A Polysulfide-Trapping Interface for Electrochemically Stable Sulfur

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A polysulfide-trapping interface for electrochemically stable sulfur cathode development Sheng-Heng Chung, Pauline J. Han, and Arumugam Manthiram ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12012 • Publication Date (Web): 29 Jan 2016 Downloaded from http://pubs.acs.org on February 2, 2016

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ACS Applied Materials & Interfaces

A polysulfide-trapping interface for electrochemically stable sulfur cathode development Sheng-Heng Chung, Pauline Han, and Arumugam Manthiram  Materials Science and Engineering Program & Texas Materials Institute The University of Texas at Austin, Austin, TX 78712, USA Abstract Lithium-sulfur (Li-S) cells have a strong edge to become as an inexpensive, highcapacity rechargeable battery system. However, currently several prohibitive challenges occur within the sulfur core, especially the polysulfide-diffusion problem. To address these scientific issues, we present here a boron-doped multiwall carbon nanotube coated separator (B-CNTcoated separator). The B-CNT-coated separator creates a polysulfide trap between the pure sulfur cathode and the polymeric separator as a “polysulfide-trapping interface,” stabilizing the active material and allowing the dissolved polysulfides to activate the bulk sulfur cores. Therefore, the dissolved polysulfides change from causing fast capacity fade to assisting with the activation of bulk sulfur clusters in pure sulfur cathodes. Moreover, the hetero-atom-doped polysulfidetrapping interface is currently one of the missing pieces of carbon-coated separators, which might inspire further studies in its effect and battery chemistry. Li-S cells employing B-CNTcoated separators (i) exhibit improved cyclability at various cycling rates from 0.2C to 1.0C rate and (ii) attain a high capacity retention rate of 60% with a low capacity fade rate of 0.04% cycle1

after 500 cycles. We believe that our B-CNT-coated separator could light up a new research

area for integrating hetero-atom-doped carbon into the flexible, light-weight, carbon-coated separator. Keywords: lithium-sulfur batteries, electrochemistry, separator, cell configuration, coating

*

Corresponding author. Tel: +1-512-471-1791; fax: +1-512-471-7681.

E-mail address: [email protected] (A. Manthiram)

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1. Introduction The sulfur cathode has many advantages including a high theoretical charge-storage capacity (1675 mA h g-1), low production cost, and materials available in high abundance. These features give lithium-sulfur (Li-S) batteries an edge as an inexpensive, high-capacity rechargeable battery.1,2 However, there are currently several prohibitive challenges that occur within the sulfur core. Commercialization of Li-S batteries is hindered by the low electrochemical utilization of sulfur due to its insulating nature and the electrochemical instability due to severe polysulfide diffusion (Li2Sx: x = 4 – 8) during intermediate discharge/charge states.2-5 The polysulfide diffusion increases the polarization within the cathode as the propagating polysulfides build gradually on the electrode surface as nonconductive Li2S2/Li2S agglomerates.5-8 Over the last few years, the insulating nature of the active material has been mostly resolved by encapsulating sulfur into numerous conductive/porous carbon matrices.2,5,9,10 A high publication dynamics in engineering carbon hosts,9,10 hetero-atom-doped carbons,9-14 and porous/conductive hybrid carbons9,10,13,15-18 has improved polysulfide retention with numerous sulfur-carbon nanocomposites. However, the conventional cell configuration may inhibit further progress with sulfur-carbon nanocomposites.2,3,19-21 The conventional cell configuration is unable to use the sulfur cathode to its highest potential because the high capacity of sulfur is generated through

a

unique

electrochemical

sulfur(solid)-polysulfides(liquid)-sulfides(solid)

conversion

reaction.1,6-8 The polysulfide intermediates readily dissolve into the organic electrolytes currently used in Li-S cells. The polysulfide dissolution prompts the rearrangement of active material during the initial cycling process, allowing the rearranged active martial to pick electrochemically favorable positions.2,22-24 However, this means that the active material trapped


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within pockets of any nanocomposites might be pulled into the electrolyte from their engineered sulfur cages, resulting in irreversible active-material loss.6-8 A new platform for developing Li-S cells is to better adopt rather than suppress the inherent materials characteristics.2,20,23 Therefore, a large effort has been poured into stabilizing the active material in a porous carbon current collector. The conductive/porous carbon matrix can then function as an inner electron pathway and as an active-material stockroom during the lithiation/delithiaton of sulfur cathodes.4,21,27-30 There has been a focused effort on improving polysulfide entrapment in order to ameliorate the capacity fade rate (< 0.20% cycle-1).27-32 As an example, carbon interlayers3,4,26-30 and carbon-coated separators4,5,31,32 both create a polysulfide trap between the sulfur cathode and the polymeric separator as a “polysulfide-trapping interface.” 3-5

