Nanosheets Encapsulated in Carbon Nanofibers as Binder-free Anode for Superior Lithium Storage

MoS2 Nanosheets Encapsulated in Carbon Nanofibers as Binder-free Anode for Superior Lithium Storage Yakai Denga, Liang Zhana*, Yanli Wanga and Shubin Yangb* a State Key Laboratory of Chemical Engineering,East China University of Science and Technology,Shanghai 200237,China; b Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education, School of Materials Science and Engineering, Beihang University, Beijing 100191, China *Corresponding authors. Liang Zhan, E-mail: zhanliang@ecust.edu.cn; Shubin Yang, Email: yangshubin@buaa.edu.cn


Introduction
Lithium ion batteries (LIBs) have been regarded as one of the most important rechargeable energy storage devices with broad applications in hybrid and electric vehicles owing to their high potentials and environmental friendliness [1][2][3].Owing to the low theoretical capacity of graphite (372 mAh g-1) [4], the reversible capacities of commercial graphite based anode materials cannot satisfy the increasing requirements for high-performance LIBs.
Therefore, many endeavours have been concentrated on exploring novel anode materials, such as transition metal oxides [5], molybdenum disulfide (MoS2) [6], stannum (Sn) [7], and silicon (Si) [8].Among the novel anode candidates, MoS2 has been recognized as one of the most promising and attractive one for LIBs, owing to its high theoretical capacity (670 mAh g-1), relatively low discharge potential, low-cost, safety and environment-friendly [9][10][11].However, bulk MoS2 suffers from a low electrical conductivity and volume expansion during cycle process, leading to poor electrochemical performance [10].To overcome these shortcomings, researchers focus on fabricating single-layer or few layered MoS2 by chemical vapor deposition [11,12], chemical exfoliation [13] and mechanical exfoliation [14,15].Unconsideration the extremely low yield, the single-layer MoS2 or MoS2 nanosheets are easy to restack, leading to the structural instability during cycling, as a result, obvious volume expansion will occur and bring up rapid capacity fading [16].Thus researcher further focus their attentions on 3 of 18 combining MoS2 nanostructures with carbonaceous materials (such as carbon nanotubes [17], grapheme [18,19], and conductive polymers [20]) to resolve the low electrical conductivity, volume change and restacking.However, how to develop a simple and efficient approach to fabricate MoS2/carbon nanostructures is still a big challenge.
Herein, we develop an efficient approach to fabricate one-dimensional MoS2/carbon nanofibers (denoted as 1D MoS2/CNFs) by electrospinning.Exfoliated MoS2 nanosheets with small lateral size were encapsulated in carbon nanofibers.The distinctive structure of 1D MoS2/CNFs can improve the electrical conductivity of pure MoS2 nanosheets, but also can prevent the restacking of nanosheets.Importantly, the free-standing MoS2/CNFs can be easily cut into flexible film and directly used as binder-free anode for lithium storage.The unique structures offer the resultant 1D MoS2/CNFs exhibit excellent electrochemical performance.For instance, the resultant 1D MoS2/CNFs has a high volumetric capacity of 700 mAh g -1 at 100 mA g -1 after 50 cycles and good high-rate performance (450 mAh g -1 at 1000 mA g -1 after 200 cycles).We believe that this efficient method can be further extended to fabricate other active nanomaterials encapsulated in carbon nanofiber or carbon nanotube for broad applications in batteries, supercapacitors and catalysts.and S (j) elemental mapping (Figure 2a and 2b).Additionally, two-dimensional MoS2 nanosheets also cannot be observed among the nanofibers (Figure 2c), but they can be detected and are well encapsulates in the matrix of CNFs (Figure 2d).After carbonization, the PAN molecules have transformed into amorphous carbon (Figure 2e), and the typical lattice fringes with an interplanar spacing of 0.63 nm are observed among the amorphous carbons (Figure 2f), indexed to the (002) facets of MoS2 [20], which has the same characteristics as the exfoliated MoS2 nanosheets (Figure S3).To further explore the chemical composition of the 1D MoS2/CNFs, XRD measurement was performed.As shown in Figure 3a, there are several strong peaks at was obtained at relative pressures P/P0 from 0 to 1 (Figure 3b).The specific surface area of 1D MoS2/CNFs was also obtained with 23.94 m 2 g -1 on the basis of the nitrogen adsorption/desorption isotherms.Interestingly, the pore size distribution indicates that there exists micropores and mesopores in MoS2/CNFs (Figure 3c), which should related to the layered and restacked MoS2 nanosheets as well as the interlaced structure of CNFs.Thermogravimetric analysis was also performed to confirm the content of MoS2 in the carbon nanofibers (Figure 3d).The elemental components and contents of 1D MoS2/CNFs were elucidated by XPS measurement.Based on the XPS surveys, it is distinct that carbon, molybdenum, sulfide and oxygen species are assumed in the MoS2/CNFs (Figure 4a), and the detail contents are illustrated in Table S1.The fitted C1s peaks are demonstrated in Figure 4b, the peaks located at 284.5, 285.5 and 287 eV are related to C-C, C-O and C=C bonds of amorphous carbon, respectively [23].The high-resolution Mo 3d spectrum can be fitted to three types at the binding energies of 236.7, 233.4 and 230 eV, corresponding to the Mo 6+ , Mo 3d 3/2 and Mo 3d 5/2 peaks (Figure 4c).The Mo 3d 3/2 and Mo 3d corresponding to the S 2p 1/2 and S 2p 3/2 peaks, represent to the S 2-in MoS2 [18].The O 1s peaks observed in the spectrum are mainly caused by the oxygen in the testing environment.Coulombic efficiency is 69.2%.The irreversible capacity loss is mainly attribute to the formation of solid electrolyte interphase (SEI) film [25].The reversible capacity of MoS2/CNFs remains stable at 700 mAh g -1 at 100 mA g -1 after 50 cycles (Figure 5a), which is much higher than that of bulk MoS2 (360 mAh g -1 , Figure 5b).Figure 5c indicates that the MoS2/CNFs also shows better cycling performance than that of bulk MoS2.For the exfoliated MoS2 nanosheets, although the initial capacity is as high as1050 mAh g -1 , there is only 450 mAh g -1 after 50 cycles (Figure S4).The capacity fading is mainly caused by the restack and structure destruction of the MoS2 nanosheets.

