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Synthesis and electrochemical properties of lithium methacrylate-based self-doped gel polymer electrolytes
Electrochimica Acta 54 (2009) 4540–4544
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Synthesis and electrochemical properties of lithium methacrylate-based self-doped gel polymer electrolytes Wan-Chul Kang, Hyoun-Gyu Park, Kyung-Chan Kim, Sang-Woog Ryu ∗ Department of Industrial Engineering Chemistry, College of Engineering, Chungbuk National University, Cheongju, Chungbuk 361-763, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 January 2009 Received in revised form 4 March 2009 Accepted 14 March 2009 Available online 27 March 2009 Keywords: Self-doped Gel polymer electrolyte Ion disassociation BF3–LiMA Ionic conductivity
a b s t r a c t In this study, a strategy for synthesizing lithium methacrylate (LiMA)-based self-doped gel polymer electrolytes was described and the electrochemical properties were investigated by impedance spectroscopy and linear sweep voltammetry. LiMA was found to dissolve in ethylene carbonate (EC)/diethyl carbonate (DEC) (3/7, v/v) solvent after complexing with boron trifluoride (BF3 ). This was achieved by lowering the ionic interactions between the methacrylic anion and lithium cation. As a result, gel polymer electrolytes consisting of BF3 –LiMA complexes and poly(ethylene glycol) diacrylate were successfully synthesized by radical polymerization in an EC/DEC liquid electrolyte. The FT-IR and AC impedance measurements revealed that the incorporation of BF3 into the gel polymer electrolytes increases the solubility of LiMA and the ionic conductivity by enhancing the ion disassociations. Despite the self-doped nature of the LiMA salt, an ionic conductivity value of 3.0 × 10−5 S cm−1 was achieved at 25 ◦ C in the gel polymer electrolyte with 49 wt% of polymer content. Furthermore, linear sweep voltammetry measurements showed that the electrochemical stability of the gel polymer electrolyte was around 5.0 V at 25 ◦ C. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Recent interest in gel polymer electrolytes may be attributed to their excellent electrochemical properties and their enhanced safety over liquid electrolytes in electrochemical devices. Since the first demonstration of polymer gel electrolytes by Feullade and Perche in 1975 [1], many gel polymer systems, including poly(vinyl chloride) (PVC) [2,3], poly(vinylidene fluoride) (PVdF) [4,5], poly(acrylonitrile) (PAN) [6,7], and poly(methyl methacrylate) (PMMA) [8–10] have been reported as prospective alternatives for lithium secondary batteries. Efforts to design gel polymer electrolytes with a high ionic conductivity led to an investigation of the effects of various lithium salts, including lithium hexafluorophosphate (LiPF6 ), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium perchlorate (LiClO4 ), and lithium tetrafluoroborate (LiBF4 ). Among these salts, LiPF6 was the most widely utilized salt in liquid electrolyte systems because of its high ionic conductivity and its compatibility with solid electrolyte interfaces (SEI) on graphite anodes [11]. However, LiPF6 can be easily decomposed to several side products by UV radiation or heating above 30 ◦ C. These factors are considered to be triggers for chain breakage in a poly(ethylene oxide) (PEO)-based polymer electrolyte [12]. LiTFSI has been developed as an alternative salt to gel polymer electrolytes because of
∗ Corresponding author. Tel.: +82 43 261 2490 fax: +82 43 262 2380. E-mail address: [email protected] (S.-W. Ryu). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.03.050
its good compatibility with polymer chains and its enhanced stability at high temperatures [13]. Though it provides relatively high ionic conductivity in polymer electrolytes, the aluminum current collector can be corroded by LiTFSI, especially at high temperatures. Monomer-type lithium salts for self-doped systems include lithium methacrylate (LiMA) and lithium acrylate (LiA) is often used in solid polymer electrolytes to produce a single-ion conductor by immobilizing the counter anion to the polymer backbone, thereby preventing the concentration gradient of the salts [14,15]. Improved electrochemical stability can be expected with the use of a single-ion conducting polymer electrolyte, as decomposition usually accompanies the mobile anion species. However, the ionic conductivity of these electrolytes is lower than the bi-ionic saltdoped conductor by an order of magnitude [16–19]. In order to enhance the ionic conductivity of single-ion conducting polymer electrolytes, a gel-type liquid pathway might be used for lithium ion conduction. A gel polymer electrolyte with high ionic conductivity might be obtained if LiMA is dissolved in a liquid electrolyte. However, the monomer-type lithium salts do not dissolve in a common liquid electrolyte [ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC) or propylene carbonate (PC)] because of their strong ionic associations. LiMA can easily be dissolved in the solvents N,N-dimethylformamide (DMF) [20] and dimethylsulfoxide (DMSO) [21]; however, these solvents are not used in practical applications. Thus, relatively little is known about LiMAbased gel polymer electrolytes and their electrochemical properties in lithium secondary batteries. The purpose of this study is to
W.-C. Kang et al. / Electrochimica Acta 54 (2009) 4540–4544
describe the new synthetic strategy to prepare LiMA-based gel polymer electrolytes in EC/DEC-based liquid electrolytes using BF3 . Due to the reduced ionic interactions between methacrylic anions and the lithium cations, there is good disassociation of the ionic species, which is expected to result in enhanced solubility. In addition, the BF3 –LiMA salt can provide high electrochemical stability because the counter anion is tethered to the backbone chain. The effect of BF3 on the ion conductivity was investigated by creating DMF-based control samples. 2. Experimental 2.1. Materials Poly(ethylene glycol) diacrylate (PEGDA, Mn : 575 g mol−1 ) was purchased from Aldrich and passed through an aluminum column to remove radical inhibitors prior to polymerization. Methacrylic acid (MA, 99%), boron trifluoride–tetrahydrofuran complex (BF3 –THF), DMF (anhydrous), and 2,2-azobis(2methylpropionitrile) (AIBN, 98%) were purchased from Aldrich and used without further purification. Lithium methacrylate was prepared by the reaction of lithium hydroxide (LiOH) and methacrylic acid in methanol solvent according to the method described in the literature [20]. Liquid electrolyte consisting of EC/DEC (3/7, v/v) was kindly supplied by Samsung. 