The optimal design of a polysulfide-trapping interface attains high tortuosity within an

optimized thickness of carbon interlayers27-29 or a fine-tuned porosity within carbon coatings.24,31,32 The high tortuosity of carbon network creates difficult polysulfide-transport pathways while ascertaining good conductive pathways with low resistivity.17,27-32 Therefore, the conductive polysulfide-trapping interface facilitates cathode active-material stabilization and hence allows us to be one step closer to the realization of Li-S batteries.3,27-30 We have demonstrated before that employing carbon-coated separators with simple fabrication techniques23,24 and tunable nanopore sizes24,31 can effectively improve polysulfidetrapping ability.31,32 The integration of the polysulfide-trapping interface with a polymeric membrane provides the resulting carbon-coated separator with remarkable mechanical strength.24,31,32 To the best of our knowledge, the investigation into polysulfide-trapping interface consisting of hetero-atom-doped carbon has thus far not been reported. Also, in comparison to the extensive discussion of nitrogen-doped carbon, which is capable of enhancing the



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conductivity and redox accessibility of Li-S battery chemistry,11,13,14,33 studies employing borondoped carbon remain relatively rare. The first boron-doped carbon utilized in Li-S research was hetero-atom-doped carbon hosts by the Guo group.12 Their boron-doped carbon (0.93% Boron content) possesses (i) a high conductivity based on the same mechanism as in nitrogen-doped carbon and (ii) a chemisorption toward polysulfides because of a lower electronegativity of boron over carbon.12,33,34 In consideration of these two reasons, we look into the multilayer boron-doped polysulfide-trapping interface in order to gain insight concerning the electrochemical stability of pure sulfur cathode. With this boron-doped multiwall carbon nanotube (B-CNT) coated separator study, a more comprehensive picture can be obtained on the development of carbon-coated separators to match the abundant sulfur-carbon nanocomposite studies that have continuously attracted progress since 2009. In addition to employing lightweight carbon coatings as the multilayer polysulfide-trapping interface, a ceramic coating layer and a Nafion coating layer have also been able to inhibit the severe polysulfide diffusion.35-38 Moreover, besides functioning as the polysulfide-trapping interface, an aluminum oxide coating with a thickness of 4 μm has served as an ion-conducting skeleton for the deposition of the dissolved polysulfides.35 A montmorillonite coating with a thickness of 25 μm (mass of 1.65 mg cm-2) introduced repulsive forces toward polysulfide anions and made the functionalized membrane as an ion selective separator.36 Several successful studies, on the other hand, have reported the use of a Nafion coating layer onto various commercial separators (Celgard) as an ion selective interface. The Nafion layers coated onto a monolayer polypropylene membrane (Nafion coating: thickness = 1 – 5 μm; mass = 0.25 mg cm-2) and a composite polyethylene/polypropylene/polyethylene membrane (Nafion coating: thickness = ~ 1 μm; mass = 0.7 – 3.5 mg cm-2) turned the porous network of the Celgard separators into


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abundant SO3-groups-coated channels. These ion-selective channels allow the hopping of lithium ions but block polysulfide anions, which inhibits the migration of polysulfides from the cathode side of the cell to the anode side.37,38 In this paper, we investigate the feasibility of a functional B-CNT coating on a polypropylene separator as the polysulfide-trapping interface in Li-S cells. By way of limiting the contribution from surface area (42.5 m2 g-1), pore volume (0.23 cm3 g-1), and micropores (micropore volume: 0.01 cm3 g-1), the B-CNT-coated separator mainly exhibits synergy occurring between the interwoven multilayer coatings and the B-CNTs. The lightweight multilayer B-CNT coating provides the cathode/separator interface with high tortuosity and, therefore, makes it difficult for polysulfides to migrate, forcing their entrapment.31,32 The BCNTs are able to provide (i) a polar surface for chemisorbing the entrapped polysulfides12,33 and (ii) a fast electron-transfer interface for reactivating the trapped active material.33 With the enhanced polysulfide-retention capability, the functional multilayer B-CNT coating with a thickness of 6.5 μm and mass of 0.06 mg cm-2 is able to stabilize the pure sulfur cathode for a long lifespan of 500 cycles.

