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Few-layer MoS2 nanosheets incorporated into hierarchical porous carbon for lithium-ion batteries
Accepted Manuscript Few-layer MoS2 nanosheets incorporated into hierarchical porous carbon for Lithium-ion batteries Haiyan Wang, Dayong Ren, Zhengju Zhu, Petr Saha, Hao Jiang, Chunzhong Li PII: DOI: Reference:
S1385-8947(15)01657-5 http://dx.doi.org/10.1016/j.cej.2015.11.105 CEJ 14503
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
10 October 2015 29 November 2015 30 November 2015
Please cite this article as: H. Wang, D. Ren, Z. Zhu, P. Saha, H. Jiang, C. Li, Few-layer MoS2 nanosheets incorporated into hierarchical porous carbon for Lithium-ion batteries, Chemical Engineering Journal (2015), doi: http:// dx.doi.org/10.1016/j.cej.2015.11.105
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Few-layer MoS2 nanosheets incorporated into hierarchical porous carbon for Lithium-ion batteries Haiyan Wang a, Dayong Ren a, Zhengju Zhu a, Petr Saha b , Hao Jiang a,*, Chunzhong Li a,* a
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and
Engineering, East China University of Science and Technology, Shanghai 200237, China b
Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Trida T. Bati 5678, 760
01 Zlin, Czech Republic Email: [email protected] (Prof. H. Jiang) and [email protected] (Prof. C. Li) Abstract In order to overcome the serious stacking and poor conductivity of graphene-like MoS2 nanosheets, we have developed the synthesis of few-layer MoS2 nanosheets incorporated into biomass-derived hierarchical porous carbon frameworks (labelled as MoS2 /C hybrids) utilizing the strong water-absorbing power of auricularia from its inherent rich porous structure. The as-obtained MoS2/C hybrids, when applied as lithium-ion batteries anode materials, show an improved specific capacity of 707.4 mAh g-1 compared with the commercial MoS2 nanosheets (580.2 mAh g-1) and the corresponding hierarchical porous carbon (215.5 mAh g-1). More meaningfully, they possess an impressive cycle life, almost without capacity fading even after 500 cycles at 1600 mA g-1. The intriguing performance is mainly attributed to the well-dispersion of few-layer MoS2 nanosheets into hierarchical porous carbon. We believe this work will provide a new insight on the design and synthesis of novel carbon-based electrode materials for potential applications in lithium-ion batteries and other clean energy devices. Keywords: few-layer MoS2, biomass, hierarchical porous carbon, lithium ion batteries.
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1. Introduction Lithium-ion batteries (LIBs) with high energy density and design flexibility are considered to be a priority candidate of power sources for future electric vehicles [1-3]. Graphite has been generally used as the commercial anode materials, while its low lithium storage capacity (372 mAh g-1) hardly meets the increasing demand of high-energy density. So exploring novel LIBs anode materials with higher capacity, longer cycle life is in sore need. Molybdenum disulfide (MoS2), a typical two-dimensional material, attracts great interests in energy storage field in recent years, which exists strong covalent forces within S-Mo interactions and weak van der Waals bonds between S-Mo-S layers. According to the 4-electron transfer reaction (MoS2 + 4Li + + 4e- → Mo + 2Li2S), MoS2 can deliver a high theoretical capacity of 670 mAh g-1 [4-6]. However, its low electrical conductivity, easy stacking and restacking, as well as volume expansion during lithium insertion/extraction will result in poor cyclability and rate capability when used as LIBs anode materials [7-9]. To overcome these drawbacks, recent studies have demonstrated that constructing MoS2/carbon hybrid structures is a good choice by hybridizing MoS2 nanosheets with various carbons, such as carbon nanotubes, graphene and so forth, which availably enhances the electron conductivity and also promotes rapid electron transfer [10-16]. In terms of the reported structures, MoS2 nanosheets are mostly decorated out of the surface of carbon matrix with strong tendency to restack during the charge/discharge process, which suffers severe structural instability, resulting in poor rate capability and cycling stability. A possible way to solve this problem is to fabricate few-layer MoS2-C incorporated structure hybrids [17-21]. For example, Zhu et al. [21] prepared few-layer MoS2 nanoplates embedded in carbon nanowires by electrospinning and showed much enhanced rate performance and cycling stability. Despite the feasibility of the incorporated structure,
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state of the art methods always involve toxic and corrosive organic chemicals of precursors, high risks during operation and limited production rate in industrial manufacture, thus hindering the applicability [22,23]. Therefore, developing an environmental friendliness, facile and scalable route to achieve the few-layer MoS2-C incorporated structure is still a challenge. Auricularia, a kind of worldwide abundant fungus, naturally contains plenty of meso/macropores which is endowed with volumetric swelling characteristics after absorbing various solutions [24,25]. Inspired by this, herein, we developed a simple method to fabricate few-layer MoS2 nanosheets incorporated into hierarchical porous biomass carbon framework. Such a novel architecture embraces the following features. First, the incorporation system effectively prevents the MoS2 aggregation and buffers the volume expansion, which ensures the structure integrity to obtain long cycling life. Furthermore, the intimate contact between MoS2 nanosheets and carbon matrix benefits fast ion/electron transportation across the interface. Second, few-layer MoS2 nanosheets obtained from the confinement growth in hierarchical porous carbon offer more active sites, lowering the barrier for lithium intercalation and also amplifying the conversion reaction. Third, the three-dimensional (3D) carbon framework with porous structure not only enhances the conductivity but also provides large contact area between the electrode and electrolyte. Thereby, when evaluated as anode materials for LIBs, the MoS2/C hybrids exhibit high specific capacity, excellent cycling stability and impressive rate capability.
