Study of the formation of a solid electrolyte interphase (SEI) in ionically crosslinked polyampholytic gel electrolytes

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Electrochimica Acta 53 (2008) 4414–4419

Study of the formation of a solid electrolyte interphase (SEI) in ionically crosslinked polyampholytic gel electrolytes Wanyu Chen, Ziwei Ou, Haitao Tang, Hong Wang, Yajiang Yang ∗ Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, China Received 23 October 2007; received in revised form 10 January 2008; accepted 13 January 2008 Available online 20 January 2008

Abstract An ionic complex of anionic and cationic monomers was obtained by protonation of (N,N-diethylamino)ethylmethacrylate with acrylic acid. A novel ionically crosslinked polyampholytic gel electrolyte was prepared through the free radical copolymerization of the ionic complex and acrylamide in a solvent mixture of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate (1:1:1, v/v) containing 1 mol/L of LiPF6 . The impedance analysis indicated that the ionic conductivity of the polyampholytic gel electrolyte was rather close to that of solution electrolytes in the absence of a polymer at the same temperature. The temperature dependence of the conductivity was found to be well in accord with the Arrhenius behavior. The formation processes of the solid electrolyte interphase (SEI) formed in both gel and solution electrolytes during the cycles of charge–discharge were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The cyclic voltammetry curves show a strong peak at a potential of 0.68 V and an increase of the interfacial resistance from 17.2  to 35.8  after the first cycle of charge–discharge. The results indicate that the formation process of SEI formed in both gel and solution electrolytes was similar which could effectively prevent the organic electrolyte from further decomposition and inserting into the graphite electrode. The morphologies of SEI formed in both gel and solution electrolytes were analyzed by field emission scanning electron microscopy. The results indicate that the SEI formed in the gel electrolyte showed a rough surface consisting of smaller solid depositions. Moreover, the SEI formed in the gel electrolyte became more compact and thicker as the cycling increased. © 2008 Elsevier Ltd. All rights reserved. Keywords: Ionically crosslinked polyampholytic gel electrolytes; Ionic conductivity; SEI

1. Introduction In general, a lithium ion battery system is composed of a graphitic anode, a LiCoO2 , LiMn2 O4 or LiNiO2 cathode and organic electrolytes containing lithium salts [1]. During the first charge–discharge cycles of the battery, a layer of solid electrolyte interphase (SEI) is formed on the surface of graphite anode, which originates from the decomposition of organic electrolyte. The formation of the SEI leads to an irreversible loss of capacity on the initial charge–discharge cycles of the lithium-ion cells. However, the formed SEI can separate the graphite electrode from the organic electrolyte and thereby prevents the organic electrolyte from further intercalating into the graphite electrode. Consequently, the SEI maintains the structural stability of the

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0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.01.029

graphite electrode [2,3]. It is clear that the formation of SEI greatly influences the properties of lithium ion batteries, such as its capacity [4]. At present, molecular dynamics (MDs) simulations [5], X-ray photoelectron spectroscopy [6], scanning electron microscopy, transmission electron microscopy [7], cyclic voltammetry (CV) [8–10] and electrochemical impedance spectroscopy (EIS) [11,12] are widely used for the investigation of the components of SEI and its buildup mechanism. In addition, most of the studied SEIs have been formed in organic liquid electrolytes in these reports. Zhang et al. [13] studied the influences of various lithium salts and carbonate solvents on the formation of SEIs using EIS. The results indicated that the lithium salts can be arranged in order of resistance of SEI (Rsei ) as follows: LiBF4 > LiSO3 CF3 > lithium bis(oxalato)borate (LiBOB) > LiPF6 , and solvents can be arranged in order of resistance of SEI (Rsei ): 1-methyl-2pyrrolidinone (NMP) > ethyl methyl carbonate (EMC) > methyl

