Ultrathin Honeycomb-like Carbon as Sulfur Host Cathode for High

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Ultrathin honeycomb-like carbon as sulfur host cathode for high performance lithium-sulfur batteries Taohong Wang, Yuhua Yang, Ling Fan, Longlu Wang, Ruifang Ma, Qingfeng Zhang, Jianguo Zhao, Junmin Ge, Xianlu Lu, Xinzhi Yu, Hongguan Yang, and Bingan Lu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01498 • Publication Date (Web): 26 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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Figure 1. Schematic illustration of the synthetic procedure of PCLSN@S.

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Figure 2. SEM images of (a) PCLS, (b) PCLSN and (c-e) PCLSN@S. TEM images of (f-g) LSPCN and (h) LSPCN@S. (i) HAADF-STEM image of PCLSN@S and the corresponding EDS elemental mappings of C, N and S.


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Figure 3. (a) XRD patterns of PCLS, PCLSN, PCLSN@S and Sulfur. (b) Raman spectra of PCLS, PCLSN and PCLSN@S. (c) Isotherms and (d) pore size distributions of PCLSN.



Figure 4. (a) C 1s and (b) N 1s XPS spectrums for PCLSN. (c) S 2p XPS spectrum for PCLSN@S. (d) TGA curve of PCLSN@S in Ar atmosphere.


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Figure 5. (a) CV curves of the honeycomb-like PCLSN@S electrode at a scan rate of 0.1 mV s-1. (b) Voltage-capacity profiles and (c) Rate performances of PCLSN@S electrode at various current densities from 0.1 to 3 C. (d) Cycling performances of PCLSN@S electrode at 2 C, the coulombic efficiency is nearly 100%.



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Ultrathin Honeycomb-Like Carbon as Sulfur Host Cathode for High Performance Lithium-Sulfur Batteries Taohong Wang,a,b Yuhua Yang,a Ling Fan,a Longlu Wang,a Ruifang Ma,a Qingfeng Zhang,a Jianguo Zhao,a,e Junmin Ge,a Xianlu Lu,a Xinzhi Yu,a Hongguan Yang,a* and Bingan Lu,a,c,d* a School of Physics and Electronics, Hunan University, Changsha, 410082, China b Physics Department, College of Science, Central South University of Forestry and Technology, Changsha, 410004, China c Fujian Strait Research Institute of Industrial Graphene Technologies, Jinjiang, 362200, China d State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Shanxi, 030001, China e School of Physics and Electronic Information, Luoyang Normal University, Luoyang, 471022, Henan, P. R. China ABSTRACT: Lithium-sulfur (Li-S) batteries have attracted great interest because of its high theoretical capacity (1675 mA h g-1). However, the low electrical conductivity of sulfur, dissolution of polysulfides and structural collapse of electrode limit its practical application. Here an ultrathin honeycomb-like porous carbon derived from loofah sponge and doped with nitrogen (PCLSN) is prepared as a stable host for sulfur nanoparticles. Attributed to the integrated honeycomb structure, hierarchical porosity, ultrathin honeycomb walls and synergistic effects between physical and chemical adsorption of polysulfides, the developed PCLSN@S cathode achieves a high initial specific capacity of 1379 mA h g-1 at 0.1 C, outstanding cyclability with a small capacity decay rate of 0.044% per cycle over 970 cycles at 2 C, excellent rate performance with a high capacity of 664 mA h g-1 at 3 C and high sulfur content of PCLSN@S up to 76.1%. Our approach provides a promising route to design other 3D porous structures for high performance Li-S batteries. KEYWORDS: Lithium-sulfur batteries; ultrathin; honeycomb-like; porous carbon; loofah sponge ■ INTRODUCTION With the global warming and air pollution caused by the consumption of fossil fuels, it is necessary to 1


