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Aliphatic thioether polymers as novel cathode active materials for rechargeable lithium battery
Electrochemistry Communications 10 (2008) 1551–1554
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Aliphatic thioether polymers as novel cathode active materials for rechargeable lithium battery Jingyu Zhang, Lingbo Kong, Lizhi Zhan, Jing Tang, Hui Zhan, Yunhong Zhou, Caimao Zhan * College of Chemistry and Molecular Sciences, Wuhan University, Wuhan City 430072, PR China
a r t i c l e
i n f o
Article history: Received 1 July 2008 Received in revised form 30 July 2008 Accepted 12 August 2008 Available online 17 August 2008 Keywords: Rechargeable lithium battery Cathode active material Aliphatic thioether polymer
a b s t r a c t Two aliphatic thioether polymers, poly[methanetetryl-tetra(thiomethylene)] (PMTTM) and poly(2,4dithiopentanylene) (PDTP) were designed, synthesized, characterized and tested as cathode active materials. The chemical structure of polymers was confirmed by FT-IR, FT-Raman, and XPS spectral analysis. Both polymers were found to have electrochemical activity as cathode materials for rechargeable lithium battery by the electrochemical tests. The specific capacity of PMTTM was 504 mA h g 1 at the third cycle and faded to 200 mA h g 1 after 10 cycles; PDTP showed low and stable specific capacity around 100 mA h g 1 even after 50 cycles. The specific capacity of fully saturated aliphatic thioether polymers demonstrated that thioether bonds offered energy storage. It was proposed that thioether bond was oxidized to form thioether cations with the help of ether solvents. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Since Liu and Visco et al. [1,2] found organic disulfide compounds could be used as cathode active materials for rechargeable lithium battery in the late of 1980s, many researches have been focused on the development of organic disulfide materials [3–14]. The electrode reactions of the disulfide compounds are based on the reversible cleavage-combination of S–S bond. However, we found recently that two organic thioether compounds, poly[1,4di(1,3-dithiolan-2-yl)benzene] and poly(2-phenyl-1,3-dithiolane), could also store energy as cathode active materials [15], and thioether function group was proposed as a functional group to offer electrode reactions: C–S–C M C–S+–C. Both the two polymers are composed of conjugated polyphenylene chains with thiolane as pendents. Since polyphenylene is well known cathode active material with specific capacity of tens mA hg 1 [16], it is essential to examine whether the thioether structure alone is enough to offer specific capacity in polymers. To investigate the redox function of thioether groups, two novel aliphatic thioether polymers, poly[methanetetryl-tetra(thiomethylene)] (PMTTM) and poly(2,4dithiopentanylene) (PDTP), were designed, synthesized, and their electrochemical activity was also tested. There are three kinds of bonds in PMTTM and PDTP: C–C, C–H, and C–S–C bonds. Since the saturated C–C and C–H bonds cannot afford redox reaction, thioether bond is the only possible electrochemical active function group among them. Therefore, we can estimate the electrochemi* Corresponding author. Tel.: +86 27 63802505; fax: +86 27 68764659. E-mail address: [email protected] (C. Zhan). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.08.012
cal activity of thioether function group by electrochemical behaviors of PMTTM and PDTP. 2. Experimental 2.1. Materials Sodium, anhydrous ethanol, perbromomethane and bibromomethane were AR purity and ethane-1,2-dithiol was 98% purity. All reagents were used as received unless otherwise indicated. The synthetic routes and structures of the polymers are described in Scheme 1a. A typical synthetic procedure of PMTTM was described as below. In a three-neck flask equipped with gas conductor, condenser, and magnetic stirrer, sodium granules (2.0 g, 87 mmol) were added to 30 ml anhydrous ethanol and stirred under nitrogen atmosphere at room temperature till sodium dissolved completely. Ethane-1,2-dithiol (3.0 ml, 35 mmol) was added to the solution and the reactants were stirred for one more hour at room temperature. Then 10 ml ethanol solution of perbromomethane (5.3 g, 16 mmol) was added dropwise to the flask and the reaction mixture was stirred overnight. The white solid was collected by filtration and washed with distilled water (50 ml 3). The product was dried in oven at 50 °C for 12 h and 1.98 g (63% yield). IR: 2915, 1411, 1182, 1108, 1051, 733, and 676 cm 1; Raman: 2928, 1398, 761, 690, 635, 510, 377, 348, and 283 cm 1. The synthetic procedure of PDTP was same to that of PMTTM; but bibromomethane was used instead of perbromomethane. The product was white solid and the yield was 57%. IR: 2924, 1426,
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J. Zhang et al. / Electrochemistry Communications 10 (2008) 1551–1554
Scheme 1. (a) Synthetic routes of PMTTM and PDTP and (b) possible charge– discharge process of thioether in ether solvent.
