Hierarchical carbon nanocages confining high-loading sulfur for high-rate lithium–sulfur batteries

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Hierarchical carbon nanocages confining Highloading sulfur for High-rate lithium-sulfur batteries Zhiyang Lyu, Dan Xu, Lijun Yang, Renchao Che, Rui Feng, Jin Zhao, Yi Li, Qiang Wu, Xizhang Wang, Zheng Hu

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S2211-2855(15)00034-8 http://dx.doi.org/10.1016/j.nanoen.2015.01.033 NANOEN693

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Nano Energy

Received date: 17 November 2014 Revised date: 4 January 2015 Accepted date: 19 January 2015 Cite this article as: Zhiyang Lyu, Dan Xu, Lijun Yang, Renchao Che, Rui Feng, Jin Zhao, Yi Li, Qiang Wu, Xizhang Wang, Zheng Hu, Hierarchical carbon nanocages confining High-loading sulfur for High-rate lithium-sulfur batteries, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2015.01.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hierarchical carbon nanocages confining high-loading sulfur for high-rate lithium-sulfur batteries Zhiyang Lyua, Dan Xua, Lijun Yanga, Renchao Cheb, Rui Fenga, Jin Zhaoa, Yi Lia, Qiang Wua,*, Xizhang Wanga,*, Zheng Hua,* a

Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, China. b Department of Materials Science and Advanced Materials Laboratory, Fudan University, Shanghai, 200438, China. *Corresponding authors. E-mail addresses: [email protected] (Q. Wu), [email protected] (X. Wang), [email protected] (Z. Hu).

KEYWORDS: hierarchical carbon nanocages; high-loading sulfur; confinement; electrochemistry; lithium–sulfur batteries

Graphical abstract hCNC

S

S@hCNC 79.8 wt%

5 μm

500 nm

5 nm

Abstract Lithium-sulfur batteries are hindered by the low utilization of sulfur, short cycle life and poor rate capability which are severe challenges today. Herein we report a new kind of carbon-sulfur composites by infusing sulfur into the novel hierarchical carbon nanocages (hCNC) with high pore volume, network geometry and good conductivity. The designed S@hCNC composite with a high sulfur loading of 79.8 wt% presents 1

the large capacity, high-rate capability and long cycle life, which could shorten the charging time for mobile devices from hours to minutes. The excellent performance derives from the unique structure of hCNC that enables the encapsulation of high-loading sulfur inside the carbon nanocages to alleviate polysulfide dissolution, meanwhile much enhance the electron conduction and Li-ion diffusion.

Introduction The increasing electrical energy demand and environmental crisis has stimulated intensive research on the electrical energy storage technologies for transportation and stationary applications [1,2]. Lithium-sulfur (Li-S) batteries are particularly attractive since the sulfur cathode exhibits a much higher theoretical specific capacity (1672 mAh g-1) than the conventional cathodes of Li-ion batteries, in addition to the low cost and environmental benignity [3-10]. To date, the large-scale commercialization of Li-S batteries is still hindered mainly by the natural insulation of sulfur and the dissolution of intermediate polysulfides (Sn2-, 3≤n≤8). The shuttle of soluble polysulfides between the electrodes could cause the gradual sulfur depletion by forming the solid precipitates (Li2S or Li2S2) on Li anode, leading to the poor utilization of active material, low Coulombic efficiency and short cycle life [3-10]. Hence, considerable efforts have been devoted to enhancing the sulfur conduction and reducing the polysulfide dissolution by wrapping sulfur with, e.g. carbon-based nanostructures [11-24] and conductive polymers [25-28]. However, most obtained composites only own the low sulfur loading (< 70 wt%), leading to a low gravimetric and volumetric energy density [3,4]. Recently, a few carbon-sulfur composites could reach a high sulfur loading over 80 wt%, but are limited by a low capacity, short cycle

