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A Systematic Effect for an Ultra-long Cycle Lithium-sulfur Battery Feng Wu, Yusheng Ye, Renjie Chen, Ji Qian, Teng Zhao, Li Li, and Wenhui Li Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b02864 • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on October 28, 2015
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A Systematic Effect for an Ultra-long Cycle Lithium-sulfur Battery †
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Feng Wu, ,‡, Yusheng Ye, , Renjie Chen,*, ,‡ Ji Qian, Teng Zhao, Li Li, ,‡ Wenhui Li
† Beijing Key Laboratory of Environmental Science and Engineering, School of Material Science & Engineering, Beijing Institute of Technology, Beijing 100081, China
‡ Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China
Keywords: systematic effect, polydopamine, high-order polysulfides, cathode, separator, lithium sulfur Rechargeable lithium-sulfur (Li-S) batteries are attractive candidates for energy storage devices because they have five times the theoretical energy storage of state-of-the-art Li-ion batteries. The main problems plaguing Li-S batteries are poor cycle life and limited rate capability, caused by the insulating nature of S and the shuttle effect associated with the dissolution of intermediate lithium polysulfides. Here, we report the use of bio-cell-inspired polydopamine (PD) as a coating agent on both the cathode and separator to address these problems (the “systematic effects”). The PD-modified cathode and separator play key roles in facilitating ion diffusion and 1 ACS Paragon Plus Environment
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keeping the cathode structure stable, leading to uniform lithium deposition and a solid electrolyte interphase. As a result, an ultra-long cycle performance of more than 3000 cycles, with a capacity fade of only 0.018% per cycle, was achieved at 2 C. It is believed that the systematic modification of the cathode and separator for Li-S batteries is a new strategy for practical applications.
Lithium-sulfur (Li-S) batteries are one of the most-studied lithium batteries1 because their theoretical capacity (1673 mAh/g) is more than five times that of lithium ion batteries.2-4 From a practical perspective, sulfur is naturally abundant, inexpensive, and nontoxic.5-7 Therefore, Li-S batteries show promise for widespread future use. Despite these advantages, the loss of electrical contact and the “shuttle effect” are still encountered when using these batteries, both of which lead to a quick reduction of the capacity. Pioneering work has been conducted to address these problems using selected cathode materials;8-13 this involves combining active materials with various carbon materials ranging from micro- or mesoporous to macroporous carbon in one- to multi-dimensions.4,
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However, polar lithium
sulfides may detach from a nonpolar carbon framework during discharge because of the low binding energy between lithium sulfides and carbon, which results in the loss of electrical contact and leads to capacity decay.9 Moreover, the shuttle effect is a serious problem that needs to be solved.22 During discharging, elemental sulfur will disconnect and be reduced into soluble intermediate long-chain polysulfides (Li2Sx, 4 ≤ x ≤ 8) and further transform into Li2S2 and Li2S. During charging, Li2S2 and Li2S will be oxidized into elemental sulfur. As a consequence of the notorious shuttle mechanism of soluble intermediate long-chain polysulfides between cathode and anode, Li2S/Li2S2 films accumulate on the anode surface and block Li+ diffusion, eventually consuming the active material. Conducting polymer membranes are one choice used to mitigate the shuttle problem.23, 24 However, mitigating the shuttle effect 2 ACS Paragon Plus Environment
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of soluble polysulfide intermediates poses a considerable challenge because conventional coating is insufficient for trapping sulfides.25-29 Moreover, current polyethylene (PE) separators have a hydrophobic surface and low surface energy, which block the diffusion of the electrolyte in the separator. The PE separator therefore impairs the cycle life directly. Research on artificial separator modification has focused on using conductive layers to interrupt the transportation pathway of soluble intermediate long-chain polysulfides (Figure S1).10, 30-33 However, it is clear that investigating each individual component only is insufficient to address various technological challenges associated with Li-S batteries.30, 31, 34 Suppressing the “shuttle effect” without sacrificing device energy density and cycle life remains a challenge. Therefore, it is necessary to investigate the behavior of the entire system. Herein, we propose a systematic modification of the cathode and separator to mitigate the shuttle effect and improve the performance of lithium sulfur cells. A polydopamine-coated sulfur-hydroxylated carbon nanotube (PD-S-HCNT) cathode and species-selective separator were used to modify the lithium-sulfur system. The cathode possesses an interfacial chemical binding and physical barrier, and the separator possesses selective penetration functions, both of which improve the performance of lithium sulfur cells (termed the “systematic effect”). Admittedly, a thin cell membrane is critical