Coaxial MoS2@Carbon Hybrid Fibers: A Low-Cost Anode Material for High-Performance Li-Ion Batteries

A low-cost bio-mass-derived carbon substrate has been employed to synthesize MoS2@carbon composites through a hydrothermal method. Carbon fibers derived from natural cotton provide a three-dimensional and open framework for the uniform growth of MoS2 nanosheets, thus hierarchically constructing coaxial architecture. The unique structure could synergistically benefit fast Li-ion and electron transport from the conductive carbon scaffold and porous MoS2 nanostructures. As a result, the MoS2@carbon composites—when serving as anodes for Li-ion batteries—exhibit a high reversible specific capacity of 820 mAh·g−1, high-rate capability (457 mAh·g−1 at 2 A·g−1), and excellent cycling stability. The use of bio-mass-derived carbon makes the MoS2@carbon composites low-cost and promising anode materials for high-performance Li-ion batteries.


Introduction
The worsening environmental problems and energy crisis have accelerated the development of electric vehicles and portable electronics, which forward an ever-growing demand for lithium-ion batteries (LIBs) with high energy density and excellent rate capability [1,2]. Among the LIB components, the commercial anode material of graphite suffers from low theoretical specific capacity (i.e., 372 mAh·g −1 ), slow reaction kinetics, and possible safety issues resulting from its low discharge voltage (<0.2 V) that may cause the formation of lithium dendrites [3][4][5]. Therefore, it is imperative to explore a high-performance anode material that can serve as an alternative replacement of the graphite-based anode [6][7][8].
In recent years, two-dimensional (2D) graphene-like nanomaterials such as transition metal dichalcogenides (TMDs) have shown great potential for their applications in energy conversion and storage fields [9][10][11][12]. As a typical TMD, MoS 2 possesses a layered structure analogous to graphite, and is composed of S-Mo-S layers separated by Van der Waals interactions [13][14][15]. The unique structure is favorable for reversible Li + insertion/extraction, thus rendering high theoretical specific capacity (670 mAh·g −1 ), which is almost double that of graphite's [16,17]. However, the practical application of MoS 2 is still hindered by its poor cycling stability and inferior rate performance due to its intrinsically low electronic conductivity and the possible structural destruction during repeated charge-discharge processes [18].
To overcome these drawbacks, combining nanostructured MoS 2 with conductive and flexible materials has been demonstrated to be an effective approach to improve the electrochemical performance. Carbon materials with large specific surface area and excellent electronic conductivity-such as graphene, carbon nanotubes, carbon nanofibers, carbon spheres, etc.-are considered to be ideal substrates in this respect [15,[19][20][21]. MoS 2 /graphene hybrid nanoflowers were developed via a simple hydrothermal method, and delivered a specific capacity of 1150 mAh·g −1 after 60 cycles [22]. Single-layered ultrasmall nanoplates of MoS 2 embedded in carbon nanofibers was used as the anode of LIBs, and showed a good cycling capability of 661 mAh·g −1 after 1000 cycles [23]. Compared with pure MoS 2 counterparts, the improved lithium storage performance can be ascribed to the synergistic effect of carbon and MoS 2 nanostructures. It should be noted that the fabrication of these carbon substrates is extremely time-consuming and costly, which would limit their widespread implementation. Moreover, non-uniform MoS 2 aggregations on the carbon backbone is another obstacle to achieving a maximum utilization of active materials [24,25]. Therefore, it remains a big challenge to explore a cost-effective MoS 2 -based anode material with excellent electrochemical performance.
In this work, we report the use of a bio-mass-derived carbon substrate from low-cost natural cotton for subsequent growth of MoS 2 nanosheets. The carbon fiber substrate provides not only a three-dimensional (3D) and open framework for homogeneous deposition of nanostructured MoS 2 , but also high electrical conductivity to improve the electrochemical reaction kinetics of MoS 2 . The as-prepared MoS 2 @carbon anode material delivers high specific capacity, excellent rate capability, and long cycling stability. Moreover, the bio-mass-derived-carbon simplifies the fabrication process, making the MoS 2 @carbon composite serve as a cost-effective anode material for high-performance LIBs.

