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Preparation and electrochemical performance of hierarchical CuS-rGO composite
Journal of Alloys and Compounds 694 (2017) 1067e1072
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation and electrochemical performance of hierarchical CuS-rGO composite Junxiang Wang a, Xianjun Lyu a, *, Lingyun Wang b, Shuang Yu b, Weixue Zhu a, Cui Han a, Xiaoqiang Cao a a b
College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China College of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2016 Received in revised form 7 September 2016 Accepted 16 October 2016 Available online 17 October 2016
Hierarchical copper sulfide (CuS)-reduced graphene oxide (rGO) composites were synthesized using a simple wet chemical method. The samples were characterized via X-ray diffraction, scanning electron microscopy, and transmission electron microscopy techniques. The electrochemical performances of hierarchical CuS-rGO composites, as anode materials for lithium-ion batteries, were also investigated in comparison with that of pure CuS. Results showed that the specific capacity of 408 mAh/g after 100 cycles and cycling stability of hierarchical CuS-rGO composites were higher than those of pure CuS. © 2016 Elsevier B.V. All rights reserved.
Keywords: Copper sulfide Reduced graphene oxide Composite Electrochemical performance Lithium-ion battery
1. Introduction Energy shortage poses a significant challenge to the present society, and thus, developing new energy sources is urgent. Lithium-ion batteries (LIBs), as a relatively mature energy storage method, are currently attracting considerable attention. The traditional graphite anode material has the advantages of high cycling stability, secure application, and long cycle life. However, its low energy density and low theoretical capacity make satisfying the increasing demand for battery capacity and fast chargingdischarging cycle difficult. Transition metal compounds, particularly sulfides [1e5], as anode materials for LIBs, have high energy density and high theoretical capacity, and thus, are receiving extensive attention from various members of the scientific community. Copper sulfide (CuS), as an important p-type semiconductor material, has many excellent physical and chemical properties. It has been widely used in many fields, such as in manufacturing solar cells [6], optical materials [7], LIBs [8] and sensors [9]. In recent years, CuS nanomaterials with different microstructures, such as
* Corresponding author. Tel.: þ86 532 86057721. E-mail address: [email protected] (X. Lyu). http://dx.doi.org/10.1016/j.jallcom.2016.10.155 0925-8388/© 2016 Elsevier B.V. All rights reserved.
nanoparticles [10], nanorods [11], nanowires [12], nanosheets [13] and hollow balls [14], have been prepared using various methods. As a negative electrode material, the theoretical capacity of CuS is 560 mAh/g [15], which is higher than that of graphite (372 mAh/g); hence, CuS will have a promising application in LIBs. However, its biggest drawback as an electrode material is that its structure can be crushed and collapsed by the large volume expansion that occurs during the cell cycle process, thereby leading to a sharp decline in cycling stability. This phenomenon is also common in other metal compound electrodes [16e18]. Researchers have developed numerous methods to prevent the influence of volume expansion on cell cycling performance; among which, the most direct means is to clad carbon. Graphene, as a widely used carbon material, has a large specific surface area, high conductivity, excellent mechanical properties, and is frequently composited with metal compounds and electrode materials to increase battery capacity and cycling stability. Sun et al. [19] prepared mesoporous hollow Co3O4 nanospheres clad in graphene lamella, which still had high cycling capacity of over 600 mAh/g under a high current density of 1 A/g after 500 cycles. Tang et al. [20] prepared 3D interconnected graphene network by a facile two-step method and reported that the composite material retained 1060 mAh/g after 200 cycles at a current density of 100 mA/g, and exhibits a superior rate capability of 670 mAh/g even under a high current density of 2 A/g. Graphene,
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which has excellent electrochemical properties, can not only improve the conductivity of the prepared composite materials, but can also function as a buffer in the process of lithium ion insertion/ extraction, restrain the volume expansion of materials, and prevent a severe drop in capacity caused by the agglomeration of nanoparticles, thereby effectively enhancing battery cycle life. In this study, hierarchical CuS-reduced graphene oxide (rGO) composites were synthesized using a simple wet chemical method, and the microstructures and morphologies were characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. A comparison with pure CuS nanomaterial showed that the electrochemical performances of CuS were obviously improved by adding graphene, and that the capacity and cycling stability of CuS-rGO composite were superior to those of pure CuS. 2. Experimental section 2.1. Preparation of the CuS-rGO composite and CuS Graphene oxide (GO) was prepared via the modified Hummers' method [21]. The product was a golden water solution, and its residual metal ions and acid radical ions were removed using a dialysis bag. Finally, GO powder was obtained through freezedrying. In a typical synthesis of the CuS-rGO composite, 15 mg GO powder, 0.170 g CuCl2$6H2O (1 mmol), and 0.133 g polyvinylpyrrolidone were dispersed into a 250 mL flask with 50 mL ethylene glycol (EG) by sonication for 2 h. Subsequently, the solution was heated in an oil bath (preheated to 150 C) for 0.5 h. Afterward, 20 mL EG (approximately 1 drop/s), including 4 mmol thiourea, was added and heating was continued for 2 h. Finally, the as-synthesized product was obtained via centrifugal separation, and washed with deionized water and ethanol to remove the residual metal ions and dried at room temperature. The preparation of CuS nanomaterials was similar with that of the CuS-rGO composites, except that GO was not added. 2.2. Characterizations The structure and composition of the samples were characterized via X-ray diffraction (XRD, Rigaku Dmax 2200) with Cu Ka radiation (l ¼ 1.5416 Å). The scanning range was run from 10 to 80 with an increasing step size of 0.02 . The morphology of the samples were characterized via scanning electron microscopy (SEM, Quanta 250 FEG, at an accelerating voltage of 5 kV), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM, JEOL JEM-2100F microscope, at an accelerating voltage of 200 kV). The electrochemical performances of the CuS-rGO composite and CuS nanomaterials were used as anode materials and separately investigated in a multichannel battery test system (LAND CT2001A model) with a voltage range of 0.05e3 V. The Electrochemical impedance of the samples was carried out using an electrochemical workstation (CHI660D) over a frequency range from 100 kHz to 0.1 Hz. The cyclic voltammetry (CV) was also performed on the electrochemical workstation with a potential range of 0.005e3 V vs. Li/Liþ at a scan rate of 0.5 mV/s.
