Electrochemical Characterization of Electrospun Nanocomposite

Article pubs.acs.org/JPCB

Electrochemical Characterization of Electrospun Nanocomposite Polymer Blend Electrolyte Fibrous Membrane for Lithium Battery O. Padmaraj,† B. Nageswara Rao,† M. Venkateswarlu,‡ and N. Satyanarayana*,† †

Department of Physics, Pondicherry University, Pondicherry 605 014, India R&D, Amara Raja Batteries Ltd., Karakambadi 517 520, India

‡

ABSTRACT: Novel hybrid (organic/inorganic) electrospun nanocomposite polymer blend electrolyte fibrous membranes with the composition poly(vinylidene difluoride-co-hexafluoropropylene) [P(VdF-co-HFP)]/poly(methyl methacrylate) [P(MMA)]/magnesium aluminate (MgAl2O4)/LiPF6 were prepared by the electrospinning technique. All of the prepared electrospun P(VdF-coHFP), PMMA blend [90% P(VdF-co-HFP)/10% PMMA], and nanocomposite polymer blend [90% P(VdF-co-HFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)] fibrous membranes were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetry, and scanning electron microscopy. The fibrous nanocomposite separator-cum-polymer blend electrolyte membranes were obtained by soaking the nanocomposite polymer blend membranes in an electrolyte solution containing 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1, v/v). The newly developed fibrous nanocomposite polymer blend electrolyte [90% P(VdF-co-HFP)/10% PMMA/6 wt % MgAl2O4/LiPF6] membrane showed a low crystallinity, low average fiber diameter, high thermal stability, high electrolyte uptake, high conductivity (2.60 × 10−3 S cm−1) at room temperature, and good potential stability above 4.5 V. The best properties of the fibrous nanocomposite polymer blend electrolyte (NCPBE) membrane with a 6 wt % MgAl2O4 filler content was used for the fabrication of a Li/ NCPBE/LiCoO2 CR 2032 coin cell. The electrochemical performance of the fabricated CR 2032 cell was evaluated at a current density of 0.1 C-rate. The fabricated CR 2032 cell lithium battery using the newly developed NCPBE membrane delivered an initial discharge capacity of 166 mAh g−1 and a stable cycle performance.

1. INTRODUCTION Secondary lithium batteries are the best power sources for fastgrowing applications in portable electronic devices, automotive electric vehicles, and aerospace technologies, among others. Because of these applications, researchers have focused on the development of high-specific-energy and high-power-density secondary lithium batteries with better electrochemical performance, safety, and reliability.1−7 Commercial Celgard separators (2320, 2325, 2340, 2400, 2500, and 2730), which are made of polyolefin (polyethylene and polypropylene) and organic liquid electrolytes, are used in most available secondary lithium batteries. Commercial Celgard separators have some drawbacks, such as low porosity, low thermal stability, poor wettability in organic liquid electrolytes, electrolyte leakage, vapor pressure, poor safety characteristics, and dendrite growth formation on the anode, that restrict the discharge capacity and cycle performance of the resulting batteries.8−10 Polymer electrolytes (PEs) can be used as better separatorcum-electrolyte membranes in place of available commercial Celgard separators as well as liquid electrolyte media to overcome the aforementioned problems. Ever since Armand and co-workers first claimed the application of a polymer electrolyte in lithium batteries, research has been expanding worldwide on account of the attractive properties of polymer electrolytes.11,12 High specific energy and power densities, low © 2015 American Chemical Society

weights, leak-proof electrolyte constructions, better safety characteristics, desired sizes and shapes, and low costs of fabrication can be expected with the use of polymer electrolytes (PEs) in secondary lithium batteries compared to liquid electrolyte media. Polymer electrolytes also play many crucial roles in all electrochemical devices such as separators to prevent physical contact between electrodes, electronic insulators, good ionic conductors, and good electrode/electrolyte interfaces and provide favorable properties such as improved electrolyte wettability; dimensional, thermal, and chemical stabilities; and electrochemical stability windows toward electrode materials.13−17 Recently, the modification of polymer electrolytes by plasticizing/blending and compositing has been done to enhance their physical and electrochemical properties to fulfill the above-mentioned requirements.14,18−28 Poly(vinylidene difluoride) (PVdF), poly(vinylidene difluoride-co-hexafluoropropylene) (PVdF-co-HFP), poly(methyl methacrylate) (PMMA), polyacrylonitrile (PAN), polystyrene (PS), and poly(ethylene oxide) (PEO) have been widely used as host polymers for the preparation of PEs.29,20−23,26 Among these Received: August 19, 2014 Revised: February 26, 2015 Published: April 13, 2015 5299