Results and discussion
A little capacity improvement could be observed in the result of MoS2 nanosheets after some cycles, which should be contributed to the activation of Li + pathways between the electrolyte and electrode during cycling [26].More importantly, the MoS2/CNFs sample exhibits distinguished high-rate capabilities at different current densities from 50 to 1000 mA g -1 (Figure 5d).The reversible capabilities of MoS2/CNFs sample is up to 530 and 450 mAh g -1 at 500 and 1000 mA g -1 , respectively, which are significantly higher than the contrast bulk MoS2 (164 mAh g -1 at 1000 mA g -1 ) and exfoliated MoS2 nanosheets (148 mAh g -1 at 1000 mA g -1 , Figure S5).And when the current rate is again reduced back to 50 mA g -1 , the reversible capacity can be recovered and maintains at 769 mAh g -1 .In addition, the MoS2/CNFs sample keep no attenuation at 1000 mA g -1 after 200 cycles (Figure 5e), indicating the excellent long cycle performance.mixture solution was poured into a syringe for electrospinning.Detailly, the diameter of the needle, the distance between needle and collector, the working voltage and the flow rate of the solution are 0.34 mm, 15 cm, 10 kV and 0.5 ml/h, respectively.Finally, the resultant 1D MoS2/PAN nanofibers were oxidized at 280 o C in air for 2 h and then carbonized at 850 o C for 3 h under Ar atmosphere.

