasing electrolyte uptake. Besides, the conductivity of GPEs is higher than that of Celgard PE. For example, the ionic conductivity of VGE-100, CGE-10, CGE-20, CGE-30 and CGE-40 were about 2.11, 3.87, 4.05, 3.15 and 2.43 mS•cm-1 at 30 °C, respectively, which are higher than that of the Celgard PE separator (0.66 mS•cm-1) and are valuable for practical applications. There are several conflicting factors that affect the ionic conductivity of the electrolyte solutions. In this study, as the ion transport in LiPF6 EC/DMC solution is mainly achieved by Li+ ions, the high GPE conductivity is caused by the higher lithium ion concentration per repeating unit and low crystallinity of the polymer network.


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Figure 5. The AC impedance spectra of (a) Celgard PE separator, (b) VGE-100 and (c) CGE-20 at the temperature range from 30°C to 90 °C. And (d) the Arrhenius plots of the ionic conductivity vs. temperature of Celgard PE; VGE-100; CGE-10; CGE-20; CGE-30 and CGE-40. Table 2. Electrolyte uptake amount, ionic conductivity and porosity of the PVdF-100, PPC-10, PPC-20，PPC-30，PPC-40 membrane, and commercial separator

Sample name

Electrolytes uptake Ionic conductivity Porosity（ amount (wt %) (mS·cm-1) at 30°C %）

Celgard

147

0.66

42

PVdF-100

422

2.11

77

PPC-10

492

3.87

80

PPC-20

501

4.05

85

PPC-30

471

3.15

85.4

PPC-40

450

2.43

87


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The Li+ ions within the GPEs were mainly solvated by the C=O and -C-F atoms because of their high electronegativity which dissociated lithium salt and hindered the movement of large PF6− anions. The Li+ and PF6− can interact with the polymer in the GPE, respectively, as shown in Figure 6c.

According to the previous literature

24,32,35

, PVdF and PPC dissociate the

electrolytic lithium salt through the formation of Li+••• (δ−) F-C (δ+) and PF6−••• (δ+) C=O (δ−) complexes in the polymer framework, respectively. The fluorine atom and carbonyl oxygen can separate the Li+ and PF6− anions in the composite PVdF/PPC gel polymer electrolyte because of the strong interaction, which prevent the reconnection between Li+ and PF6− anions simultaneously and create more free Li+. As such the ability of lithium ion transference is improved. Moreover, the ether linkages of PPC polymer chains can also facilitate Li+ transport through the interaction between the oxygen atoms in the ether linkages and the Li+ weakening the bonding between fluorine atoms and Li+. Thus, the Li+ transport is descried as the motion of free Li+ between fluorine atom of PVdF and oxygen atoms of PPC (see Figure 6d). On the other hand, the carbonate solvent molecules reduce the Li+ complexation degree with the oxygen and fluorine atoms, building the free volume phase for the segmental mobility of the polymer, which is useful for conducting Li+ ions in the polymer electrolyte and adequate for rechargeable lithium battery. Except for the electrolyte uptake, the crystallinity of polymer network also contributes to the ion conductivity. Compared with other gel polymer electrolytes blended with PPC, the ion conductivity of VGE-100 is lower than that of them owing to the high crystallinity of pure PVdF hindering the segmental mobility of the polymer and retarding the transport of Li+ ions. As shown in Figure 6(a and b), the segregation of PVdF chains by polymeric chains of PPC reduces the likelihood of crystallization in PVdF-based polymers, increases the segmental mobility of the polymer and benefit the transport of lithium ions.


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Figure 6. Conceptual illustration of the Polymer frameworks of (a) PVdF and (b) PVdF/PPC. Schematic illustration of main interaction forms between the ions and the polymer in the GPE. (c) Interaction of Li+ between PF6− and the polymer chains; (d) Transport of Li+ ions was associated with the ether linkages in the PPC and the fluorine atom in the PVdF. The thermal shrinkage of separators at elevated temperatures is very crucial for Li-ion battery application. Figure 7 (a and b) shows the digital photos of the thermal shrinking test of PPC-20 porous polymer membrane in comparison with a commercial Celgard PE separator. After being heated to 150 °C for 3 h, the thermal shrinkage of PPC-20 separator was almost negligible, while the Celgard PE separator showed significant shrinkage as high as 36%. The poor performance of the thermal shrinkage of the commercial Celgard PE separator originating from its relatively low softening temperature may make the battery more susceptible to explosion at higher temperature conditions when the internal short circuiting occurs. In comparison, the PPC-20 separator can significantly reduce this potential risk during battery operation owing to its high thermal tolerance. This indicates that the three-dimensional network and the nanofibrous structure make


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the electrospun membranes more thermally stable than that of the commercial Celgard PE separator.

