Carbon Nanofiber Elastically Confined Nanoflowers: A Highly Efficient

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Energy, Environmental, and Catalysis Applications

Carbon Nanofibers Elastically Confined Nanoflowers: A Highly-efficient Design for Molybdenum DisulfideBased Flexible Anodes Toward Fast Sodium Storage Qiao Ni, Ying Bai, Shuainan Guo, Haixia Ren, Guanghai Chen, Zhaohua Wang, Feng Wu, and Chuan Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21729 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Applied Materials & Interfaces

Carbon Nanofibers Elastically Confined Nanoflowers: A Highly-efficient Design for Molybdenum Disulfide-Based Flexible Anodes Toward Fast Sodium Storage Qiao Ni †, Ying Bai*†, Shuainan Guo†, Haixia Ren†, Guanghai Chen†, Zhaohua Wang†, Feng Wu† ‡, Chuan Wu*† ‡

†

Beijing Key Laboratory of Environmental Science and Engineering, School of

Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR China ‡

Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, PR

China KEYWORDS: flexible electrode, ex-situ electrospinning, 2D energy materials, MoS2, sodium-ion battery ABSTRACT 2D Energy Materials have been widely applied in advanced secondary batteries, among which molybdenum sulfide (MoS2) are attractive due to the potential for high capacity and good rate performance. The relatively low electronic conductivity and irreversible volume expansion of pure MoS2 still need to be improved. Here, a facile and highlyefficient ex-situ electrospinning technique is developed to design the carbon nanofibers elastically confined MoS2 nanoflowers flexible electrode. The flexible freestanding electrode exhibits enhanced electronic conductivities and ionic diffusion coefficients, leading to a remarkable high specific capacity (596 mAh g-1 at current density of 50 mA g-1) and capacity retention (with 89% capacity retention after 1100 cycles at 1 A 1

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g-1). This novel idea underscores the potential importance of fabricating various flexible devices other than sodium-ion battery.

1. INTRODUCTION Recently, flexible electronic devices continue to be an exceedingly active field in the fields of communication, implantable medical, and control technology. As one of the power sources in electronic devices, high-performance flexible batteries play an indispensable role in it. The main challenge for flexible batteries is to maintain favorable flexibility of electrodes while taking into account resource and environmental issues. For this purpose, different battery systems such as flexible solar cells,1 fuel cells,2 lithium-sulfur battery3 and lithium-ion batteries (LIBs) have been carried out.4 Among them, flexible lithium-ion batteries are currently the most active direction. However, the excessive consumption of lithium resources and the rise in price have brought increasing concerns.5, 6 Thus, it remains uncertain whether LIBs could satisfy the rising requirements for flexible electronic devices.7 Due to the extensive and easy availability of sodium resources, it can be expected that flexible SIBs will bring more possibilities to the field of flexible devices.8, 9 Currently, various flexible substrates have been applied for SIBs, including graphene-based,10 carbon cloth based,11 and carbon nanofibers (CNFs) based substrates. 12, 13

Compared with traditional fabrication processes by hard current collectors, such

carbonaceous material-based substrates can not only reduce the weight of electrodes but can also significantly improve the overall power energy density significantly. 14, 15 Besides, the preparation of flexible freestanding electrode without using of binder, 2


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conductive carbon, and hard current collector, would avoid complex interfacial reactions. Whereas, how to produce flexible film materials with high efficiency while maintaining good mechanical properties is till challenge in current research. In terms of electrode materials, two-dimensional materials especially those that undergo ion insertion/extraction reaction are widely applied in energy storage and conversion field.16, 17 Among them, molybdenum disulfide (MoS2) has received surging research interests in electrocatalysis, supercapacitor, and LIBs.18 As anode for SIBs, MoS2 has recently been explored as promising anode due to the high theoretical capacity (about 670 mAh g-1 thanks to a four-electron conversion reaction), the broad interlayer spacing between neighboring layers.19 And the exfoliated MoS2 nanosheets can further enhance the ion diffusion rates. However, the pure MoS2 electrode usually tends to decay rapidly during cycling, owing to the extremely low electronic conductivity and tremendous volume expansion.20 In this study, an ex-situ electrospinning technique was developed to fabricate MoS2 based flexible freestanding electrode as an anode for SIBs for the first time (here the MoS2 and CNFs overall composite electrode is denoted as MoS2-F). Unlike the traditional electrospinning method to prepare one-dimensional (1D) structure materials by encapsulating nanosized materials (less than 100 nm) in CNFs with an in-situ chemical routine,21-24 we fabricated the special 3D CNFs elastically confined nanoflower structure by directly dispersing the pre-prepared large-sized MoS2 powder into spinning solution. Subsequently, the flexible film can be efficiently obtained through an electrospinning process followed with suitable carbonized treatment. 3