DOI: 10.1021/jp5115477 J. Phys. Chem. B 2015, 119, 5299−5308

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The Journal of Physical Chemistry B

nanocomposite polymer blend electrolyte (NCPBE) were obtained by immersing the prepared membranes in an electrolyte solution [1 M LiPF6 in ethylene carbonate (EC)/ diethyl carbonate (DEC) (1:1, v/v)]. The electrolyte uptake behavior and ionic conductivities of PMMA blend and nanocomposite polymer blend electrolyte (NCPBE) membranes were evaluated by the electrolyte uptake method and ac impedance spectroscopy measurements. The electrochemical performance of an assembled Li/NCPBE/LiCoO2 CR 2032 coin cell using the newly developed separator-cum-NCPBE membrane was studied inside an argon filled glovebox using a battery cycle tester (BCT) at a current density 0.1 C-rate.

materials, P(VdF-co-HFP) copolymer was found to be a suitable promising material because it has good electrochemical stability, affinity to electrolyte solution, and a high dielectric constant (ε ≈ 8.4).30,31 Moreover, P(VdF-co-HFP) is a stronger Lewis base (Kb = 1.199) than the other polymers, which might help in the formation of complexes with strong Lewis acid character.32 P(VdF-co-HFP) is a semicrystalline copolymer that contains both a crystalline phase (VdF) and an amorphous phase (HFP). The crystalline phase hinders the penetration of liquid electrolyte and the migration of lithium ions in the P(VdF-co-HFP) matrix, which can lead to low ionic conductivity at room temperature. However, the VdF unit in P(VdF-co-HFP) copolymer hinders the migration of lithium ions, which might help to improve its excellent mechanical strength and chemical stability.33 Many researchers have been focusing on improving the room-temperature ionic conductivity by various methods without sacrificing their favorable properties. Therefore, in the present investigation, attempts were made to improve the room-temperature conductivity by modifying P(VdF-co-HFP) in two ways: (i) blending of PMMA into the P(VdF-co-HFP) copolymer34−37 and (ii) addition of nanocrystalline MgAl2O4 ceramic fillers to the optimized polymer blend [P(VdF-co-HFP)/PMMA] matrix to form a nanocomposite polymer blend [P(VdF-co-HFP)/PMMA/ MgAl2O4] fibrous membrane.38−40 The PMMA polymer can absorb many liquid electrolytes and has good compatibility with PVdF because of its excellent affinity with carbonate-based liquid electrolytes, which would help the membranes entrap more liquid electrolyte.35 Nanocrystalline MgAl2O4 ceramic fillers have the strongest Lewis acid character and also a high dielectric constant (ε = 8.1−8.3) among fillers, which would compete with the Lewis acid character of the Li+ ions in LiPF6 salt in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1) organic solvent for the formation of complexes with the P(VdFco-HFP)/PMMA polymer blend matrixes. The resulting nanocomposite polymer blend matrixes provide easier pathways for Li+ ions, which can enhance ionic transport and, hence, improve the ionic conductivity and electrochemical performance at room temperature of the secondary lithium battery. Different methods such as solution casting, plasticizer extraction, phase inversion, electrospinning, and hot press methods have been reported for the preparation of polymer or nanocomposite polymer electrolyte membranes.18,21,22,33−41 Among them, electrospinning is a simple and effective technique for developing thin nano-/microscale fibrous polymer membranes with highly interconnected porous structures and large surface areas.42−47 To the best of the authors’ knowledge, there are no reports on the development of electrospun nanocomposite polymer blend electrolyte [P(VdFco-HFP)/PMMA/MgAl2O4/1 M LiPF6 EC:DEC (1:1, v/v)] fibrous membranes by the electrospinning technique. Hence, the authors were motivated to develop electrospun polymer blend [90% P(VdF-co-HFP)/10% PMMA] and nanocomposite polymer blend [90% P(VdF-co-HFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)] fibrous membranes with various contents of nanocrystalline MgAl2O4 filler. The as-prepared fibrous polymer blend and nanocomposite polymer blend membranes were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). The fibrous polymer blend electrolyte (PBE) and

