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Encapsulating MWNTs into Hollow Porous Carbon Nanotubes: A Tube-in-Tube Carbon Nanostructure for High-Performance Lithium-Sulfur Batteries Yi Zhao, Wangliang Wu, Jiaxin Li, Zhichuan Xu,* and Lunhui Guan* To meet the increasing demand of high capacity energy storage systems, recently there has been intensive research on lithiumsulfur (Li-S) batteries due to their high theoretical capacity of 1672 mAh g−1. The overall electrochemical reaction in a typical Li-S battery could be denoted as S8 + 16Li ↔ 8Li2S with an average voltage of ∼2.15 V, providing a theoretical energy density of 2500 Wh kg−1, which is 3–5 times higher than that of commercial lithium ion batteries. In addition, sulfur is inexpensive, environmental benign, and abundant on earth, making it one of the most promising candidates for next-generation high energy batteries.[1] However, hampered by several technical challenges, the commercial applications of Li-S batteries are still far away.[2] The low electrical conductivity of both sulfur (5 × 10−30 S cm−1) and final discharge product (Li2S) limit the rate capabilities of batteries. The intermediate lithium polysulfides can be dissolved into electrolyte and then shuttle between cathode and anode. It forms a deposit of solid Li2S2/Li2S layer on electrode surface and so-called shuttle effect results in the low capacity and poor energy efficiency. Moreover, the difference in densities of sulfur (2.03 g cm−3) and Li2S (1.66 g cm−3) causes a large volumetric change (∼80%) of active material during discharge/charge process, which may gradually destroy the structural stability and give a rapid capacity fading. To overcome these problems, many strategies have been developed to impregnate sulfur into porous carbons or polymers to prevent the dissolution of intermediates while improve the conductivity of electrodes.[3] Among them, synthesizing various porous carbons for encapsulating sulfur has been popular.[4] Dr. Y. Zhao, W. L. Wu, J. X. Li, Prof. L. H. Guan State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter Chinese Academy of Sciences YangQiao West Road 155#, Fuzhou 350002, P.R. China E-mail: [email protected] Prof. L. H. Guan Key laboratory of Design and Assembly of Functional Nanostructures Chinese Academy of Sciences Fuzhou, Fujian 350002, China Dr. Y. Zhao, Prof. Z. C. Xu School of Materials Science and Engineering Nanyang Technological University Singapore 639798 E-mail: [email protected] Prof. Z. C. Xu Energy Research Institute @ NTU Nanyang Technological University 50 Nanyang Drive, Singapore 639798
DOI: 10.1002/adma.201401191
Adv. Mater. 2014, DOI: 10.1002/adma.201401191
Porous carbons as host materials offer a high specific surface area for electron transportation between sulfur and the host. Its porous structure and high volume also benefit the sulfur adoption and suppress the dissolution of polysulfides into electrolyte. As a result, the high capacity and good cycling retention can be achieved. However, the amorphous state and sp3-hybridized C–C bonding in these porous carbons often exhibit poor electrical conductance, which limits the electron transfer and thus restricts the rate performance to a certain extent.[5] The efforts to use carbon nanomaterials like carbon nanotubes (CNTs) and graphene to enhance the overall conductivity of electrodes have been made to overcome this problem.[4a,6] However, the relative low specific surface area of CNTs (typically < 200 m2 g−1) limits the accommodation of sulfur and the poor porosity of CNTS and graphene cannot prevent the direct contact between sulfur and electrolyte, leading to inevitable dissolution of lithium polysulfide.[7] Thus, it is necessary to design hybrid carbon materials that combines highly conductive carbon nanomaterials with porous carbons to achieve high-performance sulfur-carbon electrode.[8] For example, Xin et al. synthesized a hybrid carbon with microporous carbon layers on MWNTs to impregnate sulfur, showing good cycling stability and excellent rate performance. However, one problem for such S-hybrid carbons composites is the low sulfur content (40–60 wt%). To improve the energy density of Li-S batteries, sulfur confined within these hybrid carbons with higher loading ratio is desired. In this article, we report a novel tube-in-tube structured carbon nanomaterial (TTCN) as the host for sulfur cathode. MWNTs were encapsulated into hollow porous carbon nanotubes through a step-by-step strategy. This unique structure can enhance the electrical conductivity, hamper the dissolution of lithium polysulfide, and provide large pore volume for sulfur impregnation. As a cathode material for Li-S batteries, the obtained S-TTCN composite with 71 wt% sulfur content delivered high reversible capacity, good cycling performance as well as excellent rate capabilities. At a current density of 500 mA g−1, it still remained a high discharge capacity of 918 mAh g−1 (based on sulfur) or 652 mAh g−1 (based on S-TTCN composite) after 50 cycles. Figure 1 shows the schematic synthesis of the S-TTCN composite. Firstly, acid-treated MWNTs were coated with solid SiO2 and porous SiO2 layers via a modified stÖber method.[9] As the porogen agent and carbon precursor, an organosilicon compound (octadecyltrimethoxysilane, C18TMS) was trapped within the porous SiO2 shells (Figure S1). After chemical treatment and high temperature calcinations in Ar atmosphere (see experimental section for details), the organic moiety of C18TMS was converted into carbon.[10] Then, by etching the SiO2 layer with NaOH solution, the tube-in-tube carbon nanostructure (TTCN)
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Figure 1. Schematic illustration for the formation of S-TTCN composite: (1) Uniform coating a solid SiO2 layer and a porous SiO2 layer embedded with C18TMS molecules on MWNTs; (2) formation of porous carbon nanotube by carbonization of C18TMS; (3) etching SiO2 layers to obtain tube-in-tube carbon nanostructure (TTCN) with MWNTs encapsulated within hollow porous carbon nanotube; (4) sulfur infused into TTCN to fabricate S-TTCN composite.
