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Efficient Fabrication of Hierarchically Porous GrapheneDerived Aerogel and Its Application in Lithium Sulfur Battery Kai Zhang, Furong Qin, Yanqing Lai, Jie Li, Xiaoke Lei, Mengran Wang, Hai Lu, and Jing Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12586 • Publication Date (Web): 17 Feb 2016 Downloaded from http://pubs.acs.org on February 23, 2016

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ACS Applied Materials & Interfaces

Efficient Fabrication of Hierarchically Porous Graphene-Derived Aerogel and Its Application in Lithium Sulfur Battery Kai Zhang †, ‡, ┴, Furong Qin ‡, ┴, Yanqing Lai ‡, Jie Li ‡, Xiaoke Lei ‡, Mengran Wang ‡

, Hai Lu §, Jing Fang ‡, *

†

School of Materials Science and Engineering, Central South University, Changsha

410083, China. ‡

School of Metallurgy & Environment, Central South University, 410083, Changsha,

China. §

Engineering Research Center of High Performance Battery Materials and Devices,

Research Institute of Central South University in Shenzhen, Shenzhen 518057, China. ┴

These authors contributed equally to this work.

* Corresponding author: [email protected]

1

ACS Paragon Plus Environment


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Abstract: Hierarchically porous carbon/graphene aerogel (CGA) with relatively high surface area and pore volume is synthesized through an efficient fabrication strategy, which involves forming hydrothermal carbon layer on the pore wall as upholder and directly carbonizing the wet hydrogel from hydrothermal reaction, without using any special drying techniques. Cassava powder is used as carbon precursor which enables sustainable synthesis. Carbonizing the wet hydrothermal product is found to be a self-activation process, through which abundant pores are generated. The aerogel is used as host to encapsulating sulfur for lithium sulfur battery. Graphene, served as highly conductive scaffold, accelerates the transport of the electrons. The hierarchically porous structure is in favor of improving the electrochemical performance of lithium sulfur battery. Therefore, the cathode with high sulfur loading and high sulfur content can deliver very good performance. Keywords: Aerogel, Cassava, Graphene, Hierarchically porous, Lithium sulfur battery

1. Introduction Graphene has attracted tremendous attention since its discovery in 2004, due to its large specific surface area, superior mechanical property, and high conductivity 1. To exhibit its outstanding properties in bulk and promote its potential for practical application, it is necessary to compose the individual graphene sheets into

2


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1-3

3-dimensional macroscopic materials

. Typically, assembling macroscopic

monolithic aerogel has been regarded as one of the most attractive routes

4-9

. Besides

that, creating the hierarchical pores in the graphene-based monolith is of tremendous interests due to the importance of this structure on promoting the application performance 10-12. For the lithium sulfur (Li-S) battery, the most promising candidate for the next generation energy storage system due to the ultrahigh theoretical discharge capacity (1675 mA h g-1) and energy density (2576 wh kg-1)

13-15

, the hierarchically porous

graphene-based materials are intriguing for the construction of sulfur cathode. As we know, the application of Li-S battery is hindered by the poor cycle performance and rate capability caused by the instinct insulation of sulfur, the dissolution of the intermediates polysulfides (Li2Sx, 4≤x≤8) and the parasitic reactions between polysulfides and lithium anode

16-18

. There is no doubt that graphene-based materials

can highly improve the conductivity of the sulfur cathode, and therefore can improve the electrochemical performance

19-21

. In the hierarchical pore system, pores in

different size possess different advantages

16

. The micropores has been proved to be

good containers for accommodating and immobilizing sulfur; the mesopores not only facilitate the charge transport but also enhance the encapsulation of sulfur thus maintaining relatively high sulfur loading; while the macropores ensure good infiltration of electrolyte and suppress the migration of polysulfides due to the strong

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absorbability to the electrolyte 22. Therefore, the hierarchically porous graphene-based materials are promising matrixes for fabricating high performance sulfur cathodes. The general route for preparing the porous graphene-based aerogel is to fabricate hydrogel through wet chemical approach

5, 6, 22, 23

, and freeze-dry it with special

apparatus to maintain the porosity. For instance, Li et al. 6 prepared graphene hydrogel by heating the mix-solution of graphene oxide and ethylene diamine at 80 oC for 24 hours and then used freeze-drying technique to prepare aerogel. The resulted aerogel is ultralight and flexible, and maintains high porosity. Sui et al.23 fabricated graphene aerogel by freeze-drying the graphene hydrogel which was assembled from 12 hours of hydrothermal reaction, thus obtaining highly porous structure. However, the freeze-drying process is relatively long (generally more than 1 days) and energy costing. This challenge inevitably hampers the large scale fabrication and application of graphene-based materials. In addition, even though the macropores in the graphene aerogels are rich, the mesopores and micropores are not satisfying, simultaneously 24, 25