2. EXPERIMENTAL SECTION 2.1. Preparation of Electrospun Nanocomposite Polymer Blend (NCPB) Fibrous Membranes. Poly(vinylidene difluoride-co-hexafluoropropylene) [P(VdF-coHFP)], poly(methyl methacrylate) (PMMA), and MgAl2O4 nanosize ceramic fillers prepared by the gel-combustion method were used as the raw materials for the preparation of the electrospun polymer blend and nanocomposite polymer blend fibrous membranes by the electrospinning technique. Acetone and N,N-dimethylacetamide (DMAc) (7:3, v/v) were used as a mixed solvent. For the preparation of the electrospun nanocomposite polymer blend [P(VdF-co-HFP)/PMMA/MgAl2O4] membranes, first, an optimized 16 wt % solution of polymer blend [90% P(VdF-co-HFP)/10% PMMA] was dissolved separately in a mixed solvent of acetone/DMAc (7:3, v/v) under continuous stirring for 5 h at room temperature.40 Later, various amounts of nanocrystalline MgAl2O4 (2, 4, 6, and 8 wt %) filler (particle size < 200 nm, surface area ≈ 160 m2/g) were dispersed in the mixed solvent using an ultrasonicator and added to the optimized polymer blend [90% P(VdF-co-HFP)/ 10% PMMA] solution under constant stirring at room temperature until a lightly viscous solution was obtained. The resultant lightly viscous solution for each composition of nanocomposite polymer blend was taken into a 20 mL syringe and loaded in a syringe pump to develop nanocomposite polymer blend fibers by fixing optimized electrospinning parameters, such as a solution feed rate of 1.5 mL h−1, an applied voltage between spinneret and collector of 16 kV, a distance between the tip of the spinneret and the collector of 15 cm, a needle bore size of 24 G, and a collector drum rotation speed of 550 rpm. Electrospun polymer blend [90% P(VdF-coHFP)/10% PMMA] and nanocomposite polymer blend [90% P(VdF-co-HFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)] fibrous membranes with average thicknesses of 60−100 μm were collected on the rotating drum collector and further dried in a hot-air oven at 60 °C for 24 h to remove the solvent. 2.2. Characterization Techniques. X-ray diffraction (XRD) patterns were recorded from 10° to 80° for all prepared electrospun membranes using PANalytical X-pert PRO diffractometer (Philips) with Cu Kα radiation (λ = 0.154060 nm; 30 mA and 40 kV). Fourier transform infrared (FTIR) transmittance spectra for the prepared pure polymer, polymer blend, and nanocomposite polymer blend fibrous membranes were recorded in the range of 4000−400 cm−1 using a Nicolet 6700 spectrophotometer with 4 cm−1 resolution. The DSC curves of all of the prepared electrospun polymer blend and nanocomposite polymer blend fibrous membranes were recorded on a DSC Q20 instrument under a nitrogen atmosphere. The surface morphologies of all of the developed 5300


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The Journal of Physical Chemistry B electrospun fibrous membranes were investigated at the same magnification for comparison by scanning electron microscopy (SEM; Hitachi S4700). The electrolyte uptake capacity of the all of the electrospun membranes was determined by soaking each of them in an electrolyte solution containing 1 M LiPF6 salt in EC/DEC (1:1, v/v) solvent and weighing it at regular intervals after removing the excess liquid electrolyte by wiping it with tissue paper. The electrolyte uptake (EU) was calculated as EU =

W1 − W0 × 100% W0

battery cycle tester (BCT) (model MCV4-1/0.01/0.001-10, Bitrode, St. Louis, MO) in the potential range of 2.8−4.2 V at a current density 0.1 C-rate.

3. RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction patterns of electrospun P(VdF-co-HFP), polymer blend [90% P(VdF-co-HFP)/10%

(1)

where W1 is the mass of the wet membrane and W0 is the mass of the dry membrane. The electrical behavior and electrochemical stability of electrospun fibrous polymer electrolytes were measured using frequency response analyzer and electrochemical workstation (Novocontrol, Montabaur, Germany). The prepared polymer blend [90% P(VdF-co-HFP)/ 10% PMMA] and nanocomposite polymer blend [90% P(VdFco-HFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)] electrolyte membranes were sandwiched between two blocking stainless steel (SS) electrodes and the impedance spectra were recorded on Alpha high-frequency response analyzer in the frequency range of 1 mHz - 1 MHz. The total conductivity of each electrospun polymer blend and nanocomposite polymer blend electrolyte membranes with various amount of x wt % MgAl2O4 (x = 2, 4, 6, and 8) filler content, was calculated using the sample dimensions (thickness and area) and the resistance, evaluated from the analysis of the measured impedance data at room temperature using the equation σ (S/cm) =

t AR b

Figure 1. XRD patterns of electrospun P(VdF-co-HFP), polymer blend [90% P(VdF-co-HFP)/10% PMMA], and nanocomposite polymer blend [90% P(VdF-co-HFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)] fibrous membranes.

