MOFs-derived nitrogen-doped carbon interwoven with carbon nanotubes for high sulfur content lithium–sulfur batteries

MOFs-derived nitrogen-doped carbon interwoven with carbon nanotubes for high sulfur content lithium–sulfur batteries

Journal Pre-proof MOFs-derived nitrogen-doped carbon interwoven with carbon nanotubes for high sulfur content lithium–sulfur batteries Maru Dessie Wa...

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Journal Pre-proof MOFs-derived nitrogen-doped carbon interwoven with carbon nanotubes for high sulfur content lithium–sulfur batteries

Maru Dessie Walle, Mengyuan Zhang, Ke Zeng, Yajuan Li, YouNian Liu PII:

S0169-4332(19)32585-1

DOI:

https://doi.org/10.1016/j.apsusc.2019.143773

Reference:

APSUSC 143773

To appear in:

Applied Surface Science

Received date:

18 July 2019

Revised date:

13 August 2019

Accepted date:

22 August 2019

Please cite this article as: M.D. Walle, M. Zhang, K. Zeng, et al., MOFs-derived nitrogendoped carbon interwoven with carbon nanotubes for high sulfur content lithium–sulfur batteries, Applied Surface Science(2018), https://doi.org/10.1016/j.apsusc.2019.143773

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© 2018 Published by Elsevier.

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MOFs-derived nitrogen-doped carbon interwoven with carbon nanotubes for high sulfur content lithium–sulfur batteries

Maru Dessie Walle,a, b Mengyuan Zhang,a Ke Zeng,a Yajuan Li,a, c, *and You-Nian Liua, c, College of Chemistry and Chemical Engineering, Central South University, Changsha,

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a

Department of Materials Science, College of Science, Bahir Dar University, Bahir Dar79,

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b

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Hunan 410083, P. R. China.

Hunan Provincial Key Laboratory of Chemical Power Sources, Central South University,

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c

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Ethiopia.

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Changsha, Hunan 410083, P. R. China.



Corresponding authors: Phone/Fax: 86-731-8887 9616. E-mail addresses: [email protected] (Y. Li);

[email protected] (Y.-N. Liu)

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Abstract Despite their high energy density, low-cost, and low environmental impacts compared with the current state-of-the-art lithium-ion batteries, lithium–sulfur (Li–S) batteries still suffer from the low sulfur content, dissolution of polysulfides and capacity loss, which hinder their commercial application. To address these issues, nitrogen-doped

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cubic carbon materials (NC) interwoven with carbon nanotubes (CNT) host is

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introduced to achieve high sulfur content for lithium–sulfur batteries. The NC/CNT

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composites are derived from metal–organic frameworks (MOFs), which can

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physically confine sulfur, thus providing efficient sulfur loading. The sulfur content in the as-obtained [email protected]/CNT composite is as high as 89 wt%, while the cathode

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membrane has a sulfur loading of 3.6 mg cm–2. In addition, the NC/CNT composite

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can obstruct the dissolution and outward diffusion of polysulfides. Impressively,

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under the high sulfur content and high sulfur loading conditions, the [email protected]/CNT cell exhibits initial specific capacity of 1141 mA h g–1 at the current rate of 0.5 C (837.5 mA g–1) and the capacity retains 674.4 mA h g–1 after 120 cycles. The [email protected]/CNT cathode also shows high areal capacity (4.1 mA h cm–2). The NC/CNT composite derived from MOFs has great potential applications for high-sulfur loading cathodes.

