crosslinked polymer brushes on Poly(vinylidene

Materials Chemistry and Physics 217 (2018) 168–174

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Preparation of poly (bis[2-(methacryloyloxy)ethyl] phosphate) crosslinked polymer brushes on Poly(vinylidene fluoride) nanofibers

T

Burcu Oktay, Mustafa Hulusi Uğur, Nilhan Kayaman Apohan∗ Marmara University, Department of Chemistry, 34722, Goztepe-Istanbul, Turkey

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

ethyl phos• Bis(2-methacryloyloxy) phate was grafted onto PVDF nanofiber by ATRP.

nanofibers have a fiber diameter • The of less than 300 nm. PVDF-graft-BMEP nanofibers • The were loaded with 1 M LiPF6. conductivity of the nanofiber • Ionic was increased up to 3.41 × 10−3 S cm−1.

A R T I C LE I N FO

A B S T R A C T

Keywords: Surface-initiated ATRP PVDF nanofiber Ionic conductivity Li-ion battery

In this study, a surface modification of poly(vinylidene fluoride) (PVDF) nanofibers by atom transfer radical polymerization (ATRP) was developed to form bis[2-(methacryloyloxy)ethyl] phosphate (BMEP) crosslinked polymer brushes grafted on nanofibers for application in rechargeable lithium batteries. BMEP was polymerized by using a “grafting-from” technique on the PVDF nanofiber surface by direct initiation. FT-IR (Fourier transform infrared) spectroscopy and SEM (scanning electron microscopy) confirmed the formation of BMEP polymer brushes. Morphological analysis showed that the diameter of the nanofibers was increased by surface grafting. Nevertheless, the characteristic nanofibrillar structure was not changed significantly by ATRP. The thermal properties of the nanofibers were investigated by TGA (thermogravimetric analysis). The results showed that the thermo-oxidative stability of the nanofibers increased with the BMEP content. The conductivities of Li-salts containing nanofibers were in the range 3.41 × 10−3 S cm−1 to 8.26 × 10−4 S cm−1.

1. Introduction An increasing demand for portable electronic devices and electric vehicles has required the development of high energy storage materials with a long cycle life, a high energy density, and a high safety [1,2]. Lithium-ion batteries (LIBs) are commonly used as power sources in present-day digital products [3]. A LIB comprises an anode, a cathode, a

∗

Corresponding author. E-mail address: [email protected] (N.K. Apohan).

https://doi.org/10.1016/j.matchemphys.2018.06.044 Received 12 March 2018; Received in revised form 20 June 2018; Accepted 23 June 2018 Available online 26 June 2018 0254-0584/ © 2018 Elsevier B.V. All rights reserved.

separator, and an electrolyte. The separator allows rapid transfer of the ionic charge carrier, which is crucial for the safety and reliability of LIBs [4,5]. Polyolefin membranes, such as polyethylene, polypropylene, and their blends have been used extensively as commercial separators [2]. However, polyolefin membranes show poor thermal stability and thermal shrinkage. Consequently, they are potentially unsafe [6]. Poly (vinylidene fluoride) (PVDF) is a semi-crystalline polymer,


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N,N′-dimethylformamide (DMF, 99.5%, Merck) and acetone (99.7%, Merck) were used as solvents.

