Metal-Organic Framework Template Synthesis of NiCo2S

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Metal-organic frameworks template synthesis of NiCoS@C encapsulated in hollow nitrogen-doped carbon cubes with enhanced electrochemical performance for lithium storage Dongxia Yuan, Gang Huang, Dongming Yin, Xuxu Wang, Chunli Wang, and Limin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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ACS Applied Materials & Interfaces

Metal-organic frameworks template synthesis of NiCo2S4@C encapsulated in hollow nitrogen-doped carbon cubes with enhanced electrochemical performance for lithium storage Dongxia Yuan †, ‡, Gang Huang †, ‡, Dongming Yin †, ‡, Xuxu Wang †, Chunli Wang † and Limin Wang *† † State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, CAS, Changchun, 130022, P.R. China; ‡ University of Chinese Academy of Sciences, Beijing, 100049, P.R. China.

*Corresponding author: Prof. Limin Wang, E-mail: [email protected], Tel. +86 431 85262447 KEYWORDS : NiCo2S4; nitrogen-doped; carbon coating; electrode material; lithium storage; electrochemical performance

ABSTRACT: Owing to its richer redox reaction and remarkable electrical conductivity, bimetallic nickel cobalt sulfide (NiCo2S4) is considered as an advanced electrode material for energy storage application. Herein, nanosized NiCo2S4@C encapsulated in hollow nitrogendoped

carbon

cube

(NiCo2S4@D-NC)

has

been

fabricated

ACS Paragon Plus Environment

by

using

core@shell


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Ni3[Co(CN)6]2@polydopamine(PDA) nanocube as precursor. In this composite, the NiCo2S4 nanoparticles coated with conformal carbon layers are homogeneously embedded in 3D highconduction carbon shell from PDA. Both the inner and outer carbon coatings are helpful to increase the electrical conductivity of the electrode materials and prohibit the polysulfide intermediates from dissolving in the electrolyte. When researched as electrode materials for lithium storage, thanks to the unique structure with double layers of nitrogen-doped carbon coating, the as-obtained NiCo2S4@D-NC electrode maintains an excellent specific capacity of 480 mAh g-1 at 100 mA g-1 after 100 cycles. Even after 500 cycles at 500 mA g-1, a reversible capacity of 427 mAh g-1 can be achieved, suggesting the excellent rate capability and ultralong cycling life. Such remarkable lithium storage property indicates its potential application for future lithium ion batteries.

1. Introduction Lithium ion batteries (LIBs) have been recognized as dominant power sources for portable electronic devices, electric/hybrid electric vehicles due to the high energy density, long cycling life and environmental benignity.1, 2 However, the most used graphite anode cannot satisfy the ever-growing requirements for fast development of society and economy. Therefore, substantial efforts have been devoted to designing and exploring novel electrode material with higher capacity and longer cycle life.3-5 Recently, bimetallic NiCo2S4 have attracted much attention due to their intriguing advantages. Compared with nickel sulfides and cobalt sulfides, NiCo2S4 has much higher electronic conductivity and richer redox chemistry, which are helpful to enhance its electrochemical properties.6-8 However, there are still some problems limiting its practical application. One problem is the huge volume change during cycling leading to poor cycling


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performance. Moreover, the formation of polysulfide intermediates will further deteriorate its performance, resulting in severe loss of active material and irreversible structure destruction. Substantial efforts have been made to tackle these problems, one interesting way is to employ the ether-based electrolyte and control cut-off voltage of discharge. The ether-based electrolyte presents a higher chemical stability with the polysulfides formed by metal sulfides during cycling process than that of carbonate-based electrolyte.9-11 Another effective method is to synthesize low-dimensional nanostructured metal sulfides with unique morphology, such as hollow nanospheres12 and nanorods13. These nanostructures can help to shorten the lithium-ion diffusion length and mitigate volume change stress. Wrapping metal sulfides with a carbon layer is also an effective approach to enhance electrical conductivity and maintain structure integrity of the composite.14 What’s more, it can help to suppress the dissolution of polysulfides15. Despite these advantages, it still remains a challenge to prepare nanostructured NiCo2S4 composite with controlled morphologies and excellent cyclability as electrode material for LIBs. Metal-organic frameworks (MOFs), which are assembled by linking metal ions and organic ligands, have been widely used in various fields, such as gas storage and catalyst