and coated on a copper foil, heating for 12 h at 80 C in vacuum. The loading mass of the active material was calculated to 1.8 mg. Then the electrodes were sufficiently compacted under a tablet machine with a force of 10 MPa to prevent it from falling off. Then, the battery was assembled in a glove box filled with high-purity argon in the following order from bottom to top: positive electrode shell, anode plate, electrolyte, diaphragm, electrolyte, lithium, collector, spring leaf, and negative electrode shell. Finally, the battery was packed tightly using a tablet machine and tested after storage for 12 h at room temperature. 3. Results and discussion 3.1. Characterization of GO Fig. 1 shows the XRD pattern of the GO prepared via modified Hummers' method. The diffraction peak at 10.3 is the characteristic peak of the GO with an interlayer spacing of 0.83 nm, and no other peak is found, indicating that the GO has high purity. Fig. 2 presents the TEM image of the prepared GO. As shown in the image, the GO is uniformly distributed, and no agglomeration is found. The evident wrinkles of the GO surface are attributed to the fact that the carbon atoms linked to hydroxyl can form a distorted tetrahedron structure [22,23]. The inset is an aqueous solution of graphene. As shown in the photo, the solution is golden, highly stable, and has no agglomeration. The stable dispersion of prepared GO in water is due to the carboxyl, as well as carbonyl and epoxy groups in the structure, which provide the GO with strong hydrophily, and consequently make the GO easily dispersed into a polar medium. 3.2. Characterization of the hierarchical CuS-rGO composite and CuS Fig. 3 presents the XRD patterns of the prepared hierarchical CuS-rGO and CuS. As shown in the patterns, the prepared products have pure CuS phase and good crystalline structure, which are in good agreement with JCPDS No. 65-3561, and refer to hexagonal CuS. Simultaneously, the same diffraction peaks of the prepared hierarchical CuS-rGO and CuS imply that the GO added during the synthesis process does not change the crystal type of the product. It can not be observed of the rGO peak at about 26 because it is covered up by the strong peak of CuS. The XRD pattern of rGO is also obtained and showed in Fig. S1 (Supporting Information). An obvious peak at about 26 corresponding to the (002) plane of rGO can be found in the pattern. Fig. 4 shows the electron micrographs of the synthetic hierarchical CuS-rGO and CuS. Fig. 4a is a low magnification SEM image of
2.3. Assemblage of coin cells The samples were used as cathode materials in LIBs and their electrochemical properties were evaluated by galvanostatic charge/ discharge technique. The test electrodes were prepared by mixing the as-synthesized products, polyvinylidene fluoride, and acetylene black in methyl pyrrolidone with the mass ratio of 8:1:1 uniformly,
Fig. 1. XRD pattern of GO.
J. Wang et al. / Journal of Alloys and Compounds 694 (2017) 1067e1072
Fig. 2. TEM image of the prepared GO (inset: aqueous solution of the GO).
Fig. 3. XRD patterns of the (a) hierarchical CuS-rGO and (b) CuS.