Electrochemical measurement
Electrochemical experiments were performed using standard CR2031 type coin cells assembled in the glovebox.After the MoS2/CNFs was cut into flexible tablet, it was directly used as anode electrode.The exfoliated MoS2 and bulk MoS2 electrodes were fabricated by mixing the active material with acetylene black and poly(vinyl difluoride) (PVDF) at a weight ratio of 8:1:1.In the process of fabrication of lithium ion battery, pure lithium foil were used as counter electrode, propene polymer (PP) separator membrane as separate and 1 M LiPF6 in ethylene carbonate as electrolyte.The galvanostatic discharge/charge behavior of coin cells was tested on a battery testing

Conclusions
We have developed an efficient approach to fabricate 1D MoS2/CNFs by electrospinning.The free-standing MoS2/CNFs can be easily cut into flexible tablet and directly used as binder-free anode for lithium storage.The unique structures offer the 1D MoS2/CNFs a high gravimetric capacity (700 mAh g -1 at 100 mA g -1 ) and good high-rate performance (450 mAh g -1 at 1000 mA g -1 ) along with stable cycle property.We believe that this simple and efficient method can be further extended to fabricate various active anode material (such as transition metal oxides, Sn and Si, etc) encapsulated in one-dimensional carbon nanofibers or carbon nanotubes for broad applications in batteries, supercapacitors and catalysts.

Figure 1
Figure 1 illustrates schematically the synthetic procedure of MoS2/CNFs.Bulk

Figure 1 .Figure 2 .
Figure 1.Schematic illustration of the fabrication of 1D MoS2/CNFs Figure 2g-j shows a typical scanning transmission electron microscopy (STEM) bright field image and elemental mapping analysis of the 1D MoS2/CNFs, where carbon, molybdenum and sulfur species are all distributed in this area.More importantly, the free-standing 1D MoS2/CNFs can be Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 28 December 2016 doi:10.20944/preprints201612.0133.v1easilycut into flexible tablet and directly used as binder-free anode for lithium-ion batteries (FigureS4).

Figure 3 .
Figure 3. (a) XRD pattern of the resultant MoS2/CNFs and bulk MoS2 samples.(b) The weight loss occurs before 100 o C, attributing to the evaporation of physically adsorbed water.The main weight loss occurs in the range of 300-500 o C because of the consumption of carbon and the oxidation of MoS2 to MoO3 in air.According to the final yield of MoO3, the content of MoS2 encapsulated in the CNFs is more than 27.6%.

Figure 4 (
Figure 4 (a) XPS spectrum of the 1D MoS2/CNFs.High-resolution XPS spectra of C

preprints.org) | NOT PEER-REVIEWED | Posted: 28 December 2016 doi:10.20944/preprints201612.0133.v1
[22] o , 32.6 o and 39.6 o , corresponding to the (002), (100), (103) planes of MoS2[22].It should be noted that the peak of MoS2/CNFs at 14.2 o is much weaker than that of bulk MoS2, suggesting the bulk MoS2 has been successfully exfoliated into two-dimensional nanosheets.There is a broad and weak peak at about 25 o , which refers to the typical diffraction peak of amorphous carbon.There are no impurities are detected by XRD analysis, demonstrating the high crystallinity and phase purity of the resultant 1D MoS2/CNFs.The type-IV hysteresis loop of the isotherm with pronounced adsorptions Preprints (www.

28 December 2016 doi:10.20944/preprints201612.0133.v1
is caused by the unavoidable surface oxidation of MoS2 in carbonization process.The high-resolution S 2p spectrum can be fitted to two types at 164 and 162.7 eV, Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted:

Table S2 .
The solid electrolyte interface resistance (Rf, 4.07 Ω) and charge transfer resistance (Rct, 33.65 Ω) of MoS2/CNFs are much lower than that of bulk MoS2 (Rf,16.24Ω; Rct, 228.3 Ω).The fast diffusion of electron and high electrochemical activity for lithium storage in MoS2/CNFs sample (PAN) were dissolved in the solution by stirring at 40 o C for 12 h.Subsequently, above Preprints (www.