16-18

. Inspired

by their controllable particle size and morphology, MOFs with different chemical compositions have been recognized as promising precursors or templates to synthesize porous materials: metal oxides 19, porous carbons

20

, metal sulfides

21

metal sulfides/carbon materials

22

and so on. It is

known to all that nitrogen-doped porous carbons demonstrate superior electrochemical performance and can be easily produced by using N-rich MOFs as precursors.23,24 As one of typical Prussian Blue Analogues(PBA), Ni3[Co(CN)6]2 is rich in nickel, cobalt, carbon and nitrogen elements.25,

26

And PDA can easily deposit on any solid material surface and be

converted to N-doped carbon layer due to the existence of N-rich indole quinone units.27, 28So



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the combination of Ni3[Co(CN)6]2 and PDA can be an efficient method to synthesize nano-sized bimetallic NiCo2S4 encapsulated by double layers of nitrogen-doped carbon as electrode materials for lithium storage. In this paper, we have synthesized the composite of NiCo2S4@C nanoparticles encapsulated in hollow

nitrogen-doped

carbon

cubes

(NiCo2S4@D-NC)

by

using

core@shell

Ni3[Co(CN)6]2@PDA nanocube as the precursor. The NiCo2S4 nanoparticles would be protected by the inner and outer carbon matrix. As a proof-of-concept application, the as-prepared NiCo2S4@D-NC composite is investigated as anode material for LIBs. NiCo2S4@D-NC delivers a high specific capacity after 500 cycles at 500 mA g-1. The results indicate that NiCo2S4@D-NC composite demonstrates satisfactory electrochemical performance.

2 Experimental Section 2.1 Synthesis of Ni3[Co(CN)6]2 and Ni3[Co(CN)6]2@PDA PBA nanocubes Ni3[Co(CN)6]2 nanocubes were synthesized according to a previously reported method with some modifications29. The detailed experimental procedure was described as follow. Typically, 6 mmol (1.426 g) of NiCl2•6H2O and 9 mmol (2.647 g) of trisodium citrate were firstly dispersed into deionized water (DIW) (200 mL) to form transparent solution (denoted as solution A). And 4 mmol (1.33 g) of K3[Co(CN)6] was dispersed in DIW (200 mL) to form homogeneous solution (denoted as solution B). Solution B was then added into solution A slowly under continuous stirring for 30 min. Without any interruption, the mixture solution was aged for 12 h at ambient temperature. Finally, precipitates were collected by centrifugation and washed three times by using DIW and alcohol, respectively, then dried at 60 °C overnight. To obtain Ni3[Co(CN)6]2@PDA nanocubes, 40 mg of dopamine hydrochloride was quickly dissolved in


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100 mL of tri-butter solution (0.1 M ， pH=8.5) to form a pink solution, then 80 mg of Ni3[Co(CN)6]2 was dispersed into the pink solution under magnetic stirring for 3 h at room temperature30. The resulting black products were collected by centrifugation washed three times by using DIW and alcohol, respectively, then dried at 60 °C for overnight. 2.2 Preparation of NiCo@NC and NiCo@D-NC composites The as-prepared Ni3[Co(CN)6]2 and Ni3[Co(CN)6]2@PDA were heat-treated in Ar gas flow at 600 °C for 2 h with a ramping rate of 2 °C min-1, followed by cooling downing to ambient temperature naturally. Thereafter, the NiCo@NC and NiCo@D-NC composites were obtained, respectively. 2.3 Preparation of NiCo2S4@NC and NiCo2S4@D-NC nanocubes The sulfidation process was performed in a tubular furnace, 1.0 g of thiourea was put in the upstream side, 40 mg of NiCo@NC and 40 mg of NiCo@D-NC nanocubes were placed next to the thiourea at a downstream side, respectively. The tubular furnace temperature was firstly increased to 350 °C with a ramping rate of 1°C min-1, and then kept for 2 h at this temperature under Ar flow to yield the final products, respectively. Thiourea was as sulfur source and can be thermally decomposed to H2S gas. 2.4 Characterization and Electrochemical Measurements Phase constitutions of the produces were conducted on Powder X-ray diffraction (XRD, Bruker D8 Focus). Raman spectra were collected by a Jobin Yvon Labor spectrometer (laser, 532 nm). Microstructures and morphologies are performed by electron microscopy (SEM, Hitachi S-4800; TEM, FEI Tecnai G2 S-twin). X-ray photoelectron spectroscopy (XPS) analysis was measured



