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MOF-Derived Hollow Co3S4 Quasi-polyhedron/MWCNT Nanocomposites as Electrodes for Advanced Lithium Ion Batteries and Supercapacitors Ran Tian, Ying Zhou, Huanan Duan, Yiping Guo, Hua Li, and Hezhou Liu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00072 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 6, 2018

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ACS Applied Energy Materials

MOF-Derived Hollow Co3S4 Quasi-polyhedron/MWCNT Nanocomposites as Electrodes for Advanced Lithium Ion Batteries and Supercapacitors Ran Tian, Ying Zhou, Huanan Duan*, Yiping Guo, Hua Li, Hezhou Liu* State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China. E-mail: [email protected]; [email protected]

Abstract Transition metal sulfides/carbon nanocomposites are being extensively studied as electrode materials since a rationally designed structure incorporated with carbonaceous materials can eliminate pulverization caused by volume expansion during cycling process and promote electron transport in the electrodes. Herein, we report a cobalt sulfide/multi-walled carbon nanotube (MWCNT) nanocomposite with a novel structure where MWCNTs penetrate through hollow Co3S4 quasi-polyhedra and form conductive networks. The preparation of this unique structure involves sulfurization of ZIF-67/MWCNT precursors via solvothermal process and subsequent crystallization by thermal annealing. With the employment of TEM 3D reconstruction technology, a panoramic view of the as-prepared nanocomposites is demonstrated and the structure is thoroughly confirmed. Moreover, the hollow Co3S4/MWCNT nanocomposites exhibit high specific capacity and excellent cyclic stability as electrodes for both lithium ion batteries and supercapacitors. They

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delivered specific capacity of 1281.2 mAh g-1 after 50 cycles at 200 mA g-1 and 976.5 mAh g-1 after 500 cycles at 2 A g-1. Also, they show a high capacitance of 638.5 F g-1 at current density of 30 A g-1 and capacitance retention of 78.98% after 5000 cycles.

Keywords: Hollow structures; Co3S4; MWCNT; lithium ion batteries; supercapacitors

Introduction In recent years, the continuous shortage of fossil fuels and the urge to mitigate air pollution boost the interest in low-emission electric vehicles equipped with good-performance and long-cycle-life power supply systems.

1, 2

Lithium ion

batteries (LIBs) and supercapacitors (SCs), the two most promising energy storage devices, play pivotal roles in promoting electric automobiles.

3-6

However, further

developments of both LIBs and SCs are inhibited by inferior electrode materials: traditional graphite anode has been unable to satisfy the demands of high performance LIBs due to its low capacity density and bad cycling stability;

6-8

similarly, low energy density of carbonaceous materials limits the large-scale application of SCs.

9

So the desire to develop the next-generation electrode

materials for LIBs and SCs becomes imperative. Recently, transition metal sulfides such as MoS2, 10-12 Co3S4, WS2

16

13, 14

NiS2

15

and

have attracted researchers’ attention due to their large theoretical capacity

and excellent electrochemical activity; however, there exist some unsolved challenges including high volume change during cycling process and low electrical conductivity of the sulfides.

17, 18

Latest work has illustrated that rational design of


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nanomaterials could be a feasible approach to address the pulverization issue caused by the high volume change during cycling process. nanoflakes

22

and hollow nanostructures23,

24

17,19,20

Nanorods,

21

have been proven effective in

suppressing the pulverization during charge/discharge process. To improve the electrical conductivity of electrodes, transition metal sulfides/carbonaceous material composites have been widely studied in recent years. 25 Also, carbon-based materials such as multi-walled carbon nanotubes (MWCNTs), 26, 27 graphene, 10, 28-30 porous carbon

31-34

and carbon nanofibers 35, 36 were studied to obtain stable cyclic

performance and high rate capability. Therefore, it is of high interest to develop metal sulfides/carbonaceous material nanocomposites with an interwoven structure to improve the electrochemical performance. Metal−organic frameworks (MOFs) have recently been demonstrated as sacrificial templates to prepare transition metal oxides and sulfides with unique well-organized nanostructures.

