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Uniform incorporation of flocculent molybdenum disulfide nanostructure into three-dimensional porous graphene as an anode for highperformance lithium ion batteries and hybrid supercapacitors Fan Zhang, Yongbing Tang, Hui Liu, Hongyi Ji, Chunlei Jiang, Jing Zhang, Xiaolong Zhang, and Chun-Sing Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11705 • Publication Date (Web): 25 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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ACS Applied Materials & Interfaces

Uniform incorporation of flocculent molybdenum disulfide nanostructure into three-dimensional porous graphene as an anode for high-performance lithium ion batteries and hybrid supercapacitors Fan Zhang †, Yongbing Tang* †, Hui Liu †, Hongyi Ji †, Chunlei Jiang †, Jing Zhang †, Xiaolong Zhang †, and Chun-Sing Lee*‡ †

Functional Thin Films Research Center, Shenzhen Institutes of Advanced Technology, Chinese

Academy of Sciences, Shenzhen, 518055, China. ‡

Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films

(COSDAF), City University of Hong Kong, Hong Kong SAR, China. * Address correspondence to [email protected], [email protected]

KEYWORDS: hybrid supercapacitor, molybdenum disulfide, graphene, 3D porous composite, anode

ABSTRACT Hybrid supercapacitors (HSCs) with lithium-ion battery-type anodes and electric double layer capacitor-type cathodes, are attracting extensive attention and under wide investigation

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because of their combined merits of both high power and energy densities. However, the performances of most HSCs are limited by low kinetics of the battery-type anode which cannot match the fast kinetic of the capacitor-type cathode. In this study, we have synthesized a three-dimensional (3D) porous composite with uniformly incorporated MoS2 flocculent nanostructure onto 3D graphene via a facile solution-processed method as an anode for high-performance HSCs. This composite shows significantly enhanced electrochemical performances due to the synergistic effects of the conductive graphene sheets and the interconnected porous structure, which exhibits a high rate capability of 688 mAh/g even at a high current density of 8 A/g and a stable cycling performance (997 mAh/g after 700 cycles at 2 A/g). Furthermore, by using this composite as the anode for HSCs, the HSC shows a high energy density of 156 Wh/kg at 197 W/kg, which also remains 97 Wh/kg even at a high power density of 8314 W/kg with a stable cycling life, among the best results of the reported HSCs thus far.

INTRODUCTION With the expansion of market in high capacity energy-storage systems such as hybrid electric vehicles (HEVs), there is urgent demand to develop advanced energy storage devices with both high energy and power densities with long cycling life.1,2 In view of this, extensive researches have been devoted in recent years to improving performance of energy storage devices, such as lithium-ion batteries (LIBs) and supercapacitors (SCs).3-6 However, although LIBs can deliver high energy densities (150200 Wh/kg), their power densities are limited (less than 1000 W/kg) and their cycling lives are poor (usually below 1000 cycles) due to faradic redox reactions of active materials leading to sluggish Liion diffusion and structural degradation.7-9 On the contrary, SCs can offer high power densities (up to 10000 W/kg) and long cycling lives (>100000 cycles), but only offer relatively low energy densities

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(5-10

Wh/kg)

because

the

charging/discharging

mechanism

involves

fast

physical

adsorption/desorption of electrolyte ions on the surfaces of active materials.10 Accordingly, it is highly desirable to develop novel energy storage devices combining advantages of LIBs and SCs. Hybrid supercapacitor (HSC), a kind of asymmetric supercapacitor11,12 with a supercapacitorbattery hybrid energy storage system, has been proposed to combine the advantages of LIB and SC.1315

Generally, a HSC consists of a lithium-ion battery-type anode and an electric double layer

capacitor-type cathode in organic electrolytes containing Li salts. With the combination of the two kinds of electrodes in one system, both power and energy densities can be further enhanced, which are attributed to the fast charging/discharging rate of cathode and the high specific capacity of the anode together with the much wider working voltage window of the organic electrolytes. Various works focused on different HSC systems have been reported with encouraging results.13-22 However, for most of these reported HSCs, high energy densities are only obtained at very low current densities/power densities, and the energy densities decrease dramatically with the increase of current densities/power densities. This problem is mainly due to that the low kinetics of the battery-type anode based on the faradic lithium inter/deintercalation reactions cannot match the fast kinetic of the capacitor-type cathode based on physical adsorption/desorption of electrolyte ions.23 An effective strategy to solve this problem is to design novel anode materials with simultaneously high rate capabilities, large specific capacities and ultra-long cycling lives. Metal oxides such as Nb2O5,22 V2O5,24 MoO3,25 and TiO2,26 have been found to provide intercalation or surface redox pseudocapacitances giving fast kinetics and high power densities. However, most of these oxides have poor electric conductivities and the redox potential is relatively high (>1.5 V vs. Li+/Li), which would narrow the working voltage window (below 3.2 V) and limit their energy storage densities. Molybdenum disulfide (MoS2) has been considered as a promising anode material for LIBs 3



