- Email: [email protected]
inorganic hybrid nanocomposite fibrous polymer electrolyte for lithium battery
Polymer 55 (2014) 1136e1142
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Electrochemical studies of electrospun organic/inorganic hybrid nanocomposite fibrous polymer electrolyte for lithium battery O. Padmaraj a, B. Nageswara Rao a, Paramananda Jena a, M. Venkateswarlu b, N. Satyanarayana a, * a b
Department of Physics, Pondicherry University, Pondicherry 605 014, India Amara Raja Batteries Ltd, Karakambadi 517 520, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 October 2013 Received in revised form 10 January 2014 Accepted 14 January 2014 Available online 23 January 2014
Poly(vinylidene fluoride-co-hexafluoropropylene) P(VdF-co-HFP)/magnesium aluminate (MgAl2O4) hybrid fibrous nanocomposite polymer electrolyte membranes were newly prepared by electrospinning method. The as-prepared electrospun pure and nanocomposite fibrous polymer membranes with various MgAl2O4 filler contents were characterized by X ray diffraction, differential scanning calorimetry and scanning electron microscopy techniques. The fibrous nanocomposite polymer electrolytes were prepared by soaking the electrospun membranes in 1 M LiPF6 in EC:DEC (1:1, v/v). The fibrous nanocomposite polymer electrolyte membrane with 5 wt.% of MgAl2O4 show high electrolyte uptake, enhanced ionic conductivity is found to be 2.80 103 S cm1 at room temperature and good electrochemical stability window higher than 4.5 V. Electrochemical performance of commercial celgard 2320, fibrous pure and nanocomposite polymer electrolyte (PE, NCPE) membranes with different MgAl2O4 filler content is evaluated in Li/celgard 2320, PE, NCPE/LiCoO2 CR 2032 coin cells at current density 0.1 C-rate. The NCPE with 5 wt.% of MgAl2O4 delivers an initial discharge capacity of 158 mAhg1 and stable cycle performance compared with the other cells containing celgard 2320 separator and pure membrane. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Nano-composite MgAl2O4 filler Electrospinning Charge-discharge studies
1. Introduction Rechargeable Li-ion batteries are the best power sources for the fast growing portable electronic devices, such as laptops, notebooks, cameras, cellular phones, etc [1]. Most of the Li-ion batteries use microporous celgard (2320, 2325, 2340, 2400, 2500 & 2730) separators, which are made up of polyolefin (Polyethylene & Polypropylene) with organic liquid electrolyte [2]. Separators are the critical components, as they play a major role in determining the discharge capacity and cycle performance of the battery [3]. The commercially available celgard separators have several disadvantages, such as low porosity, low thermal stability and poor wettability in organic liquid electrolytes, which restrict the discharge capacity and cycle performance of the battery [4]. In order to overcome these problems, the researchers need to develop the separator-cum electrolyte membrane with excellent properties as well as good medium to conduct the ions between the electrodes.
* Corresponding author. Tel.: þ91 413 2654404; fax: þ91 413 2655348. E-mail address: [email protected] (N. Satyanarayana). 0032-3861/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2014.01.015
Recently, gel polymer electrolytes (GPEs) developed by various methods exhibit high ionic conductivity in the order of 103 S cm1 at room temperature but their mechanical properties are poor for lithium ion battery applications [5e16]. Now a day’s researchers have put effort to develop nanocomposite polymer electrolyte membranes with good thermal, chemical and electrochemical stabilities by incorporating some nano-sized ceramic filler, such as TiO2, SiO2, Al2O3, ZrO2, MgAl2O4, etc., [17e22]. The polymer electrolytes based on poly(vinylidene fluoride) (PVdF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-co-HFP), poly(methyl methacrylate) (PMMA) and poly(acrylonitrile) (PAN) were widely used as host polymers [23e29]. Among them, PVdF-co-HFP copolymer was found to be a suitable candidate as it has good electrochemical stability, affinity to electrolyte solution and high dielectric constant (ε z 8.4) [25,26]. Moreover, P(VdF-co-HFP) is the strongest Lewis basic polymers compared with other polymers. The Lewis acid constant (Ka) of P(VdF-co-HFP) is 0.254, and the base constant (Kb) is 1.199, which may help the formation of complexes with strongest Lewis acid character of fillers [30]. A wide range of synthesis methods, such as solution casting [7], hot press [21,22], plasticizer extraction [12], phase inversion [26], and electrospinning etc., [27e34] have been adopted for the
O. Padmaraj et al. / Polymer 55 (2014) 1136e1142
development of porous polymer membranes. Among them, electrospinning is a simple and effective technique to develop thin nano-size porous fibrous polymer membranes with highly interconnected porous structure and large surface area, which are very important factors to improve the electrolyte uptake amount, ionic conductivity and electrochemical stability [29e32]. To the best of author’s knowledge there are no reports on the development of P(VdF-co-HFP)/MgAl2O4/1 M LiPF6 EC:DEC (1:1, v/ v) nanocomposite fibrous polymer electrolyte membranes by electrospinning technique. As we know that, the MgAl2O4 ceramic filler has strongest Lewis acid character and also high dielectric constant (ε ¼ 8.1e8.3) than other fillers, which would compete with the Lewis acid character of Liþ ions in LiPF6 salt for the formation of complexes with the strongest Lewis base P(VdF-co-HFP) polymer chains. The resulting structure provides more easy pathways for Liþ ions, which can enhance ionic transport and hence, improves the conductivity [30]. The solvent EC has high dielectric constant (ε ¼ 89.6) and high viscosity (h ¼ 1.5), whereas DEC has low dielectric constant (ε ¼ 2.82) and low viscosity (h ¼ 0.748). Combination of EC and DEC (1:1, v/v) solvent has optimized dielectric constant (ε ¼ 46.21) and viscosity (h ¼ 1.124), which will provide reduced electrostatic attraction between the Liþ and PF 6 ions in a LiPF6 salt. The optimized solvent may separate (dissociate) the Liþ ions easily, which in turn improve ionic conductivity at room temperature. Hence, authors are motivated to develop electrospun fibrous pure [16 wt.% P(VdF-co-HFP)] and nanocomposite polymer membranes with various MgAl2O4 (2, 5 & 8 wt.%) fillers content. The asprepared fibrous pure and nanocomposite polymer membranes were characterized by XRD, DSC and SEM techniques. The fibrous nanocomposite polymer electrolytes (NCPEs) are prepared by soaking the electrospun membranes in 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1, v/v). The electrolyte uptake behavior and ionic conductivities of nanocomposite polymer electrolytes are evaluated through electrolyte uptake method and impedance spectroscopy measurements. Electrochemical performance of all the Li/PE, NCPE, celgard 2320/LiCoO2 CR 2032 coin cells were studied through Battery cycle tester (BCT) at current density 0.1 C-rate. 2. Experimental 2.1. Synthesis of PVdF-co-HFP/MgAl2O4 nanocomposite fibrous membranes
between the tip of the spinneret and collector is 12.5 cm, needle bore size 24 G and collector drum rotation speed is 450 rpm. The electrospun fibrous membranes with an average thickness of 80 mm were collected and dried in hot air oven at 60 C for 24 h to remove the solvent for further use. 2.2. Characterization techniques X-ray diffraction patterns (XRD) were recorded from 10 to 80 for all the prepared fibrous membranes using PANalytical X-pert PRO diffractometer (Philips) with Cu-Ka radiation (l ¼ 0.154060 nm, 30 mA and 40 kV). The DSC curves of the celgard 2320 separator and the prepared electrospun membranes were recorded on DSC Q20 instrument under nitrogen atmosphere. From the analyzed DSC curve, the crystallinity of pure and nanocomposite fibrous membranes was calculated using the following equation [13].
Xc ¼
DHs 100% DH *
(1)
where, DH* ¼ 104.7 J/g, the melting enthalpy of PVdF and DHs J/g is the melting enthalpy of prepared electrospun pure and nanocomposite membranes. The micrographs of prepared electrospun membranes were observed on scanning electron microscopy (SEM, Hitachi S4700). The electrolyte uptake behavior was observed by soaking the membranes in liquid electrolyte [1 M LiPF6 in EC:DEC (1:1, v/v)] for 1 h. The electrolyte uptake was calculated based on the following equation [13].
