Cost-effective porous carbon materials synthesized by carbonizing rice husk and K2CO3 activation and their application for lithium-sulfur batteries

In this work, we developed highly porous activated carbon (AC) materials with micro/meso porosity through carbonizing rice husk and treating them with K2CO3. Elemental sulfur was then loaded to the micropores through a solution infiltration method to form rice husk-derived activated carbon (RHAC)aS composite materials.

Journal of Science: Advanced Materials and Devices (2019) 223e229 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Cost-effective porous carbon materials synthesized by carbonizing rice husk and K2CO3 activation and their application for lithium-sulfur batteries Thanh-Tung Mai a, *, Duc-Luong Vu a, Dang- Chinh Huynh a, Nae-Li Wu b, Anh-Tuan Le c, d, ** a School of Chemical Engineering, Hanoi University of Science and Technology, Ha Noi, Viet Nam Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan Phenikaa University Nano Institute (PHENA), Phenikaa University, Hanoi 12116, Viet Nam d Faculty of Materials Science and Engineering, Phenikaa University, Hanoi 12116, Viet Nam b c a r t i c l e i n f o a b s t r a c t Article history: Received 18 March 2019 Received in revised form 25 April 2019 Accepted 25 April 2019 Available online 30 April 2019 In this work, we developed highly porous activated carbon (AC) materials with micro/meso porosity through carbonizing rice husk and treating them with K2CO3 Elemental sulfur was then loaded to the micropores through a solution infiltration method to form rice husk-derived activated carbon (RHAC)@S composite materials The as-prepared RHAC@S composites with 0.25 mg cmÀ1 and 0.38 mg cmÀ1 of sulfur loading were tested as cathodes for lithium-sulfur (Li-S) batteries The 0.25 mg cmÀ1 sulfur loaded sample showed an initial discharge capacity of 1080 mA h/g at a 0.1 C rate After 50 cycles of charge/ discharge tests at the current density of 0.2 C, the reversible capacity is maintained at 312 mA h/g The RHAC material delivered a capacity of more than 300 mA h/g at a current density of 1.7 C These results demonstrate that the RHAC porous materials are very promising as cathode materials for the development of high-performance Li-S batteries © 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Rice husk Cathode material Carbonization process Activated carbon Lithium-sulfur batteries Introduction The lithium-sulfur (Li-S) battery system is one of the promising energy storage devices for the next-generation electric power storage owing to its excellent theoretical energy density of 2600 Wh kgÀ1 which is 3e4 times higher than that of the current lithium-ion battery system [1e5] Sulfur is considered a promising cathode material due to its low cost, high theoretical capacity (1675 mA h/g), and its nontoxicity [3,4,6] Despite having several advantages over other batteries, the low electrical conductivities of sulfur and lithium sulfides and the slow redox kinetics of the active materials obstruct the practical use of LiÀS cells [7e9] To overcome these problems, a variety of polar lithium polydisulfides absorbents such as nano metal oxide [10e12], composite of sulfur and carbon * Corresponding author ** Corresponding author Phenikaa University Nano Institute (PHENA), Phenikaa University, Hanoi 12116, Viet Nam E-mail addresses: tung.maithanh@hust.edu.vn (T.-T Mai), tuan.leanh@ phenikaa-uni.edu.vn (A.-T Le) Peer review under responsibility of Vietnam National University, Hanoi [13e15], coating with conductive polymer [16,17] have been employed to improve the conductivity of the cathode, to avert the dissolution of lithium polysulfides and to reduce the shuttle effect Biomass is the most promising carbon precursor for preparing cost-effective porous carbon materials such as activated carbon materials [18,19] Activated carbons are porous materials with a well-developed pore structure, a large surface area, and a high adsorption capacity [20,21] Various biomass-derived carbon materials (e.g., cherry stone, olive stone, mangrove charcoal, rice husk, peanut shell, cotton wool) have been investigated for obtaining high electric capacities and excellent electrochemical properties when applied in lithium batteries [22e24] Agricultural byproducts are renewable resources that can be used for energy, chemicals and materials that have shown their applicability in electrochemical energy systems Due to their abundance, low cost, natural regeneration and availability in considerable amounts, these materials are environmentally friendly renewable resources [25] The residual pore volume in the nanocomposite is designed to retain pathways for the electrolyte/Li ỵ ingress and to accommodate the current mass volume expansion during cycling It is believed that for porous carbon materials, the specific surface area, https://doi.org/10.1016/j.jsamd.2019.04.009 2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 224 T.