2.2. Synthesis of BF3 –LiMA salt MA was added in slight excess to the solution of LiOH dissolved in MeOH at 25 ◦ C. The reaction was allowed to proceed under a nitrogen atmosphere. After 24 h of reaction time, the transparent solution was poured into a large amount of cold acetone to precipitate the salt. The salt was washed three times to remove the unreacted MA. The white powder was dried in a vacuum oven at 60 ◦ C for 72 h in the presence of phosphorus pentoxide (P2 O5 ). For a complex formation, a stoichiometrically equivalent amount of BF3 –THF was added directly to the LiMA powder and mixed in an argon-filled glove box at 25 ◦ C. The resulting product was dried and characterized by Fourier Transform Infrared (FT-IR) spectroscopy. In the case of the polymerization experiment, BF3 –THF was added to the LiMA salt in the presence of EC/DEC solvent as described in the next experimental section. 2.3. Preparation of gel polymer electrolytes To synthesize EC/DEC-based gel polymer electrolytes, a stoichiometrically equivalent amount of BF3 –THF was slowly added to the heterogeneous solution of LiMA in EC/DEC. After the solution turned transparent, the radical copolymerization with PEGDA was carried out in the presence of AIBN. In a typical experiment, 0.13 g (0.9 mmol) of BF3 –THF complex was added to 0.1 g (0.9 mmol) of LiMA suspended in EC/DEC (8.2 g, 3/7, v/v) solvent at 25 ◦ C. After stirring for 10 min, 1.5 g (2.6 mmol) of PEGDA, and 0.05 g of AIBN were added to the transparent solution. The solution was stored at 65 ◦ C for 6 h to allow the polymerization reaction. For DMFbased gel polymer electrolytes, LiMA was directly dissolved in DMF solvent. The other reagents were slowly added and followed the same reaction steps as described above. The obtained gel polymer electrolytes were cut into small pieces for electrochemical characterization in an argon-filled glove box.
Fig. 1. Synthetic procedure for BF3 –LiMA salt.
electrolytes were measured using bulk resistance obtained by AC impedance analysis. A Solartron 1477 frequency response analyzer (FRA) was used over a frequency range of 1 Hz to 1 MHz. The gel polymer electrolytes (1 mm thick) were sandwiched between two stainless steel electrodes and assembled into a 2016 type coin cell in an argon-filled glove box. The electrochemical stability of the gel polymer electrolytes (1 mm thick) was investigated by linear sweep voltammetry using stainless steel as a working electrode and lithium as a counter and reference electrode at room temperature. The potential was scanned from +2.0 to +7.0 V at a sweep rate of 1 mV s−1 . 3. Results and discussion 3.1. Synthesis of LiMA-based gel polymer electrolytes Generally, the LiMA units were introduced into the polymer electrolyte using two steps. First, MA was polymerized with a second monomer, for example, PEGDA in a non-aqueous solvent. Next, selective neutralization of MA was accomplished by treatment with LiOH. While it is difficult to prepare polymers in this fashion, this method was adopted due to the insolubility of LiMA in a common polymerization solvent. In order to enhance the solubility of LiMA, it was necessary to reduce the ionic interactions between the methacrylic anions and the lithium cations. ´ Florjanczyk et al. showed that lithium carboxylate-based polymer electrolytes could be synthesized by the reaction of maleic anhydride with PEO-functionalized lithium alkoxide in DMSO solvent [22]. They found that BF3 improved ion disassociation and enhanced the ionic conductivity in DMSO and PC-based gel polymer electrolytes. However, the complex reaction of a LiMA monomer with BF3 has not been directly investigated to our knowledge. For that reason, BF3 incorporated LiMA salt (BF3 –LiMA) was used in this study. Fig. 1 shows the synthetic procedure for BF3 –LiMA and Fig. 2 shows the sequential reaction from MA to LiMA and BF3 –LiMA monitored by FT-IR spectroscopy. The absorption peak of COOH was clearly seen at 1700 cm−1 . This peak represents the MA monomer. However, it disappeared completely after neutralization with LiOH. Consequently, newly formed absorption peaks
2.4. Characterization To determine the quantitative transformation of MA to LiMA and BF3 –LiMA, FT-IR spectra were measured using a JASCO 480 spectrometer (JASCO). The ionic conductivities of the gel polymer
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Fig. 2. FT-IR spectra of MA, LiMA and BF3 –LiMA.
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Fig. 3. Solubility of LiMA salt in EC/DEC solvent before (left) and after (right) BF3 –THF incorporation at 25 ◦ C.
were observed in the range of 1550–1570 cm−1 . These peaks correspond to COOLi stretching vibration after neutralization. Moreover, these peaks shifted to a higher frequency region after complexing with BF3 to form BF3 –LiMA. The peak centered at 1640 cm−1 is the stretching vibration of COOBF3 -Li. This finding is consistent with our previous results [14]. Therefore, the neutralization and complex formation of MA were accomplished with LiOH and BF3 –THF, respectively. In the next experiment, the solubility of BF3 –LiMA salt in EC/DEC solvent was investigated. As shown in Fig. 3 (left), LiMA is insoluble in EC/DEC solvent. In this solvent, the salt usually forms a heterogeneous suspension solution. However, the solution turned completely transparent after a stoichiometrically equivalent amount of BF3 –THF was added at 25 ◦ C. This change reveals the enhanced solubility of LiMA when it is complexed with BF3 . Therefore, it may be possible to synthesize LiMA-based gel polymer electrolytes in the typical liquid electrolytes of lithium secondary batteries with the use of BF3 . To verify this, radical copolymerzations of BF3 –LiMA salt and PEGDA monomer were carried out in EC/DEC solvent at 65 ◦ C as a function of polymer content. As a result, various gel polymer electrolytes were successfully prepared during 6 h of reaction time at 65 ◦ C. Polymer contents of 7, 14, 21, 28, 35, 42, and 49 wt% were obtained. The sample notation is summarized in Table 1. The radical polymerization of bi-functional monomers probably produces a network structure through chemical crosslinking because of the two double-bond nature of the compound. The gel structures produced by the polymerization of PEGDA in our study can be described as dimensionally stabilized liquid electrolytes in the crosslinked polymer matrix. It is probable that the BF3 –LiMA unit is randomly distributed along the crosslinked PEGDA chain. 3.2. Electrochemical characterization of gel polymer electrolytes The gel polymer electrolytes synthesized in this study were sandwiched between symmetric stainless steel electrodes and the bulk resistance was measured using AC impedance. As shown in Fig. 4, it is clear that the resistance is substantially larger at higher polymer contents due to the presence of polymer chains. These
Fig. 4. Nyquist plots of LiMA-based gel polymer electrolytes at 25 ◦ C.
