- Email: [email protected]
Porous graphene as cathode material for lithium ion capacitor with high electrochemical performance
Powder Technology 253 (2014) 580–583
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Short communication
Porous graphene as cathode material for lithium ion capacitor with high electrochemical performance Feiyue Tu, Suqin Liu ⁎, Tonghua Wu, Guanhua Jin, Chunyue Pan College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 13 August 2013 Received in revised form 20 November 2013 Accepted 1 December 2013 Available online 8 December 2013 Keywords: Hydrothermal Graphene Lithium ion capacitors Porous
a b s t r a c t The porous reduced graphene is prepared via an ethylene glycol assisted-hydrothermal method which is green, simple and nontoxic. The ethylene glycol assisted-hydrothermal reduced graphene (EG-RGO) delivers superior power density compared to those prepared without the assistant of ethylene glycol when used as cathode materials for the lithium ion capacitors. The EG-RGO electrode shows a high capacity of 172 mAh g−1 at 0.1 A g−1 and without capacity loss after 3000 cycles. Furthermore, the power density of the electrodes reaches 53.5 kW kg − 1 at the current density of 20 A g − 1 with an energy density of 240 Wh kg − 1 . The high electrochemical performance of EG-RGO is attributed to the high electronic conductivity of the graphene. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Graphene, the two-dimension carbon material, has attracted great interest for its excellent mechanical, electrical, thermal and optical properties, which holds a great promise for potential application in many technological fields, including nanoelectronics, hydrogen storage, solar energy cells, Li-ion batteries, supercapacitors and so on [1–7]. It can be produced by micro-mechanical exfoliation of highly ordered pyrolytic graphite, epitaxial growth, chemical vapor deposition and reduction of graphene oxide (GO) [8–11]. Among these methods, the reduction of GO is recognized as the most massive method and easy to further functionalization, which is important to the large-scale use of graphene [12]. The GO can be prepared by the oxidation of inexpensive graphite with strong acid and oxidants [13]. Within the oxidation process, the surface of GO is distributed with many oxygen-containing groups and the structural defects are also inevitable, which seriously affect the conductivity of graphene [14]. Recently, some researchers have repaired the defects of graphene by chemical vapor deposition (CVD) processing [15,16], considering that the hydrothermal method is difficult to repair the large numbers of defects in GO by hydrazine monohydrate [17]. As we know, the traditional capacitors, namely, electric double layer capacitors (EDLCs) consist of two symmetrical electrodes of activated carbon and an aqueous electrolyte, displaying very high power density and long cycling life but very low energy density compared to lithium ion batteries (LICs). Different from EDLCs, the LICs have combined the advantages of supercapacitors (high power) and lithium ion batteries ⁎ Corresponding author. Fax: +86 73188879850. E-mail address: [email protected] (S. Liu). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.12.008
(high energy) due to the high voltage durable nonaqueous electrolyte and asymmetric electrodes [18]. Recently, graphene-based LICs have been drawing people's attention for its high capacity, high energy density, high power density as well as long cycle life [19,20]. Therefore, we prepared the porous graphene assisted by the ethylene glycol as cathode materials for lithium ion capacitor. And the EG-RGO showed an enhanced electrochemical performance compared to that of hydrothermal reduced graphene oxide (RGO). 2. Experimental methods 2.1. Preparation of RGO and EG-RGO The GO was prepared by Hummer's method [13]. The RGO and EG-RGO were synthesized by the hydrothermal method [21]. Typically, a 40 mL portion of 1.0 mg mL−1 homogeneous GO aqueous dispersion was sealed into a 50 mL Teflon-lined autoclave and maintained at 180 °C for 12 h. Then the autoclave was naturally cooled to room temperature, and the as-prepared RGO block was taken into a refrigerator until it was completely iced. Finally, the iced samples were operated under freeze drying. The EG-RGO was prepared by the similar process introducing ethylene glycol in the raw materials. 2.2. Characterizations of RGO and EG-RGO The morphologies of the as-prepared samples were measured by scanning electron microscope (SEM; JSM–SEM 6360) and transmission electron microscopy (TEM; JEM-2100 F, 200 kV). The crystalline structure was characterized by X-ray diffraction (XRD; D-MAX/2550, Cu Kα). Raman spectra were obtained by Dior LABRAM-1B instrument.