. Particularly for the application in lithium sulfur battery, macropores, mesopores

and micropores are all desired. In order to fabricate more effectively, Guo proposed some effective methods to construct the graphene-based materials in bulk. For instance, Guo presented a cost-effective method to prepare the graphene-based carbon-sulfur composite, where sulfur is well distributed in the hierarchical carbon network, through a wet chemistry synthesis

26

. The as-prepared sulfur cathode can reach very high sulfur content and 4


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show good electrochemical performance. Guo also reported effective methods for fabricating graphene@microporous carbon

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, hierarchically micro/mesoporous

activated graphene 28. These methods are based on alkali activation process, by which substantial micropores/mesopores are created. The related sulfur cathodes also exhibits very good performance. Herein, we demonstrate an efficient alkali-free fabrication strategy for hierarchically porous graphene-based aerogel with rich macropores, mesopores and micropores through unconventional hydrothermal reaction and heat treatment which avoids any special drying techniques (i.e. freeze drying and supercritical drying). Cassava, not only acts as a sustainable carbon precursor, but also provides a gelatinous condition for the hydrothermal reaction to restrict the restack of reduced graphene oxide sheets. And it is expected to be fully converted into hydrothermal carbon on the pore wall in the hydrogel as support layer and micro-/mesopores donor. Therefore, it can resist the collapse of the porous structure caused by capillary motion when heating and enrich the pore system. The CGA is applied as host to accommodate sulfur and the sulfur cathode exhibits superior electrochemical performance.

2. Experimental section

2.1 Materials Preparation

2.1.1 Preparation of carbon/graphene aerogels (CGAs) 5



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Graphene oxide (GO) was synthesized by exfoliating graphite which was purchased from Tianjin Kemiou Chemical Reagent Co. (China) through Hummers method. 10 g of cassava powder was added into 150 mL of GO solution with concentration of 10 mg mL-1 in a beaker and stirred in an ultrasonic water bath (25-30 o

C) for 10 min. To further break apart the cassava particles, the turbid liquid was

subjected to high speed stirring at 22 k rpm for 5 min with PT2500E homogenizer (Kinematica, Sweden). After that, the beaker was transferred to an oil bath at 100 oC with magnetic stirring until the turbid liquid transformed to homogeneous gel. 120 mL of gel was immediately transferred into a Teflon-lined autoclave (150 mL) then placed into an oven at 180 oC for hydrothermal reaction. After water cooling, the hydrothermal products were collected and heated in a tube furnace with argon flow from room temperature to 1000 oC with a heating rate of 10 oC min-1. After 2 hours of isothermal pyrolysis, the furnace was cooled naturally, obtaining CGA. The resulted CGA from different synthetic conditions are labeled as CGA [m, t], where m represents the state (Dried or Wet) of the hydrothermal products before carbonization and t represents the hydrothermal reaction time. For example, CGA [W, 12] is the CGA obtained from carbonizing the wet hydrothermal product which is derived from 12 hours of hydrothermal reaction. And CGA [D, 12], CGA [W, 6] and CGA [D, 6] also were also synthesized. Solid carbon spheres (SCSs) derived from cassava powder and reduced graphene oxide aerogel (rGOG) were also obtained through hydrothermal reaction and 6


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carbonization. The synthetic procedures and conditions were the same with CGA [W, 12]. When using glucose as carbon precursor, the glucose was directly dissolved in the GO solution. 120 mL of the solution was transferred into a Teflon-lined autoclave (150 mL) and then placed into an oven at 180 oC for 12 hours. The dosage of glucose was the same with cassava. The dosage and concentration of GO solution were not changed.

2.1.2 Fabrication of carbon-sulfur composites Precipitated sulfur (0.65 g, Sigma-Aldrich) and carbon materials (0.35 g) were finely grinded with agate mortar. The mixture was put into a beaker (5 mL) then sealed in a Teflon-lined autoclave (20 mL) and heated to 160 oC with a heating rate of 5 oC min-1 and held for 10 hours. Whereafter, the temperature was enhanced to 250 oC with the same heating rate and held for two hours then cooled naturally. The composites were labelled as CGA [D, 6]-S, CGA [W, 6]-S, CGA [D, 12]-S, CGA [W, 12]-S, SCSs-S and rGOG-S. For preparing a CGA-Sulfur composite with a higher sulfur content, 0.75 g of elemental sulfur and 0.25 g of CGA [W, 12] were subjected to the same synthetic procedures, and the composite was denoted as CGA [W, 12]-HS.