Figure 7. The digital photos of the thermal shrinking test of Celgard PE and PPC-20 (a) before; and (b) after heat treatment. The electrochemical stability of Celgard PE and the electrospun membranes was assessed by a linear sweep voltammetry (LSV) experiment under the scanning range between 2.0−6.0 V. As shown in Fig.8, there is no obvious reaction peak up to 5 V vs. Li+/Li for the GPEs, while the oxidation potential of Celgard PE is ca. 4.7V. By LSV test, this result indicates that the gel polymer electrolyte has a good oxidation stability in the 2.0−5.0 V operating voltage environments which allows it to be more applicable for lithium-ion battery.


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Figure 8. The linear sweep voltammogram of (a) Celgard PE; (b) VGE-100; (c) CGE-10; (d) CGE-20; (e) CGE-30; and (f) CGE-40. The interfacial compatibility of the electrolyte with lithium metal electrode is an essential factor to guarantee the acceptable performance in the lithium ion batteries. Figure 9d presents the variation of the interfacial resistance of cell Li/GPE/Li based on Celgard PE, VGE-100, CGE-10, GE-20, CGE-30 and CGE-40 for a period of 7 days at room temperature. It should be noted that the interfacial resistance significantly increased with time. The AC impedance behaviors of cell Li/GPE/Li at open circuit potential with VGE-100, CGE-20 and Celgard PE separator are presented in Figure 9(a, b and c). The bulk resistance (Rb) value of both cells composed of electrospun GPEs and commercial Celgard PE separator were fairly similar (~3 Ω). However, the electrode/electrolyte interfacial resistance (Rf) value of the Celgard PE separator was significantly larger than that of all GPEs immediately after cell assembly. After a period of storage of 7 days, there was no significant increase in Rb. By contrast, the Rf values (Ω) increased following the order CGE-20 (283) < CGE-10 (315) < VGE-10 (385) < CGE-30 (400) < CGE-40 (432) < Celgard PE (761). This characteristic is influenced by the dynamic formation of a passivation layer between carbonate solvent molecules at the lithium electrode. To obtain


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fresh solvated cations with Li+ ions, lithium metal has to decompose the carbonate solvent molecules that forms a solid-electrolyte interphase (SEI) layer on the lithium-metal electrode.

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But running the cell constantly will produce more Li+ ions resulting in the growth of a SEI layer and a passivation layer on the lithium electrode surface. Hence, the Rf increased with time when the layer was growing to hinder Li+ ions transport to the lithium electrode surface. In the GPEs system, the polymer network can replace carbonate molecules to solvate Li+ ions with C=O of PPC and -C-F of PVdF on the electrospun membranes, postponing the interaction of carbonate molecules with lithium metal and suppressing the formation of SEI layer. So the smaller Rf value can be ascribed to a thinner SEI layer in the GPEs system (relative to that in the Celgard PE). Moreover, the low GPEs Rb and Rf values are beneficial for the cyclic stability and rate capacity of the lithium ion batteries.

Figure 9. The AC impedance behaviors of cell Li/GPE/Li at open circuit potential with (a) VGE100, (b) CGE-20 and (c) Celgard PE separator; The variation of interfacial resistance of cell Li/GPE/Li at various storage times with (d) different electrolytes.


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The electrochemical performances of the lithium ion battery with different electrolytes based on Celgard PE, VGE-100, and CGE-20 were further tested via coin type cells. The charge– discharge profiles (current density = 0.1C) of Li/LiFePO4 cells assembled with Celgard PE, VGE-100 and CGE-20 in the first, second, 20th, 50th and 100th cycles are exhibited in Figure 10 (a, b and c) . The Li/LiFePO4 battery assembled with CGE-20 delivered the first-cycle discharge capacity of 156 mA h g-1 between the cut-off voltage 2.5 and 4.0 V, which is 92% of the theoretical capacity of LiFePO4 (170 mA h g-1) and larger than the cell with electrolytes based on Celgard PE (145 mA h g-1) and VGE-100 (149 mA h g-1). This is due to the higher electrolyte uptake, lower crystallinity and higher ionic conductivity in the CGE-20. At the second cycle, the discharge capacity of the cells assembled with VGE-100 and CGE-20 increased to 151 mA h g-1and 159 mA h g-1, respectively, while unaltered capacity can be observed in the cell with Celgard PE separator. This finding is possibly attributed to the different activation rates of the electrolytes. Due to the physical characteristics of GPE, it allows intimate contact at the interface between electrodes and electrolytes without penetrating into the entire cathode mixture, further explaining why the fabricated cell assembled with GPE exhibited activation rate lower than the liquid electrolyte. Besides, the charge and discharge plateaus of the cells composed of VGE-100 and CGE-20 are both around 3.4 V and the voltage difference between charge and discharge curves is much smaller than the direct use of Celgard PE separator, demonstrating the good compatibility and low interfacial resistance between the asprepared gel polymer electrolyte and the electrodes. Figure10d shows the cycling performances of lithium-ion batteries assembled with Celgard PE, VGE-100, and CGE-20 at the current density of 0.1 C. The cell with CGE-20 has a higher primary capacity and better cyclic stability than the cells with VGE-100 and Celgard PE separator. After 100 cycles, the capacity retention