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life and poor rate capability [29-31]. Exploring the composites featured with high sulfur loading and excellent performance is a great challenge for Li-S batteries [3-6]. As known, in the past 30 years, carbon nanostructures have always been the focus of frontier research landmarked by the 0D fullerene (1985), 1D nanotube (1991), and 2D graphene (2004) [32]. Nowadays, 3D hierarchical carbon nanostructures are attracting increasing attention since they could integrate many advantages of individual building blocks thus present new collective effects and great potential applications [33-36]. In this study, we report the novel 3D hierarchical carbon nanocages (hCNC) with network geometry and coexisting micro-, meso-, and macro-pores. By making full use of the unique structure of the hCNC, we have realized the high-loading confinement of sulfur inside the nanocages (up to ~80 wt%), meanwhile, the polysulfide dissolution has been much alleviated. The 3D hierarchical architecture also increases the conductivity and favors the Li-ion diffusion. The so-constructed carbon-sulfur composite with high S loading of 79.8 wt% presents the superb performance as the Li-S cathode with a large capacity, high-rate capability and long cycle life, which could shorten the charging time for the mobile devices from hours to minutes.

Results and Discussion Preparation and structure of the hCNC Recently, we developed an in situ MgO template method to produce the carbon nanocages [37]. In this study, we found that the morphological character of the basic magnesium carbonate precursor can pass down to the MgO template (Figure S1). Accordingly, by selecting the precursor with 3D hierarchical structure, 3D

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hierarchical carbon nanocages (hCNC) have been first obtained with well-defined multiscale characters and coexisting micro-, meso-, and macropores. Figure 1 shows the typical scanning electron microscopy (SEM) observation for the formation of the hCNC. The basic magnesium carbonate precursor itself owns a 3D hierarchical structure (Figure 1a,b). This structural feature passes down to the MgO template upon decomposition at elevated temperature (Figure 1c,d), thereafter to the final hCNC (Figure 1e,f and Figure S2). Thus the final hCNC product keeps the unique 3D hierarchical structure of the precursor, as schematically illustrated in Figure 1g. Specifically, the micron-sized sphere-like carbon particles are composed of the nanosheets with submicron-sized interspace (Figure 1e,f). The nanosheets are of several microns in size and <100 nm in thickness (Figure 1f), which consist of the interconnected cuboidal hollow nanocages of ca. 10~50 nm in size and 4~7 well-graphitized layers in thickness as shown in Figure 2. This structure leads to the meso- and macro-porous features (Figure 2c,d). In addition, there are numerous micropore tunnels of ~0.6 nm (Figure 2c,d), as supported by the broken fringes (Figure 2b), reflecting the unsealed feature of the shells (Figure S3). The integration of the well-defined micro-meso-macropore size distributions endows the hCNC with a large specific surface area of 1276 m2 g-1 and a high micro-mesopore volume of 4.178 cm3 g-1 (Table S1), which enables the high-loading of sulfur. The network geometry of the interconnected naoncages ensures a good bulk electrical conductivity of 183 S m-1 (Figure S4). The 3D architecture also favors the structural stability, e.g., avoiding the agglomeration or restacking generally faced by 2D graphene due to the intrinsic π-π interaction [38]. The 3D multiscale hierarchical character of the hCNC is much different from the case of the randomly packed carbon nanocages (rpCNC) with

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similar nanocage blocks, as schematically demonstrated in Figure 3 (Figures S5 and S6), which could lead to a great difference in Li-S cells’ performances.

a

4MgCO3·Mg(OH)2·5H2O

b

i 5 μm

c

500 nm

Precursor decomposition MgO

d

ii 5 μm

e

500 nm

f

Carbon deposition and template removal hCNC

iii 5 μm

500 nm

g

Figure 1 SEM observation for the formation of the hCNC. (a,b) The basic magnesium carbonate precursor. (c,d) The MgO template obtained by decomposing the precursor in (a,b). (e,f) The hCNC formed by depositing carbon shell on the MgO template in (c,d) then removing MgO by acid treatment. (g) A schematic diagram for the formation of hCNC via in situ MgO template method.