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develop low-cost and environmentally friendly energy-storage systems.1-5 Lithium-sulfur (Li-S) battery is considered as one of the most promising candidates for the demand owing to its relatively nonpoisonous materials, high natural abundance, high theoretical energy density (2600 W h kg-1) and the outstanding theoretical capacity of sulfur (1675 mA h g-1 or 2800 W h L-1).6-10 Unfortunately, there are still many challenges impeding the practical utilization of Li-S batteries.11-13 Firstly, the poor electrical conductivity of both sulfur and its final discharge products lead to the low utilization of active material. Secondly, the dissolution of polysulfide intermediates (Li2Sx, x=3-8) generated in the cathode side can shuttle between the cathode and anode and react with Li metal chemically, resulting in active materials loss and dendrite issues of Li anode, causes fast capacity degradation and safety hazards.14-19 Moreover, a volumetric expansion during electrochemical lithiation and delithiation processes results in the structural collapse of the electrode.20-32 The solutions to these issues are straightforward: improve the cathode conductivity and stabilize the active material within the cathode. Carbonaceous materials, with a high conductivity, tunable pore structure, and high stability, play a particularly crucial role in the sulfur electrode.23,33 Various carbon scaffolds, including carbon black,34 activated carbons,35-36 carbon nanotubes37-40 and graphene22,41-43 have been applied to obtain carbon/sulfur composite cathodes in recent years. Despite progress resulted, the cycling stability and discharge/charge capacity are still not satisfactory. The above-mentioned carbon materials tend to pack together, leading to inhomogeneous contacts and poor links between the active material and the conducting matrix, which limit the utilization of the active material.44-47 Moreover, carbon nanotubes and graphene heavily rely on natural graphite mines which are not cost-effective, restricting their wide application.48 Besides, for the nonpolar carbon framework, polar sulfur/ lithium polysulfides 2



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may detach from it during discharge and charge processes due to the low binding energy between them, which results in severe shuttle effect and capacity decay.49 Heteroatom-doped carbon that has a polar nature is commonly used to trap the soluble lithium polysulfides from diffusing out the cathode through strong chemical binding between polysulfides molecules and carbon surface.50-52 Herein, we successfully developed an ultrathin honeycomb-like hierarchical porous carbon material derived from loofah sponge and doped with nitrogen (PCLSN), as highly efficient sulfur host for Li-S batteries. Benefiting from the unique structure, PCLSN exhibits several advantages. Firstly, the honeycomb structure requires minimum materials and has the highest density and maximum space, these properties are expected to incorporate more sulfur, increase the loading and utilization of sulfur in Li-S batteries.53-56 Secondly, the interconnected network structure with micro/meso/macro porosity can facilitate fast electron/ion transport as well as effective physical trapping of polysulfides during the sulfur redox reactions. Finally, the nitrogen doping can further enhance chemical adsorption of polysulfides, resulting in synergistic effects between physical and chemical adsorption of polysulfides.33,57 As a result, the PCLSN based cathode provides the Li-S cell with significant enhancements. The outstanding cell performance is evidenced by a high discharge capacity of 1379 mA h g-1 at 0.1 C, a low capacity fade rate of 0.044% per cycle for 970 cycles at 2 C and excellent rate performance from 0.1 to 3 C. ■ RESULTS AND DISCUSSION Synthesis of PCLSN@S nanostructures. Preparation of PCLSN@S structures is schematically shown in Figure 1. Briefly, dry loofah sponge was first soaked in potassium hydroxide (KOH) aqueous solution few days, followed by freeze drying and calcination in argon atmosphere at 800 ℃, during the annealing process, KOH served as a chemical activating reagent to obtain porous carbon material,52,58-60 which was 3


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derived from loofah sponge, denoted as PCLS. The obtained PCLS structure was washed by

Figure 1. Schematic illustration of the synthetic procedure of PCLSN@S.

