Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 40317-40323
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Interlayer-Expanded Metal Sulfides on Graphene Triggered by a Molecularly Self-Promoting Process for Enhanced Lithium Ion Storage Qingqing Wang,†,∥ Kun Rui,†,∥,# Chao Zhang,⊥ Zhongyuan Ma,† Jingsan Xu,§ Wenping Sun,# Weina Zhang,† Jixin Zhu,*,† and Wei Huang*,†,‡ †
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China ‡ Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi’an 710072, China ⊥ State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China § School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Queensland 4001, Australia # Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong, Wollongong, New South Wales 2522, Australia S Supporting Information *
ABSTRACT: A general synthetic approach has been demonstrated to fabricate three-dimensional (3D) structured metal sulfides@graphene, employing few-layered sulfide nanostructures with expanded interlayer spacing of the (002) plane (e.g., 0.98 nm for MoS2 nanoclusters and 0.65 nm for VS4 nanoribbons) and electrically conductive graphene as ideal building blocks. Here, small molecules (thioacetamide) acting as both the sulfur source and, more importantly, the structuredirecting agent adjusting the interlayer spacing are wisely selected, further contributing to a sufficient space for ultrafast Li+ ion intercalation. The appealing features of a mechanically robust backbone, ultrathin thickness, abundant exposure of interlayer edges, and good electrical conductivity in such 3D architectures are favorable for providing easy access for the electrolyte to the structures and offering a shortened diffusion length of Li+ when utilized for energy storage. As a proof of concept, the electrochemical behavior of the resulting 3D structured metal sulfides@graphene as an anode material of lithium ion batteries (LIBs) is systematically investigated. As a consequence, high specific capacities, long lifespans, and superior rate capabilities have been realized in such well-designed architectures, e.g. maintaining a specific capacity as high as 965 mAh g−1 for 120 cycles for VS4@graphene and 1100 mAh g−1 for 150 cycles for MoS2@graphene. KEYWORDS: MoS2, VS4, graphene, lithium ion battery, molecularly self-promoting process
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INTRODUCTION
As already well-known, transition metal sulfides have attracted tremendous attention due to their capacity that is much higher than that of commercial graphite anodes (372 mAh g−1) as well as enhanced safety.15 Typically, layered transition metal dichalcogenides (LTMDs) with analogous structures to graphene, e.g. MoS2, SnS2, have been reported as attractive hosts for easy ion intercalation, especially as anode materials for LIBs.16−20 Besides intercalation, the reversible conversion reaction between Li+ and sulfides forming metal nanocrystals within the Li2S matrix further contributes to substantial gain in capacity.21,22 However, poor rate capability and rapid capacity fading caused by the slow Li+ ion diffusion and low electrical conductivity have impeded the further
Rechargeable lithium ion batteries (LIBs) are extensively investigated for a wide range of potential applications from portable electronics to electric vehicles (EVs) and hybrid electric vehicles (HEVs) as power supplies.1−3 However, the achievement of LIB electrode materials with a higher energy density, better rate capability, and longer lifespan is known to be hindered by sluggish kinetics due to the unsatisfactory diffusion of ions and electrons.4,5 To address these issues, nanostructure engineering has been extensively employed to tailor various morphologies, such as nanoparticles,6−8 nanotubes,9,10 nanowires,11 and nanosheets,11,12 with shortened diffusion pathways and easier access to Li+ and electrons. In particular, atomically thin-layered electrode materials show greatly improved electrochemical performance for energy storage devices as compared to their bulk counterparts.12−14 © 2017 American Chemical Society
Received: September 11, 2017 Accepted: November 2, 2017 Published: November 2, 2017 40317
DOI: 10.1021/acsami.7b13763 ACS Appl. Mater. Interfaces 2017, 9, 40317−40323
Research Article
ACS Applied Materials & Interfaces application of metal sulfides.23,24 To this regard, rationally constructing hybrid nanostructures of sulfides with good electrically conductive backbones not only facilitates electron and mass transport but also provides efficient accommodation of volume swing during repeated Li+ intercalation/extraction, thus leading to enhanced electrochemical performance.25 Endowed with good electrical conductivity, considerable elasticity, and electrochemical activity, graphene has established itself as promising two-dimensional (2D) building blocks for layered sulfides.26−28 Nevertheless, developing an efficient approach to the controllable coupling of the two ultrathin 2D layered materials with desirable interlayer spacing remains challenging to date. Herein, we report a facile and versatile approach to synthesize ultrathin-layered sulfide nanocrystals (e.g., MoS2 nanoclusters and VS4 nanoribbons) strongly coupled with graphene nanosheets, yielding three-dimensional (3D) hybrid architectures by a hydrothermal process. Benefiting from the molecular self-promoting reaction, the ultrathin metal sulfide nanocrystals were in situ anchored on the surface of the graphene oxide (GO) nanosheets, demonstrating expanded interlayer space and intensive exposure of active edges for Li+ intercalation/extraction. Meanwhile, GO serving as the functional backbone was transformed to reduced graphene oxide (rGO) with enhanced conductivity during the hydrothermal treatment. As expected, the resulting 3D metal sulfides@ graphene hybrids exhibit high specific capacity, excellent rate capability, and superior cycling stability as electrode materials for LIBs.