2. Experimental section
2.1 Synthesis of the hierarchical porous MoS2/C hybrids The hierarchical porous MoS2/C was synthesized as follows. Typically, 0.6g ammonium thiomolybdate dissolved in 20 ml deionized water was sonicated for 30 min to form a well dispersed 3
solution with the concentration of 0.03 g ml-1. Then, 0.5g auricularia was placed into the mixture for a night long standing until the volume of the auricularia expanded completely. After freeze-drying, the swelling auricularia was finally carbonized and reduced into the MoS2/C hybrids under Ar atmosphere with a heating rate of 3 °C min-1 to 850 °C for 2 h. For the experiments with different MoS2 contents, the ammonium thiomolybdate concentration was adjusted. Hierarchical porous carbon was also prepared by using the procedure for the preparation of MoS2/C without adding ammonium thiomolybdate. The exfoliated MoS2 was prepared according to our previous work. Briefly, 0.15g of sodium molybdate and 0.3g of thioacetamide were dissolved into 30 mL of deionized (DI) water with continuous stirring. Then, the suspension was transferred into 50 ml Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 hours. The precipitates were collected by filtration, washed with DI water and ethanol and dried at 60 °C.
2.2 Characterization The morphologies were characterized by field emission SEM (S-4800) and TEM (JEOL 2100). The structure was checked by XRD (Rigaku D/max 2550VB/PC) at a scan rate of 1° min-1 . Raman spectra (Renishaw inVia Reflex) was performed at ambient temperature with a NEXUS 670 FT-IR Raman spectroscopy. The porous properties were analyzed using nitrogen adsorption and desorption isotherms that were obtained using the surface area and a porosimetry analyzer (ASAP 2010) at 77 K. Thermogravimetric analysis (NETZSCH STA409PC) was carried out with a heating rate of 10°C min–1 under flowing air.
2.3 Electrochemical Measurements
Electrochemical measurements were determined using CR 2016 type coin cells assembled in an argon-filled glovebox. The working electrode was prepared with the active material (MoS2/C), a conductive agent (carbon black, Super-P-Li), and a polymer binder (poly(vinylidene difluoride), PVDF, Aldrich) in a 4
weight ratio of around 70 : 20 : 10, and then pasted on pure Cu foil. The coating thickness on Cu foil is about 50 µm. Li foil was used as the counter electrode and a polypropylene film (Celgard-2400) was used as a separator. The electrolyte was 1 M solution of LiPF6 in a 1:1 v/v mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The cells were charged and discharged galvanostatically at different rates in the voltage range of 0.01–3.0 V on a LAND-CT2001A battery tester at room temperature. Cyclic voltammogram experiment was performed on an Autolab PGSTAT302N electrochemical workstation at scan rates of 0.2 mV s-1 .
3. Results and discussion The MoS2/C hybrids have been synthesized by absorbing ammonium thiomolybdate aqueous solution into the porous structure of auricularia and the following carbonization. The morphology and microstructure of the as-synthesized MoS2/C were characterized by SEM and TEM technique. Fig. 1a shows that the MoS2/C hybrids are assembled into a highly interconnected 3D porous network with sizes of ~ 300 nm. The high-magnification TEM observation shows that few-layer MoS2 nanosheets are well-dispersed and incorporated into carbon walls, as shown in Fig. 1b. From the high-resolution TEM image (Fig. 1c), it can also find that some of few-layer MoS2 nanosheets lay on the surface on carbon frameworks with interplanar spacing of 0.27 nm, corresponding to the (101) planes of hexagonal MoS2 crystals. It is noted that the lateral dimension and the thickness of the dispersed MoS2 nanosheets are only ~ 3.0 nm and 0.4 nm, which will be favorable for offering rich electrochemical active sites and shortening the transfer distance of electrons/ions. The corresponding selected-area electron diffraction (SAED) pattern (the inset of Fig. 1c) gives the hexagonal structure of the MoS2/C hybrids and further verified the crystallinity of MoS2. In addition, the elemental distribution of C, Mo, S elements of the MoS2/C hybrids
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was also provided in Fig. S1. The uniform distribution suggests highly dispersity of MoS2 nanosheets in hierarchical porous carbon. The crystalline structure of the MoS2/C hybrids was also examined by X-ray diffraction (XRD), as shown in Fig. 2a. All the small diffraction peaks are attributed to the hexagonal MoS2 crystals (JCPDS 37-1492) with relatively poor crystallinity mainly due to their size and layers from the TEM observation as well as the comparable amount with carbon contents. Also, we can see a broad peak at 20-30°, which will be assigned to the calcined biomass-derived carbon. Notably, the very weak (002) peak suggests the few-layer MoS2 nanosheets [26]. The Raman spectroscopy is used to prove it, as shown in Fig. 2b. The energy difference between two Raman peaks (∆k) is ~23.4 cm-1, further indicates a few-layer nature [27,28]. Furthermore, the carbon quality has also been evaluated, as shown in Fig. 2b. Two pronounced Raman peaks at 1361 cm-1 (D-band) and 1601 cm-1 (G-band) can be observed with the calculated value of ID/IG to be 0.99, showing a relatively high graphitization degree. This feature is usually good at improving the power density for LIBs. On the other hand, just like some reported MoS2 /C hybrids, our MoS2 content can also be easily controlled to some extent by changing the absorbing concentration of ammonium thiomolybdate aqueous solution, which is determined by TG measurement, as shown in Fig. S2. A total weight loss between 300°C and 500 °C is assigned to the MoS2 oxidation and the carbon combustion. The final oxidation product of MoS2 is MoO3 [17,29]. As for the above mentioned sample, the MoS2 content is ~ 54%. However, a serious stacking of MoS2 nanosheets will happen if we further increase the precursor concentration. For example, a sharp and strong (002) peak appears in the XRD pattern for the hybrids with 79% MoS2 content (Fig. S3). Therefore, we choose the hybrids with 54% MoS2 content to evaluate their electrochemical performance, aiming to investigate the importance of few-layer MoS2 nanosheets on enhancing lithium ion storage. 6
Cyclic voltammogram (CV) and galvanostatic charge/discharge profiles were performed to evaluate the electrochemical properties of the MoS2/C hybrids. Fig. 3a illustrates the CV curves conducted at a scan rate of 0.2 mV s-1 within the potential range of 0.01-3.0 V. In the first cycle, the CV curve presents two prominent reduction peaks at 1.04 and 0.41 V and two oxidation peaks at 1.60 and 2.30 V, respectively. The reduction peak at 1.04 V corresponds to Li insertion into the interlayer spacing of MoS2, accompanied by phase transformation from 2H to 1T structures of LixMoS2. The other reduction peak at 0.41 V is attributed to the conversion reaction from Lix MoS2 to Mo metal and Li2S, and then the formation of solid-electrolyte interphase (SEI) resulting from electrochemically driven electrolyte degradation [30]. In the reverse anodic scan, a weak peak at 1.60 V can be attributed to the partial oxidation of Mo metal to form MoS2 . And the broad peak at 2.30 V is described as the delithiation of Li2S (Li2S → S + 2Li + + 2e-) [31]. During the subsequent cycles, two new reduction peaks at 1.90 V, 1.23 V emerge, which corresponds to the following reactions: S + 2Li+ + 2e- → Li2S and MoS2 + xLi+ + xe- → LixMoS2, respectively [30]. Notably, the CV curves are almost identical since the second cycle, indicating a good electrochemical stability. Fig. 3b shows the galvanostatic charge/discharge curves of the MoS2 /C hybrids electrode for the first three cycles at a current density of 200 mA g-1 between 0.01 and 3.0 V. In the first discharge process, two potential plateaus at about 1.1 V and 0.6 V are observed, which is in accordance with the CV curves. The initial discharge capacity of the MoS2/C hybrids reaches 1426.4 mAh g-1 and a reversible capacity of 967 mAh g-1 was obtained with a Coulombic efficiency of 68%. The unavoidable irreversibility is due to the formation of SEI film. In the second and third cycles, the Coulombic efficiency increases to more than 95%, performing a high reversible capacity and stable cycling performance. Fig. 3c depicts the rate performance of the MoS2/C hybrids, in comparison with commercial MoS2 and the corresponding hierarchical porous carbon. The MoS2/C hybrids deliver reversible capacities of 709.6, 577.2, 475.9, 353.5, 7
307.2 and 252.2 mAh g-1 at current densities from 200, 400, 800, 1600, 3200 to 6400 mA g-1, respectively, all of which are higher than those of commercial MoS2 (580.2, 478.2, 399.8, 295.5, 209.8, 131.4 mAh g-1) and hierarchical porous carbon (215.5, 131.5, 84.5, 49, 31.2, 27 mAh g-1). Especially, when the current density goes back to 200 mA g-1 after cycling under high current densities, a reversible capacity of 630 mAh g-1 was still regained. The MoS2 /C hybrids exhibit not only higher specific capacity but also enhanced rate-capability than both commercial MoS2 and hierarchical porous carbon. The electrochemical behavior of MoS2/C-45% and MoS2/C-79% were also characterized (as shown in Fig. S5). Even with a higher dispersion of MoS2 nanosheets, the electrochemical performance of MoS2 /C-45% is hindered by the low MoS2 contents. Despite a relatively high capacity of MoS2/C-79% at a low current density, the stacking of MoS2 nanosheets happens, resulting in an inferior rate performance. Therefore, the optimized contents were further verified to MoS2/C-54%. To further understand the reason why the MoS2/C hybrids possess good electrochemical performance for lithium storage, the electrochemical impedance spectroscopy (EIS) of MoS2/C hybrids and commercial MoS2 was measured, respectively, as shown in Fig. 3d. The MoS2/C hybrids exhibit a reduced diameter of the semi-circle at high frequencies compared with commercial MoS2, indicating the greatly decreased charge-transfer resistance (Rct) at the electrode/electrolyte interface. The Warburg impedance (W) comes from the interfacial diffusion of Li+ ions at electrode/electrolyte interface，which is described by a straight sloping line in low frequency region. Apparently, the inclined line of the MoS2/C hybrids appears a larger slope than commercial MoS2. Therefore, the combination of Rct and W demonstrates the fast reaction kinetics and ionic diffusion in the MoS2/C hybrids electrode. Also, the cycling performance of the MoS2/C hybrids at a high current density of 1600 mA g-1 is shown in Fig. 4, together with the commercial MoS2 material. The capacity of MoS2/C hybrids remains about 400 mAh g-1 until the 500th cycle with a 8