the hierarchical CuS-rGO, which shows the sample is approximately 2.5 mm in diameter, and comprises a number of hierarchical CuS micro balls wrapped by graphene. Besides, the balls are welldispersed and exhibit no agglomeration. High magnification SEM image of a single CuS-rGO micro ball is presented in Fig. 4b, which is formed by 2D nanosheets with uniformed thickness that cross one another and pile together, among which numerous open porous structures are found, significantly increasing the specific surface area of the product. The crystalline structure of the hierarchical CuS-rGO was analyzed by TEM and HRTEM. As shown in Fig. 4c, the micro ball has a layered structure and the HRTEM micrograph clearly indicates the crystal lattice of the hexagonal CuS phase with the interplanar spacing of 0.303 nm, which corresponds to the (102) crystal plane of the hexagonal CuS phase, consistenting with the XRD analysis result. Fig. 4d shows the SEM image of the synthetic CuS without GO. As seen, the product morphology is characterized by layers of round pie, and almost no gap exists between the layers compared with the porous structure of CuS with GO. The possible reason of the different morphology between CuSrGO and CuS without rGO must be that: first, copper ions will be absorbed on the surface of GO by electrostatic adsorption and that can retard the reaction rate in CuS crystal growth process; second, GO will be reduced to rGO by heating in EG and tends to crumple as decreasing functional groups on its surface, that leads to the formation of the open porous structures. 3.3. Electrochemical performance of the hierarchical CuS-rGO and CuS Fig. 5 presents the results of the cycle stability tests of the hierarchical CuS-rGO and CuS at a current density of 100 mA/g. As shown in the figure, the initial discharge specific capacity of the
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hierarchical CuS-rGO is 810 mAh/g, and the second cycle discharge specific capacity remains at 546 mAh/g. Afterward, the battery charge-discharge specific capacity remains stable. After 50 cycles, the value of the hierarchical CuS-rGO is still sustained at 450 mAh/ g, which is higher than those of graphite (372 mAh/g) and the values reported in the literature [24,25]. However, the initial discharge of pure CuS is only 560 mAh/g and its capacity declines rapidly. In particular, its specific capacity only remains at 90 mAh/g after 50 cycles, which is considerably below the test result of the hierarchical CuS-rGO. The differences of electrochemical performance of these two materials are mainly related to the structure which is shown in Fig. 4. The hierarchical CuS-rGO has many open porous structures, and its specific surface area is large. During the battery testing process, such porous structure can function as a buffer to reduce the volume expansion of the material during the chargingedischarging process. Simultaneously, this open structure can come in full contact with the electrolyte, and thus, shorten electronic transmission distance to improve battery performance. Thus, the addition of graphene can not only improve the electrical conductivity of the material, but can also avoid damage to the structure and the collapse of the crystal caused by lithium ion intercalation/deintercalation, prevent CuS from agglomerating, and finally, enhance battery performance. This phenomenon has been reported more in other materials [26e28]. For pure CuS, nearly no space is found between lamellae, and the specific surface area is small. During the battery testing process, the electrode material cannot fully come in contact with the electrolyte, and thus, the redox reaction is incomplete in the circulation process. Consequently, battery performances are relatively low. Meanwhile, CuS will inevitably undergo volume expansion and agglomerate during the charging-discharging process, thereby causing a sharp decline in battery cycling performance. The electrochemical impedance test of the hierarchical CuS-rGO and CuS were also carried out after 50 cycles and the results are shown in Fig. 6. Two semicircles in the high-frequency and medium-frequency regions, and a straight line in the lowfrequency region, are found in the Nyquist spectra. The semicircle in the high-frequency region can be ascribed to the SEI or contact resistance, whereas that in the medium-frequency region is attributed to the charge transfer impedance on the electrode/ electrolyte interface, and the line in the low-frequency region corresponds to lithium ion diffusion [29,30]. The smaller the round radius is, the smaller the resistance will be. After 50 cycles, the semicircle of the hierarchical CuS-rGO is smaller than that of CuS, which indicates that the hierarchical CuS-rGO evidently has smaller resistance, better conductivity, and faster electron transfer rate; and hence, it has higher specific capacity and exhibits better performance than pure CuS. Fig. 7 shows the charge-discharge curves of the hierarchical CuS-rGO for a few representative cycles. The discharge capacities are 810, 487.6, and 451 mAh/g at a current density of 100 mA/g for the 1st, 25th and 50th cycles, respectively. As indicated in the figure, the curve shapes of the 25th and 50th cycles are approximately the same, and their specific capacities are close to each other, thereby accounting for the good cycling performance and stable cycle process of the product, and less battery capacity decline. The first discharging cycle has three plateaus, which indicates that three reactions occur during the first discharging cycle. By contrast, the 25th and 50th cycles only have one plateau and one reaction each, thereby indicating that some reactions are irreversible during the first cycle. All the aforementioned results can efficiently explain why the discharge capacity of the second cycle is 264 mAh/g lower than that of the first cycle. The charge-discharge curves for CuS at 100 mA/g is shown in Fig. S2. The corresponding discharge capacities for CuS are 559.5, 145 and 94.5 mAh/g for the
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Fig. 4. Electron micrographs of the synthetic samples: (a, b) SEM images of the hierarchical CuS-rGO, (c) TEM image of the hierarchical CuS-rGO (inset: HRTEM micrograph that corresponds to the marked area in the TEM image), and (d) SEM image of CuS (inset: high magnification SEM image of CuS).
Fig. 5. Cycling performance of the hierarchical CuS-rGO and CuS at a current density of 100 mA/g.
Fig. 6. Impedance spectra of the hierarchical CuS-rGO and CuS after 50 cycles.
reactions of the first two peaks are as follows [15,24]: 1st, 25th and 50th cycles, respectively. As the large volume expansion during the discharge/charge process, the reversible capacity of CuS fades seriously and is only 94.5 mAh/g after 50 cycles at 100 mA/g. Thus it can be summarized the CuS-rGO hybrid composite is much better than CuS as electrode materials in LIBs. The possible redox reaction in the charging-discharging process of the battery was investigated through a CV test of the hierarchical CuS-rGO, with a scanning rate of 0.5 mV/s from 0.005 to 3 V. As shown in Fig. 8, three reduction peaks can be found in the first cycle at 1.8, 1.3 and 0.5 V, which exhibit a one-to-one correspondence with the three plateaus in the charge-discharge curves. The
CuS þ xLiþ þ xe / LixCuS,
(1)
LixCuS þ (2 x)Liþ þ (2 x)e / Li2S þ Cu.