5

a

b

Ⅰ Ⅱ 10 nm

50 nm d 2000

1.5

Mesopore Micropore

1000 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/P0)

Micro-

Meso-

Macro-

10

1.0

5

0.5 0.0 0.1

dV/dlogD

Macropore

3000

dV/dlogD

Quantity 3 -1 adsorbed (cm g STP)

c

0 1 10 100 1000 Pore diameter (nm)

c

Figure 2 Characterizations of the hCNC. (a) TEM image. (b) High resolution TEM image. Arrows indicate the broken fringes. Regions I and II represent the spaces inside and in between the nanocages, respectively. (c) N2 adsorption and desorption isotherms, featured with the coexisting micro-, meso-, and macropores. (d) The corresponding pore size distributions. Micropore

Atomic scale

Mesopore

Nanoscale

Macropore

Mesoscale

Figure 3 Schematic structural characters of the hCNC and rpCNC at multiscales. The well-defined multiscale characters of the 3D hierarchical hCNC are demonstrated in the atomic scale, nanoscale and mesoscale, accompanied by the coexisting micro-, meso- and macropores. The rpCNC and hCNC have the same graphite layer with micropore tunnels at the atomic scale, the similar units of carbon nanocages at the nanoscale, but the quite different secondary structures at the mesoscale. 6

Construction and characterizations of the S@hCNC composites Figure 4 demonstrates the construction and characterizations of the S@hCNC composites. Sulfur was infused into the nanocages of hCNC by a simple melt-diffusion strategy. According to the micro-mesopore volume of hCNC (4.178 cm3 g-1) and the density of Li2S (1.66 g cm-3), the sulfur loading of 82.8 wt% is calculated for the full lithiation to Li2S within the pores (Figure 4a) [11]. Accordingly, by weighing hCNC and sulfur at the ratio of 1:5, the S@hCNC composite with the sulfur loading of 79.8 wt% was constructed, denoted as the 79.8%S@hCNC. The 89.9%S@hCNC composite with sulfur loading of 89.9 wt% was also prepared to examine the performance at a higher sulfur loading. As the counterparts of the 79.8%S@hCNC for comparison, the 81.9%S/hCNC was obtained by grinding the mixture of hCNC and sulfur but without heat treatment (thus most sulfur located outside the nanocages). And the 77.8%S@rpCNC was prepared analogous to the 79.8%S@hCNC by replacing hCNC with rpCNC (Figure S7).

7

a

c

S

200

C and S

g

f 81.9%S/hCNC 79.8%S@hCNC MgO@hCNC

100 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/P0)

h Sulfur

Intensity (a.u.)

3 -1

Quantity adsorbed (cm g STP)

C

300

e

d

0

81.9%S/hCNC 89.9%S@hCNC

Intensity (a.u.)

b

79.8%S@hCNC 500 1000 1500 -1 Raman Shift (cm )

20 40 60 2 Theta (degrees)

Figure 4 Schematic construction and characterizations of the 79.8%S@hCNC composite. (a) A schematic diagram of constructing S@hCNC composite via melt-diffusion and subsequent lithiation. To ensure the confinement of Li2S after lithiation, the partial cavity of hCNC should be left instead of fully occupied by sulfur. (b-e) TEM image (b) and the corresponding elemental mapping for C (c), S (d), and integrated C and S (e). Scale bar: 100 nm. (f) N2 adsorption and desorption isotherms. The oval highlights the different mesoporous characters. (g) Raman spectra. (h) XRD patterns.

TEM image of the 79.8%S@hCNC is similar to that of the pristine hCNC, and no discernible large sulfur particles are observed (Figure 4b). Electron energy-loss spectrum (EELS) exhibits the signs of sulfur and carbon, in accord with the energy-dispersive X-ray spectrum (EDX) (Figures S8 and S9). The elemental mapping indicates that the sulfur is homogeneously infused into the mesoporous cages (Figure 4c-e). The nitrogen adsorption and desorption isotherms show that the 79.8%S@hCNC loses the mesoporous character of the hCNC, similar to the case for the MgO@hCNC with MgO template inside the nanocages [37], while the

