In Situ Fabrication of CoS and NiS Nanomaterials Anchored on

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In-Situ Fabrication of CoS and NiS Nanomaterials Anchored on Reduced Graphene Oxide for Reversible Lithium Storage Yingbin Tan, Ming Liang, Peili Lou, Zhonghui Cui, Xiangxin Guo, Weiwei Sun, and Xuebin Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01003 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on May 26, 2016

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ACS Applied Materials & Interfaces

In-Situ Fabrication of CoS and NiS Nanomaterials Anchored on Reduced Graphene Oxide for Reversible Lithium Storage

Yingbin Tan,[a] Ming Liang,[b, c] Peili Lou,[a] Zhonghui Cui,[a] Xiangxin Guo,[a]* Weiwei Sun[b] and Xuebin Yu[c]*

a

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. b

Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China

c

∗

Department of Materials Science, Fudan University, Shanghai 200433, China.

To whom correspondence should be addressed.

E-mail: [email protected], [email protected].

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ACS Paragon Plus Environment


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ABSTRACT: CoS and NiS nanomaterials anchored on reduced graphene oxide (rGO) sheets, synthesized via combination of hydrothermal with the sulfidation process, are studied as high-capacity anode materials for the reversible lithium storage. The obtained CoS nanofibers and NiS nanoparticles are uniformly dispersed on rGO sheets without aggregation, forming the sheet-on-sheet composite structure. Such nanoarchitecture can not only facilitate ion/electron transport along the interfaces, but also effectively prevent metal-sulfide nanomaterials aggregation during the lithium reactions. Both the rGO-supported CoS nanofibers (NFs) and NiS nanoparticles (NPs) show superior lithium storage performance. In particular, the CoS NFs-rGO electrodes deliver the discharge capacity as high as 939 mA h g-1 after the 100th cycle at 100 mA g-1 with Coulombic efficiency above 98 %. This strategy for construction of such composite structure can also synthesize other metal-sulfide-rGO nanomaterials for high-capacity lithium-ion batteries. KEYWORDS: lithium-ion batteries; reduced graphene oxide; anode materials; nanofibers; metal-sulfide nanomaterials

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INTRODUCTION Performance improvement of rechargeable lithium-ion batteries (LIBs) is highly demanded by development of portable electronics and electric vehicles.

1

The

electrode materials with the long cycle lifetime as well as the high reversible capacity are always required. A large number of anode materials have been studied for this purpose. Among them, transition-metal oxides have been given intensive attention owing to their high theoretical capacities.

2-8

However, these materials usually show

large polarization and poor cycle stability because of their large volume expansion and low electrical conductivity during repetitive discharge and charge processes.

4-10

In contrast, transition-metal sulfides are promising anode materials in terms of their high reversible capacity, long cycle stability and intrinsic safety features. on the superior electrical conductivity,

13-15

11-15

Based

metal-sulfides exhibit more attractive

lithium storage performance than their corresponding oxides. However, they suffer the serious mechanical degradation and pulverization problem arising from the large volume variation, causing repaid capacity degradation and poor cycle stability. 16-18 To circumvent these obstacles, many efforts have been made on development of hybrid nanostructures through incorporating the carbonaceous matrix with the metal-sulfides.

19-24

An integrated design of metal-sulfides/carbonaceous matrix

nanocomposites may improve the electrical conductivity, offer sufficient room to buffer the volume variation, and reduce aggregation of active materials during the Li+ insertion and extraction processes, thus resulting in prolonged cycle life.

25-28

This

implies that reduction of particle size and construction of conductive framework may 3



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greatly improve the lithium storage performance of the metal-sulfides. Graphene, as a superior conductive substrate, possesses advantages including flexibility, high chemical stability, and large surface area. 29-32 Fabrication of nanosized metal-sulfides on graphene framework is expected to relieve aggregation of active materials and improve the electrical conductivity. In this study, we report preparation of rGO anchored with CoS and NiS nanomaterials via combination of hydrothermal with the sulfidation process. The formed metal-hydroxides on the surface of rGO matrix by the initial hydrothermal process in-situ react with sulfur powders to form metal-sulfides, which are uniformly distributed on the rGO networks. The metal-sulfide nanomaterials without observable agglomeration are uniformly dispersed on the surface of rGO networks, forming a sheet-on-sheet composite structure. Such nanoarchitecture leads to easy access of Li-ions as well as accommodation of the volume variation during cycles. Consequently, both the rGO-supported electrochemically active CoS nanofibers (CoS NFs) and NiS nanoparticles (NiS NPs) show good Li storage performance.