(2)
The appearance of the peak at 0.5 V is attributed to the formation of solid electrolyte interphase (SEI) layers on the surface of the electrode materials [31]. The oxidation peaks between 1.75 V and 2.5 V show that copper combines with sulfur and CuS was formed again. Multiple peaks indicate that multiple reactions occurred and a series of CuS were generated [25,32]. In the second and third
J. Wang et al. / Journal of Alloys and Compounds 694 (2017) 1067e1072
Fig. 7. Charge-discharge curves of the hierarchical CuS-rGO at a current density of 100 mA/g.
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capacities are 408, 343, and 210 mAh/g, corresponding to the current densities of 100, 500 and 1000 mA/g, respectively. Although specific capacity will decay with increasing current density, the stability of the battery is good. After decreasing during the first few cycles, capacity tends to become stable, with no rapid fading as those in pure CuS, indicating that the cycling stability of the hierarchical CuS-rGO is good during the charging-discharging process. Careful comparison of the CuS-rGO composite with literature has been carried out and the results is summarized in Supporting Information, Table S1. It can be seen that the performance of CuS-rGO composite is competitive to most reported work. Fig. S4 shows image of the CuS-rGO composite after 100 cycles. It confirms that the structural stability of the composites during cycles remain their structure. The results reveal the stability of the composite and the advantage of the open porous structures. Thus, it is reasonable that the CuS-rGO composite exhibits the excellent performance. 4. Conclusion In this work, hierarchical CuS-rGO and CuS were synthesized using a simple wet chemical method, and characterized via XRD, SEM, and TEM techniques. The electrochemical performance of the hierarchical CuS-rGO and CuS were also tested and the results show that the hierarchical CuS-rGO still has a specific capacity of 408 mAh/g after 100 cycles, which is evidently superior compared to that of CuS. Moreover, the electrochemical performance of these two materials are mainly influenced by their structures, and the hierarchical and open porous structures of CuS combined with graphene are determined to improve specific capacity and cycling stability of LIBs. Acknowledgements
Fig. 8. Cyclic voltammetry curves of the hierarchical CuS-rGO.
cycles, the SEI peak disappears, thereby indicating that the SEI has already formed during the first cycle. In addition, the decrease in the numbers and strengths of other oxidation and reduction peaks can explain that only a part of the reactions in the first cycle is reversible, which leads to a reduction in specific capacity after the first cycle. The redox peaks decrease in the cyclic voltammetry curves, which is consistent with the decline of the plateaus in the charge-discharge curves. CV curves of CuS is used to compare with the composites (Fig. S3, Supporting information). The similar curves of CuS to those of the composite demonstrate that the oxidation and reduction peaks in the composites came from CuS. Fig. 9 shows the cycling performance of the hierarchical CuSrGO at different current densities. As current density increases, the specific capacity declines. After 100 cycles, the specific
Fig. 9. Cycling performance of the hierarchical CuS-rGO at different current densities.
This work was supported by the National Natural Science Foundation of China (51204104), the Research Fund for the Doctoral Program of Higher Education of China (20133718110005), the Key Research and Development Program of Shandong Province in 2016 (2016GSF116013), and the Sponsored Research Foundation for Young Scientist of Shandong Province (BS2012CL026), and the Support Plan for Innovative Research Team of Shandong University of Science and Technology (2012KYTD102). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.10.155. References [1] T.S. Yoder, M. Tussing, J.E. Cloud, Y. Yang, Resilient carbon encapsulation of iron pyrite (FeS2) cathodes in lithium ion batteries, J. Power Sources 274 (2015) 685e692. [2] W. Qiu, J. Xia, S. He, H. Xu, H. Zhong, L. Chen, Facile synthesis of hollow MoS2 microspheres/amorphous carbon composites and their lithium storage properties, Electrochim. Acta 117 (2014) 145e152. [3] J. Zhou, J. Qin, X. Zhang, C. Shi, E. Liu, J. Li, N. Zhao, C. He, 2D space-confined synthesis of few-layer MoS2 anchored on carbon nanosheet for lithium-ion battery anode, ACS Nano 9 (2015) 3837e3848. [4] T.S. Sonia, P. Anjali, S. Roshny, V. Lakshmi, R. Ranjusha, K.R.V. Subramanian, S.V. Nair, A. Balakrishnan, Nano/micro-hybrid NiS cathodes for lithium ion batteries, Ceram. Int. 40 (2014) 8351e8356. [5] H. Wan, G. Peng, X. Yao, J. Yang, P. Cui, X. Xu, Cu2ZnSnS4/graphene nanocomposites for ultrafast, long life all-solid-state lithium batteries using lithium metal anode, Energy Storage Mater. 4 (2016) 59e65. [6] Y. Lei, X. Yang, L. Gu, H. Jia, S. Ge, P. Xiao, X. Fan, Z. Zheng, Room-temperature preparation of trisilver-copper-sulfide/polymer based heterojunction thin film for solar cell application, J. Power Sources 280 (2015) 313e319. [7] N. Sreelekha, K. Subramanyam, D. Amaranatha Reddy, G. Murali, S. Ramu, K. Rahul Varma, R.P. Vijayalakshmi, Structural, optical, magnetic and photocatalytic properties of Co doped CuS diluted magnetic semiconductor
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation and electrochemical performance of hierarchical CuS-rGO composite Junxiang Wang a, Xianjun Lyu a, *, Lingyun Wang b, Shuang Yu b, Weixue Zhu a, Cui Han a, Xiaoqiang Cao a a b
College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China College of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2016 Received in revised form 7 September 2016 Accepted 16 October 2016 Available online 17 October 2016
Hierarchical copper sulfide (CuS)-reduced graphene oxide (rGO) composites were synthesized using a simple wet chemical method. The samples were characterized via X-ray diffraction, scanning electron microscopy, and transmission electron microscopy techniques. The electrochemical performances of hierarchical CuS-rGO composites, as anode materials for lithium-ion batteries, were also investigated in comparison with that of pure CuS. Results showed that the specific capacity of 408 mAh/g after 100 cycles and cycling stability of hierarchical CuS-rGO composites were higher than those of pure CuS. © 2016 Elsevier B.V. All rights reserved.