F. Tu et al. / Powder Technology 253 (2014) 580–583
581
a
2.3. Electrochemical measurement
Intensity/ a.u.
The working electrodes were constructed by mixing the prepared powder with acetylene black, and polytetrafluoroethylene at the weight ratio of 85:10:5. The slurry was then coated onto aluminum current collector at 15 M Pa and dried at 110 °C for 12 h in a vacuum oven. The active material mass loading for all of the electrodes were 2 mg cm−2. The electrochemical properties of as-prepared samples towards lithium storage were tested in the configuration of CR 2016 type coin cells which were assembled in an argon-filled glove box (Mbraun, Unilab, Germany) with a lithium foil as the counter electrode and Celgard 2500 micropore membrane as the separator. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate/diethyl carbonate/dimethyl carbonate (1:1:1 in volume ratio). The cells were discharged and charged under constant current within the voltage range from 2 V to 4.5 V vs. Li+/Li on a battery test system (LAND CT2001A, China). And the cyclic voltammetry (CV) tests were carried out on an electrochemical workstation (CHI660, China).
RGO EG-RGO GO 10
20
30
40
50
60
2theta/ degree
b
3. Results and discussion
D
The GO transformed into RGO and EG-RGO hydrogels after hydrothermal processes by self-assemble which was agreed with the previous work [21]. As shown in the Fig. 1a and b, the RGO and EG-RGO were clearly showed porous surfaces with different size pores with the diameters from nanometer to micrometer. The graphene nanosheets were further analyzed by TEM. Both RGO and EG-RGO were nearly transparent and exhibited a very stable nature under the electron beam as shown in Fig. 1c and d. Different from RGO, the well-defined diffraction spots of EG-RGO in the selected area electron diffraction (SAED) confirmed the crystalline structure of the graphene nanosheets [22], which are displayed in the inset of Fig. 1c and d. As shown in Fig. 2a, the X-ray diffraction (XRD) patterns of GO, RGO and EG-RGO at 11.02o, 23.78o and 24.42o were observed, respectively, with the corresponding interlayer spacing of 0.802, 0.374 and 0.364 nm calculated according to Bragg's equation. The interlayer
G
Intensity\ a.u.
GO
RGO EG-RGO
1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Raman shift cm-1 Fig. 2. (a) XRD patterns and (b) Raman spectra of GO, RGO and EG-RGO.
a
b
300 nm
500 nm
c
d
Fig. 1. SEM images of (a) RGO and (b) EG-RGO, and TEM images of (c) RGO (inset, SEAD image of RGO) and EG-RGO (inset, SEAD image of EG-RGO).
582
F. Tu et al. / Powder Technology 253 (2014) 580–583
spacing of RGO and EG-RGO were much smaller than that of GO, demonstrating that most of oxygen-containing groups were reduced. The EG-RGO had smaller interlayer spacing and sharper peak than that of RGO, indicating the more exhaustive reduction of EG-RGO [14], but there are still a few oxygen-containing groups because the interlayer spacing is bigger than that of graphite (0.334 nm). The peak (100) nearly disappearing in the XRD pattern of RGO appeared in that of EG-RGO. Note that the peak (100) also appeared in the XRD patterns of unzipping carbon nanotubes suggesting that the EG-RGO was more graphitizing [23]. The ordered and disordered structures of carbon materials were intensively distinguished by Raman spectra [24,25]. Fig. 2b shows the Raman spectra of GO, RGO and EG-RGO. An obvious G band at 1585 cm−1 and a typical D band at 1332 cm−1 were observed in the GO sample. The G band and D band of RGO appeared at 1587 cm− 1, 1333 cm−1 while those of EG-RGO were centered at 1578 cm−1 and 1336 cm−1. The ID/IG ratios of GO, RGO and EG-RGO were 0.898, 1.272 and 0.263, respectively. The increasing ID/IG ratio of RGO demonstrated the increasing of the edges, other defects and disordered carbon. Herein, the smallest