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Preparation and performance analysis of PE-supported P(AN-co-MMA) gel polymer electrolyte for lithium ion battery application
Journal of Membrane Science 322 (2008) 314–319
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Preparation and performance analysis of PE-supported P(AN-co-MMA) gel polymer electrolyte for lithium ion battery application M.M. Rao a , J.S. Liu b , W.S. Li a,b,c,∗ , Y. Liang a , D.Y. Zhou a a
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China College of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, PR China c Key Lab of Technology on Electrochemical Energy Storage and Power Generation in Guangdong Universities, Guangzhou 510006, PR China b
a r t i c l e
i n f o
Article history: Received 22 April 2008 Received in revised form 2 June 2008 Accepted 5 June 2008 Available online 11 June 2008 Keywords: Lithium ion battery Gel polymer electrolyte P(AN-co-MMA) PE-supported
a b s t r a c t Copolymer, poly(acrylonitrile-co-methyl methacrylate) (P(AN-co-MMA)), was synthesized by solution polymerization with different mole ratios of monomers, acrylonitrile (AN) and methyl methacrylate (MMA). Polyethylene (PE) supported copolymer and gel polymer electrolyte (GPE) were prepared with this copolymer and their performances were characterized with FTIR, TGA, SEM, and electrochemical methods. It is found that the GPE using the PE-supported copolymer with AN to MMA = 4:1 (mole) exhibits an ionic conductivity of 2.06 × 10−3 S cm−1 at room temperature. The copolymer is stable up to 270 ◦ C. The PE-supported copolymer shows a cross-linked porous structure and has 150 wt% of electrolyte uptake. The GPE is compatible with anode and cathode of lithium ion battery at high voltage and its electrochemical window is 5.5 V (vs. Li/Li+ ). With the application of the PE-supported GPE in lithium ion battery, the battery shows its good rate and initial discharge capacity and cyclic stability. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Gel polymer electrolyte (GPE) is attractive for lithium ion batteries due to its combined advantage of liquid electrolyte (high ionic conductivity) and solid electrolyte (safety) [1]. In GPEs, polymer matrixes are required for the immobilization of electrolytes. Among the polymer matrixes that are promising for the application in GPE, polyacrylonitrile (PAN) [2], poly(vinylidene fluoride) (PVDF) [3–5], poly(methyl methacrylate) (PMMA) [6], and poly(ethylene oxide) (PEO) [7] based polymers have been most extensively studied. The GPEs with these matrixes exhibit highly ionic conductivity, usually over 10−3 S cm−1 at room temperature. However, the mechanical properties of most of them are often poor, because the impregnation of a liquid electrolyte into a polymer matrix results in softening of the polymer. The poor mechanical prop-
Abbreviations: AIBN, 2,2-azobisisobutyronitrile; AN, acrylonitrile; DAP, diallyl phthalate; DEC, diethylene carbonate; DMC, dimethyl carbonate; DMF, N,Ndimethylformamide; EC, ethylene carbonate; FTIR, Fourier transform infrared; GPE, gel polymer electrolyte; h, hour; Li, lithium; M, mole; min, minute; MMA, methyl methacrylate; P(AN-co-MMA), poly(acrylonitrile-co-methyl methacrylate); PE, polyethylene; PMMA, poly(methyl methacrylate); SEM, scanning electron microscope; SS, stainless steel; TGA, thermal gravity analysis. ∗ Corresponding author at: School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China. Tel.: +86 20 39310256; fax: +86 20 39310256. E-mail address: [email protected] (W.S. Li). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.06.004
erties may lead to internal short-circuit and safe hazards for a battery. One of the ways to solve this problem is to use a mechanical support such as PE or non-woven fabrics to prepare supported GPE [8,9]. It has been found PE-supported AN and/or MMA polymers provide the GPE with an excellent mechanical strength as well as an appropriate ionic conductivity. In this PE-supported GPE, PE contributes its mechanical strength to the GPE. In the polymer, AN contributes its chemical stability and MMA contributes its high affinity with polar solvents [10–16]. However, the ionic conductivity of the supported GPE reported in the literatures is far lower than the conventional liquid electrolyte. For example, the ionic conductivity of the PE-supported poly(acrylonitrile-co-methyl methacrylate-co-styrene) (PAMS) is only 1.1 × 10−3 S cm−1 [8]. Therefore, the ionic conductivity of PEsupported GPE should be improved for its application in lithium ion batteries. With the aim of improving the ionic conductivity of the PE-supported GPE, the mole ratio of AN and MMA monomers for the preparation of PE-supported P(AN-co-MMA) was considered in this paper. 2. Experimental 2.1. Preparation P(AN-co-MMA) was prepared by solution polymerization with AN and MMA as monomers, 2,2-azobisisobutyronitrile (AIBN) as
M.M. Rao et al. / Journal of Membrane Science 322 (2008) 314–319
315
Scheme 1. Synthesis route of P(AN-co-MMA) copolymer prepared by solution polymerization.