Keywords: high sulfur content, MOF-derived carbon, lithium–sulfur battery, N-doped carbon

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1. Introduction Lithium–sulfur (Li–S) batteries have recently attracted widespread attention because of the overwhelming advantages of sulfur, such as lightweight, high energy density and specific capacity, commercially available and low-environmental impacts,

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compared with the current state-of-the-art of the lithium-ion batteries[1, 2]. However,

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there are many obstacles which hinder the commercial application of Li–S batteries:

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(1) the poor electrical conductivity of sulfur causes sluggish reaction kinetics, which

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inevitably brings low utilization of the active materials[3]; (2) the dissolution of

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lithium polysulfides (Li2Sn, 4 ≤ n ≤ 8) in the electrolytes leads to an irreversible loss of the active materials, causes the common phenomenon called ‘‘shuttle effect’’,

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which results in low coulombic efficiency[4]; and (3) the large volume expansion

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(80%) of sulfur causes the destruction of the electrode, resulting in the degradation of the cycling performance of the cathode. Recently, carbon-based materials, such as carbon sponge [5], graphene or graphene oxides [6,7] carbon hollow spheres[8], and meso-/micro-porous carbon[9], have been employed as sulfur hosts for Li–S batteries. It is worth considering that the addition of carbon materials not only increase the conductivity of the insulator sulfur, but also provide porosity for electrolyte/ion infiltration. However, the weak interactions between the nonpolar sp2 carbons and the polar polysulfides cannot restrain the soluble polysulfides effectively, resulting in poor electrochemical performances [10,

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11]. Therefore, polar materials have been introduced to porous carbon to improve the interactions between carbons and polysulfides, efficiently trapping the polar sulfur species[12, 13]. Metal–organic frameworks (MOFs) are porous crystalline solids with exceptionally large surface areas, high porosity and flexible tunability[14], and show good promise

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in energy storage, gas storage, gas separation and catalysis[15]. Particularly,

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MOFs-derived carbon materials have emerged as particular interest as the

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multifunction application for Li–S batteries compared with other carbonaceous

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materials[16]. The MOFs-derived carbon has also nitrogen doping, which could

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obstruct the soluble polysulfides because of the polarity differences between nitrogen and the lithium polysulfides. For example, Li and co-workers prepared a

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MOFs-derived N-porous carbon on graphene nanosheets for physical confinement

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and chemical adsorption of polysulfides[17]. In addition, Chen and co-workers also fabricated a MOF-derived 3D nanoporous Co–N–C host for high sulfur loading[18,19]. However, the excessive use of the conductive matrix undoubtedly reduces the sulfur content in the cathode which lowers the energy density of lithium– sulfur battery[20]. Therefore, to obtain a cathode with high sulfur content and high areal sulfur loading yet remains a high challenge for the development of Li–S batteries. To acquire and achieve the high-energy density for the current Li−S batteries, the cathode with high sulfur content (˃70 wt% S) and sulfur loading (˃ 2 mg cm–2) are highly required.

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In this work, a novel MOFs-derived nitrogen-doped cubic carbon interwoven with carbon nanotubes (NC/CNT) for ultrahigh sulfur content cathode was designed. Firstly, due to its high surface area, NC/CNT composite enable to load ultrahigh amounts of sulfur. Secondly, NC/CNT can not only adsorb polysulfides, but also physically confine the soluble polysulfides. Thirdly, the NC/CNT composite

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facilitates fast electron and lithium-ion transport. Benefiting from its multifunction,

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the NC/CNT composite provides efficient ultrahigh sulfur content to improve the

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energy density of Li−S batteries. Notably, under ultrahigh sulfur content, the

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[email protected]/CNT composite shows a fabulous electrochemical performance.

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2.1 Preparation of ZIFs/CNT

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2.Experimental work

The preparation of ZIFs/CNT is briefly described as follow. In 15 mL methanol and

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60 mL distilled-water mixtures (1:4 by volume), 1.68 mmol of zinc nitrate (Zn(NO3)2) (Xilong Chemical Co., Guangdong, China) and 1.71 mmol of cobalt nitrate (Co(NO3)2) (Aladdin, Shanghai, China) were dissolved under the magnetic stirrer. Then, 6.09 mmol of 2-methylimidazole (C4H6N2) and 50 mg of CNT (20 – 40 nm, Aladdin, Shanghai, China) were added to the above solution. The solution was stirred for 6 h. Finally, ZIF/CNT was obtained by filtration and washed several times with methanol and dried at 70 oC.

