Radiation-crosslinked nanofiber membranes with well-designed core–shell structure for high performance of gel polymer electrolytes

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Radiation-crosslinked nanofiber membranes with well-designed core-shell structure for High performance of gel polymer electrolytes Zhenzhen Zhang, Gang Sui, Haitao Bi, Xiaoping Yang

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S0376-7388(15)00470-6 http://dx.doi.org/10.1016/j.memsci.2015.05.040 MEMSCI13722

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Journal of Membrane Science

Received date: 8 February 2015 Revised date: 28 April 2015 Accepted date: 22 May 2015 Cite this article as: Zhenzhen Zhang, Gang Sui, Haitao Bi, Xiaoping Yang, Radiation-crosslinked nanofiber membranes with well-designed core-shell structure for High performance of gel polymer electrolytes, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.05.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Radiation-crosslinked nanofiber membranes with well-designed core-shell structure for high performance of gel polymer electrolytes Zhenzhen Zhang, Gang Sui*, Haitao Bi, Xiaoping Yang State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

Abstract: The polymer nanofiber membranes with core (polyacrylonitrile, PAN)-shell (polyethylene oxide, PEO) structure were prepared by using coaxial electrospinning under different concentration of spinning solution. The polyporous nanofiber membranes were activated in liquid electrolytes and transformed into gel polymer electrolytes (GPEs). The microstructure, crystallization behavior and mechanical properties of nanofiber membranes were studied. Based on the proper collocation of polymer materials, desirable microstructure, appropriate crystallinity and moderate radiation-crosslinking, the high saturated electrolyte uptake, conservation rate and mechanical properties of polymer membranes were obtained. The electrochemical testing of the resulting GPEs revealed high ionic conductivities, good electrochemical stability and appropriate lithium-ion transference numbers, which are realized through choosing an optimal concentration of core/shell spinning solution. Moreover, the Li/GPE/LiCoO2 cells with GPEs based on the radiation-crosslinked nanofiber membranes with optimized core-shell structure showed excellent initial discharge capacities as well as remarkable cycle performance. Through the implementation of the proper collocation of two ordinary polymer materials and facile preparation techniques, comprehensive performance of the resulting GPEs was more superior to that of GPEs involved with pure PAN and commercial Celgard® 2500. The radiation-crosslinked nanofiber membranes with well-designed core-shell structure can be used as an ideal skeleton material in GPEs for lithium-ion batteries with high performance. Keywords: Nanofiber membrane; Gel polymer electrolyte; Core-shell structure; Radiation crosslinking; Electrochemical performance

1. Introduction Lithium ion battery has been widely used in portable electronic devices and electric vehicles because it has many advantages including high energy density, high working voltage, high efficiency, memory-effect-free and long cycle life [1,2]. In recent years, many approaches have been employed to develop polymer lithium-ion




batteries, which is safer than liquid lithium-ion battery, and realize the enhancement of battery capacity. However, the ionic conductivity of solid polymer electrolytes is far lower than liquid electrolytes at room temperature, which limits their practical application. Many techniques have been attempted to prepare polymer electrolytes with high ionic transport properties and low crystallinity, such as copolymerization, cross-linking, grafting, comb formation, inorganic filler addition and polymer alloy [3-7]. From a practical standpoint, the gel polymer electrolytes (GPEs) are widely adopted in commercial polymer lithium-ion battery at present [8-11], which are made from porous polymer skeleton material and absorb liquid electrolytes, therefore having a high ionic conductivity. Among many methods for the preparation of porous polymer skeleton materials, electrospinning can easily prepare membranes consisting of polymer fibers, with the porosity of 30%-90%. GPEs can be produced after the activation process of polymer membranes in liquid electrolytes. The polyporous structure in electrospun fiber membranes can provide abundant transport channels for lithium ions and a large specific surface area which can improve the interface compatibility of electrode and the electrolyte. The research works of polyacrylonitrile (PAN) in GPEs have been widely carried out in recent years due to its good thermal properties, mechanical properties, high ionic conductivity and electrochemical stability. For example, Raghavan et al.[12] have reported a PAN-based polymer electrolyte with an ionic conductivity higher than 1×10-3 S/cm at room temperature. But PAN-based GPEs show bad compatibility with lithium electrodes, and the passivation on the interface is serious [13-15]. Poly(ethylene oxide) (PEO) has ether linkages, with oxygen atoms present at a suitable inter-atomic separation to allow segmental motion of the polymeric chain which is beneficial for facile ionic conduction [16]. The suitable distance between each hanging ether oxygen in the polymer chain of PEO is important in the salt dissociation and charge transport [17]. Moreover, PEO has good compatibility with lithium electrode, and can be a promising host material for electrolytes. Oh and Amine[18] developed a kind of PEO based GPEs which exhibits ionic conductivity exceeding 1×10-3 S/cm and good battery performance. However, a typical limit of such kind GPEs lies in their poor mechanical strength. Besides, PEO is a semi-crystalline polymer, and the transport of Li+ ions mainly occurs in the amorphous regions so the crystalline region hinder the migration of lithium ions. High molecular weight PEO displays inferior fiber membrane-forming property during the electrospinning while the mechanical properties of the fiber membrane obtained by using low molecular weight PEO are quite weak owing to being partly dissolved in liquid electrolytes, which is not suitable for the skeletal material of GPEs. Some typical methods to improve the mechanical strength of nanofiber membranes include radiation treatment, inorganic particle reinforcement and thermal crosslinking [19-21]. Compared to other methods, electron beam (EB) radiation technology offers a number of advantages,




