3-MA – Felypressin

Graphene-like MoS2 Nanosheets on Carbon Fabrics as High- Performance Binder-free Electrodes for Supercapacitors and Li-Ion Batteries

ABSTRACT: Two-dimensional layer-structure materials are now of great interest in energy storage devices, owing to their graphene-like structure and high theoretical capacity. Herein, graphene-like molybdenum disulﬁde (MoS2) nanosheets were uniformly grown on carbon fabrics by using a hydrothermal method. They were evaluated as binder-free electrodes for Li- ion batteries (LIBs) and supercapacitors. As expected, long cycling life and high capacity/capacitance are achieved. When used as self-standing electrodes for LIBs, they deliver a high area capacity of ∼0.5 mAh/cm2 even after 400 cycles and remarkable rate capability in the charge/discharge potential range of 1−3 V. In addition, a three-dimensional integrated electrode of the MoS2 nanosheet exhibits a high capacitance of 103.5 mF/cm2 and long cycling stability up to at least 15 000 cycles at a current density of 3 mA/cm2 for supercapacitors. The great cycling stability of MoS2 in supercapacitors is promising in the enhancement of cycling stability through their integration with other materials as alternatives to graphene in some special ﬁelds.

1.INTRODUCTION
Nowadays, energy storage devices have attracted worldwide attention due to their vital roles as dominant mobile power sources for a range of applications, such as mobile devices, new-energy vehicles, and smart devices.1−4 To develop high- performance electrode materials for lithium-ion batteries (LIBs) and supercapacitors (SCs), metal oxide nanostructures with high speciﬁc capacity/capacitance, typically 2−3 times higher than that of the carbonous materials, have attracted much attention.5−8 However, their poor cycling stability leads to worse electrochemical performance which cannot satisfy the practical applications. Therefore, developing electrode materi- als with long cycle lives is still an urgent mission.Since graphene was discovered in 2004, many other two- dimensional (2D) layer-structure materials have also regained research interest.9−15 So far, among those two-dimensional (2D) layer-structure materials, molybdenum disulﬁde (MoS2) has played a signiﬁcant role, which has been extensively studied for many applications such as photodetectors, water treat-destroyed, resulting in faster capacity decay. To resolve this problem, a feasible way is to composite with other materials which have excellent electronic conductivity, such as graphene, carbon nanotubes, etc.20−23 Another approach is to increase the interlayer distance of MoS2, which will also introduce voids and defects into MoS2 to increase its lithium ion storage capacity. The third way is to design binder-free electrodes.Compared to traditional binder-enriched materials, the self- standing electrode does not have any additional current collector or binder, which leads to high-speed electron transport and ion diﬀusion. Therefore, the binder-free electrode can eﬀectively enhance the electrochemical perform- ance of active materials. In recent years, speciﬁc mechanical and electronic properties of three-dimensional (3D) ﬂexible binder-free energy storage devices have attracted great attention for the next generation, such as LIBs and SCs.24−27 For instance, the synthesis of graded 3D ZnCo2O4 nanowirearray/carbonless cloth binder LIB integrated electrode is reported with high reversible capacity and good cyclingments, memorizers, and especially for SCs and LIBs.

However, when evaluating as an electrode material for Li-ion batteries, due to the large volume change during the charging/ discharging cycles, the conductivity is low and the structure isperformance.28 Cheng et al. also developed an LTO/graphene foam self-standing electrode with excellent electrochemical properties.29 SCs and LIBs have many common features in improving the electrochemical properties of active materials despite their diﬀerent charge storage mechanisms. The rationally designed binder-free electrodes are among them. Very recently, the electrodes based on NiCo2O4 nanowire arrays on carbon textiles synthesized by Zhang et al. exhibited high reversible capacity/capacitance and excellent cycling ability for LIBs and SCs.30 Oxygen-deﬁcient Fe2O3 nanorods on carbon fabrics were synthesized by Lu et al., and high capacitance and excellent cycling stability are achieved when used as anodes for SCs.31In previous reports, many kinds of composites based onMoS2 were synthesized to enhance the cycling stability of MoS2. Recently, to get a more excellent anode material forLIBs, hierarchical MoS2 nanosheets/activated carbon ﬁber cloth and three-dimensional hierarchical MoS2 nano-array/ carbon cloth were synthesized and evaluated. However, the galvanostatic charging and discharging tests were performed at a potential ranging from 5 mV to 3 V and 1 mV to 3 V, respectively, and the cycling performance is not so good. Moreover, when the charge/discharge voltage was below 1 V, the carbon cloth and active carbon ﬁber can contribute a great deal of the capacity to the whole electrode, which results in the calculated capacity of active materials higher than real.32−34 Atthe same time, MoS2-based SCs are mainly binder-enriched.According to previous reports, MoS2 can be widely used in almost all commonly used electrolytes, because of the relative stability of MoS2 in acid, neutral, and alkaline aqueous solutions. When used in alkaline aqueous solution, some researchers choose Ni foam as the current collector.