2.2 Preparation of NC/CNT composite

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The as-synthesized ZIFs/CNT was put on a quartz boat and placed in a tube furnace for carbothermal reductions of ZIFs. The tube furnace was heated to 900 oC with a heating rate of 5 oC min–1 under nitrogen gas for 3 h. After cooling down to room temperature, the product was washed with 2 M HCl for 3 times to remove the metallic ions, followed by washing with distilled water to remove the possible side products

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and to make the solution neutral. Finally, the obtained NC/CNT was dried at 70 oC.

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2.3 Synthesis of [email protected]/CNT composite

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The [email protected]/CNT composite was prepared via the melt-diffusion method. First, the

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sublimed sulfur and NC/CNT were mixed thoroughly with the mass ratio of 9:1. Then,

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the mixture was transferred to a 50 mL Teflon lined autoclave and heated at 157 oC

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for 12 h to diffuse sulfur in the porous structures of NC/CNT.

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2.4 Characterization methods

The sulfur content in the [email protected]/CNT and [email protected] were determined with thermogravimetric analysis (TGA, SDT Q600 V8.0, TA Instruments, USA) at a heating rate of 10 °C min−1 under an argon atmosphere. The crystalline phases of the composites were studied with X-ray diffraction (XRD, Dmax/2550VB, Rigaku, Japan) with Cu Kα radiation. The Raman measurements were carried out using a reflex Raman system (Renishaw in Via Raman microscope, UK) equipped with a microscope

under

532

nm

laser

radiation

at

50

mW.

The

nitrogen

adsorption-desorption isotherms were measured at 77 K using an ASAP 2020 surface

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area and porosity analyzer (Micromeritics Instrument, Norcross, USA). The morphologies of the composites were characterized by scanning electron microscopy (SEM, FEI HELIOS NanoLab 600i, USA). The morphology of the composite was also investigated with a high-resolution transmission electron microscopy (TEM, FEI Titan G2 60-300, USA), which includes corrector technologies enabling resolution of

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80 pm was also applied. X-ray photoelectron spectroscopy (XPS) was carried out on

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an ESCALAB 250XI X-ray photoelectron spectrometer (ThermoFisher Scientific,

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USA) using monochromatic Al Kα radiation (Mono 500 µm) operating at a power of

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2.5 Electrochemical measurements

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150 W and the energy step size and pass energy were 0.05 and 30 eV, respectively.

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The electrochemical measurements of the [email protected]/CNT and [email protected] composites were carried out using CR2025 coin-type cells with lithium foil as an anode. The

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cathode was prepared from [email protected]/CNT or [email protected] composites. The active materials ([email protected]/CNT and [email protected]) were mixed with AB and polytetrafluoroethylene (PTFE) binder uniformly in a mass ratio of active material: AB: PTFE = 8:1:1 in an isopropyl alcohol. Then, the cathode membrane was cut and compressed at 10 MPa onto stainless-steel wire mesh with 0.8 cm in diameter. The Li–S cells were assembled with sulfur loading (2.54 – 5.60 mg cm–2). The electrolyte was lithium bis(trifluoromethane) sulfonimide (1.0 M) (LiTFSI, Novolyte Technology Co., Suzhou, China) in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (2:1 by weight) containing 1 wt % of LiNO3. The electrolyte added was 40 mL g–1 for every

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cathode. Coin cells were assembled in an Ar-filled glove box. Galvanostatic charge and discharge measurements were conducted using a LAND CT2001A battery test system (Jinnuo Electronic Co., Wuhan, China) at different rates (1 C = 1675 mA g–1) between 1.5 and 2.8 V (vs. Li/Li+). All specific capacities of the cathode were calculated according to the weight of sulfur in the active material. A CHI 660E

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Electrochemical Workstation (Chenhua, Shanghai, China) was used to perform the

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cyclic voltammetry (CV) measurement of the electrode from 1.5 to 3.0 V at the scan

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rate of 0.1 mV s–1. The electrochemical impedance (EIS) tests of the electrodes were

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carried out at open circuit voltage over the frequency range from 1.0 × 10 –2 to 1.0 ×

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105 Hz with the voltage amplitude of 5 mV.