Keywords: Copper sulfide Reduced graphene oxide Composite Electrochemical performance Lithium-ion battery
1. Introduction Energy shortage poses a significant challenge to the present society, and thus, developing new energy sources is urgent. Lithium-ion batteries (LIBs), as a relatively mature energy storage method, are currently attracting considerable attention. The traditional graphite anode material has the advantages of high cycling stability, secure application, and long cycle life. However, its low energy density and low theoretical capacity make satisfying the increasing demand for battery capacity and fast chargingdischarging cycle difficult. Transition metal compounds, particularly sulfides [1e5], as anode materials for LIBs, have high energy density and high theoretical capacity, and thus, are receiving extensive attention from various members of the scientific community. Copper sulfide (CuS), as an important p-type semiconductor material, has many excellent physical and chemical properties. It has been widely used in many fields, such as in manufacturing solar cells [6], optical materials [7], LIBs [8] and sensors [9]. In recent years, CuS nanomaterials with different microstructures, such as
* Corresponding author. Tel.: þ86 532 86057721. E-mail address: [email protected] (X. Lyu). http://dx.doi.org/10.1016/j.jallcom.2016.10.155 0925-8388/© 2016 Elsevier B.V. All rights reserved.
nanoparticles [10], nanorods [11], nanowires [12], nanosheets [13] and hollow balls [14], have been prepared using various methods. As a negative electrode material, the theoretical capacity of CuS is 560 mAh/g [15], which is higher than that of graphite (372 mAh/g); hence, CuS will have a promising application in LIBs. However, its biggest drawback as an electrode material is that its structure can be crushed and collapsed by the large volume expansion that occurs during the cell cycle process, thereby leading to a sharp decline in cycling stability. This phenomenon is also common in other metal compound electrodes [16e18]. Researchers have developed numerous methods to prevent the influence of volume expansion on cell cycling performance; among which, the most direct means is to clad carbon. Graphene, as a widely used carbon material, has a large specific surface area, high conductivity, excellent mechanical properties, and is frequently composited with metal compounds and electrode materials to increase battery capacity and cycling stability. Sun et al. [19] prepared mesoporous hollow Co3O4 nanospheres clad in graphene lamella, which still had high cycling capacity of over 600 mAh/g under a high current density of 1 A/g after 500 cycles. Tang et al. [20] prepared 3D interconnected graphene network by a facile two-step method and reported that the composite material retained 1060 mAh/g after 200 cycles at a current density of 100 mA/g, and exhibits a superior rate capability of 670 mAh/g even under a high current density of 2 A/g. Graphene,
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which has excellent electrochemical properties, can not only improve the conductivity of the prepared composite materials, but can also function as a buffer in the process of lithium ion insertion/ extraction, restrain the volume expansion of materials, and prevent a severe drop in capacity caused by the agglomeration of nanoparticles, thereby effectively enhancing battery cycle life. In this study, hierarchical CuS-reduced graphene oxide (rGO) composites were synthesized using a simple wet chemical method, and the microstructures and morphologies were characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. A comparison with pure CuS nanomaterial showed that the electrochemical performances of CuS were obviously improved by adding graphene, and that the capacity and cycling stability of CuS-rGO composite were superior to those of pure CuS. 2. Experimental section 2.1. Preparation of the CuS-rGO composite and CuS Graphene oxide (GO) was prepared via the modified Hummers' method [21]. The product was a golden water solution, and its residual metal ions and acid radical ions were removed using a dialysis bag. Finally, GO powder was obtained through freezedrying. In a typical synthesis of the CuS-rGO composite, 15 mg GO powder, 0.170 g CuCl2$6H2O (1 mmol), and 0.133 g polyvinylpyrrolidone were dispersed into a 250 mL flask with 50 mL ethylene glycol (EG) by sonication for 2 h. Subsequently, the solution was heated in an oil bath (preheated to 150 C) for 0.5 h. Afterward, 20 mL EG (approximately 1 drop/s), including 4 mmol thiourea, was added and heating was continued for 2 h. Finally, the as-synthesized product was obtained via centrifugal separation, and washed with deionized water and ethanol to remove the residual metal ions and dried at room temperature. The preparation of CuS nanomaterials was similar with that of the CuS-rGO composites, except that GO was not added. 2.2. Characterizations The structure and composition of the samples were characterized via X-ray diffraction (XRD, Rigaku Dmax 2200) with Cu Ka radiation (l ¼ 1.5416 Å). The scanning range was run from 10 to 80 with an increasing step size of 0.02 . The morphology of the samples were characterized via scanning electron microscopy (SEM, Quanta 250 FEG, at an accelerating voltage of 5 kV), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM, JEOL JEM-2100F microscope, at an accelerating voltage of 200 kV). The electrochemical performances of the CuS-rGO composite and CuS nanomaterials were used as anode materials and separately investigated in a multichannel battery test system (LAND CT2001A model) with a voltage range of 0.05e3 V. The Electrochemical impedance of the samples was carried out using an electrochemical workstation (CHI660D) over a frequency range from 100 kHz to 0.1 Hz. The cyclic voltammetry (CV) was also performed on the electrochemical workstation with a potential range of 0.005e3 V vs. Li/Liþ at a scan rate of 0.5 mV/s.
