Preparation of poly(acrylonitrile–butyl acrylate) gel electrolyte for lithium-ion batteries

Electrochimica Acta 52 (2006) 688–693

Preparation of poly(acrylonitrile–butyl acrylate) gel electrolyte for lithium-ion batteries Zheng Tian, Xiangming He ∗ , Weihua Pu, Chunrong Wan, Changyin Jiang Institute of Nuclear & New Energy Technology, Tsinghua University, Beijing 100084, PR China Received 13 April 2006; received in revised form 28 May 2006; accepted 31 May 2006 Available online 7 July 2006

Abstract Poly(acrylonitrile–butyl acrylate) gel polymer electrolyte was prepared for lithium ion batteries. The preparation started with synthesis of poly(acrylonitrile–butyl acrylate) by radical emulsion polymerization, followed by phase inversion to produce microporous membrane. Then, the microporous gel polymer electrolytes (MGPEs) was prepared with the microporous membrane and LiPF6 in ethylene carbonate/diethyl carbonate. The dry microporous membrane showed a fracture strength as high as 18.98 MPa. As-prepared gel polymer electrolytes presented ionic conductivity in excess of 3.0 × 10−3 S cm−1 at ambient temperature and a decomposition voltage over 6.6 V. The results showed that the as-prepared gel polymer electrolytes were promising materials for Li-ion batteries. © 2006 Elsevier Ltd. All rights reserved. Keywords: Microporous membrane; Acrylonitrile–butyl acrylate copolymer; Gel electrolyte; Li-ion battery

1. Introduction There is great interest in the development of highperformance sources of energy for applications such as mobile telephones, laptops, and also electric vehicles and energy/battery hybrid vehicles. Lithium-ion batteries seem to be an excellent solution, offering both reliability and high power density. The electrolyte plays key role for their performance [1–3]. Gel polymer electrolyte (GPE) has been attractive for developing Li-ion batteries due to its combined advantage of liquid electrolyte (high ionic conductivity) and solid electrolyte (free of leaking) [4]. Usually, the GPE is fabricated by dissolving the polymerization products into liquid electrolyte and cast the viscous polymer solution on to a glass or steel plate to form the gel electrolyte film in an anhydrous environment [5,6]. However, the highly viscous property of the GPE made by the solution casting method makes it inconvenient for assembling batteries, especially for large size batteries [7]. To overcome these difficulties, scientists developed phasedinversion method to make porous structure membranes [8–12]. In this way, the GPE is formed by immersing the porous

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0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.05.055

polymer membrane into a liquid electrolyte, leaving it in an anhydrous environment and heating it over 60 ◦ C until being gelled. Currently, of the various polymers that can be used for gelled electrolyte, many studies are focusing on poly(vinylidene fluoride) (PVdF)-based polymers because of their good filmforming property and solubility in some kinds of electrolyte solvents. But PVdF-based polymer electrolytes are potentially unstable against the negative electrode of Li-ion batteries. Researches show that the –CF2 -groups in PVdF-based polymers may react with lithium anode to form more stable LiF and –C CF-unsaturated bonds, which not only decay battery performance but also raise safety risk due to the energy releasing from the exothermic reactions [13,14]. Additionally, researchers do a lot of work on the poly(acrylonitrile)(PAN)-based copolymers. An acrylonitrile–methyl methacrylate copolymer, poly(acrylonitrile–methyl methacrylate), shows a very stable binder for both anode and cathode of lithium-ion batteries and acceptable ionic conductivity, as well as high electrochemical stability [7,15,16], however, considering its poor mechanical strength, it is still hard for practical application. Scientists reported recently on the properties of solid polymer electrolyte of bis(trifluoromethanesulfone)imide (LiTFSI) dissolved in poly(acrylonitrile–butyl acrylate) copolymers [17,18]. In this system, the forming flexible membranes show significant