24, 27, 37-44

For instance, Yu et al. reported the synthesis

of CoS2 nanobubble hollow prisms from ZIF-67 templates by hydrothermal method. 45 Guan and his co-workers synthesized onion-like NiCo2S4 from Co-PBA precursor by a sequential ion-exchange strategy, which led to improved electrochemical activity and stability.9 However, there is few research works carried out on MOFs-derived hollow metal sulfides/carbonaceous material nanocomposites especially synthesized via hydrothermal method because of the difficulty in incorporating carbonaceous materials during the decomposition process of MOFs. Herein, we report a MOF-template-assisted synthesis of a unique hollow Co3S4



quasi-polyhedron/MWCNT

nanocomposite

(CSC),

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which

mainly

includes

sulfurization of ZIF-67/MWCNT precursors via solvothermal process and subsequent crystallization by thermal annealing. The nanocomposites deliver a high specific capacity of 1281.2 mAh g-1 after 50 cycles at 200 mA g-1 and 976.5 mAh g-1 after 500 cycles at 2A g-1. Also, they show a high capacitance of 638.5 F g-1 at high current density of 30 A g-1 and capacitance retention of 83.3% after 2000 cycles. The excellent electrochemical performance is closely related to the unique microstructure of the nanocomposites that is scrutinized through TEM three-dimensional reconstruction technology.

Experiment sections Functionalization of MWCNTs All the chemicals are of analytical grade and were used without further purification. 1 g MWCNTs (Nanotech USA, 20~30 nm in diameter and 2~10 µm long) was added to a mixture of 100 mL concentrated H2SO4 and HNO3 (3:1 by volume), stirred and refluxed for 3 h at 80 °C; 300 mL DI water was added to dilute the acid afterwards. The MWCNT/acid suspension was filtered with a PTFE membrane with pore size of 0.45 µm and washed with DI water until the solution became neutral. The MWCNTs on the membrane were collected and dried under vacuum at 80 °C .46 Preparation of ZIF-67/MWCNTs 45 mg functionalized MWCNTs were dispersed into 25 ml methanol with 100 mg PVP by ultrasonication. 1 g Co(NO3)26H2O were then added to the solution under


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vigorous stirring. Subsequently, 30 ml of methanol solution with 1.8 g 2-Methylimdazole was rapidly poured into the above suspension and vigorously stirred for 30 min. After standing overnight, the product was collected, washed with absolute ethanol (water content ≤0.3%) for 3 times, centrifuged, and dried at 75 ℃ for 12 h. Synthesis of CSC 120 mg ZIF-67/MWCNTs were dispersed into 35 ml absolute ethanol by ultrasonication. Then 400 mg thioacetamide (TAA) was added to the suspension. The suspension was then transferred into a 100 ml Teflon-lined autoclave and maintained at 130 ℃ for 16 h. After cooling down to room temperature, the as-prepared samples were collected with centrifugation, washed with absolute ethanol, and dried at 75 ℃ for 24 h. Finally, the hydrothermal products were annealed in a tube furnace at 360 ℃ for 2 h under N2 atmosphere with a heating ramp of 3 ℃ min-1. Material Characterization X-ray diffraction (XRD) patterns were measured with Cu Kα radiation (λ = 1.5418 Å) at a step of 0.02° (Goniometer Ultima IV diffractometer). X-ray photoelectron spectroscopic (XPS) measurements were carried out using a monochromatic Al Ka X-ray source (Kratos AXIS Ultra DLD spectrometer). The surface areas were determined by nitrogen absorption and desorption measurements (Auto sorb IQ) and the Brunauer-Emmett-Teller (BET) method. Thermogravimetric analysis (TGA)



