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Cite this: Chem. Commun., 2013, 49, 11370 Received 30th August 2013, Accepted 15th October 2013 DOI: 10.1039/c3cc46642f
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Self-doped polypyrrole with ionizable sodium sulfonate as a renewable cathode material for sodium ion batteries† Limin Zhu, Yifei Shen, Mengying Sun, Jiangfeng Qian,* Yuliang Cao, Xinping Ai and Hanxi Yang*
www.rsc.org/chemcomm
A Na-host cathode is developed by grafting the polypyrrole chains with ionizable sodium sulfonate. Due to the immobile p-doping of organic anions, the self-doped polymer can act as a Na-host for reversible Na insertion–extraction reaction, thus offering a low cost and renewable organic cathode for Na ion battery applications.
Rapid development of renewable energy systems requires effective means of electric energy storage (EES). Though many types of conventional and advanced rechargeable batteries have been investigated for the grid-scale EES applications, they are all constrained by the cost and material resources, because all of them are based on the redox chemistry of rare metals.1–3 To meet the need for widespread EES applications, it is therefore a great challenge to develop low cost, renewable batteries by use of environmentally benign, inexpensive and earthabundant materials.4,5 Na-ion batteries seem to be such a new battery technology, because of their widespread availability and low cost of Na resources. In particular, if Na-ion batteries could be constructed using redox-active polymers, it would bring about a great benefit for EES applications because of their low cost and material sustainability. However, though many types of redoxactive polymer are explored for electrochemical capacitor and battery materials,6,7 only a few of them have been adopted for Na-ion batteries.8–11 A difficulty in developing the polymeric Na-storage cathodes is that the redox reactions in these polymer cathodes usually take place through a p-doping–dedoping mechanism of large electrolyte anions, which suffers from a low doping degree and slow kinetics, leading to a poor capacity utilization of the polymer chains. In addition, all the p-doped polymers are virtually not a Na-storage host and cannot act as a Na reservoir to construct a ‘‘rocking chair type’’ Na-ion battery. To enable effective Na storage in the polymeric cathodes, we propose here a new material design by self-doping
College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China. E-mail: [email protected]; Fax: +86-27-68754526; Tel: +86-27-68754526 † Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/c3cc46642f
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(graft copolymerizing) the redox polymers with an ionizable sodium organic salt, thus changing the redox reaction mechanism of the polymers from conventional p-doping–dedoping processes of large electrolyte anions to the insertion–extraction reactions of small Na+ cations. As an example, we synthesized poly(pyrrole-co-(sodium-3-(pyrrol-lyl) propanesulphonate)) (PP-PS) with ionizable Na ions and investigated its reversible Na insertion behavior. Since the sulphonate anions are chemically grafted on the polypyrrole chains, the redox reaction of the polymer must take place through the insertion and extraction of ionizable Na+ cations, thus making the copolymer a polymeric Na host cathode material. The PP-PS copolymer was synthesized by oxidative copolymerization of pyrrole and sodium 3-(pyrrol-l-yl) propanesulfonate in a two-step reaction as shown in Scheme 1. The resulting PP-PS sample appears as a dark black powder, almost insoluble in commonly used organic and aqueous electrolytes. Fig. 1 shows the SEM image and the FT-IR spectrum of the as-prepared PP-PS powder. It could be seen from Fig. 1a that the copolymer
Scheme 1
Synthetic route of the PP-PS copolymer.
Fig. 1 Structural characterization of the as-prepared PP-PS copolymer: (a) SEM image; (b) FT-IR spectrum.
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Fig. 2 Cyclic voltammograms of the PP-PS electrode in 1.0 mol L (v/v = 1 : 1) electrolyte, measured at a scan rate of 10 mV s–1.