Thanks to the sandwich structured CNFs substrates, the prepared MoS2-F electrode reveals the following advantages: (1) the flexible freestanding film can be directly cut as anodes for SIBs without binders, conductive additives, and hard current collectors, leading to a higher energy density than that of pasted plate electrodes; (2) the crosslinked CNFs can prominently enhance the mechanical property and electronic conductivity of pure MoS2; (3) The nitrogen decomposed from PAN can form more active sites on the CNFs, which much improved sodium ions diffusion kinetics. Compared with the MoS2-raw electrode, the obtained MoS2-F electrode also exhibits better cyclability and rate capability. It is worth noting that such a highlyefficient design for flexible freestanding electrode by the ex-situ electrospinning technique can fabricate flexible electrode regardless of the solubility of the raw materials in the solution and size of the active material particle, which is anticipated to be widely applied to prepare various flexible electrodes for various systems. 2. EXPERIMENTAL SECTION 2.1. Materials preparation Synthesis of MoS2-precursor 2 mmol ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O) and 60 mmol thiocarbamides (CH4N2S) were dissolved in 70 mL deionized water. Thereafter, the reaction solution was transferred into the 100 mL Tafel-line stainless steel autoclave with a heat preservation of 14h at 220 °C. Afterward, the as-collected precipitate was thoroughly washed, and dried in vacuum to get the MoS2-precursor. Synthesis of MoS2-F, MoS2-raw and MoS2@PAN Firstly, 0.9 g PAN (Mw = 150 000, 4


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Alfa) and 12 mL DMF solution were mixed with vigorously stirring to form a 7.5 wt% solution. Next, 0.7g prepared MoS2-precursor powders were added into the above solution with magnetic stirring for 6h and then a ultrasonication process (100 kHz) for 2 h at constant 30oC. Subsequently, the homogeneous colloidal solution was transferred into a 10 mL plastic syringe. Then, the push speed of the solution was controlled at 0.1 mm min−1 with a working voltage at 20 kV. The film thickness can be precisely control through a translation process. The MoS2 flexible film was finally collected at an aluminum foil. The as-spun flexible film was first stabilized in air at 280 °C for 2 h, and then carbonized at 600 °C for 6 h in high purity argon atmosphere to finally obtain the MoS2-F film. For comparison, the MoS2-raw sample was collected by carbonizing the MoS2precursor powder at 600 °C for 6 h in high purity argon atmosphere directly. For pure CNFs, the 7.5 wt% solution was applied for electrospinning according to the same conditions of MoS2-F. Next, the collected PAN film was thermally treated the same as MoS2-F. The thickness of CNFs is approximately 42 μm, and the mass is about 0.67 mg. In addition, 0.9 g PAN powders were mixed with 0.7 g prepared MoS2-precursor powders, and then the mixed powders were carbonized at 600 °C for 6 h in high purity argon atmosphere to finally obtain the MoS2@PAN. 2.2 Material characterization The X-ray diffraction (XRD) was performed between10° to 80° by Rigaku Ultima IV-185 equipped with Cu Ka radiation at a scan rate of 1° min-1 to characterize crystal structures of the samples. The microstructures of all samples were investigated by the 5



