CoS2 Nanoparticles Wrapping on Flexible Freestanding Multichannel

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CoS Nanoparticles Wrapping on Flexible Freestanding Multichannel Carbon Nanofibers with High Performance for Na-Ion Batteries Yuelei Pan, Xudong Cheng, Yajun Huang, Lunlun Gong, and Heping Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10173 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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ACS Applied Materials & Interfaces

CoS2 Nanoparticles Wrapping on Flexible Freestanding Multichannel Carbon Nanofibers with High Performance for Na-Ion Batteries Yuelei Pan, Xudong Cheng, Yajun Huang, Lunlun Gong∗, Heping Zhang∗ State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230027, PR China

ABSTRACT Exploration for stable and high-powered electrode materials is significant due to the growing demand for energy storage and also challengeable to the development and application of Na-ion batteries (NIBs). Amongst all promising electrode materials for NIBs, transition-mental sulfides have been identified as potential candidates owing to their distinct physics-chemistry characteristics. In this work, CoS2 nanomaterials anchored into Multichannel Carbon Nanofibers (MCNFs), synthesized via a facile solvothermal method with sulfidation process, are studied as flexible free-standing electrode materials for NIBs. CoS2 nanoparticles uniformly distributed in the vertical and horizontal multichannel networks. Such nanoarchitecture cannot only support space for volume expansion of CoS2 during discharge/charge process, but also facilitate ion/electron transport along the interfaces. In particular, the CoS2@MCNFs electrode delivers an impressively high specific capacity (537.5 mAh g-1 at 0.1 A g-1), extraordinarily long-term cycling stability (315.7 mAh g-1 at at 1 A g-1 after 1000 cycles), and excellent rate capacity (537.5 mAh g-1 at 0.1 A g-1 and 201.9 mAh g-1 at 10 A g-1) for sodium storage. Free-standing CoS2@MCNFs composites with mechanically flexibility provides a promising electrode materials for high-powered NIBs and flexible cells. KEYWORDS: cobalt disulfide, multichannel carbon nanofibers, flexible materials,

Corresponding author. ∗ (L.G.) E-mail address: [email protected] ∗ (H.Z.) E-mail address: [email protected]

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transition-mental sulfides, sodium-ion batteries

1. INTRODUCTION In the past several years, with the quick development of portable electronic devices and electric vehicles, electrochemical energy storage systems have attracted tremendous attention.1-2 Among these, Li-ion batteries (LIBs) and Na-ion batteries (NIBs) are two indispensable parts due to their higher lithium/sodium storage capacity and longer cycle life, enhanced safety and wide availability.3-6 Recently, how to improve the performance and promote the practical application of NIBs has come to be a hot research, which all benefit from the low cost of sodium.7-8 Its rich natural reserves, cheaper prices, suitable redox potential and similar chemistry to Li, make sodium element strategic in research of energy storage systems. While, the radius of Na+ (1.06 Å) is obviously larger than that of Li+ (0.76 Å) and consequently, there need to be more sufficient interlayer spacing for Na+, which makes it harder to identify proper electrodes materials for NIBs.7 By far, numerous alternative materials, such as carbon, metal oxides, alloys and transition-metal chalcogenides, have been developed as promising electrode materials for NIBs.9-12 Transition-metal sulfides as enormous potential anodes for sodium storage, have received significant attention due to their high theoretical capacities originating from multiple electron transfer per metal center.13-15 Douglas et al. had found that ultrafine FeS2 nanoparticles bring mechanistic advantages for batteries and present excellent capacity of 400 mAh g-1 and improved cycling.16 Furthermore, the comparisons of the electrochemical performances of CuS in various electrolytes were reported by Li et al. It is found that the sodium-ion storage performance can be greatly enhanced by using ether-based electrolyte.14,

17

Kim et al. also came to the same conclusion: Ni3S2

electrode materials in TEGDME with NaCF3SO3 (ether-based electrolyte) showed the lowest interfacial resistance and well capacity retention.18 Besides, some sulfides (MoS2 and SnS2) with special layer crystal structure and their composites with rGO (reduced graphene oxide) had been developed and applied in NIBs. Own to the


