Double-Morphology CoS2 Anchored on N-Doped Multichannel

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Double-morphology CoS Anchored on N-doped Multichannel Carbon Nanofibers as High-Performance Anode Materials for Na-Ion Batteries Yuelei Pan, Xudong Cheng, Lunlun Gong, Long Shi, Ting Zhou, Yurui Deng, and Heping Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11984 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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ACS Applied Materials & Interfaces

Double-Morphology CoS2 Anchored on N-Doped Multichannel Carbon Nanofibers as High-Performance Anode Materials for Na-Ion Batteries Yuelei Pan,a Xudong Cheng,∗a Lunlun Gong,a Long Shi,b Ting Zhou,a Yurui Deng,a Heping Zhang∗a a

State Key Laboratory of Fire Science, University of Science and Technology of

China, Hefei, Anhui 230027, PR China b

Civil and Infrastructure Engineering Discipline, School of Engineering, RMIT

University, Melbourne, VIC 3001, Australia Corresponding Authors *E-mail address: [email protected] *E-mail address: [email protected]

Keywords: Cobalt disulfide, Transition-metal sulfide, Carbon nanofibers, Sodium ion batteries, Carbon nanofibers ABSTRACT

Na-ion batteries (NIBs) have attracted increasing attention under the fact that sodium is relatively more plentiful and affordable than lithium for sustainable and large-scale energy storage systems. However, the shortage of electrode materials with outstanding comprehensive properties has limited the practical implementations of NIBs. Among all the discovered anode materials, transition-metal sulfide has been proved as one of the most competitive and promising ones due to its excellent redox reversibility and relatively high theoretical capacity. In this study, a double-morphology N-doped CoS2/multichannel carbon nanofibers composites (CoS2/MCNFs) are precisely

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designed, which overcomes the common issues such as poor long cycling life and inferior rate performance of CoS2 electrodes. The conductive 3D interconnected multi-channel nanostructure of CoS2/MCNFs provides efficient buffer zones for the release of mechanical stresses from Na+ ions intercalation/deintercalation. The synergy of the diverse structural features enables a robust frame and a rapid electrochemical reaction in CoS2/MCNFs anode, resulting in an impressive long-term cycling life of 900 cycles with a capacity of 620 mAh g-1 at 1 A g-1 (86.4% theoretical capacity) and a surprisingly high-power output. The proposed design in this study provides a rational and novel thought for fabricating electrode materials.

INTRODUCTION

In the last couple of years, as with the fast development of electric vehicles and soaring popularity of portable electronic devices, electrical energy storage technologies have attracted increasing attention from both academia and industry.1-5 Among those storage systems, Na-ions batteries (NIBs) and Li-ions batteries (LIBs) are both essential systems ascribed to the excellent ions storage capacity and superior cycle life.6,

7

However, due to the shortage and unclear distribution of lithium

resources, the production cost of LIBs keeps increasing. It is critically significant and urgent to develop an innovative rechargeable battery as alternatives. NIBs is an attractive one due to the plentiful resources of sodium in the earth, which can serve as a new low-cost energy storage technology. Although the working mechanisms of Na-ion batteries are expected to be analogous to those of Li-ion


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batteries, unfortunately the normally used materials for LIBs are not well suited for Na+ ions intercalation and deintercalation.8, 9 Therefore, there are still challenges lying ahead about seeking appropriate electrode materials for rapid Na-ions diffusion and repeated intercalation and deintercalation.10, 11 By far, a variety of electrode materials for NIBs, such as transition-metal sulfides, transition-metal diseleniums, hard carbon materials and metal oxides have been investigated.12-18 Among these, transition-metal sulfides (e.g. FeS2, MoS2, SnS, CoS2 and so on) with great potential for Na ions storage have obtained great attention owing to many advantages: (1) the smaller bandgap energy than the metal-oxygen bond, resulting in a relatively higher electrochemical activity;19, 20 (2) rapid Na+ ions diffusion velocity;21 and (3) high theoretical capacity and good thermal stability.22-24 CoS2 (existing in nature, cattierite) has been widely regarded as a potential semiconductor with lots of applications aspects, like supercapacitors, the catalyst for hydrogen evolution, solar batteries and LIBs.25-29 However, CoS2 as an electrode material applied in NIBs, has been relatively less investigated previously. Indeed, in spite of its excellent electrochemical feature and high theoretical sodium ion storage capacity (greater than 800 mAh g-1), long cycling stability and rate performance are disappointing, which is mainly because of large volume expansion/contraction when repeated charging/discharging.30-32 All of these have prohibited its wide acceptance as anodes in NIBs. So far, efforts have been made to optimize the cycling life of CoS2. One of the effective methods is to load the CoS2 on a highly conductive carbon substrate to



improve the electrical conductivity. For example, after combining CoS2 particles with multi-walled carbon nanotube (MWCNT) or graphene sheets, the composite shows a relatively higher and enhanced specific capacity.33,