EXPERIMENTAL SECTION Materials.

natural

graphite

flakes

(99

%),

Ni(NO3)2·6H2O

(99.9

%),

Co(NO3)2·6H2O (99.95 %), FeCl3 (99.99 %), oleylamine (98 %), urea (99.5 %) and S power (99.5 %) were purchased from J&K Scientific Ltd. Preparation of graphene oxide (GO). Graphite oxides were prepared based on a modified Hummer’s method. In the synthetic process, 0.5 g NaNO3 and 1.0 g graphite flakes were first grinded, and then added into the concentrated H2SO4 (98 %, 50 mL) 4


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at approximately 0 oC. After stirring 15 min, 3.1 g KMnO4 powders were dispersed in the H2SO4 solution. The mixtures were kept stirring for 2 h at a temperature below 5 °C. Subsequently, the reaction system was shifted to a 35 °C water bath and kept for 30 min. Then, the system was added with 50 mL of deionized water and kept at 98 °C under stirring for half an hour. After that, 70 mL of deionized water and 15 ml H2O2 (30 %) were subsequently poured into the solution. Finally, the suspension was centrifuged and washed by 300 mL 5% HCl and then 120 mL deionized water to remove the ions. The centrifugal block was frozen at -50 oC for 3 h and then freeze-dried for 3 days to obtain fluffy graphite oxides. 0.5 g fluffy graphite oxide powders were re-dissolved in 250 mL deionized water and exfoliated by ultrasonication to obtain 2 mg mL-1 GO solution. Synthesis of Co(OH)2-rGO composites. Co(OH)2-rGO composites were prepared by a simple solvothermal method.

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Typically, 1 mM (0.29 g) Co(NO3)2·6H2O and 4

mM (0.24 g) urea were dissolved in 50 mL of ethanol and 40 mL of GO solution (2.0 mg mL-1). After about 5 min stirring, the resulting solution was sealed and solvothermal treated at 90 °C for 8 h in a Teflon-lined autoclave (200 mL). After the reaction, the solution was centrifuged with deionized water to remove the Co2+ species, followed by freeze-drying to gain Co(OH)2-rGO composites. Synthesis of Ni(OH)2-rGO composites. 1 mM (0.291 g) of Ni(NO3)2·6H2O was added in 50 mL GO solution (2.0 mg mL-1) under magnetic stirring. After dissolving, oleylamine and ethanol (1:5 in volume) were successively added into the solution and continuously stirred for 30 min. The solution was hydrothermally treated at 180 °C for 5



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15 h in a Teflon-lined autoclave (50 mL). After the reaction, the solution was centrifuged to remove residual oleylamine and the Ni2+ species, followed by freeze-drying to gain Ni(OH)2-rGO composites. The morphology is dependent on nucleation and growth kinetics. The relatively low temperature (e.g. 90 oC) may lead to the slow nucleation process, and the interfacial tension of the nanocrystals may give rise to a spontaneous assembly of nanofibers bundles, while the relatively high temperature (e.g. 180 oC) may cause the quick diffusion of the atoms along in the interfaces, leading to formation of the nanosheets. Synthesis of FeOOH-rGO composites. 50 mL FeCl3 (0.162 g) solution was dispersed into 50 mL of GO solution (2 mg mL-1). The solution was hydrothermally treated at 80 oC for 5 h in an oil bath. After the reaction, the solution was centrifuged to remove the Fe3+ species, followed by freeze-drying to obtain the FeOOH-rGO composites. Synthesis of CoS NFs-rGO, NiS NPs-rGO and FeS-rGO composites. In a typical synthesis, 0.4 g Co(OH)2-rGO composite and 0.2 g sulfur powders were mixed and calcined at 550 °C for 3 h under N2 atmosphere. To synthesize the NiS NPs-rGO and FeS-rGO composites, the same procedure for the NiS NPs-rGO and FeS-rGO composites was applied except that 0.2 g Ni(OH)2-rGO and FeOOH-rGO composites replaced 0.2 g Co(OH)2-rGO composites. With the similar sulfidation reaction, a serial of metal-sulfides can also be fabricated, such as In2S3, MnS, Fe9S10, CuS2 and ZnS (Figure S1-S5). The HCS elemental analysis indicated that the nanocomposites consist of ∼ 75 wt % metal-sulfides nanomaterials and 25 wt % rGO. 6