Coulombic efficiency above 99%. However, the commercial MoS2 shows a rapid capacity deterioration at the identical current density with only 51 mAh g-1 remained after 150 cycles. The electrochemical performance of the exfoliated MoS2 nanosheets is also tested, which can be seen from Fig. S6 and Fig. S7. This point can further prove the impressive cycling stability of our MoS2/C hybrids. The MoS2/C hybrids show a comprehensive excellent electrochemical performance, which can be comparable to those previously reported works. For example, David et al. synthesized layered SiCN-MoS2 structure and it exhibited good performance——regaining initial charge capacity of 530 mAh g-1 when the current density returned to 100 mA g-1 after continuous cycling at 2400 mA g-1 (192 mAh g-1). Shi et al. reported the synthesis of MoSx/MWNTs nanocomposites, showing remarkably high specific capacity of 1000 mAh g-1 at 50 mA g-1 (see Table S1in ESI). These intriguing results validate the enhanced lithium storage properties and improved rate capability of the MoS2 /C hybrids, which can be attributed to its unique structure and morphology. First, the biomass carbon trapping the few-layer MoS2 nanosheets into a confined space ensures the intimate contact between the conducting carbon skeleton and the MoS2 nanosheets, favoring the fast ion transport even at higher rates. Moreover, the incorporated nanostructure benefits the strong integrity of the electrode, which can effectively avoid the stacking of MoS2 nanosheets and also alleviate the volume expansion during the charge/discharge processes, contributing to a stable cycling capacity and long life-span. Second, the few layer MoS2 nanosheets with ultrathin nature can provide abundant electroactive sites for Li storage, which enables high specific capacities, excellent rate capability. Third, 3D continuous conductive biomass carbon with abundant porous structure provides fast electronic conduction channels for MoS2. Therefore, our materials exhibit improved specific capacity, enhanced rate performance and outstanding cyclic stability.
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4. Conclusions In summary, a novel hybrid structure of few-layer MoS2 nanosheets homo-dispersively incorporated into biomass-derived hierarchical porous carbon framework has been successfully synthesized by a facile and scalable method. The unique structure can effectively avoid the stacking of MoS2 nanosheets and overcome the poor conductivity of MoS2 nanosheets. As a consequence, the MoS2/C hybrids show much better electrochemical properties than the commercial MoS2 nanosheets and the corresponding hierarchical porous carbon electrodes for reversible lithium storage. Specifically, the MoS2 /C hybrids deliver a high specific capacity of 707.4 mAh g-1 at 200 mA g-1 with intriguing rate capability (400 mAh g-1 at 1600 mA g-1 ). More importantly, they possess an excellent cycle life, almost without capacity fading even after 500 cycles. Furthermore, this approach will provide a new insight on the design and synthesis of novel carbon-based electrode materials for potential applications in lithium-ion batteries and other areas, such as supercapacitor, hydrogenation and fuel cells.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21236003, 21522602), the Shanghai Rising-Star Program (15QA1401200), the International Science and Technology Cooperation Program of China (2015DFA51220), the 111 Project (B14018), and the Fundamental Research Funds for the Central Universities.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://www.elsevier.com/.
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Captions for figures Fig. 1 (a) SEM image, (b) high-magnification and (c) high-resolution TEM images of the as-synthesized MoS2/C hybrids (inset showing the corresponding SAED pattern). Fig. 2 XRD pattern and Raman spectra of the MoS2/C-54% hybrids. Fig. 3 (a) CV curves at 0.2 mV s-1 and (b) charge–discharge curves of the MoS2/C-54% hybrids for the initial 3 cycles at a current density of 200 mA g-1, (c) rate capabilities of the MoS2/C-54% hybrids, commercial MoS2 nanosheets and the corresponding hierarchical porous carbon, (d) electrochemical impedance spectra of the MoS2 /C-54% hybrids and commercial MoS2 nanosheets. Fig. 4 Cycling behavior and Columbic efficiency of the MoS2 /C-54% hybrids and commercial MoS2 at a current density of 1600 mA g-1.
