MoS2/C/C nanofiber with double-layer carbon coating for high cycling stability and rate capability in lithium-ion batteries

MoS2 has attracted a lot of interest in the field of lithium-ion storage as an anode material owing to its high capacity and two-dimensional (2D)-layer structure. However, its electrochemical properties, such as rate capability and cycling stability, are usually limited by its low conductivity, volume variation, and polysulfide dissolution during lithiation/delithiation cycling. Here, a designed two-layer carbon-coated MoS2/carbon nanofiber (MoS2/C/C fiber) hybrid electrode with a double-layer carbon coating was achieved by a facile hydrothermal and subsequent electrospinning method. The double carbon layer (inner amorphous carbon and outer carbon fiber) shells could efficiently increase the electron conductivity, prevent the aggregation of MoS2 flakes, and limit the volume change and polysulfide loss during long-term cycling. The as-prepared MoS2/C/C fiber electrode exhibited a high capacity of up to 1,275 mAh/g at a current density of 0.2 A/g, 85.0% first cycle Coulombic efficiency, and significantly increased rate capability and cycling stability. These results demonstrate the potential applications of MoS2/C/C fiber hybrid material for energy storage and may open up a new avenue for improving electrode energy storage performance by fabricating hybrid nanofiber electrode materials with double-layer carbon coatings.


Introduction
MoS 2 is one of the typical two-dimensional (2D) transition-metal dichalcogenides (TMDs), with a layered structure loosely held together by the Van der Waals interaction. MoS 2 exhibits a unique Nano Res.
graphite-like structure in which the Mo hexagonal layers are sandwiched by two S layers, and each layer is bonded by strong covalent bonds (Mo-S) [1][2][3]. Due to its unique structure, MoS 2 demonstrates a variety of physical and chemical properties, such as photoluminescence, a bulk indirect band gap of 1.3 eV (which is similar to silicon), and excellent electrochemical performance for applications in photo/electrocatalysts, energy storage, and conversion devices [4][5][6][7][8]. Because of the weak interlayer interaction and large interlayer distance of 0.65 nm between each S-Mo-S layer, lithium ions can easily intercalate/extract through MoS 2 layers. Thus, MoS 2 is a promising anode material and its capacity can be three and a half times that of commercial graphite anodes [9][10][11][12][13]. Reversible capacity as high as 1,290 mAh/g has been reported for the MoS 2graphene composite electrode [14]. Compared to other emerging negative electrode materials, such as silicon or SnO 2 , MoS 2 generally displays much better rate capability, lower cycling degradation, as well as low cost (e.g., sub-micro MoS 2 powder retails for dollars per kg) [15]. While silicon anodes possess an initial capacity around 3,500 mAh/g when tested at low rates (e.g., 0.1 C), they retain minimal capacity at high rates (e.g., 10 C) or after long term cycling due to their 280% volumetric expansion upon full lithiation and related unstable solid electrolyte interface [12]. In addition, nanostructured MoS 2 routinely outperforms its bulk equivalent in terms of capacity retention, rate capability, and cycle life, due to shorter lithium diffusion distances and higher concentrations of active edge sites, which further enhance their lithium storage capability [11,16,17].
However, electrodes based on layered MoS 2 do not work perfectly. Although the volume change of MoS 2 is significantly less than that in silicon-based electrodes, the theoretical 103% volume expansion via the conversion reaction of MoS 2 to Li 2 S and molybdenum will still lead to depressed cycling stability and rate performance [18][19][20][21]. The intrinsically limited conductivity, restacking of MoS 2 nanosheets, side reactions between Li 2 S and electrolyte, and polysulfide dissolution during cycling can further aggravate the degradation of electrochemical performance [22][23][24].
In order to ameliorate the electrical conductivity and structural stability of MoS 2 electrodes during cycling, carbonaceous materials, such as amorphous carbon, carbon fibers, porous carbon, carbon spheres, carbon nanotubes, and graphene, have been employed to form hybrid MoS 2 -carbon systems [13,25,26]. The carbon component can enhance the electronic conductivity and concurrently buffer the volume changes of MoS 2 during the lithiation/delithiation reaction. At the same time, carbon materials can also act as a binder between the S/Li 2 S and the molybdenum to prevent the loss of polysulfide during cycling [1,23,27,28]. Based on these properties, hybrid MoS 2 -carbon systems can lead to better electrode kinetics and more stable cycling performance [29][30][31][32][33].