2.2 Characterization of materials N2 adsorption measurements were performed on ASAP 2460 analyzer (Micromeritics) at 77K. The morphology was characterized by MIRA 3 LMU 7



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(TESCAN, USA) scanning electron microscope (SEM) and Tecnai G2 F20 S-TWIX (FEI, USA) transmission electron microscope (TEM). Thermogravimetric Analysis (TGA) was performed on a synchronous thermal analyzer SDTQ600 (TA, USA). Raman spectra were collected from LABRAM-HR 800 spectrometer in the range of 3000-1000 cm-1. X-ray photoelectron spectroscopy (XPS) measurements were conducted on ESCALAB 250Xi (Thermo Fisher).

2.3 Electrochemical measurements For the preparation of electrodes, the carbon-sulfur composites (0.24 g), polyvinylidene fluoride (PVDF, Solef 6020, 0.03 g) and super P (Timical, 0.03 g) were homogenized with PT2500E homogenizer using N-methyl-2-pyrrolidone (NMP) as dispersant. The slurry was casted on a piece of Al foil and dried at 60 oC in a vacuum oven for 12 hours then punched into disks with a diameter of 13 mm. In this case, the areal sulfur loading of the sulfur cathodes (CGA [D, 6]-S, CGA [W, 6]-S, CGA [D, 12]-S, CGA [W, 12]-S, CGA [W, 12]-HS, SCSs-S and rGOG-S) was 0.9-1.2 mg cm-2.The sulfur content of the electrodes was ranging from 49.6-52.5 wt% for CGA [m, t]-S cathodes, and 62.64 wt% for CGA [W, 12]-HS. CGA [W, 12]-S cathodes with higher sulfur loading (2 and 3 mg cm-2) were also prepared through controlling the thickness of slurry.

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CGA [W, 12]-S cathodes with 4 mg cm-2 and 60.6 wt% of sulfur on the electrodes were prepared by altering the ratio of CGA [W, 12]-S composite and PVDF to 95:5, without Super P additive. 2025-type coin cells were fabricated in an argon filled glove box using Cellgard 2325-type separators and lithium as anode. The electrolyte was consisted of 1 M lithium bis(trifluoromethanesulfonimide) and 1 wt.% of LiNO3 in the mix-solvent of 1,3-dioxolane and ethylene glycol dimethyl ether ( 1:1, v/v). The galvanostatic charge-discharge tests were performed on a battery testing system (Land, China) in the voltage window of 1.8 – 3.0 V or 1.5 – 3.0 V (for the electrodes with sulfur loading of 3 mg cm-2 or higher). The Cyclic voltammetry (CV) measurements were performed on a multichannel electrochemical test system (1470E/1400A, Solartron) in the voltage range of 3.0 -1.5 V.

3. Result and discussion The principle of material design is illustrated as the upper part of figure 1. Hydrothermal carbon is firstly generated on the macroporous graphene hydrogel, acting as upholder to prevent the collapse caused by the capillary force. And then abundant mesopores/micropores are generated on the carbon layer after carbonization. The major procedures includes three steps as illustrated in the nether part of figure 1. The first step is the preparation of cassava/GO hydrogel, where the GO is homogeneous in the cassava gel. More importantly, the cassava/GO hydrogel provides

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a gelatinous condition. The next step is hydrothermal reaction where GO is self-assembled into 3D graphene hydrogel and cassava is transformed into hydrothermal carbon. Because of the high viscosity of gelatinous media, the restack of GO during the reduction is restricted. The final step is the carbonization, where the wet hydrothermal product is used instead of dry product, because the moisture could act as activating reagent for enhancing the quantity of pores of the final product 29. The morphology of CGA [W, 12], which is obtained by carbonizing the wet hydrothermal product that derived from 12 hours of hydrothermal reaction, is investigated by SEM and TEM. The results are presented in figure 2. From the SEM images (figure 2a and 2b), it is clear to see that there are abundant macropores existing in the CGA [W, 12]. The TEM images (figure 2c and 2d) also reveal that the CGA [W, 12] possesses porous structure. Compared to the TEM images of rGOG presented in figure S1a-b, the main difference is that the carbon derived from cassava can be observed from figure 2c-d. The N2 sorption measurement reveals that the CGA [W, 12] possesses relatively high surface area (679 m2 g-1) and large pore volume (0.94 cm3 g-1, from Horvath-Kawazoe model). Figure 3 presents the isotherm and the pore size distribution (PSD) plot from Density Functional Theory (DFT) model of CGA [W, 12]. The Horvath-Kawazoe (HK) PSD plot of micropores also provided in figure S2. The obvious hysteresis loop displayed in the isotherm suggests the existence of mesopores 30-32, and the PSD plots reveal the existence of micropores, mesopores and macropores, which suggests that 10