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mechanical properties, but, with very low ionic conductivity at ambient temperature, 10−5 to 10−7 S cm−1 . In this study, poly(acrylonitrile–butyl acrylate) was synthesized to make microporous gel electrolyte for Li-ion batteries. The thermal, electrochemical and mechanical properties were evaluated, showing a higher ionic conductivity than the above solid polymer electrolyte, as well as better thermal electrochemical stability and stronger mechanical strength comparing to the poly(acrylonitrile–methyl methacrylate) (poly(AN-co-MMA)) gel polymer electrolyte. 2. Experimental 2.1. Polymer synthesis and characterization Synthesis of poly(AN-co-BuA) by emulsion polymerization was reported in [19]. The synthesis of poly(AN-co-BuA) by emulsion polymerization in this study is briefly described as followings. The synthesis was conducted using distilled water as solvent in a four necks glass reactor. The reaction was carried on at 65 ◦ C with a nitrogen inlet throughout the whole experiment as a protective gas. Sodium dodecyl sulfate (SDS) was used as an emulsifier and ammonium persulfate was used as a water soluble free-radical initiator. The polymerization was continued for 4 h with vigorous agitation. Because SDS is a typical negative ion surfactant, in order to improve the emulsion effect, the pH value of the whole system was adjusted to 11. The polymer product was isolated by filtration and washed with distilled water at 60 ◦ C several times to remove impurities such as emulsifier, unreacted monomer, residual initiator. Then the copolymer was dried in a vacuum oven at 80 ◦ C for over 24 h. A white powder was obtained as the final product. Infrared spectra were derived from Nicolet FT-IR 560. Thermogravimetric analysis (TGA) and different scanning calorimetry (DSC) thermal analysis were performed to measure the thermal properties of polymers, using TA Instruments TGA 2050 and DSC 2910, respectively. 2.2. Preparation of microporous gel polymer electrolyte membrane The final product, poly(AN-co-BuA), was firstly dissolved in anhydrous N,N-dimethylformamide (DMF) to get a 10 wt.% solution by stirring and heating at 60 ◦ C. After the polymer was completely dissolved, ammonia and tetraethyl orthosilicate (TESO) were added, and the fresh born nano-scale silicon dioxide particles would proportionally disperse in copolymer to result in the enhancement of mechanical strength as well as the increase in the absorption level of electrolyte solution [10,20]. The ratio of SiO2 versus polymer kept to be 10 wt.%, which was optimized in Ref. [21]. Then the solution was cast on a glass plate using a doctor blade, followed by immersing the polymer dope into a coagulation bath filled with deionized water to precipitate the polymer, or letting water–gas–stream eject to the polymer membrane to introduce phase inversion, forming microporous structure membrane. The resulting membranes were washed again in deionized water several times,

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and were then vacuum-dried at 50 ◦ C for 10 h. The dried membrane typically has a thickness of 80–120 ␮m and a white color due to its porous structure. The membrane morphology was observed using a Hitachi JSM-6301F scanning electron microscope (SEM). Cross-sectional sample of the membranes were made by breaking them in liquid nitrogen. 2.3. Electrochemical measurements A solution of 1.0 M LiPF6 dissolved in 1:1 (volume ratio) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) was used as the liquid electrolyte. The dried polymer membrane was cut into small discs of 4.52 cm2 (diameter = 24 mm) and dipped into the liquid electrolyte for 30 s in a glove box. After wetting, the excess liquid electrolyte on the surface was removed by pressing lightly between two sheets of filter papers. Generally, the microporous membrane could absorb with a maximum 290% of liquid electrolyte versus weight of the dried membrane. To investigate ionic conductivity, the wetted membrane was sandwiched between two stainless steel (SS) plates and sealed in a cell. In the ionic conductivity–temperature experiment, each temperature required for the measurement was maintained for at least 1 h to reach thermal equilibrium. A Zahner Electrik IM6e Impedance Analyzer was employed to measure impedance, with a frequency from 1 MHz to 0.1 Hz and an AC oscillation of 10 mV. The electrochemical window was measured by linear sweep voltammetry with a stainless steal working electrode, with counter and reference electrode of lithium, at a scan rate of 1 mV s−1 , as reported in [22,23]. All assembling procedures were carried out in a dry box full of argon gas. 2.4. Mechanical measurements Mechanical measurements, tensile test, were carried out with a Gotech GT-TS-2000 apparatus, at a crosshead speed of 10 mm min−1 , using standard dumb bell type tensile bars as the testing samples. The tests were carried out at room temperature. The Gotech tensometer was connected to a computer for data collection and analysis. Tensile strength and elongation length were measured during deformation. 3. Results and discussion 3.1. Morphology of the microporous membrane Fig. 1 shows micrographs of the surface and the cross-section of a typical microporous poly(AN-co-BuA) membrane prepared by phase inversion method. It is obviously seen that the textures of the surface and body (cross-section) of the membrane are significantly different. The membrane is composed of a very dense skin and a body structure containing a large number of macrovoids. Formation of these asymmetric structures can be understood due to the mass transport in the phase inversion process. During the first period of the contact of polymer solution and nonsolvent to coagulate, the polymer dissolving in the solution rapidly precipitated and accumulated on the surface of the membrane, forming a dense skin layer. It is difficult for the inter-