However,the Ni foam can easily form NiO on the surface and has a great inﬂuence on the measurement of capacitor properties, which may result in error and exaggerated capacitance, especially when a little amount of electrode-active materials are used for the measurements.35−37 To avoid the above problem, carbon fabrics used as the current collector of the SC electrode may be a good choice, which can reﬂect the SC properties of active materials more accurately.In this work, graphene-like MoS2 nanosheets on carbon fabrics are successfully synthesized in a typical and easy hydrothermal method and directly used as self-standing electrodes for LIBs and SCs. When evaluated as anodes of LIBs, this kind of integrated electrode showed high regional capacity, long cycling stability, and excellent rate performance in a galvanostatic charging and discharging potential window ranging from 1 to 3 V. When evaluated as electrodes for SCs, the as-synthesized integrated electrodes exhibited long cycling stability until 15 000 cycles with a capacitance fading of only 19.4% in a two-electrode coin cell conﬁguration, when the current density is 3 mA/cm2.

2.RESULTS AND DISCUSSION
The graphene-like MoS2 nanosheets grown in carbon fabrics were synthesized via an easy hydrothermal method. Figure 1a shows the X-ray diﬀraction (XRD) pattern of the MoS2 nanosheets. The pattern can be well indexed to hexagonal MoS2 (JCPDS card no. 73-1508). The peak located at 2θ = 26° is a typical diﬀraction caused by carbon fabrics. The other peaks located at 2θ = 14.39, 33.62, 39.65, 44.14, 49.87, and 58.56° can be assigned to the (002), (101), (103), (006), (105), and (110) planes of the hexagonal MoS2 phase, respectively. Their morphologies were characterized through the use of ﬁeld- emission scanning electron microscopy (FESEM) and FE-transmission electron microscopy (FE-TEM). Figure 1b shows the SEM image of the as-synthesized products, which wereevenly coated high-density samples. The higher magniﬁcation of the SEM image shown in Figure 1d reveals that the samples on the carbon fabrics are graphene-like ultrathin nanosheet structures. To clearly observe the thickness of the sample on the carbon fabrics, the sectional view of the as-synthesized product is shown in Figure 1c. We can clearly estimate that the thickness of MoS2 nanosheets coated on carbon fabrics was ca. 200−300 nm. Figure 1e shows the TEM image of the MoS2 nanosheets which were scratched from the carbon fabrics, and we can clearly see a two-dimensional layer-structure, corresponding to the SEM images very well (Figure 1d).The selected-area electron diﬀraction (SAED) and high- resolution TEM (HR-TEM) analyses eﬀectively indicate the polycrystalline texture of MoS2 nanosheets (Figure 1e,g). The lattice fringes are visible, and the d-spacing of 0.2635 nm was computed to correspond well to the (101) lattice plane of the hexagonal MoS2 nanosheet. The higher magniﬁcation image of MoS2 nanosheets is shown in Figure 1f.

We can observe a well- deﬁned layered structure of MoS2 nanosheets. The thickness of the MoS2 nanosheet is about 2.5 nm, which was characterized by an atomic force microscope (AFM) in Figure 1h.The detailed elemental composition and the valence states on the surface of MoS2 nanosheets are further characterized by X-ray photoelectron spectroscopy (XPS). The investigated spectrum of the MoS2/carbon fabrics (Figure 2a) shows the existence of Mo, S, C, and O elements. Figure 2b,c show the high-resolution XPS spectra of Mo 3d and S 2p obtained fromfabrics. (a) Survey spectra and high-resolution spectra of (b) Mo 3dand S 2s, and (c) S 2p.the MoS2/carbon fabrics. The high-resolution Mo 3d peak (Figure 2b) shows two sub-peaks and a small peak at the binding energies of 229.6, 232.8, and 226.8 eV that correspond to the Mo 3d5/2, Mo 3d3/2, and S 2s states, respectively, indicating that molybdenum is in its Mo (IV) state. In Figure 2c, the peaks at the binding energies of 163.6 and 162.5 eV correspond to the S 2p1/2 and S 2p3/2 states, respectively. All of the binding energy values are well consistent with those reported for MoS2.38Li-Ion Battery Performance. Coin-type cell conﬁg- uration was used to assess the energy storage properties of MoS2/carbon fabrics for LIBs. The mass loading of MoS2 nanosheets on carbon fabrics used for LIB tests is about 2.5−3 mg. MoS2 nanosheet/carbon fabric as a cathode was compared with Li metal under the constant current circulation conditions under the potential window of 0.01−3 and 1−3 V at the current rate of 0.2 mA/cm2 at room temperature. When the discharge potential was below 1 V, the carbon fabrics will react with Li metal, and this process is nearly irreversible. As a result, the Li metal will be wasted and lead to low Coulombic eﬃciency. Furthermore, the reduction peak of MoS2 does not exist below 1 V.