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the CGA [W, 12] is hierarchically porous. These results suggest the feasibility of the synthetic route currently proposed. Using cassava as carbon precursor fits the concept of sustainable synthesis. It is found that using the gelatinous cassava results in quite large hydrothermal-carbon/graphene hydrogel, while using the soluble glucose as carbon precursor only results in a very small hydrogel (comparison are presented in figure S3) and relatively large amounts of hydrothermal carbon is not generated on the GO but deposits at the bottom of the autoclave. This indicates the restack of GO during the reduction process is restricted to some extent in the gelatinous media and demonstrates the superiority of cassava as the carbon precursor for the synthesis. It is worthy to note that GO plays a significant role on the formation of monolithic carbon, because the carbon precursor transforms into solid carbon spheres (SCSs, TEM images in figure S1c-d) without the existence of GO. However, only using the GO to prepare the rGOG through the similar procedure results in quite limit yielding (~0.04 g of rGOG from 120 mL of GO with a concentration of 10 mg ml-1). It is also demonstrated that, the concentration of cassava/GO gel is a key parameter for the synthesis. When the gel is prepared by 6 g of cassava in 150 mL of GO with a concentration of 6 mg mL-1, large SCSs in micron size are generated as evidenced by SEM images displayed in figure S4a-b, but they are very limited. Further diluting the gel (i. e. 2 g of cassava in 150 mL of GO with concentration of 2 mg mL-2), plenty of SCSs are generated with smaller size (figure S4c-d). In fact, at such low concentration of GO and cassava, this gel turns into solution, and monolithic cannot be obtained.

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Hydrothermal reaction time is found to be an important parameter. It determines whether the porous structure can sustain the capillary force during the carbonization process. When the reaction time is set to be 6 hours, the morphology and the textural properties of the final product are highly different. The SEM images (figure S5a and S5b) of CGA [W, 6] reveal that the CGA [W, 6] are compact and the TEM images (figure S5c and S5d) indicate that the graphene sheets are aggregated severely. The CGA [W, 6] possesses relatively low surface area (289 m2 g-1) and pore volume (0.23 cm3 g-1). Its isotherm plot and PSD plot from DFT model are shown in figure S6a and S6b. The PSD plot (figure S6b) reveals that the pores with diameter less than 5 nm are in the majority. The absence of large amount of pores with diameter larger than 5 nm is possibly related to the collapse of those pore caused by the capillary force. It is said that the hydrothermal carbon layer from 6 hours of hydrothermal reaction on the pore wall possesses low mechanical property to sustain the capillary force, leading to the collapse of the porous structure. To verify this, the hydrothermal products are dried at 70 oC in an air-flowing oven for comparison. Figure 4a presents the photographs of hydrothermal-carbon/graphene hydrogels obtained from 12 (left) and 6 (right) hours of hydrothermal reaction. Before drying, they are monolithic and sharing similar shape. After drying, the one from 12 hours of hydrothermal reaction cracks into several parts and maintains relatively large volume (left, figure 4b). However, the one from 6 hours of hydrothermal reaction shrinks into very small monolithic (right, figure 4b). It is worthy to note that the dried samples are with similar mass. After

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carbonization, the CGA [D, 6] and CGA [D, 12] are obtained and their morphology are observed by SEM and presented in figure 4d and 4e. It is found that the CGA [D, 6] is compact while the CGA [D, 12] are porous. N2 sorption measurement results show that the CGA [D, 6] possesses very low surface area (13 m2 g-1) and pore volume (0.012 cm3 g-1), while the CGA [D, 12] possesses relatively high surface area (95 m2 g-1) and pore volume (0.43 cm3 g-1). Their isotherm plots and DFT-PSD plots are shown in figure S6c-f. As depicted in figure S6c, the hysteresis loop also exists in the isotherm of CGA [D, 12], which indicates the existence of mesopores. From figure S6d and figure S2, it is speculated that the CGA [D, 12] possesses mesopores, macropores and few micropores. However, the CGA [D, 6] nearly is non-porous as illustrated in figure S6f and figure S2. These results strongly indicate that the hydrothermal reaction time is a key parameter for the synthesis. Based on the characterization results, another fact is demonstrated that carbonizing wet hydrothermal product is efficient to create more pores, which is a self-activated process. From HK PSD plots of micropores presented in figure S2, it can be confirmed that this self-activated process is beneficial to the generation of micropores. The comparison on the textural properties is depicted in table 1. Harkins & Jura equation is used for the t-plot analysis. The fitting lines are shown in figure S7. It is found that the samples from carbonizing wet hydrothermal products possess higher surface area and pore volume. Taking the SEM images of CGA [W,12] and CGA [D,12] presented in figure 2 and figure 4d into comparison, it is also found that 13