-T Mai et al / Journal of Science: Advanced Materials and Devices (2019) 223e229 the pore diameter distribution, the pore volume, and the sulfur filling are the critical factors for optimizing battery performances [26] Among these materials, the rice husk (RH) is one of the promising carbon precursors for producing low-cost activated carbon [27e29] The anticipated world rice production in 2012 is 489.1 million tons which means that approximately 122e163 million tons of rice husk biomass is generated globally in 2012 The significant components of RH are silica, cellulose, hemicelluloses and lignin, which yield activated carbon when pyrolyzed under an inert atmosphere [29] Recently, the activated carbon (AC) materials, derived from RH, were developed using different techniques Their potential application in energy storage systems was also demonstrated [Ref] Khu et al [30] produced the AC through carbonizing the rice husk at different temperatures (650e800  C) and activated it by NaOH The optimized AC material with a high surface area of 2681 m2 gÀ1 under activation temperature of 800  C showed its potential application in a supercapacitor with a specific capacitance of 198.4 Fg-1 in the charge/discharge mode Vu et al [31] also developed the AC with a hierarchical micro-mesoporous structure through carbonizing the RH and activating it with ZnCl2 Elemental sulfur was loaded to the micro-mesopores of activated carbon in order to demonstrate a high potential for lithium-sulfur batteries However, the BET specific surface area of as-prepared rice-husk-derived activated carbon (RHAC) materials by ZnCl2 activation resulted in a low value of, approximately, 1199 m2 gÀ1 with an average pore width of 2.24 nm and a pore volume of 0.752 cm3 gÀ1 To improve the quality of RHAC materials for Li-S battery applications we controlled the chemical activation by potassium carbonate (K2CO3) K2CO3 was selected as an activation agent due to its high activating capability, its restriction of the formation of tar and its relatively low cost In this study, we present an alternative way for synthesizing micro/mesoporous activated carbon with low cost which is easy to scale up for Li-S batteries The porous RHAC materials were obtained by carbonization of RH and chemical activation by K2CO3 The RHAC@S composites were synthesized by the method of melting diffusion The synergetic effect of the meso/microporosity and structure on the electrochemical performance of the RHAC@S cathode was investigated in detail Experimental 2.1 Preparation of activated carbon from rice husk The rice husks used as carbon precursors for the preparation of activated carbon were collected from Thai Binh province, Vietnam As indicated in Fig 1, the rice husk was initially washed using hot deionized (DI) water several times to remove impurities and was dried at 120  C in the oven for 24 h Then, the rice husks were precarbonized in a tube furnace at 350  C for two hours with a heating increase rate of  C minÀ1 For the chemical activation and for removal of silica from the rice husk, the sample was subjected to impregnation in K2CO3 solution (?w/w ¼ 1:2) together The mixture was calcined in a tubular furnace at 600  C and 800  C for three hours with a heating increase rate of  C minÀ1 under N2 atmosphere After cooling, the obtained samplea were washed with DI water, treated with aqueous M HNO3 three times and with 1M HF solution to remove some inorganic and SiO2 content in the rice husk material The final products were washed with deionized water and dried in a vacuum oven at 100  C for 24 h 2.2 Preparation of activated carbon from rice husk/sulfur composites (RHAC@S) The RHAC and Sulfur (S) composites were prepared by using a conventional melting diffusion strategy Samples with different RHAC and Sulfur with weight ratios (RHAC: S ¼ 1:0.5, and 1:0.7) were grinded and heated at 155  C for 15 h with a heating rate of  C minÀ1 under an N2 atmosphere After cooling down to room temperature, RHAC@S composites were obtained with sulfur contents of 0.25 and 0.38 mg cmÀ2 2.3 Characterizations Nitrogen adsorptionedesorption isotherms were measured using a Micromeritics ASAP2020 The specific surface areas were calculated using the Brunauer-Emmett-Teller (BET) method X-ray diffraction (XRD) was carried out with a D Max/2000 PC (Rigaku, Ltd) The surface morphologies of the composites were investigated with a scanning electron microscope (SEM, Hitachi, S4700) equipped with energy dispersive spectroscopy (EDS, OXFORD 7593-H) 2.4 Electrochemical measurement Coin cells of the 2032-type were used to study the electrochemical performance of the RHAC@S cathodes The cathodes for the battery test cells were prepared by dispersion/ dissolution of a mixture of the active material RHAC@S (60 wt%), a polyvinylidene fluoride (PVDF, KF 1300, KUREHA) binder (20 wt %) in N-methyl-2-pyrrolidene and super P carbon black (conducting agent-Timcal) (20 wt%) Next, the cathode slurry was coated on an aluminum foil and left to dry at 45  C for 24 h under nitrogen atmospheric and roll-pressed before use Lithium foil (Li) and Celgard 2400 sheets were used as the anode and separator, respectively The cells were assembled in an argon-filled glove box, and 1.0 M LiTFSI in DOL/DME (1:1 by volume) with 0.1 M LiNO3 was used as the electrolyte Studies of the charge and discharge properties of the cathodes were performed on a cell life test system (PNE solution, KOREA) These properties were measured at different current densities in the potential range of 1.8e2.8 V versus Liỵ/Li The cyclic voltammetry (CV) experiments were conducted using an electrochemical analyzer (America, Bio-logic, VSP) on the same instrument in the voltage range of 1.5e3.0 V at a scanning rate of 0.1 mV sÀ1 The impedance spectra were recorded by applying an AC voltage of mV amplitude in the frequency range of 500 mHz to kHz The specific capacity values were calculated according to the mass of sulfur Our electrochemical tests were performed at room temperature Results and discussion 3.1 Microstructure and characterization of RHAC Fig Overview of the rice cycle & activated carbon from rice husk Firstly, we examined the microstructure and characterization of the RHAC materials Here, the samples calcined at 600  C and 800  C are labeled as RHAC-600 and RHAC-800 Fig shows XRD patterns of the RHAC-600 and RHAC-800 samples The main diffraction peaks T.-T Mai et al / Journal of Science: Advanced Materials and Devices (2019) 223e229 RHAC_800 RHAC_600 Intensity (a.u.) (b) (a) 10 15 20 25 30 35 40 45 50 55 60 theta (deg.) Fig X-ray diffraction patterns of (a) RHAC-600 and (b) RHAC-800 samples of graphitic carbon could hardly be recognized in the pattern of the RHAC samples, suggesting a generally amorphous nature for the carbon material Two typical diffraction peaks at 2q values of 22.5 and 43 can be ascribed to reflections from the (002) and (110) crystal planes of graphite, and the broad peaks indicate the amorphous structure [18,19] There is almost no difference between the XRD patterns of RHAC-600 and RHAC-800, demonstrating that no graphitization occurred during the thermal treatment process To further examine the formation of activated carbon, we measured Raman spectra and BET surface areas of the RHAC-600 and RHAC800 samples as shown in the supporting information (SI) The Raman spectrum of the RHAC exhibits characteristic G- and Dbands, at 1582 cmÀ1and 1341 cmÀ1, respectively, as shown in S1 The D band (1341 cmÀ1) is attributed to the ordered/disordered carbonaceous structure of the activated carbon, while the G band (1582 cmÀ1) is due to the presence of C¼C stretching vibrations (sp2 hybridization) in activated carbon [18,19] The Raman intensity of both D- and G-bands are changed in the spectra of the RHAC-600 and RHAC-800 samples, indicating that the carbon matrix changes due to the increased carbonization temperature The intensity ratio, (ID/IG) is a measure for the zone edges of the clusters, which depend on cluster sizes and distributions In our present case, the intensity ratios (ID/IG) for RHAC are in the range of 1.00 ± 0.08 This result indicated a high percentage of structural defects in the RHAC samples which could be related to the activation process by K2CO3 It was noted that higher carbonization temperature would lead to the production of more micro/mesopores and, therefore, result in porous carbon with a higher surface area To confirm this, we shows the N2 sorption isotherms and pore size distribution of the RHAC samples at different activation temperature (600 and 800  C) As can be observed, the isotherms typically display three steps with the increase in relative pressure and indicate the existence of a pore size range from micropores to macropores The Nitrogen adsorptionedesorption curve provides qualitative information on the adsorption mechanism