Assemble Carbon Pores into Carbon Sheets: Rational Design of

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Assemble Carbon Pores into Carbon Sheets: Rational Design of Three-Dimensional Carbon Network for Lithium-Sulfur Battery Shuo Feng, Junhua Song, Chengzhou Zhu, Qiurong Shi, Dong Liu, Jincheng Li, Dan Du, Qiang Zhang, and Yuehe Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17549 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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ACS Applied Materials & Interfaces

Assemble Carbon Pores into Carbon Sheets: Rational Design of ThreeDimensional Carbon Network for Lithium-Sulfur Battery Shuo Feng, † Junhua Song, † Chengzhou Zhu, †* Qiurong Shi, † Dong Liu, † Jincheng Li, † Dan Du, † Qiang Zhang ‡ and Yuehe Lin*† † School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, 99164, USA. ‡ Department of Chemistry, Washington State University, Pullman WA, 99164, USA E-mail: [email protected]; [email protected] Keywords: Lithium-sulfur battery, three-dimensional sulfur host, physical/chemical dual adsorption, conductive network, porous carbon nanosheets

Abstract The conversion reaction based lithium-sulfur battery features an attractive energy density of 2600 Wh/kg. Nevertheless, the unsatisfied performance in terms of poor discharge capacity and cycling stability still hinders its practical applications. Recently, porous carbon materials have been widely reported as promising sulfur reservoirs to promote the sluggish reaction kinetics of sulfur conversion, tolerate volume expansion of sulfur and suppress polysulfide shuttling. However, porous carbon with simply designed nanostructure is hard to satisfy all these respects simultaneously. Herein, we applied a dual-template strategy that assembles carbon pores into carbon sheets to prepare three-dimensional (3D) nitrogen-doped porous carbon nanosheets (NPCS) as the multifunctional sulfur host for Li-S battery. By arranging carbon pores within an interconnected 3D architecture, the porous carbon sheets enable rapid electron/ion transfer. Moreover, the micro/mesopores and nitrogen dopant in N-PCS provide both physical and chemical restriction to polysulfide species. With these advances, the N-PCS/S cathode delivers a large initial discharge capacity of 1360 mAh/g at 0.1 C. When performed at 0.5 C for 1000 cycles, the cathode can still remain ~50% of its capacity with a low decay rate of 0.05% per cycle, showing the important role of 3D carbon material in Li-S battery.

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Introduction The conversion reaction based lithium-sulfur battery (LiS) features an attractive energy density of 2600 Wh/kg, which is about five times higher than that of conventional lithium-ion batteries (LIB).1-5 Nevertheless, the unsatisfied performance in terms of poor discharge capacity and cycling stability still hinders its practical applications. The insulating nature of sulfur, large volume expansion and serious shuttling effect are considered as the main obstacles in LiS.6 These days, tremendous efforts have been made to address these problems, such as coating organic/inorganic protective layer on sulfur particles,7-10 designing porous carbon matrixes11-16 and applying interlayers between cathode and separator.17-21, 49 It has been widely reported that carbon materials such as carbon nanosphere, carbon nanotube and carbon nanosheet can be used as the sulfur host to optimize LiS’s performance. Nevertheless, the limitations of these simply designed carbons are also explicit. For example, the carbon nanotube is known as an excellent conductive network for sulfur cathode. However, it has insufficient micro/meso pores to host sulfur inside, making the polysulfide dissolution inevitable.22,23 A good example of structured sulfur host is the core/yolk shell carbon spheres, which affords both void space and physical barrier to accommodate volume change and prevent polysulfides dissolution9,10 Yet, they are less capable of forming conductive networks with highly desirable electrical conductivity to promote sulfur utilization.8,16,24,25 Thus, an ideal carbon material should possess multi-functions to simultaneously resolve the abovementioned problems. To this end, it is of great importance to design porous carbons with hierarchical pore size distributions.26-31, 46-48 Among the numerous synthesis routes, carbonization of biomass such as litchi shell42,43, nori44 is an economic strategy to prepare porous carbon on a large scale.