1 M dilute hydrochloric acid (HCl) solution and deionized water. After freeze drying, the PCLS material annealed in NH3 atmosphere at 500 ℃ to form nitrogen doped PCLS product, referred as PCLSN. Finally, sulfur was impregnated into the PCLSN structure by melt-diffusion at 155 ℃ for 24 hours and 200 ℃ for 2 hours to yield PCLSN@S composite. Morphology and structure of PCLSN@S. Figure 2a and Figure S1a illustrate that the KOH etched PCLS product possesses a macroporous structure, the size of the holes is about 0.5-3.0 μm. As a comparison, Figure S1b and S1c (Supporting Information) show the framework of the carbon from loofah 4



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sponge before handled by KOH (CLS). By comparing the morphologies between PCLS and CLS, we can conclude that the surface of CLS is nonporous, the surface of PCLS is porous, which indicates that KOH is typically used as a pore-forming agent to induce a high porosity on PCLS.

Figure 2. SEM images of (a) PCLS, (b) PCLSN and (c-e) PCLSN@S. TEM images of (f-g) LSPCN and (h) LSPCN@S. (i) HAADF-STEM image of PCLSN@S and the corresponding EDS elemental mappings of C, N and S. 5


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Figure 2b and Figure S1d display typical SEM overview of the PCLSN powders, which are evidently made of honeycomb-like integrated structure, the size of the holes is maintained at about 0.5-3.0 μm. Clearly, after nitrogen doping by calcination in NH3, cavities on the surface of PCLSN material are overlapped to form 3D interconnected honeycomb structure, which facilitates an easy access to Li+ ingress/egress. Besides, micro/mesopores of PCLS and PCLSN can be seen from Figure S1e and S1f (Supporting Information). The micro/meso/macro pores are ascribed to the etching of carbon framework by redox reactions and gasification of carbon during KOH activation process, and the removal of metallic K and other K compounds produced in KOH activation process by washing with HCl dilute solution and deionized water.61 The SEM images of the PCLSN@S reveal that sulfur is uniformly dispersed in cavities of PCLSN, and there still reserves a considerable amount of void hole on the surface of PCLSN@S (Figure 2c-e) to accommodate the large volumetric expansion, thus keeping the structural integrity to reduce polysulfides dissolution. The honeycomb-like PCLSN structure could be further evidenced by low-magnification TEM image in Figure 2f, which was consistent with SEM images. The high-magnification TEM image of PCLSN in Figure 2g reveals the walls of honeycombs can be thin to 6 nm, the ultrathin walls of honeycombs allow easier transit for Li+ ion than thick physical barriers,62 which leads to excellent conductivity. Besides, PCLSN has a degree of graphitization, which further benefits the electronic conductivity of carbon.63 The TEM observation of PCLSN@S (Figure 2h) reals that the 3D network structure was maintained intact after sulfur infused into PCLSN. Figure 2i presents the high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image and the corresponding 6



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Energy-dispersive X-ray spectroscopy (EDS) elemental mapping, the mapping confirmed the presence of carbon, nitrogen and sulfur. Besides, the nitrogen and sulfur were uniformly distributed along the carbon.

Figure 3. (a) XRD patterns of PCLS, PCLSN, PCLSN@S and Sulfur. (b) Raman spectra of PCLS, PCLSN and PCLSN@S. (c) Isotherms and (d) pore size distributions of PCLSN.

The structural characteristics of PCLSN@S nanocomposite are detected by X-ray diffraction (XRD), as shown in Figure 3a. The XRD pattern of the PCLSN@S shows two broad diffraction peaks, which are 7


