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three-dimensional graphene composite for high performance lithium–sulfur batteries
Journal of Power Sources 275 (2015) 22e25
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Sulfur/three-dimensional graphene composite for high performance lithiumesulfur batteries Chunmei Xu a, Yishan Wu a, Xuyang Zhao b, Xiuli Wang b, Gaohui Du a, Jun Zhang a, *, Jiangping Tu b a Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China b Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Three-dimensional graphene is assembled by a simple hydrothermal reduction. The 3D graphene shows a hierarchical porous morphology and robust structure. Sulfur content up to 73% is loaded in 3D graphene. The composite shows high specific capacity and stable capacity retention. The unique 3D structure enables high-rate capability.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 September 2014 Accepted 3 November 2014 Available online 4 November 2014
A sulfur/graphene composite is prepared by loading elemental sulfur into three-dimensional graphene (3D graphene), which is assembled using a metal ions assisted hydrothermal method. When used as cathode materials for lithiumesulfur (LieS) batteries, the sulfur/graphene composite (S@3D-graphene) with 73 wt % sulfur shows a significantly enhanced cycling performance (>700 mAh g1 after 100 cycles at 0.1C rate with a Coulombic efficiency > 96%) as well as high rate capability with a capacity up to 500 mAh g1 at 2C rate (3.35 A g1). The superior electrochemical performance could be attributed to the highly porous structure of three-dimensional graphene that not only enables stable and continue pathway for rapid electron and ion transportation, but also restrain soluble polysulfides and suppress the “shuttle effect”. Moreover, the robust structure of 3D graphene can keep cathode integrity and accommodate the volume change during high-rate charge/discharge processes, making it a promising candidate as cathode for high performance LieS batteries. © 2014 Elsevier B.V. All rights reserved.
Keywords: Lithiumsulfur batteries Graphene Three-dimensional structure Electrochemical properties
1. Introduction
* Corresponding author. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.jpowsour.2014.11.007 0378-7753/© 2014 Elsevier B.V. All rights reserved.
Elemental sulfur delivers a high theoretical specific capacity of 1675 mAh g1 versus lithium, which is seven times higher than that of the intercalation cathode materials used in lithium-ion batteries, making lithiumesulfur (LieS) battery the promising candidate for electric vehicles and energy storage systems for renewable energy
C. Xu et al. / Journal of Power Sources 275 (2015) 22e25
[1,2]. Sulfur is also environmentally friendly, economical and abundant. However, in spite of its attracting properties, LieS battery suffers severe capacity decay because of the poor conductivity of sulfur, the dissolution of intermediate lithium polysulfide products Li2Sn (2 < n 8) along with “shuttle effect”, the precipitation of insoluble and insulating Li2S2/Li2S on the electrodes and volumetric expansion [3,4]. Considering these drawbacks, the most attractive strategy is to create nanocomposites containing sulfur, among which conductive materials have been a popular choice. Many pioneering work have been made to combine sulfur with conductive materials such as carbon nanotube [57], porous carbon [810], conducting polymers [1115], graphene and graphene oxide [3,16,17]. As one of the most promising conductive additives, graphene has significant advantages of high specific surface area, superior electron mobility and good chemical stability [18]. It has been demonstrated that graphene is an outstanding cathode support for LieS batteries to improve the electronic conductivity and the utilization of sulfur [16,17,1921]. However, graphene fails to trap the polysulfides due to its open structure, resulting in low Coulombic efficiency and limited cyclic stability. Therefore, it is urgent to construct a three-dimensional structure of graphene that provides highly conducting network for charge transfer, flexible space accommodating the volumetric expansion during cycling as well as porous morphology to immobilize the polysulfide species [3]. Though 3D graphene has been used in lithium ion batteries, to the best of our knowledge, there are few reports focusing on the application of 3D graphene in LieS batteries [22,23]. Herein, we report a simple method to prepare threedimensional graphene supported sulfur composite (S@3D-graphene) as cathode material for LieS battery. Due to its robust and highly porous 3D-connected structure, the composite showed superior electrochemical performance.