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because of its high theoretical specific capacity (670 mAh/g), layered structure which enhances Li intercalation/deintercalation, and a wide lithiation potential (0.01-3 V). However, bulk MoS2 has low electric conductivities and often suffers from poor rate performance and cycling stability. Fortunately, its electrochemical performance can be greatly improved by designing nanostructures with suitable morphologies,27-31 or by incorporating other conductive constituents such as conductive polymers,32,33 carbon nanotube,34-36 carbon nanofiber,37,38 amorphous carbon,39,40 and graphene.41-44 Nevertheless, limited by the poor rate performance and cycling stability at high current densities, there are few reports exploring MoS2-based materials for HSC applications. Recently, application of threedimensional (3D) porous graphene structures has been demonstrated to be an effective way to enhance the rate performances and cycling stabilities of anode materials,45-49 because these structures provide not only 3D porous channels for fast ion transport but also high electric conductivity via the interconnected graphene network. Herein, we have synthesized a porous composite with uniformly incorporated MoS2 flocculent nanostructure onto three-dimensional (3D) graphene foam (denoted as MoS2@3DG) via a simple solution-based process as an anode material for high-performance HSCs. Owing to the synergistic effects of the flocculent nanostructure of MoS2 and the 3DG with a highly conductive network and a 3D hierarchical mesoporous structure, the MoS2@3DG composite demonstrates superior LIB anode performance with both high specific capacity and high rate capability (688.3 mAh/g at 8 A/g) as well as long cycling life (997 mAh/g after 700 cycles at 2 A/g). Furthermore, by using MoS2@3DG as the anode for HSCs, the HSC shows a high energy density of 156 Wh/kg at 197 W/kg, which also remains 97 Wh/kg even at a high power density of 8314 W/kg with a stable cycling life, among the best results of the reported HSCs thus far. EXPERIMENTAL SECTION 4


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Synthesis of 3D Graphene Foam. The 3D graphene foam (3DG) was prepared similarly to previously reported approach.50 Briefly, graphene oxide (GO) aqueous dispersion with large sheet pieces was synthesized by a modified Hummers method using natural flake graphite (99.95% purity, average flake diameter of 300 µm) as the raw material. After centrifuged for 5 times (10,000 rpm, 30 min) to replace the solvent with ethanol, GO alcoholic dispersion with a concentration of 1 mg/mL was prepared. Then 30 mL GO alcoholic dispersion was poured into a 50 mL Teflon-lined stainless steel autoclave and heated to 180 °C for 12 h. Graphene alcogel was formed, and the solvent was exchanged to distilled water, followed by freeze-drying process to obtain 3DG foam. Synthesis of MoS2@3DG composite. First, 3DG alcogel was prepared by the aforementioned method. After absorbing excess ethanol, 151.25mg of sodium molybdate (Na2MoO4·2H2O) and 95.15mg of thiourea (CS(NH2)2) were dispersed in 30 ml distilled water. A 3DG sample was then soaked in the solution and impregnated for 12 h. Then the mixture in the Teflon-lined stainless steel autoclave was heated up to 200 °C for 24 h. The as-prepared black hydrogel was washed with distilled water to remove the unreacted salt and other impurities, and finally freeze-dried to obtain the MoS2@3DG composite. Bare MoS2 sample was prepared through a similar method without using 3DG for comparison. Synthesis of nitrogen-doped porous carbon (NPC). NPC was prepared similar to the previous reported work.51 Briefly, aniline (2.1 g) and concentrated hydrochloric acid (15 mL) were dispersed in deionized water (100 mL) with magnetic stirring in an ice-bath. After stirring for 10 min, (NH4)2S2O8 (5.13 g) was dispersed in deionized water (100 mL) and then the solution was dipped into the above mixture within 20 min drop by drop. The polymerization was carried out for 6 h in an ice-bath with the maintained magnetic stirring. Then the PANI suspension was filtered and rinsed several times with deionized water and ethanol to remove retained aniline monomer and oxidant. The precipitate was 5