PMMA], and nanocomposite polymer blend [90% P(VdF-coHFP)/10% PMMA/x wt % MgAl2O4, x = 2, 4, 6, and 8] fibrous membranes and the prepared nanocrystalline MgAl2O4 ceramic filler. In Figure 1, the fibrous P(VdF-co-HFP) copolymer membrane showed one broad, low-intensity diffraction peak at 2θ = 20.1°, which was compared with the Joint Committee on Powder Diffraction Standards (JCPDS) data (no. 00-038-1638) and confirmed the formation of the crystalline phase of the PVdF in P(VdF-co-HFP) copolymer.44,48 The observed peak intensity corresponding to the PVdF crystalline phase decreased dramatically, and a broad hump was observed in the diffraction pattern of the polymer blend [90% P(VdF-coHFP)/10% PMMA] fibrous membrane with the addition of 10% PMMA content, as shown in Figure 1. This reveals that the crystallinity of PVdF decreased in the polymer blend [P(VdF-co-HFP)/PMMA] fibrous membrane by increasing the amorphous nature of the material upon the addition of 10% PMMA, which, in turn, might enhance the ionic conductivity at room temperature. Further, addition into the optimized polymer blend [90% P(VdF-co-HFP)/10% PMMA] matrix of ceramic filler content of x wt % MgAl2O4 (x = 2, 4, 6, and 8) resulted in a decrease of the intensity of the diffraction pattern of the nanocomposite membrane up to a filler content of 6 wt % MgAl2O4. This indicates that the nanocomposite polymer blend (NCPB) fibrous membranes became increasingly amorphous in nature, as a result of the reorganization of the polymer blend matrix with the MgAl2O4 ceramic filler through the interactions of the strong Lewis acid sites of the MgAl2O4 filler and the Lewis base sites of the polymer blend [90% P(VdF-co-HFP)/10% PMMA]. Further, this interaction was confirmed by the FTIR results.36,40 The amorphous nature of the NCPB fibrous membrane can provide a more conducting

(2)

where t is the sample thickness (cm), A is the area (cm2), and Rb is the bulk resistance (Ω) of each electrolyte membrane. 2.3. Electrochemical Studies. The electrochemical stability of the prepared electrospun nanocomposite polymer blend electrolyte fibrous membrane with a 6 wt % nanocrystalline MgAl2O4 filler content was studied through cyclic voltametry (Electrochemical test station POT/GAL, Novocontrol, Germany). The measurement (Li/NCPE/SS) was carried out using stainless steel (SS) as the working electrode and lithium (Li) metal as the counter/reference electrode at a scan rate of 2 mV s−1 over the potential range of 2−5 V at room temperature. For an electrochemical measurements, prototype lithium cells were fabricated inside an argon-filled glovebox (Vacuum Atmospheres Co., Hawthorne, CA) by sandwiching the PBE or NCPBE membrane or Celgard 2320 separator between the lithium metal anode (380 μm thick, Aldrich) and the LiCoO2 cathode in a CR-2032 coin cell. The LiCoO2 cathode active material was synthesized by the combustion method, and the composite cathode was prepared by mixing 70 wt % active material (LiCoO2), 20 wt % conducting super P carbon, and 10 wt % PVdF binder (Aldrich) in N-methylpyrrolidinone (NMP) solvent. The resulting slurry was coated onto an Al foil (∼20 μm thick) using the doctor blade method and dried in a hot-air vacuum oven at 120 °C for 24 h. The dried electrode was cut into circular disks and pressed by placing between two stainless steel (SS) plates. The charge/discharge and cycle tests of the assembled Li/PBE or NCPBE or Celgard 2320/LiCoO2 CR 2032 coin cells were performed at room temperature using a 5301