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attributed to the (002) and (100) planes of carbon (JCPDS 75-1621) originated from PCLS and PCLSN carbon materials.64-66 Strong characteristic peaks of orthorhombic sulfur (JCPDS No.08−0247) in PCLSN@S indicate the successful incorporation of sulfur into PCLSN carbon.67 Figure 3b shows the Raman spectral analysis of PCLS, PCLSN and PCLSN@S. Two typical peaks were found at about 1335 cm-1 and 1589 cm-1, which are respectively assigned to the disorder-induced D band and G band arising from the symmetric stretching mode of C-C bonds from the ordered structure of graphitic crystallites of carbon.68-70 The intensity ratio of D band and G band (ID/IG) of the PCLS, PCLSN and PCLSN@S are 1.01, 1.03 and 1.08 respectively, manifesting the carbon matrix is partially graphitized, consistent with the TEM images. The characteristic peaks of sulfur were not observed in Figure 3b, which may ascribe to the fully diffusion of sulfur into pores of PCLSN. Figure 3c shows the nitrogen adsorption-desorption isotherm curves of the PCLSN, the BrunauerEmmett-Teller (BET) specific surface and total pore volume of the PCLSN are 1033.0 m2 g-1 and 0.68 cm3 g-1, respectively. The pore size distribution of the PCLSN showed in Figure 3d indicates a micro/mesoporous structure in a range between 1.2-5.0 nm with an average diameter of 2.64 nm (Table S1). The high porosity and large surface area in the PCLSN composites afford abundant space for loading of active materials and provide open channels for electrolyte penetration. The nitrogen adsorptiondesorption isotherm curves of the PCLS was shown in Figure S2. X-ray photoelectron spectroscopy (XPS) were carried out to confirm the chemical composition of PCLSN (Figure 4a-c). In the high-resolution C1s spectrum (Figure 4a), there are three peaks at 284.8, 285.7 and 287.6 eV, which were assigned to the C-C, C-N and C-O bonding respectively,71 suggesting that the coexistence of carbon, nitrogen and a small amount of oxygen in PCLSN. Figure 4b shows the 8



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fitted N 1s spectrum, which can be divided into three peaks at 398.7, 399.9 and 401.0 eV, corresponding to the pyridine N, pyrrole N and graphitic N, respectively,71-72 the pyridinic- and pyrrolic-type N are the domination as shown in Figure 4b. The content of nitrogen in PCLSN is 2.2 wt. %, these various types of doped nitrogen atoms increase the polarity of carbon surface, which

Figure 4. (a) C 1s and (b) N 1s XPS spectrums for PCLSN. (c) S 2p XPS spectrum for PCLSN@S. (d) TGA curve of PCLSN@S in Ar atmosphere.

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utilizes chemical adsorption to mitigate the shuttling effect.73 The S 2p XPS spectrum of PCLSN@S is resolved into S 2p3/2 and S 2p1/2 two peaks at 163.8 and 165.0 eV in Figure 4c respectively. The difference of binding energy between S 2p3/2 and S 2p1/2 is 1.2 eV and the intensity ratio is about 2:1.74 The C 1s and N 1s XPS spectrums of PCLSN@S were shown in Figure S3. The weight ratio of sulfur in PCLSN@S was determined to be 76.1% according to the thermogravimetric analysis (TGA) curves, as shown in Figure 4d. The high loading of sulfur is benefited from the high porosity and nitrogen atoms adoption for PCLSN. Electrochemical performance. The electrochemical performances of PCLSN@S composite are evaluated as a cathode material for Li-S batteries, the sulfur loading of electrode is about 1 mg cm-2, all the capacities were calculated based on the mass of sulfur in electrode. Figure 5a displays the cyclic voltammetry (CV) curves of the cell at a scan of 0.02 mV s-1 in the potential range of 1.6-2.8 V. During the first cathodic scan, the two reduction peaks centered at 2.27 and 1.99 V attribute to the reduction of S8 to long-chain polysulfides (Li2Sx, 4≤x≤8) and the further reduction of the long-chain polysulfides to insoluble Li2S2/Li2S, respectively.75 In the subsequent anodic scan, a main oxidation peak at 2.39 V corresponds to the reversible processes from Li2S to Li2Sx and ultimately to S8. In the third cycle, the two reduction peaks shift to higher potentials at 2.28 V and 2.00 V respectively and the anodic peak shift to lower potential at 2.35 V, suggesting an small electrochemical polarization.76 No significant changes of the curve between the third and fifth cycle indicate electrochemical performances tend to stable.