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2.6 Synthesis of lithium polysulfide

In an argon-filled glove box (MBraun, Germany), 96 mg sulfur and 46 mg lithium

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sulfide were added to 10 mL 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (v:v 1:1) solution. After stirring 10 h, we could get the 0.1 M lithium polysulfide solution (Li2Sx, x≈6). In the adsorption experiment of polysulfides, the 0.1 M lithium polysulfide solution was diluted to 2 mM, then 5 mL diluted lithium polysulfide solution and 20 mg NC or NC/CNT were sealed in vials.

3. Results and Discussion The fabrication of MOFs-derived N-doped cubic carbon interwoven with carbon nanotube (NC/CNT) hydride is illustrated in scheme 1. The commercial CNT was

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used directly without any further modification. Co(NO3)2, Zn(NO3)2 and CNT were dispersed in water/methanol (1: 4, V/V) solution, followed by the addition of an organic ligand 2-methylimidazole in the solution to obtain bimetallic ZIFs. The combination of the two metals could enhance the stability, the surface area and

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conductivity of the ZIF.

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Scheme 1. Preparation of the [email protected]/CNT composite.

During the annealing process of ZIFs at 900

o

C, the organic linker of

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2-methylimidazole is converted to N-doped carbon, in which the electropositive

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metals connected to nitrogen atoms. The NC/CNT composite is obtained after the Co and Zn ions were etched with 2 M HCl aqueous solution. Sulfur was added into NC/CNT composite by common melt-diffusion method to obtain [email protected]/CNT. The optimal temperature for melt-diffusion of sulfur into the porous carbon materials is 157 oC. Thermogravimetric analysis (TGA) was performed to determine the sulfur content (Fig. 1a). The sulfur content of the [email protected]/CNT composite reaches 89 wt%, which is much higher than that of the previous reports[17], suggesting that the micro-mesoporous structures in NC/CNT composite allows to store and confine ultrahigh sulfur content during thermal treatment. The NC/CNT composite shows a

Journal Pre-proof weight loss of 10 wt% below 50 oC due to water adsorption. The crystallographic structures of CNT, NC/CNT and [email protected]/CNT were studied using X-ray diffraction. The peaks at 2θ = 26o and 2θ = 45o for CNT and NC/CNT indicate the graphitic carbon derived from NC and CNT (Fig. 1b). The XRD patterns of

[email protected]/CNT

at 2θ = 23o, which corresponding to the (222)

composite show strong peak

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orthorhombic sulfur (PDF#85-0799) [21], (Fig. 1c). This indicates that the high

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crystallinity of sulfur on the surface of the composite.

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To investigate the graphitization level of the NC and NC/CNT, Raman spectroscopy

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measurements have been performed. The NC and NC/CNT composites exhibit the

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two types of carbon D and G bands appeared at 1338 and 1557 cm–2 (see Fig. 1d)[22, 23]. The D band corresponds to the disorder non sp2 stretches and the G band

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corresponds to the ordered sp2 stretches mode with the intensity ratios (ID/IG) of the

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0.98 and 1.38, respectively.

