Carbon Nanotube-Connected YolkâShell Carbon Nanopolyhedras

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Carbon Nanotube-Connected Yolk-Shell Carbon Nanopolyhedras with Cobalt and Nitrogen Doping as Sulfur Immobilizers for High-Performance Lithium-Sulfur Batteries Ruiqing Liu, Qi Kang, Wenhui Liu, Zhiwei Liu, Yuejiao Liu, Yizhou Wang, Jianyu Chen, Benjamin Hultman, Xiujing Lin, Yi Li, Pan Li, Zhen-Dong Huang, Xiaomiao Feng, Gang Wu, Leshu Yu, and Yanwen Ma ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01422 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 14, 2018

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ACS Applied Energy Materials

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Carbon Nanotube-Connected Yolk-Shell Carbon

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Nanopolyhedras with Cobalt and Nitrogen Doping

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as

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Lithium-Sulfur Batteries

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Ruiqing Liu

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Chen a, Benjamin Hultman b, Xiujing Lin a, Yi Li a, Pan Li a, Zhendong Huang a, Xiaomiao Feng

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a,

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a

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for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation

Sulfur

a, ‡,

Immobilizers

Qi Kang

a, ‡,

for

High-Performance

Wenhui Liu a, Zhiwei Liu a, Yuejiao Liu a, Yizhou Wang a, Jianyu

Gang Wu b,*, Leshu Yu c, Yanwen Ma a,* Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory

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Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9

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Wenyuan Road, Nanjing 210023, China.

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b Department

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University of New York, Buffalo, NY 14260, USA.

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c School

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Processing, Shangrao Normal University, Shangrao 334001, Jiangxi, China.

of Chemical and Biological Engineering, University at Buffalo, The State

of Chemistry and Environmental Science, Key Laboratory of Polymer Preparation and

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ABSTRACT: Lithium sulfur battery is regarded as a promising energy solution because of high

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energy density. However, the insulating nature and large volumetric expansion of sulfur, and the

ACS Paragon Plus Environment

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high solubility of polysulfides restrict their practical applications. Here carbon nanotube (CNT)-

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induced yolk-shell carbon nanopolyhedra, with Co-N-doping, is used as host material for sulfur.

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The CNTs are used to create a conductive network which interweaves each carbon polyhedron,

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and to induce the formation of a yolk-shell structure during the sulfur melt-diffusion process due

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to the “perforation effect”. The CNT-connected Co-N-doped carbon nanopolyhedra containing

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sulfur yolk-shell structure (S@Co-N-C/CNTs-0.5) can achieve a capacity of 712.2 mAh g−1 at

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1675 (1 C) mA g−1 after 300 cycles and 511.8 mAh g−1 at 3350 (2 C) mA g−1. The outstanding

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performance is attributed to the new paradigm, S@Co-N-C/CNTs-0.5 yolk-shell structure, which

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creates a conductive network allowing for improved electron transport and convenient electrolyte

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infiltration, and enhanced reaction kinetics for the electrochemical process synchronously. The

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significant internal void space of yolk-shell structure effectively accommodates the volume

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expansion of sulfur. Simultaneously, Co-N-doping in yolk-shell structure carbon polyhedra can

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synergistically trap polysulfides due to the strong chemical adsorption.

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KEYWORDS: lithium-sulfur batteries, metal organic frameworks, carbon nanotube, yolk-shell

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carbon nanopolyhedra, chemical adsorption

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1. Introduction

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High-energy density and stable cycle life are the most urgent demands for the rapid

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development of energy storage devices [1-3]. The lithium-sulfur (Li-S) battery, possessing a high

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theoretical specific capacity (1675 mAh g-1) and energy density (2600 Wh Kg-1), along with the

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natural abundance, low cost, and nontoxicity of elemental sulfur, has recently drawn much

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attention for use as next generation secondary batteries [4-10]. However, the commercial


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application of Li-S battery is greatly hampered by several major challenges: the low conductivity

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of sulfur and the polysulfide products (Li2S2/Li2S), large volume expansion of sulfur, high

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solubility of lithium polysulfides, and subsequent shuttle effect. These problems lead to low

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utilization of the sulfur, low coulombic efficiency, poor cycle life and rate capability [11-17].

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Considerable strategies have been proposed to alleviate the above problems. For example,

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extensive researches have been carried out to improve the conductivity of the sulfur cathode with

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conductive materials (such as porous carbon [9, 18-21], carbon nanotubes [22-24], graphene [4,

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7, 25-27], and conducting polymers [28, 29]), where the sulfur and the lithium polysulfides were

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confined within the conductive matrix. However, the physical entrapment of soluble polysulfides

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is not adequate to restrain the inevitable shuttling in the electrolyte. In addition, the volume

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expansion of the sulfur electrode is not sufficiently limited. Another solution is to design and

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fabricate special structures composed of nano-sized building blocks, such as hierarchical carbon

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nanocages [18, 30, 31], Polypyrrole-MnO2 (PPy-MnO2) coaxial nanotubes [32], unstacked

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double-layer templated graphene [33], 3D graphene-foam/reduced graphene oxide (GF-rGO)

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hybrid nested hierarchical networks [34], etc. These special structures are imbued with high-

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efficiency ion conductive networks and open ion channels which improve the sulfur content

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usage. This is due to the enhanced interior space which improves the stability of the electrode

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structure simultaneously. However, the sulfur could also diffuse out from the framwork, during

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the charge-discharge process [35].

