Conductive Carbon Network inside a Sulfur-Impregnated Carbon

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Conductive Carbon Network Inside A Sulfur-Impregnated Carbon Sponge: A Bioinspired High-Performance Cathode for Li-S Battery Xue-Li Du, Ya You, Yang Yan, Dawei Zhang, Huai-Ping Cong, Haili Qin, Chaofeng Zhang, Feifei Cao, Ke-Cheng Jiang, Yan Wang, Sen Xin, and Jian-Bo He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07607 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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ACS Applied Materials & Interfaces

Conductive Carbon Network Inside A Sulfur-Impregnated Carbon Sponge: A Bioinspired High-Performance Cathode for Li-S Battery

Xue-Li Dua,‡, Ya Youb,‡, Yang Yanc, Dawei Zhanga, Huai-Ping Conga, Haili Qina, Chaofeng Zhanga, Fei-Fei Caod, Ke-Cheng Jiange, Yan Wanga, Sen Xina,b,*, Jian-Bo Hea,*

a

School of Chemistry and Chemical Engineering, Hefei University of Technology,

Hefei 230009, P. R. China. b

Materials Science and Engineering Program & Texas Materials Institute, The

University of Texas at Austin, 1 University Station, C2201, Austin, Texas 78712, USA. c

Department of Basic Science, Dalian University of Technology · Panjin, Panjin

124221, P. R. China. d

College of Science, Huazhong Agricultural University, Wuhan 430070, China

e

Shenzhen TAFEL New Energy Technology Co. Ltd.

*

Corresponding authors. E-mail addresses: [email protected], [email protected].

‡

These authors contributed equally.

KEYWORDS: Lithium-sulfur battery, sulfur cathode, electronic conductivity, ketjen black, porous carbon, sodium alginate.

ACS Paragon Plus Environment


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ABSTRACT: A highly conductive sulfur cathode is crucial for improving the kinetic performance of a Li-S battery. The encapsulation of sulfur in porous nanocarbons is expected to benefit the Li+ migration, yet the e- conduction is still to be improved due to a low graphitization degree of conventional carbon substrate especially that pyrolyzed from carbohydrates or polymers. Aiming at facilitating the e- conduction in the cathode, here we propose to use ketjen black, a highly-graphitized nanocarbon building block to form a conductive network for electrons in a biomass-derived, hierarchically porous carbon sponge by a easily scaled-up approach at a low cost. The specifically-designed carbon host ensures a high loading and good retention of active sulfur, while also provides a faster electron transmission to benefit the lithiation/delithiation kinetics of sulfur. The sulfur cathode prepared from the carbon network shows excellent cycling and rate performance in a Li-S battery, rendering its practicality for emerging energy storage opportunities such as grids or automobiles.


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INTRODUCTION An increasing demand on energy promotes the development of advanced energy storage

systems

supercapacitors.23,

including 24

rechargeable

lithium/sodium

batteries1-22

and

Lithium-sulfur (Li-S) battery is regarded as one of the most

promising candidates due to its high theoretical energy density of 2600 W h kg-1,25-27 about 5 times greater than the Li-ion battery that built with a LiCoO2 cathode and a graphite anode (~390 W h kg-1).28 The energy advantage of Li-S battery originates from the ultrahigh specific capacities of both electrodes (Li anode: 3890 mA h g-1; S cathode: 1675 mA h g-1). The significant cost advantage, abundant natural source and environmental friendliness of S makes the battery even attractive.29 However, there are also deficiencies weakening the concept of such a battery: (1) low practical capacity and poor rate capability due to insulating properties of S and its discharge products including Li2S and Li2S2; (2) unstable sulfur electrochemistry due to dissolution and shuttle of lithium polysulfides (Li2Sx, 4≦x≦8), which leads to an abnormal Coulombic efficiency (CE) > 100% and rapid capacity fade upon cycling; (3) large volumetric variations during lithiation/delithiation of S, which leads to pulverization of cathode.30-33 Space confinement of active S into conductive substrates with high flexibility, such as porous carbons,34 and graphene (or graphene oxide) foam,35-38 carbon black/nanotube aggregates,39-41 conductive polymers,42-44 has been proved effective to improve the cathode cyclability since they can suppress the dissolution of lithium polysulfides and stand the volume variations. Additionally, the use of functional interlayers, separators and Au-S interactions for polysulfide trapping are also proved feasible in improving the cathode performance.45-47 However, the inherently low ionic and electronic conductivity of S, which lowers the practical capacity and further the power density of a Li-S battery, is still to be addressed.



