- Email: [email protected]
graphene composite as anode materials for lithium ion batteries
Journal of Physics and Chemistry of Solids 75 (2014) 1205–1209
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
One-pot facile synthesis of CuS/graphene composite as anode materials for lithium ion batteries Hua-Chao Tao a,b, Xue-Lin Yang a,b,n, Lu-Lu Zhang a,b, Shi-Bing Ni a,b a b
College of Materials and Chemical Engineering, China Three Gorges University, 8 Daxue Road, Yichang, Hubei 443002, China Collaborative Innovation Center for Microgrid of New Energy, Hubei Province, China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 May 2014 Received in revised form 7 June 2014 Accepted 11 June 2014 Available online 19 June 2014
CuS/graphene composite has been synthesized by the one-pot hydrothermal method using thiourea as the sulfur source and reducing agent. The formation of CuS nanoparticles and the reduction of graphene oxide occur simultaneously during the hydrothermal process, which enables a uniform dispersion of CuS nanoparticles on the graphene nanosheets. The electrochemical performance of CuS/graphene composite was studied as anode materials for lithium ion batteries. The obtained CuS/graphene composite exhibits a relative high reversible capacity and good cycling stability. The good electrochemical performance of CuS/graphene composite can be attributed to graphene, which improves the electronic conductivity of composite and enhances the interfacial stability of electrode and electrolyte. & 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Chalcogenides B. Chemical synthesis C. X-ray diffraction D. Electrochemical properties
1. Introduction Lithium ion batteries with high energy density and long cycle life are strongly desired for rapid growing portable electronic equipments and electric vehicles. Carbon as a commercialized anode material for lithium ion batteries has a low theoretical capacity (372 mAh g 1); many efforts have been devoted to studying safe and cheap anode materials with large reversible capacity, desirable rate capability, long cycle life and good compatibility with electrolyte [1,2]. Transition metal sulfides have been widely investigated due to their high specific capacity, low cost, safety and environmental benignity [3–10]. However, most of the metal sulfides undergo a fast reversible capacity fading during cycles, which can be attributed to large volume change during lithium ions insertion and extraction process and low electronic conductivity [11]. To overcome these problems, one of the most effective way is to design and fabricate composite with a carbon matrix which acts as electrically connecting media and a buffer layer for volume change. Graphene-based materials such as NiO/graphene [12], MoS2/ graphene [13], NiS/graphene [14], CoS2/graphene [15], FeS/graphene [16] and Bi2S3/graphene [17] have been prepared to exhibit excellent electrochemical performance because of high electronic conductivity, large specific surface area and high mechanical strength of graphene.
Among these composites, CuS/graphene composite has also been prepared using thioacetamide as reducing agent and it exhibits high peroxidase-like catalytic activity [18]. However, little research has been done on CuS/graphene composite as anode materials for lithium ion batteries. CuS as anode materials for lithium ion batteries has many advantages, such as high capacity, abundant resource and less toxicity [19]. In addition, numerous methods of chemical reduction of graphene oxide (GO) to graphene have been studied, such as hydrazine hydrate (N2H4 H2O) [20], sodium borohydride (NaBH4) [21], lithium aluminum hydride (LiAlH4) [22] and hydrohalic acid (HI and HBr) [23,24] as reducing agents. These methods are environmentally unfriendly when these reducing agents are used. In this paper, GO was reduced to graphene using thiourea as reducing agent by the hydrothermal method. Meanwhile, CuS nanoparticles were synthesized during hydrothermal process to obtain uniform dispersion on the graphene nanosheets. The reduction of GO and the deposition of CuS on graphene occur simultaneously by the one-pot method. The one-pot method to prepare CuS/ graphene composite can assure the uniform distribution of CuS nanoparticles on graphene nanosheets. The obtained CuS/graphene composite exhibits good lithium ions storage performance.
2. Experimental n Corresponding author at: College of Materials and Chemical Engineering, China Three Gorges University, 8 Daxue Road, Yichang, Hubei 443002, China. Fax: þ86 717 6397559. E-mail address: [email protected] (X.-L. Yang).
http://dx.doi.org/10.1016/j.jpcs.2014.06.010 0022-3697/& 2014 Elsevier Ltd. All rights reserved.
2.1. Preparation of CuS/graphene composite Graphite oxide was synthesized according to the modified Hummers method [25] using natural graphite as raw materials.
1206
H.-C. Tao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 1205–1209
The working electrodes were prepared by mixing active materials (CuS and CuS/graphene), acetylene black and polyvinylidene fluoride (PVDF) with a mass ratio of 70:20:10 using N-methyl-2pyrrolidone (NMP) as solvent. The resulting slurry was pasted on pure copper foil and then dried at 120 1C under vacuum for 24 h. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 and lithium foil was used as the counter electrode. Coin cells were assembled in a glove box filled with pure Ar gas. Galvanostatic discharge/charge tests were carried out on a Land CT2100 battery test system. Cyclic voltammetry (CV) measurements were performed using a CHI660C electrochemical workstation (Shanghai Chenhua) between 0.01 V and 3 V at a scan rate of 0.2 mV s 1. Electrochemical impedance spectroscopy (EIS) measurements were measured on a CHI660C electrochemical workstation in the frequency range from 0.01 Hz to 100 kHz with an amplitude of 5 mV. All of the electrochemical tests were carried out at 25 1C.
(116)
(114)
(101)
Intensity (a.u.)
2.3. Electrochemical characterization
CuS/graphene
CuS
graphite oxide 10
20
30
40
50 60 2θ (degree)
70
80
90
CuS
CuS/graphene
Intensity (a.u.)
The crystal structure of CuS and CuS/graphene was characterized by X-ray diffraction (XRD, Rigaku/mac) with Cu Ka radiation. The morphology and structure were analyzed using transmission electron microscopy (TEM, JEM-2100F) and high resolution transmission electron microscopy (HRTEM). Raman spectra were obtained with a LabRAM Aramis using 514.5 nm Ar-ion laser.