which possesses excellent chemical stability, good mechanical strength, and thermostability [7]. It also has a relatively high dielectric constant in a polymer matrix and is easily prepared in film form [8]. Thus, the use of PVDF and its copolymers as a separator for rechargeable LIB applications has been widely investigated in recent years. Various methods have been studied to prepare PVDF-based membranes, such as self-assembly, chemical vapor deposition, solution-growth, solvothermal or hydrothermal methods, and electrospinning. Among these methods, electrospinning is a versatile and easy technique [9]. However, PVDF membrane application is limited because of its inherent hydrophobicity; thus, it is often modified chemically and/or physically. Various methods are used, including grafting of functional groups, graft polymerization, or coating [10]. Free-radical graft polymerization of vinyl monomers is the most studied technique [11]. Freeradicals are produced by a free-radical initiator, ozone treatment, and/ or exposure to ionizing radiation [12]. A disadvantage of free-radical mechanisms is that uncontrolled radical products make the control of polymer brush length and architecture difficult. Moreover, the grafting density of the comonomer can be limited [13]. However, atom transfer radical polymerization (ATRP) is a type of controlled radical polymerization, which can overcome these problems [14]. ATRP combines an organic halide initiator (RX) with a catalytic system. The polymer chains extend with regularity from chemical groups containing radically transferable halogen atoms during polymerization. The halogen atoms serve as initiation sites for polymerization of the monomer [15]. There are many reports on the preparation of PVDF copolymers. Direct initiation of the secondary fluorinated site of PVDF facilitates the preparation of amphiphilic graft copolymers. For example, Hester et al. prepared amphiphilic graft copolymers from PVDF. ATRP of hydrophilic monomers was initiated at the secondary halogenated sites of PVDF [16]. Similarly, Chen et al. reported the formation of homopolymer brushes of poly((2-dimethylamino)ethyl methacrylate) (DMAEMA) and poly (ethylene glycol) monomethacrylate (PEGMA) by direct initiation from PVDF film [17]. Arslantaş et al. designed high performance PVDF-graft-copolymer membranes via ATRP of glycidyl methacrylate (GMA). The membranes were then quantitatively modified with 5-aminotetrazole by ring opening of the epoxide group [18]. Dong et al. reported the preparation of polymerizable low-molecularweight organic UV-absorber grafted onto PVDF membrane via ATRP [19]. A major challenge related to LIB technology originates from safety issues [20,21]. Short circuits, overcharging, and overheating cause explosions and fires associated with LIBs [22,23]. To solve this problem, new electrolytes have been reported that have flame-retardant and voltage-clamping properties [24–27]. Recently, organophosphorus compounds have been widely investigated as additives for LIB electrolytes [28]. In this study, we prepared a novel organophosphorus grafted PVDF nanofiber. Firstly, PVDF nanofibers with a uniform interconnected morphology were fabricated by electrospinning, and then bis[2-(methacryloyloxy)ethyl] phosphate (BMEP) as an organophosphorus monomer was polymerized by surface-initiated ATRP (SIATRP). The PVDF-graft-BMEP nanofiber mat was then loaded with 1 M LiPF6 in a mixture of EC/DMC(ethylene carbonate/dimethyl carbonate) (v/v, 1:1) to obtain a polymer electrolyte. The ionic conductivity, electrochemical stability, and thermal properties were investigated. We believe that PVDF-graft-BMEP nanofiber electrolytes exhibit a synergistic effect due to PVDF and BMEP. Moreover, the incorporation of BMEP improves the thermal properties, resulting in a safe electrolyte.

2.2. Nanofiber fabrication A 10% w/v PVDF solution was prepared in a mixture of DMF and acetone (volume ratio 2:1). A 1% w/v PCL solution was prepared in a solvent mixture with the same volume ratio. Since PVDF solution could not be spun alone, it was blended with PCL solution (PVDF:PCL = 100:1) to improve the spinning performance. Briefly, 1 mL of 1% PCL solution was added to 10 mL of 10% PVDF solution, and the mixture was stirred until homogeneous. The solution was then transferred into a syringe fitted with a stainless steel needle and attached to a power supply. A collector was placed 17 cm away from the tip of the syringe. An electrical field of 26 kV was applied between the electrospinning solutions. The fabricated nanofibers were collected in aluminum foil. The flow rate of the solution was fixed at 0.5 mL/h. 2.3. Surface functionalization of PVDF nanofibers via SI-ATRP and electrolyte membrane preparation Crosslinked BMEP brushes were grafted on the nanofiber surface using the secondary fluorine atoms of PVDF as initiating sites. PVDF nanofibers were first cut to cover an area of approximately 2 × 2 cm2. The nanofibers were cleaned in acetone for 15 min to remove organic contaminants before drying. The overall ATRP procedure was performed in an argon-filled glove box. A typical ATRP process with a [monomer]:[CuBr (catalyst)]:[Bpy (ligand)] molar ratio of 100:1:2 or 50:1:2 in methanol was performed as follows. The PVDF nanofibers were added to a 50 mL round bottom flask containing 5 mL of methanol, 6.7 mg of CuBr (0.046 mmol), 14.5 mg of Bpy (0.092 mmol), and BMEP. The reaction was performed for 24 h at room temperature. The mixture was then exposed to air to stop the reaction. The PBMEP brushes grafted on PVDF (PBMEP-g-PVDF) nanofibers were thoroughly washed with methanol to remove physically adsorbed reactants. The resultant PBMEP-g-PVDF nanofibers were dried in vacuum. Finally, nanofiber membranes were put into 1 M LiPF6/(EC + DMC) (1:1, by volume) for electrolyte preparation. An illustration of the preparation of electrolyte membrane along with the structures of the compounds used in this work can be seen in Scheme 1. 2.4. Characterization Attenuated total reflectance (ATR) FTIR spectra of the surfacefunctionalized PVDF nanofibers were obtained with a Perkin–Elmer ATR-FTIR spectrophotometer. The spectra were collected in the range from 4000 to 400 cm−1. The morphology and diameter of the nanofibers were determined by environmental scanning electron microscopy (SEM) on a Phillips XL 30 ESEM-FEG instrument. The specimens were