1185, 1143, 719, and 673 cm 1; Raman: 2934, 2901, 1401, 749, 688, 627, 504, 371, 336, and 279 cm 1. 2.2. Apparatus and procedures FT-IR spectra were recorded on a NICOLET AVATAR 360 FT-IR spectrometer with KBr pellets and FT-Raman was recorded on a Nexus 670-FT-Raman Module spectrometer. X-ray photoelectron spectroscopy (XPS) analysis data were obtained with the magnesium source radiation (1253.6 eV) operating at 10 kV and 10 mA. Thermal analysis of all samples were performed on a Diamond DSC TG-DTA 6300 instrument under nitrogen flow at a heating rate of 20 °C/min. XRD patterns of the powder specimens were obtained on a D8ADVANCE X-ray diffractometer. The cyclic voltammetry (CV) tests were carried out on CH instruments electrochemical workstation (CHI 660B) at a scan rate of 50 lV s 1 between 1.5 and 4.0 V at room temperature. The cathodes were prepared by mixing specific polymer powders with carbon black and Teflon in a weight ratio of 4:4:2, the Li film was used as the reference electrode and counter electrode with 1 M LiN(CF3SO2)2 (LiTFSI) in 1,2-dimethoxyethane (DME)/1,3-dioxolane (DOL) (1:2, by weight) as electrolyte. For electrochemical evaluation, the charge–discharge tests were performed on Land Battery test system. Working cathodes were prepared by mixing specific polymer powders with carbon black and Teflon in a weight ratio of 4:4:2. Lithium foil was used as the anode and the tests were processed at a rate of 50 mA/g in 1 M LiTFSI in DME/DOL (1:2, by weight) solution. The cut-off voltages were 1.4 and 4.2 V. 3. Results and discussion 3.1. Material characterization The polymers were insoluble and were characterized by FT-IR, FT-Raman, XPS, XRD, and TGA. PMTTM and PDTP showed similar FT-IR and FT-Raman spectra, this is in good agreement to their molecular structures. The FT-IR spectrum of PDTP (Fig. 1a) show bonds at 2924 cm 1 and shoulder at 2850 cm 1, which are stretching vibration of CH2. The presence of C–S bond in the polymer backbone is supported by the absorption peaks at 718 and 673 cm 1 (mC–S). There is no peak between 600–500 cm 1 and at 2480 cm 1, indicating that the polymers do not contain C–Br and S–H bonds. The FT-Raman spectrum of PDTP (Fig. 1b) showed the absorption peaks at 2934 and 2901 cm 1, which is attributed to
Fig. 1. (a) FT-IR spectra of PMTTM and PDTP and (b) Raman spectra of PDTP.
CH2 stretching vibration. The peaks at 688 and 627 cm 1 are assigned as C–S stretching vibration, while the peaks at 279, 335, 371 cm 1 are assigned as C–S–C skeletal deformation [17,18]. The results suggest that the polymerizations between the bromomethanes and ethane-1,2-dithiol were successful. The X-ray photoelectron spectroscopy (XPS) spectra of PMTTM and PDTP are also examined. The S2p peaks of both two polymers are located at 163.6 eV, indicating the existing of thioether sulfur in the polymers [19,20]. The S2p spectra also show that sulfur in polymers has only one valence state, which suggest the completely reaction of Na2S. The FT-IR, FT-Raman, XPS spectra results confirmed that both two polymers were assuredly obtained. Thermal gravity analysis (TGA) of PMTTM and PDTP showed 5% weight loss at 211 and 71 °C, respectively. The decomposition temperature of PMTTM closes to that of our previous reported thioether polymer [15]. However, PDTP had much lower thermal stability than PMTTM, due to existence of low molecular weight oligomer. The X-ray diffraction (XRD) patterns showed amorphous structure for PMTTM and semicrystalline for PDTP, respectively. The morphology was consistent with their molecular structure (Scheme 1a): PMTTM was hyperbranched with S atoms in 4dimension that was difficult to form order stacking; nevertheless, PDTP had linear molecular structure and easy to form regular stacking. 3.2. Electrochemical properties Fig. 2 exhibits the cyclic voltammograms (CVs) of the first cycle of PMTTM and PDTP. PMTTM and PDTP have similar CV curves,
J. Zhang et al. / Electrochemistry Communications 10 (2008) 1551–1554
Fig. 2. Cyclic voltammograms of PMTTM and PDTP cathodes vs. Li+/Li electrodes. Li film as the reference and counter electrode with 1 M LiTFSI in DME/DOL; scan rate 50 lV s 1.
