Enabling High-Areal-Capacity Lithium–Sulfur Batteries: Designing

May 9, 2017 - Xu , J.; Shui , J.; Wang , J.; Wang , M.; Liu , H.-K.; Dou , S. X.; Jeon , I.-Y.; Seo , J.-M.; Baek , J.-B.; Dai , L.Sulfur–graphene N...
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Enabling High-Areal-Capacity Lithium−Sulfur Batteries: Designing Anisotropic and LowTortuosity Porous Architectures Yiju Li,† Kun “Kelvin” Fu,† Chaoji Chen, Wei Luo, Tingting Gao, Shaomao Xu, Jiaqi Dai, Glenn Pastel, Yanbin Wang, Boyang Liu, Jianwei Song, Yanan Chen, Chunpeng Yang, and Liangbing Hu* Department of Materials Science and Engineering, University of Maryland College Park, College Park, Maryland 20742, United States S Supporting Information *

ABSTRACT: Lithium−sulfur (Li−S) batteries have attracted much attention due to their high theoretical energy density in comparison to conventional state-of-the-art lithium-ion batteries. However, low sulfur mass loading in the cathode results in low areal capacity and impedes the practical use of Li−S cells. Inspired by wood, a cathode architecture with natural, three-dimensionally (3D) aligned microchannels filled with reduced graphene oxide (RGO) were developed as an ideal structure for high sulfur mass loading. Compared with other carbon materials, the 3D porous carbon matrix has several advantages including low tortuosity, high electrical conductivity, and good structural stability, which make it an excellent 3D lightweight current collector. The Li−S battery assembled with the wood-based sulfur electrode can deliver a high areal capacity of 15.2 mAh cm−2 with a sulfur mass loading of 21.3 mg cm−2. This work provides a facile but effective strategy to develop 3D porous electrodes for Li−S batteries, which can also be applied to other cathode materials to achieve a high areal capacity with uncompromised rate and cycling performance. KEYWORDS: Li−S battery, low tortuosity, carbon matrix, polymer electrolyte, high areal capacity materials (e.g., carbon black).10 Especially, for high-massloading electrodes, active materials could be delaminated from current collectors and the high tortuosity of electrodes may hinder the ion transport and extend the ion diffusion pathway, leading to limited active material utilization. Moreover, the binder may block the electron transfer and bring in some “dead” sites, which cannot undergo electrochemical reactions.9 Consequently, it is imperative to engineer rational electrode structures for accommodating high sulfur mass loading to obtain high areal energy density without sacrificing electrochemical performance.17−22 Inspired by wood, a cathode architecture with natural, 3D aligned microchannels provides an ideal structure for high sulfur mass loading. Here, we developed a 3D aligned porous carbon matrix using carbonized wood with reduced graphene oxide (RGO) filled inside the microchannels (C-wood) for high sulfur mass loading (S@C-wood). RGO serves as an electronically conductive network to enhance the electron transport, and the oxygen functional groups on RGO nanosheets can

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echargeable Li-ion batteries (LIBs) have achieved great success in consumer electronics, electric vehicles, and stationary energy storage systems.1−5 Unfortunately, the limited improvement in well-developed LIBs can hardly sustain the increasing need for high energy density electrochemical cells. To enhance the energy density limitation of intercalation-compound-based LIBs, the Li−S batteries that undergo conversion reactions are becoming a potential alternative for next-generation energy storage systems. Sulfur holds a high capacity (1672 mAh g−1) as a cathode material, and in conjunction with a Li metal anode, Li−S batteries can deliver a much higher energy density (2600 Wh kg−1) to outperform most contemporary energy storage devices.6−10 However, although numerous nanostructured sulfur-based composite electrodes exhibited superior electrochemical performance,11−13 the mass loading of sulfur material needs to further increase in order to achieve a higher areal energy density and meet practical applications in large scale.14−16 Commonly, the conventional cathode in Li−S batteries is prepared through mechanically mixing elemental sulfur, binder, and conductive carbon together and using a doctor-blading method. The slurry-based electrode structure usually has poor contact between the nonconductive sulfur and the conductive © 2017 American Chemical Society