quently, TAA anchored on GO can further react with metal salts at 180 °C. Besides acting as the sulfur source, TAA is also employed as a small molecular structure-directing agent, giving rise to controlled in situ growth of ordered lamellar structures with an expanded interlayer. Simultaneously, GO can be further transformed to rGO by possible reduction of H2S released due to the decomposition of TAA at high temperatures,32 serving as the functional backbone with enhanced conductivity. Eventually, a 3D structured aerogel column via self-assembly of 2D ultrathin rGO nanosheet supported layered-metal-sulfide (LMS@rGO) can be obtained after freeze-drying (details can be found in the Experimental Section). The morphology and microstructure of VS4@graphene 3D architectures were systematically investigated by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The resulting product after hydrothermal reaction was a black monolithic aerogel column with a diameter of 2 cm readily depending on the volume of Teflon-lined autoclave (see inset of Figure 1a). As shown in Figure 1a,b, wrinkled rGO nanosheets with a size of several micrometers are uniformly decorated with VS4 nanocrystals that are closely integrated into the rGO substrate. Additionally, the morphology of VS4 nanocrystals can be readily tuned by adjusting the ratio of the precursor to get high mass loading of VS4 on graphene (see Experimental Section for details). Rodlike VS4 nanocrystals in large amounts rather than typical 2D morphologies can be observed, which are grown on both sides of the rGO nanosheets (Figure S1). TEM images (Figures 1c and S2) further reveal that these VS4 nanocrystals are 2D ultrathin nanoribbons anchored parallel onto the rGO substrate, with typical sizes of ca. 10 nm in width and 100 nm in length. The firm binding of VS4 with rGO nanosheets is shown by coexistence of two characteristic lattice fringes in a magnified TEM image (Figure 1d). As indicated by highresolution TEM (HRTEM) in Figure 1e, well-defined lattice fringes with an interlayer spacing of 0.65 nm can be ascribed to the (002) plane of VS4. As a consequence, well exposed (002) planes with enlarged spacing (0.56 nm for pristine VS4) are expected to be ideal for ultrafast Li+ ion intercalation related to enhanced kinetics. The composite microstructure of as-prepared MoS2@ graphene was also investigated by FE-SEM and TEM measurements as shown in Figures 2 and S3. The MoS2based aerogel column (inset photo of Figure 2a), similar to VS4@graphene, verifies the feasibility and generality of our proposed approach. As seen from FE-SEM images in Figure 2a,b, self-assembled MoS2@graphene nanoclusters consist of curled nanosheets with a diameter of 100−200 nm. TEM images (Figures 2c and S3) further confirm that ultrathinlayered MoS2 is supported on the surface of graphene. Herein, GO nanosheets acted as an excellent substrate for the selective nucleation of MoS2 and subsequent growth due to interaction between the oxygen-containing functional groups on GO sheets and TAA. Apart from attracting Mo precursors and donating sulfide for MoS2 nucleation, TAA itself acted as the structuraldirecting agent preventing the aggregation of individual MoS2 layers and more importantly stretching out between interlayers via a unique self-promoting process, without the use of exfoliation solvent (e.g., NMP, DMF) in other cases.26 As seen, an obviously enlarged interlayer distance of 0.98 nm can be obtained according to the HRTEM image (Figure 2d), larger than that of the standard interlayer spacing (0.62 nm for bulk 2H-MoS2). As indicated by the crystal structure in Figure
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RESULTS AND DISCUSSION As is depicted in Scheme 1, MoS2@graphene and VS4@ graphene 3D architectures were successfully fabricated via a facile self-assembly hydrothermal procedure featured with molecularly self-promoting reactions. During this process, thioacetamide (TAA) is anchored on GO nanosheets through electrostatic interaction and hydrogen bonds.29−31 SubseScheme 1. Schematic Illustration for On-Surface Fabrication of 3D Nanostructured Layered-Metal-Sulfide (LMS) Nanocrystalsa
a
With expanded interlayer on reduced graphene oxide (rGO) nanosheets (3D LMS@rGO) via a molecularly self-promoting process by thioacetamide (TAA). 40318
DOI: 10.1021/acsami.7b13763 ACS Appl. Mater. Interfaces 2017, 9, 40317−40323
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Figure 1. Typical FE-SEM and TEM images of VS4@graphene architectures: (a,b) FE-SEM images (inset: photo of the as-prepared sample with 3D structure); (c) TEM image; (d,e) high-resolution TEM images.