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Instrumentation and analysis. The phase composition of the as-obtained powder products was measured by an X-ray diffraction (XRD)-6000 diffractometer (Shimadzu, Japan) using Cu Kα radiation. The nanostructure and morphological characteristics of the as-prepared nanomaterials were studied by a JEM-2100F TEM (JEOL, 100 kV) and a Magellan 400 field-emission SEM (FEI, 5 kV). The energy dispersive X-ray spectroscopy, equipped in Magellan 400 SEM, was applied to detect the elemental composition of the metal-sulfide nanomaterials. Electrochemical Measurement. The standard 2025 coin cells were assembled to evaluate the electrochemical performance of the powder samples. The metal sulfides-rGO slurries were fabricated by grinding the mixture of metal sulfides-rGO nanocomposites, conductive carbon black (Super-P) and 5 wt% polyvinylidene fluoride dissolved in NMP solvents. The weight ratio of the mixture is 8:1:1. The formed slurries were deposited on current collectors (copper foils), and then dried in vacuum at 90 °C for 8 h. The typical mass loading of metal sulfides-rGO nanocomposites on electrodes was approximately 2 mg cm-2. Each coin cell assembly was operated in an Mbraun glovebox (Germany) under Ar atmosphere (O2 < 0.1 ppm, H2O < 0.1 ppm). In the coin cells, Li-metal foils were acted as the reference electrodes with celgard 2400 separates. The used electrolytes were composed of one molar LiPF6 (Sigma Aldrich) in the mixed dimethyl carbonate and ethylene carbonate solvents (1: 1 in volume). All discharge and charge measurements were carried out on a LAND CT-2001A system (Wuhan, China) at room temperature. Cyclic voltammetry tests on

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the anode materials were performed by a modular Electrochemical Workstation (Metrohm).

RESULTS AND DISCUSSION The synthesis procedure of CoS and NiS nanomaterials anchored on rGO is schematically shown in Scheme 1. In the first step, the M(OH)2 nanomaterials are first grown on the rGO surface via a simple hydrothermal process. Powder X-ray diffraction (PXRD) indicates formation of Co(OH)2 (JCPDS card no. 30-0443) and Ni(OH)2 (JCPDS card no. 74-2075) (Figure 1).

27, 34

Scanning electron microscopy

(SEM) observation reveals the obtained Co(OH)2 with the nanofiber morphology anchored on the rGO sheets (Figure 2 and Figure S6). Combined SEM with transmission electron microscopy (TEM) analysis, the thin (< 10 nm) and vimineous nature of the Co(OH)2 NFs are identified. Homogeneous distribution of Ni(OH)2 with morphology of 2D sheet in edge length of 80∼150 nm on the rGO sheets is also revealed. After sulfidation of M(OH)2 under N2 atmosphere, the rGO-supported CoS and NiS are obtained. During this process, the (OH)-ions are replaced by S-ions, yielding CoS and NiS deposited on rGO nanocomposites. Figure 3, S7 and S8 indicate that the rGO-supported CoS follows the nanostructure of the precursor, while the rGO-supported NiS turns nanoparticles (NPs). It can also be clearly seen homogeneous dispersion of the nanomaterials in the rGO supports. The typical thickness of CoS NFs is approximately 20 nm and that of NiS NPs ranges from 50 to 80 nm as indicated by SEM and TEM. The in situ transformation of M(OH)2 into 8


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metal-sulfides is confirmed by the high-resolution TEM image in combination with the fast Fourier transform (FFT). The nanocrystals with interplane spacing of 0.29 nm corresponds to CoS with the (100) lattice plane, and those with 0.298 nm corresponds to NiS with the (100) lattice plane.

28, 31

For the rGO-supported CoS NFs composites,

most peaks are attributed to the hexagonal CoS phase (JCPDS card no. 75-0605), while a few low intensity peaks are indexed to Co9S8 (Figure 1a). 35 The presence of S, Co, Ni and C elements are indicated by the EDS spectra in Figure 3e and 3f. The HCS elemental analysis reveals that the nanocomposites consist of approximately 75 wt % metal-sulfides nanomaterials and 25 wt % rGO. These results demonstrate that rGO anchored with CoS and NiS nanomaterials are successfully achieved under the present hydrothermal and sulfidation process. To evaluate the lithium storage performance of the metal-sulfides-rGO nanocomposites, electrochemical properties were investigated by employing coin cells. Figure 4a shows cyclic voltagrams (CVs) of the CoS NFs-rGO electrode between 0.01 and 3 V scanned at 0.1 mV s-1. In the first cathodic scan, three plateaus at 1.35, 1.17 and 0.7 V are clearly visible, which are ascribed to the lithium alloying with the CoS, the conversion of LixCoS into Li2S and metallic Co, and the formation of solid electrolyte interphase (SEI) film, respectively.