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dried at 60 °C for 24 h under vacuum. To obtain the NPC, the PANI powder was first carbonized at 700 °C for 2 h under Ar gas flow with a heating rate of 5 °C/min, and then activated at 800 °C for 1 h under Ar using 5 times of KOH powder as the activating agent. After cooling down to the room temperature, the NPC product was thoroughly washed with 0.1 M HCl and distilled water until a pH of 7 was obtained and then dried at 120 °C for 24 h under vacuum. Materials characterization. Morphologies of the products were characterized using transmission electron microscopy (TEM; TECNAI G2 F20 S-Twin) and scanning electron microscopy (SEM; HITACHI S-4800). Powder X-ray diffraction (XRD) analysis was carried out on a Rigaku D/Max2500 diffractometer with Cu Kα radiation. Thermogravimetric analysis (TGA) was conducted from room temperature to 750 °C at a heating rate of 10 °C/min in air or in N2. Fourier transform infrared spectroscopy (FTIR) was carried out on a Bruker-Tensor 27 IR spectrophotometer. Specific surface areas and pore size distributions of the products were measured using a Micromeritics ASAP 2020 apparatus at 77 K. Fabrication of half-cell and hybrid device. All the devices studied in this work were fabricated using the two-electrode method. For preparing MoS2@3DG composite anode, active material (MoS2@3DG), super P carbon black, and polyvinylidene fluoride (PVDF) with mass ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP) were mixed to forma uniform slurry and coated onto a Cu foil. The foil was vacuum dried at 80 °C for 24 h and pressed, then punched into 12 mm diameter electrodes with a mass loading of ~0.5 mg/cm2. For the fabrication of NPC cathode, active material (NPC), super P carbon black, and polytetrafluoroethylene (PTFE) with a mass ratio of 80:10:10 in ethanol were well mixed and rolled into thin sheets. After heated at 120 °C for 12 h in vacuum, the sheet was cut into 12 mm diameter electrodes and hot-pressed onto a carbon-coated Al foil. Both anode and cathode were dried at 120 °C for 12 h under vacuum and then transferred into an argon-filled glove box with 6


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concentrations of water and oxygen both below 0.1 ppm. For fabrication of half cells, both anode and cathode were tested using 2032 coin type cells, with lithium metal foil as the counter/reference electrode, and 1 M LiPF6 in ethylene carbonate/dimethyl carbonate/ethyl methyl carbonate (1:1:1 v/v/v) with 10 wt% fluoroethylene carbonate (FEC) as the electrolyte. Hybrid supercapacitors were assembled in coin cells with pre-activated MoS2@3DG anode (charged-discharged for 10 cycles at a current density of 0.2 A/g and ending in a lithiated state at 0.2 V) and the NPC cathode in the same Li salt containing electrolyte with a mass ratio of 3:1 (cathode/anode). Electrochemical characterization. All the electrochemical measurements were carried out at room temperature. The galvanostatic charge/discharge measurements were performed using a battery testing system (LAND CT2001A). Cyclic voltammetry (CV) measurements were performed on a CHI 600D electrochemical workstation. Electrochemical impedance spectra (EIS) were measured on an Autolab electrochemical workstation over the frequency range of 100 kHz to 10 mHz. Specific capacitance (C, F/g), energy densities (E, Wh/kg) and power densities (P, W/kg) of the hybrid supercapacitors (based on the total mass of both electrode materials) can be determined according to the following equations: C = I / [(dE / dt) · m]

(1) (2)

(3) where I (A/g) is the constant current density, V (V) is the voltage, and t1 and t2 (h) are the discharge start and end time, respectively. RESULTS AND DISCUSSION

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Characterization of the MoS2@3DG anode The strategy for fabricating the porous hierarchicalMoS2@3DG composite is briefly illustrated in Scheme 1. Firstly, a low-cost solution process wasutilized to prepare a 3DG alcogel.50 Afterwards, a 3DG hydrogel was obtained by impregnating the 3DG alcogel in an aqueous solution containing MoS2 precursor (sodium molybdate and thiourea) for 12 hours. In this process, MoO4- ions and CS(NH2)2 molecules would gradually diffuse into the porous 3DG and uniformly anchored onto the graphene sheetsvia their oxygen-containing functional groups (Figure S1), which is crucial for the homogeneous nucleation and growth of MoS2 nanostructure. Finally, MoS2@3DG with a 3D porous architecture was obtained by hydrothermal treatment and freeze-drying.