with MWNTs confined within hollow porous carbon nanotubes were obtained. Finally, S-TTCN composite was synthesized by a simple melt infiltration method.[8a] The structure and morphology of the TTCN were firstly characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 2a shows the typical SEM image of commercial MWNTs with diameters around 20–50 nm and length up to several micrometers. As shown in Figure 2b, the obtained TTCN kept the similar morphology to MWNTs with enlarged diameters up to 80–140 nm, indicating the good confinement of MWNTs inside carbon layers. The high magnification SEM image (Figure S2a) and TEM image (Figure 2c-d) show that MWNTs were encapsulated within hollow carbon shells with large internal void space. From the high magnification TEM images (Figure 2d and S2b), the thickness of the porous carbon layer was estimated to be around 10–13 nm. The internal void space and the thickness of carbon layer can be readily controlled by tuning the amount of SiO2 and C18TMS, which is simpler than other protocols.[11] The HRTEM image (Figure 2e) clearly shows the porous structure of carbon layer and the interlayer spacing of 0.34 nm for MWNTs. As a carbon host to encapsulate sulfur, the large void space of TTCN can accommodate high loading ratio of sulfur to improve the energy density of Li-S batteries. After sulfur encapsulation, the obtained S-TTCN still remained the original morphology with no sulfur particles observed outside TTCN (Figure 2f and Figure S3), indicating that sulfur was fully diffused into the pore structure of the TTCN. To further reveal the sulfur distribution within TTCN, the element mappings of carbon and sulfur were performed in Figure 2h-i, demonstrating that sulfur was homogeneously distributed within the TTCN host. To further investigate the structure of TTCN and S-TTCN, nitrogen adsorption—desorption isotherms and pore size distribution curves are shown in Figure 3a and 3b. The TTCN delivered a large surface area of 822.8 m2 g−1 and a total pore volume of 1.77 m3 g−1, which were higher than the values for other hybrid carbons.[8] It can be seen from Figure 3b that TTCN possessed micropores at 1.8 nm, mesopores at 3.6 nm,
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and a broad pore distribution from 20 to 60 nm, which were consistent with the TEM observation. These micropores and mesopores in the outer carbon layers could effectively hamper the dissolution of sulfur for long cycle retention.[12] After sulfur impregnation, surface area and pore volume of the obtained S-TTCN composite decreased to 5.6 m2 g−1 and 0.19 m3 g−1, respectively. Only a small mesoporous peak around 20 nm (the inset in Figure 3b) was observed in S-TTCN composite. Thus, we can deduce that the porous carbon layers in TTCN were fully filled with sulfur, while some void space was still remained between MWNTs and carbon layers, which would benefit the accommodation of the volume change during cycles. The thermogravimetric analysis (TGA) was performed from 30–700 °C at a heating rate of 10 °C min−1 in N2 to determine the weight content of sulfur. Seen from Figure 3c, the S-TTCN composite has a high loading ratio of 71 wt%. Meanwhile, the high sulfur evaporation temperature (200–400 °C) indicated the strong absorption between sulfur and TTCN host.[8b] Figure 3d shows the XRD patterns of the TTCN and S-TTCN composites. The TTCN exhibited a diffraction peak near 26o, which was assigned to the MWNTs. While some crystalline sulfur peaks with orthorhombic structure (JCPDS 08–0247) can be observed from S-TTCN composite. Based on the SEM and TEM characterization of S-TTCN in Figure 2, no sulfur particles were found outside TTCN, indicating that sulfur confined within the internal void space of TTCN was in crystalline state. The electrochemical performance of the S-TTCN composite, as a cathode in Li-S battery, was shown in Figure 4. The working electrode was consisted of 80 wt% S-TTCN, 10 wt% KB and 10 wt% PVDF. Figure 4a shows typical first discharge/ charge profiles of S-TTCN electrode at 500 mA g−1 between 1.95 V and 2.7 V. There were three obvious reaction plateaus in the first discharge process.[13] The first plateau at 2.32 V was attributed to a solid-liquid two phase reaction from element sulfur to Li2S8. The second sloped plateau from 2.3 to 2.1 V was due to a liquid-liquid single phase reduction from long chain Li2S8 to short-order lithium polysulfides (Li2Sn, 4 ≤ n < 8). The third long plateau at 2.1 V was corresponded to a liquid-solid
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Adv. Mater. 2014, DOI: 10.1002/adma.201401191
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COMMUNICATION Figure 2. (a) SEM image of the MWNTs. (b) SEM, (c-d) TEM, and (e) high-resolution TEM images of the TTCN, showing the tube-in-tube carbon nanostructure with large internal void space. Insets in (a) and (b) are the diameter distributions for MWNTs and TTCN, respectively. (f) TEM image, (g) Scanning transmission electron microscopy (STEM) image and the corresponding element mapping images of (h) carbon and (i) sulfur of the S-TTCN composite, indicating the homogeneous distribution of sulfur within TTCN host.