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distinctive layer structure in favor of ion intercalation and de-intercalation enables high reversible capacity.19-21 In addition, recent work showed that Na/FeS2 full cells where FeS2 was assembled as cathode, with a high energy density (> 400 Wh/Kg), far surpassing recent reports on NIBs.22 Using FeS2 as an anode would require pairing with a high-voltage cathode such as Na3V2(PO4)3 that would offer only low capacity and greater cost, whereas as a cathode, FeS2 showed greater advantages in high capacity and voltage capability enabled by Na metal plating. Cobalt sulfides, as a member of transition-mental sulfides, is a promising semiconductor material and possesses a wide range of applications like supercapacitors, dye-sensitized solar cells and lithium ion batteries.23-27 However, to our knowledge, cobalt sulfides, as electrode materials in sodium ion batteries, have been relatively less investigated. Liu et al reported the synthetic method of CoS2 with various micro/nano-structure and investigated its electrochemical performance for NIBs. Hollow CoS2 delivers a capacity of 690 mAh g-1 at 1 A g-1 in a potential range of 0.1-3.0V. However, its capacity fades quickly after 100 cycles and the potential window of 0.1-3.0 V was adjusted to 1.0-3.0 V to control its fading capacity with a stable capacity of 240 mAh g-1.28 Shadike et al prepared a CoS2/MWCNT (multi-walled carbon nanotude) nanocomposite and CoS2/MWCNT electrode delivers a high capacity of 568 mAh g-1 in ether-based electrolyte. While, the curve of long-term cycling presents a decreasing trend after 100 cycles and cannot sustain its capacity in a high current density of 1 A g-1.29 Herein, the CoS2 nanoparticles anchored into Multichannel Carbon Nanofibers (MCNFs)

composites

were

prepared

by

a

simple

solvothermal

method.

CoS2@MCNFs was designed as a self-support electrode without auxiliary additives, like carbon black, binders, and mental current collector (Al foil), which further improve the power density and volumetric energy of batteries.30 Benefiting from the innovative structure, CoS2@MCNFs electrode delivers high capacity (537.5 mAh g-1 at 0.1 A g-1), superior rate capacity (537.5 mAh g-1 at 0.1 A g-1 and 201.9 mAh g-1 at 10 A g-1), and ultralong cycle life (315.7 mAh g-1 at at 1 A g-1 after 1000 cycles) as electrode for NIBs. This design may hold great application prospect in flexible cells



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for wearable devices.

2. EXPERIMENTAL SECTION All the chemical reagents used were of analytical grade. Preparation

of

Multichannel

Carbon

Nanofibers.

A

DMF

(N,N-dimethylfor-mamide) solution (10.25 g) of 2.4 wt % PS (polystyrene) was mechanically stirred for 5 h. Then, 1.25 g of PAN (polyacrylonitrile) was added into the above solution with vigorous stirring for 12 h to form a homogeneous solution. The PAN/PS nanofibers were obtained by electrospinning with applied voltage, feeding rate and cathode-anode distance fixed at 15 KV, 8.77 μL/min and 12 cm, respectively. After that, the electrospun PAN/PS nanofibers were preoxidized at 280 °C for 3 h to improve the structural stability and form multichannel within nanofibers. The nanofibers were treated at 800 °C in Ar for 1 h in order to carbonize the PAN and finally MCNFs were obtained. 31 Preparation of CoS2@MCNFs. Typically, 0.24 g of CoCl2·6H2O and 0.17 g of thiourea were sequentially added into 70 mL ethylene glycol and were vigorous stirred for 0.5 h. Then the obtained MCNFs (35 mg) were cut into small pieces (~1×1 cm) and put into the above homogeneous solution with gently stirring at the temperature of 35 °C for 0.5 h. Afterwards, the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 12 h. After being cooled to room temperature, the as-prepared sample was rinsed with ethanol and deionized water, and then dried at 60 °C. Finally, the sample was mixed with sulfur (mass ratio of MCNFs/sulfur=3:1) and calcined under Ar at 500 °C for 0.5 h to increase the degree of crystallinity. Bare CoS2 samples were prepared with the same experimental conditions except for use of carbon substrate MCNFs. Materials Characterization. The structure information and phase purity of the as-obtained MCNFs, pure CoS2 and CoS2@MCNFs were characterized by X-ray diffraction (XRD) (Philips X’pert PRO SUPER X-ray diffractometer) using Cu-Kα radiation, Raman spectroscopy (LabRamHR, JY Company, France) and XPS test. The