34

In addition, downsizing the

particles of CoS2 to optimize the surface effect and reduce the distance of Na+ diffusion is also a useful way of benefiting Na+ ions storage and cycling stability.35 Recently, our research group also demonstrated that spherical nanoscale CoS2 anchored in the carbon nanofibers presents an outstanding cycling stability and a high rate discharge performance.36 Lately, heteroatoms doped (e.g. born, nitrogen and phosphorous) in the electrode materials is a superior method to improve the electrode cycling performance.31, 37 Among those materials, nitrogen is the most widely and intensively investigated heteroatom, which is beneficial to generate more active sites and optimize surface wettability.38 For instance, Lin et al. found that CoS2/NiS2 core-shell nanocubes show an excellent lithium/sodium ion storage capacities after coating the N-doped carbon layer.30 However, the reported CoS2/carbon composites in the literature still show less than satisfactory long-term cycling life, which may derive from their unreliable architecture and poor nanostructure strength. Therefore, it is imperative and critical to improve their electrochemical property such as by tailoring their structures and adjusting surface engineering. Herein, in this study, we designed a N-doped double-morphonology CoS2/multichannel carbon nanofibers composites (CoS2@MCNFs) that exhibits a remarkable long cycling stability and rate performance when it is served as anode


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materials for NIBs. This unique design well integrates three critical design fundamentals resulting in the outstanding electrochemical performance for Na+ ions storage: (1) Multichannel carbon nanofibers act as a suitable carbon frame are not only able to increase the electronic conductivity of the composites, but also sufficiently tolerate the repeated volume variation thereby steadying the structure during the electrochemical reaction process. (2) CoS2 crystals are controlled at the nanoscale and assembled into double morphologies, namely nanoparticle and nanosheets respectively, which are helpful for surface effect on the active materials and reducing the distance of Na+ diffusion as well as for electrolytic penetration. (3) The heteroatom nitrogen doped introduces much more active cites for the extra Na+ ions storage and enhances surface wettability. Consequently, the N-doped CoS2@MCNFs composites deliver a high specific capacity and outstanding cycling life of 900 cycles with extremely stable capacity retention for NIBs. Furthermore, the Na-ion full batteries assembled with CoS2@MCNFs and Na3V2(PO4)3@C-BN as electrodes, present a big reversible capacity reaching up to 348 mAh g-1 when tested at 0.5 A g-1.

EXPERIMENTAL SECTION

Preparation of Multichannel Carbon Nanofibers (MCNFs). Following a typical procedure, 10.25 g DMF (N,N-dimethylformamide) with 2.5 wt% polystyrene was mixed and moderately stirred around 5 hours. After that, the solution was filled with 1.22 g PAN (polyacrylonitrile) followed with mechanically stirring for about 12



hours. The original nanofibers (PAN/PS) were prepared by means of electrospinning, applied with cathode-anode distance, voltage and feeding flow rate fixed at 10 cm, 15 KV and 8.37 μL/min, respectively. Next, the obtained original nanofibers were cut into small pieces and preoxidized in air around 3 h with a constant temperature of 280 °C followed with heat treatment under the temperature of 800 °C for another 2 h in argon atmosphere for further carbonization. Preparation of CoS2@MCNFs. Generally, Co(CH3COO)2·4H2O and thiourea at a molar ratio of 1:1 were filled into 70 mL EG (ethylene glycol) and the mixed solution was continuously stirred for one hours. Then the prepared MCNFs pieces (~ 30 mg) were mixed with the above solution with gentle stirring at 40 °C for around 1 h. Then, the solution contained MCNFs was poured into autoclave followed with heat treatment under the temperature of 180 °C overnight. Then the as-prepared samples were washed with ethanol, and then dried at 50 °C in an oven. Finally, obtained black pieces were further heated in Ar atmosphere under 600 °C for about 5 h with sulfur powder based on a weight ratio of 3:1 for optimizing the degree of products crystallinity. For comparison, bare CoS2 spheres were also obtained with the absence of MCNFs, keeping other experimental conditions unchanged. Materials Characterization. Crystalline phases were identified by XRD (X-ray diffraction) testing technology using X-ray diffractometer with Philips X’pert, Raman spectrum testing using LabRamHR and XPS tests. FESEM (field-emission scanning electron microscopy) (SU-8220, HITACHI, Japan), TEM (transmission electron microscope) (JEM-2100F, JEOL) and HRTEM (high resolution TEM) were used to


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investigate morphologies of samples. TG (Thermogravimetry) was used to analyze the CoS2 proportion within the CoS2@MCNFs composites. Electrochemical Measurements. The electrodes were composed of the active materials, acetylene black, and polyvinylidene fluoride (PVDF). These materials were mixed and formed a flurry at the ratio of 8:1:1 in mass. The viscous slurry was smeared on a Cu paper and then dried in the vacuum oven over night. Coin cells were composed of CoS2@MCNFs composites acting as cathode and Na metal as anode. Electrolyte and separators for the NIB were chosen from 1.0 M NaCF3SO3/DEGDME (diethylene glycol dimethylether) and Whatman, respectively. The electrochemical testing were analyzed based on a Neware batteries tester ranging from 0.4 V to 2.9 V at the room temperature. The behavior of cyclic voltammetry were carried out at a low sweep rate of 0.1 mV/s. In contrast, the cells with pure MCNFs as well as pure CoS2 as work electrodes were assembled and investigated under the equal experimental conditions. For