2. Experimental Graphene oxide (GO) was prepared by a modified Hummer's method [24]. 3D graphene is assembled by Co2þ assistedhydrothermal reduction [25]. Briefly, 100 ml 1 g L1 GO solution was mixed with 2.2 mg CoCl2 and then transferred into a Teflon lined stainless steel autoclave and heated at 120 C for 10 h. After that, cylindrical precipitate was collected and washed 3 times by deionized water followed by vacuum freeze drying. The obtained cylindrical 3D graphene was cut to small pieces, ground with sulfur powder and heated for 12 h at 155 C to form a homogeneously mixed S@3D-graphene composite. Powder X-ray diffraction (XRD, Cu Ka radiation, Philips PW3040/60) was used to verify the structure of the composite.
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Sulfur content in the S@3D-graphene was determined by thermogravimetric/differential thermal analysis (TG/DTA) (Netzsch STA 449C thermal analyzer). The morphologies were examined by scanning electron microscopy (SEM, Hitachi S4800), and transmission electron microscopy (TEM, JEOL 2100F). The electrodes were prepared by dispersing the S@3D-graphene composite (75 wt %), acetylene carbon black (15 wt %) and polyvinylidene fluoride binder (10 wt %) in N-methyl-2-pyrrolidone solvent to form a slurry. The slurry was pasted onto aluminum foils and dried at 60 C for 12 h in a vacuum oven. Two-electrode CR2032-type coin cells with a lithium foil as the counter electrode and Celgard 2400 as the separator were assembled in an argon-filled glove box. The electrolyte solution was 1 M lithium bis(trifluoromethane sulfonimide) (LiTFSI), 0.1 M LiNO3 in a solvent of 1,3-dioxolane (DOL): 1,2-dimethoxyethane (DME) with a volume ratio of 1: 1. Cyclic voltammetry (CV) was carried out on a CHI 604D electrochemistry workstation (Shanghai Chenhua Instruments Co. Ltd.) from 1.5 to 3.0 V at a scan rate of 0.1 mV s1. The galvanostatic charge and discharge measurements were performed on a battery test system (Shenzhen Neware Tech. Ltd.) at different current densities in the voltage range from 1.5 to 3.0 V.
3. Result and discussion Fig.1a shows the thermogravimetric (TG) curves of the elemental sulfur and S@3D-graphene composite. It is found that sulfur starts to evaporate at about 220 C and losses the weight completely above 350 C. The gradual weight loss above 400 C for the S@3D-graphene composite is attributed to the decomposition of oxygen-containing functional groups on the surface of graphene. As determined by TG method, the content of sulfur in the composite is calculated to be 73%. This loading mass is quite satisfied, since it is well-accepted that the content of sulfur should exceed 50% for practical use [26]. Fig. 1b shows the XRD patterns of the elemental sulfur, the 3D graphene, and the as-produced S@3Dgraphene composite. Comparing with the elemental sulfur, the characteristic peaks of orthorhombic sulfur (JCPDS No. 77-0145) in the S@3D-graphene composite become weak, indicating poor crystallinity of sulfur which is infiltrated in the graphene networks. Besides, the broadened characteristic peak of graphene at 24.3 is also observed. The morphology of the S@3D-graphene composite is observed by SEM and TEM (Fig. 2). SEM image (Fig. 2a) of the S@3D-graphene composite shows that the 3D graphene with a highly porous hierarchical structure is composed of wrinkled and threedimensionally connected graphene nanosheets. It is worthy to mention that the as-prepared cylinder-shaped 3D graphene exhibits good flexibility and strength, suggesting a robust structure
Fig. 1. (a) Thermogravimetric curves of the S@3D-graphene composite and elemental sulfur; (b) XRD patterns of 3D graphene, elemental sulfur and the S@3D-graphene composite.
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C. Xu et al. / Journal of Power Sources 275 (2015) 22e25
Fig. 2. SEM image (a) and TEM image (b) of the S@3D-graphene composite (inset is the corresponding SAED pattern).