ChemComm
1
NaPF6 + EC/DEC
emerged as an irregular aggregate of 200–500 nm sized particles. All the IR absorptions in Fig. 1b can be attributed to the characteristic bands of polypyrrole and poly(pyrrolesulphonate). The 1556 and 1473 cm 1 bands are characteristic of the antisymmetric and symmetric ring vibrations and the 1300 and 1043 cm 1 bands reflect the C–H deformation and N–H stretching vibrations of the pyrrole rings, whereas the 1212 cm 1 band features the S–O stretching of sulphonate groups grafted on the copolymer chains. Electrochemical redox properties of the PP-PS polymer were characterized by cyclic voltammetry (CV) and galvanostatic charge–discharge cycling. Fig. 2 shows the CV curves of the PP-PS sample in 1.0 mol L 1 NaPF6 in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC). As shown in the figure, the VGCF only shows a negligible background current, suggesting that neither the carbon additive nor the electrolyte have a redox activity in this potential region of +3.3 to +3.9 V, and the pair of strong CV bands can only be attributed to the reversible p-type redox reaction. Usually, the p-type redox bands reflect the doping–dedoping reactions of electrolyte anions into/from the polymer chains. However, since our PP-PS polymer is already self-doped with immobile sulphonate anions that associate with ionizable Na+ ions, it cannot undergo a redox reaction further through p-doping–dedoping of the electrolyte anions, and therefore, the p-type redox reaction in the polymer can only be attributed to reversible insertion– extraction of the ionizable Na+ ions for counterbalancing the injection/removal of electron charges. It can also be seen in the CV curves that the cathodic and anodic peaks have almost similar peak areas since the second cycle and are kept steady for subsequent 20 cycles, suggesting a high coulombic efficiency and the cycling stability of the p-type redox reaction. To test the possibility of the copolymer as a feasible cathode material, coin-type Na/PP-PS cells were assembled with excessive Na capacity design and cycled galvanostatically at a voltage interval between 4.0 V and 2.0 V. Fig. 3 shows the charge and discharge profiles of the cells. In accord with the redox potentials of the CV peaks, the polymer electrode shows average charge and discharge potential at 3.0 V and 3.3 V, respectively, reflecting reversible p-type redox reaction. At first few cycles, the reversible discharge capacity increases from its initial value This journal is
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The Royal Society of Chemistry 2013
Fig. 3 Charge–discharge profile of coin-type Na/PP-PS cells using 1.0 mol L 1 NaPF6 + EC/DEC (v/v = 1 : 1) electrolyte. The current was set constantly at 40 mA g 1. The charge–discharge capacities were calculated according to the mass weight of the PP-PS cathode.
(75 mA g 1) to its maximum capacity of 85 mA g 1 and then is kept quite steady over 100 cycles (see Fig. S1, ESI†), demonstrating a considerable cyclability. Though this capacity is slightly lower than those of Na-host cathodes of transition metal oxides,12,13 it is much higher than the realizable redox capacities usually observed for conventional p-doped polymers14,15 and in principle, this Na storage capacity could be further improved by tailoring the polymer structure with higher Na density and lightweight organic anions. More significantly, this result reveals a new approach to use polymers for Na storage cathodes by self-doping organic Na salts. It should be mentioned that the coulombic efficiency of this PP-PS cathode at initial cycles is only about B60%, much lower than those reported for p-doped polymers. This phenomenon is difficult to explain by the present understanding of the selfdoped polymers. A possible cause for this phenomenon is the irreversible co-doping of electrolyte anions for the formation of the surface electrolyte interface on the cathode. To confirm the Na storage reaction in the polymer cathode, we conducted quantitative ICP characterization of the Na+ content in