initiator and diallyl phthalate (DAP) as cross-linking agent. Commercial AN (>99.5%) and MMA (>99.5%) were distilled in vacuum to remove the aggregation inhibitor and commercial AIBN was re-crystallized with methanol. The required amounts of AN and MMA were dissolved in anhydrous N,N-dimethylformamide (DMF) to form a homogeneous solution under N2 flow at 60 ◦ C for 30 min. Then AIBN and DAP were added in the solution. The polymerization was continued for 6 h with stirring vigorously at 60 ◦ C. The resulting solution was poured into a large excess of deionized water to yield the precipitate. The precipitate was subsequently filtered, washed with ethanol and dried in vacuum at 60 ◦ C to constant weight. Five copolymers were obtained with different mole ratios of AN to MMA, 5:1, 4:1, 3:1, 2:1, and 1:1, respectively. The main synthesis route is shown in Scheme 1. The prepared P(AN-co-MMA) copolymer was dissolved in a mixed solution of DMF and diethylene carbonate (DEC) to prepare PE-supported P(AN-co-MMA). A microporous PE separator (Celgard 2400, USA, thickness: 16 m) was then immersed in the polymer solution for 2 h, then taken out and dried in vacuum at 60 ◦ C for 24 h. To prepare PE-supported GPE, the PE-supported P(AN-co-MMA) was transferred into a glove box (Supper1220/750, Belgium) and soaked with 1 M LiPF6 in dimethyl carbonate (DMC)/diethylene carbonate/ethylene carbonate (EC) (1:1:1, v/v/v, Guangzhou Tinci High-Tech Materials Co., Ltd., battery grade) for 1 h. 2.2. Characterization The electrolyte uptake (A) of the PE-supported P(AN-co-MMA) soaked in 1 M LiPF6 (DMC/DEC/EC) (1:1:1, v/v/v) for different time was obtained by: A (%) =
W2 − W1 × 100% W1
The interfacial stability of the PE-supported GPE was determined by ac impedance spectroscopy on electrochemical instrument (CHI650B, Shanghai). In the determination of the interfacial stability, the PE-supported GPE was sandwiched between two lithium electrodes to form a symmetrical Li/PE-supported GPE/Li cell, with one lithium electrode as working electrode and the other as counter electrode and reference electrode. The electrochemical stability of the PE-supported GPE was examined for a cell of configuration Li/PE-supported GPE/SS by linear sweep voltammetry at 1 mV s−1 . To determine the battery performance, a cell Li/PE-supported GPE/LiCoO2 was set up and tested with charging/discharging instrument (PCBT-138-64D WUHAN LISUN). 3. Results and discussion 3.1. Electrolyte uptake of PE-supported P(AN-co-MMA) and ionic conductivity of GPE
Fig. 1 presents the variation of electrolyte uptake with soaking time for the PE-supported GPE preparation. It can be seen from Fig. 1 that the electrolyte uptake increases with the soaking time and keeps almost unchanged when the soaking time is over 30 min. It suggests that the best time for activating the PE-supported copolymer is 30 min. The electrolyte uptakes and ionic conductivities of the PEsupported GPE with different mole ratios of AN to MMA and the same soaking time (30 min) are shown in Table 1. It can be seen from Table 1 that the electrolyte uptake of the pure PE is low compared with the PE-supported P(AN-co-MMA). The electrolyte uptake of the PE-supported P(AN-co-MMA) with the mole ratio of AN to MMA
(1)
where W1 and W2 are the mass of the dry and wet PE-supported GPE, respectively. The ionic conductivity of the GPE was determined by ac impedance spectroscopy on electrochemical instrument (CHI650B, Shanghai) with ac amplitude of 10 mV from 100 kHz to 1 Hz. In the determination of the ionic conductivity, the PE-supported GPE was sandwiched between two parallel stainless steel (SS) discs (diameter ˚ = 16 mm). The ionic conductivity was calculated from the bulk electrolyte resistance (R) according to: =
l RS
(2)
where l is the thickness of the PE-supported GPE, and S is the contact area between PE-supported GPE and SS disc. The bulk electrolyte resistance was obtained from the complex impedance diagram. The structure and properties of the polymer were characterized by Fourier transform infrared (FTIR) (Perkin-Elmer Spectrograph) in the range of 450–4000 cm−1 (resolution: 2 cm−1 ). Its thermal stability was analyzed with thermogravimetric analyzer (NETZSCH STA 409 PC/PG). The morphology of the supported copolymer was examined with scanning electron microscope (JEOL, JSM-6380LV, Japan).
Fig. 1. Variation of electrolyte uptake in PE-supported P(AN-co-MMA) with soaking time for the GPE preparation with different mole ratios of AN to MMA.
