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

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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.

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

Journal of Science: Advanced Materials and Devices 4 (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 b Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan c Phenikaa University Nano Institute (PHENA), Phenikaa University, Hanoi 12116, Viet Nam d Faculty of Materials Science and Engineering, Phenikaa University, Hanoi 12116, Viet Nam a r t i c l e i n f o a b s t r a c t Article history: In this work, we developed highly porous activated carbon (AC) materials with micro/meso porosity Received 18 March 2019 through carbonizing rice husk and treating them with K2CO3. Elemental sulfur was then loaded to the Received in revised form micropores through a solution inﬁltration method to form rice husk-derived activated carbon (RHAC)@S 25 April 2019 composite materials. The as-prepared RHAC@S composites with 0.25 mg cm1 and 0.38 mg cm1 of Accepted 25 April 2019 Available online 30 April 2019 sulfur loading were tested as cathodes for lithium-sulfur (Li-S) batteries. The 0.25 mg cm1 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 Keywords: Rice husk RHAC material delivered a capacity of more than 300 mA h/g at a current density of 1.7 C. These results Cathode material demonstrate that the RHAC porous materials are very promising as cathode materials for the develop- Carbonization process ment of high-performance Li-S batteries. Activated carbon © 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. Lithium-sulfur batteries This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction [13e15], coating with conductive polymer [16,17] have been employed to improve the conductivity of the cathode, to avert the The lithium-sulfur (Li-S) battery system is one of the promising dissolution of lithium polysulﬁdes and to reduce the shuttle effect. energy storage devices for the next-generation electric power Biomass is the most promising carbon precursor for preparing storage owing to its excellent theoretical energy density of cost-effective porous carbon materials such as activated carbon 2600 Wh kg1 which is 3e4 times higher than that of the current materials [18,19]. Activated carbons are porous materials with a lithium-ion battery system [1e5]. Sulfur is considered a promising well-developed pore structure, a large surface area, and a high cathode material due to its low cost, high theoretical capacity adsorption capacity [20,21]. Various biomass-derived carbon ma- (1675 mA h/g), and its nontoxicity [3,4,6]. Despite having several terials (e.g., cherry stone, olive stone, mangrove charcoal, rice husk, advantages over other batteries, the low electrical conductivities of peanut shell, cotton wool) have been investigated for obtaining sulfur and lithium sulﬁdes and the slow redox kinetics of the active high electric capacities and excellent electrochemical properties materials obstruct the practical use of LiS cells [7e9]. To overcome when applied in lithium batteries [22e24]. Agricultural by- these problems, a variety of polar lithium polydisulﬁdes absorbents products are renewable resources that can be used for energy, such as nano metal oxide [10e12], composite of sulfur and carbon 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, * Corresponding author. these materials are environmentally friendly renewable resources ** Corresponding author. Phenikaa University Nano Institute (PHENA), Phenikaa [25]. The residual pore volume in the nanocomposite is designed to University, Hanoi 12116, Viet Nam. E-mail addresses: tung.maithanh@hust.edu.vn (T.-T. Mai), tuan.leanh@ retain pathways for the electrolyte/Li þ ingress and to accommo- phenikaa-uni.edu.vn (A.-T. Le). date the current mass volume expansion during cycling. It is Peer review under responsibility of Vietnam National University, Hanoi. 