that can tolerate severe volume change. TEM image (Fig. 2b) of the S@3D-graphene composite shows that sulfur particles with size ranged in 20e200 nm are confined in the pores of 3D graphene. The corresponding selected area electron diffraction (SAED, inset in Fig. 2b) is composed of the diffraction rings of graphene and the diffraction spots of sulfur, which is in accordance with the XRD results. Fig. 3a shows the first three cyclic voltammetric curves of the cell. Two reduction peaks are observed at ~2.35 V for the reduction of S8 to polysulfide (Li2Sn, 2 < n < 8), and at ~2.05 V for the further reduction to Li2S2 and Li2S, indicating the multiple reaction of sulfur with lithium ions. In addition, only one oxidation peak is found, which is attributed to the transformation of the polysulfides to S8 [1719]. Furthermore, the coincidence of the first three cyclic voltammetric curves certifies the excellent electrochemical stability of the S@3D-graphene composite. To study the storage capacity of the as-prepared material, galvanostatic discharge/charge performances were tested in the
voltage window of 3.0e1.5 V at a current density of 0.1C. Fig. 3b displays the 1st, 2nd, 3rd, 50th and 100th discharge/charge potentialecapacity profiles of the S@3D-graphene composite. It is obvious that the charge/discharge plateaus exactly resemble the redox peaks observed in the cyclic voltammetric scans. The composite delivers a first discharge capacity of 1260 mAh g1 with a sulfur utilization up to 75.2%, which is comparable with the sulfur cathodes based on sandwiched multiwalled nanotubes/graphene [22], the 3D graphene-like porous carbon [23] and the 3D nitrogen doped graphene [27]. The high electrochemical activity of sulfur in the composite could be attributed to the uniform dispersion of sulfur particles in the high conductivity 3D graphene networks that enables sufficient electrochemical reaction sites and fast electron/ ion transportation. Fig. 3c shows the cycling performance of the S@3D-graphene composite. Though a high discharge capacity of 1260 mAh g1 is obtained in the first discharge process, the capacity decreases rapidly in the first 10 cycles, which perhaps due to the erosion of
Fig. 3. Electrochemical performance of the S@3D-graphene composite: (a) First three cyclic voltammetric curves with a sweep rate of 0.1 mV s1; (b) Potential profiles; (c) Charge/ discharge capacity and Coulombic efficiency over 100 cycles at 0.1C; and (d) Charge/discharge capacity at various C-rates from 0.1 to 2C.
C. Xu et al. / Journal of Power Sources 275 (2015) 22e25
the sulfur from the outer surface of the 3D graphene. Nevertheless, the decay slows down thereafter and the capacity becomes very stable after the 40th cycle. A reversible capacity up to 700 mAh g1 is retained even for 100 cycles, which is much higher than that of the conventional sulfur/two-dimensional graphene composites (315 mAh g1 [28] or 476 mAh g1 [29] at the same current density). The superiority of the 3D graphene compared with twodimensional graphene is attributed to the unique threedimensional structure. 3D graphene provides not only the threedimensional conductive matrix for fast electron and ion transfer, but also highly porous inner space to restrain soluble polysulfides and suppress the “shuttle effect”. Rate capability is one of the key parameters for batteries that restrains their real application. Here we evaluate the rate performance of the S@3D-graphene composite by increasing the charge/ discharge current density stepwise from 0.1C to 2C every 10 cycles. As shown in Fig. 3d, when the cell is charged and discharged at 0.1C, a discharge capacity up to 1270 mAh g1 can be obtained. After the initial fast decay stage, the capacity becomes stable. When the current density is increased to 1C (1.675 A g1) and 2C (3.35 A g1), the cell works steadily and the capacity maintains at 600 mAh g1 and 500 mAh g1, respectively, which is much better than the sulfur/reduced graphene oxide composites [30]. Moreover, the discharge capacity can be recovered to 720 mAh g1 when current density is returned to 0.1C, suggesting a superior rate capability. This result demonstrates that the robust and highly porous structured graphene not only enables stable and continue pathway for rapid electron and ion transportation, but also keep cathode integrity and accommodate the volume change during high-rate charge/discharge processes. 4. Conclusion In summary, the three-dimensional graphene with a highly porous morphology was prepared by hydrothermal reduction. And element sulfur was loaded onto the 3D graphene through thermal compounding to form the S@3D-graphene composite. The S@3Dgraphene composite with a sulfur content up to 73 wt % showed high specific capacity and stable capacity retention. The unique 3D structure also enabled high-rate discharge/charge capability. The results demonstrated that the S@3D-graphene composite was a promising candidate as cathode for high-rate performance LieS batteries. Acknowledgments This work was supported by the National Natural Science Foundation of China (21203168), the Department of Science &