2. Experimental section Boron-doped-MWCNT-coated separator fabrication The thin-film B-CNT-coated separator was prepared by the same vacuum-filtration process that we used previously and suggested for fabricating a fibrous polysulfide filter onto polypropylene membranes (Celgard).31 The commercial Celgard 2500 separator has a thickness of 25 μm with high porosity. The B-CNT suspension containing 10 mg of B-CNTs (graphite: 95%, boron doping: 1 – 2%, average length: 50 µm, diameter: 20 – 40 nm, NanoTechLabs Inc.)



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and polyethylene glycol (average molecular weight: 300, Aldrich) in a 1:1 mass ratio was dispersed in 20 mL of isopropyl alcohol (IPA) and mixed overnight. The uniform mixture was then dispersed in additional 500 mL of IPA by high-power ultrasonication for 10 min. The black suspension was then vacuum-filtered onto polypropylene membranes, forming B-CNT-coated separators. The resulting B-CNT-coated separator was dried at 50 °C for 24 h in an air-oven. The used IPA was recycled for next batch utilization. The thin-film B-CNT-coating that was tightly attached to one side of polypropylene membranes had a thickness of 6.5 μm, weight of 0.06 mg cm-2, and surface electrical resistivity of 7.4 ohm square-1. The other side of the B-CNT-coated separator remained a bare polypropylene membrane with surface electrical resistivity of over megaohms square-1, serving as the electrically insulating membrane and an ultra-tough, flexible support. The resulting ultratough B-CNT-coated separators had outstanding flexibility and ductility, and remarkable adhesion between the B-CNT coating and the polypropylene membrane. The thin-film CNTcoated separator (graphite: 95%, average length: 70 – 80 µm, diameter: 20 – 40 nm, NanoTechLabs Inc.) prepared by the same vacuum-filtration process was utilized as the counterpart in the comparative electrochemical analysis.

Microanalysis and materials characterization The microstructural, morphological, and elemental analyses of the B-CNT-coated separator and sulfur cathodes were inspected by a scanning transmission electron microscopy (STEM, Hitachi S5500) and a field emission scanning electron microscope (FE-SEM) (Quanta 650 SEM, FEI) equipped with an energy dispersive X-ray spectrometer (EDX) for detecting


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elemental signals and collecting elemental mapping signals. The post-cycling analyses of the separators and cathodes were conducted at charged state. Specifically, sample cells employing BCNT-coated separator and CNT-coated separator were charged to 2.7 V and stopped at the 500th cycle. The cycled B-CNT-coated separators and cycled cathodes in sample cells were used for the inspection of the morphological and elemental changes. On the other hand, the control cells with conventional polypropylene separator were charged to 2.7 V and stopped at the 100th cycle. The cycled polypropylene separators and cycled cathodes in control cells were utilized as the reference for demonstrating the improvements from the use of B-CNT-coated separators. The cycled separators and cathodes were retrieved inside an argon-filled glove box, rinsed with a salt-free blank electrolyte for 5 min, removed the DOL/DME lotion by Kimwipes, and transported in an argon-filled sealed vessel. The prepared SEM samples were delivered within 30 min prior to microstructural and elemental analysis. The DOL/DME lotion was a saltfree blank electrolyte containing only the 1:1 volume ratio of DME/DOL solution. Nitrogen adsorption-desorption isotherms, pore-size characteristics, and porosity analyses were collected with an automated gas sorption analyzer (AutoSorb iQ2, Quantachrome Instruments) at -196 °C. Samples were outgassed at 200 °C for 360 min under the fine-powder model. Surface area was measured with a 7-point Brunauer-Emmett-Teller (BET) analysis. Poresize distributions and pore volumes were determined by the Barrett-Joyner-Halenda method (BJH, detected pore size: 3 – 300 nm), Horvath-Kawazoe method (HK, detected pore size: 0.3 – 2 nm), and a density functional theory model (DFT, detected pore size: 0.7 – 35 nm). Micropore analysis was conducted by Dubinin-Astakhov (DA) / Dubinin-Radushkevich (DR) method micropore analyses.



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Raman microscopy was performed with a WITEC Alpha300 S micro-Raman System. A 488 nm Ar laser and a 100X objective were used. Surface electrical resistivity of the B-CNTcoated separator and polypropylene membrane were measured with a resistivity system (Pro4, Lucas Labs). The measurement was based on a four-wire sense-mode configuration. A fourpoint-probe head (SP4, Lucas Labs) and a source meter (Model 2400 general-purpose sourcemeter, Keithley) were used. The analysis of the surface elements and corresponding chemical bonding states of the cycled samples was examined with X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos Analytical). The spectrometer was equipped with a monochromatic Al Ka X-ray source. The XPS samples were rinsed with a salt-free blank electrolyte for 5 min, the DOL/DME lotion was removed by Kimwipes, and transported into an argon-filled sealed chamber.