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Fig. 1. (a) TGA of [email protected]/CNT and NC/CNT, (b) XRD patterns of CNT and NC/CNT, (c) [email protected]/CNT, (d) Raman spectra of NC and NC/CNT composites, (e) N2 adsorption-desorption isotherms NC/CNT and [email protected]/CNT, and (f) pore size distributions of NC/CNT). The higher ID/IG value of NC/CNT confirms that the disorder of the carbon and graphitization degree is higher than the NC composite after the addition of CNT. The specific surface area and pore sizes of NC/CNT and [email protected]/CNT composite were determined with N2 adsorption-desorption measurements.

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As revealed in Fig. 1e, the specific surface area of NC/CNT composite was calculated using the Brunauer-Emmet-Teller (BET) and the value is 288.8 m² g–1, which is much higher than [email protected]/CNT (7.0 m² g–1). The higher specific surface area of NC/CNT composite could be attributed to the presence of micro-/meso-porous structures and arises from the carbothermal reduction. Moreover, the pore volume of NC/CNT is 0.3

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cm³ g–1, which could also help to store sulfur (see Fig. 1f)[10]. The pore size

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distribution of NC interwoven with CNT composite was determined by the

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Barrett-Joyner-Halenda (BJH) method. NC/CNT shows microporous structure

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between 1.0 – 2.0 nm and mesoporous structure between 2.0 – 50 nm[16]. Hence,

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the micro-/meso-porous structures with the high specific surface area not only provide ultrahigh sulfur content, but also chemically confine the diffusion of polysulfides with

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the aid of electronegative nitrogen.

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The morphologies of NC/CNT and [email protected]/CNT were investigated with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (see Fig. 2). As shown in Fig. 2a and b, the SEM images indicate that NC/CNT composite exhibits uniform cubic architecture. The size of each cubic is about 350 nm. The TEM image of NC/CNT reveals that the carbon cubic with numerous numbers of porous structure throughout the architecture (Fig. 2c and d), which is highly important for high sulfur loading. From the HAADF STEM of [email protected]/CNT, the elemental mapping in the composite perspicuously shows the presence of the uniform distribution of C, N, O and S elements (Fig. 2e).

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Fig. 2. (a, b) SEM images; (c, d) TEM images of NC/CNT at low and high

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magnifications. (e) HAADF STEM image and the corresponding elemental mapping

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of C, N, O and S for [email protected]/CNT composite, respectively. The uniform distribution of sulfur in the composite ensures high sulfur utilization and could also improve the charge transfer in the composite because of the contact between the conductive carbon and the active material. Fig. 3a also shows NC interwoven with CNT and the formed nanocrystalline structures.

To study the chemical interactions in [email protected]/CNT and NC/CNT composites, X-ray photoelectron spectroscopy (XPS) was carried out. As shown in Fig. 3b, the XPS survey spectra reveals that C 1s, N 1s and O 1s are the most pronounced peaks and the atomic elemental composition of C, N and O in the NC/CNT composite are 83.9%,

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5.40% and 10.7%, respectively. However, there are S 2s and S 2p peaks, and the elemental compositions after the addition of sulfur for C, N, O and S in [email protected]/CNT are 72.8%, 5.30%, 8.23% and 13.6%, respectively. The atomic percent of N becomes decreased after the addition of sulfur, suggesting that N chemically interacts with sulfur. The sulfur content of [email protected]/CNT composite is much lower than that

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determined by TGA. The high-resolution C 1s spectrum of [email protected]/CNT and NC/CNT

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composites are shown in Fig. 3c. NC/CNT and [email protected]/CNT show peak at the binding

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energies of 284.6 and 285.6 eV, corresponding to the sp2 carbon (C=C) and the C=O

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bond, respectively[22, 24].

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The high-resolution N 1s peaks in NC/CNT and [email protected]/CNT are represented in Fig.

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3d. NC/CNT shows three kinds of nitrogen, pyridinic N (398.76 eV), pyrrolic N (400.17 eV) and graphitic N (401.28 eV)[25, 26]. Whereas, the [email protected]/CNT

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composite shows only two kinds of nitrogen, pyridinic N (398.95 eV) and graphitic N (402.04 eV). In the [email protected]/CNT composite, peak intensity of the pyridinic N decreases, and the peak shifts to higher binding energy (0.19 eV) after the thermal diffusion of sulfur.