EU ¼
W1 W0 100% W0
(2)
where, W1 and W0 are the weights (g) of the wet and dry membranes respectively. The ionic conductivities of the nanocomposite polymer electrolyte membranes were evaluated at room temperature through impedance spectroscopy measurements. Each polymer electrolyte membrane was placed between stainless steel (SS) electrodes and the impedance spectra were recorded on Alpha high frequency response analyzer (Novocontrol, Germany) in the frequency range of 1 mHze1 MHz. The ionic conductivities are calculated from the analyzed impedance data and sample dimensions by using the following equation
s¼ Poly(vinylidene difluoride-co-hexafluoropropylene) [P(VdF-coHFP)] (Aldrich, AMw ¼ 4 105) and MgAl2O4 filler prepared by gel-combustion method were used as the starting materials for the preparation of fibrous nanocomposite polymer membranes by electrospinning technique. All the starting materials were dried at 60 C for 24 h in hot air oven before use. N, N-Dimethylacetamide (DMAc) and acetone were used as mixed solvents. For the preparation of fibrous nanocomposite polymer membranes, first the optimized 16 wt.% of P(VdF-co-HFP) polymer solution was prepared by dissolving it in a mixed solvent of acetone/DMAc (7:3, V/V) under continuous stirring for 3 h at room temperature [31]. Later, 1, 2, 3, 4, 5, 6 & 8 wt.% of MgAl2O4 nanoparticle filler (particle size: <100 nm, surface area: 150 m2/g) were added to the above optimized 16 wt.% P(VdF-co-HFP) polymer solution under constant stirring for 3 h at room temperature. The resultant optimized viscous solution was taken into a 10 ml syringe and loaded in a syringe pump to form nanocomposite fibers by setting the electrospinning parameters. The following electrospinning parameters are used for all the compositions: solution feed rate 1 mlh1, applied voltage between spinneret and collector is 18 kV, distance
1137
t S=cm A Rb
(3)
where, t is the sample thickness (cm), A is the area (cm) and Rb is the bulk resistance (U) of polymer electrolyte membrane. 2.3. Electrochemical studies The electrochemical stability of the prepared electrospun nanocomposite membrane was studied through cyclic voltametry (Electrochemical test station POT/GAL, Novocontrol, Germany). The measurement (Li/NCPE/SS) was carried out by using stainless steel (SS) as the working electrode and lithium (Li) metal as counter/ reference electrode at the scan rate of 1 mV s1 over the potential range of 2e5 V at room temperature. For electrochemical measurements, the prototype lithium cells were assembled inside an Argon-filled glove box [Vacuum Atmospheres Co. (VAC), USA] by sandwiching the PE, NCPE, celgard 2320 separator between lithium metal anode (380 mm thick, Aldrich) and LiCoO2 cathode in CR-2032 coin cell. The LiCoO2 cathode active material was synthesized by combustion method and the cathode electrode was prepared by
1138
O. Padmaraj et al. / Polymer 55 (2014) 1136e1142
mixing 70 wt% of active material (LiCoO2), 20 wt% of conducting super P carbon and 10 wt% of PVdF binder (Aldrich) in N-methyl pyrrolidinone (NMP) solvent. The resulting slurry was coated on to an Al-foil (w20 mm thick) using doctor blade method and dried in 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 all the prepared Li/PE, NCPE, celgard 2320/LiCoO2 cells were performed at room temperature in a battery cycle tester (BCT) (Model MCV4-1/ 0.01/0.001-10, Bitrode, USA) between the potential ranges 2.8e 4.2 V at current density 0.1 C-rate.
the conductivity and it was confirmed from the conductivity measurements. 3.2. Thermal analysis
Fig. 1 shows the XRD patterns of fibrous pure and nanocomposite polymer membranes with various MgAl2O4 filler content. It can be seen that fibrous pure [16 wt.% P(VdF-co-HFP)] polymer membrane exhibits a broad and low intensity diffraction peak at 20 , which indicates the crystalline phase of pure PVdF in P(VdF-co-HFP) host copolymer. It is confirmed by comparing the observed XRD pattern with the Joint committee on powder diffraction standards (JCPDS) data (Card No.00-038-1638). Hence, the observed diffraction pattern of fibrous pure P(VdF-coHFP) membrane exhibits semi-crystalline nature [13,23]. The intensity of PVdF diffraction peak is decreasing by addition of 2 & 5 wt.% MgAl2O4 filler as compared to the pure polymer membrane. This indicates the reduction of PVdF crystallinity in the host copolymer for the formation of complexes between strongest acidebase sites in filler and polymer segments through Lewis acidebase interactions, which in turn can enhance the ionic conductivity at ambient temperature. Further addition of 8 wt.% MgAl2O4 filler, the intensity of broad diffraction peak is increased and also some small pronounced new diffraction peaks are observed at 36 , 44 & 65 . The observed new diffraction peaks reflect the crystalline phase of MgAl2O4 nanoparticles and it is confirmed by comparing with MgAl2O4 diffraction pattern [21]. This may affect the mobility (m) of free Li ion charge carriers by blocking or hindering the pathways, which results in a decrease in
The thermal behavior and crystallinity (cc) of each composition of the nano-composite membrane was studied from each DSC curves. It is observed that the nanocomposite membrane with the addition of 5 wt.% MgAl2O4 filler has lower crystallinity compared to 2 and 8 wt.% MgAl2O4 compositions of nanocomposite membranes. Hence, the lower crystallinity composition of the nanocomposite membrane was chosen and compared its melting temperature with the melting temperature of pure polymer membrane and also commercial celgard 2320 separator. Fig. 2 shows the DSC curves of celgard 2320 separator, pure and nanocomposite [16 wt.% P(VdF-coHFP) þ 5 wt.% MgAl2O4] fibrous polymer membranes. It can be seen that each curve exhibits a sharp endothermic peak at 132 C, 141 C & 143 C, which indicates their respective melting or softening temperature. It is observed that the prepared electrospun nanocomposite polymer membranes have high thermal stability compared to the pure and celgard 2320 separator. The increase in thermal stability of MgAl2O4 added nano-composite membrane, which is attributed to the intercalation and extrafoliation of P(VdFco-HFP) copolymer matrix with MgAl2O4 fillers through Lewis acidebase interactions. Due to the strong acid sites of Mgþ & Alþ ions in MgAl2O4 fillers and strong base sites of Fluorine (F) in P(VdFco-HFP) copolymer may resists the segmental motion of polymer chains. Moreover, the melting enthalpy (DHs) from each DSC curves of 22.14 J g1 and 21.30 J g1, for pure and nanocomposite fibrous membranes respectively were used for calculating the percentage of crystallinity as shown in Fig. 2. The melting enthalpy of the electrospun pure and composite membranes can be calculated from the integral area in each melting (endothermic) DSC curve [19]. It is observed that the calculated crystallinity of 16 wt.% P(VdF-coHFP) þ 5 wt.% MgAl2O4 nanocomposite polymer membrane is found to be 20.34%, where as pure [16 wt.% P(VdF-co-HFP)] fibrous membrane exhibits 21.14% of crystallinity. The observed decrease in melting enthalpy and crystallinity with the addition of 5 wt.% of MgAl2O4 filler indicate an increase in the amorphous nature of the nanocomposite fibrous membrane. Further, it was confirmed for
Fig. 1. XRD patterns of electrospun pure and nanocomposite fibrous membranes with various MgAl2O4 filler content.
Fig. 2. DSC curves of a) celgard 2320 separator and electrospun nanocomposite fibrous membranes with various MgAl2O4 filler contents: b) 0 wt.% (pure), c) 5 wt.%.
3. Results and discussion 3.1. X-ray diffraction study
O. Padmaraj et al. / Polymer 55 (2014) 1136e1142
1139
conductivity studies. Hence, the newly developed electrospun nanocomposite [16 wt.% P(VdF-co-HFP) þ 5 wt.% MgAl2O4] fibrous polymer membrane may act as a better separator-cum electrolyte with good thermal stability as well as low crystallinity for high ionic conducting membrane compared to celgard 2320 separator. 3.3. Surface morphological study Fig. 3 shows the surface morphology of electrospun pure (Fig. 3a) and nanocomposite fibrous polymer membranes with various MgAl2O4 filler [2 wt.% (Fig. 3b), 5 wt.% (Fig. 3c) & 8 wt.% (Fig. 3d)] content. From Fig. 3, it can be seen that the membranes exhibit a three dimensional web structure with fully interconnected pores of ultrafine multi-fibers with bead free morphology [29]. The interlaying of multi-fiber layers generates a porous structure between the fibers in the electrospun membrane, which can good absorb and retain the liquid electrolyte effectively. The SEM images also depict the variation of average fiber diameters (AFD) between the pure and nanocomposite membranes with various amounts of MgAl2O4 ceramic filler as shown in Fig. 3. The AFD of electrospun membranes are strongly influenced by the spinning parameters such as solution concentration, feed rate, needle bore size, distance between the spinneret & collector and applied voltage [28]. In the present work, the effect of solution concentration was studied while the other parameters kept constant. As observed from Fig. 3aec, the AFD of pure, 2 wt.%, 5 wt.% & 8 wt.% of MgAl2O4 filler incorporated nanocomposite polymer membranes are approximately 1.5, 1, 0.5, and 1 mm, respectively. The nanocomposite polymer membrane with 5 wt.% of MgAl2O4 filler exhibits less AFD (500 nm) with uniform fiber distribution compared to other membranes. The observed difference in AFD attributes to a change in the concentration of polymer solution with different percentages of MgAl2O4 filler. 3.4. Electrolyte uptake behavior The prepared each composition of electrospun pure and nanocomposite membrane has three dimensional interlaying of multi-
Fig. 4. Variation of electrolyte uptake and conductivity of fibrous nano-composite polymer electrolyte membranes with various MgAl2O4 fillers content at room temperature.
fibers layers, which may generates the high porous structure in electrospun membranes. The fibrous pure and nanocomposite polymer electrolytes are prepared by soaking the prepared membranes in an electrolyte solution of 1 M LiPF6 in EC:DEC (1:1, v/v). Due to the acidebase interactions, the pure P(VdF-co-HFP) copolymer matrix and nanocomposite membrane P(VdF-co-HFP)/ MgAl2O4 with varying wt.% of MgAl2O4 fillers content can absorb an electrolyte solution of 1 M LiPF6 in EC:DEC (1:1, v/v). This is called electrolyte uptake. Fig. 4 shows the electrolyte uptake behavior of the electrospun pure and nanocomposite fibrous membranes with different MgAl2O4 (2, 5 & 8 wt.%) filler content. As shown in Fig. 4, the membrane exhibits 61% of increment in electrolyte uptake up to 5 wt.% of MgAl2O4 and further addition of MgAl2O4 (8 wt.%) results in a decrease of uptake behavior from 415% to 369%. The observed increase in electrolyte uptake behavior up to 5 wt.% of MgAl2O4 might be due to the small fiber diameter and hence high porosity than other membranes [20,35]. Hence, 5 wt.% of MgAl2O4
Fig. 3. SEM images of electrospun nanocomposite fibrous membranes with various MgAl2O4 filler contents: a) 0 wt.% (pure), b) 2 wt.%, c) 5 wt.% and d) 8 wt.%.