(102) (103)
2.2. Structural characterization
(110)
Graphite oxide (90 mg) was dispersed in deionized water (40 mL) via sonication for 2 h to obtain the homogeneous yellow-brown GO solution. To prepare CuS/graphene composite through the onepot method, CuSO4 5H2O (1 g, analytical grade) and thiourea (0.3 g, analytical grade) were added into the stable suspension of GO. After vigorous stirring, the solution was transferred into a Teflon-line stainless steel autoclave and reacted at 180 1C for 24 h. The black solid was washed with deionized water and alcohol to obtain CuS/graphene composite. The obtained product was dried at 80 1C under vacuum. For comparison, pure CuS was prepared under the same condition without adding the GO suspension.
GO
3. Results and discussion Fig. 1a illustrates the procedure for the fabrication of CuS/ graphene composite. GO, CuSO4 5H2O and thiourea were dispersed in deionized water under stirring and ultrasound. During the hydrothermal process, the CuS nanoparticles were obtained and GO was reduced to graphene simultaneously. The synthesis process of the CuS/graphene composite may be mainly divided into two steps. Firstly, GO incorporated with Cu2 þ ions by electrostatic force. Secondly, S2 and NH2–NH2 ions derived from the decomposition of N2H4CS, GO was reduced to graphene by NH2–NH2 ions and the S2 transformed into CuS. The reactions during the hydrothermal process can be described as follows: GO þCu2 þ -Cu2 þ –GO 2
N2H4CS-S
þ 2NH2 þC
(1) (2)
Cu2 þ þS2 -CuS
(3)
GO þNH2-graphene
(4)
The XRD patterns of graphite oxide, CuS and CuS/graphene composite are compared in Fig. 1b. There is a characteristic peak of graphite oxide at 10.51, which corresponds to (001) plane. In the case of CuS, the peaks at 27.61, 29.31, 31.81, 47.91, 53.11 and 58.71
300
600
900
1200
Raman shift
1500
1800
cm-1
Fig. 1. Schematic illustration for the fabrication process of CuS/graphene composite (a); X-ray diffraction patterns for graphite oxide, CuS and CuS/graphene composite (b); Raman spectra of GO and CuS/graphene composite (c).
can be indexed to the (101), (102), (103), (110), (114) and (116) planes of CuS (JCPDS, no.: 00-001-1281). The obtained CuS/graphene composite displays a similar XRD patterns to pure CuS, indicating that the introduction of GO does not influence the fabrication of CuS nanostructure. In addition, the characteristic diffraction peaks of graphene are not detected in the CuS/graphene composite due to its low amount and low diffraction intensity. The presence of both CuS and graphene is further confirmed by Raman spectroscopy. Fig. 1c shows the Raman spectra of GO and the CuS/graphene composite. Two characteristic peaks of the D band and G band from CuS/graphene composite are observed at about 1350 cm 1 and 1586 cm 1. One characteristic peak from the CuS at 471 cm 1 is observed in the spectrum of the CuS/graphene composite. Both XRD and Raman measurements confirm the successful integration of graphene and CuS nanoparticles. The intensity ratio of the D band
H.-C. Tao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 1205–1209
to G band (ID/IG) for GO is about 0.97. In the case of CuS/graphene composite, the ID/IG increases to 1.05 due to the decrease of the average size and increased quantity of the sp2 domains, indicating the successful reduction of GO to graphene [20]. The morphology and structure of GO, CuS and CuS/graphene composite were examined by TEM and HRTEM, as shown in Fig. 2. GO has a typical lamellar structure (Fig. 2a). The pure CuS nanoparticles with an average diameter of about 50 nm are agglomerated (Fig. 2b). The CuS nanoparticles with a size of about 50–80 nm are distributed onto the graphene nanosheets (Fig. 2c). From the HRTEM image (Fig. 2d) of CuS/graphene composite, the carbon layer with a thickness of about 5 nm and the CuS with d spacing of 0.28 nm corresponding to the (103) plane can be observed, indicating the presence of graphene and CuS. To evaluate the effect of graphene on the electrochemical properties of CuS, electrochemical measurements on CuS and CuS/graphene composite were carried out. The first two discharge (lithium ion insertion)–charge (lithium ion extraction) curves of CuS and CuS/graphene composite between 0.01 and 3 V at a current density of 50 mA g 1 are compared in Fig. 3. For the CuS electrode, the first discharge and charge capacities are 525 and 311 mAh g 1, respectively. The reversible capacity decreases to 270 mAh g 1 for the second cycle (Fig. 3a). In addition, there are two plateaus at about 2.05 and 1.68 V during the first discharge process, and two plateaus at about 1.85 and 2.25 V are observed during the first charge process, while the discharge and charge plateaus obviously shorten during the second discharge/charge process. The plateau position is the same as that in the previous study [19,26]. Compared to CuS electrode, the first discharge and charge capacities of CuS/graphene composite are 827 and 484 mAh g 1, respectively (Fig. 3b). The specific capacities were calculated based on total mass weight of CuS/graphene composite. The large irreversible capacity loss during the first cycle can be attributed to the decomposition of electrolyte and the formation of
1207
solid electrolyte interface (SEI) layer [27]. The reversible specific capacity of CuS/graphene is close to the theoretical capacities of 560 mAh g 1 for CuS. This may be attributed to graphene, which provides the high specific capacity during low potential range of 0.01–1 V. The reversible specific capacity of 473 mAh g 1 for CuS/ graphene electrode during the second cycle is close to the first reversible capacity. Fig. 4 shows the cycling stabilities of CuS and CuS/graphene in the voltage of 0.01–3 V at a rate of 50 mA g 1. The CuS/graphene composite electrode exhibits a reversible capacity of 296 mAh g 1 after 25 cycles with a capacity retention rate of 61%. The pure CuS electrode demonstrates a reversible capacity of only 50 mAh g 1 after 10 cycles. The enhanced electrochemical performance of CuS/ graphene can be attributed to the unique composite architecture of CuS nanoparticles dispersed in graphene nanosheets. In addition, graphene not only increases the electronic conductivity of composite electrode but also acts as a buffer layer that maintains the structural integrity of the electrode after large volume changes during the discharge–charge process. Moreover, graphene prevents the agglomeration of CuS nanoparticles and increases the contact area of composite and electrolyte and makes for the formation of stable SEI layer on the surface of electrode. To further confirm the enhanced electronic conductivity by graphene, EIS measurements on CuS and CuS/graphene composite were carried out before cycling. The Nyquist plots for both CuS and CuS/graphene composite electrodes consist of one depressed semicircle in the high-frequency region and a straight line in the low-frequency region (Fig. 5). The depressed semicircle is due to the charge-transfer resistance [28], and the straight line is attributed to the Li ions diffusion process with electrode [29]. The smaller semicircle for CuS/graphene indicates a lower electrochemical resistance than that of CuS. This result confirms that the interconnected graphene nanosheets can remarkably improve the electronic conductivity of composite electrode.