such as convenient and efficient, easy to control, without the introduction of additives to get the pure product, and the radiation process carrying out at room temperature even under low temperatures [22]. Since Loscertales et al. proposed a coaxial electrospinning technique [23], the preparation and application of nanofibers with core/shell structure have attracted widespread attention in the recent years. Li et al. prepared a core (polypyrrole)-shell (PAN) nanostructured conductive composites by coaxial electrospinning [24]. Liu et al. reported a core (polyimide)-shell (poly[(vinylidene fluoride)-co-hexafluoropropene]) nanofibrous separator for lithium ion batteries with improved tensile strength, thermal stability and rate capability[25]. To fully take advantage of the performance of different polymeric materials and polyporous structure, the polymer membranes consisting of nanofibers with core (PAN)-shell (PEO) structure were prepared by using coaxial electrospinning in this paper. The polymer composition, microstructure and crystallization behavior of nanofiber membranes were studied in detail. By combining with a modest amount of EB radiation, the partly crosslinked polymer nanofiber membranes with excellent electrochemical properties and mechanical properties was obtained, which can be used as high performance of skeleton material in GPEs.

2. Experimental 2.1. Materials PAN (average molecular weight of 105 g mol-1, Jilin Petrochemical Co., China.), PEO (average molecular weight of 106 g mol-1, Shanghai Liansheng Chemical Co., China.) and N,N-dimethylformamide (DMF, analytical reagents, Beijing Chemicals Co., China) were used to prepare the PAN and PEO spinning solution, respectively. 1 M lithium hexafluorophosphate (LiPF6)/ethylene carbonate (EC) : dimethyl carbonate (DMC) (1:1, vol. %) (Battery grade, MERCK) was used as the liquid electrolyte solution. Commercial Celgard® 2500 microporous monolayer membranes were purchased from Celgard LLC., USA. Due to the poor fiber membrane-forming property, the PEO raw materials were used in a comparative analysis of crystallization behavior and thermal stability in this paper. 2.2. Preparation of PAN/PEO porous nanofiber membranes PAN was vacuum dried at 60°C for 8h prior to use, and PEO vacuum dried at 40°C for 6h prior to use. For an outer fluid, homogeneous solution of PEO with various concentrations (3~14 wt%) was prepared in DMF under stirring for 10h. Similarly, homogeneous solution of PAN with concentrations of 14wt% was prepared in




DMF under stirring for 6h. The solution was loaded into two disposable plastic syringes respectively, and a dual syringe pump (JZB-1800D, Jian Yuan Medical Technology Co. Ltd., China) was used to deliver the core (PAN) and shell (PEO) fluids independently, whose flow rate was varied in the range of 0.4~0.7 ml h-1. A high voltage of 18KV was applied to the polymer solution, using a high-voltage power supply (BGG6-358, BMEI Co. Ltd., China). The distance between the needle and the collector plate was set to 18 cm. A square plate of aluminum (25cm×25cm) was used as a collector. The electrospinning was carried out at ambient temperature with a humidity of 30~40%. The PAN/PEO electrospun non-woven films were dried under vacuum at 40

for 8h. Fig.1

shows a schematic illustration of coaxial electrospinning systems. The coaxial nanofibers produced by using 3wt%, 7wt%, 10wt% and 14wt% of PEO spinning solution were designated as PAN/PEO-3%, PAN/PEO-7%, PAN/PEO-10% and PAN/PEO-14%. For comparison, electrospun pure PAN membranes were also prepared under the similar condition. Some PAN/PEO-10% nanofiber membranes were treated with 10kGy EB radiation under a dose rate of 100Gy/s, which were designated as PAN/PEO-EB. 2.3. Preparation of gel polymer electrolytes Polyporous nanofiber membranes were punched into small discs with a diameter of 22 mm. Then these small discs