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by an ESCALABMKLL XPS instrument. A JY2000 Ul-trace ICP Atomic Emission Spectrometer (ICP-AES) was used to collect the elemental analysis data. The N2 adsorption/desorption measurement and the pore size distribution were obtained by using a Micromeritics ASAP 2010 instrument.Thermogravimetric analysis (TGA) curves were measured by a 449 °C Jupiter TG analyzer with a ramping rate of 10 °C per min. CR 2025 coin-type cells were used to characterize the electrochemical performances of the composites. By mixing 70% active material, 20% carbon black and 10% poly(vinylidene difluoride) using N-methyl-2-pyrrolidone (NMP) as solvent, the working electrode was fabricated by using copper foil as current collector. The active material loading was about 1.8 mg. Based on the total mass of the composite (Nitrogen-doped carbon and metal sulfides), the specific capacity was calculated, though carbon has non-real contributions to capacities within the potential window of 1.0-3.0 V. By using 1M LiN(CF3SO2)2(LiTFSI) in 1,3-dioxolane(DOL) /dimethoxymethane(DME) (1:1 in volume) as the electrolyte, Celgard 2300 microporous membrane as separator and lithium foil as counter electrode, the coin cells were assembled in a Ar-filled glovebox and galvanostatic cycled on a Land battery tester at 1.0-3.0 V vs. Li/Li+ at room temperature. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were conducted on a BioLogic VMP3 electrochemical workstation. 3. Results and Discussion The fabrication procedure of NiCo2S4@D-NC is schematically shown in scheme 1. Uniform Ni3[Co(CN)6]2 nanocubes are synthesized by simple mixing of NiCl2 and K3[Co(CN)6] in the present of sodium citrate. Then the surface of Ni3[Co(CN)6]2 nanocubes is coated by PDA to form core-shell Ni3[Co(CN)6]2@PDA with cube-like structure. The XRD patterns of


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Ni3[Co(CN)6]2 and Ni3[Co(CN)6]2@PDA are depicted in Figure S1a(Supporting Information). All of the diffraction peaks of the XRD patterns can be indexed to Ni3[Co(CN)6]2•12H2O (JCPDS card no.89-3738). And the ICP-AES result of Ni3[Co(CN)6]2 shows that the atomic ratio of Ni/Co is about 3/2, which is consistent with the Ni/Co value in Ni3[Co(CN)6]2•12H2O. It can be seen from SEM images (Figure S2, Supporting Information), Ni3[Co(CN)6]2@PDA have a similar cube-like structure with Ni3[Co(CN)6]2, except that Ni3[Co(CN)6]2@PDA have more courser surface. Thermogravimetric analysis (TGA) curves of the precursors have three decomposition processes (Figure S3, Supporting Information). The first weight loss is ascribed to the loss of water molecules. The weight loss in the second step takes place because of the decomposition of the CN- and PDA. The slow weight loss step over 600 °C in the TGA curve is probably due to the loss of nitrogen element.31 Based on the above TGA data, the as-synthesized Ni3[Co(CN)6]2 and Ni3[Co(CN)6]2@PDA were calcined at 600 °C for 2 h at Ar atmosphere to form NiCo@NC and NiCo@D-NC nanocomposites, respectively. XRD patterns of both samples can be assigned to metallic cobalt and metallic nickel (Figure S1b, Supporting Information). SEM images of NiCo@NC and NiCo@D-NC are shown in Figure S4a and S4c. NiCo@NC exhibits irregular particle-like core-shell structure due to the collapse of framework. While the introduction of the PDA makes NiCo@D-NC remain the pristine cube-like structure. TEM images are employed to further investigate the core-shell structure. From Figure S4b and S4d, it is found that the organic ligands from MOF and the PDA are successfully transformed into carbon, and NiCo nanoparticles are well wrapped by carbon. The NiCo nanoparticles in the carbon matrix can be successfully transformed into NiCo2S4 without destroying the structure of their precursors due to the high reactivity of NiCo