assembly

of

the

full

cells,

CoS2@MCNFs

and

flower-like

Na3V2(PO4)3@C-BN were used as anode and cathode, respectively. The cathode Na3V2(PO4)3@C-BN are prepared on the basis of a literature. In a typical synthesized way of Na3V2(PO4)3@C-BN, 0.72g V2O5 and 1.52g C2H2O4·2H2O were added into 40mL de-ionized water and stirred at 70 °C for 1 h. Then, 1.84g NaH2PO4·2H2O and 0.4g glucose were added into the solution with stirring for another 10 min followed with 100mL n-propanol pouring in. The obtained solution was dried in an oven for overnight. Finally the Na3V2(PO4)3@C-BN was obtained by mixing and heating



treatment with NH4HB4O7·H2O at a weight rate of 1:3. The cathode was composed of the Na3V2(PO4)3@C-BN, carbon black and PVDF at the ratio of 7:2:1 in mass. Both anode and cathode were precycled in a half-cell model before proceeding full cell design and the weight ratio of anode and cathode were controlled to be around 6.8:1 with anode mass loading was around 0.8 mg cm-2. The charging/discharging cycling were conducted within 0.5-3.5 V. The specific capacity of electrodes were all calculated based on the total mass of composites. For full cell testing, the specific capacity were calculated based on the total mass of anode.

RESULTS AND DISCUSSION

Figure 1. The illustration of (a) preparation process for CoS2/MCNFs composites;


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(b) Na+ ions insertion and extraction processes. The MCNFs as a robust host is able to efficiently resist mechanical forces and release stresses during the intercalation/deintercalation of sodium ions. Furthermore, double morphologies of CoS2 (CoS2 nanoparticles and CoS2 nanosheets) extremely shorten the electronic transport distances and enhanced the sodium ions storage capacity. The detailed synthetic processes of N-doped CoS2@MCNFs are given in the experimental section. Figure 1a shows the schematic of the synthetic processes. In brief, the CoS2/MCNFs was prepared by the facile solvothermal method employing Co(CH3COO)2·4H2O, thiourea, ethylene glycol solution, and MCNFs as carbon substrate. After the solvothermal process, the unique N-doped CoS2@MCNFs was obtained with ultra-thin CoS2 nanosheets wrapping the outer surface of the MCNFs and CoS2 nanoparticles anchoring in the multichannel. The novel structure is expected to deliver a stable hosting and fast transportation passageway for the sodium-ion transfer and repeated intercalation/deintercalation as shown in Figure 1b, which could perform a good capacity for anode materials for NIBs, just like the artful structure of okra growing in nature achieving efficiently absorbing nutrients and emitting scrap.



Figure 2. (a) XRD patterns of N-doped CoS2@MCNFs; (b) Raman results of N-doped CoS2@MCNFs, pure multichannel carbon nanofibers and bare CoS2s; XPS of N-doped CoS2@MCNFs; (c) survey spectra; (d) Co 2p spectra; (e) S 2p spectra; and (f) N 1s spectra. Figure 2a presents the XRD patterns, in which all peaks are well matched with the CoS2 phase (JCPDS card No. 41-1471). There is no other impurities existed are observed in XRD spectrums for both of CoS2@MCNFs materials and bare CoS2 samples (in Figure S1), therefore confirming the high purity of the synthesized


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samples. Note that no (002) peak of MCNFs is detected, suggesting that the MCNFs is wrapped around CoS2 and homogeneously dispersed with a low amount.39 Raman spectroscopy is employed to further examining the obtained materials. The two samples, N-doped CoS2@MCNFs and bare CoS2 show the similar Raman bands at around 286, 314, 388 and 414 cm-1 as presented in Figure 2b, corresponding to the Eg, Tg(1), Ag and Tg(2) modes of the CoS2 crystal, respectively.40 Tg(1) modes are the various combinations of librational and stretching vibration and as for Eg, which is a single librational mode of the dumbbells. The Ag and Tg(2) modes are assigned to the in-phase and out-of-phase stretching vibrations of the sulfur atoms.40 The Raman spectra in the higher wavenumber region (1200-1800 cm-1, the inset in Figure 2b) shows the D and G bands, at approximately 1,335 cm-1 and 1,599 cm-1, respectively. In detail, D band can be attributed to the structural defects within the carbon layers and G band is due to the E2g vibrational mode present in the Sp2-bonded graphitic carbon atoms, which commonly exist in graphitic structures.39 Notably, the intensity ratio (ID/IG) in CoS2@MCNFs is calculated to be around 1.012, slight larger than that of pure MCNFs (0.977). The higher ID/IG ratio of CoS2@MCNFs indicates that extra structural defects were created by nitrogen atom incorporating into MCNFs. The proportion of CoS2 within the CoS2@MCNFs electrode is confirmed by TG tests in the air atmosphere (in Figure S2) and is calculated to be ~ 81.4 wt% based on the reaction: 3CoS2 (s) + 2O2 (g) = Co3O4 (s) + 6S (g).