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 4 (2019) 223e229 the pore diameter distribution, the pore volume, and the sulfur impregnation in K2CO3 solution (?w/w ¼ 1:2) together. The mixture ﬁlling are the critical factors for optimizing battery performances was calcined in a tubular furnace at 600 C and 800 C for three [26]. Among these materials, the rice husk (RH) is one of the hours with a heating increase rate of 3 C min1 under N2 atmo- promising carbon precursors for producing low-cost activated sphere. After cooling, the obtained samplea were washed with DI carbon [27e29]. The anticipated world rice production in 2012 is water, treated with aqueous 1 M HNO3 three times and with 1M HF 489.1 million tons which means that approximately 122e163 solution to remove some inorganic and SiO2 content in the rice million tons of rice husk biomass is generated globally in 2012. The husk material. The ﬁnal products were washed with deionized signiﬁcant components of RH are silica, cellulose, hemicelluloses water and dried in a vacuum oven at 100 C for 24 h. and lignin, which yield activated carbon when pyrolyzed under an inert atmosphere [29]. Recently, the activated carbon (AC) mate- 2.2. Preparation of activated carbon from rice husk/sulfur rials, derived from RH, were developed using different techniques. composites (RHAC@S) Their potential application in energy storage systems was also demonstrated [Ref]. Khu et al. [30] produced the AC through The RHAC and Sulfur (S) composites were prepared by using a carbonizing the rice husk at different temperatures (650e800 C) conventional melting diffusion strategy. Samples with different and activated it by NaOH. The optimized AC material with a high RHAC and Sulfur with weight ratios (RHAC: S ¼ 1:0.5, and 1:0.7) surface area of 2681 m2 g1 under activation temperature of 800 C were grinded and heated at 155 C for 15 h with a heating rate of showed its potential application in a supercapacitor with a speciﬁc 3 C min1 under an N2 atmosphere. After cooling down to room capacitance of 198.4 Fg-1 in the charge/discharge mode. Vu et al. temperature, RHAC@S composites were obtained with sulfur con- [31] also developed the AC with a hierarchical micro-mesoporous tents of 0.25 and 0.38 mg cm2. structure through carbonizing the RH and activating it with ZnCl2. Elemental sulfur was loaded to the micro-mesopores of 2.3. Characterizations activated carbon in order to demonstrate a high potential for lithium-sulfur batteries. However, the BET speciﬁc surface area of Nitrogen adsorptionedesorption isotherms were measured us- as-prepared rice-husk-derived activated carbon (RHAC) materials ing a Micromeritics ASAP2020. The speciﬁc surface areas were by ZnCl2 activation resulted in a low value of, calculated using the Brunauer-Emmett-Teller (BET) method. X-ray approximately, 1199 m2 g1 with an average pore width of 2.24 nm diffraction (XRD) was carried out with a D Max/2000 PC (Rigaku, and a pore volume of 0.752 cm3 g1. To improve the quality of RHAC Ltd). The surface morphologies of the composites were investigated materials for Li-S battery applications we controlled the chemical with a scanning electron microscope (SEM, Hitachi, S4700) equip- activation by potassium carbonate (K2CO3). K2CO3 was selected as ped with energy dispersive spectroscopy (EDS, OXFORD 7593-H). an activation agent due to its high activating capability, its re- striction of the formation of tar and its relatively low cost. 2.4. Electrochemical measurement In this study, we present an alternative way for synthesizing micro/mesoporous activated carbon with low cost which is easy to Coin cells of the 2032-type were used to study the scale up for Li-S batteries. The porous RHAC materials were ob- electrochemical performance of the RHAC@S cathodes. The tained by carbonization of RH and chemical activation by K2CO3. cathodes for the battery test cells were prepared by dispersion/ The RHAC@S composites were synthesized by the method of dissolution of a mixture of the active material RHAC@S (60 wt%), melting diffusion. The synergetic effect of the meso/microporosity a polyvinylidene ﬂuoride (PVDF, KF 1300, KUREHA) binder (20 wt and structure on the electrochemical performance of the RHAC@S %) in N-methyl-2-pyrrolidene and super P carbon black (con- cathode was investigated in detail. 