1140
O. Padmaraj et al. / Polymer 55 (2014) 1136e1142
Fig. 5. Photographic images of electrospun nanocomposite fibrous electrolyte membrane with 5 wt.% MgAl2O4 filler content: a) dry, b) wet and c) free standing wet membrane.
incorporated nanocomposite membrane was undergone to dimensional studies as shown in Fig. 5aec. The photographs show that the membrane exhibits high electrolyte uptake and sufficient mechanical strength with self standing properties even after activated in the liquid electrolyte. 3.5. Ionic conductivity Fig. 6 shows the impedance spectra of electrospun pure and nanocomposite fibrous polymer electrolyte membranes with different MgAl2O4 (2, 5 & 8 wt.%) content at room temperature. As shown in Fig. 6, all the impedance spectra show one arc and an inclined spike in the measured frequency range. The observed arc in the high frequency region and the spike in the low frequency region represent the bulk resistance and electrode/electrolyte (Double Layer Capacitance) effects of the electrospun membranes, respectively [31,36]. The intercept of inclined spike on the real axis gives the bulk resistance (Rb) of the electrospun membrane. The observed impedance spectra were fitted to an equivalent circuit using “WinFIT” software. The ionic conductivities of the electrospun membranes were calculated from the analyzed impedance data and sample dimensions. The observed bulk resistances (Rb) and ionic conductivities (s) of the electrospun membranes are given in Table 1. The variation of conductivity with MgAl2O4 filler content in polymer electrolyte membrane is also shown in Fig. 4. It can be seen that the conductivity (s) increases with the addition of MgAl2O4 filler up to 5 wt.% and further addition of MgAl2O4 (8 wt.%) results in decrease of conductivity. The observed highest
conductivity for 16 wt.% P(VdF-co-HFP) þ 5 wt.% MgAl2O4 nanocomposite membrane might be attributed to the low crystallinity and high electrolyte uptake with fully interconnected porous structure. On the other hand, electrospun membrane with 8 wt.% MgAl2O4 exhibits very low conductivity, which is due to reduce the porosity by increasing the AFD and the formation of MgAl2O4 crystalline phase in the composite membrane [34]. An increased porosity may help to increase the uptake of liquid electrolyte, which can able to enhance the ionic conductivity at room temperature. 3.6. Electrochemical studies Fig. 7 shows the electrochemical stability window of 16 wt.% P(VdF-co-HFP) þ 5 wt.% MgAl2O4 nanocomposite fibrous polymer electrolyte membrane. From Fig. 7, the observed steady increase in current density with voltage indicates the electrochemical stability limit of the fibrous polymer electrolyte membrane [31,33]. It can be found that, the newly developed nanocomposite fibrous polymer electrolyte membrane with 5 wt.% of MgAl2O4 filler content exhibits high anodic stability above 4.5 V. Hence, the newly prepared high electrochemical stability window of nanocomposite fibrous polymer electrolyte membrane should render them potentially compatible with the most high voltage cathode materials like LiCoO2 and LiMnO2, commonly used for rechargeable lithium batteries. Fig. 8 show a comparative chargeedischarge properties at a current density to corresponding 0.1 C-rate of Li/LiCoO2 cells containing nanocomposite fibrous electrolyte membrane with 5 wt.% MgAl2O4 and celgard 2320 separator. As shown in Fig. 8, the discharge capacity of 158 mAhg1 (1st cycle) & 128 mAhg1 (30th cycle) nanocomposite electrolyte membrane is compared to the discharge capacity of 149 mAhg1 (1st cycle) & 120 mAhg1 (30th cycle) celgard 2340 separator containing coin cell. The discharge capacity of cell assembled with nanocomposite polymer electrolyte higher than the cell with commercial celgard 2320 separator. The observed capacity differences from 1st to 30th cycles of Li/NCPE/ LiCoO2 cell showed an approximately 10 mAhg1 higher than the Li/celgard 2320/LiCoO2 cell, which is probably due to the difference in ionic conductivity and utilization of active material. The electrospun nanocomposite electrolyte membrane has high porosity Table 1 Electrical properties of nanocomposite fibrous polymer electrolyte membrane with various MgAl2O4 filler content.
Fig. 6. Complex impedance spectra of electrospun nanocomposite fibrous electrolyte membranes with various MgAl2O4 filler content at room temperature.
S. No
Samples (wt.%)
Resistance (Rb) U
Conductivity (S cm1)
1 2 3 4
Pure 2 5 8
4.15 3.25 3.10 7.15
1.74 2.71 2.80 1.27
103 103 103 103
O. Padmaraj et al. / Polymer 55 (2014) 1136e1142
Fig. 7. Electrochemical stability window by cyclic voltametry of electrospun nanocomposite fibrous electrolyte membrane with 5 wt.% MgAl2O4 filler content at room temperature.
with ultra-fine pore structure, which can able to absorb large amount of liquid electrolyte as compared to celgard 2320 separator. This may help to enhance the ionic conductivity by entrap the Li ions into the fibrous electrolyte membrane and hence, which leads to higher utilization of active material. It was reported that the initial discharge capacity of LiCoO2/Li-metal cell assembled with 12 wt.% SiO2/PAN based electrospun nanocomposite electrolyte activated with 1 M LiPF6 (EC/DMC) is about 139 mAhg1 at 0.5 C and 115 mAhg1 at 1 C respectively [32]. Fig. 9 shows the discharge capacity as a function of cycle number for Li/LiCoO2 cells based on celgard 2320 separator, electrospun pure and nanocomposite fibrous polymer electrolyte membrane with 5 wt.% MgAl2O4 nanoparticle filler. From Fig. 9, the discharge capacity and cycle performance was increased with addition of 5 wt.% MgAl2O4 nanoparticle filler as compared to pure fibrous membrane and celgard 2320 separator. The discharge capacity of 1st and 30th cycle of Li/NCPE/LiCoO2 cell shows 158 mAhg1 and
1141
Fig. 9. Cycle performance of Li/LiCoO2 coin cells containing electrospun nanocomposite fibrous electrolyte membranes with various MgAl2O4 filler content and celgard 2320 separator.
128 mAhg1 respectively, whereas Li/PE/LiCoO2 cell delivers 142 mAhg1 and 102 mAhg1 and Li/celgard 2320/LiCoO2 delivers 148 mAhg1 and 120 mAhg1 respectively. The observed capacity fade may be due to the structural characteristics and composition of cathode materials play a vital role on the capacity of cell and cycling performance. Hence, the electrospun nanocomposite electrolyte membrane with 5 wt.% MgAl2O4 filler has good cycle performance without any change in the property, which confirms the excellent efficiency of the membrane to conduct the ions between the electrodes [37]. Also, it has good compatibility with both the electrodes especially lithium metal. Thus, it may be concluded that the newly developed nanocomposite [16 wt.% P(VdF-co-HFP) þ 5 wt.% MgAl2O4] membrane can be useful as a better electrolyte for all rechargeable lithium battery application as well as other ionic devices. 4. Conclusion The pure [16 wt.% P(VdF-co-HFP)] and nanocomposite [16 wt.% P(VdF-co-HFP) þ x wt.% MgAl2O4, x ¼ 2, 5 & 8] fibrous polymer electrolyte membranes were prepared by electrospinning technique. The nanocomposite fibrous polymer membrane with 5 wt.% MgAl2O4 filler showed the reduction of PVdF crystallinity (Xc), high thermal stability and exhibits an uniform morphology with less average fiber diameter is 0.5 mm. The 5 wt.% MgAl2O4 incorporated nanocomposite polymer electrolyte membrane exhibits a high electrolyte uptake, high ionic conductivity and good electrochemical stability higher than 4.5 V at room temperature. The Li/ NCPE/LiCoO2 cell with 5 wt.% MgAl2O4 filler delivers the good discharge capacity with stable cycle performance compared to the celgard 2320 separator. Hence, the newly developed electrospun nanocomposite [16 wt.% P(VdF-co-HFP) þ 5 wt.% MgAl2O4] fibrous polymer electrolyte membrane can be used as a suitable electrolyte membrane for high performance rechargeable lithium batteries as well as other ionic device applications. Acknowledgments
Fig. 8. Chargeedischarge capacity of Li/LiCoO2 cells containing nanocomposite fibrous electrolyte membrane with 5 wt.% MgAl2O4 filler content and celgard 2320 separator at room temperature.