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Technology of Zhejiang Province (2013C31070), Open Fund of the Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province (2013001), and the National Undergraduate Innovation & Entrepreneurship Training Project (201310345017). References [1] S. Evers, L.F. Nazar, Acc. Chem. Res. 46 (2013) 1135e1143. [2] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nat. Mater. 11 (2012) 19e29. [3] W.D. Zhou, H. Chen, Y.C. Yu, D.L. Wang, Z.M. Cui, F.J. DiSalvo, H.D. Abruna, ACS Nano 7 (2013) 8801e8808. [4] X.M. He, J.G. Ren, L. Wang, W.H. Pu, C.Y. Jiang, C.R. Wan, J. Power Sources 190 (2009) 154e156. [5] W. Wei, J.L. Wang, L.J. Zhou, J. Yang, B. Schumann, Y.N. Nuli, Electrochem. Commun. 13 (2011) 399e402. [6] W. Zheng, Y.W. Liu, X.G. Hu, C.F. Zhang, Electrochim. Acta 51 (2006) 1330e1335. [7] L.X. Yuan, H.P. Yuan, X.P. Qiu, L.Q. Chen, W.T. Zhu, J. Power Sources 189 (2009) 1141e1146. [8] X.L. Ji, K.T. Lee, L.F. Nazar, Nat. Mater. 8 (2009) 500e506. [9] C. Lai, X.P. Gao, B. Zhang, T.Y. Yan, Z.J. Zhou, J. Phys. Chem. C 113 (2009) 4712e4716. [10] J. Zhang, J.Y. Xiang, Z.M. Dong, Y. Liu, Y.S. Wu, C.M. Xu, G.H. Du, Electrochim. Acta 116 (2014) 146e151. [11] S.H. Chung, A. Manthiram, Electrochem. Commun. 38 (2014) 91e95. [12] J.L. Wang, J. Yang, J.Y. Xie, N.X. Xu, Adv. Mater. 14 (2002) 963e965. [13] J. Wang, J. Chen, K. Konstantinov, L. Zhao, S.H. Ng, G.X. Wang, Z.P. Guo, H.K. Liu, Electrochim. Acta 51 (2006) 4634e4638. [14] Z.M. Dong, J. Zhang, X.Y. Zhao, J.P. Tu, Q.M. Su, G.H. Du, RSC Adv. 3 (2013) 24914e24917. [15] Y. Liu, J. Zhang, X.C. Liu, J.X. Guo, L.F. Pan, H.F. Wang, Q.M. Su, G.H. Du, Mater. Lett. 133 (2014) 193e196. [16] G.M. Zhou, L.C. Yin, D.W. Wang, L. Li, S.F. Pei, I.R. Gentle, F. Li, H.M. Cheng, ACS Nano 7 (2013) 5367e5375. [17] J. Zhang, Z.M. Dong, X.L. Wang, X.Y. Zhao, J.P. Tu, Q.M. Su, G.H. Du, J. Power Sources 270 (2014) 1e8. [18] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666e669. [19] K.T. Lee, R. Black, T. Yim, X. Ji, L.F. Nazar, Adv. Energy Mater. 2 (2012) 1490e1496. [20] X.Y. Zhao, J.P. Tu, Y. Lu, J.B. Cai, Y.J. Zhang, X.L. Wang, C.D. Gu, Electrochim. Acta 113 (2013) 256e262. [21] Y.S. Wu, C.M. Xu, J.X. Guo, Q.M. Su, G.H. Du, J. Zhang, Mater. Lett. 137 (2014) 277e280. [22] R.J. Chen, T. Zhao, J. Lu, F. Wu, L. Li, J.Z. Chen, G.Q. Tan, Y.S. Ye, K. Amine, Nano Lett. 13 (2013) 4624e4649. [23] Y.Y. Li, Z.S. Li, Q.W. Zhang, P.K. Shen, J. Mater. Chem. A 2 (2014) 4528e4533. [24] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [25] X. Jiang, Y.W. Ma, J.J. Li, Q.L. Fan, W. Huang, J. Phys. Chem. C 114 (2010) 22462e22465. [26] Y.S. Choi, S. Kim, S.S. Choi, J.S. Han, J.D. Kim, S.E. Jeon, B.H. Jung, Electrochim. Acta 50 (2004) 833e835. [27] C. Wang, K. Su, W. Wan, H. Guo, H.H. Zhou, J.T. Chen, X.X. Zhang, Y.H. Huang, J. Mater. Chem. A 2 (2014) 5018e5023. [28] K. Han, J.M. Shen, C.M. Hayner, H.Q. Ye, M.C. Kung, H.H. Kung, J. Power Sources 251 (2014) 331e337. [29] L.Q. Lu, L.J. Lu, Y. Wang, J. Mater. Chem. A 1 (2013) 9173e9181. [30] Y.X. Wang, S.L. Chou, H.K. Liu, S.X. Dou, J. Power Sources 244 (2013) 240e245.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Sulfur/three-dimensional graphene composite for high performance lithiumesulfur batteries Chunmei Xu a, Yishan Wu a, Xuyang Zhao b, Xiuli Wang b, Gaohui Du a, Jun Zhang a, *, Jiangping Tu b a Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China b Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Three-dimensional graphene is assembled by a simple hydrothermal reduction. The 3D graphene shows a hierarchical porous morphology and robust structure. Sulfur content up to 73% is loaded in 3D graphene. The composite shows high specific capacity and stable capacity retention. The unique 3D structure enables high-rate capability.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 September 2014 Accepted 3 November 2014 Available online 4 November 2014