field-emission scanning electron microscope (FE-SEM, HITAS-4800) and highresolution transmission electron microscopy (HR-TEM, Hitachi H-800). The Energydispersive X-ray (EDX) was applied to characterize elements distribution. Raman spectra were taken on the LabRAM HR800 equipment with a 532 nm laser. X-ray photoelectron spectroscopy (XPS) was conducted on a PHI Quantera II SXM to explore the valence of the surface samples. The binding energy was calibrated with C1s (284.6 eV). The contact angle was measured on the OCAH200 Contact Angle Meter. The mechanical behavior of the MoS2-F films was characterized by atomic force microscopy (AFM, Oxford Instrument) by the tip indentation technique. Carbon content analysis was determined with the TG-DSC analyzer from 30 °C to 600 °C in the air. 2.3 Electrochemical characterizations The prepared MoS2-F flexible film was first cut into circular discs (The radius of the circular disc is about 1.1 cm with a mass loading ~1–1.2 mg·cm–2), and then directly applied as freestanding electrodes. For MoS2-raw and MoS2@PAN, the working electrode (The mass loading is ~0.8–1.0 mg·cm–2) was fabricated by coating the slurry on a copper foil with the mass ratio of MoS2, conductive carbon (Super P, NO. EQ-LibSuper P from MTI inc. China) and binder (PVDF) of 8:1:1. Subsequently, the standard CR2025 coin cells were assembled in glovebox. Here, 1 M NaClO4 in EC–DMC (1:1 w/w) with 5% FEC was applied as a liquid electrolyte. Glass fibers (Whatman GF/C NO.1822-047) were used as separators, and sodium pieces were acted as both counter and reference electrode. Galvanostatic charge-discharge and GITT were performed on 6


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a Land testing system (CT2001A, Wuhan, China) with a potential window from 0.01 to 3.0 V (vs. Na+/Na). CV was carried out on the electrochemical workstation (CHI 660e). 3. RESULTS AND DISCUSSION The MoS2-F flexible film was prepared by an ex-situ electrospinning technique followed with carbonization process, illustrating in Figure 1. Firstly, a homogeneous spinning solution is configured by mixing the pre-prepared MoS2-raw powder and PAN with a suitable amount of DMF solution. Then the spinning process can be carried out under well-controlled conditions. Finally, the flexible MoS2-F film can be peeled off from the receiver and then obtained by a two-step annealing treatment. Furthermore, the bendable performance of MoS2-F electrode is also shown in Figure 1. More experimental details are provided in the Experimental Section. Figure 2a displays the morphology of the as-prepared MoS2-raw sample according to SEM, revealing the nanoflowers type (with average diameter of 300 nm) assembled by curled nanosheets. The morphology of MoS2-F under different magnification are presented in Figure 2b and 2c. As shown in Figure 2c, the CNFs are continuous and smooth, showed an average diameter of around 150 nm. The MoS2 nanoflowers are elastically confined by CNFs, forming the hierarchical three-dimensional (3D) network. Furthermore, the cross-section of the MoS2-F film is exhibited in Figure 2d. The film thickness is about 45 µm, and the MoS2 nanoflowers insert into the CNFs formed a sandwich architecture. Such a special structure can prevent the agglomeration of MoS2 nanoflowers and promote the infiltration of electrolyte efficiently. Specifically, it's 7



highly efficient to fabricate flexible self-supporting electrode both of ultra-lightweight and high mass-loading.25, 26 Accordingly, from the HRTEM of MoS2-raw (Figure 2e) and MoS2-F (Figure 2f), the interplanar placing of 0.65 nm and 0.71 nm can be obviously observed for MoS2-raw and MoS2-F, respectively. This interplanar placing can well correspond to (002) plane of the standard Molybdenite-2H structure. The reason for the increase in the interlayer space is easy to understand. Before electrospinning, MoS2 powder was dispersed into PAN-based electrospinning solution. In the meanwhile, the interlayer spacing of MoS2 will be inserted of certain amounts of PAN.27 During carbonizing at 600 oC, various gases such as hydrogen(H2), methane (CH4), H2O and NH3 will be released from the decomposition of PAN,28, 29 leading to the expansion of interlayer space of MoS2. This expanded interlayer spacing of MoS2F will significantly increase the diffusion rate of sodium ions and further enlarge the contact area of electrode and electrolyte.21 According to TEM energy-dispersive X-ray (EDX) mapping (Figure 2g), the Mo, S, C, and N elements are uniformly distributed in MoS2-F nanoflowers and CNFs. Here N and C elements are derived from the thermal decomposition of the PAN,30 which are not detected in the compared sample of pure MoS2-raw (Figure S1, Supporting Information). Thanks to such 3D flexible substrate of CNFs, the prepared MoS2-F flexible selfstanding film is expected to be good electrolyte wettability. Here the wettability between the electrode and electrolyte can be evaluated with contact angle according to the Laplace–Young's equation.31 Compared with MoS2-raw electrode (coated on copper foil), the contact angles were collected between electrolyte (1 M NaClO4 in EC–DMC 8