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Graphical abstract
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Research highlights 1. Few-layer MoS2 nanosheets are incorporated into hierarchical carbon frameworks. 2. A MoS2/C hybrid is prepared using strong water-absorbing power of auricularia. 3. The MoS2/C hybrids LIBs anode exhibits improved electrochemical performances.
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S1385-8947(15)01657-5 http://dx.doi.org/10.1016/j.cej.2015.11.105 CEJ 14503
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
10 October 2015 29 November 2015 30 November 2015
Please cite this article as: H. Wang, D. Ren, Z. Zhu, P. Saha, H. Jiang, C. Li, Few-layer MoS2 nanosheets incorporated into hierarchical porous carbon for Lithium-ion batteries, Chemical Engineering Journal (2015), doi: http:// dx.doi.org/10.1016/j.cej.2015.11.105
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Few-layer MoS2 nanosheets incorporated into hierarchical porous carbon for Lithium-ion batteries Haiyan Wang a, Dayong Ren a, Zhengju Zhu a, Petr Saha b , Hao Jiang a,*, Chunzhong Li a,* a
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and
Engineering, East China University of Science and Technology, Shanghai 200237, China b
Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Trida T. Bati 5678, 760
01 Zlin, Czech Republic Email: [email protected] (Prof. H. Jiang) and [email protected] (Prof. C. Li) Abstract In order to overcome the serious stacking and poor conductivity of graphene-like MoS2 nanosheets, we have developed the synthesis of few-layer MoS2 nanosheets incorporated into biomass-derived hierarchical porous carbon frameworks (labelled as MoS2 /C hybrids) utilizing the strong water-absorbing power of auricularia from its inherent rich porous structure. The as-obtained MoS2/C hybrids, when applied as lithium-ion batteries anode materials, show an improved specific capacity of 707.4 mAh g-1 compared with the commercial MoS2 nanosheets (580.2 mAh g-1) and the corresponding hierarchical porous carbon (215.5 mAh g-1). More meaningfully, they possess an impressive cycle life, almost without capacity fading even after 500 cycles at 1600 mA g-1. The intriguing performance is mainly attributed to the well-dispersion of few-layer MoS2 nanosheets into hierarchical porous carbon. We believe this work will provide a new insight on the design and synthesis of novel carbon-based electrode materials for potential applications in lithium-ion batteries and other clean energy devices. Keywords: few-layer MoS2, biomass, hierarchical porous carbon, lithium ion batteries.
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1. Introduction Lithium-ion batteries (LIBs) with high energy density and design flexibility are considered to be a priority candidate of power sources for future electric vehicles [1-3]. Graphite has been generally used as the commercial anode materials, while its low lithium storage capacity (372 mAh g-1) hardly meets the increasing demand of high-energy density. So exploring novel LIBs anode materials with higher capacity, longer cycle life is in sore need. Molybdenum disulfide (MoS2), a typical two-dimensional material, attracts great interests in energy storage field in recent years, which exists strong covalent forces within S-Mo interactions and weak van der Waals bonds between S-Mo-S layers. According to the 4-electron transfer reaction (MoS2 + 4Li + + 4e- → Mo + 2Li2S), MoS2 can deliver a high theoretical capacity of 670 mAh g-1 [4-6]. However, its low electrical conductivity, easy stacking and restacking, as well as volume expansion during lithium insertion/extraction will result in poor cyclability and rate capability when used as LIBs anode materials [7-9]. To overcome these drawbacks, recent studies have demonstrated that constructing MoS2/carbon hybrid structures is a good choice by hybridizing MoS2 nanosheets with various carbons, such as carbon nanotubes, graphene and so forth, which availably enhances the electron conductivity and also promotes rapid electron transfer [10-16]. In terms of the reported structures, MoS2 nanosheets are mostly decorated out of the surface of carbon matrix with strong tendency to restack during the charge/discharge process, which suffers severe structural instability, resulting in poor rate capability and cycling stability. A possible way to solve this problem is to fabricate few-layer MoS2-C incorporated structure hybrids [17-21]. For example, Zhu et al. [21] prepared few-layer MoS2 nanoplates embedded in carbon nanowires by electrospinning and showed much enhanced rate performance and cycling stability. Despite the feasibility of the incorporated structure,
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state of the art methods always involve toxic and corrosive organic chemicals of precursors, high risks during operation and limited production rate in industrial manufacture, thus hindering the applicability [22,23]. Therefore, developing an environmental friendliness, facile and scalable route to achieve the few-layer MoS2-C incorporated structure is still a challenge. Auricularia, a kind of worldwide abundant fungus, naturally contains plenty of meso/macropores which is endowed with volumetric swelling characteristics after absorbing various solutions [24,25]. Inspired by this, herein, we developed a simple method to fabricate few-layer MoS2 nanosheets incorporated into hierarchical porous biomass carbon framework. Such a novel architecture embraces the following features. First, the incorporation system effectively prevents the MoS2 aggregation and buffers the volume expansion, which ensures the structure integrity to obtain long cycling life. Furthermore, the intimate contact between MoS2 nanosheets and carbon matrix benefits fast ion/electron transportation across the interface. Second, few-layer MoS2 nanosheets obtained from the confinement growth in hierarchical porous carbon offer more active sites, lowering the barrier for lithium intercalation and also amplifying the conversion reaction. Third, the three-dimensional (3D) carbon framework with porous structure not only enhances the conductivity but also provides large contact area between the electrode and electrolyte. Thereby, when evaluated as anode materials for LIBs, the MoS2/C hybrids exhibit high specific capacity, excellent cycling stability and impressive rate capability.