Therefore, we test the electrochemical performance of MoS2/carbon fabrics at a potential ranging from 1 to 3 V. Figure S1 shows the charge/discharge proﬁles ofMoS2/carbon fabrics and pure carbon fabrics for the 1st, 5th, and 60th, respectively. We can see that the pure carbon fabrics exhibit high area capacity (Figure S1b). The ﬁrst-discharge reaction of MoS2/carbon fabrics comprises mainly two regions. The ﬁrst potential plateau at ∼1.1 V associated with the reaction of MoS2 and Li+ as in eq 1. The second plateau at∼0.6 V is associated with the reduction of Mo4+ into the metallic Mo embedded into a cubic Li2S matrix through theconversion process (eq 2) and the formation of a gel-like solid electrolyte interphase layer at the interface of the electrolyte and the electrode. In the charge proﬁles, the two plateaus at∼1.7 and ∼2.3 V are associated with the oxidation of cubic Mo to hexagonal MoS2 and dissociation oxidation of Li2S to sulfur, respectively. In the second and subsequent discharge proﬁles, two potential plateaus at ∼1.8 and ∼1.15 V are associated with reduction of S to Li2S and the formation of lithium intercalate of LixMoS2, respectively.MoS2 + xLi+ + xe− → LixMoS2 (1)Considering that the main lithiation/delithiation reaction of MoS2 is above 1 V after the ﬁrst cathodic process, and the potential plateau is above 2 V, the capacity contribution below 1 V cannot be taken into consideration when assembled into full battery, then, we think that the galvanostatic charge/ discharge tests of MoS2/carbon fabrics performed at a potential ranging from 1 to 3 V are more suitable.Figure 3a shows the charge/discharge proﬁles of MoS2/ carbon fabrics for the 1st, 20th, 100th, 300th, and 400th at the potential between 1 and 3 V, respectively. Because the reduction process (eq 2) may not occur in the ﬁrst discharge process, the corresponding oxidation process may not exist either. The reaction mechanism can be changed when the potential window changed. According to Figure 3a, during the ﬁrst discharge, a large plateau at about 1.25 V can be attributed to the reaction of MoS2 and Li+ as in eq 1.

The reaction of eq 2 does not exist. In the ﬁrst charge process, a large inconspicuous plateau between 1.5 and 2.5 V may be associated with the extraction of Li+ from the LixMoS2 lattice. In the subsequent cycles, only a simple insertion/extraction process occurs. It should be noted that the Li+ intercalate potential of the ﬁrst discharge process is lower than the subsequent cycles, which can be attributed to the diﬀerent discharge depth resulting in the diﬀerent Li+ insertion potential. Figure 3b shows the cycling performance of MoS2/carbon fabrics and pure carbon fabrics at a current density of 0.2 mA/cm2 in the potential window between 1 and 3 V. Because the pure carbon fabrics contribute such little capacity to the whole electrode, the reaction of Li with carbon fabrics can be neglected. After 400 cycles, the MoS2/carbon fabrics still hold a high reversible area capacity of 0.4 mAh/cm2, about 80% capacity retention after the 1st cycle, compared to the cycling performance of MoS2 carbon fabrics tested at a potential window from 0.01 to 3 V (Figure S2), which shows a capacity retention of only 63% in the 60th cycle after the 1st cycle. The enhanced cycling performance of MoS2/carbon fabrics when tested at a potential ranging from 1 to 3 V can be attributed to two reasons. First, the depth of discharge can result in the easier destruction of the electrode. Second, the current collector taking part in the electrochemical reaction may result in the active materials peeling oﬀ from the current collectors and the capacity fading rapidly.