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was found to be 93% for the VGE-100 and 90% for the Celgard PE, respectively. Compared with previous two kinds of cells, no obvious capacity degradation is observed for the cell with CGE20 and the discharge capacity reached to 157 mA h g-1 after 100 cycles. Such a high cycling performance of the cell with CGE-20 can be ascribed to the unique structure and the composition of the porous polymer membrane. The large surface area and the interconnected pores provide abundant ionic channels, high electrolyte uptake, and ionic conductivity, and the low crystallinity owing to the presence of PPC facilitate the ion transportation. These finally resulted in less capacity loss during the discharge and charge process and ensured a long and sufficient cycle life of lithium ion batteries.

Figure 10. Discharge/charge profiles of Li/GPE/LiFePO4 cells assembled with (a) Celgard PE, (b) VGE-100 and (c) CGE-20 at the current density of 0.1C between 2.5V and 4.0 V: the discharge curves in the first, second, 20th, 50th and 100th cycles. (d) Cycling performance of Li/GPE/LiFePO4 cells assembled with Celgard PE, VGE-100 and CGE-20 at 0.1C.


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Figure 11 shows the SEM images of the surface for the VGE-100 and CGE-20 membranes after 100th charge/discharge cycles. The appearance of the VGE-100 and CGE-20 were well retained without appreciable signs of mechanical stress, maintaining the original threedimensional network and hierarchically mesoporous structure. Specifically, the electrospun membranes with a porous structure readily relax the strain originated from mechanical stress during cycling. The electrospun membranes that offer ionic transport channels and provide a pathway for fast Li+ ions transportation enable to firmly maintain the shape of network and nanofibrous structure. The SEM images of the CGE-20 nanofibers show the AFD about 700nm similar to the pristine electrospun membrane before cycling, as shown in Figure 11b. By contrast, the VGE-100 nanofibers show volume expansion after cycling, which can be attributed to the presence of the swollen polymer chains.

Figure 11. Typical SEM images for (a) VGE-100 and (b) CGE-20 after 100 cycles. To further demonstrate the unique performance of the Li/LiFePO4 battery containing the CGE20, the battery performance at various current densities was also evaluated and shown in Figure 12a. The results of Celgard PE are presented for the comparison to and VGE-100 (Figure12b and c). The observed capacity of 160 mA h g-1 after 5 cycles at 0.1 C drops to 153 mA h g-1 at 0.2C. The capacity value decreased further at higher rates at 0.5, 1.0, and 2.0 C. After 25 cycles, when


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the current rate was reduced back to 0.2C, a reversible capacity of 149 mA h g-1 was obtained, showing the good rate capability of the battery with CGE-20. The sustainability and robustness of CGE-20 at high current density allows the material applicable in the high-power LIBs. In contrast, the Celgard PE and VGE-100 are exhibiting low capacity when the current rate increases to 2.0 C. This confirms that the battery using the GPE based on CGE-20 has better cycle and rate performances.

Figure 12. The rate performances of Li/GPE/LiFePO4 cells based on (a) CGE-20, (b) VGE-100 and (c) Celgard PE at various current densities (from 0.1, 0.2, 0.5, 1.0, 2.0 C).

CONCLUSIONS In summary, PVdF/PPC nanofibrous gel polymer electrolyte were successfully prepared by blending completely biodegradable poly (propylene carbonate) with tough poly (vinylidene fluoride) as the raw material through electrospinning technique along with activation in a liquid electrolyte. As a free-standing electrolyte for LIBs, the large surface area, well-developed microporous structure, sufficient electrolyte uptake, low crystallinity and appropriate porosity allow CGE-20 to exhibit a high capacity of 156 mA h g-1 in the first cycle, good cycling stability


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and rate capability. Given the simple fabrication and outstanding performance, CGE-20 represents a prospective high-performance gel polymer electrolyte for LIBs using in hybrid electric vehicles (HEVs) and electric vehicles (EVs).