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Although the use of porous carbon disperses S into “nano” sized counterparts to improve the Li+ conductivity, the carbon itself, especially that pyrolyzed from common carbohydrates (e.g., glucose or sucrose) or polymers at a relatively low temperature < 1000 oC (far less than the graphitization temperature of carbon), suffer from a poor electron conduction due to a low graphitization degree, and is thus unable to provide a sufficient electron supply for the Li-S reaction at higher rates. On the other hand, the graphitic carbon materials, such as graphene, carbon black or carbon nanotubes, are provided with a high electronic conductivity, yet suffer from an inhomogeneous S dispersion, usually forming bulk S in the composite to deteriorate the Li+ conductivity. Noted that the kinetic stability of the cathode reaction is co-determined by electronic/ionic conductivity and nanocarbon networks are effective in improving the transportation of both Li+ and e-,20, 21, 48 here we present a feasible and scalable strategy to improve the cathode performance, that is, creating a graphitic carbon network inside a porous carbon substrate. Sodium alginate (SA), a low-cost, environment-benign polysaccharide biomass, is used as carbon source to yield a hierarchically porous carbon (HPC) substrate, and kejent black (KB), a highly-graphitized nanocarbon building block is introduced into the porous carbon (KB@HPC) to form a conductive network for electron transmission. Owing to a hierarchical pore distribution from macropores to mesopores, and further to micropores in the HPC, sulfur loaded in such a carbon substrate achieves a homogenous dispersion at a nanoscale, bringing an improved electroactivity and alleviated volume expansion of S. On the other hand, the porous structure and embedded KB particles of the carbon substrate form a mixed conducting network for Li+/e-, so that the kinetics of cathode reaction is improved. As a result, the S cathode


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exhibits stable cycling performance and favorable high-rate capability in a prototype Li-S battery, proving the effectiveness of our strategy.

EXPERIMENTAL SECTION Synthesis of carbon substrates. To prepare the carbon substrates, 1 g of SA, 2 mL of deionized water (DIW), and an appropriate amount of ketjen black were premixed in a stainless steel jar, following by a 2-hour ball milling to obtain a black slurry. The slurry was dried, annealed at 800 oC (heating rate: 5 oC min-1) for 2 h in argon, and finally cooled to ambient temperature. The resultant was washed by 1 M aqueous solution of HCl, then DIW till the pH value reached neutral, and finally dried overnight to yield the target product KB@HPC. Through adjusting the mass of KB, three KB@HPC composites were prepared, with final KB contents of 50%, 25%, and 16.7% in the composites, and were separately denoted as KB@HPC1, KB@HPC3, KB@HPC5 based on the mass ratio between KB and HPC. A bare HPC substrate was also synthesized following the same method as above yet without KB. The feeding ratios of KB to SA for preparation of different KB@HPC composites were summarized in Table 1. Synthesis of S/C composites. To prepare the S/C composites, S powder (purchased from Aldrich) was mixed with the as-synthesized carbon at a mass ratio of ms:mc=6:4, to yield a S & C mixture. The mixture was then heated at 155 oC (heating rate: 1 oC min-1) for 12 h under argon protection to yield the S/C composites. The S/C composite prepared from the bare HPC were denoted as S/HPC, and those prepared from the KB@HPC1, KB@HPC3, KB@HPC5 were denoted as S/(KB@HPC1), S/(KB@HPC3) and S/(KB@HPC5), respectively.



