PMMA membranes for lithium ion batteries

Journal of Membrane Science 329 (2009) 56–59

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

The ionic conductivity and mechanical property of electrospun P(VdF-HFP)/PMMA membranes for lithium ion batteries Yanhuai Ding a,b,∗ , Ping Zhang a,b,∗ , Zhilin Long a,b , Yong Jiang a,b , Fu Xu a,b , Wei Di a a b

College of Civil Engineering & Mechanics, Xiangtan University, Hunan 411105, China Institute of Fundamental Mechanics & Material Engineering, Xiangtan University, Hunan 411105, China

a r t i c l e

i n f o

Article history: Received 29 May 2008 Received in revised form 15 September 2008 Accepted 2 December 2008 Available online 24 December 2008 Keywords: Polymer electrolyte P(VdF-HFP)/PMMA Electrospinning Ionic conduction Mechanical property

a b s t r a c t Polymer electrolytes, based on a blend of poly(methylmethacrylate) [PMMA]/poly(vinylidene difluorideco-hexafluoropropylene)[P(VdF-HFP)], were prepared by electrospinning at room temperature. The morphology, structure, ionic conduction and mechanical properties of the electrospun membranes were characterized by atomic force microscopy (AFM), FTIR spectra, X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS) and mechanical measurements. The composites showed the ionic conduction and uptake and leakage behaviors of the electrolyte solution were improved by the addition of PMMA. Also, the mechanical property of the blend membranes was better than that of the pure P(VdF-HFP) electrospun membranes. © 2008 Elsevier B.V. All rights reserved.

1. Introduction In the last decade, remarkable development of portable electric devices, such as mobile telephones, movable computers and hybrid electric vehicles, has led to a strong need of safe and highenergy lithium ion batteries. Lithium-ion polymer batteries have attracted a great deal of attention due to its higher energy density, improved safety hazards and good processability. To date, several polymer hosts have been investigated and developed including poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinyl chloride) (PVC), poly(vinylidene fluoride) (PVdF), poly(vinylidene fluoride-hexafluoro propylene) (PVdF-HFP), etc. [1,2]. Among them (PVdF-HFP) has drawn much attention of researchers, in which the amorphous HFP phase helps to capture large amount of liquid electrolytes and the PVdF crystalline phase acts as a mechanical support for the polymer matrix [3–9]. Recently, electrospinning has been developed to fabricate microporous PVdF and PVdF-HFP membranes as polymer electrolytes for lithium ion batteries [10–13]. To improve the physical properties and electrolyte affinity, a blend of PVdF-HFP/PMMA polymer electrolytes were prepared by electrospinning in this

∗ Corresponding authors at: College of Civil Engineering & Mechanics, Xiangtan University, Hunan 411105, China. Tel.: +86 732 8292247; fax: +86 732 8293240. E-mail addresses: [email protected] (Y. Ding), [email protected] (P. Zhang). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.12.024

study. The mechanical properties and electrochemical performances of the electrospun PVdF-HFP/PMMA membranes were also investigated systematically. 2. Experimental 2.1. Materials All the reagents used were analytical grade. 1:2 (w/w) ratio of PMMA (Mw 100, 000, Aldrich) and P(VdF-HFP) copolymer (Kynar Flex 2801, Atofina) were dissolved in a mixed solvent of N,N-dimethylformamide (DMF)/acetone (weight ratio of 3/7). The concentration of the mixed solution was 20 wt.%. The asprepared solution was placed in a plastic syringe equipped with a needle-diameter of 0.3 mm. PVdF-HFP/PMMA membranes was electrospinning at 30 kV with a high-voltage supply. The distance between the collector and the needle was 15 cm. The obtained PVdF-HFP/PMMA membranes were placed in vacuum for 12 h at room temperature. As comparison, electrospun PVdF-HFP membranes were also prepared as mentioned above. 2.2. Characterization The morphology of the electrospun membranes was observed by atomic force microscopy (AFM) (Multimode NS-3D, Digital Instruments). The structure was investigated by FTIR spectra (SpectrumOne, PerkinElmer Instruments). X-ray diffraction (XRD)

Y. Ding et al. / Journal of Membrane Science 329 (2009) 56–59

57

Fig. 1. Morphological feature of the electrospun PVdF-HFP and PVdF-HFP/PMMA membranes (a) pure PVdF-HFP, (b) PVdF-HFP/PMMA (2:1, w/w).

measurements were performed on an X-ray Diffractometer (Rigaku D/max-2500). For electrochemical testing, polymer electrolytes were obtained by immersing the as-prepared electrospun membranes in liquid electrolyte of 1 mol/L LiPF6 -EC/DMC (1:1 by volume) for 4 h. The ionic conductivity was measured from AC impedance by sandwiching polymer electrolyte between two stainless steel electrodes. The electrochemical measurements were performed at electrochemistry analyzer (Pgstat100, Autolab Instruments). The uptake of the electrolyte solution was investigated by soaking the electrospun membranes in the electrolyte solution with a size of D = 1.5 cm. The weights of the wetted membranes were measured against the soaking time after removal of the excess electrolyte solution on the surface of the membranes. The mechanical properties of the membranes were measured by universal testing machines (UTM, Instron Instruments). The extension rate was kept at ∼10 mm/min. The dimensions of the sheet used were ∼2 cm × 5 cm × ∼150–250 ␮m (width × length × thickness).