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Nevertheless, these porous carbons usually have irregular morphologies/nanostructures and cannot afford rapid electron/ion transfer due to their disordered pore architectures. Thus, assembling the carbon pores into a rational secondary architecture is a promising way to prepare multifunctional carbon matrix beneficial for high-performance LiS batteries.14,16,24,32-34 Recently, Pei et al. successfully prepared highly porous carbon bowls through a tetraethyl orthosilicate (TEOS) assisted resorcinol-formaldehyde (RF) coating process and showed excellent electrochemical performance.12 Their unique design satisfied most of the functions for Li-S battery. However, zerodimension carbon bowls are not able to establish an interconnected conductive framework. To solve this problem, Xin and his co-workers successfully assembled carbon pores into twodimension nanosheets by introducing graphene as a secondary template. The graphene network affords adequate conductive pathways for fast electron transfer.48 Inspired by their work, we developed a dual-template strategy to arrange carbon pores into nitrogen-doped carbon nanosheets (N-PCS). In our strategy, the Ni(OH)2 nanosheet is selected as one of the hard templates. Compared to graphene, the Ni(OH)2 nanosheet templates can be easily prepared through a rapid microwave-assisted synthesis that avoids complicated process and hazards. The interconnected Ni(OH)2 nanosheets provide an ideal 3D skeleton for further carbon pores assembly. With its unique hierarchical structure, the porous carbon sheets had a surface area as high as 1255 m2/g with large pore volume consisting of both meso/micropores. The large pore volume can accommodate sulfur and effectively tolerate volume expansion. In addition, the interconnected carbon framework offers a 3D conductive platform for fast electron/ion transport, which is beneficial to improve the utilization of the poorly conductive sulfur. With these advances, the NPCS/S cathode delivers a large initial discharge capacity of 1360 mAh/g at 0.1 C. When performed



at 0.5 C for 1000 cycles, the cathode can still remain ~50% of its capacity with a low decay rate of 0.05% per cycle, showing the important role of 3D N-PCS in lithium-sulfur battery. Results and discussions

Figure 1. (a) Synthesis procedure of nitrogen-doped porous carbon nanosheets; (b-e) TEM images of (b) Ni(OH)2 nanosheet templates; (c) RF-coated Ni(OH)2 nanosheets; (d,e) nitrogen-doped porous carbon nanosheets at different magnifications. Synthesis procedure of nitrogen-doped porous carbon nanosheets (N-PCS) is illustrated in Figure 1a. The α-Ni(OH)2 nanosheets were first prepared through a facile microwave-assisted synthesis method.35 The resultant α-Ni(OH)2 nanosheets were then coated by resorcinol-formaldehyde (RF) and silica. During the synthesis, resorcinol and formaldehyde gradually polymerized to form RF coating, while EDA acted as both nitrogen precursor and basic catalyst for silicate oligomers hydrolysis.36 The obtained RF coated α-Ni(OH)2 nanosheets exhibit a stacked structure with the


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α-Ni(OH)2 nanosheets sandwiched by RF and silica layers. After annealing at elevated and subsequent template removal, the N-PCS can be successfully obtained.

Figure 2. (a) BET adsorption-desorption measurements of N-CS, N-PCS200, N-PCS1000 and NPCS2000; (b) Pore size distribution curves of the samples in (a). The morphology of N-PCS1000 was examined by transmission electron microscopy (TEM). Figure 1b demonstrates a typical 2D structure of α-Ni(OH)2 nanosheets template. Similar to graphene sheets, the α-Ni(OH)2 sheets also display a wrinkled thin layer structure but with a much larger footprint, which stretches from a few hundred nanometers to several micrometers in length. After coated with RF/SiO2, the sheet-like morphology of α-Ni(OH)2 templates was maintained without any noticeable agglomeration or disintegration (Figure 1c). After carbonization and template removal, the derived N-PCS1000 still retains its duplicated morphology from the template (Figure 1d). The high magnification image in Figure 1e clearly shows numerous carbon pores after etching silica nanoparticles embedded in the carbon layer. Based on the microscopic analysis, we have confirmed that the as-prepared N-PCS1000 possesses not only 2D features with