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PMMA] fibrous membrane, the observed IR vibration bands are identified with the characteristic peaks of both P(VdF-coHFP) and PMMA polymer, revealing their perfect miscibility through intermolecular interactions.33,35,50 The observed welldefined IR bands in Figure 2a become featureless broad IR bands in Figure 2b upon the addition of x wt % MgAl2O4 (x = 2, 4, 6, and 8) filler to the polymer blend [90% P(VdF-coHFP)/10% PMMA] matrix. This shows the more amorphous nature of the nanocomposite polymer blend [90% P(VdF-coHFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)] fibrous membranes.35 Further, the more amorphous nature was also confirmed by the observed XRD patterns. The complete mechanism for the formation of the amorphous nature of the nanocomposite polymer blend fibrous membranes through the intercalation and exfoliation process is shown in Figure 3, in the form of schematic diagrams. Figure 4 shows the thermal behavior of the electrospun polymer blend [90% P(VdF-co-HFP)/10% PMMA] and nanocomposite polymer blend [90% P(VdF-co-HFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)] fibrous membranes. As can be seen in Figure 4, the DSC curve of the polymer blend [90% P(VdF-co-HFP)/10% PMMA] membrane exhibits two endothermic peaks at 70 and 140 °C, corresponding to the melting temperatures (Tm) of PMMA and P(VdF-co-HFP) copolymer, respectively.40,35 This indicates that homogeneous blending of 90% P(VdF-co-HFP) and 10% PMMA was accomplished through the possible chemicaloriented coordination between the “mer” units of these two polymers, which confirms their better blending.35 In Figure 4, it can be seen that each of the nanocomposite polymer blend fibrous membranes with the addition of various amounts of MgAl2O4 filler showed higher melting temperatures of 83 °C (PMMA) and 142 °C [P(VdF-co-HFP)], compared to those of the polymer blend [90% P(VdF-co-HFP)/10% PMMA] membrane. This indicates the increase in thermal stability of nanocomposite polymer blend fibrous membranes. The increase in the thermal stability of nanocomposite polymer blend fibrous membranes can be attributed to the occurrence of intercalation and exfoliation process, which might due to the Lewis acid−base interactions between the incorporated nanocrystalline MgAl2O4 filler and the polymer blend matrix.39,51 At the lowest content (2 wt %) of nanocrystalline MgAl2O4 filler in the polymer blend matrix, weak Lewis acid−base interactions can occur that can lead to the formation of an intercalated structured nanocomposite polymer blend matrix as shown in Figure 3.4c. At higher contents (more than 2 wt %) of nanocrystalline MgAl2O4 filler in the polymer blend matrix, strong Lewis acid−base interactions can occur that can lead to the formation of exfoliated nanocomposite polymer blend matrixes as shown in Figure 3.4e. An intercalated and exfoliated structured nanocomposite polymer blend matrix can lead to the formation of an increasingly amorphous structure, as confirmed by the XRD and FTIR results shown in Figures 1 and 2, respectively. Hence, the newly developed electrospun nanocomposite polymer blend [90% P(VdF-co-HFP)/10% PMMA/ 6 wt % MgAl2O4] fibrous membrane can act as a better separator-cum-electrolyte membrane with good thermal stability and low crystallinity than nanocomposite polymer blend fibrous membranes of other compositions. Figure 5 shows the surface morphologies of electrospun P(VdF-co-HFP) (Figure 5a), polymer blend [90% P(VdF-coHFP)/10% PMMA] (Figure 5b), and nanocomposite polymer blend [90% P(VdF-co-HFP)/10% PMMA/x wt % MgAl2O4 (x

path, which might help to increase the mobility (μ) of chargecarrier ions. Further, upon addition of a MgAl2O4 filler content of 8 wt %, the intensity of the broad diffraction peak increased, and also some new diffraction peaks with low intensities were observed at 36°, 44°, and 65°. The newly observed diffraction peaks were compared with the MgAl2O4 diffraction pattern and confirmed the formation of the crystalline phase of the MgAl2O4 ceramic filler. This can affect the mobility (μ) of the free Li-ion charge carriers by blocking or hindering the pathways, resulting in a decrease in the conductivity, as was confirmed by conductivity measurements. Figure 2 shows FTIR transmittance spectra of electrospun P(VdF-co-HFP), PMMA, polymer blend [90% P(VdF-co-

Figure 2. FTIR spectra of electrospun (a) P(VdF-co-HFP), PMMA, and polymer blend [90% P(VdF-co-HFP)/10% PMMA] fibrous membranes and (b) polymer blend and nanocomposite polymer blend [90% P(VdF-co-HFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)] fibrous membranes.

HFP)/10% PMMA], and nanocomposite polymer blend [90% P(VdF-co-HFP)/10% PMMA/x wt % MgAl2O4, x = 2, 4, 6, and 8] fibrous membranes. In the transmittance spectrum of the P(VdF-co-HFP) fibrous membrane (Figure 2a), the strong IR bands at 1279, 842, 510, and 472 cm−1 can be identified as characteristic peaks of PVdF (CH2CF2). The bands at 1279 and 510 cm−1 can be assigned to CF2 asymmetric stretching and bending vibrations, respectively. The band at 842 cm−1 is attributed to CH2 rocking vibrations, and the band at 472 cm−1 is assigned to CF wagging vibrations.49 In the PMMA spectrum, the observed IR peaks at 1721, 1449, and 1159 cm−1 can be assigned to CO stretching, CH3 stretching, and OCH3 stretching vibrations, respectively.50 In the spectrum of the polymer blend [90% P(VdF-co-HFP)/10% 5302