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For the design of special structure for immobilizing sulfur, metal organic frameworks (MOFs)

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can be used as precursors/templates to develop novel, tailorable structures because of their

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diverse skeletal, well-defined pore structures, functional surface and central ion, which possess

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strong, reversible interactions with polysulfides [36-38]. For example, MOFs-derived


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hierarchically porous carbon nanoplates [39], Co-N co-doped graphitic carbon (Co-N-GC) [40],

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RGO wrapped Co-doped porous carbon (RGO/C-Co) [41], these special structures have been

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successfully fabricated by choosing suitable MOFs as templates. In these special structures, “sea

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urchin”-like nanopolyhedra (Co-NCNT/NP) exhibits outstanding electrochemical performances

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as sulfur immobilizers for Li-S batteries [42]. The interconnected conductive network formed by

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outstretched NCNTs facilitate electron transportation, the hierarchical micromesoporous

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structures provide electrolyte infiltration and ion transmission channel. However, the synthesis of

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these special structures still faces challenge for facile and scalable synthesis process. For

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example, the CNTs in the CNTs/nanopolyhedra structures were prepared by CVD or under high

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temperature by the catalytic action. Nevertheless, the selection of catalysts, the diameter,

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physicochemical property and proportion of CNTs in the composites are difficult to control, and

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the precursors of nanopolyhedra were also limited.

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Meantime, considering the large volume variation (~80%) of sulfur electrodes, the yolk-shell

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structure has been proposed as an effective sulfur host [35, 43, 44]. Compared to conventional

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hollow structures and core-shell structures, the main superiority of yolk-shell structure as sulfur

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host material, is to offer more active surfaces for sulfur redox while maintaining adequate

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internal void for volume variation. In addition, the yolk-supported shell can provide a more

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robust physical architecture, and the connection between the yolk and shell can eliminate the

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poor electron transfer problem to some extent which happens to all hollow structures host.

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In this work, CNT-induced yolk-shell structure and MOFs-derived Co-N-doped carbon

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nanopolyhedra encapsulating sulfur (S@Co-N-C/CNTs) with high sulfur utilization and

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interfacial conductivity has been successfully fabricated through a facile and moderate in-situ

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synthesis and melt-diffusion method. The commercial CNTs were introduced to construct a


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convenient and universal method to structure CNTs/nanopolyhedra which could combine more

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kinds nanopolyhedra materials with CNTs. It is worth highlighting that the yolk-shell S@Co-N-

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C/CNTs formed by controlling heating temperature and heating time during the melt-diffusion

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step of sulfur impregnation, avoiding the two-stage process including hollowing and sulfur

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impregnation during the preparation process of sulfur-containing materials. As the cathode for

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Li-S batteries, S@Co-N-C/CNTs nanomaterials manifest high capacity, outstanding cycling

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durability, and rate capability. Therefore, the novel proposed architecture can effectively offer

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sufficient space to alleviate the volume associated stresses of sulfur, avoid the dissolution of

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polysulfides and improve capacity retention during charge-discharge process. This represents an

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excellent development of high-performance Li-S battery for future commercialized applications.

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2. Experimental section

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2.1 Synthesis of ZIF-67/CNTs hybrid nanopolyhedra

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Synthesis of hydroxylated carbon nanotubes (CNTs). The hydroxylated CNTs were prepared

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by a hydrothermal method, which was reported by previous report [45]. In a typical procedure,

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2.5 g pristine CNTs were dispersed in 2.0 M NaOH aqueous solution, then the solution was

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stirred for 20 min. Thereafter, the mixed solution was transferred into a hydrothermal reactor at

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180 °C for 2 h. When the reaction finished, the solution was filtered and washed for three times,

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and dried at 60 °C for 12 h. The resultant solid was ball-milling for 1 h. The hydroxylated CNTs

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were used in all the experiments, for convenience, all the hydroxylated CNTs in the texts were

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labeled as CNTs.

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Synthesis of ZIF-67. 0.2 g Co(NO3)2·6H2O was dissolved into 40 mL methanol under

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ultrasound for 10 min to form a uniform solution “A”. At the same time, 0.5 g 2-


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methylimidazole (MeIm) was dissolved into 40 mL methanol to form solution “B”. “B” was

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slowly added into “A” under constant stirring, and then it was kept at room temperature for 24 h.

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The bright purple solid was obtained through centrifugation, washed with methanol several times,

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dried at 60 °C for 12 h [41].