and porous structure of the carbonaceous materials The first step at low relative pressures less than 0.05, is a steeply increasing region which represents the condensation in small micro/mesopores Then, with a relative increase in pressure, the adsorption amount slowly increases without any notable hysteresis which signifies the progressive filling of large micro/mesopores Finally, near the saturation pressure of nitrogen, 225 the adsorption amount increases abruptly because of active capillary condensation The density functional theory (DFT) model was used to calculate the pore size distributions of the samples The increase in carbonization temperature from 600  C to 800  C would produce activated carbon with a significant micropore volume and amount of micro-porosity The RHAC sample exhibites hierarchical pores that are composed of micropores (50 nm) The BET specific surface area of RHAC-800 is calculated to be 1583.6 m2 gÀ1, and the pore volume is 0.93 cm3 gÀ1, with an average pore width of 3.2 nm In contrat, the RHAC-600 samples show a value of 913.56 m2 gÀ1 for the BET specific surface area and a value for the pore volume of 0.36 cm3 gÀ1, with an average pore width of 6.3 nm As expected for this adsorption isotherm type, these RHAC samples are predominantly of a mesoporous and microporous structure The materials with high surface area and relatively large mesopore sizes are attractive materials for lithium-sulfur batteries With the obtained excellent surface areas, the RHAC-800 sample was selected for sulfur loading for the next measurement 3.2 Microstructure and characterization of RHAC@S The morphologies of RHAC-800 and RHAC800@S samples are shown in Fig As can be seen from Fig (a), the RHAC-800 sample is filled with hollow tunnels which could be attributed to the gasification of volatiles upon activation The pores are of different sizes and different shapes However, the particles displayed nonuniformity It can be seen from the Fig (a) that the external surfaces of the activated carbons are full of cavities, are quite irregular as a result of activation with large quantities of flake structure and slitshaped micro/mesopores It has been noted that the cavities result from the evaporation of K2CO3 during carbonization, leaving empty spaces previously occupied by K2CO3 [32] From the EDS of the sample, it can be seen that the peak of silicon did not appear which can surmise that the generation of pores is due to the removal of SiO2 When sulfur is impregnated into the pores, most pores disappear and some macropores change into mesopores in the RHAC@S composite as shown in Fig 3(b) EDS-element mapping was employed to detect the chemical composition of the RHAC-800 and RHAC800@S samples EDS spectra clearly show the presence of carbon (C), oxygen (O) in RHAC-800 samples and carbon (C) and sulfur (S) in the RHAC800@S composite sample with large homogeneous distributions The XRD patterns of pure sulfur and RHAC800@S samples with various sulfur contents are shown in Fig The primary diffraction peaks of graphitic carbon are not observed in the patterns of the RHAC800@S samples, suggesting a generally amorphous nature for the carbon material The characteristic peaks of element sulfur can be found at 26.4 , 29.17, 30.76 and 35.56 and clearly confirm the successful sulfur impregnation into RHAC [33] The intensity peaks of crystalline sulfur in the XRD pattern increase with increasing Sulfur content This result confirms the successful impregnation of sulfur into the RHAC samples as well, in good agreement with the EDS analysis The N2 adsorptionedesorption isotherms of RHAC-800 and RHAC800@S samples are shown in Fig Both samples show typical type I isothermal plots with hysteresis loops that indicate the existence of mesopores [34] As mentioned above, the BET specific surface area of RHAC-800 was calculated to be 1583 m2 gÀ1 with an average pore width of 3.2 nm The high surface area and relatively large mesopore sizes are attractive because they allow the electrolyte and Li ions produced from the LieS redox reaction to penetrate into the structure [35] After impregnating Sulfur, the surface area of RHAC@S decreases because almost all pores of RHAC are filled by Sulfur 226 T.