Electrochemical analyses Electrochemical analyses were based on CR2032-type coin cells that were assembled with a pure sulfur cathode, a B-CNT-coated separator / (a bare Celgard separator and a CNTcoated separator for control cells in the comparative experiments), a lithium anode (Aldrich), and a nickel foam spacer (Pred Materials, Inc.). The B-CNT-coated separator was arranged with the functional polysulfide-trapping interface adjacent to the pure sulfur cathode and attached on it. Cell components, excluding the lithium anode, were dried in a vacuum oven for 60 min at 50 °C prior to cell assembly in an argon-filled glove box. The sulfur cathode contained 70 wt. % pure sulfur powder, 15 wt. % Super P carbon (TIMCAL), and 15 wt. % polyvinylidene fluoride binder (Grade No. L#1120, Kureha). The


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mixtures were stirred in N-methyl-2-pyrolidone (NMP, Sigma-Aldrich) for two days and then tape-casted onto an aluminum foil current collector (Kenkut) with an automatic film applicator (1132N, Sheen) at a traverse speed of 50 mm s-1, followed by evaporation of the NMP solvent for 24 h at 50 °C in an air oven. The sulfur cathode has pure sulfur as the active material with a corresponding loading of 2.4 – 2.5 mg cm-2 and with a sulfur mass of 2.7 – 2.8 mg cathode-1. The same amount of blank electrolyte was added to the sample cells and control cells as reported in our previous work.31 The blank electrolyte followed our previous work and was prepared by dissolving 1.85 M LiCF3SO3 salt (Acros Organics) and 0.1 M LiNO3 co-salt (Acros Organics) in a 1:1 volume ratio of 1, 2-dimethoxyethane (DME, Acros Organics) and 1, 3-dioxolane (DOL, Acros Organics). Electrochemical impedance spectroscopy (EIS) data were collected with a computerinterfaced impedance analyzer in the frequency range of 106 to 10-1 Hz and an amplitude perturbation of 5 mV. The impedance analysis system has a potentiostat (SI 1287, Solartron) coupled with an impedance analyzer (SI 1260, Solartron). Cyclic voltammetry (CV) measurements were evaluated with a universal potentiostat (VoltaLab PGZ 402, Radiometer Analytical). In consideration of the initial activation process (11 – 16 cycles), cells were first cycled for 20 cycles between 1.8 and 2.7 V and then the voltammograms were recorded at various scanning rates of 0.1, 0.2, and 0.5 mV s-1 and with a potential range between 1.8 and 2.8 V. Discharge/charge profiles and cyclability data were collected with a programmable 96 channel battery cycler (Arbin Instruments). The cells were initially rested for 30 min and then discharged to 1.8 V and charged to 2.7 V for a full cycle. The complete electrochemical cycling



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performance was investigated at 0.2C, 0.5C, and 1.0C rates for 500 cycles. The cycling rates were calculated based on the mass and theoretical capacity of sulfur. The calculation of the capacity fade rate and the capacity retention rate was based on the reversible discharge capacity (500 cycles) and the peak discharge capacity. This calculation method ensured that the improved cycling performance was not caused by the low utilization of sulfur in the first cycle. The activation process is discussed in detail below. The capacities of the upper-discharge plateau (QH) and the lower-discharge plateau (QL) of the cells were captured from data points in the discharge curves. The theoretical values of QH and QL are, respectively, 419 and 1256 mA h g-1. The sample cells employing B-CNT-coated separators and the control cells employing conventional polypropylene separators were investigated for, respectively, 500 and 100 cycles.

3. Results and Discussion 3.1. Configuration and characterization analyses Figure 1a and 1b show the scanning electron microscopy (SEM) observation of, respectively, the commercial B-CNTs (graphite: 95 %, boron doped: 1 – 2 %, NanoTechLabs Inc.) and the commercial polypropylene membrane (Celgard). The B-CNTs show an average length of over 50 µm (Figure S1a, Supporting Information), favorable for fabricating a longrange carbon network.31,32 Microanalysis and materials characteristics summarized in Figure S1 and S2 demonstrate that B-CNTs have limited surface area and micro-/meso-pores, which could reduce the polysulfide-trapping capability resulting from an optimized porosity.24,32 B-CNTs were coated onto polypropylene membranes through a vacuum-filtration process (Figure 1c). The B-CNT coating had a uniform thickness of 6.5 μm (Figure 1d and S3a, Supporting Information). The in-plane SEM figures (Figure 1e and S3b, Supporting Information) reveal a smooth coating that is composed of homogeneous interwoven B-CNTs. The thin-film 10 Environment ACS Paragon Plus