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Synthesis of ZIF-67/CNTs hybrid nanopolyhedra. In the synthesis procedure, 0.1 g

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hydroxylated CNTs were dispersed into 40 mL methanol under ultrasound for 10 min, which

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was followed by addition of 0.1 g Polyvinylpyrrolidone (PVP) in a beaker under ultrasonic

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treatment for 30 min. After that, 0.2 g Co(NO3)2·6H2O were added into the solution of the

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sonicated solution of 40 mL methanol and MeIm. The mixed solution was kept at room

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temperature for 24 h. The obtained sample was collected by centrifugation, washed with

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methanol for five times, and finally dried at 60 °C for 12 h.

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2.2 Synthesis of S@Co-N-C/CNTs yolk-shell structured nanopolyhedra

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Mixtures of ZIF-67/CNTs and sulfur powder (3:7, weight ratio) were sealed and heated at 155

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°C for 12 h, 155 °C for 12 h and 250 °C for 0.5 h, 155 °C for 12 h and 250 °C for 1 h, 155 °C for

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12 h and 250 °C for 2 h under Ar atmosphere. For comparison, the S@Co-N-C nanomaterials

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can be obtained when ZIF-67 and sulfur powder were heated at 155 °C for 12 h and 250 °C for

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0.5 h. For convenience, these samples were denoted as S@Co-N-C/CNTs, S@Co-N-C/CNTs-0.5,

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S@Co-N-C/CNTs-1, S@Co-N-C/CNTs-2, [email protected].

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2.3 Materials Characterization

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The crystal structure was carried out on an X-ray power diffractometer (RIGAKU, RINT-

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ULTIMA Ⅲ, Cu Kα, λ= 0.15405 nm). The morphology, structure, chemical bonding of elements

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and composition were obtained from SEM (Hitachi S-4800), TEM (FEI TalosF200X) and X-ray


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photoelectron spectroscopy (AXIS Supra) and TGA (Netzsch STA-449F3). Raman spectra were

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conducted using Renishaw inVia Reflex Raman spectroscopy with 532 nm laser excitation. N2

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adsorption-desorption isotherms were analyzed on Thermo Fisher Scientific Surfer Gas

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Adsorption Porosimeter. Before measurement, the sample was degassed at 300 °C for 6 h.

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2.4 Electrochemical Measurements

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The electrochemical measurements were employed as previous report [17]. The cathodes of

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Li-S batteries were composed of 70 wt% S@Co-N-C/CNTs nanocomposites, 20 wt% acetylene

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black, and 10 wt% PVDF. The lithium foil was used as the counter electrode and reference

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electrode. Celgard 2400 polymer membrane was applied for the separator. The electrolyte was 1

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M lithium bis-(trifluoromethanesulfonyl) imide (LiTFSI) dissolved in 1:1 mixture of 1,3-

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dioxolane (DOL) and 1,2-dimethoxyethane (DME), and with 1 wt% LiNO3. All the CR2032 coin

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cells were assembled in an argon-filled glove box, and the moisture and oxygen contents are less

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than 1 ppm. The cyclic voltammetry (CV) curves were carried out on an electrochemical

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workstation (BioLogic VMP3) in the voltage of 1.5-2.8 V with a scan rate of 0.2 mV s−1.

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Electrochemical impedance spectroscopy (EIS) was conducted in the range from 10 mHz to 100

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kHz. All the charge-discharge tests were carried out on battery testing system (LAND CT2001A)

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in the potential range of 1.7-2.8 V. The specific capacities of electrodes were calculated based on

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the mass of sulfur in the nanocomposites.

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3. Results and discussion

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The S@Co-N-C/CNTs were synthesized via in-situ synthesis and facile melt-diffusion method

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as described in the experimental section, the detailed preparation process is illustrated in Scheme

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1. The Co ions can be adsorbed effectively via electrostatic attraction on the surface of the


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hydroxylated CNTs. The ZIF-67/CNTs nanomaterials were successfully fabricated via a facile

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and scalable in-situ synthesis. Subsequently, the ZIF-67/CNTs and sulfur underwent thermal

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treatment under Ar atmosphere. In the melt-diffusion process, the ZIF-67/CNTs are carbonized

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into Co-N-C/CNTs hollow structure. Meanwhile, the sulfur diffuses into the hollow structured

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carbon polyhedra with newly attached CNTs resulting in a hierarchical porous yolk-shell

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structure consisting of small nanoparticles building blocks, labelled as S@Co-N-C/CNTs

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nanopolyhedra. The yellow nanoparticles in the sectional view represent sulfur adsorbing within

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the carbon polyhedra structure. This new structure can effectively improve the sulfur content

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utilization, overcome the volume expansion within the sulfur electrode, entrap polysulfides, and

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improve the overall reaction kinetics of the electrochemical process simultaneously, resulting in

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obvious enhancement in lithium sulfur electrochemical performance.

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Scheme 1. Synthetic route of the S@Co-N-C/CNTs nanopolyhedra by in-situ growth and melt-

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diffusion strategy.