two phase reaction from soluble Li2Sn to insoluble Li2S2 and Li2S, which contributed the major discharge capacity of the S-TTCN electrode. During the first charge process, the long oxidation plateau from 2.2 to 2.41 V was the reverse reaction, corresponding to the formation of Li2Sn (S > 2) and the final oxidation to S8. The first discharge and charge curves delivered specific capacities of 1274 and 1264 mAh g−1, corresponding to a columbic efficiency of ∼100%. Figure 4b shows the cycling performance of the S-TTCN electrode at a low current density of 500 mA g−1. As can be seen, the S-TTCN electrode exhibited excellent cycling stability and a high discharge capacity of 918 mAh g−1 was remained after 50 cycles. The discharge/charge columbic efficiency kept a high value around 98% during these cycles. Based on the overal mass of S-TTCN, this composite delivered a high capacity of 652 mAh g−1 after 50 cycles, which was higher than the values for sulfur combined with microporous and mesoporous carbons,[4c,4f ] hollow carbon spheres,[14] carbon nanofibers/nanotubes,[7b,15] graphene,[16] as well as hybrid carbons.[8a–c,17] Such good cycling performance of the
Adv. Mater. 2014, DOI: 10.1002/adma.201401191
S-TTCN electrode can be mainly attributed to the porous carbon layers, which effectively inhibit the dissolution of lithium polysulfide during cycles. The rate performance of the S-TTCN electrode was investigated as well. Figure 4c shows the discharge/charge curves under various current densities from 0.5 A g−1 to 6 A g−1. These profiles kept similar shapes with low overpotentials observed at different rates. Even at high rates, these curves still exhibited the typical reaction plateaus and delivered high capacities. Figure 4d demonstrates the excellent rate performance of the S-TTCN electrode. The discharge capacities were stabilized around 800, 750, 650, and 550 mAh g−1 when cycled at 1, 2, 4, and 6 A g−1, respectively. As the current density was reduced back to 500 mA g−1, the discharge capacity returned to 850 mAh g−1, indicating the good stability of the S-TTCN electrode during various rates. The long-term cycling stability of the S-TTCN electrode was also tested at a high rate of 2 A g−1 after being activated at 0.5 A g−1 in the initial two cycles. It can be seen from Figure 4e that these discharge/charge profiles
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Figure 3. (a) Nitrogen adsorption—desorption isotherms, and (b) pore size distribution curves of TTCN and S-TTCN composites. (c) TGA curve of S-TTCN. (d) XRD patterns of TTCN and S-TTCN composites. Insets in (b) show the magnified pore size distributions for TTCN and S-TTCN, respectively.
presented stable voltage plateaus and small capacity loss during cycles. Figure 4f demonstrates the high specific capacity as well as good cycling retention at 2 A g−1. After 200 cycles, the S-TTCN electrode still remained a high capacity of 647 mAh g−1 with a low decay rate of 0.089% per cycle. These results indicate that the design of such a TTCN structure indeed improved the performance of Li-S batteries. For comparison, MWNTs@S composite with a loading ratio of 74 wt% (Figure S4) was also synthesized. The XRD pattern of MWNTs@S (Figure S5) exhibited sharp diffraction peaks of sulfur, indicating the large particle size of sulfur outside MWNTs. As a cathode for Li-S batteries, MWNTs@S electrode showed poor capacity retention at 500 mA g−1. Seen from Figure S6, the MWNTs@S electrode suffered from rapid capacity fading in the initial cycles and only remained a discharge capacity of 660 mAh g−1 after 50 cycles. The S-TTCN exhibited better rate capabilities than MWNTs@S composite (Figure S7). We tentatively attributed the outstanding electrochemical performance of S-TTCN composite to the following reasons: Firstly, the one dimensional MWNTs and porous carbon layers offered superior electrical conductivity to facilitate fast electronic/ionic transport and enhance reaction kinetics of sulfur, giving good rate capabilities. Secondly, the carbon layers with micropores and mesopores can effectively prevent lithium polysulfide from dissolving into electrolyte, resulting in stable cycling retention and high efficiency. Finally, the large pore volume in TTCN can accommodate more sulfur to improve the energy density of batteries. The remained void space after sulfur impregnation as well as the robust TTCN framework could buffer the strain generated from the volumetric changes of sulfur during cycles and therefore keep the structure stable.