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nanoparticles, which can readily react with H2S gas produced by decomposition of thiourea under Ar atmosphere at 350 °C.32 These samples have been denoted as NiCo2S4@NC and NiCo2S4@D-NC, respectively. The corresponding XRD patterns (Figure 1a) of both samples can be assigned to the standard pattern of NiCo2S4 (JCPDS card no.20-0872) or CoNi2S4 (JCPDS card no. 24-0334). The nature of carbon from both samples is also confirmed by Raman spectra (Figure 1b), The D bands at1348 cm-1 and G bands at 1596 cm-1 indicate the existence of carbon formed during pyrolysis.33 From SEM images of NiCo2S4@D-NC (Figure 2a and 2b) and NiCo2S4@NC (Figure S5a and S5b, Supporting Information), it can be seen that both of the products well preserve the morphologies of their precursors. TEM image in Figure 2c shows that NiCo2S4@D-NC with a porous feature which inherits the core-shell structure of NiCo@D-NC. Numerous NiCo2S4 nanoparticles are uniformly distributed in inner carbon matrix, and there is existing thin carbon shell derived from PDA carbonation process. The element mapping in Figure 4d reveals that Ni, Co, N, and C are uniform distributed over the whole nanocube structure, implying coexistence of all elements and the successful formation of nitrogen-doped carbon. And the atomic ratios of Ni/Co/S in the NiCo2S4@NC and NiCo2S4@D-NC are estimated to be

3:2:6 by ICP-AES analysis, implying both the samples have the NiCo2S4 and

CoNi2S4 phase. The N-doped carbon contents of both NiCo2S4@NC and NiCo2S4@ D-NC can be measured by TGA and XRD analysis (Figure S6, Supporting Information). From the TGA and ICP-AES results, the N-doped carbon contents of NiCo2S4@NC and NiCo2S4@ D-NC can be calculated to be 8.9 % and 16.2 %, respectively. XPS tests are carried out for NiCo2S4@NC and NiCo2S4@D-NC to determine the chemical composition and chemical bonding states. As shown in Figure 3a, the signals of Ni, Co, S, N, C and O have been detected according to the full XPS spectrum of NiCo2S4@D-NC. The O


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element may be ascribed to the exposure of the sample to air. The C1s peaks (Figure 3b) can be fitted into five Gaussian peaks located at 284.6, 285.6, 286.6, 289.2 and 292.8 eV ascribed to C=C/C-C, C-S, C-N, C=O and π-π* shake up peaks, separately. The C-N bond further confirms the existence of the nitrogen-doping in carbon matrix. And the π-π* shake up peaks at 292.8 eV indicates that the existence of partial graphitic structure.34 As regards to the N 1s XPS spectrum, the binding energy at 398.4, 399.8, 401.4 eV are consistent with the pyridinic, pyrrolic and graphitic N doped in carbon matrix.35, 36 XPS spectrum of S 2p is shown in Figure 3d, the peaks at 161.3 and 162.5 eV are associated with S 2p3/2 and S 2p1/2, and the peak at 163.8 may be due to some oxidized species.37 As shown in Figure 3e and 3f, both the Ni 2p and Co 2p can be fitted with two shake-up satellites (marked as “Sat.”) and two spin-orbit doublets. For Ni 2p spectrum, the peaks at 853.0 and 870.9 eV are ascribed to Ni2+, and the peaks at 855.3 and 875.1 eV are assigned to Ni3+.38 Similarly for Co 2p spectrum, the two spin-orbit doublets at 778.1 and 793.2 eV, and 779.4 and 797.6 eV are assigned to Co3+ and Co2+, respectively.39 XPS spectra of NiCo2S4@NC are given in Figure S7 (Supporting Information)，which are similar to those of NiCo2S4@D-NC, except that the intensity of the Ni 2p and Co 2p in NiCo2S4@NC are stronger than that of NiCo2S4@D-NC. The difference may be due to most of the NiCo2S4 nanoparticles in NiCo2S4@D-NC are encapsulated by double carbon layers, which make the signals of Ni 2p and Co 2p more difficult to be detected. The porous property of NiCo2S4@D-NC has been investigated by using nitrogen adsorptiondesorption isotherm. As shown in Figure S8 (Supporting Information), the hysteresis loop at 0.51.0 V demonstrates that it can be classified as a type IV isotherm, indicating its mesoporous characteristic.40,