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coating converts the cathode side of the polypropylene membrane into a conductive interface with the surface electrical resistivity dropping down from over megaohms square-1 to 7.4 ohm square-1. This conductive interface directed toward the sulfur cathode improves the overall redox accessibility in the cathode.27-32 The hierarchical B-CNT framework consists of multilayer thinfilm coatings, creating a long-range porous architecture.5,31,32 As a result, a conductive and tortuous polysulfide-trapping interface is attached onto one side of the flexible polypropylene membrane. With the polysulfide-trapping interface arranged adjacent to the pure sulfur cathode, the B-CNT coating stabilizes the dissolved polysulfides within the cathode region of the cell.4,31,32 The strong adhesion of the coating layer on the polypropylene membrane and the good mechanical strength of the B-CNT-coated separators are proved in Figure S4 (Supporting Information). The multilayer B-CNT coating did not peel off and flake during and after the solvent washing and rinsing (isopropyl alcohol (IPA) and acetone). This demonstrates the remarkable adhesion between the polysulfide-trapping interface and the polypropylene membrane. Such strong adhesion allows the crumpled B-CNT-coated separator to retain a uniform B-CNT coating and, more importantly, allows the B-CNT-coated separator to achieve the same outstanding mechanical strength as the Celgard separator,31,32 as shown in Figure S4b (Supporting Information).

3.2. Morphological, microstructural, and elemental analyses In Figure 2a – 2c, the SEM images of post-cycled B-CNT coatings (500 cycles, charged state) indicate that the hierarchical B-CNT framework was preserved during electrochemical cycling, but the corresponding energy dispersive X-ray spectroscopy (EDX) elemental mapping



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results demonstrate stronger sulfur signals than the previously acquired EDX results of a fresh BCNT coating. Such uniform sulfur signals distributed on the B-CNT coating after cycling is a confirmation of a strong performance in trapping polysulfides at the interface.2,32 The STEM images with bright field (BF, left) and dark field (DF, right) detections for, respectively, microstructure observation and light/heavy element analysis agree with this result and further reveal the polysulfide-trapping condition. In Figure S5 (Supporting Information), the BF-STEM images and the corresponding DF detection of the uncycled B-CNTs and cycled B-CNT interface evidence that the migrating active material (heavy elements) was trapped by the polysulfide-trapping interface. The trapped active material could either recrystallize onto or infiltrate into the conductive B-CNT interface. Therefore, the trapped active material could be continuously utilized in the subsequent electrochemical cycles. On the other hand, in the SEM images of the cycled B-CNT coatings, the strong carbon signals are indicative of the unblocked electron pathways while the distinguishable oxygen and fluorine mapping results confirm proper electrolyte penetration.28,31,32 As we can predict, the balance among electrons, electrolyte, and the trapped active material allows the successive reutilization of any trapped sulfur-containing species during cycling.28 Perhaps most interestingly, the intensity of the detected sulfur signals and their corresponding mapping results show a tendency that decreases as the cycling rate increases. This might be due to an insufficient amount of time for dissolved polysulfides to diffuse out from the pure cathode while the cell is cycled at a higher rate.2,31 The corresponding low-/high-magnification inspections summarized in Figure S6, Supporting Information, agree with this tendency. The cycled cathodes (500 cycles, charged state) shown in Figure 2d – 2f exhibit similar tendency, illustrating more cavities on the cathode cycled at 0.2C rate over those cycled at 1.0C


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rate. The lack of agglomerated active material deposited on the cathode surface is a common characteristic found on cycled cathodes that were protected by B-CNT coatings. The polysulfidetrapping interface guarantees fast electron transport, smooth Li+-ion penetration, and good electrolyte wetting for facilitating the electrochemical conversion procedure.17,31,32 The above-mentioned advantages resulting from the use of B-CNT coatings are highlighted by comparing the cycled cathodes employing B-CNT-coated separators (0.2C rate, 500 cycles, charged state, Figure 2d) and conventional separators (0.2C rate, 100 cycles, charged state, Figure 2g). The surface of the cycled cathode with a bare Celgard separator has large cavities (marked in blue) caused by the loss of the active material from the cathode as compared to the cycled cathode protected by the B-CNT-coated separator (Figure 2d) and the uncycled pure sulfur cathode (Figure S7, Supporting Information).6,7,22 In addition, the diffusing polysulfides could redeposit on the surface of the cathode as a precipitation (marked in red).6,22 The activematerial loss and the inactive precipitation lead to the fast capacity fade of the cell using pure sulfur cathode and a bare Celgard separator.6-8