2. Experimental 2.1. Materials PVDF (Mn = 65.000 g mol−1), polycaprolactone (PCL, Mn = 70.000–90.000 g mol−1), copper(I) bromide (CuBr) (99.999%), 2,2′-bipyridyl (≥99%), and BMEP were supplied by Sigma–Aldrich.

Scheme 1. An illustration of the preparation of electrolyte membrane. 169


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coated with gold and then scanned by SEM. The SEM images were taken at various magnifications. Thermogravimetric analysis (TGA) was performed using a Perkin–Elmer thermogravimetric analyzer Pyris 1 TGA model. Nanofiber samples were heated from 30 °C to 750 °C at a rate of 10 °C/min in air. Ionic conductivity experiments were conducted using a Gamry Potentiostat/Galvanostat/ZRA (Gamry Series G 750, Warminster, PA, USA) with a Gamry Framework software system EIS300. EIS spectra were analyzed using Echem Analyst 5.67 software. Impedance measurements were performed on liquid electrolyte-soaked membranes sandwiched between two stainless steel electrodes (type 304 SS, 0.025 mm thick, Alfa Aesar, Karlsruhe, Germany) over the frequency range 40 Hz–100 mHz with an AC amplitude of 10 mV. The thicknesses of the electrolytes were determined before and after EIS to ensure a constant thickness throughout the experiment. Ionic conductivities were determined under an argon atmosphere using a potentiostat/galvanostat and a custom-designed glove box with a conductivity cell. The ionic conductivity was determined from the intercept of the real part of the complex impedance plot, which gives the resistance of the electrolyte, and a known area using Equation (1): Fig. 1. FTIR ATR spectra of A) PVDF nanofiber, B) BMEP monomer, C) PBMEP50-g-PVDF nanofiber, and D) PBMEP100-g-PVDF nanofiber.

1 L σ = ⎜⎛ ⎟⎞⋅⎜⎛ ⎞⎟ R A ⎝ ⎠⎝ ⎠

vibration was also observed in the spectra. The band at around 1724–1720 cm−1 belongs to the carbonyl of the methacrylate in PBMEP [34].

whereσ is the conductivity, L is the electrolyte thickness, R is the electrolyte resistance, and A is the electrolyte area. A Gamry Potentiostat/Galvanostat/ZRA (Gamry Series G 750, Warminster, PA, USA) with a Gamry Framework Software System PHE200 (Physical Electrochemistry Software, Warminster, PA, USA) was used to perform linear sweep voltammetry (LSV). This experiment was conducted to survey the electrochemical stability window of the electrolytes, employing SS as working electrode and lithium foil (Sigma–Aldrich, Steinheim, Germany) as reference and counter electrodes. The cell was assembled in the glove box under argon atmosphere. The scanning rate was set to 10 mV s−1, and the potential ranged from open circuit potential to 6.0 V (vs Li/Li+). All the electrochemical experiments were performed in an argon atmosphere in a homemade glove box at room temperature.