Although some progress has been made in enhancing electrochemical performance by fabricating hybrid MoS 2 /carbon electrode materials [34][35][36], there are still some challenges for this hybrid system. The restacking and aggregation of the nanocarbon materials, such as graphene or carbon nanotubes, or even MoS 2 nanomaterials during the fabrication process can lead to a serious loss of active surface area and sluggish ion transport kinetics. In addition, it is still difficult to coat or deposit a carbon layer with ideal thickness and compact contact interface onto nanosized MoS 2 materials. Thus, effectively controlling the carboncoating process and avoiding the aggregation of the MoS 2 /carbon is still a challenge.
Herein, we designed a two-layer carbon-coated MoS 2 /carbon hybrid nanofiber material (MoS 2 /C/C fiber), to serve as a high performance anode material for Li-ion batteries. Carbon-coated three-dimensional (3D) hierarchical MoS 2 nanospheres (MoS 2 /C spheres) were fabricated first and then embedded into ~ 250 nm diameter electrospun fibers. The double-layer carbon coating plays a key role in preventing the aggregation of MoS 2 flakes, restraining the MoS 2 volume changes, and consumption of sulfides and molybdenum during lithiation/delithiation. Due to its structural stability and enhanced electron transport, the MoS 2 /C/C fiber electrode manifests an outstanding reversible capacity and an excellent cycling performance. The double-layer carbon-coated MoS 2 composite electrode is a promising candidate anode active material for next generation lithium batteries with high capacity, high rate, and high cycling stability. Nano Res.

Preparation of MoS 2 spheres
In a typical reaction [2], 1 mmol hexaammonium heptamolybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 ·4H 2 O, AR), 14 mmol thiourea (NH 2 CSNH 2 , ≥ 99.0%), and 0.01 mmol surfactant PVP (K32) were dissolved in 40 mL of deionized water (DI water) under vigorous stirring and ultrasonication for 30 min to form a homogeneous solution. Then, the solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave and kept at 220 °C for 18 h. After being cooled to room temperature, the obtained precipitates were centrifuged and washed with DI water, acetone, and ethanol three times. The final products were dried at 70 °C under vacuum for 12 h.

Preparation of MoS 2 /C spheres
In a typical synthesis [34], 100 mg of glucose and 200 mg of the above-obtained MoS 2 spheres were dissolved in 30 mL of DI water after violent stirring and ultrasonication for 30 min. Then, the solution was transferred into a 50 mL Teflon autoclave, followed by heating at 190 °C for 10 h. The autoclave was then cooled down to room temperature and the coating product was collected from the solution, washed with DI water and ethanol, and dried at 70 °C overnight. Finally, the produced powder was carbonized at 450 °C for 3 h under a nitrogen atmosphere to obtain the MoS 2 /C spheres.

Preparation of MoS 2 /C/C fiber
The mixture solution was prepared by adding 0.5 g of polyacrylonitrile (PAN) and 0.4 M of the aboveobtained MoS 2 /C spheres in 4.5 mL of N,N-dimethyl formamide (DMF). After stirring and ultrasonication for 30 min, the mixture solution was spun by a controlled electrospinning method. The applied working voltage, flow rate, and distance between the needle and the collector were fixed at 19 kV, 0.75 mL/h, and 15 cm, respectively. The electrospun fibers were collected from the aluminum foil and then prepared for further carbonization. In a typical carbonization process, the oxidative stabilization process was first carried out using a conventional muffle furnace at 280 °C for 1 h. Then, thermal treatment of the nanofiber film was performed at 1,000 °C for 3 h, with a continuous stable high-purity nitrogen gas.

