Introduction

Lithium ion batteries have been improved using polymer nanofibrous electrolyte membrane with its highly porous structure, high electrolyte uptake and ionic conductivity to transport as much as lithium ions through it. Polymer nanofibrous electrolyte membrane provides wide electrochemical operating window and good thermal stability useful to prevent electrolyte leakage and to minimize the firing hazard for high safety of batteries as compared to liquid polymer electrolyte [1,2,3,4,5,6]. Poly(ethylene oxide) (PEO), polyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVDF), poly(vinyl alcohol) (PVA) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF–HFP) have been studied as host polymer for fabricating nanofibrous polymer electrolyte membrane [7,8,9,10,11,12,13,14,15,16]. Among these polymers, poly(vinylidene fluoride) (PVdF) has been mostly used as a semi-crystalline polymer with excellent film-forming ability, high dielectric constant and thermal stability [17]. But the crystalline domains of PVdF restrict the penetration of liquid electrolytes and the movement of lithium ions from between the electrodes during charging and discharging which show low ionic conductivity. Therefore, researchers in this field are more engaged to prepare polymer electrolyte membranes by blending or forming composites using different polymers or metal oxides to increase ionic conductivity, electrolyte uptake, and electrochemical stability than that of pure polymer electrolytes [18,19,20,21,22]. Li et al. prepared PVDF/PMMA membrane by anchoring PMMA to multiporous PVDF surface via electron beam preirradiation grafting technique and showed ionic conductivity 6.1 × 10−3 S/cm [23]. Also, Idris et al. prepared PVdF/PMMA microporous membranes using the phase-separation method with increasing percentage of PMMA and showed discharge capacity of 133 mAh/g [24]. Many different synthesis methods are used to prepare polymer electrolyte membrane by solution casting [15, 20], phase inversion [25] and electrospinning [26]. Among them, electrospinning is a simple method for preparation of nanofibrous membranes with high porosity due to tunable fiber diameter controlled by varying applied electric field, distance between syringe needle and grounded collector, polymer solution concentration, and flow rate of viscous polymer solution. Porosity is the size-dependent property; therefore, electrospun nanofibers of blended polymers synthesized by electrospinning possess high porosity which is responsible for increase in electrolyte uptake and high ionic conductivity at room temperature [27, 28]. Li et al. and Mahant et al. prepared fibrous membranes of poly(vinylidene fluoride)/poly(methyl methacrylate) (PVdF/PMMA) by electrospinning method and showed ionic conductivity 3.5 × 10−3 and 2.95 × 10−3 S/cm, respectively [29, 30]. Therefore, efforts have been made to optimize the composition of PVdF and PMMA to fabricate their composites nanofibrous membrane by electrospinning so as to increase the ionic conductivity.

In this work, the fabrication of polymer nanofibrous electrolyte membranes of PVdF–PMMA composites in different proportion (PVdF:PMMA = 100:0, 80:20 and 50:50) by electrospinning is reported to investigate the influence of PMMA on lithium ion battery performance. The effect of concentration of PMMA in composites on morphology, ionic conductivity, porosity and discharge capacity retention for lithium ion battery separator is studied and systematically compared.

Experimental

Preparation of PVdF–PMMA composites nanofibrous polymer electrolytes

PVdF–PMMA composites were prepared with varying weight ratio of PMMA in PVdF. The total polymer concentration was fixed at 15 wt%. PVdF–PMMA composites nanofibrous membranes in different proportion of PVdF and PMMA were prepared by electrospinning. In a typical procedure for the preparation of PVdF–PMMA (50:50) nanofibrous membrane, 15% PVdF–PMMA (5:5, w/w) was dissolved in a mixed solvent N,N-dimethylformamide (DMF)/tetrahydrofuran (THF) (7:3, V/V) and magnetically stirred to form a homogeneous solution and then transferred the mixed polymer solution to disposable syringe for electrospinning to get continuous nanofibers. During electrospinning, computer controlled flow rate 0.6 ml/h, electric field 20 kV and distance 18 cm between the syringe needle and grounded collector (aluminum foil) were maintained. The nanofibrous membrane on the collector plate was dried under vacuum at 70 °C for 12 h and then separated from the foil for preparation of polymer electrolyte. In the similar way, PVdF–PMMA (50:50) and PVdF–PMMA (100:00) abbreviated as pure PVdF have been prepared by electrospinning with the same operating conditions. PVdF–PMMA nanofibrous polymer electrolytes were prepared by immersing the electrospun nanofibrous membranes in 1 M LiPF6 (lithium hexaflurophosphate) in EC:DMC (1:1 v/v) (ethylene carbonate and dimethyl carbonate) solution at room temperature in a glove box under nitrogen atmosphere.

