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

n pl Hu Viet am Faculty of Materials Science and Engineering, Phenikaa University, Hanoi 12116, Viet Nam such as nano metal oxide [10e12], composite of sulfur and carbon 16,17] have been thode, to avert the the shuttle effect. rsor for preparing activated carbon materials with a area, and a high rived carbon ma- harcoal, rice husk, ted for obtaining emical properties Agricultural by- 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 þ ingress and to accommo- date the current mass volume expansion during cycling. It is believed that for porous carbon materials, the specific surface area, * 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). Contents lists availab Journal of Science: Advance journal homepage: www.el Journal of Science: Advanced Materials and Devices 4 (2019) 223e229Peer review under responsibility of Vietnam National University, Hanoi.these problems, a variety of polar lithium polydisulfides absorbents products are renewable resources that can be used for energy,1. 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 kg
1 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 Li
S cells [7e9]. To overcome [13e15], coating with conductive polymer [ employed to improve the conductivity of the ca dissolution of lithium polysulfides and to reduce Biomass is the most promising carbon precu cost-effective porous carbon materials such as materials [18,19]. Activated carbons are porous well-developed pore structure, a large surface adsorption capacity [20,21]. Various biomass-de terials (e.g., cherry stone, olive stone, mangrove c peanut shell, cotton wool) have been investiga high electric capacities and excellent electroch when applied in lithium batteries [22e24].Article history: Received 18 March 2019 Received in revised form 25 April 2019 Accepted 25 April 2019 Available online 30 April 2019 Keywords: Rice husk Cathode material Carbonization process Activated carbon Lithium-sulfur batterieshttps://doi.org/10.1016/j.jsamd.2019.04.009 2468-2179/© 2019 The Authors. Publishing services b ( 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 cm
1 and 0.38 mg cm
1 of sulfur loading were tested as cathodes for lithium-sulfur (Li-S) batteries. The 0.25 mg cm
1 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 develop- ment 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 ( r t i c l e i n f o a b s t r a c tOriginal Article Cost-effective porous carbon materials sy husk and K2CO3 activation and their ap batteries Thanh-Tung Mai a, *, Duc-Luong Vu a, Dang- Chinh Anh-Tuan Le c, d, ** a School of Chemical Engineering, Hanoi University of Science and Technology, Ha Noi, b Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan c Phenikaa University Nano Institute (PHENA), Phenikaa University, Hanoi 12116, Viet N dy Elsevier B.V. on behalf of Vietnamthesized by carbonizing rice ication for lithium-sulfur ynh a, Nae-Li Wu b, Nam le at ScienceDirect d Materials and Devices sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY license 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) mate- rials, derived from RH, were developed using different techniques. T.-T. Mai et al. / Journal of Science: Advanced224Their 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 g
1 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 g
1 with an average pore width of 2.24 nm and a pore volume of 0.752 cm3 g
1. 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 re- striction 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 ob- tained 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. 2. 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 pre- carbonized in a tube furnace at 350 C for two hours with a heating increase rate of 5 C min
1. For the chemical activation and for removal of silica from the rice husk, the sample was subjected toFig. 1. Overview of the rice cycle & activated carbon from rice husk.impregnation in K2CO3 solution (?w/w¼ 1:2) together. Themixture was calcined in a tubular furnace at 600 C and 800 C for three hours with a heating increase rate of 3 C min
1 under N2 atmo- sphere. After cooling, the obtained samplea were washed with DI water, treated with aqueous 1 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 3 C min
1 under an N2 atmosphere. After cooling down to room temperature, RHAC@S composites were obtained with sulfur con- tents of 0.25 and 0.38 mg cm
2. 2.3. Characterizations Nitrogen adsorptionedesorption isotherms were measured us- ing 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) equip- ped 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 (con- ducting 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 weremeasured at different current densities in the potential range of 1.8e2.8 V versus Liþ/Li. The cyclic voltammetry (CV) experimentswere 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 s