and coated on a copper foil, heating for 12 h at 80 C in vacuum. The loading mass of the active material was calculated to 1.8 mg. Then the electrodes were sufficiently compacted under a tablet machine with a force of 10 MPa to prevent it from falling off. Then, the battery was assembled in a glove box filled with high-purity argon in the following order from bottom to top: positive electrode shell, anode plate, electrolyte, diaphragm, electrolyte, lithium, collector, spring leaf, and negative electrode shell. Finally, the battery was packed tightly using a tablet machine and tested after storage for 12 h at room temperature. 3. Results and discussion 3.1. Characterization of GO Fig. 1 shows the XRD pattern of the GO prepared via modified Hummers' method. The diffraction peak at 10.3 is the characteristic peak of the GO with an interlayer spacing of 0.83 nm, and no other peak is found, indicating that the GO has high purity. Fig. 2 presents the TEM image of the prepared GO. As shown in the image, the GO is uniformly distributed, and no agglomeration is found. The evident wrinkles of the GO surface are attributed to the fact that the carbon atoms linked to hydroxyl can form a distorted tetrahedron structure [22,23]. The inset is an aqueous solution of graphene. As shown in the photo, the solution is golden, highly stable, and has no agglomeration. The stable dispersion of prepared GO in water is due to the carboxyl, as well as carbonyl and epoxy groups in the structure, which provide the GO with strong hydrophily, and consequently make the GO easily dispersed into a polar medium. 3.2. Characterization of the hierarchical CuS-rGO composite and CuS Fig. 3 presents the XRD patterns of the prepared hierarchical CuS-rGO and CuS. As shown in the patterns, the prepared products have pure CuS phase and good crystalline structure, which are in good agreement with JCPDS No. 65-3561, and refer to hexagonal CuS. Simultaneously, the same diffraction peaks of the prepared hierarchical CuS-rGO and CuS imply that the GO added during the synthesis process does not change the crystal type of the product. It can not be observed of the rGO peak at about 26 because it is covered up by the strong peak of CuS. The XRD pattern of rGO is also obtained and showed in Fig. S1 (Supporting Information). An obvious peak at about 26 corresponding to the (002) plane of rGO can be found in the pattern. Fig. 4 shows the electron micrographs of the synthetic hierarchical CuS-rGO and CuS. Fig. 4a is a low magnification SEM image of
2.3. Assemblage of coin cells The samples were used as cathode materials in LIBs and their electrochemical properties were evaluated by galvanostatic charge/ discharge technique. The test electrodes were prepared by mixing the as-synthesized products, polyvinylidene fluoride, and acetylene black in methyl pyrrolidone with the mass ratio of 8:1:1 uniformly,
Fig. 1. XRD pattern of GO.
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Fig. 2. TEM image of the prepared GO (inset: aqueous solution of the GO).
Fig. 3. XRD patterns of the (a) hierarchical CuS-rGO and (b) CuS.