Received: February 19, 2017 Accepted: May 9, 2017 Published: May 9, 2017 4801

DOI: 10.1021/acsnano.7b01172 ACS Nano 2017, 11, 4801−4807

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hemicellulose, and cellulose was sectioned perpendicular to the growth direction. After carbonization and activation, the shape of the porous wood slice was preserved. RGO was filled into the microchannels of the wood carbon matrix by vacuum filtration and reduction. The resultant 3D conductive wood carbon framework with low-tortuosity and open porous microchannels enhances electrolyte permeation. The aligned porous microchannels filled with continuous RGO networks can accommodate a high sulfur mass loading without blocking pathways for ion transport. The facile and low-cost procedure to manufacture the carbon matrix is appealing for many other fields of energy storage and conversion, such as flow batteries and hydrogen evolution reactions, due to the porous structure with low tortuosity and anisotropy. The morphology of the as-prepared samples is characterized using scanning electron microscopy (SEM). The top surface SEM images revealed the highly porous features of the carbonized and activated wood (Figure S1a). The 3D anisotropic and well-aligned porous structure was perfectly retained even after carbonization and activation (porosity: ∼70%) (Figure 2a−c) with a typical thickness of about 800 μm (Figure S1b). Figure 2d shows the well-defined, aligned, and elongated microchannels along the growth direction, which originate from the cellulose fibers in the original wood. After high-temperature carbonization, the excellent electronic conductivity of the C-wood framework along the microchannel direction makes it an ideal 3D current collector. Figure 2e displays numerous nanopores that are homogeneously distributed on the wall of the microchannels in the bare wood carbon matrix. The uniform nanopores result from carbon dioxide (CO2) etching effects under high temperatures. Transmission electron microscopy (TEM) images reveal that the walls of the microchannels have abundant mesopores and micropores (Figure S2). Nitrogen (N2) adsorption−desorption tests were further conducted to investigate the porosities of the bare wood carbon and C-wood (Figure S3). The specific surface areas of the bare wood carbon and C-wood are 712 and 731 m2 g−1, respectively. The high specific surface areas indicate

chemically bond with the intermediate products of sulfur to locally trap them and prevent their migration to the anode.23 The C-wood matrix has an anisotropic structure and excellent antideformation along the aligned microchannel direction for stable and safe sulfur accommodation. The low-tortuosity and open microchannels can also allow better electrolyte permeation through the composite electrode. In addition, a design of a poly(ethylene oxide) (PEO) gel composite electrolyte was used to build full cells with the S@C-wood cathode and Li metal anode. With this design, the battery showed great electrochemical performance for such 3D porous electrodes. The electrode with a sulfur mass loading of 7.8 mg cm−2 showed a high areal capacity of 6.7 mAh cm−2 at a current density of 1.56 mA cm−2 and excellent cycling stability (90% retention after 50 cycles). When the sulfur mass loading is increased to 21.3 mg cm−2, the electrode can reach an even higher areal capacity of up to 15.2 mAh cm−2.

RESULTS AND DISCUSSION The S@C-wood composite electrode was prepared by several facile steps (Figure 1). Natural wood consisting of lignin,

Figure 1. Schematic illustration of the preparation process and structure of the S@C-wood composite electrode. Anisotropic natural wood is rich in aligned microchannels along the growth direction. After carbonization and activation, the long and lowtortuosity porous microchannels were perfectly retained. The 3D aligned porous carbon matrix filled with interconnected RGO networks is an ideal host for high sulfur mass loading.