41

The chemical compositions of the N-doped CoS2@MCNFs hybrid were



identified by XPS testing. And survey spectrum of N-doped CoS2@MCNFs in Figure 2c displays five elements, C, Co, S, N and O, existed in the sample proving a high purity of as-prepared product. Note that there is a weak peak of O 1s, due to the oxidation tendencies of CoS2 in the air which is similar with that in the literature.20 The high-resolution spectra of Co 2p is displayed in Figure 2d, exhibiting two peaks locating at 794.2 and 779.1 eV and two satellite peaks (denoted as “Sat.”) at around 804.2 eV and 782.6 eV by fitting of the Co 2p spectra of N-doped CoS2@MCNFs, derived from spin-orbit of Co 2p1/2 and Co 2p3/2 for CoS2. As shown in Figure 2e, two different spin-orbit signals and one shakeup satellite peak in the S 2p high resolution spectrum are distinguished locating at 162.6, 163.8 and 168.4 eV, rooted from S 2p3/2 ,S 2p1/2 and oxidized S respectively.42 The doped S in the composites contribute to enlarge the interlayer distance due to the larger covalent radius of S atoms and improve the electrochemical activity of the conductive matrix, thus beneficial for enhancing the Na+ storage performance.43, 44 And furthermore, the XPS characterization confirms the presence of around 5.11% nitrogen in the composites consisted with N with two fitted peaks, the pyridinic-N at about 400.7 eV and pyrrolic-N at 398.6 eV as can be seen in Figure 2f. These doped-N carbon layers possess high chemical activities and beneficial for sodium kinetics.45


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Figure 3. (a) SEM images and (b) TEM images of pure multichannel carbon nanofibers (MCNFs); (c) SEM images and (d) TEM images of CoS2@MCNFs composites; (e and f) High magnification SEM images of the CoS2@MCNFs composites. The morphology and microstructures of pure MCNFs and the as-prepared



CoS2@MCNFs were tested by FESEM and TEM. A FESEM image in Figure 3a demonstrates that the pure MCNFs is fibrous with the diameter ranging from 0.5 to 0.7 µm and the TEM image (Figure 3b) of the MCNFs presents uniform parallel channels in the MCNFs with a narrow size distribution. After a facile solvothermal treatment, the MCNFs were successfully coated with an ultra-thin CoS2 nanosheets layer as can be seen in Figure 3c with low magnification. By TEM image in Figure 3d, we can vaguely see the parallel channels inside the MCNFs and the CoS2 nanosheets layer covering the MCNFs. It should be noticed that there is another morphology, CoS2 particles, generated in the parallel channels. To clearly show CoS2 particles and the layout of CoS2@MCNFs, the CoS2 nanosheets layer was peeled off by washing with a 5M hydrochloric acid solution along with the ultrasonic treatment. TEM image of samples after treatment is showed in Figure S3a, and the CoS2 nanoparticles can be seen stably anchored in the parallel channels of MCNFs with a uniform distribution, confirming the double-morphology CoS2 are formed and anchored on MCNFs. High-magnification of SEM images clearly reveal the cross-section shape characteristics (Figure 3e) and CoS2 nanosheets (Figure 3f). From Figure 3e, it is indicated that those parallel channels are retained quite completely in CoS2@MCNFs and the wrinkled CoS2 nanosheets layer with an average thickness of 180 nm grew stably at the outer surface. Impressively, the CoS2 layer is consist of extremely paper-thin CoS2 nanosheet at 1-5 nm thick (Figure 3f) stacked desultorily and it is can be expected that CoS2@MCNFs composites with unique double-morphology CoS2 structure can enhanced the sodium ions storage


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performance.

Figure 4. (a) HRTEM image and (b) energy dispersive spectrometer mapping of CoS2@MCNFs composites. A representative high-resolution TEM (HRTEM) image clearly reveals an interplanar distance of 0.248 nm in the lattice fringes, matching well with the (210) planes of cubic CoS2 (Figure 4a). The similar synthesis process without added MCNFs produces bare CoS2 nanoparticles with a rough texture, which is constructed by interconnected nanoparticles (Figure S3b). The distinct micro morphologies highlight the role of MCNFs served as solid phase substrates, which provides the active sites for CoS2 suppressing the structural agglomeration of the effective substances during the growth of CoS2 nanosheets structure. The energy dispersive spectrometer mapping of CoS2@MCNFs presented in Figure 4b confirms that Co, S, C elements were distributed uniformly among the composites. The results discussed above indicated that double-morphology CoS2@MCNFs with multi-channel carbon nanofibers as a central axis, CoS2 nanosheets as coats surrounding the axis and CoS2 nanoparticles as the filler in the channels are successfully designed.



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Figure 5. Discharge/charge behaviors of (a) CoS2@MCNFs electrode and (b) bare CoS2. (c) Cyclic voltammograms curves of CoS2@MCNFs at a low sweep rate of 0.1 mV s-1. (d) EIS (electrochemical impedance spectra) and (e) cycling life testing of CoS2@MCNFs electrode. To observe the electrochemical behavior of both CoS2@MCNFs electrode and bare CoS2 electrode, 2032-type coin cells were assembled. Effect of various electrolytes

were

firstly

investigated.