Moreover, the graphitic N also shifts to higher binding energy (0.76 eV). However, the peak of pyrrolic N disappears, suggesting that the surface area of NC could be coated with sulfur. The doped N in the composite furnishes high active sites in the material as well as high capability to adsorb the soluble polysulfides[27]. Besides, the NC/CNT composite shows a peak at the binding energy of 531.78 eV, corresponding

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to C=O (see Fig. 3e). However, the [email protected]/CNT composite shows a peak at the binding energy of 532.29 eV, which is shifted to the higher binding energy (0.51 eV). In addition, both NC/CNT and [email protected]/CNT composites show a peak at 533.30 eV, corresponding to the C–O. The XPS of S 2p spectrum of the [email protected]/CNT composite reveals that the binding energies at 164.3 and 165.5 eV, which represent the S–S and

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S–O bonds, respectively (see Fig. 3f)[28], and which confirm the covalent interactions

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between sulfur and carbon. The peak at 169.3 eV is the formation of sulfate during

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the oxidation of sulfur in the air. As a result, this covalent interaction is able to protect

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the loss of the active materials, immobilize the sulfur, prevent the shuttle effect and

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enhance the cyclic life of Li–S batteries.

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Fig. 3. (a) High resolution of TEM image of NC/CNT composite. (b) XPS survey, (c) high-resolution of C 1s, (d) N 1s, e) O 1s of [email protected]/CNT and NC/CNT composite. (f) S 2p spectrum of [email protected]/CNT. To assess the electrochemical performance of the [email protected]/CNT cathode, cyclic voltammetry, impedance, cyclic and rate performance tests were performed. The bare [email protected] cathode was also evaluated at the same conditions. Fig. 4a and b show the cyclic voltammogram curves of [email protected]/CNT and [email protected], respectively. There are

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two reduction peaks and one oxidation peak for both composites. The two cathodic peaks at 2.33 and 2.04 V, respectively, assign to the two-steps reduction of the cyclic S8 to higher order lithium polysulfides (Li2Sn, 4 ≤ n ≤ 8), which further reduce to lower order lithium polysulfides (Li2S2, Li2S)[29-31].

In the subsequent anodic scan, a sharp anodic peak observed at 2.4 V, which

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corresponds to the oxidation of the low members of the polysulfides to high orders of

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polysulfides, then to sulfur. In comparison, the potentials correspond to the reduction

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peaks and oxidation peaks of the [email protected]/CNT electrode show a high current peak and

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a positive potential shift in the reduction peaks. Hence, the results indicate the good

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reproducibility and electrochemical stability of the [email protected]/CNT cathode. In

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comparison with [email protected] cathode, the [email protected]/CNT cathode apparently exhibits higher reduction potential and lower oxidation potential, suggesting that NC decreases

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the electrode polarization[32-34]. The onset potential of the [email protected]/CNT cathode during the oxidation is 2.45 V, which is lower than [email protected] cathode (2.58 V). In addition, the onset reduction potentials of [email protected]/CNT cathode are higher than [email protected], suggesting that NC decreases the polarization of the electrode (see Fig. 4c). These also verify that the kinetics of the [email protected]/CNT electrode reaction is accelerated with the presence of nitrogen.

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Fig. 4. (a, b) Cyclic voltammogram, (c) onset potentials of [email protected]/CNT and [email protected],

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respectively. (d) The Nyquist plot of [email protected]/CNT before and after cycling.