Fig. 5. Ionic conductivities for gel polymer electrolytes as a function of polymer content at 25 ◦ C.
chains act as a barrier for lithium ion conduction in the liquid electrolyte. Interestingly, there was a rapid increase in the resistance of the GPE-6 sample (polymer content of 42 wt%). This finding may indicate that the liquid electrolyte became insufficient at a polymer content of 42 wt%. On the complex impedance plot, no semicircle was observed with any sample, including 49 wt% of polymer content. A semicircle in the Nyquist plot generally arises from coupled capacitance and resistance from the electrolyte at the interface [23]. Therefore, it can be assumed that the capacitance characteristics of the polymer chain became weaker due to the coexistence of liquid electrolytes.
Table 1 Composition and activation energy data of gel polymer electrolytes. Sample
% polymer content
Ea (kJ mol−1 )
at 25 ◦ C (10−4 S cm−1 )
at 85 ◦ C (10−4 S cm−1 )
GPE-1 GPE-2 GPE-3 GPE-4 GPE-5 GPE-6 GPE-7
7 14 21 28 35 42 49
1.90 1.95 1.95 2.61 2.70 2.88 3.75
4.84 4.35 2.93 1.97 1.22 0.38 0.30
9.23 7.04 5.98 5.24 2.50 1.12 1.19
W.-C. Kang et al. / Electrochimica Acta 54 (2009) 4540–4544
Fig. 6. Ionic conductivities of DMF-based gel polymer electrolytes with or without BF3 incorporation at 25 ◦ C.
Fig. 5 shows the ionic conductivities for the gel polymer electrolytes as a function of polymer content at 25 ◦ C. The ionic conductivity for usual salt-doped gel polymer electrolytes is around 10−3 S cm−1 at room temperature [24]. In contrast, the conductivity of LiMA-based gel polymer electrolytes is much lower than typical salt-doped system because the methacrylic anion is tethered to the backbone chain. However, the ionic conductivity reached 1.22 × 10−4 S cm−1 at 25 ◦ C in the gel polymer electrolyte that had 35 wt% of polymer content (GPE-5). A maximum ionic conductivity of 4.84 × 10−4 S cm−1 was observed for the GPE-1 sample. Despite the ionic conductivity decreased as the polymer content increased, the observed values were comparable to typical salt-doped gel polymer electrolytes. The most probable reason for this is the enhanced ion disassociation of the lithium cation from the counter anion after BF3 incorporation due to the BF3 –LiMA complex. This phenomenon ´ was suggested by Florjanczyk et al. [22]. In order to directly verify this mechanism, it was necessary to synthesize two types of gel polymer electrolytes: (1) electrolyte with BF3 and (2) electrolyte without BF3 . However, the gel polymer electrolyte of LiMA without BF3 is difficult to obtain because it is insoluble in the EC/DEC solvent. For this reason, DMF was utilized as an alternative solvent to elucidate the effects of BF3 in the LiMA-based gel polymer electrolyte. Several samples were synthesized by varying the polymer content using DMF. Fig. 6 shows the ionic conductivities for the DMF-based gel polymer electrolytes at 25 ◦ C as a function of polymer content and BF3 incorporation. It is apparent that the ionic conductivity was significantly increased by the incorporation of BF3 into the gel polymer electrolytes. For example, an ionic conductivity value of 7.9 × 10−5 S cm−1 was measured for the polymer electrolyte with 9.5 wt% polymer content. The conductivity was increased to 5.76 × 10−4 S cm−1 after BF3 incorporation, suggesting strongly that the ion disassociation of lithium cations and counter anions was enhanced by BF3 incorporation. Consequently, it can be concluded that BF3 increases the solubility of LiMA in EC/DEC solvent and enhances the conductivity of the system. Fig. 7 shows the effect of temperature on the ionic conductivity of the gel polymer electrolytes over the range of 25–85 ◦ C. The activation energy (Ea ) was calculated by using the Vogel–Tammann–Fulcher (VTF) equation [25]: = AT −1/2 exp
−Ea R(T − T0 )
4543
Fig. 7. VTF-plot of ionic conductivities for gel polymer electrolytes as a function of polymer content.