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the aerogel from carbonizing wet hydrothermal product exhibits more porous structure than that of the aerogel from carbonizing dried hydrothermal product. These results effectively demonstrate that carbonizing wet hydrothermal products leads to more porous structure. The Raman spectra of the CGA obtained from different synthetic conditions are shown in figure S8. It is clear to see all the samples present two peaks centering at ~1337 cm-1 (D band) and ~1585 cm-1 (G band), which are related to the defects and sp2 graphitic carbon layer, respectively33, 34. It is worthy to note that even though there is a certain amount of graphene existing in the aerogels, all samples exhibit higher intensity of D band than that of G band. These results indicate that the aerogels mainly are amorphous carbon with partial graphitization. XPS spectra of CGA [m, t] are displayed in figure S9 (full spectra) and figure 5. Strong signals of C1s (figure a-d) and O1s (figure e-h) are detected. Week signal of N1s (figure i-l) is also detected in those aerogels, which suggests the existence of nitrogen in the aerogels. As shown in figure a-d, the C1s spectrum can be resolved four parts. The peaks at 284.3 eV and 284.8 eV is related to sp2 and sp3 hybridized carbon. The signals of C1s in C-O and C=O are also detected at 285.6 eV and 286.7 eV. The signals of O1s are possibly from the C-O and C=O bonds at 531.7 eV and 533.3 eV. For preparing high performance Li-S batteries, the CGA [W, 12] is the best host for encapsulating the insulating sulfur, since it possesses the highest pore volume. 65 wt. % of sulfur is compounded with 35 wt. % of CGA [W, 12] through 14


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melting-infusing processes. The as-prepared carbon-sulfur composite is labelled as CGA [W, 12]-S. TGA result presented in figure S10 reveals that the sulfur content of the CGA [W, 12]-S composite is 63.3 wt.%. The TEM images of the CGA [W, 12]-S are displayed in figure 6a-c, from which there is no crystal of sulfur observed, no matter at low or high magnification. Figure 6d presents the selected area electron diffraction (SAED) pattern of the CGA [W, 12]-S. Two diffraction rings associated to graphitic carbon are observed. However, the diffraction pattern of sulfur cannot be detected, which is possibly related to the low crystallinity of sulfur. Energy Dispersive X-Ray Spectroscopy technique is used to characterize the dispersion of sulfur in the composite. As seen in figure 6e-g, sulfur is homogeneously dispersed in the CGA [W, 12]. These results suggest that sulfur is successfully infused into the CGA [W, 12] and homogeneously distributed in the porous matrix. The electrochemical behaviors of the CGA [W, 12]-S cathode are firstly evaluated by cyclic voltammetry (CV) and the curves are shown in figure 7a. Two typical cathodic peaks arising at ~2.3 V and ~2.0 V, and one anodic peak centering at ~2.4 V are observed clearly in each cyclic scan. This result is in good agreement with that of most previously reported sulfur cathodes 21, 28 35-37. The two cathodic peaks are related to the transformation of elemental sulfur to soluble polysulfides (Li2Sn, 4≤n≤8) and the formation of Li2S2 and LiS2. The anodic peak emerged in the anodic scan is related to the multistep oxidation of Li2S2 and LiS2, forming soluble polysulfides and elemental sulfur. In the first cathodic scan, there is an extra peak centered at ~1.7 V 15



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which is related to the decomposition of LiNO3, forming passivation films on the surface of lithium anode

38, 39

. This peak disappears in the subsequent cycles, which

suggests the decomposition is suppressed or weaken. It should be noted that, the CV curve in the first cycle is slightly shifted compared to the CV curves in the subsequent cycles, showing high polarization. The weaker polarization in the subsequent cycles is possibly due to the rearrangement of the active material to electrochemically favorable positions

40

. The galvanostatic charge-discharge profiles measured under

different current density are displayed at figure 7b. One long charge plateau corresponding to the oxidation peak and two discharge plateaus corresponding to the two reduction peaks in the CV curves are clear to be seen. With the increase of the current density, the gap between the charge plateau and the second discharge plateau becomes larger, which suggests the electrochemical polarization becomes larger. However, even under a relatively high current density (4 C, 1 C =1675 mA gsulfur-1), the second discharge plateau maintains above 1.9 V, which indicates the sulfur cathode possesses good accessibility for charges. Figure 7c shows the rate performance of CGA [W, 12]-S. When tested at 0.1 C, the CGA [W, 12]-S composite can deliver an initial discharge capacity of 1073 mA h g-1, but at the second cycle the discharge capacity drops to 925 mA h g-1. This sharp drop is probably related to the dissolution of polysulfides which causing the loss of active material. After 10 cycles of galvanostatic charge-discharge at 0.1 C, the current density is increased to 0.2 C. A stable discharge capacity of ~815 mA h g-1 is obtained. When further increasing the