was conducted at a heating rate of 10 °C min−1 from room temperature to 900 °C in air (SDT Q600 thermogravimetric analyzer). The morphologies of the as-prepared and cycled products were observed on a field emission scanning electron microscope (FESEM, Hitachi S4800). Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were obtained from a JEOL JEM-2100F transmission electron microscope operated at an acceleration voltage of 200 kV. The TEM-mapping and three-dimensional reconstruction images were carried out on FEI Talos F200X with a field emission gun operating at 200 kV in bright-field (BF) and high-angle annular dark-field imaging (HAADF) modes. In the 3D TEM images forming process, a mass of TEM images was automatically collected over a tilt-range from −63° to +54° using the TEM TOMOGRAPHY software. The step of the images was 2° per image from -30° to +30° and 1° per image at other angles. Subsequently, the images were filtrated, adjusted and reconstructed in Inspect 3D. At last, the reconstructed data was transferred into AVIZO to build visible images and videos. Electrochemical measurements To make LIB anode materials, CSC, Super P, and polyvinylidene fluoride (PVDF) binder with a weight ratio of 75:15:10 were mixed in a N-methyl-2-pyrrolidone (NMP) solvent. The resultant slurry was uniformly spread on a Cu foil with a loading mass of about 0.8 mg cm-2, and dried at 110℃ in a vacuum oven for 12 h. The assembly of coin cells and electrochemical testing conditions have been reported previously. 10 Briefly, CR2025-type coin cells were assembled using Li foil, Celgard


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2325 membrane, and 120 µl conventional electrolyte in an Ar-filled glovebox. The galvanostatic cycling tests were carried out in a voltage range from 3.0 to 0.01 V with

LAND

CT2001A

battery

testers.

Cyclic

voltammetry

(CV)

and

electrochemical impedance spectroscopy (EIS) measurements were performed on an electrochemical workstation (Bio-Logic VMP3, France). The electrochemical measurements of SCs were carried out on the VMP3 electrochemical workstation. The electrodes were prepared by casting the slurry (the same content as in LIBs) on the nickel foam with a loading mass of about 1.5 mg cm-2. The capacitive performance and electrochemical tests (CV and galvanostatic cycling tests) were evaluated using previously reported experimental conditions. 24

Results and Discussion The synthesis process of CSC is illustrated in Figure 1A. The functionalized MWCNTs are negatively charged due to the presence of oxygen containing groups formed during the acid treatment.47 When Co2+ ions were added to the methanol suspension of functionalized MWCNTs, they got attracted onto the surface of MWCNTs by electrostatic interaction.29 Subsequently, methanol solution of 2-Methylimdazole was added to coordinate with Co2+ so as to generate ZIF-67. On account of the electrostatic attraction between Co2+ and MWCNTs, the organic ligands and Co2+ self-assembled on MWCNTs to form the structure of ZIF-67 particles

with

MWCNTs

penetration

(ZIF-67/MWCNTs).

Afterwards,

the

ZIF-67/MWCNT precursors react with sulfide ions released during the



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decomposition of TAA in the solvothermal treatment (Figure 1B), originally forming Co-S outer shells as protective layers or scaffolds. Since the diffusion rate of Co2+ ions is higher than that of sulfide ions, the internal Co2+ ions are inclined to escape from ZIF-67 particles but finally accumulate at the outer protective shells, forming the unique hollow structure

48,49

. In this process, the oxygen containing

groups of MWCNTs are reduced while the hollow structure remains unchanged 50. After the subsequent heat treatment in N2 atmosphere, the Co-S hollow shells are crystallized into Co3S4 and the CSC is obtained. 48 The microstructures of ZIF-67/MWCNTs and CSC are confirmed by XRD measurements. The XRD patterns of ZIF-67 and ZIF-67/MWCNTs are shown in Figure 2A. All diffraction peaks of ZIF-67/MWCNTs match well with ZIF-67 except the broad peak at around 23o that is attributed to MWCNTs