2. Experimental section
2.1 Synthesis of the hierarchical porous MoS2/C hybrids The hierarchical porous MoS2/C was synthesized as follows. Typically, 0.6g ammonium thiomolybdate dissolved in 20 ml deionized water was sonicated for 30 min to form a well dispersed 3
solution with the concentration of 0.03 g ml-1. Then, 0.5g auricularia was placed into the mixture for a night long standing until the volume of the auricularia expanded completely. After freeze-drying, the swelling auricularia was finally carbonized and reduced into the MoS2/C hybrids under Ar atmosphere with a heating rate of 3 °C min-1 to 850 °C for 2 h. For the experiments with different MoS2 contents, the ammonium thiomolybdate concentration was adjusted. Hierarchical porous carbon was also prepared by using the procedure for the preparation of MoS2/C without adding ammonium thiomolybdate. The exfoliated MoS2 was prepared according to our previous work. Briefly, 0.15g of sodium molybdate and 0.3g of thioacetamide were dissolved into 30 mL of deionized (DI) water with continuous stirring. Then, the suspension was transferred into 50 ml Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 hours. The precipitates were collected by filtration, washed with DI water and ethanol and dried at 60 °C.
2.2 Characterization The morphologies were characterized by field emission SEM (S-4800) and TEM (JEOL 2100). The structure was checked by XRD (Rigaku D/max 2550VB/PC) at a scan rate of 1° min-1 . Raman spectra (Renishaw inVia Reflex) was performed at ambient temperature with a NEXUS 670 FT-IR Raman spectroscopy. The porous properties were analyzed using nitrogen adsorption and desorption isotherms that were obtained using the surface area and a porosimetry analyzer (ASAP 2010) at 77 K. Thermogravimetric analysis (NETZSCH STA409PC) was carried out with a heating rate of 10°C min–1 under flowing air.
2.3 Electrochemical Measurements
Electrochemical measurements were determined using CR 2016 type coin cells assembled in an argon-filled glovebox. The working electrode was prepared with the active material (MoS2/C), a conductive agent (carbon black, Super-P-Li), and a polymer binder (poly(vinylidene difluoride), PVDF, Aldrich) in a 4
weight ratio of around 70 : 20 : 10, and then pasted on pure Cu foil. The coating thickness on Cu foil is about 50 µm. Li foil was used as the counter electrode and a polypropylene film (Celgard-2400) was used as a separator. The electrolyte was 1 M solution of LiPF6 in a 1:1 v/v mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The cells were charged and discharged galvanostatically at different rates in the voltage range of 0.01–3.0 V on a LAND-CT2001A battery tester at room temperature. Cyclic voltammogram experiment was performed on an Autolab PGSTAT302N electrochemical workstation at scan rates of 0.2 mV s-1 .
3. Results and discussion The MoS2/C hybrids have been synthesized by absorbing ammonium thiomolybdate aqueous solution into the porous structure of auricularia and the following carbonization. The morphology and microstructure of the as-synthesized MoS2/C were characterized by SEM and TEM technique. Fig. 1a shows that the MoS2/C hybrids are assembled into a highly interconnected 3D porous network with sizes of ~ 300 nm. The high-magnification TEM observation shows that few-layer MoS2 nanosheets are well-dispersed and incorporated into carbon walls, as shown in Fig. 1b. From the high-resolution TEM image (Fig. 1c), it can also find that some of few-layer MoS2 nanosheets lay on the surface on carbon frameworks with interplanar spacing of 0.27 nm, corresponding to the (101) planes of hexagonal MoS2 crystals. It is noted that the lateral dimension and the thickness of the dispersed MoS2 nanosheets are only ~ 3.0 nm and 0.4 nm, which will be favorable for offering rich electrochemical active sites and shortening the transfer distance of electrons/ions. The corresponding selected-area electron diffraction (SAED) pattern (the inset of Fig. 1c) gives the hexagonal structure of the MoS2/C hybrids and further verified the crystallinity of MoS2. In addition, the elemental distribution of C, Mo, S elements of the MoS2/C hybrids
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was also provided in Fig. S1. The uniform distribution suggests highly dispersity of MoS2 nanosheets in hierarchical porous carbon. The crystalline structure of the MoS2/C hybrids was also examined by X-ray diffraction (XRD), as shown in Fig. 2a. All the small diffraction peaks are attributed to the hexagonal MoS2 crystals (JCPDS 37-1492) with relatively poor crystallinity mainly due to their size and layers from the TEM observation as well as the comparable amount with carbon contents. Also, we can see a broad peak at 20-30°, which will be assigned to the calcined biomass-derived carbon. Notably, the very weak (002) peak suggests the few-layer MoS2 nanosheets [26]. The Raman spectroscopy is used to prove it, as shown in Fig. 2b. The energy difference between two Raman peaks (∆k) is ~23.4 cm-1, further indicates a few-layer nature [27,28]. Furthermore, the carbon quality has also been evaluated, as shown in Fig. 2b. Two pronounced Raman peaks at 1361 cm-1 (D-band) and 1601 cm-1 (G-band) can be observed with the calculated value of ID/IG to be 0.99, showing a relatively high graphitization degree. This feature is usually good at improving the power density for LIBs. On the other hand, just like some reported MoS2 /C hybrids, our MoS2 content can also be easily controlled to some extent by changing the absorbing concentration of ammonium thiomolybdate aqueous solution, which is determined by TG measurement, as shown in Fig. S2. A total weight loss between 300°C and 500 °C is assigned to the MoS2 oxidation and the carbon combustion. The final oxidation product of MoS2 is MoO3 [17,29]. As for the above mentioned sample, the MoS2 content is ~ 54%. However, a serious stacking of MoS2 nanosheets will happen if we further increase the precursor concentration. For example, a sharp and strong (002) peak appears in the XRD pattern for the hybrids with 79% MoS2 content (Fig. S3). Therefore, we choose the hybrids with 54% MoS2 content to evaluate their electrochemical performance, aiming to investigate the importance of few-layer MoS2 nanosheets on enhancing lithium ion storage. 6
Cyclic voltammogram (CV) and galvanostatic charge/discharge profiles were performed to evaluate the electrochemical properties of the MoS2/C hybrids. Fig. 3a illustrates the CV curves conducted at a scan rate of 0.2 mV s-1 within the potential range of 0.01-3.0 V. In the first cycle, the CV curve presents two prominent reduction peaks at 1.04 and 0.41 V and two oxidation peaks at 1.60 and 2.30 V, respectively. The reduction peak at 1.04 V corresponds to Li insertion into the interlayer spacing of MoS2, accompanied by phase transformation from 2H to 1T structures of LixMoS2. The other reduction peak at 0.41 V is attributed to the conversion reaction from Lix MoS2 to Mo metal and Li2S, and then the formation of solid-electrolyte interphase (SEI) resulting from electrochemically driven electrolyte degradation [30]. In the reverse anodic scan, a weak peak at 1.60 V can be attributed to the partial oxidation of Mo metal to form MoS2 . And the broad peak at 2.30 V is described as the delithiation of Li2S (Li2S → S + 2Li + + 2e-) [31]. During the subsequent cycles, two new reduction peaks at 1.90 V, 1.23 V emerge, which corresponds to the following reactions: S + 2Li+ + 2e- → Li2S and MoS2 + xLi+ + xe- → LixMoS2, respectively [30]. Notably, the CV curves are almost identical since the second cycle, indicating a good electrochemical stability. Fig. 3b shows the galvanostatic charge/discharge curves of the MoS2 /C hybrids electrode for the first three cycles at a current density of 200 mA g-1 between 0.01 and 3.0 V. In the first discharge process, two potential plateaus at about 1.1 V and 0.6 V are observed, which is in accordance with the CV curves. The initial discharge capacity of the MoS2/C hybrids reaches 1426.4 mAh g-1 and a reversible capacity of 967 mAh g-1 was obtained with a Coulombic efficiency of 68%. The unavoidable irreversibility is due to the formation of SEI film. In the second and third cycles, the Coulombic efficiency increases to more than 95%, performing a high reversible capacity and stable cycling performance. Fig. 3c depicts the rate performance of the MoS2/C hybrids, in comparison with commercial MoS2 and the corresponding hierarchical porous carbon. The MoS2/C hybrids deliver reversible capacities of 709.6, 577.2, 475.9, 353.5, 7
307.2 and 252.2 mAh g-1 at current densities from 200, 400, 800, 1600, 3200 to 6400 mA g-1, respectively, all of which are higher than those of commercial MoS2 (580.2, 478.2, 399.8, 295.5, 209.8, 131.4 mAh g-1) and hierarchical porous carbon (215.5, 131.5, 84.5, 49, 31.2, 27 mAh g-1). Especially, when the current density goes back to 200 mA g-1 after cycling under high current densities, a reversible capacity of 630 mAh g-1 was still regained. The MoS2 /C hybrids exhibit not only higher specific capacity but also enhanced rate-capability than both commercial MoS2 and hierarchical porous carbon. The electrochemical behavior of MoS2/C-45% and MoS2/C-79% were also characterized (as shown in Fig. S5). Even with a higher dispersion of MoS2 nanosheets, the electrochemical performance of MoS2 /C-45% is hindered by the low MoS2 contents. Despite a relatively high capacity of MoS2/C-79% at a low current density, the stacking of MoS2 nanosheets happens, resulting in an inferior rate performance. Therefore, the optimized contents were further verified to MoS2/C-54%. To further understand the reason why the MoS2/C hybrids possess good electrochemical performance for lithium storage, the electrochemical impedance spectroscopy (EIS) of MoS2/C hybrids and commercial MoS2 was measured, respectively, as shown in Fig. 3d. The MoS2/C hybrids exhibit a reduced diameter of the semi-circle at high frequencies compared with commercial MoS2, indicating the greatly decreased charge-transfer resistance (Rct) at the electrode/electrolyte interface. The Warburg impedance (W) comes from the interfacial diffusion of Li+ ions at electrode/electrolyte interface，which is described by a straight sloping line in low frequency region. Apparently, the inclined line of the MoS2/C hybrids appears a larger slope than commercial MoS2. Therefore, the combination of Rct and W demonstrates the fast reaction kinetics and ionic diffusion in the MoS2/C hybrids electrode. Also, the cycling performance of the MoS2/C hybrids at a high current density of 1600 mA g-1 is shown in Fig. 4, together with the commercial MoS2 material. The capacity of MoS2/C hybrids remains about 400 mAh g-1 until the 500th cycle with a 8