The

charge/discharge

behavior

of

CoS2@MCNFs electrodes using 1.0 M NaCF3SO3 in EC/DEC (ethylene carbonate and diethyl carbonate), DEGDME (diethyleneglcol dimethylether) and PC (propylene carbonate) at 0.1 A g-1 between 0.4 to 2.9 V are showed in Figure S4. The result


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clearly indicates that CoS2@MCNFs electrode with DEGDME exhibits a larger capacity retention and smaller voltage polarization than that with EC/DEC and PC electrolytes, which can be ascribed to three critical factor: (1) a more stable SEI (solid-electrolyte interface) forming on CoS2, (2) much faster charge-transfer kinetics DEGDME and (3) better wettability among electrode materials.7,

20, 46

The

representative discharging/charging voltage profiles using both of the electrodes with 1.0 M NaCF3SO3 in DEGDME between 0.4-2.9 V are depicted in Figure 5a-b. The discharge-charge curves in both cases show a similar trend, in which discharge plateaus near 1.6 V-1.3 V and 1.1 V-0.8 V and charge plateaus near 1.8 V-1.5 V and 2.1 V-1.8 V are clearly observed, demonstrating that the introduction of MCNFs cannot change the electrochemical mechanism of CoS2. When cycled at 0.1 A g-1, CoS2@MCNFs provide an initial discharge/charge capacity of 849.59/681.37 mAh g-1 and a high coulombic efficiency 80.2%. Along charge/discharge cycling processes, charge capacity of tenth cycle slightly grow to 712.26 mAh g-1 and the phenomenon of capacity increasing of the CoS2@MCNFs for the first 10 cycles is observed, which can be ascribed to an activation and stabilization stage.34, 47 As for the bare CoS2 electrode, the initial discharge/charge capacity is 717.61/547.37 mAh g-1, resulting in a comparatively low coulombic efficiency 76.3%, furthermore there is an obvious rapid capacity fading trend in the voltage profiles for the CoS2 electrode which leads to an as low as 419.02 mAh g-1 charge capacity at the tenth cycle. And conversely, that of the CoS2@MCNFs electrode is overlapping, indicating the good cycle stability. To further investigate the discharge/charge behaviors of CoS2@MCNFs composites,



CV curves were obtained at 0.1 mV s-1. As presented in Figure 5c, two reduction peaks are located at 0.7 and 1.5 V in the initial cathodic sweep. The peak at 1.5 V is attributed to the initial insertion of sodium and the broad peak at 0.7 V is due to the displacement reactions to the formations of Co, Na2S and SEI.34 Accordingly, oxidation peaks at 1.8 and 2.1 V are detected, which are related to the formation of CoS2. From the second cycle onwards, the main reduction peaks stabled at 0.8 and 1.6 V, due to improved kinetics of the electrode.36 CV curve of bare CoS2 at 2nd cycle was showed in Figure S5. With a low scan rate, one strong and sharp reduction peak at around 1.56 V can be detected and two weak and broad reduction peaks (Peak 2 and Peak 3, labelled in Figure S5a) at 0.7 V and 1.2 V also can be observed. However, no obvious discharging plateau at 0.7 V and 1.2 V are observed in the discharging curves of bare CoS2 in Figure S5b. In fact, very weak and short discharging plateau also can be found near 0.7 V and 1.2 V in the first cycles, however, the signals disappear when cycled after the second lap, which is due to the serious dissolution and diffusion of sulfides into electrolyte and formation of insulated sodium sulfide covering on the anode. These all increase the cells internal resistance and polarization, which could vanish the voltage plateau near 0.7 V and 1.2 V gradually and eventually covered up by robust signal. The similar phenomenon can be observed in the conventional Li/S cell.48

In Figure 5a, b, the discharging voltage (in the initial three

discharging-charging cycles) of bare CoS2 is higher than that of CoS2@MCNFs electrodes, which may be due to the better surface wettability of bare CoS2 with electrolyte than that of multichannel carbon matrix. However, with the cycling


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proceeding, a portion of sulfide is transformed into sodium sulfide and deposited on the anode, thus resulting in an increase of internal cell resistance due to the insulating property of sodium sulfide. Therefore, the discharging voltage of bare CoS2 decreases and increase in charging voltage. It is obvious when it is discharged at 10th cycle in Figure S6. The discharging voltage of CoS2@MCNFs is higher than that of bare CoS2, which confirms that multichannel carbon matrix can do a great favor for restraining the sulfide dissolution and ensure long cycling stability.49 In addition, to analyze electrochemical behaviors of both electrodes, electrochemical impedance spectra (EIS) were employed and showed in Figure 5d. The plot was fitted and presented in Figure 5d. In the equivalent circuit, Zw is related to Warburg impedance, reflecting the Na-ions diffusion process in the bulk electrode.39,

50

Apparently, CoS2@MCNFs electrode shows a lower post-cycling

charge-transfer resistance (Rct=31.45 Ω) compared with that of the bare CoS2 electrode (Rct=75.92 Ω), which can be attributed to the improved electrochemical reaction kinetics own to the conducting effect of MCNFs. Consequently, all of these account for a better sodium storage property and a higher reversible capacity than the CoS2 electrode. To better illustrate the structural superiority of CoS2@MCNFs served as sodium storage anode materials, the cycling life of CoS2@MCNFs electrode along with bare CoS2 were investigated at 1 A g-1. The CoS2@MCNFs provides a discharging capacity of 539.28 mAh g-1 at 1st cycle, which slightly rises in the following cycles and then reach up to 620 mAh g-1 after 60 cycles and still stabilize at around 620 mAh g-1 surprisingly after 900 cycles (Figure 5e). It is mainly due to the