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In summary, a novel tube-in-tube carbon nanostructure with MWNTs confined within hollow porous carbon nanotubes was synthesized and used to accommodate sulfur. Due to the unique architecture of TTCN with good electrical conductivity, large pore volume and porous carbon layers, the obtained S-TTCN cathode exhibited outstanding electrochemical performance. It remained high reversible capacities of 918 mAh g−1 at 500 mA g−1 after 50 cycles and 647 mAh g−1 at 2 A g−1 after 200 cycles. Even at a high rate of 6 A g−1, it still delivered a capacity of 550 mAh g−1. This work provides a general strategy to synthesize superior hybrid carbons with large void space, which is crucial to improve the energy density and electrochemical performance of Li-S batteries. With proper chemistry, the approach may be extended to synthesize tube-in-tube structured materials with other carbon nanomaterials such as SWNTs, carbon fibers, and graphene for high performance energy storage devices.
Experimental Section Synthesis of Tube-in-Tube Carbon Nanostructure (TTCN): The pristine multi-walled carbon nanotubes (MWNTs) with diameter of 20–50 nm (purchased from Shenzhen Nanotech Por Co. Ltd) were firstly refluxed in nitric acid (65 wt%) at 140 °C for 6 hours before use. In a typical experiment, 110 mg acid-MWNTs was dispersed into 160 mL ethanol and 16 mL H2O solution and sonicated to form a homogeneous solution. Then, 2 mL NH3.H2O (25–28 wt%) and 1.36 mL tetraethoxysilane (TEOS) were added into the above dispersion under stirring and kept for 24 hours. Finally, 0.8 mL TEOS and 0.53 mL octadecyltrimethoxysilane (C18TMS) were dropped into the solution and continued to stir for 8 hours. The product was filtered and washed with ethanol and de-ionized water for several times, and then dried at 80 °C overnight.
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Adv. Mater. 2014, DOI: 10.1002/adma.201401191
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COMMUNICATION Figure 4. Electrochemical properties of the S-TTCN electrode. (a) The first and second discharge/charge curves and (b) cycling performance of S-TTCN at 500 mA g−1. (c) Discharge/charge profiles and (d) rate performance of S-TTCN at various current densities from 0.5 A g−1 to 6 A g−1. (e) Discharge/ charge curves at different cycles and (f) long-term cycling performance of S-TTCN electrode at a high rate of 2 A g−1. 500 mg above powder was treated with 30 mL H2O2 (30 wt% in water) and stirred for 15 h at room temperature. The oxidized product was filtered and redispersed into a mixture of 10 mL H2O and 0.4 mL concentrated H2SO4. This mixture was heated at 100 °C for 5 h and 160 °C for 15 h in air. Then, the obtained dark product was calcinated at 800 °C for 1 h at a heating rate of 3 K min−1 under Ar atmosphere. Finally, the tube-in-tube carbon nanostructure, denoted as TTCN, was obtained by etching the SiO2 with 2M NaOH solution at 50 °C for 12 h. Sulfur Impregnation in TTCN: The TTCN and commercial sulfur with a weight ratio of 1:4 were dispersed into 5 mL CS2 and stirred at room temperature until the CS2 was completely evaporated. The obtained mixture was ground into power and heated at 155 °C for 24 h under Ar atmosphere. The obtained product was denoted as S-TTCN. As a comparison, MWNTs@S composite was also synthesized under the same procedure. Materials Characterization: The morphology and structure characterization of the samples were carried out via transmission electron microscope (TEM, FEI Tecani G2 F20), scanning electron microscopy (SEM, JSM-6700F), X-ray diffraction (XRD, RIGAKU SCXmini), and Brunauer–Emmett–Teller surface area analyzer (BET, Micromeritics ASAP2020). Thermogravimetry analyses (TGA, NETZSCH STA449C) were measured from 30 to 700 °C at a heating rate of 10 K min−1 in N2. Electrochemical Measurements: The electrochemical performance of the samples was carried out via CR2025 coin-type test cells fabricated in
Adv. Mater. 2014, DOI: 10.1002/adma.201401191
an Ar-filled glove box. The working electrodes were consisted of 80 wt% active materials (S-TTCN, or MWNTs@S), 10 wt% conductivity agent (ketjen black, KB), and 10 wt% PVDF, which were mixed with 1-methyl2-pyrrolidinone (NMP) to form a slurry, pasted on Ni foam, and then dried at 60 °C for 12 h under vacuum. The electrolyte was 1 M LiTFSI in 1, 2-dimethoxymethane/1, 3-dioxolane (1:1 v/v) with 0.25 M LiNO3. The separator was a Celgard 2300 membrane. A lithium sheet was used as both counter and reference electrode. Cells were discharged and charged on a LAND 2001A system at room temperature. The specific capacities mentioned in this article were calculated based on the mass of sulfur.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by National Key Project on Basic Research (Grant no. 2011CB935904), the National Natural Science Foundation of China (Grant no. 21171163, 91127020), NSF for Distinguished Young Scholars of Fujian Province (Grant no. 2013J06006), the Singapore
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www.MaterialsViews.com National Research Foundation under its Campus for Research Excellence And Technological Enterprise (CREATE) programme, and the Singapore MOE Tier 1 Grant (RGT13/13). Received: March 17, 2014 Revised: May 1, 2014 Published online:
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Adv. Mater. 2014, DOI: 10.1002/adma.201401191