the cycled PP-PS electrodes at different depths of charge and discharge. In the uncharged sample, the Na+ content is 0.0511 g g 1 (per gram of electrode material), indicating that the pristine PP-PS copolymer has a chemical formula of [C4H3N]3.7[C4H2NC3H2SO3Na], which should have a theoretical Na storage capacity of 60 mA h g 1 if only taking into account the Na+ doping–dedoping reaction. When the electrode was fully charged to the upper terminal voltage of 4.0 V, the Na+ content in the electrode decreased to 0.0043 g g 1, corresponding to 0.96 Na+ extraction from the polymer. Once discharging to 2.0 V, the Na+ content in the electrode recovered to 0.0535 g g 1, implying a re-insertion of 0.956 Na+ ions into the polymer. These data demonstrate complete extraction/ insertion of the doped Na+ ions into/from the PP-PS polymer during charge–discharge processes. However, the total reversible capacity (85 mA h g 1) observed for the PP-PS polymer cannot be accounted for only from Na insertion capacity (60 mA h g 1). To test the capacitance contribution from the carbon additive in the cathode, we measured the charge–discharge capacities of the VGCF carbon (Fig. S2, ESI†) Chem. Commun., 2013, 49, 11370--11372
11371
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ChemComm and found that the carbon contributes almost no capacity at this voltage range. In fact, during the Na insertion reaction, a small amount of electrolyte anions may also be reversibly doped into the PP-PS polymer at charge and in turn dedoped from the polymer at discharge for counterbalancing the charge change of deeply activated polypyrrole chains, thus contributing their p-doping capacity to the PP-PS polymer cathode. In summary, we developed a new approach to create polymeric Na storage cathodes, simply by self-doping redox polymers with ionisable organic Na salts. Due to the immobile p-doping of organic anions, the redox reaction of the self-doping polymer can proceed through a reversible Na insertion–extraction reaction. The PP-PS copolymer developed in this work demonstrates effective Na storage with considerably high voltage, reversible capacity and cycle life. In addition, this method could be easily adapted to a wide range of polymer structures for developing high capacity Na-host cathodes, thus enabling low cost, environmentally benign Na ion batteries for widespread energy storage applications. Financial support from the National Science Foundation of China (No. 2133307 and 21273167) is greatly appreciated.
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Chem. Commun., 2013, 49, 11370--11372
Communication
Notes and references 1 B. Dunn, H. Kamath and J. M. Tarascon, Science, 2011, 334, 928. 2 V. Etacheri, R. Marom, R. Elazari, G. Salitra and D. Aurbach, Energy Environ. Sci., 2011, 4, 3243. 3 Z. Yang, J. Zhang, M. C. Kintner-Meyer, X. Lu, D. Choi, J. P. Lemmon and J. Liu, Chem. Rev., 2011, 111, 3577. ¨s, Science, 2012, 335, 1468. 4 G. Milczarek and O. Ingana 5 M. Armand and J.-M. Tarascon, Nature, 2008, 451, 652. 6 Y. Liang, Z. Tao and J. Chen, Adv. Energy Mater., 2012, 2, 742. 7 K. Oyaizu and H. Nishide, Adv. Mater., 2009, 21, 2339. 8 R. Zhao, L. Zhu, Y. Cao, X. Ai and H. X. Yang, Electrochem. Commun., 2012, 21, 36. 9 M. Zhou, Y. Xiong, Y. Cao, X. Ai and H. Yang, J. Polym. Sci., Part B: Polym. Phys., 2013, 51, 114. 10 Y. Dai, Y. Zhang, L. Gao, G. Xu and J. Xie, Electrochem. Solid-State Lett., 2010, 13, A22. 11 L. Zhao, J. Zhao, Y. S. Hu, H. Li, Z. Zhou, M. Armand and L. Chen, Adv. Energy Mater., 2012, 2, 962. 12 S. W. Kim, D. H. Seo, X. Ma, G. Ceder and K. Kang, Adv. Energy Mater., 2012, 2, 710. 13 V. Palomares, P. Serras, I. Villaluenga, K. B. Hueso, J. Carretero´lez and T. Rojo, Energy Environ. Sci., 2012, 5, 5884. Gonza ´k, K. Mu ¨ller, K. Santhanam and O. Haas, Chem. Rev., 1997, 14 P. Nova 97, 207. 15 K. S. Park, S. B. Schougaard and J. B. Goodenough, Adv. Mater., 2007, 19, 848.
This journal is
c
The Royal Society of Chemistry 2013
Published on 16 October 2013. Downloaded by McMaster University on 24/10/2014 18:04:18.