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Figure 1 a-c show the representative scanning electron microscopy (SEM) images of ZIF-

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67/CNTs and S@Co-N-C/CNTs-0.5 nanopolyhedra. Figure 1a shows that the ZIF-67/CNTs

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nanopolyhedra consists of uniform CNTs-inserted ZIF-67 regular dodecahedron with a perfectly

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smooth surface and a size of approximately 200 nm. It indicates the ZIF-67 nanopolyhedra have

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been successfully in-situ as-synthesized on the hydroxylated CNTs. It is obvious the diameter


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and length of CNTs are approximately 15 nm and 2 m (Figure S1, Supporting Information).

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Compared with the size (approximately 500 nm) of pristine ZIF-67 (Figure S2, Supporting

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Information), the ZIF-67 within the synthesized structure is smaller, because the CNTs inhibit

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the growth of ZIF-67. After the melt-diffusion stage, S@Co-N-C/CNTs-0.5 nanopolyhedra are

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obtained with uniform distribution, and the nanopolyhedra morphology is maintained with a

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slight exception of the rough and slightly collapsed surface (Figure 1b, c), which is similar to the

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morphology of ZIF-67 nanopolyhedra after heat treatment (Figure S3, Supporting Information).

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To give further insight into the morphology and microstructure of the as-prepared ZIF-67/CNTs

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and S@Co-N-C/CNTs-0.5, TEM analysis was carried out. Figure 1d reveals that the

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homogeneously distributed ZIF-67 are solid dodecahedron intertwined with CNTs. The S@Co-

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N-C/CNTs-0.5 nanopolyhedra shown in Figure 1e clearly manifests multi-walled yolk-shell

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structure. The magnified TEM image in Figure 1f confirms that the intertwined CNTs are

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inserted in Co-N-C nanopolyhedra, forming yolk-shell structured nanopolyhedra. Figure 1g

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presents the high-resolution transmission electron microscopy (HRTEM) image of CNTs

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inserted Co-N-C nanopolyhedra as blue ellipses shown, the interplanar spacing of approximate

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0.342 nm is further confirmed to the (002) planes of CNTs (Figure 1g inset). Moreover, the TEM

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elemental mappings confirm the existence and homogeneous distribution of C, Co, N, and S in

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Co-N-C/CNTs nanopolyhedra (Figure 1h-k). In contrast, the SEM and TEM images of S@Co-N-

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C after annealing are also shown (Figure S4, Supporting Information), which reveal a solid

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morphology and rough surface.


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Figure 1. (a) SEM image of ZIF-67/CNTs. (b, c) Different magnification SEM images of

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S@Co-N-C/CNTs-0.5 nanopolyhedra. (d) TEM image of ZIF-67/CNTs. (e, f) TEM images of

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S@Co-N-C/CNTs-0.5 nanopolyhedra. (g) HRTEM image of MWCNTs in S@Co-N-C/CNTs-

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0.5 nanopolyhedra. (h, i, j, k) C (blue), Co (red), N (purple), S (yellow) elemental individual

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mappings of (f).

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In order to find out the formation mechanism of the multi-walled yolk-shell structure, a series

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of comparative experiments at different temperatures were carried out. The corresponding TEM

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images of S@Co-N-C/CNTs after different annealing conditions are exhibited in Figure S5

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(Supporting Information). After heated at 155 °C for 12 h, the sulfur diffused into the pore

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structures through the melt-diffusion method (Figure S5a); when the heat treatment continued at

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250 °C for 30 minutes, the core-shell structure formation can be easily seen (Figure S5b). The


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result is significantly different from the prepared S@Co-N-C nanopolyhedra with solid core

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synthesized with the same thermal treatment, but without CNTs (Figure S4, Supporting

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Information). With a prolonging of the thermal treatment time to 1 h at 250 °C (Figure S5c), the

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chemical bonding in the ZIF-67 was further damaged and more sulfur sublimed, resulting in a

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smaller sulfur yolk inside the S@Co-N-C/CNTs nanopolyhedra. When the thermal treatment at

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250 °C was further increased to 2 h (Figure S5d), the yolk-shell structure of S@Co-N-C/CNTs

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nanopolyhedra has worn out to a certain extend, especially for the yolk, and more sulfur has

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sublimed away from the edges of the polyhedra. Meanwhile, from these images compared with

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ZIF-67/CNTs in Figure 1, it can be seen that the CNTs outside of the nanopolyhedra become

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rougher and thicker, resulting from the CNTs infiltrated with sulfur via capillary action. Most of

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the CNTs adhered to surface of the nanopolyhedra when infused with sulfur compared with the

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ZIF-67/CNTs in Figure 1d. It is therefore illustrated that the yolk-shell structure forms after half

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an hour. Based on the above experimental characterization data and the formation process, it can

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be inferred that there are two important reasons for the formation mechanism of the yolk-shell

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structure. First, it may be attributed to the “perforation effect”. The electrostatic attraction

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between ZIF-67 and hydroxylated CNTs is weak, and the organic ligands of ZIF-67 around the

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CNTs is prone to be destroied and carbonized to hollow porous structure under the thermal

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treatment. It looks like a carbon matrix is pierced through by CNTs. The effect of CNTs was

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called as “perforation effect”. Second, the binding energy of Co and S is much stronger, and the

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uniform cobalt sulfide wrapping around by porous carbon was formed during the melt-diffusion

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process. As the central ions, because of the strong binding energy between Co and S, doped Co

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ions could suppress some of the volatilization of sulfur in the central position during the thermal

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treatment process, which agreed well with the TEM mapping analysis. These two factors


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promote the formation of yolk-shell or multi-walled yolk-shell structure and can buffer the large

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volumetric expansion of sulfur.