Characterizations
X-ray diffraction (XRD) measurements were conducted with a Rigaku D/max 2550 V diffractometer using Cu Kα radiation at a wavelength of 1.5406 Å. Scanning electron microscopy (SEM) measurements were performed with a SU-8010 field emission SEM. Transmission electron microscopy (TEM), and elemental mapping were carried out using a FEI Tecnai G2 F20 TEM at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Kratos Axis Ultra multi-functional X-ray photoelectron spectroscope with a typical detection depth of ~ 5 nm. All XPS spectra were corrected using the C 1s line at 284.7 eV. The carbon content of the samples was determined through thermogravimetric and differential scanning calorimetry (TG-DSC) using a Libra/209F1 with a speed of 10 °C /min in ambient air in a temperature range of 50 to 700 °C . Raman spectroscopy was performed using an inVia-Reflex with an excitation wavelength of 785 nm. Nitrogen adsorption and desorption isotherms at 77.3 K were carried out with an ASAP 2020 physisorption analyzer (USA).

Half-cell
The electrochemical properties of MoS 2 spheres, MoS 2 /C spheres, and MoS 2 /C/C fiber were investigated using a coin-type half-cell (CR2032). For the preparation of working electrodes, active materials, Super-P carbon black, and polyvinylidene fluoride (PVDF) with a mass ratio of 70:20:10 were mixed into a homogeneous slurry by adding DMF and the obtained slurry was pasted onto pure Cu foil and dried at 60 °C overnight. Lithium metal foil works as both counter and reference electrode. The cell used LiPF 6 (1 M) in ethylene carbonate/dimethyl carbonate/diethyl carbonate (1:1:1 vol.%) as the electrolyte and a Celgard 2400 polypropylene membrane as the separator. All the batteries were assembled in an argon-filled glovebox with humidity and oxygen levels lower than 1 ppm. The charge and discharge measurements of the cells Nano Res.
were performed on a LAND CT2001A electrochemical workstation between 0.005 and 3.00 V at room temperature. Cycling voltammogram (CV) tests of the batteries were performed by a CHI660D electrochemical workstation with the same potential range.

Full-cell
The MoS 2 /C/C fiber electrode as active materials, Super-P carbon black, and PVDF with a mass ratio of 70:20:10 were mixed into a homogeneous slurry, which was then pasted onto pure copper foil as the anode electrode. The full cell was fabricated using a MoS 2 /C/C fiber electrode on Cu foil as the anode, lithium cobalt oxide (LiCoO 2 ) on aluminum foil as the cathode electrode, with the same separator and electrolyte as in the half-cell. The charge and discharge measurements of the batteries were performed on a LAND CT2001A electrochemical workstation between 0.005 and 4.20 V. The average mass loading of the half-cell was 2.0-2.4 mg/cm 2 . For the MoS 2 /C/C/LiCoO 2 full cell, the mass loadings for the anode and cathode were 4.0 and 19.9 mg/cm 2 , respectively.

Results and discussion
As illustrated in Fig. 1(a), the typical preparation process of MoS 2 /C/C fiber hybrid electrodes consists of three steps, including one step for the MoS 2 spheres preparation and two steps for the doublelayer carbon coating. In the first step, MoS 2 nanospheres were synthesized by a hydrothermal method. Then the prepared MoS 2 nanospheres were reacted with glucose and annealed under a N 2 atmosphere to get the first layer of the carbon coating. The typical morphologies of the MoS 2 and MoS 2 /C nanospheres were characterized with SEM and TEM, as shown in Figs  Nano Res. 5-10 nm. As shown in the high resolution TEM image in Fig. S2 in the ESM, the very thin carbon layer was uniformly wrapped around the MoS 2 nanospheres without damaging the 3D network. The first carbon coating layer not only facilitates the intrinsic electron conductivity of the electrode material but also limits structural damage during the electrochemical process.