ID/IG ratio of EG-RGO indicated that the EG-RGO have the least sp3 bond and defects [26,27]. The ethylene glycol might reduce the oxygen-contain groups, decrease the sp3 bond and offer the carbon radicals to repair the defects of graphene. Fig. 3a displays the 1st and the 3000th charge/discharge curves of EG-RGO at a current rate of 0.2 A g−1. The charge and discharge profiles were sloping lines without an obvious plateau. As we see, the 3000th cycle's discharge/charge curves were almost along with the 1st cycle's, indicating good cycling stability of EG-RGO. In Fig. 3b, the CV curves of
a
the batteries were nearly rectangular in shape. And the small peak at 3 V caused by the followed reaction [19]: þ
−
NC ¼ O þ Li þ e ↔NC O Li:
ð1Þ
Furthermore, the value of the peak current is proportional to the scan rate. The results above revealed that there were few oxygencontaining groups on the surface of graphene and a surface-redox limited process [28]. Fig. 3c and d give the specific capacities of RGO and EG-RGO at different current densities. The EG-RGO cells were tested at 0.1, 0.2, 1, 10, 16 and 20 A g − 1 with the corresponding capacities of 172, 160, 133, 116, 101 and 85 mAh g− 1 , respectively. And the RGO showed capacities of 155, 128, 137, 112 and 88 mAh g − 1 at 0.05, 0.1, 0.5, 2 and 4 A g− 1, respectively. As noted, at the low rate, the electrochemical performance of RGO is a little smaller than that of EG-RGO. With the increasing of the current density, the power capability of RGO decreased quickly and was inferior to that of EG-RGO. The power density of the EG-RGO was 53.5 kW kg − 1 at 20 A g− 1 with an energy density of 240 Wh kg − 1 , while the density of the RGO was 11 kW kg − 1 at 4 A g− 1 with an energy density of 260 Wh kg− 1 . Obviously, the EG-RGO electrodes showed better rates performance than the RGO. In the low current density, the lithium ion can circumvent the sp3 bond and the defects, so the capacity is dependent on the mass of graphene. However, in the high rates, the sp3 bond and defects will affect the transformation of electron as well as the lithium ion, resulting in a low conductivity of RGO, hence the rate performance of RGO is inferior to that of EG-RGO.
b
5
1.2
the 3000th cycle
-1
5 mVs
4
Current/ mA
+
Voltage vs Li/Li / V
-1
10 mVs
0.8
the 1st cycle
3
0.4
-1
1 mVs
0.0 -1
0.5 mVs
-0.4 -0.8
2 0
30
60
90
120
150
-1.2
180
2.0
2.5
4.0
4.5
d
200
200
RGO -1 0.05 A g
150
-1 0.05 A g
(a)
-1 0.1 A g
-1 0.5 A g -1 2Ag -1 4Ag
100
50
3.5
Voltage vs Li/Li /V
0
200
400
600
800
Cycle number
1000
1200
Specific Capacity/ mAh g-1
Specific capacity/ mAh g-1
c
3.0
+
Specific Capacity/ mAhg-1
-1 0.1 A g -1 0.2 A g 150 -1 1Ag
-1 0.2 Ag -1 10 A g
100
-1 20 A g
EG-RGO
50
0
-1 16 A g
0
500
1000 1500 2000 2500 3000
Cycle Number
Fig. 3. (a) The 1st and 3000th charge/discharge profiles of EG-RGO, (b) CV curves of EG-RGO at different scanning rates, and rate performance of (c) RGO and (d) EG-RGO.
F. Tu et al. / Powder Technology 253 (2014) 580–583
4. Conclusion A one-step hydrothermal method was demonstrated to prepared porous graphene assisted by ethylene glycol, which was green, facile and nontoxic. When used as electrode materials of LICs, the EG-RGO showed excellent electrochemical performance with high energy density, high power density and good stability, which is superior to the RGO. The EG-RGO showed a high capacity of 172 mAh g−1 at 0.1 A g−1 and maintained with almost no capacity loss after 3000 cycles. The power density of the EG-RGO was 53.5 kW kg−1 at 20 A g−1 with an energy density of 240 Wh kg−1. The rate performance of EG-RGO was superior to that of RGO. The EG-RGO could be a very promising candidate as electrode material of LICs for EVs. However, the mechanism of how the ethylene glycol repairs the defects of graphene is still needed for further studies.