fact that extra vacancies and structural defects on MCNFs are generated as cycle number increases, thus enhancing the insertion of much more Na+ ions and lead to a remarkable cyclic stability. On the contrary, the capacity of the bare CoS2 decreases rapidly to 337 mAh g-1 only after 100 cycles, much less than the remnant capacity of the CoS2@MCNFs (620 mAh g-1) and further proves a good long-term electrochemical stability of the CoS2@MCNFs electrode. In addition, the contribution of pure MCNFs is also investigated by galvanostatic charge/discharge testing and the representative discharge/charge voltage profiles are presented in Figure S7. The reversible specific capacity of pure MCNFs is only around 37.3 mAh g-1, which is negligible. Thus the rate of reversible capacity (based on the mass of CoS2 in the composites) to the CoS2 theoretical capacity is calculated to be about 86.4%, which demonstrate that CoS2@MCNFs has superior electrochemical reaction kinetics. The loss of irreversibility observed could be due to the slight dissolution of the reaction intermediate sulfide from surface of anode into electrolyte.

Figure 6. (a, b) Rate performance of CoS2@MCNFs electrode at a series of current densities of 0.1 to 5 A g-1. Rate performance were studied under adjoining cycles of various current densities for the CoS2@MCNFs electrode, as shown in Figure 6a-b. It is observed


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from Figure 6a that the discharging/charging profiles show limited changes from a low current density 0.1 to a high current density of 5 A g-1, indicating a low polarization of CoS2@MCNFs and a fast electrochemical reaction kinetics. Furthermore, the CoS2@MCNFs electrode delivers reversible capacities of 711.52, 681.91, 641.94, 610.20, and 569.43 mAh g-1 at the corresponding current densities of 0.1, 0.2, 0.5, 1, and 2 A g-1 (Figure 6b). Even under a current density with the 50-fold increase, a charge capacity of 508.68 mAh g-1 still can be kept at 5 A g-1. When the current density back to the initial value of 0.1 A g-1, the electrode provides a reversible capacity of 690.71 mAh g-1 and remain stable very well. To visually view the superiority of the CoS2@MCNFs’ unique nanostructure for the sodium storage, a comparison between the Na-storage performance with previously reported CoS2 electrode materials are collected in Table S1. The high specific capacity (712.26 mAh g-1 at 0.1 A g-1) as well as an outstanding cycling stability (620 mAh g-1 at 1 A g-1 for 900 cycles) the hybrid CoS2@MCNFs electrode delivered from the experimental tests are the highest compared with those reported materials in the literature. To further demonstrate the advantage of as-prepared double-morphology CoS2@MCNFs, its rate performance and long-cycling stability are compared with single-morphology CoS2@MCNFs, which has been reported in our precious work. As can be seen in Figure S8a, b, single-morphology CoS2@MCNFs composites are composed with CoS2

nanoparticles

anchoring

on

MCNFs.

The

discharge

capacity

of

single-morphology CoS2@MCNFs is around 529, 485, 404, 312, 288, and 249 mAh g-1 at the corresponding current densities of 0.1, 0.2, 0.5, 1, 2, and 5A g-1 (in Figure



S8c), which is lower than that of double-morphology CoS2@MCNFs, respectively. Although single-morphology CoS2@MCNFs electrode has a higher cycling life than that of double-morphology one, double-morphology CoS2@MCNFs electrode has an advantage in the higher reversible capacity as presented in Figure S8d. The excellent electrochemical performance the double-morphology CoS2@MCNFs demonstrate can be ascribed to the synergy between CoS2 nanosheets on the surface and CoS2 nanoparticles inside the channels. On the one hand, the nanosheets on the surface of MCNFs are in favor of lowering the barrier for Na+ inserting/extracting, therefore reducing the charge transfer resistance. More importantly, the unique wrinkled nanosheets are in favor of providing a better wettability between electrodes and electrolytes than other structures, such as nanoparticles and polyhedron, especially in the initial stage for electrolyte infiltration.51, 52 On the other hand, the nanoparticles in the multi channels of carbon nanofibers, CoS2 nanoparticles have more robust construction than CoS2 nanosheets. Furthermore, CoS2 nanoparticles possess a higher volumetric energy density than nanosheets, which can do a big favor in improving the energy density of CoS2@MCNFs composites. In general, the as-prepared double-morphology CoS2@MCNFs has the unique structural advantages and improved surface effect, resulting in an enhanced Na+ storage performance.