A sulfur/graphene composite is prepared by loading elemental sulfur into three-dimensional graphene (3D graphene), which is assembled using a metal ions assisted hydrothermal method. When used as cathode materials for lithiumesulfur (LieS) batteries, the sulfur/graphene composite (S@3D-graphene) with 73 wt % sulfur shows a significantly enhanced cycling performance (>700 mAh g1 after 100 cycles at 0.1C rate with a Coulombic efficiency > 96%) as well as high rate capability with a capacity up to 500 mAh g1 at 2C rate (3.35 A g1). The superior electrochemical performance could be attributed to the highly porous structure of three-dimensional graphene that not only enables stable and continue pathway for rapid electron and ion transportation, but also restrain soluble polysulfides and suppress the “shuttle effect”. Moreover, the robust structure of 3D graphene can keep cathode integrity and accommodate the volume change during high-rate charge/discharge processes, making it a promising candidate as cathode for high performance LieS batteries. © 2014 Elsevier B.V. All rights reserved.
Keywords: Lithiumsulfur batteries Graphene Three-dimensional structure Electrochemical properties
1. Introduction
* Corresponding author. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.jpowsour.2014.11.007 0378-7753/© 2014 Elsevier B.V. All rights reserved.
Elemental sulfur delivers a high theoretical specific capacity of 1675 mAh g1 versus lithium, which is seven times higher than that of the intercalation cathode materials used in lithium-ion batteries, making lithiumesulfur (LieS) battery the promising candidate for electric vehicles and energy storage systems for renewable energy
C. Xu et al. / Journal of Power Sources 275 (2015) 22e25
[1,2]. Sulfur is also environmentally friendly, economical and abundant. However, in spite of its attracting properties, LieS battery suffers severe capacity decay because of the poor conductivity of sulfur, the dissolution of intermediate lithium polysulfide products Li2Sn (2 < n 8) along with “shuttle effect”, the precipitation of insoluble and insulating Li2S2/Li2S on the electrodes and volumetric expansion [3,4]. Considering these drawbacks, the most attractive strategy is to create nanocomposites containing sulfur, among which conductive materials have been a popular choice. Many pioneering work have been made to combine sulfur with conductive materials such as carbon nanotube [57], porous carbon [810], conducting polymers [1115], graphene and graphene oxide [3,16,17]. As one of the most promising conductive additives, graphene has significant advantages of high specific surface area, superior electron mobility and good chemical stability [18]. It has been demonstrated that graphene is an outstanding cathode support for LieS batteries to improve the electronic conductivity and the utilization of sulfur [16,17,1921]. However, graphene fails to trap the polysulfides due to its open structure, resulting in low Coulombic efficiency and limited cyclic stability. Therefore, it is urgent to construct a three-dimensional structure of graphene that provides highly conducting network for charge transfer, flexible space accommodating the volumetric expansion during cycling as well as porous morphology to immobilize the polysulfide species [3]. Though 3D graphene has been used in lithium ion batteries, to the best of our knowledge, there are few reports focusing on the application of 3D graphene in LieS batteries [22,23]. Herein, we report a simple method to prepare threedimensional graphene supported sulfur composite (S@3D-graphene) as cathode material for LieS battery. Due to its robust and highly porous 3D-connected structure, the composite showed superior electrochemical performance.
2. Experimental Graphene oxide (GO) was prepared by a modified Hummer's method [24]. 3D graphene is assembled by Co2þ assistedhydrothermal reduction [25]. Briefly, 100 ml 1 g L1 GO solution was mixed with 2.2 mg CoCl2 and then transferred into a Teflon lined stainless steel autoclave and heated at 120 C for 10 h. After that, cylindrical precipitate was collected and washed 3 times by deionized water followed by vacuum freeze drying. The obtained cylindrical 3D graphene was cut to small pieces, ground with sulfur powder and heated for 12 h at 155 C to form a homogeneously mixed S@3D-graphene composite. Powder X-ray diffraction (XRD, Cu Ka radiation, Philips PW3040/60) was used to verify the structure of the composite.