Scheme 1.Schematic of the fabrication of the MoS2@3DG hybrid structure.

Morphologies and microstructures of the as-prepared 3DG and the MoS2@3DG composite were investigated with scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as shown in Figure 1. In Figure 1a, the 3DG framework is composed of graphene sheets interconnected into a highly porous structure, which acts as the skeleton for the deposition of MoS2. Moreover, the highly conductive 3DG skeleton50 provides a good electric contact between the current collector and deposited MoS2. A typical TEM image of the 3DG (Figure S2a) also confirms its porous

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structure, and its Raman spectrum shows characteristic peaks of graphene (Figure S2b). Figure 1b and 1c show that after forming the final MoS2@3DG structure, the 3D porous architecture is basically maintained, and the MoS2@3DG bulk sample shows a size increase of about 70% and a darker color (compare insets of Figure 1a and 1b). Figure 1b and 1c also show that the 3D porous structure is now decorated with a high density of flocculent MoS2 nanostructures in diameter of mainly 300 nm which are themselves porous assembly of nanosheets, and naturally embedded into the 3DG structure. In contrast, the bare MoS2 prepared without using a 3DG framework shows bulk morphology with much larger diameter (above 1 µm) and thicker sheets (Figure S3). A TEM image (Figure 1d) of the MoS2@3DG shows the uniform dispersion of the flocculent MoS2 nanosheets in the 3DG. Figure 1e shows a TEM image of a higher magnification and its central region is further magnified in Figure 1f which shows lattice fringes matches well to the interplanar spacing of MoS2.49 Energy dispersive spectroscopy (EDS) element mapping (Figure 1g) was also carried out to see the elemental distributions. It can be seen that S and Mo are mainly located at the bright flocculent nanoctructure sites. The MoS2@3DG nanocomposite contains approximately 63.1 wt% MoS2 nanosheets, as determined by thermogravimetric analysis (TGA, Figure S4). Crystal structure of the MoS2@3DG composite was investigated with X-ray diffraction (XRD). As shown in Figure 1h, the MoS2@3DG composite shows similar crystalline structure to the aspreparedpure MoS2, but with a lower crystallinity. The diffraction peaks at 13.8, 33, 39, and 59.8° correspond to the (002), (100), (103), and (110) planes of MoS2, respectively.40 Besides, another peak at ~24.5° is also shown in the composite, which corresponds to the (002) reflection of graphitic carbon that comes from the 3DG. The porous structure of the MoS2@3DG composite was further characterized with N2 adsorption/desorption isotherm (Figure 1i) and shows a higher BrunauerEmmett-Teller (BET) specific surface area of 44.21 m2/g than that of MoS2 (13.51 m2/g). The

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MoS2@3DG composite shows a type IV adsorption and a type H3 hysteresis loop, indicating the existence of mesopores, which is mainly originated from the mesoporous channels in the composite. Pore size distribution (inset of Figure 1i) further reveals a hierarchically porous structure with a majority of mesopores as well as some micropores and macropores. These mesopores and macropores in the 3D structure could be useful for energy storage applications, which can not only act as "tanks" for electrolyte to reduce diffusion length and facilitate transportation of Li+ ions, but also buffer the large volume changes during the charge/discharge processes.

Figure 1. (a) SEM image of the 3DG sample (inset: photograph of the 3DG bulk sample, scale bar: 1 cm). (b) SEM image of the MoS2@3DG composite (inset: optical photograph of MoS2@3DG bulk 10


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sample, scale bar: 1 cm). (c) SEM image of MoS2@3DG composite at higher magnification. (d,e) TEM images of the MoS2@3DG composite at different magnifications. (f) HRTEM image of the MoS2@3DG composite showing lattice fringes of MoS2 nanosheets. (g) TEM image and EDS elemental mapping (S, Mo and C) of the MoS2@3DG sample. (h) XRD patterns of MoS2, 3DG and MoS2@3DG. (i) N2 adsorption/desorption isotherms and the corresponding pore size distribution (inset) of the MoS2@3DG composite.