-T Mai et al / Journal of Science: Advanced Materials and Devices (2019) 223e229 Fig SEM images and EDS elemental mapping of (a,b,c) RHAC-800 and (a’,b’,c’) RHAC800@S samples 3.3 Electrochemical characterizations of RHAC@S cathode material (b) -2 Intensity(a.u.) 0.25 (mg cm ) (c) -2 0.38 (mg cm ) (a) 10 Sulfur 15 20 25 30 35 40 45 50 2theta (deg.) Fig XRD patterns of (a) pure S and RHAC800@S composites with sulfur loading content of (b) 0.25 mg cmÀ2 and (c) 0.38 mg cmÀ2 The electrochemical performance of the novel RHAC800@S composites as cathode material for Li-S batteries has systematically been investigated As shown in Fig 6, a cyclic voltammogram (CV) of the RHAC800@S (0.25 mg cmÀ2) composite is employed to perform the electrochemical reaction mechanism The pair of sharp redox peaks indicate that during charge/discharge the electrochemical reduction and oxidation of elemental sulfur (S8) proceeds in two stages The first peak at 2.4 V (vs Liỵ/Li) in the C-V curves is due to the reduction of elemental sulfur to lithium polysulfide anions (Li2Sn, n ¼ ~ 8), and the second peak at 2.05 V comprises the reduction of polysulfide ions to insoluble Li2S2 and Li2S [36] The oxidation process in the LieS cell occurs in one stage The narrow oxidation peak around 2.5 V is mainly attributed to the oxidation of Li2Sn (n > 2) into polysulfides [36e38] The first charge and discharge profiles of the RHAC800@S composite electrodes with different loaded sulfur content, shown in Fig 7, are in good agreement with the C-V curves All the discharge curves of the RHAC800@S composite electrodes have shown two 400 2.0 350 1.5 Current / mA Quantity Adsorbed (cm /g) 2.5 300 250 200 RHAC @ S RHAC 150 100 0.5 0.0 -0.5 -1.0 50 0.0 1.0 -1.5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Relative Pressure (P/Po) Fig N2 adsorptionedesorption isotherms of RHAC-800 and RHAC800@S samples 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 + Potential / V (vs Li/Li ) Fig Cyclic voltammetry curves of RHAC800@S electrode with 0.25 mg cmÀ2 of sulfur loading at a scan rate of 0.1 mV sÀ1 in a voltage range 1.5e3.0 V T.-T Mai et al / Journal of Science: Advanced Materials and Devices (2019) 223e229 800 -1 -1 Discharge capacity (mAh.g ) voltage plateau regions, corresponding to the multistep reduction reaction of sulfur during the discharge process Moreover, the upper plateau at approximately 2.3 V is caused by the conversion of elemental sulfur into higher-order lithium polysulfides (Li2Sn, n 8), while the lower plateau at about 2.1 V is attributed to the conversion of higher-order lithium polysulfides to lower-order lithium polysulfides (Li2Sn, n < 4) In this conversion solid products of Li2S2 and Li2S can precipitate due to their low solubility in the electrolyte [9,33] The cathodes with different sulfur loadings exhibite capacities of about 1080 mA h/g in the first cycle, indicating a high utilization of active sulfur This could be due to the sufficient contact between the sulfur and the electrolyte because of the excellent electrolyte adsorption capability of highly porous activated carbon materials After the initial loss of capacity resulting from the decomposition of the electrolyte and the formation of a solid electrolyte interphase (SEI) layer, the capacities at 0.1 C current rate decrease to 900 and 819 mA h/g for the cathodes with 0.25 and 0.38 mg cmÀ2 sulfur loading, respectively The cycling performance of the RHAC800@S composites has been evaluated and is shown in Fig The cycling performance of all the samples at a rate of 0.2 C between 1.8 and 2.8 V of the cutoff voltage is shown in Fig It is clear that the discharge capacity 700 600 500 400 300 200 -2 0.38 (mg cm ) -2 0.25 (mg cm ) 100 decays drastically upon cycling for all samples For cells with 0.25 mg cmÀ2 sulfur loading, the capacity approaches 1080 mA h/ g in the first cycle After the activating process at a low current rate, the capacity stabilizes at 750 mA h/g and retains at 358 mA h/ g after 50 cycles a 47.73% capacity retention The capacity of cells with 0.35 mg cmÀ2 sulfur loading shows nearly no degradation compared with that of 0.25 mg cmÀ2 The capacity stabilizes at 680 mA h/g after activating the process and retains at 312 mA h/g after 50 cycles with 45.88% capacity retained The fast capacity decay in the first few cycles can be attributed to the volumetric expansion and re-distribution of the active-sulfur during the initial lithiation process [7] As mentioned, an increase in the carbon content of the RHAC800@S composite electrodes leads to higher discharge capacities in each cycle because of the high electron conductivities of the electrodes provided by the carbon, which may promote the electrochemical reactions of sulfur with lithium [39] The rate properties of the RHAC800@S samples at various current densities in the voltage range of 1.8 Ve2.8 V (vs Liỵ/Li) at room temperature were tested and are shown in Fig The cell with 0.25 mg cmÀ2 sulfur loading has a good