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Acknowledgment This work was financially supported by the National Natural Science Foundation of China (No. 51372278).
[21] [22] [23] [24] [25]
References [26] [1] [2] [3] [4]
A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. A.K. Geim, Science 324 (2009) 1530. X. Li, X. Wang, L. Zhang, S. Lee, H. Dai, Science 319 (2008) 1229. Z. Zeng, Z. Yin, X. Huang, H. Li, Q. He, G. Lu, F. Boey, H. Zhang, Angew. Chem. Int. Ed. 50 (2011) 11093.
[27] [28]
583
X. Wang, L. Zhi, K. Muellen, Nano Lett. 8 (2008) 323. H. Lee, J. Ihm, M.L. Cohen, S.G. Louie, Nano Lett. 10 (2010) 793. C. Liu, Z. Yu, D. Neff, A. Zhamu, B.Z. Jang, Nano Lett. 10 (2010) 4863. 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) 666. P.W. Sutter, J.-I. Flege, E.A. Sutter, Nat. Mater. 7 (2008) 406. A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M.S. Dresselhaus, J. Kong, Nano Lett. 9 (2009) 30. S. Park, J. An, J.R. Potts, A. Velamakanni, S. Murali, R.S. Ruoff, Carbon 49 (2011) 3019. S. Park, R.S. Ruoff, Nat. Nanotechnol. 4 (2009) 217. W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. F. Banhart, J. Kotakoski, A.V. Krasheninnikov, ACS Nano 5 (2011) 26. M. Cheng, R. Yang, L. Zhang, Z. Shi, W. Yang, D. Wang, G. Xie, D. Shi, G. Zhang, Carbon 50 (2012) 2581. V. Lopez, R.S. Sundaram, C. Gomez-Navarro, D. Olea, M. Burghard, J. Gomez-Herrero, F. Zamora, K. Kern, Adv. Mater. 21 (2009) 4683. H. Wang, J.T. Robinson, X. Li, H. Dai, J. Am. Chem. Soc. 131 (2009) 9910. K. Naoi, S. Ishimoto, J.-i. Miyamoto, W. Naoi, Energy Environ. Sci. 5 (2012) 9363. B.Z. Jang, C. Liu, D. Neff, Z. Yu, M.C. Wang, W. Xiong, A. Zhamu, Nano Lett. 11 (2011) 3785. J.H. Lee, W.H. Shin, M.-H. Ryou, J.K. Jin, J. Kim, J.W. Choi, ChemSusChem 5 (2012) 2328. Y. Xu, K. Sheng, C. Li, G. Shi, ACS Nano 4 (2010) 4324. G. Wang, J. Yang, J. Park, X. Gou, B. Wang, H. Liu, J. Yao, J. Phys. Chem. C 112 (2008) 8192. J. Yan, T. Wei, B. Shao, Z. Fan, W. Qian, M. Zhang, F. Wei, Carbon 48 (2010) 487. A. Ferrari, Solid State Commun. 143 (2007) 47. C. Casiraghi, S. Pisana, K.S. Novoselov, A.K. Geim, A.C. Ferrari, Appl. Phys. Lett. 91 (2007) 233108. H.-J. Shin, W.M. Choi, S.-M. Yoon, G.H. Han, Y.S. Woo, E.S. Kim, S.J. Chae, X.-S. Li, A. Benayad, D.D. Loc, F. Gunes, Y.H. Lee, J.-Y. Choi, Adv. Mater. 23 (2011) 4392. D. Luo, G. Zhang, J. Liu, X. Sun, J. Phys. Chem. C 115 (2011) 11327. S.W. Lee, N. Yabuuchi, B.M. Gallant, S. Chen, B.-S. Kim, P.T. Hammond, Y. Shao-Horn, Nat. Nanotechnol. 5 (2010) 531.