tion log(T1/2 ) vs. 1/(T − T0 ) was used to calculate Ea and the results are listed in Table 1. As expected, the minimum value for Ea (1.90 kJ mol−1 ) was found at the lowest polymer content, which also had the highest conductivity, both at 25 and 85 ◦ C. The results also show that an increase in polymer content makes ion transport more difficult because the activation energy increases. However, it should be noted that Ea varied between 1.90 and 3.75 kJ mol−1 , which are quite low values, compared to a general gel polymer electrolyte. In fact, these values are close to a typical liquid electrolyte system. For example, Paillard et al. reported that the activation energy of LiTFSI salt dissolved in liquid poly(ethylene glycol) dimethylether (Mn : 500 g mol−1 ) was between 5.66 and 6.77 kJ mol−1 , depending on the salt concentration [26]. Based on this study, we can assume that there are extraordinary weak interactions between the polymer chain and the lithium cation. This behavior may be explained by the enhanced ion disassociation of the lithium cation and counter anion, which was directly caused by the BF3 complex. These findings are consistent with the results of the Nyquist plot in the previous section. The lithium cations moved easily through the liquid electrolyte phase and interacted only minimally with the polymer chain. The electrochemical stability of the gel polymer electrolytes was characterized by linear sweep voltammetry and the results are shown in Fig. 8. For the GPE-1 sample, the decomposition
In this equation, is the correlate conductivity, A is the preexponential factor, R is the gas constant, and T0 is the ideal glass transition temperature. The slope of the straight line of the func-
Fig. 8. Linear sweep voltammetry of gel polymer electrolytes with a stainless steel working electrode and a lithium reference and counter electrode at scan rate of 1 mV s−1 .
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of the electrolyte initiated at around 4.5 V and increased gradually with the applied voltage. In contrast, there was almost no current response up to 5.0 V in the case of GPE-3, -5, and 7. These samples had polymer contents of 21, 35, and 49 wt%, respectively. Furthermore, the decomposition current was found to decrease with increasing polymer content beyond 5.0 V. While a gold or platinum electrode at a high temperature would be necessary to measure the electrochemical stability with more accuracy, these results can be attributed to the polymer matrix making it difficult for the liquid electrolyte to decompose at voltages less than 5.0 V. In addition, it could also help explain the good electrochemical stability that the counter anion was tethered to the polymer backbone and no common mobile anion existed. 4. Conclusions In this study, we presented a novel strategy for obtaining LiMAbased gel polymer electrolytes in EC/DEC solvent by incorporating BF3 . BF3 –LiMA complex salt was found to be soluble in EC/DEC solvent due to the reduced ionic interactions between the lithium cations and counter anions and this enhanced ion disassociation led to an increase in the ionic conductivity. A value of 3.0 × 10−5 S cm−1 was observed at 25 ◦ C for electrolytes with 49 wt% polymer content. The electrochemical stability of the gel polymer electrolytes improved with increasing polymer content. The improved electrochemical stability seems to be related to the absence of mobile anions as well as to the oxidative stable of the polymeric interface. It can be concluded that the incorporation of BF3 increases the solubility of LiMA, enhances the ionic conductivity of the system, and improves the electrochemical stability of the electrolytes. Further studies concerning the effect of BF3 in gel polymer electrolytes are currently underway.
Acknowledgement This work was supported by the research grant of the Chungbuk National University in 2006. References [1] G. Feullade, P. Perche, J. Appl. Electrochem. 5 (1975) 63. [2] S. Rajendran, M.R. Prabhu, M.U. Rani, J. Power Sources 180 (2008) 880. [3] E.M. Shembel, O.V. Chervakov, L.I. Neduzhko, I.M. Maksyuta, Y.V. Polischuk, D.E. Reisner, P. Novak, D. Meshri, J. Power Sources 96 (2001) 20. [4] M.-S. Park, S.-H. Hyun, S.-C. Nam, S.B. Cho, Electrochim. Acta 53 (2008) 5523. [5] E.M. Stolarska, L. Niedzicki, R. Borkowska, A. Zalewska, W. Wieczorek, Electrochim. Acta 53 (2007) 1512. [6] Y.-H. Liang, C.-C. Wang, C.-Y. Chen, J. Power Sources 176 (2008) 340. [7] H. Akashi, K. Sekai, K. Tanaka, Electrochim. Acta 43 (1998) 1193. [8] J.P. Sharma, S.S. Sekhon, Solid State Ionics 178 (2007) 439. [9] C.S. Kim, S.M. Oh, Electrochim. Acta 46 (2001) 1323. [10] S.S. Sekhon, Deepa, S.A. Agnihotry, Solid State Ionics 136–137 (2000) 1189. [11] D.F. Shriver, D.P. Siska, Chem. Mater. 13 (2001) 4698. [12] C. Vincent, B. Scrosati, Modern Batteries: An Introduction to Electrochemical Power Sources, John Wiley and Sons, Inc., New York, 1997. [13] Y. Tada, M. Sato, N. Takeno, Y. Nakacho, K. Shigehara, Chem. Mater. 6 (1994) 27. [14] S.-W. Ryu, P.E. Trapa, S.C. Olugebefola, J.A. Gonzalez-Leon, D.R. Sadoway, A.M. Mayes, J. Electrochem. Soc. 152 (1) (2005) A158. [15] Y.-G. Lee, J.-K. Park, J. Power Sources 97–98 (2001) 616. [16] H.R. Allcock, D.T. Welna, A.E. Maher, Solid State Ionics 177 (2006) 741. [17] X.-G. Sun, C.A. Angell, Solid State Ionics 175 (2004) 743. [18] J.M.G. Cowie, G.H. Spence, Solid State Ionics 123 (1999) 233. [19] N. Byrne, D.R. MacFarlane, M. Forsyth, Electrochim. Acta 50 (2005) 3917. [20] T. Czerniawski, Eur. Polym. J. 36 (2000) 635. [21] N.-S. Choi, Y.M. Lee, B.H. Lee, J.A. Lee, J.-K. Park, Solid State Ionics 167 (2004) 293. ´ [22] Z. Florjanczyk, W. Bzducha, N. Langwald, J.R. Dygas, F. Krok, B. Misztal-Faraj, Electrochim. Acta 45 (2000) 3563. [23] M. Sivakumar, R. Subadevi, S. Rajendran, H.-C. Wu, N.-L. Wu, Eur. Polym. J. 43 (2007) 4466. [24] D.-W. Kim, J. Power Sources 87 (2000) 78. [25] J.-L. Souquet, M. Duclot, M. Levy, Solid State Ionics 85 (1996) 149. [26] E. Paillard, C. Iojoiu, F. Alloin, J. Guindet, J.-Y. Sanchez, Electrochim. Acta 52 (2007) 3758.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Synthesis and electrochemical properties of lithium methacrylate-based self-doped gel polymer electrolytes Wan-Chul Kang, Hyoun-Gyu Park, Kyung-Chan Kim, Sang-Woog Ryu ∗ Department of Industrial Engineering Chemistry, College of Engineering, Chungbuk National University, Cheongju, Chungbuk 361-763, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 January 2009 Received in revised form 4 March 2009 Accepted 14 March 2009 Available online 27 March 2009 Keywords: Self-doped Gel polymer electrolyte Ion disassociation BF3–LiMA Ionic conductivity