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(1:1 w/w) with 5% FEC) and prepared electrode. As displayed in Figure 3a and 3b, the contact angle of MoS2-F is nearly 0o, which means that the electrolyte can completely spread out in MoS2-F electrode. However, the contact angle of MoS2-raw (~10.8o) is obviously larger than that of MoS2-F, indicating a partially wetted by the electrolyte. The outstanding electrolyte wettability is anticipated to promote electrolyte penetration performance and enable fast dynamic of sodium ions. XRD patterns of MoS2-F, MoS2-raw, and CNFs are presented in Figure 4a. The peaks of MoS2-F and MoS2-raw can be well indexed to the standard card of hexagonal MoS2 (JCPDS #37−1492) well, indicating a well-preserved crystal structure of MoS2. The wide diffraction peak of MoS2-F at around 25o should be ascribed to the graphene carbon, corresponding to the diffraction peak of the compared sample of CNFs. The magnified (002) diffraction peak of MoS2-F shows a slight shift to the lower angle than that of MoS2-raw (Figure 4b). According to Bragg’s formula (d = 0.5λ/sin(θ)), this shift demonstrates the enlarged interlayer space of MoS2 after electrospinning.32 The Raman spectra of MoS2-F, MoS2-raw, and CNFs are displayed in Figure 4c. The distinct peak at around 1360 cm-1 can be indexed to the D-band of carbon materials, and the 1584 cm-1 one should be ascribed to the G-band.32 The peak separation (Δk) between E12g and A1g peaks can be taken to evaluate the number of layers in MoS2. Here the detected value of Δk for MoS2-F is 27.53 cm-1, which is smaller than 30.42 cm-1 of MoS2-raw. This obvious diminished number of layers can be attributed to the expanded interlayer spacing (Figure 4d), leading to a weaker van der Waals force between neighbor interlayers and a stronger out-of-plane vibration.25 Note that, those 9



results are comparable to the above TEM and XRD observation results. To get further insight into the chemical configurations and bonds between MoS2 and CNFs, XPS was performed. It is observed that the survey spectra display peaks of Mo, S, N, C and O elements without other impurities (Figure 5a). As to the highresolution of deconvoluted Mo 3d spectrum (Figure 5b), two characteristic peaks at 232.14 and 229.08 eV can be assigned to the Mo4+ 3d3/2 and Mo4+ 3d5/2, respectively. Here we point out that the peak at 235.17 eV should come from the surface oxidation of Mo4+ to Mo6+ in the air.34A pair of peaks in the 163.05 and 161.83 eV correspond to the divalent sulfur ions (S2+) in S 2p1/2 and S 2p3/2, respectively (Figure 5c).32 As shown in Figure 5d, the C 1s spectrum can be deconvoluted into three regions, indexing to NC=O (288.60 eV), C=N (285.83 eV) and C-C (284.60 eV) bonds, respectively.35 The sp2 carbon atoms with surface N-doping can provide an ideal template for the van der Waals epitaxy of MoS2 because there is no dangling bond.36 Besides, the fitting curves of N 1s spectrum can be deconvoluted into graphitic N (400.07 eV), pyrrolic N (398.96 eV), pyridinic N (398.17 eV), and Mo-N (395.55 eV) (Figure 5e).37 The calculated content percentages of pyridinic N and graphite N is as high as 45.65% and 41.64%, respectively (Figure 5f). The pyridinic N usually forms on edges of defects sites by substituting carbon atom with N atom,38 and the enriched defects will provide more electrochemically active sites, increasing the Na+ diffusion rate potentially.39 For another, graphite N is a good electronic donor, which will further enhance the electronic conductivity of the overall electrode. For the purpose of further characterize the surface state and mechanical property 10