CuS graphene
Fig. 2. TEM images of GO (a), CuS (b) and CuS/graphene composite (c) and HRTEM image of CuS/graphene composite (d).
1208
H.-C. Tao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 1205–1209
300
3.0
CuS/graphene CuS
250
2.0 1.5
200 15
150
-Z" (ohm)
1st 2nd -Z" (ohm)
Voltage V vs. Li/Li+
2.5
100
1.0
5 0 0
50
0.5
10
5 10 Z' (ohm)
15
0
0.0 0
100
200
300
400
500
0
600
50
100
Specific capacity mAh g-1
150 200 Z' (ohm)
250
300
Fig. 5. Nyquist plots of CuS and CuS/graphene composite in an open-circuit potential before cycling.
3.0 1.5
1st 2nd
2.0
1.0
1.5
0.5
Current (mA)
Voltage V vs. Li/Li+
2.5
1.0 0.5 0.0
1st 2nd
0.0 -0.5 -1.0
0
200 400 600 800 Specific capacity mAh g-1
1000 -1.5
Fig. 3. The first two discharge–charge curves of CuS (a) and CuS/graphene composite (b) between 0.01 V and 3 V at a rate of 50 mA g 1.
0.0
0.5
1.0
1.5
2.0
Potential V vs. Li/Li
2.5
3.0
+
Fig. 6. Cyclic voltammetry profiles of the first two cycles for CuS/graphene composite from 0.01 V to 3 V vs. Li/Li þ at a scan rate of 0.2 mV s 1.
Specific capacity mAh g-1
1000
Solid-Li ion insertion Open-Li ion extraction
800
the formation of a stable SEI layer. The electrochemical reactions of CuS electrode during the discharge process can be expressed as follows:
600
CuSþ xLi þ þ xe -LixCuS
(5)
LixCuSþ (2 x)Li þ þ(2 x)e -Li2Sþ Cu
(6)
CuS/graphene
400
Two anodic peaks at 2.0 and 2.45 V can be attributed to the reversible electrochemical reaction of (5) and (6). These results are well consistent with the discharge–charge curves.
CuS
200 0 0
5
10 15 Cycle number
20
25
Fig. 4. Cycling stabilities of CuS and CuS/graphene composite at a rate of 50 mA g 1 in the voltage range of 0.01–3 V.
Cyclic voltammograms of CuS/graphene composite are shown in Fig. 6. The cathodic peaks at about 1.92 and 1.48 V in the first scan shift to 2.0 and 1.42 V in the second cycle, which correspond to lithium ions intercalation into the CuS lattices and the decomposition of CuS into metallic Cu and Li2S, respectively [30]. Another cathodic peak at about 0.6 V is attributed to the decomposition of electrolyte and the formation of SEI film on the surface of electrode. This peak disappears during the second cycle, indicating
4. Conclusions A hybrid based on CuS and graphene has been successfully synthesized by the one-pot facile hydrothermal process. The CuS/ graphene composite exihibits obvious improved cycling stability compared to bare CuS. The enhanced cycling stability is mainly due to the good electronically conducting channels offered by graphene and the unique structure of composite. These results confirm the potential application of CuS/graphene for practical Li ion batteries with high-energy density. We expect that this facile preparation method could be extended to other graphene-based electrode materials in electrochemical energy storage and conversion.