41

The BET surface area is calculated to 39.7 m2 g-1. And the pore size

distribution curve (Figure S8, Supporting Information) has a sharp peak at 10.9 nm and a broad



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peak at 31.2 nm, calculated by using Barrett-Joyner-Halenda (BJH) method, which further demonstrates that NiCo2S4@D-NC possess the mesoporous structure . The high surface area and porosity can not only offer extra active sites for lithium storage but also effectively promote mass diffusion and ion transport in lithiation/delithiation processes. CV measurements at a sweep rate of 0.1 mV s-1 are conducted to evaluate the electrochemical properties of NiCo2S4@NC and NiCo2S4@D-NC as anode materials for LIBs. CV curves of the NiCo2S4@D-NC in the first three cycles are depicted in Figure 4a. In the first cycle, two reduction peaks around 1.60 and 1.11 V are ascribed to Li+ insertion into the NiCo2S4 lattices, and then conversion reaction of Li+ with NiCo2S4 to form Ni, Co and amorphous Li2S matrix.42, 43

The oxidation peak located at 1.98 V can be related to the oxidation of metallic Ni and Co to

nickel sulfides and cobalt sulfides.44 And the peak at 2.40 V may be attributed to the formation of polysulfide intermediates.45 In the following cycles, the weak cathode peak at 1.11 V up shifts to 1.30 V, while the widen peak around 1.60 V splits into two peaks at 1.69 and 1.80 V at higher voltage range. This phenomenon is probably attributed to the irreversible phase change of NiCo2S4 at the first cycles. The anodic peak locations in the subsequent process keep almost unchanged, but the peak intensity is stronger than the corresponding cathode one, which may be caused by the formation of polysulfide intermediates during cycling.

The dissolution of

polysulfide intermediates may also cause the over-charge, resulting in the active material loss and the electrode degradation.46-49 The behaviors of the redox reaction with similar features are observed in the CV curves of the NiCo2S4@NC (Figure S9a, Supporting Information). Galvanostatic charge-discharge measurements are measured in the ether-based electrolyte at the potential range of 1.0-3.0 V. all specific capacities provided in this paper are calculated by using the total mass of the composites. Representative charge-discharge curves of the