the hierarchical CuS-rGO, which shows the sample is approximately 2.5 mm in diameter, and comprises a number of hierarchical CuS micro balls wrapped by graphene. Besides, the balls are welldispersed and exhibit no agglomeration. High magnification SEM image of a single CuS-rGO micro ball is presented in Fig. 4b, which is formed by 2D nanosheets with uniformed thickness that cross one another and pile together, among which numerous open porous structures are found, significantly increasing the specific surface area of the product. The crystalline structure of the hierarchical CuS-rGO was analyzed by TEM and HRTEM. As shown in Fig. 4c, the micro ball has a layered structure and the HRTEM micrograph clearly indicates the crystal lattice of the hexagonal CuS phase with the interplanar spacing of 0.303 nm, which corresponds to the (102) crystal plane of the hexagonal CuS phase, consistenting with the XRD analysis result. Fig. 4d shows the SEM image of the synthetic CuS without GO. As seen, the product morphology is characterized by layers of round pie, and almost no gap exists between the layers compared with the porous structure of CuS with GO. The possible reason of the different morphology between CuSrGO and CuS without rGO must be that: first, copper ions will be absorbed on the surface of GO by electrostatic adsorption and that can retard the reaction rate in CuS crystal growth process; second, GO will be reduced to rGO by heating in EG and tends to crumple as decreasing functional groups on its surface, that leads to the formation of the open porous structures. 3.3. Electrochemical performance of the hierarchical CuS-rGO and CuS Fig. 5 presents the results of the cycle stability tests of the hierarchical CuS-rGO and CuS at a current density of 100 mA/g. As shown in the figure, the initial discharge specific capacity of the
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hierarchical CuS-rGO is 810 mAh/g, and the second cycle discharge specific capacity remains at 546 mAh/g. Afterward, the battery charge-discharge specific capacity remains stable. After 50 cycles, the value of the hierarchical CuS-rGO is still sustained at 450 mAh/ g, which is higher than those of graphite (372 mAh/g) and the values reported in the literature [24,25]. However, the initial discharge of pure CuS is only 560 mAh/g and its capacity declines rapidly. In particular, its specific capacity only remains at 90 mAh/g after 50 cycles, which is considerably below the test result of the hierarchical CuS-rGO. The differences of electrochemical performance of these two materials are mainly related to the structure which is shown in Fig. 4. The hierarchical CuS-rGO has many open porous structures, and its specific surface area is large. During the battery testing process, such porous structure can function as a buffer to reduce the volume expansion of the material during the chargingedischarging process. Simultaneously, this open structure can come in full contact with the electrolyte, and thus, shorten electronic transmission distance to improve battery performance. Thus, the addition of graphene can not only improve the electrical conductivity of the material, but can also avoid damage to the structure and the collapse of the crystal caused by lithium ion intercalation/deintercalation, prevent CuS from agglomerating, and finally, enhance battery performance. This phenomenon has been reported more in other materials [26e28]. For pure CuS, nearly no space is found between lamellae, and the specific surface area is small. During the battery testing process, the electrode material cannot fully come in contact with the electrolyte, and thus, the redox reaction is incomplete in the circulation process. Consequently, battery performances are relatively low. Meanwhile, CuS will inevitably undergo volume expansion and agglomerate during the charging-discharging process, thereby causing a sharp decline in battery cycling performance. The electrochemical impedance test of the hierarchical CuS-rGO and CuS were also carried out after 50 cycles and the results are shown in Fig. 6. Two semicircles in the high-frequency and medium-frequency regions, and a straight line in the lowfrequency region, are found in the Nyquist spectra. The semicircle in the high-frequency region can be ascribed to the SEI or contact resistance, whereas that in the medium-frequency region is attributed to the charge transfer impedance on the electrode/ electrolyte interface, and the line in the low-frequency region corresponds to lithium ion diffusion [29,30]. The smaller the round radius is, the smaller the resistance will be. After 50 cycles, the semicircle of the hierarchical CuS-rGO is smaller than that of CuS, which indicates that the hierarchical CuS-rGO evidently has smaller resistance, better conductivity, and faster electron transfer rate; and hence, it has higher specific capacity and exhibits better performance than pure CuS. Fig. 7 shows the charge-discharge curves of the hierarchical CuS-rGO for a few representative cycles. The discharge capacities are 810, 487.6, and 451 mAh/g at a current density of 100 mA/g for the 1st, 25th and 50th cycles, respectively. As indicated in the figure, the curve shapes of the 25th and 50th cycles are approximately the same, and their specific capacities are close to each other, thereby accounting for the good cycling performance and stable cycle process of the product, and less battery capacity decline. The first discharging cycle has three plateaus, which indicates that three reactions occur during the first discharging cycle. By contrast, the 25th and 50th cycles only have one plateau and one reaction each, thereby indicating that some reactions are irreversible during the first cycle. All the aforementioned results can efficiently explain why the discharge capacity of the second cycle is 264 mAh/g lower than that of the first cycle. The charge-discharge curves for CuS at 100 mA/g is shown in Fig. S2. The corresponding discharge capacities for CuS are 559.5, 145 and 94.5 mAh/g for the
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Fig. 4. Electron micrographs of the synthetic samples: (a, b) SEM images of the hierarchical CuS-rGO, (c) TEM image of the hierarchical CuS-rGO (inset: HRTEM micrograph that corresponds to the marked area in the TEM image), and (d) SEM image of CuS (inset: high magnification SEM image of CuS).
Fig. 5. Cycling performance of the hierarchical CuS-rGO and CuS at a current density of 100 mA/g.
Fig. 6. Impedance spectra of the hierarchical CuS-rGO and CuS after 50 cycles.
reactions of the first two peaks are as follows [15,24]: 1st, 25th and 50th cycles, respectively. As the large volume expansion during the discharge/charge process, the reversible capacity of CuS fades seriously and is only 94.5 mAh/g after 50 cycles at 100 mA/g. Thus it can be summarized the CuS-rGO hybrid composite is much better than CuS as electrode materials in LIBs. The possible redox reaction in the charging-discharging process of the battery was investigated through a CV test of the hierarchical CuS-rGO, with a scanning rate of 0.5 mV/s from 0.005 to 3 V. As shown in Fig. 8, three reduction peaks can be found in the first cycle at 1.8, 1.3 and 0.5 V, which exhibit a one-to-one correspondence with the three plateaus in the charge-discharge curves. The
CuS þ xLiþ þ xe / LixCuS,
(1)
LixCuS þ (2 x)Liþ þ (2 x)e / Li2S þ Cu.