Figure 2. Typical FE-SEM and TEM images of MoS2@graphene architectures: (a,b) FE-SEM images (inset: photo of the as-prepared 3D samples architectures); (c−e) TEM, corresponding high-resolution TEM images and demonstration of the MoS2 crystal structure.
2e, the significantly enlarged distance in the c-axis direction is beneficial to the reaction kinetics with favored Li+ intercalation/ extraction and higher lithium storage, contributing to improved electrochemical performance.26 X-ray diffraction (XRD) was performed to uncover the crystallinity and composition of the as-prepared sulfides@ graphene composites (Figure 3a). As the XRD pattern of VS4@ graphene depicts, diffraction peaks corresponding to (110), (020), (202), (121), (202), (114), and (224) are observed, which can be well-indexed to monoclinic VS4 (JCPDS No. 72− 1294).33 More importantly, a significantly different XRD pattern from that of bulk 2H-MoS2 (JCPDS No. 77−1716) is presented for as-prepared MoS2@graphene composites. In contrast to the characteristic peak at 14.38° for pristine 2H-
MoS2, a strong peak shifts to a lower angle of 8.6°, demonstrating a remarkably expanded interlayer in good agreement with HRTEM results (Figure 2d) and previous reports.34,35 Meanwhile, a new peak emerges at 17.3° sharing a diploid relationship with the aforementioned one, which is revealed by calculated d-spacing values according to Bragg’s law (1.02 nm for 8.6° and 0.51 nm for 17.3°). These results indicate the formation of a new layered structure with an expanded interlayer as compared to 0.62 nm for the (002) plane of the pristine 2H-MoS2. Moreover, the absence of a characteristic peak related to rGO sheets at approximately 26° indicates the ultrathin feature of graphene without severe stacking,36 which is also confirmed by TEM results in Figures 1 and 2. However, the presence of rGO in both hybrid samples 40319
DOI: 10.1021/acsami.7b13763 ACS Appl. Mater. Interfaces 2017, 9, 40317−40323
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ACS Applied Materials & Interfaces
Figure 3. (a) XRD patterns and (b) Raman spectra of VS4@graphene and MoS2@graphene architectures. XPS of (c,d) VS4@graphene architectures, indicating the presence of V4+ and S22− and (e,f) MoS2@graphene architecture, indicating the presence of Mo4+ and S2−.
Figure 4. Electrochemical performance of VS4@graphene and MoS2@graphene architectures: (a) rate capability of VS4@graphene at different current densities from 200 to 10 000 mA g−1; (b) cycling performance of VS4@graphene at current densities of 1000 and 3000 mA g−1; (c) rate capability of MoS2@graphene at different current densities from 200 to 10 000 mA g−1; (d) cycling performance of MoS2@graphene at current densities of 100 and 500 mA g−1.