Figure 2. Morphological characterization of the C-wood matrix. (a) Photograph of a natural wood block. (b) Photograph of a carbonized and activated natural wood slice that is cut perpendicular to the growth direction. (c) SEM image of the top surface and cross-section of the bare wood carbon matrix without RGO. (d, e) Enlarged SEM images of the microchannels, demonstrating numerous nanopores on the wall. (f, g) SEM images of the top surface and cross-section of the C-wood composite. (h) Enlarged SEM image of the interconnected RGO networks. 4802

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conformally accommodated on the surface of the RGO nanosheets without any agglomeration (Figure 3e). The elemental mappings by energy dispersive spectroscopy (EDS) show that sulfur is well dispersed in the RGO networks (Figure 3f). TEM elemental mappings further confirm the homogeneous sulfur distribution on a single RGO nanosheet with no bulky agglomerates (Figure S5). The oxygenic functional groups on GO nanosheets can immobilize sulfur species and maintain intimate touching of the conductive substrate with sulfur.23 The elemental mapping of oxygen suggests plenty of oxygenic functional groups distributed on the RGO nanosheets (Figure 3g), which can effectively prevent polysulfides from dissolving throughout the electrode. Figure S6a and b reveal the morphologies and elemental mappings of S@C-wood composite electrodes with higher sulfur loadings of 15.2 and 21.3 mg cm−2, respectively. We can see that sulfur is also uniformly accommodated in the interconnected RGO networks. Even at a high sulfur mass loading of 21.3 mg cm−2, there still exists void space in the aligned microchannels, which benefits the electrolyte penetration and tolerates sulfur volume expansion during the electrochemical reaction. For the bare wood carbon without being filled with RGO networks, the sulfur is merely dispersed on the surface inside the aligned microchannels (S@bare wood carbon) (Figure S7). The crystal structure of as-prepared bare wood carbon, Cwood, pure sulfur, and S@C-wood was revealed by X-ray diffraction (XRD) (Figure 3h). For bare wood carbon and Cwood, two broad diffraction peaks occur at ∼25° and ∼44°, which are assigned to the typical (002) and (100) plane reflection of carbon materials, respectively.25,26 Except for the diffraction peak located at ∼25°, the sharp and strong diffraction peaks observed in S@C-wood composite manifest the orthorhombic structure of sulfur (JCPDS card no. 080247).27 Raman spectroscopy was conducted to further study the structural characteristics of the samples (Figure 3i). The peaks at ∼1350 and ∼1580 cm−1 correspond to the D and G band, respectively. The intensity ratio of ID/IG represents the degree of disorder of the carbon materials.28 The relatively low ID/IG ratio indicates the amorphous nature of the bare wood carbon matrix surface, which is caused by CO2 activation. The resultant hierarchically nanoporous walls along the microchannels contribute to trapping polysulfides and alleviating “shuttle effects” to some extent. After sulfur infiltration into the C-wood composite carbon matrix, the characteristic peak intensity of sulfur becomes weak, indicating good accommodation and confinement of sulfur in the 3D porous C-wood matrix.29 To evaluate the electrochemical performance of the asprepared electrodes, a Li−S cell was assembled using a S@Cwood electrode as the cathode, lithium foil as the anode, and a PEO gel composite membrane as both the gel electrolyte and separator (Figure 4a). The brief preparation process of the PEO gel composite membrane is illustrated in Figure S8. The PEO gel composite was uniformly covered on a piece of Celgard 2400 to form a free-standing membrane. The PEO gel composite membrane serves as a barrier that can effectively hinder the polysulfides from releasing from the porous C-wood composite carbon matrix and alleviate the attack of polysulfides at the anode.30 By contrast, a S@bare wood carbon electrode is also used as a cathode in the assembled Li−S cell. The typical sulfur mass loading of the S@C-wood and S@ bare wood carbon electrodes is about 7.8 mg cm−2, which is much higher than most slurry-based cathodes in normal Li−S