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morphologies of the materials were analyzed by Field-emission scanning electron microscopy (FESEM) (JSM-6700, JEOL, Japan), transmission electron microscope (TEM) (JEM-2100F, JEOL, Japan) and high resolution TEM (HRTEM). Thermogravimetry (TG) was employed to determine the CoS2 contents in CoS2@MCNFs and the testing was carried out in air. For ex situ SEM and XRD measurements, the coin cells with the self-supported CoS2@MCNFs electrodes were disassembled in an argon-filled glove box (MBRAUN LABMASTER 130) after cycling. Subsequently, the electrode was washed in dimethyl carbonate (DMC) for several times to remove the residual electrolyte. Electrochemical Measurements. Coin cells for NIBs were assembled with CoS2@MCNFs as work electrodes directly and metallic sodium sheets as counter. Glass fibers were used as the separator, and 1.0 M NaCF3SO3 in diethylene glycol dimethylether (DEGDME) was served as the electrolyte. The galvanostatic charge/discharge behaviors was evaluated on a Neware BTS-610 test system at a voltage window of 0.4 V to 2.9 V at room temperature. Cyclic voltammetry measurements were performed on a CHI 660D electrochemical workstation (Chenhua Instrument Company Shanghai, China) at a scan rate of 0.1 mV/s. For comparison, the active material (bare CoS2), carbon black and carboxyl methyl cellulose were mixed with a weight ratio of 8:1:1 forming a homogenously slurry, and 1.0 M NaCF3SO3 in diethylene glycol dimethylether (DEGDME) was served as the electrolyte. The CoS2 electrode was characterized and tested under the same condition. The capacity of the electrode was calculated based on the total weight of samples, including MCNFs and CoS2. The total mass of electrode applied for battery testing was around 0.8 mg and the content of CoS2 within the electrode is calculated to be ~55.13 wt.%, which was confirmed by TG testing.



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3. RESULTS AND DISCUSSION

Figure 1 Illustration of the synthesis process of the CoS2@MCNFs electrode. Electrospinning of precursor nanofibers of PAN/PS, followed by carbonization to form the MCNFs paper. After facile solvothermal process, the CoS2 nanoparticles were anchored in the MCNFs. (a) Schematic illustration of the formation of CoS2/MCNFs. (b-c) Digital images of CoS2/MCNFs. The CoS2/MCNFs composite was synthesized by a solvothermal method using MCNFs as matrix and CoS2 as components and the synthesis process is shown in Figure 1a. In brief, we obtained MCNFs by electrospinning followed with carbonization in Ar. Subsequently, MCNFs along with a mixed solution of CoCl2, thiourea (Tu), and ethylene glycol were sealed in an autoclave and heated at 180 °C for 12 h. In the process of reaction, MCNFs supplied a great amount of structural defects used as nucleation sites directly for in-situ growth of CoS2 nanoparticles. As a result, CoS2 nanoparticles grew inside the channels as well as anchoring on the surface of MCNFs. Finally, the CoS2@MCNFs were obtained by mixing up CoS2@MCNFs with sulfur and calcinating at 500 °C under Ar for 2h in order to retain the unique structure and promote the crystallization of the CoS2.32 This special structure is expected to provide fast sodium-ion transport pathways and high accessibility to the electrolyte. The CoS2@MCNFs composites demonstrate good


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flexibility (Figure 1b and c) and were used as flexible free standing cathode for NIBs directly.

Figure 2 (a) XRD pattern of CoS2@MCNFs composites; (b) Raman spectra of CoS2@MCNFs composites, bare CoS2 and MCNFs; (c) XPS spectra of CoS2@MCNFs composites; (d) S 2p XPS spectra; (e) N 1s XPS spectra; (f) Schematic model of N in the MCNFs. Figure 2a shows the XRD patterns of the CoS2@MCNFs, which is almost same with that of bare CoS2 (Figure S1a). This indicating that all of the XRD diffraction peaks can be attributed to the CoS2 (JCPDS card no. 75-605). Raman spectroscopy is introduced to confirm the carbon structure information of MCNFs, pure CoS2 and



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CoS2@MCNFs. The Raman spectra of them are presented in Figure 2b. Two fairly weak peaks at about 380cm-1 and 425 cm-1 can be detected from both Raman spectra of CoS2 and CoS2@MCNFs, which is in accordance with the date of CoS2 single crystal.