COMMUNICATION
Cite this: Chem. Commun., 2013, 49, 11370 Received 30th August 2013, Accepted 15th October 2013 DOI: 10.1039/c3cc46642f
View Journal | View Issue
Self-doped polypyrrole with ionizable sodium sulfonate as a renewable cathode material for sodium ion batteries† Limin Zhu, Yifei Shen, Mengying Sun, Jiangfeng Qian,* Yuliang Cao, Xinping Ai and Hanxi Yang*
www.rsc.org/chemcomm
A Na-host cathode is developed by grafting the polypyrrole chains with ionizable sodium sulfonate. Due to the immobile p-doping of organic anions, the self-doped polymer can act as a Na-host for reversible Na insertion–extraction reaction, thus offering a low cost and renewable organic cathode for Na ion battery applications.
Rapid development of renewable energy systems requires effective means of electric energy storage (EES). Though many types of conventional and advanced rechargeable batteries have been investigated for the grid-scale EES applications, they are all constrained by the cost and material resources, because all of them are based on the redox chemistry of rare metals.1–3 To meet the need for widespread EES applications, it is therefore a great challenge to develop low cost, renewable batteries by use of environmentally benign, inexpensive and earthabundant materials.4,5 Na-ion batteries seem to be such a new battery technology, because of their widespread availability and low cost of Na resources. In particular, if Na-ion batteries could be constructed using redox-active polymers, it would bring about a great benefit for EES applications because of their low cost and material sustainability. However, though many types of redoxactive polymer are explored for electrochemical capacitor and battery materials,6,7 only a few of them have been adopted for Na-ion batteries.8–11 A difficulty in developing the polymeric Na-storage cathodes is that the redox reactions in these polymer cathodes usually take place through a p-doping–dedoping mechanism of large electrolyte anions, which suffers from a low doping degree and slow kinetics, leading to a poor capacity utilization of the polymer chains. In addition, all the p-doped polymers are virtually not a Na-storage host and cannot act as a Na reservoir to construct a ‘‘rocking chair type’’ Na-ion battery. To enable effective Na storage in the polymeric cathodes, we propose here a new material design by self-doping
College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China. E-mail: [email protected]; Fax: +86-27-68754526; Tel: +86-27-68754526 † Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/c3cc46642f
11370
Chem. Commun., 2013, 49, 11370--11372
(graft copolymerizing) the redox polymers with an ionizable sodium organic salt, thus changing the redox reaction mechanism of the polymers from conventional p-doping–dedoping processes of large electrolyte anions to the insertion–extraction reactions of small Na+ cations. As an example, we synthesized poly(pyrrole-co-(sodium-3-(pyrrol-lyl) propanesulphonate)) (PP-PS) with ionizable Na ions and investigated its reversible Na insertion behavior. Since the sulphonate anions are chemically grafted on the polypyrrole chains, the redox reaction of the polymer must take place through the insertion and extraction of ionizable Na+ cations, thus making the copolymer a polymeric Na host cathode material. The PP-PS copolymer was synthesized by oxidative copolymerization of pyrrole and sodium 3-(pyrrol-l-yl) propanesulfonate in a two-step reaction as shown in Scheme 1. The resulting PP-PS sample appears as a dark black powder, almost insoluble in commonly used organic and aqueous electrolytes. Fig. 1 shows the SEM image and the FT-IR spectrum of the as-prepared PP-PS powder. It could be seen from Fig. 1a that the copolymer
Scheme 1
Synthetic route of the PP-PS copolymer.
Fig. 1 Structural characterization of the as-prepared PP-PS copolymer: (a) SEM image; (b) FT-IR spectrum.
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Fig. 2 Cyclic voltammograms of the PP-PS electrode in 1.0 mol L (v/v = 1 : 1) electrolyte, measured at a scan rate of 10 mV s–1.