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Material characterization: Scanning electron microscopic (SEM) images were collected on a Zeiss Hitachi SU8020 scanning electron microscope. Transmission electron microscopic (TEM) images were collected on a Hitachi H-7650 at an accelerating voltage of 100 kV. X-ray diffraction (XRD) patterns, Raman spectra and X-ray photoelectron spectroscopic (XPS) spectra were collected for structural and elemental analysis of materials according to the previous work.49 A thermal analyzer (Model: TA-SDT Q600) was used to perform thermogravimetic analysis (TGA) on the

S/C

composites

(heating

rate:

10

o

C

min-1,

atmosphere:

N2).

N2

adsorption/desorption isotherms were collected on a Quantachrome Autosorb-IQ, with a degassing process carried out at 120 oC for the carbon sample and at room temperature for the S/(KB@HPC3) to avoid any sublimation of sulfur, and a non-local density functional theory (NLDFT) was employed to calculate the cumulative pore volumes (CPVs) and differential pore-size distributions (DPDs) of the carbon substrates and S/C composites. Electrochemical Characterizations. The as prepared S/C composites, ketjen black and poly-(vinyl difluoride) (pVDF) were mixed at a mass ratio of mS/C:mKB:mpVDF = 80:5:15 dissolved in N,N-dimethyl pyrrolidone (NMP) to form a uniform black slurry. The slurry was then coated on an Al foil, dried and sliced to yield the S cathodes (diameter: 12 mm). The cathodes were then paired with Li foil as anodes to assemble Swagelok-type Li-S cells in a glove box filled with argon. A microporous polypropylene fmembrane (Celgard, USA) was employed as the separator. An ether electrolyte consisting of 1 M lithium bis(trifluoromethane) sulfonamide in 1,3-dioxolane/dimethoxymethane

(v:v

=

1:1)

was

used.

Galvanostatic

discharge-charge (GDC) cycling tests of the assembled batteries were performed on an Arbin BT-1 system (voltage range: 1-2.8 V vs Li+/Li). The C rate was calculated


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based on the weight of active sulfur on the cathode. Cyclic voltammograms (CVs) and Nyquist plots were obtained on an electrochemical workstation (Model: Autolab PG302N).

RESULTS AND DISCUSSIONS To prepare the carbon substrate, the SA (Figure S1a) and the KB nanoparticles with a high graphitization degree (Figure S1b, c) were thoroughly mixed by ball-milling, yielding a grey-color precursor (Figure S1d). The precursor was then annealed under an argon atmosphere to obtain the KB@HPC substrate according to the Scheme 1. Through adjusting the addition amount of KB, three KB@HPC substrates were prepared, and were separately denoted as KB@HPC1, KB@HPC3, KB@HPC5 based on the mass ratios between KB and HPC (the KB@HPC1 is provided with the highest KB content). For comparison, we have also annealed the SA under the same conditions as those for the KB@HPC yet without the addition of KB, yielding the bare HPC substrate. According to Figure 1a and Figure S2a-c, both the three KB@HPC substrates and the bare HPC substrate show almost the same sponge-like morphology, with macropores of ~500 nm (inset of Figure 1a) observed throughout these carbons. However, the bulk sizes of these carbons gradually decrease from HPC to KB@HPC5, then KB@HPC3 and finally to KB@HPC1, as the KB ratio increases in the composite. The TEM images in Figure 1b confirms a uniform distribution of KB nanoparticles in the HPC of the KB@HPC3 composite (while the TEM image of the bare HPC shows a semitransparent morphology without any embedded black KB particles according to Figure S2d), forming a continuous KB network. In contrast, the KB@HPC5 composite with the lowest KB ratio does not form a continuous KB network, while the KB@HPC1 composite with the highest KB ratio shows significant