Dr. NS gratefully acknowledges DST, AICTE, UGC, CSIR and DRDO, Govt. of India for financial support through major research
1142
O. Padmaraj et al. / Polymer 55 (2014) 1136e1142
project grants. OP is thankful to UGC for the award of BSR fellowship for pursuing the doctoral degree. Authors also acknowledge CIF, Pondicherry University, for providing FTIR, DSC & SEM facilities.
References [1] Tarascon JM, Armond M. Issues and challenges facing rechargeable lithium batteries. Nature 2001;414:359e67. [2] Zhang SS. A review on the separators of liquid electrolyte Li-ion batteries. J Power Sources 2007;164:351e64. [3] Arora PK, Zhang ZM. Battery separators. Chem Rev 2004;104:4419e62. [4] Abraham KM, Alamgir M. Polymer electrolytes reinforced by celgard membranes. J Electrochem Soc 1995;142:683e7. [5] Armand M. Polymer solid electrolytes-an overview. Solid State Ionics 1983;9 & 10:745e54. [6] Koksbang R, Olsen II, Shackle D. Review of hybrid polymer electrolytes and rechargeable lithium batteries. Solid State Ionics 1994;69:320e35. [7] Croce F, Curini R, Martinelli A, Persi L, Ronci F, Scrosati B, et al. Physical and chemical properties of nanocomposite polymer electrolytes. J Phys Chem B 1999;103:10632e8. [8] Derrien Gaelle, Hassoun Jusef, Sacchetti Simone, Panero Stefania. Nanocomposite PEO-based polymer electrolyte using a highly porous, super acid zirconia filler. Solid State Ionics 2009;180:1267e71. [9] Croce F, Appetecchi GB, Persi L, Scrosati B. Nanocomposite polymer electrolytes for lithium batteries. Nature 1998;394:456e8. [10] Weston JE, Steele BCH. Effects of inert fillers on the mechanical and electrochemical properties of lithium salt-poly (ethylene oxide) polymer electrolytes. Solid State Ionics 1982;7:75e9. [11] Rajendiran S, Kannan R, Mahendran O. Ionic conductivity studies in poly(methyl methacrylate)-polyethylene oxide hybrid polymer electrolytes with lithium salts. J Power Sources 2001;96:406e10. [12] Cui Zhen-Yu, Xu You-Yi, Zhu Li-Ping, Wang Jian-Yu, Xi Zhan-Yu, -Zhu Bao-Ku. Preparation of PVDF/PEO-PPO-PEO blend microporous membranes for lithium ion batteries via thermally induced phase separation process. J Memb Sci 2008;325:957e63. [13] Liang Yinzheng, Cheng Sichen, Zhao Jianmeng, Zhang Changhuan, Sun Shiyuan, Zhou Nanting, et al. Heat treatment of electrospun polyvinylidene fluoride fibrous membrane separators for rechargeable lithium-ion batteries. J Power Sources 2013;240:204e11. [14] Prasanth Raghavan, Aravindan Vanchiappan, Srinivasan Madhavi. Novel polymer electrolyte based on cob-web electrospun multi component polymer blend of polyacrylonitrile/poly(methyl methacrylate)/polystyrene for lithium ion batteries. J Power Sources 2012;202:299e307. [15] Kim Jae-Kwang, Cheruvally Gouri, Li Zin, Ahn Jou-Hyeon, Kim Ki-Won, Ahn Hyo-Jun. Preparation and electrochemical characterization of electrospun, microporous membrane-based composite polymer electrolytes for lithium batteries. J Power Sources 2008;178:815e20. [16] Manuel Stephen A. Review on gel polymer electrolytes for lithium batteries. Eur Polym J 2006;42:21e42. [17] Manuel Stephen A, Nahm KS. Review on composite polymer electrolytes for lithium batteries. Polymer 2006;47:5952e64. [18] Choi Byoung-Koo, Kim Young-Wan, Shin Kyoung-Hee. Effects of ceramic fillers on the electrical properties of (PEO)16 LiClO4 electrolytes. J Power Sources 1997;68:357e60. [19] Cui Wei-Wei, Tang Dong-Yan, Gong Zai-Lin. Electrospun poly(vinylidene fluoride)/poly(methyl methacrylate) grafted TiO2 composite nanofibrous membrane as polymer electrolyte for lithium-ion batteries. J Power Sources 2013;223:206e13.
[20] Fu Dong, Luan Ben, Argue Steve, Bureau Martin N, Davidson Isobel J. Nano SiO2 particle formation and deposition on polypropylene separators for lithium-ion batteries. J Power Sources 2012;206:325e33. [21] Angulakshmi N, Nimma Elizabeth R, Swaminathan V, Suk Nahm Kee, Manuel Stephan A. Physical and electrochemical properties of MgAl2O4-incorporated polymer electrolytes composed of poly(ethylene oxide) and LiClO4. Sci Adv Mater 2011;3:702e8. [22] Angulakshmi N, Nahm KS, Nair Jijeesh R, Gerbaldi C, Bongiovanni R, Penazzi, et al. Cycling profile of MgAl2O4-incorporated composite electrolytes composed of PEO and LiPF6 for lithium polymer batteries. Electrochim Acta 2013;90:179e85. [23] Kim Kwang Man, Park Nam-Gyu, Ryu Kwang Sun, Chang Soon Ho. Characterization of poly(vinylidene fluoride-co-hexafluoropropylene) based polymer electrolyte filled with TiO2 nanoparticles. Polymer 2002;43:3951e7. [24] Wang Yifei, Ma Xiaoyan, Zhang Qilu, Tian Na. Synthesis and properties of gel polymer electrolyte membrane based on novel comb-like methyl methacrylate copolymers. J Memb Sci 2010;349:279e86. [25] Kim Kwang Man, Ko Jang Myoun, Park Nam-Gyu, Ryu Kwang Sun, Chang Soon Ho. Characterization of poly(vinylidene fluoride-co-hexafluoropropylene)based polymer electrolyte filled with rutile TiO2 nanoparticles. Solid State Ionics 2003;161:121e31. [26] Pu Weihu, He Yiangming, Wang Li, Jiang Changyin, Wan Chunrong. Preparation of PVDFeHFP microporous membrane for Li-ion batteries by phase inversion. J Memb Sci 2006;272:11e4. [27] Choi SW, Jo SM, Lee WS, Kim YR. An electrospun poly(vinylidene fluoride) nanofibrous membrane and it battery application. Adv Mater 2003;15:2027e 32. [28] Wu Na, Cao Qi, Wang Xianyou, Li Sheng, Li Xiaoyun, Deng Huayang. In situ ceramic fillers of electrospun thermoplastic polyurethane/poly(vinylidene fluoride) based gel polymer electrolytes for Li-ion batteries. J Power Sources 2011;196:9751e6. [29] Cho TH, Sakai T, Tanase S, Kimura K, Kondo Y, Tarao T, et al. Electrochemical performances of polyacrylonitrile nanofiber-based Nonwoven separator for lithium-ion battery. Electrochem Solid State Lett 2007;10:A159e62. [30] Shi Baoli, He Mingjia. Lewis acidebase property of P(VDF-co-HFP) measured by inverse gas chromatography. J Appl Polym Sci 2008;107:1642e6. [31] Li Xin, Cheruvelly Gouri, Kim Jae-Kwang, Choi Jae-Won, Ahn Jou-Hyeon, Kim Ki-Won, et al. Polymer electrolytes based on an electrospun poly(vinylidene fluoride-co-hexafluoropropylene) membrane for lithium batteries. J Power Sources 2007;167:491e8. [32] Jung Hong-Ryun, Ju Dong-Hyuk, Lee Wan-Jin, Zhang Xiangwu, Kotek Richard. Electrospun hydrophilic fumed silica/polyacrylonitrile nanofiber-based composite electrolyte membranes. Electrochim Acta 2009;54:3630e7. [33] Li Xiaoyun, Cao Qi, Wang Xianyou, Jiang Shenghui, Deng Huayang, Wu Na. Preparation of poly(vinylidene fluoride)/poly(methyl methacrylate) membranes by novel electrospinning system for lithium ion batteries. J Appl Polym Sci 2011;122:2616e20. [34] Cheruvally Gouri, Kim Jae-Kwang, Choi Jae-Won, Ahn Jou-Hyeon, Shin YongJo, Manuel James, et al. Electrospun polymer membrane activated with room temperature ionic liquid: novel polymer electrolytes for lithium batteries. J Power Sources 2007;172:863e9. [35] Stephen AM, Nahm KS, Kulandainathan MA, Ravi G, Wilson J. Poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) based composite electrolytes for lithium batteries. Eur Polym J 2006;42:1728e34. [36] Scrosati B, Croce F, Persi L. Impedance spectroscopy study of PEO-based nanocomposite polymer electrolytes. J Electrochem Soc 2000;147(5):1718e21. [37] Raghavan P, Shao X, Shin C, Baek D-H, Chaoi J-W, Manuel J, et al. Preparation and electrochemical characterization of polymer electrolytes based on electrospun poly(vinylidene fluoride-co-hexafluoropropylene)/polyacrylonitrile blend/composite membranes for lithium batteries. J Power Sources 2010;195:6088e94.