the highly porous properties of the bare wood carbon and Cwood. In the hierarchically porous carbon materials, micropores can immobilize and trap the dissolved polysulfides and the mesopores can enhance the sulfur encapsulation and improve the electrolyte penetration.10,24 After vacuum filtration, the RGO nanosheets were absorbed into the microchannels (Figure 2f). The SEM image of the cross-section distinctly exhibits that the parallel channels were filled with continuous RGO networks (Figure 2g). The interconnected RGO nanosheets with appropriate cavities can facilitate both electron transport and electrolyte immersion (Figure 2h). Figure S4 highlights the joint where the RGO nanosheets tightly bond with the wall of the microchannels, which enables fast electron transport between the RGO networks and the bare monolithic carbon matrix. The C-wood composite carbon matrix has a high porosity and a low tortuosity, which contribute to the infiltration of sulfur into the open microchannels. Figure 3a shows a

Figure 3. Characterizations of the S@C-wood composite electrode. (a) Schematic illustration of sulfur infiltration into the microchannels of the C-wood composite in an argon atmosphere. (b, c) Photographs showing molten sulfur adsorbed into the C-wood composite at 155 °C. (d, e) SEM images of the S@C-wood composite. (f, g) Elemental mappings of sulfur and oxygen in the S@C-wood composite. (h) XRD patterns of the bare wood carbon, C-wood, pure S, and S@C-wood composite. (i) Raman spectra of the pure S, bare wood carbon, C-wood, and S@C-wood composite.

schematic illustration to demonstrate how sulfur infiltrates into the microchannels of the C-wood matrix during the heating treatment. Sulfur powders were evenly distributed on the surface of the C-wood composite, which was completely absorbed into the carbon matrix after melting at 155 °C (Figure 3b and c). The obtained areal sulfur mass loading is as high as 7.8 mg cm−2. After infilrating with sulfur, the RGO networks inside the porous microchannels retain the original interconnected morphology and tightly bond with the 3D bare wood carbon skeleton (Figure 3d). Moreover, sulfur is 4803

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Figure 4. Electrochemical performance of the S@C-wood composite electrodes. (a) Schematic illustration of the Li−S cell with the PEO gel composite electrolyte (with a representative sulfur mass loading of 7.8 mg cm−2). (b, c) CVs and GCD curves of the S@C-wood composite electrode at different cycles. (d, e) Galvanostatic charge−discharge curves and rate performance of the S@C-wood composite electrode at various current densities.

back to 0.156 mA cm−2, the specific capacity recovers to 1158 mAh g−1 (9.0 mAh cm−2). These results demonstrate the outstanding rate performance of the S@C-wood composite electrode. Excellent electrolyte absorption by electrode materials, especially for high sulfur mass loading electrodes, is beneficial for retaining soluble polysulfides within the electrode matrices and ensuring stable electrochemical reactions.34−36 We conducted loading tests to demonstrate the superior antideformation and impressive electrolyte accommodation ability of our 3D porous C-wood matrix. RGO sponge (length × width × thickness: 2.0 × 2.2 × 1.2 cm) and C-wood samples (length × width × thickness: 2.4 × 1.8 × 1.2 cm) were both fully soaked with electrolyte. As shown in Figure 5a, the structure of the C-wood matrix is nondeformable and the electrolyte is well contained inside the porous carbon host under a pressure of 22.7 kPa. By contrast, the RGO sponge itself is greatly compressed and the electrolyte overflows from the RGO sponge. The excellent antideformation along with the direction of the microchannels and favorable electrolyte confinement make the S@C-wood composite electrode promising for applications in the field of planar batteries. Here, we demonstrated a large piece of free-standing S@Cwood composite electrode, which is used for assembling a planar battery (Figure 5b,c). The GCD curves of the S@Cwood composite electrode with different sulfur mass loadings at 1.56 mA cm−2 are displayed in Figure 5d. Two well-defined discharge and one charge plateau agree with the CV curves in Figure 4b. The S@C-wood composite electrodes with sulfur