were vacuum-dried at 50°C for 6 h and transferred into an argon filled glove box (moisture level < 10

ppm). GPEs were prepared by immersing small discs in 1 M LiPF6/EC : DMC (1:1, vol. %) for about 24 h at room temperature, the GPEs were obtained after removal of excess liquid electrolytes with a filter paper. 2.4. Characterization The structure of nanofiber membranes was characterized by Fourier transform infrared spectroscopy (FT-IR, EQUINOX 55, Germany). The morphology of nanofiber membranes was examined by scanning electron microscope (SEM, Hitachi S-4700, Japan) at an accelerating voltage of 20 kV. The diameter range of the nanofiber membranes was measured by image visualization software Image J 1.34s (National Institutes of Health, USA), and about 200 fibers were investigated to calculate average fiber diameter. Verification of core-shell structure of nanofiber membranes was characterized by using transmission electron microscopy (TEM, Hitachi




H800, Japan). Structure and crystallization properties of the nanofiber membranes were investigated by using an X-ray diffractometer (XRD, 2500VB2+PC, Rigaku Corporation, Japan). Differential scanning calorimetry (DSC) analysis was performed using a TA Instruments DSC Q200 in a nitrogen atmosphere from -120°C to 170°C at a heating rate of 20°C min−1. The crystallinity (Ȥc) was calculated based on the following Eq.(1) from the DSC curves [26].

χc =

∆H m × 100% ∆H m*

(1)

where ǻHm and ǻHm* represent the fusion enthalpy of nanofiber membranes and PEO with 100% crystallinity, respectively. The value of ǻHm* is 213.7 J/g [27]. The thermal stability of nanofiber membranes was analyzed by using a thermogravimetric analyzer (Q50, TA Instruments, USA) under air atmosphere, from room temperature to 800°C at a heating rate of 10°C min−1. The mechanical strength of the polymer was measured by universal testing machines (UTM, Instron Instruments). The extension rate was kept at 10 mm min-1. The dimensions of the sheet used were 1 cm×5 cm×0.3cm (width× length×thickness). To measure the porosity, the prepared membranes with different ratios of PAN to PEO were immersed into n-butanol for 2h. The porosity (P) was obtained by Eq.(2).

P=

ma / ρ a ( ma / ρ a ) + ( m p / ρ p )

(2)

where ȡa and ȡp are the density of n-butanol and the dry membrane, respectively; ma and mp are the mass of the n-butanol-incorporated membrane and the dry membrane, respectively[28]. Saturated electrolyte uptake (A) of nanofiber membranes was measured and calculated by Eq.(3):

A(%) = (W1 − W0 ) / W0 ×100%

(3)

where W0 and W1 are the mass of the dry and the saturated membrane, respectively. The GPEs were placed in an argon filled glove box without sealing, and were weighed every 24 h for 15 days, thus, conservation rate in weight with storage time were obtained. All the electrochemical properties were measured at room temperature. The electrochemical stability window was examined using the method of linear sweep voltammetry in the cell Li/GPE/SS at a scan rate of 1 mV s-1, potential voltage ranging from 2 V to 7 V. The lithium-ion transference number was measured using the method of chronoamperometry (CA) in the cell Li/GPE/Li with a polarization voltage of 5 mV. The impedance spectra were obtained by scanning in the frequency range of 0.1 Hz-200 kHz using symmetrical Li/GPE/Li cell. The total






storage period was 40 days. The charge/discharge performance of the Li/GPE/LiCoO2 cells was galvanostatically measured on a LAND CT2001A battery tester in the potential range of 2.7-4.2 V at current densities of 0.1 C. The ionic conductivity was determined by AC impedance spectroscopy in the range of 0.1 Hz-100 KHz using the cell inserted GPEs into two parallel stainless steel (SS) discs (Zahner Zennium electrochemical analyzer, Germany). The ionic conductivity could be calculated by Eq.(4):

δ = L /( Rb ⋅ S )

(4)

where į is the ionic conductivity, Rb the bulk resistance, L and S the thickness and area of the films, respectively. The electrochemical stability was determined by linear sweep voltammetry (LSV) of Li/GPE/SS cells at a scan rate of 5 mV s−1 over the range of 2-7 V on the CHI604B electrochemical workstation, China. Transference number (TLi+) was measured using the method of chronoamperometry (CA) in the cell Li/GPE/Li with a polarization voltage of 5mV on Autolab PGSTAT 302N, Metrohm, Switzerland. TLi+ was calculated by Eq. (5) [29]:

TLi + = ( I 0 − I ss ) / I 0

(5)

where Iss is the steady-state current, I0 the initial current. These two values can be read from the chronoamperogram of Li/GPE/Li cells. The charge/discharge performance of the Li/GPE/LiCoO2 cells was galvanostatically measured on a LAND CT2001A battery tester in the potential range of 2.7-4.2 V at current densities of 0.1 C. The C-rate capability was also measured at current densities of 0.1, 0.5, 1, 5 and 0.1 C rate, respectively.