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current density, the reversible capacity of ~760 mA h g-1 at 0.5 C, ~720 mA h g-1 at 1 C and ~660 mA h g-1 at 2 C is obtained. Even at ultrahigh current density of 4 C, a substantial reversible capacity of ~600 mA h g-1 is retained. When the current density turns back from 4 C to 0.5 C, the discharge capacity can be recovered to ~750 mA h g-1, which implies the good reversibility of the sulfur cathode. Since the areal sulfur loading on the sulfur cathode is one of significant parameters that affects the rate capacity. The rate performance of the sulfur cathode with a substantial areal sulfur loading of ~2 mg cm-2 is investigated. It is found that the rate performance is slightly poorer. As seen in figure S11, the capacity at 0.2 C, 0.5 C, 1 C and 2 C is ~769, ~711, ~650 and ~522 mA h g-1, respectively. These data indicate that the CGA [W, 12]-S composite possesses superior rate capability. For comparison, other materials are also used to encapsulate the elemental sulfur through the same synthetic procedures. The corresponding carbon/graphene-S composites with similar sulfur content are signed as CGA [D, 12]-S, CGA [W, 6]-S and CGA [D, 6]-S, and the sulfur content is also characterized through TGA (figure S10). The rate performance is illustrated as figure 7d and it is found that the CGA [D, 12]-S shows better rate performance than the other two samples, but it is still poorer than that of the CGA [W, 12]-S. The cycle performance of the cathodes with sulfur loading of 0.9-1.2 mg cm2 is tested under the current density of 0.5 C as Figure 8a depicts. In the first cycle, the initial discharge capacity of the CGA [W, 12]-S, CGA [D, 12]-S, CGA [W, 6]-S and CGA [D, 6]-S is 1020 mA h g-1, 944 mA h g-1, 708 mA h g-1, 635 mA h g-1, 17



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respectively. After the 100th cycle, the corresponding discharge capacity maintains at 780, 646, 552 and 517 mA h g-1. Since the CGA [W, 12] possesses significant amount of micropores, mesopores, macropores and relatively high pore volume, the electrochemical performance of CGA [W, 12]-S is better than the other CGA-S composites. On one hand, the micropores is benefited to immobilizing sulfur by strong physical absorption. Due to the absence of micropores and small pore volume of CGA [D, 12], the corresponding sulfur cathodes show poorer electrochemical performance. On the other hand, the mesopores provide large pore volume to enhance the encapsulation of sulfur and facilitate the charge transport according to some references16, 28 In addition, the macropores ensure good infiltration of electrolyte and suppress the migration of polysulfides due to the strong absorbability to the electrolyte. Because the CGA [D, 12] possesses significant amount of mesopores and macropores (figures S6d) and the CGA [D, 6] is nearly non-porous, the electrochemical performance of the CGA [D, 12]-S is better than that of the CGA [D, 6]-S. It is noticed that even though the CGA [W, 6] possesses significant amount of micropores than CGA [D, 6], the electrochemical performance of corresponding sulfur cathodes show no significant improvement. It is possibly attributed to the quite small pore volume, which could not provide sufficient space for the sulfur confinement, and the sluggish Li+ diffusion process28 41. It should be noted that the performance of SCSs-S and rGOG-S with the similar sulfur content (figure S10) is

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very poor as displayed in figure S12. The results demonstrate the superiority of the CGAs for accommodating sulfur. The cycle performance of the CGA [W, 12]-S cathodes with higher sulfur loading have also been investigated at current density of 0.5 C. As shown in figure 8b, after 100 cycle the capacity maintains at 787 mA h g-1 when the sulfur loading increased to 2 mg cm-2. However, when the sulfur loading increased to 3 mg cm-2, the reversible capacity decrease visibly, which maintains at 577 mA h g-1. Under the current density of 1 C, as depicted in figure 8c, the performance of CGA [W, 12]-S cathode with sulfur loading of 2 mg cm-2 is slightly poorer than that of the CGA [W, 12]-S cathode with sulfur loading of 1 mg cm-2. After 200 cycles the discharge capacity of CGA [W, 12]-S cathode with sulfur loading of 1 and 2 mg cm-2 is 629 and 587 mA h g-1, respectively. Based on the CGA [W, 12], a carbon-sulfur composite with higher sulfur content is prepared (labelled as CGA [W, 12]-HS). The TGA result shows that the sulfur content of the CGA [W, 12]-HS composite is as high as 78.3 wt.% (figure S10) . The cycle performance is investigated and displayed in figure 8d. When measured under the current density of 0.5 C and 1 C, the discharge capacity maintains at 619 and 589 mA h g-1, respectively, after 200 cycles. In order to increase the sulfur loading (mg cm-2) and sulfur content (wt%) of the cathode simultaneously, cathodes only with 95 wt% of CGA [W, 12] and 5 wt% of PVDF are prepared. The cycle performance is illustrated in figure 9. At the first cycle, the cathode delivers initial capacity of 1119 mA h g-1 at 0.1 C and after 70 cycle the 19