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Table 1 Electrolyte uptake of PE-supported P(AN-co-MMA) and conductivity of PEsupported GPE Molar ratio (AN/MMA) 5:1 Uptake (%) Conductivity (×10−3 S cm−1 )
4:1
103 150 1.22 2.06
Pure PE
3:1
2:1
1:1
146 1.8
149 1.26
152 0.82
48 0.49
5:1 is 103 wt% and increases to 150 wt% when the mole ratio is 4:1. As the ratio decreases further, the electrolyte uptake of the PE-supported P(AN-co-MMA) is almost not influenced by the mole ratio. The electrolyte uptake of the PE-supported P(AN-co-MMA) may be related to its pore structure and affinity to electrolyte [17,18]. The copolymerization of two monomers forms porous structure in the copolymer by cross-linking. AN helps to form larger pores and MMA provides stronger affinity to electrolyte. When the mole ratio of AN to MMA is 5:1, the pore sizes of the copolymer are too larger to retain the electrolyte, resulting in the smaller electrolyte uptake. As the amount of AN decreases in the copolymer, the pores become smaller and its ability retaining electrolyte increases, resulting in the larger electrolyte uptake. However, when the pore sizes are too small, the available volume of the pores decreases. This might reduce the electrolyte uptake. On the other hand, the amount of MMA should increase the electrolyte uptake due to its affinity. The trade-off between the pore size and the affinity results in the less dependence of the electrolyte uptake on the mole ratio of AN to MMA when it is lower than 4:1. It can be noted from Table 1 that the ionic conductivity of the GPE decreases with the mole ratio of AN to MMA decreasing from 4:1 to 1:1, although their electrolyte uptake is nearly the same. This may be ascribed to pore connectivity of the PE-supported P(AN-coMMA). AN contributes to ionic conductivity with appropriate pore structure [19,20]. The lower mole ratio of AN to MMA may cause that the pores in the PE-supported P(AN-co-MMA) are not interconnected, resulting in the lower ionic conductivity[21]. The ionic conductivity of the GPE with AN to MMA = 4:1 is 2.06 × 10−3 S cm−1 , which is larger than other ratios and the similar supported GPEs [8,15,22]. 3.2. Structure of P(AN-co-MMA)
Fig. 2. FTIR spectra for AN (a), MMA (b) and P(AN-co-MMA) with mole ratio of AN to MMA 4:1 (mole) (c) in the range of 450–4000 cm−1 .
of other similar copolymers, such as poly(methyl methacrylate-covinyl acetate) who is stable at the temperature lower than 300 ◦ C but whose ionic conductivity is lower than 2 × 10−3 S cm−1 . 3.4. SEM images of PE-supported P(AN-co-MMA) Fig. 4 presents the SEM image of PE-supported P(AN-co-MMA) and the pure PE membrane. It can be found from Fig. 4(a) that PE membrane has a uniform submicropore structure. Different from the PE membrane, the PE-supported P(AN-co-MMA) membrane shows its far larger cross-linked pores. This is important for the formation of GPE, because the pores can enlarge the contact areas between the polymer and electrolyte and retain much more electrolyte. Moreover, high affinity with electrolyte of PE-supported P(AN-co-MMA) due to the presence of polar functional groups in AN and MMA can provide PE-supported GPE with high ionic conductivity. This explains why PE-supported P(AN-co-MMA) owns highly ionic conductivity. Compared to the conventional liquid electrolytes in lithium-ion cells, the PE-supported P(AN-co-MMA) electrolyte is expected to increase the electrochemical stability of the lithium-ion battery.
Fig. 2 presents the FTIR spectra of AN, MMA and P(AN-co-MMA). The monomer AN shows its characteristic absorption peaks at 2240 cm−1 and 1623 cm−1 , which correspond to –C N and –C C groups, respectively. The monomer MMA is characteristic of the adsorption peaks at 1730 cm−1 and 1639 cm−1 , which correspond to the –C O and –C C groups, respectively. By comparing the FTIR spectra of the copolymer with those of monomers, it can be found that the P(AN-co-MMA) keeps the absorption peaks at 1732 cm−1 for C O and 2243 cm−1 for –C N and loses the absorption peak at 1623 or 1635 cm−1 for C C. This indicates that the copolymer maintains the main characteristics of the monomers and the monomers are co-polymerized through the breaking of double bonds C C in both monomers AN and MMA. 3.3. Thermal stability of P(AN-co-MMA) The thermal stability of the P(AN-co-MMA) was analyzed by thermogravimetry under N2 atmosphere, from room temperature to 600 ◦ C at a heating rate of 10 ◦ C min−1 . The result is shown in Fig. 3. It can be seen that there is no mass loss for the temperature up to 270 ◦ C. This indicates that the P(AN-co-MMA) is stable at the temperature lower than 270 ◦ C. This thermal stability is as good as that
Fig. 3. TGA curve for P(AN-co-MMA) with mole ratio of AN to MMA 4:1 (mole) from room temperature to 600 ◦ C at a heating rate of 10 ◦ C min−1 .