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agglomeration of KB nanoparticles inside the HPC (Figure S2e-f). From the XRD patterns, it is clear that the bare HPC without KB embedding shows an amorphous state, as confirmed by the broad peak at 20-30o in Figure S3a. Upon elevating the KB content in the KB@HPC composite, a sharp peak at ~26o, which is identified as a characteristic XRD peak of graphitic carbon, gradually appears and intensifies (Figure 1f and S3b-c).50 This result agrees with the Raman spectra (Figure 2a and Figure S4), which also reveals a gradually increased G-/D-band intensity ratio from the bare HPC to the KB@HPCn (the value of “n” decreases from 5 to 3 and finally to 1), confirming an improved graphitization degree of carbon composite.51 The porous structure of these carbon substrates were characterized by N2 adsorption/desorption isotherms. According to Figure 2b, c and S5, all the carbon materials have similar hierarchically porous structures consisting of micropores, mesopores and macropores, as demonstrated by their combined Type I/IV isotherms and DPDs. The bare HPC shows a high specific surface area (SSA) of 808 m2 g-1 according to the Brunauer-Emmett-Teller (BET) method and concentrated pore distributions at ~0.5 nm and ~4 nm based on the NLDFT model, which brings and a CPV of 0.83 cm3 g-1 (Figure S5a, d). Doping by KB introduces more interfaces into the HPC substrate, so that the KB@HPCn composite shows a continuously increased BET SSA from 1030 m2 g-1 (n=5) to 1115 m2 g-1 (n=3) and finally to 2265 m2 g-1 (n=1). However, the CPV of the KB@HPC5 (0.71 cm3 g-1) is slightly lower than the bare HPC, which may be due to a “filling” of KB into the mesopores of the HPC, as demonstrated by a significantly weakened distribution in the mesopore region (Figure S5e). Due to the porous nature of KB, a further increment in the KB content leads to formation of new nanopores inside the composite, and the KB@HPC3 has an increased CPV of 0.86 cm3 g-1 (Figure 2c). The KB@HPC1, as provided with the


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highest KB content, also holds the largest CPV of 2.23 cm3 g-1 (Figure S5f) due to its cracked morphology (Figure S2c). Given a S density of ~2.0 g cm-3, even the lowest theoretical S loading in these carbon substrates can reach 59 wt-% (in case of KB@HPC5), suggesting a favorable practicality of using these carbons as hosts for S cathode. Based on these carbon host materials, a facial and scalable mixing-heating process was applied to enable penetration of S (at its molten state) into these carbons, yielding four S/C composites denoted as S/HPC and S/(KB@HPCn) (n=1, 3, 5), respectively. Taking the KB@HPC3 as an example, Figure 1c and 1d show the typical S loading process in the carbon substrate. Wherein, bulk S particles in the S/C mixture disappear after heating, leaving the pristine morphology of carbon. The TEM and high-resolution TEM (HRTEM) images in Figure 1e also reveal the disappearance of crystal S in the product. The above results are consistent with the XRD patterns (Figure 1f and S3) and Raman spectra (Figure 2a and S4) collected during the S loading of the four carbons substrates, which demonstrate largely vanished S signals after heating (except Figure S3b, in which weak XRD signals of S can still be seen after heating since the KB@HPC5 does not have sufficient pore volume to accommodate S). However, the energy dispersive X-ray (EDX) spectra and the elemental mappings collected by both SEM (Figure 3a-e) and TEM (Figure 3f, g) confirm a rich and uniform S distribution in the carbon substrate. These results clearly depict the amorphous nature of the loaded S. The S encapsulation into C substrate (taking the KB@HPC3 as an example) is further confirmed by the N2 adsorption/desorption test (Figure 2b, c), which show significantly suppressed adsorption in the isotherms and almost vanished DPD in the micropore and mesopore region. Meanwhile, the BET SSA and the CPV of the S/C composite (S/(KB@HPC3))