3.3. Electrochemical analyses and cell performance The B-CNT network introduces chemisorption and multilayer trapping interface toward polysulfides. In addition to the polysulfide-trapping capability, the conductive B-CNT coating improves the electrochemical reaction capability of pure sulfur cathodes.12,37,38 Thus, Figure 3a – 3c show the overlapping discharge/charge curves of cells employing B-CNT-coated separators at various cycling rates over 500 cycles. During the discharge process, the upper-discharge plateau at 2.3 V demonstrates the reduction of sulfur to polysulfides.7,17 The overlapping upper-discharge plateaus are retained so that the utilization and retention rates of the corresponding upper-plateau



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discharge capacity (QH) are, respectively, 81% at 0.2C rate and above 53% after 500 cycles (Figure 3d and 3f), indicating the excellent polysulfide retention.23,26 The lower-discharge plateau at 2.1 V corresponds to the transformation from polysulfides to Li2S2/Li2S.6,7,26 The long, flat lower-discharge plateaus are maintained and are able to achieve a high retention rate of the corresponding lower-plateau discharge capacity QL (RQL > 65%, Figure 3e and 3f). These curves demonstrate that the cathode exhibits a thorough reduction of the trapped active material and excellent electrochemical reversibility.11,13 During cell charge, the two continuous charge plateaus at 2.3 – 2.4 V represent the oxidation reactions from Li2S2/Li2S reverting back to Li2S8/S.25,26 Similar electrochemical reversibility is shown in cells cycled at 0.5C and 1.0C rates. Although the polarization increases with the cycling rate, the electrochemical barrier has insignificant ramifications to the electrochemical reversibility. It is found that an activation process (11 – 16 cycles) is necessary for the cells coupling the light-weight B-CNT coatings (0.06 mg cm-2) and pure sulfur cathodes (2.41 mg cm-2) (Figure 3d, 3f, and S8a, Supporting Information).8,12 During these initial activation processes, sulfur particles convert to polysulfides which become electrochemically active while they are dissolved into the electrolyte.2,8 The dissolved polysulfides are stabilized within the cathode region of the cell because of the functional polysulfide-trapping interface and hence are able to assist with the activation of bulk sulfur clusters in pure sulfur cathodes.12,31,32 As a result, the cell performance exhibits an increasing tendency in the electrochemical utilization of sulfur and a high Columbic efficiency (over 100%). After the initial activation processes, the cells cycled at various cycling rates attain the peak capacities and retain stable Columbic efficiency (99 ± 1 %) in the subsequent electrochemical cycling. The polysulfide dissolution and entrapment in the architectural web allow for the rearrangement of the remaining active material to go into


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electrochemically stable sites.19,24 The activation of sulfur clusters12,31,32 and the in-situ activematerial rearrangement19,24 reduce the charge-transfer resistance from 110 ohm (before cycling) to 40 – 60 ohm (after 500 cycles, Figure S8b, Supporting Information) and, therefore, extends the stable cell cyclability to over 500 cycles. Figure 3g shows the cycling performance of the cells configured with the B-CNT-coated separators at 0.2C, 0.5C, and 1.0C rates. The resulting cells are able to power a white lightemitting diode (LED). The Li-S cell upgraded with a polysulfide-trapping interface improves the first discharge capacity from 550 to 675 mA h g-1 at 0.2C rate. The activation process for sulfur cores during the initial 11 cycles can be seen. These demonstrate a 23% improvement on the initial electrochemical utilization of bulk sulfur and a 26% additional capacity contribution from the activated sulfur core.8,12 Thus, the peak discharge capacity attains 849 mA h g-1, corresponding to a volumetric capacity of 526 mA h cm-3, and an electrode capacity approaching 600 mA h g-1 (Figure S9, Supporting Information). Control cells with polypropylene membranes also show the activation process, but the severe polysulfide diffusion triggers the cathode degradation.6,23 The activation process ceases and is replaced with the fast capacity fade in just 3 cycles (Figure S10, Supporting Information).6,7 Moreover, the cycle life of cells employing B-CNT-coated separators is extended from 63 cycles (70 h, control cell) to 500 cycles (1810 h, sample cell) with a high capacity retention rate of 60% and a low capacity fade rate of 0.04% cycle-1. The 8-time enhancement of the cyclability and the corresponding 25-time improvement of the operation time are based on the same capacity retention level (60% of the peak discharge capacity). In addition, the high conductivity of B-CNT coating allows the cell to operate stably at 0.2C and 1.0C rates over 500 cycles. The