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Fig. 5. Linear voltammograms on stainless steel for PE-supported GPE with the copolymer of AN to MMA 4:1 (mole) (a), compared with liquid electrolyte (b), scan rate: 1 mV s−1 .
the passive film on lithium and its diameter, R2 − R1 , as denoted in Fig. 6 for the fresh cell, is the resistance of the passive film [26,27]. It can be seen from Fig. 6 that the resistance of the passive film increases within two weeks but keeps almost unchanged after two weeks. This suggests that it takes two weeks for the formation and stabilization of the passive film on Li. The resistance of the PEsupported GPE is hardly related to the time, indicating that lithium does not change the performance of the PE-supported GPE, i.e., the PE-supported GPE shows its good compatibility with lithium. 3.6. Battery performance
Fig. 4. SEM images of pure PE membrane (a) and PE-supported P(AN-co-MMA) with mole ratio of AN to MMA 4:1 (mole) membrane (b).
3.5. Electrochemical stability Fig. 5 presents the linear voltammogram obtained for the cell Li/PE-supported GPE/SS, compared with the cell Li/PE (liquid electrolyte)/SS. It can be seen from the curve (b) of Fig. 5 that the liquid electrolyte decomposes at about 4.5 V (vs. Li/Li+ ). However, the PE-supported GPE does not decompose until 5.5 V (vs. Li/Li+ ), as shown by the curve (a) of Fig. 5. It is obvious that the PEsupported GPE has a far better electrochemical stability than liquid electrolyte. This electrochemical stability is also better than the GPE with unsupported P(AN-MMA), whose electrochemical window is 5 V(vs. Li/Li+ )[23]. This suggests that the PE-supported GPE can be compatible with high-voltage electrode materials and can be used in lithium ion batteries. Interfacial stability with electrode is an essential factor to guarantee acceptable performance in the Lithium ion batteries [24,25]. To understand the stability of the interface between Li and PEsupported GPE, a cell Li/PE-supported GPE/Li was set up and ac impedance spectroscopy was used to monitor the change in impedance with time. Fig. 6 presents the impedance spectra of the cell for different time. The real part of the impedance at the highest frequency represents the resistance of the PE-supported GPE, R1 as denoted in Fig. 6. The semicircle at high frequencies reflects
Fig. 7 shows the discharge curves of the battery Li/PE-supported GPE/LiCoO2 with the copolymer of AN to MMA 4:1 at different current rates according to the LiCoO2 . It can be seen that the battery at C/10 rate can achieve a good capacity, 128 mAh g−1 . Both the voltage and the capacity of the battery are found to be decreased gradually with increasing current rate but keep relatively high values, for example, at C/5 rate the battery keeps 96.8% and at 1.0 C rate keeps 89% of discharge capacity at C/10 rate.
Fig. 6. Impedance spectra of cell Li/PE-supported GPE/Li at open circuit potential, with the copolymer of AN to MMA 4:1 (mole).
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trolyte prepared with the mole ratio of AN to MMA being 4:1 has highest ionic conductivity and has good electrochemical compatibility with anode and cathode of lithium ion battery. With the application of this gel polymer electrolyte in lithium ion battery, the battery shows its initial discharge capacity as good as and its cyclic stability better than the battery with liquid electrolyte.
Nomenclature A C l R S v V Fig. 7. Rate discharge performance of battery Li/PE-supported GPE/LiCoO2 at room temperature, with the copolymer of AN to MMA 4:1 (mole).
electrolyte uptake capacity the thickness of the gel polymer electrolyte resistance the contact area between PE-supported GPE and SS disc volume voltage
Greek letters ionic conductivity ˚ diameter
References
Fig. 8. Cyclic stability of battery Li/PE-supported GPE/LiCoO2 with the copolymer of AN to MMA 4:1 (mole) (a), compared with battery Li/PE (liquid electrolyte)/LiCoO2 (b).