The MoS 2 /C nanospheres were subsequently mixed with PAN and electrospun into nanofibers. As schematically illustrated in Fig. S3 in the ESM, the asprepared nanofibers were pre-oxidized and then carbonized by high temperature thermal treatment. The as-prepared MoS 2 /C/C fibers exhibited a homogeneous diameter distribution and smooth surface. As shown in the SEM image in Fig. 1(d), the diameters of the MoS 2 /C/C fibers were mainly in the range of 250 to 280 nm. It is worth noting that no obvious MoS 2 nanosheets were found at the surface of the fibers, implying the embedding of MoS 2 /C spheres inside the nanofibers. The low magnification TEM images in Figs. 2(a) and 2(b) clearly show the MoS 2 /C spheres embedded into the carbon fiber networks. To further characterize the microstructure, we performed high-resolution TEM. A very clear two-layer carbon coating on MoS 2 spheres was observed. Furthermore, the high-resolution TEM energy dispersive spectroscopy (EDS) elemental mapping is shown in Figs. 2(e)-2(g), which exhibit the elemental distribution in the MoS 2 /C/C fiber sample in the same region as that shown in Fig. 2(d). All of the molybdenum, sulfur, Nano Res. and carbon are distributed in specific regions of the 3D MoS 2 /C/C fiber network, which further proves the successful preparation of the MoS 2 /C/C fiber hybrid nanostructure.
The crystalline structure and chemical composition of the resultant MoS 2 , MoS 2 /C nanospheres, and MoS 2 /C/C hybrid fiber were further studied by XRD and XPS. The XRD pattern of the MoS 2 nanospheres shows three dominant peaks of hexagonal crystalline 2H-MoS 2 (JCPDS: 37-1492). These three peaks, corresponding to the (002), (100), and (110) planes, emerge at 13.5°, 33.3°, and 58.9° [16], respectively. After thin layer carbon coating, the XRD pattern of the MoS 2 /C spheres does not show any obvious changes from pure the MoS 2 spheres sample, except for the appearance of a broad peak at ~ 22° associated with amorphous carbon and a decrease in the intensity of the (100) plane peak of MoS 2 , due to the thin layer carbon coating [36,37]. For the XRD pattern of MoS 2 / C/C fiber, the peak shown around 25° is ascribed to the outer layer carbon coating. The sharpened peaks corresponding to the (002) and (130) planes at 13.5° and 39.4°, respectively, are related to the high temperature thermal treatment and the shrinkage of the interlayer spacing of MoS 2 [38,39].
XPS measurements were carried out to further clarify the components of these samples and the interfacial properties between MoS 2 flakes and the carbon layer. Overall, Fig. 2(i) shows the full XPS spectra of pure MoS 2 , MoS 2 /C, and MoS 2 /C/C fiber materials. The spectrum from the pure MoS 2 spheres clearly exhibits peaks assigned to Mo 3d and 3p, S 2p, and O 1s. Both MoS 2 /C and MoS 2 /C/C fiber also include the C 1s peak, and the intensity ratio of C 1s for MoS 2 /C/C fibers is much higher than that of MoS 2 /C spheres.
The high-resolution XPS spectra of MoS 2 , MoS 2 /C nanospheres and MoS 2 /C/C fibers are shown in Figs. S4-S6 in the ESM. Two peaks are observed at 228.6 and 231.9 eV, which can be attributed to the Mo 3d 5/2 and Mo 3d 3/2 binding energies, respectively, and are ascribed to the characteristic peaks of Mo 4+ in MoS 2 flakes [40]. The peaks at 232.1 and 235.3 eV are related to the Mo 3d 5/2 and 3d 3/2 of Mo 6+ (typical of the Mo-O-C bond) [26]. The peaks at 162.5 and 161.3 eV are attributed to the S 2p 1/2 and S 2p 3/2 peaks, which are related to the sulfur in MoS 2 [13]. The C 1s spectra can be decomposed into a dominant peak at 284.7 eV for C=C/C−C, and another broad peak related to oxygen-containing functional groups, which include C-O-Mo bonds [26]. The C-O-Mo bonds can be formed by bonding Mo atoms to oxygen-containing functional groups of the carbon precursor. The interfacial interaction of the C-O-Mo bond between MoS 2 and inner carbon layer should be beneficial for the electrochemical performance of the MoS 2 /C/C fiber electrodes. As shown in Fig. S7 in the ESM, we further characterized the structure of MoS 2 /C and MoS 2 /C/C hybrid electrodes using Raman spectroscopy. For MoS 2 /C, the two distinct peaks at 377 and 404 cm -1 correspond to the E 1 2g vibration mode (the in-plane displacement of Mo and S atoms) and the A 1g vibration mode (out-of-plane symmetric displacements of S atoms along the c-axis), respectively. After embedding MoS 2 /C spheres into carbon fiber, these two MoS 2 related peaks disappeared due to the thick outer carbon