10

. As shown in

Figure 2B, the diffraction peaks of CSC match well with those of Co3S4 crystals (JCPDS 42-1448). The peak around 35o is attributed to small amount of CoO (JPCDS 42-1300) impurities. The CoO impurities come from the dehydration of Co(OH)2 that is formed from the reaction between ZIF-67 and the presence of water in absolute ethanol (water content ≤0.3%).24 FESEM and TEM images are taken to analyze the morphology and the microstructure of the as-synthesized materials. It can be seen from the FESEM (Figure S1A) and TEM (Figure S1B) images that the ZIF-67 crystals show rhombic dodecahedron morphology with the average size of 300 nm. The SEM (Figure 3A) and TEM (Figure 3B) images of the ZIF-67/MWCNTs demonstrate that the


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MWCNTs penetrate into the internal space of the ZIF-67 rhombic dodecahedra and are interwoven to form conductive networks. Figure S2A and Figure S2B are the FESEM and TEM images of the Co-S hollow shells prepared by the hydrothermal process of ZIF-67. The hollow structure is well defined and generally maintains the polyhedron shape of ZIF-67. The morphology and the microstructure of the CSC is shown in Figure 3C-E. The FESEM image in Figure 3C reveals the well maintained quasi-polyhedron shape of cobalt sulfides embedded with MWCNTs. From the TEM images in Figure 3D and E, it can be clearly seen that MWCNTs penetrate into the inner space of the hollow Co3S4, confirming the in-situ growth of ZIF-67 particles on MWCNTs and the successful sulfurization of MOF templates described above. The HRTEM image (Figure 3F) reveals that the Co3S4 shells in the nanocomposites are well crystallized with crystal lattice fringes of 0.29 nm, corresponding to the (311) plane of Co3S4. The elemental composition of the CSC is analyzed by TEM EDS mapping in Figure 4B–E. The distribution of C element exhibits the configuration of MWCNT networks (Figure 4B). And Figure 4C and 4E give the evidence of the homogeneous distribution of Co and S elements on the hollow shell. In Figure 4D, there is a little O element existing in the nanocomposites, which can be attributed to the formation of a small quantity of CoO due to the presence of water in absolute ethanol and the oxygen containing groups of MWCNTs. 24 Figure 4G shows the combined picture with C, Co, S, O elements and HAADF images, which clearly reveals the composition of the CSC.



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To better study the unique structure of the CSC, TEM tomography is applied where ensemble of TEM images of the sample from multiple tilt angles are ‘back-projected’ to form a 3D reconstruction. 51 The images obtained at -50o, 0o and 50o (Figure 5A-C) and the video (Video S1) shows that the hollow shell of the CSC nanocomposites with the general outline of the ZIF-67 crystal. In addition, by rotating the sample, it clearly shows that the MWCNTs traverse the hollow Co3S4 shell. The 3D reconstruction image (Figure 5D) and videos (Video S2 and S3) of the CSC nanocomposites confirm that the stick-like MWCNTs was distinctly inserted into the shell and form a novel structure which is beneficial for improving the electrochemical performance. XPS is carried out to investigate the elemental composition and the electronic structure of the CSC. The XPS full spectrum (Figure S3A) reveals that the nanocomposites mainly contain C, O, Co and S elements, which is consistent with the TEM mapping results. The high-resolution spectrum of Co 2p (Figure S3B) is composed of Co 2p3/2 and Co 2p1/2 and can be divided into five peaks, 2p3/2 of Co2+ (778.9eV) and Co3+ (781.2eV) ions, 2p1/2 of Co2+ (794.0eV) and Co3+ (797.2eV) ions, as well as the corresponding satellite peak (802.3eV).