notable peaks located at 232.4 and 228.9 eV are attributed to Mo 3d3/2 and Mo 3d5/2, respectively, indicating the dominance of Mo4+ in the products. Whereas Figure 3f exhibits peaks at 162.9 and 161.7 eV attributed to S 2p1/2 and S 2p3/2 orbitals, which is consistent with the oxidation state of sulfur (−2) in MoS2.40,41 The Brunauer−Emmett−Teller (BET) surface area of the 3D VS4@graphene and MoS2@graphene architecture determined from N2 adsorption−desorption isotherms (Figure S5) is 81.0 and 143.6 m2 g−1, respectively. In addition, VS4 and MoS2 loading ratios in the 3D hybrid architectures are determined to be 90.8 and 92.8%, respectively, according to thermogravimetric analysis (TGA) curves (Figure S6). The lithium storage properties of 3D VS4@graphene and MoS2@graphene hybrid architectures were evaluated as shown in Figure 4. A multiple-current galvanostatic measurement was conducted to investigate the high rate performance of VS4@
can be determined by Raman spectra in Figure 3b. The characteristic D and G peaks are observed at around 1360 and 1575 cm−1, respectively.37 Moreover, higher intensity ratios of the D to G bands (ID/IG) for sulfides@graphene hybrids compared to GO (e.g., 1.29 for VS4@graphene, 1.25 for MoS2@graphene, and 0.83 for GO) indicate the reduction of GO to rGO (Figure S4).38 The chemical states of the VS4@graphene and MoS2@ graphene architectures were analyzed by X-ray photoelectron spectroscopy (XPS). Two peaks arising from V 2p1/2 and V 2p3/2 are located at 524.6 and 517.4 eV, confirming the presence of V4+ in the VS4@graphene hybrids (Figure 3c). Meanwhile, the existence of S22− species in the hybrids is demonstrated by two characteristic peaks ascribed to S 2p1/2 (165.3 eV) and S 2p3/2 (164.0 eV) according to the S 2p core level analysis in Figure 3d.39 As can be seen from Figure 3e, two 40320
DOI: 10.1021/acsami.7b13763 ACS Appl. Mater. Interfaces 2017, 9, 40317−40323
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ACS Applied Materials & Interfaces
intercalation of Li+ into MoS2 and the formation of LixMoS2 (Figure S10b). A more intense peak at 0.54 V is believed to be attributed to the conversion reaction of LixMoS2 to Mo and Li2S along with the formation of a solid electrolyte interface (SEI).51 In the following charging process, two anodic peaks centered at 1.86 and 2.24 V are associated with the oxidation of metallic Mo and the delithiation of Li2S (e.g., Li2S → 2Li+ + S + 2e−), respectively.38,52 From the second sweep, a new peak concerning the lithiation of S emerged at around 1.45 V, representing the redox process of Li2S/S couple. Additionally, the overlapping of CV profiles during subsequent cycles suggests good reversibility and stability of the hybrid electrode. The excellent electrochemical properties of 3D LMS@rGO were further analyzed by electrochemical impedance spectroscopy (EIS), carried out after four cycles at the anode potential of 1.5 V (vs Li/Li+). The Nyquist plots (Figure S11) for both samples demonstrate a semicircle in the high-medium frequency region and an inclined line in the low frequency. A small charge-transfer resistance related to the depressed semicircle can be obtained for VS4@graphene and MoS2@ graphene, indicating a favored charge-transfer reaction for Li+ and thus enhanced Li storage properties.53 Moreover, the plot slope of Z′ (real component of the impedance) vs ω−1/2, i.e. Warburg factor, can be obtained to illustrate the speed of lithium diffusion, which is calculated to be 86.5 for VS4@ graphene and 29.2 for
[email protected] Therefore, the lithium diffusion coefficients at 25 °C were calculated to be 4.62 × 10−12 and 3.64 × 10−12 cm2 s−1 for VS4@graphene and MoS2@graphene, respectively. Herein, both the small chargetransfer resistance and high lithium diffusion coefficient contribute to enhanced Li storage properties.