which could be attributed to their similar molecular structure. Both two polymers have narrow oxidation peaks and broad reduction peaks, indicating simple oxidative reaction and complicated reduction. The oxidation peak of PMTTM appears at 2.20 and 3.9 V vs. Li/Li+ and the reduction peak between 2.00 and 1.89 V vs. Li/Li+. The oxidation peak of PDTP appears at 2.16 and 3.9 V vs. Li/Li+ and the reduction peak between 1.85 and 1.72 V vs. Li/Li+. The cycling capacity of the polymers is shown in Fig. 3. PMTTM displayed specific capacity of 504 mA h g 1 at the third cycle, which decreased rapidly and maintained the value around 200 mA h g 1 at the 16th cycle. Similarly, PDTP exhibited a maximal specific capacity of 174 mA h g 1 and a stabilized specific capacity about 100 mA h g 1 between 16th and 50th cycles. The cycling capacity of aliphatic thioether polymers suggested that polymer with thioether function groups alone can offer energy storage without disulfide bonds or conjugated polymeric main chain. The cathode activity of aliphatic thioether polymers is significant and has not been reported in literature to our knowledge. There are only three kinds of bonds in PMTTM and PDTP: C–C, C–H, and C–S–C bonds. As we known, the saturated C–C and C–H bonds cannot afford redox reaction; hence we can figure out that thioe-
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ther bond is electrochemical active function group to offering energy storage. The oxidation of thioether have been studied and reported in literatures [21–24], which illuminated the formation and existence of thioether cation. The oxidation of thioether can be facilitated by electro-donating group [21], and thioether cation can be stabilized by electron donation [23]. The specific capacity results of PMTTM and PDTP were tested in ether solvent of DME and DOL. As we know ether is electron donor, we guess that ether solvents may be helpful to the oxidation of thioether group (Scheme 1b). If the electrode reaction is a single electron redox reaction for one S atom, the theoretic specific capacity of PMTTM and PDTP should be 570 and 505 mA h g 1, respectively. Obviously, the maximal specific capacity of PMTTM was close to its theoretic specific capacity. But the specific capacity of PDTP was far less than its theoretic value. We presumed this difference was caused by the different morphology of polymers: the amorphous PMTTM was favorable to the transformation of Li+ while the crystalline PDTP might not be conductive [25]. These results suggested that amorphous structure is better than crystalline for polymeric cathode active material in lithium battery. The charge–discharge curve of PMTTM showed two charge plateaus (Fig. 3), which were similar to those of aromatic thioether polymers [15]. The voltage of the discharge plateaus of PMTTM decreased from 2.25 V at the 7th cycle to 1.75 V at the 50th cycle, while the voltage of charge increased from 7th to 50th cycle. For aliphatic thioether polymers, limitations such as low voltage efficiency and voltage fading exist, and we guess this is caused by the electric insulation property of fully saturated polymers. 4. Conclusions In this study, we designed, synthesized and characterized two aliphatic thioether polymers, poly[methanetetryl-tetra(thiomethylene)] and poly(2,4-dithiopentanylene). Their electrochemical behaviors were examined and main conclusions were made as below: (1) Fully saturated aliphatic thioether polymers PMTTM and PDTP showed good charge–discharge properties, and high specific capacity. The results suggest that thioether was the electrode reaction group, precisely, S atom offered electron storage during charge–discharge process. (2) For aliphatic thioether polymers, amorphous PMTTM possessed higher specific capacity than semicrystalline PDTP, indicating amorphous structure was beneficial to the electron storage process. References
Fig. 3. Specific capacity of the polymers vs. cycle numbers (current density 50 mA g 1). The inset shows the charge–discharge curves of PMTTM.