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In marked contrast to CoS2, CoS2@MCNFs composite presents two

characteristic peaks at about 1351 and 1583 cm-1, which correspond to D band and G band, respectively. The ID/IG ratio of CoS2@MCNFs is 0.997, which is slightly higher than that of MCNFS (ID/IG ratio=0.978). It is indicated that additional ordered aromatic structure defects are introduced through solvothermal process. XPS test was carried out to further examine the chemical states of Co, S and N in CoS2@MCNFs. As can be seen in Figure 2c, peaks of four elements Co, S, C, N and O in the XPS spectrum were determined as expected for CoS2@MCNFs composite. The peaks of Co 2p spectrum at 779.2 eV and 794.5 eV are ascribed to Co 2p3/2 and Co 2p1/2 spin-orbit peaks of CoS2, relatively.

34

The high resolution spectrum of S 2p are

presented in Figure 2d, and the wide peak ranging from 162 eV to 166 eV could be deconvoluted into two small peaks centered at 163.4 eV and 164.7 eV, corresponding to S 2p2/3 and S 2p1/2.35 In addition, the peak at about 168.5 eV corresponds to sulfur oxide in CoS2 because CoS2 has tendencies to oxidation in air which was consistent with that in the reported literature.36-37 As a result, the XPS spectra clearly verifies the formation of CoS2 in MCNFs by a one-pot solvothermal reaction. Besides, the peak at around 400 eV in Figure 2c and the deconvolution of the N 1s spectrum in Figure 2e identified the presence of nitrogen in MCNFs, which derived from raw materials PAN and thiourea. The N content in CoS2@MCNFs composite is ~4.3 wt.%. Three fitted peaks at about 398.4, 400.4 and 402.1 eV are assigned to pyridinic-N (N-6), pyrrolic-N (N-5) and quaternary nitrogen (N-Q), respectively.38-40 The N-containing surface functional groups existed on the MCNFs surface are illustrated in Figure 2f. Lots of topological defects on the graphene layer are introduced due to the nitrogen-doping and further form a disordered carbon structure, which are not only beneficial for enhancing sodium storage properties but also accelerating rate of Na+ transfer.38 The weight composition of CoS2@MCNFs composite was determined by thermogravimetry analyses (TGA) in the air. Based on the chemical reaction of 3CoS2


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(s)

+ 2O2 (g) = Co3O4 (s) + 6S (g),32 the remained solid material after TGA is Co3O4 with

a weight percentage of 35.95% as can be seen in the TGA curve (Figure S1b). Based on this, the content of CoS2 within the composite is calculated to be ~55.13 wt.% and the areal loading of CoS2 is about 1.54 mg cm-2. Figure 3 shows FESEM and TEM images of the MCNFs, CoS2 and CoS2@MCNFs composite. Figure 3a presents a typical image of the Multichannel Carbon Nanofibers (MCNFs) with a diameter about 600 nm and a smooth surface. Figure 3b is the TEM image of MCNFs and multichannel structure about 40 nm in diameter of each channel in MCNFs can be clearly observed. As can be seen from Figure 3c, the CoS2 nanoparticles with an average size of 20-30 nm are uniformly anchoring on the whole MCNFs. The detailed microstructure feature and morphology of CoS2@MCNFs were further elucidated by TEM as shown in Figure 3d, and the CoS2 nanoparticles arrange very tightly with a small gap to each other. The fracture surface in Figure 3e gives the information of regular multichannel structure directly and the CoS2 nanoparticles not only anchor on the surface of MCNFs, but also in the channels. In contrast, the CoS2 nanoparticles prepared without MCNFs (Figure 3f) tend to grow into micron particles and form agglomerate.



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Figure 3 (a) SEM and (b) TEM images of MCNFs; (c and e) SEM and (d) TEM images of CoS2@MCNFs composites; (f) SEM images of bare CoS2.