Results and Discussion
The crystalline structure of the as-prepared materials was investigated by X-ray diffraction (XRD). As shown in Figure 1a, both MoS 2 @carbon and pure MoS 2 exhibit similar XRD profiles. The distinct diffraction peaks with 2θ at around 14 • , 33 • , 39 • , 49 • , 59 • , and 60 • correspond to the (002), (100), (103), (105), (008), and (107) crystal planes of MoS 2 with 2H phase, respectively [26][27][28]. Raman spectra were recorded to further elucidate the component and structure of the MoS 2 @carbon composite ( Figure 1b). It is clearly revealed that the two distinct peaks at around 374 and 406 cm −1 can be ascribed to the in-plane E 1 2g and out-of-plane A 1g modes of the hexagonal MoS 2 crystal, respectively [29,30]. In addition, the two broad Raman peaks located at 1350 and 1583 cm −1 are readily assigned to the D and G bands of carbon, respectively, indicating the presence of the carbon component in the composite [22,31]. The high I G /I D intensity ratio (1.05) reveals good graphitization degree of the carbon substrate, which is favorable for enhancing the electrical conductivity of the MoS 2 /C composites [2,32].
The composition and chemical states of the as-prepared MoS 2 @carbon were characterized by X-ray photoelectron spectroscopy (XPS). The survey spectrum in Figure 2a indicates the existence of main elements of C, Mo, and S as well as a trace of O element absorbed on the surface [33]. The core level spectrum of Mo 3d ( Figure 2b) and S 2p (Figure 2c) show characteristic peaks with binding energies located at 232.6, 229.5, 163.5, and 162.2 eV, which belong to the Mo 3d 3/2 , Mo 3d 5/2 , S 2p 1/2 , and S 2p 3/2 components of MoS 2 , respectively [34,35]. The C 1s XPS peak (Figure 2d) can be fitted C-C and C-O-C components. All these results reveal the successful preparation of MoS 2 on the carbon fiber [23].   Intensity (a.u.) Binding Energy (eV)  Figure 3 shows the field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the pure carbon and MoS2@carbon composites. Figure 3a exhibits the typical morphology of pure cotton-derived carbon substrate, which is composed of one-dimensional (1D) fibers with smooth surface (inset) and an average diameter of 7 μm. In addition, the carbon fibers form a randomly-entangled 3D network, which is favorable for solution flux and thus uniform growth of MoS2. Figure 3b shows a panoramic view of the MoS2@carbon composites, which inherit   Intensity (a.u.) Binding Energy (eV)  Figure 3 shows the field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the pure carbon and MoS2@carbon composites. Figure 3a exhibits the typical morphology of pure cotton-derived carbon substrate, which is composed of one-dimensional (1D) fibers with smooth surface (inset) and an average diameter of 7 μm. In addition, the carbon fibers form a randomly-entangled 3D network, which is favorable for solution flux and thus uniform growth of MoS2. Figure 3b shows a panoramic view of the MoS2@carbon composites, which inherit  Figure 3 shows the field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the pure carbon and MoS 2 @carbon composites. Figure 3a exhibits the typical morphology of pure cotton-derived carbon substrate, which is composed of one-dimensional (1D) fibers with smooth surface (inset) and an average diameter of 7 µm. In addition, the carbon fibers form a randomly-entangled 3D network, which is favorable for solution flux and thus uniform growth of MoS 2 . Figure 3b shows a panoramic view of the MoS 2 @carbon composites, which inherit the 1D fibrous morphology of carbon substrates. Figure 3c shows the uniform and conformal deposition of MoS 2 nanostructures on the carbon surface, thus constructing coaxial configuration. The higher-resolution SEM image (inset in Figure 3c) reveals porous MoS 2 nanosheets aligned vertically on the fiber surface. The 1D/2D hierarchical architecture of the MoS 2 @carbon composites is beneficial for electrolyte penetration, and enlarges electrode/electrolyte interface area. The