[1] M. Liu, S.J. Visco, L.C. De Jonghe, J. Electrochem. Soc. 136 (1989) 2570–2575. [2] S.J. Visco, C.C. Mailhe, L.C. De Jonghe, M.B. Armand, J. Electrochem. Soc. 136 (1989) 661–664. [3] H. Tsutsumi, K. Okada, T. Oishi, Electrochim. Acta 41 (1996) 2657–2659. [4] H. Tsutsumi, K. Okada, K. Fujita, T. Oishi, J. Power Sources 68 (1997) 735–738. [5] K. Naoi, K. Kawase, M. Mori, M. Komiyama, J. Electrochem. Soc. 144 (1997) L173–L175. [6] H. Tsutsumi, Y. Oyari, K. Onimura, T. Oishi, J. Power Sources 92 (2001) 228– 233. [7] H. Uemachi, Y. Iwasa, T. Mitani, Electrochim. Acta 46 (2001) 2305–2312. [8] L.M. Huang, T.C. Wen, C.H. Yang, Mater. Chem. Phys. 77 (2002) 434–441. [9] J.M. Pope, T. Sato, E. Shoji, N. Oyama, K.C. White, D.A. Buttry, J. Electrochem. Soc. 149 (2002) A939–A952. [10] T. Inamasu, D. Yoshitoku, Y. Sumi-otorii, H. Tani, N. Ono, J. Electrochem. Soc. 150 (2003) A128–A132. [11] L.J. Xue, J.X. Li, S.Q. Hu, M.X. Zhang, Y.H. Zhou, C.M. Zhan, Electrochem. Commun. 5 (2003) 903–906. [12] Y. Su, Y. Niu, Y. Xiao, M. Xiao, Z. Liang, K. Gong, J. Poly. Sci. Part A: Polym. Chem. 42 (2004) 2329–2339. [13] M. Amaike, T. Iihama, Synth. Metals 156 (2006) 239–243. [14] S. Deng, L. Kong, G. Hua, T. Wu, D. Li, Y. Zhou, Z. Li, Electrochim. Acta 51 (2006) 2589–2593. [15] J.Y. Zhang, L.B. Kong, L.Z. Zhan, J. Tang, H. Zhan, Y.H. Zhou, C.M. Zhan, J. Power Sources 168 (2007) 278–281.
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[16] Petr Novák, Klaus Müller, K.S.V. Santhanam, Otto Haas, Chem. Rev. 97 (1997) 207–281. [17] D.W. Scott, H.L. Finke, J.P. Mcctjllough, M.E. Gross, K.D. Williamaon, G. Waddington, H.M. Huffman, J. Am. Chem. Soc. 73 (1951) 261. [18] M. Hayashi, T. Shimanouchi, S. Mizushima, J. Chem. Phys. 26 (1957) 608. [19] J. Yi, S.H. Goh, A.T.S. Wee, Macromolecules 34 (2001) 7411–7415. [20] T. Grzybek, R. Pietrzak, H. Wachowska, Energy & Fuels 18 (2004) 804–809.