H.-C. Tao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 1205–1209
Acknowledgments The authors acknowledge the financial supports from the National Natural Science Foundation of China (NSFC, 51272128, 51302152, and 51302153) and the Education Office of Hubei Province (D20131303 and Q20111209). The authors are grateful to Dr. Jianlin Li at the China Three Gorges University for his kind support to our research. References [1] N. Yan, L. Hu, Y. Li, Y. Wang, H. Zhong, X. Hu, X. Kong, Q. Chen, Co3O4 nanocages for high-performance anode material in lithium-ion batteries, J. Phys. Chem. C 116 (2012) 7227–7235. [2] S. Nam, S. Kim, S. Wi, H. Choi, S. Byun, S. Choi, S. Yoo, K.T. Lee, B. Park, The role of carbon incorporation in SnO2 nanoparticles for Li rechargeable batteries, J. Power Sources 211 (2012) 154–160. [3] C. Xu, Y. Zeng, X. Rui, N. Xiao, J. Zhu, W. Zhang, J. Chen, W. Liu, H. Tan, H.H. Hng, Q. Yan, Controlled soft-template synthesis of ultrathin C@FeS nanosheets with high-Li-storage performance, ACS Nano 6 (2012) 4713–4721. [4] C. Zhang, H.B. Wu, Z. Guo, X.W. Lou, Facile synthesis of carbon-coated MoS2 nanorods with enhanced lithium storage properties, Electrochem. Commun. 20 (2012) 7–10. [5] J. Ma, Z. Liu, J. Lian, X. Duan, T. Kim, P. Peng, X. Liu, Q. Chen, G. Yao, W. Zheng, Ionic liquids-assisted synthesis and electrochemical properties of Bi2S3 nanostructures, Cryst. Eng. Commun. 13 (2011) 3072–3079. [6] Y. Gu, Y. Xu, Y. Wang, Graphene-wrapped CoS nanoparticles for high-capacity lithium-ion storage, ACS Appl. Mater. Interfaces 5 (2013) 801–806. [7] C.H. Lai, M.Y. Lu, L.J. Chen, Metal sulfide nanostructures: synthesis, properties and applications in energy conversion and storage, J. Mater. Chem. 22 (2012) 19–30. [8] B. Qu, M. Zhang, D. Lei, Y. Zeng, Y. Chen, L. Chen, Q. Li, Y. Wang, T. Wang, Facile solvothermal synthesis of mesoporous Cu2SnS3 spheres and their application in lithium-ion batteries, Nanoscale 3 (2011) 3646–3651. [9] J.O. Lee, J.U. Seo, J.H. Song, C.M. Park, C.K. Lee, Electrochemical characteristics of ternary compound CoSbS for application in Li secondary batteries, Electrochem. Commun. 28 (2013) 71–74. [10] X. Yin, C. Tang, M. Chen, S. Adams, H. Wan, H. Gong, Hierarchical porous Cu2ZnSnS4 films for high-capacity reversible lithium storage applications, J. Mater. Chem. A 1 (2013) 7927–7932. [11] C.M. Park, Y. Hwa, N.E. Sun, H.J. Sohn, Stibnite (Sb2S3) and its amorphous composite as dual electrodes for rechargeable lithium batteries, J. Mater. Chem. 20 (2010) 1097–1102. [12] Y.J. Mai, S.J. Shi, D. Zhang, Y. Lu, C.D. Gu, J.P. Tu, NiO–graphene hybrid as an anode material for lithium ion batteries, J Power Sources 204 (2012) 155–161. [13] L. David, R. Bhandavat, G. Singh, MoS2/graphene composite paper for sodiumion battery electrodes, ACS Nano 8 (2014) 1759–1770.
1209
[14] Q. Pan, J. Xie, S. Liu, G. Cao, T. Zhu, X. Zhao, Facile one-pot synthesis of ultrathin NiS nanosheets anchored on graphene and the improved electrochemical Listorage properties, RSC Adv. 3 (2013) 3899–3906. [15] J. Xie, S. Liu, G. Cao, T. Zhu, X. Zhao, Self-assembly of CoS2/graphene nanoarchitecture by a facile one-pot route and its improved electrochemical Li-storage properties, Nano Energy 2 (2013) 49–56. [16] L. Fei, Q. Lin, B. Yuan, G. Chen, P. Xie, Y. Li, Y. Xu, S. Deng, S. Smirnov, H. Luo, Reduced graphene oxide wrapped FeS nanocomposite for lithium-ion battery anode with improved performance, ACS Appl. Mater. Interfaces 5 (2013) 5330–5335. [17] Z. Zhang, C. Zhou, L. Huang, X. Wang, Y. Qu, Y. Lai, J. Li, Synthesis of bismuth sulfide/reduced graphene oxide composites and their electrochemical properties for lithium ion batteries, Electrochim. Acta 114 (2013) 88–94. [18] G. Nie, L. Zhang, X. Lu, X. Bian, W. Sun, C. Wang, A one-pot and in situ synthesis of CuS–graphene nanosheet composites with enhanced peroxidase-like catalytic activity, Dalton Trans. 42 (2013) 14006–14013. [19] J.S. Chung, H.J. Sohn, Electrochemical behaviors of CuS as a cathode material for lithium secondary batteries, J. Power Sources 108 (2002) 226–231. [20] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon 45 (2007) 1558–1565. [21] Y. Si, E.T. Samulski, Synthesis of water soluble graphene, Nano Lett. 8 (2008) 1679–1682. [22] A. Ambrosi, C.K. Chua, A. Bonanni, M. Pumera, Lithium aluminum hydride as reducing agent for chemically reduced graphene oxides, Chem. Mater. 24 (2012) 2292–2298. [23] S.F. Pei, J.P. Zhao, J.H. Du, W.C. Ren, H.M. Cheng, Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids, Carbon 48 (2010) 4466–4474. [24] Y. Chen, X. Zhang, D. Zhang, P. Yu, Y. Ma, High performance supercapacitors based on reduced graphene oxide in aqueous and ionic liquid electrolytes, Carbon 49 (2011) 573–580. [25] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. [26] Y. Du, Z. Yin, J. Zhu, X. Huang, X.J. Wu, Z. Zeng, Q. Yan, H. Zhang, A general method for the large-scale synthesis of uniform ultrathin metal sulphide nanocrystals, Nat. Commun. 3 (2012) 1177. [27] Y. Liu, K. Hanai, J. Yang, N. Imanishi, A. Hirano, Y. Takeda, Silicon/carbon composites as anode materials for Li-ion batteries, Electrochem. Solid-State Lett. 7 (2004) A369–A372. [28] R. Ruffo, S.S. Hong, C.K. Chan, R.A. Huggins, Y. Cui, Impedance analysis of silicon nanowire lithium ion battery anodes, J. Phys. Chem. C 113 (2009) 11390–11398. [29] C. Wang, A.J. Appleby, F.E. Little, EIS study of hydrogen insertion under restricted diffusion conditions: I. Two-step insertion reaction, J. Electroanal. Chem. 497 (2001) 3–17. [30] Y. Han, Y.P. Wang, W. Gao, Y. Wang, L. Jiao, H. Yuan, S. Liu, Synthesis of novel CuS with hierarchical structures and its application in lithium-ion batteries, Powder Technol. 212 (2011) 64–68.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
One-pot facile synthesis of CuS/graphene composite as anode materials for lithium ion batteries Hua-Chao Tao a,b, Xue-Lin Yang a,b,n, Lu-Lu Zhang a,b, Shi-Bing Ni a,b a b
