Characterization of fibrous gel polymer electrolyte for lithium polymer batteries with enhanced electrochemical properties

Characterization of fibrous gel polymer electrolyte for lithium polymer batteries with enhanced electrochemical properties

    Characterization of fibrous gel polymer electrolyte for lithium polymer batteries with enhanced electrochemical properties Dong-Won K...

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    Characterization of fibrous gel polymer electrolyte for lithium polymer batteries with enhanced electrochemical properties Dong-Won Kang, Jae-Kwang Kim PII: DOI: Reference:

S1572-6657(16)30260-0 doi: 10.1016/j.jelechem.2016.05.029 JEAC 2661

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

16 March 2016 18 May 2016 18 May 2016

Please cite this article as: Dong-Won Kang, Jae-Kwang Kim, Characterization of fibrous gel polymer electrolyte for lithium polymer batteries with enhanced electrochemical properties, Journal of Electroanalytical Chemistry (2016), doi: 10.1016/j.jelechem.2016.05.029

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ACCEPTED MANUSCRIPT Characterization of fibrous gel polymer electrolyte for

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properties

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lithium polymer batteries with enhanced electrochemical

Dong-Won Kang, Jae-Kwang Kim*

Department of Solar & Energy Engineering, Cheongju University,

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Cheongju, Chungbuk 360-764, Republic of Korea

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* Corresponding author: Tel.: +82 43 229 8557; fax: +82 43 229 7322.

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E-mail address: [email protected]

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ACCEPTED MANUSCRIPT Abstract This study highlights physical properties that are associated with the electrochemical

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properties of lithium batteries. Although the electrochemical properties of electrospun

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gel polymer electrolytes (GPEs) have been studied and understood in great detail, the physical properties of GPEs, such as ion interaction and phase transformation, have largely

been

ignored.

A

nano-fibrous

poly(vinylidene

fluoride-co-

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hexafluoropropylene) (PVdF-HFP) polymer matrix for GPEs was prepared by electrospinning. The ionic conductivity of the GPE was approximately 2.7 × 10−3 S

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cm−1 at 30 °C. The phase transition of the PVdF-HFP matrix as well as interaction between solvent or cation and polymer matrix or anion was investigated by Raman

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spectroscopy. Interaction between PF6− anion and PVdF-HFP matrix and phase transformation of the polymer matrix were confirmed. The LiFePO4/GPE/Li cell

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showed high discharge capacities of 134.6 mAh g−1, 131.3 mAh g−1, and 113.5 mAh

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g−1 at high current densities of 1 C, 2 C, and 3 C, respectively. Moreover, this cell exhibited excellent cycle stability with high capacity retention. In particular, this

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nano-fibrous GPE is suitable for application in polymer batteries and is promising as a polymer electrolyte for scaled-up lithium batteries. Keywords: Nano-fibrous PVdF-HFP matrix; gel polymer electrolyte; ion interaction; rate capability.

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ACCEPTED MANUSCRIPT Introduction The lithium secondary battery is promising for mobile energy storage and is currently

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utilized in a wide range of devices, from small-sized electronic devices like cell-

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phones, laptops, and cameras to hybrid electric vehicles (HEVs). In addition, zeroemission, full-electric vehicles (EVs) are expected to be a commercial reality in the near future. This widespread applicability of lithium secondary batteries encourages

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battery technology breakthroughs aimed at optimizing their performance and consequently, enhancing their market competitiveness and penetration. Accordingly,

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market shares for lithium secondary batteries have grown exponentially in the last decade [1,2]. However, the full potential of this technology is far from reached; there

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is still room for improvement and materials research is needed to obtain next-

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generation lithium secondary batteries. The electrolyte for lithium secondary batteries must be an ionic conductor

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capable of solvating and transporting Li+ ions. Ideally, an electrolyte should have the