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The XRD patterns of pure sulfur, CNTs, pristine ZIF-67, ZIF-67/CNTs, and S@Co-N-

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C/CNTs-0.5 are shown in Figure 2a. As-prepared ZIF-67 had the traditional characteristic peaks

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[46]. It’s worth noting that there are no distinct CNTs peaks in the ZIF-67/CNTs composites

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because of the weak diffraction intensity when compared with ZIF-67. After calcination of ZIF-

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67/CNTs with sulfur, the characteristic peaks of ZIF-67 become undetectable, indicating the

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structure of ZIF-67 is destroyed in the calcination. On the other hand, the peaks at 23.0°, 25.7°,

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26.6°, 27.6°, 28.8°, 31.3°, and 42.6° of S@Co-N-C/CNTs-0.5 demonstrated that the sulfur

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successfully infiltrated into carbon skeleton during the annealing processes. The Raman spectra

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of CNTs, ZIF-67/CNTs, and S@Co-N-C/CNTs-0.5 composites were produced to investigate

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each structure (Figure 2b and Table S1). Two typical peaks at 1323 cm-1 (D band) and 1590 cm-1

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(G band) are due to defective carbon and graphitic carbon of CNTs. The intensity of ZIF-

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67/CNTs is lower than that of CNTs. With the restriction of sulfur in the S@Co-N-C/CNTs-0.5,

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the intensities of D and G bands also got weaker than pure CNTs. The lower intensities are

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attributed to the weakened signal of D, G band from CNTs in these composites as the CNTs

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contents are lowered compared with pure CNTs. However, the ID/IG ratio is larger, indicating an

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increase of sp3 carbons on the carbon nanotubes, which implies that sulfur interrupts the C=C sp2

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bond in CNTs.

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N2 adsorption/desorption isotherms for the ZIF-67, [email protected], ZIF-67/CNTs and

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S@Co-N-C/CNTs-0.5 are shown in Figure 2c. The BET surface areas of ZIF-67 and ZIF-

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67/CNTs are 1158 and 1000 m2 g-1, after sulfur infiltration, the BET specific surface areas of

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[email protected] and S@Co-N-C/CNTs-0.5 are 9 and 46 m2 g-1, indirectly verifying the sufficient


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confinement of sulfur. The ZIF-67/CNTs shows the pore size distribution peaks are at 2.5, 3.0,

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and 3.9 nm according to Figure S6 (Supporting Information), revealing the existence of abundant

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mesoporous. To a certain extent, these pore structures effectively confine the dissolution of

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polysulfides. In the thermogravimetric analysis (TGA) (Figure 2d), the curves display weight

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loss around 155 °C because of sulfur sublimation [7]. It reveals the sulfur content of S@Co-N-

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C/CNTs-0.5 nanocomposites is 65.7%. For comparison, the other TGA curves are also given,

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and the sulfur content of S@Co-N-C/CNTs nanocomposites is 73.4%.

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Figure 2. (a) XRD patterns of pristine sulfur, CNTs, ZIF-67, ZIF-67/CNTs, and S@Co-N-

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C/CNTs-0.5. (b) Raman spectra of CNTs, ZIF-67/CNTs, and S@Co-N-C/CNTs-0.5. (c) N2


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adsorption/desorption isotherms for the ZIF-67, [email protected], ZIF-67/CNTs, and S@Co-N-

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C/CNTs-0.5. (d) The thermogravimetric analysis (TGA) of S@Co-N-C/CNTs nanopolyhedra.

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X-ray photoelectron spectroscopy (XPS) of S@Co-N-C/CNTs-0.5 was carried out to

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investigate internal chemical bonds and valence states. The XPS spectra of the whole spectrum

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of S@Co-N-C/CNTs-0.5 nanopolyhedra is shown in Figure S7 (Supporting Information). The

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deconvoluted XPS spectra of S@Co-N-C/CNTs-0.5 at C 1s region show five peaks at 284.52 eV,

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285.59 eV, 286.49 eV, 288.62 eV, and 288.28 eV (Figure 3a), respectively, corresponding to C-

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C, C-O, C=O, and -COO. C-C indicates that this nanocomposite has a rich carbon network.