(2)
The appearance of the peak at 0.5 V is attributed to the formation of solid electrolyte interphase (SEI) layers on the surface of the electrode materials [31]. The oxidation peaks between 1.75 V and 2.5 V show that copper combines with sulfur and CuS was formed again. Multiple peaks indicate that multiple reactions occurred and a series of CuS were generated [25,32]. In the second and third
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Fig. 7. Charge-discharge curves of the hierarchical CuS-rGO at a current density of 100 mA/g.
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capacities are 408, 343, and 210 mAh/g, corresponding to the current densities of 100, 500 and 1000 mA/g, respectively. Although specific capacity will decay with increasing current density, the stability of the battery is good. After decreasing during the first few cycles, capacity tends to become stable, with no rapid fading as those in pure CuS, indicating that the cycling stability of the hierarchical CuS-rGO is good during the charging-discharging process. Careful comparison of the CuS-rGO composite with literature has been carried out and the results is summarized in Supporting Information, Table S1. It can be seen that the performance of CuS-rGO composite is competitive to most reported work. Fig. S4 shows image of the CuS-rGO composite after 100 cycles. It confirms that the structural stability of the composites during cycles remain their structure. The results reveal the stability of the composite and the advantage of the open porous structures. Thus, it is reasonable that the CuS-rGO composite exhibits the excellent performance. 4. Conclusion In this work, hierarchical CuS-rGO and CuS were synthesized using a simple wet chemical method, and characterized via XRD, SEM, and TEM techniques. The electrochemical performance of the hierarchical CuS-rGO and CuS were also tested and the results show that the hierarchical CuS-rGO still has a specific capacity of 408 mAh/g after 100 cycles, which is evidently superior compared to that of CuS. Moreover, the electrochemical performance of these two materials are mainly influenced by their structures, and the hierarchical and open porous structures of CuS combined with graphene are determined to improve specific capacity and cycling stability of LIBs. Acknowledgements
Fig. 8. Cyclic voltammetry curves of the hierarchical CuS-rGO.
cycles, the SEI peak disappears, thereby indicating that the SEI has already formed during the first cycle. In addition, the decrease in the numbers and strengths of other oxidation and reduction peaks can explain that only a part of the reactions in the first cycle is reversible, which leads to a reduction in specific capacity after the first cycle. The redox peaks decrease in the cyclic voltammetry curves, which is consistent with the decline of the plateaus in the charge-discharge curves. CV curves of CuS is used to compare with the composites (Fig. S3, Supporting information). The similar curves of CuS to those of the composite demonstrate that the oxidation and reduction peaks in the composites came from CuS. Fig. 9 shows the cycling performance of the hierarchical CuSrGO at different current densities. As current density increases, the specific capacity declines. After 100 cycles, the specific
Fig. 9. Cycling performance of the hierarchical CuS-rGO at different current densities.
This work was supported by the National Natural Science Foundation of China (51204104), the Research Fund for the Doctoral Program of Higher Education of China (20133718110005), the Key Research and Development Program of Shandong Province in 2016 (2016GSF116013), and the Sponsored Research Foundation for Young Scientist of Shandong Province (BS2012CL026), and the Support Plan for Innovative Research Team of Shandong University of Science and Technology (2012KYTD102). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.10.155. References [1] T.S. Yoder, M. Tussing, J.E. Cloud, Y. Yang, Resilient carbon encapsulation of iron pyrite (FeS2) cathodes in lithium ion batteries, J. Power Sources 274 (2015) 685e692. [2] W. Qiu, J. Xia, S. He, H. Xu, H. Zhong, L. Chen, Facile synthesis of hollow MoS2 microspheres/amorphous carbon composites and their lithium storage properties, Electrochim. Acta 117 (2014) 145e152. [3] J. Zhou, J. Qin, X. Zhang, C. Shi, E. Liu, J. Li, N. Zhao, C. He, 2D space-confined synthesis of few-layer MoS2 anchored on carbon nanosheet for lithium-ion battery anode, ACS Nano 9 (2015) 3837e3848. [4] T.S. Sonia, P. Anjali, S. Roshny, V. Lakshmi, R. Ranjusha, K.R.V. Subramanian, S.V. Nair, A. Balakrishnan, Nano/micro-hybrid NiS cathodes for lithium ion batteries, Ceram. Int. 40 (2014) 8351e8356. [5] H. Wan, G. Peng, X. Yao, J. Yang, P. Cui, X. Xu, Cu2ZnSnS4/graphene nanocomposites for ultrafast, long life all-solid-state lithium batteries using lithium metal anode, Energy Storage Mater. 4 (2016) 59e65. [6] Y. Lei, X. Yang, L. Gu, H. Jia, S. Ge, P. Xiao, X. Fan, Z. Zheng, Room-temperature preparation of trisilver-copper-sulfide/polymer based heterojunction thin film for solar cell application, J. Power Sources 280 (2015) 313e319. [7] N. Sreelekha, K. Subramanyam, D. Amaranatha Reddy, G. Murali, S. Ramu, K. Rahul Varma, R.P. Vijayalakshmi, Structural, optical, magnetic and photocatalytic properties of Co doped CuS diluted magnetic semiconductor
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