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butyrate (MB). Also Rsei increased significantly after vinylene carbonate was used as an additive in the electrolyte. Tasaki [14] studied the decomposition product and decomposition mechanism of various carbonate solvents in common organic electrolytes using density functional theory (DFT) and MDs simulations. The results indicated that the reductions of these carbonates are exothermic when the carbonates received the first and second electron in the status of solutions. Lee and Pyun [15] reported the effect of temperature on the formation of SEIs. The content of ROCO2 Li and Li2 CO3 in the SEI increased with an increase of temperature. For ROCO2 Li, the increase of content could be attributed to direct reduction of the organic electrolyte. For Li2 CO3 , the increase of content was probably due to the transformation of ROCO2 Li to Li2 CO3 . In recent years, polymer gel electrolytes have attracted great attention. Kang et al. [16] studied the cycling efficiency of lithium in polyethylene oxide (PEO) gel electrolytes. The results indicated that the star-shaped PEO was able to effectively form stable SEIs. To our knowledge, the buildup mechanism of SEIs from polymer gel electrolytes, particularly for ionically crosslinked polymer gel electrolyte, has not been studied previously. In the present work, an ionic complex of anionic and cationic monomers was obtained by protonation of (N,N-diethylamino) ethylmethacrylate (DEA) with acrylic acid (AAc). Through free radical copolymerization of the ionic complex and acrylamide (AAm) in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and EMC (1:1:1, v/v) containing 1 mol/L of LiPF6 , the polymer gel electrolyte [poly(AAc-DEA-AAm), designated as PADA] was prepared. It is a polymer gel electrolyte crosslinked by electrostatic interactions between negatively and positively charged groups. Its conductivity and the formation of SEI were investigated by EIS, CV and field emission scanning electron microscopy (FE-SEM). 2. Experimental 2.1. Materials Acrylic acid (AA, Aldrich) and (N,N-diethylamino)ethylmethacrylate (DEA, Aldrich) were distilled before use. Azobisisobutylonitrile (AIBN, Aldrich) and acrylamide (AAm, Aldrich) were recrystallized before use. Anhydrous EC, DMC, EMC, graphite electrode, LiPF6 (analytic reagents) and lithium metal foil (purity >99.9%) were kindly provided by Wuhan Lisun Power Source Co. Ltd. All materials were stored in a glove box for further use. 2.2. Preparation of PADA gel electrolytes and measurements of ionic conductivity The preparation of PADA gels has been described previously [17,18]. In the present work, the preparation of PADA gel electrolytes was carried out in a glove box filled with dry argon. Briefly, 0.36 mL of DEA, 0.12 mL of AA, 0.2 g of AAm and AIBN as initiator (0.8 wt% of the total monomers) were added to a test cell equipped with platinum electrodes (Pt 260, Shang-

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hai Leici). Eight milliliters of a mixture of EC, DMC and EMC (1:1:1, v/v) containing 1 mol/L of LiPF6 (designated as EDE solution electrolyte) was added to the test cell. The polymerization was carried out at 60 ◦ C under stirring for 2.5 h. Ionic conductivities of PADA gel electrolytes and EDE solution electrolytes were determined from complex impedance spectra measured by the IM6e electrochemical station (Zhanner Elektrik). The data were treated by Zview software. The ionic conductivity (σ) of the PADA gel electrolytes can be calculated by the equation below: σ=

1 d Rs S

Herein σ is the conductivity, Rs is the bulk resistance, d is the distance of the two platinum electrodes and S is the interface area between platinum electrode and PADA gel electrolyte, d/S = 2. The frequency of the measurements was in the range from 50 mHz to 1 MHz. The impedance was measured at different temperature from −30 ◦ C to 60 ◦ C using a cryohydrate bath or oil bath. 2.3. SEI formation Similarly, the above solution mixtures for polymerization were added to a glass-cell equipped with three electrodes. The graphite electrode was used as the working electrode and two lithium sheets were used as the counter electrode and the reference electrode, respectively. After the polymerization, the test cell was discharged–charged at constant current (0.2 mA/cm2 , 10 mV–1.5 V vs. Li/Li+ ) using a system of electrochemical corrosion (CS300, Huazhong University of Science and Technology). The complex impedance spectra of PADA gel electrolytes were measured by the IM6e electrochemical station (Zhanner elektrik) after discharge–charge cycles. The data were treated by Zview software. The frequency of the measurements was in the range of 10 mHz–100 kHz and the amplitude was 10 mV. Cyclic voltammograms were measured by CS300 in the range of 0–1.5 V versus Li+ /Li at the scan rate of 1 mV/s. After discharge–charge cycles, the graphitic anode was taken out of the test cell and rinsed repeatedly with tetrahydrofuran to remove the gel on the surface of the graphitic anode. The surface morphology of the graphitic anode was observed with FE-SEM (Sirion 200, FEI). 3. Results and discussion 3.1. Comparison of ion conductivity for PADA gels and EDE solution electrolytes Fig. 1 shows the relationships between the ion conductivities and temperature of PADA gel electrolytes and EDE solution electrolytes. Obviously, the plots of their ion conductivities versus 1/T show good linear correlations in the range of temperatures from −30 ◦ C to 60 ◦ C, which implies that the data are well in accord with Arrhenius behavior. It also indicates that the conductive behavior of Li+ in both PADA gel electrolytes and EDE solution electrolytes is still governed by thermal motion