9

Meantime, oxygen-containing groups could also effectively anchor polysulfides [15]. In the S 2p

10

spectra (Figure 3b), the peaks located at 163.58 eV and 164.85 eV are attributed to S 2p3/2 and S

11

2p1/2 [47, 48], confirming the existence of S8 molecules. The peak at 163.58 eV indicates the

12

existence of a C-S chemical bond. In addition, the relatively broad high energy peak located at

13

168.72 eV can also be observed, which is attributed to the formation of chemical bond between

14

Co and S. The Co 2p3/2 spectrum of S@Co-N-C/CNTs-0.5 nanopolyhedra reveals two

15

deconvoluted peaks at 779.5 and 780.4 eV (Figure 3c), corresponding to Co3+ and Co2+,

16

respectively. The results confirm that the formation of Co3S4 during thermal treatment process

17

[49], which can trap the soluble polysulfides. The strong binding force of Co-S also facilitates

18

the assembly of the yolk-shell structure within the S@Co-N-C/CNTs-0.5 nanopolyhedra. The

19

three deconvoluted characteristic peaks of N 1s band located at 398.75, 400.21, and 401.25 eV,

20

represent pyridinic, pyrrolic, and graphitic N species, respectively (Figure 3d) [42, 50], which

21

indicates the nitrogen doping in carbon matrix. Nitrogen-doping in porous carbons can not only

22

help confine soluble polysulfides, but also boost formation of bonding between sulfur and

23

oxygen functional groups, thus leading to a strong anchoring effect for sulfur species, and further


14

Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

facilitating an improvement for both the cycle performances and rate capabilities of lithium

2

sulfur batteries [51].

3

4 5

Figure 3. High-resolution (a) C 1s, (b) S 2p, (c) Co 2p and (d) N 1s XPS spectra of S@Co-N-

6

C/CNTs-0.5 nanopolyhedra.

7

The electrochemical performances of the S@Co-N-C/CNTs nanocomposites are shown in the

8

subsequent study. In Figure 4a, the CV curves have two reduction peaks at about 2.02 and 2.30

9

V, indicating two-step reduction of sulfur. Meantime, an oxidation peak at 2.44 V reveals the

10

typical oxidation of sulfur. As the cycle proceeds, the difference of the peak values between

11

cathodic peak and anodic peak becomes smaller, indicating the improvement in reversibility [52,


15


Page 16 of 31

1

53]. Figure 4b shows the charge-discharge curves of the S@Co-N-C/CNTs nanocomposites. It is

2

observed that there are two plateaus at about 2.02 and 2.30 V in typical discharge process. The

3

voltage plateau at about 2.30 V is corresponding to the transformation from sulfur to high-order

4

polysulfides and the voltage plateau at about 2.02 V reveals the deposition of low-order

5

polysulfides [4, 53]. The charge plateau corresponds to the transformation from Li2S2/Li2S to

6

Li2S8/S8, which agrees well with the CV analysis. Importantly, the S@Co-N-C/CNTs

7

nanocomposite electrode shows high initial specific discharge and charge capacity values of

8

1267.2 and 1315 mAh g-1 at 0.5 C. After 100th cycles, the electrode delivers a discharge capacity

9

of 876.7 mA h g-1, although the charge-discharge curves have mild decay, duing to the

10

polarization of electrode. Figure 4c shows the EIS spectra of S@Co-N-C/CNTs-0.5

11

nanopolyhedra electrodes after the 4th charge cycle. The resistance of ca. 200 ohms before cyclic

12

voltammetry and the resistance of ca. 140 ohms after cyclic voltammetry indicate that the

13

S@Co-N-C/CNTs-0.5 nanopolyhedra electrodes have excellent electrical conductivity

14

throughout the charge and discharge processes.

15

Figure 4d exhibits rate capabilities of S@Co-N-C/CNTs nanopolyhedra at different current

16

densities. The S@Co-N-C/CNTs-0.5 electrode can deliver a discharge capacity of 1133.7 mAh

17

g−1 at 167.5 mA g−1. With the increase of current density to 335, 837.5 and 1675 mA g−1, the

18

discharge capacities of electrode are 922.5, 805.8 and 673.8 mAh g−1, respectively. Even at 3350

19

mA g−1, the capacity also can reach 511.8 mAh g−1. When the current density is reduced back to

20

335 mA g−1, the specific discharge capacity of 874.8 mAh g−1 was recovered, displaying good

21

rate capabilities. For comparison, the [email protected] is capable of delivering much lower

22

capacities at each current density, respectively. Comparatively, the other materials obviously

23

exhibit a worse rate capacity than that of S@Co-N-C/CNTs-0.5 cathode, which demonstrates the


16

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

important role of the CNTs for the electrochemical performance improvement of S@Co-N-

2

C/CNTs cathode. The outstanding rate capability is due to the yolk-shell structure induced by

3

CNTs, which structure the conductive networks for electron transfer and electrolyte permeation,

4

and improve the reaction kinetics of electrochemical process synchronously allowing an

5

improved accommodation for the volumetric expansion of sulfur. It is also clearly seen that

6

S@Co-N-C/CNTs-0.5 shows higher capacity than S@Co-N-C/CNTs even at low current rate

7

(0.1 C) from Figure 4d. This is because the less the content of S, the better the conductivity of

8

S@Co-N-C/CNTs-0.5 composites. Meantime, under higher thermal treatment tempreture, the

9

conductivity of carbon framwork will be better. So, the electron transfer will be more convenient

10

and the sulfur utilization will be higher, although the S content in S@Co-N-C/CNTs-0.5 is lower

11

than that of S@Co-N-C/CNTs.