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 2. Experimental nitrogen atmospheric and roll-pressed before use. Lithium foil (Li) and Celgard 2400 sheets were used as the anode and separator, 2.1. Preparation of activated carbon from rice husk 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 The rice husks used as carbon precursors for the preparation of used as the electrolyte. activated carbon were collected from Thai Binh province, Vietnam. Studies of the charge and discharge properties of the cathodes As indicated in Fig. 1, the rice husk was initially washed using hot were performed on a cell life test system (PNE solution, KOREA). deionized (DI) water several times to remove impurities and was These properties were measured at different current densities in the dried at 120 C in the oven for 24 h. Then, the rice husks were pre- potential range of 1.8e2.8 V versus Liþ/Li. The cyclic voltammetry carbonized in a tube furnace at 350 C for two hours with a heating (CV) experiments were conducted using an electrochemical analyzer increase rate of 5 C min1. For the chemical activation and for (America, Bio-logic, VSP) on the same instrument in the voltage removal of silica from the rice husk, the sample was subjected to range of 1.5e3.0 V at a scanning rate of 0.1 mV s1. 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 RHAC materials. Here, the samples calcined at 600 C and 800 C are labeled as RHAC-600 and RHAC-800. Fig. 2 shows XRD patterns Fig. 1. Overview of the rice cycle & activated carbon from rice husk. of the RHAC-600 and RHAC-800 samples. The main diffraction peaks
T.-T. Mai et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 223e229 225 the adsorption amount increases abruptly because of active capil- RHAC_800 lary condensation. The density functional theory (DFT) model was RHAC_600 (b) 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 Intensity (a.u.) 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 g1, and the pore (a) volume is 0.93 cm3 g1, with an average pore width of 3.2 nm. In contrat, the RHAC-600 samples show a value of 913.56 m2 g1 for the BET speciﬁc surface area and a value for the pore volume of 0.36 cm3 g1, 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 sample was selected for sulfur 10 15 20 25 30 35 40 45 50 55 60 loading for the next measurement. 2 theta (deg.) 3.2. Microstructure and characterization of RHAC@S Fig. 2. X-ray diffraction patterns of (a) RHAC-600 and (b) RHAC-800 samples. 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 of graphitic carbon could hardly be recognized in the pattern of the is ﬁlled with hollow tunnels which could be attributed to the gasi- RHAC samples, suggesting a generally amorphous nature for the ﬁcation of volatiles upon activation. The pores are of different sizes carbon material. Two typical diffraction peaks at 2q values of 22.5 and different shapes. However, the particles displayed non- and 43 can be ascribed to reﬂections from the (002) and (110) uniformity. It can be seen from the Fig. 3 (a) that the external sur- crystal planes of graphite, and the broad peaks indicate the amor- faces of the activated carbons are full of cavities, are quite irregular as phous structure [18,19]. There is almost no difference between the a result of activation with large quantities of ﬂake structure and slit- XRD patterns of RHAC-600 and RHAC-800, demonstrating that no shaped micro/mesopores. It has been noted that the cavities result graphitization occurred during the thermal treatment process. To from the evaporation of K2CO3 during carbonization, leaving empty further examine the formation of activated carbon, we measured spaces previously occupied by K2CO3 [32]. From the EDS of the Raman spectra and BET surface areas of the RHAC-600 and RHAC- sample, it can be seen that the peak of silicon did not appear which 800 samples as shown in the supporting information (SI). The can surmise that the generation of pores is due to the removal of Raman spectrum of the RHAC exhibits characteristic G- and D- SiO2. When sulfur is impregnated into the pores, most pores disap- bands, at 1582 cm1and 1341 cm1, respectively, as shown in S1. The pear and some macropores change into mesopores in the RHAC@S D band (1341 cm1) is attributed to the ordered/disordered carbo- composite as shown in Fig. 3(b). EDS-element mapping was naceous structure of the activated carbon, while the G band employed to detect the chemical composition of the RHAC-800 and (1582 cm1) is due to the presence of C¼C stretching vibrations (sp2 RHAC800@S samples. EDS spectra clearly show the presence of car- hybridization) in activated carbon [18,19]. The Raman intensity of bon (C), oxygen (O) in RHAC-800 samples and carbon (C) and sulfur both D- and G-bands are changed in the spectra of the RHAC-600 (S) in the RHAC800@S composite sample with large homogeneous and RHAC-800 samples, indicating