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81.9%S/hCNC with sulfur outside the nanocages keeps the mesoporous character (Figure 4f and Figure S7). These results indicate that the sulfur is infused into the nanocages of hCNC for the 79.8%S@hCNC. In Raman spectra, the sulfur signal is negligible for the 79.8%S@hCNC compared with the obvious one for the 81.9%S/hCNC (Figure 4g), in agreement with the different locations of the sulfur in these two counterparts. Raman spectrum for the 89.9%S@hCNC presents intensive sulfur signal, indicating the partial sulfur occupation outside the nanocages [15]. In XRD patterns, only trace signal of sulfur are detected for the 79.8%S@hCNC composite (Figure 4h), reflecting the amorphism of the highly dispersive sulfur nanoparticles [39], as supported by the high resolution TEM observation (Figure S8). In contrast, the sharp diffraction peaks of sulfur appear for the 89.9%S@hCNC and 81.9%S/hCNC, indicating the existence of crystalline sulfur particles outside the nanocages, in consistence with the Raman results. All these characterizations demonstrate that the amorphous sulfur nanoparticles are encapsulated into the nanocages for the 79.8%S@hCNC. Such well-confined structure could greatly enhance the electron conduction and suppress the polysulfide dissolution, which favors the rate capability and cyclability as revealed below. Electrochemical performance The electrochemical performance of the 79.8%S@hCNC composite is evaluated as presented in Figure 5, by fabricating coin cells (2032) with a metallic Li counter electrode. The cells are cycled in the range of 1.7–2.8 V versus Li+/Li and the specific capacities are based on the mass of sulfur. The cyclic voltammogram demonstrates two typical cathodic peaks of Li-S cells at ~2.30 and ~1.95 V (Figure 5a) [40]. In the initial charge/discharge potential profiles at a 0.2 A g-1 rate, the discharge curve displays two typical plateaus corresponding to the formation of long-chain

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polysulfides (Li2Sn, 3≤n≤8) at ~2.30 V and short chain Li2S2 and Li2S at ~2.10 V (Figure 5b), in agreement with cathodic peaks in the cyclic voltammogram. A large discharge capacity of 1214 mAh g-1 is achieved, indicating a high utilization of sulfur. The 79.8%S@hCNC composite presents a high-rate capability and long cyclability (Figure 5c). At the high rate of 1.0 A g-1 and over 300 cycles, the discharge capacity reaches a large value of 558 mAh g-1 with a low degradation rate of 0.16% per-cycle. Even at the higher rate of 3 A g-1, the discharge capacity still reaches ~580 mAh g-1 after activation process with a high Coulombic efficiency (Figure S10). Furthermore, the 79.8%S@hCNC composite exhibits excellent performance at low temperature of 0 o

C, delivering a high capacity of 419 mAh g-1 after 100 cycles at a 0.5 A g-1 rate

(Figure S11). As summarized in Table 1, the electrochemical performance of the 79.8%S@hCNC is superior to those for the 89.9%S@hCNC and 81.9%S/hCNC, which results from the smaller charge transfer resistance (Rct) and the better electrical conductivity for the former due to the different locations of the sulfur (Figure 5d,e and Figures S12 and S13). It is noted that, before cycling, the Nyquist plots for either the 79.8%S@hCNC or the 81.9%S/hCNC show only one semicircle. After cycling, the Nyquist plot for the 79.8%S@hCNC still has only one semicircle with decreased diameter. In contrast, the Nyquist plot for the 81.9%S/hCNC exhibits two semicircles, corresponding to the passivation film (in high frequency region) and the Rct (in medium-to-low frequency region) (Figure 5e). The formation of the passivation film comes from the deposition of dissolved polysulfides onto the electrode [41,42]. This result indicates the effective confinement of the dissolved polysulfides inside the nanocages for the 79.8%S@hCNC (Figure S14).