to maintain the cell stability and the cell membrane proteins can transport selective ions through cell membranes. Inspired by this special membrane, a polydopamine (PD)35 membrane was used to encapsulate a sulfur-functional hydroxylated carbon nanotube (S-HCNT) cathode and PE separator, as shown in Scheme 1. With this design, the cathode can be expected to maintain the cathode stability, and the modified separator can rapidly transport Li+ but obstruct polysulfide penetration from cathode to anode. This systematic modification yields excellent capacity retention, high capacity, and high Coulombic efficiency. Figure 1a shows a schematic of the synthesis of cathode materials. To achieve good surface activity, CNTs were subjected to modified hydrothermal treatment, aided by ultra-sonication, to graft hydroxyl groups homogeneously on CNTs.36 After 3 ACS Paragon Plus Environment
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that, sulfur was encapsulated by a reaction of sodium thiosulfate with dilute hydrochloric acid and deposited onto the HCNTs under ultra-sonication, which prevented the formation of sulfur clusters. Inspired by muscles, a PD membrane was coated onto the S-HCNT material to release the surface tension of the cathode and to manipulate the surface interfacial reaction. A scanning electron microscope (SEM) image of as-synthesized PD-coated S-HCNTs shows that the surfaces of the tubes are smooth and the diameters are approximately 50–120 nm (Figure 1b). After hydroxylation, no discernible morphology changes were observed in SEM images, but there was significantly increased oxygen content in the energy dispersive X-ray spectroscopy (EDX) data (Figure S2). The corresponding high-resolution transmission electron microscopy (TEM) images of functionalized CNTs reveal that sodium hydrate treatment roughens the exterior CNT walls (Figure S3).37 Oxygen atoms are thought to form stronger bonds with lithium sulfides than with carbon atoms at the interface. Moreover, the modified hydrothermal process roughened the surface of HCNTs, which contributes to sulfur deposition (Figure 1c, d). Hydrochloric acid dropping speed (20 µL/min) and ultra-sonication were controlled to ensure conformal deposition on the HCNTs. A TEM image confirms the coaxial morphology of PD-S-HCNTs composites after coating with PD (Figure S6). Sulfur is located between the PD membrane and the electric HCNTs. Because of the specific structure, the designed coaxial cathode can serve as two lines of defense to suppress polysulfides from being dissolved into the electrolyte and penetrating across the separator. First, the –OH groups on HCNTs not only increase the binding energy between HCNTs and sulfur, but also easily form interfacial connections with Li2S/Li2S2 after discharge. Second, the self-supporting PD film can prevent polysulfides from dissolving into the electrolyte. Furthermore, the hydrophilic PD membrane enhances the ductility and wetting properties (Figure 2e) of the PD-S-HCNTs cathode. High-resolution TEM shows that the PD is well coated on the S-HCNTs, and the diameters of sulfur particles are ~40 nm (Figure 1f and g). The PD membranes around each sulfur particle can form physical barriers and allow the 4 ACS Paragon Plus Environment
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cathode to discharge/charge without deforming the overall morphology, so that the secondary particles are not ruptured during cycling (Figure 1h). The PE separator mechanism is presented in Figure 2a. When using the PE separator, large lithium dendrites and large lithium volume changes easily break solid electrolyte interphase (SEI) layers. As a result, short circuiting and cell failure occur because of the irregular deposition of Li dendrites. Lithium pulverization can also consume the electrolyte rapidly.38 Thus, the modification of the separator to diminish Li dendrite growth appears to be a promising approach. Figure 2b schematically shows the dipping synthesis used to modify the PE separator with PD. Here, PD is in-situ polymerized onto the two-sided separator, where the inner section is the PE separator and the outer sides are composed of PD. The morphology and porous structure of the PE separator becomes dense after coating (Figure 2c). In this architecture, the PD layer increases the binding force between separator and lithium polysulfides. Li ions are allowed to pass through the modified separator, while polysulfides in the electrolyte are prevented from passing through the PD-modified separator to react with metallic lithium. Therefore, this PD-modified separator serves as a third line of defense to suppress polysulfides form penetrating through the separator. These three lines of defense are arranged from the “inside” to the “outside”, including functionalizing the CNTs host, PD coating of the HCNTs-S cathode, and PD coating of the separator. These lines of defense make a systematic contribution to improving the performance of lithium sulfur cells. More importantly, after PD coating, the hydrophobic PE separator becomes hydrophilic. When dropping 0.04 mL of electrolyte on the bare PE separator, the electrolyte is repelled and forms a large contact angle (144°). Interestingly, the electrolyte contact angle with the PD separator decreases significantly and the PD separator is wetted thoroughly (Figure S8). According to the compatibility principle, the presence of oxygen-containing functional groups and nitrogen heteroatoms in membranes improves their surface wettability, which enhances polar interactions with the electrolyte solution.39 This hydrophilic property is also confirmed by photographs of the PD-S-HCNTs cathode 5 ACS Paragon Plus Environment