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NiCo2S4@D-NC are displayed in Figure 4b, which are measured at 100 mA g-1. For NiCo2S4@D-NC electrode, The initial discharge capacity and charge capacity are 526 and 503 mAh g-1, respectively, yielding a high coulombic efficiency of 96 %, which surpasses the values reported in literatures,42, 50 This phenomenon may be ascribed to the fact that the cut-off voltage of discharge is upgraded to 1.0 V, as some irreversible side reactions such as the formation of solid electrolyte interface (SEI) layer caused by the decomposition of electrolytes may take place when the voltage is below 1.0 V.11 In the initial discharge curve, there is a long sloping curve between 1.8 and 1.2 V, followed by a flat plateau around 1.2 V. Two plateaus at about 1.9 and 2.4 V can be observed in the first charge curve. The subsequent discharge curves display three plateaus at voltage of 1.9, 1.7 and 1.36 V, which are good consistent with the results of CV analysis. The cycling performance of NiCo2S4@D-NC electrode at 100 mA g-1 is depicted in Figure 4c, after the first 30 cycle, it is found that the charge and discharge capacities have a rising trend, then a high reversible discharge capacity of 480 mAh g-1 can be obtained after 100 cycles. The capacity increase behavior upon cycling process is also observed for other metal sulfides and oxides, which may be attributed to the better infiltration of electrolyte during the gradual activation process.51-53 The NiCo2S4@D-NC electrode exhibits superior rate capability as well. As depicted in Figure 4d, it can be seen that the NiCo2S4@D-NC manifests excellent capacity retention with the increase of current density, and the reversible discharge capacities of 434, 407, 378, 364 and 353 mAh g-1 are remained when the current densities varied from100, 200, 500, 800 and1000 mA g-1, separately. What’s more, the reversible capacity recovered to 427 mAh g-1 when the current density reduced to 100 mA g-1. Cycling performance of the NiCo2S4@NC composite at 100 mA g-1 is given in Figure S9b(Supporting Information). The initial discharge and charge capacities of NiCo2S4@NC are 585 and 565 mAh g-1, which are



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higher than those of NiCo2S4@NC. But the cycling performance is fluctuating, after 70 cycles the capacity begins to drop gradually, which may be caused by the irreversible structure transformation and the loss of the active material. Although its cycling performance is not very good, the NiCo2S4 @NC also exhibits an excellent rate capability (Figure S 9c, Supporting Information). The cycling property of the NiCo2S4@D-NC electrode is further investigated at 500 mA g-1, from Figure 5a, we can see that a high reversible discharge capacity of 427 mAh g-1 can be obtained up to 500 cycles. Interesting, for the first 15 cycles, the coulombic efficiency values are larger than 100 %, and similar phenomena have been found in our reported work,4,10 This could be accounted for the fact that an activation process is necessary for metal sulfides to establish the stable electrochemical reaction system.52 And the low charge-discharge current density is more helpful to the activation process, so the electrode materials should be activated at a low current density before they cycled at the high current density.4 After the initial activation process, the cells remain stable coulombic efficiency in the subsequently cycling process. The long cycling performance of NiCo2S4@NC at 500 mA g-1 has also been studied (Figure S9d, Supporting Information). The reversible capacity stabilizes at around 409 mAh g-1 after the first 300 cycles, and then the discharge capacity quickly fades to 195 mAh g-1 at the 500th cycle. The capacity fading of NiCo2S4@NC after 300 cycles may be attributed to the severe structure reconstruction and the loss of the active materials.22 As the NiCo2S4 nanoparticles are well wrapped by the carbon layer, which can not only help to alleviate the volume change of NiCo2S4 nanoparticles during cycling, but also suppress the loss of active material, the charge and discharge capacities of NiCo2S4@NC almost keep stable at the first 300 cycles. However, as the cycle number


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increases, the structure will be destroyed slowly, resulting in the active mass loss and leading to the severe capacity fading. 54-56 Figure 5b exhibits the EIS curves of NiCo2S4@NC and NiCo2S4@D-NC. It can be found that NiCo2S4@D-NC cell has a much lower charge transfer resistance than that of NiCo2S4@NC cell, indicating the NiCo2S4@D-NC has much faster charge transfer performance and enhanced conductivity. To evaluate the role of carbon shell and voids in the cubes played on the electrochemical performance of NiCo2S4@D-NC. We also studied the morphology evolution of NiCo2S4@D-NC at different lithiation states at the first charge-discharge cycle as displayed in Figure S10(Supporting Information). Obviously, as a buffer zone, the voids have the ability to effectively accommodate the large volume variations of NiCo2S4 nanoparticles at different lithiation states. To further illustrate the fact that the outer carbon shells and the void can help to preserve the morphology during cycling process, SEM images and TEM images of NiCo2S4@NC and NiCo2S4@D-NC after 20 charge-discharge cycles are given in Figure 6. The structure of NiCo2S4@NC is destroyed to some degree, while the cubic-like structure of NiCo2S4@D-NC can be well maintained, suggesting the introduction of outer carbon layer and the voids can help to preserve the cube-like structure. Thus, NiCo2S4@D-NC electrode demonstrates superior rate capability and cycling performance. Based on the above results, the rich chemical compositions and unique double N-doped carbon layers protection structure of NiCo2S4@D-NC can attribute to remarkable electrochemical properties.57-59 Firstly, NiCo2S4 can provide richer electrochemical reaction with the contributions of Ni and Co ions. Secondly, the inner carbon matrix can prevent NiCo2S4 nanoparticles aggregation and suppress the loss of active materials. Thirdly, the outer carbon layer can further improve the conductivity, block the polysulfide intermediates dissolution. And