Electrochemical performances of the MoS2@3DG composite as an anode material for LIBs were further investigated based on a half-cell configuration. Figure 2a displays cyclic voltammetry (CV) curves for the first three and the tenth cycles of the MoS2@3DG electrode over a potential window of 0.01-3 V vs. Li+/Li at a scan rate of 0.5 mV/s. It can be seen that its CV behavior is basically consistent with those of MoS2-based materials reported previously.39-52 In the first cathodic sweep, the MoS2@3DG shows two distinct peaks at around 1.03 and 0.38 V. The peak at 1.03 V corresponds to the intercalation of Li+ ions into the interlayer of MoS2 with the phase transition from trigonal prism to an octahedral structure, and the subsequent peak at 0.38 V corresponds to the further conversion reaction: MoS2 + 4Li+ + 4e−→ Mo + 2Li2S.36 In the 2nd, 3rd and 10th cycle, the cathodic peaks at 1.03 and 0.38 V disappear, and two new peaks at about 1.89 and 1.20 V emerge, suggesting the irreversibility of the above reaction, and the presence of a multi-step Li+ intercalation mechanism.34,48 Besides, part of the irreversible current from 0.75 to 0.2 V may be attributed to the reduction of electrolyte for solid-electrolyte interface (SEI) film formation. In the first anodic sweep, there are two oxidation peaks at 1.73 and 2.30 V, which can be attributed to the partial oxidation of Mo to MoS2, and the oxidation of Li2S into S, respectively.39

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Figure 2b shows the galvanostatic charge/discharge curves of MoS2@3DG for the first three cycles at a current density of 100 mA/g. The correlative potential plateaus are in accordance with the above CV results. The initial discharge and charge capacity of MoS2@3DG are 1405.9 and 1022.3 mAh/g respectively, with a Coulombic efficiency of 72.7%. The irreversible capacity loss in the first cycle is mainly caused by irreversible processes including the formation of the SEI film and the electrolyte decomposition.48 In the second and third cycles, charge capacities of 977.0 and 980.1 mAh/g were observed respectively with little degradation comparing to the initial capacity. The Coulombic efficiency reaches 96% after the third cycle, indicating high reversible capacities of the MoS2@3DG. Rate performance of the MoS2@3DG composite and the pure MoS2 sample are tested under different current densities from 0.1 to 8 A/g, as shown in Figure 2c. It can be seen that the MoS2@3DG exhibits a high rate capability, with corresponding discharge capacities of 1022.3, 936.7, 911.5, 856.7, 803.7, and 784.0 mAh/g at 0.1, 0.2, 0.5, 1, 2, and 4 A/g, respectively. The MoS2@3DG electrode also shows a high volumetric capacity of ~730 mAh/cm3 at 0.1 A/g (see calculation details in supporting information), which is also higher than graphite electrode (~620 mAh/cm3).53 Notably, when the current density increases to 8 A/g, which means charged/discharged in only 5 minutes, the MoS2@3DG still keeps a high rate capability of 688.3 mAh/g. In contrast, the bare MoS2 and the bare 3DG (Figure S5) can only deliver 25 and 370 mAh/g respectively at the same current density. When the current density is reduced back to 0.2 A/g after high rate cycling at 8 A/g, the MoS2@3DG resumes a high capacity of 942.2 mAh/g and keeps basically constant in the following ten cycles, demonstrating a superior rate capability and good capacity recovery. The good electrochemical performance of the MoS2@3DG composite electrode compared to that of the pure MoS2 electrode is also supported by the electrochemical impedance spectroscopy (EIS) analyses. Figure 2d shows Nyquist plots of the MoS2@3DG and the MoS2 electrodes tested after cycling for 50 cycles at 2 A/g at