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Figure 4 (a) HRTEM and (b) corresponding EDS elemental mapping of Co, S and C of the CoS2@MCNFs composites; (c) N2 adsorption/desorption isotherms and (d) pore size distribution curve of MCNFs and CoS2@MCNFs. In order to clarify whether there are CoS2 nanoparticles growing inside the channels of MCNFs. The CoS2@MCNFs samples were soaked in HCl (37 wt%) solution with ultrasonic treatment for 3 h. The CoS2 nanoparticles on the surface of MCNFs were partially washed away and CoS2 nanoparticles in the channels were retained for the protection of MCNFs as can be seen in Figure S2. In figure S2a and S2b, the FESEM and TEM images of the washed CoS2@MCNFs samples demonstrate that CoS2 nanoparticles grow on the MCNFs surface as well as in the multichannel compared with the unwashed samples. The unique structure not only benefit the electrochemical performance of electrodes, but also increase the active materials’ loading amount in the MCNFs. The high resolution TEM (HRTEM) image presented in Figure 4a reveals the (200) lattice plane of CoS2, with corresponding lattice spacing of 0.276 nm in the HRTEM.41 The energy dispersive X-ray spectroscopy (EDS) mapping of the CoS2@MCNFs composite (Figure 4b) indicates that C, Co and S Selements were homogeneously distributed over the hybrid materials.



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To further demonstrate the pore structure of MCNFs and CoS2@MCNFs, N2 adsorption/desorption isotherms were investigated. Figure 4c and 4d show the Type I N2 adsorption/desorption curves and pore size distribution (PSD) of bare MCNFs and CoS2@MCNFs, respectively. The Brunauer-Emmett-Teller (BET) specific surface area of MCNFs is 183.307 m2 g-1 (Figure 4c) with an average pore size ~1.5 nm (Figure 4d). After CoS2 nanoparticles grown on the MCNFs, the BET specific surface area of MCNFs increases to 452.215 m2 g-1 due to the impregnating of the ball-shaped CoS2 nanoparticles with a larger BET surface. In addition, a dramatic increase in PSD curve in the microporous region (0.5-1.5 nm) occurred (Figure 4d). This change indicates that growth of CoS2 nanoparticles on the MCNFs and the solvothermal process can introduce much more structural defects in the MCNFs, which create more additional active sites for Na-ion storage. Besides, the expanding of BET surface increase the interfacial area between electrode materials and electrolytes and the electrochemical reaction are facilitated. All of the above results prove that CoS2@MCNFs hybrid materials with distinctive structure were prepared by facile in-situ solvothermal method.


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Figure 5 Electrochemical performance of CoS2@MCNFs composites and bare CoS2. Charge-discharge cycling curves of (a) CoS2@MCNFs composites and (b) bare CoS2 in the voltage range of 0.4-2.9 V at a current density of 1 A g-1; (c) CV curves of the CoS2@MCNFs composites at a scan rate of 0.1 mV s-1; (d-e) Rate performance of CoS2@MCNFs composites; (f) Rate performance of bare CoS2; (g) Long-term Cycling performance (left) and Coulombic efficiency (right) of CoS2@MCNFs composites at 1 A g-1. CR2032-type coin cells were fabricated to evaluate the electrochemical performance of the as prepared samples, and the free-standing films (CoS2@MCNFs) were used as cathode materials for NIB directly. Figure S3a shows the