batteries. Figure 4b shows the cyclic voltammograms (CVs) of the S@C-wood composite electrode at the first, second, and fifth cycle at a scan rate of 0.02 mV s−1 within a voltage window of 1.0−3.5 V. Two typical cathodic peaks at 2.3 and 1.9 V correspond to the reduction of elemental sulfur (S8) to longchain soluble lithium polysulfides (Li2Sx) (4 < x < 8) and the formation of insoluble short-chain Li2S2/Li2S, respectively. The anodic peak at 2.4−2.5 V is assigned to the lithium polysulfides/sulfides converting into elemental sulfur.31 In the initial two cycles, the area of the CVs of the S@C-wood composite electrode slightly increases, indicating an activation process, which is attributed to the reaccommodation of active material to electrochemically active sites during the first cycle.32 The galvanostatic charge−discharge (GCD) curves of the S@C-wood composite electrode at various cycles are shown in Figure 4c. Two discharge plateaus and one charge plateau agree well with the redox peaks in the CVs.33 After several cycles, the potential difference between the charge and discharge plateaus decreases, thereby manifesting the improved electrochemical reversibility. Figure 4d exhibits the GCD curves of the S@Cwood composite electrode at various current densities from 0.156 to 7.8 mA cm−2. No apparent overcharge occurred during the charge state, which suggests that the shuttle effect of sulfur polysulfides was effectively prevented. The reversible discharge capacity can reach up to 1274 mAh g−1, corresponding to 9.9 mAh cm−2 at 0.156 mA cm−2. With an increase in current density, the specific capacity gradually decreases to 1138 mAh g−1 (8.8 mAh cm−2), 992 (7.7), 857 (6.7), 726 (5.7), and 509 (4.0), respectively (Figure 4e). Once the current density goes 4804

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confine the soluble polysulfides, ensuring stable cycling performance. We compared the areal capacity of our S@Cwood composite electrode with other recently reported sulfur electrodes.17,38−43 Our S@C-wood composite electrode with a high sulfur mass loading of 21.3 mg cm−2 possesses an areal capacity of 15.2 mAh cm−2, which is among the highest compared with other reported Li−S batteries (Figure 5f). Note that the sulfur mass loading can still be increased in the 3D Cwood matrix; we will further improve the areal energy density by optimizing the electrode structure in the future.

CONCLUSION Inspired by wood, we developed a 3D aligned porous carbon matrix using carbonized wood with RGO filled inside the lumens and voids. The porous matrix with low tortuosity, high electric conductivity, and good structural stability is an ideal 3D current collector for a high sulfur mass loading. The elongated and open microchannels can allow better electrolyte permeation through the composite electrode. The C-wood matrix also demonstrates excellent antideformation along the direction of the microchannels and favorable electrolyte confinement capacity, which contributes to immobilizing the electrolyte and impeding the migration of polysulfides. The assembled Li− S battery combined with a PEO gel composite electrolyte can deliver a high areal capacity of 15.2 mAh cm−2 with a sulfur mass loading of 21.3 mg cm−2. This work presents a simple but effective wood-inspired strategy to develop composite electrodes for Li−S batteries, which can also be applied in many other energy storage and conversion applications.

Figure 5. Absorption ability and antideformation demonstration and electrochemical performance of S@C-wood composite electrodes with high sulfur mass loadings. (a) Photographs showing the press test of the S@RGO sponge (length × width × thickness: 2.0 × 2.2 × 1.2 cm) and S@C-wood composite (length × width × thickness: 2.4 × 1.8 × 1.2 cm) soaked with electrolyte. (b) Photographs of a large piece of S@C-wood composite (length × width × thickness: 2.0 × 1.8 × 0.1 cm). (c) Lighting up a lightemitting diode using the assembled planar cell. The use of the logo is from University of Maryland, College Park. (d) GCD curves of the S@C-wood composite electrode with different sulfur mass loadings. (e) Cycling performance of the S@C-wood composite electrode with different sulfur mass loadings. The achieved areal capacities of our S@C-wood composite electrodes are much higher than those of normal Li−S cathodes. (f) Comparison of our areal capacity with other reported results.