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Short communication
Porous graphene as cathode material for lithium ion capacitor with high electrochemical performance Feiyue Tu, Suqin Liu ⁎, Tonghua Wu, Guanhua Jin, Chunyue Pan College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 13 August 2013 Received in revised form 20 November 2013 Accepted 1 December 2013 Available online 8 December 2013 Keywords: Hydrothermal Graphene Lithium ion capacitors Porous
a b s t r a c t The porous reduced graphene is prepared via an ethylene glycol assisted-hydrothermal method which is green, simple and nontoxic. The ethylene glycol assisted-hydrothermal reduced graphene (EG-RGO) delivers superior power density compared to those prepared without the assistant of ethylene glycol when used as cathode materials for the lithium ion capacitors. The EG-RGO electrode shows a high capacity of 172 mAh g−1 at 0.1 A g−1 and without capacity loss after 3000 cycles. Furthermore, the power density of the electrodes reaches 53.5 kW kg − 1 at the current density of 20 A g − 1 with an energy density of 240 Wh kg − 1 . The high electrochemical performance of EG-RGO is attributed to the high electronic conductivity of the graphene. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Graphene, the two-dimension carbon material, has attracted great interest for its excellent mechanical, electrical, thermal and optical properties, which holds a great promise for potential application in many technological fields, including nanoelectronics, hydrogen storage, solar energy cells, Li-ion batteries, supercapacitors and so on [1–7]. It can be produced by micro-mechanical exfoliation of highly ordered pyrolytic graphite, epitaxial growth, chemical vapor deposition and reduction of graphene oxide (GO) [8–11]. Among these methods, the reduction of GO is recognized as the most massive method and easy to further functionalization, which is important to the large-scale use of graphene [12]. The GO can be prepared by the oxidation of inexpensive graphite with strong acid and oxidants [13]. Within the oxidation process, the surface of GO is distributed with many oxygen-containing groups and the structural defects are also inevitable, which seriously affect the conductivity of graphene [14]. Recently, some researchers have repaired the defects of graphene by chemical vapor deposition (CVD) processing [15,16], considering that the hydrothermal method is difficult to repair the large numbers of defects in GO by hydrazine monohydrate [17]. As we know, the traditional capacitors, namely, electric double layer capacitors (EDLCs) consist of two symmetrical electrodes of activated carbon and an aqueous electrolyte, displaying very high power density and long cycling life but very low energy density compared to lithium ion batteries (LICs). Different from EDLCs, the LICs have combined the advantages of supercapacitors (high power) and lithium ion batteries ⁎ Corresponding author. Fax: +86 73188879850. E-mail address: [email protected] (S. Liu). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.12.008
(high energy) due to the high voltage durable nonaqueous electrolyte and asymmetric electrodes [18]. Recently, graphene-based LICs have been drawing people's attention for its high capacity, high energy density, high power density as well as long cycle life [19,20]. Therefore, we prepared the porous graphene assisted by the ethylene glycol as cathode materials for lithium ion capacitor. And the EG-RGO showed an enhanced electrochemical performance compared to that of hydrothermal reduced graphene oxide (RGO). 2. Experimental methods 2.1. Preparation of RGO and EG-RGO The GO was prepared by Hummer's method [13]. The RGO and EG-RGO were synthesized by the hydrothermal method [21]. Typically, a 40 mL portion of 1.0 mg mL−1 homogeneous GO aqueous dispersion was sealed into a 50 mL Teflon-lined autoclave and maintained at 180 °C for 12 h. Then the autoclave was naturally cooled to room temperature, and the as-prepared RGO block was taken into a refrigerator until it was completely iced. Finally, the iced samples were operated under freeze drying. The EG-RGO was prepared by the similar process introducing ethylene glycol in the raw materials. 2.2. Characterizations of RGO and EG-RGO The morphologies of the as-prepared samples were measured by scanning electron microscope (SEM; JSM–SEM 6360) and transmission electron microscopy (TEM; JEM-2100 F, 200 kV). The crystalline structure was characterized by X-ray diffraction (XRD; D-MAX/2550, Cu Kα). Raman spectra were obtained by Dior LABRAM-1B instrument.