rate performance with capacities of 1041, 650, 486, 395 and 305 mA h/g at current densities of 0.1, 0.2, 0.5, 0.9 and 1.7C, respectively For the cells with 0.35 mg cmÀ2 sulfur loading, the capacities are 992 mA h/g at 0.1C, 570 mA h/g at 0.2C, 412 mA h/g at 0.5C, 317 mA h/g at 0.9C, and 210 mA h/g at 1.7C The excellent rate performance indicates an excellent stability of the RHAC800@S sample during testing at different rates The EIS of the cells after the first cycling was measured to characterize the resistance of the electrode As indicated in Fig 10, the Ohmic resistance (Ro) from the high-frequency intercept on the real axis is composed of the ionic resistance of the electrolyte, the intrinsic strength of the active materials and the contact resistance of the interface between the electrodes and current collectors As shown in Fig 10, EIS of the RHAC800@S sample is composed of one depressed semicircle in the high-frequency region and of a short inclined line (Warburg impedance) in the low-frequency region The charge transfer resistance (Rct) of the carbon/sulfur electrode, originating from the interactions between the electrode and electrolyte solvent, result in the semicircle in the high-frequency region Our study indicates that the formation of a resistive film on the electrode surface in a non-aqueous organic solution can be considered to be a common phenomenon [40] The Warburg Discharge capacity (mAh.g ) Fig Initial charge-discharge profiles of RHAC800@S at a current density of 167.5 mA h/g in a voltage range 1.8e2.8 V 10 15 20 25 30 35 40 45 50 Cycle number Fig Cycling performance of the RHAC800@S samples at 335 mA h/g in a voltage range of 1.8e2.8 V 227 1200 0.1C 1000 0.2C 800 0.5C 600 0.9C 1.7C 400 -2 0.25 (mg cm ) -2 0.38 (mg cm ) 200 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Cycle number Fig Rate capability performance of RHAC800@S samples at different C-rates in a voltage range of 1.8e2.8 V 228 T.-T Mai et al / Journal of Science: Advanced Materials and Devices (2019) 223e229 -2 -Z" (ohm) 180 0.38 (mg cm ) -2 0.25 (mg cm ) 150 120 90 60 30 0 50 100 150 200 250 300 350 400 450 Z' (ohm) Fig 10 Nyquist plot of RHAC800@S electrode in the frequency range 500 mHz to kHz impedance (Wo) is related to the lithium ion diffusion within the cathode From the equivalent circuit, the Rct value is evaluated to be 170 U for cells with 0.25 mg cmÀ2 sulfur loading, which value increases to 260 U when the sulfur loading increases to 0.38 mg cmÀ2 With the rise of sulfur loading, the polarization becomes more considerable, indicating slower dynamics and increasing electrode resistance For the cells with 0.25 mg cmÀ2 sulfur loading, the polarization and charge transfer resistant is smallest Conclusion We developed a hierarchically micro/mesoporous structure of activated carbon from rice husk via a simple carbonization process in combination with the K2CO3 activation technique The RHAC-800 sample showed an amorphous nature with a high surface area (SBET) of 1583 m2 gÀ1 and a pore volume of 0.93 cm3 gÀ1 As a result, when evaluated as a cathode material for lithium-sulfur batteries, the RHAC800@S composites with sulfur loading of 0.25 mg cmÀ2 exhibite a high discharge capacity of 1080 mA h/g, as well as an excellent cycle stability and a high rate capability We believe that our results will open new avenues for the development of highperformance Li-S batteries at using cost-effective porous carbon materials Acknowledgements The authors are grateful to Project NDT.19.TW/16 (Ministry of Science and Technology, Vietnam) and project 15/FIRST/1.a/HUST and MOST 105-E002-012-MY12 (Ministry of Science and Technology, Taiwan) for financial support Appendix A Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2019.04.009 References [1] L Ma, K.E Hendrickson, S Wei, L.A Archer, Nanomaterials: Science and applications in the lithium-sulfur battery, Nano Today (2015), https://doi.org/ 10.1016/j.nantod.2015.04.011 [2] Y.S Su, Y Fu, B Guo, S Dai, A Manthiram, Fast, reversible lithium storage with a sulfur/long-chain-polysulfide redox couple, Chem Eur J 19 (2013) 8621e8626, https://doi.org/10.1002/chem.201300886 [3] J Kim, D.J Lee, H.G Jung, Y.K Sun, J Hassoun, B Scrosati, An advanced lithium-sulfur battery, Adv Funct Mater 23 (2013) 1076e1080, 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zeolite-templated carbon, carbon aerogels, carbon nanotubes,... predominantly of a mesoporous and microporous structure The materials with high surface area and relatively large mesopore sizes are attractive materials for lithium-sulfur batteries With the obtained