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corresponding high capacity retention rates are above 60% and the corresponding capacity fade rates are as low as 0.03% cycle-1. The comparative electrochemical analysis shown in Figure 3h compares the cyclability of the cells employing the B-CNT-coated separators and the CNT-coated separators. The nonporous CNTs have a surface area of 44.85 m2 g-1, pore volume of 0.38 cm3 g-1, and micropore volume of 0.01 cm3 g-1. The nonporous CNTs utilized in the CNT coating have a similar microstructure and morphology as compared to the nonporous B-CNTs. Although the use of nonporous CNTs as the polysulfide-trapping interface still improves the cell performance, the lack of high porosity,28,29 micropore traps,24,31 and chemisorption sites on the polysulfide-trapping interface limit the improvement on the long-term cycle stability. Without the chemisorption sites, the cells employing the CNT-coated separators exhibit longer activation process and lower capacity retention rate as compared to the cells employing the B-CNT interface. The electrochemical data are summarized in Table S1 (Supporting Information). The effect of the chemisorption sites was further investigated by X-ray photoelectron spectroscopic (XPS) analysis focusing on the sulfur spectra. The XPS data of the cycled B-CNT and CNT coatings indicate that the binding energy between B-CNTs and the sulfur species slightly increases as compared to that between CNTs and the trapped active material (Figure S11, Supporting Information). The increase in binding energy has been found in the case of B-doped carbon/sulfur nanocomposite.12 This signifies the good affinity towards the dissolved polysulfides and the strong interaction with the trapped active material contributed by the boron modified polarized surface. As a result, the B-CNT coatings could provide the cells with better long-term cycle stability than the CNT coatings at various cycling rates.


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3.4. Effect of the polysulfide-trapping interface with different configurations In addition to studying the different separators and coating materials, the polysulfidetrapping interfaces with three different configurations were investigated. The first configuration is the original configuration. The cathode side B-CNT coating has the B-CNT coating placed on the cathode side of the polypropylene membrane. The second configuration, in contrast, has the B-CNT coating on the anode side of the polypropylene membrane, which is the anode side BCNT coating. The third configuration has the B-CNT interface sandwiched in between the two layers of polypropylene membranes, named as the sandwiched B-CNT interface. Figure 4a and 4b show the cyclability and discharge/charge voltage profiles of the cells employing the polysulfide-trapping interface with various configurations. The electrochemical performances reveal that the cathode side B-CNT coating (black line) and the sandwiched BCNT interface (blue line) have very similar cycling performance and electrochemical characteristics, while the anode side B-CNT coating (red line) could not operate successfully in the cells. The similar cyclability between the first and the third configurations demonstrates that the suppressed polysulfide is mainly enabled by the adsorption and inhibition effects from the BCNT interface. On the other hand, the good conductivity of the polysulfide-trapping interface should improve the redox reaction of the trapped active material so that they are able to be utilized in the following electrochemical cycles rather than becoming inactive precipitation on the polysulfide-trapping interface.

3.5. Electrochemical analyses and mathematical calculation Figure 4c displays the cyclic voltammogram (CV) results at various scanning rates (0.1, 0.2, and 0.5 mV s-1). In consideration of the initial activation process, the cells were first cycled for



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20 cycles between 1.8 and 2.7 V and then the CV plots were collected between 1.8 and 2.8 V, which might reach a better and fair analysis. The CV plots display two cathodic peaks (C1: 1.9 – 2.0 V and C2: 2.3 – 2.4 V) and continuous anodic peaks (A1: 2.3 – 2.5 V) at various scanning rates, which are in agreement with the discharge/charge voltage profiles (Figure 3a – 3c) and shows a two-step redox reaction during the lithiation/delithiaton processes of sulfur cathodes.6,7 Figure 4b shows the mathematical calculation of the diffusion coefficient (DLi+). The corresponding values of DLi+ are DLi+ (C1) = 6.6 x 10-9 cm2 s-1, DLi+ (C2) = 7.8 x 10-9 cm2 s-1, and DLi+ (A1) = 3.3 x 10-8 cm2 s-1. The experimental DLi+ of the cell employing the B-CNT-coated separator is close to the value previously reported in the cell employing a bare Celgard separator (DLi+ = 2 x 10-8 – 9 x 10-9 cm2 s-1).39 This indicates that the B-CNT coating might block only the severe polysulfide diffusion rather than the smooth Li-ion diffusion.38 As a reference, the abovementioned DLi+ is based on the CV plots at various scanning rates and is calculated via the Randles-Sevick equation: Ipeak = 268600 × e1.5 × Area × DLi+ 0.5 × Concn.Li × rate0.5 . In the equation, Ipeak is the peak current (A), e is the number of electrons, Area is the cathode area (cm2), Concn.Li is the concentration of Li+ added in the electrolyte (mol mL-1), and rate is the CV scanning rate (V s-1). The Ipeak and the rate0.5 in the CV at various scanning rate are calculated and then linear fitted.