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conduction properties of a liquid and the mechanical stability of a solid, along with high chemical stability. The anode, the cathode, and the electrolyte must be compatible with each other to provide a high degree of safety, high cycle ability, and high charge and discharge rates. To enhance the safety of lithium batteries, a polyethylene oxide (PEO)-based solid polymer electrolyte is widely studied as a promising host polymer because of its good thermal properties, interfacial stability, and its ability to coordinate with a large number of inorganic cations. However, the PEO-based polymer electrolyte shows very low ionic conductivity at temperatures below room temperature due to its high crystallinity [3–6]. Several attempts have been made to increase the ionic conductivity 3

ACCEPTED MANUSCRIPT of these polymer electrolytes. Among them, the gel polymer electrolytes (GPEs) have received much attention of late, which can be regarded as an intermediate between

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typical liquid electrolytes and dry solid-polymer electrolytes. The GPEs possess high ionic conductivities >10−4 S cm−1 at room temperature and the liquid guest is trapped

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in the polymer host, thereby preventing the leakage of the liquid electrolyte. However, although the electrochemical properties of electrospun gel GPEs have been studied and understood in great detail, their physical properties, such as ion interaction and

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phase transformation, have largely been ignored.

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Several preparation techniques have been tried for the GPE polymer matrix, such as casting, phase inversion, and electrospinning [2,7–13]. Among these methods,

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electrospinning provides good mechanical strength and high ionic conductivity to the polymer matrix. Moreover, the polymer matrix should absorb the liquid electrolyte

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without leakage, be chemically compatible with the electrode materials, and adhere

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well to the electrode. Among various polymers that meet these requirements, poly(vinylidene fluoride) (PVdF) and its copolymer, poly(vinylidene fluoride-co-

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hexafluoropropylene) (PVdF-HFP) are very promising because of their good mechanical and electrochemical stabilities and their affinity to electrolyte solutions [14,15]. The fibrous polymer matrix of PVdF-HFP possesses different properties depending on the electrospinning parameters [11], and these properties can enhance the electrochemical performance of the lithium batteries. In this study, we investigate the physical properties of PVdF-HFP based GPEs, in terms of their optimized synthesis parameters (polymer solution concentration, supply voltage, etc.) and the rate capability of the corresponding LiFePO4 cell. The phase transition of the PVdF-HFP matrix, as well as the interaction 4

ACCEPTED MANUSCRIPT between the solvent and cation and the between cation and anion, is investigated. Changes with the incorporation of liquid electrolyte are observed. When PVdF-HFP

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matrix is incorporated into the LE, the α-phase of PVdF-HFP increases and the concentration of Li-coordinated PF6− decreases slightly. GPE prepared with optimized

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synthesis parameters significantly enhances the rate capability of the lithium polymer

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battery.

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Experimental

A microporous matrix of PVdF-HFP (Kynar 2801) was prepared by electrospinning

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as per the procedure standardized in our previous studies [11, 12]. A 16-wt.% solution of PVdF-HFP in a mixed solvent of acetone and N,N- dimethylacetamide (7/3, w/w)

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was electropsun by applying a voltage of 18 kV at room temperature. A thin film of

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∼80 μm thickness was collected on an aluminum foil. The electrospun membrane was vacuum dried at 60 °C for 12 h before further use. The GPE was prepared by the

electrospun

membrane

in

a

1

M

solution

of

lithium

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immersing

hexafluorophosphate (LiPF6) in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, v/v). Activation of the membrane to prepare the GPE was performed in an argon-filled glove box under <10 ppm moisture level. The LiFePO4 cathode material having 3 wt.% carbon coating was prepared by mechanical activation under optimized conditions of milling time, heating temperature, heating time, and carbon coating [16– 18]. The ionic conductivities of the liquid electrolyte and the GPE were measured from −70 °C to 80 °C in a gold-plated cell, over a frequency range of 10−1–106 Hz 5

ACCEPTED MANUSCRIPT using a Novocontrol broadband dielectric spectrometer. Raman spectra of the PVdFHFP matrix, the liquid electrolyte, and the GPE were recorded on a Bruker IFS 66