Characterizations

The surface morphology of electrospun nanofibrous membranes was investigated by scanning electron microscope (CARL ZEISS EVO-18). Fourier transform infrared (FTIR) spectra of electrospun nanofibrous membranes were obtained on α-Bruker model. X-ray diffraction (XRD) patterns of electrospun nanofibrous membranes were on Rigaku Miniflex II Desktop X-ray diffractometer. The crystallinity of electrospun nanofibrous membranes was obtained using differential scanning calorimetry (DSC) (Mettler Toleno DSC 822 e) with heating rate 10 °C per min under N2 atmosphere. Thermogravimetric analysis (TGA) was done by Perkin Elmer STA 6000 at the heating rate 10 °C min−1 from room temperature to 700 °C under N2 atmosphere. Porosity of electrospun nanofibrous membranes was calculated by weighing membrane before and after absorbing n-butanol and knowing the density of n-butanol. The electrolyte uptake of polymer electrolyte membranes was calculated by soaking the membrane in lithium hexaflurophosphate (LiPF6), ethylene carbonate (EC) and dimethyl carbonate (DMC) solution. The ionic conductivity (\( \sigma \)) of polymer electrolyte membranes was determined through an ionic conductivity cell, by sandwiching a given polymer electrolyte membrane between two stainless steel blocking electrodes (SS/polymer electrolyte membrane/SS, SS: stainless steel) using Zahner Zennium Electrochemical Analyzer at room temperature in frequency range between 10 mHz and 100 kHz with AC amplitude of 10 mV.

Results and discussion

Figure 1 shows SEM images and histograms of electrospun (a) PVdF, (b) PVdF/PMMA (80–20), and (c) PVdF–PMMA (50–50) nanofibrous membranes. It can be seen from SEM images that the average fiber diameter decreases with increasing PMMA percentage from 0 to 50% in the composites. The average fiber diameter of PVdF, PVdF–PMMA (80:20) and PVdF–PMMA (50:50) is found to be 647, 183 and 179 nm, as shown in histograms Fig. 1d–f, respectively. This unique porous structure is beneficial for the electrolyte to penetrate and retain electrolyte effectively, and it facilitates an electrolyte to diffuse smoothly into the cell assembly.

Fig. 1
figure 1

a SEM image of electrospun PVdF nanofibrous membrane. b SEM image of electrospun PVdF–PMMA (80:20) nanofibrous membrane. c SEM image of electrospun PVdF–PMMA (50:50) nanofibrous membrane. d Histogram of electrospun PVdF nanofibrous membrane. e Histogram of electrospun PVdF–PMMA (80:20) nanofibrous membrane. f Histogram of electrospun PVdF–PMMA (50:50) nanofibrous membrane