25

It can be found that the

cathodic peaks shift to approximately 1.30 and 1.56 V in the successive cycle. No obvious changes in the second and third cycles indicate good capacity retention of the electrode material. In the following cathodic scan, only one oxidation peak centred at approximately 2.2 V is observed, corresponding to the change of metallic Co to CoS. 9



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Figure 4b presents CV cycles of the NiS NPs-rGO electrodes during initial scans at 0.1 mV s-1. In the cathodic scan process, peaks ranging from 0.2 to 1.6 V are ascribed to the Li/NiS reactions including the produce of Li2S and metallic Ni and the formation of the SEI film. In the anodic scan process, two visible peaks at approximately 2.05 V and 2.24 V correspond to conversion of the metallic Ni to NiS. 17, 32

After the first cycle, the curves turn reproducible, implying the high cycle

stability of Li+ storage in the NiS NPs-rGO composites. The stable cyclic performance is an important performance indicator to evaluate an electrode material. The discharge and charge cycle stability of CoS NFs-rGO was examined at operated current density of 100 mA g-1. In the initial cycle, a Coulombic efficiency (CE) of 69.5 % is achieved based on the discharge and charge capacities (1869 and 1299 mAh g-1) (Figure 5a). This irreversible capacity loss mainly results from the irreversible generation of Li2O and formation of SEI films, which is frequently addressed in many other conversion anodes.

15-24

In the subsequent 2nd

cycle, the CE increases to 91.5 %. After 100 cycles, the specific capacity maintains 939 mAh g-1 with CE above 98 %, which represents superior cyclic performance compared to previously reported metal-sulfide-based electrodes (Table S1). The rate capabilities of the CoS NFs-rGO composites were further studied. The CoS-rGO composite electrodes exhibit the high specific capacity as well as the good cycle stability at various current densities (Figure 5b). Along with increase of the current density, the specific capacity moderately decays. The corresponding discharge capacities of the CoS NFs-rGO composites are 1094 and 570 mA h g-1 when being 10


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cycled for 10 times at 100 and 2000 mA g-1, respectively. Delightedly, as cycles back at 100 mA g-1, the specific capacity value regains above 1065 mAh g-1, suggesting good reversibility of the electrode materials. Figure 5c reveals the reversible specific capacities of the NiS NPs-rGO electrodes at 100 mA g-1 during cycles. For the initial capacities of the NiS NPs-rGO composites, 1722 and 1114 mA h g-1 are achived. The corresponding CE is 64.7 %. After 100 cycles, the reversible capacity decreases slowly to 521 mA h g-1 with high CE of 99.2 %. The rate capabilities of the NiS NPs-rGO electrodes are shown in Figure 5d. When being cycled for 10 times at 100, 200, 500, 1000 and 2000 mA g-1, the corresponding discharge capacities are 933, 680, 541, 447, and 311 mA h g-1, respectively. The discharge capacity value retains as high as 584 mAh g-1 after 30 cycles as cycles back at 100 mA g-1, implying good rate capability of the NiS NPs-rGO composites. It is clear that the layered nanocomposites have the stable cycleability. To get insight into the correlation between the microstructure and the electrochemical performance, TEM analysis of metal-sulfides-rGO was carried out after 50 cycles. As shown in Figure 6, the CoS NFs and NiS NPs uniformly disperse on the rGO sheets after 50 cycles and no obvious aggregation occurs. Concerning that rGO can effectively relieve the nanomaterials agglomeration,

15, 16, 28-32

the superior

electrochemical properties of metal-sulfides-rGO can be well explained by three aspects: (i) The rGO sheets can mitigate the volume variation of the CoS NFs and NiS NPs during the lithium reaction; (ii) The rGO sheets reduce stacking of the CoS NFs and NiS NPs and improve the capacity; (iii) The layered nanoarchitecture facilitates 11



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the electrolyte access, as well as enlarges the contact area between the CoS-rGO nanocomposites and the electrolyte, resulting in improved transport kinetics of the lithium ions. These factors lead to high reversible capacity and rate capability, and enhanced cyclic stability of the metal-sulfides-rGO composites. Besides the CoS NFs-rGO and the NiS NPs-rGO, FeOOH-rGO nanomaterials have been also successfully converted to the FeS-rGO nanomaterials via the same sulfidation procedure. The XRD patterns of the obtained sample (Figure S9) can be indexed to FeS (JCPDS card no. 80-1028). 27 Figure S10 displays the electrochemical properties of the FeS-rGO composites. The cell shows the reversible capacity as high as 735 mA h g-1 at 100 mA h g-1 with good rate performance. After 100 cycles, the discharge capacity retains 485 mAh g-1 when the applied current density being returned to 100 mA g-1.