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Encapsulating MWNTs into Hollow Porous Carbon Nanotubes: A Tube-in-Tube Carbon Nanostructure for High-Performance Lithium-Sulfur Batteries Yi Zhao, Wangliang Wu, Jiaxin Li, Zhichuan Xu,* and Lunhui Guan* To meet the increasing demand of high capacity energy storage systems, recently there has been intensive research on lithiumsulfur (Li-S) batteries due to their high theoretical capacity of 1672 mAh g−1. The overall electrochemical reaction in a typical Li-S battery could be denoted as S8 + 16Li ↔ 8Li2S with an average voltage of ∼2.15 V, providing a theoretical energy density of 2500 Wh kg−1, which is 3–5 times higher than that of commercial lithium ion batteries. In addition, sulfur is inexpensive, environmental benign, and abundant on earth, making it one of the most promising candidates for next-generation high energy batteries.[1] However, hampered by several technical challenges, the commercial applications of Li-S batteries are still far away.[2] The low electrical conductivity of both sulfur (5 × 10−30 S cm−1) and final discharge product (Li2S) limit the rate capabilities of batteries. The intermediate lithium polysulfides can be dissolved into electrolyte and then shuttle between cathode and anode. It forms a deposit of solid Li2S2/Li2S layer on electrode surface and so-called shuttle effect results in the low capacity and poor energy efficiency. Moreover, the difference in densities of sulfur (2.03 g cm−3) and Li2S (1.66 g cm−3) causes a large volumetric change (∼80%) of active material during discharge/charge process, which may gradually destroy the structural stability and give a rapid capacity fading. To overcome these problems, many strategies have been developed to impregnate sulfur into porous carbons or polymers to prevent the dissolution of intermediates while improve the conductivity of electrodes.[3] Among them, synthesizing various porous carbons for encapsulating sulfur has been popular.[4] Dr. Y. Zhao, W. L. Wu, J. X. Li, Prof. L. H. Guan State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter Chinese Academy of Sciences YangQiao West Road 155#, Fuzhou 350002, P.R. China E-mail: [email protected] Prof. L. H. Guan Key laboratory of Design and Assembly of Functional Nanostructures Chinese Academy of Sciences Fuzhou, Fujian 350002, China Dr. Y. Zhao, Prof. Z. C. Xu School of Materials Science and Engineering Nanyang Technological University Singapore 639798 E-mail: [email protected] Prof. Z. C. Xu Energy Research Institute @ NTU Nanyang Technological University 50 Nanyang Drive, Singapore 639798
DOI: 10.1002/adma.201401191
Adv. Mater. 2014, DOI: 10.1002/adma.201401191
Porous carbons as host materials offer a high specific surface area for electron transportation between sulfur and the host. Its porous structure and high volume also benefit the sulfur adoption and suppress the dissolution of polysulfides into electrolyte. As a result, the high capacity and good cycling retention can be achieved. However, the amorphous state and sp3-hybridized C–C bonding in these porous carbons often exhibit poor electrical conductance, which limits the electron transfer and thus restricts the rate performance to a certain extent.[5] The efforts to use carbon nanomaterials like carbon nanotubes (CNTs) and graphene to enhance the overall conductivity of electrodes have been made to overcome this problem.[4a,6] However, the relative low specific surface area of CNTs (typically < 200 m2 g−1) limits the accommodation of sulfur and the poor porosity of CNTS and graphene cannot prevent the direct contact between sulfur and electrolyte, leading to inevitable dissolution of lithium polysulfide.[7] Thus, it is necessary to design hybrid carbon materials that combines highly conductive carbon nanomaterials with porous carbons to achieve high-performance sulfur-carbon electrode.[8] For example, Xin et al. synthesized a hybrid carbon with microporous carbon layers on MWNTs to impregnate sulfur, showing good cycling stability and excellent rate performance. However, one problem for such S-hybrid carbons composites is the low sulfur content (40–60 wt%). To improve the energy density of Li-S batteries, sulfur confined within these hybrid carbons with higher loading ratio is desired. In this article, we report a novel tube-in-tube structured carbon nanomaterial (TTCN) as the host for sulfur cathode. MWNTs were encapsulated into hollow porous carbon nanotubes through a step-by-step strategy. This unique structure can enhance the electrical conductivity, hamper the dissolution of lithium polysulfide, and provide large pore volume for sulfur impregnation. As a cathode material for Li-S batteries, the obtained S-TTCN composite with 71 wt% sulfur content delivered high reversible capacity, good cycling performance as well as excellent rate capabilities. At a current density of 500 mA g−1, it still remained a high discharge capacity of 918 mAh g−1 (based on sulfur) or 652 mAh g−1 (based on S-TTCN composite) after 50 cycles. Figure 1 shows the schematic synthesis of the S-TTCN composite. Firstly, acid-treated MWNTs were coated with solid SiO2 and porous SiO2 layers via a modified stÖber method.[9] As the porogen agent and carbon precursor, an organosilicon compound (octadecyltrimethoxysilane, C18TMS) was trapped within the porous SiO2 shells (Figure S1). After chemical treatment and high temperature calcinations in Ar atmosphere (see experimental section for details), the organic moiety of C18TMS was converted into carbon.[10] Then, by etching the SiO2 layer with NaOH solution, the tube-in-tube carbon nanostructure (TTCN)
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Figure 1. Schematic illustration for the formation of S-TTCN composite: (1) Uniform coating a solid SiO2 layer and a porous SiO2 layer embedded with C18TMS molecules on MWNTs; (2) formation of porous carbon nanotube by carbonization of C18TMS; (3) etching SiO2 layers to obtain tube-in-tube carbon nanostructure (TTCN) with MWNTs encapsulated within hollow porous carbon nanotube; (4) sulfur infused into TTCN to fabricate S-TTCN composite.