that the carbon matrix changes distributions. due to the increased carbonization temperature. The intensity ratio, The XRD patterns of pure sulfur and RHAC800@S samples with (ID/IG) is a measure for the zone edges of the clusters, which depend various sulfur contents are shown in Fig. 4. The primary diffraction on cluster sizes and distributions. In our present case, the intensity peaks of graphitic carbon are not observed in the patterns of the ratios (ID/IG) for RHAC are in the range of 1.00 ± 0.08. This result RHAC800@S samples, suggesting a generally amorphous nature for indicated a high percentage of structural defects in the RHAC sam- the carbon material. The characteristic peaks of element sulfur can ples which could be related to the activation process by K2CO3. It be found at 26.4 , 29.17, 30.76 and 35.56 and clearly conﬁrm the was noted that higher carbonization temperature would lead to the successful sulfur impregnation into RHAC [33]. The intensity peaks production of more micro/mesopores and, therefore, result in of crystalline sulfur in the XRD pattern increase with increasing porous carbon with a higher surface area. To conﬁrm this, we shows Sulfur content. This result conﬁrms the successful impregnation of the N2 sorption isotherms and pore size distribution of the RHAC sulfur into the RHAC samples as well, in good agreement with the samples at different activation temperature (600 and 800 C). As can EDS analysis. be observed, the isotherms typically display three steps with The N2 adsorptionedesorption isotherms of RHAC-800 and the increase in relative pressure and indicate the existence of a RHAC800@S samples are shown in Fig. 5. Both samples show typical pore size range from micropores to macropores. The Nitrogen type I isothermal plots with hysteresis loops that indicate the exis- adsorptionedesorption curve provides qualitative information on tence of mesopores [34]. As mentioned above, the BET speciﬁc sur- the adsorption mechanism and porous structure of the carbona- face area of RHAC-800 was calculated to be 1583 m2 g1 with an ceous materials. The ﬁrst step at low relative pressures less than average pore width of 3.2 nm. The high surface area and relatively 0.05, is a steeply increasing region which represents the conden- large mesopore sizes are attractive because they allow the electrolyte sation in small micro/mesopores. Then, with a relative increase in and Li ions produced from the LieS redox reaction to penetrate into pressure, the adsorption amount slowly increases without any the structure [35]. After impregnating Sulfur, the surface area of notable hysteresis which signiﬁes the progressive ﬁlling of large RHAC@S decreases because almost all pores of RHAC are ﬁlled by micro/mesopores. Finally, near the saturation pressure of nitrogen, Sulfur.
226 T.-T. Mai et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 223e229 Fig. 3. 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) 0.25 (mg cm ) -2 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 cm2) composite is employed to Intensity(a.u.) perform the electrochemical reaction mechanism. The pair of sharp (c) -2 redox peaks indicate that during charge/discharge the electro- 0.38 (mg cm ) 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 reduction of polysulﬁde ions to insoluble Li2S2 and Li2S [36]. The oxidation process in the LieS cell occurs in one stage. The (a) Sulfur narrow oxidation peak around 2.5 V is mainly attributed to the oxidation of Li2Sn (n > 2) into polysulﬁdes [36e38]. 10 15 20 25 30 35 40 45 50 The ﬁrst charge and discharge proﬁles of the RHAC800@S com- 2theta (deg.) posite electrodes with different loaded sulfur content, shown in Fig. 7, are in good agreement with the C-V curves. All the discharge Fig. 4. XRD patterns of (a) pure S and RHAC800@S composites with sulfur loading curves of the RHAC800@S composite electrodes have shown two content of (b) 0.25 mg cm2 and (c) 0.38 mg cm2. 2.5 Quantity Adsorbed (cm /g) 400 2.0 3 350 1.5 Current / mA 300 1.0 250 0.5 200 RHAC @ S 0.0 150 RHAC -0.5 100 -1.0 50 -1.5 0 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 + Relative Pressure (P/Po) Potential / V (vs. Li/Li ) Fig. 6. Cyclic voltammetry curves of RHAC800@S electrode with 0.25 mg cm2 of sulfur Fig. 5. N2 adsorptionedesorption isotherms of RHAC-800 and RHAC800@S samples. loading at a scan rate of 0.1 mV s1 in a voltage range 1.5e3.0 V.