AUTHOR INFORMATION Corresponding Author *

Tel: +86 20 8528 0323; Fax: +86 20 8528 5026; E-mail: [email protected] (X.Y.Yu)

**

Tel: +86 135 6034 4587; Fax: +86 20 8528 5026; E-mai: [email protected] (W.Y.Zhou)

ACKNOWLEDGMENT This research was financial supported by the Guangdong Science and Technology Planning Project (No. 2015A020209147 and 2014A010105038), the Guangdong Natural Science Foundation (grant No. 9151064201000039), the National Natural Science Foundation of China (Nos. 51003034 and 31101854), the Key Academic Program of the 3rd phase ‘211 Project’ (No. 2009B010100001), the State Key Laboratory of Motor Vehicle Biofuel Technology (No. 2013025) ， the Opening Project of the Key Laboratory of ApplicationTechnology of Environmental Photocatalysis of Hunan Province(ccsu-KF-1402) and the Air Force Office of Scientific Research (No. FA9550-12-1-0159). REFERENCES (1) Wang, S.; Kuo, P.; Hsieh, C.; Teng, H. Design of Poly (acrylonitrile)-Based Gel Electrolytes for High-Performance Lithium Ion Batteries. Acs Appl. Mater. Inter. 2014, 6, 19360-19370.


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TOC graphic


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Figure 1. FTIR spectra of (a) PPC; (b)PVdF/PPC; (c)PVdF. 230x182mm (300 x 300 DPI)



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Figure 2. SEM images of the electrospun membranes (a) PVdF-100; (b) PPC-10; (c) PPC-20; (d) PPC-30 and (e) PPC-40. And (f) the variation of electrolyte uptake of Celgard PE and the electrospun membranes. 95x107mm (300 x 300 DPI)


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Figure 3. Photographs of (a) PVdF-100 and PPC-40 before trapping the liquid electrolyte; and (b) VGE-100 and CGE-40 after trapping the liquid electrolyte. 75x90mm (300 x 300 DPI)



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Figure 4. DSC curves of the electrospun membrane (a) PVdF-100; (b) PPC-10; (c) PPC-20; (d) PPC-30; (e) PPC-40. 109x83mm (300 x 300 DPI)


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Figure 5. The AC impedance spectra of (a) Celgard PE separator, (b) VGE-100 and (c) CGE-20 at the temperature range from 30°C to 90 °C. And (d) the Arrhenius plots of the ionic conductivity vs. temperature of Celgard PE; VGE-100; CGE-10; CGE-20; CGE-30 and CGE-40. 243x184mm (300 x 300 DPI)



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Figure 6. Conceptual illustration of the Polymer frameworks of (a) PVdF and (b) PVdF/PPC. Schematic illustration of main interaction forms between the ions and the polymer in the GPE. (c) Interaction of Li+ between PF6− and the polymer chains; (d) Transport of Li+ ions was associated with the ether linkages in the PPC and the fluorine atom in the PVdF. 145x90mm (300 x 300 DPI)


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Figure 7. The digital photos of the thermal shrinking test of Celgard PE and PPC-20 (a) before; and (b) after heat treatment. 90x107mm (300 x 300 DPI)



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Figure 8. The linear sweep voltammogram of (a) Celgard PE; (b) VGE-100; (c) CGE-10; (d) CGE-20; (e) CGE-30; and (f) CGE-40. 251x185mm (300 x 300 DPI)


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Figure 9. The AC impedance behaviors of cell Li/GPE/Li at open circuit potential with (a) VGE-100, (b) CGE20 and (c) Celgard PE separator; The variation of interfacial resistance of cell Li/GPE/Li at various storage times with (d) different electrolytes. 120x90mm (300 x 300 DPI)



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Figure 10. Discharge/charge proﬁles of Li/GPE/LiFePO4 cells assembled with (a) Celgard PE, (b) VGE-100 and (c) CGE-20 at the current density of 0.1C between 2.5V and 4.0 V: the discharge curves in the ﬁrst, second, 20th, 50th and 100th cycles. (d) Cycling performance of Li/GPE/LiFePO4 cells assembled with Celgard PE, VGE-100 and CGE-20 at 0.1C. 118x90mm (300 x 300 DPI)


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Figure 11. Typical SEM images for (a) VGE-100 and (b) CGE-20 after 100 cycles. 176x67mm (300 x 300 DPI)



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Figure 12. The rate performances of Li/GPE/LiFePO4 cells based on (a) CGE-20, (b) VGE-100 and (c) Celgard PE at various current densities (from 0.1, 0.2, 0.5, 1.0, 2.0 C). 203x153mm (300 x 300 DPI)


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