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Figure 2 shows the FTIR spectra of electrospun PVdF, PVdF–PMMA (80–20) and PVdF–PMMA (50–50) nanofibrous membranes. It is observed from the figure that for pure PVdF, the characteristic bands are appeared at 1727, 1401, 1077, 1005, 884, 690 and 621 cm−1. The band at 1727 cm−1 assigned to stretching vibration of C=C. The band at 1401 cm−1 is related to stretching vibration of CF2 group. The bands appearing at 1077 and 1005 cm−1 are appeared due to scissoring vibration of CF2 group and stretching vibration of CH group, respectively. A band at 884 cm−1 which shows the characteristic peak of vinylidene group of PVdF is assigned to C–F stretching. The bands at 690 and 621 cm−1 are due to crystalline phase of PVdF and indicate out of plane C–H bending and bending vibration of CF2. The characteristic bands for PVdF–PMMA (80:20) are appeared at 1721, 1398, 1181, 1027, 884, 695 and 621 cm−1. The band at 1721 cm−1 is identified as C=O stretching vibration in the pendant group (–COOCH3) of PMMA. The characteristic peaks at 1398 and 884 cm−1 are due to C–H bending vibration and –CH2– stretching vibration [31]. It is observed that the intensity and peak position are slightly shifted due to blending of PMMA. These changes clearly indicate the miscibility of PVdF and PMMA. For PVdF–PMMA (50:50), it is found that new bands at 1446 and 1528 cm−1 are appeared. The band at 1446 cm−1 corresponds to CH2 wagging deformation of pure PVdF membrane. It can be seen that intensity of this peak increases with the addition of PMMA. The peak corresponding to 1734 cm−1 in the PVDF/PMMA blend membrane is due to the carbonyl (C=O) group present in PMMA. This result suggests that there is a specific interaction between the carbonyl groups of PMMA and the CH2 groups of PVDF and indicates the formation of a PVDF/PMMA blend membrane.

Fig. 2
figure 2

FTIR spectra of a PVdF, b PVdF–PMMA (80:20), and c PVdF–PMMA (50:50) nanofibrous membranes

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X-ray diffraction pattern helps to provide the influence of PMMA on structure of PVdF in the sample. Figure 3 shows the XRD patterns of PVdF, PVdF–PMMA (80–20) and PVdF–PMMA (50–50) nanofibrous membranes. PVdF membrane has sharp peaks at 20.7° (110) and 26.60° (022) corresponding to orthorhombic α-phase of PVdF [30]. These peaks have been suppressed in the PVdF–PMMA composites. PVdF–PMMA (80:20) shows a semicrystalline nature whereas PVdF–PMMA (50:50) shows amorphous nature. Thus, by adding more PMMA in PVdF composites, the crystallinity of PVdF decreases and PVdF–PMMA (50:50) blend becomes more amorphous which enhances greater ionic diffusion causes more ionic conductivity of the polymer electrolyte membrane.

Fig. 3
figure 3

XRD patterns of a PVdF, b PVdF–PMMA (80:20), and c PVdF–PMMA (50:50) nanofibrous membranes

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Figure 4 shows the differential scanning calorimetric (DSC) curves of PVdF, PVdF–PMMA (80–20) and PVdF–PMMA (50–50) nanofibrous membranes. In DSC curves, melting temperature (Tm) goes on decreasing from a to c. It is observed from figure that Tm for PVdF: PMMA (100:00) is 173 °C; it goes on decreasing with the incorporation of PMMA in PVdF. The value of Tm for PVdF–PMMA (80:20) and PVdF–PMMA (50:50) is 170 and 156 °C. The decrease in melting temperature is the indication of decreased crystallinity of polymer matrix in the polymeric system. It further leads to the improvement in the segmental motion of the polymeric chain and, thereby, enhances the ionic conductivity of polymer blend electrolytes. The absence of sharp glass transition and any additional peak for PVdF–PMMA blends confirm the miscibility of PMMA with PVdF in the polymer blends. The crystallinity (χc%) of PVdF, PVdF–PMMA (80–20) and PVdF–PMMA (50–0) nanofibrous membranes was calculated from the crystalline melting heat of perfectly crystalline PVdF (\( \Delta H_{f}^{*} = 105 \) J/g), the melting enthalpy of the electrospun membranes \( (\Delta H_{f} ) \) in DSC curves determined from the integral area of the baseline using Eq. (1) [27] and listed in Table 1.