10

2.4 2.0 0

300 600 900-1 1200 Capacity (mAh g )

100 80 60 40 20 0 300

1200 1Ag

-1

800

2Ag

-1 -1

3Ag

400 0 0

50

100

150 200 Cycle number

e

-1

1200 800





400


0 0

1200

-Z'' (ohm)



250

Coulombic efficiency (%)

1.6

  800  400


150 
100 50 0 0 150 300

-Z'' (ohm)

-1

Capacity (mAh g )

-1

0.2 A g

2.8

+

2.0 2.5 Potential (V)

c

Capacity (mAh g )

Voltage (V vs.Li /Li)

1.5

d

b

1st 2nd 5th 10th

1.0 0.5 0.0 -0.5 -1.0

Current (mA)

a

Z' (ohm)

0 20

40 60 80 Cycle number

100

0

500

1,000 Z' (ohm)

1,500

Figure 5 Electrochemical performance of the 79.8%S@hCNC composite. (a) The cyclic voltammogram at a scan rate of 0.2 mV s-1 for the first ten cycles. (b) The charge/discharge potential profiles for the first cycle at a current density of 0.2 A g-1. (c) High-rate capability, long cyclability and Coulombic efficiency at different high current densities. (d) Comparison of the long cyclability at a rate of 1 A g-1 for the four different carbon-sulfur cathodes. (e) Nyquist plots for the four cathodes before cycling. Inset is the Nyquist plots for the cathodes after 100 cycles at 1 A g-1. In (d,e):

① 79.8%S@hCNC, ② 89.9%S@hCNC, ③ 77.8%S@rpCNC, ④

81.9%S/hCNC.

The structural advantage of the hCNC could be learnt by comparing the 79.8%S@hCNC and 77.8%S@rpCNC which own the comparable sulfur loading and similar building blocks of carbon nanocages, but the quite different secondary structure at the mesoscale (Figures 3 and 5d,e; Table 1). The much improved capacity and rate capability of the 79.8%S@hCNC over the 77.8%S@rpCNC demonstrates the great contribution of the unique network geometry of hCNC (Figures S15-S17 and Tables S2 and S3), which facilitates the electron conduction as well as the Li-ion

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diffusion as theoretically simulated (Figures S18). The experimental and theoretical results suggest an effective exploration of advanced materials in the viewpoint of mesoscale science which is an exciting frontier attracting increasing attention [43,44].

Table 1 Comparison of the detailed electrochemical data for the C-S composites.

Sample

Tested at a rate of 1 A g-1 for 100 cycles.

Capacity maintenances with increasing discharge current density (A g-1)

First dischar ge capacit y (mAh g-1)

0.2

Last dischar ge capacit y (mAh g-1)

Capacity degradat ion rate per-cycl e

First specif ic energ y (Wh kg-1)

Last specif ic energ y (Wh kg-1)

0.5

1.0

2.0

3.0

79.8%S@hC 100 65.1 56.8 49.6 44.5 1095.0 669.7 0.39% 1748 1069 NC % % % % % 89.9%S@hC 52.3 44.8 37.5 31.3 100 901.1 410.8 0.54% 1620 739 NC % % % % % 81.9%S/hCN 100 70.5 61.4 17.6 12.7 659.2 357.1 0.46% 1079 585 C % % % % % 77.8%S@rp 100 52.3 45.6 39.8 35.7 821.2 502.3 0.39% 1277 782 CNC % % % % % Note: 1. Specific energy is calculated by Cdischarge ×V×f. Here, Cdischarge is the discharge capacity, V is the average voltage of the cell (V is fixed at 2.0 V for convenient comparison), and f is the weight ratio of sulfur in the C-S composites. 2. Capacity maintenances are calculated in reference to the first discharge capacity of ten cycles at different current densities (see Figures S13g and S17i).

Based on the high-rate capability of the 79.8%S@hCNC, we have obtained the Li-S cells with fast charging and long cycle life as shown in Figure 6. At the charging rate of 3 A g-1 with limited charging time of 6 minutes, the charging capacity stabilizes at 300 mAh gsulfur-1. This fast charging Li-S cell reaches a stable discharge capacity of 300 mAh gsulfur-1 at a low rate of 0.2 A g-1 for 500 cycles. Controlling the charging time could inhibit the formation of long-chain dissoluble polysulfides during charging, as reflected in the absent plateau at ~2.3 V in the discharge curve (Inset in Figure 6), therefore leading to excellent cycle life [45,46]. Assuming the sulfur takes 12

up 35 wt% of a cell [9], the fast Li-S cell will deliver the specific energy of ~218 Wh kg-1, larger than that for the current Li-ion batteries (<200 Wh kg-1) (see Experimental section) [5]. Even at a higher charging rate of 5 A g-1 with a shorter charging time of 3 minutes, the cell also exhibits excellent performance with capacities of 250 mAh gsulfur-1 over 400 cycles (Figure S19). This result indicates the charging time for the mobile devices could be shortened from hours to minutes, which is greatly significant

2.7

+

-1

Capacity (mAh g )

900

Voltage (V vs.Li /Li)

and attractive.