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discharge capacity is retained at 683 mA h g-1 (figure 9a). Under the current density of 0.5 C, the performance becomes poorer as illustrated in figure 9b. Even though the first three cycles is tested at 0.1 C so as to activate the cathode, discharge capacity gradually increases until 35th cycle, where the cathode becomes stable. At 100th cycle, the reversible capacity is 542 mA h g-1. These results suggest the CGA [W, 12] is a promising material for high energy density lithium sulfur batteries. Firstly, graphene serves as highly conductive scaffold which could accelerate the electron transport thus minimizing the polarization during charge-discharge processes. Secondly, attributed to the existence of the hierarchical pore system, it can provide strong physical adsorption to capture the polysulfides, sufficient space to confine the active species and fast Li+ transport. Therefore, the cathodes based on CGA [W, 12] as host could exhibit good electrochemical performance even when the high sulfur content/loading reaches a substantial value.

4. Conclusion Hierarchically porous CGA is successfully synthesized through a green and scalable synthetic route without using any special drying techniques. The hydrocarbon derived from sustainable carbon precursor cassava is preferentially formed on the surface of the graphene sheets during the hydrothermal reaction. Longer hydrothermal reaction time leads to better mechanical property of the hydrothermal carbon layer. Therefore, it can serves as upholder to prevent collapse of the porous structure during the evaporation of water. In addition, using wet hydrothermal product for 20


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carbonization could lead to higher porosity of the CGA due to the self-activation process, through which plenty of micropores are generated. The hierarchically porous CGA is used to prepare carbon/graphene-S composites. The cathodes present superior electrochemical performance even with relatively high sulfur content and sulfur loading. The superior performance could be benefited from the hierarchically porous structure and the highly conductive graphene scaffold. The results demonstrate the hierarchically porous CGA is a promising material for high performance Li-S batteries.

Acknowledgment The authors acknowledge the financial support from the China Postdoctoral Science Foundation funded project (2015M570686), National Natural Science Foundation of China (Grant no. 51404304 and 51574288), Natural Science Foundation of Hunan Province (14JJ2001), and the Fundamental Research Funds for the Central Universities of Central South University (2015zzts186). The authors also acknowledge Dr. Liyuan Zhang for the helpful advices on material synthesis.

Supporting Information TEM images of rGOG and SCSs; HK PSD; Photos of glucose-derived hydrogel; SEM images of CGAs from different concentration; TEM and SEM images of CGA [W, 6]; isotherm and DFT PSD of [D, 12], [D, 6] and [W, 6]; fitting line for t-plot analysis; Raman and XPS spectra; TGA results; rate performance of CGA [W, 12]-S 21



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electrode (2 mgsulfur cm-2); Cycle performance of SCSs-S and rGOG-S. These materials can be found at http://pubs.acs.org.

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Table 1. Comparison on surface area, t-plot micropore area and t-plot external surface area of activated carbon materials.

ST [a]

StM [a]

V [a] [b]

VtM [a]

(m2 g-1)

(m2 g-1)

(m2 g-1)

(cm3 g-1)

(cm3 g-1)

CGA [W,12]

680

363

317

0.94

0.17

CGA [W,6]

289

121

168

0.23

0.05

CGA [D,12]

95

1

94

0.43

0.001

CGA [D,6]

13

5

8

0.012

0.002

materials

StE [a]

[a] ST, StM, StE, V, VtM represent total surface area, t-plot micropore area, t-plot external surface area, pore volume and t-plot micropore volume. [b] from Horvath-Kawazoe model