a b s t r a c t In this study, a strategy for synthesizing lithium methacrylate (LiMA)-based self-doped gel polymer electrolytes was described and the electrochemical properties were investigated by impedance spectroscopy and linear sweep voltammetry. LiMA was found to dissolve in ethylene carbonate (EC)/diethyl carbonate (DEC) (3/7, v/v) solvent after complexing with boron trifluoride (BF3 ). This was achieved by lowering the ionic interactions between the methacrylic anion and lithium cation. As a result, gel polymer electrolytes consisting of BF3 –LiMA complexes and poly(ethylene glycol) diacrylate were successfully synthesized by radical polymerization in an EC/DEC liquid electrolyte. The FT-IR and AC impedance measurements revealed that the incorporation of BF3 into the gel polymer electrolytes increases the solubility of LiMA and the ionic conductivity by enhancing the ion disassociations. Despite the self-doped nature of the LiMA salt, an ionic conductivity value of 3.0 × 10−5 S cm−1 was achieved at 25 ◦ C in the gel polymer electrolyte with 49 wt% of polymer content. Furthermore, linear sweep voltammetry measurements showed that the electrochemical stability of the gel polymer electrolyte was around 5.0 V at 25 ◦ C. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Recent interest in gel polymer electrolytes may be attributed to their excellent electrochemical properties and their enhanced safety over liquid electrolytes in electrochemical devices. Since the first demonstration of polymer gel electrolytes by Feullade and Perche in 1975 [1], many gel polymer systems, including poly(vinyl chloride) (PVC) [2,3], poly(vinylidene fluoride) (PVdF) [4,5], poly(acrylonitrile) (PAN) [6,7], and poly(methyl methacrylate) (PMMA) [8–10] have been reported as prospective alternatives for lithium secondary batteries. Efforts to design gel polymer electrolytes with a high ionic conductivity led to an investigation of the effects of various lithium salts, including lithium hexafluorophosphate (LiPF6 ), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium perchlorate (LiClO4 ), and lithium tetrafluoroborate (LiBF4 ). Among these salts, LiPF6 was the most widely utilized salt in liquid electrolyte systems because of its high ionic conductivity and its compatibility with solid electrolyte interfaces (SEI) on graphite anodes [11]. However, LiPF6 can be easily decomposed to several side products by UV radiation or heating above 30 ◦ C. These factors are considered to be triggers for chain breakage in a poly(ethylene oxide) (PEO)-based polymer electrolyte [12]. LiTFSI has been developed as an alternative salt to gel polymer electrolytes because of
∗ Corresponding author. Tel.: +82 43 261 2490 fax: +82 43 262 2380. E-mail address: [email protected] (S.-W. Ryu). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.03.050
its good compatibility with polymer chains and its enhanced stability at high temperatures [13]. Though it provides relatively high ionic conductivity in polymer electrolytes, the aluminum current collector can be corroded by LiTFSI, especially at high temperatures. Monomer-type lithium salts for self-doped systems include lithium methacrylate (LiMA) and lithium acrylate (LiA) is often used in solid polymer electrolytes to produce a single-ion conductor by immobilizing the counter anion to the polymer backbone, thereby preventing the concentration gradient of the salts [14,15]. Improved electrochemical stability can be expected with the use of a single-ion conducting polymer electrolyte, as decomposition usually accompanies the mobile anion species. However, the ionic conductivity of these electrolytes is lower than the bi-ionic saltdoped conductor by an order of magnitude [16–19]. In order to enhance the ionic conductivity of single-ion conducting polymer electrolytes, a gel-type liquid pathway might be used for lithium ion conduction. A gel polymer electrolyte with high ionic conductivity might be obtained if LiMA is dissolved in a liquid electrolyte. However, the monomer-type lithium salts do not dissolve in a common liquid electrolyte [ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC) or propylene carbonate (PC)] because of their strong ionic associations. LiMA can easily be dissolved in the solvents N,N-dimethylformamide (DMF) [20] and dimethylsulfoxide (DMSO) [21]; however, these solvents are not used in practical applications. Thus, relatively little is known about LiMAbased gel polymer electrolytes and their electrochemical properties in lithium secondary batteries. The purpose of this study is to
W.-C. Kang et al. / Electrochimica Acta 54 (2009) 4540–4544
describe the new synthetic strategy to prepare LiMA-based gel polymer electrolytes in EC/DEC-based liquid electrolytes using BF3 . Due to the reduced ionic interactions between methacrylic anions and the lithium cations, there is good disassociation of the ionic species, which is expected to result in enhanced solubility. In addition, the BF3 –LiMA salt can provide high electrochemical stability because the counter anion is tethered to the backbone chain. The effect of BF3 on the ion conductivity was investigated by creating DMF-based control samples. 2. Experimental 2.1. Materials Poly(ethylene glycol) diacrylate (PEGDA, Mn : 575 g mol−1 ) was purchased from Aldrich and passed through an aluminum column to remove radical inhibitors prior to polymerization. Methacrylic acid (MA, 99%), boron trifluoride–tetrahydrofuran complex (BF3 –THF), DMF (anhydrous), and 2,2-azobis(2methylpropionitrile) (AIBN, 98%) were purchased from Aldrich and used without further purification. Lithium methacrylate was prepared by the reaction of lithium hydroxide (LiOH) and methacrylic acid in methanol solvent according to the method described in the literature [20]. Liquid electrolyte consisting of EC/DEC (3/7, v/v) was kindly supplied by Samsung. 2.2. Synthesis of BF3 –LiMA salt MA was added in slight excess to the solution of LiOH dissolved in MeOH at 25 ◦ C. The reaction was allowed to proceed under a nitrogen atmosphere. After 24 h of reaction time, the transparent solution was poured into a large amount of cold acetone to precipitate the salt. The salt was washed three times to remove the unreacted MA. The white powder was dried in a vacuum oven at 60 ◦ C for 72 h in the presence of phosphorus pentoxide (P2 O5 ). For a complex formation, a stoichiometrically equivalent amount of BF3 –THF was added directly to the LiMA powder and mixed in an argon-filled glove box at 25 ◦ C. The resulting product was dried and characterized by Fourier Transform Infrared (FT-IR) spectroscopy. In the case of the polymerization experiment, BF3 –THF was added to the LiMA salt in the presence of EC/DEC solvent as described in the next experimental section. 