interconnected carbon nanosheets but also incorporates a considerable amount of carbon pores within. To investigate the influence of pore size and volume of N-PCS, controlled samples were prepared by adjusting the amount of TEOS from 0, 200 to 2000 µL (see experimental section). The morphologies of the four samples were characterized by scanning electron microscopy (SEM) under the same magnification. Compared to N-CS1000, the chunky agglomeration of nanosheets can be easily observed for N-CS (Figure S1a) and N-PCS200 (Figure S1b) with much larger dimension (> 10 µm in length). In contrast, N-PCS1000 and N-PCS2000 reveal a well-maintained 2D morphology instead of bulky carbon cluster (Figure S1c, d). Apparently, the silica template not only acts as a pore-forming agent, its concentration also affects the morphology of the porous carbon sheet after removal. At a relatively low TEOS content (e.g. N-CS and N-PCS200), the RF coating with sparsely embedded silica particles is mechanically susceptible to collapsing into bulk carbon during carbonization. As the TEOS content increases, however, the silica tends to form an interconnected skeleton that is robust enough to prevent RF coating from shrinking and hence to maintain the uniform 2D carbon structure.

In order to explore the effect of TEOS concentration on porosity, nitrogen adsorption desorption measurements were conducted. Figure 2a plots the nitrogen adsorption/desorption curves of NCS, N-PCS200, N-PCS1000 and N-PCS2000. The Brunauer-Emmett-Teller (BET) surface areas are calculated, without adding TEOS, N-CS only has a surface area of 153 m2/g, which agrees with its bulky structure shown in SEM images (Figure S1a). In contrast, with 200 µL TEOS addition, the surface area of N-PCS200 is increased to 554 m2/g. When increasing TEOS to 1000 µL, a surface area as high as 1255 m2/g can be obtained by N-PCS1000, almost 8 times higher than that


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of N-CS. The surface area can be further boosted to 1414 m2/g by adding 2000 µL TEOS. However, the doubled TEOS concentration does not result in a significant enhancement of surface area compared to N-PCS1000. In addition to surface area, the pore volume is another important factor affecting LiS performance. Large pore volume is often desirable to enable high sulfur loading and accommodate the swing volume of active materials. The TEOS content also effectively alter the pore volume of the different N-PCSs. Without the presence of TEOS, the N-CS offered a pore volume of 0.15 cm3/g. After adding a different amount of TEOS, the pore volumes of N-PCS200, N-PCS1000 and N-PCS2000 can be improved to 0.54, 0.79 and 0.94 cm3/g, respectively. More importantly, most pore sizes reside from 1 to 5 nm for all N-PCS samples (Figure 2b), indicating the cavities created by silica and Ni(OH)2 include both micro and mesopores. These pore sizes are beneficial for tolerating volume expansion and physically immobilizing polysulfides. Comparing to other works,12,14 the N-PCS shows a unique hierarchical porous structure which is beneficial to prevent polysulfides shuttling.

Figure 3. (a) TGA curve of N-PCS1000/S; (b) TEM image of N-PCS1000/S; (c) STEM image of N-PCS1000/S and (d-g) corresponding elemental mapping of N-PCS1000/S.