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decrease to 28 m2 g-1 and 0.049 cm3 g-1, respectively (Figure 2c). The S content in the S/C composites, as represented by the S/(KB@HPC3), is ~60 wt-% according to the TGA (Figure 3h). An XPS analysis was also employed on these S/C composites. According to the S2p spectrum in Figure S6a, the S/HPC composite shows two peaks at 164.12 eV and 165.32 eV, which separately correspond to the S 2p3/2 and 2p1/2 peaks of elemental S.49 As the KB content increases, the S 2p3/2 and 2p1/2 peaks slightly yet continuously shift to a lower binding energy (Figure 2d, S6b, c). This result suggests a gradually reduced oxidation degree of S, and could be possibly ascribed to a decreased oxygen content in the carbon substrate due to introduction of graphitic carbon component. The S/C composites were then prepared into cathodes for their electrochemical tests in prototype Li-S batteries. Figure 4a and Figure S7 show the CV profiles of these S cathodes. During the anodic process, all the four cathodes show two distinct peaks at 2.35 V (vs Li+/Li) and 2.1 V, and a broad peak at < 1.9 V. These three peaks, according to the former pioneer works, correspond to the reversible stepped reductions of amorphous S8 in C mesopores or macropores and reduction of S chains or small S clusters confined in C micropores.1, 27-29, 30, 31, 36, 49, 52 In the reverse cathodic process, two peaks at ~2.15 V and 2.45 V appear, corresponding to the delithiation of Li2S to form S chains/small S clusters and amorphous S8 again in C micropores and mesopores.49 Among the four S cathodes, the S/HPC cathode shows the smallest reduction and oxidation peaks upon the initial scanning (Figure S7a), which implies a poor electrochemical activity of the cathode. The introduction of KB significantly increases the electroactivity of the S cathodes, as evidenced by much intensified reduction and oxidation peaks during the initial CV scans (Figure 4a, S7b and S7c). The addition of KB also helps to stabilize the electrochemistry of S cathode, as


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confirmed by highly overlapped CVs of the S/(KB@HPC5) and S/(KB@HPC3) cathodes (Figure 4a and S7b) after the initial scans. Further, the S/(KB@HPC3) cathode also shows the highest current densities, in both the anodic and the cathodic processes, among the four cathodes. However, as the KB continues to rise, the electrochemistry of the S cathode becomes unstable, as proved by a continuous shrinkage in CV profiles of S/(KB@HPC1) upon cycling (Figure S7c). GDC cycling tests were also employed to compare the performance of the above cathodes in Li-S batteries. In correspondence to the CV profiles, all the four cathodes show two plateaus at 2.35 and 2.1 V, and a slopped profile at < 1.8 V (Figure 5a) during their initial discharge at 0.5 C (unless otherwise specified, the GDC rates and specific capacities throughout the work are calculated based on active S mass). Among the four cathodes, the S/(KB@HPC3) cathode shows the highest initial discharge (charge) capacity of 1783 (1581) mA h g-1 and Coulombic efficiency of 88.6%, and the smallest polarization of 0.2 V (Figure 5a). Since S has a theoretical specific capacity of 1675 mA h g-1, the excessive initial discharge capacity may be largely ascribed to the contribution of the carbon substrate,49, 53 the ultrahigh initial discharge/charge capacity of the S/(KB@HPC3) cathode would suggest an almost complete lithiation/delithiation of S in the composite. This result provides solid evidence to prove the highest electroactivity of the S/(KB@HPC3) cathode among all these cathodes. From the second cycle, the S/(KB@HPC3) cathode shows highly consistent GDC profiles (Figure 4b), with its CE maintaining around 100% and small capacity decays observed during each cycle, hence bringing an even significant capacity advantage over the three control cathodes after 5 cycles (Figure 5b). After 100 GDC cycles at 0.5 C, the S/(KB@HPC3) cathode can still retain a reversible charge capacity 963 mA h g-1, which is significantly higher than capacities delivered