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Electrochemical studies of electrospun organic/inorganic hybrid nanocomposite fibrous polymer electrolyte for lithium battery O. Padmaraj a, B. Nageswara Rao a, Paramananda Jena a, M. Venkateswarlu b, N. Satyanarayana a, * a b
Department of Physics, Pondicherry University, Pondicherry 605 014, India Amara Raja Batteries Ltd, Karakambadi 517 520, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 October 2013 Received in revised form 10 January 2014 Accepted 14 January 2014 Available online 23 January 2014
Poly(vinylidene fluoride-co-hexafluoropropylene) P(VdF-co-HFP)/magnesium aluminate (MgAl2O4) hybrid fibrous nanocomposite polymer electrolyte membranes were newly prepared by electrospinning method. The as-prepared electrospun pure and nanocomposite fibrous polymer membranes with various MgAl2O4 filler contents were characterized by X ray diffraction, differential scanning calorimetry and scanning electron microscopy techniques. The fibrous nanocomposite polymer electrolytes were prepared by soaking the electrospun membranes in 1 M LiPF6 in EC:DEC (1:1, v/v). The fibrous nanocomposite polymer electrolyte membrane with 5 wt.% of MgAl2O4 show high electrolyte uptake, enhanced ionic conductivity is found to be 2.80 103 S cm1 at room temperature and good electrochemical stability window higher than 4.5 V. Electrochemical performance of commercial celgard 2320, fibrous pure and nanocomposite polymer electrolyte (PE, NCPE) membranes with different MgAl2O4 filler content is evaluated in Li/celgard 2320, PE, NCPE/LiCoO2 CR 2032 coin cells at current density 0.1 C-rate. The NCPE with 5 wt.% of MgAl2O4 delivers an initial discharge capacity of 158 mAhg1 and stable cycle performance compared with the other cells containing celgard 2320 separator and pure membrane. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Nano-composite MgAl2O4 filler Electrospinning Charge-discharge studies
1. Introduction Rechargeable Li-ion batteries are the best power sources for the fast growing portable electronic devices, such as laptops, notebooks, cameras, cellular phones, etc [1]. Most of the Li-ion batteries use microporous celgard (2320, 2325, 2340, 2400, 2500 & 2730) separators, which are made up of polyolefin (Polyethylene & Polypropylene) with organic liquid electrolyte [2]. Separators are the critical components, as they play a major role in determining the discharge capacity and cycle performance of the battery [3]. The commercially available celgard separators have several disadvantages, such as low porosity, low thermal stability and poor wettability in organic liquid electrolytes, which restrict the discharge capacity and cycle performance of the battery [4]. In order to overcome these problems, the researchers need to develop the separator-cum electrolyte membrane with excellent properties as well as good medium to conduct the ions between the electrodes.
* Corresponding author. Tel.: þ91 413 2654404; fax: þ91 413 2655348. E-mail address: [email protected] (N. Satyanarayana). 0032-3861/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2014.01.015
Recently, gel polymer electrolytes (GPEs) developed by various methods exhibit high ionic conductivity in the order of 103 S cm1 at room temperature but their mechanical properties are poor for lithium ion battery applications [5e16]. Now a day’s researchers have put effort to develop nanocomposite polymer electrolyte membranes with good thermal, chemical and electrochemical stabilities by incorporating some nano-sized ceramic filler, such as TiO2, SiO2, Al2O3, ZrO2, MgAl2O4, etc., [17e22]. The polymer electrolytes based on poly(vinylidene fluoride) (PVdF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-co-HFP), poly(methyl methacrylate) (PMMA) and poly(acrylonitrile) (PAN) were widely used as host polymers [23e29]. Among them, PVdF-co-HFP copolymer was found to be a suitable candidate as it has good electrochemical stability, affinity to electrolyte solution and high dielectric constant (ε z 8.4) [25,26]. Moreover, P(VdF-co-HFP) is the strongest Lewis basic polymers compared with other polymers. The Lewis acid constant (Ka) of P(VdF-co-HFP) is 0.254, and the base constant (Kb) is 1.199, which may help the formation of complexes with strongest Lewis acid character of fillers [30]. A wide range of synthesis methods, such as solution casting [7], hot press [21,22], plasticizer extraction [12], phase inversion [26], and electrospinning etc., [27e34] have been adopted for the
O. Padmaraj et al. / Polymer 55 (2014) 1136e1142
development of porous polymer membranes. Among them, electrospinning is a simple and effective technique to develop thin nano-size porous fibrous polymer membranes with highly interconnected porous structure and large surface area, which are very important factors to improve the electrolyte uptake amount, ionic conductivity and electrochemical stability [29e32]. To the best of author’s knowledge there are no reports on the development of P(VdF-co-HFP)/MgAl2O4/1 M LiPF6 EC:DEC (1:1, v/ v) nanocomposite fibrous polymer electrolyte membranes by electrospinning technique. As we know that, the MgAl2O4 ceramic filler has strongest Lewis acid character and also high dielectric constant (ε ¼ 8.1e8.3) than other fillers, which would compete with the Lewis acid character of Liþ ions in LiPF6 salt for the formation of complexes with the strongest Lewis base P(VdF-co-HFP) polymer chains. The resulting structure provides more easy pathways for Liþ ions, which can enhance ionic transport and hence, improves the conductivity [30]. The solvent EC has high dielectric constant (ε ¼ 89.6) and high viscosity (h ¼ 1.5), whereas DEC has low dielectric constant (ε ¼ 2.82) and low viscosity (h ¼ 0.748). Combination of EC and DEC (1:1, v/v) solvent has optimized dielectric constant (ε ¼ 46.21) and viscosity (h ¼ 1.124), which will provide reduced electrostatic attraction between the Liþ and PF 6 ions in a LiPF6 salt. The optimized solvent may separate (dissociate) the Liþ ions easily, which in turn improve ionic conductivity at room temperature. Hence, authors are motivated to develop electrospun fibrous pure [16 wt.% P(VdF-co-HFP)] and nanocomposite polymer membranes with various MgAl2O4 (2, 5 & 8 wt.%) fillers content. The asprepared fibrous pure and nanocomposite polymer membranes were characterized by XRD, DSC and SEM techniques. The fibrous nanocomposite polymer electrolytes (NCPEs) are prepared by soaking the electrospun membranes in 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1, v/v). The electrolyte uptake behavior and ionic conductivities of nanocomposite polymer electrolytes are evaluated through electrolyte uptake method and impedance spectroscopy measurements. Electrochemical performance of all the Li/PE, NCPE, celgard 2320/LiCoO2 CR 2032 coin cells were studied through Battery cycle tester (BCT) at current density 0.1 C-rate. 2. Experimental 2.1. Synthesis of PVdF-co-HFP/MgAl2O4 nanocomposite fibrous membranes
between the tip of the spinneret and collector is 12.5 cm, needle bore size 24 G and collector drum rotation speed is 450 rpm. The electrospun fibrous membranes with an average thickness of 80 mm were collected and dried in hot air oven at 60 C for 24 h to remove the solvent for further use. 2.2. Characterization techniques X-ray diffraction patterns (XRD) were recorded from 10 to 80 for all the prepared fibrous membranes using PANalytical X-pert PRO diffractometer (Philips) with Cu-Ka radiation (l ¼ 0.154060 nm, 30 mA and 40 kV). The DSC curves of the celgard 2320 separator and the prepared electrospun membranes were recorded on DSC Q20 instrument under nitrogen atmosphere. From the analyzed DSC curve, the crystallinity of pure and nanocomposite fibrous membranes was calculated using the following equation [13].
Xc ¼
DHs 100% DH *
(1)
where, DH* ¼ 104.7 J/g, the melting enthalpy of PVdF and DHs J/g is the melting enthalpy of prepared electrospun pure and nanocomposite membranes. The micrographs of prepared electrospun membranes were observed on scanning electron microscopy (SEM, Hitachi S4700). The electrolyte uptake behavior was observed by soaking the membranes in liquid electrolyte [1 M LiPF6 in EC:DEC (1:1, v/v)] for 1 h. The electrolyte uptake was calculated based on the following equation [13].