3. Results and discussion 3.1. Morphology and structure of nanofiber membranes The morphology of coaxial electrospun membranes with different PEO content was examined using SEM, as shown in Fig. 2. The insert image provides the diameter distribution of nanofibers. It can be seen that the electrospun nanofibers interlaced with each other randomly and formed a three-dimensional network structure. In Fig. 2, the average diameter of electrospun pure PAN nanofibers was 280nm. After 3wt% of the PEO shell spinning solution was used, the average diameter of coaxial nanofibers decreased to near 200nm. Because the viscosity of low concentration of PEO solution was very small, the thin fiber and a small amount of discrete beads were formed due to the capillary breakup of the spinning jet under the action of surface tension and Coulomb




force of the electric field. [30] With the enhanced concentration of the PEO spinning solution, the average diameter of coaxial nanofibers showed a slightly increasing trend and the microporous structure of the fiber membrane was uniform. When the PEO concentration reached 14wt%, the diameter of nanofibers obviously increased and the fiber diameter distribution was wide. Fig 3 shows the TEM images of the electrospun nanofibers with different PEO content. It can be seen in the coaxial nanofibers a very thin PEO shell covered on PAN core. As the concentration of PEO spinning solution was enhanced, the PEO shell thickness was increased slightly while the surface became rough, which may be due to the hygroscopic nature of PEO. The PEO shell can absorb moisture from the atmosphere during spinning and the subsequent collection process of the fibers [31], and it led to the formation of a rough surface, which was beneficial to the improvement of electrochemical performance and interface properties with electrode. The XRD patterns of the PEO and nanofiber membranes are given in Fig 4. In the XRD curve of pure PAN, no diffraction peak can be found, which suggested that it was the amorphous structure. While the pure PEO showed two distinct diffraction peaks in the XRD curve. The peak at 19.2 °corresponded to the (120) diffraction planes, and the diffraction peak near 23.4 ° belonged to (032) and (112) crystal planes. In the case of 3wt% of the PEO spinning solution was involved in the preparation of coaxial nanofibers, only a broad peak occurred in the XRD curve. With the enhancement of concentration of the PEO spinning solution, the same two diffraction peaks at about 19.2° and 23.4° were displayed, as those of pure PEO, but the peak intensities became weak. Therefore, the degree of crystallinity of PEO was affected in the nanofibers with core-shell structure. Owing to no obvious shift of all diffraction peak positions being found, there was a negligible change in the crystalline form and the average interlayer spacing of crystallites of PEO. Fig 5 shows the DSC curves of PEO and nanofiber membranes. It can be seen that the pure PEO melting range was between 40°C to 100°C with clear melting doublets, which may reflect the transition of folded crystals to the straight chain during raising temperature of PEO. The upper melting peak can be attributed to the straight chain crystals, and the lower temperature peak can be attributed to the folded chain crystals. After the PEO was incorporated into the coaxial nanofibers, the melting doublets were still there except the samples with low content of PEO (3wt% of the PEO spinning solution was involved), which implied the presence of imperfect crystallization and recrystallization in electrospun nanofibers [32]. As shown in table 1, the degree of crystallinity of PEO in the PAN/PEO electrospun nanofibers declined significantly, compared to the pure PEO. This conformed to the XRD analysis results. One reason might be that the presence of PAN phase hindered the crystallization behavior of the PEO. On the other hand, electrospinning process inhibited the crystallization of