layer. In addition, both MoS 2 /C spheres and MoS 2 /C/C fiber hybrid materials also displayed another two peaks at the bands around 1,360 (D band) and 1,590 (G band) cm -1 . The D band can be assigned to sp 3 -hybridized disordered carbon or defective carbon, and the G band is associated with sp 2 -hybridized graphitic carbon. The relative intensity ratio of I D /I G can roughly indicate the ratio of disordered carbon to graphitic carbon. The calculated values of I D /I G for MoS 2 /C and MoS 2 /C/C hybrid materials are 0.88 and 0.97, respectively, suggesting that MoS 2 /C/C fiber contained a higher ratio of sp 3 -hybridized carbon after the high temperature annealing. Figure S8 in the ESM reveals the thermogravimetric analysis of pure MoS 2 , MoS 2 /C spheres, and MoS 2 /C/C fiber. The mass loss can be attributed to the decomposition of MoS 2 and carbon using Eqs. (1) and (2) As shown in Fig. S8 in the ESM, in the case of pure MoS 2 , 10% of the weight loss happened at approximately 350 °C owing to the oxidation of MoS 2 to MoO 3 under an air atmosphere. In contrast, MoS 2 /C and MoS 2 /C/C hybrid materials present a larger Nano Res.
weight loss of ~ 22% and 63%, respectively. The oxidative decomposition of carbon materials is thought to be the main reason for the different weight losses between MoS 2 /C and MoS 2 /C/C hybrid materials and pure MoS 2 material. Thus, the carbon contents in MoS 2 /C and MoS 2 /C/C hybrid materials are estimated to be 13.3% and 58.9%, respectively. MoS 2 , a well-known material, could offer some exciting electrochemical properties for lithium storage as an anode material. Intercalation of lithium into MoS 2 is known to occur within the voltage range of 0-3 V [13]. In the discharge process, a lithiation mechanism occurs at 1. The reaction of elemental sulfur and lithium at 2.2 V vs. Li/Li + can be described as in Eq. (5) The theoretical specific capacity of the reaction from MoS 2 to Li x MoS 2 (Eq. (3)) is 167 mAh/g, corresponding to the intercalation of one lithium ion per molybdenum atom. If the lithiation process goes deeper, attributed to the reversible conversion reaction of MoS 2 to Li 2 S and metallic molybdenum as shown in Eq. (4), the theoretical specific charge capacity can reach up to 669 mAh/g. However, the reaction represented by Eq. (5) could yield a high theoretical capacity of 1,675 mAh/g, which is one of the reasons to produce MoS 2 /carbon composites to achieve a high capacity over 1,000 mAh/g [42].
Based on the 3D network structure and double-layer carbon coating, MoS 2 /C/C fiber could be a promising active material for high-performance energy storage. We systematically evaluated the electrochemical performance of a MoS 2 /C/C fiber electrode as the anode of a Li-ion battery. Figure 3(a) shows the first three consecutive CV curves of the MoS 2 /C/C fiber electrode with a scanning rate of 0.2 mV/s. In the first discharge process, a cathodic peak at 1.05 V (vs. Li/Li + ) corresponds to the reaction in Eq. (3). The peak at 0.42 V can be assigned to the transformation of Li x MoS 2 into metallic Mo nanoparticles and Li 2 S via a conversion reaction represented by Eq. (4). A wide shoulder peak emerges in the range of 0.6-0.8 V, which can be attributed to the formation of solid electrolyte interphase (SEI). The formation of an SEI layer can also make a significant contribution to the overall charge storage capacity [36,43]. The intensities of these three peaks decreased in the following cycles. The two reduction peaks at 1.10 and 1.90 V can be attributed to formation of Li x MoS 2 and Li 2 S according to Eqs. (3) and (5). In the reverse anodic process, there are two oxidation peaks around 1.43 and 1.71 V, which are associated with the oxidation of Mo to Mo 4+ and/or Mo 6+ [15]. The dominant peak at 2.3 V could be assigned to the delithiation process of Li 2 These two peaks were in accordance with the peaks for pure MoS 2 , providing further evidence of the charge storage origin. During the subsequent cycles, the reversible reaction is based on Eq. (7) Lithiation: MoS +6Li S 6e Mo 3Li S Delithiation: During subsequent discharge cycles, the obvious peak at 0.42 V almost disappeared while two new peaks were repeatedly observed at 1.9 and 1.1 V. These two peaks correspond to the conversion of S to polysulfides and then to Li 2 S, which is consistent with other results in literature [44]. The oxidation peak at around 2.3 V, which is due to the oxidation of Li 2 S to S and lithium ions [45,46], is quite repeatable Nano Res.