Fig. 8 presents the cyclic stability of the battery Li/PEsupported GPE/LiCoO2 , compared with the battery Li/PE (liquid electrolyte)/LiCoO2 , charged and discharged with a constant current of 0.48 mA cm−2 (C/5 rate) between 4.2 and 3.0 V. It can be seen from Fig. 8 that the battery Li/PE-supported GPE/LiCoO2 has the initial capacity as high as the battery Li/PE (liquid electrolyte)/LiCoO2 and the cyclic stability of the battery Li/PE-supported GPE/LiCoO2 is better than that of the battery Li/PE (liquid electrolyte)/LiCoO2 . After 50 cycles, the battery Li/PE-supported GPE/LiCoO2 keeps 86.3% of its initial discharge capacity but the battery Li/PE (liquid electrolyte)/LiCoO2 keeps only 83% of its initial discharge capacity. 4. Conclusions The electrolyte uptake of PE-supported P(AN-co-MMA) and the ionic conductivity of the gel polymer electrolyte prepared with this supported copolymer are related to the ratio of the monomers, AN and MMA, for the copolymer preparation. The gel polymer elec-
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Preparation and performance analysis of PE-supported P(AN-co-MMA) gel polymer electrolyte for lithium ion battery application M.M. Rao a , J.S. Liu b , W.S. Li a,b,c,∗ , Y. Liang a , D.Y. Zhou a a
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China College of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, PR China c Key Lab of Technology on Electrochemical Energy Storage and Power Generation in Guangdong Universities, Guangzhou 510006, PR China b
a r t i c l e
i n f o
Article history: Received 22 April 2008 Received in revised form 2 June 2008 Accepted 5 June 2008 Available online 11 June 2008 Keywords: Lithium ion battery Gel polymer electrolyte P(AN-co-MMA) PE-supported
a b s t r a c t Copolymer, poly(acrylonitrile-co-methyl methacrylate) (P(AN-co-MMA)), was synthesized by solution polymerization with different mole ratios of monomers, acrylonitrile (AN) and methyl methacrylate (MMA). Polyethylene (PE) supported copolymer and gel polymer electrolyte (GPE) were prepared with this copolymer and their performances were characterized with FTIR, TGA, SEM, and electrochemical methods. It is found that the GPE using the PE-supported copolymer with AN to MMA = 4:1 (mole) exhibits an ionic conductivity of 2.06 × 10−3 S cm−1 at room temperature. The copolymer is stable up to 270 ◦ C. The PE-supported copolymer shows a cross-linked porous structure and has 150 wt% of electrolyte uptake. The GPE is compatible with anode and cathode of lithium ion battery at high voltage and its electrochemical window is 5.5 V (vs. Li/Li+ ). With the application of the PE-supported GPE in lithium ion battery, the battery shows its good rate and initial discharge capacity and cyclic stability. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Gel polymer electrolyte (GPE) is attractive for lithium ion batteries due to its combined advantage of liquid electrolyte (high ionic conductivity) and solid electrolyte (safety) [1]. In GPEs, polymer matrixes are required for the immobilization of electrolytes. Among the polymer matrixes that are promising for the application in GPE, polyacrylonitrile (PAN) [2], poly(vinylidene fluoride) (PVDF) [3–5], poly(methyl methacrylate) (PMMA) [6], and poly(ethylene oxide) (PEO) [7] based polymers have been most extensively studied. The GPEs with these matrixes exhibit highly ionic conductivity, usually over 10−3 S cm−1 at room temperature. However, the mechanical properties of most of them are often poor, because the impregnation of a liquid electrolyte into a polymer matrix results in softening of the polymer. The poor mechanical prop-
Abbreviations: AIBN, 2,2-azobisisobutyronitrile; AN, acrylonitrile; DAP, diallyl phthalate; DEC, diethylene carbonate; DMC, dimethyl carbonate; DMF, N,Ndimethylformamide; EC, ethylene carbonate; FTIR, Fourier transform infrared; GPE, gel polymer electrolyte; h, hour; Li, lithium; M, mole; min, minute; MMA, methyl methacrylate; P(AN-co-MMA), poly(acrylonitrile-co-methyl methacrylate); PE, polyethylene; PMMA, poly(methyl methacrylate); SEM, scanning electron microscope; SS, stainless steel; TGA, thermal gravity analysis. ∗ Corresponding author at: School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China. Tel.: +86 20 39310256; fax: +86 20 39310256. E-mail address: [email protected] (W.S. Li). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.06.004
erties may lead to internal short-circuit and safe hazards for a battery. One of the ways to solve this problem is to use a mechanical support such as PE or non-woven fabrics to prepare supported GPE [8,9]. It has been found PE-supported AN and/or MMA polymers provide the GPE with an excellent mechanical strength as well as an appropriate ionic conductivity. In this PE-supported GPE, PE contributes its mechanical strength to the GPE. In the polymer, AN contributes its chemical stability and MMA contributes its high affinity with polar solvents [10–16]. However, the ionic conductivity of the supported GPE reported in the literatures is far lower than the conventional liquid electrolyte. For example, the ionic conductivity of the PE-supported poly(acrylonitrile-co-methyl methacrylate-co-styrene) (PAMS) is only 1.1 × 10−3 S cm−1 [8]. Therefore, the ionic conductivity of PEsupported GPE should be improved for its application in lithium ion batteries. With the aim of improving the ionic conductivity of the PE-supported GPE, the mole ratio of AN and MMA monomers for the preparation of PE-supported P(AN-co-MMA) was considered in this paper. 2. Experimental 2.1. Preparation P(AN-co-MMA) was prepared by solution polymerization with AN and MMA as monomers, 2,2-azobisisobutyronitrile (AIBN) as
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315
Scheme 1. Synthesis route of P(AN-co-MMA) copolymer prepared by solution polymerization.