with MWNTs confined within hollow porous carbon nanotubes were obtained. Finally, S-TTCN composite was synthesized by a simple melt infiltration method.[8a] The structure and morphology of the TTCN were firstly characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 2a shows the typical SEM image of commercial MWNTs with diameters around 20–50 nm and length up to several micrometers. As shown in Figure 2b, the obtained TTCN kept the similar morphology to MWNTs with enlarged diameters up to 80–140 nm, indicating the good confinement of MWNTs inside carbon layers. The high magnification SEM image (Figure S2a) and TEM image (Figure 2c-d) show that MWNTs were encapsulated within hollow carbon shells with large internal void space. From the high magnification TEM images (Figure 2d and S2b), the thickness of the porous carbon layer was estimated to be around 10–13 nm. The internal void space and the thickness of carbon layer can be readily controlled by tuning the amount of SiO2 and C18TMS, which is simpler than other protocols.[11] The HRTEM image (Figure 2e) clearly shows the porous structure of carbon layer and the interlayer spacing of 0.34 nm for MWNTs. As a carbon host to encapsulate sulfur, the large void space of TTCN can accommodate high loading ratio of sulfur to improve the energy density of Li-S batteries. After sulfur encapsulation, the obtained S-TTCN still remained the original morphology with no sulfur particles observed outside TTCN (Figure 2f and Figure S3), indicating that sulfur was fully diffused into the pore structure of the TTCN. To further reveal the sulfur distribution within TTCN, the element mappings of carbon and sulfur were performed in Figure 2h-i, demonstrating that sulfur was homogeneously distributed within the TTCN host. To further investigate the structure of TTCN and S-TTCN, nitrogen adsorption—desorption isotherms and pore size distribution curves are shown in Figure 3a and 3b. The TTCN delivered a large surface area of 822.8 m2 g−1 and a total pore volume of 1.77 m3 g−1, which were higher than the values for other hybrid carbons.[8] It can be seen from Figure 3b that TTCN possessed micropores at 1.8 nm, mesopores at 3.6 nm,
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and a broad pore distribution from 20 to 60 nm, which were consistent with the TEM observation. These micropores and mesopores in the outer carbon layers could effectively hamper the dissolution of sulfur for long cycle retention.[12] After sulfur impregnation, surface area and pore volume of the obtained S-TTCN composite decreased to 5.6 m2 g−1 and 0.19 m3 g−1, respectively. Only a small mesoporous peak around 20 nm (the inset in Figure 3b) was observed in S-TTCN composite. Thus, we can deduce that the porous carbon layers in TTCN were fully filled with sulfur, while some void space was still remained between MWNTs and carbon layers, which would benefit the accommodation of the volume change during cycles. The thermogravimetric analysis (TGA) was performed from 30–700 °C at a heating rate of 10 °C min−1 in N2 to determine the weight content of sulfur. Seen from Figure 3c, the S-TTCN composite has a high loading ratio of 71 wt%. Meanwhile, the high sulfur evaporation temperature (200–400 °C) indicated the strong absorption between sulfur and TTCN host.[8b] Figure 3d shows the XRD patterns of the TTCN and S-TTCN composites. The TTCN exhibited a diffraction peak near 26o, which was assigned to the MWNTs. While some crystalline sulfur peaks with orthorhombic structure (JCPDS 08–0247) can be observed from S-TTCN composite. Based on the SEM and TEM characterization of S-TTCN in Figure 2, no sulfur particles were found outside TTCN, indicating that sulfur confined within the internal void space of TTCN was in crystalline state. The electrochemical performance of the S-TTCN composite, as a cathode in Li-S battery, was shown in Figure 4. The working electrode was consisted of 80 wt% S-TTCN, 10 wt% KB and 10 wt% PVDF. Figure 4a shows typical first discharge/ charge profiles of S-TTCN electrode at 500 mA g−1 between 1.95 V and 2.7 V. There were three obvious reaction plateaus in the first discharge process.[13] The first plateau at 2.32 V was attributed to a solid-liquid two phase reaction from element sulfur to Li2S8. The second sloped plateau from 2.3 to 2.1 V was due to a liquid-liquid single phase reduction from long chain Li2S8 to short-order lithium polysulfides (Li2Sn, 4 ≤ n < 8). The third long plateau at 2.1 V was corresponded to a liquid-solid
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Adv. Mater. 2014, DOI: 10.1002/adma.201401191
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COMMUNICATION Figure 2. (a) SEM image of the MWNTs. (b) SEM, (c-d) TEM, and (e) high-resolution TEM images of the TTCN, showing the tube-in-tube carbon nanostructure with large internal void space. Insets in (a) and (b) are the diameter distributions for MWNTs and TTCN, respectively. (f) TEM image, (g) Scanning transmission electron microscopy (STEM) image and the corresponding element mapping images of (h) carbon and (i) sulfur of the S-TTCN composite, indicating the homogeneous distribution of sulfur within TTCN host.