after the first cycle, indicating the good cycling stability of the MoS 2 /C/C fiber hybrid electrode, as further revealed in the following electrochemical tests.
The specific capacity and rate capability of the MoS 2 /C/C fiber electrode were tested by galvanostatic charge-discharge with different currents from 0.1 to 1.0 A/g. It is evident that the plateaus on the charge-discharge curves are consistent with the respective peaks on the CV curves. Similar charge and discharge plateau with different applied current densities revealed the good rate stability of the MoS 2 /C/C fiber electrode. Figure 3(c) shows the superior rate capability of the MoS 2 /C/C fiber electrode compared to that of pure MoS 2 spheres and MoS 2 /C spheres electrodes. The measured reversible capacities of the MoS 2 /C/C fiber electrode are 1301.2, 765.9, 680.8, 622.8, and 605.4 mAh/g at current densities of 0.1, 0.3, 0.5, 0.8, and 1.0 A/g, respectively. In contrast, the pure MoS 2 sphere electrode showed a rapid capacity drop with increasing current density. Although the rate capability of the MoS 2 /C spheres electrode showed a minor improvement compared with pure MoS 2 sphere, it was still much lower than that of the double-layer carbon coated MoS 2 /C/C fiber electrode.
The cycling performance of the MoS 2 /C/C fiber electrode was evaluated at a current density of 0.2 A/g as shown in Fig. 3(d). The respective performance data for pure MoS 2 and MoS 2 /C sphere electrodes were also included for comparison. The initial discharge capacities of pure MoS 2 , MoS 2 /C, and MoS 2 /C/C fiber electrodes were 1,301, 1,281, and 1,275 mAh/g, respectively. The corresponding initial Coulombic efficiencies of these electrodes were 68.9%, 75.0%, and 85.0%. Such improved Coulombic efficiency can be related to the increased conductivity after double-layer carbon coating. The irreversible capacity loss during the initial few cycles is inevitable, because of the formation of the SEI layer [43,49]. In the subsequent cycles, the Coulombic efficiency of the MoS 2 /C/C fiber electrode gradually increased to 99.2%, superior to that of the MoS 2 and MoS 2 /C electrodes with 93.3% and 98.2%, respectively. After 150 cycles, the MoS 2 /C/C fiber electrode retained 83.3% (1,062 mAh/g) efficiency, which is much higher than the 40.9% (525 mAh/g) of the MoS 2 /C electrode and the 17.9% (234 mAh/g) of the pure MoS 2 electrode. Due to the direct exposure of MoS 2 to the electrolyte, it is inevitable to introduce the irreversible reaction of MoS 2 to dissoluble metallic Mo and Li 2 S, which will lead to serious structure and electrochemistry fading during the charge and discharge process. Due to the partial blocking of this irreversible reaction, the cycling stability of MoS 2 /C increased by over 20% after a thin layer of carbon coating. For the MoS 2 /C/C electrode, the double-layer carbon coating could not only suppress the irreversible reaction, but also limit the volume change during the lithiation/delithiation process. In order to evaluate the long-term stability of the MoS 2 /C/C fiber electrode, we ran the electrode with a high current density of 1 A/g for 800 cycles. After the long-term cycling, 92.6% of the initial charge capacity remained, which means only a 0.009% capacity loss per cycle. The Coulombic efficiency of the MoS 2 /C/C fiber electrode reached 98.8% in the third cycle and stabilized between 98.8%-100% until the 800th cycle, illustrating its high Coulombic efficiency and cycling stability.