PEO, due to the fast solidifying process and high drawing rate during spinning. [33] The degree of crystallinity of PEO showed an increasing trend with the enhanced content of PEO in the nanofibers. In Fig 5, the glass transition temperature (Tg) of pure PEO was -61.1°C. At the same time, the Tg of PEO in PAN/PEO membranes obviously decreased, which meant that the movement of polymer chain segments and migration of lithium ions were likely to be more flexible. A new step around 115°C in the DSC curves of the electrospun nanofibers can be assigned to the glass transition region of PAN phase. Fig 6 shows TGA curves of PEO and nanofiber membranes. By comparing the weight loss as a function of temperature, the thermal stability of electrospun nanofibers can be analyzed. Although there was a low onset temperature of thermal degradation around 100°C for the samples containing PAN, no obvious weight losses were found within the temperature range from room temperature to 200°C for all samples. Pure PEO showed dramatic 80% weight loss before the temperature rose to 480°C, due to the decomposition of the PEO segments. PAN had a 23% weight loss at 280°C, which was caused by the cyclization and oxidation reaction taking place in the molecular chain. Then the weight gradually declined due to the removal of non-carbon atoms in the molecular structure. By comparison with pure PAN and PEO, a shift of the rapid degradation region towards higher temperature can be seen for the nanofibers with core (PAN)-shell (PEO) structure, excepting the samples with high content of PEO (14wt% of the PEO spinning solution was involved). For the nanofibers with core-shell structure, there existed some interaction, such as dipole or hydrogen-bond interaction between two phases. The PEO shell covered on the surface of PAN, cutting off the contaction between the PAN and the air, consequently the oxidation reaction were blocked to a certain degree. Moreover, the PAN hindered the crystallization behavior of the PEO, under the heating conditions, and the melting of crystals and recrystallization occurred, as shown in DSC analyses. These factors made the thermal stability of the PAN/PEO membranes improved to some extent. When the concentration of the PEO spinning solution reached 14wt%, some interstices may be produced in the electrospun nanofibers under the thermal stress due to the presence of rough fiber surface and weak interface interaction, which caused a decline in thermal stability of the resulting fiber membrane. Considering the above analyses, the electrospun nanofibers with proper core-shell structure can be very stable over a wide temperature range. 3.2 Mechanical properties of nanofiber membranes Good mechanical properties are indispensable for high performance electrolyte materials. The tensile properties of the pure PAN and PAN/PEO nanofiber membranes were tested, as shown in Fig 7. It can be seen




that tensile strength and tensile modulus of samples with low content of PEO (3wt% and 7wt% of the PEO spinning solution was involved) was relatively high, compared to those of pure PAN. This can be partly attributed to the high porosity of the pure PAN membranes, as well as the smaller fiber diameter of the PAN/PEO samples arising from high drawing ratio during the electrospinning process. But with the increase of PEO concentration in the spinning solution, the tensile properties of the PAN/PEO nanofiber membranes showed a declining trend. In order to improve the mechanical properties of electrolyte materials, and not to introduce other harmful impurities, the radiation crosslinking technology was adopted in this work. After electron beam radiation treatment, the tensile properties of PAN/PEO nanofiber membranes can be significantly increased due to the radiation crosslinking of the molecular chain. As shown in Fig 7, enhancements of 267% and 196% in tensile strength and tensile modulus of the PAN/PEO-10% samples had been obtained respectively after 10kGy electron beam radiation. Fig 8 shows the FT-IR spectra of PAN/PEO-10% and PAN/PEO-EB. After EB radiation, the intensity of peak at 1130cm-1, which is attributed to the stretching vibrations of C-O-C groups in PEO molecules, obviously decreased. At the same time, the peak around 3450 cm-1, belonging to the stretching vibrations of -OH, displayed a substantial growing intensity. The peak at 2240 cm-1 and 1670 cm-1 are assigned to stretching vibrations of -CN bond and -NH bond of PAN. For the samples after radiation treatment, the peak intensity of -NH stretching at 1670 cm-1 became very weak. Combining with the above mechanical properties, it can be inferred that a certain degree of crosslinking reaction occurred inside the nanofiber membranes after EB radiation. SEM image of PAN/PEO-EB membrane is shown in Fig 9. The insert image provides the diameter distribution of nanofibers. It can be seen that the nanofiber morphology was maintained after 10kGy radiation, but the average diameter of nanofibers increased from 250 nm to 330 nm. In the electrospinning process, the polymer chains were stretched, and then solidified and formed nanofibers in a very short time under the electric field force. During the subsequent EB radiation, the polymer chain conformational entropy was changed with the increase of temperature in material system [22] and the relaxation of molecular chains caused fiber diameter to increase slightly. The XRD patterns of the PAN/PEO-10% membrane before and after EB radiation are shown in Fig 10 (a). In the XRD curves, two distinct diffraction peaks at 19.2 ° and 23.4 ° were still displayed after 10kGy radiation, but the peak intensities were weak. After radiation treatment, therefore, the crystal form of PEO did not change, but




the degree of crystallinity had been affected. Fig 10 (b) gives the DSC curves of the PAN/PEO-10% membrane before and after EB radiation. The degree of crystallinity and Tg obtained from DSC test are summarized in Table 2. It can be seen that the degree of crystallinity of PAN/PEO-10% membrane decreased from 30.9% to 15.8% after 10kGy radiation. In addition, the Tg of PAN and PEO can be nearly unchanged.