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Figure 5. (a) CV curves of the honeycomb-like PCLSN@S electrode at a scan rate of 0.1 mV s-1. (b) Voltage-capacity profiles and (c) Rate performances of PCLSN@S electrode at various current densities from 0.1 to 3 C. (d) Cycling performances of PCLSN@S electrode at 2 C, the coulombic efficiency is nearly 100%. 11


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Figure 5b reports the discharge/charge profiles at different current rates from 0.1 to 3.0 C (1 C = 1675 mA g-1). A high initial discharge capacity of 1379 mA h g-1 was achieved at 0.1 C, indicating high sulfur utilization. The discharge curves exhibit two typical plateaus around 2.3 and 2.1 V, corresponding to the reduction of sulfur to long chain polysulfides and the formation of insoluble Li2S2/Li2S, being well consistent with the CVs. The rate capability of PCLSN@S electrode was assessed under current densities ranging from 0.1 to 3.0 C, as shown in Figure 5c. When cycled at 0.1, 0.3, 0.5, 1.0, 2.0 and 3.0 C, the PCLSN@S cathode can deliver impressive discharge capacities of 1379, 1104, 1006, 925, 805, and 664 mA h g-1, respectively. As the current density switches back to 0.1 C, the discharge capacity restores to 1097 mA h g-1, indicating excellent electrochemical reversibility. As displayed in Figure 5d, the PCLSN@S nanoarchitecture exhibited stable cycling performance over 970 cycles at 2 C. A reversible specific capacity of 406 mA h g-1 was retained with capacity retentions of 60.4% and capacity decay of 0.044% per cycle for 970 cycles. The average coulombic efficiency was 99.6% over 970 cycles, nearly 100%, which shows little shuttle effect and high cycling stability.77 To stand out the excellent properties of PCLSN@S cathode, the electrochemical performances of CLS@S and PCLS@S cathodes are also investigated (Figure S4-S7). Furthermore, Table S2 shows that the electrochemical performance of PCLSN@S cathode is better than most of the reported lithium sulfur batteries. This outstanding performance for Li-S batteries based on the PCLSN composite should be attributed to the following factors. Firstly, the 3D interwoven honeycomb-like carbon framework with abundant 12



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micro/meso/macro pores not only provides the fast ion and electron transfer channels, but also suppresses the shuttling effect during the discharge/charge cycles by physical confinement. Secondly, nitrogen functional groups further increase the adsorption towards the Li polysulfides by chemical bonding for prolonged cycling life. The synergy effects between the physical and chemical adsorption improve the sulfur utilization. Thirdly, the porous structure can present sufficient empty space against the large volumetric expansion without causing the structure crack and fracture. Furthermore, the ultrathin and graphitization characteristics of the carbon improve the electronic conductivity, thus obtaining high rate capability.75,77-78 ■ CONCLUSIONS In conclusion, we have rationally designed an integrated ultrathin honeycomb-like PCLSN carbon with hierarchical porous structure as a highly effect sulfur host for Li-S batteries. The strategy utilizes dry loofah sponge as the carbon precursor, KOH as the pore-forming agent, and NH3 as the nitrogen dopant. With the help of this 3D cross-linked honeycomb structure, the Li-S batteries exhibited high sulfur utilization up to 76.1%, high specific capacities of 1379 mA h g-1 at 0.1 C, good rate capabilities from 0.1 to 3 C especially excellent cycling stabilities with a capacity decay of only 0.044% per cycle over 970 cycles at 2 C current. This work provides a promising route to design other 3D porous structures for highperformance Li-S batteries. ■ EXPERIMENTAL SECTION Materials. Dry loofah sponge was bought from Taobao. All other reagents were of analytical grade and used without further purification. Synthesis of CLS, PCLS, PCLSN and PCLSN@S nanostructures. 10 g dry loofah sponge was 13