To prepare sulfur/carbon composite, sulfur was infused into the carbon structure through a typical melt-diffusion method. The SEM images of N-CS/S and N-PCS1000/S were shown in Figure S2 (a, b). After sulfur infiltration, bulk sulfur particles with irregular size can be found on N-CS/S surface due to its insufficient pore volume for sulfur accommodation (Figure S2 a). On the contrary, the morphology of N-PCS1000/S was well maintained without any sulfur agglomerations on carbon nanosheets’ surface (Figure S2b). To further demonstrate the advantages of having large surface area and pore volume, the same amount of N-CS/S and N-PCS1000/S (with same sulfur loading) were characterized by X-ray diffraction (XRD). As it displays in Figure S3, NPCS1000/S shows a much weaker sulfur signal than that of N-CS/S. The weakened signal indicates that most of the sulfur was penetrated inside the carbon structure instead of aggregating on the surface. According to the thermogravimetric analysis (TGA) (Figure 3a), sulfur content was kept at 60% in S/C composite. The sulfur distribution in N-PCS1000 was revealed by energy dispersive spectroscopy (EDS). According to the elemental mapping, carbon (Figure 3e), nitrogen (Figure 3f) and sulfur (Figure 3g) signals are perfectly overlapped, indicating a uniform sulfur distribution in a nitrogen-doped carbon matrix. The successful nitrogen doping was further verified by X-ray photoelectron spectroscopy (XPS) (Figure S4). Three peaks centered at 397.8, 400.1 and 400.9 eV can be assigned for pyridinic nitrogen, pyrrolic nitrogen and graphitic nitrogen. Based on the previous reports,14,37 it is confirmed that these various nitrogen species can provide effective chemical adsorption to polysulfides and suppress polysulfide dissolution during cycling.

The galvanostatic discharge and charge profiles were investigated at 0.1 C (1 C=1675 mA/ g) from 1.8-2.8 V for different N-PCSs. As it shows in Figure 4a, all samples demonstrate a two-plateau discharge behavior which is in agreement with the typical multi-step reduction of sulfur.5 The first


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plateau at 2.3 V is corresponding to S8-Li2Sx (6≤X≤8) and the second plateau at 2.1 V is related to the reduction of polysulfides from Li2Sx (2≤x≤6) to Li2S. 4 At 0.1 C, N-CS, N-PCS200, N-PCS1000 and N-PCS2000 can deliver initial discharge capacities of 1001, 1200, 1360 and 1162 mAh/g, respectively. The discharge capacities were gradually improved as the surface area increased. This phenomenon can be explained by the increased contact area between conductive carbon and sulfur, leading to more efficient sulfur utilization. Comparing to other control samples with lower surface areas, N-PCS1000 demonstrated the highest sulfur utilization, which is ~ 81% of the theoretical discharge capacity of sulfur. However, it is intriguing to notice that the further increment of the surface area did not result in higher sulfur utilization. N-PCS2000 showed the second lowest discharge capacity. The decreased capacity associated with larger surface area carbon is mainly caused by the redundant carbon pores, which leads to excessive grain boundaries and lower electrical conductivity.38 Besides, for porous carbons, micropores usually have less accessibility for fast ions diffusion due to its limited pore size.45 As it describes in Figure 2, N-PCS1000 and N-PCS2000 share a similar surface area while N-PCS2000 has less amount of mesopores but more micropores. It is believed that the abundant micropores created in N-PCS2000 can retard lithiumion transfer and result in lower sulfur utilization. Thus, the trade-off between surface area and electrical conductivity/ion diffusion makes it important to carefully design carbon matrix sulfur host material. In this work, N-PCS1000 delivered the largest initial capacity among four samples, suggesting the optimized balance between surface area and electrical conductivity.



Figure 4. (a) Charge/discharge curves of N-CS, N-PCS200, N-PCS1000 and N-PCS2000; (b) cycling performance of the four cathodes (a) at 0.5 C; (c) performance summarization of the four cathodes after 200 cycles at 0.5 C; (d) charge/discharge curves of N-PCS1000 at different cycles; (e) long-term cycling test of N-PCS1000 at 0.5 C.