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Fourier-transform spectrometer, equipped with an FRA 106 Raman module and using the 1064-nm line of a Nd:YAG laser as the excitation source. The laser power was set

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to 300 mW and the resolution was 2 cm−1. Differential scanning calorimetry (DSC) was performed using a Q1000 apparatus (TA Instruments) at a heating rate 10 °C min1

with a 25 mL min−1 He flow rate. Typically, the samples were cooled at 20 °C min−1

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from 40 °C to −120 °C and heated at 10 °C min−1 up to 180 °C. The interfacial

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resistance (Rf) of the LiFePO4/GPE/Li cell was measured with an IM6 frequency analyzer by the impedance response over the frequency range 10 mHz to 2 MHz.

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Lithium ion transference numbers were measured using the DC polarization method with the Bruce and Vincent correction [19]. The frequency range for the

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electrochemical impedance spectroscopy measurements was 500 kHz–100 mHz, with

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a 10-mV ac signal. Polarization was performed with a 20-mV dc signal. For electrochemical measurements, the LiFePO4 powder, carbon black, and

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the PVdF (Aldrich) binder were mixed in the ratio 87:5:8 by weight in N-methyl pyrrolidone (NMP) and the viscous slurry was cast on an aluminum foil and dried at 95 °C under vacuum for 12 h. The film was cut into circular discs of area 0.95 cm2 and mass ~3.0 mg for use as the cathode. Cyclic voltammetry (CV) measurements of the LiFePO4/GPE/Li cell were performed at a scan rate of 0.1 mV s−1 over 2.0–4.5 V. Electrochemical performance was tested using an automatic galvanostatic chargedischarge unit, WBCS3000 battery cycler, between 2.5 and 4.0 V. The experiments were performed at current densities of 0.1 C (0.042 mA cm−1), 1 C (0.42 mA cm−1), 2 C (0.84 mA cm−1), 3 C (1.26 mA cm−1), and 5 C (2.1 mA cm−1). 6

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Results and discussion The photograph and the morphology of the prepared PVdF-HFP matrix are presented

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in Fig. 1. The polymer matrix, with 16 wt.% polymer concentration at 18 kV (Fig. S1), is made up of a network of interlaid fibers with diameters < 800 nm. The interlaying of the fibers imparts sufficient mechanical strength to the membrane for

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safe handling. The presence of fully interconnected micron-sized pores in the

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membrane structure makes it ideal for application as a host matrix for GPEs. The fibrous PVdF-HFP matrix could theoretically accommodate >250 wt.% liquid electrolyte. However, 60 wt.% liquid electrolyte (LE) was fed into 40 wt.% polymer

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matrix to prevent the LE leakage and reduction in mechanical strength. The

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temperature dependence of the ionic conductivity of the GPE and the LE (based on 1M LiPF6 in EC/DMC) is shown in Fig. 2. The conductivities of both systems

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increase with increasing temperatures over the investigated temperature range. Although the ionic conductivity of the LE was more than 2 orders of magnitude

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higher than that of the GPE over the entire temperature range (−40 to 80 °C), the ion conductivity of the GPE ( >10−4 S cm−1) was high enough for high-power battery applications [20,21]. An ionic conductivity of ~3.0 × 10−4 S cm−1 was achieved at −10 °C in the GPE, and it increased to ~2.7 × 10−3 S cm−1 at 30 °C. The latter compares well with the reported values of GPEs with various matrices (Table S1). The ionic conductivity of GPE also depends on the liquid electrolyte uptake. Although the GPE studied here was prepared with low liquid electrolyte uptake (60 wt.%), its ionic conductivity is almost as high as the reported value of GPE with >200

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ACCEPTED MANUSCRIPT wt.% liquid electrolyte uptake. The ionic conduction mechanism for both electrolytes followed the Vogel-Tammann-Fulcher (VTF) behavior.