12

Figure 4e shows the cycling performances of S@Co-N-C/CNTs nanopolyhedra with different

13

annealing conditions at 0.5 C in the potential range of 1.7-2.8 V. There are large differences

14

between S@Co-N-C/CNTs and [email protected]. After 110 cycles, these S@Co-N-C/CNTs

15

electrodes show better cycle performances that the discharge capacity retentions are 572.5 mAh

16

g-1 (S@Co-N-C/CNTs), 873.6 mAh g-1 (S@Co-N-C/CNTs-0.5), 505.4 mAh g-1 (S@Co-N-

17

C/CNTs-1), 503.3 mAh g-1 (S@Co-N-C/CNTs-2), respectively. However, while the S@Co-N-C-

18

0.5 cathode decayed quickly during cycling, the capacity of S@Co-N-C/CNTs-0.5 after 100

19

cycles was much improved. It exhibits initial discharge capacities of 1267.2 and 968.7 mAh g-1,

20

which are about 75.7% and 57.8% of the theoretical value, respectively, indicating a high sulfur

21

utilization of S@Co-N-C/CNTs-0.5 cathode. And the coulombic efficiency of S@Co-N-

22

C/CNTs-0.5 is near 100%, which is more stable than that of [email protected]. Figure 4f shows the

23

cycle performances of S@Co-N-C/CNTs and [email protected] at 1 C. After 300 cycles, the stable


17


Page 18 of 31

1

capacities of these electrodes are 496.8 mAh g-1 (S@Co-N-C/CNTs), 712.2 mAh g-1 (S@Co-N-

2

C/CNTs-0.5), 396.5 mAh g-1 (S@Co-N-C/CNTs-1), 328.5 mAh g-1 (S@Co-N-C/CNTs-2), and

3

57.7 mAh g-1 ([email protected]), respectively. By comparison experiments, it was found that the

4

best cell performance was obtained by heating at 155 °C for 12 h and 250 °C for 0.5 h. The

5

electrochemical performances of S@Co-N-C/CNTs-0.5 are higher than those of different

6

S@MOF and MOF-derived carbon-sulfur composites (Table S2, Supporting Information). These

7

results reveal the presence of hydroxylated CNTs can enhance the cycle performance and sulfur

8

utilization on account of excellent conductivity, strong chemical/mechanical stability, and

9

synergistic interaction with Co-N-C framework. Specifically, the CNTs are inserted in the ZIF-

10

67 polyhedra, forming a yolk-shell structure with nano-sized building blocks. The significant

11

hollow space of yolk-shell structure can accommodate the volumetric expansion of the sulfur

12

electrode effectively although the sulfur content has reduced to a certain extent. The

13

electrochemical performances of yolk-shell structure (S@Co-N-C/CNTs-0.5) are better than

14

those of other hollow-structured counterparts (S@Co-N-C/CNTs-1 and S@Co-N-C/CNTs-2). It

15

may be attributed to the main superiority of yolk-shell structure, offerring more active surfaces

16

for sulfur redox while maintaining adequate internal void for volume variation, providing a more

17

robust physical architecture by the yolk-supported shell, eliminating the poor electron transfer

18

between the yolk and shell to some extent. With the extension of heat treatment time (Figure S5,

19

Supporting Information), it can be seen that the yolk-shell structure is worn out to a certain

20

extend, especially for the yolk. Although the inner hollow cavity becomes bigger, the

21

interconnection in the nanopolyhedra becomes weaker, which reduced the structural stability and

22

electrical contact of the sulfur cathode. On the other hand, the mergence of embedded Co and N

23

doping within the yolk-shell structure carbon polyhedra can effectively synergistically trap


18

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1

lithium polysulfides due to the strong chemical adsorption, thus benefitting the cycle stability of

2

Li-S battery.

3


19


Page 20 of 31

1

Figure 4. (a) The CV curves for S@Co-N-C/CNTs-0.5 nanopolyhedra cathode at 0.2 mV s-1. (b)

2

The typical voltage profiles of S@Co-N-C/CNTs-0.5 nanopolyhedra electrodes at 0.5 C. (c) EIS

3

of S@Co-N-C/CNTs-0.5 nanopolyhedra electrodes before and after cyclic voltammetry. (d, e, f)

4

The rate performances and cycling performances of different S@Co-N-C/CNTs nanopolyhedra

5

electrodes and S@Co-N-C nanopolyhedra electrode.