EXPERIMENTAL SECTION Preparation of Wood Carbon. In brief, a piece of natural basswood slice was cut perpendicular to the growth direction. The wood slice was precarbonized under 260 °C for 6 h and subsequently carbonized at 1000 °C for 6 h in an argon atmosphere.44 Next, the sample was activated in a CO2 atmosphere at 800 °C to get the final bare wood carbon. Before activation, the bare wood carbon is about 27.0 mg cm−2 with a thickness of 800 μm (length × width × thickness: 1.0 × 1.0 × 0.08 cm). After activation using CO2, the weight of the bare wood carbon is about 18.3 mg cm−2. The percent of weight loss is ∼32.2%, which indicates the bare wood carbon is highly activated. Preparation of Wood Carbon/RGO Composite. The graphene oxide (GO) was first synthesized using an improved Hummers method.25 A 160 mg amount of prepared GO powder was dispersed into 10 mL of deionized water, and the mixture was then probesonicated for 1 h to get the GO solution. The as-prepared bare wood carbon (length × width × thickness: 1.0 × 1.0 × 0.08 cm) was placed on a filter funnel, and the obtained homogeneous GO solution was added onto the surface of a bare wood carbon slice. The GO solution was absorbed into the bare wood carbon matrix along with the microchannels after vacuum filtration using a turbo pump. The bare wood carbon filled with GO solution was then treated by freeze-drying for 48 h. After freeze-drying, the wood carbon/GO composite was transferred into a furnace tube and heated at 600 °C for 1 h to obtain the wood carbon/RGO composite, which is denoted as C-wood. The mass densities of the RGO and bare wood carbon are about 1.6 and 18.3 mg cm−2, respectively. Preparation of S@C-Wood Composite Electrode. The S@Cwood composite electrode was synthesized using a “melt and diffusion” method. Typically, a layer of sulfur powder (Sigma-Aldrich) was uniformly covered on the top surface of a C-wood composite and then sealed in a Teflon-lined stainless steel reaction vessel at 155 °C for 12 h under an argon atmosphere. After the heating treatment, the free-standing S@C-wood composite electrode was obtained. The bare wood carbon without being filled with RGO and directly composited with sulfur is denoted as S@bare wood carbon.

mass loadings of 7.8, 15.2, and 21.3 mg cm−2 can deliver a high specific areal capacity of 6.7, 12.6, and 15.2 mAh cm−2, respectively. We further explored the cycling performance of the S@Cwood composite electrodes with various sulfur mass loadings at a current density of 1.56 mA cm−2 (Figure 5e and Figure S9). The S@C-wood composite electrode with a sulfur mass loading of 7.8 mg cm−2 exhibits a superior capacity retention of 90.0% after 50 cycles. As a comparison, the S@bare wood carbon electrode without being filled with RGO networks exhibits a relatively poor cycling stability (only 54.1% after 50 cycles) (Figure S10). It indicates that the interconnected long-range RGO networks serve as important reservoirs to accommodate a high sulfur mass loading, except for the porous walls on the parallel microchannels. The RGO networks filled in the elongated microchannels contribute to firmly immobilizing the electrolyte inside the C-wood composite carbon matrix, which can effectively impede the migration of polysulfides to the anode and alleviate shuttling effects.37 For the high sulfur mass loading of 21.3 mg cm−2, the specific areal capacitance can still reach up to 11.2 mAh cm−2 after 50 cycles. These results further suggest that the structure of the 3D porous C-wood composite matrix can effectively prevent electrolyte leakage and 4805