graphene hybrids (Figure 4a). Meanwhile, representative discharge−charge profiles in the voltage window from 0.01 to 3.0 V at different current densities were displayed in Figure S7. The VS4@graphene electrode delivers a reversible capacity as high as 950 mAh g−1 at a current density of 200 mA g−1, while a considerable capacity of ca. 300 mAh g−1 can be retained at a high current density of 10 000 mA g−1. When the current density returned to 200 mA g−1, the capacity can still reach as high as 920 mAh g−1. Cycling performance was also evaluated at 1000 and 3000 mA g−1 and is shown in Figure 4b. Initial reversible capacities of 751 and 612 mAh g−1 are obtained at 1000 and 3000 mA g−1, respectively. Interestingly, the VS4@ graphene electrode exhibits a rising capacity during the cycling process, with a specific capacity reaching 965 mAh g−1 at a current density of 1000 mA g−1 after 120 cycles. The capacity increase can be attributed to the gradually expanded and exfoliated interlayers, providing more active Li+ storage sites and a low energy barrier for Li+ intercalation/extraction.42 On the other hand, the unsatisfactory capability and rapid capacity fading of pure VS4 were attributed to the irreversible volume change as well as poor electric conductivity (Figure S8a). Coincidentally, the heterostructure composed of graphene supported MoS2 with a further expanded interlayer leads to excellent lithium storage properties. Figures 4c and S9 demonstrated the rate capability of MoS2@graphene, which is around 900, 800, 734, 650, 540, 400, 320, and 232 mAh g−1 at a current density of 200, 500, 1000, 2000, 3000, 5000, 7000, and 10 000 mA g−1, respectively. After undergoing a high rate, the specific capacity rapidly returned to 930 mAh g−1 at 200 mA g−1, demonstrating excellent reversibility. Moreover, MoS2@ graphene demonstrates predominant cycling behavior with retained capacities of 1100 and 720 mAh g−1 at a current density of 100 and 500 mA g−1 after 150 cycles, respectively (Figure 4d). By comparison, the pure MoS2 obtained under identical conditions demonstrated severe capacity decay during cycling (Figure S8b). Clearly, the 3D VS4@graphene and MoS2@graphene architectures display enhanced electrochemical performance in LIBs benefiting from their unique structural features. On one hand, the expanded interlayers of metal sulfides with exposed active sites contribute to facilitated ion/ electron transport kinetics. As the cycles increase, the expanded interlayer with more active sites released as well as the formation of gel-like polymeric layer, was considered to facilitate the reversible intercalation/extraction of more Li+, delivering increasing capacities.43−45 Additionally, rGO in the heterostructure not only effectively buffers the volume variations during repeated discharge/charge but also prevents self-aggregation of the sulfides, thus ensuring sufficient surface area and ion exchange channels for electrochemical reactions.46 Cyclic voltammetry (CV) studies were further conducted to elucidate the electrochemical reactions at a scan rate of 0.2 mV s−1 between 0.01 and 3.0 V (vs Li/Li+). As shown in Figure S10a, the reduction peak at 1.2 V during the initial cathodic scan should be assigned to the intercalation of Li+ resulting in the formation of Li3+xVS4.47 The smaller peak at 0.51 V is likely related to the further reduction to metallic V and Li2S.48 Moreover, the first anodic scan for VS4@graphene exhibits two pronounced peaks at 1.98 and 2.45 V, corresponding to the delithiation of Li2S and the formation of amorphous VS4.49,50 During the following cycles, two redox pairs stabilized at around 1.98 and 2.45 V with decreasing intensities can be observed, which is consistent with previous reports.49 Similarly, a small reduction peak around 1.44 V can be ascribed to the
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CONCLUSIONS In summary, a general and facile molecularly self-promoting synthesis route has been developed for the preparation of 3D nanostructured layered-metal-sulfides@graphene with a larger interlayer spacing of the (002) plane (e.g., 0.98 nm for MoS2 nanoclusters and 0.65 nm for VS4 nanoribbons). This generic hydrothermal approach can be further used to synthesize a series of 3D structured metal sulfides@graphene hybrids, i.e. in situ growth of ultrathin metal sulfides with a large interlayer distance on wrinkled graphene nanosheet backbones. Endowed with enlarged contact area, better conductivity, expanded interlayer distance, and more active site exposure, the resulting composites display superior electrochemical performance and a longer lifespan, shedding light on the structural engineering of layered materials as electrodes in LIBs.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13763. Experimental details and supporting Figures S1− S11(PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (J.Z.) *E-mail:
[email protected] (W.H.) ORCID
Jingsan Xu: 0000-0003-1172-3864 Jixin Zhu: 0000-0001-8749-8937 40321
DOI: 10.1021/acsami.7b13763 ACS Appl. Mater. Interfaces 2017, 9, 40317−40323
Research Article
ACS Applied Materials & Interfaces Author Contributions ∥
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Q.W. and K.R. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21501091), the NSF of Jiangsu Province (55135065), the Recruitment Program of Global Experts (1211019), the “Six Talent Peak” Project of Jiangsu Province (XCL-043), the National Key Basic Research Program of China (973) (2015CB932200), and the China Postdoctoral Science Foundation (2016M600404, 2017T100360). The authors thank Prof. Fengwei Huo for valuable discussions.
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DOI: 10.1021/acsami.7b13763 ACS Appl. Mater. Interfaces 2017, 9, 40317−40323
Research Article
ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.7b13763 ACS Appl. Mater. Interfaces 2017, 9, 40317−40323