To provide more direct evidence, the morphologies ofMoS2/carbon fabrics tested at a potential ranging from 0.01 to 3 V (a, b, c) and 1 to 3 V (d, e, f) after the cycles were compared. Figure 3c shows the rate performance of MoS2/ carbon fabrics at various current densities from 0.2 to 4 mA/ cm2 in a potential window of 1−3 V. When the current density increases to 4 mA/cm2, the MoS2/carbon fabric electrode still exhibits a high area capacity of 0.29 mAh/cm2, and when returning back to 0.2 mA/cm2, the capacity of MoS2/carbon fabrics is recovered with a capacity retention of about 86.7%. The ﬁrst two charge/discharge curves of pure carbon fabricsabove 1 V are shown in Figure S3. As shown in Figure S3, pure carbon fabrics contribute a low area capacity of less than 0.015 mAh/cm2 between 1 and 3 V. Figure 4a−c show the SEM images of MoS2/carbon fabrics tested at a potential ranging from 0.01 to 3 V after 60 cycles, and it was observed that the structures of the carbon fabrics and graphene-like nanosheets were completely destroyed. The materials are partly peeled oﬀ from the carbon fabrics. Figure 4d−f show the SEM images of MoS2/carbon fabrics tested at a potential ranging from 1 to 3 V after 400 cycles. Compared to the electrode that is tested at 0.01−3 V, the morphologies of MoS2 were not destroyed completely, and thus we can see the nanosheet structure clearly.Supercapacitor Performance. The MoS2/carbon fabrics were also applied as self-standing electrodes of supercapacitors. The mass loading of MoS2 on carbon fabrics used for SC tests is about 1.8−2.2 mg/cm2. The capacitive properties of the MoS2/carbon fabrics were ﬁrst tested with cyclic voltammetry (CV) and galvanostatic charge−discharge measurements in three-electrode conﬁgurations. Figure 5adisplays the CV curves of the MoS2/carbon fabrics at scan speeds varying from 10 to 50 mV/s. The pseudocapacitancedisplayed in Figure 5b.

The representative capacitance of the electrode material is calculated as followsbehavior can be conﬁrmed obviously from the fact that the CV curve shape is diﬀerent from the electrical double-layerCarea= IΔt/(ΔVS)capacitance, which is a nearly rectangular CV curve. Notably, redox peaks between −0.8 and 0 V can be seen in all CV curves due to the Faraday eﬀect. As the scanning rate increases from 10 to 50 mV/s, the peak current rises up, but the CV curve shape shows no signiﬁcant change. This result suggests that rapid redox reactions towards electrochemical energy storage can be performed with MoS2/carbon fabrics.Constant current charge/discharge proﬁles were conducted at current densities ranging from 1 to 8 mA/cm2, which arewhere I represents the constant-discharge current, S means the geometrical area of the electrode, Δt is the discharged time after IR drop, and ΔV is the voltage drop upon discharge. The speciﬁc capacitance of the MoS2/carbon fabric electrode calculated from each discharge curve is about 191.9, 152.3,136.7, 127.4, 119.3, and 92.6 mF/cm2 at 1, 2, 3, 4, 5, and 8mA/cm2, respectively, as shown in Figure 5c. Figure 5d shows the cycling performance of MoS2/carbon fabrics at a current density of 2 mA/cm2. After 2000 charge/discharge cycles, speciﬁc capacitance attenuation is negligible, indicating thatthe MoS2/carbon fabric electrode has superior cycling stability performance. The symmetric two-electrode system adopted to evaluate the capacitive performance in this study due to the three-the electrode still retains a high speciﬁc capacitance of about83.7 mF/cm2 (about 80.6% capacitance retention).