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Z. Tian et al. / Electrochimica Acta 52 (2006) 688–693

Fig. 1. Micrographs of the porous membrane prepared by phase inversion from the solutions of poly(AN-co-BuA). (a) Surface image and (b) cross-section image.

nal solvent molecules to quickly escape from the membrane body. Consequently, the rate of phase-inversion is slowed down, and large-scale cavities in the membrane body are formed. The formation of the cavities or pores can be ascribed to the effect of syneresis pressure on the growth of polymer precipitation. 3.2. Mechanical strength of gelled microporous membrane The engineering deformation–engineering stress curves obtained from poly(AN-co-BuA) gel polymer electrolyte membrane, poly(AN-co-MMA), and PVdF-HFP are shown in Fig. 2. For both the industrial assembling method and practical application safety, mechanical properties are key factors. As being discussed above, the PVdF-based polymers have the most outstanding film- or membrane-forming properties, which, however, have serious safety problems and low cycle efficiency performance due to their chemical reactions with electrodes. From Fig. 2, PVdF-HFP gel membrane had the weakest fracture strength, only 2.5 MPa at ambient temperature, and kept nearly constant stress until fracture due to its similar flexible properties to poly(ethylene). Comparing to the result reported in

Ref. [24], the gelification for PVdF-HFP membrane significantly increased the membranes ductibility and decreased the fracture stress. According to the plasticizer effect theory, the introduction of gelification also had the same effect on the poly(AN-co-BuA), which had a higher fracture stress and lower deformation ratio before gelification. Different compositions of poly(AN-co-BuA) gel membrane showed different mechanical properties. poly(AN-co-BuA)-1 composing of 13.3:1 (AN:BuA, molar ratio) has the strongest fracture strength, while the weakest ductibility; poly(AN-coBuA)-2 sample with a ratio of 6.7:1 (AN:BuA) shows a higher ductibility and a weaker fracture strength; and poly(AN-coBuA)-3 composing of 4.4:1 (AN:BuA) shows the best ductibility and lowest fracture strength of these three samples. It can be concluded that poly(AN-co-BuA) shows an increase in ductibility with butyl acrylate content, as well as a decrease in fracture strength. This phenomenon can be explained by the rigidity of nitrile group and flexibility of butyl group of BuA. And as shown in Fig. 2, the poly(AN-co-MMA) gel membrane (line 4) with a composition of 4.5:1 (AN:MMA, molar ratio) has a fracture strength of 4.88 MPa, being only nearly one fourth of that of poly(AN-co-BuA), as well as a 26% elongation. Considering not only the fracture strength but also other parameters of the gel polymer electrolyte membrane, the sample with a composition of 4.4:1 (AN:BuA, molar ratio) had the best comprehensive performance. And the further measurements and tests are all carried out with this composition samples. 3.3. FT-IR spectra and thermal measurement

Fig. 2. Engineering deformation–engineering stress curves of different gel membranes.

FT-IR spectra of the polymer matrix poly(AN-co-BuA) is shown in Fig. 3. The matrix poly(AN-co-BuA) sample shows a characteristic ester absorption band at 1730 cm−1 (carbonyl group), a broad band occurring at around 2850–3050 cm−1 (the –CH-stretch mode of the main chain), a band at 2240 cm−1 (the –CN stretch mode), and the symbol bands at the range of 1360–1174 cm−1 arose from the –C–O-group of poly(ANco-BuA). The bands at 1454 and 939 cm−1 , respectively, show the characteristic signs of methylene and butyl group of the main chain. No band at 1635 cm−1 assigned to the C C stretch

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Fig. 3. FT-IR spectra of the polymer matrix poly(AN-co-BuA).