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of the MoS2-F flexible film, atomic force microscopy (AFM) was tracked to characterize the 3D surface morphology and Young’s modulus of MoS2-F. Figure 6a presents the topography image of simultaneously obtained morphology for MoS2-F by AFM. The surface topography of the film can be well in line with SEM images: the cross-linked CNFs elastically confined MoS2 nanoflowers. Corresponding, the mechanical behavior of the MoS2-F films was characterized on by the tip indentation technique on AFM, the collected Young’s modulus is mainly distributed in ~2.0-2.6 Gpa (Figure 6b), showing a favorable mechanical performance.40,

41

Such good

mechanical performance can be attributed to the highly cross-linked CNFs derived from the carbonization of the cross-linked polymer of PAN. The composition of the MoS2-F has been analyzed by thermogravimetric analysis (TGA) (Figure 6c), the MoS2-F underwent significant weight loss of about 52.7% mainly below 430 oC. Considering the combustion of PAN and fully oxidation of MoS2 to MoO3,42 the calculated content of MoS2 in the as-prepared MoS2-F film is approximately 52.6% (assuming that the final product is MoO3). Therefore, the residuum can be ascribed to the content carbon.43 The relatively large amount of carbon in the flexible film could boost the elasticity of the MoS2-F electrode. Note that, it is challenge to prepare the film both of excellent mechanical properties and high mass-loading of active materials. To the best of our knowledge, the content of active materials in the film can be actually regulated by changing the content of PAN in the electrospinning solution. Besides, we can improve the mass ratio of active material and CNFs via preparing large-sized particle instead of the nanosized ones, or applying different polymers (with less carbon redundant after 11



calcination). The results of measured electronic conductivity indicate that MoS2-F is as high as 7.59 E-04 S cm-1, which is about 38-fold higher than that of MoS2-raw (1.99 E05 S cm-1), as shown in Figure 6d. The enhanced electronic conductivity should be mainly attributed to the conductive N-doped CNFs, which endows impressively effects on improving cyclic and rate performance. In order to evaluate the electrochemical properties of MoS2-F as anodes for sodium-ion batteries, sodium metal was used as the counter electrode to assemble the half-cell. Figure 7a shows the CV curves of the initial three cycles for MoS2-F electrode at the scan rate of 0.1 mV s−1 with the potential window from 0.01 to 3.0 V (vs. Na+/Na). For the first cathodic process, the irreversible peak at around 1.08 V can be indexed to the decomposition of the electrolyte, leading to the formation of SEI for the overcharge at high potential.44, 45 The peak above 0.4 V indicates the phase transformation from MoS2 to NaxMoS2 (MoS2 + xNa+ → NaxMoS2), and the peak below 0.4 V can be attributed to the formation of Na2S and Mo (NaxMoS2 + (4-x)Na+ → 2Na2S + Mo).46 For the anodic process, the peak at about 0.40 V and 1.76 V can be ascribed to the reconstruction of NaxMoS2 (2Na2S + Mo →NaxMoS2 + (4-x)Na+) and formation of MoS2 (NaxMoS2 → 1T-MoS2 + xNa+).47, 48 The subsequent two cycles are almost completely coincident, demonstrating the high reversibility of MoS2-F electrode. Figure 7b and Figure S2a show the galvanostatic charge-discharge curves of MoS2-F and MoS2-raw at a current density of 50 mA g-1 between 0.01 and 3 V (vs. Na+/Na). The initial discharge and charge specific capacity of MoS2-F are 978 and 598 mAh g-1 with an initial coulombic efficiency (ICE) approximately 61.1%. The large irreversible 12