3.CONCLUSIONS
In summary, graphene-like MoS2 successfully grown on carbon ﬁnanosheets have been supercapacitor use. Moreover, the cycling performance in a three-electrode system can be aﬀected by some other aspects, such as the active materials peeling oﬀ from the current collectors. Figure 6a presents the CV curves of the two- electrode device at scan rates from 10 to 300 mV/s with a potential window between −0.8 and 0.8 V. Not only because of the surface electrosorption of Li+ cations but also because of the fast, reversible successive surface redox reactions of MoS2, the measured CV curves show quasirectangular shapes. It can be seen from the absence of redox peaks that the super- capacitor is charged and discharged at a pseudoconstant rate throughout the voltammetric cycle. From the galvanostatic charge−discharge measurements at current densities between 0.5 and 8 mA/cm2, we obtained the accurate electrochemical values of the symmetric two-electrode system (Figure 6b). The capacitance of the electrode can be calculated according to the following equation Carea = 2IΔt/(ΔVS) where I is the constant discharge current, S is the geometrical area of the electrode, Δt is the discharged time after IR drop, and ΔV is the voltage drop upon discharge, respectively.39−41 The area capacitance values of the electrode were about 159.38, 134.5, 114.5, 103.5, 93.5, 85.63, and 71 mF/cm2 at 0.5, 1, 2, 3, 4, 5, and 8 mA/cm2 that are calculated from the charge/discharge curves, respectively, as shown in Figure 6c. Compared with the three-electrode conﬁgurations, the capacitance of the electrode is lower in a two-electrode system, which may be a normal phenomenon in the supercapacitor test. Figure 6d shows the cycling performance of the two-electrode system at 3 mA/cm2. After 15 000 cycles, ber fabrics with strong adhesion by a simple hydrothermal method. The MoS2/carbon fabrics are self-standing electrodes for energy storage devices. When used as an anode for Li-ion batteries, graphene-like MoS2/ carbon fabrics give a high reversible area capacity of 0.5 mAh/ cm2, long cycling stability (80% capacity retention after 400 cycles), and excellent rate performance. A high speciﬁc capacitance of about 95 mF/cm2 at 3 mA/cm2 and extraordinary cycling stability (near 81% capacitance retention after 15 000 cycles) can be achieved as an electrochemical capacitor electrode. Because of the excellent cycling stability of MoS2, we can composite with other electrode materials that have large speciﬁc capacitance but poor cycling stability to improve the electrochemical properties of electrode materials just like graphene and partly replace graphene in some special aspects.

4.METHODS
Graphene-like MoS2 nanosheets were obtained by a typical and simple hydrothermal method. Since all reagents used are analytical without further puriﬁcation. To directly grow MoS2 nanosheets on carbon fabrics, commercial carbon fabrics which are used as substrates were ﬁrst cleaned with acetone, ethanol, and deionized water, and then the carbon fabrics were cut into the needed sizes (2 cm × 4 cm) and immersed in 0.5 M NaOH solution for about 1 h. After the above pretreatment, a piece of the above carbon fabrics was placed in a Teﬂon-lined stainless-steel autoclave (50 mL) containing a mixture of 0.3 g Na2MoO4·2H2O, 0.4 g CH4N2S, and 35 mL deionized water. The sealed autoclave was heated in an oven at 240 °C for 24 h. Immediately after the completion of the reaction, the reacted carbon cloth was washed with deionized water and ethanol, and then dried in a vacuum oven at 80 °C for 12 h.Characterization. The X-ray diﬀraction (XRD) pattern was obtained by using an X-ray diﬀractometer (X’Pert PRO, PANalytical B.V., the Netherlands) with Cu Kα radiation (λ = 0.154 nm). X-ray photoelectron spectrometry was performed on a VG Multi-lab 2000 system with a monochromatic Al Kα X-ray source. The morphologies and structures of the as-synthesized products were charac- terized by a ﬁeld-emission scanning electron microscope (Nova NanoSEM 450, FEI, the Netherlands), a ﬁeld-emission transmission electron microscope (Tecnai G2 F30, FEI, the Netherlands), and an atomic force microscope (AFM, SPM9700, Shimadzu).Electrochemical Measurements. The carbon fabric- supported MoS2 nanosheets directly acted as the working electrode. Lithium metal was used as the counter electrode, and a polypropylene ﬁlm (Celgard-2300) was used as the separator. The electrolyte used was 1.0 M LiPF6 solution with a mixture of ethylene carbonate and dimethyl carbonate (EC/ DMC, 1:1 in volume). Galvanostatic charging and discharging tests were conducted using a battery testing system (Land, China) at a potential ranging from 1 to 3 V. The SC tests were performed on a CHI 660C electrochemical workstation in an aqueous LiOH electrolyte (1 M). The three-electrode cell tests were conducted with Pt foil as the counter electrode and a saturated calomel electrode as the reference electrode. The two-electrode SC tests were conducted in a two-electrode coin cell (CR2032) conﬁguration 3-MA assembled in air, where MoS2/ carbon fabrics serve as both the cathode and anode, glassy ﬁbers serve as the separator. Coin cells were assembled in an argon-ﬁlled glovebox for a lithium-ion battery.