Poly(methyl methacrylate) reinforced poly(vinylidene fluoride) composites electrospun nanofibrous polymer electrolytes as potential separator for lithium ion batteries
Fabrication of nanofibrous polymer electrolyte membranes of poly(vinylidene fluoride) (PVdF) and poly(methyl methacrylate) (PMMA) 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 of PVdF membrane as separator. As-fabricated polymer electrospun nanofibrous membranes were characterized by SEM, FTIR, XRD, TGA and DSC for morphology, structure, crystallinity and thermal stability. PVdF–PMMA (50:50) polymer electrolyte membrane showed ionic conductivity 0.15 S/cm and electrolyte uptake 290% at room temperature. After 50 cycles, the discharge capacity 140 mAh/g of Li/PE/LiFePO4 cells with PVdF–PMMA (50:50) as polymer electrolyte (PE) membrane was found to be retained around 93.3%. The electrolyte uptake, ionic conductivity, and discharge capacity retention were improved by optimizing the proportion of PMMA in PVdF. Nanofibrous PVdF–PMMA (50:50) polymer electrolyte membrane was found to be a potential separator for lithium ion batteries.
KeywordsPoly(vinylidene fluoride) Poly(methyl methacrylate) Polymer electrolyte Electrospinning Nanofibers Lithium ion batteries
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 . 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 . 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 . Many different synthesis methods are used to prepare polymer electrolyte membrane by solution casting [15, 20], phase inversion  and electrospinning . 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.
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.
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
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
∆H f (J/g)
Rb (\( \Omega \))
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.
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).
This work was supported by the Department of Science and Technology (DST, New Delhi, India) Support under DST-FIST Program, Grant no. SR/FST/PSI-178/2012(C).
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