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in Figure 1d. After PD is coated on the hydrophobic S-HCNTs, the PD-S-HCNTs become hydrophilic. This enhanced hydrophilicity improves the electrolyte uptake speed and helps to increase the ionic conductivity. Fourier transform infrared (FTIR) spectroscopy at 3162 and 1400 cm−1 also confirms the enrichment of the -OH groups of the PD separator and of the PD-S-HCNTs cathode, which facilitates the formation of a stable SEI layer38. N-H vibrations from the amino groups at 1572 cm−1 confirm that PD is successfully included in both the cathode and modified separator. To verify the protective effect of the PD separator, the anode morphologies of various cells after cycling were characterized by SEM. Without the PD coating on the separators in batteries, Li dendrites were sharp and spread all over the surface of the Li metal anode (Figure 2d left, middle and inset). Longitudinal lithium dendrites break SEI layers and may even lead to short-circuiting. However, no obvious Li dendrites were formed on the anode surface after 1000 cycles when using the PD-modified separator. This suggests that the PD-modified separator can be used to form a more stable SEI layer and soften Li dendrite growth during charge/discharge (Figure 2d right). The smooth anode surface facilitates more efficient Li ion transfer at the interfaces. Specifically, the favorable hydrophilicity of the PD layer facilitates uniform Li ionic deposition, thereby effectively reducing the localized reaction of Li ions. More importantly, barriers are formed to capture polysulfides when using the bilaterally coated PD-modified separator in a Li-S system by isolating them in the separator or cathode area to suppress the “shuttle effect”. Cyclic voltammogram tests were conducted to investigate the electrochemical behavior of cells with a PD-S-HCNTs cathode, PD-modified separator, and Li metal anode (Figure 3a). The sharp charge/discharge peaks corroborate the rapid electron/ion transfer process in our cells. Two cathodic peaks near 2.32 V and 2.08 V and two anodic peaks near 2.28 V and 2.37 V were observed, associated with the coupled conversion between elemental sulfur and lithium polysulfides and between lithium polysulfides and Li2S2/Li2S. The roles of sulfur-based cathodes and species-selective separators are confirmed by the improved cycle performance of the 6 ACS Paragon Plus Environment
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PD-S-HCNTs/PD-modified separator (PD-S-HCNTs/PD) system, compared with the batteries assembled with S-CNTs/PE separator (S-CNTs/PE), S-HCNTs/PE separator (S-HCNTs/PE), and PD-S-HCNTs/PE separator (PD-S-HCNTs/PE) structures. Here, controlled amounts of electrolyte (0.1 mL) and sulfur content were used, and all the tests were conducted in a 30 °C incubator.40 At 0.2 C (Figure 3b), values of the initial reversible discharge capacity of the four configurations are similar, i.e., ~1280 mAh/g. The capacity decay rates of the S-CNTs/PE (0.84% per cycle), S-HCNTs/PE (0.62% per cycle), PD-S-HCNTs/PE (0.49% per cycle), and PD-S-HCNTs/PD (0.04% per cycle) systems decreased gradually, as the sulfur species are more effectively captured in the systematically modified hybrid system. Analyzing these results, it is clear that the introduction of functional groups doubled the capacity, followed by a further 46% increase using the PD coating on the cathode. Finally, with the PD-coated separator, another 53% increase in capacity was achieved. Therefore, the PD-coated separator plays a key role in enhancing the utilization of active materials, while both the CNTs surface modification and PD-coated cathode have similar effects on retarding polysulfide dissolution. Because of this systematic effect, the PD-S-HCNTs/PD system retards the fast capacity decay for 100 cycles and shows superior capacity retention compared with the conductive polymer modification reported to date.24, 34, 41, 42
To elucidate the role of the cathode, a PD-S-HCNTs cathode was assembled with a PE separator and discharged at 0.1 C. The PD-S-HCNTs/PE cell delivers a high specific capacity of 1320.6 mAh/g and shows a stable cycle performance, with 58% capacity retention after 1000 cycles, corresponding to a small capacity decay rate of 0.05% per cycle (Figure 3c), which confirms that the designed cathode can inhibit the “shuttle effect” at the cathodic area successfully. More importantly, when combining the PD-S-HCNTs cathode with a PD-modified separator, the cell shows a high reversible capacity of 769.7 mAh/g, even after 1000 cycles. The effect of the PD separator is remarkable. We hypothesize that the cycling stability of the PD-S-HCNTs/PD system can be linked to the strong chemical covalent bonding of 7 ACS Paragon Plus Environment