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Sulfur content in the S@3D-graphene was determined by thermogravimetric/differential thermal analysis (TG/DTA) (Netzsch STA 449C thermal analyzer). The morphologies were examined by scanning electron microscopy (SEM, Hitachi S4800), and transmission electron microscopy (TEM, JEOL 2100F). The electrodes were prepared by dispersing the S@3D-graphene composite (75 wt %), acetylene carbon black (15 wt %) and polyvinylidene fluoride binder (10 wt %) in N-methyl-2-pyrrolidone solvent to form a slurry. The slurry was pasted onto aluminum foils and dried at 60 C for 12 h in a vacuum oven. Two-electrode CR2032-type coin cells with a lithium foil as the counter electrode and Celgard 2400 as the separator were assembled in an argon-filled glove box. The electrolyte solution was 1 M lithium bis(trifluoromethane sulfonimide) (LiTFSI), 0.1 M LiNO3 in a solvent of 1,3-dioxolane (DOL): 1,2-dimethoxyethane (DME) with a volume ratio of 1: 1. Cyclic voltammetry (CV) was carried out on a CHI 604D electrochemistry workstation (Shanghai Chenhua Instruments Co. Ltd.) from 1.5 to 3.0 V at a scan rate of 0.1 mV s1. The galvanostatic charge and discharge measurements were performed on a battery test system (Shenzhen Neware Tech. Ltd.) at different current densities in the voltage range from 1.5 to 3.0 V.
3. Result and discussion Fig.1a shows the thermogravimetric (TG) curves of the elemental sulfur and S@3D-graphene composite. It is found that sulfur starts to evaporate at about 220 C and losses the weight completely above 350 C. The gradual weight loss above 400 C for the S@3D-graphene composite is attributed to the decomposition of oxygen-containing functional groups on the surface of graphene. As determined by TG method, the content of sulfur in the composite is calculated to be 73%. This loading mass is quite satisfied, since it is well-accepted that the content of sulfur should exceed 50% for practical use [26]. Fig. 1b shows the XRD patterns of the elemental sulfur, the 3D graphene, and the as-produced S@3Dgraphene composite. Comparing with the elemental sulfur, the characteristic peaks of orthorhombic sulfur (JCPDS No. 77-0145) in the S@3D-graphene composite become weak, indicating poor crystallinity of sulfur which is infiltrated in the graphene networks. Besides, the broadened characteristic peak of graphene at 24.3 is also observed. The morphology of the S@3D-graphene composite is observed by SEM and TEM (Fig. 2). SEM image (Fig. 2a) of the S@3D-graphene composite shows that the 3D graphene with a highly porous hierarchical structure is composed of wrinkled and threedimensionally connected graphene nanosheets. It is worthy to mention that the as-prepared cylinder-shaped 3D graphene exhibits good flexibility and strength, suggesting a robust structure
Fig. 1. (a) Thermogravimetric curves of the S@3D-graphene composite and elemental sulfur; (b) XRD patterns of 3D graphene, elemental sulfur and the S@3D-graphene composite.
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C. Xu et al. / Journal of Power Sources 275 (2015) 22e25
Fig. 2. SEM image (a) and TEM image (b) of the S@3D-graphene composite (inset is the corresponding SAED pattern).