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3.2. Formation of SEI in PADA gels and EDE solution electrolytes

Fig. 1. Arrhenius plot for ion conductivities of PADA gel electrolytes () and EDE solution electrolytes () vs. temperature ranging from −30 ◦ C to 60 ◦ C.

of the ions [19]. As shown in Fig. 1, the ion conductivities of PADA gel electrolytes and EDE solution electrolytes exhibited only small differences within one order of magnitude at the same temperature. For instance, the ionic conductivity of PADA gel electrolytes is 2.9 × 10−3 S cm−1 and the ionic conductivity of EDE solution electrolytes is 7.8 × 10−3 S cm−1 at 20 ◦ C. This minor difference can be ascribed to the ionically crosslinked three-dimensional network in the PADA gel and the lower concentration of polymer (8 wt%) in the PADA gel electrolyte. The large amount of organic liquid as a continuous phase was immobilized in a three-dimensional network structure of PADA gel. Therefore, the transfer and diffusion of Li+ and negatively charged ions within the PADA gel electrolyte were similar to that in the liquid electrolyte. In other words, the presence of PADA polymer in the system seems to exert no significant influence on the conductivity behavior of the ions. In addition, the ionic conductivity of PADA gel electrolytes still remained 3.9 × 10−4 S cm−1 even at −30 ◦ C. By contrast, the ion conductivities of conventional polymer gel electrolytes are usually in the range of 10−5 S cm−1 to 10−8 S cm−1 at this temperature [20,21].

Fig. 2 shows the cyclic voltammetry curves of PADA gels and EDE solution electrolytes at room temperature. In general, SEI between the graphite electrode and the electrolyte arises from decomposition products of organic electrolytes during the first intercalation of lithium into graphite [6]. As shown in Fig. 2A, a strong reduction current peak appeared at a potential of ca. 0.7 V (p) after first cycle of charge–discharge, which reflects the formation of SEI between the graphite anode surface and the EDE solution electrolyte [12]. After a second cycle, this peak rapidly became weaker and gradually disappeared during subsequent cycles. This indicates that SEI formed after two cycles prevents further decomposition of the electrolyte. Fig. 2B shows a similar phenomenon for the PADA gel electrolyte, namely, a strong reduction current peak appeared at a potential of ca. 0.7 V (P1) at the first cycle and disappeared completely at the10th cycle. This shows that there was no significant difference in the buildup mechanism of SEI in both PADA gels and EDE solution electrolytes. Herein, it is necessary to discuss the stability of AA-based polymer at low potentials. Usually, the instability of AA-based polymer is arises from the proton of carboxyl group. In the present work (PADA gel), coordination of protons in AA by amine in DEA can increase the electrochemical stability of protons. Furthermore, another component (acrylamide, AAm) as co-monomer took part in the polymerization of ionic complex. The interaction between macromolecular chains may occur and result in more stable proton because pendent amide groups on the polymer chains can also interact with AA. Yang et al. [22] and Tasaki [14] studied the reductions of various carbonate electrolytes. The results indicated that EC was more easily reduced than others. In the present work, both the PADA gel electrolytes and EDE solution electrolytes contained EC solvent. It is assumed that EC was more preferentially reduced on the graphite surface to form SEI although both DEC and EMC also participate in SEI formation. Hence the strong reduction peak at ca. 0.7 V at the first cycle can be attributed to

Fig. 2. Cyclic voltammograms of graphite anode in EDE solution electrolyte (A) and PADA gel electrolyte (B).

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Fig. 3. Impedance spectra of graphitic electrodes in EDE solutions (plots A1 and A2) and PADA gel electrolytes (plots B1 and B2). Plot A2 and B2 are the enlarged view of high frequency ranges in the plots A1 and B1. The frequency varied from 10 mHz to100 kHz. (䊉) Before cycle. () After 1st cycle. () After 3rd cycles. () After 10th cycles.