Fig. 4
figure 4

DSC curves of a PVdF b PVdF–PMMA (80:20) c PVdF–PMMA (50:50) nanofibrous membranes

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Table 1 Melting enthalpy (∆H f ), crystallinity (χc), bulk resistance (Rb) and ionic conductivity (σ) of PVdF, PVdF/PMMA (80:20) and PVdF/PMMA (50:50) nanofibrous membranes
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$$ \chi_{\text{c}} (\% ) = \frac{{\Delta H_{f} }}{{\Delta H_{f}^{*} }} \times 100. $$
(1)

The evaluated crystallinity of PVdF–PMMA (50–50) membrane was found to be 24.12% as compared to 41.75% for PVdF–PMMA (80–20) and 50.75% for pure PVdF membrane. Hence, the prepared PVdF–PMMA (50–50) membrane has the lowest crystallinity means amorphous nature as confirmed from XRD. Lowest crystallinity of PVdF–MMA (50–50) nanofibrous membrane represents the membrane suitability towards potential separator in lithium ion battery due to high electrolyte uptake and ionic conductivity which helped for more migration of lithium ions through it.

Figure 5 shows thermogravimetry (TGA) of PVdF, PVdF–PMMA (80–20) and PVdF–PMMA (50–50) nanofibrous membranes. It can be seen from the figure that the onset decomposition temperature of PVdF is 454 °C with a weight loss of 8%. The second decomposition occurs at 510 °C with a weight loss of about 70%. There are two decompositions on the TG curve of PVdF:PMMA (80:20). The first decomposition occurs at 368 °C with a weight loss of about 9% which may be due to decomposition of PMMA in the membrane. The second decomposition occurs at 430 °C with a weight loss of about 30%. A small weight loss 4% is observed for PVdF: PMMA (50:50) with first decomposition temperature at 288 °C and the second decomposition for the same composition occurs at 330 °C with a weight loss of about 30% which may be due to increase in content of PMMA in the membrane. These results indicate that the PVdF–PMMA membrane has a good thermal stability with minimum weight  % loss.

Fig. 5
figure 5

Thermogravimmetry (TGA) of a PVdF, b PVdF–PMMA (80–20) and c PVdF–PMMA (50–50) nanofibrous membranes

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Porosity of the prepared PVdF, PVdF–PMMA (80–20) and PVdF–PMMA (50–50) nanofibrous membranes was determined by n-butanol uptake method. Nanofibrous membranes were immersed in n-butanol for 2 h. The porosity % of PVdF, PVdF–PMMA (80:20) and PVdF–PMMA (50:50) nanofibrous membrane was found to be 78, 83 and 87%, respectively. The high porosity was reflected to explain the high electrolyte uptake of polymer electrolytes. The electrolyte uptake behavior of the electrospun PVdF, PVdF–PMMA (80:20) and PVdF–PMMA (50:50) membrane was recorded after every 5 min by soaking the electrospun membranes in the liquid electrolyte 1 M LiPF6/EC:DMC (1:1 v/v) solution for 30 min. The electrolyte uptake was calculated by measuring weight of electrospun membrane before and after absorbing the electrolyte solution [27]. The PVdF, PVdF–PMMA (80:20) and PVdF–PMMA (50:50) membranes showed an electrolyte uptake of about 260, 275 and 290% within 30 min (Fig. 6). It is observed from the data that electrolyte uptake % increases with increasing PMMA percentage. The unique fibrous structure of the membranes induces fast liquid penetration to form the polymer electrolyte with interpenetrating polymer network within 30 min. The higher electrolyte uptake of PVdF–PMMA (50:50) membrane could make an improvement of ionic conductivity suitable as separator for lithium ion battery. The porosity, electrolyte uptake and ionic conductivity of PVdF, PVdF–PMMA (80:20) and PVdF–PMMA (50:50) nanofibrous membranes are shown in Fig. 7. The Nyquist plot of PVdF, PVdF–PMMA (80:20) and PVdF–PMMA (50:50) nanofibrous polymer electrolyte membranes at room temperature is shown in Fig. 8. The bulk resistance (Rb) of the polymer electrolyte membranes was determined from the intercept of the extended impedance plots with the x axis from the Nyquist plots. The bulk resistance was found to be decreasing with increasing % of PMMA in PVdF. The enhancement in ionic conductivity was attributed mainly due to higher electrolyte uptake obtained by porous structure and decrease in crystallinity by adding PMMA in PVdF. Bulk resistance and ionic conductivity of PVdF, PVdF–PMMA (80:20) and PVdF–PMMA (50:50) nanofibrous polymer electrolyte membranes are listed in Table 1. The nanofibrous membranes obtained by the process of electrospinning had fully interconnected porous structures with decrease in diameter by increasing PMMA percentage in PVdF.