2.3. Preparation of gel polymer electrolytes To synthesize EC/DEC-based gel polymer electrolytes, a stoichiometrically equivalent amount of BF3 –THF was slowly added to the heterogeneous solution of LiMA in EC/DEC. After the solution turned transparent, the radical copolymerization with PEGDA was carried out in the presence of AIBN. In a typical experiment, 0.13 g (0.9 mmol) of BF3 –THF complex was added to 0.1 g (0.9 mmol) of LiMA suspended in EC/DEC (8.2 g, 3/7, v/v) solvent at 25 ◦ C. After stirring for 10 min, 1.5 g (2.6 mmol) of PEGDA, and 0.05 g of AIBN were added to the transparent solution. The solution was stored at 65 ◦ C for 6 h to allow the polymerization reaction. For DMFbased gel polymer electrolytes, LiMA was directly dissolved in DMF solvent. The other reagents were slowly added and followed the same reaction steps as described above. The obtained gel polymer electrolytes were cut into small pieces for electrochemical characterization in an argon-filled glove box.
Fig. 1. Synthetic procedure for BF3 –LiMA salt.
electrolytes were measured using bulk resistance obtained by AC impedance analysis. A Solartron 1477 frequency response analyzer (FRA) was used over a frequency range of 1 Hz to 1 MHz. The gel polymer electrolytes (1 mm thick) were sandwiched between two stainless steel electrodes and assembled into a 2016 type coin cell in an argon-filled glove box. The electrochemical stability of the gel polymer electrolytes (1 mm thick) was investigated by linear sweep voltammetry using stainless steel as a working electrode and lithium as a counter and reference electrode at room temperature. The potential was scanned from +2.0 to +7.0 V at a sweep rate of 1 mV s−1 . 3. Results and discussion 3.1. Synthesis of LiMA-based gel polymer electrolytes Generally, the LiMA units were introduced into the polymer electrolyte using two steps. First, MA was polymerized with a second monomer, for example, PEGDA in a non-aqueous solvent. Next, selective neutralization of MA was accomplished by treatment with LiOH. While it is difficult to prepare polymers in this fashion, this method was adopted due to the insolubility of LiMA in a common polymerization solvent. In order to enhance the solubility of LiMA, it was necessary to reduce the ionic interactions between the methacrylic anions and the lithium cations. ´ Florjanczyk et al. showed that lithium carboxylate-based polymer electrolytes could be synthesized by the reaction of maleic anhydride with PEO-functionalized lithium alkoxide in DMSO solvent [22]. They found that BF3 improved ion disassociation and enhanced the ionic conductivity in DMSO and PC-based gel polymer electrolytes. However, the complex reaction of a LiMA monomer with BF3 has not been directly investigated to our knowledge. For that reason, BF3 incorporated LiMA salt (BF3 –LiMA) was used in this study. Fig. 1 shows the synthetic procedure for BF3 –LiMA and Fig. 2 shows the sequential reaction from MA to LiMA and BF3 –LiMA monitored by FT-IR spectroscopy. The absorption peak of COOH was clearly seen at 1700 cm−1 . This peak represents the MA monomer. However, it disappeared completely after neutralization with LiOH. Consequently, newly formed absorption peaks
2.4. Characterization To determine the quantitative transformation of MA to LiMA and BF3 –LiMA, FT-IR spectra were measured using a JASCO 480 spectrometer (JASCO). The ionic conductivities of the gel polymer
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Fig. 2. FT-IR spectra of MA, LiMA and BF3 –LiMA.
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Fig. 3. Solubility of LiMA salt in EC/DEC solvent before (left) and after (right) BF3 –THF incorporation at 25 ◦ C.
were observed in the range of 1550–1570 cm−1 . These peaks correspond to COOLi stretching vibration after neutralization. Moreover, these peaks shifted to a higher frequency region after complexing with BF3 to form BF3 –LiMA. The peak centered at 1640 cm−1 is the stretching vibration of COOBF3 -Li. This finding is consistent with our previous results [14]. Therefore, the neutralization and complex formation of MA were accomplished with LiOH and BF3 –THF, respectively. In the next experiment, the solubility of BF3 –LiMA salt in EC/DEC solvent was investigated. As shown in Fig. 3 (left), LiMA is insoluble in EC/DEC solvent. In this solvent, the salt usually forms a heterogeneous suspension solution. However, the solution turned completely transparent after a stoichiometrically equivalent amount of BF3 –THF was added at 25 ◦ C. This change reveals the enhanced solubility of LiMA when it is complexed with BF3 . Therefore, it may be possible to synthesize LiMA-based gel polymer electrolytes in the typical liquid electrolytes of lithium secondary batteries with the use of BF3 . To verify this, radical copolymerzations of BF3 –LiMA salt and PEGDA monomer were carried out in EC/DEC solvent at 65 ◦ C as a function of polymer content. As a result, various gel polymer electrolytes were successfully prepared during 6 h of reaction time at 65 ◦ C. Polymer contents of 7, 14, 21, 28, 35, 42, and 49 wt% were obtained. The sample notation is summarized in Table 1. The radical polymerization of bi-functional monomers probably produces a network structure through chemical crosslinking because of the two double-bond nature of the compound. The gel structures produced by the polymerization of PEGDA in our study can be described as dimensionally stabilized liquid electrolytes in the crosslinked polymer matrix. It is probable that the BF3 –LiMA unit is randomly distributed along the crosslinked PEGDA chain. 3.2. Electrochemical characterization of gel polymer electrolytes The gel polymer electrolytes synthesized in this study were sandwiched between symmetric stainless steel electrodes and the bulk resistance was measured using AC impedance. As shown in Fig. 4, it is clear that the resistance is substantially larger at higher polymer contents due to the presence of polymer chains. These
Fig. 4. Nyquist plots of LiMA-based gel polymer electrolytes at 25 ◦ C.