TEM image in Figure 3e exhibits ultrathin nanosheet-like MoS 2 structures [36]. The high-resolution TEM (HRTEM) image in Figure 3f shows a typical edge view of the MoS 2 nanosheets with clear lattice fringes. The thickness of the MoS 2 nanosheets is estimated to be about 10 nm, and the interlayer spacing of 0.65 nm corresponds to the (002) plane of the MoS 2 [37]. For comparison, pure MoS 2 nanoflowers with an average diameter of 500 nm were prepared as a control sample (Figure 3d). the 1D fibrous morphology of carbon substrates. Figure 3c shows the uniform and conformal deposition of MoS2 nanostructures on the carbon surface, thus constructing coaxial configuration. The higher-resolution SEM image (inset in Figure 3c) reveals porous MoS2 nanosheets aligned vertically on the fiber surface. The 1D/2D hierarchical architecture of the MoS2@carbon composites is beneficial for electrolyte penetration, and enlarges electrode/electrolyte interface area. The TEM image in Figure 3e exhibits ultrathin nanosheet-like MoS2 structures [36]. The high-resolution TEM (HRTEM) image in Figure 3f shows a typical edge view of the MoS2 nanosheets with clear lattice fringes. The thickness of the MoS2 nanosheets is estimated to be about 10 nm, and the interlayer spacing of 0.65 nm corresponds to the (002) plane of the MoS2 [37]. For comparison, pure MoS2 nanoflowers with an average diameter of 500 nm were prepared as a control sample (Figure 3d).   To evaluate electrochemical properties, the MoS2@carbon composite was investigated as an anode material for lithium-ion batteries. As shown in Figure 4a,b, the MoS2@carbon composite electrode shares identical CV curve shapes to pure MoS2 electrodes for the initial five cycles at a scanning rate of 0.1 mV·s −1 in the potential range of 0.01-3.0 V (versus Li/Li + ). Specifically, in the first cycle, there are two cathodic peaks appearing at about 1.5 V and 0.35 V. The broad peak at about 1.5 V corresponds to Li + intercalation into the layered structure of the MoS2 to form LixMoS2. The 0.35 V peak is attributed to the further conversion reaction from LixMoS2 into metallic Mo and Li2S [18,31,38]. In the reverse anodic scan, the oxidation peaks at 1.8 V and 2.3 V are associated with the stepwise oxidation of Mo into Mo 4+ /Mo 6+ and the oxidation of Li2S into sulfur or polysulfides, respectively [39]. To evaluate electrochemical properties, the MoS 2 @carbon composite was investigated as an anode material for lithium-ion batteries. As shown in Figure 4a,b, the MoS 2 @carbon composite electrode shares identical CV curve shapes to pure MoS 2 electrodes for the initial five cycles at a scanning rate of 0.1 mV·s −1 in the potential range of 0.01-3.0 V (versus Li/Li + ). Specifically, in the first cycle, there are two cathodic peaks appearing at about 1.5 V and 0.35 V. The broad peak at about 1.5 V corresponds to Li + intercalation into the layered structure of the MoS 2 to form Li x MoS 2 . The 0.35 V peak is attributed to the further conversion reaction from Li x MoS 2 into metallic Mo and Li 2 S [18,31,38]. In the reverse anodic scan, the oxidation peaks at 1.8 V and 2.3 V are associated with the stepwise oxidation of Mo into Mo 4+ /Mo 6+ and the oxidation of Li 2 S into sulfur or polysulfides, respectively [39]. In the subsequent cycles, the reduction peaks at 1.4 V and 2 V are ascribed to the reduction of Mo 6+ /Mo 4+ and the formation of Li 2 S, respectively. Galvanostatic charge/discharge cycling measurements were carried out to investigate the lithium storage capacity of the MoS 2 @carbon composites. As shown in Figure 4c, two voltage plateaus at 1.5 V and 0.35 V are observed in the first discharge curve, indicating the two-step lithiation reaction of MoS 2 . In the following discharge curves, the plateaus observed in the first discharge shift towards the higher potentials of 2 V and 1.4 V due to the improved reaction kinetics after the first lithiation. During the charging process, the MoS 2 @carbon composite shows two potential plateaus