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Raman spectroscopy was performed to confirm the formation of the α-phase

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after the incorporation of LE. The Raman spectrum was recorded for the pure fibrous PVdF-HFP matrix and the GPE and its possible assignments are given in Fig. 3a. Drastic increase in the intensity of the 795 cm−1 peak corresponding to combination of

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the CH2 rocking (r) and CF2 stretching (ν) vibrations, confirm the α-phase of crystalline PVdF-HFP [22,23]. At the same time, the peak at 838 cm−1 (out-of-phase r

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CH2 and ν CF2 of PVdF-HFP corresponding to the β-phase) was markedly diminished. This indicates transition of the crystalline PVdF-HFP chain from the β-

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phase with all-trans conformation to the α-phase with TGTG− conformation. The change also suggests separation of hydrophilic and hydrophobic sites, which can

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influence the morphology and functionality of the membrane. Interaction between the

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EC solvent and the Li+ cation appears in the satellite band (at 725 cm−1) of ringbreathing fundamentals of the carbonate ring. The satellite band is influenced by the

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anion type and the lithium salt concentration [24]. The influence of the polymer matrix on the interaction between EC and Li+ was investigated with relative intensity (Fig. 3b). Two components are used to describe the band, one component (~718 cm−1) related to free EC and a second component (~730 cm−1) corresponding to Li+ ions coordinated EC. The band profiles are constant irrespective of polymer matrix addition. We therefore conclude the absence of strong interaction between the EC and the PVdF-HFP membrane and the interaction between EC and Li+ cation is almost the same on both the LE and the GPE. To study the interaction between the polymer matrix and the anion, the symmetry vibration of the PF6− anion was observed in Fig. 3c. The solid lines represent the bandfit of the experimental spectra (circles), using 8

ACCEPTED MANUSCRIPT Lorentzian profiles. The absorption at ~743 cm−1, corresponding to stretching of the free PF6− anion, is the strongest. The band at ~744 cm−1 corresponds to contact ion

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coordination between the Li+ cation and the PF6− anion, while the band at ~741 cm−1 is due to higher-order aggregation [25]. The concentration of the Li+-coordinated

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anion is strongly influenced by the concentration of the lithium salt and the LE temperature [26]. When PVdF-HFP matrix was incorporated into the LE, the concentration of Li-coordinated PF6− decreases slightly, confirming the interaction

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between the polymer matrix and the PF6− anion. The lithium transport numbers of LE

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and GPE were 0.28 and 0.32, respectively. Thus, an improvement in the transference number was observed for the GPE, because of free Li+ ions generated by the

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interaction between the polymer matrix and the PF6− anion. The DSC thermograms of LE and GPE (Fig. 4a) shows glass transition

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temperatures (Tg) at approximately −94 °C and −70 °C, respectively, which can be

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attributed to the EC/DMC solvent. For GPE, the higher Tg indicates that the incorporation of polymer matrix imparts a plasticizing effect in the solvent. Fig. 4b

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shows the crystallization and melting transitions of LE and GPE. During the heating scan from −120 °C to 150 °C, LE exhibited a sharp exothermic peak at −45 °C corresponding to phase transition, and a melting peak at −3.4 °C. For GPE, an endothermic peak was observed at 12 °C but no phase transition peak was observed. In addition, when fibrous polymer matrix is added, melting temperatures increased because the phase containing the EC/DMC has a swollen gel composition [27]. The combustion tests of a commercial separator (Celgard 2430) and the GPE are shown in Fig. 5. A porous nonwoven separator to enhance the flame retarding ability of GPE has been explored recently [19, 28–31]. When the commercial 9

ACCEPTED MANUSCRIPT separator was exposed to fire, the separator shrank immediately and caught on fire in a short time (< 2s); it was completely burned in 3 s. On the other hand, the GPE

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shows flame retarding ability although the swollen liquid electrolyte evaporated when exposed to fire. The electrospun PVdF-HFP membrane did not catch fire when put in

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the flame. The flame retarding ability is important for the safety of the lithium battery and using the fibrous polymer matrix can improve lithium battery safety.