[21] R.S. Glass, J.R. Duchek, J.T. Klug, G.S. Wilson, J. Am. Chem. Soc. 99 (1977) 7349– 7350. [22] W.K. Musker, P.B. Roush, J. Am. Chem. Soc. 98 (1976) 6745–6746. [23] D.W. Werst, J. Am. Chem. Soc. 113 (1991) 4345–4346. [24] G.S. Wilson, D.D. Swanson, J.T. Klug, R.S. Glass, M.D. Ryan, W.K. Musker, J. Am. Chem. Soc. 101 (1979) 1040–1042. [25] W.H. Meyer, Adv. Mater. 10 (1998) 439–448.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Aliphatic thioether polymers as novel cathode active materials for rechargeable lithium battery Jingyu Zhang, Lingbo Kong, Lizhi Zhan, Jing Tang, Hui Zhan, Yunhong Zhou, Caimao Zhan * College of Chemistry and Molecular Sciences, Wuhan University, Wuhan City 430072, PR China
a r t i c l e
i n f o
Article history: Received 1 July 2008 Received in revised form 30 July 2008 Accepted 12 August 2008 Available online 17 August 2008 Keywords: Rechargeable lithium battery Cathode active material Aliphatic thioether polymer
a b s t r a c t Two aliphatic thioether polymers, poly[methanetetryl-tetra(thiomethylene)] (PMTTM) and poly(2,4dithiopentanylene) (PDTP) were designed, synthesized, characterized and tested as cathode active materials. The chemical structure of polymers was confirmed by FT-IR, FT-Raman, and XPS spectral analysis. Both polymers were found to have electrochemical activity as cathode materials for rechargeable lithium battery by the electrochemical tests. The specific capacity of PMTTM was 504 mA h g 1 at the third cycle and faded to 200 mA h g 1 after 10 cycles; PDTP showed low and stable specific capacity around 100 mA h g 1 even after 50 cycles. The specific capacity of fully saturated aliphatic thioether polymers demonstrated that thioether bonds offered energy storage. It was proposed that thioether bond was oxidized to form thioether cations with the help of ether solvents. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Since Liu and Visco et al. [1,2] found organic disulfide compounds could be used as cathode active materials for rechargeable lithium battery in the late of 1980s, many researches have been focused on the development of organic disulfide materials [3–14]. The electrode reactions of the disulfide compounds are based on the reversible cleavage-combination of S–S bond. However, we found recently that two organic thioether compounds, poly[1,4di(1,3-dithiolan-2-yl)benzene] and poly(2-phenyl-1,3-dithiolane), could also store energy as cathode active materials [15], and thioether function group was proposed as a functional group to offer electrode reactions: C–S–C M C–S+–C. Both the two polymers are composed of conjugated polyphenylene chains with thiolane as pendents. Since polyphenylene is well known cathode active material with specific capacity of tens mA hg 1 [16], it is essential to examine whether the thioether structure alone is enough to offer specific capacity in polymers. To investigate the redox function of thioether groups, two novel aliphatic thioether polymers, poly[methanetetryl-tetra(thiomethylene)] (PMTTM) and poly(2,4dithiopentanylene) (PDTP), were designed, synthesized, and their electrochemical activity was also tested. There are three kinds of bonds in PMTTM and PDTP: C–C, C–H, and C–S–C bonds. Since the saturated C–C and C–H bonds cannot afford redox reaction, thioether bond is the only possible electrochemical active function group among them. Therefore, we can estimate the electrochemi* Corresponding author. Tel.: +86 27 63802505; fax: +86 27 68764659. E-mail address: [email protected] (C. Zhan). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.08.012
cal activity of thioether function group by electrochemical behaviors of PMTTM and PDTP. 2. Experimental 2.1. Materials Sodium, anhydrous ethanol, perbromomethane and bibromomethane were AR purity and ethane-1,2-dithiol was 98% purity. All reagents were used as received unless otherwise indicated. The synthetic routes and structures of the polymers are described in Scheme 1a. A typical synthetic procedure of PMTTM was described as below. In a three-neck flask equipped with gas conductor, condenser, and magnetic stirrer, sodium granules (2.0 g, 87 mmol) were added to 30 ml anhydrous ethanol and stirred under nitrogen atmosphere at room temperature till sodium dissolved completely. Ethane-1,2-dithiol (3.0 ml, 35 mmol) was added to the solution and the reactants were stirred for one more hour at room temperature. Then 10 ml ethanol solution of perbromomethane (5.3 g, 16 mmol) was added dropwise to the flask and the reaction mixture was stirred overnight. The white solid was collected by filtration and washed with distilled water (50 ml 3). The product was dried in oven at 50 °C for 12 h and 1.98 g (63% yield). IR: 2915, 1411, 1182, 1108, 1051, 733, and 676 cm 1; Raman: 2928, 1398, 761, 690, 635, 510, 377, 348, and 283 cm 1. The synthetic procedure of PDTP was same to that of PMTTM; but bibromomethane was used instead of perbromomethane. The product was white solid and the yield was 57%. IR: 2924, 1426,
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J. Zhang et al. / Electrochemistry Communications 10 (2008) 1551–1554
Scheme 1. (a) Synthetic routes of PMTTM and PDTP and (b) possible charge– discharge process of thioether in ether solvent.