N-PCS1000/S also demonstrated a superior performance at high current density. As exhibited in Figure 4b, when cycled at 0.5 C, an initial capacity of 776, 878, 993 and 751 mAh/g can be achieved by N-CS, N-PCS200, N-PCS1000 and N-PCS2000, respectively. After 200 cycles, NCS exhibited a severe capacity decay. On the contrary, the cycling performances of N-PCS200, 1000, 2000 were noticeably better due to their larger surface areas and pore volumes, which effectively slow down the kinetics of polysulfide dissolution. The detailed electrochemical


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performances of these four samples are summarized in Figure 4c. After 200 cycles, N-CS only maintained 67.3% of its initial capacity while N-PCS200, N-PCS2000 showed obviously enhanced capacity retention of 77.9% and 80.6%, respectively. It is important to notice that although N-CS and N-PCS2000 shared a similar initial capacity, N-PCS2000 held a higher retention at the end of the test, indicating the advantage of porous nanostructure on suppressing irreversible sulfur dissolution. Among all tested cathode, N-PCS1000 not only delivered the largest initial capacity but also showed the highest retention (81%) and an average Coulombic efficiency of 99.6%. Moreover, comparing to the samples with lower sulfur utilization, more polysulfides were generated in N-PCS1000 cell during cycling. However, the higher polysulfide concentration did not result in severe shuttling effect, which is evidenced by the better cycling stability and higher Coulombic efficiency. Instead, N-PCS1000 showed the best cycling capability and highest Coulombic efficiency, indicating a much improved electrochemical stability. As it displays in Figure 4d, there was only a slightly capacity fading in the first five cycles. After 200 cycles, NPCS1000 can still deliver 811 mAh/g, showing a well-maintained discharge voltage plateau at 2.3 and 2.1 V with no obvious increment of polarization. The improved performance in terms of capacity and stability can be ascribed to its high conductivity and unique hierarchical structure, which enables N-PCS1000’s strong ability of restraining polysulfides dissolution and shuttling effect. To investigate the long-term cycling capability of N-PCS1000/S, the cell was performed for 1000 cycles at 0.5 C. As it presented in Figure 4e, the fading rate of N-PCS1000/S significantly decreases after the initial capacity decay caused by the mechanical failure of PVDF binder.39, 40 At 300th cycle, it can remain 72.7% of its initial discharge capacity with an average Coulombic efficiency as high as 99.6%. From 300th to 600th cycle, the capacity becomes more stable and 81.5%



of its capacity can be maintained at this step. After cycling, N-PCS1000/S has a discharge capacity of 493 mAh/g, maintaining ~ 50% of its initial discharge capacity with an ultra-low fading rate of 0.05% per cycle. The excellent performance indicates the prominent ability of N-PCS1000 on improving electrode’s cycling capability even without adding any polysulfides additives in electrolyte.41

Figure 5. (a) Charge and discharge curves of N-PCS1000 at various current densities; (b) cycling performance of N-CS, N-PCS200 and N-PCS2000 at various current densities; (c) cycling performance of N-PCS1000 at 1 C; (d) charge/discharge profiles of N-PCS1000/S and CNT/S thick cathode; (e) cycling performance of N-PCS 1000/S and CNT/S thick cathode


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To further exhibit the superiority of N-PCS1000, the cell was examined at various current densities. As it plots in Figure 5a, N-PCS1000/S delivers a capacity of 1318, 1073, 883, 805, and 696 mAh/g at 0.1, 0.2, 0.5, 1 and 2 C, respectively. When the current rate was switched back to 0.2 C, a discharge capacity 1012 mAh/g can be recovered. Compared with N-CS/S and N-PCS200/S (Figure 5b), N-PCS1000 showed a much-enhanced sulfur utilization, especially at a high current rate, indicating the excellent overall-conductivity of N-PCS1000 cathode. Significantly, when cycled at 1 C, N-PCS1000/S presented both high initial discharge capacity (996 mAh/g) and outstanding stability. From 100th to 400th cycle, N-PCS1000/S represented a capacity retention as high as 91.7%, corresponding to an ultra-low fading rate of ~0.027% per cycle. Figure S5 shows the STEM image and corresponding EDS mappings of N-PCS1000 after cycling, the sulfur signal well overlaps with carbon signal, suggesting the excellent stability of N-PCS1000/s electrode and greatly suppressed shuttling effect. In order to achieve higher energy density, the sulfur loading in N-PCS1000/S composite was then increased to 80% (64% in electrodes). To fully demonstrate the important role of N-PCS1000, a carbon nanotube/sulfur composite (CNT/S) with the same sulfur content was also prepared. As shown in Figure 5d, when increase sulfur loading to 3.2 mg/cm2, the CNT/S cathode not only has a poor initial capacity of 233 mAh/g but also exhibits a serious cell polarization. More interesting, an activation process can be observed in CNT/S cathode that its capacity gradually reached the maximum value and then decayed quickly after 25 cycles (Figure 5e). As we discussed before, even though CNT was reported as the high-conductivity carbon material, it lacks meso/micro pores to have intimate contact with sulfur. Instead, the sulfur will directly deposit on CNT surface and finally form a thick insulating layer which retards electron transfer. As a result, the CNT/S cathode will suffer from a sluggish kinetics in the initial cycles. As the polysulfides dissolving into