600

Charge Discharge

2.4 2.1 1.8 0

100 200 300 -1

Capacity (mAh g )

300 Charge 6 minutes

0

0

100

200 300 Cycle number

400

500

Figure 6 Long cyclability of the Li-S cell with limited charging time of 6 minutes at the charging/discharging rate of 3 A g-1/0.2 A g-1. Inset is the corresponding charge/discharge curves of the second cycle. The high sulfur loading up to ~80 wt%, which is much higher than 40-70 wt% in most literatures to date [3,4], could meet the requirement for practical applications [47]. In comparison with the state-of-the-art high-sulfur-loading carbon-sulfur composites, the 79.8%S@hCNC presents the high-level performance with the large discharge capacity, high-rate capability and long cyclability when using similar electrochemical evaluation (Table S4). The excellent performance derives from the large pore volume and 3D hierarchical architecture of the hCNC. Such unique structure makes the 79.8%S@hCNC composite simultaneously own the advantages of high-loading-sulfur confinement, fast Li-ion diffusion as well as good conductivity,

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which is unusual for the most C-S composites to date. The unique structure of the hCNC also suggests the potential in many other fields, such as lithium air batteries cathodes, supercapacitors, fuel cells and heterogeneous catalysis, where high specific area, good conductivity and rapid medium diffusion are generally required. It is worth to mention that very recently there are a few reports about the C-S composites with high sulfur loading [48-50]. By doping the carbon materials with nitrogen [48,49] or treating the C-S composite with cetyltrimethyl ammonium bromide and employing the new ionic liquid-based electrolyte [50], the corresponding Li-S cells’ performances could be much improved (Table S4). This suggests a promising further optimization of Li-S batteries by multifaceted approaches.

Conclusion We report the novel 3D hierarchical carbon nanocages with high pore volume, network geometry and good conductivity. By making full use of the unique structure of the hCNC, we have realized the high-loading encapsulation of sulfur inside the nanocages. The designed 79.8%S@hCNC composite presents the large capacity, high-rate capability and long cycle life, which could shorten the charging time for the mobile devices from hours to minutes. The excellent performance results from the confinement of the high-loading sulfur inside the unique carbon nanocages, which alleviates the polysulfide dissolution while much enhances the electron conduction and Li-ion diffusion. The well correlation of the superb performance with the unique architecture, together with the convenient constructing strategy, suggests a new alternative to develope the advanced cathode materials of Li-S batteries.