vibration of the monomer is present, indicating that C C is transformed to C–C completely. The TGA and DSC thermograms of poly(AN-co-BuA) are shown in Fig. 4. The decomposition temperature for poly(ANco-BuA) is over 290 ◦ C, which means to ensure a wider range of safe working condition. And the glass transition temperature (Tg ) of dried polymer product is below 80 ◦ C, nearly 20 ◦ C lower than that of poly(AN-co-MMA), reflecting a lower degree of crystallinity due to its flexible butyl group attached to the polymer main body. 3.4. Ionic conductivity and electrochemical window of MGPEs After immersing into the liquid electrolytes and getting heated at 60 ◦ C for gellification, the membrane becomes very flexible and transparent. Fig. 5 compares the ionic conductivities of the poly(AN-co-BuA) membrane and liquid electrolyte which were measured from low temperature to high temperature (heating process) and then back to the low temperature (cooling process). Before this series of measurement, all the poly(ANco-MMA) electrolyte membranes have not been gelled. From Fig. 5, it can be seen that gel polymer electrolyte membranes composed of poly(AN-co-BuA) has a higher ionic

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Fig. 5. Arrhenius plot of ionic conductivity for various electrolytes. (1) Poly(AN-co-MMA) MGPE swollen with 320 wt.% of liquid electrolyte (
), (2) poly(AN-co-BuA) MGPE swollen with 190 wt.% of liquid electrolyte ((): cooling process; (): heating process) and (3) poly(AN-co-BuA) MGPE swollen with 290 wt.% of liquid electrolyte ((䊉): cooling process; (): heating process).

conductivity in a wide range of temperature compared to electrolyte membranes composed of poly(AN-co-MMA), which is consistent with the conclusion of the above thermal analysis (Tg ). This phenomenon can be explained by the butyl group attached to main skeleton of the polymer. The butyl groups longer than the methyl groups are able to offer a larger average room or cavity for the segment movement, and make it easier for the main chain to rotate, extend and contract resulting in decreasing the glass temperature of the polymer significantly. The main chain of poly(AN-co-BuA) is more flexible than that of poly(AN-coMMA). For the poly(AN-co-BuA) electrolyte membranes with high amount (290 wt.%) of the liquid electrolyte (EC/DEC), nearly the same ionic conductivities are observed during heating and cooling processes. It can be said that there are not significant conductivity differences before and after gelification of the polymer electrolyte. It is because that the liquid electrolyte is excessively remained in the membrane predominating the conducting activities, and the mobility of Li+ ions is not totally coupled to the movement of the polymer segment [25]. The slight difference in the low range of temperature is due to the behavior of pure

Fig. 4. Thermo curves of different polymers. (a) TGA curve of poly(AN-co-BuA) and (b) DSC curves of poly(AN-co-BuA) and poly(AN-co-MMA).

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liquid electrolyte during the heating process. For the membranes with low amount (190 wt.%) of the liquid electrolyte, the behavior of the conductivity versus temperature curse is irregular in the temperature range between 40 and 60 ◦ C in the heating process. This phenomenon is due to the change of liquid electrolyte existing state. Initially, the liquid electrolyte plays the main part in conducting ion. As temperature increasing, the polymer is gradually swollen to form a gel electrolyte, which is a thermal and dynamic process. In this process, the polymer chain’s movement varies irregularly, so the ionic conductivity is not following specific routine. After the gelification at 60 ◦ C, the liquid electrolyte had totally been swollen by the polymer matrix to form a real gel electrolyte membrane. Therefore, in the coming heating and the next cooling process, the curve of temperature versus conductivity shows characters of gel polymer electrolyte. The electrochemical stability of different gel polymer electrolyte membranes was evaluated by linear sweep voltammetric measurements, as shown in Fig. 6. The equipment for measurement is a sandwiched composition, which contains a microporous gel polymer electrolyte membrane, swollen with 190 wt.% of liquid electrolyte, between a lithium electrode and an inert stainless-steel electrode. The voltage was swept from the opencircuit voltage of the cell towards more positive values until a large current change due to electrolyte decomposition at the inert electrode interface occurred. According to the research [26], the decomposition voltage is determined by the type of the lithium salt in liquid electrolyte. However, in this work the results show that, using the same liquid electrolyte solution LiPF6 -EC/DEC, the poly(AN-co-BuA) system has a far higher decomposition voltage, as much as 6.6 V, than the poly(AN-co-MMA) system only 5 V versus Li. This phenomenon means that using the same electrolyte salt and solvent the poly(AN-co-BuA) was much more resistant to the oxidation at a high-voltage than poly(ANco-MMA). It may be due to the chemical stability or inert property of the butyl group attached to the copolymer skeleton. Comparing to only one carbon group, the butyl group, containing four carbons and another four single bonds, has stronger capacity to share large amount of the ions or electrons trans-