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surface functional groups, in contrast to the free shuttling of soluble polysulfides between the cathode and separator district during cycling. The rate performance of a PD-S-HCNTs/PD coin cell is shown in Figure 3d. At a current rate of 2 C, a reversible initial capacity of 940.8 mAh/g was achieved, which can be attributed to the nano-sized sulfur particles. The rate capability of the coin cell decreases gradually with increasing current rate, but the system exhibits a highly improved cycling stability. After 300 cycles, a high capacity of 858.7 mAh/g was retained even at a high current rate of 4 C, with only an 8.7% decrease compared with that at 2.0 C. Even at an extremely high discharge rate of 5 C, the capacity still reached 798.1 mAh/g. Also, the capacity recovers to 840.8 mAh/g when the current rate decreases to 2 C. After a further 100 cycles, the coin cell was left in an open circuit for 30 days and then discharged/charged at 3 C; the discharge capacity decreased to 728.5 mAh/g at this time. More surprisingly, it recovered to 755.6 mAh/g after 15 electrolyte re-soaking cycles and then displayed a very stable cycle performance, which means that the restriction of the “shuttle effect” improved. The Coulombic efficiency of the coin cell is ~98% from 2 C to 5 C, which indicates that the shuttle phenomenon is controlled effectively because of the systematic effect of the cathode and separator modification. After a 400-time increment of current density from 2 C to 5 C and a 200-time increment of current density from 2 C to 3 C, the discharge capacity remains at 741.4 mAh/g, which reveals that the rate performance is improved by intimate contact of sulfur between the HCNTs and PD, and the effective PD-modified separator barrier. The galvanostatic charge-discharge behavior of the coin cell at 2 C is evaluated at 1.7-3.0 V versus Li+/Li (Figure 3e and f). The integration of the cathode and PD separator makes cells stable during cycling. The corresponding capacity retentions for the 500th, 1000th, 1500th, and 2000th cycles are 89.9% 81.9%, 75.4% and 67%, respectively. These results confirm that the PD-S-HCNTs/PD cell shows efficient electrolyte infiltration and efficient polysulfide confinement and alleviates active material loss, thereby ensuring excellent electrochemical stability. To quantify the trapped effect of sulfur species, we have calculated the binding 8 ACS Paragon Plus Environment
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energies of sulfur species with various configurations (Figure 4). Heteroatoms with lone electron pairs can bind with LixS (0 < x ≤ 2), whereas oxidized heteroatoms interact weakly with LixS clusters. For HCNTs (Figure 4b), oxygen atoms bind strongly with Li2S/Li2S2 and give stronger binding energies of 0.25–0.60 eV than with CNTs. It is worth mentioning that the number of oxygen atoms is greater than the calculated result, so the real binding energies are much stronger. For PD (Figure 4c), we find that the nitrogen and oxygen atoms bind strongly with Li2S and Li2S2 and give stronger binding energies of 3.35–3.70 eV. HCNTs and PD show stronger binding effects on high-order polysulfides (S32- and S42-), mostly because hydroxyl and amino groups can induce asymmetrical charge distribution on the side containing sulfur atoms, consequently resulting in larger polarizations and stronger electrostatic interactions.9, 43 Figure 4d-f compare the average loading of sulfides absorbed in PE and PD separators for the same degree of polymerization. Blue and white colors indicate accessible Connolly volumes. The PE separator shows no ability to absorb sulfides, whereas one PD module can adsorb 114.430, 114.390, 39.706, and 59.913 units of Li2S, LiS, S32-, and S42-, respectively. The free volume holes of PD contribute considerably to the trapping properties of the membranes, as shown in the inset Table of Figure 4d. Considering that PD has additional interchain interactions, more energy is needed to overcome the attractive forces between the chains, which permit a higher selectivity of diffusion to restrain sulfides but allow the penetration of lithium ions. Nitrogen-/oxygen-containing functional groups and a free volume are vital for trapping intermediate polysulfides and final products (Li2S/Li2S2). It can be seen that high binding energies and strong interchain interactions lead to low microscopic diffusivity of polysulfides and effective confinement of polysulfides in the cathode area. To further demonstrate the significance of the systematically modified system, we cycled the cell for 3161 cycles at 2 C (Figure 5a). The cycle performance can be separated into three periods: (1) inevitable dissolution; (2) stability; and (3) 9 ACS Paragon Plus Environment
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undesirable decay. The curve-fitting equations and differential equations are shown. After 3161 cycles, the Coulombic efficiency of the cell remains at ~99% and the cathode surface gives rise to new phenomena. In the first 500 cycles, the capacity decreased to 844.3 mA/g, corresponding to a capacity loss of 10.23%. The capacity decay rate was 0.020% loss per cycle in the first 500 cycles, as sulfur was inevitably dissolved into the electrolyte and simultaneously formed a SEI layer. When a cathode sample was collected after 500 cycles, the cathode surface comprised a few single ~800-nm-sized plates (Figure 5b). After 500 cycles, a few dendrites were formed on the anode (Figure 5c). The Li-S cells show consistently stable, high Coulombic efficiency cycling characteristics, which can be attributed to more uniform Li deposition at the PD-modified separator/lithium anode interface and