that can tolerate severe volume change. TEM image (Fig. 2b) of the S@3D-graphene composite shows that sulfur particles with size ranged in 20e200 nm are confined in the pores of 3D graphene. The corresponding selected area electron diffraction (SAED, inset in Fig. 2b) is composed of the diffraction rings of graphene and the diffraction spots of sulfur, which is in accordance with the XRD results. Fig. 3a shows the first three cyclic voltammetric curves of the cell. Two reduction peaks are observed at ~2.35 V for the reduction of S8 to polysulfide (Li2Sn, 2 < n < 8), and at ~2.05 V for the further reduction to Li2S2 and Li2S, indicating the multiple reaction of sulfur with lithium ions. In addition, only one oxidation peak is found, which is attributed to the transformation of the polysulfides to S8 [1719]. Furthermore, the coincidence of the first three cyclic voltammetric curves certifies the excellent electrochemical stability of the S@3D-graphene composite. To study the storage capacity of the as-prepared material, galvanostatic discharge/charge performances were tested in the
voltage window of 3.0e1.5 V at a current density of 0.1C. Fig. 3b displays the 1st, 2nd, 3rd, 50th and 100th discharge/charge potentialecapacity profiles of the S@3D-graphene composite. It is obvious that the charge/discharge plateaus exactly resemble the redox peaks observed in the cyclic voltammetric scans. The composite delivers a first discharge capacity of 1260 mAh g1 with a sulfur utilization up to 75.2%, which is comparable with the sulfur cathodes based on sandwiched multiwalled nanotubes/graphene [22], the 3D graphene-like porous carbon [23] and the 3D nitrogen doped graphene [27]. The high electrochemical activity of sulfur in the composite could be attributed to the uniform dispersion of sulfur particles in the high conductivity 3D graphene networks that enables sufficient electrochemical reaction sites and fast electron/ ion transportation. Fig. 3c shows the cycling performance of the S@3D-graphene composite. Though a high discharge capacity of 1260 mAh g1 is obtained in the first discharge process, the capacity decreases rapidly in the first 10 cycles, which perhaps due to the erosion of
Fig. 3. Electrochemical performance of the S@3D-graphene composite: (a) First three cyclic voltammetric curves with a sweep rate of 0.1 mV s1; (b) Potential profiles; (c) Charge/ discharge capacity and Coulombic efficiency over 100 cycles at 0.1C; and (d) Charge/discharge capacity at various C-rates from 0.1 to 2C.
C. Xu et al. / Journal of Power Sources 275 (2015) 22e25
the sulfur from the outer surface of the 3D graphene. Nevertheless, the decay slows down thereafter and the capacity becomes very stable after the 40th cycle. A reversible capacity up to 700 mAh g1 is retained even for 100 cycles, which is much higher than that of the conventional sulfur/two-dimensional graphene composites (315 mAh g1 [28] or 476 mAh g1 [29] at the same current density). The superiority of the 3D graphene compared with twodimensional graphene is attributed to the unique threedimensional structure. 3D graphene provides not only the threedimensional conductive matrix for fast electron and ion transfer, but also highly porous inner space to restrain soluble polysulfides and suppress the “shuttle effect”. Rate capability is one of the key parameters for batteries that restrains their real application. Here we evaluate the rate performance of the S@3D-graphene composite by increasing the charge/ discharge current density stepwise from 0.1C to 2C every 10 cycles. As shown in Fig. 3d, when the cell is charged and discharged at 0.1C, a discharge capacity up to 1270 mAh g1 can be obtained. After the initial fast decay stage, the capacity becomes stable. When the current density is increased to 1C (1.675 A g1) and 2C (3.35 A g1), the cell works steadily and the capacity maintains at 600 mAh g1 and 500 mAh g1, respectively, which is much better than the sulfur/reduced graphene oxide composites [30]. Moreover, the discharge capacity can be recovered to 720 mAh g1 when current density is returned to 0.1C, suggesting a superior rate capability. This result demonstrates that the robust and highly porous structured graphene not only enables stable and continue pathway for rapid electron and ion transportation, but also keep cathode integrity and accommodate the volume change during high-rate charge/discharge processes. 