initiator and diallyl phthalate (DAP) as cross-linking agent. Commercial AN (>99.5%) and MMA (>99.5%) were distilled in vacuum to remove the aggregation inhibitor and commercial AIBN was re-crystallized with methanol. The required amounts of AN and MMA were dissolved in anhydrous N,N-dimethylformamide (DMF) to form a homogeneous solution under N2 flow at 60 ◦ C for 30 min. Then AIBN and DAP were added in the solution. The polymerization was continued for 6 h with stirring vigorously at 60 ◦ C. The resulting solution was poured into a large excess of deionized water to yield the precipitate. The precipitate was subsequently filtered, washed with ethanol and dried in vacuum at 60 ◦ C to constant weight. Five copolymers were obtained with different mole ratios of AN to MMA, 5:1, 4:1, 3:1, 2:1, and 1:1, respectively. The main synthesis route is shown in Scheme 1. The prepared P(AN-co-MMA) copolymer was dissolved in a mixed solution of DMF and diethylene carbonate (DEC) to prepare PE-supported P(AN-co-MMA). A microporous PE separator (Celgard 2400, USA, thickness: 16 m) was then immersed in the polymer solution for 2 h, then taken out and dried in vacuum at 60 ◦ C for 24 h. To prepare PE-supported GPE, the PE-supported P(AN-co-MMA) was transferred into a glove box (Supper1220/750, Belgium) and soaked with 1 M LiPF6 in dimethyl carbonate (DMC)/diethylene carbonate/ethylene carbonate (EC) (1:1:1, v/v/v, Guangzhou Tinci High-Tech Materials Co., Ltd., battery grade) for 1 h. 2.2. Characterization The electrolyte uptake (A) of the PE-supported P(AN-co-MMA) soaked in 1 M LiPF6 (DMC/DEC/EC) (1:1:1, v/v/v) for different time was obtained by: A (%) =
W2 − W1 × 100% W1
The interfacial stability of the PE-supported GPE was determined by ac impedance spectroscopy on electrochemical instrument (CHI650B, Shanghai). In the determination of the interfacial stability, the PE-supported GPE was sandwiched between two lithium electrodes to form a symmetrical Li/PE-supported GPE/Li cell, with one lithium electrode as working electrode and the other as counter electrode and reference electrode. The electrochemical stability of the PE-supported GPE was examined for a cell of configuration Li/PE-supported GPE/SS by linear sweep voltammetry at 1 mV s−1 . To determine the battery performance, a cell Li/PE-supported GPE/LiCoO2 was set up and tested with charging/discharging instrument (PCBT-138-64D WUHAN LISUN). 3. Results and discussion 3.1. Electrolyte uptake of PE-supported P(AN-co-MMA) and ionic conductivity of GPE
Fig. 1 presents the variation of electrolyte uptake with soaking time for the PE-supported GPE preparation. It can be seen from Fig. 1 that the electrolyte uptake increases with the soaking time and keeps almost unchanged when the soaking time is over 30 min. It suggests that the best time for activating the PE-supported copolymer is 30 min. The electrolyte uptakes and ionic conductivities of the PEsupported GPE with different mole ratios of AN to MMA and the same soaking time (30 min) are shown in Table 1. It can be seen from Table 1 that the electrolyte uptake of the pure PE is low compared with the PE-supported P(AN-co-MMA). The electrolyte uptake of the PE-supported P(AN-co-MMA) with the mole ratio of AN to MMA
(1)
where W1 and W2 are the mass of the dry and wet PE-supported GPE, respectively. The ionic conductivity of the GPE was determined by ac impedance spectroscopy on electrochemical instrument (CHI650B, Shanghai) with ac amplitude of 10 mV from 100 kHz to 1 Hz. In the determination of the ionic conductivity, the PE-supported GPE was sandwiched between two parallel stainless steel (SS) discs (diameter ˚ = 16 mm). The ionic conductivity was calculated from the bulk electrolyte resistance (R) according to: =
l RS
(2)
where l is the thickness of the PE-supported GPE, and S is the contact area between PE-supported GPE and SS disc. The bulk electrolyte resistance was obtained from the complex impedance diagram. The structure and properties of the polymer were characterized by Fourier transform infrared (FTIR) (Perkin-Elmer Spectrograph) in the range of 450–4000 cm−1 (resolution: 2 cm−1 ). Its thermal stability was analyzed with thermogravimetric analyzer (NETZSCH STA 409 PC/PG). The morphology of the supported copolymer was examined with scanning electron microscope (JEOL, JSM-6380LV, Japan).
Fig. 1. Variation of electrolyte uptake in PE-supported P(AN-co-MMA) with soaking time for the GPE preparation with different mole ratios of AN to MMA.