EU ¼
W1 W0 100% W0
(2)
where, W1 and W0 are the weights (g) of the wet and dry membranes respectively. The ionic conductivities of the nanocomposite polymer electrolyte membranes were evaluated at room temperature through impedance spectroscopy measurements. Each polymer electrolyte membrane was placed between stainless steel (SS) electrodes and the impedance spectra were recorded on Alpha high frequency response analyzer (Novocontrol, Germany) in the frequency range of 1 mHze1 MHz. The ionic conductivities are calculated from the analyzed impedance data and sample dimensions by using the following equation
s¼ Poly(vinylidene difluoride-co-hexafluoropropylene) [P(VdF-coHFP)] (Aldrich, AMw ¼ 4 105) and MgAl2O4 filler prepared by gel-combustion method were used as the starting materials for the preparation of fibrous nanocomposite polymer membranes by electrospinning technique. All the starting materials were dried at 60 C for 24 h in hot air oven before use. N, N-Dimethylacetamide (DMAc) and acetone were used as mixed solvents. For the preparation of fibrous nanocomposite polymer membranes, first the optimized 16 wt.% of P(VdF-co-HFP) polymer solution was prepared by dissolving it in a mixed solvent of acetone/DMAc (7:3, V/V) under continuous stirring for 3 h at room temperature [31]. Later, 1, 2, 3, 4, 5, 6 & 8 wt.% of MgAl2O4 nanoparticle filler (particle size: <100 nm, surface area: 150 m2/g) were added to the above optimized 16 wt.% P(VdF-co-HFP) polymer solution under constant stirring for 3 h at room temperature. The resultant optimized viscous solution was taken into a 10 ml syringe and loaded in a syringe pump to form nanocomposite fibers by setting the electrospinning parameters. The following electrospinning parameters are used for all the compositions: solution feed rate 1 mlh1, applied voltage between spinneret and collector is 18 kV, distance
1137
t S=cm A Rb
(3)
where, t is the sample thickness (cm), A is the area (cm) and Rb is the bulk resistance (U) of polymer electrolyte membrane. 2.3. Electrochemical studies The electrochemical stability of the prepared electrospun nanocomposite membrane was studied through cyclic voltametry (Electrochemical test station POT/GAL, Novocontrol, Germany). The measurement (Li/NCPE/SS) was carried out by using stainless steel (SS) as the working electrode and lithium (Li) metal as counter/ reference electrode at the scan rate of 1 mV s1 over the potential range of 2e5 V at room temperature. For electrochemical measurements, the prototype lithium cells were assembled inside an Argon-filled glove box [Vacuum Atmospheres Co. (VAC), USA] by sandwiching the PE, NCPE, celgard 2320 separator between lithium metal anode (380 mm thick, Aldrich) and LiCoO2 cathode in CR-2032 coin cell. The LiCoO2 cathode active material was synthesized by combustion method and the cathode electrode was prepared by
1138
O. Padmaraj et al. / Polymer 55 (2014) 1136e1142
mixing 70 wt% of active material (LiCoO2), 20 wt% of conducting super P carbon and 10 wt% of PVdF binder (Aldrich) in N-methyl pyrrolidinone (NMP) solvent. The resulting slurry was coated on to an Al-foil (w20 mm thick) using doctor blade method and dried in 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 all the prepared Li/PE, NCPE, celgard 2320/LiCoO2 cells were performed at room temperature in a battery cycle tester (BCT) (Model MCV4-1/ 0.01/0.001-10, Bitrode, USA) between the potential ranges 2.8e 4.2 V at current density 0.1 C-rate.
the conductivity and it was confirmed from the conductivity measurements. 3.2. Thermal analysis
Fig. 1 shows the XRD patterns of fibrous pure and nanocomposite polymer membranes with various MgAl2O4 filler content. It can be seen that fibrous pure [16 wt.% P(VdF-co-HFP)] polymer membrane exhibits a broad and low intensity diffraction peak at 20 , which indicates the crystalline phase of pure PVdF in P(VdF-co-HFP) host copolymer. It is confirmed by comparing the observed XRD pattern with the Joint committee on powder diffraction standards (JCPDS) data (Card No.00-038-1638). Hence, the observed diffraction pattern of fibrous pure P(VdF-coHFP) membrane exhibits semi-crystalline nature [13,23]. The intensity of PVdF diffraction peak is decreasing by addition of 2 & 5 wt.% MgAl2O4 filler as compared to the pure polymer membrane. This indicates the reduction of PVdF crystallinity in the host copolymer for the formation of complexes between strongest acidebase sites in filler and polymer segments through Lewis acidebase interactions, which in turn can enhance the ionic conductivity at ambient temperature. Further addition of 8 wt.% MgAl2O4 filler, the intensity of broad diffraction peak is increased and also some small pronounced new diffraction peaks are observed at 36 , 44 & 65 . The observed new diffraction peaks reflect the crystalline phase of MgAl2O4 nanoparticles and it is confirmed by comparing with MgAl2O4 diffraction pattern [21]. This may affect the mobility (m) of free Li ion charge carriers by blocking or hindering the pathways, which results in a decrease in
The thermal behavior and crystallinity (cc) of each composition of the nano-composite membrane was studied from each DSC curves. It is observed that the nanocomposite membrane with the addition of 5 wt.% MgAl2O4 filler has lower crystallinity compared to 2 and 8 wt.% MgAl2O4 compositions of nanocomposite membranes. Hence, the lower crystallinity composition of the nanocomposite membrane was chosen and compared its melting temperature with the melting temperature of pure polymer membrane and also commercial celgard 2320 separator. Fig. 2 shows the DSC curves of celgard 2320 separator, pure and nanocomposite [16 wt.% P(VdF-coHFP) þ 5 wt.% MgAl2O4] fibrous polymer membranes. It can be seen that each curve exhibits a sharp endothermic peak at 132 C, 141 C & 143 C, which indicates their respective melting or softening temperature. It is observed that the prepared electrospun nanocomposite polymer membranes have high thermal stability compared to the pure and celgard 2320 separator. The increase in thermal stability of MgAl2O4 added nano-composite membrane, which is attributed to the intercalation and extrafoliation of P(VdFco-HFP) copolymer matrix with MgAl2O4 fillers through Lewis acidebase interactions. Due to the strong acid sites of Mgþ & Alþ ions in MgAl2O4 fillers and strong base sites of Fluorine (F) in P(VdFco-HFP) copolymer may resists the segmental motion of polymer chains. Moreover, the melting enthalpy (DHs) from each DSC curves of 22.14 J g1 and 21.30 J g1, for pure and nanocomposite fibrous membranes respectively were used for calculating the percentage of crystallinity as shown in Fig. 2. The melting enthalpy of the electrospun pure and composite membranes can be calculated from the integral area in each melting (endothermic) DSC curve [19]. It is observed that the calculated crystallinity of 16 wt.% P(VdF-coHFP) þ 5 wt.% MgAl2O4 nanocomposite polymer membrane is found to be 20.34%, where as pure [16 wt.% P(VdF-co-HFP)] fibrous membrane exhibits 21.14% of crystallinity. The observed decrease in melting enthalpy and crystallinity with the addition of 5 wt.% of MgAl2O4 filler indicate an increase in the amorphous nature of the nanocomposite fibrous membrane. Further, it was confirmed for
Fig. 1. XRD patterns of electrospun pure and nanocomposite fibrous membranes with various MgAl2O4 filler content.
Fig. 2. DSC curves of a) celgard 2320 separator and electrospun nanocomposite fibrous membranes with various MgAl2O4 filler contents: b) 0 wt.% (pure), c) 5 wt.%.
3. Results and discussion 3.1. X-ray diffraction study
O. Padmaraj et al. / Polymer 55 (2014) 1136e1142
1139
conductivity studies. Hence, the newly developed electrospun nanocomposite [16 wt.% P(VdF-co-HFP) þ 5 wt.% MgAl2O4] fibrous polymer membrane may act as a better separator-cum electrolyte with good thermal stability as well as low crystallinity for high ionic conducting membrane compared to celgard 2320 separator. 3.3. Surface morphological study Fig. 3 shows the surface morphology of electrospun pure (Fig. 3a) and nanocomposite fibrous polymer membranes with various MgAl2O4 filler [2 wt.% (Fig. 3b), 5 wt.% (Fig. 3c) & 8 wt.% (Fig. 3d)] content. From Fig. 3, it can be seen that the membranes exhibit a three dimensional web structure with fully interconnected pores of ultrafine multi-fibers with bead free morphology [29]. The interlaying of multi-fiber layers generates a porous structure between the fibers in the electrospun membrane, which can good absorb and retain the liquid electrolyte effectively. The SEM images also depict the variation of average fiber diameters (AFD) between the pure and nanocomposite membranes with various amounts of MgAl2O4 ceramic filler as shown in Fig. 3. The AFD of electrospun membranes are strongly influenced by the spinning parameters such as solution concentration, feed rate, needle bore size, distance between the spinneret & collector and applied voltage [28]. In the present work, the effect of solution concentration was studied while the other parameters kept constant. As observed from Fig. 3aec, the AFD of pure, 2 wt.%, 5 wt.% & 8 wt.% of MgAl2O4 filler incorporated nanocomposite polymer membranes are approximately 1.5, 1, 0.5, and 1 mm, respectively. The nanocomposite polymer membrane with 5 wt.% of MgAl2O4 filler exhibits less AFD (500 nm) with uniform fiber distribution compared to other membranes. The observed difference in AFD attributes to a change in the concentration of polymer solution with different percentages of MgAl2O4 filler. 3.4. Electrolyte uptake behavior The prepared each composition of electrospun pure and nanocomposite membrane has three dimensional interlaying of multi-
Fig. 4. Variation of electrolyte uptake and conductivity of fibrous nano-composite polymer electrolyte membranes with various MgAl2O4 fillers content at room temperature.
fibers layers, which may generates the high porous structure in electrospun membranes. The fibrous pure and nanocomposite polymer electrolytes are prepared by soaking the prepared membranes in an electrolyte solution of 1 M LiPF6 in EC:DEC (1:1, v/v). Due to the acidebase interactions, the pure P(VdF-co-HFP) copolymer matrix and nanocomposite membrane P(VdF-co-HFP)/ MgAl2O4 with varying wt.% of MgAl2O4 fillers content can absorb an electrolyte solution of 1 M LiPF6 in EC:DEC (1:1, v/v). This is called electrolyte uptake. Fig. 4 shows the electrolyte uptake behavior of the electrospun pure and nanocomposite fibrous membranes with different MgAl2O4 (2, 5 & 8 wt.%) filler content. As shown in Fig. 4, the membrane exhibits 61% of increment in electrolyte uptake up to 5 wt.% of MgAl2O4 and further addition of MgAl2O4 (8 wt.%) results in a decrease of uptake behavior from 415% to 369%. The observed increase in electrolyte uptake behavior up to 5 wt.% of MgAl2O4 might be due to the small fiber diameter and hence high porosity than other membranes [20,35]. Hence, 5 wt.% of MgAl2O4
Fig. 3. SEM images of electrospun nanocomposite fibrous membranes with various MgAl2O4 filler contents: a) 0 wt.% (pure), b) 2 wt.%, c) 5 wt.% and d) 8 wt.%.