College of Materials and Chemical Engineering, China Three Gorges University, 8 Daxue Road, Yichang, Hubei 443002, China Collaborative Innovation Center for Microgrid of New Energy, Hubei Province, China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 May 2014 Received in revised form 7 June 2014 Accepted 11 June 2014 Available online 19 June 2014
CuS/graphene composite has been synthesized by the one-pot hydrothermal method using thiourea as the sulfur source and reducing agent. The formation of CuS nanoparticles and the reduction of graphene oxide occur simultaneously during the hydrothermal process, which enables a uniform dispersion of CuS nanoparticles on the graphene nanosheets. The electrochemical performance of CuS/graphene composite was studied as anode materials for lithium ion batteries. The obtained CuS/graphene composite exhibits a relative high reversible capacity and good cycling stability. The good electrochemical performance of CuS/graphene composite can be attributed to graphene, which improves the electronic conductivity of composite and enhances the interfacial stability of electrode and electrolyte. & 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Chalcogenides B. Chemical synthesis C. X-ray diffraction D. Electrochemical properties
1. Introduction Lithium ion batteries with high energy density and long cycle life are strongly desired for rapid growing portable electronic equipments and electric vehicles. Carbon as a commercialized anode material for lithium ion batteries has a low theoretical capacity (372 mAh g 1); many efforts have been devoted to studying safe and cheap anode materials with large reversible capacity, desirable rate capability, long cycle life and good compatibility with electrolyte [1,2]. Transition metal sulfides have been widely investigated due to their high specific capacity, low cost, safety and environmental benignity [3–10]. However, most of the metal sulfides undergo a fast reversible capacity fading during cycles, which can be attributed to large volume change during lithium ions insertion and extraction process and low electronic conductivity [11]. To overcome these problems, one of the most effective way is to design and fabricate composite with a carbon matrix which acts as electrically connecting media and a buffer layer for volume change. Graphene-based materials such as NiO/graphene [12], MoS2/ graphene [13], NiS/graphene [14], CoS2/graphene [15], FeS/graphene [16] and Bi2S3/graphene [17] have been prepared to exhibit excellent electrochemical performance because of high electronic conductivity, large specific surface area and high mechanical strength of graphene.
Among these composites, CuS/graphene composite has also been prepared using thioacetamide as reducing agent and it exhibits high peroxidase-like catalytic activity [18]. However, little research has been done on CuS/graphene composite as anode materials for lithium ion batteries. CuS as anode materials for lithium ion batteries has many advantages, such as high capacity, abundant resource and less toxicity [19]. In addition, numerous methods of chemical reduction of graphene oxide (GO) to graphene have been studied, such as hydrazine hydrate (N2H4 H2O) [20], sodium borohydride (NaBH4) [21], lithium aluminum hydride (LiAlH4) [22] and hydrohalic acid (HI and HBr) [23,24] as reducing agents. These methods are environmentally unfriendly when these reducing agents are used. In this paper, GO was reduced to graphene using thiourea as reducing agent by the hydrothermal method. Meanwhile, CuS nanoparticles were synthesized during hydrothermal process to obtain uniform dispersion on the graphene nanosheets. The reduction of GO and the deposition of CuS on graphene occur simultaneously by the one-pot method. The one-pot method to prepare CuS/ graphene composite can assure the uniform distribution of CuS nanoparticles on graphene nanosheets. The obtained CuS/graphene composite exhibits good lithium ions storage performance.
2. Experimental n Corresponding author at: College of Materials and Chemical Engineering, China Three Gorges University, 8 Daxue Road, Yichang, Hubei 443002, China. Fax: þ86 717 6397559. E-mail address: [email protected] (X.-L. Yang).
http://dx.doi.org/10.1016/j.jpcs.2014.06.010 0022-3697/& 2014 Elsevier Ltd. All rights reserved.
2.1. Preparation of CuS/graphene composite Graphite oxide was synthesized according to the modified Hummers method [25] using natural graphite as raw materials.
1206
H.-C. Tao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 1205–1209
The working electrodes were prepared by mixing active materials (CuS and CuS/graphene), acetylene black and polyvinylidene fluoride (PVDF) with a mass ratio of 70:20:10 using N-methyl-2pyrrolidone (NMP) as solvent. The resulting slurry was pasted on pure copper foil and then dried at 120 1C under vacuum for 24 h. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 and lithium foil was used as the counter electrode. Coin cells were assembled in a glove box filled with pure Ar gas. Galvanostatic discharge/charge tests were carried out on a Land CT2100 battery test system. Cyclic voltammetry (CV) measurements were performed using a CHI660C electrochemical workstation (Shanghai Chenhua) between 0.01 V and 3 V at a scan rate of 0.2 mV s 1. Electrochemical impedance spectroscopy (EIS) measurements were measured on a CHI660C electrochemical workstation in the frequency range from 0.01 Hz to 100 kHz with an amplitude of 5 mV. All of the electrochemical tests were carried out at 25 1C.
(116)
(114)
(101)
Intensity (a.u.)
2.3. Electrochemical characterization
CuS/graphene
CuS
graphite oxide 10
20
30
40
50 60 2θ (degree)
70
80
90
CuS
CuS/graphene
Intensity (a.u.)
The crystal structure of CuS and CuS/graphene was characterized by X-ray diffraction (XRD, Rigaku/mac) with Cu Ka radiation. The morphology and structure were analyzed using transmission electron microscopy (TEM, JEM-2100F) and high resolution transmission electron microscopy (HRTEM). Raman spectra were obtained with a LabRAM Aramis using 514.5 nm Ar-ion laser.