Based on the above discussion, the outstanding electrochemical properties of the MoS 2 /C/C fiber electrode may be derived from the synergistic effects between the MoS 2 nanosphere network and the double-layer carbon coating. First, the introduction of the inner layer conductive carbon coating can prevent the restacking of MoS 2 nanosheets and increase the conductivity of the MoS 2 nanospheres, especially at the interface of MoS 2 nanosheets. Second, the robust outer layer carbon could effectively tolerate the volume change and prevent the loss of metallic molybdenum and sulfides during cycling. The conductive matrix of the carbon fiber can also provide a fast electron transport path for the whole electrode. Third, the fibrous network structure and high surface area of the MoS 2 /C/C fiber electrode (371.6 m 2 /g) also increased the area exposed to the electrolyte, facilitating ion diffusion and transport for high capacity and good rate capability (Fig. S9 in the ESM). the lithiation process and electrode structure evolution after long-term cycling. As demonstrated in the schematic and shown in SEM images, the pure MoS 2 spheres electrode showed serious structure fading and formed into continuous film after long-term cycling because of direct exposure of MoS 2 to the electrolyte [15,27]. This could lead to the profound decomposition of MoS 2 nanospheres and the dissolution of metallic Mo and Li 2 S. The MoS 2 /C electrode also exhibited obvious structure change, but part of the nanosphere structures was still observed at the up surface of the electrode, due to the thin layer of carbon coating protection [34]. After coating with outer layer carbon fiber, the double-layer carbon coating could limit the volume change of the whole electrode and the dissolution of metallic Mo and sulfides during the lithiation/delithiation cycles. Thus, even after long-term cycling, the MoS 2 /C/C fiber electrodes could still maintain the original fibrous network structure, which further explains why this double-layer carbon-coated MoS 2 material could display such an enhanced rate performance, high reversible capacity, and long-term cycle life [50].
To further demonstrate the promising properties of MoS 2 /C/C fiber as the anode material of Li-ion batteries, we fabricated a full cell with a LiCoO 2 electrode as the cathode. Figure 5(a) indicates the working potential of LiCoO 2 is 3.0-4.2 V, while MoS 2 /C/C shows a working potential of 0.0-3.0 V, which shows the great potential of these materials to form high performance full batteries and be used as the next general lithium-ion battery. Figure 5(b) exhibits the charge and discharge voltage profiles of the full battery, which show a potential from 0 to 4.2 V, a capacity of 155 mAh/g at 0.1 A/g, and close to the theoretical capacity of LiCoO 2 . The total electrochemical reactions of the full cell can be explained by Eq. (8) A red LED was lit up for more than 60 min by the assembled full battery (Fig. 5(c)), which indicates great potential and practical application, and suggests the MoS 2 /C/C fiber is a promising high-performance anode material. Figure S10 in the ESM shows the cycling performance of the MoS 2 /C/C//LiCoO 2 full cell. The full cell retained 77% capacity after 80 cycles. After the initial 10 cycles, the capacity decreased to 107 mAh/g (83%) and the Coulombic efficiency stayed at 94%. However, the cell rapidly became more stable for the subsequent cycles, and achieved nearly 98% Coulombic efficiency for all following cycles.

Conclusions
In summary, we report a facile hydrothermal and subsequent electrospinning method to fabricate a double protective carbon layer coated MoS 2 fibrous electrode material, denoted as MoS 2 /C/C fiber. Attributed to its intriguing double-layer carbon shell structure, the MoS 2 /C/C fiber exhibits a series of remarkable electrochemical properties as the anode material of a Li-ion battery, such as a high first cycle specific capacity (1,275 mAh/g at 0.2 A/g), outstanding rate capability, and good capacity stability after long-term cycling (92.6% capacity retained after 800 cycles at 1 A/g). The high-energy storage performance of the MoS 2 /C/C fiber material can be attributed to the design of the double carbon layer shell protection (inner amorphous carbon and outer carbon fiber). Furthermore, such a double-layer carbon coating strategy could prove useful in other areas of the energy storage field for improving rate capability and cycling stability.