1140
O. Padmaraj et al. / Polymer 55 (2014) 1136e1142
Fig. 5. Photographic images of electrospun nanocomposite fibrous electrolyte membrane with 5 wt.% MgAl2O4 filler content: a) dry, b) wet and c) free standing wet membrane.
incorporated nanocomposite membrane was undergone to dimensional studies as shown in Fig. 5aec. The photographs show that the membrane exhibits high electrolyte uptake and sufficient mechanical strength with self standing properties even after activated in the liquid electrolyte. 3.5. Ionic conductivity Fig. 6 shows the impedance spectra of electrospun pure and nanocomposite fibrous polymer electrolyte membranes with different MgAl2O4 (2, 5 & 8 wt.%) content at room temperature. As shown in Fig. 6, all the impedance spectra show one arc and an inclined spike in the measured frequency range. The observed arc in the high frequency region and the spike in the low frequency region represent the bulk resistance and electrode/electrolyte (Double Layer Capacitance) effects of the electrospun membranes, respectively [31,36]. The intercept of inclined spike on the real axis gives the bulk resistance (Rb) of the electrospun membrane. The observed impedance spectra were fitted to an equivalent circuit using “WinFIT” software. The ionic conductivities of the electrospun membranes were calculated from the analyzed impedance data and sample dimensions. The observed bulk resistances (Rb) and ionic conductivities (s) of the electrospun membranes are given in Table 1. The variation of conductivity with MgAl2O4 filler content in polymer electrolyte membrane is also shown in Fig. 4. It can be seen that the conductivity (s) increases with the addition of MgAl2O4 filler up to 5 wt.% and further addition of MgAl2O4 (8 wt.%) results in decrease of conductivity. The observed highest
conductivity for 16 wt.% P(VdF-co-HFP) þ 5 wt.% MgAl2O4 nanocomposite membrane might be attributed to the low crystallinity and high electrolyte uptake with fully interconnected porous structure. On the other hand, electrospun membrane with 8 wt.% MgAl2O4 exhibits very low conductivity, which is due to reduce the porosity by increasing the AFD and the formation of MgAl2O4 crystalline phase in the composite membrane [34]. An increased porosity may help to increase the uptake of liquid electrolyte, which can able to enhance the ionic conductivity at room temperature. 3.6. Electrochemical studies Fig. 7 shows the electrochemical stability window of 16 wt.% P(VdF-co-HFP) þ 5 wt.% MgAl2O4 nanocomposite fibrous polymer electrolyte membrane. From Fig. 7, the observed steady increase in current density with voltage indicates the electrochemical stability limit of the fibrous polymer electrolyte membrane [31,33]. It can be found that, the newly developed nanocomposite fibrous polymer electrolyte membrane with 5 wt.% of MgAl2O4 filler content exhibits high anodic stability above 4.5 V. Hence, the newly prepared high electrochemical stability window of nanocomposite fibrous polymer electrolyte membrane should render them potentially compatible with the most high voltage cathode materials like LiCoO2 and LiMnO2, commonly used for rechargeable lithium batteries. Fig. 8 show a comparative chargeedischarge properties at a current density to corresponding 0.1 C-rate of Li/LiCoO2 cells containing nanocomposite fibrous electrolyte membrane with 5 wt.% MgAl2O4 and celgard 2320 separator. As shown in Fig. 8, the discharge capacity of 158 mAhg1 (1st cycle) & 128 mAhg1 (30th cycle) nanocomposite electrolyte membrane is compared to the discharge capacity of 149 mAhg1 (1st cycle) & 120 mAhg1 (30th cycle) celgard 2340 separator containing coin cell. The discharge capacity of cell assembled with nanocomposite polymer electrolyte higher than the cell with commercial celgard 2320 separator. The observed capacity differences from 1st to 30th cycles of Li/NCPE/ LiCoO2 cell showed an approximately 10 mAhg1 higher than the Li/celgard 2320/LiCoO2 cell, which is probably due to the difference in ionic conductivity and utilization of active material. The electrospun nanocomposite electrolyte membrane has high porosity Table 1 Electrical properties of nanocomposite fibrous polymer electrolyte membrane with various MgAl2O4 filler content.
Fig. 6. Complex impedance spectra of electrospun nanocomposite fibrous electrolyte membranes with various MgAl2O4 filler content at room temperature.
S. No
Samples (wt.%)
Resistance (Rb) U
Conductivity (S cm1)
1 2 3 4
Pure 2 5 8
4.15 3.25 3.10 7.15
1.74 2.71 2.80 1.27
103 103 103 103
O. Padmaraj et al. / Polymer 55 (2014) 1136e1142
Fig. 7. Electrochemical stability window by cyclic voltametry of electrospun nanocomposite fibrous electrolyte membrane with 5 wt.% MgAl2O4 filler content at room temperature.
with ultra-fine pore structure, which can able to absorb large amount of liquid electrolyte as compared to celgard 2320 separator. This may help to enhance the ionic conductivity by entrap the Li ions into the fibrous electrolyte membrane and hence, which leads to higher utilization of active material. It was reported that the initial discharge capacity of LiCoO2/Li-metal cell assembled with 12 wt.% SiO2/PAN based electrospun nanocomposite electrolyte activated with 1 M LiPF6 (EC/DMC) is about 139 mAhg1 at 0.5 C and 115 mAhg1 at 1 C respectively [32]. Fig. 9 shows the discharge capacity as a function of cycle number for Li/LiCoO2 cells based on celgard 2320 separator, electrospun pure and nanocomposite fibrous polymer electrolyte membrane with 5 wt.% MgAl2O4 nanoparticle filler. From Fig. 9, the discharge capacity and cycle performance was increased with addition of 5 wt.% MgAl2O4 nanoparticle filler as compared to pure fibrous membrane and celgard 2320 separator. The discharge capacity of 1st and 30th cycle of Li/NCPE/LiCoO2 cell shows 158 mAhg1 and
1141
Fig. 9. Cycle performance of Li/LiCoO2 coin cells containing electrospun nanocomposite fibrous electrolyte membranes with various MgAl2O4 filler content and celgard 2320 separator.
128 mAhg1 respectively, whereas Li/PE/LiCoO2 cell delivers 142 mAhg1 and 102 mAhg1 and Li/celgard 2320/LiCoO2 delivers 148 mAhg1 and 120 mAhg1 respectively. The observed capacity fade may be due to the structural characteristics and composition of cathode materials play a vital role on the capacity of cell and cycling performance. Hence, the electrospun nanocomposite electrolyte membrane with 5 wt.% MgAl2O4 filler has good cycle performance without any change in the property, which confirms the excellent efficiency of the membrane to conduct the ions between the electrodes [37]. Also, it has good compatibility with both the electrodes especially lithium metal. Thus, it may be concluded that the newly developed nanocomposite [16 wt.% P(VdF-co-HFP) þ 5 wt.% MgAl2O4] membrane can be useful as a better electrolyte for all rechargeable lithium battery application as well as other ionic devices. 4. Conclusion The pure [16 wt.% P(VdF-co-HFP)] and nanocomposite [16 wt.% P(VdF-co-HFP) þ x wt.% MgAl2O4, x ¼ 2, 5 & 8] fibrous polymer electrolyte membranes were prepared by electrospinning technique. The nanocomposite fibrous polymer membrane with 5 wt.% MgAl2O4 filler showed the reduction of PVdF crystallinity (Xc), high thermal stability and exhibits an uniform morphology with less average fiber diameter is 0.5 mm. The 5 wt.% MgAl2O4 incorporated nanocomposite polymer electrolyte membrane exhibits a high electrolyte uptake, high ionic conductivity and good electrochemical stability higher than 4.5 V at room temperature. The Li/ NCPE/LiCoO2 cell with 5 wt.% MgAl2O4 filler delivers the good discharge capacity with stable cycle performance compared to the celgard 2320 separator. Hence, the newly developed electrospun nanocomposite [16 wt.% P(VdF-co-HFP) þ 5 wt.% MgAl2O4] fibrous polymer electrolyte membrane can be used as a suitable electrolyte membrane for high performance rechargeable lithium batteries as well as other ionic device applications. Acknowledgments
Fig. 8. Chargeedischarge capacity of Li/LiCoO2 cells containing nanocomposite fibrous electrolyte membrane with 5 wt.% MgAl2O4 filler content and celgard 2320 separator at room temperature.