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potential of ~2.48 V. In accordance with other MoS2-based anodes,39,49 these impedance spectra are both composed of a medium-to-high frequency depressed semicircle and a low-frequency sloping straight line. The semicircle corresponds to the interfacial charge transfer reaction (Rct), while the sloping line corresponds to the Warburg impedance (Zw) which is related to the solid-state diffusion of Li+ into the active materials.47 The Nyquist plots of both MoS2@3DG and MoS2 were fitted by the appropriate electric equivalent circuit (Figure S6). According to the fitting results, the Rct value (28.2 Ω) of the MoS2@3DG electrode is much lower than that of the MoS2 electrode (90.3 Ω), indicating that this 3D architecture with graphene as the skeleton can greatly enhance the electric conductivity and electrochemical reaction kinetics of the MoS2@3DG electrode. To further investigate the cycling performance of the MoS2@3DG electrode, galvanostatic charge/discharge measurement was conducted at a high current density of 2 A/g for 700 cycles, and the result is shown in Figure 2e. The charge capacity first stabilizes at around 840 mAh/g during the first 150 cycles, then gradually increases until the 400th cycle, and finally stabilizes to be 997 mAh/g after 700 cycles. Besides, the Coulombic efficiency of the MoS2@3DG electrode is nearly 100% during the whole cycling measurement except for the first few cycles. As a comparison, the MoS2 electrode shows a poor cycle performance (Figure S7a), where the specific capacity quickly fades from 724 to 33 mAh/g after only 50 cycles. As for the 3DG electrode, it displays a good cycle performance, which is due to the highly conductive network and mesoporous 3D structure, but with a relatively low capacity of 470 mAh/g after 300 cycles (Figure S7b). The phenomenon of the specific capacity increase with cycling for the MoS2@3DG electrode could be attributed to the growth of a gel-like polymeric layer and possible electrochemical activation of the composite, which is also observed in some metal oxide/sulfide based hybrid electrodes with nanostructures.54,55 Notably, the initial charge capacity is as high as 833 mAh/g at 2 A/g even without any prior charge/discharge

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activation process at low current densities. Hence, our MoS2@3DG electrode not only possesses superior cycling performance, but also much higher capability at fast charge/discharge rates, revealing synergistic effects of 3DG and flocculent MoS2 for enhancing the electrochemical performance. In addition, the capacity of the MoS2@3DG at 2 A/g after 700 cycles is more than two times of the graphite-based anodes (i.e. 372 mAh/g), and is among the best values of the previously reported MoS2-based electrode materials (Table S1).

Figure 2. (a) Cyclic voltammetry curves of the MoS2@3DG composite at a scan rate of 0.5 mV/s. (b) Galvanostatic charge/discharge curves of MoS2@3DG at a current density of 100 mA/g. (c) Rate

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capability of MoS2@3DG and MoS2 at different current densities. (d) Nyquist plots of MoS2@3DG and MoS2 after 50 cycles at 2 A/g. (e) Cycling performance of MoS2@3DG at a current density of 2 A/g. Characterization of the NPC cathode Nitrogen-doped porous carbon (NPC) is chosen as the cathode material for HSCs, which was synthesized by using polyaniline as a nitrogen-containing carbon precursor followed by carbonization and chemical activation, similar to the previous work.51 The as-prepared NPC shows a random branch-like rod morphology with a porous structure (Figure S8a). Moreover, the NPC has a specific surface area of 2398 m2/g and well-controlled pore size distribution of 0.5-3 nm (Figure S8b, S8c). The electrochemical performance of the NPC was evaluated in a Li half-cell system over a potential range of 2.0 to 4.5 V (vs Li+/Li) as shown in Figure S8d-S8f. As expected, the NPC has a much better electrochemical performance than the commercial activated carbon (hereafter referred as AC). CV curves of NPC electrode (Figure S8d) are relatively rectangular in shape and the charge/discharge curves are nearly straight lines, close to the ideal capacitive behavior. Linear charge/discharge profiles were also observed in Figure S8e at all current densities, indicating capacitive behavior with the adsorption/desorption of ions on the surface. The NPC electrode exhibits a relatively high capacity of ~ 97.9 mAh/g at 1 A/g, good rate capability of ~ 95.6 mAh/g at 10 A/g and good cycle stability with capacity retention of 90% after 2,000 cycles at 1 A/g and high Coulombic efficiency (Figure S8f). In contrast, the AC material only shows capacities of 60.4 mAh/g at 1 A/g (Figure S9). Hybrid supercapacitor performance based on MoS2@3DG anode and NPC cathode With the above outstanding electrochemical performances, the as-prepared MoS2@3DG composite and the NPC should be promising anode and cathode materials respectively for high-