more stable SEI formation on top of the lithium anode. In the next 1500 cycles, the discharge capacity decreased from 847.8 mA/gS to 631.5 mA/gS, corresponding to a capacity reduction of 23.00% and 0.015% loss per cycle. During this period, the PD membrane serves as a buffer during charge/discharge. Remarkable, single nanoplates on the cathode surface began to aggregate, and small nanoflowers (2 µm) were formed. The ideal interfacial PD layer for the Li metal anode is chemically stable in a highly reducing environment and is mechanically strong to release tension.35, 44 The stable PD membrane and SEI on the lithium anode surface lead to an excellent cycle life during deep cycling; a much lower sulfur compound content of the SEI layer is shown in Figure 6. The sulfur content of the anode SEI layer of the PD-S-HCNTs/PD system (2.65%) is much lower than that of the CNTs-S/PE system (11.96%). In the last 1161 cycles, some of the exposed active materials were dissolved into the electrolyte; thus, the discharge capacity decayed more rapidly than in the previous period. This result corresponds to a capacity reduction of 26.61% and 0.023% loss per cycle. After 3000 cycles, contiguous nanoflowers composed of 5–20 µm nanoplates formed large nanoflower structures. The dependence on cycle times suggests the following mechanism for 10 ACS Paragon Plus Environment
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nanoflower self-assembly (Figure 5d). In the early stage, sulfur forms predominantly complex high-order polysulfides with Li+. In the second stage, the nucleation of nanoflowers grows with the deeper reduction of polysulfides to low-order polysulfides. During this process, PD serves as an existing nucleus and the soluble high-order polysulfides can be easily transformed into solid Li2S2/Li2S. The growth of nanoflowers occurs at individual S-species binding sites on the cathode surfaces and causes separate petals to appear. In the last stage, substantial deposition is indicated further in the SEM images and EDX mapping, which indicates large agglomerates, and petals of Li2SxOy are formed. At last, a flower-like structure completely forms by anisotropic growth. In this growth process, PD induces agglomerate nucleation to form many petals. Further, instead of forming a poorly conductive Li2S2/Li2S layer on HCNTs, PD binds these petals together with its adhesive properties, which can decrease charge transfer resistance and SEI resistance for improved cycle performance. To validate the electrochemical activity of the nanoflower, the HCNTs-S-PD@PD cell was fully charged back and disassembled. Surprisingly, no nanofolowers were found after fully charged to 3.0V (Figure S13). The HCNTs-S-PD composite cathode surface transforms into complanate structure as it was before cycled and proceeds to further cycling. This result means that the attractive petals of the nanoflowers were decomposed and formed uniform composites at cathode surface (Figure S13), which demonstrates that the nanoflowers show electrochemical activity during cycling. Thus, our systematically modified Li-S batteries perform well. Table S1 summarizes the electrochemical properties of different structures in the Li-S battery. Our systematically modified cells have much better cycle performance and capacity retention than most individual component modifications. In summary, we demonstrate here the “systematic effect” of a modified cathode and separator that have the ability to control polysulfide transportation and improve the performance of lithium sulfur cells. The cell was cycled at a high rate (2 C) up to 3161 times, accompanied by an almost ~99% Coulombic efficiency and 0.023% capacity loss per cycle. These results suggest that the PD-S-HCNTs/PD system, with 11 ACS Paragon Plus Environment
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its multiple lines of defense to inhibit the shuttle effect, high electrolyte uptake speed, and impressive stability, holds great promise for future environmental and commercial Li-S applications. The systematic design and species-selectable separator developed in this work may open a new avenue of confinement and separation (such as lithium-air cells, fuel cells, and lithium ion batteries, and enable the preparation of a superior system in photonics, water treatment, and drug delivery applications.
ASSOCIATED CONTENT
Supporting Information. Experimental procedures and additional characterization. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *(R.C)
[email protected] (R. Chen). Phone: +86-10-6891-2508
Author Contributions ⊥
F.W. and Y. Y. contributed equally to this work. F. W., Y. Y. and R. C. designed the
study. Y. Y., J. Q., and W. L. performed the experiments. Y. Y., F. W., and R. C. analyzed the data and performed the discussion and conclusion. Y. Y. and R. C. performed the sequence mapping. Y. Y., F. W., R. C., J. Q., T. Z. and L. L. wrote the manuscript. Notes
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The authors declare no competing financial interest. Teng Zhao currently is a Ph.D. candidate at University of Cambridge, UK. ACKNOWLEDGMENT This work was supported by the National Science Foundation of China (21373028), Major achievements Transformation Project for Central University in Beijing, National Key Program for Basic Research of China (2015CB251100) and Beijing Science and Technology Project (D151100003015001).