F. Tu et al. / Powder Technology 253 (2014) 580–583
581
a
2.3. Electrochemical measurement
Intensity/ a.u.
The working electrodes were constructed by mixing the prepared powder with acetylene black, and polytetrafluoroethylene at the weight ratio of 85:10:5. The slurry was then coated onto aluminum current collector at 15 M Pa and dried at 110 °C for 12 h in a vacuum oven. The active material mass loading for all of the electrodes were 2 mg cm−2. The electrochemical properties of as-prepared samples towards lithium storage were tested in the configuration of CR 2016 type coin cells which were assembled in an argon-filled glove box (Mbraun, Unilab, Germany) with a lithium foil as the counter electrode and Celgard 2500 micropore membrane as the separator. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate/diethyl carbonate/dimethyl carbonate (1:1:1 in volume ratio). The cells were discharged and charged under constant current within the voltage range from 2 V to 4.5 V vs. Li+/Li on a battery test system (LAND CT2001A, China). And the cyclic voltammetry (CV) tests were carried out on an electrochemical workstation (CHI660, China).
RGO EG-RGO GO 10
20
30
40
50
60
2theta/ degree
b
3. Results and discussion
D
The GO transformed into RGO and EG-RGO hydrogels after hydrothermal processes by self-assemble which was agreed with the previous work [21]. As shown in the Fig. 1a and b, the RGO and EG-RGO were clearly showed porous surfaces with different size pores with the diameters from nanometer to micrometer. The graphene nanosheets were further analyzed by TEM. Both RGO and EG-RGO were nearly transparent and exhibited a very stable nature under the electron beam as shown in Fig. 1c and d. Different from RGO, the well-defined diffraction spots of EG-RGO in the selected area electron diffraction (SAED) confirmed the crystalline structure of the graphene nanosheets [22], which are displayed in the inset of Fig. 1c and d. As shown in Fig. 2a, the X-ray diffraction (XRD) patterns of GO, RGO and EG-RGO at 11.02o, 23.78o and 24.42o were observed, respectively, with the corresponding interlayer spacing of 0.802, 0.374 and 0.364 nm calculated according to Bragg's equation. The interlayer
G
Intensity\ a.u.
GO
RGO EG-RGO
1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Raman shift cm-1 Fig. 2. (a) XRD patterns and (b) Raman spectra of GO, RGO and EG-RGO.
a
b
300 nm
500 nm
c
d
Fig. 1. SEM images of (a) RGO and (b) EG-RGO, and TEM images of (c) RGO (inset, SEAD image of RGO) and EG-RGO (inset, SEAD image of EG-RGO).
582
F. Tu et al. / Powder Technology 253 (2014) 580–583
spacing of RGO and EG-RGO were much smaller than that of GO, demonstrating that most of oxygen-containing groups were reduced. The EG-RGO had smaller interlayer spacing and sharper peak than that of RGO, indicating the more exhaustive reduction of EG-RGO [14], but there are still a few oxygen-containing groups because the interlayer spacing is bigger than that of graphite (0.334 nm). The peak (100) nearly disappearing in the XRD pattern of RGO appeared in that of EG-RGO. Note that the peak (100) also appeared in the XRD patterns of unzipping carbon nanotubes suggesting that the EG-RGO was more graphitizing [23]. The ordered and disordered structures of carbon materials were intensively distinguished by Raman spectra [24,25]. Fig. 2b shows the Raman spectra of GO, RGO and EG-RGO. An obvious G band at 1585 cm−1 and a typical D band at 1332 cm−1 were observed in the GO sample. The G band and D band of RGO appeared at 1587 cm− 1, 1333 cm−1 while those of EG-RGO were centered at 1578 cm−1 and 1336 cm−1. The ID/IG ratios of GO, RGO and EG-RGO were 0.898, 1.272 and 0.263, respectively. The increasing ID/IG ratio of RGO demonstrated the increasing of the edges, other defects and disordered carbon. Herein, the smallest