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by the control cathodes (Figure 5c). Ex-situ SEM and EDX characterizations further show that the S/(KB@HPC3) cathode, after 100 cycles at 0.5 C, still keeps its integrity (Figure S8a) with a uniform and strong S signal detected in the C matrix (Figure S8b-e) to prove the fine S retaining ability of the cathode. The S/(KB@HPC3) cathode also exhibits an favorable rate performance. Upon elevating the rate to 1 C and further to 2 C, the cathode still preserves 1322 (CE = 88.4%) and 1290 mA h g-1 (CE = 79.2%) upon the initial charge (Figure 5d, e). Meanwhile, the GDC profiles at each rate remain stable, so that the cathode can deliver an acceptable polarization of < 0.5 V at 2 C (Figure 4c). The S/(KB@HPC3) cathode also shows notable cycling performance at higher rates, which stably outputs specific capacities of 738 (555) mA h g-1 after 200 (300) GDC cycles at 1 (2) C (i.e., average capacity losses of 0.22% (0.19%) per cycle at 1 (2) C), significantly surpassing the rest cathodes (Figure 5d, e). Herein, the impressive performance of the S/(KB@HPC3) cathode over the control cathodes originates from its structural merits. With the integral and hierarchically porous HPC substrate, a homogeneous dispersion of electroactive S is realized throughout the composite cathode to benefit the Li+ migration, while the intermediate products of Li-S electrochemical reaction (Li2Sn, n=2-8), can be effectively confined in the conductive carbon substrate without dissolving into the electrolyte or lose electric contact with the current collector. Meanwhile, the embeded KB nanopartiles built a highly conductive network in the S-loaded HPC to enable fast electron transmissions, so that a “kinetically stable” cathode is obtained. The KB content in the carbon substrate also affects the performance of the S/(KB@HPC) cathode. When the KB content is low (e.g., in the KB@HPC5 composite), the KB nanoparticles cannot form an effective network inside the HPC,


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but remain dispersive as separated dots (Figure S2e). In that case, it is hard to achieve a seamless electron transmission in the cathode. Besides, the introduction of an extra amount of KB (e.g., in the KB@HPC1 composite) leads to cracks and pulverizations of carbon substrate, which inevitably deteriorates the structural integrity of the cathode and creating excessive interface as proved by the N2 adsorption/desorption test. In that case, the interface resistance may be significantly raised, and the polysulfide intermediates may easily dissolve from the open-structured carbon substrate. This reasonably explains why the S/(KB@HPC1) cathode cannot deliver a satisfactory electrochemical performance. Moreover, according to the electrochemical impedance spectra (EIS) in Figure S8, the newly-assembled Li-S/(KB@HPC3) battery, with an appropriate KB and HPC mass ratio in the cathode, shows the smallest charge transfer resistance, and the resistance value is further reduced after 10 GDC cycles at 0.5 C, suggesting an electrochemical activation of S occurred on the cathode. All these results give a rational explication for its optimized electrochemical performance.`

CONCLUSION To conclude, we have sucessfuly embedded graphitized KB nanoparticles inside a biomass-derived pyrolysis HPC sponge, yielding a favorable substrate to enable a high-performance S cathode with high S loading for rechargeable Li-S batteries. Through precisely controling the KB content in the composite carbon substrate, the KB forms a highly conductive network in the HPC without sacrificing the integrity of the carbon substrate. The built-in KB network, in combination with the hierachically porous structure of the HPC sponge, forms a seamless conducting network for Li+/e-, which benefits the lithiation/delithiation kinetics and electrochemical stability of the