two phase reaction from soluble Li2Sn to insoluble Li2S2 and Li2S, which contributed the major discharge capacity of the S-TTCN electrode. During the first charge process, the long oxidation plateau from 2.2 to 2.41 V was the reverse reaction, corresponding to the formation of Li2Sn (S > 2) and the final oxidation to S8. The first discharge and charge curves delivered specific capacities of 1274 and 1264 mAh g−1, corresponding to a columbic efficiency of ∼100%. Figure 4b shows the cycling performance of the S-TTCN electrode at a low current density of 500 mA g−1. As can be seen, the S-TTCN electrode exhibited excellent cycling stability and a high discharge capacity of 918 mAh g−1 was remained after 50 cycles. The discharge/charge columbic efficiency kept a high value around 98% during these cycles. Based on the overal mass of S-TTCN, this composite delivered a high capacity of 652 mAh g−1 after 50 cycles, which was higher than the values for sulfur combined with microporous and mesoporous carbons,[4c,4f ] hollow carbon spheres,[14] carbon nanofibers/nanotubes,[7b,15] graphene,[16] as well as hybrid carbons.[8a–c,17] Such good cycling performance of the
Adv. Mater. 2014, DOI: 10.1002/adma.201401191
S-TTCN electrode can be mainly attributed to the porous carbon layers, which effectively inhibit the dissolution of lithium polysulfide during cycles. The rate performance of the S-TTCN electrode was investigated as well. Figure 4c shows the discharge/charge curves under various current densities from 0.5 A g−1 to 6 A g−1. These profiles kept similar shapes with low overpotentials observed at different rates. Even at high rates, these curves still exhibited the typical reaction plateaus and delivered high capacities. Figure 4d demonstrates the excellent rate performance of the S-TTCN electrode. The discharge capacities were stabilized around 800, 750, 650, and 550 mAh g−1 when cycled at 1, 2, 4, and 6 A g−1, respectively. As the current density was reduced back to 500 mA g−1, the discharge capacity returned to 850 mAh g−1, indicating the good stability of the S-TTCN electrode during various rates. The long-term cycling stability of the S-TTCN electrode was also tested at a high rate of 2 A g−1 after being activated at 0.5 A g−1 in the initial two cycles. It can be seen from Figure 4e that these discharge/charge profiles
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Figure 3. (a) Nitrogen adsorption—desorption isotherms, and (b) pore size distribution curves of TTCN and S-TTCN composites. (c) TGA curve of S-TTCN. (d) XRD patterns of TTCN and S-TTCN composites. Insets in (b) show the magnified pore size distributions for TTCN and S-TTCN, respectively.
presented stable voltage plateaus and small capacity loss during cycles. Figure 4f demonstrates the high specific capacity as well as good cycling retention at 2 A g−1. After 200 cycles, the S-TTCN electrode still remained a high capacity of 647 mAh g−1 with a low decay rate of 0.089% per cycle. These results indicate that the design of such a TTCN structure indeed improved the performance of Li-S batteries. For comparison, MWNTs@S composite with a loading ratio of 74 wt% (Figure S4) was also synthesized. The XRD pattern of MWNTs@S (Figure S5) exhibited sharp diffraction peaks of sulfur, indicating the large particle size of sulfur outside MWNTs. As a cathode for Li-S batteries, MWNTs@S electrode showed poor capacity retention at 500 mA g−1. Seen from Figure S6, the MWNTs@S electrode suffered from rapid capacity fading in the initial cycles and only remained a discharge capacity of 660 mAh g−1 after 50 cycles. The S-TTCN exhibited better rate capabilities than MWNTs@S composite (Figure S7). We tentatively attributed the outstanding electrochemical performance of S-TTCN composite to the following reasons: Firstly, the one dimensional MWNTs and porous carbon layers offered superior electrical conductivity to facilitate fast electronic/ionic transport and enhance reaction kinetics of sulfur, giving good rate capabilities. Secondly, the carbon layers with micropores and mesopores can effectively prevent lithium polysulfide from dissolving into electrolyte, resulting in stable cycling retention and high efficiency. Finally, the large pore volume in TTCN can accommodate more sulfur to improve the energy density of batteries. The remained void space after sulfur impregnation as well as the robust TTCN framework could buffer the strain generated from the volumetric changes of sulfur during cycles and therefore keep the structure stable.