3. Results and discussion Fig. 1 shows the morphological feature of the electrospun PVdF-HFP and PVdF-HFP/PMMA membranes. All the image of the electrospun membranes shows a 3D structure with a random fiber orientation that is evenly distributed on the substrate. It has been

reported the electrospun membranes have 1–2 orders of magnitude more surface area than that of the continuous films [14]. As shown in Fig. 1a, the diameters of the PVdF-HFP nanofibers were approximately 100–250 nm and most of the fibers was not interconnected. The surface of the nanofibers was very smooth due to its homogeneous polymeric texture. It was clearly shown in Fig. 1b that by the addition of PMMA, the nanofibers were frequently interconnected by fusing the attaching point of two or three individual fibers. Furthermore, the average diameter of the fibers increased slightly and has been measured on the range of 200–350 nm. Comparison with the pure PVdF-HFP nanofibers, the surface of the composite nanofibers became rough. It was suggested such results resulting from the increasing interface of PVdF-HFP and PMMA phase could enhance the ionic conduction. Fig. 2 shows the FTIR spectra of the as-prepared electrospun membranes. With addition of PMMA, CF2 asymmetric stretching vibration and CF3 rocking vibration became weak, which could be ascribed to the steric interaction between the PMMA and PVdF-HFP molecular chains. The crystallinity of the as-prepared electrospun membranes were determined by XRD. Fig. 3 represents the XRD patterns of the electrospun PVdF-HFP and PVdF-HFP/PMMA membranes. For the pure electrospun PVdF-HFP membranes, the diffraction peaks at 2 = 18.4◦ and 20.1◦ depicted the reflections of the (0 2 0) and (1 0 0) planes of typical a-type crystal structure, respectively. The intensities of these two peaks decreased and a large hump was observed in the pattern of electrospun PVdF-HFP/PMMA membranes. That is,

58

Y. Ding et al. / Journal of Membrane Science 329 (2009) 56–59

Fig. 2. FTIR spectra of the as-prepared electrospun membranes.

the crystallinity of PVdF-HFP decreased with the addition of PMMA because the moving of PVdF-HFP chains in crystallization was hindered by the large group CH3 OCO– in PMMA chains. Moreover, the crystallinity has been depressed by the hydrogen bonds between the PMMA and PVdF-HFP chains. The uptake and leakage behavior of the electrolyte solution of the electrospun fibrous membranes is shown in Fig. 4. The spacenetwork pore structure of electrospun membranes is thought to enable much faster penetration of electrolyte solution into the membranes [11,13]. Pure PVdF-HFP membrane showed an uptake electrolyte of 273% and 349% after 1 and 240 min, respectively. For composite membrane, uptake capability was improved due to the increasing of amorphous phase. The composite membrane showed an uptake electrolyte of about 377% after 240 min. The leakage of electrolyte solution is very important for the cycling performance of lithium ion batteries. To investigate the leakage of electrolyte, a situation was simulated as same as working batteries by placing the electrospun membrane between two filter papers. As shown in the figure, the loss of the electrolyte solution increased gradually until the leakage time reached 2 h. The electrospun PVdF-HFP and PVdF-HFP/PMMA membranes retained 76% and 87% of the initial absorption ratio, respectively. Both the membranes showed good uptake and leakage properties sufficient for real lithium ion batteries application. But the composite membranes exhibited a more uptake and a less leakage than pure PVdF-HFP membrane because of its lower crystallinity.

Fig. 3. XRD patterns of the electrospun PVdF-HFP and PVdF-HFP/PMMA membranes.

Fig. 4. Uptake and leakage behavior of the electrospun fibrous membranes.

Fig. 5 shows the ionic conductivity of PVdF-HFP and PVdFHFP/PMMA polymer electrolytes. As was mentioned above, the electrospun composite membranes showed good uptake and leakage properties. It was suggested its ionic conductivity was higher than electrospun PVdF-HFP membranes. The ionic conductivity of electrospun composite membrane reached 1.99 × 10−3 S cm−1 at room temperature. To verify the ion conductivity in the battery system, full cells were assembled with as-prepared electrospun PVdF-HFP/PMMA and commercial celgardTM 2400 as electrolyte. Raw LiFePO4 and lithium metal were used as cathode and anode materials, respectively. The prototype cells were subjected to cycle tests with cutoff voltages of 2.4 and 4.1 V at a constant current of 0.1C rate. The cycle performance of the prototype cells was shown in Fig. 6. The initial discharge capacities of both cells were between 140 and 145 mAh g−1 . The full cells with electrospun PVdF-HFP/PMMA electrolyte showed a stable discharge behavior and little capacity loss under constant current conditions, which still retained a capacity of 133.5 mAh g−1 after 150 cycles. However, the cell with celgardTM 2400 showed a remarkable capacity fading. The capacity decreased to 115 mAh g−1 after 150 cycles. The improved cycle performance of the prototype cells with electrospun PVdF-HFP/PMMA could be partially ascribed to the low leakage in the sandwich structure of the cell. The mechanical properties of electrospun membranes are presented in Fig. 7. The produced membranes were approximately

Fig. 5. Arrhenius plots of the as-prepared electrolytes.