52

This implies that the Co

element in Co3S4 is made up of Co2+ and Co3+ ions. Thermogravimetric analysis is used to accurately determine the content of MWCNTs (Figure S4). According to the TGA data, the Co3S4 content in the sample is estimated to be 73.45 wt.% .53 The porous features of ZIF-67/MWCNTs and CSC are examined by nitrogen adsorption–desorption analysis. As displayed in Figure 6, the ZIF-67/MWCNTs


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shows a BET specific surface area of 1050 m2 g−1 with the pore diameter distribution concentrated in the range of 2-5 nm, indicating that the porous structure of ZIF-67 remains with the incorporation of MWCNTs. The nitrogen adsorption–desorption isotherms of the CSC show a typical IV-type isotherm with a H3 type hysteresis loop, suggesting the mesoporous nature of the nanocomposites.54 Besides this, the nanocomposite shows a BET specific surface area of 112 m2 g−1, which is much higher than the hollow Co3S4 (59 m2 g-1). The large specific surface area gives rise to large contact surface between electrode and electrolyte to enhance the ion-electron exchange and thus the electrochemical activity. 55 The pore size distribution calculated by Discrete Fourier Transform (DFT) method confirms the presence of mesopores of 2–15 nm in size. The hierarchical porous characteristics can buffer the volume change during charge/discharge process and facilitate the transport of Li+ with abundant micropores and mesopores serving as pathways for electrolyte transport. 56 The electrochemical behavior of CSC as anode for LIBs was firstly studied by CV and galvanostatic cycling tests. Figure 7A shows the CV profiles in the first five cycles of the CSC from 0.01 to 3 V vs. Li+/Li at a scan rate of 0.5 mV s−1. Clearly, the obvious reduction peak at around 0.9 V in the first cycle can be attributed to the lithium insertion into Co3S4 to form Li2S and Co nano-particles. The weak and broad peak located at about 0.5 V can be associated with the irreversible formation of solid electrolyte interface (SEI) layer. The peak near 0.1 V corresponds to the lithium-intercalation of MWCNTs. The two main anodic peaks at around 2.1 V and 2.35 V correspond to the oxidation of Co metal back to Co3S4.25After the first two



cycles, the two reduction peaks centered at 1.7 V and 1.3 V exhibit a remarkable shift from the original peaks, and the CV curves thereafter turn to be stable, suggesting enhanced reversibility of lithiation/delithiation process upon cycling. On the other hand, the two anodic peaks located at 2.1 V and 2.35 V remain constant from the first cycle.57 This result is consistent with the representative charge/discharge profiles measured between 0.01 and 3 V at a current density of 200 mA g-1 in Figure S5A, where the lengthy discharge plateau at 1.2 V in the first cycle corresponds to the lithiation reaction of Co3S4. The obvious charge plateau around 2.0V can be attributed to the oxidation reaction of Co nanocrystals. 45 Figure 7B shows the specific capacity in the galvanostatic charge/discharge measurements at a current density of 200 mA g-1. The initial specific discharge and charge capacities are 1644.2 mAh g-1 and 1055 mAh g-1 with the Coulombic Efficiency (CE) of 64.17%. The large irreversible capacity in the first cycle is due to the incomplete recovery of the Co3S4, the decomposition of electrolyte, and the formation of irreversible SEI layer.

58

In the second cycle, the specific discharge and

charge capacity become 1246.3 mAh g-1 and 1134.7 mAh g-1, respectively. The reason for the higher charge capacity in the 2nd cycle than in the 1st cycle is due to the unstable incipient electrochemical reaction,

59

which is confirmed by the higher

charge plateau in Figure S5A. After the first cycle, CSC shows relatively stable cycling performance. The CE after the fifth cycle is all above 96%, and a reversible capacity of 1281.2 mAh g-1 is retained after 50 cycles at 200 mA g-1. The slight augment of specific capacity after 20 cycles is attributed to the reversible formation of


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a gel-like polymeric layer providing excess capacity through a “pseudo capacitance-type behavior”, which commonly takes place in electrodes based on transition metal compounds.