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To compare the resistance contributions of the [email protected]/CNT electrode before and after cycling, electrochemical impedance test was performed (Fig. 4d). The impedance test of [email protected]/CNT cathode before cycling reveals that single depressed semicircle in the high-frequency region and a sloped-line in the low-frequency region. Particularly, the semicircle in the high-frequency region is corresponding to the internal resistance of the electrolyte and the separator while the semi-circles from the high to middle-frequency region are corresponding to the non-linear contact charge transfer resistance between the electrolyte and electrode interfaces[35, 36]. However, the impedance of the [email protected]/CNT cathode after cycling exhibits two depressed

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semicircles in high and medium frequency regions and an inclined line in the low-frequency region. The [email protected]/CNT electrode shows lower charge transfer resistance after cycling. In addition, the inclined line is corresponding to the Warburg region (Li-ion diffusion) in the cathode. In comparison with the previous report[37-39], the faster reaction kinetic characteristics of the [email protected]/CNT electrode porous and N-doped material, which renders the rapid

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can be attributed to the

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electron transport in the cathode.

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The galvanostatic charge/discharge tests were investigated to evaluate the

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performance of [email protected]/CNT cathode at different sulfur loading with the current rate

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of 0.5 C (837.5 mA g–1) (Fig. 5a). Benefited from the high utilization of sulfur, the cell with 3.6 mg cm–2 sulfur loading and areal capacity of 4.1 mA h g–1 delivers the

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initial specific capacity of 1141 mA h g–1 , which stabilizes at 686.0 mA h g–1 with

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capacity retention and an average capacity decay rate of 61.4% and 0.3% after 120 cycles, respectively. The sulfur utilization for this cathode is 68%. When the sulfur loading was increased to 5.6 mg cm–2, the [email protected]/CNT cathode displays low initial specific capacity (818.0 mA h g–1) and retains a capacity of 468.2 mA h g–1 with capacity retention of 57.2% after 67 cycles. The sulfur utilization is 48%. In this case, the capacity fading is very high due to the high areal loading of sulfur in the cathode, which is impeded by the activation process. The results confirmed that NC/CNT provides efficient high sulfur loading and physical confinement, which could chemically adsorb the polysulfides with the help of the electronegative nitrogen.

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Fig. 5. (a) Cyclic performances, (b) charge-discharge profiles of [email protected]/CNT at the

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current rate of 0.5 C. (c) Cyclic performance of [email protected]/CNT at 1 C. (d) Rate

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capability of [email protected]/CNT and [email protected]

In addition, the NC/CNT composite accelerates electron transfer and lithium ion for polysulfide subsequent conversion reactions. This diminishes the shuttle effect of soluble polysulfides, resulting in very good cyclic performance. The galvanostatic charge-discharge profile of the [email protected]/CNT cathode with 5th, 20th, 50th and 100th cycles were determined at the current rate of 0.5 C as depicted in Fig. 5b. The [email protected]/CNT cathode pronounces two typical discharge plateaus and one charge plateau, which are the typical electrochemical properties of sulfur. At the high and steep plateau about a potential of 2.3 V, cyclo-S8 opens and changes to long chain polysulfides Li2Sn (4 ≤ n ≤ 8) and the solid Li2S is formed at a potential of 2.1 V. The

Journal Pre-proof cycling performance of [email protected]/CNT cathode at the current density of 1675 mA g–1 is depicted on Fig. 5c. The initial discharge capacity of [email protected]/CNT cathode with the high sulfur loading of 2.54 mg cm–2 is 892.8 mA h g–1 and maintains a reversible capacity of 571.0 mA h g–1 with a capacity retention of 64% after 120 cycles. The [email protected]/CNT cathode also displays stable and high coulombic efficiency (≥ 97%).