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Figure 7. Ex situ XRD patterns (a) between 1.0 and 2.9 V, and ex situ XRD patterns (b)

and HRTEM images (c-d) between 0.4 and 2.9 V. To comprehensively clarify the reversible Na-ions stoargae mechanism of the CoS2@MCNFs, HRTEM and ex situ XRD were investigated within the two potential ranges of 1.0-2.9 V and 0.4-2.9 V. Figure 7a exhibits the XRD patterns of the CoS2@MCNFs electrode after ten cycles at two different charge/discharge states at 1.0 V and 2.9 V, among which signals at around 21.5° and 23.9° derive from parafilm, which is used to protect the materials from the air. Except for the peaks of the parafilm and MCNFs (at 26.3°), the characteristic peaks at 32.4° and 36.3° of the CoS2 were detected in both of the discharge state at 1.0 V and charge state at 2.9 V, while no diffraction peak of the Na2S and Co were detected. This indicates that no Na2S and Co crystalline phase was formed within this potential state of 1.0 V. In the meantime, this also confirms the insertion of the Na+ ions process and the formation



of intermediate NaxCoS2 phase in the discharging process. The CoS2 characteristics peaks can be clearly observed in XRD patterns of charged status at 2.9 V in Figure 7a and matches well to the standards card for cubic phase CoS2 (JCPDS card No. 41-1471), which indicates that there is limited influence on the original structure of the CoS2@MCNFs when it is in the charging process with repeating Na-ions insertion/extraction. For further investigation, the potential window was adjusted to 0.4-2.9 V and the ex situ XRD testing results of CoS2@MCNFs electrodes after ten cycles were showed in Figure 7b. After been fully discharged to 0.4 V, the XRD signals of Co and Na2S were finally found with the peaks locating at 44.8°(Co, JCPDS No. 88-2325) and 39.2°(Na2S, JCPDS No. 47-1698), respectively. This indicated that the conversion reaction from NaxCoS2 phase to Na2S and Co occurred below 1.0 V with the consecutive Na-ions insertion process. When it was charged back to 2.9 V, one weak but sharp characteristic peak of CoS2 can be detected at 32.8°, confirming a superior reversibility of the CoS2@MCNFs’ Na-ions inserting/extracting reactions. The HRTEM images were also employed to clarify the electrochemical reaction mechanism of CoS2 as showed in Figure 7b, c. When discharged to 0.4 V, the lattice with an interplanar spacing of 0.20 nm and 0.23 nm are consistent well with the (111) crystal plane of Co and (220) crystal plane of Na2S, respectively. As fully charged to 2.9 V, the (101) plane of CoS2 appears, further demonstrating the process of initial insertion of Na-ions and typical conversion reaction at low potential window. This coincides well with the analysis gained from the CV curves of Figure 5c, as well as XRD patterns in Figure 7b, c. The reaction of Na-ions storage mechanism for CoS2


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can be deduced as following CoS2 + 4Na + 4e ↔ Co + 2Na2S

(1)

Figure 8. Electrochemical testing of CoS2@MCNFs//Na3V2(PO4)3@C-BN sodium-ion full cell: (a) Illustration of full cell structure; and (b) Long cycling performance of full cell at 0.5 A g-1. The photo in (b) shows the full cell which powers a white LED. To realize the industry applications of using the CoS2@MCNFs as an electrode and demonstrate its excellent electrochemical performance, the full batteries were assembled by using CoS2@MCNFs and Na3V2(PO4)3@C-BN (XRD and SEM patterns showed in Figure S9) as electrodes. Figure 8a shows the cosntruction of the



full batteries, including the anode, cathode, a separator and NaCF3SO3-based electrolyte. The Na3V2(PO4)3@C-BN cathode materials were fabricated by a facile self-assembly process,38 showing a discharging capacity of 103.3 mAh g-1 (when cycled at 1 C) (Figure S10 and S11). The active materials of the cathode and anode were equilibrated with a mass ratio of 6.8:1. The cycling performance at 0.5 A g-1 between 0.4 and 3.5 V is depicted in Figure 8b. It shows that the discharge capacity at 10th cycle is 356 mAh g-1 based on the mass of the anode. A capacity of 348 mAh g-1 is obtained after 100 cycles, demonstrating its excellent cyclability. Figure S11 shows the charge/discharge curves of the full cell at 0.5 A g-1 at 1st, 5th, 50th, and 100th cycles between 0.5-3.5 V. The initial discharging and charging capacities are 418 mAh g-1and 521 mAh g-1 calculated based on the total mass of the anode. The irreversible capacity loss at the initial cycle could be caused by the electrolyte decomposition and SEI formation. With cycling going on, the full cell stabilized and a reversible capacity of around 348 mAh g-1 can be obtained even after 100 cycles, presenting the excellent cycling stability. In addition, the full batteries were further tested to run a light emitting diode (LED) (in Figure 8b). A remarkable performance and cyclability for sodium storage of CoS2@MCNFs electrode were observed mainly because of the unique nanostructure with multi-channel carbon nanofibers as a central axis and CoS2 nanosheets as coats surrounding it. There are several reasons. Firstly, the multi-channel carbon nanofibers not merely serve as an outstanding conductive framework in favor of fast charge transmission but also act as a robust substrate for well dispersing and immobilizing CoS2 nanosheets, thus strengthening the structural


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stability of the hybrid materials. Secondly, CoS2 nanosheets with the ultrathin feature at the MCNFs surface and CoS2 nanoparticles in the different channels increase the contact areas with the electrolyte as well as the electrochemical effective sites, which are beneficial for rapid diffusion and storage of Na-ions. Finally, the unique structure offers sufficient free spaces, which could efficiently buffer the stress during the repeated cycling, resulting in an improvement of both capacity retention and rate performance.