4. Conclusion In summary, the three-dimensional graphene with a highly porous morphology was prepared by hydrothermal reduction. And element sulfur was loaded onto the 3D graphene through thermal compounding to form the S@3D-graphene composite. The S@3Dgraphene composite with a sulfur content up to 73 wt % showed high specific capacity and stable capacity retention. The unique 3D structure also enabled high-rate discharge/charge capability. The results demonstrated that the S@3D-graphene composite was a promising candidate as cathode for high-rate performance LieS batteries. Acknowledgments This work was supported by the National Natural Science Foundation of China (21203168), the Department of Science &
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Technology of Zhejiang Province (2013C31070), Open Fund of the Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province (2013001), and the National Undergraduate Innovation & Entrepreneurship Training Project (201310345017). References [1] S. Evers, L.F. Nazar, Acc. Chem. Res. 46 (2013) 1135e1143. [2] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nat. Mater. 11 (2012) 19e29. [3] W.D. Zhou, H. Chen, Y.C. Yu, D.L. Wang, Z.M. Cui, F.J. DiSalvo, H.D. Abruna, ACS Nano 7 (2013) 8801e8808. [4] X.M. He, J.G. Ren, L. Wang, W.H. Pu, C.Y. Jiang, C.R. Wan, J. Power Sources 190 (2009) 154e156. [5] W. Wei, J.L. Wang, L.J. Zhou, J. Yang, B. Schumann, Y.N. Nuli, Electrochem. Commun. 13 (2011) 399e402. [6] W. Zheng, Y.W. Liu, X.G. Hu, C.F. Zhang, Electrochim. Acta 51 (2006) 1330e1335. [7] L.X. Yuan, H.P. Yuan, X.P. Qiu, L.Q. Chen, W.T. Zhu, J. Power Sources 189 (2009) 1141e1146. [8] X.L. Ji, K.T. Lee, L.F. Nazar, Nat. Mater. 8 (2009) 500e506. [9] C. Lai, X.P. Gao, B. Zhang, T.Y. Yan, Z.J. Zhou, J. Phys. Chem. C 113 (2009) 4712e4716. [10] J. Zhang, J.Y. Xiang, Z.M. Dong, Y. Liu, Y.S. Wu, C.M. Xu, G.H. Du, Electrochim. Acta 116 (2014) 146e151. [11] S.H. Chung, A. Manthiram, Electrochem. Commun. 38 (2014) 91e95. [12] J.L. Wang, J. Yang, J.Y. Xie, N.X. Xu, Adv. Mater. 14 (2002) 963e965. [13] J. Wang, J. Chen, K. Konstantinov, L. Zhao, S.H. Ng, G.X. Wang, Z.P. Guo, H.K. Liu, Electrochim. Acta 51 (2006) 4634e4638. [14] Z.M. Dong, J. Zhang, X.Y. Zhao, J.P. Tu, Q.M. Su, G.H. Du, RSC Adv. 3 (2013) 24914e24917. [15] Y. Liu, J. Zhang, X.C. Liu, J.X. Guo, L.F. Pan, H.F. Wang, Q.M. Su, G.H. Du, Mater. Lett. 133 (2014) 193e196. [16] G.M. Zhou, L.C. Yin, D.W. Wang, L. Li, S.F. Pei, I.R. Gentle, F. Li, H.M. Cheng, ACS Nano 7 (2013) 5367e5375. [17] J. Zhang, Z.M. Dong, X.L. Wang, X.Y. Zhao, J.P. Tu, Q.M. Su, G.H. Du, J. Power Sources 270 (2014) 1e8. [18] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666e669. [19] K.T. Lee, R. Black, T. Yim, X. Ji, L.F. Nazar, Adv. Energy Mater. 2 (2012) 1490e1496. [20] X.Y. Zhao, J.P. Tu, Y. Lu, J.B. Cai, Y.J. Zhang, X.L. Wang, C.D. Gu, Electrochim. Acta 113 (2013) 256e262. [21] Y.S. Wu, C.M. Xu, J.X. Guo, Q.M. Su, G.H. Du, J. Zhang, Mater. Lett. 137 (2014) 277e280. [22] R.J. Chen, T. Zhao, J. Lu, F. Wu, L. Li, J.Z. Chen, G.Q. Tan, Y.S. Ye, K. Amine, Nano Lett. 13 (2013) 4624e4649. [23] Y.Y. Li, Z.S. Li, Q.W. Zhang, P.K. Shen, J. Mater. Chem. A 2 (2014) 4528e4533. [24] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [25] X. Jiang, Y.W. Ma, J.J. Li, Q.L. Fan, W. Huang, J. Phys. Chem. C 114 (2010) 22462e22465. [26] Y.S. Choi, S. Kim, S.S. Choi, J.S. Han, J.D. Kim, S.E. Jeon, B.H. Jung, Electrochim. Acta 50 (2004) 833e835. [27] C. Wang, K. Su, W. Wan, H. Guo, H.H. Zhou, J.T. Chen, X.X. Zhang, Y.H. Huang, J. Mater. Chem. A 2 (2014) 5018e5023. [28] K. Han, J.M. Shen, C.M. Hayner, H.Q. Ye, M.C. Kung, H.H. Kung, J. Power Sources 251 (2014) 331e337. [29] L.Q. Lu, L.J. Lu, Y. Wang, J. Mater. Chem. A 1 (2013) 9173e9181. [30] Y.X. Wang, S.L. Chou, H.K. Liu, S.X. Dou, J. Power Sources 244 (2013) 240e245.