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performance HSCs. Before fabricating the HSC, MoS2@3DG electrode was pre-activated for 10 cycles at 200 mA/g in a Li half-cell to achieve high efficiency and then lithiated to 0.2 V. The preactivated MoS2@3DG anode was coupled with a freshly prepared NPC cathode to fabricate the HSC named as MoS2@3DG//NPC (Figure 3a). The mass ratio of NPC to MoS2@3DG was optimized to be 3:1 (Figure 3b) due to the charge balance between the cathode and anode, and the voltage range of 0.0-4.0 V was chosen for high energy density and long cycling life.16,19 During the charge process, PF6- ions from electrolyte are absorbed in the porous structure of the NPC cathode, while Li-ions are intercalated into the MoS2@3DG anode. Discharge process reverses the charge process. The electrochemical performance of the MoS2@3DG//NPC HSC is shown in Figure 3c-3f. In Figure 3c, CV curves of the HSC show slight deviation from the ideal rectangular shape different from the symmetric supercapacitor with rectangular CV shape, which is attributed to the synergistic effect of two different energy-storage mechanisms. Moreover, the shape of CV curves is basically retained as the scan rate increases. Figure 3d shows the charge/discharge curves at different current densities (based on the total mass of both electrodes), which show little deviation from the linear slope of an ideal supercapacitor. Besides, the charge-discharge curves in the initial three cycles overlap at varied current densities from 0.1 to 1 A/g, indicating good reversibility (Figure S11). The specific capacitance values of the MoS2@3DG//NPC HSC (based on total mass of both electrode materials) are 88.3, 75.6, 68.0, 63.8, 60.0, and 55.7 F/g at the current densities of 0.1, 0.2, 0.5, 1, 2, and 4 A/g, respectively. The charge/discharge curve of the MoS2@3DG//NPC HSC was also compared with that of the HSC using MoS2@3DG and AC (MoS2@3DG//AC) at 0.2 A/g as shown in Figure 3e. The charge/discharge time of the MoS2@3DG//NPC HSC was much longer than that of the MoS2@3DG//AC HSC, indicating the electrochemical performance of the HSC with the NPC cathode is superior compared to that with the AC cathode. Furthermore, the MoS2@3DG//NPC HSC

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shows stable cycling life over 2000 cycles (from 97.2 Wh/kg to 75.4 Wh/kg, retention of 78% at 1.0 A/g) (Figure 3f), and the Coulombic efficiency of the HSC is above 90% during the whole cycling measurement. In contrast, the MoS2//NPC HSC with pure MoS2 anode shows severe energy density decay for only 100 cycles, demonstrating the important function of the 3D conductive and porous structure of MoS2@3DG anode. Besides, the FEC additive in the electrolyte also plays an important role for improving the cycling stability of the MoS2@3DG//NPC HSC at such a wide voltage range (Figure S12), which is beneficial for forming stable SEI films56.

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Figure 3. (a) Schematic illustration of the working mechanism of the MoS2@3DG//NPC HSC. (b) Ragone plot of the HSCs using MoS2@3DG as anode and NPC as cathode with various mass ratios from 2:1 to 6:1 (cathode:anode) over a voltage window of 0.0-4.0 V. (c) CV curves of the MoS2@3DG//NPC HSC over a voltage window of 0.0-4.0 V at different scan rates. (d) Galvanostatic charge/discharge curves of the MoS2@3DG//NPC HSC at different current densities from 0.5 to 4.0 A/g. (e) Galvanostatic charge/discharge curves of the MoS2@3DG//NPC HSC and the

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MoS2@3DG//AC HSC as contrast at a current density of 0.2 A/g. (f) Cycling performance of the MoS2@3DG//NPC HSC and MoS2//NPC HSC at a current density of 1.0 A/g for 2000 cycles.

Figure 4 shows a Ragone plot (energy density vs. power density based on the total mass of both electrode materials) of the MoS2@3DG//NPC HSC. At a power density of 197 W/kg, the MoS2@3DG//NPC HSC can achieve a high energy density of 156 Wh/kg, which is higher than the MoS2@3DG//AC HSC (102 Wh/kg). Even at an ultra-high power density of 8314 W/kg (discharged in 36 s), the MoS2@3DG//NPC HSC can still remain at 97 Wh/kg. Moreover, the Ragone plot also demonstrates superior electrochemical performance of the MoS2@3DG//NPC HSC compared to similar HSC systems using m-Nb2O5-C//MSP-20,22 H2Ti6O13//CMK-3,20 graphite//AC,14 and GLTO//G-SU.18 Besides, the energy density of our HSCs based on the total mass of packaged device is estimated to be ~ 30 Wh/kgcell at the power density of ~ 2500 W/kgcell (as the electrode materials account for ~ 30% of the total mass of the packaged device57), which is more than 5 times of commercial SCs and comparable with Ni-metal hydride batteries with much higher power density, demonstrating the synergistic effect of SCs and LIBs. The reasons for the outstanding electrochemical performance of the MoS2@3DG//NPC HSC are as follows. Firstly, the synergistic effect of flocculent MoS2 and 3DG leads to superior performance of MoS2@3DG electrode in terms of high reversible capacity, high-rate capability and long-life cycling stability. The 3DG conductive skeleton enables fast electron transport, while the mesoporous flocculent MoS2 nanostructure with uniform dispersion on the 3D porous graphene sheets enables rapid Li+ transport, both contributing to the outstanding highrate capability. Besides, the 3D hybrid porous structure provides a large contact area between the material and the electrolyte, which enables more active sites for Li+ insertion/extraction resulting in higher reversible capacity. Lastly, the highly porous structure of the NPC material with high specific