Coulombic efficiency above 99%. However, the commercial MoS2 shows a rapid capacity deterioration at the identical current density with only 51 mAh g-1 remained after 150 cycles. The electrochemical performance of the exfoliated MoS2 nanosheets is also tested, which can be seen from Fig. S6 and Fig. S7. This point can further prove the impressive cycling stability of our MoS2/C hybrids. The MoS2/C hybrids show a comprehensive excellent electrochemical performance, which can be comparable to those previously reported works. For example, David et al. synthesized layered SiCN-MoS2 structure and it exhibited good performance——regaining initial charge capacity of 530 mAh g-1 when the current density returned to 100 mA g-1 after continuous cycling at 2400 mA g-1 (192 mAh g-1). Shi et al. reported the synthesis of MoSx/MWNTs nanocomposites, showing remarkably high specific capacity of 1000 mAh g-1 at 50 mA g-1 (see Table S1in ESI). These intriguing results validate the enhanced lithium storage properties and improved rate capability of the MoS2 /C hybrids, which can be attributed to its unique structure and morphology. First, the biomass carbon trapping the few-layer MoS2 nanosheets into a confined space ensures the intimate contact between the conducting carbon skeleton and the MoS2 nanosheets, favoring the fast ion transport even at higher rates. Moreover, the incorporated nanostructure benefits the strong integrity of the electrode, which can effectively avoid the stacking of MoS2 nanosheets and also alleviate the volume expansion during the charge/discharge processes, contributing to a stable cycling capacity and long life-span. Second, the few layer MoS2 nanosheets with ultrathin nature can provide abundant electroactive sites for Li storage, which enables high specific capacities, excellent rate capability. Third, 3D continuous conductive biomass carbon with abundant porous structure provides fast electronic conduction channels for MoS2. Therefore, our materials exhibit improved specific capacity, enhanced rate performance and outstanding cyclic stability.
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4. Conclusions In summary, a novel hybrid structure of few-layer MoS2 nanosheets homo-dispersively incorporated into biomass-derived hierarchical porous carbon framework has been successfully synthesized by a facile and scalable method. The unique structure can effectively avoid the stacking of MoS2 nanosheets and overcome the poor conductivity of MoS2 nanosheets. As a consequence, the MoS2/C hybrids show much better electrochemical properties than the commercial MoS2 nanosheets and the corresponding hierarchical porous carbon electrodes for reversible lithium storage. Specifically, the MoS2 /C hybrids deliver a high specific capacity of 707.4 mAh g-1 at 200 mA g-1 with intriguing rate capability (400 mAh g-1 at 1600 mA g-1 ). More importantly, they possess an excellent cycle life, almost without capacity fading even after 500 cycles. Furthermore, this approach will provide a new insight on the design and synthesis of novel carbon-based electrode materials for potential applications in lithium-ion batteries and other areas, such as supercapacitor, hydrogenation and fuel cells.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21236003, 21522602), the Shanghai Rising-Star Program (15QA1401200), the International Science and Technology Cooperation Program of China (2015DFA51220), the 111 Project (B14018), and the Fundamental Research Funds for the Central Universities.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://www.elsevier.com/.
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Captions for figures Fig. 1 (a) SEM image, (b) high-magnification and (c) high-resolution TEM images of the as-synthesized MoS2/C hybrids (inset showing the corresponding SAED pattern). Fig. 2 XRD pattern and Raman spectra of the MoS2/C-54% hybrids. Fig. 3 (a) CV curves at 0.2 mV s-1 and (b) charge–discharge curves of the MoS2/C-54% hybrids for the initial 3 cycles at a current density of 200 mA g-1, (c) rate capabilities of the MoS2/C-54% hybrids, commercial MoS2 nanosheets and the corresponding hierarchical porous carbon, (d) electrochemical impedance spectra of the MoS2 /C-54% hybrids and commercial MoS2 nanosheets. Fig. 4 Cycling behavior and Columbic efficiency of the MoS2 /C-54% hybrids and commercial MoS2 at a current density of 1600 mA g-1.
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Graphical abstract
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Research highlights 1. Few-layer MoS2 nanosheets are incorporated into hierarchical carbon frameworks. 2. A MoS2/C hybrid is prepared using strong water-absorbing power of auricularia. 3. The MoS2/C hybrids LIBs anode exhibits improved electrochemical performances.
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