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capacities can be interpreted as the formation of the SEI and sodium ions insert into the vacancies of carbon, irreversibly.49-51 Although the ICE of MoS2-F seems a little lower than that of MoS2-raw (ICE=69.5%), it keeps a relatively stable average operating voltage in the following cycles. Note that, here we calculated the mass of the active material according to the total weight of the MoS2-F. As displayed in Figure 7c, the electrode of MoS2-F delivers a high specific capacity of about 525 mAh g–1 even after 70 cycles at the current density of 50 mA g-1, and the coulombic efficiencies remain over 97%. Whereas, the specific capacity of MoS2-raw dramatically decays, only remains 96 mAh g-1 after 70 cycles (Figure S2b). What’s more, the compared pure CNFs can only deliver a relatively low reversible capacity of 142 mAh g-1 at the first cycle (Figure S3a). The rate performance of MoS2 was also evaluated in the range of different current densities and shown in Fig. 7d. At the current density of 50, 100, 200, 500, 1000 and even 2000 mA g-1, the maximal charge capacities for MoS2-F are 596, 475, 427, 331, 251 and 186 mAh g-1, respectively, which are obviously higher than that of MoS2-raw (Figure S2c) and CNFs samples obviously (Figure S3b). Once the current density returns to 50 mA g−1, the charge capacity can still up to 526 mAh g−1, suggesting excellent Na+ diffusion kinetics and superior structural stability of MoS2-F. Especially, even at a high current density of 1 A g-1, the MoS2-F can still keep the reversible specific capacities of 243 mAh g-1 after an ultralong cycle life of 1100 cycles with 89% capacity retention, demonstrating the outstanding rate performance and cycling stability. Note that, there exists a capacity raising process in the front 400 cycles, which can be 13



ascribed to the active process during high current density.25, 52 Such capacity raising processes were commonly observed in many carbon‐based metal oxide composites, which can be responsible for the enhanced kinetics of Na+ between active material and electrolyte, and the formation of gel-like polymeric layer.53, 54 Since our prepared MoS2 particle is a nanoflower assembled with stacked nanosheets, in the high current density (1A g-1 in this work), the intercalation process of sodium ions will be more difficult than that in the low current density process.55 However, the capacity of MoS2-raw dramatically decays after about 50 cycles at 1 A g-1 (Figure S2d). To demonstrate the special effects of the PAN-derived carbon nanofibers on the electrochemical performance, another contrast experiment was conducted by directly mixing PAN powders with MoS2-raw and then preparing the anode (denoted as MoS2@PAN) for SIBs. However, the capacity is rapidly fading from 422 to 63 mAh g-1 after 70 cycles even at 50 mA g-1(Figure S3c), and only a low capacity of about 40 mAh g-1 can be obtained at high rate of 1 A g-1 (Figure S3d). Compared with the MoS2-F electrode, the traditional grinding process will result in the morphology damage severely, and particle agglomeration of the nano-sized MoS2 particle, which can be responsible for the poor dynamic and low capacity of MoS2@PAN. Furthermore, even under a high current density of 2 A g-1, a stable reversible specific capacity of 205 mAh g-1 can be obtained after 300 cycles with a short active process, demonstrating the outstanding rate performance and cycling stability (Figure S4). Compared with the recent reported references, this long cycle stability and high capacity is pretty competitive (Table S1). To get insight into the reversibility, the morphology of MoS2-F and MoS2-raw 14


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after 1100 cycles are also studied. As revealed in Figure S5a and S5b, the original appearance and hierarchical morphology of MoS2-F still retain, and the overall structure of carbon nanofibers elastically confined MoS2 nanoflowers still homogeneous distributes. However, the particles of MoS2-raw are aggregated severely over repeated sodiation/desodiation precesses, which could responsible for the rapid capacity decay of MoS2-raw electrode. Such results disclose that the designed 3D highly cross-linked N-doped CNFs play a significant role in maintaining the structural integrity of the MoS2-F electrode. Combined with the N-doped defect sites, it can further boost more storage space for sodium ions. Therefore, the stable 3D networks can greatly improve the electronic conductivity of MoS2, leading to superior electrochemical performance.56 To further explain the reasons for the improved electrochemical characteristics of MoS2-F, cyclic voltammetry (CV) at various scanning rates was conducted. It has been reported that capacitive storage will be beneficial to high rate performance, and Cyclic voltammetry (CV) is a useful tool to separate diffusion and capacitive-controlled contributions.57 As revealed in Figure 8a, the shape at different sweep rates keeps well from 0.1 to 0.7 mV s−1. Based on the equation (1):