Fig. 5. Ionic conductivities for gel polymer electrolytes as a function of polymer content at 25 ◦ C.
chains act as a barrier for lithium ion conduction in the liquid electrolyte. Interestingly, there was a rapid increase in the resistance of the GPE-6 sample (polymer content of 42 wt%). This finding may indicate that the liquid electrolyte became insufficient at a polymer content of 42 wt%. On the complex impedance plot, no semicircle was observed with any sample, including 49 wt% of polymer content. A semicircle in the Nyquist plot generally arises from coupled capacitance and resistance from the electrolyte at the interface [23]. Therefore, it can be assumed that the capacitance characteristics of the polymer chain became weaker due to the coexistence of liquid electrolytes.
Table 1 Composition and activation energy data of gel polymer electrolytes. Sample
% polymer content
Ea (kJ mol−1 )
at 25 ◦ C (10−4 S cm−1 )
at 85 ◦ C (10−4 S cm−1 )
GPE-1 GPE-2 GPE-3 GPE-4 GPE-5 GPE-6 GPE-7
7 14 21 28 35 42 49
1.90 1.95 1.95 2.61 2.70 2.88 3.75
4.84 4.35 2.93 1.97 1.22 0.38 0.30
9.23 7.04 5.98 5.24 2.50 1.12 1.19
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Fig. 6. Ionic conductivities of DMF-based gel polymer electrolytes with or without BF3 incorporation at 25 ◦ C.
Fig. 5 shows the ionic conductivities for the gel polymer electrolytes as a function of polymer content at 25 ◦ C. The ionic conductivity for usual salt-doped gel polymer electrolytes is around 10−3 S cm−1 at room temperature [24]. In contrast, the conductivity of LiMA-based gel polymer electrolytes is much lower than typical salt-doped system because the methacrylic anion is tethered to the backbone chain. However, the ionic conductivity reached 1.22 × 10−4 S cm−1 at 25 ◦ C in the gel polymer electrolyte that had 35 wt% of polymer content (GPE-5). A maximum ionic conductivity of 4.84 × 10−4 S cm−1 was observed for the GPE-1 sample. Despite the ionic conductivity decreased as the polymer content increased, the observed values were comparable to typical salt-doped gel polymer electrolytes. The most probable reason for this is the enhanced ion disassociation of the lithium cation from the counter anion after BF3 incorporation due to the BF3 –LiMA complex. This phenomenon ´ was suggested by Florjanczyk et al. [22]. In order to directly verify this mechanism, it was necessary to synthesize two types of gel polymer electrolytes: (1) electrolyte with BF3 and (2) electrolyte without BF3 . However, the gel polymer electrolyte of LiMA without BF3 is difficult to obtain because it is insoluble in the EC/DEC solvent. For this reason, DMF was utilized as an alternative solvent to elucidate the effects of BF3 in the LiMA-based gel polymer electrolyte. Several samples were synthesized by varying the polymer content using DMF. Fig. 6 shows the ionic conductivities for the DMF-based gel polymer electrolytes at 25 ◦ C as a function of polymer content and BF3 incorporation. It is apparent that the ionic conductivity was significantly increased by the incorporation of BF3 into the gel polymer electrolytes. For example, an ionic conductivity value of 7.9 × 10−5 S cm−1 was measured for the polymer electrolyte with 9.5 wt% polymer content. The conductivity was increased to 5.76 × 10−4 S cm−1 after BF3 incorporation, suggesting strongly that the ion disassociation of lithium cations and counter anions was enhanced by BF3 incorporation. Consequently, it can be concluded that BF3 increases the solubility of LiMA in EC/DEC solvent and enhances the conductivity of the system. Fig. 7 shows the effect of temperature on the ionic conductivity of the gel polymer electrolytes over the range of 25–85 ◦ C. The activation energy (Ea ) was calculated by using the Vogel–Tammann–Fulcher (VTF) equation [25]: = AT −1/2 exp
−Ea R(T − T0 )
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Fig. 7. VTF-plot of ionic conductivities for gel polymer electrolytes as a function of polymer content.
tion log(T1/2 ) vs. 1/(T − T0 ) was used to calculate Ea and the results are listed in Table 1. As expected, the minimum value for Ea (1.90 kJ mol−1 ) was found at the lowest polymer content, which also had the highest conductivity, both at 25 and 85 ◦ C. The results also show that an increase in polymer content makes ion transport more difficult because the activation energy increases. However, it should be noted that Ea varied between 1.90 and 3.75 kJ mol−1 , which are quite low values, compared to a general gel polymer electrolyte. In fact, these values are close to a typical liquid electrolyte system. For example, Paillard et al. reported that the activation energy of LiTFSI salt dissolved in liquid poly(ethylene glycol) dimethylether (Mn : 500 g mol−1 ) was between 5.66 and 6.77 kJ mol−1 , depending on the salt concentration [26]. Based on this study, we can assume that there are extraordinary weak interactions between the polymer chain and the lithium cation. This behavior may be explained by the enhanced ion disassociation of the lithium cation and counter anion, which was directly caused by the BF3 complex. These findings are consistent with the results of the Nyquist plot in the previous section. The lithium cations moved easily through the liquid electrolyte phase and interacted only minimally with the polymer chain. The electrochemical stability of the gel polymer electrolytes was characterized by linear sweep voltammetry and the results are shown in Fig. 8. For the GPE-1 sample, the decomposition
In this equation, is the correlate conductivity, A is the preexponential factor, R is the gas constant, and T0 is the ideal glass transition temperature. The slope of the straight line of the func-
Fig. 8. Linear sweep voltammetry of gel polymer electrolytes with a stainless steel working electrode and a lithium reference and counter electrode at scan rate of 1 mV s−1 .