T.-T. Mai et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 223e229 227 decays drastically upon cycling for all samples. For cells with 0.25 mg cm2 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 cm2 sulfur loading shows nearly no degradation compared with that of 0.25 mg cm2. 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 cur- Fig. 7. Initial charge-discharge proﬁles of RHAC800@S at a current density of rent densities in the voltage range of 1.8 Ve2.8 V (vs. Liþ/Li) at 167.5 mA h/g in a voltage range 1.8e2.8 V. room temperature were tested and are shown in Fig. 9. The cell with 0.25 mg cm2 sulfur loading has a good rate performance voltage plateau regions, corresponding to the multistep reduction with capacities of 1041, 650, 486, 395 and 305 mA h/g at current reaction of sulfur during the discharge process. Moreover, the up- densities of 0.1, 0.2, 0.5, 0.9 and 1.7C, respectively. For the cells per plateau at approximately 2.3 V is caused by the conversion of with 0.35 mg cm2 sulfur loading, the capacities are 992 mA h/g at elemental sulfur into higher-order lithium polysulﬁdes (Li2Sn, 0.1C, 570 mA h/g at 0.2C, 412 mA h/g at 0.5C, 317 mA h/g at 0.9C, 4 n 8), while the lower plateau at about 2.1 V is attributed to the and 210 mA h/g at 1.7C. The excellent rate performance indicates conversion of higher-order lithium polysulﬁdes to lower-order an excellent stability of the RHAC800@S sample during testing at lithium polysulﬁdes (Li2Sn, n < 4). In this conversion solid prod- different rates. ucts of Li2S2 and Li2S can precipitate due to their low solubility in The EIS of the cells after the ﬁrst cycling was measured to the electrolyte [9,33]. The cathodes with different sulfur loadings characterize the resistance of the electrode. As indicated in Fig. 10, exhibite capacities of about 1080 mA h/g in the ﬁrst cycle, indi- the Ohmic resistance (Ro) from the high-frequency intercept on the cating a high utilization of active sulfur. This could be due to the real axis is composed of the ionic resistance of the electrolyte, the sufﬁcient contact between the sulfur and the electrolyte because of intrinsic strength of the active materials and the contact resistance the excellent electrolyte adsorption capability of highly porous of the interface between the electrodes and current collectors. As activated carbon materials. After the initial loss of capacity result- shown in Fig. 10, EIS of the RHAC800@S sample is composed of one ing from the decomposition of the electrolyte and the formation of depressed semicircle in the high-frequency region and of a short a solid electrolyte interphase (SEI) layer, the capacities at 0.1 C inclined line (Warburg impedance) in the low-frequency region. current rate decrease to 900 and 819 mA h/g for the cathodes with The charge transfer resistance (Rct) of the carbon/sulfur electrode, 0.25 and 0.38 mg cm2 sulfur loading, respectively. originating from the interactions between the electrode and elec- The cycling performance of the RHAC800@S composites has trolyte solvent, result in the semicircle in the high-frequency re- been evaluated and is shown in Fig. 8. The cycling performance of gion. Our study indicates that the formation of a resistive ﬁlm all the samples at a rate of 0.2 C between 1.8 and 2.8 V of the cut- on the electrode surface in a non-aqueous organic solution can off voltage is shown in Fig. 8. It is clear that the discharge capacity be considered to be a common phenomenon [40]. The Warburg Discharge capacity (mAh.g ) Discharge capacity (mAh.g ) 800 1200 -1 -1 0.1C 700 1000 600 800 0.2C 500 400 600 0.5C 0.9C 300 1.7C 400 200 -2 -2 0.38 (mg cm ) 200 0.25 (mg cm ) 100 -2 -2 0.25 (mg cm ) 0.38 (mg cm ) 0 0 0 5 10 15 20 25 30 35 40 45 50 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Cycle number Cycle number Fig. 8. Cycling performance of the RHAC800@S samples at 335 mA h/g in a voltage Fig. 9. Rate capability performance of RHAC800@S samples at different C-rates in a range of 1.8e2.8 V. voltage range of 1.8e2.8 V.
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