CONCLUSIONS The CoS-rGO and NiS-rGO nanocomposites with superior lithium storage performance have been prepared by the hydrothermal and sulfidation process. Concerning the safety and environmental friendly, this preparation method avoids the emission of toxic gas H2S from a hydrothermal or solvothermal process. The synthetic CoS NFs and NiS NPs are homogeneous dispersed on the rGO sheets. After 100 cycles, the CoS NFs-rGO electrodes deliver a relatively high capacity of 939 mA h g-1 at 100 mA g-1. The superior electrochemical properties of the CoS-rGO nanocomposites mainly results from the sheet-on-sheet structure of the CoS-rGO nanocomposites, which possess the enlarged contact area between the CoS-rGO 12


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nanocomposites and the electrolyte and the fast Li+ transport kinetics.

ACKNOWLEDGEMENTS Guo X. X. sincerely thanks the following support of scientific research funds: the National Key Basic Research Program of China (2014CB921004), the Science Foundation for Youth Scholar of State Key Laboratory of High Performance Ceramics and Superfine Microstructures (SKL201303). Tan, Y. B. gratefully acknowledges the funds support provided by China Postdoctoral Science Foundation (2015LH0026, 2015M581667). Supporting Information Available: Figures showing XRD, SEM, TEM and discharge and charge curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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Nanotube/layered

MoS2

Nanohybrid Network for Lithium Ion Battery Anode with Superior Rate Capacity and Long-Cycle-Life. Nano Energy 2015, 16, 10-18. (22) Zhang, L.; Wu, H. B.; Yan, Y.; Wang, X.; (David) Lou, X. W. Hierarchical MoS2 Microboxes Constructed by Nanosheets with Enhanced Electrochemical Properties for Lithium Storage and Water Splitting. Energy Environ. Sci. 2014, 7, 3302-3306. (23) Stephenson, T.; Li, Z.; Olsenab, B.; Mitlin, D. Lithium Ion Battery Applications of Molybdenum Disulfide (MoS2) Nanocomposites. Energy Environ. Sci. 2014, 7, 209-231. (24) Zhao, L.; Yu, X. Q.; Yu, J. Z.; Zhou, Y. N.; Ehrlich, S. N.; Hu, Y. S.; Su, D.; Li, H.; Yang, X. Q.; Chen, L. Q. Remarkably Improved Electrode Performance of Bulk MnS by Forming a Solid Solution with FeS-Understanding the Li Storage Mechanism. Adv. Funct. Mater. 2014, 24, 5557-5566. (25) Wang, Q. F.; Zou, R. Q.; Xia, W.; Ma, J.; Qiu, B.; Mahmood, A.; Zhao, R.; Yang, C.; Xia, D. G.; Xu, Q. Facile Synthesis of Ultrasmall CoS2 Nanoparticles within Thin N-Doped Porous Carbon Shell for High Performance Lithium-Ion Batteries. Small 2015, 21, 2511-2517. 16