(1)

𝑏

i = a𝜐

Here i is measured current, υ is sweep rate, a and b are constants. As can be seen in Figure 8b, the calculated b values are all between 0.5 and 1.0, indicating a more favored capacitive-controlled contributions.58 In addition, the total current can be quantitatively 15



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detached into two parts:

𝑖(𝑉) = 𝑘1υ + 𝑘2𝜐1/2

(2)

Where i (V) is the total current at a fixed potential, k1 υ represents capacitive effects, k2 υ1/2 represents diffusion-controlled insertion. In Figure 8c, the capacitive-controlled region is shaded in the total measured current at 0.7 mV S-1. The calculated capacitive contribution is up to 57.15%. Especially, Figure 8d summarizes the pseudocapacitivecontrolled contributions and diffusion-controlled contributions to the total current of MoS2-F electrode at scan rates of 0.1, 0.3, 0.5 and 0.7 mV s-1. The capacitive contribution is 51.17, 52.42, 55.20 and 57.15%, respectively. Such high capacitive contribution should be attributed to the enlarged interlayer spacing after electrospinning and the enhanced electronic conductivity, which in turn verifies the reasons of the high rate performance.59 To get further analyze of the Na-diffusion kinetics, GITT was applied to analyze the MoS2-F and MoS2-raw electrodes at the current density of 50 mA g-1 within the potential range of 0.01-3 V in the first discharge-charge process. Figure 9a shows the GITT curves of MoS2-F and MoS2-raw electrodes in the initial charge process up to 3.0 V. The cells were charged both for 10 min and followed with an open circuit relaxation for 1 h. The apparent sodium ion diffusion coefficients (DNa+) can be calculated based on the following equation:60

4 𝑚𝑉𝑚 2 ∆𝐸𝑠 2 ∆𝐸𝜏 𝑀𝐴

𝐷𝑁𝑎 + = 𝜋𝜏

( )( ) 16


(3)

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Where M and m represent molar mass and mass of active materials; Vm is molar volume (can be calculated from cell parameters); A is the active surface area of the electrode; τ is pulse duration. ΔEs and ΔEτ are potential variations of quasi-equilibrium potential and potential variation during a constant current pulse.61, 62 The calculated DNa+ under the first charge process is clearly presented in Figure 9b. The DNa+ of MoS2-F electrode is in the magnitude order of 10−11 -10-10 cm2 s−1, which is notably higher than that of MoS2raw (10−12 -10-11 cm2 s−1). The DNa+ values for both electrodes at around 1.8 V are obviously lower than those at other potential range due to the oxidation and reduction reactions.63 The higher DNa+ of MoS2-F can be ascribed to the following reasons: (1) the expanded interlamellar spacing enable less resistant for the extraction of sodium ions; (2) the enriched defects originated from pyridinic nitrogen can provide more electrochemically active sites; (3) the outside CNFs can elastically confined MoS2 particle, which is beneficial to preventing particle agglomeration and indirectly shorten the diffusion path of Na+ ions. 4 CONCLUSIONS In summary, a simple and effective ex-situ electrospinning technique has been developed for preparing the carbon nanofibers elastically confined MoS2 nanoflowers as flexible electrode for SIBs. Thanks to the highly cross-linked and superior mechanical property of CNFs from the thermal decomposition of PAN, the MoS2-F flexible film can be directly applied as freestanding anode for SIBs without using binders, conductive additives, and hard current collectors. Compared with the pure 17