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Table 1 Electrolyte uptake of PE-supported P(AN-co-MMA) and conductivity of PEsupported GPE Molar ratio (AN/MMA) 5:1 Uptake (%) Conductivity (×10−3 S cm−1 )
4:1
103 150 1.22 2.06
Pure PE
3:1
2:1
1:1
146 1.8
149 1.26
152 0.82
48 0.49
5:1 is 103 wt% and increases to 150 wt% when the mole ratio is 4:1. As the ratio decreases further, the electrolyte uptake of the PE-supported P(AN-co-MMA) is almost not influenced by the mole ratio. The electrolyte uptake of the PE-supported P(AN-co-MMA) may be related to its pore structure and affinity to electrolyte [17,18]. The copolymerization of two monomers forms porous structure in the copolymer by cross-linking. AN helps to form larger pores and MMA provides stronger affinity to electrolyte. When the mole ratio of AN to MMA is 5:1, the pore sizes of the copolymer are too larger to retain the electrolyte, resulting in the smaller electrolyte uptake. As the amount of AN decreases in the copolymer, the pores become smaller and its ability retaining electrolyte increases, resulting in the larger electrolyte uptake. However, when the pore sizes are too small, the available volume of the pores decreases. This might reduce the electrolyte uptake. On the other hand, the amount of MMA should increase the electrolyte uptake due to its affinity. The trade-off between the pore size and the affinity results in the less dependence of the electrolyte uptake on the mole ratio of AN to MMA when it is lower than 4:1. It can be noted from Table 1 that the ionic conductivity of the GPE decreases with the mole ratio of AN to MMA decreasing from 4:1 to 1:1, although their electrolyte uptake is nearly the same. This may be ascribed to pore connectivity of the PE-supported P(AN-coMMA). AN contributes to ionic conductivity with appropriate pore structure [19,20]. The lower mole ratio of AN to MMA may cause that the pores in the PE-supported P(AN-co-MMA) are not interconnected, resulting in the lower ionic conductivity[21]. The ionic conductivity of the GPE with AN to MMA = 4:1 is 2.06 × 10−3 S cm−1 , which is larger than other ratios and the similar supported GPEs [8,15,22]. 3.2. Structure of P(AN-co-MMA)
Fig. 2. FTIR spectra for AN (a), MMA (b) and P(AN-co-MMA) with mole ratio of AN to MMA 4:1 (mole) (c) in the range of 450–4000 cm−1 .
of other similar copolymers, such as poly(methyl methacrylate-covinyl acetate) who is stable at the temperature lower than 300 ◦ C but whose ionic conductivity is lower than 2 × 10−3 S cm−1 . 3.4. SEM images of PE-supported P(AN-co-MMA) Fig. 4 presents the SEM image of PE-supported P(AN-co-MMA) and the pure PE membrane. It can be found from Fig. 4(a) that PE membrane has a uniform submicropore structure. Different from the PE membrane, the PE-supported P(AN-co-MMA) membrane shows its far larger cross-linked pores. This is important for the formation of GPE, because the pores can enlarge the contact areas between the polymer and electrolyte and retain much more electrolyte. Moreover, high affinity with electrolyte of PE-supported P(AN-co-MMA) due to the presence of polar functional groups in AN and MMA can provide PE-supported GPE with high ionic conductivity. This explains why PE-supported P(AN-co-MMA) owns highly ionic conductivity. Compared to the conventional liquid electrolytes in lithium-ion cells, the PE-supported P(AN-co-MMA) electrolyte is expected to increase the electrochemical stability of the lithium-ion battery.
Fig. 2 presents the FTIR spectra of AN, MMA and P(AN-co-MMA). The monomer AN shows its characteristic absorption peaks at 2240 cm−1 and 1623 cm−1 , which correspond to –C N and –C C groups, respectively. The monomer MMA is characteristic of the adsorption peaks at 1730 cm−1 and 1639 cm−1 , which correspond to the –C O and –C C groups, respectively. By comparing the FTIR spectra of the copolymer with those of monomers, it can be found that the P(AN-co-MMA) keeps the absorption peaks at 1732 cm−1 for C O and 2243 cm−1 for –C N and loses the absorption peak at 1623 or 1635 cm−1 for C C. This indicates that the copolymer maintains the main characteristics of the monomers and the monomers are co-polymerized through the breaking of double bonds C C in both monomers AN and MMA. 3.3. Thermal stability of P(AN-co-MMA) The thermal stability of the P(AN-co-MMA) was analyzed by thermogravimetry under N2 atmosphere, from room temperature to 600 ◦ C at a heating rate of 10 ◦ C min−1 . The result is shown in Fig. 3. It can be seen that there is no mass loss for the temperature up to 270 ◦ C. This indicates that the P(AN-co-MMA) is stable at the temperature lower than 270 ◦ C. This thermal stability is as good as that
Fig. 3. TGA curve for P(AN-co-MMA) with mole ratio of AN to MMA 4:1 (mole) from room temperature to 600 ◦ C at a heating rate of 10 ◦ C min−1 .