ID/IG ratio of EG-RGO indicated that the EG-RGO have the least sp3 bond and defects [26,27]. The ethylene glycol might reduce the oxygen-contain groups, decrease the sp3 bond and offer the carbon radicals to repair the defects of graphene. Fig. 3a displays the 1st and the 3000th charge/discharge curves of EG-RGO at a current rate of 0.2 A g−1. The charge and discharge profiles were sloping lines without an obvious plateau. As we see, the 3000th cycle's discharge/charge curves were almost along with the 1st cycle's, indicating good cycling stability of EG-RGO. In Fig. 3b, the CV curves of
a
the batteries were nearly rectangular in shape. And the small peak at 3 V caused by the followed reaction [19]: þ
−
NC ¼ O þ Li þ e ↔NC O Li:
ð1Þ
Furthermore, the value of the peak current is proportional to the scan rate. The results above revealed that there were few oxygencontaining groups on the surface of graphene and a surface-redox limited process [28]. Fig. 3c and d give the specific capacities of RGO and EG-RGO at different current densities. The EG-RGO cells were tested at 0.1, 0.2, 1, 10, 16 and 20 A g − 1 with the corresponding capacities of 172, 160, 133, 116, 101 and 85 mAh g− 1 , respectively. And the RGO showed capacities of 155, 128, 137, 112 and 88 mAh g − 1 at 0.05, 0.1, 0.5, 2 and 4 A g− 1, respectively. As noted, at the low rate, the electrochemical performance of RGO is a little smaller than that of EG-RGO. With the increasing of the current density, the power capability of RGO decreased quickly and was inferior to that of EG-RGO. The power density of the EG-RGO was 53.5 kW kg − 1 at 20 A g− 1 with an energy density of 240 Wh kg − 1 , while the density of the RGO was 11 kW kg − 1 at 4 A g− 1 with an energy density of 260 Wh kg− 1 . Obviously, the EG-RGO electrodes showed better rates performance than the RGO. In the low current density, the lithium ion can circumvent the sp3 bond and the defects, so the capacity is dependent on the mass of graphene. However, in the high rates, the sp3 bond and defects will affect the transformation of electron as well as the lithium ion, resulting in a low conductivity of RGO, hence the rate performance of RGO is inferior to that of EG-RGO.
b
5
1.2
the 3000th cycle
-1
5 mVs
4
Current/ mA
+
Voltage vs Li/Li / V
-1
10 mVs
0.8
the 1st cycle
3
0.4
-1
1 mVs
0.0 -1
0.5 mVs
-0.4 -0.8
2 0
30
60
90
120
150
-1.2
180
2.0
2.5
4.0
4.5
d
200
200
RGO -1 0.05 A g
150
-1 0.05 A g
(a)
-1 0.1 A g
-1 0.5 A g -1 2Ag -1 4Ag
100
50
3.5
Voltage vs Li/Li /V
0
200
400
600
800
Cycle number
1000
1200
Specific Capacity/ mAh g-1
Specific capacity/ mAh g-1
c
3.0
+
Specific Capacity/ mAhg-1
-1 0.1 A g -1 0.2 A g 150 -1 1Ag
-1 0.2 Ag -1 10 A g
100
-1 20 A g
EG-RGO
50
0
-1 16 A g
0
500
1000 1500 2000 2500 3000
Cycle Number
Fig. 3. (a) The 1st and 3000th charge/discharge profiles of EG-RGO, (b) CV curves of EG-RGO at different scanning rates, and rate performance of (c) RGO and (d) EG-RGO.
F. Tu et al. / Powder Technology 253 (2014) 580–583
4. Conclusion A one-step hydrothermal method was demonstrated to prepared porous graphene assisted by ethylene glycol, which was green, facile and nontoxic. When used as electrode materials of LICs, the EG-RGO showed excellent electrochemical performance with high energy density, high power density and good stability, which is superior to the RGO. The EG-RGO showed a high capacity of 172 mAh g−1 at 0.1 A g−1 and maintained with almost no capacity loss after 3000 cycles. The power density of the EG-RGO was 53.5 kW kg−1 at 20 A g−1 with an energy density of 240 Wh kg−1. The rate performance of EG-RGO was superior to that of RGO. The EG-RGO could be a very promising candidate as electrode material of LICs for EVs. However, the mechanism of how the ethylene glycol repairs the defects of graphene is still needed for further studies.