MoS2-raw, the MoS2-F exhibits much improved electrochemical properties: when cycled at a current density of 1 A g-1, the MoS2-F electrode exhibits a high reversible capacity (243 mAh g-1 after 1100 cycles) and exceptional high-rate capability (183 mAh g-1 even at 2 A g-1). Through various characterization methods (TEM, Raman, XPS, AFM, Contact angle, GITT et.al), we confirmed that the enhanced electrochemical properties can be attributed to the following reasons including the enlarged interlayer spacing, favorable electrolyte wettability, high content of pyridinic N and graphite N doped CNFs, significantly improved electronic conductivities and ionic diffusion coefficients. Such an ex-situ electrospinning technique can efficiently fabricate flexible electrode regardless of preparing particles size in nanoscale, which is anticipate to bring a unique perspective in designing and fabricating flexible electrodes at various systems beyond SIBs.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx TEM images and EDX mappings of MoS2-raw sample; electrochemical performances of MoS2-raw, pure CNFs, and MoS2@PAN sample; long-term cyclic performance of MoS2-F electrode at 2A g-1; SEM images of MoS2-F and MoS2-raw after long-term cyclic; the summary of electrochemical performances of the relevant MoS2-based anode materials for SIBs. AUTHOR INFORMATION 18


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Corresponding Author *E-mail:

[email protected] (Y. Bai)

*E-mail:

[email protected] (C. Wu)

ORCID Ying Bai: 0000-0003-3645-4357 Chuan Wu: 0000-0003-3878-179X Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (Grant No. 2015CB251100).

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Figure 1. Schematic illustration of the preparation process of MoS2-F and the corresponding flexible display.

26


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Figure 2. SEM images of MoS2-raw(a) and MoS2-F (b, c); (d) Cross section of MoS2F film; HRTEM images of MoS2-raw(e) and MoS2-F(f) and the inset in (e) and (f) is the corresponding line profile of the framed area, respectively; (g) TEM image of MoS2-F

and corresponding EDX mappings of C(red), S(green), Mo(purple) and N(yellow).

Figure 3. Contact angles between electrodes and electrolyte of MoS2-F(a) and MoS2raw(b).

27



Figure 4. (a) XRD patterns of MoS2-F, MoS2-raw, and CNFs; (b) Enlarged figure of (002) diffraction peak; (c) Raman spectra of MoS2-F, MoS2-raw, and CNFs; (d) Structural models of the MoS2 with different interlayer spacings

Figure 5. (a)XPS survey spectrum of MoS2-F; High-resolution XPS spectra of Mo 3d 28


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(b), S 2p(c), C 1s(d), N1s(e) in MoS2-F; (f) The ratio of different N types in MoS2-F.

Figure 6. (a) AFM topography image of the MoS2-F film; (b) Young’s modulus of the MoS2-F film; (c) TG curve of prepared MoS2-F in the air; (d) Electronic conductivities of prepared MoS2-F film and MoS2-raw electrode.

29



Figure 7．(a) CV curves of the initial 3 cycles for MoS2-F electrode at the scan rate of 0.1 mV s−1; (b) Charge-discharge curves of the initial 3 cycles for MoS2-F electrode; (c) Cycle performances of MoS2-F electrode with a current density of 50 mA g-1; (d) Rate performances of MoS2-F electrode; (e) Long-term cycling performances at 1A g-1 for MoS2-F electrode.

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Figure 8. (a) CV curves of MoS2-F at the scan rates from 0.1 to 0.7 mV s-1; (b) b values (line slope) according to Log(i) and Log(υ) the oxidation peaks and reduction peaks; (c) Capacitive storage contributions to total measured current at the scan rate of 0.7 mV s-1;(d)Pseudocapacitive-controlled contributions and diffusion-controlled contributions to the total current of MoS2-F electrode at scan rates from 0.1 to 0.7 mV s-1

Figure 9. (a) GITT curves of MoS2-F and MoS2-raw electrodes in the first charge process; (b) the corresponding Na+ diffusion coefficients during charging process for 31



MoS2-F and MoS2-raw electrodes.

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Carbon Nanofiber Elastically Confined Nanoflowers: A Highly Efficient

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