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In summary, a novel tube-in-tube carbon nanostructure with MWNTs confined within hollow porous carbon nanotubes was synthesized and used to accommodate sulfur. Due to the unique architecture of TTCN with good electrical conductivity, large pore volume and porous carbon layers, the obtained S-TTCN cathode exhibited outstanding electrochemical performance. It remained high reversible capacities of 918 mAh g−1 at 500 mA g−1 after 50 cycles and 647 mAh g−1 at 2 A g−1 after 200 cycles. Even at a high rate of 6 A g−1, it still delivered a capacity of 550 mAh g−1. This work provides a general strategy to synthesize superior hybrid carbons with large void space, which is crucial to improve the energy density and electrochemical performance of Li-S batteries. With proper chemistry, the approach may be extended to synthesize tube-in-tube structured materials with other carbon nanomaterials such as SWNTs, carbon fibers, and graphene for high performance energy storage devices.
Experimental Section Synthesis of Tube-in-Tube Carbon Nanostructure (TTCN): The pristine multi-walled carbon nanotubes (MWNTs) with diameter of 20–50 nm (purchased from Shenzhen Nanotech Por Co. Ltd) were firstly refluxed in nitric acid (65 wt%) at 140 °C for 6 hours before use. In a typical experiment, 110 mg acid-MWNTs was dispersed into 160 mL ethanol and 16 mL H2O solution and sonicated to form a homogeneous solution. Then, 2 mL NH3.H2O (25–28 wt%) and 1.36 mL tetraethoxysilane (TEOS) were added into the above dispersion under stirring and kept for 24 hours. Finally, 0.8 mL TEOS and 0.53 mL octadecyltrimethoxysilane (C18TMS) were dropped into the solution and continued to stir for 8 hours. The product was filtered and washed with ethanol and de-ionized water for several times, and then dried at 80 °C overnight.
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Adv. Mater. 2014, DOI: 10.1002/adma.201401191
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COMMUNICATION Figure 4. Electrochemical properties of the S-TTCN electrode. (a) The first and second discharge/charge curves and (b) cycling performance of S-TTCN at 500 mA g−1. (c) Discharge/charge profiles and (d) rate performance of S-TTCN at various current densities from 0.5 A g−1 to 6 A g−1. (e) Discharge/ charge curves at different cycles and (f) long-term cycling performance of S-TTCN electrode at a high rate of 2 A g−1. 500 mg above powder was treated with 30 mL H2O2 (30 wt% in water) and stirred for 15 h at room temperature. The oxidized product was filtered and redispersed into a mixture of 10 mL H2O and 0.4 mL concentrated H2SO4. This mixture was heated at 100 °C for 5 h and 160 °C for 15 h in air. Then, the obtained dark product was calcinated at 800 °C for 1 h at a heating rate of 3 K min−1 under Ar atmosphere. Finally, the tube-in-tube carbon nanostructure, denoted as TTCN, was obtained by etching the SiO2 with 2M NaOH solution at 50 °C for 12 h. Sulfur Impregnation in TTCN: The TTCN and commercial sulfur with a weight ratio of 1:4 were dispersed into 5 mL CS2 and stirred at room temperature until the CS2 was completely evaporated. The obtained mixture was ground into power and heated at 155 °C for 24 h under Ar atmosphere. The obtained product was denoted as S-TTCN. As a comparison, MWNTs@S composite was also synthesized under the same procedure. Materials Characterization: The morphology and structure characterization of the samples were carried out via transmission electron microscope (TEM, FEI Tecani G2 F20), scanning electron microscopy (SEM, JSM-6700F), X-ray diffraction (XRD, RIGAKU SCXmini), and Brunauer–Emmett–Teller surface area analyzer (BET, Micromeritics ASAP2020). Thermogravimetry analyses (TGA, NETZSCH STA449C) were measured from 30 to 700 °C at a heating rate of 10 K min−1 in N2. Electrochemical Measurements: The electrochemical performance of the samples was carried out via CR2025 coin-type test cells fabricated in
Adv. Mater. 2014, DOI: 10.1002/adma.201401191
an Ar-filled glove box. The working electrodes were consisted of 80 wt% active materials (S-TTCN, or MWNTs@S), 10 wt% conductivity agent (ketjen black, KB), and 10 wt% PVDF, which were mixed with 1-methyl2-pyrrolidinone (NMP) to form a slurry, pasted on Ni foam, and then dried at 60 °C for 12 h under vacuum. The electrolyte was 1 M LiTFSI in 1, 2-dimethoxymethane/1, 3-dioxolane (1:1 v/v) with 0.25 M LiNO3. The separator was a Celgard 2300 membrane. A lithium sheet was used as both counter and reference electrode. Cells were discharged and charged on a LAND 2001A system at room temperature. The specific capacities mentioned in this article were calculated based on the mass of sulfur.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by National Key Project on Basic Research (Grant no. 2011CB935904), the National Natural Science Foundation of China (Grant no. 21171163, 91127020), NSF for Distinguished Young Scholars of Fujian Province (Grant no. 2013J06006), the Singapore
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www.MaterialsViews.com National Research Foundation under its Campus for Research Excellence And Technological Enterprise (CREATE) programme, and the Singapore MOE Tier 1 Grant (RGT13/13). Received: March 17, 2014 Revised: May 1, 2014 Published online:
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Adv. Mater. 2014, DOI: 10.1002/adma.201401191