60, 61

This phenomenon is also confirmed by the

charge/discharge profile of the 50th cycle as shown in Figure S5A, where the voltage plateau is ambiguous with a gentle slope. In comparison, hollow Co3S4 shells without MWCNTs exhibit moderate initial discharge and charge capacities of 1271.1 mAh g-1and 919.2 mAh g-1 at 200 mA g-1, respectively, and a poor specific capacity retention down to 240 mAh g-1 after 50 cycles. Obviously, the introduction of MWCNTs endows CSC with higher specific surface area and better electrochemical activity, resulting in higher initial capacities. In the first ten cycles (Figure 7B), the Co3S4 electrode experiences a substantial capacity loss but the CSC electrode doesn’t because the MWCNTs can effectively accommodate the volume expansion by the scaffold effect and subsequently reduce the active material loss. Figure 7D shows the long-term cycling performance of the CSC at the current density of 2 A g-1. The gradual capacity decay in the first 50 cycles might be due to the stabilization of SEI film and irreversible trapping of lithium ions in the Co3S4 lattice at high current density.

62

After that, a slow increase in specific capacity is observed due to the

activation caused by high-rate discharge/charge and the reversible formation of a gel-like polymeric layer.

61

Remarkably, the CSC eventually delivers a reversible

capacity of 976.5 mAh g-1 after 500 cycles with CE close to 100%, which is superior to most of the cobalt sulfide nanocomposite materials in previous reports (Table S1). Rate capability of the electrode is tested through galvanostatic measurements at



Page 14 of 30

different current densities (Figure 7C). As the current densities increase from 250 to 500, 1000, 2000 and 4000 mA g-1, the specific capacity of the CSC electrode becomes 1013.3, 951.8, 835.3, 685.3, 521.2 mAh g-1, respectively. When the current density turns back to 250 mA g-1, the specific capacity recovers to 1098.1 mAh g-1. In contrast, the Co3S4 only shows 155.7, 102.7, 70.8, 49.2 and 34.6mAh g-1 at corresponding current density. In Figure S5B, the charge–discharge voltage profiles of different current densities are shown. With the increase of the current density, the plateau voltage increases due to the influence of resistance, indicating high electrochemical reversibility of the CSC. To further understand the superior electrochemical performance of the CSC, EIS analysis was employed. Figure S6 exhibits the Nyquist plots of CSC and Co3S4 electrode after 50 cycles. The compressed semicircles in the high and medium frequency range are associated with the charge-transfer resistance (Rct); the straight line in the low frequency range corresponds to the Warburg impendence (Zw).

63

The

diameter of the semi-circle in the plots of CSC electrode is remarkably reduced compared to that of Co3S4, confirming that the introduction of MWCNTs in the composites can effectively reduce the charge-transfer resistance. TEM (Figure S7A) and SEM (Figure S7B) images are taken for the electrode after 50 cycles. Clearly, the Co3S4 shells are partly aggregated but the outline remains. MWCNTs still penetrate through the particles so that the Co3S4 retains the electrochemical activity in the charge/discharge process. The CSC nanocomposites as SC electrodes are also evaluated using a


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three-electrode configuration with a Pt counter electrode and a saturated calomel electrode (SCE) reference electrode in a 2.0 M KOH aqueous solution. The CV performance of the CSC is measured with a scan rate ranging from 2 to 30 mV s-1. The redox peak in Figure 8A is associated with the reversible reaction of CoS/CoSOH. 24

The galvanostatic charge/discharge voltage-time profiles at the current densities

from 2 A g-1 to 30 A g-1 is shown in Figure 8B. The voltage plateaus corresponding to the redox peak verify the existence of pseudocapacitive performance. 64 Thanks to the unique structure with the incorporation of MWCNTs, the CSC exhibits high capacitances of 850.3, 815.6, 768.5, 696.6, and 638.5 F g-1 at current densities of 2, 5, 10, 20, and 30 A g-1, respectively, which is much better than the bare Co3S4(Figure 8C). These values are obviously superior, when compared with related research listed in table S2. Figure 8D shows the long-term cycling test at the current density of 10 A g-1. After 5000 cycles, 78.98% of the initial capacitance is preserved, showing a more stable cycling performance than Co3S4 electrode (64.89% after 2000 cycles, Figure8D). FESEM images of the CSC electrodes before (Figure S9A) and after supercapacitor tests (Figure S9B) show that the shape of the Co3S4 shells is well maintained, verifying the stability of the electrode. 64