REFERENCES 1. Yang, Y.; Zheng, G.; Cui, Y. Chem. Soc. Rev. 2013, 42, 3018-3032. 2. Ji, X.; Lee, K. T.; Nazar, L. F. Nat. Mater. 2009, 8, 500-506. 3. Wang, H.; Yang, Y.; Liang, Y.; Robinson, J. T.; Li, Y.; Jackson, A.; Cui, Y.; Dai, H. Nano Letters 2011, 11, 2644-2647. 4. Chen, R.; Zhao, T.; Wu, F. Chem. Commun. 2015, 51, 18-33. 5. Chung, W. J.; Griebel, J. J.; Kim, E. T.; Yoon, H.; Simmonds, A. G.; Ji, H. J.; Dirlam, P. T.; Glass, R. S.; Wie, J. J.; Nguyen, N. A.; Guralnick, B. W.; Park, J.; SomogyiÁrpád; Theato, P.; Mackay, M. E.; Sung, Y. E.; Char, K.; Pyun, J. Nat. Chem. 2013, 5, 518-524. 6. Qiu, Y.; Li, W.; Zhao, W.; Li, G.; Hou, Y.; Liu, M.; Zhou, L.; Ye, F.; Li, H.; Wei, Z.; Yang, S.; Duan, W.; Ye, Y.; Guo, J.; Zhang, Y. Nano Lett. 2014, 14, 4821-4827. 7. Xu, R.; Belharouak, I.; Li, J. C. M.; Zhang, X.; Bloom, I.; Bareño, J. Adv. Energy Mater. 2013, 3, 833-838. 8. Wei Seh, Z.; Li, W.; Cha, J. J.; Zheng, G.; Yang, Y.; McDowell, M. T.; Hsu, P.-C.; Cui, Y. Nat. Commun. 2013, 4, 1331. 9. Li, W.; Zhang, Q.; Zheng, G.; Seh, Z. W.; Yao, H.; Cui, Y. Nano Lett. 2013, 13, 5534-5540. 10. Yao, H.; Yan, K.; Li, W.; Zheng, G.; Kong, D.; Seh, Z. W.; Narasimhan, V. K.; Liang, Z.; Cui, Y. Energy Environ. Sci. 2014, 7, 3381-3390. 11. Chen, H.; Wang, C.; Dai, Y.; Qiu, S.; Yang, J.; Lu, W.; Chen, L. Nano Lett., 2015, 15, 5443-5448. 12. Zhao, M. Q.; Peng, H. J.; Tian, G. L.; Zhang, Q.; Huang, J. Q.; Cheng, X. B.; Tang, C.; Wei, F. Adv. Mater. 2014, 26, 7051-7058. 13. Zhang, Q.; Wang, Y.; Seh, Z. W.; Fu, Z.; Zhang, R.; Cui, Y. Nano Lett. 2015, 15, 3780-3786. 13 ACS Paragon Plus Environment
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14. Zheng, G.; Yang, Y.; Cha, J. J.; Hong, S. S.; Cui, Y. Nano Lett. 2011, 11, 4462-4467. 15. Zheng, G.; Zhang, Q.; Cha, J. J.; Yang, Y.; Li, W.; Seh, Z. W.; Cui, Y. Nano Lett. 2013, 13, 1265-1270. 16. Kim, J.; Lee, D. J.; Jung, H. G.; Sun, Y. K.; Hassoun, J.; Scrosati, B. Adv. Funct. Mater. 2013, 23, 1076-1080. 17. Elazari, R.; Salitra, G.; Garsuch, A.; Panchenko, A.; Aurbach, D. Adv. Mater. 2011, 23, 5641-5644. 18. Xin, S.; Guo, Y. G.; Wan, L. J. Acc. Chem. Res. 2012, 45, 1759-1769. 19. Jayaprakash, N.; Shen, J.; Moganty, S. S.; Corona, A.; Archer, L. A. Angew. Chem. 2011, 123, 6026-6030. 20. Manthiram, A.; Fu, Y.; Chung, S. H.; Zu, C.; Su, Y. S. Chem. Rev. 2014, 114, 11751-11787. 21. Zhou, J.; Li, R.; Fan, X.; Chen, Y.; Han, R.; Li, W.; Zheng, J.; Wang, B.; Li, X. Energy Environ. Sci. 2014, 7, 2715-2724. 22. Huang, J. Q.; Zhang, Q.; Peng, H. J.; Liu, X. Y.; Qian, W. Z.; Wei, F. Energy Environ. Sci. 2014, 7, 347-353. 23. Chen, H.; Dong, W.; Ge, J.; Wang, C.; Wu, X.; Lu, W.; Chen, L. Sci. Rep. 2013, 3, 1910. 24. Li, G. C.; Li, G. R.; Ye, S. H.; Gao, X. P. Adv. Energy Mater. 2012, 2, 1238-1245. 25. Chen, R.; Zhao, T.; Lu, J.; Wu, F.; Li, L.; Chen, J.; Tan, G.; Ye, Y.; Amine, K. Nano Lett. 2013, 13, 4642-4649. 26. Zhou, W.; Xiao, X.; Cai, M.; Yang, L. Nano Lett. 2014, 14, 5250-5256. 27. Lin, T.; Tang, Y.; Wang, Y.; Bi, H.; Liu, Z.; Huang, F.; Xie, X.; Jiang, M. Energy Environ. Sci. 2013, 6, 1283-1290. 28. Ji, L.; Rao, M.; Aloni, S.; Wang, L.; Cairns, E. J.; Zhang, Y. Energy Environ. Sci. 2011, 4, 5053-5059. 