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firstly cut into pieces and soaked in 200 ml of 3 M potassium hydroxide (KOH) aqueous solution 10 days at room temperature. Subsequently, the loofah sponge was collected and freeze dried 2 days. Then, the KOH treated loofah sponge was heated at 800 ℃ for 2 hours with a heating rate of 5 ℃ min-1 under argon protection in a tube furnace. The resultant product was washed with 1 M dilute hydrochloric acid (HCl) solution and deionized water three times. Finally, the PCLS sample was obtained. For comparison, CLS was also prepared using dry loofah sponge without KOH treatment through the same process. After freeze drying, the obtained PCLS material was annealing in NH3 atmosphere at 500 ℃ for 2 hours with a heating rate of 5 ℃ min-1, the product was referred as PCLSN. Finally, the PCLSN@S composite was made by mixing PCLSN and sulfur (1:4 by mass ratio) by strongly grinding for 20 minutes and then coheating to 110 ℃ with 30 minutes, continue heating to 155 ℃ with a heating rate of 0.5 ℃ min-1 for 24 hours, at last, continue heating to 200 ℃ with 30 minutes for 2 hours under argon protection in tube furnace. Characterization. The samples were characterized by SEM (Hitachi, S-4800, 5kV), TEM (Titan G2 60–300), XRD (Philips, X’ pertpro), Raman spectroscopy (Renishaw 2000 system), XPS (ESCALAB 250Xi), BET (Quabrasorb SI-3MP) and TGA (Netzsch STA 449). Electrochemical measurements. The electrode slurry was prepared by mixing PCLSN@S composites with acetylene black and polyvinylidene fluoride (PVDF) at the weight ratio of 7:2:2 in Nmethyl-2-pyrrolidone (NMP). The electrode with an average loading of sulfur about 1 mg cm-2 and a thickness of 18 um (Figure S8) was prepared by coating the slurry onto an aluminum foil and dried at 60 °C for 24 h in vacuum. 2032-type coin cells were applied to assemble test cells. The electrolyte was 1 M bis-(trifluoromethane) sulfonamide lithium salt (LiTFSi) in a mixed solvent of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) with a volume ratio of 1:1, including 1 wt. % LiNO3 as an electrolyte 14



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additive, the amount of electrolyte is excessive. Lithium metal foil with a thickness of 0.58 mm was used as the counter electrode, and a microporous polypropylene was acted as the separator. 2032-type coin cells were assembled in a glovebox filled under argon (H2O < 0.5 ppm, O2 < 0.5 ppm). For comparison, CLS@S, PCLS@S cathode with sulfur loading about 1 mg cm-2 and PCLSN@S cathode with sulfur loading about 4 mg cm-2 (Figure S9) were prepared using the same method. ■ ASSOCIATED CONTENT Supporting Information Available: SEM images of CLS, PCLS and PCLSN, TEM images of PCLS and PCLSN, BET curves of PCLS, XPS curves of PCLSN@S, voltage-capacity profiles of CLS@S and PCLS@S, cycling performances of CLS@S and PCLS@S, SEM pictures of the PCLSN@S cathode electrode, cycling performances of PCLSN@S electrode with a sulfur loading of 4 mg cm-2, the table for N2 adsorption-desorption isothermal analysis of the PCLS and PCLSN, the table for electrochemical performances of carbon materials-based cathodes for Li-S batteries. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (Nos. 51672078 and 21473052) and Hunan University State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body Independent Research Project (No. 71675004) Hunan Youth Talents (2016RS3025), and Foundation of State Key Laboratory of Coal Conversion (Grant No. J17-18-903), and the National Natural 15


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Ultrathin Honeycomb-like Carbon as Sulfur Host Cathode for High

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