electrolyte, more active sulfur can be exposed so that the capacity of CNT/S cathode will gradually increase. However, carbon nanotube lacks effective physical barriers to limit the detrimental contact between sulfur and electrolyte. Therefore, the serious shuttling effect will result in a quick capacity decay in the following cycles. N-PCS1000, on the other hand, is consisted of numerous meso/micro pores for sulfur accommodation. By arranging carbon pores within an interconnected 3D architecture, the porous carbon sheets enable rapid electron/ion transfer for significantly improved sulfur utilization. Hence, the thick N-PCS1000/S cathode can still deliver a large capacity of ~692 mAh/g. After 200 cycles, it holds 81.7% of its initial capacity, showing a high sulfur utilization as well as a good electrochemical stability. The average Coulombic efficiency of thick N-PCS1000/S cathode is ~90% (Figure S6). Compared to carbon nanotube, the N-PCS1000 demonstrates its unique superiorities in terms of suppressing polysulfides shuttling and promoting reaction kinetics, indicating the necessity of 3D carbon materials in high-performance lithiumsulfur batteries.

Conclusion In summary, we have demonstrated a dual-template strategy to synthesize nitrogen-doped twodimensional porous carbon nanosheets. The carbon pores have been successfully integrated into 2D carbon nanosheets. With optimized morphology and porosity, the as-e N-PCS1000 features three advantages: (1) Hierarchical carbon pores afford N-PCS1000 sufficient void space as well as strong polysulfides confinement ability. (2) Heteroatom doping provides chemical bonding with polysulfide species and hence mitigates shuttling effect. (3) Interconnected 2D carbon nanosheets establish a conductive framework for fast electron/ion transfer to improve sulfur utilization. As the result, when employing N-PCS1000 as the sulfur host, a discharge capacity as high as 1360 mAh/g


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can be achieved at 0.1 C. After 1000 cycles at 0.5 C, the cell still retains ~ 50% of its initial capacity, accounting for an ultralow fading rate of 0.05% per cycle. The successfully improved electrochemical performance indicates the importance of designing multifunctional carbon materials for high-performance lithium-sulfur batteries. Experimental Section Chemicals and reagents Formaldehyde (37% w/w, CH2O), resorcinol (99%), ethylenediamine (99%, C2H8N2) and nickel nitrate hexahydrate (98% ~ 102%, Ni(NO3)2 • 6H2O) were purchased from Alfa Aesar. Tetraethyl orthosilicate ( ≥ 99%) and ethylene glycol (99.8%) were obtained from Sigma-Aldrich. Hydrofluoric acid (40 ~ 51%, HF) was bought from J. T. Baker.

Synthesis of Ni(OH)2 Nanosheet Template The Ni(OH)2 nanosheet template was prepared through a modified microwave-assisted liquid phase growth.35 Typically, 9.3 g Ni(NO3)2•6H2O (0.032 mol) and 7.68 g (0.128 mol) were dissolved in a solvent consisting of 420 mL ethylene glycol and 60 mL deionized water (DI water) to form a transparent solution. The as prepared solution was transferred to a flask and treated under microwave irradiation at 720 W for 6 minutes. The resulting light green precipitation was centrifuged and washed with ethanol for 3 times. The Ni(OH)2 nanosheet template was obtained after freeze-drying for 12 hours.