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The equivalent circuit model to analyze the impedance spectra consists of the bulk resistance of the GPE (Rb), charge transfer resistance (Rct), Warburg impedance

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(Zw), and double-layer capacitance (CDL). The GPE exhibits a single semi-circular impedance pattern, typical of GPEs having high ionic conductivity with contributions

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from Rb in the high-frequency range and Rct in the middle-frequency range. The semicircular impedance pattern at high frequencies is due to the parallel combination of

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CDL and Rct. The inclined line at low frequencies represents the Zw values, related to

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Li+ ion diffusion in the LiFePO4 electrode. Fig. 6a shows the impedance spectra of the LiFePO4/GPE/Li cell with cycling. The Rb value at high frequency was very low and

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remained almost constant with cycling. However, the Rct values, associated with the exchange current density related to the transfer and diffusion of Li+ ions, decreased continuously with cycling because the penetration and transport of Li+ ion is improved from the solid electrolyte interface (SEI) layer formed at the first cycle. The Li+ diffusion coefficient (D) may be calculated by the following equation [32].

D



R 2T 2 2n 4 F 4 C 2 2

where R is the gas constant, T is the Kelvin temperature, n is the number of electrons transferred per molecule during oxidization, F is the Faraday constant, C is the 10

ACCEPTED MANUSCRIPT lithium ion concentration, and σ is the Warburg factor. The Warburg factor σ can be obtained from the following equation:

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The diffusion coefficient increases from 4.9 × 10−17 m2 s−1 at the first cycle to 2.8 × 10−13 m2 s−1 at the 30th cycle because penetration of the Li+ ion improved with cycling. However, the diffusion coefficient decreased to 5.2 × 10−14 m2 s−1 at the 100th cycle

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due to irreversible transport of Li+ ions after prolonged cycling. Fig. 6b shows the CV of the LiFePO4/GPE/Li cell during cycling; the oxidation and reduction processes

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occur at 3.75 and 3.1 V, respectively, during the first cycle with a peak separation (ΔV) of 0.65 V. As shown in the figure, nearly overlapping CV curves are obtained

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for the first and fifth cycles. ΔV is lower for subsequent cycles than the first cycle,

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indicating an increased efficiency for the electrochemical reaction with cycling. The area under the respective anodic and cathodic peaks remain almost constant even after

cell.

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repeated cycling, indicating an equal amount of Li+ ion insertion and extraction in the

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

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Z ''  1/ 2

The charge-discharge behaviors of the LiFePO4/GPE/Li cells at different

current densities are presented in Fig. 7a. At a low rate (0.1 C), the cell achieved a high capacity of 161.5 mAh g−1, 95% of the theoretical capacity. The capacities of GPE are higher than that of the Celgard membrane with the same LE (Fig. S2). At higher rates, the discharge capacity decreases, but remains as high as 134.6 mAh g−1 at 1 C, 131.3 mAh g−1 at 2 C, and 113.5 mAh g−1 at 3 C. Although the LiFePO4 electrode has a low carbon content of 12.9 wt% (7.9% coating and 5% carbon conductor), the values are still higher than reported liquid electrolyte-based GPE with LiFePO4 electrode at room temperature [27, 33–35]. Moreover, the voltage separation 11

ACCEPTED MANUSCRIPT (ΔV) between charge and discharge plateaus increases steadily with increasing current density, but still show a low value from 0.07 V at 0.1 C to 0.43 V at 3 C. These

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observations are the result of the increased polarization of the cell at higher current densities. This excellent rate property could be ascribed to the grafted nano-fibrous

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PVdF-HFP matrix having good affinity for both the electrolyte and the electrode. The lithium polymer cell shows stable cycle performance even at high current densities (Fig. 7b). At the end of the 50th cycle, the discharge capacities are 150.2, 116.6, 115.2,

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93.2, and 69.2 mAh g−1 at 0.1, 1, 2, 3, and 5 C, respectively. Thus, even at the high

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current densities of 1, 2, 3, and 5 C, the variation in discharge during the first 50 cycles is <17%, compared to the initial capacity. Also, the capacity fades per cycle at

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all the above current densities are as low as <~0.4% with high Coulombic efficiencies of ~99%. The GPE presents satisfactory rate performance between 0.1 C and 5 C.