Y. Ding et al. / Journal of Membrane Science 329 (2009) 56–59

59

4. Conclusion Polymer electrolytes based on a blend of PVdF-HFP/PMMA were prepared by electrospun. The crystallinity of PVdF-HFP has been depressed by addition of PMMA. The composite membrane showed an uptake and leakage for electrolyte solution of 377% and 87% after 240 min, respectively. The ionic conductivity of electrospun PVdF-HFP/PMMA electrolyte reached 2.0 × 10−3 S cm−1 at room temperature. Furthermore, the tensile strength and elongation at break was improved by the addition of PMMA. Acknowledgements The authors gratefully acknowledge the financial support by the Scientific Research Fund of Hunan Provincial Education Department of China (Nos.: 07A07108c892, 08c893) and Undergraduate Innovation Fund of Education Ministry of China (No: 081053019). Fig. 6. Cycle performance of the prototype cells.

Fig. 7. Mechanical properties of electrospun membranes.

150–250 ␮m thick, which could be increased or decreased by simply adjusting the electrospun processing parameters. To confirm the data validity, over 10 samples were measured and the coefficient of variance (CV) has been calculated. As can be seen from the figure, the electrospun PVdF-HFP membrane showed low tensile strength and elongation at break. The tensile strength increased with the addition of the PMMA. It was suggested the relative hard PMMA chains enhanced the tensile strength of PVdF-HFP that is more flexile than PMMA. Additionally, the increasing of the interfiber bonding induced by PMMA, resulted in a higher tensile strength.

References [1] A. Manuel Stephan, K.S. Nahm, Review on composite polymer electrolytes for lithium batteries, Polymer 47 (2006) 5952. [2] A. Manuel Stephan, Review on gel polymer electrolytes for lithium batteries, Eur. Polym. J. 42 (2006) 21. [3] N.T. Kalyana Sundaram, A. Subramania, Nano-size LiAlO2 ceramic filler incorporated porous PVDF-co-HFP electrolyte for lithium-ion battery applications, Electrochim. Acta 52 (2007) 4987. [4] K.M. Kim, N.G. Park, K.S. Ryu, S.H. Chang, Characteristics of P(VDF-HFP)/TiO2 composite membrane electrolytes prepared by phase inversion and conventional casting methods, Electrochim. Acta 51 (2006) 5636. [5] J.Y. Xi, X.P. Qiu, L.Q. Chen, PVDF-PEO/ZSM-5 based composite microporous polymer electrolyte with novel pore configuration and ionic conductivity, Solid State Ionics 177 (2006) 709. [6] M. Wachtler, D. Ostrovskii, P. Jacobsson, B. Scrosati, A study on PVdF-based SiO2 -containing composite gel-type polymer electrolytes for lithium batteries, Electrochim. Acta 50 (2004) 357. [7] H.P. Zhang, P. Zhang, Z.H. Li, M. Sun, Y.P. Wu, H.Q. Wu, A novel sandwiched membrane as polymer electrolytes for lithium ion battery, Electrochem. Commun. 9 (2007) 1700. [8] A. Subramania, N.T.K. Sundaram, A.R.S. Priya, G.V. Kumar, Preparation of a novel composite micro-porous polymer electrolyte membrane for high performance Li-ion battery, J. Membr. Sci. 294 (2007) 8. [9] Y.X. Jiang, Z.F. Chen, Q.C. Zhuang, J.M. Xu, Q.F. Dong, L. Huang, S.G. Sun, A novel composite microporous polymer electrolyte prepared with molecule sieves for Li-ion batteries, J. Power Sources 160 (2006) 1320. [10] S.-S. Choi, Y.S. Lee, C.W. Joo, S.G. Lee, J.K. Park, K.-S. Han, Electrospun PVDF nanofiber web as polymer electrolyte or separator, Electrochim. Acta 50 (2004) 339. [11] J.R. Kim, S.W. Choi, S.M. Jo, W.S. Lee, B.C. Kim, Electrospun PVdF-based fibrous polymer electrolytes for lithium ion polymer batteries, Electrochim. Acta 50 (2004) 69. [12] K. Gao, X. Hu, C. Dai, T. Yi, Crystal structures of electrospun PVDF membranes and its separator application for rechargeable lithium metal cells, Mater. Sci. Eng. B 131 (2006) 100. [13] J.R. Kim, S.W. Choi, S.M. Jo, W.S. Lee, B.C. Kim, Characterization and properties of P.VdF-HFP.-based fibrous polymer electrolyte membrane prepared by electrospinning, J. Electrochem. Soc. 152 (2005) A295. [14] J. Huang, S. Virji, B. Weiller, R. Kaner, Polyaniline nanofibers: facile synthesis and chemical sensors, J. Am. Chem. Soc. 125 (2003) 314.