Fig. 6
figure 6

Electrolyte uptake of a PVdF, b PVdF–PMMA (80:20), and c PVdF–PMMA (50:50) nanofibrous membranes

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Fig. 7
figure 7

Porosity, electrolyte uptake and ionic conductivity of PVdF, PVdF–PMMA (80:20) and PVdF–PMMA (50:50) nanofibrous membranes

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Fig. 8
figure 8

Nyquist plot of a PVdF, b PVdF–PMMA (80:20) and c PVdF–PMMA (50:50) nanofibrous polymer electrolyte membranes

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Battery performance was conducted by fabricating Swagelok (Li/PE/LiFePO4) cells by placing as-fabricated nanofibrous polymer electrolyte (PE) membranes between lithium metal anode and lithium iron phosphate (LiFePO4) cathode [8, 29]. The fabrication of test cells was carried out in a glove box. The electrochemical tests of the Li/PE/LiFePO4 cells were conducted in an automatic galvanostatic charge–discharge unit, battery cycler (Arbin BT 2000) at room temperature at the current rate of 0.1 °C. The charge–discharge process was performed between 2.6 and 4.3 V [30]. Figure 9 shows 1st cycle of charge and discharge curves of fabricated Swagelok (Li/PE/LiFePO4) cells. The initial discharge capacity was found to be 138.4, 144.7 and 150.3 mAh/g for separators of PVdF, PVdF/PMMA (80:20) and PVdF/PMMA (50:50) membranes, respectively. Compared with cells of different PEs, discharged capacity for the cell with PVdF–PMMA (50:50) separator was found to be relatively high cathode utilization corresponding to 88.4% of theoretical capacity. Moreover, the PVdF/PMMA (50:50) nanofibrous polymer electrolyte membrane showed a slightly higher discharge capacity than that of PVdF and PVdF/PMMA (80:20) due to higher ionic conductivity. The cycle performance of the cells with the polymer electrolytes based on PVdF, PVdF–PMMA (80:20) and PVdF–PMMA (50:50) membranes is shown in Fig. 10. Even after 50 cycles, the discharge capacity of PVdF/PMMA (50:50) membranes was found to be 140 mAh/g. This shows that 93.3% of its initial discharge capacity was retained compared to other polymer electrolytes. This indicates that the discharge capacity has a good retention even after 50 cycles. These results suggest that PVdF–PMMA (50:50) nanofibrous polymer electrolyte membrane is a potential separator for lithium ion battery.

Fig. 9
figure 9

1st cycle of charge and discharge curves of Swagelok cells with a PVdF, b PVdF–PMMA (80:20), and c PVdF–PMMA (50:50) nanofibrous polymer electrolyte membranes

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Fig. 10
figure 10

Cycle performance of Swagelok cells with a PVdF, b PVdF–PMMA (80:20) and c PVdF–PMMA (50:50) nanofibrous polymer electrolyte membranes

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Conclusions

PVdF–PMMA composite fibers with diameter in nanoscale membranes were successfully prepared by electrospinning. PVdF–PMMA composites membranes showed excellent electrochemical properties due to the interconnected porous structure. The increase in percentage of PMMA enhances electrolyte uptake and ionic conductivity of PVdF–PMMA composites membranes. Among PVdF–PMMA membranes studied, PVdF–PMMA (50:50) membrane exhibited the highest porosity, electrolyte uptake, ionic conductivity, and discharge capacity retention. These results suggested that the preparation methodology for PVdF/PMMA composites membranes by electrospinning with PVdF/PMMA (50:50) nanofibrous polymer electrolyte membrane was found to be potential and promising separator for lithium ion batteries than that of pure PVdF and PVdF–PMMA (80:20).