1. The impedance spectra were recorded by applying an AC voltage of 5 mV amplitude in the frequency range of 500 mHz to 1 kHz. The specific capacity values were calculated according to the mass of sulfur. Our electro- chemical tests were performed at room temperature. 3. Results and discussion 3.1. Microstructure and characterization of RHAC Firstly, we examined the microstructure and characterization of the RHACmaterials. Here, the samples calcined at 600 C and 800 C are labeled as RHAC-600 and RHAC-800. Fig. 2 shows XRD patterns Materials and Devices 4 (2019) 223e229of the RHAC-600 and RHAC-800 samples. Themain diffraction peaks cedof 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 amor- phous 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 RHAC- 800 samples as shown in the supporting information (SI). The Raman spectrum of the RHAC exhibits characteristic G- and D- bands, at 1582 cm
1and 1341 cm
1, respectively, as shown in S1. The D band (1341 cm
1) is attributed to the ordered/disordered carbo- naceous structure of the activated carbon, while the G band (1582 cm
1) is due to the presence of C¼C stretching vibrations (sp2 10 15 20 25 30 35 40 45 50 55 60 RHAC_800 RHAC_600 ).u.a( ytisnetnI 2 theta (deg.) (a) (b) Fig. 2. X-ray diffraction patterns of (a) RHAC-600 and (b) RHAC-800 samples. T.-T. Mai et al. / Journal of Science: Advanhybridization) 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 sam- ples 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 carbonwith 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 carbona- ceous materials. The first step at low relative pressures less than 0.05, is a steeply increasing region which represents the conden- sation 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,the adsorption amount increases abruptly because of active capil- lary 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 (<10 nm), mesopores (10e50 nm), and macropores (>50 nm). The BET specific surface area of RHAC-800 is calculated to be 1583.6 m2 g
1, and the pore volume is 0.93 cm3 g
1, with an average pore width of 3.2 nm. In contrat, the RHAC-600 samples show a value of 913.56 m2 g
1 for the BET specific surface area and a value for the pore volume of 0.36 cm3 g
1, with an average pore width of 6.3 nm. As expected for this adsorption isotherm type, these RHAC samples are predomi- nantly 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 samplewas 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. 3. As can be seen from Fig. 3 (a), the RHAC-800 sample is filled with hollow tunnels which could be attributed to the gasi- fication of volatiles upon activation. The pores are of different sizes and different shapes. However, the particles displayed non- uniformity. It can be seen from the Fig. 3 (a) that the external sur- faces of the activated carbons are full of cavities, are quite irregular as a result of activation with large quantities of flake structure and slit- shaped 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 disap- pear 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 car- bon (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. 4. 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. 5. Both samples show typical type I isothermal plots with hysteresis loops that indicate the exis- tence of mesopores [34]. As mentioned above, the BET specific sur- face area of RHAC-800 was calculated to be 1583 m2 g
1 with an average pore width of 3.2 nm. The high surface area and relatively largemesopore 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 Materials and Devices 4 (2019) 223e229 225Sulfur. (a) (c) (b) 10 15 20 25 30 35 40 45 50 0.25 (mg cm-2) 0.38 (mg cm-2) Sulfur ).u.a(ytisnetnI 2theta (deg.) Fig. 4. XRD patterns of (a) pure S and RHAC800@S composites with sulfur loading content of (b) 0.25 mg cm
2 and (c) 0.38 mg cm
2. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 50 100 150 200 250 300 350 400 debrosd A ytitnau Q (c m 3 /g ) Relative Pressure (P/Po) RHAC @ S RHAC Fig. 5. N2 adsorptionedesorption isotherms of RHAC-800 and RHAC800@S samples. Fig. 3. SEM images and EDS elemental mapping of (a,b,c) RHAC-800 and (a’,b’,c’) RHAC @S samples. T.-T. Mai et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 223e2292263.3. Electrochemical characterizations of RHAC@S cathode material 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 cm
2) composite is employed to perform the electrochemical reaction mechanism. The pair of sharp redox peaks indicate that during charge/discharge the electro- chemical reduction and oxidation of elemental sulfur (S8) proceeds in two stages. The first peak at 2.4 V (vs. Liþ/Li) in the C-V curves is due to the reduction of elemental sulfur to lithium polysulfide anions (Li2Sn, n ¼ 4 ~ 8), and the second peak at 2.05 V comprises the