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Such a GPE is attractive for large-scale battery systems requiring high safety and low

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cost with high rate capability.

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Conclusions

A nano-fibrous PVdF-HFP matrix has been activated by incorporating 1 M LiPF6 solution in EC/DMC, to prepare a gel polymer electrolyte (GPE). The presence of thin fibers with large surface areas and fully interconnected micron-sized pores in the matrix renders a high ionic conductivity of 2.7 × 10−3 S cm−1 at 30 °C, which is suitable for room-temperature applications of lithium batteries. Raman spectroscopy reveals that the PVdF-HFP matrix was converted to α-phase from β-phase by incorporation of the LE and only a weak interaction existed between the PVdF-HFP matrix and the PF6− anion. The diffusion coefficient of the Li+ ion increased with 12

ACCEPTED MANUSCRIPT cycling up to the 30th cycle, but decreased with further cycling due to the irreversible transport of Li+, which is associated with capacity fade. Good compatibility of the

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GPE with the LiFePO4 electrode was confirmed by CV studies that showed welldefined peaks for lithium insertion and extraction. A LiFePO4/GPE/Li cell gives

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initial discharge capacities of 161.5 and 113.5 mAh g−1 at 0.1 and 3 C, respectively, at room temperature, along with a stable cycling property. The room-temperature cell performance decreased at higher current densities, but still maintained a high rate

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capability. Therefore, this nano-fibrous GPE is suitable for application in polymer

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secondary batteries and is promising as a polymer electrolyte for scaled-up lithium

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Acknowledgements

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batteries.

We are thankful to Prof. Ahn of the Gyeongsang National University for his help in

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preparing the electrospun polymer membrane.

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batteries, Adv. Funct. Mater. 24 (2014) 44–52.

nanofiber-based nonwoven separators for lithium-ion batteries, J. Power Sources 226 (2013) 82–86.

X. Li, J. He, D. Wu, M. Zhang, J. Meng, P. Ni, Development of plasma-

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treated polypropylene nonwoven-based composites for high-performance

N. Shubha, R. Prasanth, H. H. Hoon, M. Srinivasan, Plastic crystalline-semi polymer

composite

electrolyte

based

on

non-woven

poly(vinylidenefluoride-co-hexafluoropropylene)

porous

membranes

for

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crystalline

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lithium-ion battery separators, Electrochim. Acta 167 (2015) 396–403.

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lithium ion batteries, Electrochim. Acta 125 (2014) 362–370. J.K. Kim, J. Manuel, G.S. Chauhan, J.H. Ahn, H.S. Ryu, Ionic liquid-based polymer

AC

gel

electrolyte

for

LiMn0.4Fe0.6PO4

cathode

prepared

by

electrospinning technique, Electrochim. Acta 55 (2010) 1366–1372 .

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H. Xie, Z. Tang, Z. Li, Y.He, Y. Liu, H. Wang, PVDF-HFP composite polymer electrolyte with excellent electrochemical properties for Li-ion batteries, J. Solid State Electrochem. 12 (2008) 1497–1502.

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ACCEPTED MANUSCRIPT N. Wu, Q. Cao, X. Wang, X. Li, H. Deng, A novel high-performance gel polymer electrolyte membrane basing on electrospinning technique for lithium

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rechargeable batteries, J. Power Sources 196 (2011) 8638– 8643.

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Figure cations

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Fig. 1. Photo and SEM image of nano-fibrous PVdF-HFP matrix for gel polymer electrolyte.

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Fig. 2. Ionic conductivity as a function of temperature for 1M LiPF6 in EC/DMC liquid electrolyte (LE) and gel polymer electrolyte (GPE).