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Experimental Section Preparation of hCNC After finding the morphological correlation between the basic magnesium carbonate precursor and MgO template (Figure S1), we selected the commercially available precursor with 3D hierarchical structure from XILONG CHEMICAL CO., LTD. This precursor was then overspread in the quartz tube reactor. After flushed with Ar flow for 20 minutes, the reactor was heated to 800 °C at the rate of 10 °C min in Ar flow. Then benzene was introduced into the tube at 60 µL min-1 through a constant-flow pump for 30 min. The reactor was thus naturally cooled down to room temperature. The as-prepared MgO@hCNC was first stirred in 6 M HCl aqueous solution for 48 h to remove the MgO template and repeatedly washed with de-ionized water, and the hCNC was obtained after dried at 80 °C for 24 h. Similarly, the rpCNC was prepared and the only difference is the use of the commercial MgO particles (20~60 nm) as the template. Construction of carbon-sulfur composites The hCNC and sulfur was mixed with the weight ratio of 1:5 or 1:10, and ground for 30 minutes in an agate mortar. The mixture was heated to 150 oC and held for 12 h. During this process, the melted sulfur easily diffused into the interior of the porous carbon nanocages of the hCNC. Then the temperature was increased to 250 oC and kept for 2 h to eliminate the sulfur on the outside surface of the nanocages by evaporation. The 79.8%S@hCNC and 89.9%S@hCNC composites with the sulfur loading of 79.8 and 89.9 wt% were obtained, respectively. Similarly, the 77.8%S@rpCNC composite was prepared by using rpCNC instead of hCNC. The 81.9%S/hCNC composite was obtained by mechanically mixing the hCNC and sulfur with a 1:5 weight ratio without undergoing the heat treatment. Characterization Scanning electron microscopy (SEM, Hitachi S4800 at 10 kV), high resolution transmission electron microscopy (HRTEM, JEM-2100F operating at 200 kV), and X-ray diffraction (Philips X’pert Pro X-ray diffractometer, Cu Kα1 radiation of 1.54056 Å) were used for sample characterization. N2 adsorption/desorption isotherms were measured on Thermo Fisher Scientific Surfer Gas Adsorption Porosimeter at 77 K. Before measurement of the hCNC, the sample was degassed at 300 oC for 4 hours. Such pretreatment was not applied in the measurement for 15

S@hCNC to prevent the sulfur volatility (Figure S20). The specific surface area was calculated using the BET (Brunauer-Emmett-Teller) method based on the adsorption data. Micro- and meso-pore size distributions were calculated by using HK (Horvath-Kawazoe, N2 on graphite at 77.2 K) and BJH (Barrett-Joyner- Halenda) methods from the adsorption branch of N2 isotherm, respectively. Macropore size distribution was obtained by using mercury porosimetry method measured on Thermo Fisher Scientific PASCAL 140 and 440. Raman spectra were collected on an R-3000HR spectrometer using a blue LED laser (λ=514 nm). Thermogravimetry analysis (Netzsch STA-449F3) was performed at a heating rate of 10 oC min-1 from room temperature to 425 oC under N2 flow. The electrical conductivity measurement was measured by a four-wire method using a source measure unit (Keithley 6430, Figure S4). Electrochemistry Working electrode was prepared by casting the slurry containing active material (80 wt%), conducting carbon (10 wt%), and polyvinylidene fluoride binder (PVDF, 10 wt%) dissolved in N-methyl-2- pyrrolidnone (NMP) onto Al foil substrate and dried in a vacuum oven at 80 oC for 12 h. The mass loading of S@hCNC was 1.0~1.5 mg cm-2. Coin cells (2032) were assembled in the Ar-filled glovebox with the oxygen and humidity level individually below 0.1 ppm. Lithium metal foil (1 mm thick) was used as a counter electrode and Celgard 2500 as the separator. The electrolyte solution was composed of 1.0 M bis-(trifluoromethane)sulfonamide lithium (LiTFSI) salt in a mixed solvent of 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) with 1:1 volume ratio containing LiNO3 (1 wt%). The charge-discharge behavior was characterized in NEWARE CT3008 multichannel battery testing unit, under different constant current densities (A g-1) with a voltage window of 1.7-2.8 V. The specific energy is calculated by Cdischarge×V×f, where V is the average voltage of the cell (2.08 V in Figure 6), and f is the weight ratio of sulfur in the battery (35%). The cyclic voltammogram was performed in the range of 1.5-2.8 V at a scan rate of 0.2 mV s-1 with a VMP3 electrochemical station (Bio-logic). The electrochemical impedance spectroscopy was measured on the open circuit voltage status with disturbance amplitude of 5 mV in the frequency range of 100 kHz-10 mHz.

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Acknowledgements This work was jointly supported by the NSFC (51232003, 21373108, 21173114, 21203092, 21473089), “973” programs (2013CB932902), Jiangsu science fund (BE2012159) and Suzhou Program (ZXG2013025). The authors would like to thank the Thermo Fisher Scientific Company for macropore measurement.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online eversion at http://dx.doi.org/10.1016/ j.nanoen.