ChemComm
1
NaPF6 + EC/DEC
emerged as an irregular aggregate of 200–500 nm sized particles. All the IR absorptions in Fig. 1b can be attributed to the characteristic bands of polypyrrole and poly(pyrrolesulphonate). The 1556 and 1473 cm 1 bands are characteristic of the antisymmetric and symmetric ring vibrations and the 1300 and 1043 cm 1 bands reflect the C–H deformation and N–H stretching vibrations of the pyrrole rings, whereas the 1212 cm 1 band features the S–O stretching of sulphonate groups grafted on the copolymer chains. Electrochemical redox properties of the PP-PS polymer were characterized by cyclic voltammetry (CV) and galvanostatic charge–discharge cycling. Fig. 2 shows the CV curves of the PP-PS sample in 1.0 mol L 1 NaPF6 in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC). As shown in the figure, the VGCF only shows a negligible background current, suggesting that neither the carbon additive nor the electrolyte have a redox activity in this potential region of +3.3 to +3.9 V, and the pair of strong CV bands can only be attributed to the reversible p-type redox reaction. Usually, the p-type redox bands reflect the doping–dedoping reactions of electrolyte anions into/from the polymer chains. However, since our PP-PS polymer is already self-doped with immobile sulphonate anions that associate with ionizable Na+ ions, it cannot undergo a redox reaction further through p-doping–dedoping of the electrolyte anions, and therefore, the p-type redox reaction in the polymer can only be attributed to reversible insertion– extraction of the ionizable Na+ ions for counterbalancing the injection/removal of electron charges. It can also be seen in the CV curves that the cathodic and anodic peaks have almost similar peak areas since the second cycle and are kept steady for subsequent 20 cycles, suggesting a high coulombic efficiency and the cycling stability of the p-type redox reaction. To test the possibility of the copolymer as a feasible cathode material, coin-type Na/PP-PS cells were assembled with excessive Na capacity design and cycled galvanostatically at a voltage interval between 4.0 V and 2.0 V. Fig. 3 shows the charge and discharge profiles of the cells. In accord with the redox potentials of the CV peaks, the polymer electrode shows average charge and discharge potential at 3.0 V and 3.3 V, respectively, reflecting reversible p-type redox reaction. At first few cycles, the reversible discharge capacity increases from its initial value This journal is
c
The Royal Society of Chemistry 2013
Fig. 3 Charge–discharge profile of coin-type Na/PP-PS cells using 1.0 mol L 1 NaPF6 + EC/DEC (v/v = 1 : 1) electrolyte. The current was set constantly at 40 mA g 1. The charge–discharge capacities were calculated according to the mass weight of the PP-PS cathode.
(75 mA g 1) to its maximum capacity of 85 mA g 1 and then is kept quite steady over 100 cycles (see Fig. S1, ESI†), demonstrating a considerable cyclability. Though this capacity is slightly lower than those of Na-host cathodes of transition metal oxides,12,13 it is much higher than the realizable redox capacities usually observed for conventional p-doped polymers14,15 and in principle, this Na storage capacity could be further improved by tailoring the polymer structure with higher Na density and lightweight organic anions. More significantly, this result reveals a new approach to use polymers for Na storage cathodes by self-doping organic Na salts. It should be mentioned that the coulombic efficiency of this PP-PS cathode at initial cycles is only about B60%, much lower than those reported for p-doped polymers. This phenomenon is difficult to explain by the present understanding of the selfdoped polymers. A possible cause for this phenomenon is the irreversible co-doping of electrolyte anions for the formation of the surface electrolyte interface on the cathode. To confirm the Na storage reaction in the polymer cathode, we conducted quantitative ICP characterization of the Na+ content in the cycled PP-PS electrodes at different depths of charge and discharge. In the uncharged sample, the Na+ content is 0.0511 g g 1 (per gram of electrode material), indicating that the pristine PP-PS copolymer has a chemical formula of [C4H3N]3.7[C4H2NC3H2SO3Na], which should have a theoretical Na storage capacity of 60 mA h g 1 if only taking into account the Na+ doping–dedoping reaction. When the electrode was fully charged to the upper terminal voltage of 4.0 V, the Na+ content in the electrode decreased to 0.0043 g g 1, corresponding to 0.96 Na+ extraction from the polymer. Once discharging to 2.0 V, the Na+ content in the electrode recovered to 0.0535 g g 1, implying a re-insertion of 0.956 Na+ ions into the polymer. These data demonstrate complete extraction/ insertion of the doped Na+ ions into/from the PP-PS polymer during charge–discharge processes. However, the total reversible capacity (85 mA h g 1) observed for the PP-PS polymer cannot be accounted for only from Na insertion capacity (60 mA h g 1). To test the capacitance contribution from the carbon additive in the cathode, we measured the charge–discharge capacities of the VGCF carbon (Fig. S2, ESI†) Chem. Commun., 2013, 49, 11370--11372
11371
View Article Online
Published on 16 October 2013. Downloaded by McMaster University on 24/10/2014 18:04:18.