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of the electrolyte initiated at around 4.5 V and increased gradually with the applied voltage. In contrast, there was almost no current response up to 5.0 V in the case of GPE-3, -5, and 7. These samples had polymer contents of 21, 35, and 49 wt%, respectively. Furthermore, the decomposition current was found to decrease with increasing polymer content beyond 5.0 V. While a gold or platinum electrode at a high temperature would be necessary to measure the electrochemical stability with more accuracy, these results can be attributed to the polymer matrix making it difficult for the liquid electrolyte to decompose at voltages less than 5.0 V. In addition, it could also help explain the good electrochemical stability that the counter anion was tethered to the polymer backbone and no common mobile anion existed. 4. Conclusions In this study, we presented a novel strategy for obtaining LiMAbased gel polymer electrolytes in EC/DEC solvent by incorporating BF3 . BF3 –LiMA complex salt was found to be soluble in EC/DEC solvent due to the reduced ionic interactions between the lithium cations and counter anions and this enhanced ion disassociation led to an increase in the ionic conductivity. A value of 3.0 × 10−5 S cm−1 was observed at 25 ◦ C for electrolytes with 49 wt% polymer content. The electrochemical stability of the gel polymer electrolytes improved with increasing polymer content. The improved electrochemical stability seems to be related to the absence of mobile anions as well as to the oxidative stable of the polymeric interface. It can be concluded that the incorporation of BF3 increases the solubility of LiMA, enhances the ionic conductivity of the system, and improves the electrochemical stability of the electrolytes. Further studies concerning the effect of BF3 in gel polymer electrolytes are currently underway.
Acknowledgement This work was supported by the research grant of the Chungbuk National University in 2006. References [1] G. Feullade, P. Perche, J. Appl. Electrochem. 5 (1975) 63. [2] S. Rajendran, M.R. Prabhu, M.U. Rani, J. Power Sources 180 (2008) 880. [3] E.M. Shembel, O.V. Chervakov, L.I. Neduzhko, I.M. Maksyuta, Y.V. Polischuk, D.E. Reisner, P. Novak, D. Meshri, J. Power Sources 96 (2001) 20. [4] M.-S. Park, S.-H. Hyun, S.-C. Nam, S.B. Cho, Electrochim. Acta 53 (2008) 5523. [5] E.M. Stolarska, L. Niedzicki, R. Borkowska, A. Zalewska, W. Wieczorek, Electrochim. Acta 53 (2007) 1512. [6] Y.-H. Liang, C.-C. Wang, C.-Y. Chen, J. Power Sources 176 (2008) 340. [7] H. Akashi, K. Sekai, K. Tanaka, Electrochim. Acta 43 (1998) 1193. [8] J.P. Sharma, S.S. Sekhon, Solid State Ionics 178 (2007) 439. [9] C.S. Kim, S.M. Oh, Electrochim. Acta 46 (2001) 1323. [10] S.S. Sekhon, Deepa, S.A. Agnihotry, Solid State Ionics 136–137 (2000) 1189. [11] D.F. Shriver, D.P. Siska, Chem. Mater. 13 (2001) 4698. [12] C. Vincent, B. Scrosati, Modern Batteries: An Introduction to Electrochemical Power Sources, John Wiley and Sons, Inc., New York, 1997. [13] Y. Tada, M. Sato, N. Takeno, Y. Nakacho, K. Shigehara, Chem. Mater. 6 (1994) 27. [14] S.-W. Ryu, P.E. Trapa, S.C. Olugebefola, J.A. Gonzalez-Leon, D.R. Sadoway, A.M. Mayes, J. Electrochem. Soc. 152 (1) (2005) A158. [15] Y.-G. Lee, J.-K. Park, J. Power Sources 97–98 (2001) 616. [16] H.R. Allcock, D.T. Welna, A.E. Maher, Solid State Ionics 177 (2006) 741. [17] X.-G. Sun, C.A. Angell, Solid State Ionics 175 (2004) 743. [18] J.M.G. Cowie, G.H. Spence, Solid State Ionics 123 (1999) 233. [19] N. Byrne, D.R. MacFarlane, M. Forsyth, Electrochim. Acta 50 (2005) 3917. [20] T. Czerniawski, Eur. Polym. J. 36 (2000) 635. [21] N.-S. Choi, Y.M. Lee, B.H. Lee, J.A. Lee, J.-K. Park, Solid State Ionics 167 (2004) 293. ´ [22] Z. Florjanczyk, W. Bzducha, N. Langwald, J.R. Dygas, F. Krok, B. Misztal-Faraj, Electrochim. Acta 45 (2000) 3563. [23] M. Sivakumar, R. Subadevi, S. Rajendran, H.-C. Wu, N.-L. Wu, Eur. Polym. J. 43 (2007) 4466. [24] D.-W. Kim, J. Power Sources 87 (2000) 78. [25] J.-L. Souquet, M. Duclot, M. Levy, Solid State Ionics 85 (1996) 149. [26] E. Paillard, C. Iojoiu, F. Alloin, J. Guindet, J.-Y. Sanchez, Electrochim. Acta 52 (2007) 3758.