1185, 1143, 719, and 673 cm 1; Raman: 2934, 2901, 1401, 749, 688, 627, 504, 371, 336, and 279 cm 1. 2.2. Apparatus and procedures FT-IR spectra were recorded on a NICOLET AVATAR 360 FT-IR spectrometer with KBr pellets and FT-Raman was recorded on a Nexus 670-FT-Raman Module spectrometer. X-ray photoelectron spectroscopy (XPS) analysis data were obtained with the magnesium source radiation (1253.6 eV) operating at 10 kV and 10 mA. Thermal analysis of all samples were performed on a Diamond DSC TG-DTA 6300 instrument under nitrogen flow at a heating rate of 20 °C/min. XRD patterns of the powder specimens were obtained on a D8ADVANCE X-ray diffractometer. The cyclic voltammetry (CV) tests were carried out on CH instruments electrochemical workstation (CHI 660B) at a scan rate of 50 lV s 1 between 1.5 and 4.0 V at room temperature. The cathodes were prepared by mixing specific polymer powders with carbon black and Teflon in a weight ratio of 4:4:2, the Li film was used as the reference electrode and counter electrode with 1 M LiN(CF3SO2)2 (LiTFSI) in 1,2-dimethoxyethane (DME)/1,3-dioxolane (DOL) (1:2, by weight) as electrolyte. For electrochemical evaluation, the charge–discharge tests were performed on Land Battery test system. Working cathodes were prepared by mixing specific polymer powders with carbon black and Teflon in a weight ratio of 4:4:2. Lithium foil was used as the anode and the tests were processed at a rate of 50 mA/g in 1 M LiTFSI in DME/DOL (1:2, by weight) solution. The cut-off voltages were 1.4 and 4.2 V. 3. Results and discussion 3.1. Material characterization The polymers were insoluble and were characterized by FT-IR, FT-Raman, XPS, XRD, and TGA. PMTTM and PDTP showed similar FT-IR and FT-Raman spectra, this is in good agreement to their molecular structures. The FT-IR spectrum of PDTP (Fig. 1a) show bonds at 2924 cm 1 and shoulder at 2850 cm 1, which are stretching vibration of CH2. The presence of C–S bond in the polymer backbone is supported by the absorption peaks at 718 and 673 cm 1 (mC–S). There is no peak between 600–500 cm 1 and at 2480 cm 1, indicating that the polymers do not contain C–Br and S–H bonds. The FT-Raman spectrum of PDTP (Fig. 1b) showed the absorption peaks at 2934 and 2901 cm 1, which is attributed to
Fig. 1. (a) FT-IR spectra of PMTTM and PDTP and (b) Raman spectra of PDTP.
CH2 stretching vibration. The peaks at 688 and 627 cm 1 are assigned as C–S stretching vibration, while the peaks at 279, 335, 371 cm 1 are assigned as C–S–C skeletal deformation [17,18]. The results suggest that the polymerizations between the bromomethanes and ethane-1,2-dithiol were successful. The X-ray photoelectron spectroscopy (XPS) spectra of PMTTM and PDTP are also examined. The S2p peaks of both two polymers are located at 163.6 eV, indicating the existing of thioether sulfur in the polymers [19,20]. The S2p spectra also show that sulfur in polymers has only one valence state, which suggest the completely reaction of Na2S. The FT-IR, FT-Raman, XPS spectra results confirmed that both two polymers were assuredly obtained. Thermal gravity analysis (TGA) of PMTTM and PDTP showed 5% weight loss at 211 and 71 °C, respectively. The decomposition temperature of PMTTM closes to that of our previous reported thioether polymer [15]. However, PDTP had much lower thermal stability than PMTTM, due to existence of low molecular weight oligomer. The X-ray diffraction (XRD) patterns showed amorphous structure for PMTTM and semicrystalline for PDTP, respectively. The morphology was consistent with their molecular structure (Scheme 1a): PMTTM was hyperbranched with S atoms in 4dimension that was difficult to form order stacking; nevertheless, PDTP had linear molecular structure and easy to form regular stacking. 3.2. Electrochemical properties Fig. 2 exhibits the cyclic voltammograms (CVs) of the first cycle of PMTTM and PDTP. PMTTM and PDTP have similar CV curves,
J. Zhang et al. / Electrochemistry Communications 10 (2008) 1551–1554
Fig. 2. Cyclic voltammograms of PMTTM and PDTP cathodes vs. Li+/Li electrodes. Li film as the reference and counter electrode with 1 M LiTFSI in DME/DOL; scan rate 50 lV s 1.