ChemComm and found that the carbon contributes almost no capacity at this voltage range. In fact, during the Na insertion reaction, a small amount of electrolyte anions may also be reversibly doped into the PP-PS polymer at charge and in turn dedoped from the polymer at discharge for counterbalancing the charge change of deeply activated polypyrrole chains, thus contributing their p-doping capacity to the PP-PS polymer cathode. In summary, we developed a new approach to create polymeric Na storage cathodes, simply by self-doping redox polymers with ionisable organic Na salts. Due to the immobile p-doping of organic anions, the redox reaction of the self-doping polymer can proceed through a reversible Na insertion–extraction reaction. The PP-PS copolymer developed in this work demonstrates effective Na storage with considerably high voltage, reversible capacity and cycle life. In addition, this method could be easily adapted to a wide range of polymer structures for developing high capacity Na-host cathodes, thus enabling low cost, environmentally benign Na ion batteries for widespread energy storage applications. Financial support from the National Science Foundation of China (No. 2133307 and 21273167) is greatly appreciated.
11372
Chem. Commun., 2013, 49, 11370--11372
Communication
Notes and references 1 B. Dunn, H. Kamath and J. M. Tarascon, Science, 2011, 334, 928. 2 V. Etacheri, R. Marom, R. Elazari, G. Salitra and D. Aurbach, Energy Environ. Sci., 2011, 4, 3243. 3 Z. Yang, J. Zhang, M. C. Kintner-Meyer, X. Lu, D. Choi, J. P. Lemmon and J. Liu, Chem. Rev., 2011, 111, 3577. ¨s, Science, 2012, 335, 1468. 4 G. Milczarek and O. Ingana 5 M. Armand and J.-M. Tarascon, Nature, 2008, 451, 652. 6 Y. Liang, Z. Tao and J. Chen, Adv. Energy Mater., 2012, 2, 742. 7 K. Oyaizu and H. Nishide, Adv. Mater., 2009, 21, 2339. 8 R. Zhao, L. Zhu, Y. Cao, X. Ai and H. X. Yang, Electrochem. Commun., 2012, 21, 36. 9 M. Zhou, Y. Xiong, Y. Cao, X. Ai and H. Yang, J. Polym. Sci., Part B: Polym. Phys., 2013, 51, 114. 10 Y. Dai, Y. Zhang, L. Gao, G. Xu and J. Xie, Electrochem. Solid-State Lett., 2010, 13, A22. 11 L. Zhao, J. Zhao, Y. S. Hu, H. Li, Z. Zhou, M. Armand and L. Chen, Adv. Energy Mater., 2012, 2, 962. 12 S. W. Kim, D. H. Seo, X. Ma, G. Ceder and K. Kang, Adv. Energy Mater., 2012, 2, 710. 13 V. Palomares, P. Serras, I. Villaluenga, K. B. Hueso, J. Carretero´lez and T. Rojo, Energy Environ. Sci., 2012, 5, 5884. Gonza ´k, K. Mu ¨ller, K. Santhanam and O. Haas, Chem. Rev., 1997, 14 P. Nova 97, 207. 15 K. S. Park, S. B. Schougaard and J. B. Goodenough, Adv. Mater., 2007, 19, 848.
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c
The Royal Society of Chemistry 2013