Dr. NS gratefully acknowledges DST, AICTE, UGC, CSIR and DRDO, Govt. of India for financial support through major research
1142
O. Padmaraj et al. / Polymer 55 (2014) 1136e1142
project grants. OP is thankful to UGC for the award of BSR fellowship for pursuing the doctoral degree. Authors also acknowledge CIF, Pondicherry University, for providing FTIR, DSC & SEM facilities.
References [1] Tarascon JM, Armond M. Issues and challenges facing rechargeable lithium batteries. Nature 2001;414:359e67. [2] Zhang SS. A review on the separators of liquid electrolyte Li-ion batteries. J Power Sources 2007;164:351e64. [3] Arora PK, Zhang ZM. Battery separators. Chem Rev 2004;104:4419e62. [4] Abraham KM, Alamgir M. Polymer electrolytes reinforced by celgard membranes. J Electrochem Soc 1995;142:683e7. [5] Armand M. Polymer solid electrolytes-an overview. Solid State Ionics 1983;9 & 10:745e54. [6] Koksbang R, Olsen II, Shackle D. Review of hybrid polymer electrolytes and rechargeable lithium batteries. Solid State Ionics 1994;69:320e35. [7] Croce F, Curini R, Martinelli A, Persi L, Ronci F, Scrosati B, et al. Physical and chemical properties of nanocomposite polymer electrolytes. J Phys Chem B 1999;103:10632e8. [8] Derrien Gaelle, Hassoun Jusef, Sacchetti Simone, Panero Stefania. Nanocomposite PEO-based polymer electrolyte using a highly porous, super acid zirconia filler. Solid State Ionics 2009;180:1267e71. [9] Croce F, Appetecchi GB, Persi L, Scrosati B. Nanocomposite polymer electrolytes for lithium batteries. Nature 1998;394:456e8. [10] Weston JE, Steele BCH. Effects of inert fillers on the mechanical and electrochemical properties of lithium salt-poly (ethylene oxide) polymer electrolytes. Solid State Ionics 1982;7:75e9. [11] Rajendiran S, Kannan R, Mahendran O. Ionic conductivity studies in poly(methyl methacrylate)-polyethylene oxide hybrid polymer electrolytes with lithium salts. J Power Sources 2001;96:406e10. [12] Cui Zhen-Yu, Xu You-Yi, Zhu Li-Ping, Wang Jian-Yu, Xi Zhan-Yu, -Zhu Bao-Ku. Preparation of PVDF/PEO-PPO-PEO blend microporous membranes for lithium ion batteries via thermally induced phase separation process. J Memb Sci 2008;325:957e63. [13] Liang Yinzheng, Cheng Sichen, Zhao Jianmeng, Zhang Changhuan, Sun Shiyuan, Zhou Nanting, et al. Heat treatment of electrospun polyvinylidene fluoride fibrous membrane separators for rechargeable lithium-ion batteries. J Power Sources 2013;240:204e11. [14] Prasanth Raghavan, Aravindan Vanchiappan, Srinivasan Madhavi. Novel polymer electrolyte based on cob-web electrospun multi component polymer blend of polyacrylonitrile/poly(methyl methacrylate)/polystyrene for lithium ion batteries. J Power Sources 2012;202:299e307. [15] Kim Jae-Kwang, Cheruvally Gouri, Li Zin, Ahn Jou-Hyeon, Kim Ki-Won, Ahn Hyo-Jun. Preparation and electrochemical characterization of electrospun, microporous membrane-based composite polymer electrolytes for lithium batteries. J Power Sources 2008;178:815e20. [16] Manuel Stephen A. Review on gel polymer electrolytes for lithium batteries. Eur Polym J 2006;42:21e42. [17] Manuel Stephen A, Nahm KS. Review on composite polymer electrolytes for lithium batteries. Polymer 2006;47:5952e64. [18] Choi Byoung-Koo, Kim Young-Wan, Shin Kyoung-Hee. Effects of ceramic fillers on the electrical properties of (PEO)16 LiClO4 electrolytes. J Power Sources 1997;68:357e60. [19] Cui Wei-Wei, Tang Dong-Yan, Gong Zai-Lin. Electrospun poly(vinylidene fluoride)/poly(methyl methacrylate) grafted TiO2 composite nanofibrous membrane as polymer electrolyte for lithium-ion batteries. J Power Sources 2013;223:206e13.
[20] Fu Dong, Luan Ben, Argue Steve, Bureau Martin N, Davidson Isobel J. Nano SiO2 particle formation and deposition on polypropylene separators for lithium-ion batteries. J Power Sources 2012;206:325e33. [21] Angulakshmi N, Nimma Elizabeth R, Swaminathan V, Suk Nahm Kee, Manuel Stephan A. Physical and electrochemical properties of MgAl2O4-incorporated polymer electrolytes composed of poly(ethylene oxide) and LiClO4. Sci Adv Mater 2011;3:702e8. [22] Angulakshmi N, Nahm KS, Nair Jijeesh R, Gerbaldi C, Bongiovanni R, Penazzi, et al. Cycling profile of MgAl2O4-incorporated composite electrolytes composed of PEO and LiPF6 for lithium polymer batteries. Electrochim Acta 2013;90:179e85. [23] Kim Kwang Man, Park Nam-Gyu, Ryu Kwang Sun, Chang Soon Ho. Characterization of poly(vinylidene fluoride-co-hexafluoropropylene) based polymer electrolyte filled with TiO2 nanoparticles. Polymer 2002;43:3951e7. [24] Wang Yifei, Ma Xiaoyan, Zhang Qilu, Tian Na. Synthesis and properties of gel polymer electrolyte membrane based on novel comb-like methyl methacrylate copolymers. J Memb Sci 2010;349:279e86. [25] Kim Kwang Man, Ko Jang Myoun, Park Nam-Gyu, Ryu Kwang Sun, Chang Soon Ho. Characterization of poly(vinylidene fluoride-co-hexafluoropropylene)based polymer electrolyte filled with rutile TiO2 nanoparticles. Solid State Ionics 2003;161:121e31. [26] Pu Weihu, He Yiangming, Wang Li, Jiang Changyin, Wan Chunrong. Preparation of PVDFeHFP microporous membrane for Li-ion batteries by phase inversion. J Memb Sci 2006;272:11e4. [27] Choi SW, Jo SM, Lee WS, Kim YR. An electrospun poly(vinylidene fluoride) nanofibrous membrane and it battery application. Adv Mater 2003;15:2027e 32. [28] Wu Na, Cao Qi, Wang Xianyou, Li Sheng, Li Xiaoyun, Deng Huayang. In situ ceramic fillers of electrospun thermoplastic polyurethane/poly(vinylidene fluoride) based gel polymer electrolytes for Li-ion batteries. J Power Sources 2011;196:9751e6. [29] Cho TH, Sakai T, Tanase S, Kimura K, Kondo Y, Tarao T, et al. Electrochemical performances of polyacrylonitrile nanofiber-based Nonwoven separator for lithium-ion battery. Electrochem Solid State Lett 2007;10:A159e62. [30] Shi Baoli, He Mingjia. Lewis acidebase property of P(VDF-co-HFP) measured by inverse gas chromatography. J Appl Polym Sci 2008;107:1642e6. [31] Li Xin, Cheruvelly Gouri, Kim Jae-Kwang, Choi Jae-Won, Ahn Jou-Hyeon, Kim Ki-Won, et al. Polymer electrolytes based on an electrospun poly(vinylidene fluoride-co-hexafluoropropylene) membrane for lithium batteries. J Power Sources 2007;167:491e8. [32] Jung Hong-Ryun, Ju Dong-Hyuk, Lee Wan-Jin, Zhang Xiangwu, Kotek Richard. Electrospun hydrophilic fumed silica/polyacrylonitrile nanofiber-based composite electrolyte membranes. Electrochim Acta 2009;54:3630e7. [33] Li Xiaoyun, Cao Qi, Wang Xianyou, Jiang Shenghui, Deng Huayang, Wu Na. Preparation of poly(vinylidene fluoride)/poly(methyl methacrylate) membranes by novel electrospinning system for lithium ion batteries. J Appl Polym Sci 2011;122:2616e20. [34] Cheruvally Gouri, Kim Jae-Kwang, Choi Jae-Won, Ahn Jou-Hyeon, Shin YongJo, Manuel James, et al. Electrospun polymer membrane activated with room temperature ionic liquid: novel polymer electrolytes for lithium batteries. J Power Sources 2007;172:863e9. [35] Stephen AM, Nahm KS, Kulandainathan MA, Ravi G, Wilson J. Poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) based composite electrolytes for lithium batteries. Eur Polym J 2006;42:1728e34. [36] Scrosati B, Croce F, Persi L. Impedance spectroscopy study of PEO-based nanocomposite polymer electrolytes. J Electrochem Soc 2000;147(5):1718e21. [37] Raghavan P, Shao X, Shin C, Baek D-H, Chaoi J-W, Manuel J, et al. Preparation and electrochemical characterization of polymer electrolytes based on electrospun poly(vinylidene fluoride-co-hexafluoropropylene)/polyacrylonitrile blend/composite membranes for lithium batteries. J Power Sources 2010;195:6088e94.