(102) (103)
2.2. Structural characterization
(110)
Graphite oxide (90 mg) was dispersed in deionized water (40 mL) via sonication for 2 h to obtain the homogeneous yellow-brown GO solution. To prepare CuS/graphene composite through the onepot method, CuSO4 5H2O (1 g, analytical grade) and thiourea (0.3 g, analytical grade) were added into the stable suspension of GO. After vigorous stirring, the solution was transferred into a Teflon-line stainless steel autoclave and reacted at 180 1C for 24 h. The black solid was washed with deionized water and alcohol to obtain CuS/graphene composite. The obtained product was dried at 80 1C under vacuum. For comparison, pure CuS was prepared under the same condition without adding the GO suspension.
GO
3. Results and discussion Fig. 1a illustrates the procedure for the fabrication of CuS/ graphene composite. GO, CuSO4 5H2O and thiourea were dispersed in deionized water under stirring and ultrasound. During the hydrothermal process, the CuS nanoparticles were obtained and GO was reduced to graphene simultaneously. The synthesis process of the CuS/graphene composite may be mainly divided into two steps. Firstly, GO incorporated with Cu2 þ ions by electrostatic force. Secondly, S2 and NH2–NH2 ions derived from the decomposition of N2H4CS, GO was reduced to graphene by NH2–NH2 ions and the S2 transformed into CuS. The reactions during the hydrothermal process can be described as follows: GO þCu2 þ -Cu2 þ –GO 2
N2H4CS-S
þ 2NH2 þC
(1) (2)
Cu2 þ þS2 -CuS
(3)
GO þNH2-graphene
(4)
The XRD patterns of graphite oxide, CuS and CuS/graphene composite are compared in Fig. 1b. There is a characteristic peak of graphite oxide at 10.51, which corresponds to (001) plane. In the case of CuS, the peaks at 27.61, 29.31, 31.81, 47.91, 53.11 and 58.71
300
600
900
1200
Raman shift
1500
1800
cm-1
Fig. 1. Schematic illustration for the fabrication process of CuS/graphene composite (a); X-ray diffraction patterns for graphite oxide, CuS and CuS/graphene composite (b); Raman spectra of GO and CuS/graphene composite (c).
can be indexed to the (101), (102), (103), (110), (114) and (116) planes of CuS (JCPDS, no.: 00-001-1281). The obtained CuS/graphene composite displays a similar XRD patterns to pure CuS, indicating that the introduction of GO does not influence the fabrication of CuS nanostructure. In addition, the characteristic diffraction peaks of graphene are not detected in the CuS/graphene composite due to its low amount and low diffraction intensity. The presence of both CuS and graphene is further confirmed by Raman spectroscopy. Fig. 1c shows the Raman spectra of GO and the CuS/graphene composite. Two characteristic peaks of the D band and G band from CuS/graphene composite are observed at about 1350 cm 1 and 1586 cm 1. One characteristic peak from the CuS at 471 cm 1 is observed in the spectrum of the CuS/graphene composite. Both XRD and Raman measurements confirm the successful integration of graphene and CuS nanoparticles. The intensity ratio of the D band
H.-C. Tao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 1205–1209
to G band (ID/IG) for GO is about 0.97. In the case of CuS/graphene composite, the ID/IG increases to 1.05 due to the decrease of the average size and increased quantity of the sp2 domains, indicating the successful reduction of GO to graphene [20]. The morphology and structure of GO, CuS and CuS/graphene composite were examined by TEM and HRTEM, as shown in Fig. 2. GO has a typical lamellar structure (Fig. 2a). The pure CuS nanoparticles with an average diameter of about 50 nm are agglomerated (Fig. 2b). The CuS nanoparticles with a size of about 50–80 nm are distributed onto the graphene nanosheets (Fig. 2c). From the HRTEM image (Fig. 2d) of CuS/graphene composite, the carbon layer with a thickness of about 5 nm and the CuS with d spacing of 0.28 nm corresponding to the (103) plane can be observed, indicating the presence of graphene and CuS. To evaluate the effect of graphene on the electrochemical properties of CuS, electrochemical measurements on CuS and CuS/graphene composite were carried out. The first two discharge (lithium ion insertion)–charge (lithium ion extraction) curves of CuS and CuS/graphene composite between 0.01 and 3 V at a current density of 50 mA g 1 are compared in Fig. 3. For the CuS electrode, the first discharge and charge capacities are 525 and 311 mAh g 1, respectively. The reversible capacity decreases to 270 mAh g 1 for the second cycle (Fig. 3a). In addition, there are two plateaus at about 2.05 and 1.68 V during the first discharge process, and two plateaus at about 1.85 and 2.25 V are observed during the first charge process, while the discharge and charge plateaus obviously shorten during the second discharge/charge process. The plateau position is the same as that in the previous study [19,26]. Compared to CuS electrode, the first discharge and charge capacities of CuS/graphene composite are 827 and 484 mAh g 1, respectively (Fig. 3b). The specific capacities were calculated based on total mass weight of CuS/graphene composite. The large irreversible capacity loss during the first cycle can be attributed to the decomposition of electrolyte and the formation of
1207
solid electrolyte interface (SEI) layer [27]. The reversible specific capacity of CuS/graphene is close to the theoretical capacities of 560 mAh g 1 for CuS. This may be attributed to graphene, which provides the high specific capacity during low potential range of 0.01–1 V. The reversible specific capacity of 473 mAh g 1 for CuS/ graphene electrode during the second cycle is close to the first reversible capacity. Fig. 4 shows the cycling stabilities of CuS and CuS/graphene in the voltage of 0.01–3 V at a rate of 50 mA g 1. The CuS/graphene composite electrode exhibits a reversible capacity of 296 mAh g 1 after 25 cycles with a capacity retention rate of 61%. The pure CuS electrode demonstrates a reversible capacity of only 50 mAh g 1 after 10 cycles. The enhanced electrochemical performance of CuS/ graphene can be attributed to the unique composite architecture of CuS nanoparticles dispersed in graphene nanosheets. In addition, graphene not only increases the electronic conductivity of composite electrode but also acts as a buffer layer that maintains the structural integrity of the electrode after large volume changes during the discharge–charge process. Moreover, graphene prevents the agglomeration of CuS nanoparticles and increases the contact area of composite and electrolyte and makes for the formation of stable SEI layer on the surface of electrode. To further confirm the enhanced electronic conductivity by graphene, EIS measurements on CuS and CuS/graphene composite were carried out before cycling. The Nyquist plots for both CuS and CuS/graphene composite electrodes consist of one depressed semicircle in the high-frequency region and a straight line in the low-frequency region (Fig. 5). The depressed semicircle is due to the charge-transfer resistance [28], and the straight line is attributed to the Li ions diffusion process with electrode [29]. The smaller semicircle for CuS/graphene indicates a lower electrochemical resistance than that of CuS. This result confirms that the interconnected graphene nanosheets can remarkably improve the electronic conductivity of composite electrode.