3.6. Polysulfide-trapping test Figure 5 shows a series of digital images of the polysulfide-trapping test. Two different ultra-high-concentration polysulfide cores (1.0M, 2 mL) were sealed by either the B-CNT-coated separator (left) or the bare polypropylene separator (right), and immersed into a blank electrolyte


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solution (20 mL) with the orifice down (Figure S12, Supporting Information). It shows the polysulfide leaking starts in 5 min in the case of the bare polypropylene separator. The B-CNTcoated separator limited the polysulfide leaking for over 12 h and remained with low polysulfide leaking after 96 h. Moreover, the anode side of the B-CNT-coated separator remained white and clean after 100 h. However, the anode side of the bare polypropylene separator was contaminated by polysulfide and turned into yellow. Such severe test demonstrates that the B-CNT coating sufficiently reduces dissolved polysulfides from penetrating through the separator membrane. It is worth to notice that the B-CNT coating retained a complete and uniform surface coating after being crumpled onto the orifice of polysulfide core, soaked in polysulfides, and torn from the polysulfide core. This reconfirms its outstanding ductility.

4. Conclusion In summary, the functional B-CNT-coated separator effectively improves the cycling stability of Li-S cells employing pure sulfur cathodes. The electrochemical cycling exhibits a stable reversible discharge capacity of 509 mA h g-1 with a high capacity retention rate of 60% and a low capacity fade rate of 0.04% cycle-1 after 500 cycles. The excellent cycle stability arises from two major factors: (i) the hierarchical B-CNT network and the polarized surface creating a functional polysulfide-trapping interface and (ii) the long-range carbon network serving as a conductive interface during electrochemical cycling. As a result, the B-CNT-coated separator lights up a new research area for integrating hetero-atom-doped carbon into the flexible, lightweight, carbon-coated separator.



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ASSOCIATED CONTENT Supporting Information Microanalysis of commercial B-CNTs and B-CNT-coated separators before and after cycling (500 cycles), electrochemical analysis of the cells employing B-CNT-coated separators, cell performance of the cells employing B-CNT-coated separators, electrochemical analysis of the cells employing the bare Celgard separator, and schematic configuration of polysulfide-trapping test. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Science Foundation Nanosystems Engineering Research Center (NERC) for Nanomanufacturing Systems for Mobile Computing and Mobile Energy Technologies (NASCENT) through award number EEC-1160494.


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FIGURE CAPTIONS

Figure 1 Microstructural analysis: (a) B-CNTs, (b) polypropylene membrane, (c) schematic configuration of B-CNT-coated separator, (d) cross-sectional SEM of B-CNT-coated separator, and (e) in-plane SEM of (d).

Figure 2 Microstructural analysis: SEM of cycled B-CNT-coated separators at various cycling rates of (a) 0.2C, (b) 0.5C, and (c) 1.0C. SEM of cycled cathodes with B-CNT-coated separator at various cycling rates of (d) 0.2C, (e) 0.5C, and (f) 1.0C. (g) SEM of cycled cathodes with conventional polypropylene separator at 0.2C rate.

Figure 3 Electrochemical analysis and cell performance: Discharge/charge profiles of Li-S cells employing B-CNT-coated separators at various cycling rates of (a) 0.2C, (b) 0.5C, and (c) 1.0C. (d) QH analysis, (e) QL analysis, (f) RQH and RQL analysis, and (g) cyclability (inset is the LED powered by the resulting cell) of Li-S cells employing B-CNT-coated separators. (h) Comparative electrochemical cyclability of the Li-S cells employing B-CNT-coated separators and CNT-coated separators at various cycling rates.

Figure 4 Configuration analysis of various B-CNT-coated separators: (a) electrochemical cyclability and (b) discharge/charge profiles of the Li-S cells employing B-CNT-coated separators with different interface configurations (black line: the cathode side B-CNT coating;



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red line: the anode side B-CNT coating; blue line: the sandwiched B-CNT interface). Electrochemical analysis: (c) CV plots of the Li-S cells employing B-CNT-coated separators at 0.1, 0.2, and 0.5 mV s-1 and (d) the linear fitting of CV curves.

Figure 5 Polysulfide-trapping test with a polysulfide core.


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 5



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TABLE OF CONTENTS


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A Polysulfide-Trapping Interface for Electrochemically Stable Sulfur

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