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(26) Wu, R. B.; Wang, D. P.; Rui, X. H.; Liu, B.; Zhou, K.; Law, A. W. K.; Yan, Q. Y.; Wei, J.; Chen, Z. In-situ Formation of Hollow Hybrids Composed of Cobalt Sulfides Embedded within Porous Carbon Polyhedra/Carbon Nanotubes for High-Performance Lithium-Ion Batteries. Adv. Mater. 2015, 27, 3038-3044. (27) Jin, F. Y.; Wang, Y. Topotactical Conversion of Carbon Coated Fe-based Electrodes on Graphene Aerogels for Lithium Ion Storage. J. Mater. Chem. A 2015, 3, 14741-14749. (28) Wang, Z. Q.; Li, X.; Yang, Y.; Cui, Y. J.; Pan, H. G.; Wang, Z. Y.; Chen, B. L.; Qian, G. D. Highly Dispersed β-NiS Nanoparticles in Porous Carbon Matrices by a Template Metal-Organic Framework Method for Lithium-Ion Cathode. J. Mater. Chem. A 2014, 2, 7912-7916. (29) Kong, S. F.; Jin, Z. T.; Liu, H.; Wang, Y. Reduction of Oxygen on Dispersed Nanocrystalline CoS2. J. Phys. Chem. C 2014, 118, 25355-25364. (30) Ma, C. Z.; Xu, J.; Alvarado, J.; Qu, B. H.; Somerville, J.; Lee, J. Y.; Meng, Y. S. Investigating the Energy Storage Mechanism of SnS2-rGO Composite Anode for Advanced Na-Ion Batteries. Chem. Mater. 2015, 27, 5633-5640. (31) Kong, D. B.; He, H. Y.; Song, Q.; Wang, B.; Lv, W.; Yang, Q. H.; Zhi, L. J. Rational Design of MoS2@graphene Nanocables: towards High Performance Electrode Materials for Lithium Ion Batteries. Energy Environ. Sci. 2014, 7, 3320-3325. (32) Geng, H.; Kong, S. F.; Wang, Y. NiS Nanorod-assembled Nanoflowers Grown on Graphene: Morphology Evolution and Li-Ion Storage Applications. J. Mater. Chem. A, 2014, 2, 15152-15158. (33) Wang, Y. C.; Zhou, T.; Jiang, K.; Da, P. M.; Peng, Z.; Tang, J.; Kong, B.; Cai, W. B.; Yang, Z. Q.; Zheng, G. F. Reduced Mesoporous Co3O4 Nanowires for Efficient Water Oxidation Electrocatalyst and Supercapacitor Electrode. Adv. Energy Mater. 2014, 4, 1400696. (34) Gao, M. R.; Sheng, W. C.; Zhuang, Z. B.; Fang, Q. R.; Gu, S.; Jiang, J.; Yan, Y. S. Efficient Water Oxidation Using Nanostructured α-Nickel-Hydroxide as an Electrocatalyst. J. Am. Chem. Soc. 2014, 136, 7077-7084. 17



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(35) Zhu, H.; Zhang, J. F.; Zhang, R. P. Y.; Du, M. L.; Wang, Q. F.; Gao, G. H.; Wu, J. D.; Wu, G. M.; Zhang, M.; Liu, B.; Yao, J. M.; Zhang, X. W. When Cubic Cobalt Sulfide Meets Layered Molybdenum Disulfide: A Core-Shell System toward Synergetic Electrocatalytic Water Splitting. Adv. Mater. 2015, 27, 4752-4759.

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Figure captions: Scheme 1. Schematic illustration for the fabrication of CoS and NiS anchored on rGO. M(OH)2 can be easily fabricated on rGO (step 2) via the hydrothermal method (step 1). On step 2, the treatment of rGO-supported M(OH)2 with sulfur power leads to the formation of CoS and NiS anchored on rGO. Figure 1. XRD patterns of a) Co(OH)2-rGO, CoS-rGO and b) Ni(OH)2-rGO, NiS-rGO. Figure 2. SEM a-c) and TEM d) images of as-synthesized Co(OH)2-rGO at various magnifications. e-g) SEM and h) TEM images of as-synthesized Ni(OH)2-rGO. Figure 3. TEM and HRTEM images of the as-synthesized CoS NFs-rGO and NiS NPs-rGO: a, b) low-magnification TEM images and c, d) HRTEM images. EDS patterns of CoS NFs-rGO e) and NiS NPs-rGO f). Figure 4. CV curves of a) CoS NFs-rGO and b) NiS NPs-rGO. Discharge and charge profiles of c) CoS NFs-rGO and d) NiS NPs-rGO with increasing cycle numbers at 100 mA g-1. Figure 5. a) discharge and charge curves of CoS NFs-rGO at the current density of 100 mA g-1; b) rate capabilities of CoS NFs-rGO at various current densities between 100 mA g-1 and 2000 mA g-1; c) discharge and charge curves of NiS NPs-rGO at 100 mA g-1; d) rate capabilities of NiS NPs-rGO at various current densities. Figure 6. TEM images of the metal-sulfides-rGO composite after 50th cycle at a current density of 100 mA g-1 with respect to a) CoS NFs-rGO and b) NiS NPs-rGO. Scheme 1 19



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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 6

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In Situ Fabrication of CoS and NiS Nanomaterials Anchored on

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