around 1.8 V and 2.3 V, which are consistent with the CV curves [22,40]. Figure 4d presents the cyclic performance of the as-prepared MoS 2 @carbon composites and pure MoS 2 powders at a current density of 100 mA·g −1 . It can be seen that the MoS 2 @carbon anode delivers an initial discharge and charge capacities of 1246 and 820 mAh·g −1 , respectively, which are higher than those of the pure MoS 2 electrode (i.e., 1100 and 689 mAh·g −1 ) and comparable to the reported MoS 2 /graphene (i.e., 1160 and 896 mAh·g −1 ) [41]. The initial capacity loss is mainly caused by the irreversible processes such as the inevitable formation of a solid-electrolyte interface (SEI) film on the electrode surface and some side reactions between Li + and active materials [42][43][44][45][46]. The Coulombic efficiency is as high as 67.6% in the first cycle (vs. 62.6% for the pure MoS 2 ), which increases to >98% from the second cycles. More importantly, the composite anode retains a high specific capacity of 869 mAh·g −1 after 100 cycles, indicating excellent cycling stability. For comparison, the specific capacity of the pure MoS 2 electrode decreases significantly to only 290 mAh·g −1 after 100 cycles. In addition, the good cyclic performance of the MoS 2 @carbon composite is further confirmed by another cell tested at a current density of 2000 mA·g −1 . As shown in Figure 4e, the composite electrode can deliver a reversible specific capacity of about 480 mAh·g −1 after 210 cycles, which is still higher than the commercial graphite anode (372 mAh·g −1 ). In the subsequent cycles, the reduction peaks at 1.4 V and 2 V are ascribed to the reduction of Mo 6+ /Mo 4+ and the formation of Li2S, respectively. Galvanostatic charge/discharge cycling measurements were carried out to investigate the lithium storage capacity of the MoS2@carbon composites. As shown in Figure 4c, two voltage plateaus at 1.5 V and 0.35 V are observed in the first discharge curve, indicating the two-step lithiation reaction of MoS2. In the following discharge curves, the plateaus observed in the first discharge shift towards the higher potentials of 2 V and 1.4 V due to the improved reaction kinetics after the first lithiation. During the charging process, the MoS2@carbon composite shows two potential plateaus around 1.8 V and 2.3 V, which are consistent with the CV curves [22,40]. Figure 4d presents the cyclic performance of the as-prepared MoS2@carbon composites and pure MoS2 powders at a current density of 100 mA·g −1 . It can be seen that the MoS2@carbon anode delivers an initial discharge and charge capacities of 1246 and 820 mAh·g −1 , respectively, which are higher than those of the pure MoS2 electrode (i.e., 1100 and 689 mAh·g −1 ) and comparable to the reported MoS2/graphene (i.e., 1160 and 896 mAh·g −1 ) [41]. The initial capacity loss is mainly caused by the irreversible processes such as the inevitable formation of a solid-electrolyte interface (SEI) film on the electrode surface and some side reactions between Li + and active materials [42][43][44][45][46]. The Coulombic efficiency is as high as 67.6% in the first cycle (vs. 62.6% for the pure MoS2), which increases to >98% from the second cycles. More importantly, the composite anode retains a high specific capacity of 869 mAh·g −1 after 100 cycles, indicating excellent cycling stability. For comparison, the specific capacity of the pure MoS2 electrode decreases significantly to only 290 mAh·g −1 after 100 cycles. In addition, the good cyclic performance of the MoS2@carbon composite is further confirmed by another cell tested at a current density of 2000 mA g −1 . As shown in Figure 4e, the composite electrode can deliver a reversible specific capacity of about 480 mAh·g −1 after 210 cycles, which is still higher than the commercial graphite anode (372 mAh·g −1 ). In addition to the higher specific capacity and cyclic performance, the MoS2@carbon anode also exhibits enhanced rate capability compared to the pure MoS2 powder. Figure 5a compares the rate