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Figure 3. Schematic diagrams of the complete mechanism for the formation of nanocomposite polymer blend fibrous membranes through the intercalation and exfoliation process. (1) (a) Two-dimensional (2-D) and (b) three-dimensional (3-D) structures and (c) 2-D schematic diagram of P(VdF-co-HFP). (2) (a) 2-D and (b) 3-D structures and (c) 2-D schematic diagram of PMMA. (3) (a) 2-D and (b) 3-D structures and (c) 2-D schematic diagram of P(VdF-co-HFP)/PMMA. (4) (a) 3-D structure of P(VdF-co-HFP)/PMMA; (b) 2-D representation of nanocrystalline MgAl2O4 fillers; and (c−e) 2-D schematic diagrams of (c) the intercalated nanocomposite polymer blend, (d) the exfoliated nanocomposite polymer blend, and (e) the agglomerated (MgAl2O4 filler) exfoliated nanocomposite polymer blend membranes.

= 2, 4, 6, and 8)] (Figure 5c−f) fibrous membranes. In Figure 5, all of the SEM images exhibit three-dimensional weblike structures with fully interconnected ultrafine multifiber layers with a bead-free morphology. The interlayering of multifiber layers generates a nano-/microporous structure between the ultrafine fibers in all of the developed electrospun polymer blend and nanocomposite polymer blend fibrous membranes that might be able to absorb and retain more electrolyte effectively.44−47 All of the SEM images show variations in the average fiber diameters (AFDs) between the copolymer, polymer blend, and nanocomposite polymer blend fibrous membranes with various amounts of MgAl2O ceramic filler, as shown in Figure 5. The AFDs of all of the electrospun membranes were measured approximately from the captured SEM images and were found to be 1.5 μm (copolymer), 1.75 μm (polymer blend), 1 μm (NCPB fibrous membrane with 2

wt % MgAl2O4), 1 μm (NCPB fibrous membrane with 4 wt % MgAl2O4), 0.75 μm (NCPB fibrous membrane with 6 wt % MgAl2O4), and 1 μm (NCPB fibrous membrane with 8 wt % MgAl 2 O4 ). The nanocomposite polymer blend fibrous membrane with a 6 wt % MgAl2O4 filler content exhibited a lower AFD (750 nm) with a uniform fiber distribution compared to the polymer blend and other nanocomposite polymer blend membranes. The observed difference in AFD can be attributed to the change in the concentration of the polymer solution upon the addition of various amounts of MgAl2O4 ceramic filler.40 It is known that the following spinning parameters strongly influence the fiber diameter and morphology of fibrous membranes: solution concentration, feed rate, needle bore size, distance between the spinneret and collector, and applied voltage. In the present work, the effect of 5303


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blend electrolyte [90% P(VdF-co-HFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)/LiPF6] fibrous membranes were prepared by soaking the polymer membranes in an electrolyte solution consisting of 1 M LiPF6 in EC/DEC (1:1, v/v). As soon as a drop of liquid electrolyte was dripped on the electrospun membranes, the drop of liquid electrolyte spread quickly on the surface of fibrous membranes and penetrated into the interior of the membranes within 1 s. This implies that the prepared electrospun membranes, having fully interconnected multifiber layers, had porous structures and that there was an electron affinity between the polymer matrixes and the liquid electrolyte that could assist the rapid absorption by and penetration into the membranes.35,52 Figure 6 shows the electrolyte uptake behavior of the electrospun polymer blend electrolyte [90% P(VdF-co-HFP)/10% PMMA/LiPF6] and nanocomposite polymer blend electrolyte [90% P(VdF-coHFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)/1 M LiPF6] fibrous membranes. In Figure 6, the nanocomposite polymer blend electrolyte fibrous membrane with a MgAl2O4 filler content of 6 wt % exhibits a high electrolyte uptake up to 200%, and further addition of MgAl2O4 filler to a content of 8

Figure 4. DSC curves of electrospun polymer blend [90% P(VdF-coHFP)/10% PMMA] and nanocomposite polymer blend [90% P(VdFco-HFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)] fibrous membranes.

solution concentration was studied while the other parameters were kept constant. Electrospun polymer blend electrolyte [90% P(VdF-coHFP)/10% PMMA/LiPF6] and nanocomposite polymer

Figure 5. SEM images of electrospun (a) pure P(VdF-co-HFP), (b) polymer blend [90% P(VdF-co-HFP)/10% PMMA], and (c−f) nanocomposite polymer blends {90% P(VdF-co-HFP)/10% PMMA/x wt % MgAl2O4 (x = (c) 2, (d) 4, (e) 6, and (f) 8]} fibrous membranes. 5304