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Author's Vitae

Zhiyang Lyu received his B.Eng. degree from Northeast Forestry University in 2009. He is currently a Ph.D. candidate under the supervision of Prof. Zheng Hu in School of Chemistry and Chemical Engineering, Nanjing University. His research interests focus on the synthesis of hierarchical carbon nanocages and their applications in energy storage especially for Li-S, Li-ion, Li-O2 batteries.

Dan Xu graduated in 2012 from School of Chemistry and Chemical Engineering, Nanjing University, China, where she continued to do research on mesoscopic material as a master student. Her current research focuses on CVD synthesis of carbon nanocages for lithium oxygen battery as well as lithium sulfur battery and lithium ion battery.

Lijun Yang received his PhD in solid mechanics from Harbin Institute of Technology in 2006, and gradually converged to chemistry after two post-doc periods in IMEC Belgium and Nanjing University. Now he is an associate researcher in Nanjing University and mainly focuses on the theoretical understanding of the mechanisms in energy conversion and storage systems, such as fuel cells, supercapacitors and lithium batteries.

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Renchao Che obtained his Ph.D. in solid state physics from Chinese Academy of Sciences in 2003. He was a research fellowship in National Institute for Materials Sciences (NIMS), Japan (2004-2007), and was appointed a professor in Laboratory of Advanced Materials, Fudan University in 2008. He mainly focuses on the investigation of nano-functional materials and radar absorption materials by advanced transmission electron microscopy techniques. Rui Feng received her M.S. degree under the supervision of Prof. Zheng Hu at School of Chemistry and Chemical Engineering, Nanjing University, China in 2014. Her research focuses on the synthesis of the carbon nanocages supported LiFePO4 nanoparticles and their application for Li-ion batteries cathodes.

Jin Zhao received his M.E. degree in Institute of Advanced Materials from Nanjing University of Posts and Telecommunications, China in 2012. He is now a Ph.D. candidate in Prof. Zheng Hu’s group in School of Chemistry and Chemical Engineering, Nanjing University. His research focuses on synthesis of carbon nanomaterials and their applications in energy storage and conversion. Yi Li obtained his Ph.D. in physical chemistry from School of Chemistry and Chemical Engineering, Nanjing University, China, in 2012. After a postdoctoral fellowship at School of Electronic Science and Engineering, Nanjing University, he was appointed a lecturer in Nanjing University of Posts and Telecommunications in 2014. His current research interests include semiconductor materials, electronics and energy devices. Qiang Wu obtained his Ph.D. from Nanjing University in 2004. He was appointed an associate professor of physical chemistry of Nanjing University in 2006. As a Hua-Ying Scholar, he visited Stanford University in professor Yi Cui’s group for one year. His scientific interests focus on semiconductor nanomaterials, energy storage, and electrocatalysis.

Xizhang Wang obtained his Ph.D. in chemistry from Nanjing University in 2001. He was appointed an associate professor in 2003 and professor of chemistry of Nanjing University in 2011. He was also a Fellow of Japan Society for the Promotion of Science (JSPS) in Tokyo University (2003-2005). His scientific interests mainly focus on nanomaterial chemistry, sustainable energy, and heterogeneous catalysis. Zheng Hu received his BS (1985) and PhD (1991) degrees in physics from Nanjing University. After two-year’s postdoctoral research in Department of Chemistry, he became an associate professor in 1993, and subsequently acquired 20

the professor position in 1999, and Cheung Kong Scholar professor in 2007. He is the owner of the highly competitive NSFC fund for outstanding young scientists of China (2005). As a guest scientist, he visited Research Center of Karlsruhe (Germany), University of Cambridge (UK), and MIT (USA) for two years. Hu is engaged in the research field of physical chemistry and materials chemistry addressing the growth mechanism, materials design and energy applications of a range of nano-/mesostructured materials, especially the carbon-based materials and group III nitrides.

Highlights 1. Novel 3D hierarchical carbon nanocages with high pore volume, network geometry and good conductivity. 2. High-loading confinement of ~80 wt% sulfur inside the nanocages. 3. Much alleviated polysulfides dissolution due to the confinement. 4. The high-rate performance for the high-sulfur-loading carbon-sulfur composites. 5. Well established correlation between the high performance and unique structure.

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