Synthesis of RF-SiO2 coated Ni(OH)2 Nanosheet The nitrogen doped porous carbon nanosheets were synthesized through a silica-assisted resorcinol-formaldehyde (RF) coating strategy.12 In a typical synthesis, 200 mg Ni(OH)2 nanosheet



templates were dispersed in a mixed solution consisting of 60 mL ethanol and 140 mL DI water and sonicated for 5 minutes. Then, 200 mg resorcinol, 0.3 mL formaldehyde, 0.32 mL ethylene diamine (EDA) and 1 mL TEOS were added in sequence. The mixture was stirring for 16 hours at room temperature. The final product of brown precipitation was obtained by filtration and dried in a freeze-drier for 12 hours.

Synthesis of Nitrogen Doped Porous Carbon Nanosheet The RF-SiO2 coated Ni(OH)2 nanosheets were further carbonized at 900 oC for 2 hours under nitrogen atmosphere. Then, the nitrogen doped porous carbon nanosheets were obtained after removal of silica by using HF solution and marked as N-PCS1000. For other control samples, 0 mL, 0.2 mL and 2 mL TEOS were added during the synthesis and labeled as N-CS, N-PCS200 and N-PCS2000, respectively.

Synthesis of S/N-PCS Composite The S/PCS composites were prepared by a simple heat treatment. Generally, 40 mg PCS and 60 mg sulfur were mechanically mixed and then heat-treated at 155 oC in a Teflon-line autoclave for 12 hours.

Material Characterization Transmission electron microscopy (TEM) images were obtained by Philips CM200 UT (Field Emission Instruments, USA). Scanning electron microscopy (SEM) images were taken by FEI SEM Quanta 200F (Field Emission Instruments, USA). X-ray Diffraction (XRD) characterization was carried out by Rigaku Miniflex 600. The tube was operated at 40 kV accelerating voltage and


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15 mA current. The sulfur loading was conducted on TA instrument from 0-600 oC under nitrogen atmosphere. Surface area and pore size distribution were characterized using Micromeritics ASAP 2020 Plus. DFT methods are applied for pore size distribution analysis.

Electrochemical measurements The obtained S/PCS composite, Super P and PVDF were mixed together with a mass ratio of 7:2:1 and dispersed in N-methyl-2-pyrrolidone solvent to form electrode slurry. The slurry was cast onto aluminium foil using Dr. Blade. The sulfur loading on the electrode was maintained around 1 mg/cm2. 2032-type coin cells were assembled in an argon-filled glove box. Lithium metal foil was used as counter electrode and the polypropylene membrane (Celgard 2400) was used as the separator. The electrolyte consisted of 1 M lithium bistrifluoromenthanesulfonylimide (LiTFSI) in a mixed solvent of 1, 3-dioxolane and 1, 2-dimethoxyethane (DOL/DME, v: v=1:1) with 0.2 M lithium nitrate (LiNO3). The thickness of coating electrode in the cell assembly is about 60 µm. The electrolyte to sulfur ratio in coin cells is controlled as 20 µL/mg s. Electrochemical performance was tested on Landt Instruments CT2001A with a voltage window of 1.8-2.8 V.

Associated Content Supporting Information SEM images of N-CS, N-PCS200, N-PCS1000 and N-PCS2000; SEM images of N-CS/S and NPCS1000/S; XRD patterns of N-CS, N-PCS1000, N-CS/S and N-PCS1000/S; XPS survey of NPCS1000; STEM image of N-PCS1000/S after cycling; Coulombic efficiency of the thick sulfur cathodes



Acknowledgements This work was supported by a start-up grant from Washington State University. We appreciate Dr. Min-kyu Song’s group for the use of their argon-filled glove box. We thank Franceschi Microscopy & Image Center at Washington State University for TEM measurements.

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Assemble Carbon Pores into Carbon Sheets: Rational Design of

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