the reduction of EC [8,12]. The main reaction for EC is [6]: 2(CH2 O)2 CO + 2e− + 2Li+ → LiO2 CO CH2 CH2 OCO2 Li + C2 H4 (g) Fig. 2 also shows that the presence of PADA seems to have no significant effect on the formation of the SEI. In addition, an extra peak is observed at ca. 0.4 V (P2) after the first cycle for the PADA gel electrolyte. This could be due to the reaction of traces of water with LiPF6 at this potential [23]. Moreover, the coulombic efficiency of the first cycle and the second cycle were found to be 63% and 85.6%, respectively. After third cycle, the coulombic efficiency increased to 91%. Fig. 3 shows the impedance spectra of EDE solutions and PADA gel electrolytes before cycle and after the 1st, 3rd and 10th cycles at room temperature. The impedance spectra at various cycle times were composed of the semicircles in the range of high frequencies and straight sloping lines in the range of low frequencies. The equivalent circuit modeled by ZView software is shown in Fig. 4. Herein, Rb is the bulk resistance of the cell, which reflects a combined resistance of the PADA gel electrolyte and the electrodes. Rsei and Csei are resistance and capacitance of the SEI on the graphite surfaces, which correspond to the semicircle of the impedance spectra in the area of high frequencies. Rct and Cdl are the charge-transfer resistance and its relative

Fig. 4. The equivalent circuit for the impedance spectra of the Li/graphite cell.

double-layer capacitance, which correspond to the semicircle in the range of medium frequencies of impedance spectra. We note that the Rct is more correlated to the kinetics of the electrochemical reactions on the electrode. The semicircles in the range of middle frequencies do not clearly appear in the impedance spectra because electrode reactions do not occur in the case of a cell potential of 1.5 V, which resulted in a high Rct . W is the Warburg impedance correlated to a combined effect of the diffusion of lithium ions on the interface of electrode–electrolyte, which corresponds to the straight line in the range of low frequencies of the impedance spectra [13,24]. In the case of EDE solutions and PADA gel electrolytes, the diameters of the semicircles in the range of high frequencies of the impedance spectra were small before the charge–discharge cycles. The values of Rsei were 15  and 17 , respectively. However, their Rsei significantly increased to respectively 25  and 36  after the first cycle. This demonstrates that the SEI was formed on the graphitic electrode surfaces [25]. The growth rates of Rsei became slow with an increase of subsequent cycle times. In the case of PADA gel electrolytes, the value of Rsei was 38  after the 10th cycle. This can be ascribed to continuous growth of SEI with an increase of subsequent cycle times. In addition, the impedance spectra of graphitic electrodes in PADA gel electrolytes are similar to that in the EDE solution electrolyte, which further suggests that the presence of PADA polymer seems to have no influence on the composition of SEI and its buildup mechanism. Fig. 5 shows FE-SEM images of the graphite electrode surfaces before and after charge–discharge cycles when EDE solution (images A–A10) and PADA gel (images B–B10) electrolytes were employed. As shown in Fig. 5A and B,

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Fig. 5. FE-SEM images of the surface of graphite electrodes. (A and B) Pristine graphite electrode surfaces in EDE solution and PADA gel electrolytes, respectively. (A1 and B1) Graphite electrode surfaces after the first charge–discharge cycle in EDE solution and PADA gel electrolytes, respectively. A1E and B1E are enlarged images of A1 and B1, respectively. (A10 and B10) Graphite electrode surfaces after 10 charge–discharge cycles in EDE solution and PADA gel electrolytes, respectively.

smooth surfaces of both graphite electrodes are observed, which implies that no SEI is formed before the cycle. After the first charge–discharge cycles, granular solids on the surfaces of both graphite electrodes were clearly observed. Fig. 5A1 shows compact granular solids with a diameter of about 3 ␮m

on surfaces of the graphite electrode when EDE solution electrolytes were employed. Contrastingly, sparse granular solids with diameters of about 0.3 ␮m were observed on the surface of the graphite electrode when PADA gel electrolytes were used (Fig. 5B1). Apparently, the insoluble granules arise from the

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decomposition of EC [14,22]. The enlarged images (Fig. 5A1E and B1E) of FE-SEM further indicate that the decomposition product of EC deposited on the surfaces of the graphite electrode was sheet-like. After 10 cycles of charge–discharge, the graphite electrodes were covered completely by integrated SEI with a certain thickness (Fig. 5A10 and B10). By comparison, the SEI formed in the PADA gel electrolyte showed a rough surface consisting of compact and small sheet-like deposition. This difference in the morphology of SEI can be attributed to the different processes of deposition. In the case of the EDE solution electrolyte, the decomposition product was directly deposited on the surface of the graphite electrode. In the case of the PADA gel electrolyte, the deposition was taking place through the three dimensional network of the polymer matrix. Thus, the SEI formed in the PADA gel electrolyte shows a rough surface consisting of small, compact solid matters. The morphology analysis of SEI suggests that the SEI formed in the PADA gel electrolyte can effectively prevent the organic electrolyte from further decomposition and intercalating into the graphite electrode. This is consistent with the CV and EIS analysis and explains the reason for the rather similar ion conductivities of the PADA gel and EDE solution electrolytes. 4. Conclusion An ionically crosslinked PADA gel electrolyte was prepared in a solvent mixture of EC, DMC and EMC (1:1:1, v/v) containing 1 mol/L of LiPF6 . The results of electrochemical impedance spectroscopic measurements indicated that its ion conductivities are rather closed to those of the corresponding liquid electrolytes in the absence of PADA polymer. The SEI formed in PADA gel electrolytes and corresponding liquid electrolytes on the surfaces of graphite electrodes has been identified by cyclic voltammetry, impedance spectroscopy and FE-SEM. The results indicate that the compact SEI formed in PADA gel electrolytes during the initial several cycles of charge–discharge is similar to that formed in liquid electrolyte, which effectively prevented the organic electrolyte from further decomposition and intercalation into the graphite electrode. However, the morphology analysis of SEI suggests that the surface of SEI formed in the PADA gel elec-