performance of the anodes. When the current density increases gradually from 100 to 2000 mA·g −1 , the MoS2@carbon anode can maintain a reversible specific capacity of 457 mAh·g −1 , revealing its excellent rate capability. As the current density returns to 100 and 250 mA·g −1 , the composite anode recovers its average specific capacities of 891 and 840 mAh·g −1 , again indicating the outstanding reversibility of the MoS2@carbon anode. In sharp contrast, the MoS2 powder anode merely delivered about 85 mAh g −1 at 2000 mA·g −1 . The better rate capability of the MoS2@carbon anode can be ascribed to the conductive carbon substrate, which offers high conductivity for rapid electron collection and transfer. A better understanding of the resistive behavior is gained from the electrochemical impedance spectra (EIS). Figure 5b shows the resulting Nyquist plots fitted by Zview software. The EIS curves of both electrodes are composed of one semicircle in the high-to-mediate frequency region In addition to the higher specific capacity and cyclic performance, the MoS 2 @carbon anode also exhibits enhanced rate capability compared to the pure MoS 2 powder. Figure 5a compares the rate performance of the anodes. When the current density increases gradually from 100 to 2000 mA·g −1 , the MoS 2 @carbon anode can maintain a reversible specific capacity of 457 mAh·g −1 , revealing its excellent rate capability. As the current density returns to 100 and 250 mA·g −1 , the composite anode recovers its average specific capacities of 891 and 840 mAh·g −1 , again indicating the outstanding reversibility of the MoS 2 @carbon anode. In sharp contrast, the MoS 2 powder anode merely delivered about 85 mAh·g −1 at 2000 mA·g −1 . The better rate capability of the MoS 2 @carbon anode can be ascribed to the conductive carbon substrate, which offers high conductivity for rapid electron collection and transfer. A better understanding of the resistive behavior is gained from the electrochemical impedance spectra (EIS). Figure 5b shows the resulting Nyquist plots fitted by Zview software. The EIS curves of both electrodes are composed of one semicircle in the high-to-mediate frequency region and a spike in the low-frequency region. The semicircle corresponds to the charge transfer resistance (R ct ), and the spike is associated with the Li-ion diffusion into the electrode [41]. Notably, the MoS 2 @carbon anode shows a smaller R ct than the pure MoS 2 anode (175 vs. 260 Ω). The decreased R ct reveals that the combination of carbon and MoS 2 components renders improved reaction kinetics for fast electrochemical reactions.
Combining the high specific capacity, good rate capability, and excellent cycling stability, the MoS 2 @carbon composite could serve as a low-cost anode material with attractive electrochemical properties. The performance enhancement of the composite could be attributed to the unique hierarchical coaxial configuration [42,47]. First, the bio-mass-derived 1D carbon fiber substrate offers high conducting pathways for fast electron transfer, while the MoS 2 nanostructures shorten the electron transport distances. In addition, the 3D scaffold and the porous MoS 2 nanosheets facilitate smooth electrolyte penetration and ion transport. Both the enhanced electron and ion transport would accelerate the reaction kinetics and thus improve the electrochemical utilization of the composite. Second, the nanoscaled size and uniform distribution of the MoS 2 nanosheets on the carbon fibers enlarge the contact area between MoS 2 and electrolyte, thereby offering more active sites for Li-ion storage. Third, the coverage of MoS 2 on carbon can help to reduce the carbon-electrolyte interaction and reduce side reactions responsible for the irreversible capacity. Fourth, the carbon matrix provides a solid backbone to prevent MoS 2 nanosheets from restacking and agglomeration during the charge/discharge processes, which ensures the structural integrity of the whole electrode for long-term cycling endurance.