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ACS Nano Preparation of Poly(ethylene oxide) Gel Composite Electrolyte. Niobium (Nb) and calcium (Ca) codoped Li7La3Zr2O12 powders, as the ceramic fillers, were first prepared using the conventional solid-state-reaction method as reported by our previous work.45 Then, 400 mg of ceramic fillers, 250 mg of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and 400 mg of PEO (molecular weight: ∼600 K) were dispersed into 10 mL of acetonitrile (ACN). The mixture was violently stirred at 60 °C to obtain a homogeneous solution. The above solution was uniformly covered onto the surface of Celgard 2400 and dried in a vacuum oven at room temperature. The obtained PEO gel composite membrane was then dipped in a 1,3-dioxolane and 1,2-dimethoxyethane (DOL:DME = 1:1 in volume) electrolyte with 1 M LiTFSI for full absorption. Materials Characterization. Morphologies of the samples were observed by a Hitachi SU-70 field emission scanning electron microscope equipped with an energy-dispersive X-ray analyzer and JEOL 2100 LaB6 TEM. The material crystalline phase was investigated by an X-ray diffractometer (Rigaku TTR III) with Cu Kα radiation (λ = 0.151 42 nm). Nitrogen adsorption−desorption tests were conducted using a Micromeritics ASAP 2020 physisorption analyzer at 77 K. The specific surface area was calculated according to the Brunauer−Emmett−Teller method using the adsorption branch of the nitrogen isotherm. The chemical bond information on as-prepared samples was revealed by a Raman spectrometer with a He−Ne laser wavelength of 633 nm (Jobin-Yvon HR800). Electrochemical Measurements. The free-standing S@C-wood composite was directly used as the working electrode. The C-wood matrix with a thickness of 800 μm has an areal density of 19.9 mg cm−2, which can serve as an ideal lightweight 3D current collector. The areal mass loading of sulfur (7.8, 15.2, and 21.3 mg cm−2) was simply controlled by the original addition before melting. For the high sulfur mass loading (21.3 mg cm−2), the sulfur content exceeds 50 wt % of the whole electrode (51.7 wt %), even when taking the 3D current collector of the bare wood carbon into account. The electrolyte volume is only 7.5 μL per 1 mg of sulfur. The lithium−sulfur batteries combining a liquid electrolyte and a gel composite electrolyte were assembled in 2032 coin cells. The liquid electrolyte was 1 M LiTFSI/ DOL + DME (1:1 by volume). The PEO gel composite covered Celgard 2400 as both a separator and gel composite electrolyte. The cyclic voltammetry was performed using a computer-controlled BioLogic potentiostat (VMP3/Z). The galvanostatic charge/discharge measurements were conducted in a voltage range of 1−3.5 V on a LAND battery tester. The specific capacity mentioned in this work was calculated based on the mass of sulfur.