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discharge/charge curves of CoS2@MCNFs in the potential window of 0.1-2.9 V at 100 mA g-1. The capacity in the first cycle is about 570 mAh g-1, but a drastic capacity fading of ~14% can be observed in the second cycle. The rate performance of this electrode is presented in Figure S3b. The capacity gently decreases from 560 mAh g-1 to 130 mAh g-1 within 150 cycles and then stabilizes at ~130 mAh g-1. To improve battery performance and control the electrode reaction, the potential operation window of 0.1-2.9 V was adjusted to 0.4-2.9 V.28 Figure 5a and 5b display the charge-discharge curves of the CoS2@MCNFs and CoS2 electrode in the 1st, 2nd, 3rd, 50th and 100th cycles between 0.4 and 2.9 V (vs. Na+/Na) at a current density of 100 mA g-1. The initial discharge and charge capacity of CoS2@MCNFs are 641.8 and 544.7 mAh g-1 with an excellent initial Coulombic efficient (ICE) of 85% (The specific capacities of CoS2@MCNFs are based on the total mass of CoS2 and MCNFs). Moreover, the reversible capacity is around 537 mAh g-1. The high ICE and reversible capacity could be ascribed to the rational hybrid design and utilization of MCNFs as the carbon substrate. While, the initial discharge and charge capacity for bare CoS2 electrode are only 541.6 and 465.7 mAh g-1. The overlapping voltage profiles of CoS2@MCNFs electrode demonstrate wonderful cycling stability. And even after cycling for 100 cycles, the charge capacity is still as large as about 507.8 mAh g-1. As for CoS2 electrode, by contrast, not only is its first charge/discharge capacity smaller than that of CoS2@MCNFs electrode, there is also a long-lasting capacity fading resulting to a low discharge/charge capacity of 292.8/291.1 mAh g-1 after 100th cycles. These results further confirm the excellent electrochemical reversibility of CoS2@MCNFs electrode. Figure 5c shows the CV curves of CoS2@MCNFs as electrode for sodium ion batteries at a scan rate 0.1 mV s−1 in the potential range of 0.4-2.9 V. During the first scanning cycle, two reduction peaks at about 0.7 and 1.1 V and two obvious oxidation peaks centered at 1.7 and 2.0 V are observed. However, the reduction peaks shift to 0.9 and 1.5 V in the subsequent cycles. The difference between the initial conversion in the first cycle and the reversible reaction may be ascribed to the formation of solid electrolyte interphase (SEI) and the interface strain induced by the carbon nanofibers layers. Obviously, the


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crisscross carbon nanofibers create the mechanical interaction with each other in the process of electrochemical reactions and modulate the chemical reactivity of CoS2 nanoparticles toward chemical conversion.28, 42 The reduction peaks occurred at 1.5 and 0.9 V should be originated from the reaction: CoS2 + xNa+ + xe- → 2NaxCoS2 and NaxCoS2 +(4-x)Na+ + (4-x)e- Co→ Co +2Na2S, respectively.28, 43 According to the literature date 44, the CoS2/Li battery shows reduction peaks at 0.6 V and 1.0 V and the peaks remain stable at 1.1 V and 1.6 V, which are consistent with the value of CoS2@MCNFs/Na battery. The discharge curve of the CoS2@MCNFs/Na battery in Figure 5c bear such remarkable similar to that of the CoS2/Li battery in voltage range from 0.01 V to 3.0 V, implying that CoS2@MCNFs/Na battery probably conducts a similar discharge reaction mechanism to the CoS2/Li battery.29 The CV curves of the subsequent cycles are almost overlapped, demonstrating a good cycling stability of the electrode. In addition, the corresponding plateau can be observed in the discharge/charge profiles of the CoS2@MCNFs with two charge plateaus at 1.6-1.8 V and 1.9-2.1 V and two discharge plateaus at 0.8-1.0 V and 1.4-1.6 V as shown in Figure 5a. Due to that excellent rate capacity of electrode could highly efficiently cut down the discharge/charge time and improve applicability of the batteries, we investigated the rate performance of CoS2@MCNFs at multiple current densities and displayed in Figure 5d and 5e. Figure 5d presents the detailed discharge/charge profiles with current densities from 0.1 to 10 A g-1, and only modest increase in the charge plateau can be observed from small current to the large one, illustrating rapid reaction kinetics and low polarization of [email protected] In addition, the CoS2@MCNFs electrode is able to deliver steady and reliable discharge capacity of 529.7, 485.6, 404.5, 312.7, 288.4, and 249.2 mAh g-1 at current densities of 0.1, 0.2, 0.5, 1, 2, and 5A g-1 (Figure 5e). Even at a 100-fold increased current density (10 A g-1), the discharge capacity of about 201.9 mAh g-1 can be maintained, showing the superior rate capacity for CoS2@MCNFs. When reducing back current density to 0.1 A g-1, CoS2@MCNFs electrode still delivers a high reversible charge capacity of around 518.9 mAh g-1. It illustrates that CoS2@MCNFs can sustain high rate cycling test without destroying its