CONCLUSION

An innovative CoS2/multichannel carbon nanofibers composites (CoS2@MCNFs) hybrid material with elaborately controlled novel configurations were successfully developed through electrospinning followed by a facile solvothermal method. The well-defined ultrathin CoS2 nanosheets and CoS2 nanoparticles are uniformly grown on MCNFs networks with powerful bonding force, which offers an optimized host for Na- ions inserting/extracting reactions. It was surprised that when using it as an anode for NIBs, the CoS2@MCNFs exhibits a high specific capacity, outstanding rate capacity, and excellent cycle life. The three-dimension conductive framework of MCNFs, double-morphology structure of ultrathin CoS2 nanosheets and nanosized CoS2 particles, as well as the unique architectural configuration along with heteroatoms doping, take charge of the extraordinary electrochemical performances of the CoS2@MCNFs. This novel nanostructure delivers good insights for Na-ions storage electrodes and even shed some light on other relevant functional nanophase



materials for Li-ion batteries and ultracapacitors.

ASSOCIATED CONTENTS

The Supporting Information is showed in Supporting Information file. XRD pattern of the bare CoS2; TGA curves of CoS2@MCNFs composites and bare MCNFs; TEM image of treated CoS2@MCNFs composites with HCl solution; SEM images of bare CoS2; Charge-discharge curves of CoS2@MCNFs electrodes using 1 M NaCF3SO3 in EC/DEC, PC, DEGDME at 0.1 A g-1 and cycling performance in the three different solvents; CV curves at 2nd cycle with three reduction peaks marked as Peak 1, Peak 2 and Peak 3 (in a scan rate of 0.07 mV/s) and charge-discharge curves (at a current density of 0.1 A g-1) of bare CoS2 electrodes; The discharge curves of CoS2@MCNFs electrode and bare CoS2 electrode (at a current density of 0.1 A g-1); Discharge/charge curves and cycling life testing of pure MCNFs at a current density of 1 A g-1; SEM and TEM of single-morphology N-doped CoS2@MCNFs composites, reported in the literature. Rate performance and long-term cycling performance (at a current density of 1 A g-1) of single-morphology N-doped CoS2@MCNFs composites and double-morphology N-doped CoS2@MCNFs composites in this work; XRD pattern and SEM image of NVP@C-BN; Charge/discharge curves and cycling performance of NVP@C-BN electrode at a current density of 1 C; Galvanostatic charge/discharge curves for the 1st, 5th, 50th, and 100th cycles at a current density of 0.5 A g-1 under the potential range of 0.5-3.5 V; Comparison of the Na-storage performance with reported CoS2 electrode materials.


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AUTHOR INFORMATION ORCID Xudong Cheng: 0000-0002-2177-3235 Heping Zhang: 0000-0002-1984-7039 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This research is supported by Anhui Programs for Science and Technology Development (No. 1604a0902175) and Fundamental Research Funds for the Central Universities (Grant No. WK2320000035). REFERENCES (1) Dunn, B.; Kamath, H.; Tarascon, J.-M., Electrical Energy Storage for the Grid: a Battery of Choices. Science 2011, 334, 928-935. (2) Wu, C.; Jiang, Y.; Kopold, P.; van Aken, P. A.; Maier, J.; Yu, Y., Peapod-Like Carbon-Encapsulated Cobalt Chalcogenide Nanowires as Cycle-Stable and High-Rate Materials for Sodium-Ion Anodes. Adv. Mater. 2016, 28, 7276-7283. (3) Yuan, B.; Sun, X.; Zeng, L.; Yu, Y.; Wang, Q., A Freestanding and Long-Life Sodium-Selenium Cathode by Encapsulation of Selenium into Microporous Multichannel Carbon Nanofibers. Small 2018, 14, 1703252. (4) Liu, D. H.; Li, W. H.; Zheng, Y. P.; Cui, Z.; Yan, X.; Liu, D. S.; Wang, J.; Zhang, Y.; Lü, H. Y.; Bai, F. Y. In Situ Encapsulating α-MnS into N, S-Codoped Nanotube-Like Carbon as Advanced Anode Material: α→β Phase Transition Promoted Cycling Stability and Superior Li/Na-Storage Performance in Half/Full Cells. Adv. Mater. 2018, 30, 1706317. (5) Fan, H.-H.; Li, H.-H.; Guo, J.-Z.; Zheng, Y.-P.; Huang, K.-C.; Fan, C.-Y.; Sun, H.-Z.; Li, X.-F.; Wu, X.-L.; Zhang, J.-P. Target Construction of Ultrathin Graphitic Carbon Encapsulated FeS Hierarchical Microspheres Featuring Superior Low-Temperature Lithium/Sodium Storage Properties. J Mater. Chem. A 2018, 6, 7997-8005. (6) Zhang, Y.; Pan, A.; Wang, Y.; Wei, W.; Su, Y.; Hu, J.; Cao, G.; Liang, S., Dodecahedron-Shaped Porous Vanadium Oxide and Carbon Composite for High-Rate Lithium Ion Batteries. ACS appl. Mater. Inter. 2016, 8, 17303-17311. (7) Zhang, K.; Hu, Z.; Liu, X.; Tao, Z.; Chen, J., FeSe2 Microspheres as a High-Performance Anode Material for Na-Ion Batteries. Adv. Mater. 2015, 27, 3305-3309.



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Table of Contents graphic


Double-Morphology CoS2 Anchored on N-Doped Multichannel

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