3.3 Saturated electrolyte uptake and conservation rate of nanofiber membranes The saturated electrolyte uptake ratio and conservation ratio of the electrospun PAN and PAN/PEO membranes at room temperature are shown in Fig 11. Liquid electrolytes can be absorbed by electrospun membranes into the mutual-linked three-dimensional pore and the partial gelation of the nanofibers. The saturated electrolyte uptake can be influenced by the polarity of the polymer and the morphology of nanofiber membranes, such as pore structure, porosity and fiber diameter. The porosities of electrospun PAN and PAN/PEO membranes are given in Table 3. The porosity of PAN/PEO-10% membrane was highest among these samples. In addition, good affinity between nanofiber membranes with PEO shell structure and liquid electrolytes existed. All of these factors led to a relatively high saturated electrolyte uptake ratio of PAN/PEO-10% membrane of 870%. Liquid electrolytes absorbed in nanofiber membranes can be lost gradually along with the extension of time. The lost liquid electrolytes mainly came from micropores among nanofibers, and the PAN/PEO-10% membrane displayed the highest conservation rate (86.8%). After EB radiation treatment, the saturated electrolyte uptake of PAN/PEO-10% membrane dropped a bit, but was higher than other samples while the conservation rate of liquid electrolytes in the EB-radiated nanofiber membrane was dramatically improved. Therefore, good electrochemical performance of PAN/PEO-EB membranes should be expected.

3.4 Basic electrochemical performance of the GPEs Lithium-ion transference number (TLi+) of the GPEs based on the pure PAN and PAN/PEO nanofiber membranes are shown in Table 3. Among them, the TLi+ value of the pure PAN samples was 0.44, nearly equaled to the reported value [34], while the TLi+ value of GPEs with PAN/PEO-10% can go up to 0.74. After treated by EB radiation, the TLi+ value of PAN/PEO GPEs was still higher than that of pure PAN GPEs. The ionic conductivity, which is a key indicator to the selection of electrolyte materials for lithium-ion batteries, is also shown in Table 3. The ionic conductivity of PAN-based GPE was 1.12×10-3S/cm, while almost all of the PAN/PEO-based GPE showed higher ionic conductivity. Especially the ionic conductivity of GPEs with




PAN/PEO-10% rose to 5.36 ×10-3S/cm after EB radiation. Liquid electrolytes absorbed in the micropores and the gelatin layer of the PAN/PEO nanofiber membranes enrich the transport channel for lithium ions, as shown in Fig12. Moreover, the existence of PAN affects the crystallization of the PEO shell. A part of ionic transport occurs through the amorphous region of the PEO and PAN. In PAN phase, the lithium ions are present around the C=O groups in EC and nitrile groups in PAN, and the segmental motion of EC and PAN can affect the movement of lithium ions. In PEO phase, the lithium ion transport can occur mainly through an association-disassociation between the ions and the PEO hosts coupling with the movement of polymer chain segment in the amorphous phase [26]. Therefore, the proper core/shell structure of nanofiber membranes can improve the transference number and ionic conductivity of the resulting GPEs. Compared to the un-radiated nanofiber membranes, crosslinking reactions caused by EB radiation not only destroyed the regularity of the molecular chains, but also reduced the degree of crystallinity of polymers in nanofiber membranes. Accordingly, the ionic conductivity of GPE with EB-radiated PAN/PEO nanofiber membranes was significantly enhanced. Good electrochemical stability is necessary to the GPEs for the practical application in lithium ion batteries. The decomposition voltage of GPE samples can be defined as the voltage value corresponding to the baseline intercept of tangent to the significantly rising stage of current curve in the linear sweep voltammogram [35]. The linear sweep voltammograms of the GPEs based on the nanofiber membranes and commercial Celgard® 2500 are shown in Fig.13. From the curves in Fig.13, it can be seen that the decomposition voltage of all samples was around 5V, which was enough for the application of the electrolytes [36]. The high electrochemical stability of GPEs with core-shell nanofiber structure can be attributed to the fact that the excellent affinity between the ether group of PEO and the oxygen atom of the carbonate ester group in liquid electrolytes [37, 38]. Besides, liquid electrolytes can be partly dissolved in the PEO shell during the gelation process, and the association of -C-O-Li+ groups and swollen fibrous matrix with large surface area also enhanced the electrochemical stability of the GPEs. Fig. 14 shows the interface impedance spectra of Li/GPE/Li cells. The interfacial resistance (Ri) of Li/ Celgard® 2500/Li cell was about 750ȍ. At the same time, the Ri value of the cells with GPEs based on electrospun PAN/PEO-10%, PAN/PEO-EB and pure PAN membranes was 610ȍ, 516ȍ and 967ȍ, respectively. The above results indicated the GPEs based on the PAN/PEO-10 % and PAN/PEO-EB nanofiber membranes had better stability and compatibility with lithium electrodes/liquid electrolytes compared to the commercial Celgard® 2500 and the GPEs based on the pure PAN membranes.