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loaded S. As a demonstration of optimized design, the S/(KB@HPC3) cathode shows a high reversible capacity of 1581 mA h g-1 upon initial charge at 0.5 C, and excellent capacity retaining abilities at various rates. The synthetic strategy is simple, scalable and inspiring, and the cathode material has a significant cost advantage over its counterparts, yet is provided with a much improved performance. In view of the practicality of the cathode material, a Li-S battery built from it should seek for better opportunities in emerging energy storage fields, e.g., grids or automobiles. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Supporting

Information

Available:

Optical

images

of

raw

materials,

morphologies of carbon substrate, XRD patterns, Raman spectra, XPS spectra of the S/C composite, ex-situ SEM image and EDX results of a cycled cathode, and other electrochemical data including CVs and EIS. Corresponding author * Email: [email protected], [email protected]. Author Contributions ‡ These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (Grant No. 21403050, 21303064, 21503031, 21503063, 21576063, 51472070), Anhui Provincial Natural Science Foundation (1608085MB32), Fundamental Research Funds for the Central Universities (Grants J2014HGBZ0126, 2014HGQC0015), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) (Grant


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20130111110025), and Wuhan Chenguang Science and Technology Project for Young Experts (Grant 2015070404010192). The electrochemistry study of the S cathode in Austin, TX, was supported by the Lawrence Berkeley National Laboratory BMR Program (Grant 7223523).



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Table 1. Feeding ratios of KB to SA for preparation of different KB@HPC composites. Carbon substrate KB@HPC1 KB@HPC3 KB@HPC5

KB (mg) 64 22 13


SA (g) 1 1 1

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Scheme 1. Typical preparation process of the S/(KB@HPC) composite.



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Figure 1. Morphological and structural characterization results of the preparation process of S/(KB@HPC3), including: (a) the SEM image of KB@HPC3 (inset shows the enlarged SEM image), (b) the TEM image of KB@HPC3 (inset shows the HRTEM image), (c) the SEM image of the S & KB@HPC3 mixture, (d) the SEM image of the S/(KB@HPC3) composite after S loading (inset shows the enlarged SEM image), (e) the TEM image of S/(KB@HPC3) (inset shows the HRTEM image), and (f), the XRD spectra of the preparation process.


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Figure 2. Structural and elemental characterization results of the S/(KB@HPC3) composite, including: (a) the Raman spectra showing the preparation process of S/(KB@HPC3), (b) the N2 adsorption/desorption isotherms and (c) the corresponding CPVs and DPDs of KB@HPC3 and S/(KB@HPC3), and (d) the XPS S2p spectrum of S/(KB@HPC3).



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Figure 3. Morphological and structural characterization results of the S/(KB@HPC3) composite, including: (a) the SEM image (the white square shows the target area), (b) the EDX spectrum collected from the target area in (a), and the EDX elemental mappings of (c) C, (d) S, and (e) O, (f) the TEM image, (g) the EDX mappings of C, S and O, and the EDX spectrum collected from (f), (h) the TGA curve.


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Figure 4. Electrochemical profiles of the S/(KB@HPC3) cathode in Li-S cells: (a) typical CV profiles collected within a potential range of 1.0-2.8 V (vs Li+/Li) at a scan rate of 0.1 mV s-1, (b) voltage profiles collected for the initial 5 GDC cycles at 0.5 C, (c) voltage profiles collected from the 5th GDC cycle at various rates increasing from 0.5 C to 2 C.



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Figure 5. Electrochemical performance comparison between various S cathodes in Li-S cells. Wherein, (a-b) show the voltage profiles collected from (a) the initial GDC cycles and (b) the 5th GDC cycles of the S/(KB@HPC1), S/(KB@HPC3), S/(KB@HPC5) and S/HPC cathodes at 0.5 C, and (c-e) shows the cycling performance of these cathodes at 0.5 C, 1 C and 2 C, respectively.


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Table of contents (TOC)


Conductive Carbon Network inside a Sulfur-Impregnated Carbon

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