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Figures and Figure captions: Figure 1. The synthetic pattern (upper) and procedures (nether) of CGA. Figure 2. The SEM (a, b) and TEM (c, d) images of CGA [W, 12]. Figure 3. The isotherm linear plot (a) and the DFT-pore size distribution plot (b) of CGA [W, 12]. Figure 4. The photographs of the hydrothermal products (a) before drying, (b) after drying and (c) after carbonization; the SEM images of the (d) CGA [D, 12] and (e) CGA [D, 6]. Figure 5. The C1s (a-d), N1s (e-h) and O1s (i-l) spectra of CGA [m, t]. (a), (e) and (i) belong to CGA [W, 12]; (b), (f) and (j) belong to CGA [D, 12]; (c), (g) and (k) belong to CGA [W, 6]; (d), (h) and (l) belong to CGA [D, 6]. Figure 6. (a) The TEM images of CGA [W, 12]-S composite; (b) the magnification of the big red square section in (a); (c) the magnification of the small red square section in (a); (d) the SAED pattern corresponding to the black circle section in (a); (e) the STEM image, the mapping of (f) carbon element and (g) sulfur element. Figure 7. (a) The CV curves of the CGA [W, 12]-S cathode measured with a swapping rate of 0.1 mV s-1; (b) the galvanostatic charge-discharge profiles of the CGA [W, 12]-S cathode tested under different current density; (c) the rate capability

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of the CGA [W, 12]-S cathode; (d) the rate performance of the CGA [D, 12]-S, CGA [W, 6]-S and CGA [D, 6]-S cathodes. Figure 8. (a) The cycle performance of the CGA [W, 12]-S, CGA [D, 12]-S, CGA [W, 6]-S and CGA [D, 6]-S cathodes with average sulfur loading of ~1.1 mg cm-2, tested under the current density of 0.5 C; (b) the cycle performance of CGA [W, 12]-S cathodes with sulfur loading of 2 and 3 mg cm-2, tested under the current density of 0.5 C; (c) Long cycle performance of CGA [W, 12]-S cathode with cathodes with sulfur loading of 1, 2 and 3 mg cm-2, tested under the current density of 1 C; (d) The cycle performance of CGA [W, 12]-HS cathode with sulfur loading of ~1.1 mg cm-2, tested under the current density of 0.5 C and 1 C. Figure 9. The cycle performance at (a) 0.1 C and (b) 0.5 C of CGA [W, 12]-S cathode with sulfur loading of 4 mg cm-2 and sulfur content of 60 wt%, prepared from 95 wt% CGA [W, 12]-S composite and 5 wt% PVDF.

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Figure 1. The synthetic pattern (upper) and procedures (nether) of CGA.

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Figure 2. The SEM (a, b) and TEM (c, d) images of CGA [W, 12].

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Figure 3. The isotherm linear plot (a) and the DFT-pore size distribution plot (b) of CGA [W, 12].

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Figure 4. The photographs of the hydrothermal products (a) before drying, (b) after drying and (c) after carbonization; the SEM images of the (d) CGA [D, 12] and (e) CGA [D, 6].

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Figure 5. The C1s (a-d), N1s (e-h) and O1s (i-l) spectra of CGA [m, t]. (a), (e) and (i) belong to CGA [W, 12]; (b), (f) and (j) belong to CGA [D, 12]; (c), (g) and (k) belong to CGA [W, 6]; (d), (h) and (l) belong to CGA [D, 6].

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Figure 6. (a) The TEM images of CGA [W, 12]-S composite; (b) the magnification of the big red square section in (a); (c) the magnification of the small red square section in (a); (d) the SAED pattern corresponding to the black circle section in (a); (e) the STEM image, the mapping of (f) carbon element and (g) sulfur element.

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Figure 7. (a) The CV curves of the CGA [W, 12]-S cathode measured with a swapping rate of 0.1 mV s-1; (b) the galvanostatic charge-discharge profiles of the CGA [W, 12]-S cathode tested under different current density; (c) the rate capability of the CGA [W, 12]-S cathode; (d) the rate performance of the CGA [D, 12]-S, CGA [W, 6]-S and CGA [D, 6]-S cathodes.

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Figure 8. (a) The cycle performance of the CGA [W, 12]-S, CGA [D, 12]-S, CGA [W, 6]-S and CGA [D, 6]-S cathodes with average sulfur loading of ~1.1 mg cm-2, tested under the current density of 0.5 C; (b) the cycle performance of CGA [W, 12]-S cathodes with sulfur loading of 2 and 3 mg cm-2, tested under the current density of 0.5 C; (c) Long cycle performance of CGA [W, 12]-S cathode with cathodes with sulfur loading of 1, 2 and 3 mg cm-2, tested under the current density of 1 C; (d) The cycle performance of CGA [W, 12]-HS cathode with sulfur loading of ~1.1 mg cm-2, tested under the current density of 0.5 C and 1 C.

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Figure 9. The cycle performance at (a) 0.1 C and (b) 0.5 C of CGA [W, 12]-S cathode with sulfur loading of 4 mg cm-2 and sulfur content of 60 wt%, prepared from 95 wt% CGA [W, 12]-S composite and 5 wt% PVDF.

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