ACKNOWLEDGMENTS This work was supported as part of the Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under award number DESC0001160. Y.L. and T.G. acknowledge the China Scholarship Council (CSC) for financial support. REFERENCES (1) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-ion Batteries: A Review. Energy Environ. Sci. 2011, 4, 3243−3262. (2) Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928−935. (3) Amine, K.; Kanno, R.; Tzeng, Y. Rechargeable Lithium Batteries and Beyond: Progress, Challenges, and Future Directions. MRS Bull. 2014, 39, 395−401. (4) Manthiram, A. Materials Challenges and Opportunities of Lithium Ion Batteries. J. Phys. Chem. Lett. 2011, 2, 176−184. (5) Goodenough, J. B.; Park, K.-S. The Li-Ion Rechargeable Battery: A Perspective. J. Am. Chem. Soc. 2013, 135, 1167−1176. (6) Zhang, J.; Shi, Y.; Ding, Y.; Zhang, W.; Yu, G. In Situ Reactive Synthesis of Polypyrrole-MnO2 Coaxial Nanotubes as Sulfur Hosts for High-Performance Lithium−Sulfur Battery. Nano Lett. 2016, 16, 7276−7281. (7) Peng, H. J.; Zhang, G.; Chen, X.; Zhang, Z. W.; Xu, W. T.; Huang, J. Q.; Zhang, Q. Enhanced Electrochemical Kinetics on Conductive Polar Mediators for Lithium−Sulfur Batteries. Angew. Chem. 2016, 128, 13184−13189. (8) Seh, Z. W.; Sun, Y.; Zhang, Q.; Cui, Y. Designing high-energy lithium−sulfur batteries. Chem. Soc. Rev. 2016, 45, 5605−5634. (9) Bresser, D.; Passerini, S.; Scrosati, B. Recent Progress and Remaining Challenges in Sulfur-Based Lithium Secondary Batteries-A Review. Chem. Commun. 2013, 49, 10545−10562. (10) Manthiram, A.; Fu, Y.; Chung, S.-H.; Zu, C.; Su, Y.-S. Rechargeable Lithium−Sulfur Batteries. Chem. Rev. 2014, 114, 11751− 11787. (11) Zheng, G.; Zhang, Q.; Cha, J. J.; Yang, Y.; Li, W.; Seh, Z. W.; Cui, Y. Amphiphilic surface modification of hollow carbon nanofibers for improved cycle life of lithium sulfur batteries. Nano Lett. 2013, 13, 1265−1270. (12) Li, N.; Zheng, M.; Lu, H.; Hu, Z.; Shen, C.; Chang, X.; Ji, G.; Cao, J.; Shi, Y. High-rate lithium−sulfur batteries promoted by reduced graphene oxide coating. Chem. Commun. 2012, 48, 4106− 4108. (13) Moon, S.; Jung, Y. H.; Jung, W. K.; Jung, D. S.; Choi, J. W.; Kim, D. K. Encapsulated monoclinic sulfur for stable cycling of Li−S rechargeable batteries. Adv. Mater. 2013, 25, 6547−6553. (14) Chang, C. H.; Chung, S. H.; Manthiram, A. Effective Stabilization of a High-loading Sulfur Cathode and a Lithium-Metal Anode in Li-S Batteries Utilizing SWCNT−Modulated Separators. Small 2016, 12, 174−179. (15) Elazari, R.; Salitra, G.; Garsuch, A.; Panchenko, A.; Aurbach, D. Sulfur-Impregnated Activated Carbon Fiber Cloth as a Binder-Free Cathode for Rechargeable Li-S batteries. Adv. Mater. 2011, 23, 5641− 5644. (16) Qie, L.; Manthiram, A. Uniform Li2S Precipitation on N, OCodoped Porous Hollow Carbon Fibers for High-Energy-Density Lithium−Sulfur Batteries with Superior Stability. Chem. Commun. 2016, 52, 10964−10967. (17) Li, Z.; Zhang, J. T.; Chen, Y. M.; Li, J.; Lou, X. W. D. Pie-like Electrode Design for High-Energy Density Lithium-Sulfur Batteries. Nat. Commun. 2015, 6, 8850. (18) Song, J.; Xu, T.; Gordin, M. L.; Zhu, P.; Lv, D.; Jiang, Y. B.; Chen, Y.; Duan, Y.; Wang, D. Nitrogen-Doped Mesoporous Carbon Promoted Chemical Adsorption of Sulfur and Fabrication of HighAreal-Capacity Sulfur Cathode with Exceptional Cycling Stability for Lithium-Sulfur Batteries. Adv. Funct. Mater. 2014, 24, 1243−1250.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01172. SEM and TEM images; N2 adsorption−desorption isotherm curves; photographs of the fabrication process of PEO gel composite membrane; cycling performance data (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail (L. Hu): [email protected]. ORCID

Chunpeng Yang: 0000-0001-7075-3356 Liangbing Hu: 0000-0002-9456-9315 Author Contributions †

Y. Li and K.“Kelvin” Fu contributed equally to this work.

Notes

The authors declare no competing financial interest. 4806

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ACS Nano

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DOI: 10.1021/acsnano.7b01172 ACS Nano 2017, 11, 4801−4807