Synthesis of the MoS 2 @Carbon Composites
All reagents were of analytical grade and used as received without further purification. In a typical procedure, natural cotton was employed as a biomass source of the carbon substrate. The cotton was first carbonized at 900 • C for 3 h (heating rate: 5 • C/min) under N 2 atmosphere in a horizontal tube furnace to obtain carbon. Subsequently, MoS 2 @carbon composites were synthesized using the one-step hydrothermal method. Typically, 0.076 g of ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 ·4H 2 O, AMT) and 1 g of thiourea was dissolved in 35 mL deionized water under magnetic stirring for 30 min. Then, 100 mg of the carbon substrate was immersed into the solution, which was transferred into a 50 mL Teflon-lined stainless steel autoclave and kept at 180 • C for 24 h. After cooling down to room temperature, the black product was washed with deionized water and ethanol several times, and finally dried at 60 • C for 12 h. MoS 2 @carbon was calcined at 800 • C for 2 h under N 2 atmosphere to increase the degree of crystallinity. For comparison, MoS 2 powder was synthesized via the same hydrothermal procedure but without the addition of carbon.

Material Characterization
The phase structure of the samples was characterized by X-ray diffraction (XRD) with Cu Kα radiation (λ = 0.15418 nm) (X'Pert Pro MPD, Philips, Almelo, The Netherlands). Raman spectral analysis was carried out on a Renishaw inVia Raman Spectrometer (Renishaw Invia RM200, London, UK) using an excitation wavelength of 532 nm. The as-prepared sample was placed on conductive resin to observe the morphology using field emission scanning electron microscopy (FE-SEM, FEI Nano SEM 450, FEI, Portland, OR, USA). The composite was dispersed in ethanol under ultrasonication for transmission electron microscopy (TEM, FEI Tecnai F30G 2 , FEI, Portland, OR, USA) investigation.
The surface elements and chemical states of the sample were examined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, Waltham, MA, USA).

Electrochemical Measurements
The electrochemical performance was evaluated using CR 2016 coin cells assembled in an argon-filled glove box. The working electrode was fabricated by casting a slurry of 70 wt % active material, 20 wt % conductive agent (carbon black), and 10 wt % binder (polyvinylidene fluoride) in N-methyl-2-pyrrolidinone (NMP) on a copper foil. Li foil, a Celgard 2400 microporous polypropylene membrane, and a solution of 1M LiPF 6 in ethylene carbonate (EC), dimethyl carbonate (DMC), and ethylmethyl carbonate (EMC) (1:1:1, volume ratio) served as the counter electrode, the separator, and the electrolyte, respectively. Solartron electrochemical workstation (1260 + 1287, Bognor Regis, UK) was employed to obtain the cyclic voltammetry (CV) in the potential range of 0.01-3.0 V (vs. Li/Li + ) at a rate of 0.1 mV·s −1 , and the electrochemical impedance spectra (EIS) in the frequency range from 1 MHz to 50 mHz. Galvanostatic charge/discharge cycling was conducted on a Land battery system (LAND, BT2013A, Wuhan, China) under different current densities.

Conclusions
A low-cost bio-mass-derived carbon material was explored as effective substrates for the uniform growth of MoS 2 nanosheets via a hydrothermal approach. The MoS 2 @carbon composite exhibits well-defined 1D/2D hierarchical coaxial architecture, which is favorable for improving the reaction kinetics and the electrode structure. When evaluated as an anode of LIBs, the MoS 2 @carbon composite delivers a high specific capacity of 820 mAh·g −1 at 100 mA·g −1 , superior rate capability with 457 mAh·g −1 retained at a high-rate current of 2000 mA·g −1 , and excellent cycling stability without noticeable capacity loss after 100 cycles. The remarkable lithium ion storage performance makes the present MoS 2 @carbon composite a promising low-cost anode material candidate for high-performance LIBs.