29. Hassoun, J.; Scrosati, B. Adv. Mater. 2010, 22, 5198-5201. 30. Zhou, G.; Pei, S.; Li, L.; Wang, D. W.; Wang, S.; Huang, K.; Yin, L. C.; Li, F.; Cheng, H. M. Adv. Mater. 2013, 26, 625–631. 31. Huang, C.; Xiao, J.; Shao, Y.; Zheng, J.; Bennett, W. D.; Lu, D.; Saraf, L. V.; Engelhard, M.; Ji, L.; Zhang, J.; Li, X.; Graff, G. L.; Liu, J. Nat. Commun. 2014, 5. 32. Wei, H.; Ma, J.; Li, B.; Zuo, Y.; Xia, D. ACS Appl. Mater. Interfaces. 2014, 6, 20276-20281. 33. Liang, X.; Hart, C.; Pang, Q.; Garsuch, A.; Weiss, T.; Nazar, L. F. Nat. Commun. 2015, 6, 5682. 34. Chung, S. H.; Manthiram, A. Adv. Mater. 2014, 26, 7352-7357. 35. Waite, J. H. Nat. mater. 2008, 7, 8-9. 36. Zu, C.; Manthiram, A. Adv. Energy Mater. 2013, 3, 1008-1012. 37. Lee, S. W.; Yabuuchi, N.; Gallant, B. M.; Chen, S.; Kim, B. S.; Hammond, P. T.; Shao-Horn, Y. Nat Nano 2010, 5, 531-537. 38. Zheng, G.; Lee, S. W.; Liang, Z.; Lee, H. W.; Yan, K.; Yao, H.; Wang, H.; Li, W.; Chu, S.; Cui, Y. Nat Nanotechnol. 2014, 9, 618-623. 14 ACS Paragon Plus Environment
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39. Yu, D.; Goh, K.; Wang, H.; Wei, L.; Jiang, W.; Zhang, Q.; Dai, L.; Chen, Y. Nat Nanotechnol. 2014, 9, 555-562. 40. Urbonaite, S.; Novák, P. J. Power Sources. 2014, 249, 497-502. 41. Zhou, W.; Yu, Y.; Chen, H.; DiSalvo, F. J.; Abruña, H. D. J. Am. Chem. Soc. 2013, 135, 16736-16743. 42. Fu, Y.; Manthiram, A. RSC Advances 2012, 2, 5927-5929. 43. Zhou, G.; Yin, L. C.; Wang, D. W.; Li, L.; Pei, S.; Gentle, I. R.; Li, F.; Cheng, H. M. ACS Nano. 2013, 7, 5367-5375. 44. Vaugheya1, J.T., Liu, G.; Zhang, J. G. MRS Bulletin, 2014, 39, 429-435.
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Insert Table of Contents Graphic and Synopsis Here
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Scheme 1. Schematic of the systematic effect of a lithium sulfur battery covering a PD-S-HCNTs cathode and PD bilaterally coated separator to manipulate the “shuttle effect”.
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Figure 1. (a) Synthesis of PD-S-HCNTs cathode and the mechanism before and after discharge. (b) SEM image of PD-S-HCNTs. (c) Schematic illustration of chemical structure of HCNTs. (d) Molecular nano-sulfur deposited on HCNTs. (e) Images of hydrophobic CNTs, hydrophilic S-HCNTs, hydrophilic PD-S-HCNTs, and scaled synthesis of homogeneous hydrophilic PD-S-HCNTs composite. (f, g) High-resolution TEM images of PD-S-HCNTs. (h) Inside view of the PD-S-HCNTs cathode composite.
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Figure 2. (a) Schematic diagrams of Li anode structure using a PE separator. (b) The modification process of PD bilaterally coated on the separator. SEM image of PE (left) and TEM image (right) of the PD layer used for the lithium sulfur separator. The FT-IR spectra (middle) of PD-S-HCNTs and PD-modified separator are also shown. (c) Schematic diagram of Li anode structure using a PD-modified separator. (d) Li anode surface morphology using a S-HCNTs/PE separator after 200 cycles (left), PD-S-HCNTs/PE separator after 1000 cycles (middle and inset), and PD-S-HCNTs/PD-modified separator after 1000 cycles (right) at 2 C for a Li-S battery.