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trolyte consists of smaller solid matters and it is rougher than the surface of SEI formed in liquid electrolyte. Acknowledgement The research work was financially supported by the National Natural Science Foundation of China (No. 20474022). References [1] Y. Wang, P.B. Balbuena, J. Phys. Chem. B 106 (2002) 4486. [2] J.S. Kim, Y.T. Park, J. Power Sources 91 (2000) 172. [3] M. Stjerndahl, H. Bryngelsson, T. Gustafsson, J.T. Vaughey, M.M. Thackeray, K. Edstr¨om, Electrochim. Acta 52 (2007) 4947. [4] Y. Wang, P.B. Balbuena, J. Phys. Chem. A 106 (2002) 9582. [5] O. Borodin, G.D. Smith, P. Fan, J. Phys. Chem. B 110 (2006) 22773. [6] S. Leroy, H. Martinez, R. Dedryve‘re, D. Lemordant, D. Gonbeau, Appl. Surf. Sci. 253 (2007) 4895. [7] A. Naji, J. Ghanbaja, P. Willmann, D. Billaud, J. Power Sources 81–82 (1999) 207. [8] Y.N. Li, J. Yang, Z. Jiang, J. Phys. Chem. Solids 67 (2006) 882. [9] L. Fransson, T. Eriksson, K. Edstr¨om, T. Gustafsson, J.O. Thomas, J. Power Sources 101 (2001) 1. [10] H. Lee, S. Choi, S. Choi, H.J. Kim, Y. Choi, S. Yoon, J.J. Cho, Electrochem. Commun. 9 (2007) 801. [11] H. Schranzhofer, J. Bugajski, H.J. Santner, C. Korepp, K.C. M¨oller, J.O. Besenhard, M. Winter, W. Sitte, J. Power Sources 153 (2006) 391. [12] H. Zheng, K. Jiang, T. Abe, Z. Ogumi, Carbon 44 (2006) 203. [13] S.S. Zhang, K. Xu, T.R. Jow, Electrochim. Acta 51 (2006) 1636. [14] K. Tasaki, J. Phys. Chem. B 109 (2005) 2920. [15] S.B. Lee, S. Pyun, Carbon 40 (2002) 2333. [16] Y. Kang, N. Cho, K.A. Noh, J. Kim, C. Lee, J. Power Sources 146 (2005) 171. [17] Y. Zhao, W. Chen, J. Xia, W. Liu, Y. Yang, H. Xu, X. Yang, Macromol. Chem. Phys. 207 (2006) 1674. [18] W. Chen, H. Tang, Z. Ou, H. Wang, Y. Yang, Electrochim. Acta 53 (2007) 2065. [19] Z. Yue, I.J. McEwen, J.M.G. Cowie, Solid State Ionics 156 (2003) 155. [20] C. Capiglia, Y. Saito, H. Yamamoto, H. Kageyama, P. Mustarelli, Electrochim. Acta 45 (2000) 1341. [21] A. Magistris, E. Quartarone, P. Mustarelli, Y. Saito, H. Kataoka, Solid State Ionics 152–153 (2002) 347. [22] C.R. Yang, Y.Y. Wang, C.C. Wan, J. Power Sources 72 (1998) 66. [23] D.F. Liu, J. Nie, W.C. Guan, H.Q. Duan, L.M. Zhuo, Solid State Ionics 167 (2004) 131. [24] S.S. Zhang, J. Power Sources 163 (2007) 713. [25] S.S. Zhang, T.R. Jow, K. Amine, G.L. Henriksen, J. Power Sources 107 (2002) 18.