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integrated structure. For comparison, rate performance of the bare CoS2 electrode was also carried out as shown in Figure 5f. The bare CoS2 electrode delivers a rather large capacity of 540 mAh g-1 in the initial few cycles, while its reversible capacity loses quickly in the period of increasing the current density from 0.1 to 10 A g-1. When the current density is reduced back to 0.1 A g-1, CoS2 electrode could not maintain the stability and displays a low capacity of around 330 mAh g-1. To directly illustrate the excellent performance of electrochemical properties and superior of CoS2@MCNFs nanostructures, the long-term cycling performance was tested. Figure 5g shows the long-term cycling performance of bare CoS2 and CoS2@MCNFs electrode at 1 A g-1. It is impressive that the CoS2@MCNFs electrode displays a high discharge capacity of about 315 mAh g-1 even after 1000 cycles, which could be benefit from the stable multi-channel nanostructures of CoS2@MCNFs materials. After the initial several cycles, the coulombic efficiency is higher than 98.6% in the following cycles, which further demonstrates the great cyclability of CoS2@MCNFs electrode.

Figure 6 (a) Ex situ XRD patterns of the CoS2@MCNFs composite at 0.6 V and 2.9 V voltage conditions, respectively. (b) SEM image of the CoS2@MCNFs electrode after cycling with two inserted images, high magnification SEM image and TEM images. To analyze the sodiation/desodiation process of CoS2@MCNFs, ex situ XRD were conducted after five cycles as can be seen in Figure 6a. Parafilm is employed to protect CoS2@MCNFs from air with its XRD peaks centered at 21.8°, 24.1° and 62.3°, respectively. Peaks of MCNFs at around 25.8° can be observed for both patterns. At 0.4 V, the signal peaks of Co and Na2S appears at around 44.5° and 45.9°, respectively. When charged to 2.9 V, a very weak signal of CoS2 at 32.5° can be detected. These results confirmed the reaction mechanism of CoS2 discussed at previous content of


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CV results. Combining the CV results and XRD patterns at different charge/discharge voltages, the reaction mechanism of CoS2 can be proposed as CoS2 + 4Na+ + 4e− ↔ Co + 2Na2S. Figure 6b gives the SEM and TEM images of CoS2@MCNFs electrode after 20 cycles. No obvious structural changes are observed except that the exterior surface of CoS2@MCNFs becomes rough compared with that of original CoS2@MCNFs composites, which may be caused by SEI formation and residual electrolyte. The intact morphology remained after cycling demonstrates the good structure stability of the novel electrode materials.

4. CONCLUSION In summary, the flexible CoS2@MCNFs nanocomposites with superior sodium storage performance have been successfully prepared by in-situ solvothermal method and sulfidation process. The well-defined CoS2 nanoparticles in a diamond of around 30 nm are evenly grown in the MCNFs with mighty adhesion, which offers an ample space as well as a structurally stable and multichannel host for Na+ intercalation and deintercalation. Impressively, CoS2@MCNFs nanocomposite, as abode for NIBs, delivers a high specific capacity of 537.5 mAh g-1 at 0.1 A g-1, excellent rate capacity (537.5 mAh g-1 at 0.1 A g-1 and 201.9 mAh g-1 at 10 A g-1), and extremely long cycle life (315.7 mAh g-1 at at 1 A g-1 after 1000 cycles) for sodium storage. The three-dimensional space in the vertical and horizontal multichannel networks, as well as the unique nanoparticles of CoS2 anchored in the MCNFs are responsible for the conspicuous electrochemical properties of the CoS2@MCNFs. This well-designed flexible free-standing electrode with outstanding electrochemical performance provides a bright prospect for flexible electrodes with high power and energy density.

ASSOCIATED CONTENT Supporting Information XRD pattern of the bare CoS2, TGA curves of CoS2@MCNFs composites and bare MCNFs, SEM image and TEM image of treated CoS2@MCNFs composites with HCl



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solution, electrochemical performance in potential window of 0.1-2.9 V, comparison of the Na-storage performance with reported Transition-metal sulfides-based electrode materials

AUTHOR INFORMATION Corresponding Author ∗ (L.G.) E-mail address: [email protected] ∗ (H.Z.) E-mail address: [email protected]

ACKOWLEDGMENTS The authors deeply appreciate the supports from Anhui Programs for Science and Technology Development (No. 1604a0902175) and Fundamental Research Funds for the Central Universities (Grant No. WK2320000032).

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CoS2 Nanoparticles Wrapping on Flexible Freestanding Multichannel

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