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Current/mA
(a)
Current/mA
4 3
0.0
-1.0 -1.5 -2.0
2
-2.5
1
-3.0 2.5
2.4
2.3
2.2
1600
1 st 2 nd 100 th 500 th 1000 th 2000 th 3161 th
-0.5
2.1
Capacity(mAh/g)
5
Potential/V
0 -1
scan rate: 0.1mV/s
ip,Ep taken
-2 cycle ip (mA) Ep (V)
-3 -4
1 st 2.193 2.268
3.0
2 nd 2.497 2.276
2.8
100 th 2.253 2.289
2.6
500 th 1.951 2.293
2.4
1000 th 1.96 2.299
2000 th 1.693 2.284
2.2
PD-S-HCNTs/PE charge PD-S-HCNTs/PE discharge
1200 1000
3161 th 0.763 2.296
2.0
800 600 400 200
1.8
PD-S-HCNTs/PD charge PD-S-HCNTs/PD discharge
(b)
1400
S-HCNTs/PE charge S-HCNTs/PE discharge
Discharge (mAh/gS)
S-CNTs/PE charge s-CNTs/PE discharge
0
1.6
0
20
40
60
80
100
Cycle number
Potential/V 2000
(c)
1600
PD-S-HCNTs@PE 0.1C
1200 800 400 0
0
100
200
300
400
500
600
700
800
900
1000
Capacity(mAh/gS)
1400
(d)
(Right axis)
1200 3.2 A g-1
1000
PD-S-HCNTs@PD modified separator
4.8 A g-1
6.4 A g-1
-1
3.2 A g
-1
8.0 A g
800
4.8 A g-1
Stand for 30 days 400
0
100
200
300
100 80 60 40
(Left axis)
600
120
400
20 0 600
500
Cycle number 3.2
3.2
2.8 2.6 2.4
1 st 2 nd 3 rd 4 th 5 th 6 th 11 th
discharge
1.80
1.75
1.70
1 st 2 nd 3 rd 4 th 5 th 6 th 11 th
11 cycles charge
1 st
11 th
880 900 920 940 960 980 1000 1020
Capacity (mAh/g)
2.2
0.31 V
2.0 scan rate： 1.8
(f)
3.0 2.8
Voltage (V)
3.0
1.85
Voltage (V)
(e)
2.6 2.4
11 th 1500cycles 489cycles 100 th 1161 cycles 500 th 1000 th 1500 th charge 2000 th 2500 th 3161 th
2.2
0.23 V
2.0
2C (3200 mA/g)
discharge
1.8
discharge
1.6
1.6 0
200
400
600
800
Capacity (mAh/g)
1000
1200
0
200
400
600
800
1000
1200
Capacity (mAh/g)
Figure 3. (a) CV profile of the PD-S-HCNTs/PD-modified separator coin cell. Capacity retention of Li-S batteries with various configurations at 0.2 C (b). (c) The discharge of the Li-S cell with a PD-S-HCNTs/PE cell at 0.1 C. (d) Capacity of the PD-S-HCNTs/PD-modified separator coin cell at various rates from 2 C to 5 C. Inset: the mechanism associated with the PD membrane. Voltage
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Coulombic effiency (%)
Cycle number
Voltage (V)
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profiles plotted for the 1st to 11th (e) and 11th to 3161st (f) cycles of the PD-S-HCNT/PD cell.
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Figure 4. Top view of Li2S, LiS, S32-, and S42- on CNTs (a), HCNTs (b), and PD (c) surfaces. The calculated corresponding binding energy is also shown. (d) Schematic diagrams for calculating free volume and the average loading (inset table) of sulfides of PE and PD separators. Simulation of the sulfides absorbed in PE (e) and PD (f) separators.
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Figure 5. (a) Cycle performance of the HCNTs-S-PD/PD cell at 2 C. Cathode (b) and lithium metallic (c) surface images of the PD-S-HCNTs/PD coin cells after 0 (①), 500 (②), 1000 (③), 2000 (④), 3000 (⑤) and 3161 (⑥) cycles at 2 C. (d) Proposed mechanism of nanoflower formation. Corresponding SEM image and its elemental mapping profiles after 3161 cycles are also shown.
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Lithium anode of S-CNTs/PE battery after 200 cycles
(b)
C
Counts (a.u.)
(a)
Counts (a.u.)
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O Element Amount(
O
C 18.91
O 26.29
F 42.85
F
S 11.96
S
S
C
Lithium anode of PD-S-HCNTs/PD battery after 1000 cycles
O
Element C Amount(% 20.17
O 42.41
F 34.17
S 2.65
S
O
C 0
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C 1
2
3
Energy/(KeV)
4
5
0
F
S 1
2
3
4
5
Energy/ KeV
Figure 6. SEM images of the lithium anode of an S-CNTs/PE cell after 200 cycles (a) and of a HCNTs-S-PD/PD cell after 1000 cycles (b). Their corresponding EDX mappings are also shown, which display the distribution and content of carbon, oxygen, fluorine, and sulfur.
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