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Fig. 5. Linear voltammograms on stainless steel for PE-supported GPE with the copolymer of AN to MMA 4:1 (mole) (a), compared with liquid electrolyte (b), scan rate: 1 mV s−1 .
the passive film on lithium and its diameter, R2 − R1 , as denoted in Fig. 6 for the fresh cell, is the resistance of the passive film [26,27]. It can be seen from Fig. 6 that the resistance of the passive film increases within two weeks but keeps almost unchanged after two weeks. This suggests that it takes two weeks for the formation and stabilization of the passive film on Li. The resistance of the PEsupported GPE is hardly related to the time, indicating that lithium does not change the performance of the PE-supported GPE, i.e., the PE-supported GPE shows its good compatibility with lithium. 3.6. Battery performance
Fig. 4. SEM images of pure PE membrane (a) and PE-supported P(AN-co-MMA) with mole ratio of AN to MMA 4:1 (mole) membrane (b).
3.5. Electrochemical stability Fig. 5 presents the linear voltammogram obtained for the cell Li/PE-supported GPE/SS, compared with the cell Li/PE (liquid electrolyte)/SS. It can be seen from the curve (b) of Fig. 5 that the liquid electrolyte decomposes at about 4.5 V (vs. Li/Li+ ). However, the PE-supported GPE does not decompose until 5.5 V (vs. Li/Li+ ), as shown by the curve (a) of Fig. 5. It is obvious that the PEsupported GPE has a far better electrochemical stability than liquid electrolyte. This electrochemical stability is also better than the GPE with unsupported P(AN-MMA), whose electrochemical window is 5 V(vs. Li/Li+ )[23]. This suggests that the PE-supported GPE can be compatible with high-voltage electrode materials and can be used in lithium ion batteries. Interfacial stability with electrode is an essential factor to guarantee acceptable performance in the Lithium ion batteries [24,25]. To understand the stability of the interface between Li and PEsupported GPE, a cell Li/PE-supported GPE/Li was set up and ac impedance spectroscopy was used to monitor the change in impedance with time. Fig. 6 presents the impedance spectra of the cell for different time. The real part of the impedance at the highest frequency represents the resistance of the PE-supported GPE, R1 as denoted in Fig. 6. The semicircle at high frequencies reflects
Fig. 7 shows the discharge curves of the battery Li/PE-supported GPE/LiCoO2 with the copolymer of AN to MMA 4:1 at different current rates according to the LiCoO2 . It can be seen that the battery at C/10 rate can achieve a good capacity, 128 mAh g−1 . Both the voltage and the capacity of the battery are found to be decreased gradually with increasing current rate but keep relatively high values, for example, at C/5 rate the battery keeps 96.8% and at 1.0 C rate keeps 89% of discharge capacity at C/10 rate.
Fig. 6. Impedance spectra of cell Li/PE-supported GPE/Li at open circuit potential, with the copolymer of AN to MMA 4:1 (mole).
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trolyte prepared with the mole ratio of AN to MMA being 4:1 has highest ionic conductivity and has good electrochemical compatibility with anode and cathode of lithium ion battery. With the application of this gel polymer electrolyte in lithium ion battery, the battery shows its initial discharge capacity as good as and its cyclic stability better than the battery with liquid electrolyte.
Nomenclature A C l R S v V Fig. 7. Rate discharge performance of battery Li/PE-supported GPE/LiCoO2 at room temperature, with the copolymer of AN to MMA 4:1 (mole).
electrolyte uptake capacity the thickness of the gel polymer electrolyte resistance the contact area between PE-supported GPE and SS disc volume voltage
Greek letters ionic conductivity ˚ diameter
References
Fig. 8. Cyclic stability of battery Li/PE-supported GPE/LiCoO2 with the copolymer of AN to MMA 4:1 (mole) (a), compared with battery Li/PE (liquid electrolyte)/LiCoO2 (b).
Fig. 8 presents the cyclic stability of the battery Li/PEsupported GPE/LiCoO2 , compared with the battery Li/PE (liquid electrolyte)/LiCoO2 , charged and discharged with a constant current of 0.48 mA cm−2 (C/5 rate) between 4.2 and 3.0 V. It can be seen from Fig. 8 that the battery Li/PE-supported GPE/LiCoO2 has the initial capacity as high as the battery Li/PE (liquid electrolyte)/LiCoO2 and the cyclic stability of the battery Li/PE-supported GPE/LiCoO2 is better than that of the battery Li/PE (liquid electrolyte)/LiCoO2 . After 50 cycles, the battery Li/PE-supported GPE/LiCoO2 keeps 86.3% of its initial discharge capacity but the battery Li/PE (liquid electrolyte)/LiCoO2 keeps only 83% of its initial discharge capacity. 4. Conclusions The electrolyte uptake of PE-supported P(AN-co-MMA) and the ionic conductivity of the gel polymer electrolyte prepared with this supported copolymer are related to the ratio of the monomers, AN and MMA, for the copolymer preparation. The gel polymer elec-
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