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surface area and narrow pore size distribution is favorable for large amount and fast adsorption/desorption of ions, providing both improved energy density and rate capability. Therefore, the MoS2@3DG//NPC HSC has good potential as a promising next-generation energy-storage device.

Figure 4. Ragone plots of the MoS2@3DG//NPC and the MoS2@3DG//AC HSCs. The energy and power densities are calculated based on the total mass of both electrode materials, and compared with other reported HSCs: m-Nb2O5-C//MSP-20,22 H2Ti6O13//CMK-3,20 graphite//AC,14 and G-LTO//GSU.18

CONCLUSIONS In summary, we have synthesized a porous composite MoS2@3DG with uniformly incorporated MoS2 flocculent nanostructure onto three-dimensional graphene as an anode for high-performance hybrid supercapacitors. Owing to the synergistic effects of the conductive graphene sheets and the

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flocculent MoS2 nanostructure together with the interconnected porous structure, this composite shows significantly enhanced electrochemical performances in terms of high rate capability of 688 mAh/g at a high current density of 8 A/g and a stable cycling performance (997 mAh/g after 700 cycles at 2 A/g). Furthermore, by using MoS2@3DG as the anode, the as-fabricated hybrid supercapacitor MoS2@3DG//NPC exhibits a high energy density of 156 Wh/kg at 197 W/kg, and remains a value of 97 Wh/kg even at a high power density (8314 W/kg) with a good cycling stability over a wide voltage window of 0.0-4.0 V. These results demonstrate that incorporating a high-performance MoS2@3DG composite anode is an effective strategy to enhance the energy and power densities of hybrid supercapacitors. ASSOCIATED CONTENT Supporting Information. Figure S1–S12: Structure characterization of pure 3DG and pure MoS2 materials; electrochemical data of 3DG and MoS2 electrodes; structure characterization and supercapacitor performance of NPC material; Ragone plot of hybrid supercapacitors with different ratios of cathode and anode; galvanostatic charge/discharge curves of the MoS2@3DG//NPC HSC at different current densities; cycling performance of the MoS2@3DG//NPC HSCs with FEC-added electrolyte and no FEC-added electrolyte. Table S1: Summary of cycling performance data for MoS2-based anodes. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Fax: +86 755 86392596. Tel: +86 755 86392592.

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*E-mail: [email protected] Fax: +852 34427826. Tel: +852 34427826. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial supports from Guangdong Innovative and Entrepreneurial Research Team Program (No. 2013C090), Natural Science Foundation of Guangdong Province (No. 2014A030310226), Science and Technology Planning Project of Guangdong Province (No.

2015A010106008),

Shenzhen

Science

and

Technology

Planning

Project

(No.

JCYJ20150401145529042), China Postdoctoral Science Foundation (No. 2015M570737), SIAT Innovation Program for Excellent Young Researchers (No. 201407), and Scientific Equipment Project of Chinese Academy of Sciences (yz201440). REFERENCES [1] Armand M.; Tarascon J. M. Building Better Batteries. Nature 2008, 451, 652-657. [2] Whittingham, M. S. Lithium Batteries and Cathode Materials. Chem. Rev. 2004, 104, 4271-4302. [3] Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845-854. [4] Goodenough, J. B.; Kim, Y. S. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587-603. [5] Zhang, L. L.; Zhao, X. S. Carbon-Based Materials as Supercapacitor Electrodes. Chem. Soc. Rev. 2009, 38, 2520-2531. [6] Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Schalkwijk, W. V. Nanostructured Materials

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Uniform Incorporation of Flocculent Molybdenum ... - ACS Publications

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