which could be attributed to their similar molecular structure. Both two polymers have narrow oxidation peaks and broad reduction peaks, indicating simple oxidative reaction and complicated reduction. The oxidation peak of PMTTM appears at 2.20 and 3.9 V vs. Li/Li+ and the reduction peak between 2.00 and 1.89 V vs. Li/Li+. The oxidation peak of PDTP appears at 2.16 and 3.9 V vs. Li/Li+ and the reduction peak between 1.85 and 1.72 V vs. Li/Li+. The cycling capacity of the polymers is shown in Fig. 3. PMTTM displayed specific capacity of 504 mA h g 1 at the third cycle, which decreased rapidly and maintained the value around 200 mA h g 1 at the 16th cycle. Similarly, PDTP exhibited a maximal specific capacity of 174 mA h g 1 and a stabilized specific capacity about 100 mA h g 1 between 16th and 50th cycles. The cycling capacity of aliphatic thioether polymers suggested that polymer with thioether function groups alone can offer energy storage without disulfide bonds or conjugated polymeric main chain. The cathode activity of aliphatic thioether polymers is significant and has not been reported in literature to our knowledge. There are only three kinds of bonds in PMTTM and PDTP: C–C, C–H, and C–S–C bonds. As we known, the saturated C–C and C–H bonds cannot afford redox reaction; hence we can figure out that thioe-
1553
ther bond is electrochemical active function group to offering energy storage. The oxidation of thioether have been studied and reported in literatures [21–24], which illuminated the formation and existence of thioether cation. The oxidation of thioether can be facilitated by electro-donating group [21], and thioether cation can be stabilized by electron donation [23]. The specific capacity results of PMTTM and PDTP were tested in ether solvent of DME and DOL. As we know ether is electron donor, we guess that ether solvents may be helpful to the oxidation of thioether group (Scheme 1b). If the electrode reaction is a single electron redox reaction for one S atom, the theoretic specific capacity of PMTTM and PDTP should be 570 and 505 mA h g 1, respectively. Obviously, the maximal specific capacity of PMTTM was close to its theoretic specific capacity. But the specific capacity of PDTP was far less than its theoretic value. We presumed this difference was caused by the different morphology of polymers: the amorphous PMTTM was favorable to the transformation of Li+ while the crystalline PDTP might not be conductive [25]. These results suggested that amorphous structure is better than crystalline for polymeric cathode active material in lithium battery. The charge–discharge curve of PMTTM showed two charge plateaus (Fig. 3), which were similar to those of aromatic thioether polymers [15]. The voltage of the discharge plateaus of PMTTM decreased from 2.25 V at the 7th cycle to 1.75 V at the 50th cycle, while the voltage of charge increased from 7th to 50th cycle. For aliphatic thioether polymers, limitations such as low voltage efficiency and voltage fading exist, and we guess this is caused by the electric insulation property of fully saturated polymers. 4. Conclusions In this study, we designed, synthesized and characterized two aliphatic thioether polymers, poly[methanetetryl-tetra(thiomethylene)] and poly(2,4-dithiopentanylene). Their electrochemical behaviors were examined and main conclusions were made as below: (1) Fully saturated aliphatic thioether polymers PMTTM and PDTP showed good charge–discharge properties, and high specific capacity. The results suggest that thioether was the electrode reaction group, precisely, S atom offered electron storage during charge–discharge process. (2) For aliphatic thioether polymers, amorphous PMTTM possessed higher specific capacity than semicrystalline PDTP, indicating amorphous structure was beneficial to the electron storage process. References
Fig. 3. Specific capacity of the polymers vs. cycle numbers (current density 50 mA g 1). The inset shows the charge–discharge curves of PMTTM.
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