3.5. Cycle performance of the Li/GPE/LiCoO2 cells The cycle performances of the Li/GPE/LiCoO2 cells using different electrolytes at 0.1C rate were tested at room temperature, as shown in Fig 15. In Fig 15(a), the cell with the GPEs based on electrospun PAN/PEO-10% membranes exhibited an initial discharge capacity of 126.5 mAhg-1, which was larger than that of the cell with GPEs based on the pure PAN membranes and Celgard® 2500. After EB radiation of electrospun PAN/PEO-10% membranes, the initial discharge capacity of the resulting cell can attain 136.7 mAhg-1. The cyclic stability of the Li/GPE/LiCoO2 cells with GPEs up to 50 cycles is shown in Fig 15(b). The test was executed between 2.7 and 4.2 V of cutoff voltage. The initial discharge capacity of cells with GPEs based on electrospun PAN/PEO-10%, PAN/PEO-EB, PAN membranes, as well as Celgard® 2500 was 126.7mAhg-1, 136mAhg-1, 117.8mAhg-1 and 125.5mAhg-1, respectively. After 50 cycles, the cells with PAN/PEO-10% membranes retained 90% of its initial discharge capacities, while a retention rate of 95% was presented for the cells with PAN/PEO-EB membranes, which was much higher than that of the cells based on the electrospun pure PAN membranes and Celgard® 2500. The excellent cycle performance of the cells with PAN/PEO-EB membranes can mainly ascribe to high saturated electrolyte uptake and conservation rate, good ionic conductivity and interface performance. Fig 15(c) displays the cycle and C-rate performances of Li/GPE/LiCoO2 cells with GPEs. The cell with GPEs based on PAN/PEO-EB membranes can achieve good discharge capacity of 134, 129, 119,105 and 132 mAh−1 at 0.1, 0.5, 1, 5 and 0.1 C rate, respectively. It can be seen from Fig 15 (c) that the cells with the pure PAN membranes and Celgard® 2500 also showed capacity loss with increasing rate current. However, the discharge capacity of the cells with GPEs based on PAN/PEO-EB membranes was highest among these samples at each current rate. This confirms that the cells containing EB-radiated PAN/PEO membranes with proper core-shell structure possess good cycle and rate performances.

4. Conclusions The polyporous polymer nanofiber membranes with core (PAN) -shell (PEO) structure were prepared by using coaxial electrospinning, and then converted to GPEs after the activation process in liquid electrolytes. The microstructure, crystallinity, thermal stability and mechanical properties of nanofiber membranes could be adjusted by changing the concentration of core-shell spinning solution. High saturated electrolyte uptake and




conservation rate were obtained for the polymer nanofiber membranes with appropriate core-shell structure, which resulted in a satisfactory electrochemical performance of the resulting GPEs, including high ionic conductivities, electrochemical stability and lithium-ion transference numbers. Through the combining with radiation crosslinking, nanofiber membranes with preferred core-shell structure showed good mechanical properties. Furthermore, the Li/GPE/LiCoO2 cells with GPEs based on the radiation crosslinked nanofiber membranes presented remarkable initial discharge capacities and cycle performance compared to those of pure PAN and commercial Celgard products. Consequently, the radiation-crosslinked polymer nanofiber membranes with well-designed core-shell structure will be used as a new type of skeletal material in high performance of GPEs.

Acknowledgment The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 51073019), the Program for New Century Excellent Talents in University (No. NCET-12-0761) and the National High-tech R&D Program of China (863 Program) (No. 2012AA03A203).

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Nomenclature




PAN

polyacrylonitrile

PEO

polyethylene oxide

GPEs

gel polymer electrolytes

EB

electron beam

DMF

N,N-dimethylformamide

EC

ethylene carbonate

DMC

dimethyl carbonate

CA

chronoamperometry

SS

stainless steel

Ȥc

crystallinity(%)

ǻHm

the fusion enthalpy of nanofiber membrane(J/g)

ǻHm*

the fusion enthalpy of PEO with 100% crystallinity(J/g)

P

porosity(%)

ȡa

density of n-butanol(g/cm3)

ȡp

density of dry membrane(g/cm3)

ma

mass of the n-butanol-incorporated membrane(g)

mp

mass of the dry membrane(g)