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(Rb) and electrode/electrolyte double layer capacitance (Cdl), respectively, of all of the electrospun polymer electrolyte membranes.53,54 The intercept of inclined spike on the real axis represents the bulk resistance (Rb) of all of the electrolyte membranes.40 The observed impedance responses were fitted to an equivalent circuit using WinFIT software. The conductivity of each electrospun membrane was calculated from the analyzed impedance data and sample dimensions. The observed bulk resistances (Rb), calculated conductivity values (σ), and electrolyte uptakes of all of the electrospun electrolyte fibrous membranes are reported in Table 1. The variation in conductivity with the electrolyte uptake with respect to the MgAl2O4 filler content in the polymer blend electrolyte [90% P(VdF-co-HFP)/10% PMMA/LiPF6] membrane is also shown in Figure 6. The increase in conductivity upon the blending of 10% PMMA in 90% P(VdF-co-HFP) polymer matrix and the incorporation of various amounts of MgAl2O4 filler in the polymer blend [90% P(VdF-co-HFP)/ 10% PMMA] matrix is due to the increase of the liquid electrolyte uptake and the amorphous-phase content, which contributes to the formation of more tunnels, thereby allowing greater Li-ion migration. Generally, the pure P(VdF-co-HFP) electrolyte comprise a crystalline phase (PVdF), an amorphous phase (HFP), and a liquid electrolyte solution in its pores.33 Because a high PVdF crystalline content prevents the migration of lithium ions, this can lead to a decrease in the conductivity at room temperature, as shown in Table 1. It is known that the conduction in the P(VdF-co-HFP)-based gel polymer electrolyte membrane depends mainly on the entrapped liquid electrolyte solution in the pore structure. The organic carbonate-based liquid electrolyte would penetrate and interact with the remaining surface hydroxyl groups of the PMMA/ MgAl2O4 matrix by Lewis acid−base interactions, which might also partly contribute to an enhancement of the conductivity of the nanocomposite polymer blend electrolyte fibrous membranes. Furthermore, the more amorphous phase can take a tight hold of the absorbed electrolyte solution and prevent leakage of the electrolyte solution.33 Hence, the developed nanocomposite polymer blend electrolyte [90% P(VdF-coHFP)/10% PMMA/6 wt % MgAl2O4/1 M LiPF6] fibrous membrane can be swollen sufficiently by the carbonate-based liquid electrolyte. Figure 8 shows the potential stability window of the electrospun nanocomposite polymer blend electrolyte [90% P(VdF-co-HFP)/10% PMMA/6 wt % MgAl2O4/1 M LiPF6] fibrous membrane. In Figure 8, the observed steady increase in current density with voltage indicates the potential stability limit of the nanocomposite polymer blend electrolyte membrane. It can be seen that the newly developed nanocomposite polymer blend electrolyte fibrous membrane with a 6 wt % MgAl2O4 filler content exhibits high anodic stability above 4.5 V. Hence, the high potential stability window of the newly prepared nanocomposite polymer blend electrolyte fibrous membrane should render it potentially compatible with most high-voltage cathode materials such as LiCoO2 and LiMnO2, which are commonly used for rechargeable lithium batteries. Figure 9 shows the charge/discharge capacity of a Li/LiCoO2 cell containing the newly developed fibrous nanocomposite polymer blend electrolyte (NCPBE) membrane with a MgAl2O4 filler content of 6 wt %. As shown in Figure 9, the discharge capacity of the nanocomposite polymer blend electrolyte membrane is 166 mAh g−1 in the first cycle and

Figure 6. Variation of electrolyte uptake and conductivity of electrospun nanocomposite polymer blend electrolyte [90% P(VdFco-HFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)/LiPF6] fibrous membranes with various contents of MgAl2O4 filler.

wt % results in a decrease of the uptake behavior. The electrolyte uptake was found to increase with the addition of 10% PMMA in the polymer blend electrolyte [90% P(VdF- coHFP)/10% PMMA/LiPF6] membrane and various contents of MgAl2O4 filler in the nanocomposite polymer blend electrolyte [90% P(VdF-co-HFP)/10% PMMA/MgAl2O4/LiPF6] membranes. This is mainly due to the excellent compatibility and affinity between [P(VdF- co-HFP)/PMMA/6 wt % MgAl2O4] and the carbonate-based liquid electrolyte in the newly developed nanocomposite polymer blend electrolyte fibrous membranes.35 Figure 7 shows the room-temperature complex impedance spectra of the electrospun polymer blend electrolyte [90%

Figure 7. Complex impedance spectra of electrospun polymer blend and nanocomposite polymer blend electrolyte [90% P(VdF-co-HFP)/ 10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)/LiPF6] fibrous membranes with various contents of MgAl2O4 filler.