6

To further study the effect of CNTs and yolk-shell structure in nanopolyhedra cathode, the

7

microstructure of S@Co-N-C/CNTs-0.5 cathode after charge-discharge cycles was investigated

8

through SEM. Compared to the fresh S@Co-N-C/CNTs-0.5 nanocomposites (Figure S8a,

9

Supporting Information), the morphologies of the cycled S@Co-N-C/CNTs-0.5 composites were

10

well-maintained with an integrated spherical morphology after 100 cycles shown in Figure S8b

11

(Supporting Information), suggesting the yolk-shell structure can accommodate the large

12

volumetric change, accompanying with aggregation of some nanopolyhedra. In the

13

nanocomposites, CNTs and carbon framework provided efficient routes for electron transfer, the

14

inner hollow structure alleviated the large volumetric change. Consequently, the cycling stability

15

of S@Co-N-C/CNTs-0.5 is greatly improved. The high surface area of the Co-N-C/CNTs

16

benefited the current diffusion. The inner spatial structure keeped the integrity of the cathode and

17

the bonding interaction between S and Co-N suppressed the diffusion of polysulfides. The

18

existence of CNTs, effective yolk-shell structure matrix of cobalt and nitrogen co-doped carbon

19

nanopolyhedra, bonding interaction between S and Co-N, and synergistic effect between them,

20

structure a stable microstructure for sulfur-containing cathode.

21 22

4. Conclusions


20

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

In conclusion, S@Co-N-C/CNTs yolk-shell nanopolyhedra have been successfully fabricated

2

through in-situ, as-synthesized, and melt-diffusion method. The sulfur is encapsulated into the

3

MOFs-derived cobalt and nitrogen co-doped yolk-shell carbon nanopolyhedra structure. Co-N-

4

C/CNTs yolk-shell structure nanopolyhedra not only are imbued with the conductive networks

5

for excellent electron transport and electrolyte infiltration, but also effectively suppress the

6

exorbitant dissolution of polysulﬁdes and buffer the volumetric expansion of sulfur. As cathode

7

materials for Li-S batteries, S@Co-N-C/CNTs-0.5 nanopolyhedra manifest high capacity,

8

outstanding cycling durability and rate capability. It can achieve a capacity of 712.2 mAh g−1 at

9

1675 (1 C) mA g−1 after 300 cycles and above 511.8 mAh g−1 at 3350 (2 C) mA g−1. The new

10

structure could effectively alleviate the excessive dissolution of polysulfides to improve the

11

capacity retention during the charge-discharge process which represents an excellent

12

development of high-energy Li-S batteries.

13

ASSOCIATED CONTENT

14

Supporting Information

15

Supplementary material (TEM image of CNTs, SEM image and TEM image of ZIF-67

16

nanopolyhedra, different magnification SEM images of ZIF-67 nanopolyhedra after heat

17

treatment, the SEM and TEM images of S@Co-N-C nanopolyhedra, the TEM images of S@Co-

18

N-C/CNTs after different annealing conditions, pore size distribution of ZIF-67, S@Co-N-C,

19

ZIF-67/CNTs, and S@Co-N-C/CNTs-0.5 nanopolyhedra, the XPS survey scanning whole

20

spectrum of S@Co-N-C/CNTs-0.5 nanopolyhedra, SEM images of an S@Co-N-C/CNTs-0.5

21

nanopolyhedra electrode before and after 100 cycles, Raman ID/IG ratio for different


21


Page 22 of 31

1

nanocomposites, summary of test for different S@MOF and MOF-derived carbon-sulfur

2

composites) is available free of charge on the ACS Publication website at DOI:.

3

AUTHOR INFORMATION

4

Corresponding Author

5

*Email: [email protected], [email protected]

6

Author Contributions

7

All authors have given approval to the final version of the manuscript. ‡These authors

8

contributed equally.

9

ACKNOWLEDGEMENTS

10

This work was supported by the NSFC (51772157, 21805140, 61504062, 51802161), Priority

11

Academic Program Development of Jiangsu Higher Education Institutions (YX03001), NSF of

12

Jiangsu Province (BK20160890, BK20160886), Jiangsu National Synergetic Innovation Center

13

for Advanced Materials (SICAM), Synergetic Innovation Center for Organic Electronics and

14

Information Displays, Jiangsu Province “Six Talent Peak” (2015-JY-015), Qing Lan Project of

15

Jiangsu Province, Natural Science Key Project of Jiangxi Province (2017ACB20040) and NUPT

16

(NY215014, NY215152), National Science Foundation (CBET-1511528, 1604392) and the

17

Sustainable Manufacturing and Advanced Robotics Technology (SMART) Community of

18

Excellence program at the University at Buffalo, SUNY.

19

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1


TOC Graphic

2 3

The S@Co-N-C/CNTs yolk-shell nanopolyhedras have been successfully fabricated via

4

introduction of CNTs, which manifest excellent electrochemical performances, especially at high

5

C-rates.

6 7


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Carbon Nanotube-Connected YolkâShell Carbon Nanopolyhedras

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