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Fig. 3. Raman spectra of the pure PVdF-HFP matrix, LE and GPE at room

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temperature.

Fig. 4. DSC thermograms of LE and GPE with magnification of the Tg (a) and Tm (b)

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region.

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(GPE).

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Fig. 5. The combustion test of commercial separator (LE) and gel polymer electrolyte

Fig. 6. Impedance behavior of the LiFePO4/GPE/Li cell as a function of cycle at 25 °C (a) and cyclic voltammetry (CV) curves during 1st, 2nd and 5th cycles (b). Fig. 7. Charge-discharge profile (a) and cycling performance (b) at different current density for LiFePO4/GPE/Li cell at room temperature.

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electrolyte.

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Fig. 1. Photo and SEM image of nano-fibrous PVdF-HFP matrix for gel polymer

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10 10 10 10

LE GPE

-2

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10

1M LiPF6 in EC/DMC

-3

-4

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10

-1

-5

-6

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Conductivity (S/cm)

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-7

3.0

3.2

3.4

3.6

1000/K

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2.8

3.8

4.0

4.2

4.4

Fig. 2. Ionic conductivity as a function of temperature for 1M LiPF6 in EC/DMC

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liquid electrolyte (LE) and gel polymer electrolyte (GPE).

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(a)

800

810

Free EC

β-phase 820

830

840

850

Li coordinated EC

710

715

720

725 -1

Raman shift (cm

730

735

)

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Raman shift (cm-1)

LE GPE

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Intensity (a.u)

α-phase

790

1M LiPF6 in EC/DMC

(b)

PVdF-HFP pure PVdF-HFP PEs

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(c)

Fig. 3. Raman spectra of the pure PVdF-HFP matrix, LE and GPE at room temperature.

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1M LiPF6 in EC/DMC

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(a)

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GPE

o

o

-94 C

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Heat flow (mW)

-70 C

-110

-100

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LE

-90

-80

o

-70

-60

-50

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Temperature ( C)

1M LiPF6 in EC/DMC

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LE GPE

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Heat flow (mW)

(b)

-60 -50 -40 -30 -20 -10

0

10

20

30

40

50

o

Temperature ( C)

Fig. 4. DSC thermograms of LE and GPE with magnification of the Tg (a) and Tm (b) region.

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GPE

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LE

20 s

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3s

Fig. 5. The combustion test of commercial separator (LE) and gel polymer electrolyte (GPE).

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500

(a)

first cycle 30 cycle 100 cycle

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300 200 100 0

0

100

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-Z`` (ohm)

400

200

300

400

500

4.0

4.5

0.6

Cycle No.

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(b)

0.2 0.0

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Current (mA)

0.4

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Z` (ohm)

1 st 2 nd 5 th

-0.2

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-0.4

-0.6 2.0

2.5

3.0

3.5

Voltage (V)

Fig. 6. Impedance behavior of the LiFePO4/GPE/Li cell as a function of cycle at 25 °C (a) and cyclic voltammetry (CV) curves during 1st, 2nd and 5th cycles (b).

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4.00

C-rate 0.1 C 1C 2C 3C

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3.50 3.25 3.00 2.75 2.50

0

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60

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(b)

180 160

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140 120 100

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Voltage (V)

80 100 120 140 160 180 200

Specific capacity (mAh/g)

200

80 60

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40 20

0

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Voltage (V)

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(a)

3.75

0

C-rate 0.1 C 1C 2C 3C 5C 10

20

30

40

50

Specific capacity (mAh/g)

Fig. 7. Charge-discharge profile (a) and cycling performance (b) at different current density for LiFePO4/GPE/Li cell at room temperature.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights  Nano-fibrous gel polymer electrolyte (GPE) was prepared by electrospinning process.

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 Ionic conductivity of the GPE was measured to be of the order of 2.7×10 −3 S c m−1 at 30 °C.

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 The interaction between PF6- anion and PVdF-HFP matrix existed.

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 The gel polymer cell exhibited excellent cycle stability and rate capability.

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