P(VdF-co-HFP)/10% PMMA/LiPF6] fibrous membrane and the nanocomposite polymer blend electrolyte [90% P(VdF-coHFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)/ LiPF6] fibrous membranes with different compositions. As shown in Figure 7, all of the impedance spectra show one arc and an inclined spike in the measured frequency range. The observed arc in the high-frequency region and spike in the lowfrequency region represent the effects of the bulk resistance 5305


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Table 1. Composition, Bulk Resistance, Conductivity and Electrolyte Uptake of Electrospun Pure, Polymer Blend, and Nanocomposite Polymer Blend Fibrous Electrolyte Membranes sample no. 1 2 3 4 5 6

resistance (Rb, Ω)

sample pure P(VdF-co-HFP)/LiPF6 90% P(VdF-co-HFP)/10% PMMA/LiPF6 90% P(VdF-co-HFP)/10% PMMA/2 wt % 90% P(VdF-co-HFP)/10% PMMA/4 wt % 90% P(VdF-co-HFP)/10% PMMA/6 wt % 90% P(VdF-co-HFP)/10% PMMA/8 wt %

38.70 4.40 5.64 5.50 3.50 4.80

MgAl2O4/LiPF6 MgAl2O4/LiPF6 MgAl2O4/LiPF6 MgAl2O4/LiPF6

conductivity (σ, S cm−1) 1.875 1.290 1.395 1.730 2.603 2.012

× × × × × ×

EU (wt %)

10−4 10−3 10−3 10−3 10−3 10−3

52 58 138 172 201 195

membrane, hence leading to higher utilization of the active material.40

4. CONCLUSIONS The electrospun polymer blend [90% P(VdF-co-HFP)/10% PMMA] fibrous membrane and nanocomposite polymer blend [90% P(VdF-co-HFP)/10% PMMA/x wt % MgAl2O4 (x = 2, 4, 6, and 8)] fibrous membranes of different compositions were prepared by the electrospinning technique. The fibrous nanocomposite polymer blend membrane with a 6 wt % MgAl 2 O 4 filler content showed a reduction of PVdF crystallinity, a lower average fiber diameter with a bead-free morphology, good thermal stability, and a high electrolyte uptake. The activated nanocomposite polymer blend electrolyte [90% P(VdF-co-HFP)/10% PMMA/6 wt % MgAl2O4/1 M LiPF6] fibrous membranes showed a high ionic conductivity with good electrochemical stability at room temperature. The fabricated CR 2032 Li/NCPBE/LiCoO2 coin cell using the newly developed nanocomposite polymer blend electrolyte fibrous membrane with a 6 wt % MgAl2O4 filler content delivered a good discharge capacity and stable cycle performance. Hence, the newly developed electrospun nanocomposite polymer blend electrolyte [90% P(VdF-co-HFP)/10% PMMA/ 6 wt % MgAl2O4/1 M LiPF6] fibrous membrane can be used as a separator-cum-polymer electrolyte membrane for highperformance rechargeable lithium batteries as well as other electrochemical device applications.

Figure 8. Potential stability window of the best properties of the electrospun nanocomposite polymer blend electrolyte fibrous membrane with a 6 wt % MgAl2O4 filler content at room temperature.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 0413-2654 404. Fax: +91 413 2655348. Notes

The authors declare no competing financial interest.

Figure 9. Charge−discharge capacity plot of a fabricated Li/LiCoO2 cell using the newly developed nanocomposite polymer blend electrolyte fibrous membrane with a 6 wt % MgAl2O4 filler content at room temperature.

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ACKNOWLEDGMENTS N.S. gratefully acknowledges DST, AICTE, UGC, CSIR, DAE, and DRDO, Government of India, for financial support through major research project grants. O.P. is thankful to UGC for the award of a BSR fellowship for pursuing a doctoral degree. The authors also acknowledge CIF, Pondicherry University, for providing DSC and SEM facilities.

142 mAh g−1 in the 30th cycle. The observed capacity difference from first to the 30th cycle of the Li/NCPBE/ LiCoO2 cell was approximately 25 mAh g−1, which is probably due to the utilization of the active material and the structural stability of the LiCoO2 cathode electrode. The newly developed electrospun nanocomposite polymer blend electrolyte [90% P(VdF-co-HFP)/10% PMMA/6 wt % MgAl2O4/1 M LiPF6] fibrous membrane has a high porosity with an ultrafine pore structure and is able to absorb/retain large amounts of liquid electrolyte as compared to a standard Celgard separator. This might help to enhance the mobility of the Li ions by entrapping them in the fibrous nanocomposite polymer blend electrolyte

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