Fig. 6. Current–voltage curves of different polymer membranes cells at 25 ◦ C (scan rate 1 mV s−1 ).

ferring through polymer main body. Moreover, a larger need of energy is needed to initiate the chemical oxidation reaction between lithium disc and poly(AN-co-BuA). This great electrochemical progress in enlarging decomposition voltage will definitely broaden lithium-ion battery application on portable electric devices. 4. Conclusions Poly(AN-co-BuA) MGPEs can be prepared for microporous membrane based gel electrolyte of Li-ion battery. The AN and BuA copolymers form flexible membranes of glass transition temperature far lower than that of poly(AN-co-MMA). The conductivity of these systems increases with the increase in gelled amount of liquid electrolyte. Conductivity at room temperature is more than 3 × 10−3 S cm−1 , and the electrochemical window is over 6.5 V. All these advantages over poly(AN-co-MMA) can be proposed that the butyl group attached to the main chain promotes the segmental motion of the polymer matrix and the ion mobility, decreasing the crystallization degree of the matrix. The high fracture stress and a proper breaking elongation ratio ensure the battery working safely. Acknowledgement The authors highly appreciate three anonymous reviewers for their revision comments. References [1] X.M. He, W.H. Pu, L. Wang, C.Y. Jiang, C.R. Wan, Prog. Chem. 18 (2006) 24. [2] L. Wang, X.M. He, W.H. Pu, C.Y. Jiang, C.R. Wan, Prog. Chem. 18 (2006) 641. [3] Z.J. Ling, X.M. He, J.J. Li, C.Y. Jiang, C.R. Wan, Prog. Chem. 18 (2006) 459. [4] J.Y. Song, Y.Y. Wang, C.C. Wan, J. Power Sources 77 (1999) 183. [5] K.M. Abraham, Z.J. Jiang, J. Electrochem. Soc. 144 (1997) L136. [6] S.S. Zhang, K. Xu, T.R. Jow, Solid State Ionics 158 (2003) 375. [7] S.S. Zhang, M.H. Ervin, K. Xu, T.R. Jow, Solid State Ionics 176 (2005) 41. [8] W.H. Pu, X.M. He, L. Wang, C.Y. Jiang, C.R. Wan, J. Membr. Sci. 272 (2006) 11. [9] W.H. Pu, X.M. He, L. Wang, Z. Tian, C.Y. Jiang, C.R. Wan, J. Membr. Sci. 280 (2006) 6. [10] X.M. He, Q. Shi, X. Zhou, C.R. Wan, C.Y. Jiang, Electrochim. Acta 51 (2005) 1069. [11] F. Boudin, X. Andrieu, C. Jehoulet, I.I. Olsen, J. Power Sources 804 (1999) 81. [12] Y. Saito, H. Kataoka, T. Sakai, S. Deki, Electrochim. Acta 46 (2001) 1747. [13] A.D. Pasquier, F. Disma, T. Bowner, A.S. Gozdz, G. Amatucci, J.M. Tarascon, J. Electrochem. Soc. 145 (1988) 472. [14] H. Maledi, G. Deng, A. Anari, J. Howard, J. Electrochem. Soc. 146 (1999) A3224. [15] H.S. Min, D.W. Kang, D.Y. Lee, D.W. Kim, J. Polym. Sci., Part B: Polym. Phys. 40 (2002) 1496. [16] H. Akashi, K.I. Tanaka, K. Sekai, J. Electrochem. Soc. 145 (1998) 881. [17] Z. Florjanczyk, E. Zygadlo-Monikowska, A. Affek, A. Tomaszewska, A. Lasinska, M. Marzantowicz, J.R. Dygas, F. Krok, Solid State Ionics 176 (2005) 2123. [18] Z. Florjanczyk, E. Zygadlo-Monikowska, W. Wieczorek, A. Ryszawy, A. Tomaszewska, K. Fredman, D. Golodnitsky, E. Peled, B. Scrosati, J. Phys. Chem. 108 (2004) 14907.

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