CuS graphene
Fig. 2. TEM images of GO (a), CuS (b) and CuS/graphene composite (c) and HRTEM image of CuS/graphene composite (d).
1208
H.-C. Tao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 1205–1209
300
3.0
CuS/graphene CuS
250
2.0 1.5
200 15
150
-Z" (ohm)
1st 2nd -Z" (ohm)
Voltage V vs. Li/Li+
2.5
100
1.0
5 0 0
50
0.5
10
5 10 Z' (ohm)
15
0
0.0 0
100
200
300
400
500
0
600
50
100
Specific capacity mAh g-1
150 200 Z' (ohm)
250
300
Fig. 5. Nyquist plots of CuS and CuS/graphene composite in an open-circuit potential before cycling.
3.0 1.5
1st 2nd
2.0
1.0
1.5
0.5
Current (mA)
Voltage V vs. Li/Li+
2.5
1.0 0.5 0.0
1st 2nd
0.0 -0.5 -1.0
0
200 400 600 800 Specific capacity mAh g-1
1000 -1.5
Fig. 3. The first two discharge–charge curves of CuS (a) and CuS/graphene composite (b) between 0.01 V and 3 V at a rate of 50 mA g 1.
0.0
0.5
1.0
1.5
2.0
Potential V vs. Li/Li
2.5
3.0
+
Fig. 6. Cyclic voltammetry profiles of the first two cycles for CuS/graphene composite from 0.01 V to 3 V vs. Li/Li þ at a scan rate of 0.2 mV s 1.
Specific capacity mAh g-1
1000
Solid-Li ion insertion Open-Li ion extraction
800
the formation of a stable SEI layer. The electrochemical reactions of CuS electrode during the discharge process can be expressed as follows:
600
CuSþ xLi þ þ xe -LixCuS
(5)
LixCuSþ (2 x)Li þ þ(2 x)e -Li2Sþ Cu
(6)
CuS/graphene
400
Two anodic peaks at 2.0 and 2.45 V can be attributed to the reversible electrochemical reaction of (5) and (6). These results are well consistent with the discharge–charge curves.
CuS
200 0 0
5
10 15 Cycle number
20
25
Fig. 4. Cycling stabilities of CuS and CuS/graphene composite at a rate of 50 mA g 1 in the voltage range of 0.01–3 V.
Cyclic voltammograms of CuS/graphene composite are shown in Fig. 6. The cathodic peaks at about 1.92 and 1.48 V in the first scan shift to 2.0 and 1.42 V in the second cycle, which correspond to lithium ions intercalation into the CuS lattices and the decomposition of CuS into metallic Cu and Li2S, respectively [30]. Another cathodic peak at about 0.6 V is attributed to the decomposition of electrolyte and the formation of SEI film on the surface of electrode. This peak disappears during the second cycle, indicating
4. Conclusions A hybrid based on CuS and graphene has been successfully synthesized by the one-pot facile hydrothermal process. The CuS/ graphene composite exihibits obvious improved cycling stability compared to bare CuS. The enhanced cycling stability is mainly due to the good electronically conducting channels offered by graphene and the unique structure of composite. These results confirm the potential application of CuS/graphene for practical Li ion batteries with high-energy density. We expect that this facile preparation method could be extended to other graphene-based electrode materials in electrochemical energy storage and conversion.