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The rate capability of the [email protected]/CNT cathode was measured (see Fig. 5d). The initial

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specific capacities of [email protected] cathode at the current rate of 0.5, 1 and 2 C for every

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10th cycles are 790, 460 and 336 mA h g−1, respectively. When the cycle is restored

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back to 0.5 C, the cathode shows a capacity of 513.0 mA h g−1. Whereas, the

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[email protected]/CNT cathode delivers 1205, 854 and 763 at the current rate of 0.5, 1 and 2 C, respectively. When the discharge capacity recovers back to 0.5 C, the [email protected]/CNT

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cathode shows a capacity of 764 mA h g−1. Due to a synergy property of NC

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interwoven with CNT, the specific capacity of [email protected]/CNT is higher than that of [email protected] cathode. The average coulombic efficiency during the rate capability for both [email protected] and [email protected]/CNT cathodes are more than 99% during the discharge-charge processes.

Fig. 6. The image of adsorption capability of NC and NC/CNT for Li2S6.

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We have also carried out an experiment for the adsorption capability of NC and NC/CNT to polysulfides. As shown in Fig. 6, when the NC and NC/CNT is added to the Li2S6 at the time = 0 h, there is no any significant difference, however, after 4 h, Li2S6 with NC and NC/CNT clearly shows a significant difference of color. The lithium polysulfide with NC/CNT shows colorless solution and confirms that the

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NC/CNT has a high capability to adsorb the soluble polysulfides. Furthermore, the

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comparison of the electrochemical performance of the [email protected]/CNT cathode with the

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current state-of-the-art of Li–S batteries is depicted on Table 1. Table 1. Comparison of the electrochemical performance of [email protected]/CNT cathode

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with the current state-of-the-art of Li–S batteries.

S

N

Materials

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Capacity (mA h g–1) Initial

n

(wt%)

na

(wt%)

Rate After

Ref (C)

cycles

54

-

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HPCN-S

-

730 a

0.5

[2]

[email protected]/S 58

-

1343

540 a

0.5

[40]

S-NPC/G

64

9.6

629.9

608 b

1.0

[17]

MPCP-S-I

43

-

-

420 c

0.05

[16]

C-S-1

37

5.55

1509

474 c

0.2

[41]

[email protected]

30

-

1100

510 c

0.1

[42]

[email protected]/CNT

89

5.40

1141

686 c

0.5 This work

[email protected]/CNT

89

5.40

893.0

571

c

1.0

Note: a) n = 50 cycles, b) n=300 cycles; c) n = 120 cycles.

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The data clearly shows that the [email protected]/CNT cathode has higher sulfur content compared with the previously works. The cyclability of this cathode at high sulfur content especially at the current rate of 0.5 C shows better performance.

Conclusion

of

A multifunctional MOFs-derived nitrogen-doped cubic carbon interwoven with CNT

ro

(NC/CNT) composite for high sulfur content host of Li–S batteries was prepared. The

-p

cubic carbon interwoven with CNT not only provides efficient sulfur loading, but also

re

chemically adsorbs the soluble polysulfides with the electronegative nitrogen. In

lP

addition, NC/CNT composite offers fast charge and lithium ion transports. The [email protected]/CNT cathode exhibits a sulfur content as high as 89 wt%, and displays the

na

specific capacity of 1141 mA h g–1 at the current rate of 0.5 C (837.5 mA g–1) and

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retains 674.4 mA h g–1 with capacity retention of 61.4% after 120 cycles. Overall, the NC/CNT cathode with high sulfur content is beneficial for improving the energy density of Li–S batteries.

Acknowledgements This work financially supported by the National Natural Science Foundation of China (Nos. 21676304, 21636010 and 21878342) and the Hunan Provincial Science and Technology Plan Project (No. 2016TP1007).

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Conflict of interest The authors declare no conflict of interest.

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Highlights 

MOFs-derived carbon interwoven with carbon nanotubes shows a high advantage to store high sulfur contents (89 wt%).



The N-doped carbon interwoven with CNT obstructs the dissolution of soluble polysulfides.

na

lP

re

-p

ro

at the current rate of 0.5 C after 120 cycles.

of

The cathode made from [email protected]/CNT maintains a capacity of 674.4 mA h g–1

Jo ur