3.2. SEM images of PVDF-based nanofibers PVDF nanofibers were prepared by electrospinning, forming a highly porous three-dimensional network. A SEM image of the PVDF nanofibers is shown in Fig. 2. This indicates that the nanofibers have a beadless morphology and a narrow size distribution with diameters ranging from 100 to 300 nm. Web morphologies and the high specific surface area of the nanofibers enhance the electrochemical activity of the electrolyte materials. The high surface area and porous structure provide short ion transport paths [9]. Surface-initiated ATRP of BMEP was performed in methanol in the presence of Cu(I)Br/2,2′-bipyridine. The molar ratios of catalyst to divinyl monomer were set as 1:50 and 1:100 for PBMEP50-g-PVDF and PBMEP100-g-PVDF, respectively, where 50 and 100 represent the initial monomer/catalyst ratios. This reaction system was successfully used for the controlled polymerization of phosphorus-containing (meth)acrylate [35]. ATRP is a useful method for designing polymer brushes from

3. Results and discussion 3.1. FTIR spectra PVDF is known to form four different crystalline structures: the orthorhombic α, β, and δ phases, and the monoclinic γ phase. The ATRFTIR spectra of the PVDF and PBMEP-g-PVDF nanofibers are shown in Fig. 1. In Fig. 1A, the bands at 760, 796, 975, and 1070 cm−1 were assigned to the α-phase, while those at 510, 839, and 876 cm−1 corresponded to the β-phase of PVDF [29]. The strong peak at 1400 cm−1 was ascribed to -CH2 in-plane blending. The peaks at 1164 and 1225 cm−1 were attributed to in-plane stretching of C–F bonds. A band at 1672 cm−1 was assigned to the carbonyl group in DMF [30]. The peak at 835 cm−1 was assigned to CF2, and that at 868 cm−1 was assigned to the CeC vibration [31,32]. The characteristic peaks of PVDF were also observed for all samples. ATR-FTIR analysis was used to characterize the PBMEP brushes. In Fig. 1B, the phosphate groups of the BMEP monomer were observed at 1250 cm−1. The bands at 1161 and 983 cm−1 were related to stretching vibrations of P=O and PeOeC, respectively. C=C double bonds were observed at 1636 and 813 cm−1 [33]. Fig. 1C and D shows the ATR-FTIR spectra of PBMEP grafted PVDF nanofibers using different monomer concentrations. It was observed that the characteristic peaks of PBMEP were retained for both spectra. The peaks at 1171 and 977 cm−1 were related to the P=O and PeOeC groups of BMEP. The peak at 1400 cm−1 was assigned to the CH2 vibration of PVDF, while the CF2 stretching vibration of PVDF was represented at 1171 cm−1. A peak at 977 cm−1 corresponding to CeC

Fig. 2. SEM image of the PVDF nanofibers. 170


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Table 1 Surface compositions of nanofibers before and after modification. BMEP50-g-PVDF

C O F P

BMEP100-g-PVDF

Before ATRP

After ATRP

Before ATRP

After ATRP

47.66 6.81 45.53 –

48.94 16.92 28.25 0.65

46.99 7.19 45.82 –

52.42 15.70 30.86 1.02

(EDS) (Fig. 4). The surface composition deduced from this spectrum is given in Table 1. In the spectrum of PVDF nanofibers, the C1s and F1s peaks originating from the main chemical elements of the PVDF are present, and O1s indicates the presence of PCL. After ATRP of BMEP, in addition to C1s, F1s, and O1s, P2s peaks appeared, indicating successful formation of PBMEP brushes on the surface of PVDF nanofibers (Fig. 4b). For PBMEP50-g-PVDF nanofibers, the C content increased from 47.66% to 48.94%. The percentage of O increased significantly after BMEP modification from 6.81% to 16.92%. The phosphorus content on the surface was 0.65%. Similarly, the percentages of C and O increased with the BMEP content on the PBMEP100-g-PVDF nanofibers. The percentages of C and O increased after grafting of BMEP on the PBMEP100-g-PVDF nanofibers. Additionally, the phosphorus content was 1.02%. The morphological changes in PVDF nanofibers after impregnation with lithium salt solution were also investigated by SEM. Fig. 5 shows a SEM image of the PVDF nanofibers. The average fiber diameter was almost the same as before. Furthermore, the fiber morphology did not change significantly. PVDF has a low affinity for electrolyte solutions [39]. Fig. 6a and b shows SEM images of the PBMEP grafted PVDF

Fig. 3. SEM images of the PBMEP-g-PVDF nanofibers (a) PBMEP50-g-PVDF and (b) PBMEP100-g-PVDF.