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Acknowledgment This work was financially supported by the National Natural Science Foundation of China (No. 51372278).
[21] [22] [23] [24] [25]
References [26] [1] [2] [3] [4]
A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. A.K. Geim, Science 324 (2009) 1530. X. Li, X. Wang, L. Zhang, S. Lee, H. Dai, Science 319 (2008) 1229. Z. Zeng, Z. Yin, X. Huang, H. Li, Q. He, G. Lu, F. Boey, H. Zhang, Angew. Chem. Int. Ed. 50 (2011) 11093.
[27] [28]
583
X. Wang, L. Zhi, K. Muellen, Nano Lett. 8 (2008) 323. H. Lee, J. Ihm, M.L. Cohen, S.G. Louie, Nano Lett. 10 (2010) 793. C. Liu, Z. Yu, D. Neff, A. Zhamu, B.Z. Jang, Nano Lett. 10 (2010) 4863. 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) 666. P.W. Sutter, J.-I. Flege, E.A. Sutter, Nat. Mater. 7 (2008) 406. A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M.S. Dresselhaus, J. Kong, Nano Lett. 9 (2009) 30. S. Park, J. An, J.R. Potts, A. Velamakanni, S. Murali, R.S. Ruoff, Carbon 49 (2011) 3019. S. Park, R.S. Ruoff, Nat. Nanotechnol. 4 (2009) 217. W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. F. Banhart, J. Kotakoski, A.V. Krasheninnikov, ACS Nano 5 (2011) 26. M. Cheng, R. Yang, L. Zhang, Z. Shi, W. Yang, D. Wang, G. Xie, D. Shi, G. Zhang, Carbon 50 (2012) 2581. V. Lopez, R.S. Sundaram, C. Gomez-Navarro, D. Olea, M. Burghard, J. Gomez-Herrero, F. Zamora, K. Kern, Adv. Mater. 21 (2009) 4683. H. Wang, J.T. Robinson, X. Li, H. Dai, J. Am. Chem. Soc. 131 (2009) 9910. K. Naoi, S. Ishimoto, J.-i. Miyamoto, W. Naoi, Energy Environ. Sci. 5 (2012) 9363. B.Z. Jang, C. Liu, D. Neff, Z. Yu, M.C. Wang, W. Xiong, A. Zhamu, Nano Lett. 11 (2011) 3785. J.H. Lee, W.H. Shin, M.-H. Ryou, J.K. Jin, J. Kim, J.W. Choi, ChemSusChem 5 (2012) 2328. Y. Xu, K. Sheng, C. Li, G. Shi, ACS Nano 4 (2010) 4324. G. Wang, J. Yang, J. Park, X. Gou, B. Wang, H. Liu, J. Yao, J. Phys. Chem. C 112 (2008) 8192. J. Yan, T. Wei, B. Shao, Z. Fan, W. Qian, M. Zhang, F. Wei, Carbon 48 (2010) 487. A. Ferrari, Solid State Commun. 143 (2007) 47. C. Casiraghi, S. Pisana, K.S. Novoselov, A.K. Geim, A.C. Ferrari, Appl. Phys. Lett. 91 (2007) 233108. H.-J. Shin, W.M. Choi, S.-M. Yoon, G.H. Han, Y.S. Woo, E.S. Kim, S.J. Chae, X.-S. Li, A. Benayad, D.D. Loc, F. Gunes, Y.H. Lee, J.-Y. Choi, Adv. Mater. 23 (2011) 4392. D. Luo, G. Zhang, J. Liu, X. Sun, J. Phys. Chem. C 115 (2011) 11327. S.W. Lee, N. Yabuuchi, B.M. Gallant, S. Chen, B.-S. Kim, P.T. Hammond, Y. Shao-Horn, Nat. Nanotechnol. 5 (2010) 531.