A

saturated electrolyte uptake(%)

W0

mass of the dry membrane(g)

W1

mass of the saturated membrane(g)

į

ionic conductivity(S/cm)

Rb

the bulk resistance(ȍ)

Ri

the interfacial resistance(ȍ)

L

thickness of the films(cm)

S

area of the films(cm2)

TLi+

transference number

Iss

the steady-state current(A)

I0

the initial current(A)

Tg

glass transition temperature( )



Table captions Table 1 Non-isothermal crystallization parameters and Tg of PEO and nanofiber membranes Table 2 Non-isothermal crystallization parameters and Tg of PAN/PEO-10% before and after EB radiation Table 3 Electrochemical properties of nanofiber membranes

Figure Captions Fig. 1. Schematic diagram of a set-up for coaxial electrospinning Fig. 2. SEM images and diameter distributions (shown as the insert images) of nanofiber membranes: (a) pure PAN (b) PAN/PEO-3% (c) PAN/PEO-7% (d) PAN /PEO-10% (e) PAN/PEO-14% Fig. 3. TEM images of nanofiber membranes: (a) pure PAN (b) PAN/PEO-3% (c) PAN/PEO-7% (d) PAN /PEO -10% (e) PAN/PEO-14% Fig. 4. The XRD patterns of PEO and nanofiber membranes Fig. 5. DSC curves of PEO and nanofiber membranes Fig. 6. TGA curves of PEO and nanofiber membranes Fig. 7. The mechanical properties of nanofiber membranes Fig. 8. FT-IR spectra of PAN/PEO-10% before and after EB radiation Fig. 9. SEM image and diameter distribution (shown as the insert image) of PAN/PEO-EB Fig. 10.The XRD patterns (a) and DSC curves (b) of PAN/PEO-10% before and after EB radiation Fig. 11. Saturated electrolyte uptake ratio and conservation ratio of nanofiber membranes Fig. 12. Schematic diagram of Lithium ion transport channel in gel polymer electrolytes Fig. 13. Linear sweep voltammogram of Li/GPE/SS cells Fig. 14. The interface impedance spectra of Li/GPE/Li cells Fig. 15. The initial discharge capacities (a), cycle performance (b) and C-rate capability (c) of Li/GPE/LiCoO2 cells (25°C, 2.7-4.2V).






Table1 Non-isothermal crystallization parameters and Tg of PEO and nanofiber membranes




Tg of PAN

ǻHf of PEO

Cristallinity of PEO

(J g-1)

%

Pure PEO

202.1

95.0

-61.1

-

PAN/PEO-3%

37.8

17.7

-64.6

116.9

PAN/PEO-7%

59.4

27.8

-65.0

116.0

PAN/PEO-10%

66.0

30.9

-65.8

115.6

PAN/PEO-14%

101.2

47.9

-69.8

115.1

Pure PAN

-

-

-

117.5

!



Tg of PEO

!

(°C

!

(°C

Table 2 Non-isothermal crystallization parameters and Tg of PAN/PEO-10% before and after EB radiation




ǻHf of PEO

Cristallinity of PEO

(J/g)

%

PAN/PEO-10%

66.0

PAN/PEO-EB

33.9

!

Tg of PEO

!

Tg of PAN

!

(°C

(°C

30.9

-65.8

115.6

15.8

-65.1

117.3



Table 3 Electrochemical properties of nanofiber membranes Porosity (%)




TLi+

į (×10-3 S/cm)

Pure PAN

75

0.44

1.12

PAN/PEO-3%

70.2

0.66

0.91

PAN/PEO-7%

71.8

0.72

2.47

PAN/PEO-10%

77.3

0.74

4.93

PAN/PEO-14%

68.8

0.56

4.36

PAN/PEO-EB

72.6

0.57

5.36



Fig. 1.






Fig. 2.




(a)





(b)


(c)





(d)



(e)



Fig. 3.

(a)

(b)

(c)

(d)




(e)



Fig. 4






Fig. 5.






Fig. 6.






Fig. 7.






Fig. 8.






Fig. 9.











Fig. 10.


(a)

(b)










Fig. 11.






Fig.12.






Fig. 13.






Fig. 14.






Fig. 15.






Highlights: 1) The optimized core-shell fiber structure collocated with moderate radiation-crosslinking. 2) The skeleton materials showed good mechanical properties. 3) The GPEs exhibited remarkable electrochemical properties. 4) The GPEs had outstanding initial discharge capacities and cycle performance. 5) High applicable value of the skeleton materials in GPEs for lithium-ion batteries was presented.






Graphic Abstract

Graphical Abstract (for review)