H.-C. Tao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 1205–1209
Acknowledgments The authors acknowledge the financial supports from the National Natural Science Foundation of China (NSFC, 51272128, 51302152, and 51302153) and the Education Office of Hubei Province (D20131303 and Q20111209). The authors are grateful to Dr. Jianlin Li at the China Three Gorges University for his kind support to our research. References [1] N. Yan, L. Hu, Y. Li, Y. Wang, H. Zhong, X. Hu, X. Kong, Q. Chen, Co3O4 nanocages for high-performance anode material in lithium-ion batteries, J. Phys. Chem. C 116 (2012) 7227–7235. [2] S. Nam, S. Kim, S. Wi, H. Choi, S. Byun, S. Choi, S. Yoo, K.T. Lee, B. Park, The role of carbon incorporation in SnO2 nanoparticles for Li rechargeable batteries, J. Power Sources 211 (2012) 154–160. [3] C. Xu, Y. Zeng, X. Rui, N. Xiao, J. Zhu, W. Zhang, J. Chen, W. Liu, H. Tan, H.H. Hng, Q. Yan, Controlled soft-template synthesis of ultrathin C@FeS nanosheets with high-Li-storage performance, ACS Nano 6 (2012) 4713–4721. [4] C. Zhang, H.B. Wu, Z. Guo, X.W. Lou, Facile synthesis of carbon-coated MoS2 nanorods with enhanced lithium storage properties, Electrochem. Commun. 20 (2012) 7–10. [5] J. Ma, Z. Liu, J. Lian, X. Duan, T. Kim, P. Peng, X. Liu, Q. Chen, G. Yao, W. Zheng, Ionic liquids-assisted synthesis and electrochemical properties of Bi2S3 nanostructures, Cryst. Eng. Commun. 13 (2011) 3072–3079. [6] Y. Gu, Y. Xu, Y. Wang, Graphene-wrapped CoS nanoparticles for high-capacity lithium-ion storage, ACS Appl. Mater. Interfaces 5 (2013) 801–806. [7] C.H. Lai, M.Y. Lu, L.J. Chen, Metal sulfide nanostructures: synthesis, properties and applications in energy conversion and storage, J. Mater. Chem. 22 (2012) 19–30. [8] B. Qu, M. Zhang, D. Lei, Y. Zeng, Y. Chen, L. Chen, Q. Li, Y. Wang, T. Wang, Facile solvothermal synthesis of mesoporous Cu2SnS3 spheres and their application in lithium-ion batteries, Nanoscale 3 (2011) 3646–3651. [9] J.O. Lee, J.U. Seo, J.H. Song, C.M. Park, C.K. Lee, Electrochemical characteristics of ternary compound CoSbS for application in Li secondary batteries, Electrochem. Commun. 28 (2013) 71–74. [10] X. Yin, C. Tang, M. Chen, S. Adams, H. Wan, H. Gong, Hierarchical porous Cu2ZnSnS4 films for high-capacity reversible lithium storage applications, J. Mater. Chem. A 1 (2013) 7927–7932. [11] C.M. Park, Y. Hwa, N.E. Sun, H.J. Sohn, Stibnite (Sb2S3) and its amorphous composite as dual electrodes for rechargeable lithium batteries, J. Mater. Chem. 20 (2010) 1097–1102. [12] Y.J. Mai, S.J. Shi, D. Zhang, Y. Lu, C.D. Gu, J.P. Tu, NiO–graphene hybrid as an anode material for lithium ion batteries, J Power Sources 204 (2012) 155–161. [13] L. David, R. Bhandavat, G. Singh, MoS2/graphene composite paper for sodiumion battery electrodes, ACS Nano 8 (2014) 1759–1770.
1209
[14] Q. Pan, J. Xie, S. Liu, G. Cao, T. Zhu, X. Zhao, Facile one-pot synthesis of ultrathin NiS nanosheets anchored on graphene and the improved electrochemical Listorage properties, RSC Adv. 3 (2013) 3899–3906. [15] J. Xie, S. Liu, G. Cao, T. Zhu, X. Zhao, Self-assembly of CoS2/graphene nanoarchitecture by a facile one-pot route and its improved electrochemical Li-storage properties, Nano Energy 2 (2013) 49–56. [16] L. Fei, Q. Lin, B. Yuan, G. Chen, P. Xie, Y. Li, Y. Xu, S. Deng, S. Smirnov, H. Luo, Reduced graphene oxide wrapped FeS nanocomposite for lithium-ion battery anode with improved performance, ACS Appl. Mater. Interfaces 5 (2013) 5330–5335. [17] Z. Zhang, C. Zhou, L. Huang, X. Wang, Y. Qu, Y. Lai, J. Li, Synthesis of bismuth sulfide/reduced graphene oxide composites and their electrochemical properties for lithium ion batteries, Electrochim. Acta 114 (2013) 88–94. [18] G. Nie, L. Zhang, X. Lu, X. Bian, W. Sun, C. Wang, A one-pot and in situ synthesis of CuS–graphene nanosheet composites with enhanced peroxidase-like catalytic activity, Dalton Trans. 42 (2013) 14006–14013. [19] J.S. Chung, H.J. Sohn, Electrochemical behaviors of CuS as a cathode material for lithium secondary batteries, J. Power Sources 108 (2002) 226–231. [20] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon 45 (2007) 1558–1565. [21] Y. Si, E.T. Samulski, Synthesis of water soluble graphene, Nano Lett. 8 (2008) 1679–1682. [22] A. Ambrosi, C.K. Chua, A. Bonanni, M. Pumera, Lithium aluminum hydride as reducing agent for chemically reduced graphene oxides, Chem. Mater. 24 (2012) 2292–2298. [23] S.F. Pei, J.P. Zhao, J.H. Du, W.C. Ren, H.M. Cheng, Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids, Carbon 48 (2010) 4466–4474. [24] Y. Chen, X. Zhang, D. Zhang, P. Yu, Y. Ma, High performance supercapacitors based on reduced graphene oxide in aqueous and ionic liquid electrolytes, Carbon 49 (2011) 573–580. [25] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. [26] Y. Du, Z. Yin, J. Zhu, X. Huang, X.J. Wu, Z. Zeng, Q. Yan, H. Zhang, A general method for the large-scale synthesis of uniform ultrathin metal sulphide nanocrystals, Nat. Commun. 3 (2012) 1177. [27] Y. Liu, K. Hanai, J. Yang, N. Imanishi, A. Hirano, Y. Takeda, Silicon/carbon composites as anode materials for Li-ion batteries, Electrochem. Solid-State Lett. 7 (2004) A369–A372. [28] R. Ruffo, S.S. Hong, C.K. Chan, R.A. Huggins, Y. Cui, Impedance analysis of silicon nanowire lithium ion battery anodes, J. Phys. Chem. C 113 (2009) 11390–11398. [29] C. Wang, A.J. Appleby, F.E. Little, EIS study of hydrogen insertion under restricted diffusion conditions: I. Two-step insertion reaction, J. Electroanal. Chem. 497 (2001) 3–17. [30] Y. Han, Y.P. Wang, W. Gao, Y. Wang, L. Jiao, H. Yuan, S. Liu, Synthesis of novel CuS with hierarchical structures and its application in lithium-ion batteries, Powder Technol. 212 (2011) 64–68.