Conclusions Hollow Co3S4 quasi-polyhedron/MWCNT nanocomposites were synthesized via a facile two-step heat treatment of ZIF-67/MWCNTs. The nanocomposite exhibits a unique structure where the MWCNTs penetrate through Co3S4 hollow shells to form conductive networks. The nanocomposites show high specific capacity and excellent



cyclic stability as anode materials in LIBs (1281.2 mAh g-1 after 50 cycles at 200mAh g-1 and 976.5 mAh g-1 after 500 cycles at 2A g-1). They also show remarkable electrochemical performance as electrodes of the SCs. The excellent electrochemical property of the CSC can be attributed to: (a) the hollow-shell structure that can mitigate the pulverization and inactivation caused by the volume expansion of Co3S4 during cycling process; (b) the improved electron transport between the MWCNTs and the hollow Co3S4 particles in the electrode to enhance and stabilize the rate performance; (c) the high surface area and abundant micropores and mesopores provide pathways for electrolyte transport to activate the electrode and increase the specific capacity. This work will inspire future research on new nanocomposites of transition metal sulfide/carbon-based material for applications in SCs, LIBs, catalysis and so on.

Corresponding Authors: Email: [email protected] (Huanan Duan) Email: [email protected] (Hezhou Liu)

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional figures about SEM/TEM/EIS/XPS/TGA characterization and additional table about the performance comparison with other cobalt sulfide based materials.


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ORCID Huanan Duan: 0000-0003-3052-3905 Notes The authors declare no competing financial interest.

Acknowledgements This work is supported by the Natural Science Foundation of China (no. 11304198), SMC-Chen Xing Young Scholar Award of SJTU, and State Key laboratory of Rare Earth Resource Utilization (RERU2016008). Instrumental Analysis Center of Shanghai Jiao Tong University, National Engineering Research Center for Nanotechnology, Prof. Peng Zhang's group, Mr. Xiaoqian Feng, and Miss Cheng Yang of SJTU are gratefully acknowledged for assisting with relevant analysis.

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Figure 1. Schematic illustration of the preparation of CSC: (A) the whole process and (B) the hollow Co3S4 structure formation.



Figure 2. XRD patterns. (A) The XRD patterns of the ZIF-67 and the ZIF-67/MWCNTs, and (B) the XRD patterns of the CSC.

Figure 3. (A) FESEM and (B) TEM images of ZIF-67/MWCNTs, (C) FESEM, (D, E)


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TEM, and (F) HRTEM images of the CSC.

Figure 4. (A) BF, (B to E) the corresponding element mapping, (F) HAADF-STEM, and (G) combined image of B to F of the CSC.

Figure 5. BF images of the CSC with the tilt angle of (A) -50o, (B) 0o and (C) 50o, and (D) the 3D reconstruction images of the CSC.



Figure 6. (A) Nitrogen adsorption-desorption isotherms and (B) the corresponding pore size distribution of the Co3S4, ZIF-67/MWCNTs and the CSC.

Figure 7. (A) CV curve of the first five cycles of the CSC, (B) cycling performance and coulombic efficiency of the CSC and the Co3S4 at 200 mA g-1, (C) rate performance of the CSC and Co3S4 from 250 mA g-1 to 4 A g-1, and (D) cycling performance and coulombic efficiency of the CSC at 2 A g-1.


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Figure 8. (A) CV curves of the CSC at various scan rates from 2 to 30 mV s-1, (B) charge/discharge voltage profiles of the CSC at various current densities from 2 to 30 A g-1, (C) specific capacitance values of the CSC and Co3S4 at various current densities, and (D) cycling stability of the CSC and Co3S4 electrode at a current density of 10 A g-1.




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MWCNT

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