various monovinyl and divinyl monomers. In ATRP of a divinyl monomer, the propagation is controlled by a fast activation/deactivation equilibrium, and active primary chains are present at a low concentration, as with monovinyl monomer. However, several vinyl groups of the monomer react with the propagating center during each active cycle [36]. During ATRP, the monomer converts to polymer chains containing pendant vinyl groups. The propagating linear chains react with vinyl groups of the free monomer as well as the pendant vinyl groups, because all the vinyl groups have similar reactivity [37]. Therefore, a branched structure is accomplished by intermolecular reactions between the pendant vinyl and propagating radicals. Kim and coworkers reported that the electrochemical properties of polymer electrolytes are mainly affected by the structural characteristics of the porous polymer matrix, such as the fiber diameter, pore size, and pore structure [38]. The effect of PBMEP crosslinked polymer brushes on the nanofiber diameter and morphology was also evaluated by SEM. SEM images of PBMEP50-g-PVDF (Fig. 3a) and PBMEP100-g-PVDF (Fig. 3b) nanofibers showed that the nanofibrillar structure was largely preserved. The PBMEP chains were evidently bonded on the PVDF fiber surfaces. However, relatively small amount of PBMEP chains completely fill the pores in the resulting PVDF fiber surface. The diameter of the fibers increased further as the PBMEP content was increased. The elemental compositions of the PVDF nanofibers before and after PBMEP grafting were analyzed by energy dispersive X-ray spectroscopy

Fig. 5. SEM image of Li salt impregnated PVDF nanofibers.

Fig. 6. SEM images of Li salt impregnated nanofibers (a) PBMEP50-g-PVDF and (b) PBMEP100-g-PVDF.

Fig. 4. EDS spectra of (A) PVDF and (B) PBMEP50-g-PVDF. 171


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Table 2 Thermal properties of nanofibers. Codes

T1 (˚C)

PVDF PBMEP50-gPVDF PBMEP100-gPVDF

T2 (˚C)

T3 (˚C)

Char yield (%)

Range

Peak

Range

Peak

Range

Peak

314–369 195–263

347 236

438–482 443–486

460 468

494–531 502–528

516 516

0.071 4.94

269–346

305

394–445

423

493–514

528

7.809

decomposition temperature with two weight loss steps. The first weight loss step of PBMEP50-g-PVDF was observed at 195–263 °C (curve b), while the first weight loss of PBMEP100-g-PVDF was observed at 269–346 °C (curve c). As shown, the first decomposition temperature of PBMEP50-g-PVDF is lower, compared to PBMEP100-g-PVDF. This is probably due to thermal decomposition of the PBMEP brushes [45]. The second weight loss step is in the range 443–486 °C for PBMEP50-g-PVDF and 394–445 °C for PBMEP100-g-PVDF, which was attributed to the decomposition of PVDF. The final weight loss of the PVDF nanofibers was observed at 516 °C. PBMEP grafted PVDF nanofibers showed final weight losses at 516 °C and 528 °C for PBMEP50-g-PVDF and PBMEP100-g-PVDF, respectively. PVDF nanofibers showed a char yield of 0.071% at 750 °C. However, that of PBMEP50-g-PVDF was 4.94% at 750 °C, which was lower than the value of 7.80% obtained for PBMEP100-g-PVDF nanofibers. The TGA data is summarized in Table 2. It is known that phosphorus polymers degrade at low temperatures because of the lower energy of the C–P bond (264 kJ/mol) [46,47]. Early degradation of phosphorus polymers creates a glassy layer above the sample and limits the generation of combustible carbon-containing gases by inhibiting oxygen transfer [48]. Hence, the grafting of PBMEP crosslinked polymer brushes from the PVDF nanofiber surface is a very effective way to increase residual char. The flame retardancy of the materials is improved by covering their surfaces with char.

Fig. 7. TGA curves of nanofibers, a) PVDF nanofiber, b) PBMEP50-g-PVDF, and c) PBMEP100-g-PVDF.

nanofiber electrolytes. The fiber diameters changed significantly by swelling during lithium salt impregnation. Thus, it is clear that as expected, there is a specific interaction between the methylene group of BMEP and the carbonyl groups of the carbonates [40,41]. The nanofiber matrix, with an appropriate porous structure and interactions between the fiber and the solvent, plays a vital role in lithium-ion transport [42,43]. The decrease in porosity due to swelling of the nanofiber membranes in the presence of the LiPF6/EC/DMC mixture, observed in Fig. 6, was attributed to the strong interactions and electrolyte absorption behavior of the PBMEP-g-PVDF nanofibers.