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the large number of voids in cubes can release the mechanical stresses and maintain structural integrity. Finally, the hierarchical porous structure can offer more active sites, shorten the transport pathways of lithium ion and electrons and buffer the stresses caused by volumetric change when cycling. 4. Conclusions

In summary, we employed a MOF-derived synthetic approach to prepared NiCo2S4 nanoparticles encapsulated in double layers of nitrogen-doped porous carbon composite. And the NiCo2S4@DNC shows superior rate capability and distinguished cycling stability as the anode material for LIBs. The promoted electrochemical performances can be attributed to compositional and structural merits. Especially, the outer carbon layer can enhance the conductivity of the composite, prohibit the dissolution of polysulfide intermediates, and enhance electrode structure integrity during cycling process. Given its excellent electrochemical performances, NiCo2S4@DNC composite represents its potential use as anode material for future LIBs.

Acknowledgement Financial support is offered by the National Nature Science Foundation of China (Grant No. 21521092).

Supporting Information. TG curves analysis of the precursors; XRD patterns and SEM images of Ni3[Co(CN)6]2, Ni3[Co(CN)6]2@PDA, NiCo@NC and NiCo@D-NC nanocubes precursors; TEM images of NiCo@NC and NiCo@D-NC nanocubes; SEM image and TEM image of NiCo2S4@NC; XPS analysis of NiCo2S4@NC; TGA curves of NiCo2S4@NC and NiCo2S4@D-NC; XRD patterns of


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NiCo2S4@D-NC after calcined at air; N2 isotherm and size distribution of NiCo2S4@D-NC; CV curves; and Electrochemical prformance of NiCo2S4@NC; TEM images of NiCo2S4@D-NC at different states.

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Scheme1. Schematic illustration of the formation process of NiCo2S4@D-NC nanocubes.

Figure 1. (a) XRD patterns and (b) Raman spectra of NiCo2S4@NC and NiCo2S4@D-NC.



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Figure 2. (a, b) SEM images, (c) TEM image and (d) element mapping measurements of NiCo2S4@D-NC

Figure 3. XPS spectra of the NiCo2S4@D-NC: (a) Survey spectrum, (b) C 1s, (c) N 1s, (d) S 2p, (e) Ni 2p, (f) Co 2p.


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Figure 4.(a) Representative CV curves of NiCo2S4@D-NC at a scan rate of 0.1 mV s-1 between 1.0-3.0 V versus Li/Li+. (b) Charge-discharge voltage profiles of NiCo2S4@D-NC for 1st, 2nd, 10th and 20th cycles in the voltage range of 1.0-3.0 V at a current density of 100 mA g-1. (c) Capacity and Columbic efficiency versus cycle number profiles of NiCo2S4@D-NC at a current density of 100 mA g-1. (d) Rate capability of NiCo2S4@D-NC electrode.



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Figure 5. (a) Long cycling performance of NiCo2S4@D-NC at the current density of 500 mA g-1 up to 500 cycles, (b) EIS curves of the NiCo2S4@NC and NiCo2S4@D-NC electrodes.

Figure 6. SEM images and TEM images of (a, c) NiCo2S4@NC and (b, d) NiCo2S4@D-NC after 20 charge-discharge cycles at the current density of 500 mA g-1.


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Metal-Organic Framework Template Synthesis of NiCo2S

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