3.3. Thermal analysis It is known that fluoropolymers are more thermally stable than hydrocarbon polymers because of the high electronegativity of fluorine and the high dissociation energy of the C–F bond [44]. The thermal properties of the nanofibers were investigated by TGA and DTG, as shown in Fig. 7 and Fig. 8, respectively. Thermal degradation ranges were determined from the corresponding derivative curves (Fig. 8). Fig. 7 shows TGA curves of PVDF, PBMEP50-g-PVDF, and PBMEP100-gPVDF nanofibers. Due to differences in their chemical structures, the nanofibers began to decompose within different temperature ranges. As shown in curve a, PVDF nanofibers exhibited weight loss in the range 430–531 °C. Conversely, the PBMEP grafted nanofibers showed a lower

3.4. Conductivity The ionic conductivity of polymer electrolytes is the most critical parameter in achieving a good electrochemical performance for LIBs. Fig. 9 presents Nyquist curves of nanofiber electrolytes determined by AC impedance at 30 °C. The ionic conductivity was obtained from the high frequency intersection of the curve with the Z axis. The L/A values for the nanofiber electrolytes, PBMEP100-g-PVDF and PBMEP50-g-PVDF, were 0.675 and 0.225 cm−1, respectively. PBMEP100-g-PVDF showed a higher ionic conductivity (3.41 × 10−3 S cm−1) than PBMEP50-g-PVDF

Fig. 8. DTG curves of nanofibers, a) PVDF nanofibers, b) PBMEP50-g-PVDF, and c) PBMEP100-g-PVDF.

Fig. 9. Typical Nyquist diagrams of nanofiber electrolytes. 172


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Fig. 10. Linear sweep voltammetry (LSV) curves of the nanofiber electrolytes (scan rate: 10 mV s−1).

(8.26 × 10−4 S cm−1). Absorbed lithium salt solution is known to be responsible for ionic conductivity [49]. Also, the interactions and compatibility between the fiber and lithium salt electrolyte solution play a major role in lithium ion transport within the matrix [42]. Hence, it was suggested that a higher BMEP ratio will increase the conductivity. As shown in SEM images, the fiber thickness of the Li doped electrolyte increases due to absorption of lithium salt solution, and better interactions occur between the methylene groups of BMEP and the carbonyl group of carbonate. A similar trend was reported previously by Song et al. [40,50]. The ionic conductivities obtained in this study are much higher than in other academic studies on polymeric separators and electrolytes [51–54]. 3.5. Linear sweep voltammetry (LSV) Fig. 10 shows the electrochemical stability window of the PVDFbased nanofiber electrolytes as a function of voltage in the range 3.0–6.0 V vs Li at room temperature. The cells were prepared with a SS electrode and lithium counter and reference electrodes [55]. As shown in the first region, the plateau is flat and straight; this low residual current prior to the breakdown voltage, with no peaks in the voltage ranges 3.0–5.15 V and 3.0–5.45 V, respectively, for PBMEP50-g-PVDF and PBMEP100-g-PVDF, confirms the high purity of the prepared nanofiber electrolytes [56,57]. Beyond 5.15 and 5.45 V, the current increases rapidly with potential and causes anodic breakdown due to decomposition of the nanofiber electrolytes. This can be attributed to differences in the absorption and storage volumes of the PBMEP50-gPVDF and PBMEP100-g-PVDF nanofiber electrolytes. In the examined voltage range, the rapid increase in current is considered as the upper limit of the electrolyte stability range. The enhanced current in the high-voltage range is generally attributed to decomposition of the electrolytes. Thus, the high stability of the nanofiber electrolytes may be due to the absence of impurities, which is a desirable feature because it allows their use in high-voltage batteries. Thus, this electrolyte can be used in lithium-ion polymer batteries, which have cell voltages around 4.5 V. 4. Conclusions Poly(bis[2-(methacryloyloxy)ethyl] phosphate) grafted PVDF nanofiber-based polymer electrolyte membranes were prepared by surface-initiated ATRP in a one-step process. PVDF nanofibers were initially prepared by electrospinning with diameters ranging from 100 to 300 nm. The secondary fluorine groups of PVDF enabled direct initiation without an additional ATRP initiator. It is concluded that the lithium salt-doped nanofiber electrolytes are sufficiently electrochemically stable for use in high-voltage batteries. 173


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